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The geology of the Moray Firth. United Kingdom Offshore Regional Report

By I J Andrews, D Long, P C Richards, A R Thomson, S Brown, J A Chesher and M McCormac

Bibliographic reference: Andrews, I J, Long, D. Richards. P C, Thomson, A R, Brown, S, Chesher, J A, and McCormac. M. 1990. United Kingdom offshore regional report: the geology of the Moray Firth. (London: HMSO for the British Geological Survey.)

British Geological Survey

The geology of the Moray Firth. United Kingdom Offshore Regional Report

I J Andrews, D Long, P C Richards, A R Thomson, S Brown, J A Chesher and M McCormac

London: HMSO, 1990. @ NERC copyright 1990 First published 1990. 0d 291132 C20 12190 ISBN 0 11 884379 6

Production of this report was funded by the Department of Energy and the Natural Environment Research Council

The coastline used on many maps and diagrams in this book is based on Ordnance Survey mapping

(Front cover) The view from Port an Righ looking northwards towards Tarbat Ness. The Great Glen Fault separates the Jurassic sediments in the foreground from Old Red Sandstone and Moine rocks forming the cliffs in the distance. Photo. J A Chesher.

(Rear cover)

Foreword

This report describes the geology of the Moray Firth, which is here taken to extend from the innermost Moray Firth eastwards to 1ºE, which is close to the United Kingdom/Norway Median Line. The area includes both the inner and outer Moray Firth sedimentary basins as well as other marginal basins north of the latitude of the Wick Fault. The report is one of a series of ten Offshore Regional Reports produced by the British Geological Survey, and complements the 1:250000 geological and geophysical maps published for the area. These maps and reports have been compiled from data collected by the Survey and others since the late 1960s. BGS surveys and the production of both maps and reports, have been largely funded by the Department of Energy.

The Moray Firth report area has been the focus of much attention from the hydrocarbon industry, and includes several producing oilfields. The data collected during the exploration and production of hydrocarbons from the area provide a vast source of information which has been interpreted for the Department of Energy by the British Geological Survey. Much of this information is now released by the Department of Energy. and supplements the considerable volume of information published by the industry itself, notably the proceedings of conferences of the Institute of Petroleum. Information from well 12/28-2 has yet to be released, and has been used in this report with the kind permission of Tenneco United Kingdom Inc.

Responsibilities of individual authors during the production of this report have been as follows:

• I J Andrews — Structure, Carboniferous, Permian and Triassic
• D Long — Oligocene, Neogene. Quaternary. sea-bed sediments and non-hydrocarbon economic geology
• P C Richards — Basement, Devonian and hydrocarbon aspects of economic geology
• A R Thomson — Lower Cretaceous; Paleocene and Eocene with a contribution by J P Dennis
• S Brown — Jurassic
• J A Chesher — Introduction, excursion guide and compilation of the report
• M McCormac — Upper Cretaceous

In addition to the work of the authors, the report draws on the pool of expertise in BGS, not only within the Marine groups, but also from the Land Survey and specialists from Stratigraphy and Tectonics. The Offshore Regional Report Series is co-ordinated by the Marine Surveys North Research Programme and edited by D Evans.

F G Larminie Director,  British Geological Surve,  14 March 1990

Chapter 1 Introduction

The area that for the purposes of this report is termed the Moray Firth lies between latitudes 57°30′N and 59°30′N, and extends from the coastline of Scotland to 1ºE (but to 0° south of 58°N). This area is significantly more extensive than the coastal embayment normally known as the Moray Firth, which is here loosely termed the inner Moray Firth. The area covers some 60 000 km² bordering the north coast of Buchan, the east coasts of Easter Ross, Sutherland, Caithness and the Orkney Islands. Although the area does not extend as far east as the United Kingdom/Norway Median Lint. (Figure 1), it is an important part of the North Sea oil province, and includes major oilfields such as Piper, Claymore and Beatrice.

The geology and topography of the adjacent onshore areas are varied. The county of Caithness on the north-west coast of the inner Moray Firth is largely built of flagstones of Devonian age which create spectacular coastal cliffs, notably at Duncansby Head marking the eastern entrance to the Pentland Firth. Farther south in Sutherland and Easter Ross, a flatter coastal zone of Mesozoic rocks borders the Moray Firth, with Moine basement rocks forming a mountainous hinterland. Around Inverness and Dornoch, Devonian rocks form relatively low-lying country with broad firths and low coastlines. On the southern side of the Moray Firth from Inverness to the mouth of the River Spey, there is a well-developed lowland plain on Devonian sediments; farther east, rolling hills formed mainly of Moine and Dalradian metasediments extend to the coast. To the south, the ancient mountain chain of the Grampian Highlands forms high ridges, with many summits exceeding 1000 m in height.

Offshore, there is an irregular sea floor adjacent to rugged coastlines, but the sea bed of the Moray Firth is generally smooth, with water depths in the western part of the area reaching some 50 to 70 m, and deepening eastwards to around 150 m (Figure 1). This gently sloping sea floor is interrupted off the north coast of Buchan and in the northeast of the report area by deep, elongated depressions where the water depth can be in excess of 200 m.

The occurrence of isolated outcrops of Permo-Triassic and Jurassic rocks along the coastal regions of the Moray Firth led to the suggestion that similar beds could be present offshore, and that the inner Moray Firth might be underlain by a large sedimentary basin (Arkell, 1933). The first regional gravity survey of the area in 1957 led to the discovery of a large, elliptical, negative gravity anomaly (Collette, 1960). The thickness of low-density material, either sediments or granite, necessary to produce this anomaly was considered to be 10 km at a density contrast of 0.3 gm/cm^3.. Although a thin sedimentary cover was envisaged by Read and MacGregor (1948), Collette (1960) considered that the main part of the anomaly was the result of a. large Caledonian granitic body similiar to those on land. Other workers, including Hallam (1965), considered it more likely that the gravity anomaly signified a major basin of Mesozoic sediments. The later observation of long-wavelength, relatively low-amplitude, magnetic anomalies (Institute of Geological Sciences, 1972) also cast doubt on the granite hypothesis. Seismic, gravity and drilling surveys in the early 1970s by the Institute of Geological Sciences, later renamed the British Geological Survey (BGS), provided further confirmation of a Mesozoic basin (Chesher et al., 1972; Chesher and Bacon, 1975; Sunderland, 1972).

Oil company exploration in the area began in the mid-1960s following successful finds of gas in the southern North Sea. The first commercial deep seismic survey was shot across the inner Moray Firth for Total in 1965. During the late 1960s, following the first round of United Kingdom Licensing, Hamilton drilled well 12/26-1 and Total drilled wells 12/23-1, 12/21-1 and 12/22-1; these wells, when released, provided information of the deep sedimentary basin fill. In January 1973, Piper became the first oilfield to be discovered in the area; by the start of 1989, eight oilfields were in production and a further three under development.

Work to elucidate the geology of the Moray Firth was undertaken by BGS as part of a programme to produce geological maps of the UK Continental Shelf. These published maps (see inside back cover) have provided much input into this report, together with published data and extensive commercial exploration material interpreted by BGS on behalf of the Department of Energy. The commercial data used are confined to those either published, released by the Department of Energy prior to January 1989, or released by oil companies earlier than required by statute.

