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Geology of the Whithorn district. Memoir for geological sheet 2. Scotland

Bibliographical reference: Barnes, R P. 1989. Geology of the Whithorn district. Memoir British Geological Survey. Sheet 2. Scotland.

British Geological Survey Scotland. London: Her Majesty's Stationery Office 1989. © Crown copyright 1989 First published 1989. ISBN 0 11 884423 7. Printed in the United Kingdom for HMSO Dd 240423 C20 6/89

Other publications of the Survey dealing with this district and adjoining districts

Books

Maps

Notes

(Front cover)

(Rear cover)

1:10 000-scale maps

(10000ScaleMaps)

The component 1:10 000-scale National Grid sheets (all within 100 km square NX) of 1:50 000 Sheet 2 are shown on the diagram above. These maps were partially resurveyed in 1983–84 by R P Barnes following the original survey (at six-inch scale) by J Craik in 1871–72. These maps are not published but they are available for reference at the British Geological Survey Office, Murchison House, Edinburgh where uncoloured dyeline copies can be purchased.

Geology of the Whithorn district

The area described in this memoir forms the southern part of a peninsula which extends southwards into the Solway Firth. The relief is subdued with the land largely used for mixed agriculture. The district, which is covered by 1:50 000 sheet 2 of the geological map of Scotland, includes three small settlements of which Whithorn is the largest. The area contains extensive and, in places, spectacular coastal sections.

The introductory chapter places the Silurian sandstone and interbedded silty mudstone succession of the Whithorn district in its regional context, as part of the Lower Palaeozoic Southern Uplands terrain. The origin of this terrain is still a matter of some controversy. Three subsequent chapters give details of the stratigraphy and sedimentology, the structure and the numerous minor intrusions. The final chapter describes the Quaternary 'drift' deposits, dominantly of glacial origin, which cover parts of the area.

(Frontispiece) Folded Kirkmaiden Formation greywacke succession, in Back Bay [NX 368 394], displaying major first fold structures and two, conjugate, sets of second fold structures (ID 3758)

(Geological succession) Geological succession in the Whithorn district.

Preface

The district described in this memoir, covered by sheet 2 of the 1:50 000 Geological Map of Scotland, was first surveyed by J Craik in 1871–72. The first edition of the map, published in 1872, was accompanied by a memoir (Sheet Explanation) published in 1873. The map was revised by B N Peach and J Horne and a second edition was published in 1923.

A partial resurvey of the district, concentrating on areas in which the Lower Palaeozoic strata are well exposed, was made by Dr R P Barnes in 1983–84. This work was carried out by the BGS as part of a continuing regional reappraisal of the Lower Palaeozoic geology of Southern Scotland. Revised Solid and Drift maps for the Whithorn district (Sheet 2) were published on a single sheet in 1987.

The memoir was written by Dr R P Barnes with contributions from Dr D E White (palaeontology) and Dr N M S Rock (petrology of the igneous rocks). Photographs were taken by Mr T S Bain and Mr F I MacTaggart. The memoir was edited by Dr P M Allen and Dr P Stone.

F G Larminie, OBE Director, British Geological Survey, Keyworth, Nottingham. NG12 5GG. 12th January 1989

Chapter 1 Introduction and geological setting

The Whithorn district of Galloway lies towards the southern end of the Wigtown peninsula, a promontory extending southwards into the Solway Firth and culminating in the high cliffs of Burrow Head. It separates Luce Bay to the west from Wigtown Bay to the east. Relief is generally subdued with the highest point, the Fell of Carleton, rising only 146 m above OD (Figure 1). However, some spectacular sea cliffs, especially along the south-west coast, provide excellent rock exposure locally. Inland a strong north-east to south-west topographic lineament reflects both the regional strike of the underlying strata and the main direction of ice flow during the most recent glaciation. Land use is almost entirely agricultural.

Geologically the district is part of the extensive outcrop of Lower Palaeozoic strata which forms the Southern Uplands of Scotland (Figure 2), an area bounded to the north by the Southern Upland Fault and to the south by unconformably overlying Devonian and Carboniferous strata (Greig, 1971). Early Geological Survey work on the Southern Uplands culminated in the publication of a memoir (Peach and Horne, 1899) in which the basic stratigraphy of the area was established. Three parallel outcrops, one of Ordovician and two of Silurian (Llandovery and Wenlock) strata (the 'Northern', 'Central' and 'Southern' belts respectively) were distinguished, each made up largely of unfossiliferous greywacke, though with interbedded fossiliferous silt stone beds in the Southern Belt, and fossiliferous black mudstone and chert sequences ('Moffat Shale'), forming narrow, discontinuous, strike-parallel outcrops in the Northern and Central belts. Peach and Horne interpreted the Moffat Shale as the base of the sequence and regarded their boat-shaped outcrop patterns as inliers formed by the exposure of anticline cores. Subsequent work radically altered this interpretation with the recognition that, whilst the Moffat Shale was indeed the base of the sequence, the younging sense in the adjacent steeply inclined greywacke beds was dominantly to the north-west, despite intense local folding, on both sides of the shale inliers. Further, the detrital components of the greywackes may be markedly different on either side of the main shale outcrops emphasising the lack of stratigraphic continuity (e.g. Kelling, 1961; Floyd, 1982). However, since the overall south-east younging trend was confirmed, with the oldest rocks in the north-west and progressively younger strata appearing southwards (e.g. Toghill, 1970), large-scale, strike-parallel faulting with downthrow to the southeast was invoked to explain the relationships. This structural pattern is now recognised as an imbricate thrust system (e.g. Webb, 1983). Meanwhile the perceived position of the Southern Uplands at the margin of a major early Palaeozoic tectonic plate encouraged comparison with the ideal accretionary prism model developed by Seely and others (1974). The result was the description of the Southern Uplands as an accretionary complex assembled above a north-west-directed subduction zone during the Ordovician and Silurian (McKerrow and others, 1977; Leggett and others, 1979). The complex was thought to have developed as successive thin layers of sediment were sheared from the surface of the downgoing plate and underthrust beneath a stack of similar slices. Such an origin for the Southern Uplands has gained widespread acceptance but debate still continues and two alternatives have been recently proposed (Murphy and Hutton, 1986; Stone and others, 1987). Both envisage the southern part of the Southern Uplands, in which the Whithorn district is situated, developing as a foreland fold and thrust belt deforming the sedimentary fill of a successor basin formed above a major suture after continental collision. There would thus be no association with subduction. The imbricate thrust geometry could form in either environment and the argument hinges on detailed sedimentology, stratigraphy and petrology.

