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The Skiddaw Group of the English Lake District Memoir for parts of sheets 22 Maryport, 23 Cockermouth, 24 Penrith, 28 Whitehaven, 29 Keswick, 30 Appleby, 31 Brough and 48 Ulverston

By A H Cooper, N J Fortey, R A Hughes, S G Molyneux, R M Moore, A W A Rushton, P Stone

Bibliographic reference: Cooper, A H, Fortey, N J, Hughes, R A, Molyneux, S G, Moore, R M, Rushton, A W A, and Stone, P. 2004. The Skiddaw Group of the English Lake District. Memoir of the British Geological Survey.

British Geological Survey

The Skiddaw Group of the English Lake District Memoir for parts of sheets 22 Maryport, 23 Cockermouth, 24 Penrith, 28 Whitehaven, 29 Keswick, 30 Appleby, 31 Brough and 48 Ulverston

A H Cooper, N J Fortey, R A Hughes, S G Molyneux, R M Moore, A W 2A Rushton, P Stone

Contributors

P M Allen, D C Cooper, J A Evans, S R Hirons, B C Webb

Keyworth, Nottingham: British Geological Survey 2004 © NERC copyright 2004 First published 2004. Printed in the UK for the British Geological Survey by Halstan & Co Ltd., Amersham.

The topographic lines used on figures and maps and the National Grid are taken from the 1:50 000Ordnance Survey maps 89 (1989), 90 (1988), 91 (1990) and 96 (1988). © Crown copyright reserved. Ordnance Survey Licence No. GD272191/2004 ISBN 0 85272 484 5

Acknowledgements

The authors and contributors to this memoir would like to express sincere thanks to numerous colleagues for assistance with field work, access to unpublished data and discussion of results and interpretations. They are too numerous to mention individually but we should like to single out the late Gareth Roycroft who died tragically in 1987 as the result of a fall whilst studying the rocks between Skiddaw and Ullock Pike. Chapters in this memoir have been written by the following authors:

Notes

Preface

The Lake District is one of England’s outstanding scenic areas, a status confirmed by its recent nomination as a World Heritage Site. In 1951 much of the area was designated as one of the first National Parks and its enduring popularity is such that it now attracts an estimated 20 million visitor days per year. The spectacular scenery arises from the combination of glacial erosion, during the last Ice Age, and the underlying geology. Lower Palaeozoic rocks underpin the mountainous heart of the Lake District and it is those in the northern sector, comprising the Skiddaw Group, which form the subject of this memoir. The account is based on a comprehensive investigative and survey programme carried out by the British Geological Survey at intervals between 1982 and 1997. It marks an evolutionary development in the Survey’s memoir series in that, rather than describing the geology of an area defined by one or more 1:50 000 map sheets, it focuses on a specified stratigraphical unit spanning several such sheets.

The impetus for a geological initiative in the Lake District arose from several factors; not least was the lack of a modern map-based interpretation for one of England’s principal outdoor leisure areas. The case was equally compelling scientifically given the importance of the region for the understanding of large-scale, Palaeozoic geological processes, including the destruction of the Iapetus Ocean and the construction of the Caledonian-Acadian orogenic belt. Previous attempts at geological interpretation had foundered largely because of a combination of intractable, homogeneous lithologies, sparse and localised macrofossil assemblages, and complex, polyphase deformation. Various long-running geological controversies had been generated by the apparently paradoxical relationships shown in some places, while elsewhere the geological detail remained largely enigmatic.

An integrated, multidisciplinary approach was applied. Field examination of exposures and traditional survey techniques were complemented by geophysical, geochemical, remote sensing, mineralogical, petrological, isotope dating, sedimentological and biostratigraphical studies. A fundamental advance was the development of directly comparable graptolite and acritarch biostratigraphical schemes, application of which has enabled the lithostratigraphy and structure to be elucidated. Further insight into the structure has been achieved through the combination of geophysical models for the concealed Lake District batholith, and the geochemistry and mineralogy of its aureoles, with mapping results over a large area. These techniques have helped to define major, regionally significant structures in the Skiddaw Group. On the microscopic scale, detailed petrographical provenance studies, supported by whole rock and isotope geochemistry, have identified the characteristics of the likely source area for the original sediment that now forms the Skiddaw Group sedimentary rocks.

This study affords a good example of the applications of publicly funded science. It directly enhances understanding of geology in one of England’s foremost National Parks, adding value to the available educational and leisure experience. In addition, it provides a land management tool both in the National Park and the surrounding areas. In view of the likely occurrence of Skiddaw Group rocks at depth across much of northern England, the study is also relevant to the understanding of the deep geological structure of a much wider region.

David A Falvey, PhD Director, British Geological Survey, Kingsley Dunham Centre, Keyworth, Nottingham NG12 5GG

The Skiddaw Group of the English Lake District — summary

The stratigraphical and structural framework of the Skiddaw Group has been a controversial issue for much of the past century. Its rocks have been generally regarded as among the most intractable in England but a recently completed resurvey programme led by the British Geological Survey has resolved many of the outstanding difficulties and provided a comprehensive geological understanding. Important advances in graptolite and acritarch biostratigraphy underpin the lithostratigraphical interpretation, which in turn allows a more informed analysis of basin geometry, structure and burial metamorphism. This memoir presents and discusses the principal scientific findings of the resurvey.

The Skiddaw Group occupies the northern part of the Lower Palaeozoic inlier of the English Lake District; smaller inliers occur to the east at Ullswater, Bampton, Cross Fell and Teesdale, and to the south at Ravenglass, Black Combe and Furness. Sporadic records from outcrop and boreholes farther afield indicate that similar strata extend beneath much of northern England. To the west, the Manx Group of the Isle of Man is a regional correlative. The group consists of turbidite mudstone and sandstone, which range from Tremadoc (or possibly Cambrian) to Llanvirn in age.

The strata were deposited in an extensional environment at the Avalonian margin of the Iapetus Ocean. A range of petrological and geochemical parameters defines the provenance of the group within an extinct continental margin volcanic arc, possibly of Precambrian age. Palaeocurrent analysis and deduced basin configuration suggest that this source area was situated farther south, although no specific location can be identified. New graptolite and acritarch biostratigraphical zonal schemes have been developed. They assist in the recognition of three distinct lithostratigraphical domains, separated by major structural lineaments. In the north, about 5000 m of predominantly silty turbidites accumulated; two major sandstone incursions, the Watch Hill and Loweswater formations, alternate with three mudstone-dominated successions, the Bitter Beck, Hope Beck and Kirk Stile formations. Farther south, across the Causey Pike Fault, within a separate younger succession that consists mainly of early Llanvirn mudstone (Tarn Moor Formation) there are sporadic interbedded volcaniclastic turbidites and bentonites, a precursor of the extensive volcanism of the late Llanvirn to Caradoc. The Llanvirn strata overlie a major olistostrome, the Buttermere Formation, which contains Tremadoc and Arenig components that were emplaced from the south in the late Arenig. The olistostrome may have resulted from continuing extensional tectonic instability, but may alternatively have been triggered by the initiation of subduction-related uplift. The inliers of the southern Lake District lie to the south of the Southern Borrowdale Lineament, a major structural and geophysical feature, and they each show unique and characteristic geological features.

By late Llanvirn or early Caradoc times, the deep marine environment in which the Skiddaw Group was deposited had been transformed into a subaerial basement for the Borrowdale and Eycott volcanic groups. The scale of the unconformity between the Skiddaw Group and the two volcanic groups indicates considerable disorganisation and erosion of the former prior to volcanicity. Further, during that phase, volcanotectonic extensional faulting seems likely to have imposed further complication on Skiddaw Group structural architecture. Volcanism ended in the late Caradoc, but it seems unlikely that the final closure of Iapetus Ocean occurred before the mid-Silurian, when convergence of Avalonia with Laurentia initiated polyphase tectonic deformation of the Skiddaw Group. Thrust imbrication probably began around late Wenlock, as the Southern Uplands thrust belt extended across the sutured Iapetus Ocean. However, there is no evidence for penetrative deformation and imposition of cleavage until the early Devonian Acadian Orogeny. A penetrative cleavage with a broadly north-east Caledonide trend was imposed at that time. It affects all of the early Palaeozoic rocks, including the Skiddaw Group, where it locally cuts an earlier, bedding-parallel fabric. Consequently, the character of the earliest tectonic fabric varies from slaty cleavage to a crenulation cleavage. Later phases of Acadian compression probably involved re-activation of thrusts and faults within the Skiddaw Group. This resulted in marked strain partitioning and the resultant development of crenulation cleavages that are difficult to correlate between domains. Granite intrusion at about 400 Ma spanned the final cleavage-forming episode.

In addition to detailed lithostratigraphical, biostratigraphy and structural interpretation, the memoir presents supporting data that include mineralisation localities, geochemical analysis and comprehensive faunal lists. A section indicating other sources of information on the geology of the region and a full reference list is also included.

(Front cover) Loweswater Fell, looking south along Mosedale to Gale Fell within the Crummock Water thermal aureole (D3821). Photographer: T S Bain.

(Succession) Skiddaw Group and overlying lithostratigraphical units in the Northern Fells and Central Fells belts of the Lake District.

Chapter 1 Introduction

This memoir gives a concise account of the Skiddaw Group of sedimentary rocks that crops out in the English Lake District and surrounding areas. Formerly called the Skiddaw Slates, the rocks are Tremadoc (possibly late Cambrian) to Llanvirn, deep-water, marine turbidite mudstones and sandstones (mainly wackes) with graptolites as the only widespread macrofossils. The largest outcrop (about 480 km2) forms the northern part of the Lower Palaeozoic, Lake District massif (Figure 1), and equivalent strata are shown by borehole records to extend westwards beneath the Carboniferous cover of the Cumbrian coastal plain. Smaller inliers occur in the south of the Lake District at Black Combe, Furness and Ravenglass, and in the east at Ullswater and Bampton. Other inliers occur farther east at Cross Fell and in Teesdale, and sporadic borehole records still farther to the north-east suggest that similar rocks underlie much of northern England.

The Skiddaw Group is named after the mountain of Skiddaw that rises to a height of 931 m to the north of Keswick. The cleaved, mud-rich lithologies that dominate the group weather readily to form the rounded topography that typifies the north of the Lake District. The highest and most rugged parts of the Skiddaw Group, at Skiddaw, Blencathra, Grasmoor (Plate 1), and at Black Combe in the south, occur where the strata have been hornfelsed by igneous intrusions. In general, the typically rounded topography of the Skiddaw Group contrasts markedly with the more craggy landscape produced by the stratigraphically overlying Borrowdale Volcanic Group and the associated intrusive granitic plutons in the central part of the Lake District. The entire Lower Palaeozoic inlier of the Lake District is fringed by a Carboniferous and Permian succession that gives rise to a comparatively subdued topography.

The largest Skiddaw Group inlier (hereafter called the Skiddaw inlier) is in the northern Lake District, and is cut by glacially sculpted valleys, many of which now contain lakes. Between Penrith and Keswick, the east–west sections of the valleys of the rivers Greta and Glenderamackin follow the line of the Causey Pike Fault (Figure 1), the tectonic junction between two contrasting Skiddaw Group successions. The fault can be traced westwards, as a line of valleys, to the north end of Ennerdale Water. Other major valleys are also fault-controlled with a general north-west–south-east trend, parallel to that of major structures such as the Lake District Boundary Fault at the Irish Sea coast, and the Vale of Eden boundary faults including the Pennine faults at Cross Fell. In the west of the Skiddaw inlier, the lakes of Buttermere, Crummock Water and Loweswater (Plate 2) all follow this north-west–south-east fault trend, as does the northern part of Lorton Vale. In the central part of the inlier, Bassenthwaite Lake and the floodplain of the River Derwent occupy another major fault-controlled valley with this trend. Derwent Water is similarly fault-bounded, but the outcrop of the Borrowdale Volcanic Group on its east and south sides also influences its shape.

Most of the Skiddaw inlier lies within the Lake District National Park, with much land owned by the National Trust. Keswick and Cockermouth are the main centres of population. Both thrive on tourism, though Cockermouth is also an important agricultural market town. Commercial forestry is economically important, with extensive plantations, for example around Whinlatter Pass, Skiddaw Dodd, Setmurthy Common and to the south of Troutbeck.

Historically, mining has contributed significantly to the local economy. Keswick itself grew up on the site of the Elizabethan copper smelters at Brigham Forge (Ward, 1876; Dewey and Eastwood, 1925). In the 17th century the Keswick pencil industry developed when graphite (plumbago) was discovered nearby in Borrowdale (Ward, 1876; Adams, 1988). The 19th and 20th centuries saw the exploitation of lead (Eastwood, 1921) and later of baryte (Young and Cooper, 1988; Tyler, 1990) but all mining activity has now ceased (mineralised localities within and peripheral to the Skiddaw Group outcrop are listed in Appendix 1).

Regional geotectonic setting

The Lake District is the largest of the Lower Palaeozoic inliers that lie immediately to the south of the inferred position of the Iapetus Suture, a zone of continental collision marking the original site of the early Palaeozoic Iapetus Ocean (Figure 1). This grew to a maximum width in excess of 1000 km in the late Cambrian and early Ordovician, but had closed completely by the late Silurian (Soper et al., 1992b and references therein; Pickering and Smith, 1995). The Iapetus Ocean was bordered to the north (Figure 2) by the palaeocontinent of Laurentia and to the south by Avalonia, a marginal component of the palaeocontinent of Gondwana (Cocks et al., 1997). During the early Ordovician the Avalonian margin of the Iapetus Ocean lay at a latitude of around 60°S (Scotese and McKerrow, 1990, 1991). The ocean waters were cold at this palaeolatitude and the widespread circum-polar ‘Atlantic’ faunal province is characterised by an abundant, but low diversity, graptolite fauna (Skevington, 1974). The limited occurrences of early Ordovician trilobites in the Skiddaw Group are also indicative of a cool, peri-Gondwanan marine environment (Fortey et al., 1989).

Closure of the Iapetus Ocean was effected by the rifting of Avalonia from Gondwana and its subsequent northwards drift towards Laurentia (Figure 2). The line of collision, the Iapetus Suture (Figure 1), now separates Laurentian terranes to the north from Avalonian terranes to the south; in Britain it coincides approximately with the Solway Firth and the Anglo–Scottish border (McKerrow and Soper, 1989). The Skiddaw Group was deposited and uplifted on the northern margin of Avalonia during its early Palaeozoic northward drift, and deformed during its eventual collision with Laurentia as the Iapetus Ocean closed (Figure 2). The major tectonic structures within the Skiddaw Group were formed during the early Devonian Acadian deformation event, caused by the convergence of the Avalonian and Armorica–Iberian microcontinents (Soper et al., 1992b).

For much of that time the area destined to become the English Lake District lay close to the junction of the Iapetus Ocean (between Laurentia and Gondwana) and the Tornquist Sea (between Baltica and Gondwana) (Figure 2). Various dynamic configurations of the three continental plates and their intervening oceans have been suggested (e.g. Soper and Hutton, 1984; Soper et al., 1987; Hutton, 1987, Pickering and Smith, 1995). Most of these palaeogeographic and tectonic reconstructions have emphasised the role of the Iapetus Ocean and subduction at its margins (McKerrow et al., 1991 and references therein; Soper et al., 1992a, b). However, some aspects of Lake District geology have been compared with the concealed Caledonides of eastern England and the Brabant Massif, which relate more to the evolution of the Tornquist Sea (Pharaoh et al., 1991).

Geological evolution

The Skiddaw Group was deposited, initially in a passive margin setting, during the (possibly) late Cambrian, Tremadoc, Arenig and early Llanvirn. Its sediment was derived from ancient, continental-margin volcanic-arc rocks and their basement in the Gondwanan orogen farther south. About 5000 m of muddy, silty and sandy turbidites were deposited, and form a succession much of which can be demonstrated north of the Causey Pike Fault in the northern part of the Skiddaw inlier. However, these thick continental margin deposits became unstable during the late Arenig, and a huge olistostrome was emplaced, now preserved to the south of the Causey Pike Fault. The succeeding post-olistostrome strata record the first volcanism coeval with Skiddaw Group deposition, with the influx of sporadic volcaniclastic turbidite deposits and bentonite ash layers during the early Llanvirn. The different lithostratigraphies described from each side of the Causey Pike Fault (Cooper et al., 1995) are summarised in (Figure 3) and correlated with the smaller, southern and eastern inliers; the chronostratigraphy follows the proposals of Fortey et al. (1995).

The olistostrome emplacement event may have been related to the rifting from Gondwana of the Avalonian microcontinent, on the northern margin of which lay the Lake District depositional basin. Alternatively, extension on the passive margin prior to the final rifting phase, or the initiation of subduction-related uplift may have caused the instability. The generation of andesitic melts by the subduction of Iapetus Ocean crust beneath the northern margin of Avalonia led to uplift of the Skiddaw Group basin during the Llanvirn. By the late Llanvirn or early Caradoc the deep marine Skiddaw Group had become the subaerial basement on to which the Borrowdale and Eycott volcanic groups were erupted.

The scale and nature of the unconformity between the two groups reflects considerable disruption and erosion of the Skiddaw Group prior to volcanicity. The disruption is believed to be partly the result of the slumping and olistostrome emplacement, and partly the result of the basin inversion processes (Hughes et al., 1993 and references therein).

The Eycott and Borrowdale volcanic groups were erupted above a southerly dipping subduction zone during the northward drift of Avalonia (Fitton et al., 1982; Kokelaar, 1988 and references therein). Palaeomagnetic results have been interpreted in terms of eruption during a single, normal polarity chron in the early Caradoc (Piper et al., 1997). Isotopic Sm–Nd dating of garnet whole-rock pairs from the Borrowdale Volcanic Group gives an average of 457 ± 4 Ma (Thirlwall and Fitton, 1983). Extensional volcanotectonic faulting is likely to have imposed further complication on Skiddaw Group structural architecture. Volcanism ended in the late Caradoc, and the Borrowdale Volcanic Group, together with the Skiddaw Group, was intruded by granite plutons at around 450 Ma (Hughes et al., 1996).

Thermal equilibration, coupled with possible further extension, followed volcanism and allowed late Caradoc to Ashgill marine transgression across the re-established passive margin. These conditions prevailed through the early Silurian, with deposition of the older parts of the Windermere Supergroup, until convergence of Avalonia with Laurentia in late Wenlock times. This enabled the Southern Uplands thrust belt to extend across the sutured Iapetus Ocean (Stone et al., 1987), initiating thrusting in the Skiddaw Group. No tectonic fabric was imposed at this stage and the evidence for uplift and structural disturbance comes from the widespread resetting of Rb–Sr isotopic systems to the 420–430 Ma interval (Stone et al., 1999 and references therein).

The timing of this putative thrust advance towards the Avalonian hinterland is compatible with the development of the foreland basin in which was deposited the younger part of the Windermere Supergroup (Kneller, 1991; Kneller et al., 1993). Evidence from the southern Lake District shows that foreland basin sedimentation continued there through the Ludlow and Přídolí followed by deformation of the basin fill during the early Devonian, Acadian orogeny. A penetrative slaty cleavage with a broadly Caledonide trend was imposed at that time on all the Lower Palaeozoic rocks, including the Skiddaw Group where locally it cuts an earlier, bedding-parallel compaction fabric. Later phases of Acadian compression probably involved re-activation of thrusts within the Skiddaw Group, with associated strain partitioning resulting in domainal crenulation cleavages (Hughes et al., 1993). Granite intrusion at about 395 Ma overlapped the main cleavage-forming episode (Soper and Roberts, 1971; Soper and Kneller, 1990; Cooper et al., 1988).

The Lake District massif is surrounded by Devonian (Old Red Sandstone lithofacies), Carboniferous and Permo–Triassic sequences that unconformably overlie the Lower Palaeozoic strata. These younger sequences thicken away from the Lake District inlier but many of the contacts are fault-controlled (Firman and Lee, 1986; Chadwick et al., 1995; Kirby et al., 2000). It is probable that normal faulting throughout the late Palaeozoic and Mesozoic added further structural complication to the Skiddaw Group, with probable re-activation of existing faults. Early Tertiary thermal uplift followed, again with possible fault re-activation, and subsequent erosion that has continued to the present-day.

History of geological investigation

Stratigraphy

The historical development of lithostratigraphy in the Skiddaw Group is summarised in (Table 1). The rocks were first described as the Skiddaw Slates by Otley (1820, 1823) and Sedgwick (1832). Systematic investigations commenced in 1872 with mapping, at 1:10 560 scale, by J C Ward of the Geological Survey. He divided the Skiddaw Slates into five lithostratigraphical divisions overlain by the ‘Volcanic Series of Borrowdale’ (Ward, 1876). During the following century, Geological Survey workers including Dixon (70–71 in Geological Survey of Great Britain, 1925), Eastwood (51 in Geological Survey of Great Britain, 1933) and Eastwood et al. (1931) showed that some of Ward’s divisions were lateral equivalents and the sequence became simplified, firstly to three units and eventually to the two-fold classification of Rose (1955). Rose believed the Loweswater Flags were overlain by the Mosser-Kirkstile Slates, and recognised the previously separated Blake Fell mudstones as a metamorphic aureole. Subsequently, Jackson (1961) added the name Hope Beck Slates for the sequence below the Loweswater Flags, so making three divisions below the ‘Latterbarrow Sandstone’. A completely different nomenclature, involving eight formations, was proposed by Simpson (1967). However, this classification took little account of palaeontological evidence and repeated many earlier mistakes; it is now regarded as untenable.

Jackson (1978), Wadge (1978) and Moseley (1984) have more recently proposed further lithostratigraphical schemes for the Skiddaw Group. All agreed about the lower part of the group, but the upper part proved more contentious. In particular, the Latterbarrow Formation (Latterbarrow Sandstone of Eastwood et al., 1931) was believed to occupy different stratigraphical positions by all three authors. Subsequent work by Allen and Cooper (1986) showed that it rests unconformably on the Skiddaw Group and belongs to the Borrowdale Volcanic Group. The Redmain Sandstone was considered by Eastwood (53 in Geological Survey of Great Britain, 1927) to be equivalent to the Latterbarrow Sandstone, but was shown by Allen and Cooper to be a distinct unit. They designated it the Redmain Formation, but made no conclusion as to where it fitted in the Skiddaw Group stratigraphy. The youngest Skiddaw Group strata, the Tarn Moor Mudstones (Wadge et al., 1972), have also been the subject of disagreement. Wadge (1978) removed them altogether and placed them at the top of his separately defined Eycott Group, whereas Jackson (1978) retained them at the top of the Skiddaw Group, but as lateral equivalents to the top of the Eycott Group. Moseley (1984) suggested that the Tarn Moor Mudstones were partly equivalent to, and partly younger than, the upper part of the Eycott Group, and also equated them with part of his Kirkstile Slate Formation.

The recent BGS work reported here shows that two different lithostratigraphical sequences, named the Northern and Central Fells belts, exist within the main Skiddaw inlier. These are geographically isolated from the Southern Lake District inliers (Figure 3). The Northern Fells and Central Fells belts can be traced eastwards to the Cross Fell Inlier where a stratigraphy similar to that of the Lake District has been proposed (Cooper and Molyneux, 1990). A recently discovered complication is the presence of a possible late Cambrian acritarch flora in Skiddaw Group mudstones from an exposure immediately below the Eycott Volcanic Group (Millward and Molyneux, 1992). The status of this occurrence remains unclear and it is not formally assigned to any particular stratigraphical division. It raises the possibility that Skiddaw Group sedimentation commenced earlier than commonly supposed. The biostratigraphy and lithostratigraphy of the Skiddaw Group are further discussed in chapters 2 and 3 respectively of this memoir.

Palaeontology

Sedgwick (1832) initially noted that the Skiddaw Slates contained no organic remains but subsequently (1848) he recorded the first graptolites from the group. The Skiddaw inlier and its abundant graptolite localities were studied throughout the 19th century. Early workers made large collections, mainly from screes, and there followed a profusion of papers by many authors including Sedgwick, Hausmann, Salter, Harkness, Nicholson, Dakyns, Aveline, Ward, Huddleston, Postlethwaite, Goodchild, Marr, Harker, Elles, Wood and Read. Lapworth (1879) and Marr (1894) made notable early contributions to the palaeontology of the Skiddaw Group and its faunal zonation, based on graptolites. The subsequent zonal refinements of Elles (1898) and Elles and Wood (1901–1918) culminated with the widely applied scheme of Elles (1933). This zonation was further refined by Jackson (1962) and Cooper et al. (1995).

Progress in understanding the Skiddaw Group has also been made through the use of microfossils. The first discoveries were of Llanvirn microfossil assemblages in the Cross Fell inlier (Lister et al., 1969), and correlations were made later with the Skiddaw and Teesdale inliers (Wadge et al., 1967; Lister and Holliday, 1970). In the Skiddaw inlier, the Watch Hill Grits had been previously equated with the Arenig Loweswater Flags, but the discovery of Tremadoc microfossil and graptolite assemblages in the former (Molyneux and Rushton, 1988; Rushton, 1985) established them as a separate stratigraphical unit, and extended the age range of the Skiddaw Group down to the Tremadoc. Recent biostratigraphical work, based largely on graptolites and acritarchs, with a limited number of trilobites, has been partly presented by Cooper et al. (1995) and is discussed further in Chapter 2 of this memoir.

Orr (1996) has described the extensive trace fossil ichnofauna. It is of limited stratigraphical value, but is similar to that described from other early Ordovician, deep marine sequences thought to have been deposited in high southerly latitudes along the margin of Avalonia.

Structure

The interpretation of the Skiddaw Group structure and its relationship to the overlying Borrowdale and Eycott volcanic groups has been a long-running geological controversy, with faulted, conformable and unconformable relationships all invoked at various times. From Harkness (1863) to Green (1917) and Hollingworth (1955), a conformable relationship was favoured, though Dakyns (1869) preferred an unconformity. Simpson (1967) recognised polyphase deformation in the Skiddaw Group and concluded that it was overlain unconformably by the Borrowdale Volcanic Group. Helm (1970) and Helm and B Roberts (1971) developed this interpretation. Other workers, including Soper (1970) and Soper and D Roberts (1971) for the Borrowdale Volcanic Group and Downie and Soper (1972) for the Eycott Volcanic Group, challenged the unconformity interpretation. The controversy raged throughout much of the 1970s until more evidence of pre-Borrowdale Volcanic Group deformation (D Roberts, 1971, 1977a, b; Jeans, 1971, 1972; Webb, 1972, 1975) and unconformity (Wadge, 1972) led to a general acceptance that the relationship was indeed an unconformable one (Soper and Moseley, 1978).

Questions remained about the scale of the unconformity and the degree of tectonism that predated it, in particular concerning the relationships between the tectonic folding events and the multiple cleavages present in the Skiddaw Group (Helm, 1970; D Roberts, 1971, 1976; Helm and B Roberts, 1971). It is now recognised that much of the pre-Borrowdale Volcanic Group deformation is actually of sedimentary and early post-sedimentary origin. On a regional scale, uplift and considerable erosion of the Skiddaw Group preceded the eruption of the Borrowdale and Eycott volcanic groups as the deep marine Skiddaw Group strata were uplifted to become the subaerial basement for volcanism. The scale of the unconformity beneath the volcanic rocks is now known to be considerable, with the full range of Skiddaw Group stratigraphy immediately subjacent to various parts of the volcanic sequence (Millward and Molyneux, 1992).

The overall position of the Lake District within the plate tectonic and terrane configuration of the Iapetus Ocean has influenced modern thinking of the tectonic history of the Skiddaw Group; key references include Soper et al. (1987, 1992a, 1992b), Soper and Woodcock (1990), Cooper et al. (1993), Kneller et al. (1993) and Hughes et al. (1993). Final closure of the ocean occurred in the mid- to late Silurian, and Stone et al. (1999) have speculated that thrusting and uplift associated with Avalonia–Laurentia collision was responsible for the widespread resetting of Rb–Sr radiometric ages to the 420–430 Ma interval. The age of the regional cleavage was demonstrated to be early Devonian by Soper and Roberts (1971) and Soper and Kneller (1990), from the relationship of that fabric with the approximately 395 Ma Skiddaw and Shap granites respectively. Subsequent crenulation cleavages were thought by Hughes et al. (1993) and Stone et al. (1999) to be largely domainal, arising through strain partitioning across reactivated earlier thrusts. The structural evolution of the Skiddaw Group is discussed in greater detail in Chapter 4 of this memoir.

Complementary map series

The overview of the Skiddaw Group presented in this memoir complements a series of geological maps published at the 1:50 000 scale (see p.117–118 for further details). The main Skiddaw inlier spans the sheets 23 Cockermouth, 28 Whitehaven, 29 Keswick and 37 Gosforth. The Ullswater and Bampton inliers lie mainly on the sheet 30 Appleby, and the Black Combe and Furness inliers lie on the sheet 48 Ulverston. The Cross Fell inlier is included within the sheets 24 Penrith, 30 Appleby and 31 Brough-under-Stainmore. Lorton and Loweswater (NY 12), together with the Black Combe, Furness and Cross Fell inliers, are areas of particular geological interest and are illustrated in special 1:25 000 scale maps. Also available are the British Geological Survey publications and Technical Reports listed in pp.118–120.

Chapter 2 Biostratigraphy

The efficacy of biostratigraphy depends to a large extent on the completeness of the information employed. The present chapter, which reviews the distribution of fossils throughout the Skiddaw Group, is thought to be the most complete survey yet undertaken. The resultant biostratigraphy stems from the integration of schemes that were developed using the distribution of macrofossils, especially graptolites, and microfossils, in particular acritarchs. These two disciplines are discussed separately below.

Correlation of the graptolite and acritarch zones with the stages of the Tremadoc, Arenig and Llanvirn series is shown in (Figure 4). Correlation with the Tremadoc stages is fairly secure for both fossil groups. The position of the Araneograptus murrayi Biozone at the top of the Tremadoc places it securely in the Migneintian Stage, while the Tremadoc acritarch microfloras from the Skiddaw Group can be referred to the Cressagian and Migneintian on the basis of comparisons with microfloras from Shropshire. Correlation with the stages of the Arenig is more tenuous because their type areas in south Wales are predominantly in shelly rather than graptolitic facies, and the acritarch assemblages show only a limited similarity. The base of the Moridunian coincides with the base of the Arenig Series, the base of the Whitlandian is thought to correlate approximately with the base of the Didymograptus simulans Biozone, and that of the Fennian with the base of the Isograptus victoriae victoriae Biozone.

Macrofossils

Graptolites are the most important group of macrofossils in the Skiddaw Group. Their presence throughout almost the whole succession makes the Skiddaw Group a biostratigraphical standard for the graptolitic facies in England and Wales, and more generally for the Arenig Series on the western margin of the Gondwanan continent.

Although graptolites have been collected from the Skiddaw Group for about 150 years it has not proved easy to arrive at a satisfactory biostratigraphy. The rocks are in general sparsely fossiliferous and most of the graptolites found are ill-preserved examples of taxonomically troublesome groups. Considerable difficulty has lain also with the lack of good superpositional evidence (compare Marr, 1894). The present review has benefitted from the results of the comprehensive mapping programme and from recent advances in the understanding of Arenig graptolite faunas in other parts of the world.

Eastwood et al. (1968, pp.24–33) gave a good account of the history of study of Skiddaw Group graptolites and a critique of the graptolite zonation. In 1898, Elles used only the generalised divisions ‘Lower’, ‘Middle’ and ‘Upper’ Skiddaw slates, but by 1933 had elaborated a scheme of 10 zonal and subzonal divisions (Elles, 1898, 1933); her ideas were based largely on theoretical patterns in the evolution of graptolites, such as stipe attitude and stipe reduction. A fresh approach by Jackson (1962), based empirically on new collections and their inferred superposition, showed that Elles’ divisions were not all tenable, and he reduced the Arenig part of her scheme to four divisions. Jackson’s scheme was adopted by Eastwood et al. (1968).

The faunal succession reported here (Figure 4) is an extension of the revision initiated by Cooper et al. (1995, p.189). It is in part based empirically on graptolites collected from the lithostratigraphical succession arrived at by mapping, and partly on correlation with biostratigraphical successions in other regions of the world, ably summarised by Cooper and Lindholm (1990). Those authors compiled a ‘composite standard sequence’ of graptolite appearances, and, as discussed below, in so far as the succession from the Skiddaw Group can be compared, it shows a good general agreement.

In the Skiddaw Group the general character of the graptolite faunas is determined largely by the didymograptids, which commonly form the main component of the assemblages. The taxonomy of pendent, declined, deflexed and horizontal didymograptids is difficult — these are the very groups excluded from Cooper and Lindholm’s compilation — and in consequence many identifications are doubtful and correlations based upon them correspondingly uncertain. Nevertheless, a practical biostratigraphy of the Skiddaw Group has to take account of the didymograptids, and this is reflected in the choice of didymograptid taxa to characterise most of the zonal units. Note that the generic name Didymograptus is used here in a broad sense. Several generic names are available for didymograptids, but as some (for example Corymbograptus) are without satisfactory definition and others (such as Acrograptus) are widely misinterpreted, they cannot be applied with confidence to most of the Skiddaw Group specimens.

The stratigraphical distribution of the Skiddaw graptolites is described, zone by zone, below. In certain areas, such as the Lorton Fells, it is possible to represent the vertical ranges of many taxa and place their first appearances in superpositional order, as shown in (Figure 6), (Figure 7), (Figure 8), (Figure 10) and (Figure 12), wherein the numbers refer to the selected graptolite localities listed in Appendix 2. However, the finding of the rarer taxa is so chancy that the ranges must not all be taken as definitive. Many taxa are known from isolated localities that cannot be placed in superpositional context. These are assigned to the graptolite zones discussed below 500 m and appear in (Table 2), which is a complete list of Skiddaw graptolites showing their zonal distribution.

Graphical correlation of the graptolitic succession

Cooper and Lindholm (1990) used six of the best known early Ordovician graptolitic successions to compile a ‘composite standard succession’ of first appearances of about 100 graptolite taxa, based chiefly on easily identified forms. About 30 of those taxa are recognised in the Skiddaw Group, and the inferred order of their appearance was compared by graphical correlation with Cooper and Lindholm’s (1990, fig. 4) composite standard sequence, using the technique that they described. The taxa show a similar order of appearance (Figure 5a).

In comparing the Skiddaw succession with Cooper and Lindholm’s composite standard, the following assumptions and approximations were made: (a) Didymograptus nicholsoni of Cooper and Lindholm represents D. kurcki and/or D. infrequens of the present account. (b) Isograptus caduceus gibberulus is not in the composite standard, but is very similar to I. caduceus australis (No. 22 in (Figure 5b) and is thought to occupy an equivalent place in the sequence of bioevents; it occurs with I. caduceus cf. imitatus (No. 20 in (Figure 5b) at Ullock Pike. (c) The first species of Cryptograptus to appear in the Skiddaw Group are C. hopkinsoni and C. antennarius, whereas the composite standard refers primarily to C. marcidus and C. schaeferi. (d) The first appearance of Pseudoclimacograptus in the composite standard is plotted against P. cumbrensis, which, however, is now transferred to Undulograptus. Regrettably, several of the taxa used in Cooper and Lindholm’s (1990) compilation (notably the isograptids) are at best rare in the Skiddaw Group, so their ranges therein are not known with certainty.

In (Figure 5b) the taxa in the composite standard are numbered on the horizontal axis and their inferred order in the Skiddaw Group is shown (by name and number) on the vertical axis. The line of correlation is relatively secure at either end but is less well constrained in its mid-part, mainly because the precise superposition of several first appearances in the Loweswater and Kirk Stile formations could not be ascertained; such approximate records are bracketed together and all recorded as from one general level.

In the first iteration (Figure 5b) four taxa (Nos 3, 6, 8 and 9) lie high above the line of correlation. They are assumed to have ‘unfilled ranges’ (i.e. they might be expected to be found eventually at lower levels in the Skiddaw Group), and are omitted from further analysis.

Five other taxa (Nos 14, 15, 21, 26 and 29) that occur below the line of correlation are allocated new places in the composite standard. Of these, Pseudobryograptus (No. 15) gave a problematical result in Cooper and Lindholm’s (1990, p.505) analysis also. Pseudotrigonograptus ensiformis (No. 21), on the other hand, was recorded in all six of Cooper and Lindholm’s standard sequences, and should have a stable position in the composite standard. The shift demanded by the present correlation suggests that some of the early Skiddaw Group specimens may be incorrectly identified, though they seem too large to be referable to the earlier species P. minor. In contrast, Pseudisograptus angel (No. 26) occurred in only one of Cooper and Lindholm’s standard sequences, so its position in their composite standard is not very secure and the shift indicated here can be accepted more readily.

A second iteration showed that Didymograptus distinctus (No. 16) and Cardiograptus (No. 24) have unfilled ranges, whereas the remainder of the correlation is essentially unchanged.

Araneograptus murrayi Biozone

The Araneograptus murrayi Biozone was introduced by Cooper et al. (1995, p.189) for the oldest graptolitic strata in the Skiddaw Group. It is characterised by A. murrayi ((Plate 3)i) and also contains the declined didymograptids Didymograptus cf. sinensis and the D. sp. of Molyneux and Rushton (1988, figs 9a-c). It is the only zone in which large dictyonematids have been found. Cooper and Lindholm (1990, fig. 4) recorded the first appearance of A. murrayi at their biohorizon 11 (Figure 5a). The upper part of our murrayi Biozone presumably includes equivalents of the Baltic copiosus Biozone (Lindholm, 1991), which cannot be separately distinguished in the Skiddaw Group.

The zone ranges through the Bitter Beck and Watch Hill formations and extends into the base of the Hope Beck Formation. It may occupy some 400 to 1000 m of strata. The base is not seen but the top is overlain at Trusmadoor by strata of the phyllograptoides Biozone.

Localities

Trusmadoor Localities 4 and 5, (Figure 6), Locality 1 (Bitter Beck) and Localities 2 and 3 (west of Cockermouth) all yielded Araneograptus murrayi.

Tetragraptus phyllograptoides Biozone

The Tetragraptus phyllograptoides Biozone was originally proposed for part of the Scandinavian succession and was adopted for part of the Skiddaw Group by Cooper et al. (1995, p.189). T. phyllograptoides itself has not been found in England, but some of its associates at the top of its Scandinavian range — Didymograptus protobalticus, D. rigoletto (Figure 9) o, q and Clonograptus multiplex, which occurs in the immediately overlying beds — are found at a number of closely spaced localities near the south-east end of Trusmadoor (Localities 9, 10; Maletz et al., 1991). Tetragraptus spp. and small horizontal Didymograptus occur in the same area, together with one Tetragraptus (Pendeograptus) cf. fruticosus (Figure 9)p). Cooper and Lindholm (1990, figure 4) record the first appearance of D. protobalticus at their biohorizon 16.

The phyllograptoides Biozone is recognised only in one area of the Skiddaw Group, at Trusmadoor (Figure 6), where it lies immediately above the murrayi Biozone. It has been proved through a few tens of metres of strata near the base of the Hope Beck Formation. The top of the zone has not been recognised, because a large thickness in the middle part of the Hope Beck Formation has not yielded diagnostic graptolites.

Didymograptus varicosus Biozone

Cooper et al. (1995, p.190) introduced the term varicosus Biozone as a revised concept based on the deflexus Biozone of earlier workers, extending it to include newly discovered faunas from the upper part of the Hope Beck Formation. D. varicosus (as figured by Mu et al., 1979, pl. 32, figs 1–4) is by far the commonest graptolite in the zone (Figure 9)m and extends as a rarity into the zone above. A large declined species resembling D. balticus (but known only from poorly preserved material) is confined to the zone. Horizontal didymograptids, such as D. cf. decens, are very rare. The only species to provide a clear connection with Cooper and Lindholm’s (1990) list of biohorizons is Didymograptus filiformis (Figure 9)d, which appears at horizon 23 and is found at or near the lowest fossiliferous levels in the varicosus Biozone.

The varicosus Biozone is developed in the upper 200 m or more of the Hope Beck Formation, and is present in the lower half of the Loweswater Formation, possibly attaining a thickness of 800–900 m in the Lorton Fells (Figure 7). An unknown thickness of strata separating the lowest fauna of the varicosus Biozone from the phyllograptoides Biozone in the Hope Beck Formation has not yielded stratigraphically useful fossils, so the position of the base of the zone is uncertain. The varicosus Biozone is clearly overlain by the simulans Biozone in Hope Gill (Figure 8).

Localities

The lowest fossiliferous beds are in Hope Beck (Locality 12) and Blaze Beck (Locality 11). The base of the Loweswater Formation lies within the varicosus Biozone at Dodd (Locality 28) and west of Blaze Bridge (Locality 13). Faunas higher in the succession were obtained from Scawgill (Locality 15), Hope Gill (Locality 29), Brown How, Whinlatter (Locality 18), Jonah’s Gill (Locality 39) and Mungrisdale (Locality 40). Faunas of the varicosus Biozone occur in some clasts in the Buttermere Formation olistostrome, for example on Whiteless Pike (Locality 141). Sparse faunas from Swinside (Localities 31, 32) may represent either the varicosus or the simulans Biozone; their stratigraphical position (Figure 8) suggests the varicosus Biozone.

Didymograptus simulans Biozone

Cooper et al. (1995) introduced the simulans Biozone to replace the misleading term nitidus Biozone of earlier workers. The simulans Biozone is characterised by slender didymograptids such as D. simulans, the true D. deflexus ((Plate 3)j), D. gracilis, the declined forms usually assigned to D. nicholsoni, and the tiny pendent D. (Didymograptellus) minutus (Figure 9)k, l. Azygograptus abounds locally, its distribution apparently being controlled by high-energy environmental conditions (Beckly and Maletz, 1991). The base of the zone is taken at the appearance of D. simulans (Figure 9)g and/or a densely thecate form of ‘D. nicholsoni’, here referred to D. infrequens (Figure 9)e, f. The more sparsely thecate form of ‘D. nicholsoni’ figured by Elles and Wood (1901, pl.2, figs 4a, 4b) has a different proximal development and is assigned to D. kurcki (Figure 9)a. Horizontal didymograptids are very rare and none resembles D. nitidus.

The base of the zone may lie near Cooper and Lindholm’s (1990, fig. 4) biohorizon 28 (‘Acrograptus’ nicholsoni), above which Didymograptus gracilis and Isograptus primulus (the latter represented by a single specimen from Barf) appear in due order (Figure 9)b, c. However, Azygograptus eivionicus (Figure 9)i appears at least as low as the lowest D. gracilis in the Skiddaw Group, whereas it appears later than D. gracilis in Cooper and Lindholm’s analysis. The appearance of Pseudophyllograptus angustifolius and Tetragraptus (Pendeograptus) pendens in the simulans Biozone marks a relatively late advent in the Skiddaw Group, for they lie 10 or more biohorizons higher than is shown in Cooper and Lindholm’s compilation.

A passage from the varicosus to simulans Biozone is evident in the Hope Gill section (Localities 29–30), although the strata are poorly fossiliferous. The simulans Biozone has been recorded through at least 500 m of strata in the upper part of the Loweswater Formation and the lower part of the Kirk Stile Formation, the richest assemblages being from Barf (Locality 25, (Figure 7). The simulans Biozone is overlain by the victoriae Biozone at Hopegill Head (Locality 70) and north of Barf (Localities 60–61), but interpretation of the sequence is complicated by the reappearance (or persistence) of assemblages typical of the simulans Biozone in the sandstonerich unit of Broom Fell, Lord’s Seat and Woodend Brow, as discussed below.

Localities

Sparse faunas have been collected from Burnbank Fell and Darling How (Localities 36–38) around Loweswater, and from Hope Gill (Locality 30), the ridge from Hopegill Head to Whiteside (Locality 33) and on Hobcarton End (Locality 34). North of the Whinlatter Pass are richer faunas from Aiken Beck (Locality 16), Brown How (Locality 19) and around Barf (Localities 24–26). Farther north, the Loweswater Formation is fossiliferous at Tom Rudd Beck (Locality 23) and Ling Fell (Locality 21). The simulans Biozone is developed at Carl Side, Skiddaw (Locality 35), and at Mungrisdale (Localities 41–42). Some clasts in the Buttermere Formation olistostrome contain graptolites of the simulans Biozone, at Sleet Hause (Locality 150), Stoneycroft Gill (Locality 152) and Swinside near Portinscale (Locality 153). Graptolites from Buttermere Quarry (Locality 138) are from an olistolith whose age appears to lie close to the varicosus–simulans biozonal boundary.

Isograptus victoriae Biozone

This zone is newly proposed for the lower part of the gibberulus Biozone. As used by earlier authors such as Fortey et al. (1990), the gibberulus Biozone was a rather extensive unit (Figure 4). Having revised the identifications of Isograptus species and reviewed their distribution in the Skiddaw Group, it is now possible to suggest a refinement of the biostratigraphical succession.

In the lower part of the Kirk Stile Formation, Isograptus victoriae victoriae and Didymograptus hirundo (Figure 13) Selected graptolites from Llanvirn strata of the Skiddaw Group. All specimens except for l are in the collections of the British Geological Survey. All magnified 5 except k and l. The parallel lines indicate the trace of the cleavage in the plane of bedding. a. Archiclimacograptus angulatus magnus (Berry), AJ 1008, cited by Skevington in Wadge et al. 1972, p. 63, as a species of Pseudoclimacograptus. Tarn Moor Formation (murchisoni Biozone), Tarn Moor Tunnel, 808?818 m from the south portal. b. Paraglossograptus sp., RX 4960. Tarn Moor Formation (artus Biozone), Mosedale Beck (Locality 160). c. Cryptograptus schaeferi Lapworth, RX 3189. Tarn Moor Formation (artus Biozone), Cawell Beck (Locality 157). d. Climacograptus bifomis Mu & Lee?, RX 3195. Tarn Moor Formation (close to the base of the murchisoni Biozone), Aik Beck (Locality 165). e.f. Amplexograptus confertus (Lapworth), AJ 858 and AJ 877, Tarn Moor Formation (murchisoni Biozone), Tarn Moor Tunnel, 2311?2344 m from the south portal. g. Undulograptus cumbrensis (Bulman), RX 5463. Skiddaw Group mudstones (artus Biozone?), quarry east of Greenscoe Farm, Low Furness (Locality 174). h. Archiclimacograptus cf. caelatus (Lapworth), RX 3724. Tarn Moor Formation (upper part of the artus Biozone?), Aik Beck (Locality 166). i. Didymograptus artus Elles & Wood, Zv 713. Kirk Stile Formation (artus Biozone), north of Hazelhurst (Locality 134). j. Didymograptus spinulosus Perner, RX 2503A. Kirk Stile Formation (artus Biozone), Outerside (Locality 80). k. Didymograptus murchisoni speciosus Ekstr�m, RX 3194, 4. Tarn Moor Formation (close to the base of the murchisoni Biozone), Aik Beck (Locality 165). l, m. Didymograptus (Acrograptus) affinis Nicholson. l, proximal end, Natural History Museum Q.5858a, 10. Tarn Moor Formation (artus Biozone), Aik Beck (Locality 164). m, RX 3194, 5. Tarn Moor Formation (close to the base of the murchisoni Biozone), Aik Beck (Locality 165). n. Holmograptus lentus (Holm), distal fragment, Zv 752. Kirk Stile Formation (artus Biozone), north of Hazelhurst (Locality 134). o. Didymograptus acutidens Elles & Wood, PC 9084. Skiddaw Group mudstones (artus Biozone), Park Farm, Askam in Furness (Locality 175)." data-name="images/P936906.jpg">(Figure 11)k, r appear at about the same level, as is also the case in southern Sweden (Cooper and Lindholm, 1990, figure 1, III, and biohorizons 39 and 41). This is the lowest level in the Skiddaw Group at which horizontal didymograptids dominate the fauna, and at which specimens assigned to the long-ranging taxa D. extensus linearis and D. uniformis lepidus (Figure 13) Selected graptolites from Llanvirn strata of the Skiddaw Group. All specimens except for l are in the collections of the British Geological Survey. All magnified 5 except k and l. The parallel lines indicate the trace of the cleavage in the plane of bedding. a. Archiclimacograptus angulatus magnus (Berry), AJ 1008, cited by Skevington in Wadge et al. 1972, p. 63, as a species of Pseudoclimacograptus. Tarn Moor Formation (murchisoni Biozone), Tarn Moor Tunnel, 808?818 m from the south portal. b. Paraglossograptus sp., RX 4960. Tarn Moor Formation (artus Biozone), Mosedale Beck (Locality 160). c. Cryptograptus schaeferi Lapworth, RX 3189. Tarn Moor Formation (artus Biozone), Cawell Beck (Locality 157). d. Climacograptus bifomis Mu & Lee?, RX 3195. Tarn Moor Formation (close to the base of the murchisoni Biozone), Aik Beck (Locality 165). e.f. Amplexograptus confertus (Lapworth), AJ 858 and AJ 877, Tarn Moor Formation (murchisoni Biozone), Tarn Moor Tunnel, 2311?2344 m from the south portal. g. Undulograptus cumbrensis (Bulman), RX 5463. Skiddaw Group mudstones (artus Biozone?), quarry east of Greenscoe Farm, Low Furness (Locality 174). h. Archiclimacograptus cf. caelatus (Lapworth), RX 3724. Tarn Moor Formation (upper part of the artus Biozone?), Aik Beck (Locality 166). i. Didymograptus artus Elles & Wood, Zv 713. Kirk Stile Formation (artus Biozone), north of Hazelhurst (Locality 134). j. Didymograptus spinulosus Perner, RX 2503A. Kirk Stile Formation (artus Biozone), Outerside (Locality 80). k. Didymograptus murchisoni speciosus Ekstr�m, RX 3194, 4. Tarn Moor Formation (close to the base of the murchisoni Biozone), Aik Beck (Locality 165). l, m. Didymograptus (Acrograptus) affinis Nicholson. l, proximal end, Natural History Museum Q.5858a, 10. Tarn Moor Formation (artus Biozone), Aik Beck (Locality 164). m, RX 3194, 5. Tarn Moor Formation (close to the base of the murchisoni Biozone), Aik Beck (Locality 165). n. Holmograptus lentus (Holm), distal fragment, Zv 752. Kirk Stile Formation (artus Biozone), north of Hazelhurst (Locality 134). o. Didymograptus acutidens Elles & Wood, PC 9084. Skiddaw Group mudstones (artus Biozone), Park Farm, Askam in Furness (Locality 175)." data-name="images/P936906.jpg">(Figure 11) q, j occur. All specimens referred to I. caduceus gibberulus occur at higher levels in the Kirk Stile Formation. The victoriae Biozone is therefore taken from the level at which I. victoriae and D. hirundo appear, extending up to the base of the gibberulus Biozone. In the Lorton Fells, the zone may occupy a thickness of over 1000 m. Although the general position of the zone can be determined, I. victoriae itself is rare and there are uncertainties about the position of the lower and upper boundaries of the zone.

Lower boundary

Isograptus victoriae victoriae and D. hirundo occur in the Kirk Stile Formation west of Hopegill Head, apparently some 300 m above the Loweswater Formation (Locality 70, (Figure 8) although a fault interrupts the section there. To the west, above Gasgale Crags where the section is unfaulted, the lower beds of the Kirk Stile Formation contain Pseudophyllograptus angustifolius, but have yielded no graptolites diagnostic of the victoriae Biozone. At Wythe Gill, Kirk Fell (Locality 43), a large specimen of the I. victoriae group (cf. I. v. maximus) has been found about 250 m above the Loweswater Formation (Figure 7). Farther east, near Thornthwaite (Ullister Hill), however, the Loweswater Formation passes up through some 600 m of Kirk Stile Formation from which no example of the I. victoriae group is known, followed by a sandstone-rich unit 100 m thick, before a level with I. v. victoriae is reached near Ladies Table (Locality 60). The faunas of the intervening strata consist very largely of Pseudophyllograptus angustifolius, or, where sandstone dominates the succession, taxa that occur typically in the simulans Biozone, for example D. deflexus, D. infrequens, D. kurcki or D. simulans (Figure 7); localities 47, 50, 55). The victoriae Biozone has not been recognised east of Bassenthwaite Lake, although its presence may be predicted on the flank of Skiddaw between Carl Side and Ullock Pike.

Upper boundary

The top of the victoriae Biozone coincides with the appearance of I. caduceus gibberulus, marking the base of the gibberulus Biozone. The passage from the victoriae to the gibberulus Biozone occurs between Hopegill Head and Grisedale Pike, although part of the succession there is cut out by the Gasgale Thrust.

Localities

Isograptus victoriae has been found at Hopegill Head (Locality 70), Wythe Gill, Kirk Fell (Locality 43), and Wythop Woods north of Ladies Table (Locality 60). The commonest associated species are D. hirundo and Ps. angustifolius. Some localities have horizontal didymograptids that are assigned to the zone, but only with doubt because of the lack of distinctive elements in the fauna (e.g. Localities 65–67 in the Lorton Fells). Definitive faunas of the victoriae Biozone have not been obtained from clasts in the Buttermere Formation, but material of victoriae to gibberulus age occurs at several localities, such as Ill Gill (Locality 143), Ramps Gill (Locality 144) and Littledale Edge (Locality 147).

Isograptus caduceus gibberulus Biozone

This is a newly formulated restriction of the gibberulus Biozone of Fortey et al. (1990). Earlier workers followed Elles (1933) in supposing that Isograptus caduceus gibberulus appeared at a lower horizon than Didymograptus hirundo, and that “I. c. gibberulus, and ‘I. gibberulus’ generally, ranged into the D. hirundo Zone” (Jenkins, 1982, p.221). This was corrected by Fortey et al. (1990, p.128), who indicated that D. hirundo appeared at as low a level as I. victoriae and I. gibberulus. They treated all the strata with I. caduceus gibberulus and I. victoriae victoriae as representing the gibberulus Biozone, and reserved the term hirundo Biozone for the strata above the range of I. c. gibberulus but below the artus Biozone. Further work, summarised in (Figure 7) and (Figure 8), has now shown that the principal diagnostic taxon, I. caduceus gibberulus (Figure 13) Selected graptolites from Llanvirn strata of the Skiddaw Group. All specimens except for l are in the collections of the British Geological Survey. All magnified 5 except k and l. The parallel lines indicate the trace of the cleavage in the plane of bedding. a. Archiclimacograptus angulatus magnus (Berry), AJ 1008, cited by Skevington in Wadge et al. 1972, p. 63, as a species of Pseudoclimacograptus. Tarn Moor Formation (murchisoni Biozone), Tarn Moor Tunnel, 808?818 m from the south portal. b. Paraglossograptus sp., RX 4960. Tarn Moor Formation (artus Biozone), Mosedale Beck (Locality 160). c. Cryptograptus schaeferi Lapworth, RX 3189. Tarn Moor Formation (artus Biozone), Cawell Beck (Locality 157). d. Climacograptus bifomis Mu & Lee?, RX 3195. Tarn Moor Formation (close to the base of the murchisoni Biozone), Aik Beck (Locality 165). e.f. Amplexograptus confertus (Lapworth), AJ 858 and AJ 877, Tarn Moor Formation (murchisoni Biozone), Tarn Moor Tunnel, 2311?2344 m from the south portal. g. Undulograptus cumbrensis (Bulman), RX 5463. Skiddaw Group mudstones (artus Biozone?), quarry east of Greenscoe Farm, Low Furness (Locality 174). h. Archiclimacograptus cf. caelatus (Lapworth), RX 3724. Tarn Moor Formation (upper part of the artus Biozone?), Aik Beck (Locality 166). i. Didymograptus artus Elles & Wood, Zv 713. Kirk Stile Formation (artus Biozone), north of Hazelhurst (Locality 134). j. Didymograptus spinulosus Perner, RX 2503A. Kirk Stile Formation (artus Biozone), Outerside (Locality 80). k. Didymograptus murchisoni speciosus Ekstr�m, RX 3194, 4. Tarn Moor Formation (close to the base of the murchisoni Biozone), Aik Beck (Locality 165). l, m. Didymograptus (Acrograptus) affinis Nicholson. l, proximal end, Natural History Museum Q.5858a, 10. Tarn Moor Formation (artus Biozone), Aik Beck (Locality 164). m, RX 3194, 5. Tarn Moor Formation (close to the base of the murchisoni Biozone), Aik Beck (Locality 165). n. Holmograptus lentus (Holm), distal fragment, Zv 752. Kirk Stile Formation (artus Biozone), north of Hazelhurst (Locality 134). o. Didymograptus acutidens Elles & Wood, PC 9084. Skiddaw Group mudstones (artus Biozone), Park Farm, Askam in Furness (Locality 175)." data-name="images/P936906.jpg">(Figure 11)d, occurs at a higher stratigraphical level than I. victoriae. Many of the taxa of the victoriae Biozone range up into the gibberulus Biozone, for example D. extensus linearis, D. cf. Goldschmidti, D. hirundo and D. uniformis lepidus (Figure 13) Selected graptolites from Llanvirn strata of the Skiddaw Group. All specimens except for l are in the collections of the British Geological Survey. All magnified 5 except k and l. The parallel lines indicate the trace of the cleavage in the plane of bedding. a. Archiclimacograptus angulatus magnus (Berry), AJ 1008, cited by Skevington in Wadge et al. 1972, p. 63, as a species of Pseudoclimacograptus. Tarn Moor Formation (murchisoni Biozone), Tarn Moor Tunnel, 808?818 m from the south portal. b. Paraglossograptus sp., RX 4960. Tarn Moor Formation (artus Biozone), Mosedale Beck (Locality 160). c. Cryptograptus schaeferi Lapworth, RX 3189. Tarn Moor Formation (artus Biozone), Cawell Beck (Locality 157). d. Climacograptus bifomis Mu & Lee?, RX 3195. Tarn Moor Formation (close to the base of the murchisoni Biozone), Aik Beck (Locality 165). e.f. Amplexograptus confertus (Lapworth), AJ 858 and AJ 877, Tarn Moor Formation (murchisoni Biozone), Tarn Moor Tunnel, 2311?2344 m from the south portal. g. Undulograptus cumbrensis (Bulman), RX 5463. Skiddaw Group mudstones (artus Biozone?), quarry east of Greenscoe Farm, Low Furness (Locality 174). h. Archiclimacograptus cf. caelatus (Lapworth), RX 3724. Tarn Moor Formation (upper part of the artus Biozone?), Aik Beck (Locality 166). i. Didymograptus artus Elles & Wood, Zv 713. Kirk Stile Formation (artus Biozone), north of Hazelhurst (Locality 134). j. Didymograptus spinulosus Perner, RX 2503A. Kirk Stile Formation (artus Biozone), Outerside (Locality 80). k. Didymograptus murchisoni speciosus Ekstr�m, RX 3194, 4. Tarn Moor Formation (close to the base of the murchisoni Biozone), Aik Beck (Locality 165). l, m. Didymograptus (Acrograptus) affinis Nicholson. l, proximal end, Natural History Museum Q.5858a, 10. Tarn Moor Formation (artus Biozone), Aik Beck (Locality 164). m, RX 3194, 5. Tarn Moor Formation (close to the base of the murchisoni Biozone), Aik Beck (Locality 165). n. Holmograptus lentus (Holm), distal fragment, Zv 752. Kirk Stile Formation (artus Biozone), north of Hazelhurst (Locality 134). o. Didymograptus acutidens Elles & Wood, PC 9084. Skiddaw Group mudstones (artus Biozone), Park Farm, Askam in Furness (Locality 175)." data-name="images/P936906.jpg">(Figure 11)q, n r j. D. uniformis uniformis (Figure 13) Selected graptolites from Llanvirn strata of the Skiddaw Group. All specimens except for l are in the collections of the British Geological Survey. All magnified 5 except k and l. The parallel lines indicate the trace of the cleavage in the plane of bedding. a. Archiclimacograptus angulatus magnus (Berry), AJ 1008, cited by Skevington in Wadge et al. 1972, p. 63, as a species of Pseudoclimacograptus. Tarn Moor Formation (murchisoni Biozone), Tarn Moor Tunnel, 808?818 m from the south portal. b. Paraglossograptus sp., RX 4960. Tarn Moor Formation (artus Biozone), Mosedale Beck (Locality 160). c. Cryptograptus schaeferi Lapworth, RX 3189. Tarn Moor Formation (artus Biozone), Cawell Beck (Locality 157). d. Climacograptus bifomis Mu & Lee?, RX 3195. Tarn Moor Formation (close to the base of the murchisoni Biozone), Aik Beck (Locality 165). e.f. Amplexograptus confertus (Lapworth), AJ 858 and AJ 877, Tarn Moor Formation (murchisoni Biozone), Tarn Moor Tunnel, 2311?2344 m from the south portal. g. Undulograptus cumbrensis (Bulman), RX 5463. Skiddaw Group mudstones (artus Biozone?), quarry east of Greenscoe Farm, Low Furness (Locality 174). h. Archiclimacograptus cf. caelatus (Lapworth), RX 3724. Tarn Moor Formation (upper part of the artus Biozone?), Aik Beck (Locality 166). i. Didymograptus artus Elles & Wood, Zv 713. Kirk Stile Formation (artus Biozone), north of Hazelhurst (Locality 134). j. Didymograptus spinulosus Perner, RX 2503A. Kirk Stile Formation (artus Biozone), Outerside (Locality 80). k. Didymograptus murchisoni speciosus Ekstr�m, RX 3194, 4. Tarn Moor Formation (close to the base of the murchisoni Biozone), Aik Beck (Locality 165). l, m. Didymograptus (Acrograptus) affinis Nicholson. l, proximal end, Natural History Museum Q.5858a, 10. Tarn Moor Formation (artus Biozone), Aik Beck (Locality 164). m, RX 3194, 5. Tarn Moor Formation (close to the base of the murchisoni Biozone), Aik Beck (Locality 165). n. Holmograptus lentus (Holm), distal fragment, Zv 752. Kirk Stile Formation (artus Biozone), north of Hazelhurst (Locality 134). o. Didymograptus acutidens Elles & Wood, PC 9084. Skiddaw Group mudstones (artus Biozone), Park Farm, Askam in Furness (Locality 175)." data-name="images/P936906.jpg">(Figure 11)m appears to be confined to the gibberulus Biozone, and specimens assigned to D. nitidus have been collected from it but are not definitely known in the victoriae Biozone. Pseudophyllograptus is less common than below.