Geological summary

The onshore geology (Figure 2) consists chiefly of metamorphic Dalradian and Moine rocks intruded by several granitic masses and unconformably overlain by nonmarine Devonian (Old Red Sandstone) deposits of the Orcadian Basin. The Maine and Dalradian rocks underwent polyphase folding and metamorphism during the early Palaeozoic Caledonian orogeny. This process had largely ceased before the Old Red Sandstone (ORS) sediments were deposited, although these show local intense folding and thrusting. The Devonian and earlier rocks are affected by a number of major north-north-east-trending dislocations, most notably the Great Glen Fault.

The basement rocks offshore consist of metasediments with Dalradian affinities, and granitic rocks similar in age to those on land. Unconformably overlying these rocks are continental ORS deposits of the Orcadian Basin; these are interpreted mainly as the deposits of a shallow evaporitic lake, aeolian dune sands, and alluvial sediments. Carboniferous rocks mainly occur in the outer Moray Firth as a series of coal-bearing, fluviodeltaic sediments of Viséan and Namurian age.

The offshore area of the Moray Firth has been a depositional centre from Devonian times to the present, with up to 16 km of sediment preserved in the deepest parts of the basin. A linked system of discrete extensional basins developed during the Early Devonian, and acted as loci for sediment accumulation through into the Carboniferous. During Permian times, the sedimentation pattern changed radically, and the Moray Firth area became virtually a single depocentre which survived till the onset of Mesozoic rifting at the end of the Triassic Period. Permian strata occur over a large area of the Moray Firth, but their distribution is markedly different from that of the Carboniferous due to Hercynian uplift and associated erosion. This may also account for the absence from the area of Upper Carboniferous rocks. The Permian was a time of deposition of Rotliegend continental deposits, followed by Zechstein shallow-marine carbonates and sulphates in Triassic times there was a return to arid, continental conditions and the deposition of shallow playa-lake sediments.

During the Early Jurassic there was a spread of marine deposits into the area, followed by regressive, paralic facies sediments during much of the Middle Jurassic when a largely subaerial volcanic centre developed in the extreme east of the report area. Marine conditions were widely reestablished in the Late Jurassic and continued into the Cretaceous. The Late Jurassic and Early Cretaceous saw the main phase of block-faulting and graben formation both in the Moray Firth and in the northern North Sea as a whole. The Permo-Triassic Moray Firth basin fragmented into a complex of small half grabens and sub-basins, mainly along pre-existing fault trends. At the same time, a larger-scale differentiation of the area gave rise to separate western and eastern domains, here termed the Inner and Outer Moray Firth basins respectively, with fundamentally different structural characters. A period of overall subsidence and relatively minor faulting followed in Late Cretaceous and Tertiary times; this was centred on the graben in the east of the area.

Throughout Cretaceous and earliest Paleocene times, the area was largely submerged by incursions of the Boreal Ocean, and thick sequences of calcareous argillites, chalk and chalk-marl were deposited. Later, Paleocene and Eocene clastic sediments of submarine fan and deltaic origin were laid down in a slowly subsiding basin. Although the Oligocene and Neogene were times of continued subsidence and infilling of the North Sea Basin, there was uplift and regression in the western part of the area.

A complex Quaternary history is reflected offshore by thick Quaternary deposits. These record early deltaic sedimentation followed by predominantly glacial and glaciomarine conditions during the middle and late Pleistocene. There is also a thin cover of variable Holocene sediments.

Chapter 2 Basement

Moine and Dalradian rocks crop out over much of northeast Scotland and although not always found along the coastal tract of the Moray Firth, they do occur to the north of Helmsdale on the Tarbat Peninsula, on the Black Isle, and along the southern coast east of Buckie (Figure 3). A small inlier of Lewisian Gneiss crops out on the Cromarty coast at Rosemarkie (Rathbone and Harris, 1980).

The Moine metasediments north of the Great Glen Fault comprise mostly psammitic and pelitic schists that were originally deposited as sands and muds. The rocks were probably first metamorphosed during the Precambrian Grenville orogeny, but a late Precambrian 'Moravian' metamorphic event at about 750 Ma also affected the Moine rocks, at least in the south-western part of their outcrop. The Moine was further deformed and metamorphosed during the Caledonian orogeny.

The late Precambrian Dalradian metasediments were mainly deposited under shallow-marine conditions, either on a continental shelf or marginal to a deeper marine basin. They make up the Dalradian Supergroup, which is subdivided into four major groups reflecting differing tectonostratigraphic environments. The earliest, the Grampian Group, consists largely of psammite, and the overlying Appin and Argyll groups are composed of interbedded limestones, quartzites and pelitic schists. The uppermost Southern Highland Group consists of metagreywackes and volcaniclastic rocks, and is locally intruded by basic sills. Anderton et al. (1979) have described the depositional environments represented by the Dalradian as ranging from shallow-tidal to deep-water, submarine-fan facies. The Port Askaig Tillite, a Precambrian glacial deposit, is a notable stratigraphic unit marking the mid point of the Dalradian Supergroup.

The Dalradian rocks were metamorphosed and deformed during the Caledonian orogeny from 600 to 440 Ma (Rogers et al., 1989). The Dalradian of north-east Scotland is notable for the intrusion at around 500 Ma of large, layered, basic igneous bodies. This large input of basic magma was coincident with the peak of metamorphism in the Buchan region.

Large granitic bodies were intruded into Dalradian and Moine metasediments both at the start of uplift and during the main regional uplift at the waning phase of the Caledonian orogeny. 'Older^. granites intrude Dalradian rocks in north-east Scotland; these were emplaced at 470 Ma. 'Newer' granites were emplaced in Late Silurian to Early Devonian times (430 to 400 Ma), and now form much of the Grampian Highlands to the south of the Mora) Firth: they are generally of calc-alkaline type and some are compositionally zoned. Early, highly foliated, augen granites intrude both the Moine and Dalradian sequences, but are volumetrically minor and occur adjacent to the study area only near Portsoy (Figure 3) and at Carn Chuinneag (25 km west of Tain).

Basement rocks are commonly recognized on deep seismic profiles offshore beneath sedimentary sequences, but have been penetrated in only a small number of wells. Metasediments of Dalradian affinity have been recorded in wells 12/23-1, 13/19-2 and 13/29-2 (Figure 3). In well 12/23-1, a deformed and fractured granitoid rock underlies fine-grained, quartz-biotite gneiss, which in turn is succeeded by Old Red Sandstone. Whole-rock K-Ar age determinations on the gneiss suggest an uplift date of around 443±10 Ma (Frost et al., 1981).

In contrast, the Dalradian rock in well 13/19-2 on the Halibut Platform is a fractured. migmatitic, sillimanite-bearing, schistose semipelite with abundant veins of calcite, quartz and pyrite. A K-Ar dating gave an age of 430±13 Ma, which suggests that its formation may relate to the same uplift event as that ascribed to the gneiss in well 12/23-1.

The third presumed Dalradian sequence, in well 13/29-2, consists of green to greyish green, fractured, chloritic schist on which no radiometric age determinations have been carried out. This is overlain by about 8 m of rock described on the composite log as weathered granite, but which may actually be an erosional product from the nearby granite pluton (Figure 3).