Whatever its origin the Whithorn district is typical of the Southern Uplands, containing a sequence of interbedded greywacke sandstones, siltstones and shales with bedding generally steeply inclined and striking north-east to southwest. The sequence has been divided into three lithostratigraphical units (Figure 3): the Kirkmaiden Formation (KMN) forms the northern part of the area and is succeeded southwards by the Carghidown Formation (CGD), (both parts of the Hawick Group), with a small area of strata assigned to the Riccarton Group (RCN) present in the extreme south-east of the promontory. Greywacke is dominant in all three units and lithostratigraphic division is based on the presence of interbedded red mudstone in the Carghidown Formation and distinctive laminated siltstone within the Riccarton Group succession.

Limited biostratigraphical control in the Whithorn district is provided by sparse graptolite faunas. These show the presence of upper Llandovery (griestoniensis Zone) strata in the Kirkmaiden Formation and lower Wenlock strata (centrifugus–riccartonensis zones) in the Riccarton Group at Burrow Head. The boundary between the Carghidown Formation and the Riccarton Group is interpreted as a conformable southward-younging transition. This implies that the Carghidown Formation is of latest Llandovery (crenulata Zone) or earliest Wenlock (centrifugus Zone) age in this area and thus younger than the Kirkmaiden Formation (Table 1). The thickness of strata comprising these units is very difficult to assess. Probably three Wenlock graptolite zones are represented in a very small area of exposure of Riccarton Group strata compared with the two or possibly three zones present in a very large area of Hawick Group strata. Exposures of the Carghidown Formation at the Scares, (Barnes and others, 1988) small islands in the centre of Luce Bay, are along strike from the mainland outcrop of the Kirkmaiden Formation and suggest that these two formations are in part lateral facies variants (Figure 4). They may represent as little as 1 km total thickness.

Chapter 2 The Silurian turbidite succession

Lithological characteristics

The strata cropping out in the Whithorn district have the sedimentological characteristics of the Hawick and Riccarton groups exposed along strike to the north-east. The local subdivisions of the Hawick Group, the Kirkmaiden and Carghidown formations, were both defined by Rust (1965) and are distinguished from each other by the presence of red mudstone interbedded with the greywackes of the Carghidown Formation. Red mudstone is absent from the Riccarton Group, these strata being characterised by the local occurrence of a distinctive laminated siltstone lithology interbedded with the greywackes.

Greywacke (a poorly sorted sandstone) is the dominant lithology in all these divisions forming beds ranging from a few centimetres to several metres in thickness. Each bed represents the sediment deposited from a single turbidity current flow. The sedimentological divisions within each bed can be described using notation based on the work of Bouma (1962) who recognised that the deposit from a turbidity current typically comprises one or more of five divisions:Ta massive, graded;Tb discontinuous parallel laminated;Tc cross laminated;Td finely parallel laminated andTe pelagic interturbidite muds.

A submarine fan system affords the most likely depositional environment for turbidite sequences; different facies within that environment can be deduced from a combination of sedimentary features such as grain size, bed thickness and organisation. These have been formalised into a classification scheme for turbidite sequences by Pickering and others (1986) which can usefully be applied to the rocks of the Whithorn district. The relevant part of the classification scheme is summarised in (Table 2).

The turbidite sequences of the Whithorn district can be assigned to two principal facies which make up alternating members ranging from less than one to tens of metres in thickness (Figure 5), (Table 2).

Facies C (principally C2.2) members are commonly made up of a random assortment of beds of different thickness, although groups of beds of uniform thickness, or upward-thinning sequences, are locally recognisable. The thick beds occur either singly or in groups in an otherwise relatively thinly bedded sequence. Greywacke beds are usually less than 50 cm thick but range from a few centimetres to about 2 m; silty mudstone partings occur but are thin (usually less than 10 cm) or discontinuous. Beds have parallel, sharp tops and bases and are laterally continuous along strike. The greywacke is usually medium- or fine-grained although rarely it does include coarse-grained detritus. Ta, Tb and Tc divisions are variably developed, usually alone or in Tab, Tac and Tbc combinations. The lowest division, most commonly Tb, characteristically constitutes most of the bed thickness. Where present the Td division is represented by the silty mudstone partings. Pelagic mudstone ( Te) is only seen at two localities as thin dark laminae. Greywacke beds commonly have groove or flute casts at their bases and may contain mudstone rip-up clasts. Free-standing ripples, usually symmetrical with amplitudes of 2–3 cm and wavelengths of about 20 cm, are common on the tops of greywacke beds. They cut through the laminations in the top of the beds and some have a drape of reworked sediment on the lee side. Thick greywacke beds (1–2 m) are massive or poorly graded with fine-grained tops and may include coarse-grained detritus. Internal grain-size variations occur in places suggesting composite units. Some of the thick greywacke beds are erosive, cutting sharply down into the underlying beds and thinning rapidly laterally within a few metres; these were deposited in narrow channels. These thickly bedded greywackes are transitional between Facies C2.1 and B2.1.

Facies D (principally Facies D2.3 but transitional to Facies C2.3) is dominated by silty mudstone, commonly finely laminated, with a variable proportion (up to 50%) of thinly bedded (1 cm) siltstone and fine-grained greywacke beds (up to 5 cm thick). Individual greywacke beds are parallel-sided, and laterally continuous Tc units. The Facies D members range in thickness from 10 cm to 10 m, separating Facies C members and locally forming the upper part of upward-thinning and fining sequences. Trace fossils are abundant in places (Benton, 1982).

The association of Facies C and D is typical of a middle fan environment in idealised turbidite fan models (e.g. Walker and Mutti, 1973). The interdigitation of the different facies results from the migration of broad shallow channels, in which Facies C turbidites were deposited, across the surface of the fan. Facies D sediments were deposited in interchannel or overbank environments. The orientation of sole marks in Facies C in all of the formations mapped (Figure 6) varies mainly between east and north-east with flutes indicating a current direction from the east. In the Carghidown Formation a few sole marks, from localities on the east coast, trend south-east with one set of flutes indicating a current from that direction. The current directions represented by ripples on the tops of beds are variable (Figure 6) and commonly at a high angle to the sole mark orientation. These currents, dominantly from the south or east, reworked the fine-grained tops of previously deposited turbidites.