The gibberulus Biozone has been recognised around Grisedale Pike (Localities 72–76), in strata younger than the victoriae Biozone south of the Gasgale Thrust (see above). Jenkins (1982, fig. 2H–J) figured I. caduceus cf. imitatus from the north-west side of the summit, whereas

I. c. gibberulus has been found 500 m farther south-east ((Figure 8), Locality 76). The strata there are folded and the stratigraphical separation of these localities is not known, but the regional younging direction is south-eastwards. These taxa may correspond to biohorizons 44 and 46 in Cooper and Lindholm’s compilation (1990, fig. 4), well above biohorizons 39–41 at which I. victoriae victoriae and D. hirundo appear.

Isograptus caduceus gibberulus has been found at various places between Ullock Pike (Locality 100) and Kiln Pots (Locality 107) on the west flank of Skiddaw, associated with horizontal didymograptids. To the north of a line extending approximately west-south-west to east-north-east, between Localities 107 and 108 from [NY 2375 2958] to [NY 2410 2970], however, the fauna changes abruptly to that of the cucullus Biozone (Figure 10), between Localities 107 and 108). The type locality for I. caduceus gibberulus is Randel Crag (Locality 115), where it is associated with I. victoriae divergens (Jenkins, 1982). Farther north, at White Horse (Locality 116), a more doubtful example of I. c. gibberulus is associated with Pseudisograptus angel (for which this is the type locality), but no species typical of the cucullus Biozone are present at this locality. P. angel ((Figure 13) Selected graptolites from Llanvirn strata of the Skiddaw Group. All specimens except for l are in the collections of the British Geological Survey. All magnified 5 except k and l. The parallel lines indicate the trace of the cleavage in the plane of bedding. a. Archiclimacograptus angulatus magnus (Berry), AJ 1008, cited by Skevington in Wadge et al. 1972, p. 63, as a species of Pseudoclimacograptus. Tarn Moor Formation (murchisoni Biozone), Tarn Moor Tunnel, 808?818 m from the south portal. b. Paraglossograptus sp., RX 4960. Tarn Moor Formation (artus Biozone), Mosedale Beck (Locality 160). c. Cryptograptus schaeferi Lapworth, RX 3189. Tarn Moor Formation (artus Biozone), Cawell Beck (Locality 157). d. Climacograptus bifomis Mu & Lee?, RX 3195. Tarn Moor Formation (close to the base of the murchisoni Biozone), Aik Beck (Locality 165). e.f. Amplexograptus confertus (Lapworth), AJ 858 and AJ 877, Tarn Moor Formation (murchisoni Biozone), Tarn Moor Tunnel, 2311?2344 m from the south portal. g. Undulograptus cumbrensis (Bulman), RX 5463. Skiddaw Group mudstones (artus Biozone?), quarry east of Greenscoe Farm, Low Furness (Locality 174). h. Archiclimacograptus cf. caelatus (Lapworth), RX 3724. Tarn Moor Formation (upper part of the artus Biozone?), Aik Beck (Locality 166). i. Didymograptus artus Elles & Wood, Zv 713. Kirk Stile Formation (artus Biozone), north of Hazelhurst (Locality 134). j. Didymograptus spinulosus Perner, RX 2503A. Kirk Stile Formation (artus Biozone), Outerside (Locality 80). k. Didymograptus murchisoni speciosus Ekstr�m, RX 3194, 4. Tarn Moor Formation (close to the base of the murchisoni Biozone), Aik Beck (Locality 165). l, m. Didymograptus (Acrograptus) affinis Nicholson. l, proximal end, Natural History Museum Q.5858a, 10. Tarn Moor Formation (artus Biozone), Aik Beck (Locality 164). m, RX 3194, 5. Tarn Moor Formation (close to the base of the murchisoni Biozone), Aik Beck (Locality 165). n. Holmograptus lentus (Holm), distal fragment, Zv 752. Kirk Stile Formation (artus Biozone), north of Hazelhurst (Locality 134). o. Didymograptus acutidens Elles & Wood, PC 9084. Skiddaw Group mudstones (artus Biozone), Park Farm, Askam in Furness (Locality 175)." data-name="images/P936906.jpg">(Figure 11)i) is therefore assigned to the gibberulus Biozone, placing its appearance a little lower than biohorizon 53 of Cooper and Lindholm (1990), which they give as the level at which Cryptograptus and diplograptids appear. No diplograptid or cryptograptid has been found at White Horse or any other locality in this zone; earlier records prove on re-examination to be based on examples of Pseudotrigonograptus.

The parts of the Skiddaw Group succession correlated with the gibberulus Biozone are faulted and folded, and no safe estimate of the thickness could be made. On the north-west flank of Skiddaw it may occupy more than 400 m of strata.

Localities

Besides the localities around Grisedale Pike and Skiddaw mentioned above, the gibberulus Biozone has been recognised at Winnah (Locality 84), Abbey Gate (Locality 87), Sandybeck (Locality 86) and Knott Head (Localities 90, 91). Its presence is inferred south of Skiddaw, on Doups and east of Lonscale Fell (Localities 94, 96), on the basis of badly preserved specimens of D. uniformis. Clasts in the Buttermere Formation at Rowling End (Locality 151) contain D. uniformis and are considered to have come from the gibberulus Biozone.

Aulograptus cucullus Biozone

The uppermost graptolite zone in the British Arenig has long been termed the Didymograptus hirundo Zone (Elles, 1933; Jackson, 1962), and was considered to be characterised by the stratigraphical range of D. hirundo itself. When it was found that D. hirundo appeared much lower in the Kirk Stile Formation than had previously been supposed, Fortey et al. (1990, p.128) modified the definition of the hirundo Zone to embrace the partial range of

D. hirundo above the range of I. caduceus gibberulus. However, D. hirundo is seldom found at this level, and the uppermost Arenig is better characterised by Aulograptus and other graptolites, including diplograptids (Fortey et al., 1990). Of these, the distinctive Aulograptus cucullus is chosen to name the zone.

The fauna of the cucullus Biozone includes several taxa not found below: Acrograptus aff. affinis, Didymograptus sparsus ((Figure 13) Selected graptolites from Llanvirn strata of the Skiddaw Group. All specimens except for l are in the collections of the British Geological Survey. All magnified 5 except k and l. The parallel lines indicate the trace of the cleavage in the plane of bedding. a. Archiclimacograptus angulatus magnus (Berry), AJ 1008, cited by Skevington in Wadge et al. 1972, p. 63, as a species of Pseudoclimacograptus. Tarn Moor Formation (murchisoni Biozone), Tarn Moor Tunnel, 808?818 m from the south portal. b. Paraglossograptus sp., RX 4960. Tarn Moor Formation (artus Biozone), Mosedale Beck (Locality 160). c. Cryptograptus schaeferi Lapworth, RX 3189. Tarn Moor Formation (artus Biozone), Cawell Beck (Locality 157). d. Climacograptus bifomis Mu & Lee?, RX 3195. Tarn Moor Formation (close to the base of the murchisoni Biozone), Aik Beck (Locality 165). e.f. Amplexograptus confertus (Lapworth), AJ 858 and AJ 877, Tarn Moor Formation (murchisoni Biozone), Tarn Moor Tunnel, 2311?2344 m from the south portal. g. Undulograptus cumbrensis (Bulman), RX 5463. Skiddaw Group mudstones (artus Biozone?), quarry east of Greenscoe Farm, Low Furness (Locality 174). h. Archiclimacograptus cf. caelatus (Lapworth), RX 3724. Tarn Moor Formation (upper part of the artus Biozone?), Aik Beck (Locality 166). i. Didymograptus artus Elles & Wood, Zv 713. Kirk Stile Formation (artus Biozone), north of Hazelhurst (Locality 134). j. Didymograptus spinulosus Perner, RX 2503A. Kirk Stile Formation (artus Biozone), Outerside (Locality 80). k. Didymograptus murchisoni speciosus Ekstr�m, RX 3194, 4. Tarn Moor Formation (close to the base of the murchisoni Biozone), Aik Beck (Locality 165). l, m. Didymograptus (Acrograptus) affinis Nicholson. l, proximal end, Natural History Museum Q.5858a, 10. Tarn Moor Formation (artus Biozone), Aik Beck (Locality 164). m, RX 3194, 5. Tarn Moor Formation (close to the base of the murchisoni Biozone), Aik Beck (Locality 165). n. Holmograptus lentus (Holm), distal fragment, Zv 752. Kirk Stile Formation (artus Biozone), north of Hazelhurst (Locality 134). o. Didymograptus acutidens Elles & Wood, PC 9084. Skiddaw Group mudstones (artus Biozone), Park Farm, Askam in Furness (Locality 175)." data-name="images/P936906.jpg">(Figure 11)a), D. acutidens, D. nicholsoni planus, Cryptograptus spp. ((Figure 13)c) and diplograptids (Eoglyptograptus, Undulograptus, ((Figure 13) Selected graptolites from Llanvirn strata of the Skiddaw Group. All specimens except for l are in the collections of the British Geological Survey. All magnified 5 except k and l. The parallel lines indicate the trace of the cleavage in the plane of bedding. a. Archiclimacograptus angulatus magnus (Berry), AJ 1008, cited by Skevington in Wadge et al. 1972, p. 63, as a species of Pseudoclimacograptus. Tarn Moor Formation (murchisoni Biozone), Tarn Moor Tunnel, 808?818 m from the south portal. b. Paraglossograptus sp., RX 4960. Tarn Moor Formation (artus Biozone), Mosedale Beck (Locality 160). c. Cryptograptus schaeferi Lapworth, RX 3189. Tarn Moor Formation (artus Biozone), Cawell Beck (Locality 157). d. Climacograptus bifomis Mu & Lee?, RX 3195. Tarn Moor Formation (close to the base of the murchisoni Biozone), Aik Beck (Locality 165). e.f. Amplexograptus confertus (Lapworth), AJ 858 and AJ 877, Tarn Moor Formation (murchisoni Biozone), Tarn Moor Tunnel, 2311?2344 m from the south portal. g. Undulograptus cumbrensis (Bulman), RX 5463. Skiddaw Group mudstones (artus Biozone?), quarry east of Greenscoe Farm, Low Furness (Locality 174). h. Archiclimacograptus cf. caelatus (Lapworth), RX 3724. Tarn Moor Formation (upper part of the artus Biozone?), Aik Beck (Locality 166). i. Didymograptus artus Elles & Wood, Zv 713. Kirk Stile Formation (artus Biozone), north of Hazelhurst (Locality 134). j. Didymograptus spinulosus Perner, RX 2503A. Kirk Stile Formation (artus Biozone), Outerside (Locality 80). k. Didymograptus murchisoni speciosus Ekstr�m, RX 3194, 4. Tarn Moor Formation (close to the base of the murchisoni Biozone), Aik Beck (Locality 165). l, m. Didymograptus (Acrograptus) affinis Nicholson. l, proximal end, Natural History Museum Q.5858a, 10. Tarn Moor Formation (artus Biozone), Aik Beck (Locality 164). m, RX 3194, 5. Tarn Moor Formation (close to the base of the murchisoni Biozone), Aik Beck (Locality 165). n. Holmograptus lentus (Holm), distal fragment, Zv 752. Kirk Stile Formation (artus Biozone), north of Hazelhurst (Locality 134). o. Didymograptus acutidens Elles & Wood, PC 9084. Skiddaw Group mudstones (artus Biozone), Park Farm, Askam in Furness (Locality 175)." data-name="images/P936906.jpg">(Figure 11)) f, h, b. One of the most widely distributed and easily recognised species is Aulograptus cucullus (Figure 13) Selected graptolites from Llanvirn strata of the Skiddaw Group. All specimens except for l are in the collections of the British Geological Survey. All magnified 5 except k and l. The parallel lines indicate the trace of the cleavage in the plane of bedding. a. Archiclimacograptus angulatus magnus (Berry), AJ 1008, cited by Skevington in Wadge et al. 1972, p. 63, as a species of Pseudoclimacograptus. Tarn Moor Formation (murchisoni Biozone), Tarn Moor Tunnel, 808?818 m from the south portal. b. Paraglossograptus sp., RX 4960. Tarn Moor Formation (artus Biozone), Mosedale Beck (Locality 160). c. Cryptograptus schaeferi Lapworth, RX 3189. Tarn Moor Formation (artus Biozone), Cawell Beck (Locality 157). d. Climacograptus bifomis Mu & Lee?, RX 3195. Tarn Moor Formation (close to the base of the murchisoni Biozone), Aik Beck (Locality 165). e.f. Amplexograptus confertus (Lapworth), AJ 858 and AJ 877, Tarn Moor Formation (murchisoni Biozone), Tarn Moor Tunnel, 2311?2344 m from the south portal. g. Undulograptus cumbrensis (Bulman), RX 5463. Skiddaw Group mudstones (artus Biozone?), quarry east of Greenscoe Farm, Low Furness (Locality 174). h. Archiclimacograptus cf. caelatus (Lapworth), RX 3724. Tarn Moor Formation (upper part of the artus Biozone?), Aik Beck (Locality 166). i. Didymograptus artus Elles & Wood, Zv 713. Kirk Stile Formation (artus Biozone), north of Hazelhurst (Locality 134). j. Didymograptus spinulosus Perner, RX 2503A. Kirk Stile Formation (artus Biozone), Outerside (Locality 80). k. Didymograptus murchisoni speciosus Ekstr�m, RX 3194, 4. Tarn Moor Formation (close to the base of the murchisoni Biozone), Aik Beck (Locality 165). l, m. Didymograptus (Acrograptus) affinis Nicholson. l, proximal end, Natural History Museum Q.5858a, 10. Tarn Moor Formation (artus Biozone), Aik Beck (Locality 164). m, RX 3194, 5. Tarn Moor Formation (close to the base of the murchisoni Biozone), Aik Beck (Locality 165). n. Holmograptus lentus (Holm), distal fragment, Zv 752. Kirk Stile Formation (artus Biozone), north of Hazelhurst (Locality 134). o. Didymograptus acutidens Elles & Wood, PC 9084. Skiddaw Group mudstones (artus Biozone), Park Farm, Askam in Furness (Locality 175)." data-name="images/P936906.jpg">(Figure 11)e, although, like many of the associated taxa, it ranges up into the artus Biozone at the base of the Llanvirn. Some species common at lower levels persist as rarities into this zone: D. extensus linearis, D. nitidus and D. hirundo, together with Xiphograptus svalbardensis. Compared with Cooper and Lindholm’s (1990) sequences of biohorizons, species of the cucullus Biozone appear at horizons 53 to 55, although a single juvenile specimen of Cardiograptus (biohorizon 52) was found at the top of the zone at Outerside.

The cucullus Biozone at Force Crag (Locality 78) and on Outerside (Locality 79) clearly overlies the gibberulus Biozone at Grisedale Pike (Localities 72–76), although the stratigraphical separation is not known (Figure 8). The gibberulus Biozone of Kiln Pots (Locality 107) and south of Sand Beds (Locality 104) is succeeded so sharply by the cucullus Biozone faunas of Ling How, Raven Crag (north of Sand Beds) and Watches (Localities 108–112; (Figure 10) that a faulted contact may be suspected.

The cucullus Biozone is succeeded by the artus Biozone on Outerside (Localities 79, 80) and Souther Fell (Fortey et al., 1990; Localities 133, 134), and probably also in Mosedale Beck, Threlkeld (Localities 159, 160). The thickness through which the cucullus Biozone is developedappearstobegreat,possiblyseveralhundred metres, but is uncertain because of the prevalence of folding and faulting.

Localities

Coledale (Force Crag and Outerside; Localities 78, 79), Seathwaite How Quarry, Embleton (Locality 170), Ling How to Watches (Localities 108–113), Dead Beck (Localities 117–119) and Skiddaw Dodd (Locality 97). The zone has been proved at many localities over a large area from the south side of Blencathra to the River Glenderamackin, Bannerdale and Souther Fell (Localities 124–130). It may be present in the lowest beds of the Tarn Moor Formation in Mosedale Beck, Threlkeld Common (Locality 159; (Figure 12), and it is the oldest zone detected at Black Combe (Localities 171–173; Rushton and Molyneux, 1989).

Didymograptus artus Biozone

The artus Biozone was proposed by Fortey and Owens (1987, p.90) to replace the misleading older term ‘bifidus’ Zone. Fortey et al. (1990) discussed criteria for its recognition, mainly the appearance of Didymograptus (D.) artus and (D.) spinulosus (Figure 13)i, j. Elles’ (1933) contention that the base of the zone is recognisable at Outerside and Hazelhurst, on the east of Souther Fell, was upheld by Fortey et al. (1990, table 2) who gave faunal lists. In Cooper and Lindholm’s compilation, the base of the artus Biozone is recognised at horizon 56. The zone is present in the uppermost beds of the Kirk Stile Formation, to a thickness of no more than a couple of hundred metres. The top of the zone is not seen.

The artus Biozone is well developed in the Tarn Moor Formation (Cooper et al., 1995, p.202). The lower beds are exposed in Mosedale Beck, Threlkeld Common, where there appears be a passage from the cucullus to the artus Biozone (Figure 12). To the west of Ullswater, outcrops near Glencoyne [NY 384 187] have yielded evidence of the Llanvirn, whereas underground borings at Greenside Mine entrance at [NY 363 178] revealed strata near the Arenig–Llanvirn (cucullus–artus) boundary.

East of Ullswater, strata exposed in Aik Beck have yielded faunas of the artus Biozone, and at one locality (Locality 165) there is a suggestion of strata near or above the artus–murchisoni zonal boundary. The artus Biozone is present in the Bampton inlier, at Thornship Beck (the type-locality for D. artus itself) and at Keld Gill, but the best known fauna is that from the Tailbert–Lanshaw Tunnel, described by Skevington (1970), in which the presence of Nicholsonograptus fasciculatus was taken to indicate a high level in the artus Biozone. The total thickness is not known.

The Kirkland Formation of the Cross Fell Inlier includes representatives of both the lower and upper parts of the artus Biozone (Burgess and Holliday, 1979).

Localities

Outerside (Locality 80), Hazelhurst (Locality 134), Mosedale Beck (Localities 158,160) and below the Borrowdale Volcanic Group unconformity in Matterdale Beck (Locality 161); Cawell Beck (Locality 157); Aik Beck (Localities 163, 164), Ullswater Inlier; the River Lowther (Locality 167) and elsewhere in the Bampton inlier (Localities 168, 169); Furness (Localities 174?, 175); the Cross Fell and Teesdale inliers.

Didymograptus murchisoni Biozone

Although the murchisoni Biozone has long been recognised in England and Wales, it has sometimes proved difficult to distinguish from the underlying zone using pendent didymograptids (Strachan, 1960, p.111). There is a putative record of the zone in Aik Beck (Locality 165) where Didymograptus murchisoni cf. speciosus (Figure 13)k was found. Work in the Builth area (Davies et al., 1997, p.11) has indicated that D. speciosus appears there in the lower part of the murchisoni Biozone, so the examples from Aik Beck, although more doubtful, may suggest the presence of strata near or above the artus–murchisoni zonal boundary.

The only positive record of the murchisoni Biozone in the Skiddaw Group is in the Tarn Moor Tunnel (Wadge et al., 1969, 1972). Skevington (in Wadge et al., 1972) emphasised the presence of biserial graptolites (Amplexograptus (Figure 13)e, f and Pseudoclimacograptus) and, by comparison with their distribution in Sweden, inferred that the murchisoni Biozone was present. There may be more than 300 m of beds referable to the murchisoni Biozone in the Tarn Moor Tunnel, but the stratigraphical relationships of the zone were not seen and its total thickness is unknown.

Macrofossils other than graptolites

All groups of macrofossils other than graptolites are rare, with the exception of the supposed phyllocarid crustacean Caryocaris. Carapaces of Caryocaris ((Plate 3)a) are found commonly throughout the Skiddaw Group, but are not known to have biostratigraphical value. However, Rushton and Williams (1997) suggested that the tail-pieces (which are found much more rarely) may show differences of stratigraphical significance. Specimens from early Arenig strata have five pairs of marginal spinules, those from the late Arenig have three pairs ((Plate 3)d), and those from the Llanvirn have only one pair.

The trilobites of the Skiddaw Group, reviewed by Fortey et al. (1989), are mainly bathypelagic cyclopygids and atheloptic (small-eyed or blind, deep-water benthic) forms ((Plate 3)b, c, e–h). The most diverse assemblage, however, is the Migneintian (upper Tremadoc) Angelina sedgwickii Biozone fauna, consisting of ten genera, recorded by Rushton (1988) from the River Calder (Locality 136). This fauna may be from an olistolith in the Buttermere Formation, but could be from an older, in situ deposit (see Chapter 3). Several other Skiddaw Group trilobite species also occur in the Arenig Series of south Wales (Fortey and Owens, 1987), two being known from the Whitlandian and about ten from the Fennian, whilst a few of the other species have been recorded from the early Llanvirn rocks of Wales. Their stratigraphical distribution was given by Cooper et al. (1995, figs 4, 15). The biogeographical affiliation of the trilobites shows that the Skiddaw Group was deposited on the edge of the Gondwanan continent (Fortey et al., 1989), and indicates a deep-marine environmental setting.

Other body fossils are very rare. Sponge spicules have been found at a few localities, for example Barf (Locality 25), Knott Head (Locality 91) and east of Crag Wood (Locality 123). Some lingulate brachiopods were described by Postlethwaite (1897) and reviewed by Cocks (1978, pp.9, 13–15, 169), and two further specimens were collected during this survey. Two specimens of Mollusca are known, a single bivalve (possibly byssate) from Lonscale Crags [NY 2924 2689] and an orthoconic nautiloid from Blease Gill, Blencathra [about [NY 315 265]. An example of a supposedly benthic hyolith, collected from a clast in the Buttermere Formation in Sail Beck [NY 1880 1818], Buttermere, may have been derived down-slope from a relatively shallow-water site. Similarly, at Beck Grains (Locality 137), graptolitic and trilobitic mudstones, which may also be part of a clast in the Buttermere Formation, contain a thin layer resembling a shelly lag. This yielded a conulariid, orthoid brachiopods, an orthothecid hyolith and two ostracods.

Trace fossils are ubiquitous in the Skiddaw Group, mostly of kinds that extend along rather than across the planes of bedding. Orr (1996) described some 20 ichnotaxa, mainly of fodinichnia, and concluded that the relatively diverse ichnofauna provides further indications of a deep-marine environment.

Microfossils

The application of micropalaeontology to the biostratigraphy of the Skiddaw Group is a comparatively recent development, spanning the last three decades. The earliest accounts dealt with acritarchs and chitinozoa from the Skiddaw Group of the Cross Fell and Teesdale inliers (Wadge et al., 1967, 1969; Lister et al., 1969; Lister and Holliday, 1970), and resulted in the recognition of four stratigraphical assemblages in the Cross Fell succession (Lister in Arthurton and Wadge, 1981). More recent collections from the Skiddaw Group have shown that acritarchs in particular have an important role to play in deciphering Skiddaw Group stratigraphy. They often complement the graptolites, and in some instances, for example in the lowest part of the succession in the Northern Fells Belt of the Skiddaw Inlier, provide more abundant and reliable biostratigraphical evidence.

There remains a general lack of published acritarch biozonal schemes for the lower Ordovician, although informal schemes exist for all or part of the Tremadoc–Llanvirn interval in Shropshire, south Wales, eastern Newfoundland, northern Norway, Morocco, Algeria and Alberta (references in Molyneux et al., 1996). Molyneux (in Cooper et al., 1995) documented a succession of acritarch assemblages in the Skiddaw Group, and these are used here as the basis of a biozonation (Figure 4). The biozones are best regarded as assemblage biozones (sensu Whittaker et al., 1991) since the criteria needed to define other types of biozone are often not met. For example, the sampling interval is relatively coarse, and the exact superpositional relationships between isolated samples are uncertain because of structural and stratigraphical complexities. These problems make it difficult to ascertain the precise level at which individual taxa appear or disappear in the succession, criteria that are needed to define the various types of biozones based on ranges of taxa. The biozones are instead characterised by a particular assemblage of acritarch taxa, the upper and lower boundaries of each biozone being located generally but not precisely in the rock succession. The order of succession of the acritarch biozones has been determined from several complementary lines of evidence, including superpositional evidence based on mapping, the controls provided by graptolites in the Skiddaw Group, and comparison with acritarch assemblages elsewhere.

The biozonal scheme effectively covers only the latest Tremadoc and Arenig parts of the Skiddaw Group. No systematic study has been made of acritarch microfloras from Llanvirn strata, although a limited amount of information is available. Microfloras older than the late Tremadoc Araneograptus murrayi Biozone are also excluded from the biozonal scheme. Microfloras of older Tremadoc and possible Cambrian age have been recorded from the Skiddaw Group, but most if not all of the older Tremadoc assemblages are reworked into a late Arenig olistostrome deposit, while the superpositional relationship between the possible Cambrian rocks and the other parts of the Skiddaw Group succession remains unclear.

Of all the acritarch biozones recognised in the Skiddaw Group, two contain particularly distinctive assemblages. The Cymatiogalea messaoudensis–Stelliferidium trifidum Biozone, spanning the Tremadoc–Arenig boundary, is generally easy to recognise because of the distinctive nature of its eponymous and other characteristic taxa. Apart from the Lake District, the assemblage comprising the biozone has been recognized in south Wales, southern Spain, on the island of Rügen off the north coast of Germany, in the Czech Republic and in Turkey, while elements of the microflora are known from Algeria (Servais and Molyneux, 1997). The messaoudensis–trifidum microflora appears to have considerable biostratigraphical significance in lower Ordovician successions of Gondwana and peri-Gondwana. The Upper Arenig Frankea hamata–Striatotheca rarirrugulata Biozone is also readily recognisable, and its distinctive microflora has been reported from the Isle of Man and Brabant Massif, in addition to the Lake District (Molyneux and Leader, 1997).

The following account provides details of the composition and occurrence of each acritarch microflora and biozone in the Skiddaw Group, along with the evidence for its age. The stratigraphical ranges of selected taxa are shown in (Figure 14).

Cambrian(?) acritarchs

Four samples, collected from the Skiddaw Group about 1.35 km north-north-west of Eycott Hill [NY 3820 3074] (Millward and Molyneux, 1992), yielded acritarch microfloras dominated by Timofeevia spp., including T. phosphoritica. Timofeevia ranges into the lower part of the Arenig, but it is more common in microfloras of Middle and Late Cambrian age. The predominance of this genus suggests a possible Cambrian age for the samples, although the evidence is not conclusive and an early Ordovician age cannot be discounted. Nevertheless, there are no diagnostic Tremadoc or Arenig marker species. Other acritarchs include Cristallinium?, Micrhystridium and poorly preserved acanthomorph acritarchs, all of which would be consistent with either a Cambrian or early Ordovician age.

Cressagian (Lower Tremadoc) acritarchs

Several localities east of Buttermere have yielded Cressagian acritarch microfloras, all of which are considered to be from blocks of early Tremadoc age in the Buttermere Formation olistostrome (Cooper et al., 1995). On Goat Crag [NY 1928 1640], [NY 1894 1631], [NY 1895 1635], microfloras from the Goat Gills Member are dated as early Tremadoc, equivalent to either the flabelliformis or tenellus graptolite Biozone of the Welsh Borderland, by the presence of Acanthodiacrodium cf. ubui, Priscotheca tumida? and Stelliferidium cortinulum? (see acritarch ranges given by Rasul, 1979; Molyneux et al., 1996). Microfloras from the undifferentiated Buttermere Formation on the northern slope of High Snockrigg [NY 1857 1706], at Newlands Hause [NY 1930 1762], [NY 1927 1751], and in the upper part of Swinside Gill between [NY 1921 1784] and [NY 1901 1777], also contain Cressagian acritarchs, with some or all of Acanthodiacrodium angustum, Acanthodiacrodium ubui, Cymatiogalea bellicosa, Cymatiogalea spp., Dasydiacrodium cilium?, Micrhystridium diornamentum, Priscotheca tumida?, Stelliferidium cortinulum and Vulcanisphaera spp.

Migneintian (Upper Tremadoc) acritarchs

Poorly preserved Migneintian acritarchs were reported from Skiddaw Group outcrops on the River Calder around [NY 0687 1174] by Molyneux and Rushton (1984). Subsequent collecting has established that Tremadoc rocks occur over a distance of about 0.45 km along the river [NY 0685 1172] to [NY 0706 1210]. The most diverse collections are from the southernmost exposures [NY 0685 1172] to [NY 0687 1174], where the acritarchs are associated with trilobites of the Migneintian Angelina sedgwickii Biozone (Molyneux and Rushton, 1984; Rushton, 1988; Appendix 2, Locality 136). Among the acritarchs, ?Cymatiogalea membrana and Stelliferidium fimbrium provide independent evidence for a late Tremadoc age, based on their ranges in the Sheinton Shales of Shropshire (Rasul and Downie, 1974; Rasul, 1979; Molyneux and Rushton, 1984). There is no macrofaunal control on the age of the acritarchs from the more northerly collections [NY 0693 1188] to [NY 0694 1190] and [NY 0706 1210], but Stelliferidium fimbrium and S. trifidum? again suggest a late Tremadoc age.

Possible Migneintian acritarch assemblages also occur in the Buttermere Formation east of Buttermere. Stelliferidium fimbrium? and Vulcanisphaera frequens? suggest a late Tremadoc age for microfloras from High Snockrigg [NY 1814 1710], the foot of Swinside Gill [around [NY 1875 1780] and Sail Beck [NY 1876 1791], based on the level at which these acritarchs occur in Shropshire (Rasul, 1979) and Poland (Górka, 1967). Stelliferidium trifidum suggests a similar age for microfloras collected from High Hole Beck [NY 199 173] and crags [NY 1922 1743] south of Newlands Hause. Also along Sail Beck [NY 1881 1819], the association of Arbusculidium cf. destombesii with S. fimbrium? suggests a possible mid- to late Tremadoc age.

Undifferentiated Tremadoc acritarch microfloras

Acritarch microfloras from Goat Crag [NY 1886 1655], [NY 1928 1640], High Snockrigg [NY 1814 1710] and Swinside Gill [NY 1902 1776], all localities in the Buttermere Formation east of Buttermere, comprise species of Acanthodiacrodium (including A. angustum?), Cymatiogalea (C. cristata, C. cuvillieri, C. multarea?), Polygonium, Priscotheca, Stelliferidium and Vulcanisphaera (V. africana, V. britannica?). The association of these taxa is characteristic of the Tremadoc, but all range through the series.

A similar microflora, indicating an undifferentiated Tremadoc age, was obtained from Skiddaw Group rocks beneath the Eycott Volcanic Group, 0.45 km south-south-east of the summit of Eycott Hill [NY 3877 2909] (Millward and Molyneux, 1992).

Stratigraphical occurrence of Tremadoc acritarchs

The Tremadoc microfloras listed above show no direct evidence for recycling, i.e. there is no mixing of Tremadoc and younger species. Nevertheless, microfloras from the Buttermere area are considered to be recycled because of their occurrence in the Buttermere Formation, an olistostrome emplaced during the late Arenig. The Tremadoc acritarchs from the River Calder might also be recycled, if the beds exposed there represent a raft within the Buttermere Formation, but could be from older in situ deposits (see Chapter 3). Similarly, it is uncertain whether the Tremadoc (and possible Cambrian) acritarchs from the Skiddaw Group on Eycott Hill are from an in situ succession, or whether they are from blocks within an olistostrome.

In other instances, there is clear evidence that Tremadoc acritarchs are recycled into younger deposits. Recycling of Tremadoc acritarchs is evident, for example, in additional samples from the Buttermere Formation of Goat Crag [NY 1894 1631], [NY 1900 1646], High Snockrigg [NY 1857 1706], [NY 1841 1707], Swinside Gill [NY 1876 1779] and Sail Beck [NY 1880 1819], where they occur in each case with Arenig acritarchs. Perhaps the best example, however, is from a debris flow within the Kirk Stile Formation around Beckgrains Bridge [NY 1904 3552], near the northern edge of the Skiddaw inlier, where Tremadoc species such as Acanthodiacrodium angustum, Cymatiogalea bellicosa, C. membrana, Stelliferidium trifidum and Vulcanisphaera spp. occur in the same samples as late Arenig acritarchs of the hamata–rarirrugulata Biozone. It is notable that the Tremadoc component includes both Cressagian (lower Tremadoc) (C. bellicosa) and Migneintian (upper Tremadoc) (C. membrana, S. trifidum) acritarchs, and that the late Arenig age indicated by the hamata–rarirrugulata microflora approximates to the age of emplacement of the Buttermere Formation.

Cymatiogalea messaoudensis–Stelliferidium trifidum Assemblage Biozone

The acritarchs of the Cymatiogalea messaoudensis–Stelliferidium trifidum Biozone are the oldest Skiddaw Group acritarchs that can be placed in superpositional order. The biozone spans the Bitter Beck, Watch Hill and lower part of the Hope Beck formations in the Northern Fells Belt (Figure 4). The base of the biozone has not been located in the Skiddaw Group, but its top, coinciding with the base of the overlying trifidum–bohemicum Biozone, is estimated to lie approximately 400 m above the base of the Hope Beck Formation (Figure 15) in the valley of the River Derwent around [NY 1354 3269], north of Watch Hill, near the northern edge of the Skiddaw Inlier. Acritarchs indicating the messaoudensis–trifidum Biozone occur in the murrayi graptolite Biozone at localities 2 [NY 1120 2982] and 3 around [NY 1420 3122] of Molyneux and Rushton (1988; Appendix 2, Localities 3 and 1, respectively) and at Trusmadoor [NY 2777 3362] (Cooper et al., 1995; Appendix 2, Locality 4), but also occur in close proximity to graptolites of the phyllograptoides Biozone in Burn Tod Gill (around [NY 2797 3340]; Appendix 2, Localities 9, 10). The graptolite evidence thus shows the messaoudensis–trifidum Biozone to span the murrayi–phyllograptoides biozonal boundary, which may be taken as the Tremadoc–Arenig series boundary.

Two features of the messaoudensis–trifidum microflora are readily apparent. The first is the presence of a number of distinctive species, including Acanthodiacrodium? dilatum, Caldariola glabra, Cymatiogalea deunffii ((Plate 4)h), C. messaoudensis ((Plate 4)a; name corrected from messaoudii by Servais and Molyneux, 1997), Stellechinatum sicaforme s.l. ((Plate 4)j), Vavrdovella areniga s.l. and Vogtlandia coalita, some of which are restricted to this assemblage. Together with Stelliferidium trifidum, which is often relatively abundant, they give the messaoudensis–trifidum microflora its unique and readily recognisable character. Furthermore acritarch species that are generally more typical of Tremadoc assemblages, such as Cymatiogalea cristata, Stelliferidium trifidum ((Plate 4)b) and species of Acanthodiacrodium and Vulcanisphaera, are associated with forms that are more characteristic of post-Tremadoc assemblages, including species of Coryphidium, Peteinosphaeridium, Pirea, Striatotheca and Veryhachium. The messaoudensis–trifidum microflora thus appears to be transitional between Tremadoc and Arenig microfloras.

Molyneux (in Cooper et al., 1995) divided the messaoudensis–trifidum microflora into five components (subassemblages), based on the successive appearance of species. The levels of these appearances can be located near the northern edge of the Skiddaw inlier where the structure is relatively simple, the succession generally dipping northwards at about 30 to 40° from Bitter Beck across Watch Hill to the River Derwent. Possible complications arise, however, from repetition of strata across strike faults, minor folding including slump folds, and poor exposure. The sub-assemblages of Cooper et al. (1995) are used here to define sub-biozones of the messaoudensis–trifidum Biozone.

Sub-biozone 1 is represented in the Bitter Beck Formation, but its base has not been located. It comprises the messaoudensis–trifidum Biozone, but without Peteinosphaeridium, Rhopaliophora and specimens of Striatotheca that have processes developed in two mutually perpendicular planes. No acritarch species provides positive evidence for the presence of the sub-biozone. Although some acritarchs do not range above sub-biozone 1, for example Acanthodiacrodium tuberatum, they are too rare to be used as positive indicators. Hence, sub-biozone 1 is distinguished on negative evidence, namely the absence of the taxa listed above.

Microfloras assigned to sub-biozone 1 have been obtained from the Bitter Beck Formation:

  1. in Bitter Beck between St Helen’s Bridge [NY 1420 3122] and a point east-north-east of Greenlands [NY 1484 3138] (localities 3 and 4 of Molyneux and Rushton, 1988; (Figure 15), MPA 24347 to 24356)
  2. farther upstream in Bitter Beck, south of Setmurthy Common [NY 1657 3120], [NY 1635 3104], [NY 1629 3107], [NY 1650 3096] (Figure 15), MPA 33478 to 33484
  3. from outcrops in Bitter Beck [NY 1284 3065] and the River Cocker [NY 1184 2959] to [NY 1222 3026] in Cockermouth

The St Helen’s Bridge locality has also yielded Araneograptus murrayi, indicating the murrayi Biozone (Appendix 2, Locality 1). The composition of a microflora from beds assigned to the Bitter Beck Formation near Park Wood, Bassenthwaite [NY 2496 3312], is also consistent with sub-biozone 1.

The overlying sub-biozones of the messaoudensis–trifidum Biozone are based on the appearance of acritarch species along a traverse from Watch Hill to the River Derwent. As noted above, sub-biozone 2 is discriminated from the underlying sub-biozone 1 by the presence of Peteinosphaeridium, Rhopaliophora ((Plate 4)d) and certain species of Striatotheca. In the relatively uncomplicated, northward-dipping succession north-east of Cockermouth, the lowest sample to contain these forms was collected from the southern slopes of Watch Hill [NY 1557 3146], and is estimated to be about 10 m above the base of the Watch Hill Formation (Figure 15), MPA 28677). Using this evidence, Molyneux (in Cooper et al., 1995) reported the base of sub-biozone 2 to coincide approximately with the base of the Watch Hill Formation, but there is an unsampled stratigraphical interval, estimated to be about 70 m, between the sample from Watch Hill and samples from the Bitter Beck Formation in Bitter Beck. Furthermore, two specimens of Peteinosphaeridium and a single Rhopaliophora have been recorded from the north bank of the River Derwent at Cockermouth [NY 1135 3117], in a faulted inlier of the Bitter Beck Formation. The distance of these specimens beneath the Watch Hill Formation is unknown, as is their level of occurrence in relation to other Bitter Beck microfloras. It follows, therefore, that there is some uncertainty over the stratigraphical relationship between this microflora from the River Derwent and other microfloras from the Bitter Beck Formation that have been assigned to sub-biozone 1. For the moment, however, Peteinosphaeridium and Rhopaliophora are taken to indicate that the boundary between sub-biozone 2 (above) and sub-biozone 1 (below) perhaps ought to be moved to a level within the upper part of the Bitter Beck Formation.

The base of sub-biozone 3 is placed at the first appearance of Micrhystridium aff. acuminosum ((Plate 4)f) and Striatotheca prolixa ((Plate 4)c), estimated to be about 110 m above the base of the Watch Hill Formation on Dodd Crag [NY 1560 3156] (Figure 15), MPA 28678). There is, however, an unsampled stratigraphical interval of about 100 m between the lowest sample yielding these species and the highest microflora assigned to sub-biozone 2. The first undoubted specimens of Veryhachium trispinosum s.l. appear within the sub-biozone, and the first true specimens of Coryphidium have been recorded from the top of the sub-biozone. The microfloras recorded from Watch Hill by Molyneux and Rushton (1988, localities 5 and 6) represent sub-biozone 3.

The base of sub-biozone 4 is placed at the first appearance of Coryphidium aff. elegans of Molyneux and Leader (1997; (Plate 4)g, equivalent to C. cf. elegans of Cooper and Molyneux, 1990; Cooper et al., 1995) in the Watch Hill Formation on the northern slopes of Watch Hill. There is some repetition of strata across strike faults, but the lowest record of C. aff. elegans is estimated to be about 380 m above the base of the Watch Hill Formation in Setmurthy Plantation [NY 1534 3236] (Figure 15), MPA 28684). An unsampled stratigraphical interval of about 50 m separates this level from the highest microflora assigned to sub-biozone 3. Adorfia prolongata andCymatiogalea granulata ((Plate 4)e) appear within subbiozone 4.

Sub-biozone 5 is distinguished by the presence of Acanthodiacrodium aff. angustum and Marrocanium simplex. It is represented by microfloras from the Hope Beck Formation on the south bank of the River Derwent e.g. around [NY 1429 3319], [NY 1440 3297], [NY 1486 3335], [NY 1688 3288], where its lowest occurrence is about 90 m above the base of the formation (Figure 15), MPA 28692), 1.65 km east of Hewthwaite Hall. As the highest sample assigned to sub-biozone 4 was collected just below the top of the Watch Hill Formation, from a disused quarry [NY 1516 3272] near Hewthwaite Hall, there is clearly a stratigraphical interval at the base of the Hope Beck Formation for which acritarch data are lacking. Arbusculidium filamentosum appears within sub-biozone 5.

Acritarch floras assigned to the various sub-biozones have been recorded elsewhere in the Skiddaw inlier, notably around Cockermouth and in the Uldale Fells, north of Skiddaw. In the latter area, they have provided biostratigraphical constraints in an area of structural complexity. Characteristic elements of the messaoudensis–trifidum Biozone are also recycled into sedimentary rocks at the base of the Eycott Volcanic Group (Millward and Molyneux, 1992), providing evidence for the scale of uplift and erosion that occurred prior to deposition of the latter.

Around Cockermouth, the Watch Hill Formation exposed in the banks of the River Cocker [NY 1222 3029] to [NY 1224 3035] and in the Cockermouth by-pass [NY 1120 2982] (locality 2b of Molyneux and Rushton, 1988) has yielded microfloras representing sub-biozone 4 or older.

In the Uldale Fells, microfloras assigned to sub-biozone 3 have been recorded from the northern end of the col at Trusmadoor [NY 2777 3362], from a block bearing Araneograptus murrayi (Appendix 2, Locality 4); those assigned to sub-biozone 4 have been obtained from the Watch Hill Formation at the head of Frozenfell Gill [NY 2909 3362] and on Meal Fell [NY 2825 3378], [NY 2828 3376]; and microfloras representing sub-biozone 5 occur in the Hope Beck Formation on the north bank of Burn Tod Gill [NY 2798 3341], [NY 2797 3340], [NY 2795 3341], close to specimens of Didymograptus rigoletto representing the phyllograptoides Biozone (Maletz et al., 1991; Appendix 2, Localities 9, 10). Acritarch floras from the Hope Beck Formation in the River Ellen [NY 2880 3395], [NY 2865 3430] represent either sub-biozone 5 or a slightly higher level.

Microfloras from other localities in the Uldale Fells could represent either the messaoudensis–trifidum Biozone or the overlying trifidum–bohemicum Biozone. Those from the Watch Hill and Hope Beck formations on Orthwaite Bank [NY 2587 3319], around [NY 2575 3330], for example, comprise A.? dilatum, C. glabra, S. trifidum, Vogtlandia coalita and Veryhachium lairdii, all of which range through the messaoudensis–trifidum Biozone and into the trifidum–bohemicum Biozone. Similarly, microfloras from Red Gill [NY 2930 3478], [NY 2932 3472] and Silver Gill [NY 2987 3409] comprise long-ranging forms, consistent with either the messaoudensis–trifidum or trifidum–bohemicum Biozone.

At the eastern edge of the Skiddaw Inlier, microfloras obtained from Trout Beck [NY 3864 2697] to [NY 3833 2699] may represent sub-biozones 4 and 5, but their stratigraphical context is unclear. Nearby localities (e.g. Appendix 2, Localities 154, 155) have yielded younger Arenig acritarchs and graptolites, including a possible cucullus Biozone graptolite, and it is possible that the Trout Beck microfloras are from an outcrop of the Buttermere Formation. In the same area, a microflora reported by Millward and Molyneux (1992) from the Skiddaw Group beneath the Eycott Volcanic Group, about 1 km north-north-west of the summit of Eycott Hill [NY 3828 3041], should probably be assigned to sub-biozone 4, given the association of Cymatiogalea granulata with C. cristata and S. sicaforme sicaforme. Some of the microfloras recorded by Cooper and Molyneux (1990) from the Catterpallot Formation of the Cross Fell Inlier, containing Coryphidium aff. elegans but without Acanthodiacrodium aff. angustum or Marrocanium simplex, might also be assigned to sub-biozone 4.

It is notable that, with the possible exception of the Trout Beck occurrences, all the Lake District records of the messaoudensis–trifidum microflora are from the Northern Fells Belt. There are no records of the microflora from the Buttermere Formation olistostrome in the Central Fells Belt, for example, although both older Tremadoc and younger Arenig microfloras are well represented there.

Stelliferidium trifidum–Coryphidium bohemicum Assemblage Biozone

The Stelliferidium trifidum–Coryphidium bohemicum Biozone is represented by microfloras from the Hope Beck Formation of Howfoot Wood [NY 1354 3269], [NY 1352 3270], estimated to be about 400 m above the base of the formation (Figure 15), MPA 28719, MPA 28722). The most significant difference between this and the underlying messaoudensis–trifidum Biozone is the replacement of Coryphidium aff. elegans, which characterises the upper part of the latter, by C. bohemicum?, which persists into the middle Arenig (Molyneux and Leader, 1997). The highest specimens of C. aff. elegans occur about 360 m above the base of the Hope Beck Formation (Figure 15), MPA 28710, 28713). Skiddaw Group specimens of Coryphidium bohemicum? resemble the type material of C. bohemicum, but were questionably assigned to the species by Molyneux and Leader (1997) because of their poor preservation.

Stelliferidium trifidum is rarer in this biozone than in the messaoudensis–trifidum Biozone. Striatotheca principalis parva has its first appearance in the trifidum–bohemicum Biozone, while a number of other species, including Acanthodiacrodium? dilatum, Micrhystridium aff. acuminosum, Stellechinatum sicaforme, Striatotheca prolixa and Vogtlandia coalita, have their last appearances. Definite specimens of Cymatiogalea messaoudensis are absent from the trifidum–bohemicum microflora of Howfoot Wood, but a single questionable specimen has been recorded in a sample from the south bank of the River Derwent [NY 1429 3319], 0.9 km north-east of Howfoot Wood, and is associated with Acanthodiacrodium aff. angustum, Stelliferidium fimbrium?, Stelliferidium trifidum?, and species of Coryphidium?, Pirea, Polygonium, Stelliferidium and Timofeevia. With the exception of A. aff. angustum, all these taxa are present in the trifidum–bohemicum microflora from Howfoot Wood. The stratigraphical relationship between the River Derwent and Howfoot Wood microfloras is unclear. The presence of A. aff. angustum in the former suggests affinity with sub-biozone 5 of the messaoudensis–trifidum Biozone, but the sample containing the River Derwent microflora appears to lie at a relatively high level in the Hope Beck Formation, possibly just above the Howfoot Wood samples, which would put it in the trifidum–bohemicum Biozone.

Other Lake District microfloras that can be assigned to the trifidum–bohemicum Biozone are scarce. Acritarch assemblages from the Hope Beck Formation in the River Ellen of the Uldale Fells [NY 2822 3421], [NY 2858 3427], in which Coryphidium bohemicum? is associated with Micrhystridium aff. acuminosum, accompanied by a variety of other forms including Polygonium spp., Stelliferidium trifidum, Veryhachium lairdii s.l. and Veryhachium trispinosum s.l., are assigned to the biozone. So too may be a microflora from Trout Beck [NY 3815 2697] in which M. aff. acuminosum is associated with Striatotheca principalis parva, although the stratigraphical context is unclear (see discussion of the messaoudensis–trifidum Biozone, above).

Coryphidium bohemicum Assemblage Biozone

The Coryphidium bohemicum Assemblage Biozone is represented by microfloras from the top 10 m of the Hope Beck Formation and the lowest 90 m of the Loweswater Formation in Jonah’s Gill [NY 1902 3399] to [NY 1904 3420] (Figure 15), MPA 28725 to 28736). Coryphidium bohemicum? occurs throughout, but Stelliferidium trifidum is only questionably present. Vulcanisphaera? sp. A is also present, and characterises microfloras from this biozone. This is a distinctive acritarch with a spherical vesicle and short processes that divide distally into long, recurved, tangential filaments. Specimens of Polygonium spp. are very abundant. The bohemicum Biozone lacks a number of species that are prominent in lower biozones, as noted above.

So far, the bohemicum Biozone has only been recognised with certainty in Jonah’s Gill, where graptolites of the Didymograptus varicosus Biozone have also been recorded from the Loweswater Formation (Appendix 2, Locality 39).

Lower Arenig (Moridunian) acritarch microfloras from the Lorton Fells and Buttermere

The Hope Beck and lower parts of the Loweswater formations in the Lorton Fells are considered to correlate with the succession containing the trifidum–bohemicum and bohemicum biozones north of the Watch Hill Thrust, but have generally yielded sparse microfloras that lack the diversity and distinctive elements of the latter. Consequently, neither the trifidum–bohemicum Biozone nor the bohemicum Biozone have been recognised with certainty in the Lorton Fells.

More diverse microfloras reappear in lower Arenig (Moridunian) rocks of the Buttermere Formation, south of the Causey Pike Fault, and at least some comparisons can be made with the microfloras obtained north of the Watch Hill Thrust.

Lorton Fells

Hope Beck Formation

One microflora, estimated to be about 200 m below the top of the Hope Beck Formation in Hope Beck [NY 1730 2348] (Figure 16), comprises undetermined acanthomorph acritarchs, Micrhystridium spp., small sphaeromorph acritarchs and Veryhachium minutum, but is too poor to assign to any of the acritarch biozones recognised in the Arenig part of the Skiddaw Group north of the Watch Hill Thrust. This microflora may be from about the same level as a varicosus Biozone graptolite collected from Hope Beck (Appendix 2, Locality 12), or slightly higher.

A second microflora from the Hope Beck Formation of Swinside Plantation [NY 1840 2403], estimated to be about 360 m below the top of the formation (Figure 16), contains sparse, poorly preserved material, including acanthomorph acritarchs, sphaeromorph acritarchs, Acanthodiacrodium?, Micrhystridium spp., Stelliferidium?, Veryhachium lairdii and V. minutum?. Also present, however, is one specimen that can be assigned to Coryphidium bohemicum?, together with several fragments and poorly preserved specimens that probably represent the same species. The presence of C. bohemicum? is significant, since its occurrence suggests correlation of the Hope Beck Formation at this locality with the upper part of the Hope Beck Formation north of the Watch Hill Thrust, that is above sub-biozone 5 of the messaoudensis–trifidum Biozone (see discussion of sub-biozone 5, the Stelliferidium trifidum-Coryphidium bohemicum Biozone and the Coryphidium bohemicum Biozone).

Loweswater Formation

Acritarch microfloras from the lower part of the Loweswater Formation in the Lorton Fells are sparse and of low diversity, and Molyneux (in Cooper et al., 1995) suggested that this part of the formation should be regarded as an interzone between the bohemicum Biozone and the Stelliferidium aff. pseudoornatum Biozone. This solution is not entirely satisfactory, as the superpositional relationship between the bohemicum and interzonal microfloras cannot be established because of their geographical separation. Nevertheless, two of the microfloras considered here, from Ladyside Pike and Scawgill Bridge Quarry, lie below the lowest microfloras assigned to the S. aff. pseudoornatum Biozone, and are now assigned questionably to the bohemicum Biozone (Figure 16), (Figure 17). The ‘interzonal’ microflora from Burnbank Fell (Cooper et al., 1995, p.192) is now inferred to lie within the S. aff. pseudoornatum Biozone.

The microflora from the north-west ridge of Ladyside Pike [NY 1798 2332], collected approximately 200 m above the base of the Loweswater Formation between the summit and Swinside, comprises undetermined acanthomorph acritarchs, Coryphidium bohemicum?, Coryphidium sp., Lophosphaeridium? sp., Micrhystridium sp. cf. M. aff. acuminosum, Micrhystridium spp. and sphaeromorph acritarchs. This microflora occurs close to sparse graptolite faunas of the varicosus or, less likely the simulans Biozone (Appendix 2, Localities 31, 32).

The microflora from Scawgill Bridge Quarry [NY 1775 2585], collected from the varicosus Biozone (Appendix 2, Locality 15) and estimated to be about 370 m above the base of the Loweswater Formation, contains Acanthodiacrodium?, undetermined acanthomorph acritarchs, Coryphidium sp., Lophosphaeridium? sp., Micrhystridium sp. cf. M. aff. acuminosum, Micrhystridium spp., Stelliferidium spp. and sphaeromorph acritarchs.

The composition of the Scawgill Bridge Quarry and Ladyside Pike microfloras, particularly their low diversity and general lack of diagnostic elements, is typical of the interzonal assemblages reported by Molyneux (in Cooper et al., 1995). The occurrence of C. bohemicum? (Ladyside Pike) is consistent with an early to mid-Arenig age.

The microflora from the Loweswater Formation of Burnbank Fell [NY 1156 2156] (Molyneux and Rushton, 1988, p.50) comprises ?Caldariola glabra, Coryphidium sp., Cymatiogalea deunffii, Peteinosphaeridium sp., Polygonium spp., Stellechinatum spp., Stelliferidium spp., Veryhachium lairdii s.l. and Vulcanisphaera cirrita. The stratigraphical position of this microflora within the formation is uncertain, but graptolite collections from Burnbank Fell are reported to indicate the simulans Biozone (see Appendix A, Localities 36, 37). Correlation with the simulans Biozone would imply that the acritarch assemblage is from a level within the Stelliferidium aff. pseudoornatum Biozone, the base of which may lie within the varicosus Biozone (see below).

Buttermere

Buttermere Formation

The association of Micrhystridium aff. acuminosum with Coryphidium and Striatotheca principalis parva? in the Buttermere Formation of Long How around [NY 1726 1730] and Buttermere Quarry [NY 1732 1726] invites comparison with the trifidum–bohemicum Biozone. Also present, however, are forms that resemble Stelliferidium aff. pseudoornatum, while graptolites from Buttermere Quarry (Appendix 2, Locality 138) indicate a level close to the varicosus–simulans boundary (Cooper et al., 1995). Hence, M. aff. acuminosum seems to range higher in the Central Fells than in the Northern Fells, although there is a possibility that it is reworked in the Buttermere Formation olistostrome.

Stelliferidium aff. pseudoornatum Assemblage Biozone

The microflora of the Stelliferidium aff. pseudoornatum Biozone is equivalent to the Stelliferidium sp. nov. assemblage of Cooper et al. (1995). Its most distinctive form is a species of Stelliferidium with short conical processes that resemble those of S. pseudoornatum, but the specimens from this microflora have better developed striations than the latter species. The microflora is generally of low diversity, and the most common forms are morphologically simple acanthomorph acritarchs, such as Polygonium, and sphaeromorph acritarchs. These are features that it has in common with underlying microfloras from the Lorton Fells. Specimens of Coryphidium, including C. bohemicum?, occur in nearly all samples from the biozone. Other taxa, including Acanthodiacrodium cf. simplex, Acanthodiacrodium? spp., Caldariola glabra?, Micrhystridium sp. cf. M. aff. Acuminosum, Micrhystridium cf. aremoricanum, Micrhystridium spp., Peteinosphaeridium?, Stelliferidium? spp., Striatotheca spp. and Veryhachium spp., have more sporadic occurrences.

The S. aff. pseudoornatum Biozone has been recognised on Whinlatter Crag around [NY 2021 2448], in Tom Rudd Beck [NY 1712 2819], on Embleton High Common [NY 1697 2794], and at Barf [NY 2180 2645], in each case occurring in the upper part of the Loweswater Formation, and probably in the upper 200 m of the formation (Figure 17). On Embleton High Common (Appendix 2, Locality 22) and at Barf (Locality 25), acritarchs of the pseudoornatum Biozone occur in proximity to graptolites of the simulans Biozone. The pseudoornatum microflora has also been recorded from Hodgson How Quarry [NY 2442 2361] where it occurs in beds assigned to one of two sandstones in the Kirk Stile Formation (Figure 17). The base of the Isograptus victoriae Biozone is placed below the lower sandstone in the Lorton Fells (Figure 7), so the S. aff. pseudoornatum Biozone is inferred to extend above the base of that graptolite biozone. A further, questionable record of the S. aff. pseudoornatum Biozone is from the Loweswater Formation in Hope Gill [NY 1805 2262], estimated to be about 500 m above the base of the formation (Figure 16), and close to a graptolite fauna of the varicosus Biozone (Appendix 2, Locality 29). Neither the base nor the top of the S. aff. pseudoornatum Biozone have been located in the Skiddaw Group, but these occurrences of the S. aff. pseudoornatum microflora in the Northern Fells Belt suggest that the S. aff. pseudoornatum Biozone spans the simulans Biozone, extending into the underlying and overlying graptolite zones.

In addition to the records from the Northern Fells Belt, the S. aff. pseudoornatum microflora has been recorded from the Central Fells Belt near Buttermere, where it occurs in blocks of mid-Arenig age that were redeposited in the Buttermere Formation olistostrome. The microflora occurs in the Robinson Member of the Buttermere Formation on Goat Crag [NY 1900 1646], [NY 1887 1650], [NY 1887 1665], and in the undifferentiated Buttermere Formation on Robinson [NY 1998 1715], north of Buttermere [NY 1751 1733], [NY 1763 1750], [NY 1756 1765], in Squat Beck [around [NY 1795 1775], [NY 1782 1789], on the northern slopes of High Snockrigg [NY 1857 1706], and possibly on Scale Knott west of Crummock Water [NY 1538 1751], [NY 1536 1754]. Microfloras from the Buttermere Formation of Low Bank [NY 1721 1801] and Knott Rigg [NY 1942 1827] appear to be transitional between the S. aff. pseudoornatum microflora and the overlying Frankea hamata–Striatotheca rarirrugulata microflora (see below).

Frankea hamata–Striatotheca rarirrugulata Assemblage Biozone

The Frankea hamata–Striatotheca rarirrugulata Biozone is the highest acritarch biozone recognised in the Arenig part of the Skiddaw Group. Together with the messaoudensis–trifidum microflora, the hamata–rarirrugulata microflora constitutes one of the more readily recognisable acritarch microfloras from the Skiddaw Group, and is certainly the most widespread, occurring in the Northern Fells and Central Fells belts of the Skiddaw Inlier, the Black Combe Inlier, and the Cross Fell Inlier. It is more diverse than the middle Arenig microfloras from the Hope Beck and Loweswater formations. Polygonomorph acritarchs, notably species of Frankea, Striatotheca and Veryhachium, are important and characteristic elements, two of the more easily determined species giving their names to the biozone. Also characteristic is Coryphidium aff. bohemicum of Molyneux and Leader (1997), a form that replaces the middle Arenig C. bohemicum?. Stelliferidium aff. pseudoornatum does not range into the hamata–rarirrugulata Biozone, although it is associated with Frankea hamata? and Coryphidium aff. bohemicum? in a microflora from the Buttermere Formation of Knott Rigg [NY 1942 1827]. The latter thus appears to be transitional between the pseudoornatum and hamata–rarirrugulata microfloras. A second microflora from the Buttermere Formation, from Low Bank [NY 1721 1801], contains Coryphidium bohemicum? and Frankea hamata?, and may also be transitional between the pseudoornatum and hamata–rarirrugulata microfloras.

All the samples assigned to the hamata–rarirrugulata Biozone contain at least one of Coryphidium aff. bohemicum, Frankea hamata ((Plate 4)k) or Striatotheca rarirrugulata ((Plate 4)i, m), 75 per cent of samples contain at least two of these taxa, and 38 per cent of samples contain all three. Other characteristic taxa include Frankea breviuscula, Frankea sartbernardensis, Marrocanium simplex, Polygonium spp., Striatotheca principalis parva, Striatotheca principalis principalis, Veryhachium lairdii and Veryhachium trispinosum; except for Polygonium these are all polygonomorph acritarchs. Taxa that occur more sporadically, but which are nevertheless considered to be important, include Acanthodiacrodium cf. simplex, Arkonia, Aureotesta clathrata, Micrhystridium sp. A of Rushton and Molyneux (1989; (Plate 4)n), Orthosphaeridium bispinosum, Stellechinatum cf. celestum, Striatotheca frequens, Striatotheca cf. quieta, Veryhachium aff. lairdii of Rushton and Molyneux (1989), and ?Vogtlandia flosmaris.