Deep seismic profiles cannot be used to correlate strongly folded sequences, and there are not enough well records of metamorphic rocks in the Moray Firth area to allow speculation concerning the relationship of drilled sections to specific Dalradian groups. However, the age dates obtained from offshore metamorphic rocks are clustered between 460 and 420 Ma (Frost et al., 1981), and are consistent with those determined onshore and elsewhere in the northern North Sea. They reflect widespread uplift that punctuated, and finally marked the end of significant tectonic and metamorphic phases which affected both the Moine and Dalradian sequences.

Two wells in the inner Moray Firth terminated in granitic rocks. As well as the deformed and fractured granitoid rock recovered in 12/23-1, a coarsely crystalline granite with large crystals of quartz, feldspar and biotite has been recorded in well 12/21-3. These wells lie at the western flank of the Smith Bank High, a later positive feature which may have developed as a result of the presence of underlying granite. Other granites have been postulated on the basis of gravity modelling to lie at depth in the inner Moray Firth; the Central Ridge between the Great Glen Sub-Basin and Lossiemouth Sub-Basin may be underlain by granite (Dimitriopolis and Donato, 1981), but no wells have proved its occurrence.

In the outer Moray Firth, the South Halibut Granite has been penetrated in wells 13/24-2, 14/26-2 and 20/1-2 (Figure 3). A number of whole-rock Rb-Sr determinations suggest that the granite is 416 ± 52 Ma old. Despite the poor resolution of the daring, it is most likely that the granite compares in age with the 'Newer' granites onshore. It is a fine- to coarse-grained, equigranular granite that is typically fractured, with some fractures infilled by calcite. The main mineral constituents are quartz, oligoclase and orthoclase, with minor muscovite, epidote, chlorite, zircon and magnetite. The fresh granite is generally overlain by weathered granite or granite 'wash' up to 27 m thick; this may extend beyond the limits of the pluton, as in well 13/29-2. Pebbles of this granite occur in Old Red Sandstone conglomerates in nearby wells (Richards, 1985b), indicating that the granite was being eroded shortly after its emplacement during the final stages of the Caledonian orogeny.

The limited information from wells that penetrate basement rocks offshore in the Moray Firth suggests that the northern North Sea basement is an extension of metamorphic and granitic rocks seen onshore. This lends some support to the view of Frost et al. (1981) that the Scottish and Norwegian Caledonides may in some way link across the North Sea beneath the Mesozoic sequence.

Chapter 3 Post-Caledonian structural development

The existence of sedimentary basins in the Moray Firth area was not demonstrated until the early 1960s. Relatively little was known of the structure of the area until the 1970s when well information and good quality seismic data became available, The structural development of the Moray Firth area has been complex: the Precambrian basement with its established north-easterly trending Caledonian tectonic patterns has been overlain by sedimentary basins of different ages whose dispositions have been significantly controlled by the underlying basement structures. Since the Caledonian orogeny, the area has been a site of sedimentary accumulations during several intervals of geological time. The Moray Firth area includes over 5 km of Devonian (Old Red Sandstone), 1.5 km of Lower Carboniferous. 1.5 km of Permo-Triassic, 3.5 km of Jurassic to Early Cretaceous sediments in the inner Moray Firth with 3 km of Middle Jurassic volcanic rocks present only in the outer Moray Firth, and 4 km of sediment deposited from the Late Cretaceous to the present. Although the ORS is volumetrically important, most detail concerning the offshore structure relates to Permian and younger rocks.

During the early stages of post-Hercynian basin development, the Moray Firth formed an embayment on the western margin of the Northern Permian Basin, which occupied the northern North Sea. In simple terms, the basin later developed during phases of crustal extension in the Late Triassic to Early Cretaceous into the western arm of a three-armed graben system which included the Viking and Central grabens (Figure 4). Maximum extension and crustal thinning occurred beneath the Central and Viking grabens, whereas the Witch Ground Graben was either the restricted 'failed north-western arm' of this system or the north-western extension of the Central Graben, in the inner Moray Firth during this interval, maximum extension was approximately parallel to the Great Glen Fault, and resulted in considerable reactivation of many pre-existing Caledonian faults. This added a dextral strike-slip component to the otherwise extensional faulting style in this part of the basin, which developed without crustal thinning. In the outer Moray Firth, normal faults developed without the same strong Caledonian control, and greater extension was accomplished through attenuation of the crust. A subsequent period of thermal relaxation, which caused subsidence without major faulting during the Late Cretaceous and Tertiary, was centred on the outer Moray Firth and the North Sea grabens. Basin development was interrupted during the Tertiary by a phase of uplift in the west or the Moray Firth area.

The Moray Firth area can be divided into inner and outer basins which show differences in fault trends, structural style and basin development history (Figure 4) and (Figure 5). This structural subdivision of the Moray Firth area can be recognised on the Bouguer gravity anomaly map (Figure 6). The gravity values not only reflect variations in sedimentary thickness in the individual sub-basins, but also regional crustal structure. In general, the inner basin is characterised by a deep gravity low, and considerably higher values occur in the outer basin. Because of this difference, the area of negative anomalies can be conveniently equated with the Inner Moray Firth Basin.

Zones of steep gravity gradient, such as those in the north-west of the Inner Moray Firth Basin, correspond to major faults identifiable on seismic data. The western margin of the Inner Moray Firth Basin is delineated by the Helmsdale Fault and the northern margin by the Wick Fault. Farther east is the Fladen Ground Spur, a southerly extension of the East Shetland Platform that separates the less well-defined Outer Moray Firth Basin from the South Viking Graben, in the south, the Banff Fault separates the basins from the basement rocks of Buchan, and the Peterhead Ridge isolates them from the Aberdeen Platform (Figure 5).

The basinal areas have experienced largely continuous sedimentation since at least Permian times, accumulating up to 7 km of sediment, whereas some intrabasinal highs have remained areas of little or no deposition. The thickest sequence of Permian and Mesozoic sediment in the study area was probably deposited in parts of the Inner Moray Firth Basin, but a considerable thickness of Upper Cretaceous and perhaps Tertiary sediment was removed from this region during subsequent uplift. Tilted fault-blocks, horsts and half grabens further divide these basins into many separate centres of deposition (Figure 5). In addition, four 'marginal basins', the East and West Fair Isle basins, the East Orkney Basin and the Dutch Bank Basin, are located within and around the East Shetland Platform north of about 58º40′N.

Thickness of the crust

The Mohorovičić Discontinuity, or Moho, is the seismic discontinuity at the base of the crust separating it from the upper mantle. It dips steadily from about 27 km depth at the north coast of Scotland to about 32 km under the Midland Valley (Bamford et al., 1978; Bamford, 1979). The presence of thinned crust beneath the grabens of the North Sea was first proposed by Collette (1960) from the results of a gravity survey. Variations in the regional gravity field after removing the effects of Mesozoic sedimentary basin fill reveal differences in the underlying deep crustal structure. It has been deduced that the crust is thinned by some 5 km beneath the Witch Ground and Buchan grabens (Figure 7) and by over 10 km in the Central Graben (Christie and Sclater, 1980; Donato and Tully, 1981; Barr, 1985; Zervos, 1987). The depth to the Moho beneath the Moray Firth area, as shown by refraction seismic studies and gravity modelling, can be shown to decrease from west to east. In the Inner Moray Firth Basin there is no significant crustal thinning; the crust is estimated to be 26 to 27 km thick at the western limit of Upper Cretaceous rocks. The Moho is also at the more usual depth of some 30 km beneath the East Shetland Platform and under the Aberdeen Platform (Figure 8). The shallowest Moho in the area lies beneath the Witch Ground Graben at a depth of about 23 km.