Greywacke petrography

The greywackes from each of the three stratigraphical divisions are very similar in composition. There is some variation in the relative proportions of the principal detrital components, partly in relation to grain size, but it cannot be correlated with stratigraphy.

The sandstones arc mostly fine- to medium-grained, calcareous, lithic greywackes. They form poorly sorted deposits consisting of angular to subrounded grains with up to 40 per cent silt matrix. Coarse-grained sandstone may be better sorted, grading into lithic arenite. The grains are dominantly of quartz, with carbonate, lithic fragments, feldspar and mica also present. Ferromagnesian minerals are absent. Feldspar, including potassium feldspar and plagioclase, is usually about 10 per cent of the sand fraction. The proportion of mica varies considerably from about 3 per cent in medium-grained sandstone at the base of greywacke beds to 15 per cent in fine-grained greywacke common in the upper parts of beds. Carbonate forms up to 15 per cent of the rock and probably originated largely as detrital grains which, although now recrystallised, are still sometimes recognisable. The partial replacement by carbonate of other grains and matrix confirms that some redistribution has taken place; in rare cases the matrix of coarse-grained sandstone consists almost entirely of crystalline carbonate. Lithic detritus increases from about 15 per cent in medium-grained sandstone to predominance in very coarse-grained sandstone (lithic arenite). It consists of volcanic rock fragments, polycrystalline quartz, granitic detritus and fine-grained sedimentary material. The volcanic detritus is usually basic to intermediate in composition with acid rock types only rarely present. Among the polycrystalline quartz grains are quartz arenite, intergrown quartz crystals of vein and granitic origin and some highly strained quartzites. Coarse-grained intergrowths of quartz and feldspar are probably of granitic origin. Sedimentary rock fragments are mainly siltstone, some of which are cleaved. Red or pink stained cherty material is common in some samples from the Carghidown Formation.

'Red mica' is a common minor constituent locally in fine-to medium-grained greywacke in the southern outcrops of the Kirkmaiden Formation, throughout the Carghidown Formation and also rarely in the Riccarton Group. It occurs as coarse-grained flakes orientated parallel to bedding. The red colour is caused by hematite coatings on the mica flakes and is associated with patchy hematite staining of the rock. Red mica does not occur continuously in any one bed and its presence may be a diagenetic effect.

Accessory minerals include green tourmaline, zircon and garnet, the last two being abundant in one sample from the base of a greywacke bed in the Riccarton Group. Scattered opaque minerals are ubiquitous and probably include both detrital and diagenetic materials.

Detailed stratigraphy

The correlation between the lithostratigraphy and biostratigraphy of the three divisions recognised in the Whithorn area is summarised in (Table 1).

Hawick Group: Kirkmaiden Formation (KMN)

These strata are best exposed in the coastal section southwards from Back Bay [NX 369 393] (e.g. frontispiece), where thinly developed Facies C and D members are well represented. Facies C is dominant to the north. Thick Tbc greywacke beds are exposed near Monreith [NX 359 407.] and around the Fell of Barhullion [NX 375 418]. Some very thickly bedded, massive greywackes are exposed between Cairndoon [NX 380 389] and Fell of Carleton [NX 400 376], in an otherwise medium-to thin-bedded sequence.

At Kirkmaiden [NX 365 401] and Back Bay [NX 371 394] thin black mudstone laminae in the silty mudstone between greywacke beds contain sparse graptolite faunas. Slender monograptids of vomerinus type were first reported from these localities by Rust (1965). Collections made by the BGS include the species: Monoclimacis cf. griestoniensis (Nicol) (Files and Wood, 1911), Monograptus priodon (Bronn) and cf. M. spiralis (Geinitz), indicative of the Mcl. griestoniensis Zone (upper Llandovery).

Hawick Group: Carghidown Formation (CGD)

The boundary between the Carghidown and Kirkmaiden formations is drawn at the northern limit of the occurrence of red mudstone beds. There is no evidence of a major structural break although strata in the vicinity of this boundary are not well exposed. Hence the junction between the two formations is interpreted as a conformable transition marking the appearance of red mudstone in the Hawick Group sequence (Figure 4).

The Carghidown Formation is well exposed in the extensive coastal sections on both sides of the southern part of the Wigtown peninsula. It is dominated by Facies C, with sporadic occurrences of very thickly bedded greywackes, especially on the east coast. The distinctive red mudstone beds are thin and rare in the northern part of the outcrop area but their thickness increases (up to 2 m) and they become more common southwards. Red mudstones typically occur in Facies D members, interbedded with the typical grey-green silty mudstone, against which bed-parallel margins are diffuse (often mottled), or interlaminated. The red mudstone beds are internally structureless and laterally persistent. A primary origin for the red colouration is supported by the occurrence of patches of red mudstone mixed with green in the matrix of slumped units (e.g. north of Chapel Port West [NX 478 361]. Discrete zones of disrupted bedding, probably due to soft sediment deformation are widespread in the Carghidown Formation in sequences up to tens of metres thick. Between Miller's Port [NX 478 390] and Sheddock Hole [NX 477 396], for example, pinch and swell structures in single beds vary, through open flexures, to local mélange development with lenses and irregular blocks of greywacke in a mudstone matrix.

The Carghidown Formation is unfossiliferous but fossils of earliest Wenlock age from the overlying Riccarton Group (see below) and late Llandovery fossils from the Kirkmaiden Formation suggest that it is also of late Llandovery age.

Riccarton Group

The junction of the Hawick and Riccarton groups is transitional; it is marked by the disappearance of red mudstone and the incoming of dark grey, finely laminated siltstone beds. At Burrow Head [NX 453 341] this occurs within a broad belt of southward-younging strata within which the southernmost red mudstone bed and the northernmost dark grey siltstone bed are separated by a 30 m section containing a 6 m-wide zone of strata disrupted by faults and minor folds. Rust (1965) proposed a major fault at this locality to facilitate his overall interpretation of the stratigraphy but the faults may equally well be relatively minor, with the Carghidown Formation conformably overlain by the Riccarton Group. This is consistent with the relationships to the northeast in Kirkcudbrightshire where Clarkson and others (1975) have described a transitional facies, including both red mudstone and laminated siltstone (the Ross Formation) between the Hawick and Riccarton groups. Laminated siltstone is also exposed on the Isle of Whithorn [NX 480 359], in an intensely deformed zone which also contains red mud-stone beds. The two rock types may be in tectonic juxtaposition here, or together they may comprise an equivalent of the Ross Formation.