In the Northern Fells Belt, the hamata–rarirrugulata Biozone succeeds the Stelliferidium aff. pseudoornatum Biozone in the Kirk Stile Formation. The precise level at which the microfloral change takes place remains uncertain, although evidence from the Central Fells Belt suggests that the hamata–rarirrugulata microflora occurs in the former gibberulus Biozone (sensu Fortey et al., 1990, equivalent to the victoriae and gibberulus biozones of this account). As the S. aff. pseudoornatum Biozone is inferred to range above the base of the victoriae Biozone, the base of the hamata–rarirrugulata Biozone must lie within either the upper part of the victoriae Biozone or the gibberulus Biozone.

The upwards extent of the hamata–rarirrugulata Biozone has not been determined, mainly because no systematic palynological examination of Llanvirn Skiddaw Group rocks has been undertaken, but the hamata–rarirrugulata Biozone correlates with the cucullus Biozone in the Northern Fells Belt and the Black Combe Inlier. As there is evidence for a change in the composition of acritarch microfloras across the Arenig–Llanvirn boundary, the top of the hamata–rarirrugulata Biozone might eventually be located close to the base of the Llanvirn.

In the Northern Fells Belt, occurrences of the hamata–rarirrugulata microflora in the Kirk Stile Formation near Simonscales Mill [NY 1208 2879], [NY 1203 2885] are considered to lie above graptolite faunas from Whinnah, Sandybeck and Abbey Gate that have been assigned to the I. caduceus gibberulus Biozone (Appendix 2, Localites 84, 86, 87). Other records in the Northern Fells Belt are from:

  1. strata beneath the unconformity at the base of the Eycott Volcanic Group in the Over Water spillway [NY 2586 3552] (Millward and Molyneux, 1992, fig. 5; Appendix 2, Locality 122), south-west of Overwater Hall [NY 2412 3461] and near Whitfield Cottage [NY 2217 3503]
  2. the River Ellen in the Uldale Fells [NY 2796 3421], [NY 2799 3422], [NY 2805 3423], [NY 2807 3424]
  3. Watches [NY 2406 3026] (Appendix 2, Locality 112)
  4. around Beckgrains Bridge [NY 1904 3552]

The Watches microflora is from the cucullus Biozone. Additional samples from Watches around [NY 2411 3069] to [NY 2408 3027] yielded sparse material, without the more characteristic elements of the hamata–rarirrugulata microflora, but the presence in these samples of ?Arkonia virgata, Striatotheca cf. quieta, S. principalis principalis and Veryhachium aff. lairdii (sensu Rushton and Molyneux, 1989) is consistent with a late Arenig age.

In the Central Fells Belt, the hamata–rarirrugulata microflora has been recorded from a number of localities around Buttermere, in each case from the Buttermere Formation olistostrome. These include Squat Beck [NY 1788 1775], [NY 1785 1776], [NY 1796 1777], [NY 1794 1778], High Snockrigg [NY 1842 1708], Swinside Gill [NY 1900 1777] to [NY 1895 1776], Sail Beck [NY 1880 1819], Ramps Gill [NY 1915 1855] to [NY 1930 1853], Knott Rigg [NY 1943 1827], [NY 1943 1822] and localities in Buttermere village [NY 1763 1704], [NY 1751 1703]. The proximity of the hamata–rarirrugulata microflora to Tremadoc assemblages in Swinside Gill and Sail Beck constitutes part of the evidence for the chaotic nature of the Buttermere Formation. The hamata–rarirrugulata microflora also occurs in Ya Gill [NY 0737 1241] to [NY 0758 1234] and at Beck Grains [NY 0776 1128], close to the Tremadoc rocks of the River Calder and possibly from a south-westerly continuation of the Buttermere Formation outcrop, and from Skiddaw Group rocks at Causeway Foot [NY 2907 2197], below the sub-Borrowdale Volcanic Group unconformity described by Wadge (1972). Graptolite faunas from Ramps Gill and Beck Grains indicate either the victoriae Biozone or the gibberulus Biozone (Appendix 2, Localities 137, 144).

From the Black Combe Inlier, assemblages reported by Rushton and Molyneux (1989) from the south side of Black Combe between Whicham and Whicham Mill [SD 1289 8299], [SD 1292 8306], [SD 1415 8365], [SD 1526 8502], Whirl Pippin [SD 1664 8571], Knott Hill [SD 1701 8750], and south of the Whicham Valley on Lacra Bank [SD 1435 8192], [SD 1454 8180], [SD 1455 8180, SD 1480 8191] and near Bankside [SD 1576 8313], can be assigned to the hamata–rarirrugulata Biozone. Graptolite faunas collected from the Black Combe Inlier indicate the cucullus Biozone (Appendix 2, Localities 171 to 173). One sample from the Murton Formation of the Cross Fell Inlier, collected south of Murton Pike [NY 7342 2239], has also yielded the hamata–rarirrugulata microflora, and graptolites collected from the Murton Formation indicate a late Arenig or possibly younger age (Cooper and Molyneux, 1990).

Llanvirn acritarch assemblages

No systematic examination of acritarch assemblages from the Llanvirn part of the Skiddaw Group, principally the Tarn Moor Formation of the Central Fells Belt and the Kirkland Formation of the Cross Fell Inlier, has been undertaken during the resurvey. Nevertheless, there are limited data from the Kirkland Formation and from the Tarn Moor Formation of the Bampton inlier. Although there is no direct graptolite control on the acritarch microfloras from the Tarn Moor Formation, they are presumed to be from the artus Biozone. Arkonia spp. ((Plate 4)o), Stellechinatum celestum ((Plate 4)l), Striatotheca frequens and S. quieta are generally present. Although these or comparable forms may be present in the hamata–rarirrugulata microflora (see above), they are much more evident in the Tarn Moor Formation microfloras. Dicrodiacrodium?, Frankea hamulata and F. longiuscula are also present in the Tarn Moor Formation, but have not been recorded from the hamata–rarirrugulata microflora. This evidence, together with unpublished data summarised by Downie (1984), suggests a change in the acritarch microfloras across the Arenig–Llanvirn boundary in the Lake District, but the precise level at which that change takes place has not been determined. From a biostratigraphical point of view, one implication is that it may be possible to recognise a distinctive acritarch assemblage above the hamata–rarirrugulata microflora in the Skiddaw Group, and therefore to position the top of the hamata–rarirrugulata Biozone in the succession. At the moment, it appears likely that the top of the hamata–rarirrugulata Biozone lies close to the base of the Llanvirn.

Chapter 3 Lithostratigraphy

Lithostratigraphy and locality details are presented in this chapter. Sedimentological interpretations, palaeocurrent analyses, interpretations of basin configuration and provenance analyses are provided in Chapter 4. The history of the development of Skiddaw Group is discussed in Chapter 1 (Table 1). Sedgwick (1832) first defined the Skiddaw Slates; Jackson (1961) formally defined the Skiddaw Group, including the Latterbarrow Formation within it. This unit is now regarded as part of the overlying Borrowdale Volcanic Group (Allen and Cooper, 1986).

Two distinct lithostratigraphical sequences, namely the Northern Fells Belt and the Central Fells Belt (Figure 3), are recognised within the Skiddaw Group of the Skiddaw and Cross Fell inliers (Cooper and Molyneux, 1990). The sequences are separated by the Causey Pike Fault (described in detail in Chapter 5).

The Skiddaw Group of the Northern Fells Belt comprises the Bitter Beck, Watch Hill, Hope Beck, Loweswater and Kirk Stile formations (Figure 3), (Figure 18), and is unconformably overlain by the Eycott Volcanic Group (Millward and Molyneux, 1992). The Northern Fells Belt in the Cross Fell Inlier comprises the Catterpallot Formation (Cooper and Molyneux, 1990), the correlative of the Watch Hill and possibly Bitter Beck formations (Figure 3). The Redmain Formation of Allen and Cooper (1986), though geochemically distinctive, appears to represent a highly weathered part of the Loweswater Formation preserved immediately below the basal Carboniferous red beds and unconformity. It is not a mappable unit and it is recommended that its use be discontinued.

In the Central Fells Belt, the Buttermere and Tarn Moor formations (Figure 3), (Figure 18) are unconformably overlain by the Borrowdale Volcanic Group, with the Latterbarrow Formation at its base in the west. The Central Fells Belt in the Cross Fell Inlier (Figure 1), (Figure 3) comprises the Murton and Kirkland formations, defined by Burgess and Wadge (1974) and revised by Cooper and Molyneux (1990). Strata equivalent to the Kirkland Formation are also present in the Teesdale inlier (Figure 1); Johnson, 1961; Burgess and Holliday, 1979).

The Black Combe and Furness inliers of the southern Lake District are geographically isolated from the main exposures of the Central Fells Belt. They lie along, and south of, a major structural line, interpreted by Kneller and Bell (1993) as an Acadian mountain front, in which the major fold structure is the Westmorland Monocline (Figure 1). In the Black Combe area, the steep limb of this fold coincides with a major geophysical lineament, termed the Southern Borrowdales Lineament by Lee (1989). This lineament also coincides with the approximate position of the southern margin of the Borrowdale Volcanic Group volcanotectonic rift system, and the margin of the Lake District batholith (Cooper et al., 1993; Kneller and Bell, 1993).

The area south of this line was termed the Southern Fells Belt by Cooper et al. (1993), but this memoir follows Cooper et al. (1995) and uses the term ‘Southern Lake District inliers’ to encompass the outcrops at Black Combe and Furness, and also the small Ravenglass inlier. The Skiddaw Group in these inliers is largely undivided (Rushton and Molyneux, 1989). The overlying Borrowdale Volcanic Group, which includes the mudstone-tuff sequence of the Whinney Bank Formation at its base (Figure 3), attenuates abruptly southwards and is very thin at Furness, where the Skiddaw Group is mostly overlain by upper Ordovician (Cautleyan) mudstone and limestone (Rose and Dunham, 1977) of the Dent Group.

In the descriptive account below, turbidites are classified using the Bouma (1962) scheme. The sedimentological interpretations of Chapter 4 (based largely upon the work of Moore, 1992) use the more detailed turbidite classification of Pickering et al. (1986). Grain size is classified according to the Udden–Wentworth scheme (see, for example, Leeder, 1982); bed and lamina thickness are classified according to the Ingram and Campbell schemes respectively (as used by Tucker, 1982). The sandstone classification of Leeder (1982) is used throughout. Wackes and arenites are distinguished where detailed petrographical studies have been carried out; where no detailed petrographical data are available the rocks are termed sandstones. Compositional data, both petrographical and geochemical, are given in Appendix 3.

Northern Fells Belt

In the Northern Fells Belt, the Skiddaw Group comprises five formations overlain by the Eycott Volcanic Group. In ascending order these formations are the Bitter Beck Formation, Watch Hill Formation, Hope Beck Formation, Loweswater Formation and the Kirk Stile Formation. Type areas, typical facies and key sections of each formation are described below. Possible Cambrian strata, found at one locality near Eycott Hill, are not assigned to any particular formation but for convenience are described below with the Bitter Beck Formation.

Bitter Beck Formation

The mudstones of the Bitter Beck Formation were first recognised as a distinct unit by Molyneux and Rushton (1988) and formally defined by Cooper et al. (1995). The type area is Bitter Beck, east of Cockermouth [NY 1399 3113] to [NY 1500 3133], where the upper part of the formation is exposed. North of there, the formation passes upwards transitionally into the Watch Hill Formation. There is no complete section through the Bitter Beck Formation because the lowest strata are thrust southwards by about 5 km, over the Kirk Stile Formation, along the Watch Hill Thrust (Hughes et al., 1993, fig. 3). The formation above the Watch Hill Thrust in the type area is at least 500 m thick.

The Bitter Beck Formation comprises mainly thinly laminated dark grey mudstone, silty mudstone and siltstone with minor amounts of wacke sandstone. The beds are distal turbidites, and comprise mostly graded siltstone beds with homogeneous mudstone laminae (Bouma Tde units). The sandstone component is pale grey, fine grained, and forms, generally, thin beds with parallel- to wavy-lamination and irregular bases and tops. In places along Bitter Beck and near Cockermouth [NY 114 312], slump folding is common with dislocations subparallel to bedding. Between Elva Plain Farm [NY 175 316] and Higham Hall School [NY 180 316], the lowest part of the formation includes up to 20 per cent of generally thin- to medium-bedded, fine-grained wacke.

Biostratigraphical correlation

The Bitter Beck type section between [NY 1420 3122] and [NY 1480 3137] has provided both graptolites and acritarchs indicative of the late Tremadoc murrayi Biozone and the messaoudensis–trifidum acritarch sub-Biozone 1.

Key Sections

Bitter Beck

The formation in its type area is moderately well exposed along Bitter Beck which runs past Greenlands Farm to St Helens Bridge [NY 1499 3134] to [NY 1419 3119]. Here it is mainly laminated dark grey siltstone with abundant laminae and sporadic very thin discontinuous beds of very fine-grained wacke. Disharmonic folds and shears, probably of slump origin, are common and are cut by a discordant cleavage. The Bitter Beck Formation here has yielded both acritarchs and graptolites (Molyneux and Rushton, 1988).

River Derwent, Papcastle

West of Cockermouth, below Papcastle, the Bitter Beck Formation is exposed on the outside of a tight meander of the River Derwent [NY 1142 3116]. The rock is dark grey, laminated siltstone, with 40 per cent very fine-grained sandstone in laminae to very thin beds, and about 10 per cent mudstone. Complex disharmonic folds are common, and are cross-cut by the cleavage, with some refolding by folds with an axial planar cleavage. Some parts of the sequence are completely disrupted and consist of floating blocks of siltstone and sandstone in a sheared matrix. The section has yielded diverse, abundant and reasonably well-preserved microfossils of the messaoudensis–trifidum Biozone, possibly sub-Biozone 2.

River Cocker, Cockermouth

The formation is exposed along the River Cocker from the old railway bridge [NY 1222 3027] to Double Mills [NY 1183 2959]. At the railway bridge the gradational contact of the Bitter Beck Formation with the overlying Watch Hill Formation can be inferred to within a few metres. Here the Bitter Beck Formation comprises mainly dark grey laminated siltstone with abundant light grey siltstone laminae. This passes southwards (down-sequence) into siltstone with abundant laminae and very thin beds of fine- to coarse-grained lithic and quartz-rich wacke. Microfossil samples from this section yielded material comparable with that derived from both the Bitter Beck and Watch Hill formations to the east.

Eycott Hill

Just below the unconformity at the base of the Eycott Volcanic Group on Eycott Hill [NY 382 305], small, isolated exposures of siltstone and mudstone with possible soft sediment folds have yielded acritarch assemblages, including an assemblage that may be older than those found elsewhere in the Bitter Beck Formation (Millward and Molyneux, 1992; Chapter 2). These acritarch assemblages indicate ages ranging from mid- or late Cambrian to latest Tremadoc or earliest Arenig (see Millward and Molyneux, 1992, for locality details). Field relationships there are not clear, and much younger Skiddaw Group rocks are present about 1.5 km to the west (on the flank of Souther Fell [NY 36 29]).

Two explanations for the presence of older strata in the area are possible. They may either represent beds equivalent to and older than the Bitter Beck Formation seen elsewhere, or the succession may be part of an olistostrome deposit, similar to the debris flows present in the Kirk Stile Formation at Beckgrains Bridge [NY 1904 3552] or the Buttermere Formation of the Central Fells Belt. Both of these examples contain Tremadoc acritarchs, but in the former they occur in the same samples as acritarchs of the hamata–rarirrugulata Biozone and are clearly recycled. In the latter, they occur in strata that are juxtaposed against beds that yield the hamata–rarirrugulata Biozone and so are interpreted as occurring in slumped blocks. No assemblages of mixed ages have been recorded from the vicinity of Eycott Hill, and no juxtaposition of assemblages of disparate ages occurs there. The exposures are inconclusive and either interpretation is possible. The strata are shown as ‘Skiddaw Group, undivided’ on the 1:50 000 geological sheets 23 Cockermouth and 29 Keswick.

Watch Hill Formation

This sandstone-dominated formation, defined by Cooper et al. (1995), is equivalent to the Watch Hill Grits of previous authors and the ‘Grits Group’ of Eastwood et al. (1968). Watch Hill [NY 1495 3189], 3 km east-north-east of Cockermouth, is the type area (Figure 18). Small exposures of the formation are present between Watch Hill [NY 148 318] and Setmurthy Plantation [NY 159 318], and in the tract north-north-eastwards from Setmurthy Common [NY 165 319] towards the valley of the River Derwent. Contrary to the interpretation of Banham et al. (1981), sedimentary way-up evidence shows the sequence in the type area to be normal, dipping and younging northwards (Cooper and Hughes, 1993).

Around Watch Hill, the formation is folded and faulted, but estimated to be between 550 and 800 m thick. It thins westwards to about 100 m at Cockermouth [NY 122 303] and eastwards to 40 m around Great Sca Fell [NY 291 339]. The suggested presence of contemporaneous lavas in the Watch Hill Formation (Jackson, 1961) has now been discounted and the Watch Hill felsite is recognised as a sill (Hughes and Kokelaar, 1993).

The formation comprises mainly medium- to thick-bedded wacke sandstones. Bouma Tabcd cycles are common, and indicate deposition from turbidity currents. The Watch Hill and Bitter Beck formation boundary is gradational and probably a lateral facies change (see Chapter 4), marked by an increase in the proportion of sandstone and a decrease in mudstone. The base of the Watch Hill Formation is defined at the level at which sandstone and siltstone predominate. In the type area, the lower boundary lies just below the quarry at the foot of Watch Hill [NY 1557 3146] and the position of the upper boundary with the overlying Hope Beck Formation is inferred to within about 20 m near Hewthwaite Hall [NY 1516 3276]. At Cockermouth, about 80 m of strata are exposed in the River Cocker, and the lower boundary of the formation can be seen near the old railway viaduct [NY 1222 3029]. The upper boundary is not exposed, but is inferred to lie just to the north of the Lorton Street road bridge [NY 1225 3040] between exposures of the Watch Hill and Hope Beck formations.

Several kilometres east of the type area, pebbly lithic wackes appear in the Watch Hill Formation between Little Cockup [NY 2588 3323] and Great Sca Fell [NY 2900 3387]. In the same general area, the sandstone sequence is attenuated at Trusmadoor [NY 2787 3363] and Meal Fell [NY 2827 3369] where the formation is present as two leaves of sandstone separated by mudstone. The sandstones are lithologically different from those of the type area (Appendix 3), and are also coarser grained with more clay matrix.

Biostratigraphical correlation

Molyneux and Rushton (1988) have published detailed biostratigraphical evidence that indicates a latest Tremadoc age for the formation; further details are given in Chapter 2. The interval inferred from graptolite faunas is within the murrayi–Biozone. The acritarch microfloras span the messaoudensis–trifidum Biozone sub-Biozones 2, 3 and 4.

Key Sections

Watch Hill

Exposures in the type area occur on Watch Hill around Dodd Crag [NY 156 315]. The oldest exposed rocks here crop out in a small disused quarry [NY 1552 3148] below the crag, and are probably less than 15 m above the base of the formation. The sequence consists of wacke sandstones (approximately 40 per cent) along with siltstones and mudstones (approximately 60 per cent). The wackes are pale greenish grey, medium to very coarse grained, mainly lithic (with dark grey siltstone lithoclasts up to 1.5 cm across), and are thin to medium bedded. At the west end of Dodd Crag, the bases of some sandstone beds have rare flute casts, which confirm that the sequence is the correct way up. Higher up the hillside the proportion of sandstone increases to around 50 per cent and the beds are of medium thickness (up to 0.3 m).

Elva Plain

At Elva Hill [NY 1724 3204] about 5 m of strata are exposed, of which some 90 per cent consists of lithic wacke, thinly to thickly bedded and ranging up to very coarse grained. The beds are typically 0.20 to 0.25 m thick, but vary from 0.1 m to 0.60 m, with the thinner beds showing marked lateral thickness variations ((Plate 5)). The typical wacke bed is massive in its lower part with a parallel-laminated upper part containing mudstone lithoclasts. Possible flute casts and other irregularities occur on the sandstone bases, but the direction of palaeocurrent flow is rarely discernible;thesequenceisthecorrectwayup.Thesiltstoneand silty mudstone interbeds are wavy- and parallel-laminated, up to 0.20 m thick, and also show lateral thickness variations.

River Cocker and Tom Rudd Beck, Cockermouth

Exposres of the Watch Hill Formation occur along the River Cocker and Tom Rudd Beck, in Cockermouth. Up to 50 m of strata are exposed along the river, from near the road bridge [NY 1225 3039] to near the old railway bridge [NY 1222 3029]. Coarse- and very coarse-grained lithic wackes dominate in beds up to 1 m thick, interbedded with dark grey siltstone. Graded bedding shows they are the correct way up, younging northwards. The wackes are commonly boudinaged with shear planes, subparallel to bedding, that bound folded siltstone interbeds. Along Tom Rudd Beck [NY 1237 3032] to [NY 1227 3032] there are similar lithologies and structures to those seen in the adjacent river section.

Cockermouth By-Pass

Here the formation is about 100 m thick and exposed in road cuttings [NY 1119 2986] along the Cockermouth by-pass road. Poor exposures show mainly thin-bedded (up to 0.1 m) brown weathered siltstones with sporadic lithic wackes similar to those low on Watch Hill. Graptolites and microfossils from the road section indicate a late Tremadoc age (Chapter 2).

Little Cockup

On Little Cockup [NY 2588 3323], the Watch Hill Formation is exposed in vertical north-west younging strata. The lowest part of the formation is transitional with the underlying Bitter Beck Formation. The lowest parts of the Watch Hill Formation comprise a mixture of dark grey siltstone interbedded with wacke in thin to medium beds. Fine- to medium-grained, cross-laminated (Tc) quartz-rich wackes, and matrix-rich, lithic wackes in graded units (Ta) with quartz and lithic granules and small pebbles are present. This sequence is about 100 m thick and is overlain to the north-west by a massive wacke unit about 30 m thick. This massive wacke is a lithic-rich, small pebble conglomerate with abundant rounded and subangular quartz pebbles and abundant mudstone and siltstone flakes or rip-up clasts up to 5 cms across. Higher in the sequence, the beds are thick to very thick (0.5 to 1.5 m), commonly lenticular, with erosional bases that show scour and groove structures. Normal grading is present, and mudstone clasts are generally more abundant in the lower parts of the beds.

The massive wackes around Little Cockup [NY 2588 3323] are lithologically different to those of Watch Hill; granule grain sizes dominate, and there is more clay matrix than in those to the west. In the coarser clast fraction, polycrystalline quartz of metamorphic origin is more abundant than at Watch Hill and monocrystalline quartz and feldspars (untwinned types dominant over plagioclase) are less abundant. The lithic clast assemblage is dominated by rounded clasts of mudstone and siltstone. Volcanic clasts are common and are dominated by rhyolite and feldspar-phyric fragments. Clasts of granite and quartz-feldspar hornfels are also present. The fine sand fraction of these rocks consists dominantly of strained and unstrained monocrystalline quartz grains with lesser amounts of plagioclase and untwinned feldspar grains. The high lithic content of these wackes makes them distinct from other Watch Hill Formation samples, but this may be a function of grain size.

Trusmadoor

At the north-west end of the gap of Trusmadoor, above the north-east side [NY 2787 3363], about 20 m of laminated siltstone and mudstone with about 15 to 25 per cent of interbedded wacke passes southwards into fossiliferous siltstone and mudstone, assigned to the Hope Beck Formation. The wackes are medium to coarse grained, lithic-rich, and occur in thin to medium beds (up to 0.3 m) with parallel and convolute laminations (Tbc units). Load casts indicate they are the correct way up, and younging southwards. The sequence here represents a highly attenuated Watch Hill Formation around 30 m thick.

At Trusmadoor [NY 2777 3363], the large Dictyonema pulchellum and D. sp. described by Rushton (1985) are now assigned to Araneograptus murrayi, following Lindholm (1991), and indicate the murrayi Biozone. Acritarchs from Trusmadoor are of the messaoudensis–trifidum sub-Biozone 3, suggesting a possible lateral equivalence with the Watch Hill Formation farther east. The beds at this locality were placed in the Bitter Beck Formation by Cooper et al. (1995), but are now reassigned to the Watch Hill Formation.

Meal Fell

On the summit of Meal Fell [NY 2827 3369] the Watch Hill Formation is overturned, dips to the south-south-west, and youngs northwards. Two leaves of sandstone are present, each about 20 m thick, separated by about 20 m of siltstone and mudstone. The lowest sandstone on the summit is a massive, homogeneous, medium-grained, lithic-rich wacke, and is possibly a channel-fill deposit. The upper (northern) [NY 2830 3375] sandstone leaf is a medium- to coarse-grained, lithic wacke with small pebbles and mudstone clasts. The medium beds (0.1 to 0.3 m) show grading and indistinct basal flute marks confirming inversion. Acritarchs indicate a level within the upper part of the formation.

Burn Tod

The Watch Hill Formation is exposed [NY 2757 3305] on both sides of Burn Tod Gill. It dips steeply and youngs to the northeast, with local overturned beds dipping to the south-west. Approximately 50 m of strata are exposed in the stream section and adjacent slopes. Mediumto very thick-bedded (0.3 to 1.3 m thick) lithic wackes dominate, containing small lithic and quartz granules and pebbles. Mudstone and siltstone form about 10 per cent of the sequence in units 0.5 to 1 m thick. Sedimentary structures in the wackes include graded bedding (Ta), parallel lamination (Tb), load casts, flame structures, washouts and flute casts.

Great Sca Fell

Around Great Sca Fell [NY 290 338] very coarse to small pebble lithic wackes, 40 to 100 m thick, occur in thick and very thick beds. They contain abundant rounded and deformed clasts of siltstone and mudstone, plus quartz and rock fragments. Sedimentary structures are rare, and way-up evidence is lacking so the relationship between these beds and the surrounding graptolite-bearing localities is uncertain. Microfossil evidence confirms equivalence to a level within the Watch Hill Formation.

Hope Beck Formation

The Hope Beck Slates, proposed by Jackson (1961), were formalised as the Hope Beck Formation by Cooper et al. (1995). The type area is around Hope Beck [NY 168 239]. The lower boundary with the Watch Hill Formation is tentatively identified at the north-west end of the Trusmadoor section and inferred to within 20 m near Hewthwaite Hall [NY 1516 3276]. The transitional upper boundary with the overlying Loweswater Formation, described by Jackson (1961, 1978), occurs on the east flank of Dodd [NY 171 233]. Jackson (1961) used the presence of arenites ‘3 inches’ (7.6 cm) or more in thickness as the criterion for separating the Hope Beck and Loweswater formations. No continuous sequence through the Hope Beck Formation is exposed and the succession is poorly fossiliferous. The top 400 m is well exposed in the vicinity of Dodd [NY 166 231] and Swinside Plantation [NY 185 241]. Within the region the formation is probably between 600 and 800 m thick.

The Hope Beck Formation consists mainly of dark grey siltstone and mudstone in laminated and very thin beds, with up to 5 per cent of sandstone mainly in thin and medium beds; sporadic pebbly mudstone beds are also present. The siltstone is commonly graded, passing up into homogeneous mudstone. Bioturbation is common in these lithologies, occurring mainly as burrows subhorizontal to bedding, some of which are filled with faecal pellets. The sandstones are mainly quartz-rich lithic wackes with some quartz wackes, generally medium to coarse grained, but including numerous beds with granules and pebbles. The clast assemblage includes abundant monocrystalline plutonic quartz and common polycrystalline metamorphic quartz. Lithic grains include intraformational mudstone and siltstone, crenulated phyllite, quartz-mica schist and crushed chlorite clasts. The high quartz content and paucity of volcanic clasts in these sandstones contrasts markedly with those of the Watch Hill Formation (Appendix 3).

The sandstone beds exhibit sedimentary structures indicative of turbidite deposition with Tabc , Tbc and Tc Bouma units widely present; the siltstones and mudstones are turbidite Td units and probable hemipelagic Te units respectively. The pebbly mudstones are matrix-supported (50 per cent matrix) and may represent debris-flow deposits. The pebbles include contorted intraformational mudstone, siltstone and very fine-grained sandstone that is rich in heavy minerals. Rare pebbles of volcanic lithologies include feldspar-phyric basalt and volcaniclastic rock.

Biostratigraphical correlation

Graptolites occur sparingly in the Hope Beck Formation. Near the base, the presence of Didymograptus protobalticus and D. rigoletto (Maletz et al., 1991) near the junction of Trusmadoor and Burn Tod Gill [NY 2795 3339], associated with the messaoudensis–trifidum sub-Biozone 5, indicates the upper part of the phyllograptoides Biozone as recognised in Scandinavia. Near the top, localities in Blaze Beck [NY 1771 2565] to [NY 1809 2539] yielded faunas referable to the varicosus Biozone (Figure 7). Outcrops of the formation in the valley of the River Derwent have yielded acritarchs of the messaoudensis–trifidum sub-Biozone 5 and the trifidum–bohemicum Biozone. An early Arenig age is indicated consistently.

Key Sections

Hope Beck and Dodd

Hope Beck is the type section of the formation, but because of the gentle dip only a small thickness of it is exposed there. The majority of the formation is exposed on the flanks of Dodd up to the gradational boundary with the overlying Loweswater Formation.

The lowest part of the formation exposed in Hope Beck [NY 1686 2384] shows about 1 m of mainly medium- to thick-laminated grey siltstone and mudstone plus two sandstone interbeds. The latter are thin (0.08 to 0.25 m), slightly lenticular beds of fine-grained wacke with cross and convolute laminations (Tc ), indicating the correct way up.

Exposures in a small gorge [NY 1762 2352] to [NY 1731 2342] along the Hope Beck stream section show gently dipping, dark grey, laminated siltstone and mudstone with up to 5 per cent laminated very fine-grained wackes. The highest exposure [NY 1739 2329] shows 0.32 m of wacke with graded, parallel-laminated and cross-laminated bedding structures (Bouma units Tabc), confirming that the sequence is the correct way up. The contact with the overlying Loweswater Formation, defined by Jackson (1961), is nearby on the slope leading up to Dodd [NY 171 233].

Swinside Plantation

Forestry track exposures in Swinside Plantation [NY 1834 2424] to [NY 1860 2362] show mainly grey and dark grey, laminated siltstone and mudstone with abundant laminae of very fine-grained wacke. Also present are sporadic thin beds (up to 0.1 m) of very fine-grained wacke with convolute- and cross-laminated sedimentary structures (Tc). In Swinside Plantation [NY 1840 2403], the Hope Beck Formation contains up to 40 per cent of sandstone. These sandstones vary from thin to medium bedded (0.07–0.30 m), with two lithologies of different petrographical and sedimentary character. Quartz-rich lithic wackes of fine to medium grain size dominate; these typically display grading, parallel-lamination and cross-lamination (Tabc units). A few turbidite beds are medium- and coarse-grained matrix-rich wackes, containing lenticular bodies of pebbly mudstone conglomerate, but otherwise massive and homogeneous.

Trusmadoor

Trusmadoor is an important section through the Watch Hill–Hope Beck formation boundary, and is the only place in Britain where the Tremadoc–Arenig transition can be recognised in graptolite-bearing rocks (Fortey et al., 1991; see Chapter 2). The Watch Hill Formation yielded graptolites and acritarchs typical of the topmost part of the Tremadoc (murrayi Biozone); the overlying laminated siltstones and mudstones present at the base of the Hope Beck Formation near the junction of Trusmadoor and Burntod Gill [NY 2795 3339] contain graptolites and acritarchs indicating the basal Arenig phyllograptoides Biozone. The base of the Arenig Series therefore occurs in the Trusmadoor section although the formation and series boundary may not coincide.

Loweswater Formation

The Loweswater Flags of previous authors were formally renamed the Loweswater Formation by Cooper et al. (1995). The formation is present mainly in the western part of the Skiddaw inlier, with a small faulted inlier in the east at Mungrisdale [NY 164 209] (Roberts, 1992a). The formation is composed mainly of quartz-rich wacke and has a fairly constant thickness of approximately 900 m in the Lorton Fells area [NY 188 255] and at Hope Gill Head [NY 186 224] but decreases northwards to an estimated 450 m around Jonah’s Gill [NY 190 343]. Thickness in the faulted inlier at Mungrisdale cannot be determined with certainty, but at least 250 m of strata are present.

The lower boundary is best exposed on Dodd [NY 171 233], and is transitional with the underlying Hope Beck Formation. Jackson (1961) loosely defined an upper boundary between Thornthwaite and Whiteside. It was designated by Cooper et al. (1995) as the transition seen in a small quarry on the west flank of Whiteside End [NY 1660 2169]. Here, bed thickness decreases abruptly upwards over a few metres, and the proportion of mudstone in the succession increases, to form a mainly laminated facies (beds of 0.5 to 1 cm thickness) of sandstone and mudstone with sporadic very thin and thin (1 to 7 cms) beds of quartz-rich wacke. The top of the formation is taken at the highest thin sandstone bed in the dominantly sandstone part of the succession.

The sandstones of the Loweswater Formation are mainly quartz-rich feldspathic wackes with some quartz-rich lithic wackes. The basal part of the formation has thin, mainly fine-grained wacke beds, with cross and convolute laminations, interbedded with 30 per cent to 50 per cent of siltstone and mudstone; Bouma Tcde units dominate. Thicker wacke beds appear about 50 m above the formation base. These are fine to medium grained, mainly parallel-laminated and ripple cross-laminated sandstones (Tbc turbidite units) (Plate 6), (Plate 7). Bed thickness increases gradually upwards to a maximum of 1 m near the middle of the formation; concurrently the maximum grain size increases to very coarse grained, though most beds are still fine to medium grained (Plate 8). Complete Ta-e units are present, with the argillaceous parts comprising 10 per cent to 20 per cent of the succession. The upper part of the formation is a mirror image of the lower part, bed thickness and grain size decreasing gradually while the percentage of siltstone and mudstone increases. High in the succession, the arenaceous beds are mainly cross- and convolute-laminated Tc units (Plate 9).

The Loweswater Formation wackes average 39 per cent matrix (Appendix 3), and are quartz-rich (about 40 per cent), with feldspar more abundant than lithic fragments. Quartz is predominantly of strained monocrystalline type, with some large unstrained monocrystalline clasts. The plagioclase feldspars are dominantly andesine and form about 10 per cent of the rock. The lithic grains constitute about 5 per cent and are mainly sandstone, [NY 1638 2097], north-west side of Grasmoor. Sandstone and siltstone Tbc units, showing signs of soft-sediment faulting and boudinage (L3179). Scale: width of veiw approximately 0.5 m. siltstone and mudstone. They also include subordinate volcaniclastic and siliceous pyroclastic fragments, clasts of fine-grained volcanic and metamorphic rocks, and detrital mica.

About 10 per cent of the sandstone beds in the formation contain more than 50 per cent matrix, and are informally distinguished as ‘high-matrix wackes’. They are typified by poorly developed sedimentary structures, smooth bases, normal grading, and have ripple cross-laminations sporadically preserved at their tops. These beds are commonly strongly cleaved because of their high matrix content. The detrital grains in the ‘high-matrix wackes’ are generally unaltered and of similar type and proportion to the other Loweswater Formation wackes. Rather different, mature quartz wackes are present locally in Jonah’s Gill [NY 181 343].

Loweswater Formation sandstone beds are fairly continuous, but commonly show a slightly lenticular form when traced for distances of 100 m or so, as for example at Scawgill Bridge Quarry [NY 1770 2578]. Sporadic channel structures are also present and can be seen on Darling Fell, Loweswater [NY 1265 2218]. Here, channels filled with cross-stratified (0.5 to 1 m scale), medium- to coarse-grained sandstone are cut into the underlying beds. A similar, but smaller, channel is present near Swinside Plantation [NY 1778 2482].

Bioturbation is widespread in the Loweswater Formation and comprises mainly subhorizontal, branched and unbranched, meandering, subcylindrical burrows, 0.3 to 1.5 cm in diameter, mostly varieties of Dictyodora. Looping burrows assigned to Palaeochorda are also present, as are a few near-vertical cylindrical burrows and sporadic circular and meandering burrows. Body fossils of the organisms responsible for the burrows are absent. The ichnofauna has been described and illustrated by Orr (1996).

Biostratigraphical correlation

Mudstone interbeds near the base of the succession, for example west of Blaze Bridge [NY 1790 2510], have yielded graptolites indicative of the varicosus Biozone, including Didymograptus varicosus, D. aff. balticus, D. filiformis, and Tetragraptus species including rare T. fruticosus. The fauna of the varicosus Biozone ranges through much of the Loweswater Formation, but the upper beds have yielded graptolites indicative of the simulans Biozone, including D. simulans, D. infrequens, Pseudophyllograptus angustifolius and Azygograptus ellesi (Beckly and Maletz, 1991, p.913). Acritarchs are scarce in the lower part of the formation, except in Jonah’s Gill where the Coryphidium bohemicum Biozone occurs. The Stelliferidium aff. pseudoornatum Biozone has been recorded from the upper part of the formation, for example on Embleton High Common, in Tom Rudd Beck, and at Barf. A mid-Arenig age is indicated.

Key Sections

Loweswater Fell

Loweswater Fell gives its name to the formation which is well exposed on its southern side, Darling Fell [NY 128 226]. The rocks here form a recumbent anticline with a basal thrust along the southern and eastern side (Webb and Cooper, 1988). Approximately the upper 500 to 600 m of the formation are exposed on the hillside for about 1200 m to the east of a disused quarry [NY 1256 2225]. The sequence is dominated by quartz-rich wackes, typically fine to medium grained, but becoming coarse grained in places. The majority of the beds are thin to medium, but in approximately the middle of the formation, on the overturned limb of the anticline [NY 1355 2196] they reach 2 m in thickness. The medium, thick and very thick beds commonly show graded bedding, parallel lamination and cross or convoluted lamination (Tabc).

A large channel structure is present on the west flank of the Darling Fell anticline [NY 1265 2218]. The channel cuts down into the underlying beds and is filled with parallel-laminated and cross-laminated (0.5 to 1 m scale), medium- to coarse-grained sandstone; the foreset beds show a wide range of orientations. The channel-fill sandstones form an outcrop about 150 m long, subparallel to the palaeoflow direction which is approximately to the north-north-west; the sides of the channel are nearly vertical (Moore, 1992).

Burnbank Fell

The sequence on Burnbank Fell is similar in style to that seen on Loweswater Fell to the north and also contains a major channel structure within the sandstones. The channel [NY 1173 2151] is filled with a very thick (about 5 m) bed of matrix-rich wacke with little internal structure apart from normal grading (Ta) from very coarse grained at the base to medium grained at the top; faint parallel lamination (Tb) is also present locally in the upper part of the bed (Moore, 1992).

Whiteside End

Whiteside End shows the full sequence of the Loweswater Formation dipping steeply southwards on the northern flank of the Gasgale Syncline above the Gasgale Thrust. In the northern part of the section there is an interbedded transition from the underlying Hope Beck Formation. The bedding thickness increases and the amount of siltstone and mudstone decreases; the lower beds are dominantly fine grained, thin to medium bedded, quartz-rich wackes with cross-laminations and parallel laminations (Tbc). Passing southwards along the hill side, the bed thickness increases to a maximum of around 1 m in about the middle of the formation (Plate 8). Here basal turbidite units range up to very coarse grained, and grade up into laminated and cross-laminated siltstone (Tabc). The upper part of the succession is a mirror image of the lower part, with a gradual decrease in bed thickness, and an increase in the amount of mudstone and siltstone (Plate 9). The upper boundary of the formation is taken at the top of this sequence. It is best seen in a small disused quarry on the west flank of Whiteside End [NY 1660 2169] (as described above), at the highest thin sandstone bed in the sandstone-dominated part of the succession.

Hope Beck and Ladyside Pike

Some 285 m of the Loweswater Formation are well exposed along Hope Beck [NY 1800 2270] to [NY 1821 2480]. The sequence dip is near vertical, younging to the south; laterally it extends through a series of minor folds up to the summit of Ladyside Pike [NY 1849 2275]. The base is not exposed along the beck, but the lowest part of the sequence [NY 1800 2270] comprises thin-bedded, fine-grained, quartz-rich wackes with abundant cross-laminated beds and sporadic parallel-laminated beds (Tbc). Upstream 100 m from here [NY 1805 2261], bed thickness increases to medium, and grain size increases to medium. Matrix-rich and quartz-rich wackes are present. The matrix-rich varieties commonly have a calcite cement that weathers to a soft brown crust up to several centimetres thick. Both types occur in complete turbidite Ta-e units with interbeds of probable hemipelagic mudstone. Near the middle of the Loweswater Formation the thickest and coarsest beds are exposed in a waterfall [NY 1809 2256]. Here the wackes are thick-bedded (up to 0.73 m), quartz-rich, up to coarse-grained at the base but grading up into medium- and fine-grained, laminated and cross-laminated siltstone (Tabc units). From the waterfall southwards, the beds decrease in thickness becoming dominantly thin, then very thin with increasing amounts of interbedded siltstone and mudstone; in the sandstones the dominant sedimentary structure is cross-lamination (Tc). The top of the Loweswater Formation along Hope Beck is faulted [NY 1831 2241] against the overlying Kirk Stile Formation.

Scawgill Bridge Quarry, Sware Gill and Kirk Fell

Scawgill Bridge Quarry [NY 1770 2578] and the adjacent stream section of Sware Gill [NY 1755 2776] to [NY 1744 2603] are two readily accessible sections in the Loweswater Formation (Plate 10), (Plate 11). Moore (1992) presented detailed sedimentological logs of the Loweswater Formation in Scawgill Bridge Quarry. These logs, and their interpretations, are reproduced in Chapter 4.

The sequence in the lower part of Sware Gill is similar to that in the upper part of Scawgill Bridge Quarry. Higher up the gill, beds become thinner (less than 0.2 m) and consist of fine-grained quartz wacke. Here the dominant sedimentary structures are cross-lamination and convolute lamination (Tc). In the topmost 8 m of the stream section [NY 1773 2603] the proportion of sandstone decreases to around 40 per cent, with very thin to thin beds, containing traces of cross-lamination, interbedded with siltstone and mudstone.

In Sware Gill [NY 1743 2621] and on Kirk Fell [NY 1737 2600] and Greystones [NY 1769 2618], the upper part of the Loweswater Formation comprises thin and very thin beds of fine-grained, quartz-rich wackes with cross-lamination (Tc) alternating with 40 per cent siltstone. Flat-lying, recumbent sedimentary slump folds with associated shear planes are present here, and are deformed by the tectonic cleavage.

Embleton High Common, Tom Rudd Beck, Ling Fell and Sale Fell

Embleton High Common [NY 169 281], Tom Rudd Beck [NY 1635 2899] to [NY 1710 2817], Ling Fell [NY 179 286] and Sale Fell [NY 194 297] all lie along the major Sale Fell Anticline, an upright isoclinal fold with nearly vertical strata on both limbs. Beds on the northern limb dip and young mainly to the north; those on the southern limb are commonly overturned, with northerly dips and southward younging directions. Abundant sedimentary structures provide way-up evidence here, with a particularly good example of flute casts in an old quarry on the south-west side of Embleton High Common [NY 1679 2794] (Plate 12).

Tom Rudd Beck, east of Embleton High Common, reveals both flanks of the Sale Fell Anticline. The anticlinal axis crosses the stream [NY 1688 2849], where the dip and younging directions change. From there southwards to [NY 1710 2814] there is approximately 60 per cent exposure within a stratigraphical thickness of 107 m of the Loweswater Formation (Moore, 1992, enclosure 7). The sequence is dominantly siltstone and mudstone, with about 30 per cent thin-bedded, very fine- to fine-grained quartz-rich wackes; the latter are mainly cross-laminated with sporadic convolute-laminated beds (Tcd units). Some graded beds of medium- to fine-grained wacke up to 0.5 m thick (Ta) occur near the top of the section. The large amounts of siltstone and mudstone in the formation are a local development interpreted by Moore (1992) as an interlobe facies.

Exposures on the north and south flanks of Ling Fell [NY 179 286] are mainly of thin- to medium-bedded, fine- to medium-grained, quartz-rich wacke with parallel and cross-lamination (Tbc). Sporadic medium to thick beds of medium- to fine-grained, matrix-rich lithic wackes are also present, showing graded bedding and some parallel lamination (Tab).

The south-western flank of Sale Fell exposes the Loweswater Formation and the Sale Fell Anticline. The north limb of the anticline, through Fisher Wood [NY 1820 2945], shows approximately 250 m of beds. They are dominantly thin to medium bedded (0.1 to 0.3 m), fine- to medium-grained quartz-rich wacke with cross-lamination (Tc); up to 20 per cent siltstone and mudstone is also present in thin interbeds. In about the middle of the sequence [NY 1818 2946] up to 20 per cent of medium to thick beds of coarse-grained, matrix-rich lithic wacke displaying graded and parallel bedding (Tab) are present. Similar beds are also exposed in the small car park [NY 1855 2932] and on Sale Fell, at Dodd Crag [NY 1888 2947].

Barf

The Barf crags are important Loweswater Formation exposures, long famous for the graptolites collected from their screes. However, these screes have been collected so intensively that fossils are now rare and reference has to be made to museum collections. The Barf crags include an upstanding white-painted rock pillar, about 3 m high, called the Bishop of Barf [NY 2180 2647]. At Barf, the formation is folded into a major recumbent anticline overturned towards the south-south-east. In the lower crags, to about 50 m above the Bishop, the beds are inverted, dip north-north-west and young southwards. The higher crags, Slape Crag [NY 2157 2668] and the summit [NY 2145 2675] show the correct way up. The top of the Loweswater Formation is exposed immediately south of Barf. The upper sequence at the Bishop is mainly thin- to medium-bedded, cross-laminated (Tc), fine-grained quartz-rich wackes, with up to 50 per cent of siltstone and mudstone. The beds are dominantly medium bedded on Slape Crag, and include both parallel- and cross-laminated beds (Tbc); these thin northwards with cross-lamination becoming dominant. In a disused quarry [NY 2191 2677] at the foot of Barf are exposed about 20 m of mainly medium to thick, overturned beds (up to 0.5 m thick) and a mixture of graded to laminated turbidite units (Tabc) with laminated and cross-laminated beds (Tbc).

Carsleddam, Doups and Broad End

On the west flank of Skiddaw, the Loweswater Formation is present in an anticlinal fold between Carsleddam [NY 2573 2741] and Carl Side [NY 2552 2809]. To the south-west, the anticline is bounded by a north-west-trending fault with massive quartz and baryte mineralisation developed at White Stones [NY 2532 2714]. To the north-east, across another fault, the anticline extends to Broad End [NY 261 280] and the side of Little Man [NY 2645 2798] where the rocks are hornfelsed in the aureole of the Skiddaw granite. Within this anticlinal area only the upper 300 to 400 m of the Loweswater Formation are exposed. There is a gradual increase in wacke bed thickness towards the core of the anticline, with the proportion of siltstone and mudstone in the sequence falling to around 20 per cent. The wackes are mainly fine to medium grained and quartz-rich, in dominantly thin to medium beds. The widespread development of parallel and cross-lamination is indicative of Tbc turbidite units.

Jonah’s Gill

The Jonah’s Gill section [NY 190 343] (Hughes, 1995b) is the most northerly exposure of the Loweswater Formation, and with a thickness of around 450 m, is much thinner than in the south. Jonah’s Gill is separated from the southern exposures by the Watch Hill Thrust Fault that has a probable southerly directed displacement of between 4 and 10 km (see Chapter 5). The apparent northward thinning of the formation may therefore be exaggerated by tectonic shortening of the original geometry of the depositional system. In Jonah’s Gill, the lowest beds are approximately 60 per cent laminated, thin to medium bedded, fine to medium grained and quartz-rich. These are mainly cross-laminated with some parallel lamination (Tbc), and are interbedded with about 40 per cent mudstones and siltstones. Upstream the proportion of sandstone increases to around 70 per cent and the beds are mainly thin to medium with parallel and cross-lamination (Tbc). Sporadic beds with graded lower parts are also present (Tabc).

Mungrisdale

The fault-bounded outcrop of sandstone around Raven Crags, Mungrisdale [NY 363 306] to [NY 360 311] is the most easterly exposure of the Loweswater Formation. No formation boundaries are exposed, the sequence is considerably folded and faulted (Roberts, 1977a, b, 1990, 1992a) and as little as 250 m of strata may be present. These comprise mainly very thin- to medium-bedded, fine- to medium-grained, quartz-rich wackes with common cross-lamination (Tc). Thin interbeds of siltstone and mudstone comprise 20 to 40 per cent of the sequence.

Kirk Stile Formation

The laminated mudstones and siltstones overlying the Loweswater Formation were originally referred to as Kirk Stile Slates and Mosser Slates. They were subsumed into the Kirk Stile Slates (Jackson, 1978) and formally designated the Kirk Stile Formation by Cooper et al. (1995). The Kirk Stile Formation definition follows Jackson’s (1978) usage; the base is defined at the top of the Loweswater Formation on Whiteside End (see above). The formation is not, however, conformably overlain by the Latterbarrow Formation as stated by Jackson (1978).

The Kirk Stile Formation is named after the inn and church at Kirk Stile, Loweswater [NY 131 209], but the surrounding area is poorly exposed and unfossiliferous. Consequently, Cooper et al. (1995) suggested the area from the formational base on Whinlatter Pass [NY 203 245], through Sleet How [NY 206 228] to the youngest strata on Outerside [NY 214 216], as a stratotype for the formation. No precise upper boundary can be defined for the formation, which is unconformably overlain by the Eycott Volcanic Group (Millward and Molyneux, 1992). The formation is between 1500 and 2500 m thick, but some of this variation may be due to stacking of slumped masses.

The Kirk Stile Formation consists typically of thin- laminated to very thin-bedded, dark grey siltstone and mudstone. Grading is commonly present, and the beds represent distal, low-density turbidite deposits and hemipelagites (Tde). At about 600 m and 1300 m above the base of the formation there are locally developed, lenticular sandstone-rich units (about 80 and 120 m thick respectively) that include 20 to 30 per cent of lithic wacke. The lower sandstone unit can be traced from near Darling How Plantation [NY 1840 2701] eastwards through Lord’s Seat [NY 2044 2657] to Bassenthwaite Lake [NY 2162 2805]. The upper unit is centred on Hogg Park [NY 207 287]. The sandstones are typically very thin to thin bedded, with parallel and ripple cross-laminations (Tbc). The lithic wackes of the Kirk Stile Formation are highly altered, with much of the detrital mineralogy obscured by clay-mineral replacement. Strained quartz is the most abundant grain-type; feldspar is rare, and lithic grains are relatively common. The dominant lithic clasts are volcaniclastic siltstone and altered mafic volcanic lithologies.

In the upper part of the formation, sporadic thick beds of sedimentary breccia occur in association with slumped strata. The slumped beds commonly occur in units 2 to 40 m thick, bounded by less disturbed and undisturbed beds, showing the slumping to be synsedimentary or early post-sedimentary. The breccias are best seen on Outerside [NY 2132 2140] where they are hornfelsed in the Crummock Water aureole. These breccias are the ‘Outerside fluxoturbidite’ of Jackson (1978). They include clasts up to about 0.2 m in diameter, mainly of siltstone and mudstone but with a few quartz-rich wackes. The clasts vary from subrounded to angular, set in a silty mudstone matrix (Plate 13).

Another unit of massive breccia, more than 40 m thick, occurs at Beckgrains Bridge [NY 1904 3552]. It includes mudstone, siltstone and a few sandstone clasts, up to 0.5 m in diameter and forming 5 to 20 per cent of the rock, all set in a folded and sheared mudstone matrix. To the west along Sunderland Gill [NY 1805 3498] to [NY 1864 3514], the breccia appears to be underlain by a slumped and deformed succession of wackes, siltstones and mudstones.

Biostratigraphical correlation

Graptolites recorded from the lowest parts of the Kirk Stile Formation are of limited biostratigraphical value and are only tentatively referred to the mid-Arenig simulans Biozone. Graptolites indicating the victoriae Biozone are present in the immediately overlying part of the formation. Higher still, the gibberulus Biozone is recognised by the appearance of Isograptus caduceus gibberulus, commonly associated with Didymograptus extensus linearis and D. uniformis lepidus, at a level about 700 m or more above the base of the formation, and thence through much of the higher part. The youngest part of the formation so far proved lies on the north-east flank of Outerside [NY 211 215] and on Souther Fell near Hazelhurst [NY 362 289], where the graptolite faunas of the cucullus (formerly hirundo) and artus biozones are adjacent and show that the succession lies across the Arenig–Llanvirn boundary (Fortey et al., 1990, table 2).

The Stelliferidium aff. pseudoornatum acritarch Biozone has been recorded at Hodgson How [NY 2442 2361] from beds that are considered to be of gibberulus Biozone age (Beckly and Maletz, 1991), and are inferred to represent part of the Kirk Stile Formation. Elsewhere, for example in the Overwater Spillway [NY 2586 3552], the Southerndale–Watches area [NY 2406 3026] and in the River Cocker [NY 1208 2879], the Kirk Stile Formation has yielded acritarchs of the hamata–rarirrugulata Biozone. Around Beckgrains Bridge [NY 1904 3552], the breccia has yielded acritarchs of the hamata–rarirrugulata Biozone, but also contains recycled Tremadoc acritarchs such as Acanthodiacrodium angustum, Cymatiogalea bellicosa, Saharidia fragilis and species of Acanthodiacrodium, Cymatiogalea and Vulcanisphaera. Moreover, the hamata–rarirrugulata Biozone microflora from Beckgrains Bridge contains Striatotheca frequens and Stellechinatum cf. celestum, which suggest that its stratigraphical position may be close to the Arenig–Llanvirn boundary. If so, the breccia might be younger than the similar Buttermere Formation olistostrome in the Central Fells Belt of the Lake District. This resembles the Beckgrains deposit in so far as it contains a mixture of Tremadoc and Arenig acritarchs, but has so far failed to yield latest Arenig or Llanvirn fossils, and is inferred to have been emplaced close to the gibberulus–cucullus biozonal boundary.

Key Sections

Skiddaw

The Kirk Stile Formation is well exposed on the west flank of Skiddaw, on the ridge leading up to Randel Crag and on the crag itself [NY 2544 2945]. This is a historically important fossil locality that has yielded graptolites from the gibberulus Biozone. The lithologies here typify much of the Kirk Stile Formation and comprise fairly homogeneous dark grey siltstone with mudstone laminae, giving the rock a faint, finely banded appearance. Towards the top of Randel Crag, the strata are hornfelsed in the thermal aureole of the Skiddaw granite and the weakly cleaved siltstones contain chiastolite crystals up to 2 mm long. This lithology forms much of the mountain side eastwards to the top of Skiddaw Man [NY 2604 2908].

Ullock Pike

The Edge, Ullock Pike and Longside Edge provide extensive exposures of the Kirk Stile Formation although north of The Edge [NY 2412 2951] the sequence is disrupted by faulting. The Edge has yielded graptolites of the cucullus Biozone that assigns the section to the upper part of the formation. Lithologically it is mainly laminated dark grey siltstone and mudstone with commonly developed cone-in-cone nodules. Disharmonic slump folds are widespread e.g. [NY 2397 2971]. To the southeast [NY 2412 2951], the sequence has yielded numerous graptolites attributable to the gibberulus Biozone, suggesting the lower part of the formation. There, most of the exposures are of laminated and very thin-bedded dark grey siltstone with subordinate mudstone laminae.

Skiddaw Dodd

Forestry tracks on Skiddaw Dodd expose many sections in the Kirk Stile Formation. A north-east–south-west-trending syncline passes approximately through the summit of the hill [NY 2442 2736], and on both limbs graptolites representative of the gibberulus and cucullus biozones are present, indicating the middle and upper parts of the formation. The lithology of most of Skiddaw Dodd comprises laminated dark grey siltstones, with subordinate mudstones. Locally, (as west of Watch Crag [NY 2380 2780]) the siltstones are highly pyritic with cubic crystals of pyrite up to 1 cm across and veins of pyrite. Near the axis of the syncline, the highest beds exposed also contain sporadic cone-in-cone nodules, up to 0.2 m in diameter and 0.05 m thick, flattened in the plane of the bedding (as in Dodd Wood [NY 2462 2740]).

Wythop, Lords Seat and Broom Fell

Here, the formation is siltstone-dominated, but contains up to 10 per cent of laminated, very thin beds of very fine-grained wackes. The middle of the formation at Wythop, Lords Seat and Broom Fell includes a 120 m-thick sequence of thin-bedded, fine-grained quartz-rich wacke, commonly with cross-lamination (Tc). The unit is traceable from Ladies Table to Hogg Park [NY 2070 2870], and also occurs at Lords Seat [NY 2044 2657], Broom Fell [NY 1940 2719] and south-east of Greystones [NY 1785 2643]. At one locality [NY 2094 2920] graptolites are indicative of the victoriae Biozone. In some places, the formation contains graptolites typical of the simulans Biozone.

Hope Gill Head and Gasgale Crags

The contact between the Loweswater and Kirk Stile formations is faulted in Hope Beck [NY 1831 2242]. South of here the dip is southerly into the axis of the syncline through Hope Gill Head [NY 1863 2229]. The Kirk Stile Formation hereabouts is mainly composed of laminated siltstone and mudstone with very thin interbeds of cross-laminated, fine-grained wacke (Tc) present locally. The wacke beds form up to 20 per cent of the sequence at Hope Gill Head [NY 1857 2216], along Gasgale Crags [NY 1743 2196], and beside the Whiteside footpath [NY 1690 2162]. Below Whin Ben [NY 1641 2098] the basal Kirk Stile Formation contains disharmonic slump folds hornfelsed by the Crummock Water thermal aureole (Cooper, 1986, 1990a, 1992).

Grasmoor

On Grasmoor, the Kirk Stile Formation is hornfelsed in the Crummock Water metamorphic aureole (Cooper et al., 1988). The protolith was dark grey, laminated siltstone and mudstone with subordinate amounts of sandstone near the base of the formation (described above from Gasgale). The siltstone and mudstone have been bleached to a pale grey colour and indurated to produce a massive hard hornfels, but sedimentary structures are well preserved (Plate 7) Over much of Grasmoor and Lad Hows [for example [NY 1729 2859] well-developed disharmonic slump folds are preserved, ranging in scale from a few centimetres to many metres across.

Outerside and Causey Pike

Outerside spans the gradational northern boundary of the Crummock Water aureole. The beds here are dominantly laminated siltstone and mudstone dipping to the south, but with small-scale slump folds; within the aureole these rocks are hard, pale grey hornfels (Cooper et al., 1988). In addition to the laminated siltstones, localised beds of breccia are also present [NY 2132 2140] (Plate 13) (described above in the introduction to the Kirk Stile Formation). On the north-east face of Outerside [NY 2144 2169] the beds have yielded abundant graptolites, indicating that this part of the sequence straddles the Arenig–Llanvirn boundary (Fortey et al., 1990). On the top of Causey Pike, the hornfelsed beds of the Kirk Stile Formation are thrust over wackes of the Robinson Member (Buttermere Formation, see below) by the southerly directed Causey Pike Thrust. As at Outerside, the Kirk Stile Formation comprises laminated siltstone and mudstone, but is now a hard, pale grey hornfels with tourmaline veins (Cooper, 1990a, b)

Beckgrains Bridge

At Beckgrains Bridge [NY 1904 3552], debris-flow deposits in excess of 40 m thick occur within the Kirk Stile Formation. They comprise rounded clasts of mudstone, siltstone, and sandstone in a mudstone matrix. The clast axes are 0.03 to 0.5 m long with a preferred orientation in a north-south vertical plane, parallel to the probable tectonic fabric in the matrix. The breccia here yielded microfossils showing it to be a mixture of Tremadoc and late Arenig material. The mixed microfossil assemblages occur together in the same samples, indicating intense mixing of sedimentary material during redeposition, probably during the late Arenig.

River Caldew

Slump-folded siltstones and mudstones of the Kirk Stile Formation, hornfelsed by the Skiddaw granite, are exposed in the River Caldew [NY 331 325] to [NY 325 328]. The original sedimentary rock appears to have been laminated siltstone and mudstone, with a few lenticular laminae of very fine-grained, cross-laminated sandstone. These lithologies have been metamorphosed mainly to cordierite-biotite hornfels (Eastwood et al., 1968; Merriman et al., 1991), with additional alteration to quartz-muscovite hornfels adjacent to the Grainsgill Greisen (Roberts, 1983). The beds display complex, disharmonic folds with sheared limbs. These structures were described in detail by Roberts (1971), and reinterpreted by the same author (1990, 1992b) as slump folds (see Chapter 5).

Central Fells Belt

Buttermere Formation

The Buttermere Formation (Cooper et al., 1995) is an olistostrome deposit, at least 1500 m thick. It includes disrupted and folded mudstone, siltstone and sandstone turbidite olistoliths ranging in age from Tremadoc to late Arenig. Most of the formation is undivided, but two members are present in the middle part in the Buttermere area. These are the Goat Gills Member, a sedimentary breccia, and the overlying sandstone-dominated Robinson Member. The Robinson Member has also been identified in the vicinity of Causey Pike (Cooper, 1990c, figs 24, 26) and at Swinside, near Braithwaite. Webb and Cooper (1988) and Webb (1990) described the large-scale slump folds in the succession and informally proposed the names of the Buttermere Formation and the Robinson Member. Webb (1992) presented further details and the definition of the Goat Gills Member. The slump folds are discussed further and illustrated in Chapter 5.