Structure of the Inner Moray Firth Basin

The Inner Moray Firth Basin is subdivided into a series of north-easterly trending half-grabens and fault-block highs, with maximum sedimentary thickness in the half-grabens associated with the Great Glen. Helmsdale and Smith Bank faults (Figure 5). Deviations from the dominant north-ease Caledonian trend are minor, but the Great Glen fault crosses the basin on a north-north-east alignment, and both the northern and southern bounding faults run east-north-east (Figure 5).

Two types of fault are recognised in the Inner Moray Firth Basin (Figure 9): (1) major, long-lived faults with synsedimentary movement, some of which have throws exceeding 3 km. These are often associated with influxes of sand into the basin. (2) Faults with several episodes of block-faulting on a scale of 10 to 100 m. These faults, especially those with smaller throws, are essentially planar but range from vertical to low angled.

Many of the larger faults exhibit a listric geometry on seismic profiles (Barr. 1985: Barr et al., 1985), and features characteristic of strike-slip movement are also recognisable. These include flower geometries (Figure 10), en-échelon faults and folds, scissor faults, pull-apart systems, inversion structures, lateral offsets in stratigraphy, and ordered combinations of extensional and compressional faults that include major thrusts and listric faults (Bird et al., 1987).

The Great Glen Fault crosses the Inner Moray Firth Basin from Tarbat Ness to Wick in a north-north-easterly direction. Evidence for the amount and direction of major late Caledonian and Hercynian strike-slip movement along the fault is apparently conflicting. Nevertheless, the consensus is that overall relative dextral movement of 25 to 30 km has occurred since the Middle Devonian, following a pre-Devonian history of major, but little-understood, transcurrent movement (Kennedy, 1946; Holgate, 1969: Garson and Plant. 1972: Donovan et al., 1976; Speight and Mitchell, 1979). During the Mesozoic there was little normal movement along the Great Glen Fault, for this sense of movement was concentrated on the Helmsdale Fault whose throw exceeds 3 km near its intersection with the Great Glen Fault off Wick. However, structures observed in the near-surface Lower Cretaceous sediments along the line of the Great Glen Fault (Figure 10) indicate that they were influenced by strike-slip movement along a controlling fault line at depth (McQuillin et al., 1982).

The Great Glen Sub-Basin (Figure 5) and (Figure 9) contains a thick succession with a regional dip to the north, and its most westerly sediments crop out at Brora and Helmsdale. North-west of the Beatrice Oilfield there is a listric fault that soles out in a detachment within Upper Jurassic mudstones: this fault separates the underlying Permian to Middle Jurassic fault pattern parallel to the Great Glen Fault from an entirely independent set of Late Jurassic faults oriented almost at right angles to the earlier pattern (Bird et al., 1987).

En-echelon segments of the Smith Bank Fault system extend some 100 km north-eastwards from the Beatrice Oilfield and have throws of up to 2.5 km (Figure 9). The fault may be developed along a Caledonian structure and has experienced several periods of normal movement, as well as probable oblique-slip movement. Although this fault was inactive during the Permo-Triassic, there is evidence for Early Jurassic movement, and major synsedimentary faulting occurred in the late Oxfordian to Ryazanian, (Linsley et al., 1980; Andrews and Brown. 1987).

The Smith Bank Graben originated as a half-graben in the Jurassic, but the basin geometry was modified by subsequent Cretaceous movement on the West Bank Fault. A maximum of 5 to 6 km of Permian to Early Cretaceous sediment is preserved in the Smith Bank Graben. South-eastwards from the West Bank High, Mesozoic sediments thicken slowly towards the Banff Fault within the Banff Sub-Basin (Figure 9); this latter half-graben imparts a degree of symmetry to the Inner Moray Firth Basin as a whole.

The arcuate Lossiemouth Fault throws down to the south-east and merges with the north-westerly downthrowing West Bank Fault through a complicated zone of horsts and grabens where strike-slip displacement is thought to have occurred. The Lossiemouth Fault plane has a well-defined listric geometry, with an upper, steep section passing downwards into a planar feature which dips southeastwards at 25° to 30º within the basement (Figure 9). This basement structure is interpreted as a Caledonian thrust that remained a line of weakness during Mesozoic extension. At this time it became the focus for normal movement and allowed the formation of the Lossiemouth Sub-Basin through relaxation (Barr, 1985). The Lossiemouth Sub-Basin contains up to 4 km of Permian and younger sediments, mainly Late Jurassic and Early Cretaceous in age.

The area between the Wick Fault and the Smith Bank High, termed the Wick Sub-Basin (Figure 5) and (Figure 9), contains considerable evidence of dextral, oblique-slip fault movement during the Jurassic and Early Cretaceous. The Wick Sub-Basin consists of two east-west-trending parts; a southern extensional half-graben where dips exceed 45º. and a northern, isolated, fault-bounded basin. Both are interpreted as being controlled by strike-slip fault movement (Roberts et al., 1990). To the north, the Wick Fault has a vertical throw of up to 4 km separating the Old Red Sandarac of the Caithness Ridge from the Inner Moray Firth Basin. However, where the fault steps northwards at around 2° 25′W, there is a series of contractional anticlines and associated flower structures (Roberts et al., 1990). The Little Halibut Fault at the eastern limit of the Inner Moray Firth Basin was not active in the Permo-Triassic, for thicknesses of contemporaneous sediment are comparable on either side of the fault. It did however experience south-westerly downthrow of up to 1.6 km during the Early Cretaceous.

The major highs within the Inner Moray Firth Basin are the Smith Bank and West Bank highs. which lie on opposing sides of the Smith Bank Graben. The base of the Permian is as shallow as 2100 m at the crest of the Smith Bank High, and some 1800 m at the West Bank High. The east-north-easterly-trending Central Ridge is a less significant feature which developed as a tilted fault block adjacent to the Lossiemouth Fault. The Central Ridge probably owes its existence to the buoyancy of an underlying Caledonian granite (Dimitropoulos and Donato. 1981).

The inner Moray Firth area has experienced several periods of uplift, the most recent of which. in the Tertiary, had a profound effect on the sea-bed outcrop pattern. An estimated maximum uplift of 500 to 1000 in can be inferred by restoring the dip of a Lower Cretaceous channel which is at present 'flowing uphill', and by comparing seismic velocities (that are a function of lithology, compaction and depth of burial) in the Inner and Outer Moray Firth basins (Bird et al., 1987; Roberts et al., 1990).

Structure of the Outer Moray Firth Basin

As with the inner basin, the Outer Moray Firth Basin is composed of several elements, of which the most significant are the Halibut Horst, the Witch Ground Graben and the Buchan Graben (Figure 5). North-westerly trending faults in the Witch Ground Graben and north of the Halibut Horst are likely to be Hercynian structures which extend from the Central Graben (Figure 4), whereas faults running approximately east-west in the vicinity of the Halibut Horst and Peterhead Sub-Basins are the result of complex interaction between Caledonian and Hercynian structures. Due to their very deep burial, listric fault planes in the outer Moray Firth are generally not imaged on seismic sections, but such geometries can be inferred by the pretence of roll-over anticlines, as in the case of the south-western bounding fault of the Witch Ground Graben (Beach, 1984).