The Riccarton Group is dominantly composed of Facies C members, up to 50 m thick and commonly of uniform bed thickness, separated by thin Facies D members. The dark grey laminated siltstone beds, which characterise this group, are sparsely distributed and usually interbedded with silty mudstone of Facies D. They range from 1 cm to 1 m in thickness and have sharp contacts against silty mudstone. The siltstone is carbonaceous with fine laminations locally disrupted by bioturbation.

Graptolite faunas from the siltstone beds indicate an early Wenlock age. The earliest Wenlock graptolite zone of Cyrtograptus centrifugus was recorded at a cliff top exposure [NX 4538 3411] at Burrow Head, where C. centrifugus Boucek, Monoclimacis vomerinus cf. vomerinus (Nicholson), M. vomerinus cf. basilicus (Lapworth), and Monograptus cf. danbyi Rickards were collected. This zone was also recognised on the Isle of Whithorn e.g.[NX 4800 3597]. At Burrow Head the Cyrtograptus murchisoni Zone may be present at the base of the cliff [NX 4638 3434]. At a cliff top exposure close by [NX 4633 3437] Monograptus cf. riccartonensis Lapworth was collected, indicating the M. riccartonensis Zone. The C. centrifugus to M. riccartonensis zones have also been recorded in the Ross Formation of the Riccarton Group on the east side of Kirkcudbright Bay (Kemp and White, 1985).

Chapter 3 Structure

Introduction

The earliest description of the structural geology of the Whithorn area (Rust, 1963; 1965) invoked five phases of folding: F1, gently to moderately plunging tight folds; F2, horizontal or gently plunging, steeply inclined open folds; F3, steeply plunging and vertical open to close folds; F4, recumbent open folds and F5, minor, steeply plunging dextral kink bands. Rust related the dominant cleavage to his F3 structures with an earlier F1 fabric only recognisable in the cores of F1 folds. Weir (1968) described a similar structural chronology from the coastal sections farther east along strike between Creetown and Gatehouse of Fleet. In a revision of the structure of these and other areas, Stringer and Treagus (1981) recognised that Rust's F1 and F3 episodes could be combined in a single event postdated by a second fold-phase comprising Rust's F2 and F4. They related the regional cleavage to the earlier of their two fold episodes and examined its non-axial planar aspect (see below) in detail (Stringer and Treagus, 1980; 1981).

The attitude of bedding in the Whithorn area is summarised in the stereograms on (Figure 7). The moderate, northward dip in the north (stereograms A and I) gradually changes southwards through vertical strata (B, C, G and F) to a steep southward dip (D and E). Two structural domains (Figure 3), described below as Zones 1 and 2, are principally defined by differences in the plunge of the first folds in the two areas although they also reflect differences in later structures. The junction between these zones can be located just north of Sheddock [NX 474 397] on the east coast, and approximately placed in an area of poor exposure near Glasserton [NX 418 375] in the west. These localities are joined by an irregular belt of boulder clay, apparently filling a hollow in bedrock. Just north of the (?minor) east–west fault at Shed-dock numerous minor thrusts occur, dipping gently northwards. These are considered to be minor faults associated with the main, zone-boundary structure which is interpreted as a thrust dipping gently to the north and with a southward transport direction.

The overall southward younging succession, determined from the biostratigraphy, is difficult to reconcile with the structure. Zone 1 is predominantly northward younging and although Zone 2 is intensely folded the overall southward vergence of structures suggests that this area also is broadly northward younging. At least two major early thrusts must be present (Figure 4), probably strike-parallel and associated with early, southward-verging structures, their direction of transport having been towards the south. Although minor faults of this type are recognised, the major reverse faults cannot be positively identified in the Whithorn area.

Folding

The first phase of deformation is by far the most important throughout the district. A second, relatively minor event, is represented in both zones. Major folds are rare in Zone 1 and the steeply inclined strata young predominantly northwards despite an abundance of variable minor structures. By contrast folds of intermediate to major size are well developed in Zone 2 although minor structures are relatively rare. Large-scale folds are exceptionally well exposed in the cliff section from Back Bay [NX 368 394] (cover photograph) to Cairndoon [NX 376 388] and in the Port Allen [NX 478 410] coastal section. Folds are described following the terminology of Fleuty (1964).

Zone 1: First deformation (D1)

Folds in Zone 1 are principally tight to isoclinal and upright to inclined. They show a wide range of hinge plunge (see stereograms C to H, (Figure 7)) and are associated with a slaty cleavage. Axes vary from linear to strongly curved in vertical or steeply dipping axial planes so that their plunge may change from horizontal to steep or vertical. Steep and vertically plunging folds commonly form sinistral pairs with short limbs up to 100 m long. Some steeply plunging folds are downward facing, suggesting that the plunge variation is partly due to rotation of fold axes. Cleavage is typically congruous but non-axial planar in gently and moderately plunging folds where it strikes up to 20° clockwise of the axial planes. Generally the cleavage becomes more closely axial planar as fold plunge increases.

At two localities (Isle of Whithorn Bay [NX 477 356] and Cairn Head [NX 486 383]) gently plunging folds are refolded by steeply plunging, sinistral fold pairs. The early, gently plunging folds may be slump folds. Elsewhere however, folds of both attitudes are related to a single cleavage and at these localities their interference may indicate relatively early and late phases of a single (D1) deformational event during which the stress regime changed with increasing sinistral shear.

D1 folds show an overall similar style, although slickensides on bedding planes and mineral lineations in fine, bed-parallel veins are perpendicular to fold axes and testify to bedding plane slip in the early stages of fold development. Subsequent flattening, seen in deformed concretions, tightened the interlirnb angles. No appreciable extensional strain has been recognised. Where the folds affect strata of widely differing bed thicknesses they are commonly disharmonic.

Zone 1: Post-D1 deformation

A few southward-verging, open to close folds clearly fold the first cleavage and are ascribed to a second deformational event (D2). Their relationship to other post-D1 structures in Zone 1 cannot be demonstrated. Axial planes dip steeply to the south and fold axes plunge gently north-east or southwest. A weak crenulation cleavage is usually only developed in the axial regions of the folds. D2 structures are best developed in the section between Port of Counan [NX 419 362] e.g. (Plate 1a) and Port Castle Bay [NX 426 358] where short limbs dip gently south-east and range in length from a few centimetres to 20 m.