The formation is best seen east of Buttermere village, around Sail Beck [NY 170 175] to [NY 188 190], High Snockrigg [NY 187 169] and Robinson [NY 202 169]. The formation is bounded to the north-north-west by the Causey Pike Fault and to the east by a fault which juxtaposes the Tarn Moor Formation. To the south and south-east, the formation persists up to the unconformity beneath the overlying Borrowdale Volcanic Group. To the west, the extent of the formation is unclear, but exposures in the River Calder [NY 060 115] may be referable to the Buttermere Formation or may represent the underlying succession (see below). The base and top of the formation are not seen.

Most of the Buttermere Formation olistoliths are of dark grey mudstone, commonly with pale grey siltstone and very fine- to medium-grained sandstone laminae. Other olistoliths are homogeneous dark grey mudstone and medium-grained wacke-type sandstone. The olistoliths vary in size from a few millimetres up to a kilometre or more across; most are in the 5 to 10 m range. Instructive, water-polished slab exposures are seen in Sail Beck [NY 188 178] to [NY 188 180]. Here, angular to subrounded, dark grey, banded siltstone and mudstone olistoliths up to 8 m long are enclosed in a pale grey, sandy and silty mudstone matrix. Some of the olistoliths are angular and were at least partly lithified before being incorporated in the deposit; others are ragged with injection structures into and from the matrix, suggesting they were in a plastic state when redeposited. Very fine quartz grains present throughout the argillaceous matrix around the olistoliths seem likely to have been part of a disaggregated sandstone. From Hindscarth [NY 2156 1652] to the Newlands valley [NY 234 199], some 400 m of siltstone and mudstone olistostrome deposits overlie and appear to be younger than the Robinson Member. The clasts and matrix are intensely deformed by minor folds and shears, many of which were generated during emplacement of the olistostrome (Webb and Cooper, 1988).

Similar slump-folded mudstones, siltstones and sandstones with Tremadoc trilobites and acritarchs (Molyneux and Rushton, 1984; Rushton, 1988) occur along the River Calder [NY 0687 1178]. These beds may be part of the pre-Buttermere Formation succession, or a raft of older material within it. Similarly, the Arenig strata (possibly gibberulus Biozone) at Beck Grains [NY 0776 1128] (Allen and Cooper, 1986) may be from an olistolith or be part of a coherent sequence.

Goat Gills Member

The Goat Gills Member is a local breccia approximately in the middle part of the Buttermere Formation. It contains rounded and angular siltstone and fine-grained sandstone clasts, generally around 0.025 m but up to 0.2 m across, set in a silty mudstone matrix (Webb, 1990, 1992). The clasts are identical to lithologies in the overlying Robinson Member (see below) and bedding is commonly clearly visible within them. There is little evidence of clast stratification or imbrication within the breccia. Locally, within-clast bedding appears to be traceable through adjacent clasts suggesting at least partial in situ disruption rather than complete debris flow. The Goat Gills Member crops out in the core of the major anticlinal slump fold at Goat Gills [NY 190 161], and extends some 600 m north-westwards across Goat Crag (Webb and Cooper, 1988; Webb, 1992). It is up to 100 m thick, but its base is not seen. Along the upper limb of the anticline, the breccia passes into highly disrupted and contorted siltstone and mudstone with sporadic sandstone boudins of sedimentary origin. Both the breccia and the overlying disturbed unit have yielded microfloras of Tremadoc age.

Robinson Member

The Robinson Member is a series of large sandstone-rich olistoliths, each ranging up to 1 km across, also in the middle part of the Buttermere Formation. The olistoliths are mainly quartz-rich lithic wackes and granule conglomerates interbedded with siltstone and mudstone; lithologically they are similar to the Loweswater Formation of the Northern Fells Belt.

The wacke beds are typically very thick (up to 0.2 m), but coarse- and granule-grained beds locally reach 2.5 m thick. Turbidite sedimentary structures are common, and include graded bedding, convolute laminations and ripple-drift bedding (Tabc). The bases of the sandstones are mainly planar, but flute marks are sporadically preserved (Plate 14). Re-orientated palaeocurrents are unreliable because of the structural complexity of the olistostrome deposits.

The sandstones are typically quartz-rich lithic wackes with a matrix content of 28 to 57 per cent. Subangular to subrounded clasts are dominantly of strained quartz with moderate amounts of feldspar and sedimentary lithic fragments. In the granule conglomerates, the quartz clasts are subrounded and rounded. The sandstones are compositionally similar to those in the Loweswater and Hope Beck formations of the Northern Fells Belt, and are probably of similar age.

The Robinson Member was highly disrupted during emplacement of the Buttermere olistostrome. Major rafts of the member, 1 km or more in length, are preserved on the inverted limbs and in the hinge regions of major anticlines passing through Goat Crag, the Snockrigg area, Moss Force and through Robinson and Littledale (Webb and Cooper, 1988; Webb, 1990, 1992). Except where the Goat Gills Member is present, the Robinson Member is enclosed within an envelope of Buttermere Formation that is rich in sandstone boudins; this was informally annotated by Webb (1992, fig. 3.5) as ‘Mélange with sandstone (Robinson Member)’. However, the sandstone-rich envelope is probably best included in the background Buttermere Formation as it does not constitute a readily mappable unit.

At Goat Crag [NY 1895 1624], the Robinson Member directly overlies the Goat Gills Member. The junction appears to be locally conformable, but the lowest beds of the Robinson Member are, in general, highly disturbed and, in Goat Gills [NY 1914 1630], are truncated against the underlying unit. A maximum thickness of around 250 m of the Robinson Member is exposed in Littledale [NY 211 173], where its base is not seen.

Other large rafts of sandstone occur below Causey Pike [NY 2196 2074] (Cooper, 1990a, b). Here the wackes are thin, medium and thick bedded with parallel and ripple cross-lamination (Tbc). Dips are steep but normal to the north-east of Causey Pike, whereas farther south [NY 219 207] the beds are overturned, with complex disharmonic slump folds on a 20 to 150 m scale (Cooper, 1990c, fig. 26). Sandstones present at Swinside [NY 243 225] are probably also part of the Robinson Member.

Biostratigraphical correlation of the olistoliths

Fossils from the Buttermere Formation (including trilobites as well as graptolites and acritarchs if the River Calder exposures are included) indicate ages ranging from early Tremadoc to late Arenig. In addition to graptolites from the gibberulus to simulans biozones, specimens from Buttermere Quarry (situated within a large siltstone and mudstone olistolith of the Buttermere Formation) indicate a level close to the varicosus–simulans biozonal boundary. Those from Scope Beck indicate no more than a varicosus or simulans Biozone age. The Arenig Stelliferidium aff. pseudooratum and hamata–rarirrugulata acritarch assemblages are readily identifiable in the Buttermere Formation. Tremadoc microfloras are also widespread. They include acritarchs such as Acanthodiacrodium ubui, Cymatiogalea bellicosa, Dasydiacrodium cilium?, Micrhystridium diornamentum, Priscotheca tumida? and Stelliferidium cortinulum, indicating an early Tremadoc age (Rasul, 1979) for beds at Newlands Hause, in the upper part of Swinside Gill, on Goat Crag and on High Snockrigg. The acritarchs from Goat Crag are from the Goat Gills Member, some component of which is therefore early Tremadoc. Other species, notably Stelliferidium fimbrium? and Vulcanisphaera frequens?, suggest a late Tremadoc age (Rasul, 1979) for strata in Sail Beck, the lower part of Swinside Gill, and on High Snockrigg. These are in addition to the late Tremadoc assemblage from the River Calder (Molyneux and Rushton, 1984).

Three features of the Buttermere Formation merit further comment. Firstly, there is no evidence of latest Arenig rocks of the cucullus Biozone; the youngest rocks in the olistostrome are from the gibberulus Biozone or within the range of the hamata–rarirrugulata Biozone. This has implications for the timing of emplacement of the olistostrome (see below).

Secondly, there is no evidence for the messaoudensis–trifidum Biozone in the Buttermere Formation. The only evidence that beds of earliest Arenig age might be included in the olistostrome is derived from Robinson, where a doubtful specimen of the graptolite Azygograptus validus (Figure 9)n; see also Beckly and Maletz, 1991), associated with Didymograptus filiformis, may represent a level below the varicosus Biozone. Farther south, the succession passes upwards into poorly laminated silty mudstone on the north-west flank of Hindscarth [NY 211 164]; this has yielded Pseudophyllograptus, possibly of mid-Arenig age. This evidence suggests that the Robinson Member is of early and mid-Arenig age, and was subsequently disrupted and emplaced into the Buttermere Formation. On acritarch evidence, however, the Robinson Member on Goat Crag is correlated with the Stelliferidium aff. pseudoornatum Biozone. The absence of the messaoudensis–trifidum Biozone from the Buttermere Formation is consistent with the origin of the olistostrome on the upper continental slope or outer continental shelf, where deposition of beds of latest Tremadoc and earliest Arenig age might have been restricted or absent during a time of eustatic regression (Fortey, 1984).

Thirdly, a remarkable feature of the Buttermere Formation is the juxtaposition of beds of widely varying ages. In Swinside Gill [NY 1901 1777] to [NY 1900 1777] and ‘Second Gill’ [NY 1881 1819] to [NY 1880 1819], Tremadoc acritarch assemblages occur within a few metres of the late Arenig hamata–rarirrugulata Biozone. There is no mixing of the assemblages to indicate recycling in the normally accepted sense, as occurs at Beckgrains Bridge in the Northern Fells Belt. The close proximity of rocks with such disparate ages, and the chaotic distribution of the biostratigraphical records, indicates that a great deal of mixing occurred during emplacement of the olistostrome. The clasts are of sufficient size that most of the samples collected for micropalaeontological analysis contain evidence of only a single age. One sample from the foot of Swinside Gill [NY 1876 1779] contains a mixture of Tremadoc and Arenig acritarchs, suggesting that it was collected from the disaggregated matrix around the olistoliths. Even so, the Buttermere Formation is not an entirely chaotic deposit and retains some relict stratigraphy. Sandstones of the Robinson Member, although highly disrupted, probably lie mainly at a single, stratigraphical level. These problems are discussed more fully in Chapter 2.

Age and emplacement direction of the olistostrome

The Buttermere Formation contains blocks of material of Tremadoc to late Arenig age and is overlain by the Tarn Moor Formation of latest Arenig and Llanvirn age. The age of olistostrome emplacement is thus inferred to be in the late Arenig, possibly near the boundary between the gibberulus and cucullus biozones. From the geometry of the major slump folds, Webb and Cooper (1988) inferred that the olistostrome was emplaced by down-slope movement towards the north-north-west. Furthermore, from the formation thickness and olistolith sizes it is apparent that the bulk of the formation around Buttermere was emplaced in one massive slumping episode (Webb and Cooper, 1988). However, the brecciated and sheared nature of the deposit makes it impossible to distinguish any subsidiary slump masses that may be present within it.

Key Sections

Ill Gill, Ard Crags and Knott Rigg

Slump folding and brecciation in the Buttermere Formation are exposed along Ill Gill near Keskadale Farm [NY 2074 1935] to [NY 2002 1898], and in the adjacent sections along Ard Crags [NY 2055 1965] and the ridge of Knott Rigg [NY 1985 1908]. Most of the rock is laminated siltstone and mudstone with complex disharmonic folds (commonly with lobate forms). These chaotic slump-folds developed through partial brecciation (with folding, and floating fold hinges in a mudstone matrix) and some complete brecciation into debris-flow beds. In areas with this complex disruption, the cleavage is the dominant fabric in the rock, and it is on weathered cleavage surfaces that the sedimentary structures are best seen. Good examples of the breccia in Ill Gill [NY 2074 1935] show clasts mainly around 0.1 m across, but reaching a maximum of around 1 m. They are of mudstone, siltstone and/or sandstone, surrounded either by a matrix of siltstone and mudstone fragments, or by siltstone deformed into complex disharmonic folds. One water-worn surface in Ill Gill [NY 2065 1926] shows siltstone with very chaotic disharmonic folding and refolding with abundant injection structures, isolated hinges of folds, and balls or clasts of siltstone, all shown up by slight colour variations in the pale grey to dark grey siltstone. Elsewhere in the gill [NY 2037 1917], the siltstone includes fine sandstone laminae that pick out numerous shear planes, either subparallel to the bedding or cutting obliquely through the sequence.

Newlands Hause, Moss Force, Swinside Gill and Sail Beck

Near Newlands Hause, a sandstone olistolith within the Buttermere Formation is displayed in the middle of Moss Force waterfall [NY 1930 1740] (Plate 15). The olistolith is wacke-type sandstone, about 5 m high; it is surrounded by siltstone and mudstone that is brecciated and sheared. Similar rafts or olistoliths of sandstone occur throughout the crags adjacent to Moss Force. At Newlands Hause and along Swinside Gill leading down to Sale Beck [NY 1909 1777] to [NY 1877 1780], there are numerous exposures of sheared, slump-folded and brecciated siltstone and mudstone. Upstream in Sail Beck, from the confluence with Swinside Gill [NY 1873 1778] to [NY 1877 1793], the water-worn surfaces of the rock in the stream expose mudstone olistoliths as described in the main section above.

High Snockrigg

On High Snockrigg, the sandstones of the Robinson Member are folded into a major recumbent fold with inverted strata in the lower part of the face [NY 1855 1675]. The sandstones are thin- and medium-bedded (0.1 to 0.3 m) fine- to medium-grained, quartz-rich wackes with parallel and cross-lamination (Tbc). On the slopes below the main crags, siltstone and mudstone with abundant boudinaged and disaggregated beds of sandstone form a breccia that surrounds the major sandstone olistolith that caps High Snockrigg.

Lambing Knott and Muddock Crags

At Lambing Knott [NY 194 154] (Plate 23) and on Muddock Crags [NY 191 156], the Robinson Member wacke sandstones display complex, disharmonic angular folds with considerable evidence of flow, soft-sediment deformation, disruption and brecciation of the bedding. The sandstone beds are similar to those of the Loweswater Formation, comprising mainly fine- to medium-grained quartz-rich wackes in medium beds, with parallel and cross-lamination (Tbc).

Robinson and Robinson Crags

From Lambing Knott up the hillside to the peak of Robinson, the Robinson Member appears in abundant small outcrops and as surface debris of mainly fine-grained quartz-rich wacke. The latter is present as beds up to 0.05 m thick that are interbedded with siltstone and mudstone. To the north, on Robinson Crags [NY 2010 1713], fine-grained quartz-rich wacke beds generally range from 0.1 to 0.3 m in thickness; they are disposed in a major recumbent anticline, developed during the slump fold deformation.

Goat Crag

On Goat Crag, the Buttermere Formation and its component members are affected by large-scale slump folding (Webb and Cooper, 1988; Webb, 1990; 1992), and have a complex disrupted outcrop pattern (Figure 18); (Plate 24). The Goat Gills Member lies below the Robinson Member as shown in (Figure 33). The Robinson Member in the vicinity of Goat Crag is accessible on the spur between Goat Gills and Hassnesshow Beck, and along Hassnesshow Beck [NY 1904 1618] where thin, medium and thick (0.05 to 0.6 m) beds of fine- to medium-grained quartz-rich wacke are exposed. These beds have parallel, convolute and cross-lamination (Tbc). They were inverted by the early large-scale slump folding and they have been refolded and cut by the main east-west-trending tectonic folding and cleavage.

Littledale

In Littledale, the Robinson Member is present as a large elongate olistolith, about 1.4 km long, exposed on Littledale Crags [NY 2148 1755], Blea Crag [NY 2135 1758] and the flank of Robinson [NY 2093 1729]. On Blea Crag, a massive quartz-rich wacke sandstone bed with well-developed basal flute marks is exposed [NY 2135 1758] (Plate 14). On Littledale Crags, the Robinson Member is near horizontal at the top of the crags, but about 40 to 50 m below the ridge crest, it is folded and steeply dipping. The lower part of this sequence, above the reservoir, [NY 215 177] forms cliffs with massive sandstone beds up to 2.5 m thick.

Causey Pike

On the south side of Causey Pike, sandstones of the Robinson Member are present below the Causey Pike Thrust Fault (Cooper, 1990b, 1990c). These sandstones comprise a large olistolith, and have experienced considerable slump deformation with folds up to several hundreds of metres across; many parts of the sequence are inverted [NY 2214 2070] and tightly folded [NY 2196 2074] (Figure 38). Lithologically the sandstones are mainly fine- to medium-grained quartz-rich wackes in thin, medium and thick beds (0.03 to 0.5 m) displaying parallel, cross and convolute laminations (Tbc). Sporadic, well-developed flute marks are also present on the bases of some of the units [NY 2191 2073]. On the north side of the Causey Pike ridge, between Stonycroft Gill [NY 2372 2118] and the footpath to Sleet Hause [NY 2275 2100], similar wacke sandstones are exposed below the Causey Pike Thrust Fault. In contrast with the strata to the south, all of this northern sequence is the correct way up. Eastwards, the disrupted sandstones of Causey Pike become increasingly more brecciated, folded, sheared and boudinaged along the ridge to Rowling End [NY 2292 2075] where they merge laterally into the surrounding undivided Buttermere Formation.

Lingholm

At Lingholm, water-worn exposures of the Buttermere Formation beside Derwent Water show soft-sediment folding and shearing. The lithology is laminated, medium to dark grey siltstone and mudstone in very thin, graded beds [NY 2551 2229] to [NY 2537 2160]. Throughout this area these beds are disrupted by disharmonic slump folding with well-developed shears commonly cutting across bedding at a shallow angle, or becoming parallel to bedding in the way described by Webb and Cooper (1988). In addition, mudstone and siltstone breccia, with angular clasts up to 0.02 m across set in a siltstone matrix, occurs interbedded with the sheared siltstone and mudstone; the breccia is well exposed at Kitchen Bay [NY 2510 2136].

Trout Beck

Along Trout Beck [NY 3700 2691] to [NY 3862 2698] the most easterly sequence assigned to the Buttermere Formation is preserved in a fault slice. The dominant lithologies are dark grey mudstone and siltstone with sporadic thin, fine-grained sandstone beds and rare beds of intraformational breccia up to 0.5 m thick. The strata are commonly isoclinally folded and disrupted in a style suggestive of slump folding. Association with the Buttermere Formation is supported by the evidence of graptolites and acritarchs that range from late Tremadoc or early Arenig age at the eastern end of the section [NY 3864 2697] to [NY 3853 2705], through mid-Arenig (simulans Biozone) [NY 3837 2704] to possible late Arenig (cucullus Biozone) in the west [NY 3781 2703]. This variation, potentially across the whole of the Arenig, is apparently contained in no more than 200 m thickness of strata. If the youngest age determination is correct, it suggests a possible slightly younger age for the emplacement of the Buttermere Formation hereabouts than to the west.

River Calder

The sequence along the River Calder [NY 068 117] lies about 200 m north of the unconformity beneath the overlying Latterbarrow Formation. Allen and Cooper (1986) recorded the strata here as dominantly siltstone and mudstone with a few beds, up to 1 m thick, of fine-grained feldspathic wacke with graded bedding, parallel, convolute and cross-lamination structures (Tabc). Rushton (1988) noted slump-folded, flat-lying ‘Skiddaw Slate’ [NY 0687 1178], succeeded downstream by grey mudstone, siltstones and thin beds of sandstone, generally dipping to the west or north-west at 20° to 30°. The strata just downstream of the slump folds have yielded trilobites of the late Tremadoc Angelina sedgwickii Biozone (Molyneux and Rushton, 1984; Rushton, 1988). The fauna suggests an outer shelf environment, a situation shallower than that inferred for the bulk of the Skiddaw Group. If the material belongs to the Buttermere Formation it could have been transported from this shallower environment during olistostrome emplacement and redeposition.

Tarn Moor Formation

The Tarn Moor Formation was proposed by Cooper et al. (1995) as the formal name for the succession of latest Arenig to early Llanvirn mudstone and siltstone, with minor volcaniclastic turbidite and bentonite beds, that occurs in the eastern part of the Central Fells Belt. The Tarn Moor Formation is present in the south-eastern part of the Skiddaw inlier and forms all of the Ullswater and Bampton inliers (Figure 18).

The term Tarn Moor Mudstone was introduced by Wadge et al. (1972) for mudstones of D. murchisoni Biozone age proved in the Tarn Moor Tunnel aqueduct. These strata were included in the Eycott Group, but Jackson (1978) included them in the Skiddaw Group, along with the early Llanvirn succession proved by Skevington (1970) in the nearby Tailbert–Lanshaw Tunnel aqueduct. Moseley (1984) named the sequence the Tarn Moor Mudstone Formation and placed it near the top of the Skiddaw Group as a lateral equivalent in part of the Eycott Group. Millward and Molyneux (1992) showed that correlation with the Eycott Group is unjustified, so the proposal of Jackson (1978) is now regarded as more tenable although no formal description has been previously published. The Tarn Moor Formation equates with the Kirkland Formation of the Cross Fell Inlier (Burgess and Wadge, 1974; Cooper and Molyneux, 1990).

The base of the Tarn Moor Formation has not yet been proved, but its age and outcrop pattern indicate that it lies above the Buttermere Formation. If this is so, its lower boundary is likely to be unconformable on the highly disrupted Buttermere Formation, although the time-break involved would be very small. The youngest strata known occur in the Tarn Moor Tunnel (Wadge et al., 1969); the top of the Tarn Moor Formation is unconformably overlain by the Borrowdale Volcanic Group (Wadge, 1972).

The thickness of the formation is difficult to determine, but is probably around 1000 to 1500 m. A broad tripartite division of the formation is suggested by the outcrop pattern, and by biostratigraphical evidence. The lower beds in the Skiddaw inlier are laminated and thickly laminated mudstone with subordinate siltstones. The overall character is similar to the Kirk Stile Formation of the Northern Fells Belt, with which its age partly overlaps. The oldest proved beds are at Causeway Foot [NY 2907 2197], where acritarchs of the hamata–rarirrugulata Biozone occur, and Birkett Beck [NY 3280 2469] where cucullus Biozone graptolites and some trilobites have been found. Both these localities are of late Arenig age, but the early Llanvirn artus Biozone occurs in Cawell Beck [NY 3416 2582] and Mosedale Beck [NY 3554 2416] to [NY 3546 2307]. At the latter locality, possible latest Arenig and definite earliest Llanvirn strata are close together (see discussion in Chapter 2). A similar stratigraphical level may be present in Greenside Mine [NY 363 178], in the western part of the Ullswater inlier beneath the Borrowdale Volcanic Group. There, laminated mudstones from trial underground borings yielded late Arenig to early Llanvirn graptolites.

The middle part of the formation is typified by mudstone with up to 5 per cent of bentonite and volcaniclastic interbeds. The bentonite forms very thin laminae and discrete beds up to a few centimetres thick. Volcaniclastic sandstone beds may locally dominate up to about 12 m of the succession. These beds are commonly graded at the base and some have parallel- or cross-laminated upper parts forming Tab or Tabc turbidite units. In the east of the Skiddaw inlier, tuffaceous sandstones are present in Matterdale Beck [NY 3888 2346], and occur interbedded with mudstones that have yielded artus Biozone graptolites (Wadge, 1972). In the Ullswater inlier, along Aik Beck [NY 4728 2248] to [NY 4730 2227], the mudstones contain thin volcaniclastic sandstone beds and sporadic bentonite layers. Graptolites from here, including Acrograptus affinis and Didymograptus cf. murchisoni speciosus, indicate a Llanvirn (Abereiddian) age, mainly within the artus Biozone but apparently just ranging up into the murchisoni Biozone. Quartz-rich sandstones with little or no volcaniclastic material also occur in this middle part of the formation in the Ullswater inlier and are locally abundant (as at Glencoyne Park [NY 388 198]).

In the Bampton inlier, the proportion of volcanic beds apparently increases to a maximum near Bampton [NY 530 166] and Shap [NY 548 144]. Here Dakyns et al. (1897) recorded abundant ‘ash’ beds, generally 2 to 4 m thick, but some are up to 12 m thick. However, most of these largely tuffaceous bodies have been reinterpreted by Bell (1997) as being of intrusive origin and therefore linked with the Borrowdale Volcanic Group. Bell also recognised a unit of volcaniclastic sandstone and mudstone (the Tailbert Formation) unconformably overlying the Skiddaw Group but intervening between it and the lowest extrusive rocks of the Borrowdale Volcanic Group. Previous inclusion of this unit within the Tarn Moor Formation reinforced the apparent south-eastwards increase in the volcaniclastic component, which may therefore be spurious. Apart from sporadic bentonites, Bell recorded only one likely volcaniclastic interbed, 1 m thick and probably emplaced by mass flow, in Tailbert Gill [NY 5343 1533]. Within the Bampton inlier, an artus Biozone age is indicated by graptolite faunas from Thornship and Keld gills. Mudstones associated with sporadic tuff units in the Tailbert–Lanshaw Tunnel (Skevington, 1970) have also yielded graptolites of the artus Biozone.

The highest proven part of the formation is in the Tarn Moor Tunnel at the eastern end of the Ullswater inlier. Here Wadge et al. (1969, 1972) recorded mudstones of the murchisoni Biozone faulted against probable artus Biozone mudstones. Compared to the rest of the formation, these highest mudstones of the Tarn Moor Formation are darker (dark grey to black) in colour. Volcaniclastic interbeds have not been recorded from this stratigraphical level although there is a possibility that the Tailbert–Lanshaw Tunnel fauna, from mudstone with volcaniclastic interbeds, may range up to the murchisoni Biozone (Skevington, 1970).

Key sections

Mosedale Beck (Skiddaw inlier)

Mosedale Beck has cut a 600 m section [NY 3556 2386] to [NY 3567 2438] through a thick sequence of landslipped till, producing an incised narrow gorge in the underlying Tarn Moor Formation. The exposed rock varies from soft, fissile, laminated, dark grey mudstone to homogeneous mudstone; in places bedding is very regular, elsewhere the rock is severely folded, locally with a strong crenulation cleavage. The rock is commonly pyritic [NY 3555 2415], weathering to give rusty patches on the surface [NY 3560 2422] or breaking along a network of ferruginous joints. The section has yielded well-preserved graptolites of the Didymograptus artus Biozone. However, the southern part of the section [NY 3559 2418] to [NY 3558 2411] yielded graptolites which span the Arenig–Llanvirn boundary, and it is possible that Arenig strata could also be present.

Matterdale Beck (Skiddaw inlier)

The section at Matterdale Beck [NY 3888 2346] has been described by Wadge (1972) who also recorded the angular unconformity with the overlying Borrowdale Volcanic Group at this locality (Plate 16, (Plate 17). The formation here comprises dark grey silty mudstones with thin pale grey siltstone laminae and fine-grained volcaniclastic sandstone beds. Nearby, beside the track leading to the stream section, an exposure [NY 3900 2358] reveals cleaved, medium-grained, tuffaceous sandstone with a mudstone matrix. In the stream section, the Tarn Moor Formation contains graptolites indicative of the early Llanvirn artus Biozone.

Aik Beck (Ullswater inlier)

Along Aik Beck, a 500 m-long section is exposed in mainly fissile, dark grey, laminated mudstone containing graptolites; they are from the artus Biozone with a few towards the northern end [NY 4731 2237] suggesting the murchisoni Biozone. Bioturbation is sparse but widespread, mostly as burrows filled with faecal pellets. Where the beck flows through Parkfoot Plantation [NY 4697 2320] to [NY 4711 2304] there are dark grey, silty mudstones with sandy laminae, and thin beds of grey siltstone and fine-grained sandstone. Farther upstream (southwards) [NY 4725 2270] to [NY 4731 2233], exposures are of thin-bedded, parallel-laminated, dark grey mudstone and siltstone with thin interbeds of pale grey, commonly wavy laminated sandstone (Tcturbidite units). Near the southern end of the section [NY 4730 2231] volcaniclastic sandstone and bentonite appear in the sequence; some mudstones here are green and yellow in colour, which may also indicate a volcaniclastic component. Volcaniclastic beds range up to 15 cm thick and contain abundant altered ferromagnesian minerals and small elongate muscovite crystals which may originally have been glass shards (Hughes, 1995a). One lenticular bed [NY 4730 2228] of tuffaceous sandstone up to 0.05 m thick was observed to have a bentonitic top.

Tarn Moor Tunnel (Ullswater inlier)

The Tarn Moor Tunnel was driven from Ullswater to divert water into Haweswater reservoir, and extends south-east from the Ullswater inlier [NY 4766 2270] towards the Bampton inlier [NY 4949 2069] (Wadge et al., 1969, 1972). The north of the tunnel was inaccessible but the spoil removed from it consisted entirely of weathered ‘brown mudstone’ that Wadge et al. (1972) compared with the sequence at Aik Beck. About 50 m of these beds were seen in situ before the tunnel abruptly entered about 65 m of dark grey, thinly banded, sandy and silty mudstones. This part of the sequence yielded graptolites of the murchisoni Biozone (Skevington in Wadge et al., 1972), and is the youngest so far recorded from the Skiddaw Group.

Tailbert-Lanshaw Tunnel (Bampton inlier)

The Tailbert-Lanshaw Tunnel [NY 5345 1515] to [NY 5660 1179] was excavated in the Bampton inlier as part of the Haweswater aqueduct. Excavated material was mainly grey-black mudstone that yielded graptolites (Skevington, 1964, 1970) from the artus Biozone. Skevington (1970, citing observations by M J C Nutt) also recorded green tuffaceous rocks interbedded with the mudstones for about 30 m near the northern end of the tunnel. This accords with the presence nearby in Tailbert Gill [NY 5343 1533] of a lenticular (possibly boudinaged) bed, up to 1 m thick, of very coarse-grained volcaniclastic sandstone, interbedded with the dark grey siltstones and mudstones.

Keld Gill (Bampton inlier)

Exposures in Keld Gill are of dark to pale grey, laminated mudstone alternating with sporadic siltstone interbeds, commonly up to 1 cm thick, and rarer, graded, fine-grained sandstone to siltstone beds up to 5 cm thick. Graptolites from two localities [NY 5414 1345] and [NY 5407 1343] both prove the artus Biozone. The Keld Gill section [NY 5405 1342] exposes the junction between the Skiddaw and Borrowdale Volcanic groups (Bell, 1997). Here, steeply inclined mudstone, with a strong fabric parallel to bedding, loses integrity upwards into a prominent weathered surface. This surface is overlain by 50 cm of pale grey, structureless mudstone with abundant bioturbation traces, which is subjacent to gently dipping tuffaceous rocks of the Borrowdale Volcanic Group.

Shap Abbey (Bampton inlier)

The section along the River Lowther (and adjacent exposures) at Shap Abbey [NY 5488 1540] to [NY 5482 1437] has historical importance because of the description by Dakyns et al. (1897) of a major volcanic component in the succession. At the north of the section near the stepping stones [NY 5485 1500] an exposure of massive, medium-grained, reddish stained tuff is apparently faulted against purple-stained mudstone to the south. Southwards, exposure is poor, but dark grey, laminated silty mudstone is seen in several places [NY 5480 1470] and intermittently south for 100 m]. Crags west of the river [NY 5480 1435] expose another tuff unit (or possibly the same one seen by the river) apparently overlying siltstones, though the contact is poorly exposed. This tuff unit is 10 to 12 m thick and comprises several beds varying in composition from a conglomeratic lithology to medium-grained tuff; the former contains lithic volcanic clasts, subordinate mudstone clasts, and chert clasts, all ranging up to 10 cms in diameter with abundant rounded quartz grains in the matrix. These beds may be volcaniclastic turbidites or debris flows within the dominantly mudstone and siltstone sequence, but exposure is very poor and Bell (1997) preferred an intrusive origin, suggesting agglomerate/tuff vent fills.

Tailbert Formation

The Tailbert Formation was defined by Bell (1997) in the Bampton inlier. It comprises over 200 m of distinctive volcaniclastic sandstone with subordinate siltstone and, towards the top of the formation, mudstone. For the most part the sandstone is relatively massive and featureless, commonly forming beds about 1 m in thickness, but locally it contains large clasts (up to 50 cm across but mostly of centimetre scale) of mudstone, tuff and/or volcaniclastic siltstone. The detrital components of the sandstone itself are well sorted, but are angular to subangular; the grains consist of plagioclase, quartz (commonly strained), calcite (polycrystalline) and volcaniclastic sandstone, with much rarer grains of pilotaxitic lava and apatite. Some beds have loaded bases; ripple cross-bedding has been reported from one locality.

The formation has a restricted outcrop, largely faultdefined, in the south-west of the Bampton inlier, a type area is designated at Tailbert Bank, 500 m west and north-west of Tailbert Farm [NY 5335 1446]. The formation is unconformably overlain by the Borrowdale Volcanic Group but also appears to lie unconformably above the Tarn Moor Formation. Hence, its stratigraphical association is ambiguous and its inclusion within the Skiddaw Group is tentative.

Southern Lake District inliers

The Skiddaw Group inliers of Black Combe, Low Furness and Ravenglass are geographically isolated to the south of the main Skiddaw inlier (Figure 1). Their relationships to the Central Fells Belt rocks are unclear, but Black Combe and Furness are biostratigraphically equivalent to the lower parts of the Tarn Moor Formation (and to the highest Kirk Stile Formation of the Northern Fells Belt). All three inliers are for the most part stratigraphically undivided (Figure 3). Additional details for Black Combe and Furness were given by Rose and Dunham (1977) and Johnson et al. (2001). Around Black Combe the Skiddaw Group is overlain by the Borrowdale Volcanic Group, but the latter attenuates southwards and is largely missing in the Furness inliers where the late Ordovician Dent Group unconformably overlies the Skiddaw Group strata. The Ravenglass inlier occurs as a faulted sliver, adjacent to the western margin of the Eskdale granitic pluton and entirely within the footwall of the Lake District Boundary Fault (Akhurst et al., 1997, pp.38–39).

Biostratigraphical correlation

The Skiddaw Group biostratigraphy at Black Combe was reviewed by Rushton and Molyneux (1989). They concluded that graptolite faunas from the north side of the Whicham valley indicated a biostratigraphical level close to, but below, the Arenig–Llanvirn boundary, within the hirundo (now cucullus) Biozone. There were no definitive Llanvirn species and the acritarch assemblages were also compatible with a late Arenig age.

Graptolites in the Low Furness inlier were originally considered to indicate the late Arenig hirundo Biozone (Skevington, cited in Rose and Dunham, 1977). Reassessment of these faunas, and specimens from a new quarry [SD 225 762] near Greenscoe House, suggests the late Arenig (Fennian) to early Llanvirn (Abereiddian) hirundo (i.e. cucullus) to artus Biozone. The evidence for Llanvirn graptolites is based on the species Didymograptus (D.) cf. spinulosus and Didymograptus cf. robustus, and is more in agreement with the age derived from acritarch and chitinozoa assemblages (Downie, in Rose and Dunham, 1977). The available evidence suggests that the Skiddaw Group in the Low Furness inlier is a little younger than that in the Black Combe inlier.

Black Combe

This inlier forms a north-east-trending ridge rising to 600 m above sea level at Black Combe [SD 135 855] and covers an area of about 50 km2. The succession of siltstones and mudstones was divided by Helm (1970) into three stratigraphical units on the basis of colour. The paler colours in these rocks are now recognised to be products of secondary metasomatism and metamorphism. The unaltered rocks correspond to the Whicham Blue Slates of Helm, whereas both his Fellside Mudstones and Townend Olive Slates are altered, the latter being also affected by a pervasive cleavage fabric and intruded by numerous granitic sheets. The only subdivision now recognised is the sandstone of the Knott Hill Formation (Johnson et al., 2001) which crops out in a fault-bounded block, adjacent to the unconformity beneath the overlying Borrowdale Volcanic Group, on the east side of the inlier.

The thickness of the Skiddaw Group in the Black Combe inlier is uncertain because bedding is generally poorly defined and commonly obscured by a pervasive slaty cleavage. Where it is discernible, the bedding reveals that the silty mudstone is folded on a metre to decimetre scale with no consistency in the amount and direction of dip. This structural complexity and the lack of stratigraphical control make thickness estimates speculative. In the Northern Fells Belt, Cooper et al. (1995) estimate that the Skiddaw Group is up to 5000 m thick with late Arenig strata comprising almost half this thickness. A general comparison based on age equivalence would therefore suggest that up to about 2000 m of Skiddaw Group strata might be preserved at Black Combe, including the 300 m of Knott Hill Formation sandstone.

Dark grey, unaltered silty mudstone crops out on the south-east side of the inlier and locally contains sandy laminae which make bedding clearly discernible; the laminae are particularly common between Whicham Mill [SD 152 851] and Knott Hill [SD 174 873]. They generally form less than 10 per cent of the succession, but at the latter locality they increase in abundance upwards in what is thought to be a gradational passage into the Knott Hill Formation. Pyrite-bearing nodules, possibly of diagenetic origin, occur locally throughout the dark silty mudstone succession. The boundary between the dark, unaltered and pale, metasomatised mudstones is gradational over a few tens of metres in a north-east-trending zone across the south-east slopes of Black Combe. It is inclined north-west at about 40°, subparallel to the slaty cleavage, and was initiated when the metasomatised mudstone was superposed over the less-altered rocks during thrusting and cleavage formation in the early Devonian (see Chapter 5). Bedding and any other original depositional features are rarely preserved in the pale metasomatised silty mudstone that, overall, becomes lithologically more uniform north-westwards, as the intensity of the metasomatic alteration and slaty cleavage increases.

Knott Hill Formation

The Knott Hill Formation comprises up to 300 m of fine-grained sandstone that crops out in the east of the inlier on Knott Hill [SD 174 873], between the Swinside fault zone and the unconformity with the overlying Borrowdale Volcanic Group. The sandstone lies mainly within the metasomatic alteration zone and is consequently pale in colour. Its stratigraphical position is uncertain. It appears to be the youngest part of the Skiddaw Group succession in Black Combe, although mudstone at a lower (diagenetic) grade of burial metamorphism is isolated within the Swinside fault zone (Johnson et al., 2001; Chapter 6). The base of the Knott Hill Formation is gradational as sandy laminae in the underlying silty mudstone increase in abundance and thickness upwards over 10 to 15 m; it is formally defined as the point where sandy laminae predominate over silty mudstone. The sandstone near the base is laminated and thinly bedded (Plate 18), but higher up the bedding is less well developed and the sandstone is locally massive. Generally, the formation dips at between 25° and 40° to the west-north-west. Poor evidence suggests that the beds are the correct way up, but locally overturned in intrafolial slump folds. Good examples of soft-sediment deformation on a decimetre scale occur 70 m south of Knott Hill summit.

Furness inliers

The Skiddaw Group is poorly exposed in two contiguous inliers on the Furness peninsula (Figure 1), where it is unconformably overlain by the Dent Group of Ashgill age (Rose and Dunham, 1977; Kneller et al., 1994; Johnson et al., 2001).

There is no indication of the structure or thickness of the Skiddaw Group hereabouts because the inlier is extensively drift covered, with few natural exposures. Most information has been gathered from quarry sections, temporary exposures and boreholes.

The rocks are dominantly dark grey mudstones with sporadic siltstone laminae; bedding is commonly difficult to see. Rose and Dunham (1977) studied the Furness Brick Company’s Askam Shale Quarries at Park Farm [SD 218 755] which were subsequently abandoned and landscaped. The quarries exposed uniform, relatively soft mudstone containing some clastic mica but little or no quartz, and no sandy interbeds. When freshly exposed the mudstone was almost coal-black, but pyritic inclusions weathered to deep ferruginous stains. A faint lamination was apparent in places, but strong cleavage, intense crushing and minor contortions made it difficult to see the dip, which appeared to be steep to the east.

When the Askam quarries were abandoned, the company opened a new quarry south-east of Greenscoe House [SD 225 764]. Here the rock is lithologically uniform dark grey and black silty mudstone with ferruginous staining on joint surfaces. Silty and micaceous laminae are locally present and reveal minor folds. Recent excavations south-west of Greenscoe Craggs [SD 221 761] for the A595 road exposed Skiddaw Group mudstones overlain by upper Ordovician Dent Group strata, with no intervening volcanic rocks. At the steeply dipping contact, bedding in the Dent Group appeared to be parallel to the contact, but bedding in the Skiddaw Group mudstone was indistinct.

Ravenglass inlier

Strictly this comprises two small, adjacent inliers east of the village of Ravenglass [SD 093 965] in an area of poor exposure spanning the Lake District Boundary Fault. The Skiddaw Group strata are contained within the fault zone as tectonised slivers, thermally metamorphosed by the adjacent Eskdale granitic pluton. Hornfelsed black mudstone contains metamorphic biotite and chlorite with sporadic tourmaline and chiastolite; some indurated and recrystallised sandstone is also present. It is not possible to correlate the strata with any particular part of the Skiddaw Group.

Cross Fell Inlier

Details of the Skiddaw Group in the Cross Fell inlier can be found in Arthurton and Wadge (1981), Burgess and Holliday (1979), and Burgess and Wadge (1974). Stratigraphical nomenclature was updated by Cooper and Molyneux (1990), who also showed that the Causey Pike Fault could be traced eastwards to Cross Fell where it separates distinct sequences comprising the Catterpallot Formation in the north and the Kirkland and Murton formations in the south.

Catterpallot Formation

The Catterpallot Formation was proposed by Cooper and Molyneux (1990) for the strata to the north of the Causey Pike Fault. Arthurton and Wadge (1981) included these strata in the Murton Formation, described below. The Catterpallot Formation crops out in the north of the Cross Fell inlier and includes at least 1000 m of turbiditic siltstones and feldspathic or lithic wackes with subordinate intraformational conglomerate beds. The wackes form 20 to 40 per cent of the succession and generally range from 0.05 to 1.5 m in thickness, exceptionally reaching 5 m. Grading, parallel and cross-lamination define Tabc turbidite units. Many beds have sharp, erosional bases with sporadic flute casts indicating palaeocurrent flow from the south. Angular to subangular grains of quartz and feldspar dominate the wacke clast population, accompanied by a wide range of sedimentary and igneous lithologies. The conglomeratic interbeds consist of rounded sandstone and siltstone clasts in a grey mudstone matrix. They are interpreted as slump breccias (Arthurton and Wadge, 1981) and are well developed in Melmerby Beck [NY 6303 3693] where a 4 m-thick example is exposed. No macrofossils have been recovered from the Catterpallot Formation, but the microfossils indicate a probable latest Tremadoc age (Cooper and Molyneux, 1990; see Chapter 2) suggesting biostratigraphical equivalence with the Watch Hill and Bitter Beck formations in the Skiddaw inlier of the Lake District.

Murton Formation

The Murton Formation in its confirmed outcrop to the south of the Causey Pike Fault was fully described by Burgess and Holliday (1979). It consists dominantly of pale to dark grey, laminated siltstones with subordinate pale grey sandstone beds, 0.5 to 5 cm thick. The contacts between the Murton and Kirkland formations are faulted; several hundred metres of strata must be present, but the true thickness is not known. Many of the exposures show complex polyphase folding with abundant sheared folds and shear planes suggesting a slump origin. Since the beds have also been tectonically folded with development of slaty and crenulation cleavages, a systematic lithostratigraphy has so far been impossible to determine. Graptolites are rare in the Murton Formation and a single trilobite occurrence has been reported (Fortey et al., 1989). Acritarchs are more widespread and were first recorded by Lister et al. (1969). A reappraisal of the fossil evidence by Cooper and Molyneux (1990) suggested a late Arenig age, probably within the cucullus Biozone. This age, and the overall lithofacies, suggests a general association with the Buttermere Formation olistostrome of the Central Fells Belt in the Skiddaw inlier.

Kirkland Formation

The Kirkland Formation (Burgess and Wadge, 1974) comprises grey silty mudstones, with subordinate brown-weathering, pyritic mudstones and dark, blue-grey, calcareous mudstones. These were recorded by Burgess and Holliday (1979) as being interbedded with many thick tuff beds and rare, thin and altered basalt or andesite sheets, probably lavas. The volcanic rocks have since been reinterpreted: the tuffs are now recognised as being largely water-laid as volcaniclastic turbidite deposits (Cooper and Molyneux, 1990) and the igneous rocks are now interpreted as sills intruded into wet, partly lithified sediment (Hughes and Kokelaar, 1993). The volcaniclastic beds are commonly fine or medium grained but may become coarse locally. The component detritus is broadly andesitic with spilitic grains also common; red jasper and black mudstone clasts are prominent locally. Many beds show some grading and cross-bedding and are interpreted as Tac turbidites. They range in thickness from a few centimetres to apparently over 20 m (Burgess and Holliday, 1979, pp.6–7).

The total thickness of the formation cannot be estimated accurately as no boundaries are exposed, but at least 1000 m of strata may be present. Burgess and Wadge (1974) considered that a two-fold lithological division was possible; a lower sequence of volcaniclastic turbidites interbedded with mudstone and siltstone, and an upper sequence dominated by black graptolitic mudstone. From both of these putative divisions the Kirkland Formation has yielded abundant graptolites representing the artus Biozone; acritarch evidence is compatible with this early Llanvirn age. Lithologically and biostratigraphically, the Kirkland Formation is similar to the Tarn Moor Formation in the Central Fells Belt of the Skiddaw inlier.

Teesdale Inlier and boreholes in Northern England

The exposures of the Skiddaw Group in the Teesdale inlier (Figure 1) are very small and restricted to a few riverside examples. The largest outcrop at Cronkley Pencil Mill [NY 8485 2955] (formerly worked for slate to make slate pencils) is dark grey mudstone that breaks along the cleavage to produce rod-like fragments. The exposure here has yielded graptolites and acritarchs of probable early Llanvirn age (Johnson, 1961; Lister and Holliday, 1970) suggesting a correlation with the Kirkland Formation of the Cross Fell inlier. Near to the Teesdale inlier, the Wrentnall Shaft [NY 8105 3054] at Cow Green Mine proved grey and purple altered slates below the basal unconformity of the Carboniferous sequence at a depth of 104 m.

The Skiddaw Group has also been proved in a few deep boreholes on the Alston Block to the east of the Cross Fell and Teesdale inliers (Figure 1). Altered greenish grey cleaved mudstones were proved at a depth of 470 m in the bottom of the Allenheads No 1 Borehole [NY 8604 4539] at Allenheads Village (Burgess, 1971). East of here, and beyond the subcrop of the Weardale granite (Dunham et al., 1965), the Roddymoor Borehole [NZ 1512 3634] proved the Skiddaw Group (Woolacot, 1923) at a depth of 861.8 m. Pale and dark grey argillaceous rocks with highly contorted bedding and thick quartz veins were recovered for another 25 m from that depth to the bottom of the hole. Two strong crenulation cleavages were reported. This description accords with the appearance of a sample of core held by the Hancock Museum in Newcastle upon Tyne. Woolacot also noted that the pale grey shale contained thin garnetiferous bands, becoming locally schistose. However, no confirmation of this description has so far been possible and the detailed logs of the borehole do not mention garnetiferous rock, only argillaceous grey shale on one log and ‘greenish slate or phyllite, top 12 ft greenish striped, remainder black graphitic’ on another.

Ingleton Group

South-east of the Lake District, strata commonly correlated with the Skiddaw Group crop out at the head of Ribblesdale in the Craven inliers of Chapel le Dale and Ribblesdale. These form the Ingleton Group, an interbedded succession of grey-green turbiditic sandstone and siltstone (Tabc) with sporadic intraformational conglomerate (Arthurton et al., 1988, pp.4–5). There is much evidence locally for soft-sediment deformation and slumping (Leedal and Walker, 1950). Poor exposure and isoclinal folding make any estimate of total thickness difficult, but at least 600 m of strata are present. The sandstones are feldspathic and lithic wackes, generally fine to medium grained but in places very coarse grained. The range of lithic clasts present includes intrabasinal material and fine-grained igneous rocks. No fossil evidence of age has been found and the only age control is a Rb–Sr isochron of 505 ± 7 Ma obtained by O’Nions et al. (1973) from ‘slates’ and interpreted by them as the age of dewatering and folding. Biostratigraphical evidence was obtained from the nearby Beckermonds Scar borehole [SD 8635 8016] which penetrated probable Ingleton Group beneath 260 m of Carboniferous cover (Wilson and Cornwell, 1982). A sample from a depth of 396.2 m contained a probable Arenig acritarch microflora. The early Ordovician strata in the borehole were also notable for their high content of detrital magnetite (Berridge, 1982).

The overall lithofacies and age of the Ingleton Group appears broadly comparable to the Skiddaw Group. However, one significant difference is the more juvenile character of the Ingleton Group clastic detritus (Stone and Evans, 1997) as deduced from neodymium isotope ratios (discussed more fully in Chapter 4). These data strongly suggest that the compositional characteristics of the Ingleton Group (of probable Arenig age) could only be matched in Llanvirn components of the Skiddaw Group, such as parts of the Tarn Moor Formation.

Chapter 4 Depositional environment

Sedimentology

Jackson (1961) was the first author to interpret the rocks of the Skiddaw Group as a sequence of turbidites, but little detail was added to that general interpretation until the late 1980s. Webb and Cooper (1988) recognised the Buttermere Formation (Chapter 3) as an olistostrome deposit, consisting of sand- and mud-grade turbidite rocks redeposited by a submarine slumping event. Moore (1992) undertook the first detailed modern sedimentological analysis of the Skiddaw Group, and the data and interpretations presented below are largely a summary of his work. Moore worked mostly within the Skiddaw inlier where exposure is best, and focused on the sandstone-dominated parts of the sequence, namely the Watch Hill and Loweswater formations. The presence of a penetrative cleavage within mudstones and siltstones makes it difficult, if not impossible, to interpret the sedimentary structures in the parts of the succession where those lithologies are dominant.

As in Chapter 3, grain size is classified according to the Udden–Wentworth scheme (see, for example, Leeder, 1982), bed and lamina thickness are classified according to the Ingram and Campbell schemes respectively (as used by Tucker, 1982), and the sandstone classification follows Leeder (1982).

Facies and facies associations

Moore recognised and described twenty-one facies classes belonging to six main facies groups (A to F) using a slightly modified version of the Pickering et al. (1986) turbidite classification scheme. The typical facies and depositional processes of each facies class are summarised in (Table 3), and their stratigraphical distribution is summarised in (Table 4): the reader is referred to Pickering et al. (1986) and Moore (1992) for further details. Substantial parts of the sequence were not classified because of the difficulties presented by penetrative cleavage in finer grained lithologies, and the Tarn Moor Formation was not included in Moore’s work. Using additional information from palaeocurrents and slump-fold orientations, Moore recognised four principal facies associations from these twenty-one facies classes.

Proximal channel/levee facies association

This association was recognised within the Watch Hill Formation (Figure 19). The facies classes A2.5, A2.7, B1.1, B1.2, B2.2 and B2.3 (Table 3) occur in fining-upward units ((Figure 19)a, Elva Quarry 0.0 to 4.5 m), and are interpreted as coarse-grained sandstones deposited within channels. There is no indication of the lateral extent of these channels, but the regular thickness (about 4 m) of fining-upward cycles is believed to correspond to channel depth. Increasing ((Figure 19)b and decreasing (Figure 19)a proportions of coarser and thicker class B beds are believed to indicate variations in channel proximity.

Facies classes C2.7 with subordinate C2.6, C2.5 and facies group B represent levees to the proximal channels. Facies groups C and D are interpreted as interchannel deposits dominated by mudstone deposited from suspension (Figure 19)a.

Palaeocurrents. In the Watch Hill area, directional flute casts and prod marks show a wide spread of orientations between north and south-west, with a mean towards 285° (Figure 20). Ripple current directions show a mean towards 023°, and their divergence from sole structure orientations is believed to indicate the spreading of overbank flows.

Generally westward palaeoflow in the Watch Hill area, coupled with a general westward thinning of the formation (from about 520 m at Watch Hill to about 200 m at Cockermouth) is interpreted as distal (downcurrent) thinning. One implication of this interpretation is that the contact between the Bitter Beck and Watch Hill formations is, at least in part, a lateral facies change.

Directional sole structures from the Watch Hill Formation at Great Cockup (Figure 20) indicate palaeo-flow towards the north-east (mean towards 071°), a difference of some 210° between here and Watch Hill. Moore (1992) offered two explanations for this difference. He suggested that the palaeoflow directions at Great Cockup might indicate flow along an east–west-orientated linear trough, with an important source of sediment entering the basin between here and Watch Hill. Alternatively, he suggested that the variation might indicate radial flow upon a submarine fan.

Distributary channel facies association

This association is recognised in the Loweswater Formation at Darling Fell [NY 1257 2224] and [NY 1173 2151], and is transitional from the depositional lobe facies association (see below). Thin- to thick-bedded turbidites (facies classes C2.5, C2.3, C2.2 and C2.1), amalgamated thin sandstone beds (C2.6), and medium to thick sandstone beds (facies group B) make up the depositional lobe ((Figure 21), 0.0 to 7.9 m). This is overlain by the distributary channel facies association, which consists of thick sandstone beds (B1.1, B2.1, C2.1) with lobe geometries, incised by channel-fill sandstones ((Figure 21), 7.9 m upwards).

Channels are 0.5 to 2.0 m deep with steep margins. Channel-fill sandstones (B2.1, B2.2, B1.1) are medium to coarse grained, and may be structureless, horizontally laminated, or cross-laminated ((Figure 21), 9.9 m upwards). Channel dimensions are difficult to determine, but channel facies were developed over a minimum distance of 1 km perpendicular to flow direction.

Palaeocurrents. Palaeocurrent directions for the distributary channel facies association at Darling Fell are shown in (Figure 20). Directional sole structures indicate that erosive currents flowed towards the north-west (mean towards 322°). Ripple directions show a bimodal distribution (means towards 027° and 250°), and are interpreted as representing overbank flows or spreading of flows at channel terminations.

Depositional lobe facies association

This association, also recognised in the Loweswater Formation, consists mainly of thin to medium beds of fine-grained sandstone (facies group C2, with some C1; (Figure 22), 0.5 to 15.5 m, 19.2 to 26.5 m, 28.8 to 39.2 m), separated by intervals of facies group D2 representing the interlobe facies association (see below).

Palaeocurrents. Sole mark and ripple current directions for this facies association across the Skiddaw inlier are shown in (Figure 20) and (Figure 23)a. Generally the data indicate a palaeoflow towards the north, but in detail the pattern is more complex. Most notably, at Scawgill Bridge, Low Fell, Hope Gill and at Darling Fell, there is a marked divergence between palaeocurrent directions derived from sole structures (which indicate palaeoflow towards north-west or north-north-west) and those from ripples (which indicate palaeoflow towards north-north-east and west-south-west).

Moore presented a number of theoretical explanations of the observed palaeocurrent divergence. He concluded that the pattern could be explained by topographical elements (related to intrabasinal faults) controlling turbidity current flow (Figure 23)b. In this model, the turbidity currents which produced sole structures with north-westerly palaeoflow are believed to have been constrained within linear troughs that controlled channel orientation. The north-north-east and west-south-west-directed palaeoflows derived from ripples are interpreted as reflected waves generated at the trough margins that were capable of crossing the trough axes. This topic is discussed in further detail below (basin configuration).

Interlobe facies association

This facies association, again recognised also within the Loweswater Formation, is dominated by facies group D2 (Figure 24). Siltstone turbidites predominate, and a lower fan or interchannel setting is indicated. A transitional lobe-fringe facies between the interlobe facies association and the depositional lobe facies association is exposed in Scawgill Bridge Quarry (Figure 24), 28.4 to 42.7 m).

Sea level changes and sedimentation

Various authors (for example Pickering et al. 1989) have discussed the relationships between sea-level changes (either eustatic or local) and the supply of siliciclastic material to sedimentary basins, concluding that supply increases during low stands of sea level and decreases during high stands. Nicoll et al. (1992) presented detailed sea-level curves for the late Cambrian to Arenig intracratonic carbonate-clastic shelf sequences of Scandinavia and Australia, but it has not been possible to prepare similar high resolution curves for the siliciclastic sequences of the Skiddaw Group. Woodcock et al. (1999) interpreted gross sedimentation patterns in the Manx Group (Isle of Man) in terms of relative sea-level fluctuations, and attempted an event stratigraphic correlation with the Skiddaw Group.

A speculative sea-level curve for the Skiddaw Group is presented in (Figure 25), where it is compared with the global sea-level curve of Fortey and Cocks (1988).The mudstone-siltstone-dominated Bitter Beck Formation is believed to represent a Tremadoc high stand of sea level, prior to the short-lived but extensive regression across the base of the Arenig Series. Thereafter, the sandstone turbidite sequence of the Watch Hill Formation coincides with a lowering of sea level during the latest Tremadoc times, possibly coinciding with that regression. The transgression that preceded the high stand during deposition of the Hope Beck Formation is very similar to the global trend towards higher sea levels during the early Arenig. The low stand phase correlated with the Loweswater Formation and the subsequent high stand of the Kirkstile Formation also correlate closely with global trends. The global late Arenig low stand may have been a factor leading to instability and the emplacement of the Buttermere Formation olistostrome. However, the large size, frequency and wide distribution of the mass wasting, slump events seem likely to require additional, tectonic factors. Woodcock et al. (1999) argued that such tectonic events might be related to the rifting of Avalonia from Gondwana, but the earliest stages of subduction-related magma genesis may also have contributed to the instability (see Chapter 5).

Summary of depositional history

The Skiddaw Group comprises a sequence of submarine turbidite sediments. Mudstone turbidites dominate the sequence, but the Watch Hill and Loweswater formations record major events of siliciclastic input to the basin.

The late Tremadoc Bitter Beck Formation is interpreted as a turbidite fan with sandstones present either as channel levees or as small depositional lobes (Moore 1992). The restricted amount of coarse clastic material is believed to indicate a high sea level stand, or sediment starvation due to intrabasinal topography.

The proximal channel/levee facies association that dominates the latest Tremadoc to early Arenig Watch Hill Formation indicates a major input of coarse clastic detritus, probably during a relatively low sea level stand. Deposition was either in an east–west trough or on a meandering fan with generally northward palaeoflow. The transitional contact with the Bitter Beck Formation may be simply a lateral facies change. It is not known whether sedimentation was restricted to channels and levees or whether the channels were pathways to more extensive depositional lobes. Depositional lobes related to the channel facies have not been recognised, but the presence of very fine- and fine-grained sandstone in overbank deposits suggests that sand was transported through the channel systems to be deposited elsewhere.

The early to mid-Arenig Hope Beck Formation is a mudstone-dominated turbidite fan. The interlobe facies association is recognised, with some lobe-fringe sandstones and rare pebbly mudstones are also present. The formation is believed to have been deposited during a relatively high sea level stand, and its transition with the overlying Loweswater Formation represents a relative lowering of sea level.

The mid-Arenig Loweswater Formation represents the acme of turbidite fan activity within the Skiddaw Group, with interlobe, depositional lobe and distributary channel facies associations recognised. Palaeocurrent interpretations, thickness variations and the widespread occurrence of convolute laminations (believed to have had a seismic trigger) suggest deposition in a tectonically active, extensional depositional basin with north-north-west-trending normal faults separating north-easterly tilted fault blocks.

The late Arenig to early Llanvirn Kirk Stile Formation is a mudstone-dominated turbidite fan containing rare sand bodies that represent the final phase of lobe development. Widespread slump folds and debrites are the products of soft-sediment deformation during late Arenig or early Llanvirn times, and are roughly contemporaneous with emplacement of the Buttermere Formation olistostrome. Tectonic activity or a lowering of sea level are possible causes of the widespread soft-sediment deformation.

Basin configuration

Webb and Cooper (1988) presented an interpretation of basin configuration based on an analysis of slump fold facing directions. North of the Causey Pike Fault such folds face mainly towards the south-east, and are therefore believed to have been emplaced towards the south-east. The origin of some of the larger slump folds in this area is controversial (Hughes et al., 1993, see Chapter 5) but there remains evidence of a likely palaeoslope inclined towards the south-east, at least during the Tremadoc and early Arenig.

Conversely, south of the Causey Pike Fault, slump folds within the Buttermere Formation olistostrome indicate emplacement towards the north-west (Webb and Cooper, 1988). These authors believed that the contrasting scale and facing directions of the slump folds on either side of the Causey Pike Fault suggested a narrow linear basin with a steep, faulted southern margin and axis that corresponded approximately with the fault.

There is little in common between the Webb and Cooper (1988) model of basin configuration and that of Moore (1992). Moore carried out a detailed analysis of slump fold geometry throughout the main inlier, and concluded that a westerly facing palaeoslope was present throughout the main inlier, when the main slumping event and olistostrome emplacement occurred. North of the Causey Pike Fault, slumping towards the south-east was recognised only rarely, for example at Gasgale Gill, and was there thought to have been influenced by local structural controls.

Moore’s interpretation of the depositional basin in the mid-Arenig is based largely on detailed palaeocurrent analysis (discussed above), supplemented by information on lateral thickness variations within the Loweswater Formation and the interpretation of possible synsedimentary faults. Moore proposed an extensional basin setting for the Loweswater Formation submarine fan, in which north-north-west-trending normal faults separated fault blocks tilted towards the north-east (Figure 23). Furthermore, he believed the widespread occurrence of convolute laminations within the Loweswater Formation to be indicative of contemporaneous tectonic activity.