The Witch Ground Graben trends in an east-southeasterly direction and is bordered by structural highs both to the south-west and north-east (Figure 5) and (Figure 9). Although often described as a simple half-graben, its geometry varies along its length (Boote and Gustav, 1987), and it is this complexity and the presence of many tilt-block highs which provide traps for hydrocarbons. At the Claymore Oilfield, minor half-grabens are controlled by faults bordering the Halibut Platform. Halibut Horst and the Claymore structure, but farther south-east the Witch Ground Graben is more symmetrical, with a major fault on the north-eastern margin. Tilted blocks dip in opposite directions on either side of the graben, and the Claymore and Piper oilfields represent mirror-image tilt-blocks on either flank. Further horst-blocks occur within the axis of the graben, including those at the Tartan and Galley oilfields (Figure 5).

The Witch Ground Graben developed intermittently from the Triassic until the end of the Barremian (Early Cretaceous), but was a major centre of deposition in the Late Jurassic, and more especially during the Early Cretaceous (Beach, 1984). The addition of a thick Tertiary succession gives a maximum post-Permian sediment thickness of 6 to 7km in the southern part of the graben.

The Witch Ground Graben is bordered to the west and south-west by the uplifted blocks of the Halibut Platform, the Halibut Horst and the Renee Ridge (Figure 5). The area termed the Halibut Platform includes several half-grabens containing Late Jurassic and Early Cretaceous sediments, but remained a relatively high area between the Inner Moray Firth Basin and the Witch Ground Graben during this interval. It was subsequently downthrown relative to the Halibut Horst. The Halibut Horst is a major east- west-trending basement ridge whose bounding faults were active from at least the Jurassic, or possibly from the Permian, through to the early Tertiary. It is locally onlapped by Upper Cretaceous chalk, but at its centre, where basement lies at only 400 m depth, granite and Devonian sediments are overlain directly by Paleocene deposits, South-west of the Witch Ground Graben, faults with interfering easterly, south-easterly and east-northeasterly trends form a complex tilted fault block termed the Renee Ridge. This ridge extends westwards to merge with the Halibut Horst, and is partly composed of a Middle Jurassic volcanic pile overlain by chalk.

To the north-east of the Witch Ground Graben, the Piper Shelf is a transitional area between the graben and the Fladen Ground Spur. The latter is a south-easterly plunging zone of mainly Old Red Sandstone strata with locally preserved Carboniferous rocks; these are overlain by thin Cretaceous and Tertiary sediment. The Fladen Ground Spur was formed in the Late Jurassic, and its western margin coincides with the 'subcrop' of the Zechstein reflector beneath the base Cretaceous unconformity. A Jurassic initiation for the structure is implied, for within the Permo-Triassic succession there are no facies changes in the direction of the Fladen Ground Spur, but the area was a source of Upper Jurassic conglomerates found in the south Viking Graben (Harms et al., 1981).

The presence of the Caledonian South Halibut Granite to the south of the Halibut Horst (Figure 5) is reflected by the presence of a broad north-easterly trending high during several phases of the Outer Moray Firth Basin's history. The effect of the granite was evident as early as the Devonian, but most significant was its uplift in the Middle Jurassic when it separated the Inner Moray Firth Basin from the Buchan Graben, and its cover was stripped off. Subsequently its influence waned, and during later Jurassic and Cretaceous time, thick sediments were deposited fairly continuously across it to form the eastern part of the Banff Sub-Basin. The arcuate Bosies Bank Fault moved mainly in Late Jurassic time; it has a maximum throw of 1.5 km at top Permian level and cuts the western margin of the granite.

The area of the Peterhead Sub-Basins (Figure 5) is dominated by east-north-easterly trending faults which control south-easterly thickening half-grabens of Late Jurassic to Early Cretaceous age between the Banff and Peterhead faults (Figure 9). Farther east, the Peterhead Fault becomes a less prominent feature and there are step faults into the Buchan Graben, which is itself extensively broken by faulting. South of the Peterhead Ridge, the Aberdeen Platform is an area where generally there has been little fault movement since Early Permian time, except in two grabens trending east-north-east and north-north-east respectively where pre-Permian to Late Jurassic faulting is evident. Eocene faulting seen on section 4S in (Figure 9) was due to collapse of the underlying Permian salt following dissolution, and is not the result of diastrophic forces. This type of faulting is extensively seen farther south-east on the western side of the Central Graben.

Structure of the marginal basins

The East Shetland Platform, whose southerly limit is seen in the extreme north of the study area (Figure 4), forms a broad zone of shallow basement and Old Red Sandstone outcrop with a thin Tertiary cover in the east. Between the Inner and Outer Moray Firth basins and the East Shetland Platform, there are four marginal basins which developed mainly during the Permo-Triassic. These are termed the East Orkney (or Lindsay) Basin, the Dutch Bank Basin, and the East and West Fair Isle basins.

The East Orkney Basin lies to the north of the Caithness Ridge and has a northerly dipping, half-graben geometry controlled by large listric faults on its northern margin (not evident on (Figure 9) due to vertical exaggeration) which trend both to the north-east and north-west. The thickest basin fill is estimated as some 4 km adjacent to the major faults in the north, where the Cretaceous succession combines with the thickest Permo-Triassic sequence.

The Dutch Bank Basin forms a large embayment to the north of the Witch Ground Graben, and contains up to 2 or 3 km of Permian and younger sediments (Figure 5). The western arm has a half-graben geometry, with strata rising to the south-west onto a basement high, the West Fladen High, on which Jurassic rocks are only patchily preserved. Some Triassic faulting is suggested by the increased thickness of Triassic redbeds in a local graben structure (Figure 9), but the West Fladen High was formed in Late Jurassic to Early Cretaceous times, when much fault movement took place. Farther east, the basin is dominated by a Permo-Triassic fill beneath the unconformable Cretaceous.

The elongate, north-north-easterly trending West Fair Isle Basin extends as far south as the Wick Fault (Figure 5). In the north it is a simple half-graben controlled on the eastern margin by a major normal fault that is the continuation from Shetland (Figure 4) of either the Walls Boundary Fault or the Nesting Fault (BGS Fair Isle Solid Geology sheet). Between the Orkney Isles and the Caithness Ridge the structure is complex: the basin exhibits rapid variations in thickness as might be expected in the area where throw is transferred from the westerly downthrowing bounding fault on the eastern margin to the easterly downthrowing Great Glen Fault on the western side in the south. A basin fill of Permo-Triassic redbeds (BGS Caithness Solid Geology sheet) was unaffected by faulting during sedimentation, which confirms the age of the faulting as post-Triassic, and suggests that the basin forms a remnant of a formerly more extensive area of deposition. To the east, in the very north of the study area, is the southern part of a poorly understood basin of Permo-Triassic redbeds termed the East Fair Isle Basin (BGS Fair Isle Solid Geology sheet).

Structural development of the Moray Firth

Studies in the Moray Firth have highlighted differences between the inner and outer basins, particularly in terms of deep structure and structural style. The major distinctions are that the sedimentary fill of the Inner Moray Firth Basin overlies crust of normal thickness and shows widespread evidence of strike-slip fault movement (McQuillin et al., 1982; Barr, 1985; Bird et al., 1987), whereas in the outer basin there is thinned crust, a rifted zone, and a thick post-rift sedimentary sequence (Donato and Tully, 1981; Beach, 1984). These differences necessitate separate mechanisms of basin development, although being in such close proximity the formation of both basins must have been intimately related.