Two other distinct sets of structures, locally developed in Zone 1, probably postdate D2. These are rare, northward-verging, open folds, and steeply plunging minor folds. The former plunge gently north-east with steeply dipping axial planes. They fold the first cleavage and are associated with a weak crenulation cleavage. Short limbs are usually less than 2 m long. Although these folds are comparable in style and attitude to D2 folds they cannot be related because they have opposite senses of vergence without intervening major folds or faults. Steeply plunging dextral open minor folds, with vertical axial planes striking between 170° and 020°, are especially common in the section from Burrow Head to Port of Counan where short limbs are typically 10 to 20 cm long but range up to 1 rn. Larger-scale gentle and open flexures of bedding may also be related. Small flexures with fractured axial planes are common and have the appearance of kink bands. Rare minor folds of the same style, but with axial planes striking north-cast and a sinistral sense, represent a conjugate set of structures.

Zone 2: First deformation (D1)

Zone 2 is characterised by folds with a uniform plunge to the north-east (stereograms A,B and I, (Figure 7)); these are close to isoclinal and inclined throughout most of the area, but become dominantly isoclinal and more upright to the south on the west side of the area (stereogram B). A slaty cleavage is congruous with the folds but is typically non-axial planar, striking up to 20° clockwise of the axial plane. Steeply plunging D1 structures occur in the Monreith area (Figure 3) and are well exposed at Black Rocks [NX 358 408]. They vary from minor folds with short limbs a few centimetres long, locally intensely developed in very thinly bedded strata, to larger-scale folds in more thickly bedded greywackes. These folds are close to tight, vertical or steeply plunging, upright sinistral pairs with a strong cleavage striking 080° to 100°.

Zone 2: Post-D2 deformation

Two sets of structures plunging gently north-east are grouped as D2 because of their conjugate geometry: (i) South-cast verging open structures, with axial planes dipping steeply south-east and a weak crenulation cleavage, are widespread. Gently dipping short limbs are up to 100 m long. (ii) Locally developed open to close folds of relatively small scale (short limbs less than 5 m), have recumbent or gently dipping axial planes and are associated locally with an intense crenulation cleavage. Both sets of structures are well displayed in the Kirkmaiden [NX 364 401] and Back Bay [NX 368 394] (see frontispiece) sections where they fold bed-parallel dykes (Plate 1). Recumbent D2 folds interfere with steeply plunging D1 folds at Black Rocks [NX 358 408]. A weak subhorizontal crenulation of the first cleavage, developed throughout the southernmost part of Zone 2, is correlated with the recumbent set of D2 structures exposed further north.

One other structural style occurs at one locality only. In the foreshore at Port Allen [NX 479 409], minor, open to close, moderate to steeply plunging conjugate structures have a box-fold geometry.

Regional fold relationships

The deformational events in both zones are probably equivalent, but the differences between the resulting structures, that characterise the two zones, require some variation in the stress regime, either temporal or spatial. The steeply plunging D1 folds common in Zone 1 also occur in Zone 2 in the Monreith area at the south-west end of a narrow strike-parallel belt, which is also exposed on the east coast of the Wigtown peninsula. These folds require a component of sinistral shear during deformation. It is possible that the two areas in which they occur represent belts of major (D1) sinistral shear, superimposed on an overall stress regime dominated by north-west to south-east thrusting. D2 is locally developed in both zones as steeply inclined, southward-verging folds which are coaxial with D1 in Zone 1 and have the same sense of asymmetry. The recumbent conjugate set of D2 folds, developed only in the Kirkmaiden area, requires the principal shortening direction during D, to have been north-west to south-east in the plane of bedding. This is consistent with the development of Dy during approximately bed-parallel thrusting and suggests that D1 and D2 are closely related.

The D1 deformation, including the imposition of the regional slaty cleavage, predates the numerous minor intrusions present throughout the area but some of the dykes are folded by D2 structures. Equivalent intrusions elsewhere in the Southern Uplands are now recognised to range from late Wenlock to early Devonian in age (see Chapter 4 of this volume; Rock and others, 1986).

Faulting

Faults are very common in the coastal sections. In the majority of cases no estimate of throw is possible but movement directions may commonly be interpreted from drag folding or slickensides. Five principal groups of faults are recognised:

  1. Early faults related to D1 folding.
  2. Gently- to moderately-dipping thrust faults
  3. North-south vertical sinistral faults
  4. North-west to south-east vertical dextral faults
  5. Strike-parallel vertical sinistral faults.

Group i

Changes in the direction of younging commonly occur at bed-parallel faults. These faults may be equivalent to the minor strike-parallel reverse faults, associated with D1 structures, which can be recognised in well exposed sections. Some steeply, plunging fold pairs in zone 1 are associated with zones of shear which locally replace fold hinges. Two, postulated, major early strike-parallel reverse faults (Figure 4), throwing down to the south, belong to this group.

Group ii

Early fold structures are deformed at a number of localities by reverse faults, striking approximately 060° and dipping moderately to gently northwards or, less commonly, southwards. These faults may have developed during D2 deformation. Fault planes are commonly curved and throws are usually of the order of a few metres. The inferred fault (p.9), separating structural zones 1 and 2, is probably a major structure of this group.

Group iii

Vertical faults trending between 350° and 010° with a sinistral horizontal throw, indicated by slickensides or slickencrysts in quartz-carbonate veneers, occur throughout the area. They are particularly well exposed around Burrow Head. Lamprophyre dykes are intruded along some of these faults whilst others displace dykes.

Group iv

Vertical faults trending between 100° and 140° have a dextral horizontal throw. Flexuring of bedding adjacent to these faults together with subhorizontal slickensides and slickencrysts demonstrate the essentially horizontal dextral movement. Numerous dextral faults, with individual horizontal displacements usually between 1 and 10 m, form a fracture zone which dominates the coastal section from Port of Counan [NX 418 362] to Burrow Head. Fault planes are commonly marked by breccias, up to 1 m thick, developed even where movement is small. These faults displace and are intruded by lamprophyre dykes suggesting a similar age range to the north-south vertical sinistral faults, to which they are conjugate.