Geotectonic getting

Irrespective of the basin geometry, support for an extensional environment is also provided by the illite crystallinity and clay mineral assemblages of the Skiddaw Group mudstones. These have been considered by Fortey et al. (1993) and by Merriman and Frey (1999) and are discussed in Chapter 6. The early burial metamorphism, characterised by late diagenetic to low anchizonal grades, seems likely to have occurred under a relatively high geothermal gradient (>35°C/km). This probably related to high heat flow during a phase of crustal thinning with associated lithospheric extension. Similar results have been reported from the Manx Group (Roberts et al., 1990). However, there remains some uncertainty over the relationship between burial metamorphism in the depositional basin and thermal metamorphism during subsequent volcanism. The former occurred during extension in the Tremadoc to Llanvirn interval; the latter occurred during Llanvirn to Caradoc, supra-subduction zone basin uplift and volcanicity, with eruption of the Borrowdale and Eycott Volcanic groups, when a high geothermal gradient would also be expected.

Plant et al. (1991) noted a number of similarities between some aspects of the regional geochemical characteristics of the Skiddaw Group and those of the upper part of the late Precambrian, Dalradian Supergroup in the Scottish Highlands. Both show enhanced levels of the gold pathfinder elements (As, Sb and Bi) and were believed to have acted as crustal reservoirs for later ore-forming processes. The geotectonic setting of the Dalradian is well established as an ensialic, tectonically controlled, extensional basin (Anderton, 1982) and a similar environment is applicable to the Skiddaw Group. The comparison holds only for the extensional basin phase and subsequent differences in terrane evolution must be stressed. The Dalradian basin continued to extend until oceanic-style volcanism occurred prior to orogenesis; the Skiddaw Group basin was uplifted above a developing subduction zone to become the foundations of a continental margin volcanic arc.

Provenance

Petrographical assessment of sedimentary provenance

A range of specific lithologies can be identified petrographically from Skiddaw Group sandstone grain assemblages. In all samples examined, the commonest grain type is strained monocrystalline quartz, of plutonic origin but showing signs of tectonism. Lithic fragments of Bitter Beck granite, the dominance of untwinned and potassium feldspar over plagioclase, and the common occurrence of tourmaline, provide further evidence for the erosion of unroofed acid plutons in the source region. Pyroclastic and volcaniclastic fragments present throughout the group suggest some erosion of arc-related deposits. Fragments of cleaved granite, slate and quartz-mica schist suggest recycling from an orogenic source. The detrital components are summarised in (Figure 26) in terms of the relative proportions of quartz, feldspar and lithic components. This allows comparison with the average proportional composition of sands and sandstones from known modern geotectonic environments. There is clear compositional variation between individual formations with data spread across the quartzose ends of the recycled orogen and continental block fields. However, many of the wacke sandstones contain sufficient arc-related lithic material to pull their compositions into the mixed provenance field. Moore (1992) gives a more exhaustive treatment of petrographic provenance analysis.

Bitter Beck Formation

Sandstones are sparse in this formation but appear relatively uniform in composition. They are feldspathic wackes, generally with a dominance of untwinned plagioclase over other feldspar types. Quartz is the commonest detrital material, almost invariably monocrystalline and strained. Rare phyllitic grains comprise the lithic component. Tourmaline, spinel and zircon are present as detrital mineral grains. A cratonic interior provenance is suggested (Figure 26)a.

Watch Hill Formation

The sandstones of this formation are variable in grain size with a tendency for the coarser grained lithologies to have a higher proportion of lithic detritus. This is illustrated by the comparison between medium-grained lithologies from Watch Hill and coarse-grained lithologies from Great Cockup (Figure 26)b. Despite these differences, most of the examples can be classified as lithic wackes with a few straying into the feldspathic field.

In the Watch Hill specimens, an early quartz cement is present which has inhibited matrix formation and alteration of labile components ((Plate 19)a). The quartz grains are mostly strained and of plutonic origin but some unstrained, volcanic quartz is also present. Polycrystalline quartz grains are largely of metamorphic origin ((Plate 19)b). Alkali feldspar, as microcline and perthite, is dominant over plagioclase ((Plate 19)c) which is mainly andesine. Lithic clasts include sedimentary, igneous and metamorphic types. Well-rounded clasts of mudstone and siltstone are unlikely to have an intrabasinal origin but contorted intraformational mudstone clasts are also present. The volcanic component is very variable and includes pyroclastic fragments with relict shards, and a volcaniclastic assemblage of angular feldspar fragments, spilitic basalt ((Plate 19)d), rhyolite, microgranite with graphic texture ((Plate 19)e), and altered glass with feldspar phenocrysts. Significantly, the range of volcanic grains is also seen to be strained and cleaved with alteration due to metamorphism. Unequivocal metamorphic lithologies present include phyllite and muscovite schist. Detrital minerals are mainly zircon ((Plate 19)f), tourmaline and spinel. The provenance assessment from (Figure 26) is of a relatively quartzose recycled orogen.

The coarse-grained lithic wackes from Great Cockup have very little early quartz cement and are more matrix-rich. Polycrystalline quartz is also more abundant than in the Watch Hill samples with monocrystalline quartz and feldspar correspondingly less abundant. The lithic types have a similar range in both areas but are more abundant at Great Cockup. From (Figure 26) the provenance would appear to have shifted towards the lithic end of a recycled orogen relative to the Watch Hill material. These differences may be partly the result of the grain-size variation and the lithic wackes from both areas are believed to be derived from the same, geologically mixed hinterland.

Hope Beck Formation

The sandstones of this formation are quartz-rich lithic wackes. Monocrystalline quartz is generally strained and of plutonic origin, the polycrystalline quartz is of metamorphic origin and feldspar is rare. Lithic grains include intraformational mudstone and siltstone, phyllite, quartz-muscovite schist, felsite and contorted chloritic rock. Additional accessory components from pebbly mudstone interbeds include feldspar-phyric basalt, rhyolite and volcaniclastic sandstone. The inferred provenance ((Figure 26)c) spans the quartzose recycled orogen and the craton interior categories.

Loweswater Formation

The wackes forming this unit are relatively quartzose and feldspathic. Many of the quartz grains show marginal pressure solution with new quartz precipitated in strain shadows and veins ((Plate 20)a); locally this obscures the original texture. The detrital quartz grains are mostly monocrystalline and strained, although there is a tendency for the largest grains to be unstrained, with a little accompanying polycrystalline metamorphic quartzite, chert and felsite. Some grains showing the intergrowth of monocrystalline quartz and chlorite are interpreted as of vein origin ((Plate 20)b). Feldspar is present as andesine plagioclase, microcline and perthite, but much is untwinned and many grains show signs of alteration or replacement. The lithic components are largely sedimentary and many are probably intrabasinal; however, well-rounded sandstone and mudstone grains, and metapelite grains carrying a pre-existing crenulation fabric ((Plate 20)c), most probably have an extrabasinal origin. Accessory lithic components include various volcaniclastic and pyroclastic lithologies, quartz-feldspar porphyry, basalt, graphitic granite, hornfels, quartz-mica schist ((Plate 20)d) and mylonite ((Plate 20)e). Detrital muscovite is largely replaced by chlorite ((Plate 20)f), but some rare biotite is also present. Provenance indications (Figure 26)d are of derivation from a quartzose recycled orogen with some dissected arc and continental block material pulling average compositions towards a mixed field.

A variation from the average Loweswater Formation composition is seen in the Jonah’s Gill area [NY 191 342] where the wackes are relatively more mature. They have a very high quartz content and a more restricted range of lithic accessories, which are predominantly sedimentary intraformational clasts. For these examples a craton interior provenance is indicated by (Figure 26)d.

Kirk Stile Formation

Sandstones within this formation are relatively uncommon and those examined were variable in terms of grain size, carbonate alteration and deformation. They are lithic or feldspathic wackes and most are fairly quartzose. Monocrystalline quartz is usually strained although there is a tendency for the more rounded grains to be clear and unstrained; polycrystalline quartz can be referred to both metamorphic quartzite and sedimentary chert. Feldspar is a widespread accessory, although it is rare locally and is very variable in character. The principal lithic clasts are either phyllitic or comprise very dark mudstone, thought by Moore (1992) to be a volcaniclastic sediment. The mudstone clasts contain rounded silt-size grains of quartz and feldspar interpreted by Moore as arising from reworking rather than direct volcanic input to the source strata. The overall provenance assessment (Figure 26)e shows a range from recycled orogen sources to a craton interior with only a slight suggestion of volcanic input.

The debris-flow beds within the formation, notably at Beckgrains Bridge [NY 190 355], contain rounded sandstone clasts of quartzose feldspathic wacke. These show a similar compositional range to that seen in the Loweswater Formation, including the quartzose wackes of Jonah’s Gill (Figure 26)f.

Buttermere Formation

Very large sandstone rafts within this olistostrome unit make up the Robinson Member. The component sandstones are quartz-rich lithic wackes; some have relatively little feldspar but one sample examined was feldspar-rich. Bedding laminae of detrital heavy minerals, particularly zircon, were observed in another sample. The lithic clast range includes granite and hornfels but is dominated by sedimentary intrabasinal material. There is an overall similarity with the wackes of the Hope Beck Formation and the Jonah’s Gill variety of the Loweswater Formation. However, the Robinson Member is derived probably from the south and there need be no direct link with those lithologies of the Northern Fells Belt. Provenance indications (Figure 26)f support a source within a quartzose recycled orogen or cratonic interior.

Tarn Moor Formation

A significant detrital volcaniclastic component has been described from both mudstone and sandstone of this formation (Cooper and Molyneux, 1990; Hughes, 1995a) and its correlative in the Cross Fell inlier, the Kirkland Formation (Arthurton and Wadge, 1981). Altered ferromagnesian minerals are widely present, forming discrete laminae in some places. Small, elongate secondary muscovite grains occur and probably represent original shards that imply a direct volcanic contribution to the sequence. This is in keeping with the occurrence of sporadic bentonite interbeds, rather than (or at least in addition to) sedimentary reworking of a volcanic provenance. The volcaniclastic sandstones are crystal-lithic wackes, albeit relatively quartz rich. Many sandstones in the sequence are devoid of volcanic material and are quartzose lithic wackes in which the lithic fraction is exclusively sedimentary and probably of intrabasinal origin. A mixed provenance is indicated with an active volcanic zone developing on what had previously been either a quartzose recycled orogen or craton interior.

Whole rock geochemical assessment of sedimentary provenance

Whole rock geochemical data has been utilised in sedimentary provenance studies in similar ways to the quantitative petrographic information. Element ratios and abundance, and various discriminant functions derived from either ratios or absolute abundances, have been compared with the range of values shown by material from established sedimentary environments. In practice, there are great differences between the mobilities of different elements under different conditions. This introduces inherent uncertainties into any provenance analysis but, in an exhaustive assessment of the various techniques, Moore (1992) consistently established a provenance for the Skiddaw Group within an ancient volcanic arc founded on continental crust. Moore also noted a consistent hint of a passive margin provenance environment and found that evidence for coeval volcanism was restricted to the younger parts of the group. These trends can be summarised by the comparison of Skiddaw Group analytical data (summarised in Appendix 3) with average multielement patterns for different tectonic settings (Floyd et al. 1991). This confirms (Figure 27) that both continental arc/active margin and passive margin provenance characteristics are present in Skiddaw Group rocks.

In (Figure 27) (a, b) Skiddaw Group analytical data from four of the Northern Fells formations, normalised to average upper continental crust, are compared with average values from ‘standard’ provenances. Elements are arranged from left to right in order of decreasing ocean residence times (Floyd et al., 1991) and form two relative mobility groups, a mobile group to the left and a stable group to the right. Important features to note are the positive V–Cr–Ni–Ti–Sc anomalies associated with mafic input so that only the passive margin environment shows depletion, and the positive Hf–Zr anomaly restricted to the passive margin setting. In all of the formations of the Skiddaw Group the greatest variation occurs within the more mobile elements. There is a general similarity between all of the profiles with the Hope Beck and Watch Hill formations being particularly close. There is no precise fit with the type examples but the positive V–Cr–Ni anomalies, particularly for the Loweswater and Kirk Stile formations, coupled with negative (albeit slightly elevated) Zr, is indicative of some arc component in the source.

Moore (1992) also investigated the rare earth element (REE) geochemistry of the Skiddaw Group. He noted a general uniformity across the five formations of the Northern Fells and a broad similarity with average post-Archaean shale. The principal variation came from Loweswater Formation mudstones associated with high-matrix wackes. In these rocks, unusually little enrichment in the light REEs (relative to chondrite normalisation) was taken as an indication of a larger volcanic component than was present elsewhere. The general similarity with average post-Archaean shale probably indicates that the provenance area had an overall composition equivalent to average continental crust and was not dominated by a single rock type. The lack of variation between the formations of the Northern Fells suggests that they shared a common provenance, which must therefore have been consistently available throughout the Tremadoc to Llanvirn interval.

Comparison with the characteristic patterns for source types is aided if the formation averages are normalised with respect to each source type and then plotted as multi-element spectra, when a near-horizontal line at 1 would confirm a close similarity. This is done in (Figure 27) (c, d) with normalisation to continental arc/active margin and passive margin, respectively. For the more mobile elements the Skiddaw Group shows closest similarity with the passive margin setting but the more stable, and probably more reliable, elements correspond more closely to the continental arc source. There is no definitive match and a combination of influence by both these end-members seems likely. An important trend that is confirmed is the apparent increase in the basic component in the younger stratigraphical units, the Loweswater and particularly the Kirk Stile formations. Finally, it must be stressed that the geochemically defined, standard provenance parameters are highly generalised and are compared with a relatively small Skiddaw Group dataset. The conclusions are indicative, supporting other evidence, rather than independently definitive.

Neodymium isotope chemistry

Neodymium isotopes are particularly useful in the assessment of provenance because of the generally stable behaviour of the REEs during sediment transport, diagenesis and low-grade metamorphism. The effects of any possible mobility and redistribution during diagenetic processes are mitigated by the use of large rock samples. The Nd composition is usually presented in the form of epsilon units (εNd) calculated from the isotopic ratios at the time of deposition of the sediment. This method enables direct comparison of the isotope compositions of the detritus and thus allows assessment of the provenance character at the time of its erosion and reworking. The Nd values are relatively robust and evolved at approximately -1 unit per 100 million years; they thus provide a good indication of the average relative maturity of sediment at the time of its deposition. Several recent studies of the British paratectonic Caledonides provide further background and details of methodology (Thorogood, 1990; McCaffrey, 1994; Stone and Evans, 1995, 1997).

The εNd values obtained from each of the Skiddaw Group formations are plotted in (Figure 28) with separate results cited for representative wackes and mudstones. The maximum values recorded are from the Watch Hill Formation and indicate that it received the highest level of juvenile volcanogenic detritus. This interpretation receives some support from the petrography but taken together, the εNd wacke data from the Northern Fells Belt contradicts the whole rock geochemical trends in showing an increase in maturity, and a corresponding decrease in any juvenile component, into the younger formations. The wacke trend can most readily be interpreted as an initial increase in juvenile input, from the Bitter Beck to the Watch Hill formations, followed by a decline in juvenile input and increasing maturity through into the Kirk Stile Formation. This could be viewed in terms of the progressive unroofing of a volcanic arc to expose the underlying continental basement. Values of Nd from the Kirk Stile Formation, around–8, are compatible with derivation from a Proterozoic source. It is interesting to note that a similar Nd value was obtained from a quartzose lithic wacke of the Tarn Moor Formation suggesting the interaction of two opposing provenance trends:

The crystal-lithic wackes of the Tarn Moor Formation would be expected to have far higher εNd values reflecting their high juvenile content. The Ingleton Group is a useful comparison in this respect with values up to about–1 indicating a very high juvenile content and discounting any possibility of a direct correlation between its source and that for most of the Skiddaw Group.

Two other aspects of the Nd data summarised in (Figure 28) both emphasise the importance of the Causey Pike Fault, and support the proposition that it might have played an early role in partitioning the basin. Firstly, wacke samples from south of the fault all have relatively low Nd values, similar to those of the Kirk Stile Formation. This includes the Robinson Member of the Buttermere Formation, which has a likely early to middle Arenig age for its original deposition; the strata of similar age in the Northern Fells Belt contain significantly more juvenile material. Conversely, the Knott Hill Formation of the Black Combe inlier also has a relatively low Nd value and is of a similar late Arenig age as the Kirk Stile Formation. Secondly, and more consistently, there is also a contrast across the Causey Pike Fault in the mudstone data. For the most part, the mudstone εNd values are lower than those recorded from the wackes of the same formation. This is a widely reported phenomenon thought to reflect the more distal nature of the mudstone, its likely derivation from a much broader provenance, and the greater degree of homogenisation of the detrital components in the hemipelagic environment. The uniformity of mudstone εNd values across the Northern Fells Belt is therefore no surprise, and illustrates the broadly Proterozoic character of the Avalonian hinterland on which were superimposed the more juvenile local variations reflected in the wackes. The background provenance therefore remained fundamentally unchanged throughout the Tremadoc and Arenig. The contrast across the Causey Pike Fault is striking and indicates an abrupt shift to a more mature regional provenance (although still broadly Proterozoic) for the mainly Llanvirn Tarn Moor Formation. This is an unexpected result in view of the increased juvenile volcaniclastic and bentonitic input seen therein. It emphasises that the volcanic events were probably brief, sporadic and gave rise to discrete beds; they did not contribute to the background Tarn Moor Formation sediment. The mudstone trend continues to the south where εNd values from late Arenig strata in the Black Combe inlier are also relatively low.

In summary, wackes to the north have a relatively larger juvenile component than coeval wackes to the south; mudstones to the north, of Tremadoc to earliest Llanvirn age, have a relatively larger juvenile component than late Arenig to Llanvirn mudstones to the south. The contrast in Nd values emphasises the importance of the Causey Pike Fault and reinforces its interpretation as an early, basin partitioning structure across which there has been subsequent shortening of the basin architecture.

Summary of provenance indicators

The Skiddaw Group comprises deep-water slope and basin deposits laid down in one or more extensional basins developed peripheral to the Gondwanan margin. By the late Arenig, the extensional rifting had led to the separation of the Avalonian terrane from Gondwana (Figure 2). During the same early Palaeozoic time, subduction of the Iapetus Ocean was established beneath Avalonia/Gondwana. The earliest subduction-related volcanic rocks in Wales are of Tremadoc age (Kokelaar, 1988 and references therein) which raises the possibility that the Skiddaw Group basin developed in a back-arc or inter-arc environment. In that situation the Skiddaw Group would be expected to contain much juvenile volcanic material. However, volcanism does not appear to have commenced in the Lake District region until some 20 to 25 Ma later (Cooper and Molyneux, 1990; Hughes and Kokelaar, 1993) with the first bentonite and volcaniclastic turbidite interbeds in the early Llanvirn Tarn Moor Formation. The Skiddaw Group provenance was deduced by Cooper et al. (1995) to be largely from an old, inactive and probably Precambrian, continental arc terrane lying to the south-east of the depositional basin.

The petrographical evidence is fairly consistent in suggesting a source for most of the Skiddaw Group sandstone in an old, inactive volcanic arc founded on continental crust. An additional source of sediment was available during deposition of the Loweswater Formation when mature quartz sand was perhaps derived from the craton interior and sorted in a shelf environment before resedimentation by turbidite flow. The overall consistency of the provenance during the Tremadoc and Arenig, at least for the Northern Fells rocks, is confirmed by the geochemical data, particularly the REEs. The latter also suggest that the provenance had an overall composition equating to average continental crust.

The petrographical evidence, and to a lesser extent the geochemistry, have been interpreted in terms of an increased volcanic input in the younger strata, culminating with the volcaniclastic interbeds in the Tarn Moor Formation. Neodymium isotope results tend to contradict this trend, showing a general decrease in the juvenile component of Northern Fells sandstones from the Tremadoc through into the Arenig. Furthermore, the non-volcaniclastic sandstones of the Tarn Moor Formation appear to continue this trend through into the Llanvirn. Neodymium isotope data from the mudstones show a comparable contrast across the Causey Pike Fault; a uniform Tremadoc to Arenig provenance signature in the Northern Fells contrasts with a significantly more mature character in the Tarn Moor Formation on the south side of the fault. Relatively sparse data suggest that the more mature characteristics also apply to the sandstone rafts of the Buttermere Formation olistostrome and both the sandstones and mudstones of the Skiddaw Group in the Black Combe inlier.

The overall pattern that emerges suggests the interaction of contrasting trends. An increase in volcaniclastic input with time, from the Tremadoc, through the Arenig and into the Llanvirn, was countered by increasing maturity of the provenance as a whole. This effect was coupled with increasing southward maturity in coevally deposited sediment.

Chapter 5 Structure

Rastall (1910) commented that ‘The unravelling of the structure of the Skiddaw Slates would be a task of immense difficulty, and would probably not repay the expenditure of the necessary time’. Hopefully the latter part of this assertion is wrong. As with many complex geological problems, numerous conflicting interpretations of the structure of the Skiddaw Group have been published over the last 80 years. There can be little doubt that the interpretations presented here will be refined in the future.

The Lake District Lower Palaeozoic inlier (Figure 1) is the northernmost part of the Caledonide fold belt south of the Iapetus Suture Zone. The palaeogeographical position of the inlier, on the northern margin of eastern Avalonia and near to the Tornquist Sea convergence zone (Chapter 1), makes an understanding of its structural history an important factor in early to mid-Palaeozoic plate tectonic reconstructions (for example Soper et al., 1987, 1992b; Pickering et al., 1988; Pickering and Smith 1995). Within the inlier, the deep marine strata of the Skiddaw Group form the basement to the subduction-related Borrowdale and Eycott Volcanic groups (largely erupted during the early Caradoc), and the overlying marine strata of the Windermere Supergroup (of late Caradoc to Přídolí age).

Summary of previous structural research

The first ‘modern’ account of the highly complex, polyphase deformation of the Skiddaw Group was by Simpson (1967), with subsequent work by Helm (1970), Helm and Roberts (1971) and Jeans (1971) (see review by Soper and Moseley, 1978). Early, pre-cleavage folding of the Skiddaw Group, described by Roberts (1971), Jeans (1972) and Webb (1972), was reinterpreted as slump folding produced by soft-sediment deformation and olistostrome emplacement by Webb and Cooper (1988) and by Webb (1990, 1992). A major, pre-cleavage, recumbent fold (the Gillbrae Nappe) was also described by Banham et al. (1981) from the northern part of the Skiddaw Inlier. It was thought to have developed by ‘gravity gliding’. However, more recent work in the area, as described in this memoir, has not substantiated the existence of such a large-scale fold.

The complexity of the structure led Simpson (1967) and Helm (1970) to propose a mid-Ordovician, pre-volcanic tectonic event; Helm described six phases of deformation in the Black Combe inlier. Recognition that the regional cleavage (the oldest tectonic fabric in the Skiddaw Group) was common to all the rocks of the inlier and therefore post-Přídolí in age seemed to preclude such an event. However, interest in the possibility of pre-volcanic deformation was re-awakened by the appreciation of the magnitude of the unconformity beneath the Borrowdale and Eycott Volcanic groups (Millward and Molyneux, 1992; Cooper and Hughes, 1993).

The main tectonic deformation occurred during the early Devonian Acadian orogeny, and was described by Soper et al. (1987). This was the earliest cleavage-forming event to affect the Skiddaw Group and produced a regional, north-east- to east-trending cleavage. This is the ‘late-Caledonian’ cleavage of Soper (1970) and Soper and Roberts (1971), and is generally axial planar to large-scale, upright to steeply inclined folds. Soper and Roberts (1971) also noted that the Skiddaw granite postdates the regional S1 cleavage, but predates later crenulation fabrics, so constraining their ages. Hughes et al. (1993) and Stone et al. (1999) have reviewed the structural history of the Skiddaw Group.

Principal structures and deformation events

The sequence of deformation events that has affected the Skiddaw Group is summarised in (Table 5). The earliest deformation was syndepositional and generated folds that are superficially similar to some of the tectonic folds. Indeed, the mechanisms of formation of some of the major folds remain controversial (see below).

The earliest large-scale deformation event produced large-scale slump folding, shearing and brecciation, and was related to olistostrome emplacement in the late Arenig. This was followed by fault-block rotation and tilting, as regional uplift of the Skiddaw Group was initiated during the late Llanvirn in response to subduction and magma genesis prior to the main volcanic episode. The Eycott and Borrowdale Volcanic groups, along with the older parts of the adjacent Carrock Fell Complex, were probably then subjected to further large-scale folding, and were preserved by synvolcanic subsidence before the mid-Caradoc (Piper et al., 1997).

The Skiddaw Group was affected by pervasive tectonism during the early Devonian Acadian event (Soper et al., 1987), which deformed all the Lower Palaeozoic parts of the Lake District. Crenulation of the regional S1 cleavage by later fabrics (with associated folds) indicates that the Acadian tectonic event consisted of more than one pulse of activity. For the most part, the overlying volcanic groups and Windermere Supergroup are affected by only the first of these events (D1).

The main deformation event (D1) produced the regional north-east to east-trending penetrative cleavage and its associated folds in the Skiddaw Group. The intensity of the S1 cleavage is variable, and domains of contrasting intensity appear to be terminated or offset laterally by north-trending wrench faults.

The post-S1 crenulation fabrics and their related folds are highly variable in intensity and distribution, but include a gently dipping to subhorizontal fracture cleavage (axial planar to minor recumbent folds), and an upright, northerly trending fracture cleavage. These crenulation fabrics are, at least in part, related to late Acadian reverse/thrust faults.

Pre-Acadian deformation

Soft-sediment slump folding

Evidence for syndepositional slumping is widespread in the Skiddaw Group. The folds produced are intrafolial, disharmonic, recumbent and lobate, generally affecting strata thicknesses of 0.1 to 10 m; a typical example is illustrated in (Plate 21). The slump folds are commonly separated from adjacent beds by shears, subparallel to the bedding. They are deformed by all the later tectonic phases.

In some places, a pre-tectonic compaction fabric that is generally parallel or subparallel to bedding (discussed below) cuts the slump folds. Because of their intrafolial nature, the axial planes of the slump folds are commonly very close to the overall bedding attitude. Locally the compaction fabric may appear, therefore, to be axial planar to the slump folds. The compaction fabric is so strongly developed in the argillaceous lithologies that it is locally crenulated by the tectonic fabric (S1) produced during the earliest tectonic deformation.

Key structures and sections

Loweswater Fell

Webb and Cooper (1988) interpreted the Loweswater Anticline as a very large slump fold. Moore (1992) disagreed with this interpretation, believing the Loweswater Anticline to be an Acadian tectonic structure. Hughes et al. (1993) cast further doubt on the Webb and Cooper interpretation by demonstrating a tectonic origin for the apparent north-eastern extension of the Loweswater Anticline, the Sale Fell Anticline.

The interpretation of the Loweswater Anticline at Loweswater Fell [NY 13 22] remains contentious. In that area it is a major, north-west-trending, lobate, recumbent fold, overturned to the east and south-east; the fold is floored by an irregular thrust fault (Figure 29). The lobate nature of the fold means that its axis swings from a westerly trend overlooking Loweswater lake to a northerly trend on the north side of Darling Fell [NY 13 23]. The typically east-north-east-trending S1 cleavage is absent, but a north-west-trending cleavage is present, and cuts across the fold trend. Webb and Cooper (1988) believed that the geometry and style of the folding is compatible with the tightening, flattening and refolding of pre-existing north-trending slump folds about the north-west-trending cleavage. In contrast, the apparent north-eastern extension of the Loweswater Anticline in the Sale Fell area [NY 19 29] is a normal, upright, tight, north-east-trending F1 fold with a well-developed axial planar S1 cleavage fabric.

River Caldew

Mudstones and siltstones of the Kirk Stile Formation are hornfelsed by the Skiddaw granite along the River Caldew [NY 327 325] in the north-east of the Skiddaw inlier. Pre-Acadian and

Acadian deformation structures are present, and the folding was fully described by Roberts (1971, 1990, 1992b). Pre-Acadian soft-sediment deformation produced recumbent, north–south-trending folds; these were refolded during the Acadian tectonic event, and are now steeply plunging. The style of the slump folds is complex and variable (Plate 22). Concentric, similar, chevron and conjugate fold elements are all present, although close to tight, similar folds form the most dominant style.

Interference patterns caused by Acadian refolding of the soft-sediment fold structures are also present, with good examples exposed at the junction of Grainsgill Beck [NY 3276 3258]. Roberts (1992b) described these interference structures, although he noted that refolding during a continuum of slumping activity could also have occurred. In this context, Roberts cited one unusual problem of the Caldew section, which is the apparent absence of cleavage associated with the Acadian folds in the hornfelsed rock; elsewhere in the aureole well-preserved, upright, east–west-trending, S1 cleavage is present.

The River Caldew structures were also studied in detail by Moore (1992), who concluded that the facing directions of pre-Acadian soft-sediment fold structures indicate a palaeoslope inclined towards the west.

Buttermere Formation

This olistostrome deposit comprises approximately 1.5 km of slumped and redeposited Tremadoc to late Arenig sedimentary rocks (see also Chapters 3 and 4). Olistostrome emplacement is believed to have occurred during the late Arenig. Webb (1975) studied the complex deformation within the Buttermere Formation in detail, but it was first recognised as an olistostrome by Webb and Cooper (1988) and Webb (1990, 1992).

The olistostrome is not entirely chaotic in character. In the southern part of its outcrop it contains a relict stratigraphy in which the sandstones of the Robinson Member are disposed in relatively tight, overturned, west-north-west-facing slump folds. The degree of disruption of the Robinson Member olistoliths appears to be controlled by their position within the major fold structures (Figure 30). The larger rafts of sandstone are almost entirely restricted to the inverted limbs and the anticlinal hinge regions. Shear planes are more common on the non-inverted limbs, with sandstone either absent or represented only by sporadic small rafts and boudins up to a few metres in length. Boudinage is rarely seen on the inverted limbs. Webb and Cooper (1988) related these differences to variations in the stress regime during folding. Minor slump folds are widespread in the Robinson Member and for the most part are congruous with the major structures; they demonstrate the lobate, axial curvature of the major folds and confirm the westerly facing direction (Figure 31).

Small-scale slump folds are widespread and intense in the more argillaceous parts of the Buttermere Formation (Plate 23). Few parts of the sequence are undisturbed, and complex anastomosing shear zones separate folds tens of metres. The folds occur most commonly as minor structures with amplitudes of 1 to 2 m. Larger slump folds, a few tens of metres in amplitude, affect the Robinson Member on the inverted fold limb near the western end of Goat Crag and along Hassnesshow Beck [NY 189 164]. Within this area, the slump folds are cut by the Acadian S1 slaty cleavage, best developed in the siltstone and mudstone lithologies.(and rocks of markedly different biostratigraphical ages). The folds show marked plunge variation from near-vertical to subhorizontal. Webb (1975) showed that the plunge of the folds had been exaggerated tectonically, and defined sub-areas in which plunge was predominantly either to the north-east or the south-west. He proposed an en echelon arrangement of major folds to juxtapose the different sets of congruous minor structures (Figure 32).

Key structures and sections

Goat Crag, Robinson and Hindscarth (Buttermere Formation)

Large slump folds are present on Goat Crag [NY 189 164], Robinson [NY 199 167] and Hindscarth [NY 215 165]. The trace of a major anticlinal slump fold descends from near the top of Goat Crag, south-eastwards through Goat Gills (Figure 33). This anticline is overturned towards the west so that the beds cropping out over most of the hillside dip eastwards and are inverted. Minor slump folds with amplitudes of a few metres or less are also common. They are intrafolial periclines with curvilinear hinges, which plunge south-eastwards and are congruous with the major fold. On the inverted limb of the major slump fold, sandstone turbidites of the Robinson Member form an extensive outcrop. Although extensively folded by slumping (Plate 24), the sandstones are not pervasively disrupted here, except near their junction with the stratigraphically underlying Goat Gills Member, which crops out in the core of this major anticline. Near to the junction, sandstone beds are irregularly and disharmonically folded and sheared, and only very locally does the junction appear to be undisturbed. On the more gently dipping, normal limb of the major anticline, the Robinson Member is highly disrupted and forms a matrix-supported breccia, with sandstone rafts and boudins, enclosed in sheared and folded mudstone. Sandstone rafts are well exposed near the summit of Goat Crag, where they range in length and thickness from a little more than 1 m to several tens of metres. The folds occur most commonly as minor structures with amplitudes of 1 to 2 m. Larger slump folds, a few tens of metres in amplitude, affect the Robinson Member on the inverted fold limb near the western end of Goat Crag and along Hassnesshow Beck [NY 189 164]. Within this area, the slump folds are cut by the Acadian S1 slaty cleavage, best developed in the siltstone and mudstone lithologies.

Hassness (Buttermere Formation)

Interference between the early slump folds and the later tectonic folds is well displayed in the crags around Hassness, at the foot of Goat Crag. The gently plunging slump folds are visible in the small cliffs [NY 185 158] overlooking the lake, and the tectonic folds, with axial planar S1 cleavage, can be seen plunging steeply in the glacially scoured rock surface above the cliff.

Buttermere Village (Buttermere Formation)

Details of the Skiddaw Group at Buttermere village are given by Jeans (1973), Webb (1975) and Moseley (1992). Complex slump-folds within siltstones and mudstones of the Buttermere Formation are well exposed on ice-smoothed rock surfaces around Buttermere Village [NY 175 169]. In contrast, no folding is evident in nearby Buttermere Quarry [NY 1734 1726] where the rocks dip and young to the south-east. Since the surrounding strata are intensely folded and refolded, the strata in the quarry are regarded as part of a large, internally undeformed raft. The slump-folds are mainly small scale, tight and complex, with a variable trend (commonly northerly) and a steep plunge where they are refolded by the main east-northeast- trending F1 tectonic folds. The have an associated axial planar S1 cleavage.

Causey Pike (Robinson Member)

Intensely folded wackes of the Robinson Member crop out on the south-facing slope below Causey Pike [NY 2196 2074] (Figure 38). The strata are mainly inverted, but to the north-east [NY 2270 2115] the beds are normal and less deformed.

Compaction fabric

A compaction fabric is widespread within the Skiddaw inlier. The fabric is produced by a crystallographic alignment of chlorite-mica stacks and phyllosilicate grains (Fortey et al. 1993), and is commonly an accentuation of the primary sedimentary lamination. Generally, it is most strongly developed in argillaceous lithologies, and probably results from burial, either prior to, or during, pre-volcanic uplift. The high heat flow thought to have been prevalent at the time (Merriman and Frey, 1999, and discussed further in Chapter 6) may have catalysed the formation of this fabric by accelerating recrystallisation of clay minerals (Merriman and Peacor, 1999).

The nature of the S1 cleavage varies with the intensity of the compaction fabric. Where the compaction fabric is strongly developed, S1 is commonly a crenulation cleavage; where the compaction fabric is weak or absent, S1 cleavage is commonly a penetrative, slaty fabric.

Pre-volcanic uplift and erosion

The broadly contemporaneous Eycott and Borrowdale Volcanic groups rest unconformably on the Skiddaw Group (Moseley, 1974; Wadge, 1978, Millward and Molyneux, 1992). At one time the Eycott Volcanic Group was thought to be significantly older than the Borrowdale Volcanic Group on the basis of an acritarch flora described from sedimentary interbeds at the base of the former (Downie and Soper, 1972). This evidence has since been re-assessed by Millward and Molyneux (1992), who were unable to substantiate the original interpretation and instead favoured a common, late Llanvirn to Caradoc age.

Both volcanic groups were erupted in a mainly sub-aerial environment with only rare marine incursions (Branney 1988; Millward and Molyneux 1992). This situation contrasts markedly with the deep marine depositional environment of the Skiddaw Group, and requires complete basin inversion during the post-mid-Llanvirn but pre-Caradoc interval (about 464 to 458 Ma on the Tucker and McKerrow (1995) timescale).

The scale of the unconformity beneath the volcanic rocks demonstrates the magnitude of the stratigraphical disruption, caused by the combination of gravity-driven massmovementandthesubsequentbasininversion.A wide range of biostratigraphical ages has been determined in sequences immediately subjacent to the overstepping unconformity. By comparison with the appropriate stratigraphical level in a hypothetically complete Skiddaw Group succession, the extent of the missing sequence can be suggested. At the northern margin of the Skiddaw inlier the age of strata below the Eycott Volcanic Group unconformity ranges from possible Cambrian to Llanvirn (Millward and Molyneux, 1992). Around Over Water [NY 2592 3552], up to 1500 m of Skiddaw Group strata are apparently missing (Millward and Molyneux, 1992) and beneath the unconformity here the Skiddaw Group is N inverted (Plate 25). The presence of possible Cambrian acritarchs below the Eycott Volcanic Group in the Eycott Hill area [NY 3826 3036] can be interpreted to suggest that up to 5000 m of Skiddaw Group were eroded from this area (Millward and Molyneux, 1992).

Farther south, beneath the Borrowdale Volcanic Group, various Arenig to Llanvirn biostratigraphical horizons occur close to the unconformity (Cooper and Hughes, 1993). Again, it can be interpreted that up to 5000 m of Skiddaw Group strata were removed locally. However, in this case the Borrowdale Volcanic Group rests unconformably on the Buttermere Formation, which had already been stratigraphically disordered during emplacement.

At the unconformity beneath the Eycott Volcanic Group, there is stratigraphical coherence over wide areas of the outcrop, despite the variation of stratigraphic level subjacent to the volcanic rocks. Much of this variation therefore seems most likely to have been caused by fault block rotation prior to volcanicity. Rotation could have been either extensional or compressional, and was probably superimposed both on disrupted zones caused by the earlier slump movements and on areas that had escaped such disruption. Pre-volcanic erosion cut across this severely disordered sedimentary pile and in some places may have removed only very small quantities of the Skiddaw Group to produce the sub-unconformity relationships seen.

It should be stressed that there is no evidence for any penetrative tectonic fabric imposed during pre-volcanic uplift and stratal disorganisation. This has been a long-running controversy in Lake District geology, as discussed earlier in this chapter. A pre-volcanic deformation event related to collision or accretion, which produced tectonic folds or fabrics, is therefore highly unlikely. The only pre-volcanic but postdepositional fabric present in the Skiddaw Group is the widespread compaction fabric.

Branney and Soper (1988) suggested that uplift of the Skiddaw Group was due to buoyancy effects associated with the generation of andesitic melts. This hypothesis was further explored by Hughes et al. (1993), who noted that mantle contamination, thermal expansion, and volumetric expansion due to hydration of rising magma may all have contributed to the buoyancy of rising andesitic melts above the subduction zone, causing the regional uplift of the Skiddaw Group. Structures that were produced in the Skiddaw Group specifically during this uplift have not been identified to date, but the scale and nature of the unconformity shows that the effects of pre-volcanic uplift were widespread and profound.

Synvolcanic deformation

The Skiddaw Group was the basement to Caradoc supra-subduction zone, rift system volcanism. A network of volcanotectonic, caldera-related faults has been identified within the Borrowdale Volcanic Group (Branney and Soper, 1988) and it is highly likely that similar structures affected parts of the Skiddaw Group. Proven displacements on the volcanotectonic faults within the Borrowdale Volcanic Group commonly exceed 400 m (Branney and Kokelaar, 1994).

Volcanotectonic disruption of the Skiddaw Group during the Caradoc is certain to have occurred, and probably involved reactivation of the existing fault framework. The present-day elevation of the Eycott Volcanic Group is significantly lower than that of the Borrowdale Volcanic Group, despite Acadian thrusting of the former. One possible implication of this relationship is that, between the two volcanic groups, there is a volcanotectonic structure with a significant downthrow to the north. Major structures such as the Causey Pike Fault would probably have acted as a focus for such movement, and it is particularly tempting to suggest that the major faults in the Skiddaw Group near its contact with the Eycott Volcanic Group are re-activated synvolcanic faults. Certainly, the evidence of palaeomagnetic data (Piper et al., 1997) indicates that the large-scale tilting of the Eycott Volcanic Group probably occurred in the mid-Caradoc.

Large-scale deformation of subvolcanic basement sequences due to the loading effects and gravitational spreading of stratovolcanoes has been described from modern settings. Beneath Mount Etna, for example, such deformation has produced a range of compressional structures including thrust faults and anticlinal folds (Borgia et al., 1992). It is possible, if not probable, that the Skiddaw Group basement was deformed in a similar way by the loading effect of the overlying Borrowdale and Eycott volcanic groups, but structures attributable specifically to this type of deformation have not been recognised.

Post-volcanic convergence of Avalonia and Laurentia

Following the cessation of volcanicity in the late Caradoc, Avalonia continued to drift northwards until its collision with Laurentia (Figure 2), probably during the Wenlock (p.4). Destruction of the Iapetus Ocean during this interval was effected by subduction at the northern margin of the ocean, where an accretionary complex developed at the leading edge of Laurentia. Following continental convergence, this over-rode the Avalonian margin and advanced southwards as a foreland fold and thrust belt preceded by a foreland basin. This process has been widely referred to as ‘soft’ collision and did not result in collision orogenesis.

Stone et al. (1987), Kneller (1991), Kneller et al. (1993) and Hughes et al. (1993) have discussed the initiation of the foreland basin in southern Scotland and its propagation on to Avalonia. The Skiddaw Group was clearly affected by this process, in which south-directed thrusts/reverse faults dominated the structural style. The principal such structures within the main Skiddaw inlier (the Watch Hill, Loweswater, Gasgale and Causey Pike thrusts: (Figure 34) may have been initiated at this stage, although thrust reactivation of existing structures seems highly probable in view of the earlier structural history of the Skiddaw Group.

The southward younging of the thrust deformation front is well established in the Southern Uplands (southern Scotland) (Barnes et al., 1989), and may have continued into the Lake District. However, no tectonic structures can be identified within the Skiddaw Group that were produced unequivocally by pre-Acadian thrust deformation. The timing of this event may nevertheless be indicated by the widespread resetting of the Rb–Sr isotopic systems in adjacent igneous rocks. Pertinent age determinations are summarised in (Figure 35) and are discussed by Stone et al. (1999).

An analogous situation was studied in the Welsh Lower Palaeozoic basin by Evans et al. (1995) who concluded that the resetting was chemically controlled during hydration. In the northern Lake District, uplift associated with the thrust development would have opened fractures, increased permeability and facilitated the formation of secondary hydrated minerals, thus resetting the Rb–Sr system.

An acritarch flora from the upper part of the Borrowdale Volcanic Group was considered to indicate a Caradoc eruption age (Molyneux, 1988), but more recently acquired data now suggest that the key species occurs below the Caradoc. Nevertheless a Sm–Nd garnet whole-rock age of 457 ± 4 Ma (Thirlwall and Fitton, 1983) is in broad agreement with a Caradoc age. Palaeomagnetic results have been interpreted (Piper et al., 1997) as showing that the volcanic sequences were erupted during a single normal-polarity chron occupying the early Caradoc. These data contradicted apparently unequivocal Rb–Sr ages of 423 ± 3 and 432 ± 3 Ma that had been previously obtained by Rundle (1987). Similarly, for the Eskdale and Ennerdale granitic intrusions, Rundle (1979) obtained precise Rb–Sr results of 429 ± 4 and 420 ± 4 Ma respectively, whereas U–Pb zircon dates obtained by Hughes et al. (1996) were 450 ± 3 and 452 ± 4 Ma, respectively. The U–Pb dates associated the plutons firmly with the later stages of volcanicity.

A different picture emerges from the younger, post-cleavage, Shap and Skiddaw granites. The Rb–Sr age from Shap is 394 ± 3 Ma (Wadge et al., 1978) by comparison with a U–Pb discordia zircon age of 390 ± 4 Ma (Pidgeon and Aftalion, 1978). The Skiddaw granite yielded a K–Ar age of 399 ± 8 Ma (Shepherd et al., 1976, recalculated by Rundle, 1982) and a Rb–Sr age of 393 ± 5 Ma (Shepherd and Darbyshire, 1981); both are within error of a poorly constrained preliminary U–Pb zircon age of 406 ± 12 Ma. There is thus no evidence for any resetting of the younger granites by either thermal or hydration effects.

From the Skiddaw Group itself a Rb–Sr isochron of 427 ± 34 Ma has been reported by Evans (1996) from late Arenig, Kirk Stile Formation mudstone at Dodd Wood [NY 2363 2785]. Despite the large error there is no overlap with the depositional age of about 480 Ma, which must be younger than all of the heterogeneous detrital components in the rock. In mudstones, such homogenisation and resetting is a dehydration reaction linked to re-equilibration of the Rb–Sr system during the diagenetic, or low-grade metamorphic, illite to smectite transition (Evans et al., 1995).

This substantial body of evidence clearly indicates a regional resetting of Rb–Sr isotopic systems at between 430 and 420 Ma. The exact cause of the resetting is unclear, but a tectonic or thermal event seems probable, and the initiation of thrusting within the Skiddaw Group during post-volcanic convergence of Avalonia and Laurentia is a clear possibility. Hughes et al. (1996) suggested that resetting of Rb–Sr isotopic ages for the Eskdale and Ennerdale intrusions (at 429 ± 4 Ma and 420 ± 4 Ma, respectively) was due to intra-Ludlow tectonism.

Acadian deformation

The entire Lower Palaeozoic sequence of the Lake District and its surrounding inliers was affected by the Acadian deformation event in the early Devonian. In the Borrowdale and Eycott Volcanic groups and the Windermere Supergroup, a single cleavage is almost ubiquitous, but later crenulation cleavages are rare and are developed only locally. In contrast, in the Skiddaw Group, the north-east- to east-trending regional Acadian cleavage (S1) and its related fold structures are commonly crenulated by complex, later Acadian cleavage fabrics with related fold structures. These have a wide range of attitudes, are markedly domainal, and are related at least in part to a set of north-dipping Acadian reverse/thrust faults present in the Skiddaw inlier. They include open and gently plunging, gently inclined folds with an associated, shallow-dipping crenulation cleavage and north-west-trending, upright folds with an axial planar crenulation cleavage.

Main deformation (D1)

The main episode of Acadian deformation produced large-scale folds (F1) in the Skiddaw Group. These major folds are readily recognised where there is strong stratigraphical and lithological control, such as in the Loweswater Formation. They are more difficult to identify in the mudstone-dominated sequences of the Hope Beck and Kirk Stile formations.

F1 folds are mostly tight to isoclinal, with gently plunging hinges and steeply inclined or vertical axial planes, and trend between north-east and east-north-east. Major F1 folds have amplitudes and wavelengths up to several kilometres, and include the anticlines at Sale Fell [NY 1900 2967], Scawgill [NY 179 257], Skiddaw [NY 2637 2792] and Barf [NY 2141 2688], along with synclines at Wythop [NY 2080 2845] and Dodd [NY 2440 2735] (Figure 36). Fold style varies from upright and isoclinal (Sale Fell and Skiddaw), to open and periclinal (the Scawgill Anticline). The Barf Anticline was refolded by later Acadian deformation into a recumbent structure overturned to the south-east. The limbs of all the major structures have minor, congruous, open to isoclinal folds, with amplitudes and wavelengths up to several hundreds of metres.

The same Acadian event produced a regional cleavage (S1) (Plate 26) that trends generally between north-east and east-north-east (Figure 37), and is continuous into the overlying volcanic groups. This cleavage is almost always axial planar to the F1 folds. The S1 and post-S1 cleavage data set collected by BGS geologists during the re-survey of the Skiddaw inlier was compiled in Cooper (1997).

Transecting fold-cleavage relationships are normal in the Windermere Supergroup of the southern Lake District, but have been seen only in minor fold structures in the Skiddaw Group (for example on the southern limb of the Sale Fell anticline — see below). The S1 cleavage fabric is most commonly of penetrative, slaty, pressure solution type (Plate 26), (Plate 27), but occurs also as a spaced fracture fabric, and as a crenulation fabric where the earlier compaction fabric is strongly developed.

The S1 cleavage has a fairly consistent north-east to east-north-east trend in the Skiddaw inlier, where it dips mostly towards the south at 60 to 90° (Figure 37), e.g. area e. In the central part of the Skiddaw inlier is a large, fault-bounded area of northerly dipping cleavage (Figure 37), areas c, d, g, and other, more localised zones of northerly dipping cleavage also occur. The central part includes most of the area north of the Crummock Water aureole, extending eastwards into Dodd [NY 246 271] and west to the Vale of Lorton [NY 154 246] (Figure 37), areas d and g). It is bounded by the Causey Pike Thrust Fault to the south, the Derwent Water Fault to the east, and the Lorton Vale faults to the west. The northerly dipping S1 cleavage within this area may be a result of large-scale reorientation of the cleavage in the hanging wall of the Causey Pike Thrust Fault, between lateral ramps coincident with the Lorton and Derwent Water faults.

In addition to the large scale re-orientation of the S1 cleavage, more localised disturbance can be demonstrated at several localities. In the area of Great Cockup [NY 273 333] and Meal Fell [NY 283 338], and also near Mungrisdale, the main fold hinges and associated cleavage swing away from their usual east-north-east orientation to a north-west trend (Figure 37), area B). At Meal Fell, this appears to have been caused by compression of the already folded and cleaved rocks into a reentrant formed by the more rigid Eycott Volcanic Group and Carrock Fell Complex gabbro and granophyre (see British Geological Survey, 1:10 000 Sheet NY 32 SE, 1994). At Mungrisdale, the anomalous north-west orientation (Figure 37), area f (Roberts, 1992a) may be related to movement on the adjacent Carrock End Fault, again with juxtaposition of the rigid Eycott Volcanic Group.

In the Black Combe inlier, the Skiddaw Group strata lie in the footwall of a major south-east-directed thrust, possibly an exposed part of the South Lakes ramp of Lee (in Millward et al., 2000). The thrust dips to the north-west at about 30° and the S1 cleavage is generally sub-parallel to it with dips of between 25° and 40° to the north-west. The S1 cleavage commonly cuts a well-developed and pervasive bed-parallel compaction fabric and so appears widely as a crenulation cleavage.

The north-western part of the outcrop is pervasively metasomatised and tight, reclined F1 folds plunge down the dip of cleavage, which is axial planar (Bell, 1992). Most of the folds face westwards. The tight, or even isoclinal style of the folding means that over wide areas S1 and the bed-parallel compaction fabric are subparallel. To the south-west, in the non-metasomatised part of the outcrop, the S1 cleavage is much less intensely developed than in the north, although with the same attitude, and the associated reclined folds are more open. The metasomatism front therefore coincides with a significant strain boundary. In the high-strain zone to the north, the F1 folds are tighter, the S1 fabric intensifies and the earlier bedding compaction fabric is rotated to become parallel to S1. Some of the F1 folds in this zone are also markedly curvilinear. Bell (1992) has described the reclined folds, both in the high strain zone and farther south, as sheath folds formed within a shear zone. The hinge orientation would be compatible with thrusting towards the south-east.

In the Ullswater inlier (Figure 37), area n, the S1 cleavage strikes approximately north-east and dips steeply to the south-east. It is axial planar to folds which range from tight to open in style, steep to gentle in plunge, and occur at a variety of scales (Hughes, 1995a). Farther south-east, in the Bampton inlier, there is a widespread bedding-parallel fabric, ranging from a slaty cleavage to a spaced shaly fissility. It is abruptly cut at the unconformity beneath the overlying Borrowdale Volcanic Group, as at Keld Gill [NY 5397 1340]. The Acadian S1 cleavage is only sporadically developed as a spaced to slaty fabric, striking north-east and mostly dipping steeply towards the north-west (Bell, 1997; (Figure 37), area p. Farther east, at Cross Fell, the S1 cleavage is slaty, steeply inclined and strikes about east-south-east–west-north-west (Arthurton and Wadge, 1981; Burgess and Holliday, 1979). It is axial planar to tight to isoclinal, asymmetric F1 folds, the hinges of which plunge variably within the axial plane.

The exact relationships between the F1 folds (and related cleavage fabrics) and the Acadian thrusts/reverse faults within the Skiddaw inlier remain unclear, due to a combination of factors including the great complexity and domainal nature of the cleavages, the presence of a compaction fabric which is difficult to distinguish from S1, and the overlap of the orientations of S1 and post-S1 crenulation fabrics in some areas. Hughes et al. (1993) believed that at least some of the post-S1 crenulation fabrics were related to thrusting, implying that S1 and its related folds predate the thrusting event. In contrast, Webb (1999) suggested that the cleavage within thrust-related hanging-wall anticlines and foot-wall synclines is S1, implying that the thrusting was part of the D1 event.

The distribution of the various cleavage fabrics through the Skiddaw Group inliers may also have been influenced by basement controls. The S1 cleavage is fairly uniformly developed although distinctly domainal in intensity, while post-S1 cleavages are much more localised in the Skiddaw and Black Combe inliers. These are situated above the northern and southern margins of the Lake District batholith where compression may have been focussed. Conversely, the Ullswater and Bampton inliers, where the later cleavages are very rare, overlie the main body of the batholith and were perhaps protected by it from the later stages of compression. The later cleavages were not developed in the Furness inliers which may have lain to the south of a thrust-controlled deformation front during the post-D1 deformation phases.

Key structures and sections

The principal D1 structures of the main Skiddaw inlier are shown in (Figure 36) and described below.

Sale Fell Anticline

The tight to isoclinal Sale Fell Anticline is a large-scale fold extending from Harrot [NY 1586 2756] to the north end of Bassenthwaite Lake [NY 200 301]. The fold is developed in the Loweswater Formation, and sedimentary structures give abundant younging evidence. At its western end on Harrot, through Embleton High Common and Ling Fell, most of the northern limb dips steeply northwards and is the correct way up. In contrast, the southern limb is largely inverted, with south-younging beds dipping steeply northwards. This way-up variation can be seen along Tom Rudd Beck [NY 1638 2897] to [NY 1711 2814]. Numerous minor folds associated with the main structure occur on Dodd Crag [NY 1884 2944], at the western end of Sale Fell. High on Sale Fell, the structure of the axial region of the fold is complex; the beds are inverted on both limbs, but dip inwards towards the axis of the fold. The presence of numerous large quartz veins striking parallel to the fold axes here (and on Ling Fell) indicates intense deformation in the hinge zone. Two small lamprophyre intrusions [NY 1928 2968]; [NY 1930 2974] are also approximately coincident with the axis of the Sale Fell Anticline. The penetrative S1 cleavage is mainly axial planar and is particularly well developed on its southern limb. Transecting fold–cleavage relationships are present in a minor fold on the southern limb of the anticline [NY 2040 3004], where S1 is approximately 10° clockwise of the fold axial plane.

Wythop Syncline

Between the Sale Fell and Scawgill anticlines, the Wythop Syncline [NY 2080 2845] is poorly exposed in the Kirk Stile Formation and associated sandstones. Along its northern limb, limited sedimentary way up evidence suggests that the steeply dipping beds are overturned, like those on Sale Fell to the north. The strata on the southern limb are generally the correct way up and more gently dipping.

Scawgill Anticline

In contrast to the Sale Fell Anticline, the Scawgill Anticline is an open structure, with no structural inversion on the fold limbs. It has an outcrop width of about 4 km, largely due to the southern limb, which generally dips south at less than 50°, being repeated by faulting. In contrast, faulting in many places cuts out the northerly dipping northern limb. The axis of the fold is intensely faulted along Blaze Beck [NY 1800 2579], west of the Whinlatter Pass.

The western end of the Scawgill Anticline is marked by a belt of anomalously north–south striking beds, dipping westwards towards the Vale of Lorton. Their attitude probably relates to a lateral ramp structure and wrench faulting along the Vale of Lorton, which have also affected the fold structures of the Lorton Fells to the west of the valley. The overall effect of this westerly dipping zone is to produce a broad dome-like structure. At the southern limits of the fold, the bedding becomes slightly overturned (dipping north and younging south) in the upper reaches of Hope Beck [NY 1821 2248], close to the Gasgale Thrust Fault.

Skiddaw Anticline

The Skiddaw Anticline is best defined within the sandstones of the Loweswater Formation. It can be traced east-north-eastwards from near White Stones [NY 2530 2736] above Carsleddam and across the flank of Skiddaw Little Man (where the rock becomes progressively more hornfelsed), passing about 200 m north of the summit [NY 2651 2798]. From here it can be traced eastwards across several northerly trending faults, each with a sinistral offset, to cross the hillside near Skiddaw House [NY 2781 2813]. The fold is close to tight, has a gentle south-westerly plunge and an upright axial plane. Its southern limb is overturned in the Kirk Stile Formation on the south face of Carsleddam, and this overturned limb can be traced across the Dodd Fault on to the hillside of Doups [NY 2565 2652] above Millbeck.

Dodd Syncline

The mudstones and siltstones of the Kirk Stile Formation are folded into a closed syncline on Skiddaw Dodd, with the axis of the fold passing just to the north of Dodd summit [NY 2440 2735]. The syncline can be traced up the flank of Long Side to the north of Ullock Pike [2487 2844], but it is offset about 400 m northwards (relative to its trend at Dodd) by a sinistral wrench fault [NY 2434 2787] that runs through the valley between Dodd and Ullock. The S1 cleavage is axial planar to this structure and to the numerous associated minor folds. The intensity of the cleavage fabric is very variable hereabouts; belts of intense slaty cleavage up to 200 m wide alternate with belts of weakly developed, fracture cleavage.

Folds and faults between Great Cockup and Meal Fell

North of the Watch Hill Thrust Fault in the Uldale Fells, the Watch Hill Formation and adjacent strata are folded into a series of tight upright to recumbent folds. Much of the sequence is inverted, for example the strata at Meal Fell [NY 2825 3375]. There is a style similarity and physical continuity with D1 structures elsewhere, but the trend of the bedding, folds, cleavage and many of the faults hereabouts is north-west–south-east, swinging to the more normal north-east–south-west trend only in the area from Great Cockup [NY 2732 3334] westwards. Where the Watch Hill Fault is cut out by later faulting, south of Burn Tod [NY 2845 3251] (Figure 37), area b, the change in strike from north-west to north-east is abrupt across a normal fault. Compared with much of the Skiddaw Group inlier, the attitude and orientation of the folds and cleavage in this area are anomalous. The observed geometry may have been caused by sinistral transpressive deformation of the Skiddaw Group against the adjacent rigid mass of the Carrock Fell Complex and Eycott Volcanic Group.

Later Acadian structures

Post-S1 crenulation cleavages fabrics and related minor folds are common within the Skiddaw inlier. They are very variable in their attitude and intensity, domainal in their distribution (Plate 27), (Plate 28); (Figure 37), and occur in some areas as conjugate sets. Crenulation fabrics are generally absent from the Borrowdale and Eycott Volcanic groups and the Windermere Supergroup. However, a post-S1 crenulation fabric is present both in the Skiddaw Group and in argillaceous rocks at the very base of the Borrowdale Volcanic Group in Borrowdale (Hughes et al., 1993). Elsewhere, crenulation fabrics within the Borrowdale and Eycott Volcanic groups and the Windermere Supergroup are apparently related to local faults.

The difficulty of interpretation of these crenulation fabrics within the Skiddaw Group (in particular, establishing their chronology) is exacerbated by a number of factors including the presence of a locally strong compaction fabric (pp.87, 88) which in some areas closely resembles S1, overlap of the orientations of discrete fabrics, and by their probable large-scale re-orientation within fault blocks.

Roberts (1977b) carried out a detailed structural investigation of the Skiddaw Group in the Blencathra–Mungrisdale area in the north-east of the Skiddaw inlier, and recognised three discrete cleavages (Figure 37), area f. There, the east-north-east trending, southerly dipping S1 regional Acadian cleavage was shown to be crenulated by a gently inclined fabric (with related open, minor fold structures). Both these cleavages were in turn shown to be crenulated by a generally north-trending, upright cleavage fabric.

Roberts’ two crenulation cleavages are common throughout much of the Skiddaw inlier, and the chronology of an earlier (but post-S1), gently inclined crenulation fabric, itself crenulated by a later, upright, north-to north-west-trending fabric also seems to be applicable to much of the inlier. The exception to the rule is in the Loweswater Fells (Figure 37), part of area g, where Webb (1999) noted that the regional S1 Acadian cleavage and the upright, north-west-trending cleavage are mutually exclusive. Webb suggested that these two cleavages may be part of a conjugate set, and if this is so they must be contemporaneous. Alternatively, it is possible that the upright, north-west-trending cleavage in the Loweswater Fells is coaxial, but not cogenetic with the upright, north- to north-west-trending fabric recognised elsewhere in the Skiddaw inlier.

Because of this remaining uncertainty, all the crenulation cleavages are grouped collectively in this account as ‘post-S1’ crenulation cleavage fabrics (Plate 28). However, it is recognised that throughout much of the Skiddaw inlier, the two main post-S1 cleavage fabrics are an earlier, gently inclined crenulation fabric and a later, upright, north- to north-west-trending fabric.

The most widespread post-S1 structure is a subhorizontal to gently inclined crenulation cleavage (Figure 37), areas d, e, for example) that is axial planar to open, gently plunging minor folds (Plate 27). In the Skiddaw inlier, at least some of these gently dipping crenulation fabrics are associated with minor folds related to southerly directed thrust/reverse faults (see for example Roberts, 1992a). This thrusting event may be responsible for the re-orientation of the main S1 cleavage in the area north of the Crummock Water aureole and southerly directed thrust faulting at Watch Hill, Gasgale Crags and Causey Pike (pp.99, 100).