The post-Caledonian formation of the Moray Firth sedimentary basins occurred in four phases:

1. A period of extension and intermittent subsidence during the Devonian and Carboniferous, controlled by alignments inherited from the Caledonian orogeny.
2. A phase of subsidence during the Permo-Triassic. This was not associated with North Sea extension and rifting, and therefore does not fit the present development models (see mechanism sections below).
3. A period of north-east to south-west extension occurred during the Jurassic to Early Cretaceous. In the Inner Moray Firth Basin and the marginal basins, faulting was related to conspicuous strike-slip movement along existing Caledonian dislocations, whereas in the Outer Moray Firth Basin, larger-scale extension was accommodated by normal faulting, and isostatically compensated by thinning of the crust.
4. During the final phase, in Late Cretaceous and Tertiary times, thermal relaxation subsidence in the outer Moray Firth contrasted with episodes of uplift in the Inner Moray Firth Basin and the marginal basins.

Mechanism for the Inner Moray Firth Basin

The formation of the Inner Moray Firth Basin (Figure 11) is best considered in terms of the oblique-slip movement of a network of rigid blocks moving in reponse to strike-slip fault movement on the Great Glen Fault (McQuillin et al., 1982). The mechanism used to explain the creation of this movement is a dextral shift of some 8 km parallel to the Great Glen Fault which resulted in a 7 to 8 per cent post-Triassic extension of the crust between the Wick and Banff faults (McQuillin et al., 1982; Barr, 1985). This mechanism relies on the Great Glen and Wick faults being effectively pinned at their intersection, such that little or no relative movement occurred farther north. The total displacement was distributed across the basin through a series of smaller oblique displacements on intrabasinal faults, as evidenced by the large number of strike-slip fault structures noted within the basin (Barr, 1985; Bird et al., 1987; Roberts et al., 1990).

The apparent extension without crustal thinning may be due to extension taking place along listric normal faults which continue down to a subhorizontal detachment surface estimated to have been at a depth of 20 to 25 km (Barr, 1985), close to the base of the crust. Such a mechanism would leave the crust below the detachment little affected, with the bulk of the extension having been transferred into neighbouring areas of major lithospheric stretching, such as the North Sea grabens.

The lack of crustal thinning in the Inner Moray Firth Basin is a feature that is also used to explain the later phase of Tertiary uplift (Beach, 1985). During the extension phase, the average density of the crust was reduced by the addition of low-density sediments. However, because the crust remained thick, inversion occurred as the crust rose in an attempt to regain isostatic equilibrium. Another suggestion is that phases of sinistral strike-slip movement along the Great Glen Fault led to compression and uplift, or inversion, in the Inner Moray Firth Basin (Threlfall, 1981; Bird et al, 1987).

Mechanism for the Outer Moray Firth Basin

Several observations suggest that the development of the Outer Moray Firth Basin is more analogous to that of the Viking and Central grabens than to that of the Inner Moray Firth Basin. The important characteristics of the basin are: 1) the presence of a volcanic centre and uplifted Central North Sea Arch during the Middle Jurassic at the junction between the Witch Ground Graben and the Viking and Central grabens. 2) The occurrence of thinned crust beneath the rifted basin depocentre (Figure 7) and (Figure 8) with a thick, unfaulted, Tertiary basin fill. The development of the basin is best explained in terms of the McKenzie (1978) model of basin formation. This theoretical model far the origin of sedimentary basins invokes suet-thing and thinning of the crust by large-scale extension. This causes an initial rifting episode of normal faulting and rapid subsidence in the upper crust. Wernicke (1981) suggested that this usually occurs along pre-existing faults that have a listric geometry and sole out at depth into decoupling zones. Extension creates a thermal anomaly by the upwelling of dense, hot, mantle material to replace the thinned crust: when extensional stresses stop, this anomaly decays and the lithosphere subsides over a wider area than that affected by the rifting. This thermal relaxation subsidence occurs without faulting to allow the formation of a saucer-shaped sedimentary basin and a classic 'steer's head' profile (White and McKenzie, 1988).

In the case of the Outer Moray Firth Basin, the rifting phase was in Late Jurassic and Early Cretaceous time, and involved fault-block rotation, footwall uplift and half-graben formation. The amount of post-Triassic extension in the Witch Ground Graben has been estimated to be probably of the order of 30 to 50 per cent, but may be as high as 100 per cent. (Christie and Sclater, 1980; Smythe et al,. 1980; Wood. 1981; Barton and Wood, 1984; Beach, 1984). The thermal-sag phase took place in the Late Cretaceous and Tertiary when a thick cover of sediment was deposited in and beyond the immediate graben area.

Summary of the Post-Caledonian history of the area

The basin development in the Moray Firth area can be considered in five major phases which occurred during the Devonian, Carboniferous, Permo-Triassic, Jurassic to Early Cretaceous, and Late Cretaceous to Tertiary intervals.

Devonian

Following the Caledonian orogeny, large thicknesses of Old Red Sandstone were deposited in the postorogenic Orcadian Basin. The area of deposition of these fluvial and lacustrine deposits possibly extended as far as Norway. The formation of this and other basins may have resulted from extension and relaxation of thrusts formed during the earlier mountain building episode (Norton et al., 1987).

Carboniferous

The occurrence of coal-bearing Lower Carboniferous strata in the outer Moray Firth suggests that a tectonically controlled structure analagous to the Midland Valley of Scotland may have developed in the south-east of the study area (Johnson and Dingwall, 1981). However, the present distribution of rocks of this age is only a preserved remnant, and gives no indication as to the original fault control on deposition.

Permo-Triassic

Late Carboniferous to Early Permian uplift, erosion, and dyke injection (Russell and Smythe, 1983) is seen to represent the northernmost effects of the Hercynian orogeny, or alternatively the marginal effects of a supposed 'Pinto-Atlantic' rifting episode to the north and west of Scotland (Smythe, 1989). It was followed by the deposition of Lower Permian elastic sediments, Upper Permian carbonates and evaporites. Subsequently, Triassic mudstones and sandstones were laid down in a single basin covering the whole of the Moray Firth area.

There is uncertainty as to whether there was large-scale synsedimentary fault activity in the area during the Permo-Triassic, although such movement took place to the west of Orkney (Kirton and Hitchen. 1987). Current views on the topic are contradictory; Frosrick et al. (1988) have proposed that the Inner Moray Firth Basin developed during the Triassic in an analogous fashion to the East African Rift system, and Roberts et al. (1989) have argued that while a rift did form, it was of Permian age. Neither view is supported here because seismic and well evidence indicate only local fault control on sedimentation during these times in the Moray Firth. Wedges of sediment cited as evidence of rifting by the above authors are considered here to be variously Devonian or Late Jurassic in age.

Nevertheless, the north-eastern extension of the Highland Boundary Fault (Figure 4) did play a major role in controlling the distribution of platform sedimentation in the Moray Firth, separating it from the basinal area of sedimentation to the south-east (Threlfall, 1981). In the vicinity of the Halibut Horst there is a clastic Zechstein succession, which suggests that there was a contemporaneous high nearby. There is also a dolomite breccia, interpreted as the deposit of a debris flow which may have been triggered by faulting. In the Witch Ground Graben, a period of uplift and erosion occurred prior to the probable initiation of the graben and the deposition of Upper Triassic elastics in the western part of the graben (Harker et al., 1987).