Group v

The most impressive faults in the area are strike-parallel (approximately 060°) fractures which form marked topographic features in areas where drift is thin or absent. Discrete fracture planes have several generations of slickencryst development on quartz/carbonate veneers. These dominantly show a horizontal displacement although moderate to steeply plunging fibres suggest a vertical component to some phases of movement. The main faults, such as those at Isle of Whithorn, are typically flanked by zones of sinistral shear on bedding, marked by slickencrysts in fine bed-parallel veins and minor displacements of oblique lamprophyre dykes. Age relationships observed at the coast, and the lateral continuity of these faults, suggest that this is the most recent major phase of faulting.

Locally faults may occur which do not fit into any of the above groups; two examples are worthy of note. Between Cairndoon [NX 380 389] and Carleton [NX 392 380] a set of faults trending 080°–090° forms marked scarp features and displaces axial plain traces of folds in a manner suggesting downthrow to the south. In parts of the east coast section minor faults trending 110°–130° have a sinistral sense of displacement.

The age relationships between fold and fault structures, in part deduced from the manner in which they affect minor intrusions, are summarised in (Table 3) (see also Barnes and others, 1986).

Mineralisation

Fault zones containing traces of iron (siderite) and copper (chalcopyrite and malachite) have been explored at Mary Mine [NX 439 348], Tonderghie. Similar sparse mineralisation occurs in other fault zones in the area hut no other attempts at exploitation are known. Numerous baryte veins in the Burrow Head to Port Castle Bay [NX 426 358] coastal section are also related to faulting and some contain minor sulphide mineralisation. The age of the mineralisation is uncertain.

Chapter 4 Minor intrusions

Dykes are a common feature of the coastal sections throughout the Whithorn district and it is likely that their absence from the 1:50 000 solid geology map over much of the inland part of the area simply reflects the relatively poor exposure there. Many more dykes are present than it has been possible to show on the map. The majority of the dykes are basic, Mg- and K-rich lamprophyres and appinites but a variety of more intermediate-acid lithologies are also present including some with a restricted andesitic composition and others which arc broadly dacitic (Barnes and others, 1986). With one exception the dykes are considered to be members of the late Silurian to early Devonian 'Caledonian' swarm. The exception is a single dolerite dyke at Burrow Head which consists of fresh, fine-grained plagioclase and pyroxene and is probably part of a regional Tertiary dyke swarm.

Intrusions range up to several metres in thickness, although the majority are between 10 cm and 1 m. The principal dyke orientation (Figure 8) is parallel to bedding, requiring an extension of about 1 per cent perpendicular to strike to accommodate the intrusive swarm. Locally dykes may follow the various fault trends. A few intrusions are elliptical in outcrop plan with a maximum larger dimension of 130 m.

Petrography

Many of the 'Caledonian' dykes are highly altered to carbonate, chlorite, sericite and other secondary minerals such that they can only be broadly defined mafic–intermediate (K) or intermediate-felsic (J). Less altered dykes can be readily assigned to one of the following four groups in terms of their compositional and textural characteristics.

i Acid porphyrite (FPt) and microgranodiorite (FMd)

These rocks are dacitic in composition (c.63–67 wt per cent SiO2) and consist of generally sericitised plagioclase, subordinate alkali feldspar, essential quartz, altered hornblende and biotite. Phenocrysts of plagioclase and sometimes biotite distinguish acid porphyrite from microgranodiorite which is aphyric.

ii Porphyrite (P)

These rocks are of andesitic composition (c. 57–63 wt per cent SO2), consisting of phenocrysts of generally sericitised plagioclase, and occasionally of biotite or hornblende, in a matrix of the same minerals (Plate 2)a. Quartz and alkali feldspar are minor constituents.

iii Lamprophyre (L)

These intermediate to mafic (colour index 33–67) dyke rocks of K-andesite (shoshonitic) composition have generally high alkalis (especially K) for their silica content (45–57 wt per cent Si02). They contain phenocryst combinations of olivine, pyroxene, hornblende and/or biotite in a matrix of feldspars, minor quartz and mafic minerals. Olivine, if present, is confined to phenocrysts, and felsic minerals to the groundmass. Hornblende and biotite together reach essential amounts modally. Panidiomorphic texture (wholly euhedral phenocrysts), globular felsic structures and pseudohexagonal castellated biotite crystals are typical. The group is subdivided into kersantite (LK) containing phlogopitic biotite and plagioclase (Plate 2)b; minette (LM) containing biotite and orthoclase, and spessartite (LS) with hornblende and plagioclase (Plate 2)c.

iv Appinite (Y)

These are coarse-grained equivalents of the hornblende lamprophyres (Plate 2)d. They are generally non-porphyritic and show well developed mafic and felsic segregations.

The lamprophyres and appinites represent magmas which originated deep in the mantle (Barnes and others, 1986; Rock and others, 1986); the different groups probably resulted from slight variations in melting conditions and differing degrees of modification in the upper mantle and crust. Their overall geochemical homogeneity suggests relatively short-lived melting and intrusion events. Most of the felsic lithologies (FePt, FMd and P) are petrogenetically unrelated to the lamprophyre suites and were probably produced during melting events in the lower crust or upper mantle. Only a few of the felsite dykes may represent felsic differentiates of the lamprophyric magmas.

History of intrusion events

The age of the intrusions can only be inferred by comparison with other areas of the Southern Uplands. Very few lamprophyre dykes intrude the major Lower Devonian granitic plutons (c.400 Ma) whereas the plutons are intruded by numerous felsite dykes. Lamprophyre dykes are emplaced into Wenlock sedimentary rocks in SW Scotland and give radiometric ages of 395–418 Ma (Rock and others, 1986); elsewhere they intrude Lower Devonian conglomerates and lavas. Thus the dyke swarm probably ranges from late Silurian to early Devonian in age.