The north to north-westerly trending crenulation cleavage is related to sporadic, small-scale, minor, upright open folds. These structures are strongly developed in the Derwent Water area, and in a belt running north-west from there, parallel to the Lorton Vale and Derwent Water wrench faults (Figure 37), areas c, g, h). This cleavage is much weaker, though still present, in the area to the east of Bassenthwaite Lake but is obscured by the anomalous north-west re-orientation of the S1 cleavage in the area of Meal Fell (Figure 37), area b). Webb (1999) reported that S1 and the upright, north-west-trending cleavage are mutually exclusive in the Loweswater Fells. He suggested that, in that area at least, the two sets of structures may be a conjugate set produced simultaneously by dextral shear along north-west-trending transfer faults. Roberts (1977b) also reported that this cleavage exists as a conjugate set in the Blencathra–Mungrisdale area.

Post-S1 structures are largely absent from the Ullswater, Bampton and Furness inliers, but are common at Black Combe. Here, two discrete deformations of the main S1 fabric can be demonstrated (Helm, 1970; Bell, 1992). Rare upright and open folds of S1, on a variety of scales, plunge gently to the north-west, but are cut by a locally intense crenulation cleavage which strikes approximately north-east and dips steeply towards the north-west. The development of this cleavage seems to have been closely associated with the emplacement of granitic sills, as seen in the Grassgill Beck section [SD 134 877] to [SD 139 874]. The cleavage is axial planar to tight, angular chevron folds which vary in amplitude and wavelength from a few centimetres up to many tens of metres. Despite this intensity of post-D1 deformation in the Black Combe inlier, only the S1 slaty cleavage is seen in the Furness inliers less than 10 km farther to the south-east.

Farther east, within the Cross Fell inlier, a fairly widespread post-S1 crenulation cleavage is axial planar to open, asymmetric folds and a later crenulation cleavage is upright and axial planar to approximately east-west-trending open folds (Burgess and Holliday, 1979; Arthurton and Wadge, 1981). Superimposed on all of these fabrics are later zones of intense folding (but without cleavage) up to 5 m wide, north-north-west-trending, and dipping steeply east. They may possibly be related to north-north-west-trending faults of the Pennine Fault system.

Throughout the Skiddaw and Black Combe inliers there are localised crenulation cleavages that do not fit easily into any of the categories described above and have ambiguous relationships with them. These may represent local re-orientations of the fabrics or their anomalous development in unusual and probably fault-influenced stress regimes. Alternatively, they may have arisen as conjugate developments of the more widespread fabrics, or may even be significantly younger and entirely post-Acadian. Conjugate cleavages with kink-band geometry are present within a few metres of the contact with the Borrowdale Volcanic Group on Narrow Moor as at [NY 2350 1729], [NY 2379 1726] (Hughes, 1994).

Key structures and sections

Buttermere Church

This accessible exposure, next to the road at Buttermere Church [NY 1765 1717], shows the interaction of the S1 and post-S1 cleavages. Steeply plunging F1 folds with an axial planar S1 cleavage (variably slaty and crenulation as the well-developed compaction fabric is cut) are exposed on the polished surface of a roche moutonnée. On an adjacent vertical face next to the road, the steeply dipping beds and cleavage are seen to be refolded by open recumbent folds with a weak, subhorizontal and axial planar, post-S1 crenulation cleavage (Webb, 1975; Moseley, 1992).

Ladyside Pike and Hopegill Head

Steeply dipping Kirk Stile Formation sandstones and mudstones crop out on the southern limb of the Scawgill Anticline between Ladyside Pike and Hopegill Head [NY 186 223] (Cooper, 1990b). Complex slump folds are deformed by tectonic folds with an upright S1 cleavage, axial planar between steep limbs. These are cut across by a well-developed, shallow dipping, post-S1 crenulation cleavage. The intersections of the various planes cause intense fragmentation of the rock hereabouts. Slabs of Kirk Stile Formation on Hopegill Head [NY 1857 2225] have a well-developed, cross-cutting crenulation fabric. Its intersection with bedding surfaces produces a steeply plunging, north-west-trending lineation.

Barf

The structure here is clearly recognisable in the sandstone beds of the Loweswater Formation. The strata are steeply dipping and form the southern limb of the F1 Scawgill Anticline, but they are also refolded and overturned to the south-east into a recumbent post-F1 anticline, folded about a gently inclined, northerly dipping axial planar crenulation cleavage. On the upper parts of Barf above the standing stone called ‘The Bishop’ [NY 2180 2646] and below Slape Crags [NY 2157 2669], the face of the hill, in the nose of the recumbent anticline, has suffered some recent movement along the bedding planes. Associated collapse and tightening of the fold structures has produced voids in the hinges of some of the recumbent folds. Zones of deformation to the north of Barf itself [NY 2172 2714] contain a strong local development of a gently inclined, post-S1 fracture cleavage.

Mungrisdale

A crenulation cleavage is widespread in Kirk Stile Formation siltstones on the northern spur of Souther Fell [NY 360 300], south-west of Mungrisdale. The fabric dips moderately to the south, and locally is axial planar to small overturned folds. The crenulation fabric cuts across upright, tight F1 folds (with wavelengths of several hundred metres) and an associated, steeply dipping slaty cleavage.

Timing and causes of Acadian deformation

The multiple episodes of deformation affecting the Skiddaw Group suggest that the Acadian event was protracted, perhaps spanning several millions of years. The final closure of the Iapetus Ocean and the overthrusting of Avalonia by Laurentia proceeded from late Llandovery times onwards (see for example Soper and Woodcock, 1990), but the first penetrative deformation of the Skiddaw Group did not occur until very much later.

In the Lake District Lower Palaeozoic inlier, the regional cleavage (S1) affects all rocks from the possibly Cambrian and Tremadoc parts of the Skiddaw Group up to the Přídolí beds at the top of the Windermere Supergroup.

The Přídolí ranges from 419 to 417 Ma according to Tucker and McKerrow (1995), so the Acadian event must be 417 Ma or younger in age. The regional cleavage predates the Skiddaw granite dated at 399 ± 8 Ma (Rb–Sr Rundle, 1981) and 392 ± 4 Ma (K–Ar, Shepherd et al., 1976) (Figure 35), though a later crenulation cleavage postdates intrusion of the granite (Soper and Roberts, 1971). Formation of the regional slaty cleavage has also been demonstrated as broadly synchronous with intrusion of the Shap granite (Soper and Kneller, 1990) dated at 390 ± 6 Ma (Pidgeon and Aftalion, 1978). This body of evidence was used by Soper et al. (1987) to confirm that the main Acadian slaty cleavage was developed during the Emsian Stage of the early Devonian. Further evidence has been obtained from the Skiddaw inlier, where the Crummock Water aureole, dated at 401 ± 3 Ma (Rb–Sr, Cooper et al., 1988) post-dates the S1 cleavage (Hughes et al., 1993). This suggests that the age of the regional slaty cleavage formation may have been a little earlier than Emsian.

The main Acadian cleavage has been directly dated farther south in the Craven (Ribblesdale) inliers, where metamorphic white mica from a Ludlow bentonite gave ages of 397 ± 7 Ma and 418 ± 3 Ma (K–Ar and Ar–Ar respectively, Merriman et al., 1995). By comparison with the timescale of Tucker and McKerrow (1995), the evidence again suggests that Acadian deformation commenced a little earlier than the Emsian.

Soper et al. (1992b) suggested that the deformation event was caused by the initial impingement of Armorica–Iberia (another rifted Gondwanan continental fragment) on to the southern margin of eastern Avalonia. The main Acadian cleavage (S1 ) forms a regional arc, apparent in the Skiddaw Group as a swing in strike from north-east–south-west in the west to east–west in the eastern part of the Lake District (and north-west–south-east in the Cross Fell inlier), but more fully developed in the Windermere Supergroup outcrop. This is believed to result from transpressive deformation at an irregular collision margin (Soper et al., 1987), with a basement block moving from the south as a rigid indenter and creating the cleavage arc around its northern margin.

Faulting

Major thrust faults

Stratigraphical evidence confirms the presence of a network of reverse/thrust faults across the main Skiddaw inlier (Figure 34), (Figure 36), possibly linked at depth to the structures underlying the Southern Borrowdales lineament. Strictly speaking, for the most part these faults are reverse faults at the surface, but are probably listric and become thrust faults at depth. It is likely that the thrusts were initiated during the 420 to 430 Ma interval when the Rb–Sr isotopic systems were reset (p.92). Further movement occurred during the Acadian event around 395 to 400 Ma.

Watch Hill Thrust

The Watch Hill Thrust (Figure 34), (Figure 36) extends across the north of the Skiddaw Group inlier and has carried the oldest strata (Bitter Beck Formation) southwards over the younger Kirk Stile Formation. It has an east-north-east trend and dips north at around 30°. From the geometry of the beds on either side of the fault, and the thickness of the sequence that is cut by it, a southerly translation of at least 4 to 5 km seems likely. In the west the Watch Hill Thrust is offset by the Lorton Vale Fault (discussed further below); south of Burn Tod [NY 2845 3251], the Watch Hill Thrust is offset by later normal faulting.

Loweswater Thrust

Above Loweswater [NY 136 218], the Loweswater Thrust (Figure 34), (Figure 36) is represented by a narrow zone, about 5 m thick, in which sandstone beds are sheared and boudinaged. No discrete thrust plane is exposed, but the zone dips gently to the north-west and has translated the Loweswater Formation towards the south-east, over the Kirk Stile Formation. The Loweswater Anticline lies in the hanging wall of the thrust and its axial plane, which dips to the north-west more steeply than the thrust plane, must converge with the latter at depth. The Loweswater Thrust can be traced for about 10 km to the north-east towards Ling Fell [NY 179 285]. There, the Sale Fell anticline affects the Loweswater Formation strata in the hanging wall, with Kirk Stile Formation strata again in the footwall.

Gasgale Fault

Along Gasgale Crags [NY 175 218], the Loweswater Formation and the lowest part of the Kirk Stile Formation are thrust southwards over younger beds of the Kirk Stile Formation. Along the path that leads up to Whiteside End [NY 1690 2159] the position of the east-west-trending Gasgale Fault can be inferred to within a few metres and the reverse movement on the fault is here interpreted to be about 250 m southwards (Cooper, 1992). North of the thrust fault, minor upright F1 folds have a congruous axial planar S1 cleavage, which rotates progressively towards the thrust plane to dip eventually northwards, parallel to the fault, at about 45°. In the same area, the main S1 cleavage is crenulated by a gently dipping fabric with associated minor recumbent folds.

The Gasgale Fault (Figure 34), (Figure 36) terminates in the west at the Lorton Vale Fault, and in the east at another north-trending fault that runs along Bassenthwaite Lake. These structures are believed to be thrust transfer faults (Webb, 1999).

Causey Pike Fault

The Northern Fells and Central Fells belts of the Skiddaw inlier are separated by the Causey Pike Fault ((Figure 34), (Figure 36)) and the coincident Crummock Water aureole (Figure 1), (Figure 18), (Figure 36). This fault marks a major stratigraphical junction with at least 2 km vertical downthrow to the south. It can be traced across the full width of the Skiddaw inlier and reappears farther east in the Cross Fell inlier (Cooper and Molyneux, 1990). The aureole is recognised only within the Skiddaw inlier, where it has an elongate shape some 15 km long and less than 3 km wide. It is associated with a marked linear gravity anomaly, interpreted as arising from a subsurface linear granitic body, the ‘Grasmoor granite’ (Cooper et al., 1988). The aureole has been dated at 401 ± 3 Ma (Rb–Sr, Cooper et al., 1988), and postdates the S1 cleavage. The Crummock Water aureole is believed to have formed above a large granitic body. The linear geometry of the geophysical anomaly and the metamorphic aureole suggest that the granite may have been emplaced originally into a shear zone. Evidence for the direction of movement along this zone is scant, but the slight deflection of crenulation cleavage orientation in the southern part of Blencathra [NY 300 250] to [NY 360 280] and north of Grasmoor [NY 175 215] (Figure 37), areas f and g) hints at a sinistral component of movement. The fault was certainly subject to southerly directed reverse and thrust faulting that postdates the Crummock Water aureole because the top of Causey Pike is formed of resistant, hard hornfels thrust over the softer, sheared mudstones and sandstones of the upper part of the Robinson Member (Figure 38).

Southern Borrowdales lineament

The junction between the Central and Southern Fells belts is marked by the Southern Borrowdales lineament (Lee, 1989) (Figure 1). This appears to have been a long-lived structure, active during Borrowdale volcanism and Acadian tectonism, when it was associated with the formation of the Westmorland Monocline (Kneller and Bell, 1993). The Skiddaw Group is exposed in the Black Combe inlier at the western end of this lineament. Bell (1992) speculated that its presence was related to south-directed thrusting up a blind footwall ramp in the hinge zone of the Westmorland Monocline. Exposures of the Skiddaw Group on the northern side of Black Combe are intensely cleaved, sheared and metasomatised with quartz-tourmaline mineralisation (Johnson, 1992). On the south side of Black Combe the Skiddaw Group siltstones are also affected by the regional S1 cleavage and subsequent crenulation cleavages, in a style similar to that seen in the Skiddaw Group farther north in the main inlier. As discussed previously, Bell (1992) regarded the F1 reclined folds in the Black Combe inlier as sheath folds indicative of thrust movement towards the south-east within a broad shear zone.

Wrench and normal faults

In the Skiddaw inlier are several major northerly trending wrench fault systems that appear to have had a long history of movement (Figure 36); some behaved as transfer faults during Acadian thrusting. The most important of these are the Derwent Water Fault [NY 2680 2000], the Dodd Fault [NY 2492 2740] and the northward extension of the Coniston Fault [NY 3142 2276]. Faulting with a north- to north-westerly trend is also developed along the Vale of Lorton [NY 1590 2140], extending northwards to Cockermouth [NY 1390 2900], but because of extensive drift cover these faults are largely inferred from stratigraphical displacements. The Carrock End Fault [NY 357 325], which partly controls the north-east margin of the Skiddaw inlier, may also be part of this suite.

All of these faults form part of an arcuate suite which trends north-north-east through the Windermere Supergroup, curves to a more northerly trend in the Borrowdale Volcanic Group and then to a north-north-west trend as it passes into the Skiddaw Group. The component faults are commonly associated with mineralisation, notably either lead (for example the veins at Thornthwaite Mine [NY 2231 2579]), or baryte (for example along the Dodd Fault [NY 2351 2854]; [NY 2400 2828] and [NY 2557 2691]). The Dodd Fault has several subparallel branches and across it is a left-lateral displacement of about 400 m; the fault zone is intensively quartz-veined in places, especially at White Stones [NY 2532 2719].

Normal faults occur throughout the main inlier with two main trends, east-north-east and north-north-west. The east-north-east-trending normal faults are responsible for the repetition of the stratigraphy across the Scawgill Anticline and for minor displacements of the Darling Fell fold structures. Faults belonging to this set, but with a more south-westerly trend, repeat the sequence along Ullock Pike [NY 2404 2939]. In the north of the inlier, a major normal fault at Over Water [NY 2476 3492] juxtaposes the Kirk Stile and Hope Beck formations suggesting a downthrow to the north of around 600 m. A normal fault with an apparently large southerly throw lies to the south of Burn Tod [NY 2853 3258] and Knott [NY 3020 3283] where it juxtaposes the Kirk Stile and Bitter Beck formations. However, because this fault appears to truncate the leading edge of the Watch Hill Thrust Fault [NY 2715 3270], it has a much smaller throw than would be estimated solely from the juxtaposed stratigraphy.

Normal faults with a north-north-west trend are sub-parallel to, and appear to be partly related to, the major wrench faults. The throw on these faults ranges up to many hundreds of metres and they are commonly associated with mineralisation, especially those that are close to the wrench faults. Some of the faults are likely to have long histories of movement extending into Carboniferous times or later, because when traced laterally, many of them continue to cut Carboniferous strata. On the regional scale, they are probably associated in this respect with the major late Palaeozoic basin boundary structures such as the Lake District and Vale of Eden boundary faults.

Chapter 6 Metamorphism

The Skiddaw Group displays a complex pattern of very low grade (late diagenetic to epizonal) metamorphism accompanied by contact metamorphism around granitic intrusions (Figure 39). Late diagenetic grades are found in relatively soft mudstones, many with only a single poorly developed cleavage, which make up the Tarn Moor Formation (Llanvirn) and Bitter Beck Formation (Tremadoc). Fortey et al. (1993) considered that the late diagenetic rocks preserve syndepositional burial metamorphism augmented by effects of burial beneath the Borrowdale Volcanic Group. In other parts of the outcrop, anchizonal to epizonal regional metamorphism occurs in mudstones which are cut by up to three sets of Acadian cleavage. However, much of the Buttermere Fells area, containing anchizonal to epizonal metamorphism, is also believed to be underlain, at depths between 4 and 6 km, by granitic rocks associated spatially with the Lake District batholith (Lee, 1986). Their precise age is uncertain (they may conceivably be late Ordovician or early Devonian) and some question remains as to the degree of influence these intrusions exerted on the regional metamorphic pattern in the Skiddaw Group (Fortey, 1989; Fortey et al., 1993).

In addition to the concealed granite plutons, igneous bodies within and adjacent to the Skiddaw Group include granites and suites of minor intrusions broadly associated with formation of the late Ordovician Borrowdale Volcanic Group (Fortey et al., 1994; Hughes et al., 1996), together with granites emplaced during the early Devonian (Acadian) orogenic event. A narrow contact metamorphic aureole occurs in Skiddaw Group rocks adjacent to the late Ordovician Ennerdale intrusion in the main inlier (Clark, 1963). A 4 km-wide zone of pale, spotted slate forming the northern part of the Black Combe Skiddaw Group inlier is also interpreted as a low-grade contact metamorphic aureole (Bell, 1992) that is also developed in Borrowdale Volcanic Group country rocks adjacent to the late Ordovician Eskdale granodiorite. Acadian contact metamorphism is represented by the zoned hornfels aureole around the Skiddaw granite (Eastwood et al., 1968) and the Crummock Water aureole (Jeans, 1973; Cooper et al., 1988), an extensive zone of pale spotted slate overlying a concealed granite, the ‘Grasmoor granite’ (Lee, 1986).

Burial and regional metamorphism

The regional-scale variation of grade in the Skiddaw Group outwith the evident contact aureoles has been determined by examining the mineralogy and white mica (illite) crystallinity of very fine grained (< 2 µm) fractions separated from Skiddaw Group mudstones and siltstones.

Thomas (1986; also see Oliver et al., 1984) employed the Weber index of white mica crystallinity (Weber, 1967) to study the pattern and extent of this variation. Later, Fortey (1989) and Fortey et al. (1993) confirmed the general pattern observed by Thomas in an investigation which employed a somewhat higher sampling density (one sample to between 1 and 2 km2) and measured white mica crystallinity according to the Kubler Index (KI), which is the full-width at half mean height ( Δ2 Θ) of the white mica 10 Å (00l) XRD peak (Kubler, 1968; Frey, 1987) measured under standard experimental conditions as set out by Roberts et al. (1991). KI decreases with the growth of white mica during metamorphism (Merriman et al., 1990), providing a measure of grade in sub-greenschist facies pelitic rocks. However, the KI may also be lowered by strain-induced crystal growth (Roberts and Merriman, 1985; Roberts et al., 1991) or increased by stress-induced disaggregation of the crystallites (Roberts et al., 1990). In addition, the presence of interstratified paragonite, as reported in many Skiddaw Group mudstones (Fortey, 1989), creates a shoulder in the XRD trace between about 9.6 and 9.7 Å which broadens the 10 Å muscovite XRD peak, resulting in over-estimation of KI and loss in precision of grade determinations (Frey, 1969). Nevertheless, from the samples judged not to contain significant interstratified paragonite (on the basis of the shape of the 10 Å peak), it is clear that the range of metamorphic grade across the Skiddaw Group outcrop is from late-diagenetic to anchizonal with epizonal (approximately lowest greenschist) grades seen locally.

Skiddaw inlier and Ullswater and Bampton inliers

The principal areas of anchizonal grade rocks in the Skiddaw inlier include the Arenig strata north and south of the Causey Pike Fault in the Buttermere and Lorton fells, and also a zone which extends around the margin of the Skiddaw massif (Figure 39). In these areas, the < 2 µm fractions of silty mudstone and siltstone are dominated by 2M white mica, chlorite and quartz, together with accessory rutile and zircon. XRD patterns indicate that in many samples the white mica is interstratified muscovite-paragonite in which the potassic member is dominant. However, from examination of the shape of 10 Å peaks, interstratified paragonite was judged to be absent from a significant number of samples which therefore provide an accurate indication of grade. This is particularly true of samples from the Loweswater Formation (Fortey, 1989). In all the apparently paragonite-free rocks, relatively strong 14 Å and weak 7 Å peaks suggest the presence of an iron-rich variety of chlorite.

Optical and backscattered electron microscope observations of thin sections of interlayered siltstone-mudstone of anchizonal grade confirm the characteristic presence in the siltstone component of a bedding-parallel fabric; it is cut by an oblique cleavage that is more strongly developed in the accompanying mudstone layers. Both these fabrics are cut by opaque iron-oxide seams developed along a later, close-spaced fracture cleavage ((Plate 29)a). Ovoid bodies of intergrown chlorite and white mica (chlorite-mica stacks) up to 0.5 mm wide occur extensively, locally forming greater than 50 per cent of individual siltstone and fine-grained sandstone layers that are less than a millimetre thick ((Plate 29)b). Milodowski and Zalasiewicz (1991) concluded that similar chlorite-mica stacks, widespread in pelitic rocks in the Welsh Basin, were formed by diagenetic alteration of detrital grains of biotite and other mafic silicates, possibly of volcanic origin. Skiddaw Group siltstones also contain siliceous spheroids less than 0.5 mm wide, of probable biogenic origin. Oblate, siliceous nodules with cone-in-cone structure, some greater than 0.1 m in diameter, are widespread in the Kirk Stile Formation and have been described by Jeans (1973).

Variation in KI measurements from the anchizonal areas, for instance around Hope Gill [NY 175 230], Low Fell [NY 136 229] and Embleton Common [NY 165 275], is such that upper anchizonal to epizonal values were obtained from rocks of the Loweswater Formation whereas low to middle anchizonal values were obtained from the Kirk Stile Formation (Fortey, 1989). Epizonal grades were observed in the mudstone tops to turbidite sandstone units in the Loweswater Formation at Aiken Beck [NY 190 262]. Although in the Kirk Stile Formation the variation is undoubtedly influenced by the presence of interstratified paragonite, results for nonparagonitic samples from this formation indicate that grade is lower than in the underlying Loweswater Formation. This contrast is possibly related to stratigraphy, but may also be influenced by differences in intensity of deformation. The pattern in the Loweswater–Buttermere area was interpreted by Fortey et al. (1993) as being related to folding and south-east-directed late Acadian thrusting (Hughes et al., 1993). The highest grades (upper anchizonal to epizonal) were observed in anticlinal structures overlying the Loweswater, Gasgale Gill and Causey Pike thrusts, whereas lower grades (late diagenetic to lower anchizonal) were observed in rocks underlying the thrusts.

The areas of late-diagenetic and lowest anchizonal rocks include a broad zone extending from around Watch Hill as at [NY 166 314] to the north-west margin of the Skiddaw Group outcrop in the main inlier. Such rocks also occupy a small area around the River Calder in the western extreme of the main inlier for example [NY 074 126] and a broad swathe that includes the south-eastern part of the main inlier and much of the Ullswater and Bampton inliers. These areas correspond approximately with the area where Lee (1986) thought that outlying portions of the Lake District batholith were deeply buried (>8 km depth) or absent. However, in the Bampton inlier late diagenetic rocks overlie granite at depths of less than 2 km (Lee, 1986), possibly a mid-Devonian body related to the nearby Shap granite.

The areas of late diagenetic rocks include both the oldest (Bitter Beck Formation) and youngest (Tarn Moor Formation) parts of the Skiddaw Group. Arenig strata are also affected, for instance at Troutbeck [NY 385 271]. In the late diagenetic rocks, < 2 µm fractions contain 1M muscovite, chlorite and minor quartz. There is little evidence from the XRD patterns of paragonite. A steeply inclined cleavage is present locally ((Plate 29)c), but in general it is weak or absent, indicating only limited tectonic deformation. Fortey et al. (1993) proposed that the late diagenetic grades around Watch Hill represent the accumulated effect of prolonged (Arenig to late Silurian) burial. During this period, syndepositional burial was followed by uplift and erosion that preceded further burial of the Skiddaw Group beneath the Borrowdale and Eycott volcanic groups. The late diagenetic grades of the Llanvirn Tarn Moor Formation may have developed during this second, mid-Ordovician subvolcanic burial.

Minute spheroids are common in weakly deformed late diagenetic mudstone of the Tarn Moor Formation at Mosedale Beck as at [NY 355 232] ((Plate 29)d). They appear in thin section to be formed of a carbonate mineral. In addition, a sample of Arenig sandstone from the Troutbeck stream section (sample (E71362): [NY 3844 2705]) preserves comparably minute (about 100 µm diameter) concentrically zoned carbonate spheroids associated with areas of apparently undeformed, grain-supporting calcite cement. Carbonaceous spheroids that occur in strongly deformed beds of the Manx Group (Isle of Man) have been identified as acritarchs (Power and Barnes, 1999).

A further feature recorded in the Tarn Moor Formation is the presence, in a minority of the samples, of pyrophyllite or, rarely, kaolinite. The former is more typically formed at anchizonal grades, and so its presence appears to be anomalous. Fractures cemented by pyrophyllite are present in Skiddaw Group mudstones at Mosedale Beck and Aik Beck [NY 473 224], both close to the base of the Borrowdale Volcanic Group, suggesting that they may represent geothermal alteration during the Caradoc volcanism.

Cross Fell and Teesdale inliers

Strata correlative with parts of the Skiddaw Group of the main inlier occur in the Cross Fell inlier at the western boundary of the Alston Block (Burgess and Wadge, 1974; Burgess and Holliday, 1979; Arthurton and Wadge, 1981; Cooper and Molyneux, 1990); the mineralogical characteristics are similar. Metamorphic grade determined by white mica crystallinity ranges from middle to upper anchizonal in Arenig siltstone and silty mudstone in the north-western (Catterpallot Formation) and south-eastern (Murton Formation) parts of the inlier (Figure 39)c. The anchizonal Arenig rocks of the Catterpallot Formation are juxtaposed with late diagenetic mudstones forming the upper part of the early to mid-Llanvirn Kirkland Formation across the Causey Pike Fault (locally known as the Catterpallot Fault). Kirkland Formation mudstones also occur in the extreme south-east of the inlier.

Complex, mostly post-Acadian faults restrict the outcrop continuity but, in its broad metamorphic characteristics, the Cross Fell inlier appears to extend the pattern established farther west in the main Skiddaw inlier.

Llanvirn mudstones of the Teesdale inlier, overlying the concealed Weardale granite (Dunham et al., 1965), are assigned to the Kirkland Formation. No measurements of metamorphic grade are available from them, although Burgess and Holliday (1979) record the presence of cleavage.

Black Combe and Low Furness inliers

The Skiddaw Group rocks of the Black Combe inlier (Figure 39)d comprise a northern zone of pale, spotted, contact metamorphosed rocks (corresponding with the Town End Slate and Fellside Mudstones of Helm, 1970) and a southern zone, lacking evidence of contact metamorphism, comprising the southern flank of the Black Combe massif and the area south-east of the Whicham valley (corresponding with the Whicham Blue Slates of Helm, 1970). Mudstone samples from the southern zone yielded mid- to upper anchizonal KI values, and are thus interpreted to represent anchizonal regional metamorphism. The inlier lies within the south-east-facing Westmorland Monocline, and the regional metamorphism is probably of Acadian age.

The small, poorly exposed, Low Furness inlier (including the Greenscoe area) contains dark grey Llanvirn Skiddaw Group mudstones of late diagenetic grade. The fractions less than 2 µm contain 1 µmmica and chlorite, with minor quartz; pyrophyllite and kaolinite have also been identified. They thus resemble the Tarn Moor and Kirkland formations, greatly extending the regional extent of late-diagenetic Llanvirn mudstones in north-west England. They are cut by andesitic intrusions and agglomerate necks assigned to the Borrowdale Volcanic Group. Soper (1970) described a single, steeply inclined, south-south-west-striking, closely spaced strain-slip cleavage that cuts the Skiddaw Group mudstones, the volcanic rocks, and Ashgill to Přídolí strata of the unconformably overlying Windermere Supergroup. In the latter, grade ranges from mid-anchizonal (for example Ashgill strata immediately overlying the Low Furness inlier) to epizonal. The grade of the Skiddaw Group rocks is therefore lower than that of the overlying strata, yet there is no evidence of a tectonic mechanism to explain this inversion. Johnson et al. (2001) propose that during their some 70 Ma of pre-Acadian history the Skiddaw Group rocks became lithified and dewatered during burial metamorphism. As a result, although they responded to Acadian tectonic stress by developing a cleavage fabric, prograde mineral reactions were slow, hindered by relatively high re-activation thresholds, with the result that the late-diagenetic burial mineralogy was preserved.

Contact metamorphism

In the Skiddaw inlier, extensive areas of contact metamorphism are present, surrounding the outcrop of the Skiddaw granite and, in the Crummock Water area, over an inferred concealed granitic intrusion whose presence is supported by gravimetric readings (Lee, 1986; Cooper et al., 1988). There are, in addition, more limited aureoles in proximity to the Ennerdale intrusion (Clark, 1963; Hughes and Fettes, 1994) and to minor intrusions (Fortey et al., 1994). These intrusions are not described in detail here, but their important characteristics (and those of other intrusions) are summarised in (Table 6).

Contact metamorphism around the Skiddaw granite

Small outcrops in valley bottoms within the Skiddaw massif expose the roof of an approximately circular cupola of mid-Devonian biotite-granite, the Skiddaw granite, dated as 399 ± 8 Ma by Rundle (1981) by recalculation of K–Ar mineral ages given by Shepherd et al. (1976). The Skiddaw Group forms the host rock, except on the northern flank of the granite where the host rocks are members of the gabbroic Carrock Fell complex, of probable late Arenig age (468 ± 10 Ma, K–Ar dating of hornblende: Rundle, 1979). Skiddaw Group sandstones, siltstones and silty mudstones display a concentric zoned pattern of contact metamorphism extending some 3 km from the principal granite outcrop in the River Caldew [NY 313 313]. An inner zone of biotite–cordierite–andalusite hornfels is surrounded by an outer zone of andalusite (chiastolite)-hornfels. In the inner zone, cordierite and andalusite crystals attain 2 mm or more in diameter, but neither staurolite nor sillimanite has been reported. Eastwood et al. (1968) recorded garnet in metasomatised hornfels at Grainsgill [NY 316 330], and suggested that it formed during an early phase of contact metamorphism related to emplacement of the Carrock Fell gabbro.

The outer, chiastolite-slate zone is characterised by swarms of prismatic andalusite porphyroblasts in dark grey slightly hornfelsed pelite. Soper and Roberts (1971) observed that these cut across and are to some extent synchronous with the steep, north-east-trending, S1 Acadian slaty cleavage, but are themselves deformed on the low-angle S2 cleavage. Besides providing a lower limit of 399 ± 8 Ma to the age of S1, this relationship also demonstrates that S2 is appreciably younger than S1, separated by the granite emplacement. Eastwood et al. (1968) recorded the presence of chloritoid in samples (for example (E14985)) from the outer zone [NY 282 317], and of garnet associated with this chloritoid and with biotite. Garnet crystals have also been recorded more recently in fine-grained pelitic, muscovite-chlorite altered hornfels at Knott ((E51860) and (E71372): [NY 296 331]).

The Grainsgill outcrop of the Skiddaw granite has undergone strong griesen-like alteration, principally at Carrock Fell Mine [NY 323 330] in the vicinity of a set of steep, north–south-trending, quartz-dominated, hydrothermal veins with polymetallic (high in W-As) mineralisation (Shepherd et al., 1976; Ball et al., 1985). The alteration extends into the adjacent Skiddaw Group rocks as widespread muscovite and chlorite formed at the expense of biotite, with pinitisation (white mica alteration) of cordierite and andalusite. Tourmaline is present in the most strongly altered rocks (Eastwood et al., 1968), and tourmalinised hornfels forms selvages to steep quartz-tourmaline veins ((E71373); (E71374)) at Snab [NY 305 311]. Roberts (1983) recorded loss of Na and K from the altered granite and retrogressive K-metasomatism in hornfels up to about 200 m from the granite. Cooper and Bradley (1990) recorded a progressive decrease in ammonium concentrations in the Skiddaw Group within the inner hornfels zone, as the granite is approached, from values near 1000 ppm NH4+ in the outer hornfels zone and in unhornfelsed rocks, to values near 250 ppm, comparable with that in the altered granite. The unaltered granite itself contains less than 30 ppm NH4+, suggesting loss of volatile components from the hornfels and a degree of concomitant enrichment of the granite during the hydrothermal process. D C Cooper (oral communication, 1990) concluded from a geochemical analysis of samples from a north-west–south-east traverse across the aureole and granite, that minor enhancement of As could be detected in the altered Skiddaw Group rocks.

Crummock Water aureole

An elongated area of pale grey (bleached), pervasively spotted slates, some 24 by 3 km in area, extending approximately westwards from Causey Pike [NY 218 209] to near Croasdale Beck [NY 096 177] (Figure 39), is known as the ‘Crummock Water Aureole’ (Cooper et al., 1988). Though formerly regarded as a distinctive lithostratigraphical unit, the ‘Blake Fell Mudstone’ of Dixon (70–71 in Geological Survey of Great Britain, 1925), it has been re-interpreted as a zone of low-grade contact metamorphism (Eastwood et al., 1931; Jeans, 1973; Cooper et al., 1988) within which Skiddaw Group rocks of the Kirk Stile Formation are indurated and altered, with loss of the finely divided dark pigment. Pelitic rocks in this zone are characterised by chloritic spots less than a millimetre across ((Plate 29)e). In thin section, many of the spotted metapelites display a felted (lepidoblastic) fabric, generally bedding-parallel, of white mica flakes accompanied by quartz, chlorite, rutile and albite. Cleavage is generally absent, but exceptionally a weak, pre-hornfels, cross-cutting crenulation fabric and a weak, post-hornfels fracture cleavage may be present. Some of the XRD traces obtained from whole-rock less than 2 µm fractions (Fortey, 1989) gave epizonal KI values, but in others the shape of the 10 Å peak indicates the presence of paragonite, both interstratified with muscovite and as discrete crystallites. In extreme cases, the 9.6 to 9.7 Å shoulder is comparable with the 10 Å peak itself, indicating a paragonite content comparable with that of the potassic mica. Fine-grained biotite is present locally in the matrix of the rocks, but does not define a distinctive biotite-grade zone within the aureole (Jeans, 1973; Fortey, 1989). The spots consist mostly of finely intergrown chlorite and white mica. Unaltered chiastolite was reported from Blake Fell [NY 109 197] by Eastwood et al. (1931), and derivation of chlorite-mica spots by alteration of andalusite in the Crummock Water area was inferred by Jeans (1973). Cordierite has not been reported.

Cooper et al. (1988) concluded that development of the aureole involved retrogressive hydrothermal alteration of originally biotite-andalusite-bearing hornfels. They demonstrated that the altered rocks display substantial gains in As, B, K and Rb, with losses of Cl, Ni, S, Zn, H2O and C (ammonium was not analysed). There is also evidence of smaller, more localised depletion in Cu, Fe, Li and Mn, and gains in Ca, F and Si, whilst Co, Pb and the REEs show evidence of at least local redistribution. Many of the chalcophile elements show evidence of initial widespread depletion and subsequent local enrichment.

The aureole is traversed by a series of steep fractures, with local brecciation, which are sealed by quartz veins that also contain white mica, chlorite, tourmaline, accessory rutile and, rarely, albite. Individual fractures, typically varying from hairlines up to 1 to 2 cm thick, are in places joined by ladder-like sets of short, subhorizontal, lateral veins. A major set of such fractures, trending approximately east–west, some greater than 1 km in length, is accompanied by a weakly developed north–south-trending set. At exposure, the veins are accompanied by wallrock alteration zones of hard, fine-grained, black quartz-tourmaline rock which create vein-like sheets up to 1 m thick. Slender apophyses of tourmalinite may be seen to extend laterally along bedding for several metres. Good examples occur east of Crummock Water around Rannerdale [NY 166 186], on Melbreak [NY 148 186] and farther west around Croasdale Beck [NY 097 177]. The tourmalinite consists of blue-green zoned, dravitic tourmaline crystals embedded in quartz, with accessory amounts of rutile, zircon and albite. Fine banding preserves the bedding of the original metasedimentary rock ((Plate 29)f), indicating their replacement origin, but the original phyllosilicate minerals have been completely destroyed, with addition of B and removal of many elements including La and Ce (Fortey and Cooper, 1986). The presence of rutile in fractures indicates at least millimetre-scale transport of Ti, but Zr appears to have remained almost uniquely immobile.

Gravimetric traverses indicated a concealed, elongated, high-level intrusion of granitic rock, the ‘Grasmoor granite’, along the trace of the Causey Pike Fault at the southern margin of the aureole; its upper surface lies between 1.5 and 0.5 km below the present land surface (Cooper et al., 1988). Dating of indurated pelite from the aureole by whole-rock Rb–Sr (Cooper et al., 1988) gave an estimated Acadian age of 395 ± 12 Ma, within the error range of the Skiddaw granite (Rundle, 1981). This is regarded as a good estimate of the age of the concealed granite, even though strictly it refers to the metasomatic overprint. The bleaching and veining postdate S1 structures but predate S2. Tourmalinisation is attributable to release of a B-rich volatile fraction from the granite when fractures developed in the overlying hornfels carapace. The source of the boron may be from fractionation of the granite itself, but hydrothermal extraction from the Skiddaw Group host rocks, which are generally B-enriched (Cooper et al., 1988; British Geological Survey, 1992), is a viable alternative. The location of the gravity anomaly under the southern margin of the aureole has been attributed to post-hornfels displacement on the Causey Pike Fault, here expressed as a thrust that marks the southern boundary of the aureole (the Causey Pike Thrust). Strongly spotted hornfels (E71329) occurs in the immediate hanging wall to the thrust at the summit of Causey Pike itself [NY 218 209].

Contact metamorphism related to other intrusions in the Skiddaw inlier

The Crummock Water aureole is distinct and separate from the zone of contact metamorphism that surrounds the Ennerdale intrusion (Eastwood et al., 1931; Clark, 1963), and attains a width of at least 2 km in Borrowdale Volcanic Group rocks south of Ennerdale itself. The indurated Skiddaw Group rocks in this aureole were described by Eastwood et al. (1931) as silicified, with abundant albite and chlorite, the latter forming spots locally. Minor constituents include tourmaline, rutile and biotite.

Baking and bleaching also occur around the margin of the Threlkeld–St John’s microgranite. Induration was noted by Fortey et al. (1994) adjacent to members of the Scawgill Bridge group of microdioritic to meladioritic (appinitic) minor intrusions and also around the Embleton diorite and associated intrusions. Few minor intrusions or instances of contact metamorphism are described from the Ullswater and Bampton inliers.

Cross Fell inlier

Skiddaw Group rocks of the Cross Fell inlier are crossed by sets of doleritic, microgranitic and lamprophyric minor intrusions. Arthurton and Wadge (1981) recorded pale, baked rocks close to these intrusions, with development of chloritic spots close to dolerite bodies. They also noted weakly spotted Arenig mudstone and siltstone in the extreme northern and north-eastern part of the inlier, which they considered lay within the aureole of the buried mid-Devonian Weardale granite (Dunham et al., 1965). Thermal effects attributed to cooling of the Weardale granite were also detected by increased vitrinite reflectance in Lower Carboniferous shales in the cover rocks (Creaney, 1980), emphasising the long duration of the thermal metamorphic regime.

Black Combe and Furness inliers

In the Black Combe inlier, the pale and spotted mudstones formerly designated as the Town End Slate and Fellside Mudstones (Helm, 1970) lie within a zone of low-grade contact metamorphism and bleaching some 4 km wide (Bell, 1992). The rocks display lepidoblastic white mica fabrics accompanied by swarms of chloritic spots less than a millimetre across, and thus resemble the rocks in the Crummock Water aureole. Tourmalinite veining is present, for instance at Anna Crag [SD 143 865] and Grey Stones [SD 160 876]. At some localities for example [SD 163 890]; [SD 174 875] the elongated chloritic spots occur aligned along a strong cleavage almost perpendicular to bedding, indicating tectonic deformation of originally more spherical spots. The north-east-trending southern margin of the pale mudstones is subparallel with the southern boundary of the Eskdale intrusion, which is separated from the Skiddaw Group strata by a relatively narrow strip of Borrowdale Volcanic Group rocks metamorphosed to biotite-hornblende hornfels. The pale mudstones may thus be an outer zone of this late Ordovician contact aureole. However, the width of the aureole thus defined seems incompatible with both the steep southern flank of the intrusion, as interpreted from gravimetric measurements by Lee (1986), and the presence of southward-directed overthrusting at the northern boundary of the Skiddaw Group inlier (Johnson et al., 2001). The geophysical data do not eliminate the possibility of another intrusion beneath Black Combe. The inlier contains distinctive sets of microgranodiorite, rhyolite, andesite and basalt minor intrusions (Johnson, 1992; Johnson et al., 2001; see (Table 6)). The microgranodiorites take the form of south-east-dipping sheets within a shear zone regarded as Acadian in age (Bell, 1992). The others, which display a greater variety of form and orientation, occur within a 4 km-wide belt to either side of the boundary of the pale mudstones. None of these sets of intrusions is in itself of sufficient size to account for the extent of contact metamorphism. Hence, the origin of contact metamorphism in the Skiddaw Group at Black Combe remains unresolved, save that it predates the main Acadian cleavage in the area.

The contact metamorphosed mudstones north-west of Knott Hill for example [SD 160 875]; [SD 165 877] are cut by the prominent north–south Windy Slack and Swinside faults. The strata between the faults contains the best preserved fossils in the Black Combe inlier (E W Johnson, personal communication) which was thought to be indicative of low metamorphic grade. Samples from two localities [SD 1652 8772]; [SD 1700 8750] gave apparently confirmatory, late-diagenetic KIs, compatible with the prevalence of 1Md white mica in these rocks. However, the XRD-traces also indicated the presence of rectorite (interlayered smectite-illite) and of 2M1 white mica (in addition to the 1Md polytype) which strongly suggests retrogressive alteration of formerly higher grade rocks by the action of aqueous fluids, perhaps generated during late fault movements. The presence of well-preserved fossils in this area would therefore seem to be unrelated to metamorphic grade.

Regional assessment

The Skiddaw Group presents a complex pattern of metamorphism in which the effects of several processes can be identified. Metamorphic evolution spanned three principal episodes during the Lower Palaeozoic and Devonian.

  1. Tremadoc–Llanvirn: deposition and burial in an extensional continental margin basin formed during detachment of Eastern Avalonia from Gondwana
  2. Caradoc: subvolcanic burial and granite intrusion in an inter-arc setting related to subduction of the Iapetus Ocean
  3. Mid-Silurian to mid-Devonian: burial beneath the Windermere Supergroup foreland basin sequence following convergence of Eastern Avalonia and Laurentia, with basin inversion and granite emplacement during the Acadian orogeny

Primary burial (episode i) can be recognised where late-diagenetic mudstones show only weak deformation and a strong bedding fabric. However, the presence of pyrophyllite and kaolinite in such rocks, especially where pyrophyllite veining is present, implies a later element of alteration during burial beneath the Borrowdale and Eycott Volcanic groups (episode ii). Regional metamorphism can be recognised where anchizonal and epizonal grades occur in mudstones with one or more cross-cutting cleavages, and is associated with Acadian tectonism (episode iii). Hornfelsic aureoles surround intrusions emplaced during both episode ii (Ennerdale intrusion, Threlkeld microgranite) and episode iii (Skiddaw granite). In addition, the weak but extensive contact metamorphism of the Crummock Water aureole is attributed to a concealed, episode iii granite, while similar effects at Black Combe may have been caused by the Eskdale intrusion (episode ii) or a concealed intrusion of uncertain (but pre- or early Acadian) age. Finally, retrogressive changes associated with passage of hydrous fluids on fracture systems are apparent in greisenitic alteration at Grainsgill, tourmalinisation at Crummock Water and Black Combe, and in localised late-diagenetic grades near Knott Hill, Black Combe.

Skiddaw Group metapelites share many mineralogical characteristics with metapelites of comparable age from the Isle of Man (Roberts et al., 1990) and north-central Wales (Merriman and Roberts, 1985; Roberts et al., 1991). These include the common presence of interstratified muscovite-paragonite and chlorite-mica stacks which, in contrast, are not typical of metapelites from the Silurian sequences of the southern Lake District (Windermere Supergroup) and the Scottish Southern Uplands. The b0 parameter in white mica measured by XRD from 19 Skiddaw Group pelitic samples falls in the range 8.98 to 9.00 Å, whereas the range for 29 Windermere Supergroup samples is 9.02 to 9.05 Å. This suggests a difference in geothermal gradients (Guidotti and Sassi, 1986) such that the Lower Ordovician rocks were metamorphosed under a field gradient of 35 to 40°C/km, consistent with successive extensional margin and volcanic arc-basin settings, whereas the Silurian rocks were buried under a gradient of 20 to 25°C/km, consistent with a late collision, foreland basin setting.

The pattern of variation in metamorphic grade prompts consideration of two important areas of uncertainty. First is consideration of factors that may have determined the degree of variation in the development of regional metamorphism. In (Figure 39), a broad swathe of regionally metamorphosed, cleaved rocks is flanked by areas where burial metamorphic effects are preserved in weakly deformed rocks. The question is why burial effects should be preserved in some parts of the Skiddaw Group whereas tectonic effects dominate in others. In the Borrowdale Volcanic Group, zones of strongly cleaved, slaty rocks enclose a central belt of weaker cleavage that overlies the late Ordovician Lake District batholith. Firman and Lee (1986) concluded that the Lake District batholith buttressed the central belt so that Acadian tectonic stress was partitioned into flanking zones. Fortey (1989) suggested that the Skiddaw Group might be analogous in that the areas of weakly deformed, late-diagenetic rocks may overlie relatively rigid ‘basement’ which shielded them from Acadian deformation. On the basis of gravimetric data (Lee, 1986), such ‘basement’ is unlikely to be part of the Lake District batholith, and pre-Tremadoc metamorphic rocks may be more plausible.

The second issue is that of the influence on the metamorphism of a granite beneath the Buttermere Fells. In this area, cleaved anchizonal Skiddaw Group strata overlie an inferred granitic body of uncertain age buried some 4 to 6 km deep (Lee, 1986). If the granite is a pre-Acadian (probably late Ordovician) component of the Lake District batholith then it might be expected to have formed a pre-cleavage contact aureole in the overlying Skiddaw Group and also to have shielded the Skiddaw Group during Acadian deformation. However, there is no evidence for the presence of such contact metamorphism, for instance at Robinson [NY 201 169] or Grisedale Pike [NY 199 226]. If the concealed granite is of Acadian (early Devonian) age, then it might be expected to have created a contact aureole younger than the main Acadian cleavage, as at Skiddaw. Once again, evidence of such contact metamorphism is lacking in the Buttermere Fells. It may be that the depth to the concealed granite was such that, during cooling, temperatures reached at the level of the present surface were insufficient to bring about porphyroblast development; indeed, they may not have been significantly greater than those reached during the cleavage-forming metamorphism. Only the ‘Grasmoor granite’ penetrated high enough to produce a contact aureole (Crummock Water) visible at the present surface. The age of the more deeply concealed granite and its role in the metamorphism of the Skiddaw Group remain unresolved.

Chapter 7 Regional correlation

As described earlier in this memoir, the Lake District Lower Palaeozoic inlier exposes part of the northern margin of the Avalonian micro-continent. The stratigraphical belts of the Skiddaw Group, and major structural lines in the main Skiddaw inlier, can be traced laterally eastwards to the Cross Fell inlier (Chapters 1 and 3; Cooper and Molyneux, 1990). The gross Lower Palaeozoic structural framework may be traceable westwards across west Cumbria and the Irish Sea by its expression as reactivated Upper Palaeozoic and Mesozoic, basin margin synsedimentary faults, as illustrated by Jackson et al. (1987, 1995). In the eastern Irish Sea, these faults suggest that the Northern Fells Belt extends westwards, at least to the Isle of Man, with the Lagman Fault (the south-east margin of the Ramsey–Whitehaven ridge) possibly linking with the Causey Pike Fault (Figure 40) through the complex transfer system of the Lake District Boundary–St. Bees fault zones (Akhurst et al., 1997).

Additional difficulties are introduced into both the along-strike correlations outlined above, and any cross-strike correlations south-east towards the Ingleton Group, by the likely combination of thrust and transcurrent movements on the major faults. For example, Cooper et al. (1995) estimated a minimum sinistral movement of 70 km on the Causey Pike Fault, probably preceded by south-directed thrust movement and then followed by renewed southerly directed thrusting, locally of about 0.5 to 1 km; southward displacement of 4 to 5 km has been estimated for the Watch Hill Thrust (Hughes et al., 1993).

Correlation with the Ingleton Group

South and east of the Black Combe and Furness inliers, the Skiddaw Group is concealed beneath younger strata, but similar lithofacies reappear in the Craven inliers of Ribblesdale and Chapel le Dale (Figure 1). They are assigned to the Ingleton Group and are comparable with wackes of Arenig age proved in the nearby Beckermonds Scar Borehole [SD 8635 8016] (Wilson and Cornwell, 1982; Chapter 3). The Ingleton Group rocks of the Chapel le Dale inlier have a south-easterly palaeocurrent derivation, broadly similar to that of the Loweswater Formation, which is also of similar Arenig age. However, their sedimentary petrographical compositions are sufficiently different to preclude any common provenance (Leedal and Walker, 1950; Moore, 1992), with the Ingleton Group containing much more juvenile volcaniclastic detritus than the Skiddaw Group. This is particularly evident from a comparison of the neodymium isotope ratios from each group (Figure 28); pp.78–81 which show a significantly higher proportion of juvenile material in the Ingleton Group as compared to the Skiddaw Group.

The Ingleton Group crops out at the south-west edge of the Askrigg structural block and lies in the footwall of the North Craven Fault. This is part of the major Craven fault system that throws down to the west, and places the Craven inliers in a somewhat analogous structural position to the Cross Fell inlier farther north (Figure 1). These major basement faults introduce further uncertainties into the original relationship of the Ingleton and Skiddaw groups.

Correlation with the Manx Group

The Manx Group of the Isle of Man is comparable in age, lithology and facies to the Skiddaw Group (Figure 41). It comprises a thick sequence of mudstone, siltstone and sandstone turbidites with sedimentary breccias (Gillott 1956a, b; Simpson 1963; Woodcock et al., 1999), and has its closest similarities with the Skiddaw Group of the Northern Fells Belt (Cooper et al., 1995).

Simpson (1963) erected a succession of twelve formations based entirely on interpreted relationships and without biostratigraphical control. Though the Manx Group has so far proved to be only sparsely fossiliferous, the limited biostratigraphical evidence that is available does not support the order of superposition in Simpson’s (1963) scheme. Instead it suggests that formations towards the top of Simpson’s succession are among the oldest exposed on the island, whereas the lowest formation in Simpson’s scheme should be placed higher in the succession. A revised succession was suggested by Cooper et al. (1995). Although still based on the concept of a single succession it took account of the biostratigraphy and outcrop pattern, while conserving Simpson’s order of superposition as far as possible. A more radical lithostratigraphical revision, proposed by Woodcock et al. (1999), divided the Manx Group into separate though partially overlapping successions in seven discrete structural tracts. More recent work (British Geological Survey, 2001; Chadwick et al., 2001) focused on the principal uncertainties of the Woodcock et al. (1999) model, and allowed rationalisation of the stratigraphy.

Biostratigraphical evidence

The Manx Group has yielded both graptolites and acritarchs, although the former are sparse and generally poorly preserved. Acritarchs are more widespread, occur in assemblages that closely resemble those from the Skiddaw Group, and establish a very similar biostratigraphical range to the latter (Molyneux, 1979, 1999, 2001). The most recent work on acritarch microfloras from the Manx Group accompanied the resurvey of the island by the British Geological Survey (2001), and added a number of new localities to those known previously.

The Glen Dhoo and Lonan formations (sensu British Geological Survey, 2001) have both yielded acritarch assemblages comparable with those from low in the Skiddaw Group of the Northern Fells. The assemblages from the Glen Dhoo Formation range in age from probable Tremadoc to early Arenig (Chadwick et al., 2001), and include assemblages comparable with those from the highest part of the messaoudensis-trifidum and the trifidum-bohemicum biozones of the Skiddaw Group. This in turn indicates correlation with the lower part of the Hope Beck Formation and with the phyllograptoides or possibly varicosus biozones. The Glen Dhoo Formation probably also includes older strata (Figure 41). Similarly, acritarchs obtained from strata beneath the Keristal Member of the Lonan Formation, although sparse and poorly preserved, suggest a Tremadoc age, while an assemblage from the Santon Member, higher in the Lonan Formation, indicates the upper part of the messaoudensis-trifidum Biozone or the trifidum-bohemicum Biozone (Figure 41). A prolific but fragmentary graptolite fauna has also been reported from the Santon Member (Rushton, 1993), but gives only a general Arenig age.

Above the Glen Dhoo and Lonan formations, the Creg Agneash, Mull Hill, Maughold, Barrule, Injebreck and Glen Rushen formations generally yield barren palynology samples, but sparse, low diversity acritarch assemblages do occur (Chadwick et al. 2001). Such assemblages, containing many of the taxa recorded from the Manx formations and interspersed with a high proportion of barren samples, are characteristic of the Hope Beck and lower part of the Loweswater formations in the Lorton Fells of the Lake District (lower Arenig, Didymograptus varicosus Biozone). The similarity between palynological assemblages from these parts of the Manx and Skiddaw successions is worth noting, and may have some stratigraphical significance.

Dendroid graptolites from the Cronk Sumark Slates, a lithostratigraphical term not used in the recent BGS survey, were thought to occur at the lowest exposed level in the group, below the Glen Dhoo Formation (for example, see Cooper et al., 1995; Woodcock et al., 1999). However, their locality is now considered to lie within the Injebreck Formation, albeit close to the faulted contact with the Glen Dhoo Formation (Chadwick et al., 2001; British Geological Survey, 2001). Rushton (1993) concluded that the specimens, which are poorly preserved, could indicate either a Tremadoc or an Arenig age.

A further parallel between the Manx and Skiddaw successions is the presence of the Stelliferidium aff. pseudoornatum Biozone in both. In the Manx Group, the assemblage indicating the biozone occurs mainly in the Creggan Moar Formation, indicating a mid to early late Arenig in age and correlations with the upper part of the Loweswater Formation and lower part of the Kirk Stile Formation, and with the top of the varicosus Biozone, the whole of the Didymograptus simulans Biozone, and some part of the overlying Isograptus victoriae Biozone. The base of the S. aff. pseudoornatum Biozone in the Manx Group may lie in the Glen Rushen Formation. The incoming of the S. aff. pseudoornatum assemblage in the Lake District, in the upper part of the Loweswater Formation, marks a change in acritarch recovery, from a largely barren succession below to a more acritarch-rich succession above. Most samples from the upper part of the Loweswater Formation yielded acritarchs, although yields from individual samples and diversity may be low. The same is true in the Manx Group. The Creggan Moar Formation has one of the highest recovery rates in the Manx Group, contrasting with a higher frequency of barren samples in the underlying formations. The pattern suggests a possible change in the environment, promoting increased acritarch productivity. If the changing pattern of acritarch recovery can be shown to result from a synchronous event in the Manx–Skiddaw basin, it would strengthen the correlation between the Creggan Moar and upper part of the Loweswater Formation.

The Stelliferidium aff. pseudoornatum Biozone is overlain by the hamata-rarirrugulata Biozone in both the Isle of Man and the Lake District. In the Isle of Man, the biozone is present in the Creggan Moar Formation at Creggan Moar, and in the Lady Port Formation. Its presence indicates a late Arenig age (upper part of the Isograptus victoriae or Isograptus caduceus gibberulus Biozone to the Aulograptus cucullus Biozone), and correlation with the Kirk Stile Formation.

Lithostratigraphical evidence

The general compositional similarity between the wackes of the Manx and Skiddaw groups has been emphasised in several petrographical studies (for example Moore, 1992; Cooper et al., 1995). It is reinforced by geochemical similarities reported by Barnes et al. (1999) between the Lonan Formation and the Loweswater Formation of the Skiddaw Group (Figure 41). Wacke formations in the latter (Watch Hill and Loweswater formations) are also similar to the Lonan Formation in terms of their neodymium isotope ratios as reported by Stone and Evans (1997). The Nd values of -5.1 and -6.0 from the Lonan Formation overlap with those from both the Watch Hill Formation (-4.1 to -5.5) and Loweswater Formation (-5.8 to -6.4) (Figure 28). This affinity of compositional characteristics is strongly indicative of a shared provenance for the lithic wackes of the Lonan Formation and those of the Skiddaw Group.

Debrites (the products of debris-flow processes) are distinctive lithological components of both the Manx Group and Skiddaw Group (Figure 41). The Manx Group debrites comprise mudstone-supported angular and rounded clasts, dominantly of mudstone but also including sandstone clasts. Lithologically, these debrites are very similar to those of the Buttermere Formation olistostrome (emplaced in the late Arenig) and to the probable coeval breccia beds in the Kirk Stile Formation (late Arenig to earliest Llanvirn). Simpson (1963) recognised two units of 'slump-breccia' in the Manx Group, Ballanayre and Sulby, placing them at widely separate levels in the succession, the former near the base and the latter near the top. However, from mica crystallinity studies, Roberts et al. (1990) concluded that the Ballanayre slump-breccia was high in the Manx succession. This conclusion is consistent with the late Arenig age of the Lady Port Formation, with which the Ballanayre slump-breccia is conformably interrelated (Simpson, 1963; Woodcock and Morris, 1999). The Ballanayre debrites are therefore similar in age to the debrites of the Kirk Stile Formation and the Buttermere Formation olistostrome. Their correlation implies a very widespread phase of slope failure and mass flow in the original sedimentary basin. In comparison, the Sulby slump-breccia is now included in the Injebreck Formation (Chadwick et al., 2001; British Geological Survey, 2001), implying a probable correlation with the Loweswater Formation (Figure 41).

A significant difference between the Manx and Skiddaw groups is the presence in the former of thick beds of very quartz-rich wacke and quartz arenite. These form the Creg Agneash and Mull Hill formations in the Woodcock et al. (1999) scheme, but were included by Simpson (1963) in his Maughold Banded Group (for example the Mull Hill Quartzite) and, to a much lesser extent, in his Lonan Flags. There are no such thick, quartzitic developments with in the Skiddaw Group and the closest analogues would seem to be the unusually mature quartz-wackes within the Loweswater Formation at Jonah’s Gill.

Another contrast between the Skiddaw and Manx groups lies in the presence or absence of a contemporaneous extrusive volcanic component. In the Manx Group, the ‘Peel volcanic rocks’ were recorded as andesitic lava, tuff and agglomerate with sedimentary intercalations (Simpson, 1963) which have yielded an early Arenig acritarch microflora (Molyneux, 1999). An occurrence of volcaniclastic rock, the ‘Ballaquane tuff’, has been described by McConnell et al. (1999) as being of likely late Arenig age because of its apparent interbedding with the Creggan Moar Formation, although it may represent a sliver of older rock along the faulted contact between the Creggan Moar and Niarbyl (Silurian) formations (BGS, 2001; Chadwick et al., 2001). Hughes and Kokelaar (1993) re-interpreted two supposed Skiddaw Group lavas in the Lake District and Cross Fell inlier as sills. Thus, the oldest primary volcanic material in the Skiddaw Group is in the volcaniclastic and bentonite beds of the Llanvirn Tarn Moor Formation (see Chapter 3 for description), but even there the neodymium ratios from the dominant siltstones show no widespread influx of juvenile material (Figure 28). This absence from the Skiddaw Group of volcanic rocks with equivalent ages to those in the Manx Group at Peel and Ballaquane, coupled with the presence of sills in the Skiddaw and Cross Fell inliers (Hughes and Kokelaar, 1993), suggested to Cooper et al. (1995) that the extrusive volcanic origin of the Manx Group examples was in doubt. However, volcanic rocks of Arenig age have been described from the Ribband Group (see below), a possible correlative of the Manx Group in south-east Ireland, and on the regional scale it seems that it is the Skiddaw Group which is anomalous in this respect.

Correlation with Eastern Ireland

Several terranes have been postulated in eastern Ireland (reviewed in Murphy et al., 1991), among which are the Grangegeeth terrane in the north, the Bellewstown terrane, and the Leinster terrane in the south; the latter may itself be composite (Max et al., 1990).