Jurassic to Early Cretaceous

During the Middle Jurassic, much of the Moray Firth area, particularly the east, was influenced by uplift of what Ziegler (1982) has termed the Central North Sea Arch. Deposition occurred along the northern and western fringes of this uplifted area, including the Inner Moray Firth Basin, which probably is the only part of the Moray Firth area that underwent continuous deposition from Early Jurassic to Late Cretaceous times. During the Middle Jurassic, the axis of the Central North Sea Arch foundered in the course of volcanic activity, and thicknesses of Middle Jurassic strata on the arch were controlled by contemporaneous faulting (Woodhall and Knox, 1979). As the Central North Sea Arch subsided in Callovian time, it was transgressed by a diachronous, paralic, shallow-marine sequence.

During Early to Middle Jurassic time, active faulting in the Inner Moray Firth Basin was concentrated on the few long-lived faults. However the major control on present thickness of this stratigraphic interval has been pre-Callovian erosion resulting from late Middle Jurassic uplift in the eastern part of the area (Andrews and Brown, 1987).

A more extensive phase of block-faulting occurred during the late Oxfordian, perhaps lasting from mid-Oxfordian to early Kimmeridgian times (Linsley et al., 1980). The earlier timing is demonstrated in the Smith Bank Graben where upper Oxfordian shaly mudstone drapes numerous small, tilted fault-blocks. Faulting also influenced the deposition of the late Oxfordian to Kimmeridgian Piper Formation and produced changes in both the alignment of sand bars and the thickness of units across faults (Maher, 1980).

The final collapse of the Central North Sea Arch to form a deep graben coincided with a major phase of crustal extension, and with widespread synsedimentary faulting and block rotation. The evidence for the onset of this phase of faulting comes from the lower Kimmeridgian onshore, where the Kintradwell Boulder Beds were deposited immediately adjacent to the Helmsdale Fault (Lam and Porter, 1977). The timing of the acme of fault movement, as recognised by the dating of maximum sand input in wells on the downthrown side of faults, varied in detail across the basin, although it generally occurred during the Volgian (Andrews and Brown, 1987; Harker et al., 1987). In addition to the sedimentological evidence for movement, the degree of extension can be expressed in terms of block rotation; at the Claymore Oilfield, dips of 25° and 10° have been measured at the base and top respectively of the Jurassic sandstones (Maher and Harker, 1987).

During the Early Cretaceous, shaly mudstone onlapped much of the fault-block topography formed during the Jurassic, so that a pronounced unconformity was developed on long-established highs. This is often termed the 'Cimmerian' unconformity after Ziegler (1975). On the flanks of the Witch Ground Graben, the base of the Lower Cretaceous latest Ryazanian to Barremian sequence (Harker et al., 1987) rests unconformably on rocks ranging in age from Palaeozoic to Jurassic. Elsewhere, on starved highs in the outer Moray Firth, complete but condensed sequences occur in the Lower Cretaceous. In basinal areas, sedimentation was continuous from Late Jurassic to Early Cretaceous time.

Renewed tectonism, fault-block rotation and crestal erosion occurred locally in the Witch Ground Graben during the Early Cretaceous, leading to further elastics being shed from the Halibut Horst and adjacent blocks. The sandstones are of late Valanginian to Barremian age, but faulting subsequently waned and only limited sand pulses occurred during the Aptian (Beach, 1984; Boote and Gustav, 1987). Biostratigraphic data reveal episodic onlap north-eastwards on to the Fladen Ground Spur during the Early Cretaceous, but the relative local importance of global eustatic changes of sea level and tectonic control at that time is not known (Beach, 1984).

Early Cretaceous faulting in the Inner Moray Firth Basin occurred not only on the long-lived Great Glen and Smith Bank faults, but also along a set of minor, listric, east–west-trending faults which often sole out in Upper Jurassic shaly mudstone. During this interval, the West Bank Fault created, and developed, the horst structure of the West Bank High from a former asymmetrically tilted block. The most significant Early Cretaceous tectonic expression was renewed fault activity on the Little Halibut Fault, where extensive faulting is indicated by the presence of 1.6 km of Valanginian to Albian sandstones.

Late Cretaceous to present

In the Outer Moray Firth Basin, the Late Cretaceous and Tertiary were times of little fault activity during which a saucer-shaped basin formed. This basin was part of a larger zone of post-rifting thermal subsidence centred on the Central and Viking grabens. The Late Cretaceous was predominantly an interval of transgression and pelagic chalk deposition, although regressions in the Turonian and Senonian caused erosion on structural highs in and around the Witch Ground Graben and at the Buchan Oilfield (Burnhill and Ramsay, 1981).

Fault movement related to salt collapse occurred during the Eocene in the south-east of the area, and a final phase of uplift in the Inner Moray Firth Basin probably took place during the late Tertiary. In the outer Moray Firth, subsidence continued through the Tertiary and Quaternary, whilst to the west the Highlands of Scotland were uplifted to form a plateau surface. This tilting affected the Inner Moray Firth Basin where about 1 km of Upper Cretaceous and Tertiary sediments were removed.

Chapter 4 Devonian (Old Red Sandstone)

The Old Red Sandstone (ORS) of Scotland is a Devonian nonmarinc succession deposited in two major depositional basins, he Midland Valley Graben between the Highland Boundary Fault and the Southern Upland Fault (Figure 4), and the Orcadian Basin which broadly lies beneath and around the Moray Firth. Deposits of the Orcadian Basin are exposed onshore along the south coast of the Moray Firth, in Sutherland, Caithness and the Orkney Islands (Figure 12) as well as farther north in Shetland. The ORS is also very widely distributed offshore where it has been sampled at sea bed to the north of 59°N (BGS Orkney and Fair Isle Solid Geology sheets) and has been drilled in BGS boreholes BH71/22, BH72/17 and BH81/28 which recovered undated sandstones. It has been encountered at depth in over 25 released commercial wells offshore.

ORS deposition in northern Britain occurred when the area was part of the southern hemisphere desert belt. The terrain e n which the Orcadian Basin occurs moved from approximately 20°S to about 15°S during the Devonian ('fading. 1985). Sedimentation was dominated by shallow, partly evaporitic lakes, aeolian dunes and alluvial systems. The ORS in Scotland is regarded as being of continental origin, although some form of connection. possibly a river, may have linked the lake of the Orcadian Basin with a Middle Devonian sea in the Central Graben south-east of the Moray Firth area (Mykura. 1983).

Reconstructions of the geometry and extent of the Orcadian Basin broadly suggest that its western margin lay slightly west of the present north-western coast of the Moray Firth, and that its eastern margin lay in western Norway (Ziegler. 1982). The position of the southern margin of the basin, which appears to cut across the structural grain, may be defined by a deep-seated fracture (Watson, 1985). Over 5000 m of ORS sediments were deposited in the Orcadian Basin as a result of the collapse of the overthickened crust of the Caledonian mountain chain. Localised half-grabens may have formed within the basin area, controlled by extensional reactivation along easterly dipping Caledonian thrusts (Astin, 1985; McClay et al., 1986; Norton et al., 1987). Around the margins of the Orcadian Basin, the ORS rests unconformably on Precambrian basement.