The polyphase intrusion history of the Whithorn district is recorded by the complex field relationships exhibited by the dykes with respect to each other and to structural features. Cross-cutting relationships between dykes are rare although biotite (kersantite) and hornblende (spessartite) lamprophyres are mutually cross-cutting in several localities, notably just south of Port Castle Bay [NX 428 356]. Rust (1965) reported that lamprophyres locally cut felsite dykes. All dykes postdate D1 folds and the regional cleavage. Bed-parallel lamprophyre dykes are folded by both recumbent and steeply inclined D2 folds at several localities in the Back Bay [NX 368 394] to Cairndoon [NX 376 388] coastal section (e.g. (Plate 1b) and (Plate 1c) although one dyke in this section is possibly contemporaneous with folding (Plate 1b). Dykes are both cut by, and intruded along and parallel to, the conjugate north–south sinistral and north-west to south-east dextral fault systems at numerous localities throughout the area. Intrusion contemporaneous with fault movement is illustrated by a biotite lamprophyre dyke, along a minor dextral fault south of Port Castle Bay, in which a strong fabric is consistent with dextral shear during emplacement. Strike-parallel sinistral faults displace dykes, and the associated shear zones cause numerous small displacements on bed-parallel fractures. The steeply plunging kink bands also deform lamprophyre dykes.

The likely order of tectonic and intrusive events is summarised in (Table 3). The principal phases of intrusion probably represent periods of extension between compressive tectonic episodes. No dykes have been seen to cut D2 folds hence event 2.iii in (Table 3) cannot be proven, although many of the bed-parallel dykes may have been intruded at this time. Rare, apparently syntectonic dykes indicate that intrusion may have been a continual process. It is apparent that biotite and hornblende lamprophyre magmas were continuously available as these dykes exhibit the complete range of age relationships. Felsite dykes are more difficult to date because of their sparse distribution. Rust (1965) related the fabrics commonly observed in porphyrite/porphyry dykes to the regional cleavage and suggested that they must be relatively early (pre- or syn-D1). However, these fabrics, though typically subparallel to dyke margins, may vary widely in orientation even within a single intrusion and they are only subparallel to the regional cleavage in bed-parallel dykes. The intensity of the development of these fabrics is also variable and overall the fabric seems more likely to be of magmatic origin. A few north-west to south-east trending felsite dykes post-date the dextral fault set but bed-parallel felsite dykes are probably relatively early.

The earliest dykes in the Whithorn area pre-date D2 deformation which, it is argued earlier (p.11), must be closely related temporally to D1 and to thrusting. Therefore dyke emplacement was contemporaneous with the later stages of thrusting. If the host rocks were deformed within a subduction-related accretionary prism as described by Leggett and others (1979) this implies crustal extension sub-parallel to the assumed direction of subduction at a time of regional compression. This apparent incompatibility, taken in conjunction with the chemistry of the dykes, is inconsistent with syntectonic emplacement into trench sediments above a subduction zone. The crux of the problem is the need to pass magma, derived from deep in the mantle, through an active subduction zone without contamination and intrude it into a compressive forearc setting only a few kilometres behind the trench. The argument is more fully developed elsewhere (Barnes and others, 1986; Stone and others, 1987) with the dyke evidence seeming to weaken the accretionary prism model for the Southern Uplands. It may be more readily explicable in terms of transpression within a foreland fold and thrust belt.

Chapter 5 Quaternary deposits

During the Quaternary Period the Whithorn district was scoured by part of a large, composite ice sheet. The dominant north-east to south-west trend of roches moutonnées, glacial striae and drumlins, coupled with common erratics of granite derived from the Galloway plutons, all indicate a source for the ice in the high ground of central south-west Scotland. However, rare erratics of the distinctive Ailsa Craig microgranite indicate that ice flowing generally southward, and originating in the Scottish Highlands, occasionally encroached onto at least the western fringe of the promontory. Glaciation came to an end with climatic amelioration about 12 000 years ago and outwash from the melting ice deposited extensive spreads of sand and gravel in places. The rise in sea level at that time, as huge volumes of water were released to the oceans, countered by subsequent isostatic uplift, is marked by raised beaches fringing the present coastline.

Boulder clay

Boulder clay (or till) fills small hollows and forms gently undulating spreads throughout the area but is topographically most significant in the distinctive whale-backed drumlins. The thickest boulder clay deposits occur along the west coast where they are draped over the ancient coastline and locally include interbedded sand and gravel. In places the boulder clay can be divided into two varieties; a friable reddish-brown clay containing many locally derived rock fragments resting on a stiff blue-grey clay with fewer lithoclasts but a much higher proportion of exotic erratics. At one locality, the old tile works at Monreith [NX 358 415], a marine fauna of lamellibranchs, forams and crustacea fragments was recovered from the lower blue-grey clay (Craik and Geikie, 1873). These were most likely carried onshore by ice which had previously scoured an area of the seabed. In general terms the lower boulder clay can be described as a lodgement till, deposited and compressed beneath an advancing ice sheet. By contrast the upper, less well consolidated boulder clay probably formed as an ablation till deposited as the ice melted in situ.

Glacial or fluvioglacial sand and gravel

These deposits are of three types: mounds with kame morphology (e.g. Arbrack [NX 461 375]), caps to low hills (e.g. in Monreith Park [NX 357 425]) and flat-topped outwash fans of which relics occur on the coast west of Cairndoon [NX 373 392], at Burrow Head [NX 451 342] and at Port Allen [NX 477 409]. Although the kames are unsorted and without stratification, the other deposits are typically well bedded. Beds of clean fine sand, commonly laminated, occur between beds of variably sorted gravel in which grain size ranges up to 40 cm. Clasts are dominantly well rounded, consisting of greywacke and granite, with some, more angular, shaly material. Bedding in the Burrow Head deposit dips 10° to the south and gravel horizons include lenticular clasts with a well developed grain imbrication. This suggests that the depositing currents flowed southwards.

Raised beaches and blown sand

Where the coastline comprises thick boulder clay deposits it is commonly fringed by a well developed raised beach terrace with a shingle surface at about 5 m above sea level. A raised, wave-cut platform with thin shingle deposits occurs at the same elevation cut into bedrock on the east coast of the peninsula, backed by a discontinuous low feature marking the former coastline.

Blown sand rests on the raised beach which fringes the Point of Lag [NX 365 395]. It is also spread sparingly over the boulder clay slopes north-west of Monreith.

Tufa

Patches of pale grey tufa up to several metres across drape the cliffs above the high water mark between Port Castle Bay [NX 426 3581 and Burrow Head. It is usually a ragged, cellular deposit with stalactitic structures on the underside where not in contact with the rock. In places it contains sparse shell fragments. The top surfaces have been smoothed by water flow, although there is no present drainage of water over them. The age of the tufa is unknown but it is probably of fairly recent origin.