Arenig strata are unproved in the Grangegeeth terrane, but the Llanvirn succession comprises volcanic tuffs with subordinate interbedded graptolitic mudstones of artus and possibly murchisoni Biozone age (Murphy et al. 1991 and references therein). The graptolites are mainly scandent forms with ‘Atlantic province’ affinities, like those of the Lake District. The occurrence of Llanvirn volcanic units interbedded with graptolitic mudstones invites comparison with the Tarn Moor Formation of the Central Fells belt, but the Irish succession contains a much higher proportion of volcanic rocks. The unconformably overlying Grangegeeth Group of volcanic conglomerates, tuffs and shales has yielded abundant Caradoc shelly faunas. The volcanic rocks have tholeiitic affinities and geochemically compare most closely with the mainly Caradoc Eycott Volcanic Group (Millward and Molyneux, 1992) which unconformably overlies the Northern Fells belt of the Skiddaw Group in the Lake District. However, the shelly fauna from the Grangegeeth Group has Laurentian affinities, whereas the Eycott Volcanic Group is overlain by the Drygill Shales that have a Longvillian (mid-Caradoc) shelly fauna with Anglo–Welsh, and therefore Avalonian or Gondwanan, affinities (Dean, 1963).

The Bellewstown terrane is separated from the Grangegeeth Terrane by the Slane Fault, which Todd et al. (1991) suggested was part of the Iapetus Suture fault complex. The Llanvirn rocks contain pendent didymograptids and a shelly fauna of Gondwanan affinity, comparable to the deep-water Gondwanan trilobites described from the Skiddaw Group by Rushton (1988) and Fortey et al. (1989). The Bellewstown succession was interpreted as representing an island site within the Iapetus Ocean (Harper et al., 1990). It is unconformably capped by volcaniclastic turbidites, mudflows and mudrocks of latest Llanvirn to Caradoc age, with shelly faunas of Anglo–Welsh affinity that contrast with those of Grangegeeth. The volcanic successions have calc-alkaline affinities and Murphy (1987) compared them with the Borrowdale Volcanic Group of the Lake District (Central Fells Belt).

The Ordovician of the Leinster terrane (Murphy et al., 1991) shows fewer similarities with the Lake District succession than the other terranes. The succession along the northern edge of the terrane, in the Balbriggan and Herbertstown inliers, comprises unfossiliferous red and green mudstones overlain by grey mudstones and siltstones. These are unconformably overlain by andesitic volcanic rocks, capped by mudstones and siltstones that have yielded a mid-Caradoc fauna (Romano, 1980). Graptolites from the south-eastern part of the Leinster terrane include representatives of the gibberulus Biozone and an equivalent of the obscure interval below the varicosus Biozone in the Lake District (Rushton, 1997). These are broadly comparable with the biozonal range of the Hope Beck to Kirk Stile formations of the Skiddaw Group. In both areas the dominantly marine mudstone/ siltstone succession is unconformably overlain by a mainly Caradoc volcanic assemblage, the Borrowdale Volcanic Group in the Lake District and the Duncannon Group (Parkes, 1992) in Leinster.

McConnell et al. (1999) have drawn attention to similarities in the Ribband and Manx Group lithostratigraphies, implying a link into the Skiddaw Group. All three groups, Ribband, Manx and Skiddaw, were regarded as containing a significant quartzo-feldspathic wacke sandstone incursion in the varicosus Biozone; these are respectively the Palace Member of the Oaklands Formation, the Santon Member of the Lonan Formation and the Loweswater Formation. The ambiguous volcanic units in the Manx Group (Peel and Ballaquane) were correlated with better defined equivalents in the Ribband Group (Dowery Hill and Donard–Kilcarry respectively). This strengthens the interpretation of the Isle of Man examples as in situ volcanic arc rocks, and emphasises the anomalously passive depositional environment of the Skiddaw Group. A possibility exists that the Skiddaw Group rocks relate more to reworking of continental-derived deltaic material fed into a separate part of a major fan complex.

McConnell et al. (1999) also stressed the stratigraphical importance of an unusual and distinctive ‘coticule package’ of spessartine quartzite (coticule) and tourmalinite, believed to be the product of an exhalative event of wide geographical extent within the Caledonian–Appalachian orogen (Kennan and Kennedy, 1983). The package has been described from the Maulin Formation of the Ribband Group (Figure 41) and is associated with thinly bedded manganese-ironstone (Maughold, Creggan Moar and Lady Port formations) and stratiform tourmalinite (Injebreck Formation) in the Manx Group (stratigraphical units after Woodcock et al., 1999). Tourmaline veins in the Crummock Water aureole were cited (by Woodcock et al.) as a link with the Kirk Stile Formation of the Skiddaw Group. However, very little bedded tourmalinite has been recorded from that aureole, and the tourmalinite, as noted, is associated with vein material.

Correlation with North America

In North America, rocks assigned an Avalonian origin occur along the Atlantic seaboard of Canada and the eastern United States from Newfoundland (the eponymous Avalon peninsula) arguably as far south as the Gondwanan terrane of Florida (Nance and Thompson, 1996 and references therein). In a broad sense, the Skiddaw Group correlates with components of the Appalachian Gander Zone (van Staal et al., 1998, and references therein). Most of the recorded Avalonian sequences are older than the Skiddaw Group, but the St John Group of southern New Brunswick ranges up to the early Ordovician (Tremadoc) black shales of the Reversing Falls Formation, which were deposited on a deep marine shelf (Tanoli and Pickerill, 1988; Landing et al., 1997). The age, lithology and likely depositional environment all invite broad comparison with the Irish, Isle of Man and Skiddaw Group sequences.

Information sources

Further geological information held by the British Geological Survey and relevant to the Skiddaw Group is listed below. Sources include published maps, memoirs and reports and open-file maps and reports, together with borehole records, mine plans, fossils, rock samples, thin sections, hydrogeological data and photographs. Attention is drawn to the regularly updated Catalogue of Maps and Books issued by the British Geological Survey, and to the Geoscience Data Index system available in British Geological Survey libraries and on the web site at www.bgs.ac.uk. This is a developing computer-based system that carries out searches of indexes to collections and digital databases for specified geographical areas. It is based on a geographic information system linked to a relational database management system. Results of the searches are displayed on maps on the screen.

Maps

The original, primary geological survey of the Lake District Skiddaw Group inliers (Skiddaw, Ullswater, Bampton, Black Combe and Furness) was undertaken between 1872 and 1895 at a scale of six inches to one mile (1:10 560). It was largely the work of J C Ward with contributions by W T Aveline and R Russell. The northern and western fringes of the area were resurveyed at 1:10 560 scale, between 1921 and 1937, by E E L Dixon, T Eastwood, S E Hollingworth, W C C Rose, B Smith and F M Trotter and published at a scale of one inch to one mile (1:63 360) in 1929 (Whitehaven), 1937 (Gosforth and Bootle) and 1960 (Cockermouth). These sheets were subsequently reprinted without revision at the 1:50 000 scale. Copies of the manuscript maps arising from these earlier surveys may be consulted at the British Geological Survey library in Edinburgh. Geological 1:10 000 scale National Grid maps covering the Lake District Skiddaw Group outcrop, in whole or in part, are listed below, together with the initials of the surveyors responsible for the Skiddaw Group work and the dates of their survey. The surveyors were: P M Allen, R S Arthurton, R P Barnes, A M Bell, A H Cooper, D J Fettes, R A Hughes, E W Johnson, D E Roberts, G J Roycroft, N J Soper, P Stone, A J Wadge and B C Webb. Copies of the manuscript maps arising from this recent phase of investigations have been deposited in the British Geological Survey libraries in Edinburgh and Keyworth, Nottingham for public reference; they may also be inspected at the Survey’s Information Office in the Natural History Museum, South Kensington, London. Copies may be purchased directly from the British Geological Survey as uncoloured dyeline copies.

Sheet

Name

Surveyor

Date

Skiddaw Inlier

NY 01 NE

Kirkland

RAH

1995

NY 01 SW

Egremont

RAH, RPB

1990

NY 01 SE

Kinniside

PMA, DJF

1984–85, 1996

NY 11 NW

Floutern Tarn

RAH, DJF

1993

NY 11 NE

Buttermere

BCW, AHC

1985, 1992

NY 12 NW

un-named

BCW

1983–86

NY 12 NE

un-named

AHC

1982–86

NY 12 SW

un-named

BCW

1983–86

NY 12 SE

un-named

PMA, AHC, BCW

1982–86

NY 13 NE

Bothel

AHC

1989

NY 13 SW

Cockermouth

AHC

1986–89

NY 13 SE

Embleton

RAH

1989

NY 21 NW

Derwent Fells

AHC, RAH

1992, 1993

NY 21 NE

Grange

AHC

1992–93

NY 22 NW

Thornthwaite

AHC

1985–89

NY 22 NE

Skiddaw

GJR, AHC

1980–87, 1993

NY 22 SW

Braithwaite

AHC

1986–91

NY 22 SE

Keswick

RSA , GJR, AHC

1970, 1980–87, 1986–90

NY 23 NW

Ireby

AHC

1989

NY 23 NE

Aughertree

AHC

1989–90

NY 23 SW

Bassenthwaite

RAH

1990

NY 23 SE

Great Cockup

RPB, AHC, RAH

1989–91

NY 32 NW

Blencathra

DER, RPB, AHC

1973, 1990, 1994

NY 32 NE

Eycott Hill

PS

1994–95

NY 32 SW

Threlkeld and

AHC

1994

NY 32 SE

Matterdale End and Dockray

AHC, PS

1994–95

NY 33 SW

Carrock Fell

RPB

1990

NY 33 SE

Mosedale

PS

1990–91

Ullswater Inlier

NY 31 NE

Glenridding

RAH

1995

NY 42 SW

Little Mill Fell

AJW, RAH

2001

NY 42 SE

Pooley Bridge

AJW, RAH

2001

Bampton Inlier

NY 51 NW

Bampton

AMB

2001

NY 51 SW

Swindale

AMB

2001

NY 51 SE

Hardendale

AMB

2001

Black Combe Inlier

SD 18 NW

Black Combe and Bootle

AMB, EWJ

1989, 1993

SD 18 NE

Duddon Bridge

EWJ, AMB

1988–89, 1993,

SD 18 SW

Silecroft

AMB, EWJ

1989, 1993

SD 18 SE

Millom Park

AMB, EWJ

1989, 1993

Furness Inlier

SD 27 NW

Askam

NJS

1992–93

Books and reports

Memoirs and reports relevant to west Cumbria arranged by topic. Most are not widely available but may be purchased from BGS or consulted at BGS and other libraries.

General geology

Memoirs and Sheet Explanations

Reports

Mineral resources

Non-mineral resource assessment

Land use planning

APPLETON, J D. 1995. Potentially harmful elements from natural sources and mining areas: characteristics, extent and relevance to planning and development in Great Britain. British Geological Survey Technical Report, WP/95/3.

APPLETON, J D. 1995. Radon, methane, carbon dioxide, oil seeps and potentially harmful elements from natural sources and mining areas: relevance to planning and development in Great Britain. British Geological Survey Technical Report, WP/95/4.

APPLETON, J D, and BALL, T K. 1995. Radon and background radioactivity from natural sources: characteristics, extent and relevance to planning and development in Great Britain. British Geological Survey Technical Report, WP/95/2.

APPLETON, J D, HOOKER, P J, and SMITH, N J P. 1995. Methane, carbon dioxide and oil seeps from natural sources and mining areas: characteristics, extent and relevance to planning and development in Great Britain. British Geological Survey Technical Report, WP/95/1.

Seismicity

Biostratigraphy

Remote sensing

Popular publications

Other data sources

Material collections

Addresse for data sources

References

Most of the references listed below are held in the Libraries of the British Geological Survey at Edinburgh and Keyworth, Nottingham. Copies of the references can be purchased subject to the current copyright legislation. BGS Library catalogue can be searched online at: http://geolib.bgs.ac.uk

ADAMS, J. 1988. Mines of the Lake District Fells. (Lancaster: Dalesman Books.)

AKHURST, M C, and 24 others. 1997. The Geology of the west Cumbria district. Memoir of the British Geological Survey, Sheets 28, 37 and 47 (England and Wales).

ALLEN, P M, and COOPER, D C. 1986. The stratigraphy and composition of the Latterbarrow and Redmain sandstones, Lake District, England. Geological Journal, Vol. 21, 59–76.

ANDERTON, R. 1982. Dalradian deposition and the late Precambrian-Cambrian history of the N Atlantic region: a review of the early evolution of the Iapetus Ocean. Journal of the Geological Society of London, Vol. 139, 421–431.

ARTHURTON, R S, JOHNSON, E W, and MUNDY, D J C. 1988. Geology of the country around Settle. Memoir of the British Geological Survey, Sheet 60 (England and Wales).

ARTHURTON, R S, and WADGE, A J. 1981. Geology of the country around Penrith. Memoir of the Geological Survey of Great Britain, Sheet 24 (England and Wales).

BALL, T K, FORTEY, N J, and SHEPHERD, T J. 1985. Mineralisation at the Carrock Fell tungsten mine, Northern England: paragenetic, fluid inclusion and geochemistry study. Mineralium Deposita, Vol. 20, 57–65.

BANHAM, P H, HOPPER, F M W, and JACKSON, J B. 1981. The Gillbrea Nappe in the Skiddaw Group, Cockermouth, Cumbria, England. Geological Magazine, Vol. 118, 509–516.

BARNES, R P, LINTERN, B C, and STONE, P. 1989. Timing and regional implications of deformation in the Southern Uplands of Scotland. Journal of the Geological Society of London, Vol. 146, 905–908.

BARNES, R P, POWER, G M, and COOPER, D C. 1999. The definition of sandstone-bearing formations in the Isle of Man and correlation with adjacent areas — evidence from sandstone geochemistry. 121–138 in In sight of the suture: the geology of the Isle of Man in its Iapetus Ocean context.

WOODCOCK, N H, FITCHES, W R, QUIRK, D G, and BARNES, R P (editors). Geological Society of London, Special Publication, No. 160.

BECKLY, A J, and MALETZ, J. 1991. The Ordovician graptolites Azygograptus and Jishougraptus in Scandinavia and Britain. Palaeontology, Vol. 34, 887–925.

BELL, A M. 1992. The stratigraphy and structure of the Skiddaw Group in the Black Combe inlier: geological notes to accompany 1:25 000 sheet SD18. British Geological Survey Technical Report, WA/92/70.

BELL, A M. 1997 The Skiddaw Group and its contact with surrounding rocks in the Bampton Inlier, Cumbria. British Geological Survey Technical Report, WA/97/9.

BERRIDGE, N G. 1982. Petrography of the pre-Carboniferous rocks of the Beckermonds Scar Borehole in the context of the magnetic anomaly at the site. Proceedings of the Yorkshire Geological Society, Vol. 44, 89–98.

BORGIA, A, FERRARI, L, and PASQUARE, G. 1992. Importance of gravitational spreading in the tectonic and volcanic evolution of Mount Etna. Nature, Vol. 357, 231–235.

BOUMA, A H. 1962. Sedimentology of some flysch deposits: a graphic approach to interpretation. (Amsterdam: Elsevier.)

BRANNEY, M J. 1988. Subaerial explosive volcanism, intrusion, sedimentation, and collapse in the Borrowdale Volcanic Group, SW Langdale, English Lake District. Unpublished PhD thesis, University of Sheffield.

BRANNEY, M J, and KOKELAAR, B P. 1994. Volcanotectonic faulting, soft-state deformation and rheomorphism of tuffs during development of a piecemeal caldera, English Lake District. Bulletin of the Geological Society of America, Vol. 106, 507–530.

BRANNEY, M J, and SOPER, N J. 1988. Ordovician volcano-tectonics in the English Lake District. Journal of the Geological Society of London, Vol. 145, 367–376.

BRITISH GEOLOGICAL SURVEY. 1992. Regional Geochemistry of the Lake District and adjacent areas. (Keyworth, Nottingham: British Geological Survey.)

BRITISH GEOLOGICAL SURVEY. 2001. Isle of Man. Sheets 36, 45, 56 and 57. Solid and Drift Geology. 1:50 000. (Keyworth, Nottingham: British Geological Survey.)

BURGESS, I C. 1971. Synopses of logs of Allenheads No 1 Borehole ([8604 4539]; OD 406.76 m) and No 2 Borehole (8715 4505]; OD 536.45 m). Annual Report for the Institute of Geological Sciences for 1970, 33.

BURGESS, I C, and HOLLIDAY, D W. 1979. Geology of the country around Brough-under-Stainmore. Memoir of the Geological Survey of Great Britain, Sheet 31 and parts of sheets 25 and 30 (England and Wales).

BURGESS, I C, and WADGE, A J. 1974. The geology of the Cross Fell area. (London: HMSO for the Institute of Geological Sciences.)

CAMERON, D G, and 6 others. 1993. Mineral exploration in the Lower Palaeozoic rocks of south-west Cumbria. Part 1: Regional surveys. British Geological Survey Technical Report WF/93/4, Mineral Reconnaissance Programme, Report 128.

CAUNT, S. 1984. Geochemical aspects of the Threlkeld Microgranite. Transactions of the Leeds Geological Association, Vol. 10, 89–100.

CHADWICK, R A, HOLLIDAY, D W, HOLLOWAY, S, and HULBERT, A G. 1995. The Northumberland–Solway basin and adjacent areas. Subsurface Memoir of the British Geological Survey.

CHADWICK, R A, and 16 others. 2001. Geology of the Isle of Man and its offshore area. British Geological Survey Research Report, RR/01/06.

CLARK, L. 1963. The geology and petrology of the Ennerdale Granophyre, its metamorphic aureole and associated mineralization. Unpublished PhD thesis, University of Leeds.

COCKS, L R M. 1978. A review of British Lower Palaeozoic brachiopods, including a synoptic revision of Davidson’s Monograph. Monograph of the Palaeontographical Society, Publication No. 549, part of Vol. 131 for 1977.

COCKS, L R M, MCKERROW, W S, and VAN STAAL, C R. 1997. The margins of Avalonia. Geological Magazine, Vol. 134, 627–636.

COOPER, A H. 1986. Excursion to the Skiddaw Group, Hope Beck. Proceedings of the Cumberland Geological Society, Vol. 4, 419–420.

COOPER, A H. 1990a. The Skiddaw Group of Causey Pike. Proceedings of the Cumberland Geological Society, Vol. 5, 222–230.

COOPER, A H. 1990b. The Skiddaw Group of Hope Beck, Hopegill Head and Gasgale Gill. 67–74 in The geology of the Lake District. MOSELEY, F (editor). (London: The Geologists’ Association.)

COOPER, A H. 1990c. The Skiddaw Group of Stoneycroft Gill, Causey Pike and Outerside. 82–89 in The geology of the Lake District. MOSELEY, F (editor). (London: The Geologists’ Association.)

COOPER, A H. 1992. Gasgale Crags and Whiteside. 64–67 in Caledonian Structures in Britain south of the Midland Valley. TREAGUS, J E (editor). Geological Conservation Review Series, No. 3.

COOPER, A H. 1997. Cleavage data and stereograms for the Skiddaw Group : 1:50 000 sheets 23 (Cockermouth) and 29 (Keswick). British Geological Survey Technical Report, WA/97/72.

COOPER, A H, and HUGHES, R A. 1993. Discussion of Millward, D, Molyneux, S G. 1992 ‘Field and biostratigraphic evidence for an unconformity at the base of the Eycott Volcanic Group in the English Lake District’. Geological Magazine, Vol. 130, 271–272.

COOPER, A H, MILLWARD, D, JOHNSON, E W, and SOPER, N J. 1993. The early Palaeozoic evolution of northwest England. Geological Magazine, Vol. 130, 711–724.

COOPER, A H, and MOLYNEUX, S G. 1990. The age and correlation of the Skiddaw Group (early Ordovician) sediments in the Cross Fell inlier (northern England). Geological Magazine, Vol. 127, 147–157.

COOPER, A H, and 5 others. 1995. The stratigraphy, correlation, provenance and palaeogeography of the Skiddaw Group (Ordovician) in the English Lake District. Geological Magazine, Vol. 132, 185–211.

COOPER, D C, and BRADLEY, A D. 1990. The ammonium content of granites in the English Lake District. Geological Magazine, Vol. 127, 579–586.

COOPER, D C, and 6 others. 1988. The Crummock Water aureole: a zone of metasomatism and source of ore metals in the English Lake District. Journal of the Geological Society of London, Vol. 145, 523–540.

COOPER, R A, and LINDHOLM, K. 1990. A precise worldwide correlation of early Ordovician graptolite sequences. Geological Magazine, Vol. 127, 497–525.

CREANEY, S. 1980. Petrographic texture and vitrinite reflectance variation on the Alston Block, North-East England. Proceedings of the Yorkshire Geological Society, Vol. 42, 553–580.

DAKYNS, J R. 1869. Notes on the geology of the Lake District. Geological Magazine, Vol. 6, 56–68, 116–117.

DAKYNS, J R, TIDDEMAN, R H, and GOODCHILD, J G. 1897. The geology of the country between Appleby, Ullswater and Haweswater. Memoir of the Geological Survey of Great Britain.

DAVIES, J R, and 5 others. 1997. Geology of the country around Llanilar and Rhayader. Memoir of the British Geological Survey, Sheets 178 and 179 (England and Wales).

DEAN, W T. 1963. The Stile End Beds and Drygill Shales (Ordovician) in the east and north of the English Lake District. Bulletin of the British Museum (Natural History), Geology Series, Vol. 9, 49–65.

DEWEY, H, and EASTWOOD, T. 1925. Copper ores of the Midlands, the Lake District and the Isle of Man. Special Reports on the Mineral Resources of Great Britain. Memoir of the Geological Survey of Great Britain, Vol 30.

DICKINSON, W R. 1970. Interpreting detrital modes of greywacke and arkose. Journal of Sedimentary Petrology, Vol. 40, 695–707.

DICKINSON, W R, and 8 others. 1983. Provenance of North American Phanerozoic sandstones in relation to tectonic setting. Bulletin of the Geological Society of America, Vol. 94, 222–235.

DOWNIE, C. 1984. Acritarchs in British stratigraphy. Geological Society of London Special Report, No. 17.

DOWNIE, C, and SOPER, N J. 1972. Age of the Eycott Volcanic Group and its conformable relationship to the Skiddaw Slates in the English Lake District. Geological Magazine, Vol. 109, 259–268.

DUNHAM, K C, DUNHAM, A C, HODGE, B L, and JOHNSON, G A L. 1965. Granite beneath Viséan sediments with mineralisation at Rookhope, northern Pennines. Quarterly Journal of the Geological Society of London, Vol. 121, 383–417.

EASTWOOD, T. 1921. The lead and zinc ores of the Lake District. Special Reports on the Mineral Resources of Great Britain. Memoir of the Geological Survey of Great Britain, Vol. 22.

EASTWOOD, T, DIXON, E E L, HOLLINGWORTH, S E, and SMITH, B. 1931. The geology of the Whitehaven and Workington district. Memoir of the Geological Survey of Great Britain, Sheet 28 (England and Wales).

EASTWOOD, T, HOLLINGWORTH, S E, ROSE, W C C, and TROTTER, F M. 1968. Geology of the country around Cockermouth and Caldbeck. Memoir of the Geological Survey of Great Britain, Sheet 23 (England and Wales).

ELLES, G L. 1898. The graptolite fauna of the Skiddaw Slates. Quarterly Journal of the Geological Society of London, Vol. 54, 463–539.

ELLES, G L. 1933. The Lower Ordovician graptolite faunas with special reference to the Skiddaw Slates. Summary of Progress of the Geological Survey of Great Britain for 1932, 94–111.

ELLES, G L, and WOOD, E M R. 1901–1918. A monograph of British graptolites. Monographs of the Palaeontographical Society.

EVANS, J A. 1996. Basin development across Iapetus, Project No. 40079. Final report on Rb–Sr whole rock dating of mudrock suites from the Lake District and Southern Uplands. NERC Isotope Geosciences Laboratory Report Series, No. 87.

EVANS, J A, MILLAR, I L, and NOBLE, S R. 1995. Hydration during uplift is recorded by reset Rb–Sr whole-rock ages. Journal of the Geological Society of London, Vol. 152, 209–212.

FIRMAN, R J, and LEE, M K. 1986. The age and structure of the concealed English Lake District batholith and its probable influence on the subsequent sedimentation, tectonics and mineralisation. 117–127 in Geology in the Real World — the Kingsley Dunham volume. NESBITT, R W, and NICHOL, J (editors). (London: Institution of Mining and Metallurgy.)

FITTON, J G, THIRLWALL, M F, and HUGHES, D J. 1982. Volcanism in the Caledonian orogenic belt of Britain. 611–636 in Andesites. THORPE, R S (editor). (New York: Wiley.)

FLOYD, P A, SHAIL, R, LEVERIDGE, B E, and FRANKE, W. 1991. Geochemistry and provenance of Rhenohercynian synorogenic sandstones in relation to setting. 173–188 in Developments in sedimentary provenance studies. MORTON, A C, TODD, S P, and

HOUGHTON, P D W (editors). Geological Society of London, Special Publication, No. 57.

FORTEY, N J. 1989. Low grade metamorphism in the Lower Ordovician Skiddaw Group of the Lake District, England. Proceedings of the Yorkshire Geological Society, Vol. 47, 325–337.

FORTEY, N J, COOPER, A H, HENNEY, P J, COLMAN, T, and NANCARROW, P H A. 1994. Appinitic intrusions in the English Lake District. Mineralogy and Petrology, Vol. 51, 355–375.

FORTEY, N J, and COOPER, D C. 1986. Tourmalinisation in the Skiddaw Group around Crummock Water, English Lake District. Mineralogical Magazine, Vol. 50, 171–226.

FORTEY, N J, ROBERTS, B, and HIRONS, S R. 1993. Relationship between metamorphism and structure in the Skiddaw Group, English Lake District. Geological Magazine, Vol. 130, 631–638.

FORTEY, R A. 1984. Global earlier Ordovician transgressions and regressions and their biological implications. 37–50 in Aspects of the Ordovician system. Bruton, D L (editor).

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FORTEY, R A, and 9 others. 1991. Progress and problems in the selection of stratotypes for the bases of Ordovician series in the type area U.K. 5–25 in Advances in Ordovician geology. BARNES, C R, and WILLIAMS, S H (editors). Geological Society of Canada, Paper 90–9.

FORTEY, R A, BECKLY, A J, and RUSHTON, A W A. 1990. International correlation of the base of the Llanvirn Series, Ordovician System. Newsletters in Stratigraphy, Vol. 22, 119–142.

FORTEY, R A, and COCKS, L R M. 1986. Marginal faunal belts and their structural implications, with examples from the Lower Palaeozoic. Journal of the Geological Society of London, Vol. 143, 151–160.

FORTEY, R A, and COCKS, L R M. 1988. Arenig to Llandovery faunal distributions in the Caledonides. 233–246 in The Caledonian–Appalachian Orogen. HARRIS, A L, and FETTES, D J (editors). Geological Society of London, Special Publication, No. 38.

FORTEY, R A, HARPER, D A T, INGHAM, J K, OWEN, A W, and RUSHTON, A W A. 1995. A revision of Ordovician series and stages from the historical type area. Geological Magazine, Vol. 132, 15–30.

FORTEY, R A, and OWENS, R M. 1987. The Arenig Series in south Wales. Bulletin of the British Museum (Natural History), Vol. 41, 69–307.

FORTEY, R A, OWENS, R M, and RUSHTON, A W A. 1989. The palaeogeographic position of the Lake District in the early Ordovician. Geological Magazine, Vol. 126, 9–17.

FREY, M. 1969. A mixed-layer paragonite/phengite of low-grade metamorphic origin. Contributions to Mineralogy and Petrology, Vol. 24, 63–65.

FREY, M. 1987. Very low grade metamorphism of clastic sedimentary rocks. 9–58 in Low temperature metamorphism. FREY, M (editor). (Glasgow: Blackie.)

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GILLOTT, J E. 1956a. The structural geology of the Manx Slates. Geological Magazine, Vol. 93, 301–313.

GILLOTT, J E. 1956b. Breccias in the Manx Slates: their origin and stratigraphic relations. Liverpool and Manchester Geological Journal, Vol. 1, 370–380.

GÓRKA, H. 1967. Quelques nouveaux Acritarches des silexites du Trémadocien supérieur de la region de Kielce (Montagne de Ste Croix, Pologne). Cahiers de Micropaléontologie, Série 1, No 6.

GREEN, J F N. 1917. The age of the chief intrusions of the Lake District. Proceedings of the Geologists’ Association, Vol. 28, 1–30.

GUIDOTTI, C V, and SASSI, F P. 1986. Classification and correlation of metamorphic facies by means of muscovite bo data from low-grade metapelites. Neues Jahrbuch für Mineralogie Abhandlungen, Vol. 153, 363–380.

HARKNESS, R. 1863. On the Skiddaw Slate Series (with a note on the Skiddaw fossils, by J W Salter). Quarterly Journal of the Geological Society of London, Vol, 19, 113–139.

HARPER, D A T, PARKES, M A, HÖEY, A N, and MURPHY, F C. 1990. Intra-Iapetus brachiopods from the Ordovician of eastern Ireland: implications for Caledonide correlation. Canadian Journal of Earth Sciences, Vol. 27, 1757–1761.

HELM, D G. 1970. Stratigraphy and structure in the Black Combe inlier, English Lake District. Proceedings of the Yorkshire Geological Society, Vol. 38, 105–148.

HELM, D G, and ROBERTS, B. 1971. The relationship between the Skiddaw and Borrowdale Volcanic Groups in the English Lake District. Nature Physical Sciences, Vol. 232, 181–183.

HITCHEN, C S. 1934. The Skiddaw Granite and its residual products. Quarterly Journal of the Geological Society of London, Vol. 90, 158–200.

HOLLINGWORTH, S E. 1955. The geology of the Lake District — a review. Proceedings of the Geologists’ Association, Vol. 65, 385–402.

HOWE, M P A. 1999. The Silurian fauna (graptolite and nautiloid) of the Niarbyl Formation, Isle of Man. 177–187 in In sight of the suture: the geology of the Isle of Man in its Iapetus Ocean context. WOODCOCK, N H, QUIRK, D G, FITCHES, W R, and BARNES, R P (editors). Geological Society of London, Special Publication, No. 160.

HUGHES, R. 1994. The Skiddaw Group rocks and the Skiddaw Group–Borrowdale Volcanic Group contact in the area between Robinson and Catbells, Cumbria. British Geological Survey Technical Report, WA/94/69.

HUGHES, R. 1995a. The Skiddaw Group and its contact with the Borrowdale Volcanic Group in the Ullswater inlier. British Geological Survey Technical Report, WA/95/59.

HUGHES, R. 1995b. The Skiddaw Group rocks of the Embleton area. British Geological Survey Technical Report, WA/95/19.

HUGHES, R, and FETTES, D. 1994. Geology of the 1:10 000 sheet NY 11 NW (Floutern Tarn). British Geological Survey Technical Report, WA/94/70.

HUGHES, R A, COOPER, A H, and STONE, P. 1993. Structural evolution of the Skiddaw Group (English Lake District) on the northern margin of eastern Avalonia. Geological Magazine, Vol. 130, 621–629.

HUGHES, R A, EVANS, J A, NOBLE, S R, and RUNDLE, C C. 1996. U–Pb chronology of the Ennerdale and Eskdale intrusions supports sub-volcanic relationships with the Borrowdale Volcanic Group (Ordovician, English Lake District). Journal of the Geological Society of London, Vol. 153, 33–38.

HUGHES, R A, and KOKELAAR, P. 1993. The timing of Ordovician magmatism in the English Lake District and Cross Fell inliers. Geological Magazine, Vol. 130, 369–377.

HUTTON, D H W. 1987. Strike-slip terranes and a model for the evolution of the British and Irish Caledonides. Geological Magazine, Vol. 124, 405–425.

INGERSOLL, R V, BULLARD, T F, FORD, R L, GRIMM, J P, PICKLE, J D, and SARES, S W. 1984. The effect of grainsize on detrital modes: a test of the Gazzi-Dickinson point-counting method. Journal of Sedimentary Petrology, Vol. 54, 103–116.

JACKSON, D E. 1961. Stratigraphy of the Skiddaw Group between Buttermere and Mungrisdale, Cumberland. Geological Magazine, Vol. 98, 515–528.

JACKSON, D E. 1962. Graptolite zones in the Skiddaw Group in Cumberland, England. Journal of Paleontology, Vol. 36, 300–313.

JACKSON, D E. 1978. The Skiddaw Group. 79–98 in The Geology of the Lake District. Moseley, F (editor). Occasional Publication of the Yorkshire Geological Society, No. 3.

JACKSON, D I, and 5 others. 1995. United Kingdom offshore regional report: the geology of the Irish Sea. (London: HMSO for British Geological Survey.)

JACKSON, D I, MULHOLLAND, P, JONES, S M, and WARRINGTON, G. 1987. The geological framework of the East Irish Sea Basin. 191–203 in Petroleum geology of North West Europe: proceedings of the 3rd Conference. BROOKS, J, and GLENNIE, K (editors). (London: Graham & Trotman.)

JEANS, P J F. 1971. The relationship between the Skiddaw Slates and the Borrowdale Volcanics. Nature Physical Sciences, Vol. 234, 59.

JEANS, P J F. 1972. The junction between the Skiddaw Slates and Borrowdale Volcanics in Newlands Beck, Cumberland. Geological Magazine, Vol. 109, 25–28.

JEANS, P J F. 1973. The structure, metamorphism and stratigraphy of the Skiddaw Slates east of Crummock Water, Cumberland. Unpublished PhD thesis, University of Birmingham.

JENKINS, C J. 1982. Isograptus gibberulus (Nicholson) and the isograptids of the Arenig Series. Proceedings of the Yorkshire Geological Society, Vol. 44, 219–248.

JOHNSON, E W. 1992. Geology of the Stoupdale area of Black Combe, south-west Cumbria. British Geological Survey Technical Report, WA/92/71.

JOHNSON, E W, and 14 others. 2001. Geology of the country around Ulverston. Memoir of the British Geological Survey, Sheet 48 (England and Wales).

JOHNSON, G A L. 1961. Skiddaw Slates proved in the Teesdale Inlier. Nature, Vol. 90, 996–997.

KENNAN, P S, and KENNEDY, M J. 1983. Coticules — a key to regional correlation along the Appalachian–Caledonian orogen? 355–361 in Regional trends in the geology of the Appalachian–Caledonian–Hercynian–Mauritanide Orogen. SCHENK, P E (editor). (Dordrecht: D Reidel Publishing Company.)

KIRBY, G A, and 9 others. 2000. The structure and evolution of the Craven Basin and adjacent areas. Subsurface Memoir of the British Geological Survey.

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KNELLER, B C, and BELL, A M. 1993. An Acadian mountain front in the English Lake District: the Westmorland Monocline. Geological Magazine, Vol. 130, 203–213.

KNELLER, B C, KING, L M, and BELL, A M. 1993. Foreland basin development and tectonics on the northwest margin of eastern Avalonia. Geological Magazine, Vol. 130, 691–697.

KNELLER, B C, SCOTT, R W, SOPER, N J, JOHNSON, E W, and ALLEN, P M. 1994. Lithostratigraphy of the Windermere Supergroup, Northern England. Geological Journal, Vol. 29, 219–240.

KOKELAAR, B P. 1988. Tectonic controls of Ordovician arc and marginal volcanism in Wales. Journal of the Geological Society of London, Vol. 145, 759–765.

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LAPWORTH, C. 1879. On the tripartite division of the Lower Palaeozoic rocks. Geological Magazine, Vol. 6, 1–15.

LEE, M K. 1986. A new gravity survey of the Lake District and three-dimensional model of the granite batholith. Journal of the Geological Society of London, Vol. 143, 425–435.

LEE, M K. 1989. Upper crustal structure of the Lake District from modelling and image processing of potential field data. British Geological Survey Technical Report, WK/89/1.

LEEDAL, G P, and WALKER, G P L. 1950. A restudy of the Ingletonian Series of Yorkshire. Geological Magazine, Vol. 87, 57–66.

LEEDER, M R. 1982. Sedimentology — process and product. (London: George Allen and Unwin.)

LINDHOLM, K. 1991. Ordovician graptolites from the early Hunneberg of southern Scandinavia. Palaeontology, Vol. 34, 283–327.

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WOODCOCK, N H, and 6 others. 1999. Revised lithostratigraphy of the Manx Group, Isle of Man. 45–68 in In sight of the suture: the geology of the Isle of Man in its Iapetus Ocean context.

WOODCOCK, N H, QUIRK, D G, FITCHES, W R, and BARNES, R P (editors). Geological Society of London, Special Publication, No. 160.

WOOLACOT, D. 1923. On a boring at Roddymoor Colliery, near Crook, Co Durham. Geological Magazine, Vol. 60, 50–62.

YOUNG, B, and COOPER, A H. 1988. The geology and mineralisation of Force Crag Mine, Cumbria. Proceedings of the Cumberland Geological Society, Vol. 5, 5–11.

Appendix 1 Mineralised localities in and adjacent to the Skiddaw Group

Locality

Grid reference

Principal ores

1

Roughton Gill

[NY 303 344]

Pb, Zn, Cu, As

2

Carrock Mine

[NY 323 329]

W, Mo, Pb, Zn, Cu

3

Carrock End

[NY 353 335]

Cu, Pb

4

Bannerdale

[NY 335 295]

Pb, Zn

5

Threlkeld (Halls Fells)

[NY 325 260]

Pb, Zn, Ba

6

Glenderaterra

[NY 296 274]

Pb, Zn

7

Robin Hood

[NY 228 328]

Sb

8

Loweswater

[NY 145 215]

Pb, Zn

9

Crag Fell and Croasdale

[NY 100 180]

Hematite

10

Kinniside

[NY 041 148]

Pb, Ba

11

Knockmurton

[NY 095 190]

Hematite

12

Sosgill

[NY 103 237]

Pb

13

Thornthwaite

[NY 223 258]

Pb, Zn

14

Barrow Mine

[NY 232 222]

Pb, Zn

15

Brandlehow

[NY 250 196]

Pb, Zn

16

Goldscope

[NY 226 185]

Pb, Cu, As

17

Force Crag

[NY 200 215]

Baryte, Pb, Zn

18

Black Combe

[NY 135 855]

Cu, Pb, Zn, As

Appendix 2 Skiddaw Group graptolite localities

No.

Locality

Grid reference†

Zone

Formation

Report no.

1

St Helen’s Bridge

[NY 420 3122]

murrayi

Bitter Beck

89/114 (4, 5)

2

Harrot Hill

[NY 1060 3014]

murrayi

Watch Hill

89/114 (2)

3

Cockermouth bypass

[NY 1119 2986]

murrayi

Watch Hill

95/136 (1)

4

Trusmadoor

[NY 2777 3363]

murrayi

Watch Hill

89/116 (36)

5

Trusmadoor

[NY 2782 3360]

murrayi

Hope Beck

89/116 (37)

6

Trusmadoor

[NY 2792 3354]

phyllograptoides ?

Hope Beck

89/116 (38)

7

Trusmadoor

[NY 2797 3348]

phyllograptoides ?

Hope Beck

89/116 (39)

8

Trusmadoor

[NY 2774 3354]

phyllograptoides

Hope Beck

89/116 (40, 41)

9

Burn Tod Gill

[NY 2799 3340]

phyllograptoides

Hope Beck

89/116 (43)

10

Burn Tod Gill

[NY 2795 3339]

phyllograptoides

Hope Beck

89/116 (44), 96/258

11

Blaze Beck

[NY 1781 2558] to [NY 1783 2533]

varicosus

Hope Beck

96/127 (2–6)

12

Hope Beck

[NY 1695 2376]

varicosus

Hope Beck

96/57 (2)

13

W of Blaze Bridge

[NY 179 251]

varicosus

Loweswater

96/127 (8, 9)

14

Ling Fell (W of summit)

[NY 1731 2868]

varicosus ?

Loweswater

96/127 (30)

15

Scawgill Bridge Quarry

c.[NY 178 258]

varicosus

Loweswater

96/127 (10–13)

16

E of Scawgill Quarry

c.[NY 181 259]

simulans

Loweswater

96/127 (14, 15, 17)

17

Aiken Beck

[NY 1839 2612]

varicosus or simulans ?

Loweswater

96/127 (18)

18

Whinlatter

[NY 1883 2503]

varicosus

Loweswater

96/127 (20)

19

Whinlatter (W end)

[NY 1876 2517]

simulans

Loweswater

96/127 (19)

20

Aiken

c.[NY 199 260]

simulans

Loweswater

96/127 (21, 22)

21

Ling Fell (S of summit)

c.[NY 1801 2844]

simulans ?

Loweswater

96/127 (26, 27)

22

Embleton High Common

[NY 1696 2793]

simulans

Loweswater

96/127 (31)

23

Tom Rudd Beck

[NY 1638 2898]

simulans

Loweswater

96/127 (33)

24

Barf (around summit)

c.[NY 214 268]

simulans ?

Loweswater

93/74 (21, 26, 27)

25

Barf (around Slape Crag)

c.[NY 217 265]

simulans

Loweswater

93/74 (22–24)

26

Barf (N of summit)

c.[NY 2145 2706]

simulans

Loweswater

93/74 (29–32)

27

W of Woodend Brow Quarry

c.[NY 217 277]

simulans

Loweswater

93/74 (36–39)

28

Dodd, Hopebeck

c.[NY 167 231]

varicosus

Loweswater

96/57 (7–9)

29

Hope Gill

c.[NY 1810 2257]

varicosus

Loweswater

96/57 (11, 12)

30

Hope Gill

[NY 1824 2245]

simulans

Loweswater

96/57 (13)

31

Swinside

[NY 1791 2335]

varicosus or simulans

Loweswater

96/57 (14)

32

Swinside

c.[NY 1798 2330]

varicosus or simulans ?

Loweswater

96/57 (15, 16)

33

W of Hopegill Head

c.[NY 178 223]

simulans ?

Loweswater

96/57 (18, 19)

34

Hobcarton End

c.[NY 196 241]

simulans ?

Loweswater

96/57 (20, 21)

35

Carl Side (S of summit)

[NY 2549 2785]

simulans

Loweswater

93/244 (9)

36

Burnbank Fell

[NY 1154 2172]

simulans

Loweswater

95/138 (2)

37

Burnbank Fell

[NY 1146 2156]

simulans

Loweswater

95/138 (3)

38

Darling Fell

[NY 1268 2205]

simulans

Loweswater

95/138 (4)

39

Jonah’s Gill

[NY 1904 3414] to [NY 1910 3436]

varicosus

Loweswater

91/218 (2, 3)

40

Mungrisdale Quarry

[NY 3633 3058]

varicosus

Loweswater

89/122 (1)

41

Undercrag, Mungrisdale

[NY 362 306]

simulans

Loweswater

89/122 (2, 3)

42

Raven Crag, Mungrisdale

[NY 3610 3105]

simulans ?

Loweswater

89/122 (5), 91/219 (2)

43

Wythe Gill

[NY 1683 2641]

victoriae

Kirk Stile

96/127 (34)

44

Greystones

c.[NY 176 266]

victoriae ?

Kirk Stile

96/127 (36–39)

45

Widow Hause

[NY 1811 2716]

victoriae ?

Kirk Stile

96/127 (40)

46

W of Broom Fell

c.[NY 189 270]

victoriae ?

Kirk Stile

96/127 (42–43)

47

Broom Fell–Lord’s Seat

c.[NY 197 267]

victoriae ?

Kirk Stile

96/127 (55–57)

48

SW of Lord’s Seat

c.[NY 199 262]

victoriae ?

Kirk Stile

96/127 (58–59)

49

Spout Force

[NY 1825 2625]

victoriae ?

Kirk Stile

96/127 (44)

50

SW of Broom Fell

c.[NY 190 268]

victoriae ?

Kirk Stile

96/127 (45–48)

51

NW of Lord’s Seat

[NY 2013 2676]

victoriae ?

Kirk Stile

93/74 (18)

52

W of Lord’s Seat

[NY 2028 2667]

victoriae ?

Kirk Stile

93/74 (19)

53

W of Woodend Brow

[NY 2098 2770]

victoriae ?

Kirk Stile

93/74 (41)

54

Darling How Plantation

c.[NY 180 264]

victoriae ?

Kirk Stile

96/127 (49–53)

55

Woodend

[NY 2161 2776]

victoriae ?

Kirk Stile

93/74 (40)

56

W of Woodend

[NY 2103 2737]

victoriae ?

Kirk Stile

93/74 (34)

57

W of Woodend

[NY 2122 2726]

victoriae ?

Kirk Stile

93/74 (33)

58

Ullister Hill

c.[NY 210 255]

victoriae ?

Kirk Stile

93/74 (2, 4, 6–9)

59

Seat How

c.[NY 213 257]

victoriae–gibberulus

Kirk Stile

93/74 (11–16)

60

Wythop Woods

[NY 2094 2920]

victoriae

Kirk Stile

93/74 (51)

61

Wythop Woods

c.[NY 211 290]

victoriae–gibberulus

Kirk Stile

93/74 (50)

62

Beck Wythop

[NY 2118 2828]

victoriae–gibberulus

Kirk Stile

93/74 (43)

63

Beck Wythop

c.[NY 210 283]

victoriae–gibberulus

Kirk Stile

93/74 (45–47)

64

S of Sale Fell

[NY 2016 2939]

victoriae ?

Kirk Stile

93/74(48)

65

S of Sale Fell

c.[NY 197 295]

victoriae ?

Kirk Stile

96/127 (60–63)

66

Bladder Keld

c.[NY 177 282]

victoriae ?

Kirk Stile

96/127 (65–72)

67

Embleton High Common

c.[NY 173 278]

victoriae ?

Kirk Stile

96/127 (74–75)

68

W of Hopegill Head

[NY 177 221] to [NY 180 214]

simulans ?

Kirk Stile

96/57 (25–32)

69

Hope Gill, below Hopegill Head

[NY 183 224] to [NY 186 226][NY 186 226]

simulans ?

Kirk Stile

96/57 (33, 38)

70

Hope Gill, below Hopegill Head

around 1857 2224

victoriae

Kirk Stile

96/57 (34–37)

71

WSW of Grisedale Pike

[NY 1962 2240]

gibberulus ?

Kirk Stile

96/57 (49)

72

N of Grisedale Pike

[NY 1984 2265]

gibberulus

Kirk Stile

96/57 (49)

73

Summit of Grisedale Pike

[NY 1984 2253]

gibberulus ?

Kirk Stile

96/57 (48)

74

SW of Grisedale Pike

[NY 1966 2222]

gibberulus

Kirk Stile

96/57 (47)

75

S & SE of Grisedale Pike

c.[NY 202 224]

gibberulus ?

Kirk Stile

93/148 (1, 3)

76

500 m SE of Grisedale Pike

[NY 2013 2218]

gibberulus

Kirk Stile

93/148 (2)

77

Long Crag

[NY 198 218]

gibberulus

Kirk Stile

96/57 (51, 52)

78

Force Crag

[NY 197 214]

cucullus

Kirk Stile

96/57 (55)

79

Outerside (NW face)

around [NY 211 216]

cucullus

Kirk Stile

93/148 (4–9)

80

Outerside (E face)

[NY 2143 2155]

artus

Kirk Stile

93/148 (11)

81

E of Felldyke

[NY 0880 1975]

victoriae or gibberulus

Kirk Stile

93/192 (7)

82

S of Carling Knott

[NY 1198 1986]

victoriae or gibberulus

Kirk Stile

93/169 (3)

83

Low Fell, Loweswater

[NY 1390 2286]

gibberulus ?

Kirk Stile

95/138 (7)

84

Winnah

[NY 1062 2420]

gibberulus

Kirk Stile

95/138 (8)

85

Mosser Beck

[NY 1199 2445]

simulans–low victoriae

Kirk Stile

95/138 (11)

86

Sandy Beck Gill, near Wood Farm

[NY 1294 2620]

gibberulus

Kirk Stile

95/136 (3)

87

Abbey Gate

[NY 1280 2783]

gibberulus

Kirk Stile

95/136 (2)

88

Hodgson How

[NY 2437 2364]

victoriae or gibberulus

Kirk Stile

93/150 (7)

89

Heavy Sides

[NY 217 235]

gibberulus ?

Kirk Stile

93/149 (10–12)

90

Knott Head, trackside

[NY 2241 2412]

gibberulus

Kirk Stile

93/149 (24)

91

Knott Head, quarry

[NY 2235 2444]

gibberulus

Kirk Stile

93/149 (26)

92

Sale Fell

[NY 1935 2960]

simulans

Loweswater

96/127 (24)

93

Latrigg

[NY 2812 2432]

zone uncertain

Kirk Stile

93/262 (1, 2)

94

E of Lonscale Fell

[NY 2924 2704]

gibberulus

Kirk Stile

93/244 (23)

95

Jenkin Hill

[NY 2740 2688]

gibberulus or cucullus

Kirk Stile

93/244 (19)

96

Doups

[NY 253 267]

gibberulus

Kirk Stile

93/244 (11)

97

Skiddaw Dodd

c.[NY 2455 2692]

cucullus

Kirk Stile

93/242 (2)

98

S of Carlside Tarn

c.[NY 2570 2795]

victoriae or gibberulus

Kirk Stile

93/244 (5)

99

NNW of Ullock Pike

[NY 2440 2885]

gibberulus ?

Kirk Stile

93/78 (6)

100

NW of Ullock Pike

around [NY 242 290]

gibberulus

Kirk Stile

93/78 (7–9)

101

Head of Sandbeds Gill

[NY 2395 2900]

gibberulus

Kirk Stile

89/371 (2)

102

Sandbeds Gill

[NY 2838 2900]

gibberulus

Kirk Stile

89/371 (1)

103

The Edge and N of Sandbeds Gill

[NY 239 292] to [NY 241 293][NY 241 293][NY 241 293]

gibberulus

Kirk Stile

89/371 (5–7)

104

Sand Beds

2397 2934

gibberulus

Kirk Stile

89/371 (8)

105

Raven Crag, N of Sand Beds

2381 2946

gibberulus

Kirk Stile

89/371 (10, 11)

106

Raven Crag, below Kiln Pots

[NY 2375 2956]

gibberulus

Kirk Stile

89/371 (12, 13)

107

Kiln Pots

[NY 2407 2962]

gibberulus

Kirk Stile

89/371 (15)

108

Raven Crag, below Kiln Pots

c.[NY 2375 2965]

cucullus

Kirk Stile

89/371 (14, 16)

109

N of Raven Crag

c.[NY 238 297]

cucullus

Kirk Stile

89/371 (18–20)

110

S of Ling How

c.[NY 241 298]

cucullus

Kirk Stile

89/371 (21–23)

111

Ling How

[NY 239 299]

cucullus

Kirk Stile

89/371 (24–26)

112

Watches

[NY 239 304] to [NY 242 303][NY 242 303]

cucullus

Kirk Stile

89/116 (14–17)

113

N of Watches

[NY 2398 3056]

cucullus

Kirk Stile

89/116 (13)

114

Southerndale Beck

[NY 2415 3067]

cucullus

Kirk Stile

89/116 (18)

115

Randel Crag

[NY 2535 2945]

gibberulus

Kirk Stile

89/364 (3–5)

116

White Horse Fell

[NY 2506 3052]

gibberulus

Kirk Stile

96/134

117

Dead Beck

c.[NY 263 315]

cucullus

Kirk Stile

89/116 (24–25)

118

Dead Beck

c.[NY 263 314]

cucullus

Kirk Stile

89/116 (26–27)

119

Dead Beck

c.[NY 264 315]

cucullus

Kirk Stile

89/116 (28–29)

120

Scalegill Beck

[NY 201 356]

gibberulus or cucullus

Kirk Stile

89/116 (2–3)

121

Scalegill Beck

[NY 206 355]

cucullus

Kirk Stile

89/116 (4)

122

Over Water Spillway

[NY 259 355]

zone uncertain

Kirk Stile

89/116 (7)

123

River Ellen, E of Crag Wood

[NY 2722 3438]

victoriae or gibberulus

Kirk Stile

89/116 (34)

124

Gate Gill, Blencathra

c.[NY 3173 2636]

cucullus

Kirk Stile

94/70 (4, 5)

125

Hallsfell Top

c.[NY 325 269]

cucullus

Kirk Stile

94/70 (7–9)

126

Doddick Fell

c.[NY 338 273]

cucullus

Kirk Stile

94/70 (13–16)

127

Scales Fell

[NY 3318 2771]

cucullus

Kirk Stile

94/70 (22)

128

Glenderamackin Valley

[NY 3455 2806]

cucullus

Kirk Stile

94/70 (25–27)

129

The Tongue, Bannerdale

[NY 3500 3000]

cucullus

Kirk Stile

91/219 (9)

130

Souther Fell (NW end)

[NY 3529 2952]

cucullus?

Kirk Stile

91/219 (10)

131

Souther Fell (summit)

[NY 355 296]

artus

Kirk Stile

91/219 (13)

132

Hazelhurst

[NY 3595 2890]

cucullus

Kirk Stile

89/122 (9)

133

Hazelhurst

[NY 3608 2900]

cucullus

Kirk Stile

89/122 (10)

134

N of Hazelhurst

[NY 3628 2929]

artus

Kirk Stile

98/122 (110

135

Wilton

c.[NY 036 111]

victoriae–gibberulus

Buttermere

93/190 (1–4)

136

River Calder

[NY 0687 1174]

upper Tremadoc

Buttermere

93/191 (2)

137

Beck Grains

[NY 776 1128]

victoriae–gibberulus

Buttermere

93/191 (9)

138

Buttermere Quarry

[NY 1732 1726]

varicosus–simulans

Buttermere

93/168 (3)

139

Rowantree Beck

[NY 1776 1816]

gibberulus ?

Buttermere

93/168 (7)

140

SW of Whiteless Pike

[NY 1766 1872]

varicosus or simulans

Buttermere

93/168 (12)

141

Whiteless Pike (summit)

[NY 1797 1894]

varicosus

Buttermere

93/168 (13)

142

Dry Gill

[NY 1838 1829]

victoriae or gibberulus

Buttermere

93/168 (14)

143

Ill Gill

[NY 2015 1900]

victoriae or gibberulus

Buttermere

93/60 (9–10)

144

Ramps Gill

c.[NY 192 185]

victoriaeor gibberulus

Buttermere

93/168 (16–18)

145

High Snockrigg

[NY 1863 1682]

varicosus or simulans

Buttermere

93/168 (2)

146

Robinson

[NY 2010 1700]

phyllograptoides or varicosus

Buttermere

93/60 (2)

147

Littledale Edge

[NY 2132 1595]

victoriae–gibberulus ?

Buttermere

93/60 (7)

149

Scope Beck

[NY 2187 1821]

varicosus or simulans ?

Buttermere

93/60 (12)

150

Sleet Hause

c.[NY 222 207]

simulans

Buttermere

93/148 (14–16)

151

Rowling End

[NY 2297 2031]

gibberulus

Buttermere

93/150 (2, 3)

152

Stonycroft Gill

[NY 2284 2122]

simulans

Buttermere

93/150 (1)

153

Swinside

[NY 2431 2250]

simulans

Buttermere

93/150 (6)

154

Troutbeck, Threlkeld Common

[NY 3782 2704]

cucullus ?

Buttermere

93/265 (5)

155

Troutbeck

[NY 3837 2704]

zone uncertain

Buttermere

93/265 (6)

156

Birkett Beck

[NY 3280 2469]

cucullus ?

Tarn Moor

93/263 (2)

157

Cawell Beck

[NY 3416 2582]

artus

Tarn Moor

95/18 (1)

158

Mosedale Beck, Threlkeld

[NY 3546 2307]

artus

Tarn Moor

93/264 (1)

159

Mosedale Beck, horizons A-H

[NY 3558 2422]

cucullus

Tarn Moor

95/66 (2)

160

Mosedale Beck, horizons I-M

[NY 3558 2411]

artus

Tarn Moor

95/66 (3–7)

161

Matterdale Beck

[NY 3888 2346]

artus

Tarn Moor

93/264 (9)

162

Greenside Lead Mine

c.[NY 363 178]

cucullus–artus boundary

Tarn Moor

95/68 (2)

163

Aik Beck

[NY 4732 2200]

artus

Tarn Moor

95/37 (1)

164

Aik Beck

[NY 4732 2212]

artus

Tarn Moor

95/37 (3, 4)

165

Aik Beck

[NY 4731 2237]

basal murchisoni ?

Tarn Moor

96/260 (1, 2)

166

Aik Beck

[NY 4727 2257]

upper artus ?

Tarn Moor

95/37 (10)

167

River Lowther, Shap

[NY 5482 1450]

cucullus or artus

Tarn Moor

see 96/260

168

Thornship Beck

[NY 5504 1310]

artus

Tarn Moor

96/260 (4)

169

Keld Gill

[NY 5405 1344]

artus

Tarn Moor

96/260 (3)

170

Seathwaite How Quarry

c.[NY 175 308]

cucullus

Kirk Stile

89/114 (12)

171

Anna Crag, Black Combe

[SD 142 867]

cucullus

Skiddaw Group

86/227 (2)

172

White Hall Knott

[SD 1582 8574]

cucullus

Skiddaw Group

86/227 (3)

173

Hook Knott

[SD 1643 8577]

cucullus

Skiddaw Group

86/227 (4)

174

Askam brick quarry

[SD 2250 7622]

artus ?

Skiddaw Group

95/1 (5)

175

Park Farm brick quarry

c.[SD 218 754]

artus

Skiddaw Group

95/1 (1–4)

Appendix 3 Compositional data for the Skiddaw Group: geochemical and modal analysis

Geochemical analysis of Skiddaw Group rock samples

The majority of analysed samples (Table 7) were collected (where rock exposure allowed) at 50 to 70 m intervals along two long traverses across the principal outcrop of the Skiddaw Group prior to geological mapping. The more westerly traverse ran from the junction with the Borrowdale Volcanic Group near Dale Head [NY 2188 1547] across Robinson, Newlands Hause, Whiteless Pike, Crag Hill, Grizedale Pike, Seat How (Thornthwaite), and Barf to Peel Wyke at the north-western end of Basenthwaite Lake [NY 2044 3076]. The eastern traverse ran from the A66 road east of Scales [NY 3499 2708] across Mungrisdale Common, the River Caldew, Knott, and Trusmadoor to Stockdale [NY 2607 3484] near to the Eycott Volcanic Group junction. These samples, which included altered (bleached) and contact-metamorphosed rocks, were supplemented by others collected to give improved coverage of specific lithologies and formations, principally from Hope Beck, Bitter Beck, Watch Hill, Mosedale Beck (Matterdale), Aik Beck (Pooley Bridge), the area west of Shap village and Black Combe.

Analysed samples comprised about 2 kg of fresh rock chips with no visible vein material which were jaw crushed to about 5 mm diameter. A 100–150 g split of the crush was tema-milled in an agate pot and splits of the resulting powder analysed for the elements reported. Hand specimens were also collected at each site and these, together with splits of the crushed and milled material, are stored in the BGS National Geological Records Centre at Keyworth. B, Be and Li were determined by Inductively Coupled Argon Plasma Spectrometry (analysts: B Tait and L Ault) in the BGS laboratories. The remaining elements were determined by X-ray Fluorescence Spectrometry using fused glass beads for major elements and pressed powder pellets for trace constituents. The majority of samples were analysed by this method at the University of Nottingham (analysts P K Harvey and B P Atkin) with some, principally from the Tarn Moor Formation, determined in the BGS laboratories at Keyworth (analysts A S Robertson, M N Ingham, P H Miles and A G Scothern).

Modal analysis of Skiddaw Group rock samples

Thin sections of sandstone samples were point-counted (100–300 points per slide) by R M Moore using the Gazzi-Dickinson method (Dickinson, 1970) to eliminate variation of modal composition with grain size due to breakage of lithic fragments into constituent grains (Ingersoll et al., 1984). The method requires that components of lithic clasts > 0.0625 mm diameter are recorded as separate grains. The aim of the analysis was to record the mineralogy at deposition, hence altered grains were counted as the original phase if relict fragments could be identified (Table 8), (Table 8), (Table 10), (Table 11), (Table 12).