Peat and alluvium

Spreads of peat grading into peaty clay occur in low-lying areas above boulder clay and filling hollows in bedrock. They probably originated in a lacustrine environment. The large deposits, such as Whithorn Moss [NX 409 422] and Glasserton [NX 424 390], may be several metres thick; small deposits are usually less than 1 m thick. Irregular thin spreads of peat (e.g. Craigdhu [NX 392 403] resting on an uneven bedrock surface probably represent relics of once more extensive peat deposits.

The only drainage channels of any significance arc those of the Monreith Burn, the Ket and the Drummulin Burn. Thin alluvial deposits occur in the last two and comprise coarse sand and gravel with some dispersed larger clasts in a silty sand matrix. Clasts vary from well rounded to angular and primarily consist of locally derived greywacke and mudstone although they include more exotic chert, arenite and granitic material which was probably reworked from the glacial deposits. An abandoned oxbow meander in the Drummulin Burn near Isle of Whithorn [NX 475 368] has accumulated peaty clay.

References

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Figures, plates and tables

Figures

(Figure 1) Main physical features of the Whithorn district and the area to the north

(Figure 2) The 1:50 000 Sheet 2 district in its regional context within the Southern Uplands of Scotland

(Figure 3) A summary map of the solid geology of 1:50 000 Sheet 2, omitting dykes

(Figure 4) Postulated detail of the Hawick Group in Luce Bay between the 1:50 000 Sheet 2 and Sheet I districts. Two major, strike-parallel, reverse faults (early thrusts) must be inferred in the Whithorn district to reconcile the stratigraphy and structure. These faults separate three structural blocks. In Block 1, a lateral (NE–SW) transition from Kirkmaiden to Carghidown formations is suggested by the presence of red mudstone beds, in the Hawick Group, at the Scares and Mull of Galloway. Vertical transitions from Carghidown to Kirkmaiden formations in Block 2 and from Carghidown Formation to Riccarton Group in Block 3, are consistent with a gradual restriction, southwards with time, in the area of deposition of red mudstones as shown in the sections. Detail of the Mull of Galloway is from McCurry in Barnes and others (1987)

(Figure 5) Representative measured sections within the Carghidown and Kirkmaiden formations. Interstratified 'packets' of thinly bedded greywacke and silty mudstone (facies D2.3) and medium-bedded greywackes (facies C2.2) include thick greywacke beds and red mudstone beds in the Carghidown Formation. Facies D2.3 sequences are stylised, apart from one such sequence, in the Carghidown Formation section, which is expanded to show the internal detail

(Figure 6) Palaeocurrent data from coastal sections in the Kirkmaiden and Carghidown formations and the Riccarton Group, together with summary diagrams showing the total data from the Kirkmaiden and Carghidown formations. Current azimuths from sole marks (grooves and flutes) are shown in the outer circles and for ripples on the tops of greywacke beds in the inner circles. All data have been corrected for rotation during D1 folding

(Figure 7) Stereographic projections summarising measurements of bedding and first fold axes in the areas as shown. Plots were made in the lower hemisphere in equal area projection

(Figure 8) Orientation data for minor intrusions in different areas shown on half-rose diagrams with the total data for the Whithorn area shown in the central diagram

Plates

(Frontispiece) Folded Kirkmaiden Formation greywacke succession, in Back Bay [NX 368 394], displaying major first fold structures and two, conjugate, sets of second fold structures (ID 3758)

(10000ScaleMaps) The component 1: 10 000-scale National Grid sheets

(Plate 1a) Steeply inclined, open D2 fold pair plunging gently north-east and verging southwards, with a steep fracture cleavage visible in the hinges. Short limb approximately 20 m long. Port of Counan [NX 419 361] (MNS 4682)

(Plate 1b) Recumbent D2 fold in thinly bedded greywackes, utilised by a bed-parallel 1 m thick kersantite dyke to step sideways. Folding only affects strata between the two parts of the dyke. Dyke terminations are parallel to the fold axis, suggesting synchronous folding and intrusion. Callies Port [NX 372 391] (MNS 4681)

(Plate 1c) Bed-parallel 1.5 m thick kersantite dyke folded by a recumbent minor D2 fold. Back Bay [NX 371 393] (MNS 4680)

(Plate 2) a. Porphyrite ( x 32, XP) (S73660), from a 4–8 rn thick dyke north of Portyerrock [NX 478 342]. Plagioclase phenocrysts in a matrix of intergrown plagioclase laths with interstitial chlorite (black) (PMS 504) b. Kersantite ( x 27, PPL) (S72849), from a 2 m dyke at Port of Counan [NX 419 362]. Scattered biotite laths and clusters of pyroxene crystals in a feldspar matrix (PMS 502)c. Spessartite ( x 32, PPL) (S72848), from a 20 m diameter intrusion near Glasserton [NX 409 376]. Euhedral amphibole and apatite (e.g. bottom centre) in a matrix of poikilitic plagioclase laths (cloudy) with minor quartz (clear) (PMS 503)d. Appinite ( x 27, PPL) (S72829), from a 20 m diameter intrusion near Glasserton [NX 409 376]. Euhedral amphibole crystals in a coarse-grained plagioclase matrix partly replaced by epidote (PMS 501)

Tables

(Table 1) Silurian stratigraphical succession in the Whithorn area Series

(Table 2) A classification scheme for the turbidite sequences of the Whithorn district based on the proposals of Pickering and others (1986) and Walker and Mutti (1973)

(Table 3) Chronological relationships between tectonic and intrusive events. The intrusive 'events' probably represent peaks of activity in a period during which all of the main magma types were available. B and H, biotite and hornblende lamprophyre dykes; F, felsitc dykes

Tables

Table 3 Chronological relationships between tectonic and intrusive events.

The intrusive 'events' probably represent peaks of activity in a period during which all of the main magma types were available. B and H, biotite and hornblende lamprophyre dykes; F, felsitc dykes

Age Event Deformation Intrusion
Early Devonian 2(vii) Strike-parallel sinistral shear zones and N–S vertical kink bands
(vi) Bed-parallel dykes (B)
(v) NW–SE and N–S (fault-parallel) dykes (B,H, and F)
-----?----- (iv) Conjugate NW–SE dextral and N–S sinistral faulting
(iii) ?Bed-parallel dykes
(ii) Minor folding (D2)
(i) Bed-parallel dykes (B,H and F)
Wenlock 1 Main folding (D1) and cleavage development