Tables

(Table 7) Median values for chemical analyses of Skiddaw Group rock samples, omitting altered (bleached) and contact metamorphosed samples

1

2

3

4

5

6

7

8

9

10

11

12

13

SiO2

57.35

58.73

78.99

54.72

77.83

55.34

70.28

55.93

68.77

57.09

73.29

52.43

57.59

Al2O3

20.69

20.05

8.31

22.02

7.96

22.19

12.08

21.94

11.64

20.63

10.09

24.71

21.61

TiO2

1.10

0.96

0.44

1.12

0.55

1.16

0.73

1.13

0.67

1.08

0.61

1.03

1.09

Fe2O5

9.20

8.53

5.68

9.69

6.87

8.34

7.52

9.33

8.38

9.92

8.12

9.72

8.65

MgO

1.80

2.12

1.07

1.93

1.29

1.90

1.61

1.83

1.74

1.88

1.42

1.85

1.71

CaO

0.20

0.29

0.37

0.17

0.22

0.23

0.34

0.24

0.60

0.18

0.19

0.37

0.31

Na2O

0.88

1.05

1.23

0.93

0.09

0.75

1.70

0.86

0.86

0.88

0.22

0.73

0.81

K2O

2.99

3.20

1.02

3.18

0.79

3.95

1.35

3.12

1.36

2.98

1.24

3.17

3.82

MnO

0.23

0.16

0.14

0.23

0.31

0.22

0.27

0.18

0.44

0.34

0.28

0.39

0.08

P2O5

0.17

0.13

0.13

0.16

0.12

0.22

0.20

0.17

0.24

0.17

0.13

0.25

0.16

LOI

4.33

4.45

2.00

4.80

2.46

4.41

2.95

4.51

3.56

4.42

2.38

5.87

4.07

As

105

21

3

29

14.5

17

14

16

26

16.5

5

25

31

B

2.7

95

19

88

26

73

27

84

23

59

16

-

104

Ba

64

636

211

694

214.5

772.5

302

672

302.5

678.5

260

480

742

Be

29

2.6

0.9

3.5

1.3

3.2

1.3

3.0

1.0

3.0

1.0

-

3.5

Ce

132

87

45

104

48.5

96

51

102

61.5

86.5

61

98

92

Co

115

24

15

21

19

19.5

18

17

23

23

15

29

8

Cr

19

117

41

134

48.5

121.5

61

120

60.5

115.5

62

105

130

Cu

52

28

13

35

13

25.5

24

26

29.5

30.5

15

32

24

Ga

28

25

9

28

9.5

28

14

27

15.5

26

13

-

28

La

92

45.5

15

45

21.5

473.5

21

47

23.5

43

20

60

40

Li

26

108.5

46

116

95.5

109

76

111

84.5

96

46

-

49

Mo

16

<1

<1

<1

<1

<1

<1

<1

<1

<1

<1

1

<1

Nb

139

17.5

9

20

10

20

13

22

12.5

20

12

17

21

Ni

107

52

18

61

24

53

34

52

44

55

29

52

39

Pb

34

13.5

12

19

34.5

18.5

18

20

17.5

14

11

11

39

Rb

182

137

45

144

41

178.5

65

146

55.5

145

63

139

160

Sr

20

91.5

32

113

30

119

80

121

58

104

21

104

65

S

<1

245

<10

<10

1022.5

<10

1279

<10

1689.5

25.5

5

485

Th

656

13.5

6

13

6.5

14

8

15

8

14

7

17

15

U

41

2.5

<2

3

2

3

2

3

2

3

2

4

3

V

86

125.5

57

139

58

145

78

140

81.5

129

65

130

131

Y

16

35

20

36

25

38.5

27

36

29.5

36

29

28

36

Zn

12

78

42

99

82

87.5

84

91

79

96.5

52

103

90

Zr

3

178.5

138

168

187.5

177.5

202

176

193

176

203

124

200

n

308

16

13

46

10

28

33

178

17

97

11

30

5

(Table 8) Means and ranges of petrographical data (%) for the Watch Hill Formation

a.Watch Hill (n=7)

mean

minimum

maximum

strained monocrystalline quartz

45.76

38.00

53.00

unstrained monocrystalline quartz

3.07

0.50

10.00

polycrystalline quartz

7.43

3.00

12.00

plagioclase feldspar

3.14

0.67

5.50

untwinned feldspar

10.07

6.00

15.50

volcanic lithic fragments

2.00

0.00

9.00

metavolcanic lithic fragments

1.17

0.00

5.50

sedimentary lithic fragments

9.36

1.67

12.00

metasedimentary lithic fragments

2.95

0.50

7.00

detrital mica

0.14

0.00

0.50

pyrite

1.50

0.50

3.00

carbonate cement

0.00

0.00

0.00

clay matrix

12.95

4.00

26.00

chlorite

0.38

0.00

1.33

heavy minerals

0.07

0.00

1.50

matrix

15.05

5.50

28.67

b.Great Cockup (n=4)

mean

minimum

maximum

strained monocrystalline quartz

24.75

23.00

26.50

unstrained monocrystalline quartz

4.12

2.50

6.50

polycrystalline quartz

12.00

8.00

18.50

plagioclase feldspar

1.37

0.50

2.50

untwinned feldspar

4.25

2.00

7.00

volcanic lithic fragments

4.62

2.50

7.00

metavolcanic lithic fragments

2.50

0.50

5.50

sedimentary lithic fragments

21.00

12.00

27.50

metasedimentary lithic fragments

0.25

0.00

0.50

detrital mica

0.62

0.50

1.00

pyrite

0.12

0.00

0.50

carbonate cement

0.00

0.00

0.00

clay matrix

23.62

19.00

27.50

chlorite

0.12

0.00

0.50

heavy minerals

0.62

0.00

1.00

matrix

25.12

24.00

38.50

(Table 9) Means and ranges of petrographical data (%) for the Hope Beck Formation (n=8)

mean

minimum

maximum

strained monocrystalline quartz

44.71

29.00

56.00

unstrained monocrystalline quartz

2.23

0.00

8.00

polycrystalline quartz

3.77

0.00

7.50

plagioclase feldspar

0.52

0.00

2.00

untwinned feldspar

3.42

0.00

7.00

volcanic lithic fragments

0.52

0.00

2.50

metavolcanic lithic fragments

0.50

0.00

1.50

sedimentary lithic fragments

3.06

0.00

8.00

metasedimentary lithic fragments

1.25

0.00

4.00

detrital mica

0.77

0.00

3.00

pyrite

0.81

0.00

2.00

carbonate cement

0.44

0.00

3.00

clay matrix

37.25

19.00

52.00

chlorite

0.58

0.00

2.00

heavy minerals

0.17

0.00

5.00

matrix

40.02

20.00

52.00

(Table 10) Means and ranges of petrographical data (%) for the Loweswater Formation, excluding high-matrix greywackes and greywackes from Jonah’s Gill (n=26)

mean

minimum

maximum

strained monocrystalline quartz

36.95

23.50

56.00

unstrained monocrystalline quartz

2.97

0.00

9.50

polycrystalline quartz

5.61

2.00

10.50

plagioclase feldspar

3.03

0.00

6.50

untwinned feldspar

7.33

0.00

14.50

volcanic lithic fragments

0.61

0.00

2.50

metavolcanic lithic fragments

0.72

0.00

4.50

sedimentary lithic fragments

2.29

0.00

5.00

metasedimentary lithic fragments

1.44

0.00

3.67

detrital mica

1.47

0.00

7.00

pyrite

0.79

0.00

4.00

carbonate cement

2.88

0.00

32.50

clay matrix

32.43

11.50

53.67

chlorite

1.06

0.00

8.00

heavy minerals

0.39

0.00

1.50

matrix

39.00

22.00

55.00

(Table 11) Means and ranges of petrographical data (%) for the Loweswater Formation

a.high-matrix greywackes (n=5)

mean

minimum

maximum

strained monocrystalline quartz

21.47

18.00

29.33

unstrained monocrystalline quartz

4.53

1.67

7.33

polycrystalline quartz

5.53

3.33

7.00

plagioclase feldspar

2.20

1.00

3.00

untwinned feldspar

7.47

3.33

10.67

volcanic lithic fragments

0.87

0.33

2.33

metavolcanic lithic fragments

1.40

0.67

3.00

sedimentary lithic fragments

2.33

1.00

4.67

metasedimentary lithic fragments

1.47

1.00

2.67

detrital mica

2.13

1.33

3.00

pyrite

0.93

0.00

2.00

carbonate cement

0.80

0.00

4.00

clay matrix

46.20

42.00

50.33

chlorite

1.80

0.33

3.00

heavy minerals

0.87

0.00

1.67

matrix

52.73

50.33

56.67

—

b.greywackes from Jonah’s Gill (n=4)

mean

minimum

maximum

strained monocrystalline quartz

45.33

40.67

48.33

unstrained monocrystalline quartz

4.00

1.67

5.00

polycrystalline quartz

3.67

1.33

8.33

plagioclase feldspar

0.75

0.33

1.33

untwinned feldspar

3.83

2.00

7.33

volcanic lithic fragments

0.00

0.00

0.00

metavolcanic lithic fragments

0.00

0.00

0.00

sedimentary lithic fragments

0.67

0.00

1.00

metasedimentary lithic fragments

0.67

0.00

1.33

detrital mica

0.58

0.33

1.33

pyrite

0.50

0.00

1.67

carbonate cement

14.42

4.33

23.33

clay matrix

24.75

10.33

38.33

chlorite

0.42

0.00

0.67

heavy minerals

0.42

0.00

1.00

matrix

41.08

32.33

49.33

(Table 12) Means and ranges of petrographical data (%) for the Kirk Stile Formation (n=4)

mean

minimum

maximum

strained monocrystalline quartz

30.12

20.00

42.00

unstrained monocrystalline quartz

1.12

0.00

4.00

polycrystalline quartz

3.00

1.00

6.00

plagioclase feldspar

1.50

0.00

4.00

untwinned feldspar

3.00

0.50

6.00

volcanic lithic fragments

0.37

0.00

1.50

metavolcanic lithic fragments

0.75

0.00

2.00

sedimentary lithic fragments

3.12

0.00

8.50

metasedimentary lithic fragments

2.25

0.00

7.00

detrital mica

1.46

0.00

3.50

pyrite

0.75

0.00

2.50

carbonate cement

11.87

0.00

47.00

clay matrix

35.67

13.50

48.67

chlorite

3.87

0.00

13.00

heavy minerals

1.12

0.00

2.50

matrix

54.75

51.00

61.00

Fossil inventory for the Skiddaw Group

The list below comprises microfossils and macrofossils recorded from the Skiddaw Group during the course of the resurvey, and as such is an indication of the biodiversity of the group. To satisfy the rules and recommendations of the International Codes of Botanical and Zoological Nomenclature, authors of species referred to in the memoir are included.

Acritarchs

Graptolites

Trilobites

Other macrofossils

Brachiopoda

Mollusca (s.l.)

Arthropoda


Figures, plates and tables

Figures

Skiddaw Group and overlying lithostratigraphical units in the Northern Fells and Central Fells belts of the Lake District.

(Figure 1) Distribution of Skiddaw Group inliers, principal faults and named boreholes. AB Allenheads Borehole; BF Butterknowle Fault; BS Beckermonds Scar Borehole; CF Craven Faults; CPF Causey Pike Fault; DF Dent Fault; GF Gilcrux Fault; LF Lunedale Fault; MF Maryport Fault; NFF Ninety Fathom Fault; PF Pennine Faults; RD Roddymoor Borehole; RK Rookhope Borehole; SBF Stublick Fault; STF Stockdale Fault.

(Figure 2) Palaeocontinental reconstructions (from Van Staal et al., 1998) from early Ordovician (475 Ma) to early Silurian (435 Ma), showing the northwards drift of Avalonia and the closure of Iapetus.Av Avolonia; Arm Armorica

(Figure 3) Stratigraphical belts and an outline lithostratigraphy of the Skiddaw Group.

(Figure 4) Correlation of graptolite and acritarch zones in the Skiddaw Group.

(Figure 5a) Comparison of Cooper and Lindholm’s (1990) succession of biohorizons with the zonal sequence developed in the Skiddaw Group. The sequence of biohorizons is on the left and the first appearances in the ‘composite standard sequence’ are shown for those species that are known from the Skiddaw Group.

(Figure 5b) Graphical correlation of the first appearances in (Figure 5)a with the earliest known appearances in the Skiddaw Group. First and second iterations (see text).

(Figure 6) Distribution of graptolites in the section at Trusmadoor, Uldale Fells. The locality numbers correspond to those in Appendix 2. La2, La3 Lancefieldian stages 2 and 3 Be1 Bendigonian 1

(Figure 7) Distribution of graptolites in the Lorton Fells, north of the Whinlatter Pass. The inferred superposition of the principal localities is shown alongside the lithostratigraphical column; locality numbers correspond to those in Appendix 2. The numbers of supplementary localities are shown against the ranges of relevant species. Questionably identified specimens are shown by means of ?, cf. or aff. In the lithostratigraphical column, S’ and S” represent the sandstone-rich units in the Kirk Stile Formation.

(Figure 8) Distribution of graptolites in the Lorton Fells, south of the Whinlatter Pass. For explanation see Figure 7. The general superposition of the principal localities is considered correct but the succession is interrupted by faults.

(Figure 9) Selected graptolites from the lower divisions of the Skiddaw Group. Specimens prefixed RX are in the collections of the British Geological Survey, Keyworth. SM indicates the Sedgwick Museum, Cambridge. All magnified 5 except Fig. p which is 3. The parallel lines in Figures j–l indicate the trace of the cleavage in the plane of bedding. a. Didymograptus kurcki Törnquist, SM X.26164. Loweswater Formation (simulans Biozone), Barf (Locality 25). This taxon was figured by Elles & Wood 1901, pl.2, figs 4 a, b, as D. nicholsoni.; b Didymograptus gracilis Törnquist, RX 2558. Loweswater Formation (simulans Biozone), Barf Locality 25).; c. Isograptus cf. primulus Harris, Natural History Museum P.7232. Loweswater Formation (simulans Biozone), Barf (Locality 25).; d. Didymograptus filiformis Tullberg, RX 1212. Loweswater Formation (varicosus Biozone), west of Blaze Bridge (Locality 13). ; e, f. Didymograptus infrequens Kraft. e, obverse view , RX 230, f, reverse view RX 229. Loweswater Formation (simulans Biozone), east of Scawgill Bridge Quarry (Locality 16).; g. Didymograptus simulans Elles & Wood, RX 1227. Loweswater Formation (simulans Biozone), Hope Gill (Locality 69).; h. Didymograptus deflexus Elles & Wood, SM A.17713 (paratype), Loweswater Formation (simulans Biozone), Barf (Locality 25).; i. Azygograptus eivionicus Elles, RX 2211. Loweswater Formation (simulans Biozone), Tom Rudd Beck (Locality 23).; j. Didymograptus similis (Hall), RX 1600. Buttermere Formation (near the varicosus-simulans zonal boundary), Buttermere Quarry (Locality 138).; k. l. Didymograptellus? minutus (Törnquist, possibly attributable to the subsp. pygmaeus Monsen. Both on block RX 1590. Buttermere Formation (near the varicosus-simulans zonal boundary), Buttermere Quarry (Locality 138).; m. Didymograptus varicosus Wang, SM X.10309. Loweswater Formation (varicosus Biozone), Scawgill Bridge Quarry (Locality 15).; n. Azygograptus validus Törnquist?, RX 1273. Block from the phyllograptoides or varicosus Biozone, Buttermere Formation, Robinson (Locality 146).; o. Didymograptus rigoletto Maletz, Rushton & Lindholm, SM A.55054 Hope Beck Formation (phyllograptoides Biozone), Burn Tod Gill (Locality 10). ;p. Tetragraptus (Pendeograptus) cf. fruticosus (Hall), SM A.55120a, 3. Hope Beck Formation (phyllograptoides Biozone), Burn Tod Gill (Locality 10).; q. Didymograptus protobalticus Monsen, RX 3000. Hope Beck Formation (phyllograptoides Biozone), Trusmadoor (Locality 8).

(Figure 10) Distribution of graptolites on the west side of Skiddaw, in a transect extending from Carl Side, through Ullock Pike to Watches. For explanation see (Figure 7). On account of folding the precise superposition of the fossil localities and their relative stratigraphical separation are less certain than those shown in Figure 7) and (Figure 8). Several species have been collected from ‘Carl Side’ but their exact localities are not recorded. Their presence is recorded by ? near the boundary between the Loweswater and Kirk Stile formations.

(Figure 13) Selected graptolites from Llanvirn strata of the Skiddaw Group. All specimens except for l are in the collections of the British Geological Survey. All magnified 5 except k and l. The parallel lines indicate the trace of the cleavage in the plane of bedding. a. Archiclimacograptus angulatus magnus (Berry), AJ 1008, cited by Skevington in Wadge et al. 1972, p. 63, as a species of Pseudoclimacograptus. Tarn Moor Formation (murchisoni Biozone), Tarn Moor Tunnel, 808?818 m from the south portal. b. Paraglossograptus sp., RX 4960. Tarn Moor Formation (artus Biozone), Mosedale Beck (Locality 160). c. Cryptograptus schaeferi Lapworth, RX 3189. Tarn Moor Formation (artus Biozone), Cawell Beck (Locality 157). d. Climacograptus bifomis Mu & Lee?, RX 3195. Tarn Moor Formation (close to the base of the murchisoni Biozone), Aik Beck (Locality 165). e.f. Amplexograptus confertus (Lapworth), AJ 858 and AJ 877, Tarn Moor Formation (murchisoni Biozone), Tarn Moor Tunnel, 2311?2344 m from the south portal. g. Undulograptus cumbrensis (Bulman), RX 5463. Skiddaw Group mudstones (artus Biozone?), quarry east of Greenscoe Farm, Low Furness (Locality 174). h. Archiclimacograptus cf. caelatus (Lapworth), RX 3724. Tarn Moor Formation (upper part of the artus Biozone?), Aik Beck (Locality 166). i. Didymograptus artus Elles & Wood, Zv 713. Kirk Stile Formation (artus Biozone), north of Hazelhurst (Locality 134). j. Didymograptus spinulosus Perner, RX 2503A. Kirk Stile Formation (artus Biozone), Outerside (Locality 80). k. Didymograptus murchisoni speciosus Ekstr�m, RX 3194, 4. Tarn Moor Formation (close to the base of the murchisoni Biozone), Aik Beck (Locality 165). l, m. Didymograptus (Acrograptus) affinis Nicholson. l, proximal end, Natural History Museum Q.5858a, 10. Tarn Moor Formation (artus Biozone), Aik Beck (Locality 164). m, RX 3194, 5. Tarn Moor Formation (close to the base of the murchisoni Biozone), Aik Beck (Locality 165). n. Holmograptus lentus (Holm), distal fragment, Zv 752. Kirk Stile Formation (artus Biozone), north of Hazelhurst (Locality 134). o. Didymograptus acutidens Elles & Wood, PC 9084. Skiddaw Group mudstones (artus Biozone), Park Farm, Askam in Furness (Locality 175)." data-name="images/P936906.jpg">(Figure 11) Selected graptolites from the higher Arenig divisions of the Skiddaw Group. All specimens are from the Kirk Stile Formation except for q, and all are in the collections of the British Geological Survey, Keyworth, except for c and r. All magnified 5 except for a, k, n, and p, which are 4. The parallel lines indicate the trace of the cleavage in the plane of bedding. a. Didymograptus sparsus Hopkinson, Ro 133, 4. cucullus.Biozone, Seathwaite How Quarry, Embleton (Locality 170). b. Undulograptus cumbrensis (Bulman), RX 2942. cucullus d. Biozone, Scalegill (Locality 121). c. Aulograptus climacograptoides (Bulman), Tullie House Museum, Carlisle, 126–1978, No. 94. cucullus Biozone, Outerside (Locality 79?). d. Isograptus caduceus gibberulus (Nicholson), TMcH 55. gibberulus Biozone, ‘north-west flank of Skiddaw’, presumably at or near Randel Crag (Locality 115). e. Aulograptus cucullus (Bulman), Ht 410. cucullus Biozone, Hazelhurst (locality 132). f. Undulograptus austrodentatus cf. anglicus (Bulman), Ro 26. cucullus Biozone, Dead Beck (Locality 117). g. Cryptograptus antennarius (Hall), Ht 389. cucullus Biozone, Hazelhurst (Locality 132). h. Undulograptus sinicus Mu & Lee, Ht 395. cucullus Biozone, Hazelhurst (Locality 132). i. Pseudisograptus angel Jenkins, Ht 553. gibberulus Biozone, White Horse Fell (Locality 116). j. Didymograptus unifomis lepidus Ni, GSM 46366. gibberulus Biozone, Randel Crag (Locality 115). k. Isograptus victoriae victoriae Harris, RX 397, 4. Kirk Stile Formation (victoriae Biozone), Hope Gill (Locality 70). l. Azygograptus lapworthi Nicholson, GSM 46365. victoriae or gibberulus Biozone, Hodgson How Quarry (Locality 88). m. Didymograptus unifomis uniformis Elles & Wood, RX 2927. gibberulus Biozone, Abbey Gate (Locality 87). n. Didymograptus cf. goldschmidti Monsen, RX 3844, a long specimen in which the right stipe shows 140 thecae, 4. gibberulus Biozone, White Horse Fell (Locality 116). o. Didymograptus distinctus Harris & Thomas, FMT 286. gibberulus Biozone, Randel Crag (Locality 115). p. Pseudotrigonograptus ensiformis (Hall), AT 3364, 4. gibberulus Biozone, Winnah (Locality 84). q. Didymograptus extensus linearis Monsen, RX 1406. Block from the victoriae or gibberulus Biozone, Buttermere Formation, Beck Brow, Wilton (Locality 135). r. Didymograptus hirundo Salter, proximal part of Keswick Museum KESMG 4136. gibberulus Biozone, locality “RC”, presumably Randel Crag (Locality 115).

(Figure 12) Distribution of graptolites in the section in Mosedale Beck, 700 m south-west of Lobbs, Threlkeld Common. It is a dip section and the strata dip steeply. The principal fossil horizons are shown by the letters A to N, the base of the artus Biozone being taken at horizon I, where Holmograptus lentus appears.

(Figure 13) Selected graptolites from Llanvirn strata of the Skiddaw Group. All specimens except for l are in the collections of the British Geological Survey. All magnified X5 except k and l. The parallel lines indicate the trace of the cleavage in the plane of bedding. a. Archiclimacograptus angulatus magnus (Berry), AJ 1008, cited by Skevington in Wadge et al. 1972, p. 63, as a species of Pseudoclimacograptus. Tarn Moor Formation (murchisoni Biozone), Tarn Moor Tunnel, 808–818 m from the south portal. b. Paraglossograptus sp., RX 4960. Tarn Moor Formation (artus Biozone), Mosedale Beck (Locality 160). c. Cryptograptus schaeferi Lapworth, RX 3189. Tarn Moor Formation (artus Biozone), Cawell Beck (Locality 157). d. Climacograptus bifomis Mu & Lee?, RX 3195. Tarn Moor Formation (close to the base of the murchisoni Biozone), Aik Beck (Locality 165). e.f. Amplexograptus confertus (Lapworth), AJ 858 and AJ 877, Tarn Moor Formation (murchisoni Biozone), Tarn Moor Tunnel, 2311–2344 m from the south portal. g. Undulograptus cumbrensis (Bulman), RX 5463. Skiddaw Group mudstones (artus Biozone?), quarry east of Greenscoe Farm, Low Furness (Locality 174). h. Archiclimacograptus cf. caelatus (Lapworth), RX 3724. Tarn Moor Formation (upper part of the artus Biozone?), Aik Beck (Locality 166). i. Didymograptus artus Elles & Wood, Zv 713. Kirk Stile Formation (artus Biozone), north of Hazelhurst (Locality 134). j. Didymograptus spinulosus Perner, RX 2503A. Kirk Stile Formation (artus Biozone), Outerside (Locality 80). k. Didymograptus murchisoni speciosus Ekström, RX 3194, 4. Tarn Moor Formation (close to the base of the murchisoni Biozone), Aik Beck (Locality 165). l, m. Didymograptus (Acrograptus) affinis Nicholson. l, proximal end, Natural History Museum Q.5858a, 10. Tarn Moor Formation (artus Biozone), Aik Beck (Locality 164). m, RX 3194, 5. Tarn Moor Formation (close to the base of the murchisoni Biozone), Aik Beck (Locality 165). n. Holmograptus lentus (Holm), distal fragment, Zv 752. Kirk Stile Formation (artus Biozone), north of Hazelhurst (Locality 134). o. Didymograptus acutidens Elles & Wood, PC 9084. Skiddaw Group mudstones (artus Biozone), Park Farm, Askam in Furness (Locality 175).

(Figure 14) Ranges of key acritarch taxa in the Skiddaw Group.

(Figure 15) Stratigraphical occurrence of acritarch taxa in the lower part of the Skiddaw Group in the Northern Fells Belt. The section is based on a traverse from Bitter Beck [NY 1650 3096] northwards across Watch Hill to the valley of the River Derwent [NY 1354 3269], [NY 1352 3270]. The section is repeated by faulting. Data from the top of the Hope Beck Formation and base of the Loweswater Formation are from Jonah’s Gill [NY 1902 3399] to [NY 1904 3420].

(Figure 16) Acritarch occurrences in the Skiddaw Group (Northern Fells Belt) in the Lorton Fells, south and west of Whinlatter Pass and north of Gasgale.

(Figure 17) Acritarch occurrences in the Skiddaw Group (Northern Fells Belt) in the Lorton Fells, north and east of Whinlatter Pass.

(Figure 18) Geological map of the Skiddaw, Ullswater and Bampton inliers in the northern Lake District. BB Bitter Beck; Bu Buttermere; CPF Causey Pike Fault; D Dodd; GT Gasgale Thrust; HB Hope Beck; K Kirk Stile; L Loweswater; LT Loweswater Thrust; T Tarn Moor Tunnel; WE Whiteside End; WH Watch Hill; WHT Watch Hill Thrust

(Figure 19) Sedimentary logs of the Watch Hill Formation, showing the proximal channel–levee facies association (from Moore, 1992). See p.71 for key.

(Figure 20) Palaeocurrent data for the Watch Hill and Loweswater formations, Skiddaw inlier (from Moore, 1992). a Watch Hill; b Great Cockup; c Darling Fell; d Low Fell; e Liza Beck; f Dove Crag; g Hope Gill; h Scawgill Bridge; i Barf; j Woodend Brow; k Ling Fell; l Sale Fell; m Jonah’s Gill; n Mungrisdale

(Figure 21) Sedimentary logs through the Loweswater Formation at Darling Fell [NY 125 225], showing the distributary channel facies association (from Moore, 1992). Sections are approximately 86 m apart from north-west (left) to south-east (right). See p.71 for key.

(Figure 22) Sedimentary log of the Loweswater Formation at Fisher Wood, Sale Fell, showing the depositional lobe facies association. Base of log at [NY 1846 2942] (from Moore, 1992). See p.71 for key.

(Figure 23) a. Palaeocurrent patterns for the Loweswater Formation (from Moore, 1992). b. Possible basin geometry during deposition of the Loweswater Formation (from Moore, 1992). c. Localities a Darling Fell; b Low fell; c Liza Beck d Dove Crag e Hope Hill; f. Scawgill Bridge g Barf; h Woodend Brow i Ling Fell; j Sale Fell.

(Figure 24) Sedimentary logs of the Loweswater Formation at Scawgill Bridge Quarry, showing the interlobe facies association. Base of log at [NY 1777 2577] (from Moore, 1992).

(Figure 25) Comparison of global sea-level curve (from Fortey and Cocks, 1986) and a sea-level curve for the Skiddaw Group, based mainly on the supply of siliciclastic sediment to the basin.

(Figure 26) Ternary provenance discrimination diagrams (after Dickinson et al., 1983) for the formations of the Skiddaw Group based on the proportions of detrital quartz, feldspar and lithic grains. a Bitter Beck Formation b Watch Hill Formation c Hope Beck Formation d Loweswater Formation e Kirk Stile Formation f Buttermere Formation

(Figure 27) Element abundance diagrams (spidergrams) for the Skiddaw Group. a. Characteristic patterns for different tectonic settings, after Floyd et al. (1991) b. Mean sandstone patterns for the Kirk Stile, Loweswater, Hope Beck and Watch Hill formations c. Skiddaw Group means normalised to average continental arc/active margin values d. Skiddaw Group means normalised to average passive margin values Nb/Nb* ratio actual normalised Nb abundance/calculated normalised Nb abundance extrapolated between Ni and Ti

(Figure 28) Variation in Nd with Skiddaw Group stratigraphy. Values for the Ingleton Group are included for comparison. grey tone mudstone data; • sandstone/wacke data

(Figure 29) Simplified section through the Loweswater Anticline (after Webb and Cooper, 1988).

(Figure 30) Structural domains of the major soft-sediment slump folds (after Webb and Cooper, 1988). 1. Pure shear extension with simple shear leads to boudinage with minor folds. 2. Pure shear compression with simple shear leads to minor folds alone. 3. Pure shear contraction with antithetic simple shear leads to antithetic minor folds. 4. Zone of extension and possible boudinage due to tangential longitudinal strain.

(Figure 31) Equal area stereogram plot of minor slump fold axes from the Robinson area, with plan view of overall axial curvature and westerly vergence (after Webb and Cooper, 1988).

(Figure 32) Minor slump folds in the Buttermere olistostrome (after Webb and Cooper, 1988). a. Minor fold axes from the Buttermere area b and c. Minor fold axes from the subareas shown. d. Possible relationship between the subareas.

(Figure 33) Sketch view of Goat Crag from the south-west, showing outcrop patterns and axial-plane traces in the slump-folded Buttermere Formation olistrome (after Webb and Cooper, 1988).

(Figure 34) Schematic cross-section through the Skiddaw Group outcrop in the Northern Fells of the Skiddaw inlier (after Hughes et al., 1993).

(Figure 35) A summary of radiometric ages relevant to the 430–420 Ma resetting event.

(Figure 36) The distribution of the major faults and folds in the Skiddaw inlier.

(Figure 37) Cleavage data for the Skiddaw, Ullswater and Bampton inliers. The data are presented as poles to cleavage planes, and are contoured where possible. Most of the data were collected during the BGS resurvey, and can be found in BGS Technical Report, WA/97/72. Insets a to m include poles to all cleavage planes. All poles in inset n relate to S1; poles in inset p include some bedding data where bedding and S1 are indistinguishable.a NY13SE; b NY23SE; c NY12NE; d NY22NW; e NY22NE; f Mungrisdale/Blencathra area (data from Roberts 1977b); g NY12SE; h NY22SW; j NY22SE; k NY32NW/SW; l part of NY21NW (data from Webb 1975);m NY21NE; n Ullswater inlier (data from Hughes, 1995a); p Bampton inlier (data from Bell, 1997).

(Figure 38) Sketch of the structure visible on the south side of Causey Pike. The wackes of the Robinson Member are complexly folded and disrupted. The hornfels of the Crummock Water aureole is thrust faulted over the Robinson Member by the Causey Pike Fault.

(Figure 39) Metamorphic map of the Skiddaw Group in the Skiddaw, Ullswater, Bampton, Black Combe and Cross Fell inliers. Granites: Enn Ennerdale; Esk Eskdale; Sk Skiddaw; Th Threlkeld

(Figure 40) Principal structural elements of the East Irish Sea Basin, west of the Lake District (after Jackson et al., 1995).

(Figure 41) Possible correlation between the Skiddaw, Manx and Ribband groups (after BGS, 2001; Ribband Group data from McConnell et al., 1999). Mn-rich rocks

(Figure 42) Mineralised localities.

Plates

(Front cover) Cover photograph Loweswater Fell, looking south along Mosedale to Gale Fell within the Crummock Water thermal aureole (D3821). Photographer: T S Bain.

(Plate 1) View north-west to Grasmoor, Wandhope and Crag Hill [NY2020 1720] (D3846).

(Plate 2) View south-south-east from Low Fell, Loweswater [NY 1361 2194] (D3822).

(Plate 3) Selected macrofossils from the Skiddaw Group. Specimens whose numbers are prefixed by SM are in the Sedgwick Museum, Cambridge and c is in the Natural History Museum, London. All other specimens are in the biostratigraphy collections of the British Geological Survey, housed at Keyworth. a, d. Caryocaris wrightii Salter. a, latex cast of carapace, CS 98, 3. Kirk Stile Formation, cucullus Biozone, Outerside (Locality 79). d, tail-piece, RX 1105, 5. Kirk Stile Formation, victoriae Biozone?, Darling How (Locality 54). b. Pricyclopyge binodosa binodosa (Salter), SM A.32819, 2.5. Kirkland Formation, artus Biozone, Eller Gill, Cross Fell inlier. P. binodosa (s.l.) is the trilobite most widely found in the Skiddaw Group. Photograph supplied by Dr R M Owens. c. Placoparia cambriensis (Hicks), Natural History Museum, London, It 14197, 2.5. Kirkland Formation, artus Biozone, Eller Gill, Cross Fell inlier. This genus is recorded from the lower Llanvirn at Outerside (Locality 80) and Matterdale Beck (Locality 161). Photograph supplied by Dr R M Owens. e. Cyclopyge sp. Composite photograph of RX 1438 and counterpart RX 1438A, 6. Block of victoriae or gibberulus Biozone in the Buttermere Formation, Beck Grains (Locality 137). f. Prospectatrix brevior Rushton, latex cast of RX 916, 3. Upper Tremadoc block in the Buttermere Formation, River Calder (Locality 136). g. Girvanopyge sp. Latex cast of pygidium, SM A.40438, 6. Tarn Moor Formation, artus Biozone, Thornship Beck (Locality 168). Photograph supplied by Dr R M Owens. h. Pliomerid with 11 thoracic segments, cast of Ht 1267, 6. Hope Beck Formation, zone uncertain, from the River Derwent, ESE of Kirkhouse [NY 1690 3290]. i. Araneograptus murrayi (Hall), RX 3099, x1. Watch Hill Formation, murrayi Biozone, Trusmadoor (Locality 4). j. Didymograptus deflexus Elles & Wood, latex cast of holotype, SM A.17712, 4.5. Loweswater Formation, simulans Biozone, Barf (Locality 25).

(Plate 4) Acritarchs from the Skiddaw Group. All specimens are 1200, and all are housed in the MPK collection of the British Geological Survey, Keyworth, Nottingham, UK. a. Cymatiogalea messaoudensis Jardiné et al., 1974. MPK 5357. Watch Hill Formation, Watch Hill, Skiddaw inlier (Northern Fells Belt). Late Tremadoc–early Arenig. b. Stelliferidium trifidum (Rasul) Fensome et al., 1990.MPK 5371. Bitter Beck Formation, Bitter Beck, Skiddaw inlier (Northern Fells Belt). Late Tremadoc. c. Striatotheca prolixa Molyneux in Molyneux and Rushton, 1988. MPK 5938. Catterpallot Formation, Cross Fell inlier. Late Tremadoc–early Arenig. d. Rhopaliophora palmata (Combaz and Peniguel) Playford and Martin 1984. MPK 5959. Catterpallot Formation, Cross Fell inlier. Late Tremadoc–early Arenig. e. Cymatiogalea granulata Vavrdová, 1966. MPK 10859. Catterpallot Formation, Cross Fell inlier. Late Tremadoc–early Arenig. f. Micrhystridium aff. acuminosum Cramer and Díez, 1977. MPK 5936. Catterpallot Formation, Cross Fell inlier. Late Tremadoc–early Arenig. g. Coryphidium aff. elegans Cramer et al. (sensu Molyneux and Leader, 1997). MPK 5951. Catterpallot Formation, Cross Fell inlier. Late Tremadoc–early Arenig. h. Cymatiogalea deunffii Jardiné et al., 1974. MPK 5355. Watch Hill Formation, Watch Hill, Skiddaw inlier (Northern Fells Belt). Late Tremadoc–early Arenig. i. Striatotheca rarirrugulata (Cramer et al.) Eisenack et al., 1976. MPK 2434. Skiddaw Group, Black Combe inlier. Late Arenig. j. Stellechinatum sicaforme sicaforme Molyneux in Molyneux and Rushton 1988. MPK 5366. Bitter Beck Formation, Bitter Beck, Skiddaw inlier (Northern Fells Belt). Late Tremadoc. k. Frankea hamata Burmann, 1970. MPK 10860. Tarn Moor Formation, Bampton inlier. Llanvirn. l. Stellechinatum celestum (Martin) Turner, 1984. MPK 5968. Kirkland Formation, Cross Fell inlier. Llanvirn. m. Striatotheca rarirrugulata (Cramer et al.) Eisenack et al., 1976. MPK 5964. Murton Formation, Cross Fell inlier. Late Arenig. n Micrhystridium sp. A of Rushton and Molyneux, 1989. MPK 2447. Skiddaw Group, Black Combe inlier. Late Arenig. o. Arkonia virgata Burmann, 1970. MPK 5966. Kirkland Formation, Cross Fell inlier. Llanvirn.

(Plate 5) Medium-, thin- and thick-bedded wackes and siltstones of the Watch Hill Formation. Elva Hill [NY 1724 3204], near Cockermouth (MNS8729). Scale: hammer 30 cms long.

(Plate 6) The Loweswater Formation at Swinside, Whinlatter Pass [NY 1781 2486], showing thick-bedded parallel- and cross-laminated wacke units (Tbc), with truncated cross-laminations indicating structural way-up (MNS8725). Scale: hammer 30 cms long.

(Plate 7) The Loweswater Formation at Liza Beck [NY 1638 2097], north-west side of Grasmoor. Sandstone and siltstone Tbc units, showing signs of softsediment faulting and boudinage (L3179). Scale: width of view approximately 0.5 m.

(Plate 8) The Loweswater Formation on the west flank of Whiteside [NY 1605 2237]. Typical, medium-bedded, medium- to coarse-grained sandstone beds, dipping and younging towards the south-south-east (D3804).

(Plate 9) Thin-bedded sandstone and siltstone at the top of the Loweswater Formation; south flank of Whiteside End [NY 1660 2169] (D3832). Hammer is 30 cm long.

(Plate 10) The Loweswater Formation at Scawgill Bridge Quarry [NY 1776 2584], Whinlatter Pass. Medium- and thick-bedded wackes, showing typical lateral continuity and even nature of bedding (D3823).

(Plate 11) The Loweswater Formation at Scawgill Bridge Quarry [NY 1776 2584], Whinlatter Pass. Thin- to medium-bedded Tb units, passing upwards into cross- and convolute-laminated Tc units (D3826). Scale: hammer head 9.6 cms long.

(Plate 12) Flute casts on base of wacke bed, Loweswater Formation, Embleton High Common, [NY 1696 2793] (D3845). Scale: hammer head 9.6 cms long.

(Plate 13) The Outerside debrite at Outerside [NY 2132 2140], showing subrounded clasts of bedded siltstone and a few sandstone clasts in a sandstone matrix (MNS8710). Scale: the chisel of the hammer head is 9 cms long.

(Plate 14) Flute clasts on base of massive wacke bed, Robinson Member. Blea Crag, Scope Beck, Newlands [NY 2135 1758] (D3845). Scale: hammer head is 9.6 cms long.

(Plate 15) Sandstone olistolith, about 5 m high, in the Buttermere Formation at Moss Force, Newlands Hause, near Buttermere [NY 1930 1740] (MNS8712).

(Plate 16) The unconformity below the Borrowdale Volcanic Group in Matterdale Beck is an irregular surface dipping at 25–30º to the south-south-west, and is eroded across steeply dipping Skiddaw Group mudstones. The contact is exposed on the left by the hammer, and crosses the stream below the waterfall that is formed by basal Borrowdale Volcanic Group conglomerates. [NY 3888 2345] (L2035).

(Plate 17) The junction between the Skiddaw Group and the Borrowdale Volcanic Group, Matterdale Beck. Clastic rocks of the basal Borrowdale Volcanic Group, here consisting of mudstone pebbles in a mud matrix, rest unconformably upon the steeply dipping Skiddaw Group mudstones. The hammer head rests upon the irregular contact [NY 3888 2345] (L2036). Scale: hammer is 30 cms long.

(Plate 18) Thinly bedded sandstone facies of the Knott Hill Formation, Knott Hill [SD 1752 8712] (GN186). Scale is 15 cms long.

(Plate 19) Photomicrographs illustrating texture and composition of sandstones from the Watch Hill Formation. a. Quartz grains, mudstone lithic fragments and subordinate feldspar grains in a quartz-cemented, medium-grained sand. Sample CU284/1. Watch Hill Formation. Plane polarised light, magnification x25. b. Subrounded clast of metamorphic polycrystalline quartz with strong crystal orientation. Undulose extinction in subcrystals and cross-cutting cleavage. Other clasts include rounded mudstone with weathered rim (top left), chert (top right) and quartz grains of very fine sand size in a clay matrix — almost certainly derived from pelite lithic grains. Sample CU299. Watch Hill Formation. Plane polarised light, magnification x40. c. Plagioclase grain with quartz and feldspar grains and contorted detrital mica flake in a matrix of dominantly secondary clay minerals derived from the breakdown of pelitic lithic fragments, one outline of which is discernible in the upper left corner. Sample CU326. Watch Hill Formation. Plane polarised light, magnification x40. d. Subrounded clast of basalt with randomly orientated plagioclase laths and traversed by a thin chlorite vein. Sample CU326. Watch Hill Formation. Plane polarised light, magnification x40. e. Subrounded clast of quartz and feldspar with graphic texture. Other grains include quartz, chert, subhedral zircon with patchy clay matrix and quartz cement. Sample CU284/3. Watch Hill Formation. Crossed polars, magnification x100. f. Subhedral zircon in a clast of quartzite. Sample CU326. Watch Hill Formation. Crossed polars, magnification x250.

(Plate 20) Photomicrographs illustrating texture and composition of sandstones from the Loweswater Formation. a. Quartz grains with subordinate feldspar and lithic fragments in moderately sorted greywacke of medium sand grain size. Quartz grains display pressure solution and quartz is precipitated in strain shadows and in a vein. Thus primary textural attributes of individual grains and the host rock are obscured. Sample CU35. Loweswater Formation. Plane polarised light, magnification x25. b. Monocrystalline quartz corroded by clay growth at its margins with inclusions of euhedral crystal stacks of chlorite interpreted to be derived from vein quartz.Sample CU30. Loweswater Formation. Plane polarised light, magnification x100. c Crenulated metapelite clast in a clay matrix. Sample CU42. Loweswater Formation. Crossed polars, magnification x250. d. Clast of quartz-muscovite schist with very fine sand size grains of quartz and plagioclase and detrital muscovite in a clay matrix. Sample M90-19. Loweswater Formation. Crossed polars, magnification x100. e. Clast of quartz mylonite with a monocrystalline quartz grain in a matrix of silt grade quartz grains and phyllosilicates and secondary opaque pyrite (top centre). Sample CU30. Loweswater Formation. Crossed polars, magnification x100. f. Muscovite pellet partially replaced by chlorite with fine sand size quartz and plagioclase grains. Sample M18. Loweswater Formation. Plane polarised light, magnification x100.

(Plate 21) Soft-sediment folding in hornfelsed siltstones within the Crummock Water aureole. Lad Hows [NY 1729 1925] (D 3829). Scale: hammer is 30 cms long.

(Plate 22) Slump folds in the Kirk Stile Formation within the thermal aureole of the Skiddaw granite in the River Caldew above Swineside [NY 3299 3260] (A6673). Scale bar is about 10 cms long.

(Plate 23) Disharmonic slump folding in the Buttermere Formation at Lambing Knott [NY 194 154] (MNS8717).

(Plate 24) Slump folds in the Robinson Member within the inverted limb of the Goat Crag Anticline [NY 1888 1629] (D3843). Scale: hammer (centre right) is 30 cms long.

(Plate 25) Unconformity at the base of the Eycott Volcanic Group, Chapel House Reservoir [NY 2582 3551]. The steeply dipping fabric beneath the unconformity is bedding in the Skiddaw Group (D 4457). Scale: hammer head is 9.6 cms long.

(Plate 26) Regional S1 cleavage (inclined from top left to bottom right) with associated minor flexures of the bedding-parallel fabric (steeply inclined from top right to bottom left). Long Side [NY 2425 2859], Kirk Stile Formation (MNS8735). Width of view approximately 0.35 m.

(Plate 27) Regional S1 cleavage, inclined from top left to bottom right, folded by minor open folds with axial planar crenulation cleavage, inclined from top right to bottom left. North-east of Hollows Farm, Borrowdale [NY 249 174]. Buttermere Formation (MNS8736). Scale: hammer is 30 cms long.

(Plate 28) Thinly laminated siltstone/sandstone with bedding subparallel to the regional S1 cleavage, inclined from top left to bottom right, cut by a later crenulation cleavage, inclined from top right to bottom left (E65304). Field of view 3 mm. South side of Long Barrow [NY 0416 1360], Kirk Stile Formation (MNS9163).

(Plate 29)a Interbedded mudstone and siltstone; the former displays microscopically striped penetrative cleavage intersected by spaced crenulation-fracture cleavage exploited by later-formed hematite films, Kirk Stile Formation [NY 2720 2370]. Magnification 10 objective, plane polarised light (E71336). b Chlorite-mica stacks and spheroids in silty mudstone of the Kirk Stile Formation [NY 2126 2330] intersected by spaced fracture cleavage exploited by films of hematite. Magnification 20 objective, plane polarised light (E62215). c Silty mudstone in the Tarn Moor Formation, Mosedale Beck [NY 3546 2308], intersected by a single weak crenulation cleavage. The rock has a strong bedding-parallel fabric but minute iron-stained spheroids show little flattening. Magnification 10 objective, plane polarised light (E60822). d Abundant microspheroids in mudstone with strong bedding-parallel fabric, interbedded with sandstone (not seen), Skiddaw Group, Troutbeck [NY 3823 2704]. Backscattered-electron micrograph. Energy-dispersive X-ray analysis (EDAX) indicates that the spheroids consist of chloritic cores surrounded by shells of sideritic carbonate (scale in image) (E71360). e Intense micro-spotting in bleached, indurated mudstone interbedded with siltstone, Kirk Stile Formation slate, Crummock Water aureole [NY 2674 1927]. Magnification 2.0 objective, plane polarised light (E71282). f Tourmalinised mudstone intersected by quartz-tourmaline fracture-veining, Kirk Stile Formation pelite, Crummock Water aureole [NY 1775 1977]. Magnification 2.0 objective, plane polarised light (E71289).

Tables

(Table 1) Stratigraphical schemes used for the Skiddaw Group in the Lake District.

(Table 2) Occurrence of graptolite species by zone in the Skiddaw Group.

(Table 3) Summary of main facies and depositional processes of facies classes recognised by Moore (1992).

(Table 4) Stratigraphical distribution of facies classes (summarised from Moore 1992). Classification scheme modified by Moore after Pickering et al. (1986).

(Table 5) Summary of tectonic events.

(Table 6) Summary data for the principal intrusive bodies and suites within and adjacent to the Skiddaw Group

(Table 7) Median values for chemical analyses of Skiddaw Group rock samples, omitting altered (bleached) and contact metamorphosed samples.

(Table 8) Means and ranges of petrographical data (%) for the Watch Hill Formation.

(Table 9) Means and ranges of petrographical data (%) for the Hope Beck Formation (n=8).

(Table 10) Means and ranges of petrographical data (%) for the Loweswater Formation, excluding high-matrix greywackes and greywackes from Jonah’s Gill (n=26).

(Table 11) Means and ranges of petrographical data (%) for the Loweswater Formation.

(Table 12) Means and ranges of petrographical data (%) for the Kirk Stile Formation (n=4).

Tables

(Table 1) Stratigraphical schemes used for the Skiddaw Group in the Lake District

Ward (1876, p.47)

SKIDDAW SLATES

Black Slates of Skiddaw Gritty beds of Gatesgarth, Latterbarrow, Tongue Beck, Watch Hill and Great Cockup

Dark Slates

Sandstone series of Grasmoor and Whiteside

Dark Slates of Kirk Stile

Dixon (1925, p.27)

SKIDDAW SLATES

Mosser Striped Slates (including Skiddaw or Watch Hill Grit) Loweswater Flags

Kirk Stile Slates

Blake Fell Mudstones

Eastwood et al. (1931, p.28)

SKIDDAW SLATES

Latterbarrow Sandstone Mosser and Kirkstile Slates, with Watch Hill Grit in upper part

Loweswater Flags and Blakefell Mudstone

Rose (1955)

SKIDDAW SLATES

Mosser–Kirkstile Slates Loweswater Flags (with Watch Hill Grits)

Jackson (1961)

SKIDDAW SLATES

Latterbarrow Sandstone Mosser–Kirkstile Slates Loweswater Flags (with Watch Hill Grits)

Hope Beck Slates

Simpson (1967)

Latterbarrow Sandstone unconformity

SKIDDAW SLATES

Sunderland Slates

Watch Hill Grits and Flags Mosser Slates

Loweswater Flags Kirkstile Slates

Blakefell Mudstones Buttermere Flags Buttermere Slates

Jackson (1978)

SKIDDAW GROUP

Latterbarrow Sandstone Tarn Moor and other Llanvirn mudstones Kirk Stile Slates

Loweswater Flags (with Watch Hill Grits)

Hope Beck Slates

Wadge (1978)

EYCOTT GROUP

Tarn Moor Mudstones (High Ireby and Binsey volcanic formations)

SKIDDAW GROUP

Latterbarrow Sandstone Kirkstile Slates

Loweswater Flags Hope Beck Slates

Moseley (1984)

SKIDDAW GROUP

Latterbarrow Sandstone Formation

Tarn Moor Mudstone Formation

Kirkstile Slate Formation Loweswater Flags Formation Hope Beck Slates Formation

Molyneux and Rushton (1988)

Latterbarrow Sandstone

unconformity

SKIDDAW GROUP

Kirk Stile Slates Loweswater Flags Hope Beck Slates Watch Hill Grits Tremadoc beds of the River Calder

This memoir and Cooper et al. (1995)

SKIDDAW GROUP––

NORTHERN FELLS BELT

Kirk Stile Formation

Loweswater Formation

Hope Beck Formation

Watch Hill Formation

Bitter Beck Formation

This memoir and Cooper et al. (1995)

SKIDDAW GROUP

CENTRAL FELLS BELT

Tarn Moor Formation

Buttermere Formation

SKIDDAW GROUP–

SOUTHERN LAKE DISTRICT INLIERS

Skiddaw Group of the Black Combe and Furness inliers

(Table 2). Occurence of graptolite species by zone in the Skiddaw Group

SPECIES

murrayi

phyllograptoides

varicosus

simulans

victoriae

gibberulus

cucullus

artus

murchisoni

Araneograptus murrayi

x

Didymograptus cf. sinensis

x

x

Didymograptus [declined form]

x

Clonograptus multiplex

x

Didymograptus cf. demissus

x

Didymograptus cf. holmi

x

Didymograptus protobalticus

x

Didymograptus rigoletto

x

Tetragraptus (Pendeograptus) fruticosus

cf.

x?~

~?

Tetragraptus amii

?

?

x

x

x

Tetragraptus pseudobigsbyi

x

x

x

x

x

Tetragraptus quadribrachiatus

x

x

x

x

x

x

Azygograptus validus ?

~

~

Didymograptus filiformis

~

x

x

Clonograptus sp.

x

Dichograptus octobrachiatus

x ~

~x

~

~x

x

Dichograptus sedgwickii

x

x

~

~

Dichograptus separatus

?

x

Didymograptus aff. balticus

x

Didymograptus vacillans attenuatus

x

Didymograptus cf. decens

x

x

Didymograptus deflexus

?~

~x

x

Didymograptus cf. ellesi

x

Didymograptus cf. kunmingensis

x

Didymograptus varicosus

x~

~

Loganograptus sp.

x

Pseudobryograptus sp.

x

Tetragraptus reclinatus

x

x

x

x

x ~

~

Trochograptus diffusus

x ~

~

?~

~?

Didymograptus minutus

?~

~?x

Didymograptus similis

~

~

Didymograptus v-fractus

~

~

Etagraptus ? tenuissimus

~

~

Holograptus deani

~

~x

?

Adelograptus ? divergens

x

Azygograptus eivionicus

x

Azygograptus ellesi

x

Didymograptus cf. goldschmidti

?

x

x~

Didymograptus gracilis

x

Didymograptus infrequens

x

x

Didymograptus kurcki

x

x

Didymograptus simulans

x

x

Didymograptus sp. [extensiform]

x

x

Isograptus cf. primulus

x

Loganograptus logani

x

x

x

Pseudobryograptus cumbrensis

x

x

Pseudophyllograptus angustifolius

x

x

x

x

x

Pseudotrigonograptus ensiformis

x

x

x

x ~

~?

Pseudotrigonograptus minor

x

x

x

Schizograptus reticulatus

x

Schizograptus tardifurcatus

x

~

Tetragraptus crucifer

x

x

Tetragraptus serra

x

x

x

x

Tetragraptus (Pendeograptus) pendens

x

Tetragraptus (Pendeograptus) postlethwaitei

x

Azygograptus lapworthi

?~

~? ~

~

Didymograptellus exilis ?

~

~

Tetragraptus reclinatus abbreviatus

~

~

Didymograptus extensus linearis

x

x

?

Didymograptus hirundo

x

x

x

Didymograptus uniformis lepidus

x

x

Isograptus victoriae cf. maximus

x

Isograptus victoriae victoriae

x

Phyllograptus densus

x

Tetragraptus headi

x

x

Didymograptus cf. praenuntius

~

~

Phyllograptus typus

~

~

cf.

Didymograptus distinctus

x

Didymograptus nitidus

x

x

Didymograptus uniformis

x

Didymograptus cf. uniformis

x

Didymograptus v-fractus volucer

x

x

Isograptus caduceus gibberulus

x

Isograptus caduceus cf. imitatus

x

Isograptus caduceus subsp. [large]

x

Isograptus victoriae cf. divergens

x

Pseudisograptus angel

x

Pseudisograptus sp. A [of Jenkins]

x

Pseudoisograptus bigsbyi

cf.

x

?

“Thamnograptus” doveri

x

Xiphograptus svalbardensis

x

?

Didymograptus sparsus

~

~x

?

Acrograptus affinis

x

x

x

Aulograptus cucullus

x

x

sp.

Aulograptus climacograptoides

x

cf.

Cardiograptus sp.

x

Cryptograptus antennarius

x

x

Cryptograptus hopkinsoni

x

Didymograptus acutidens

x

x

x

Didymograptus cf. extensus

x

Didymograptus nicholsoni planus

x

Didymograptus protobifidus

x

x

Didymograptus sp. ‘a’

x

x

Eoglyptograptus dentatus

x

x

Eoglyptograptus shelvensis

x

cf.

Tetragraptus cf. lui

x

Undulograptus austrodentatus

x

x

Undulograptus cumbrensis

x

x

Undulograptus sinicus

x

Glossograptus sp.

~

~

Cryptograptus schaeferi

~

~x

?

Amplexograptus confertus

x

x

Azygograptus coelebs

x

Climacograptus angustatus ?

x

Climacograptus tailbertensis

x

Didymograptus acutus

x

Didymograptus artus

x

cf.

Didymograptus cf. dubitatus

x

Didymograptus euodus

aff.

Didymograptus cf. indentus

x

Didymograptus miserabilis

x

Didymograptus nicholsoni

x

cf.

Didymograptus pakrianus

cf.

?

Didymograptus robustus

x

cf.

Didymograptus spinulosus

x

?

Didymograptus stabilis

x

Diplograptus ellesi

x

Diplograptus hollingworthi

x

Glossograptus acanthus

x

Glossograptus armatus

x

Holmograptus lentus

x

Janograptus cf. peruviensis

x

Janograptus petilus

x

Nicholsonograptus fasciculatus

x

sp.

Paraglossograptus sp.

x

Tetragraptus sp.

x

Trichograptus fragilis

x

Undulograptus austrodentatus cf. anglicus

x

x

Climacograptus biformis ?

x

Didymograptus murchisoni geminus

x

Didymograptus murchisoni speciosus

x

Phyllograptus ? nobilis

x

Pseudoclimacograptus angulatus magnus

x

Pseudoclimacograptus angulatus micidus

?

Pseudoclimacograptus scharenbergi

x

Pterograptus elegans

?

X Species occurs cf. aff. ? identification qualified or uncertain, but horizon definite ~ horizon uncertain

(Table 3) Summary of main facies and depositional processes of facies classes recognised by Moore (1992)

Facies class

Main facies

Depositional processes

A

Gravels, muddy gravels, gravelly muds, pebbly sands, >5% gravel

A1.3

matrix-supported, thin pebbly mudstones

debris-flow deposits

A1.4

matrix-supported pebbly sandstones

high-concentration turbidity currents

A2.5

pebbly sandstones in medium beds with low-angle - cross stratification; traction carpets present

tractional bedload beneath high-concentration turbidity currents

A2.7

normally graded pebbly sandstones

grain deposition from high-concentration turbidity currents

B

Sands, with >80% sand grade and <5% pebble grade

B1.1

structureless, medium to thick beds of fine- to medium-grained wackes

rapid deposition from high-concentration turbidity currents

B1.2

thin-bedded, coarse-grained sandstones

thin turbidity currents

B2.1

medium beds of mostly ungraded, medium- to very coarse-grained sandstones

rapid deposition from high-concentration turbidy currents

B2.2

thick beds of ungraded, coarse-grained with sole structures and convolute laminations

high-concentration turbidity currents

B2.3

variably bedded, normally graded sandstones

grain deposition from suspension

C

Sand-mud couplets and muddy sands, with 20–80% sand grade and <80% mud grade

C1.1

high matrix wackes in medium to thick, poorly, sorted beds, with crude normal grading

muddy, high-concentration turbidity currents or debris flows

C2.1

thick- and very thick-bedded sandstone-mudstone couplets

decreasing concentration turbidity currents

C2.2

medium-bedded sandstone-mudstone couplets with normal grading

decreasing concentration turbidity currents

C2.3

thin-bedded sandstone-mudstone couplets

low concentration turbidity currents

C2.5

thin or very thin beds of fine- or very fine-grained sandstone with up to 60% mudstone-siltstone interbeds

dilute turbidity currents (sand) and deposition from suspension (mud)

C2.6

thin beds, commonly amalgamated, of very fine- to fine- grained sandstone, cross-laminated throughout

low volume turbidity currents

C2.7

thin and very thin beds of medium- to coarse-grained sandstone, with mudstone-siltstone laminae

low volume turbidity currents

D

Silts, silty muds and silt-mud couplets, with >80% mud, >40% silt, and 0-20% sand

D2.1

thin and very thin beds of siltstone and very fine- grained sandstone with mudstone interbeds

low concentration turbidity currents

D2.2

thick, lenticular laminae or siltstone and very fine-grained sandstone

low concentration turbidity currents

D2.3

horizontal laminae of siltstone and mudstone

low concentration turbidity currents

E

Muds and clays, with >95% mud, <40% silt and <5% sand and coarser grade

E2.2

laminated clay

accumulation of flocculated clay

F

Chaotic deposits with variable grain or clast size

F2.1

slump folds and sediment slides

gravity induced soft-sediment deformation

(Table 4) Stratigraphical distribution of facies classes (summarised from Moore 1992). Classification scheme modified by Moore after Pickering et al. (1986).

Formation

Principal facies recognised by Moore (1992)

Buttermere Formation

F2.1

Kirk Stile Formation

Dominated by facies classes D and E, with also F2.1

Loweswater Formation

B1.1, B2.1, B2.2, C1.1, C2.1, C2.2, C2.3, C2.5, C2.6, D2.1, D2.2, D2.3

Hope Beck Formation

Dominated by facies classes D and E, with also A1.3

Watch Hill Formation

A1.4, A2.5, A2.7, B1.1, B1.2, B2.1, B2.2, B2.3, C2.5, C2.6, C2.7, E2.2

Bitter Beck Formation

Dominated by facies classes D and E, with also C2.3, C2.5, C2.7, F2.1

(Table 6) Summary data for the principal intrusive bodies and suites within and adjacent to the Skiddaw Group

Name [references]

Form

Rock type

Alteration

Age

Skiddaw granite [1, 2, 3, 4, 5, 6]

three separate restricted outcrops exposing roof of major cupola

perthite-phyric biotite-granite

muscovite- chlorite, becoming intense and greisenitic around polymetallic mineralisation

granite 392 ± 4 Ma (K–Ar. micas); greisen 383 ± 4 Ma (K–Ar, mica)

Threlkeld–St John’s microgranite [4,7, 8]

upper part of minor sub-volcanic cupola

plagiophyric microgranite with almandine crystals of probable xenocrystic origin

albite, sericite, calcite chlorite, quartz alteration

early Caradoc, 459 ± 25 Ma (recalculation4 of earlier Rb–Sr whole rock determination of 445 ± 15 Ma8)

Ennerdale granophyre [5,9, 10, 11]

major laccolithic sub volcanic intrusion

granophyric biotite- hornblende- microgranite; minor basic components

chloritisation of mafic minerals

early Caradoc, 452 ± 4 Ma (U–Pb, zircon)

Embleton diorite suite [1, 4, 12]

stocks, sills, outcropping north of the Embleton Valley

sparsely phyric hornblende- microdiorite to microgranite

moderate, sericite chlorite alteration

probably early Caradoc, 444 ± 24 Ma (Rb–Sr, whole rock)

Scawgill

Bridge microdiorite and related ‘appinitic’ minor intrusions [1, 4, 12]

dykes, sills, plugs, many deformed and cleaved

porphyritic augite- hornblende microdiorite (e.g. Scawgill Bridge intrusion), meladiorite, basalt, tonalite

variable: strong muscovite, chlorite, calcite, epidote alteration; intense chlorite-quartz rich alteration

early Caradoc, 458 ± 9 Ma (K–Ar age on three hornblende concentrates from the Great Cockup ‘Picrite’)

Sale Fell and other calc-alkaline lamprophyre intrusions [1, 4, 12, 13]

dykes

minette and kersantite

strong chlorite- muscovite alteration

early Devonian, 402 ± 9 (K–Ar age on single biotite concentrate from Sale Fell)

Minor intrusions in the Cross Fell inlier [13, 14, 15]

sills up to 50 m thick; dykes

dolerite sills, calc- alkaline lamprophyre dykes; microgranite and quartz-porphyry dykes

variable

probable Ordovician and mid-Devonian ages but no isotopic data available

Pre-Acadian minor intrusions in the Black Combe inlier [16, 17, 18]

deformed dykes and sills

groupings of highly altered basaltic, andesitic and rhyolitic rocks

strongly altered

probably Caradocian but no isotopic data available

Microgranodiorite sheets in the Black Combe inlier [16, 17, 18]

cleaved, NE- trending, inclined sheets

microgranodiorite with feldspar and quartz phenocrysts

strongly altered

probably early to mid-Devonian, but no isotopic data available
References