A three-fold subdivision of the ORS into Lower, Middle and Upper was established in the last century by Murchison (1859), and still forms the basis of ORS stratigraphy. A summary of some of the stratigraphic terms used onshore can be found in (Figure 13). Although a division into Lower, Middle or Upper ORS can be made in some wells offshore, a large number of sequences remain undifferentiated because the rocks are of a predominantly arenaceous nature and lack fossil material.

Lower Old Red Sandstone

Sequences onshore around the margins of the Orcadian Basin comprise associations of lenticular conglomerate, finc-grained sandstone, and locally high proportions of siltstone and mudstone. These sediments were probably deposited in isolated, mainly playa-lake basins fringed by active fault scarps. On the southern side of the Moray Firth, Lower ORS deposits are absent other than a fluvial sequence associated with playa-lakes that is preserved in the Turriff Basin at New Aberdour (Mykura, 1983).

Lower ORS rocks have been recognised in four boreholes offshore: 12/27-1, 12/27-2, 12/28-2 and 13/19-1 (Figure 12) and (Figure 14). All these wells show similar Lower ORS successions dominated by a siltstone or mudstone lithology. The sequence is locally in excess of 976 m thick and comprises predominantly grey to reddish brown siltstone and mudstone with minor, greyish brown, very fine-grained. silty, calcareous sandstone. The sediments are in most places micaceous, particularly on flat parting surfaces. Lamination in the siltstone is often picked out by a colour banding reflecting a slight grain-size sorting. Many of the laminae are dolomitic and some thin siltstone beds are possibly wave rippled.

The four borehole sections have yielded miospores which confirm an Early Devonian age. The miospore assemblages include specimens of Emphanisporites,annulatus McGregor. E. rotatus McGregor. E. robusius McGregor, Calamospora sp., Retusotriletes sp., R. simplex Naumova, and Apiculiretuspora sp.

Deposition in fairly low-energy, nonmarine environments is suggested for the sediments by their fine-grained nature, their relative abundance of terrestrially derived miospores, and their lack of marine microorganisms. Comparison with onshore sections suggests that deposition occurred in intermontane lacustrine basins, and the presence of wave ripples in some siltstone beds implies that the lakes may not have been very deep. These lakes were probably formed in fairly small, isolated basins (Richards, 1985a; Watson, 1985), but their distribution is difficult to ascertain because of the generally poor quality of seismic data at the relevant depth. It is possible, however, that they formed at the same sites as later Mesozoic basins (Norton et al., 1987).

Some Lower ORS siltstone sequences, such as those in 12/28-2, 12/27-2 and 13/19-1, are overlain by unfossiliferous, sandstone-dominated units that may be Early or Middle Devonian in age (Figure 14). Comparisons can be drawn with either the Lower ORS sections at Mealfuarvonie, west of Loch Ness (Mykura and Owens. 1983) or with the Middle ORS in Ross and Cromarty.

Middle Old Red Sandstone

Middle ORS rocks crop out onshore in Orkney, Caithness, Easter Ross and along parts of the southern shore of the Moray Firth, as well as on Shetland and Fair Isle farther north. They occur mainly as lacustrine, aeolian and fluvial facies (Mykura. 1983: Astin, 1985). The lacustrine facies is probably the better known, and hasgiven rise to the popular term 'Orcadian Lake' to describe the Middle Devonian Orcadian Basin. These lacustrine sediments are known as the Caithness Flags on the Scottish Mainland. and as the Stromness. Rousay and Eday flags on Orkney (Figure 13). They occur in sequences up to 4 km thick and include. fish-bearing beds, of which the most notable is the Achanarras Limestone. Complete units of the Caithness Flags onshore generally comprise four lithological types in a rhythmic association (Donovan. 1980):

1. At the base of the cycle are laminated, carbonate-rich siltstones with fish remains and nonglacial varves that record deposition in eutrophic, stratified lakes with seasonal algal blooms.
2. Above are alternating dark grey, carbonaceous shaly mudstones and coarse-grained, cross-laminated siltstones which were laid down in shallow, occasionally dessicated, lakes.
3. Alternations of dark grey, carbonaceous siltstones and shaly mudstones with synaeresis cracks that are typical of shallow-lake sedimentation.
4. At the top of the cycle are grey or olive-green shaly mudstones with siltstones and symmetrically ripple-bedded, fine-grained sandstones that show subaerial shrinkage cracks. These are interpreted as shallow, ephemeral-lake deposits.

These rhythmic cycles are interpreted as the result of climatically controlled long-term rises and falls of lake levels in an enclosed basin (Donovan, 1980). The sediments in each cycle can be up to 60 m thick, but are more usually in the 5 to 10 m range (Mykura, 1983).

Although a relatively large number of interbedded sandstone and siltstone sequences of ORS affinity occur offshore in the Moray Firth, unequivocal palynological identification of Middle Devonian strata has been made in only a small number of wells. Seismic reflection data are of little help in trying to determine the regional distribution of Middle ORS strata offshore, but can be used to define individual sub-basins (Norton et al., 1987).

Well 13/22-1 contains a 267 m-thick Middle ORS sequence similar to those onshore; it comprises interbedded, varicoloured siltstone, silty mudstone, and sandstone which are mostly fine grained but locally medium to coarse grained or gravelly (Figure 15). Spore assemblages recovered from the upper part of the section include Rhabdosporites parvulus Richardson, Auroraspora minuta Richardson, and Dibolisporites echinaceus (Eisenack) Richardson, and are similar to those found onshore in the Eday Group flagstones. The middle part of the section includes Retusotriletes distinctus Richardson and Ancyrospora ancyrea (Eisenack) Richardson and resembles assemblages in the Lower Caithness Flags and the Achanarras Limestone onshore (Richardson. 1964).

Middle ORS rocks have also been recorded in well 13/24-1. The sequence here is very different to that in 13/22-1 asonly thin mudstone beds up to 2 m thick are recorded in an otherwise predominantly sandy and conglomeratic sequence containing granite clasts. The only other identifications of Middle ORS deposits made in the offshore area of the Moray Firth have been in wells 14/19-10 and 14/19-11 on the Halibut Platform south of the Claymore Oilfield. Here, the sequence is composed of interbedded sandstone, siltstone and mudstone, and is in excess of 870 m thick.

Although Middle ORS identifications are few, a tentative palaeogeographic reconstruction can be made based on Mykura's (1983) concept updated with offshore data. (Figure 16) shows the possible extent of the Orcadian Lake during early Givetian time when the Upper Caithness Flags, Rousay Flags or Upper Stromness Flags were being deposited. The position of the north-western Moray Firth coastline in (Figure 16) is displaced along the Great Glen Fault in order to allow for a post-Middle Devonian dextral 20 shift of 30 km to its present position. The 13/22-1 well section records the presence of a permanent Middle ORS lake, whereas the 13/24-1 sequence probably represents an alluvial plain, located south of the main lake, that was transgressed infrequently at times of highest lake levels. The presence of mudstones, siltstones and sandstones in the Halibut Platform area indicates that the lake extended at least that fat east. Duncan and Hamilton (1988) have estimated that the lake was a closed system up to 122 m deep, with saline– brackish water and a wave fetch that may have been up to 225 km.

The presence of late Givetian palynomorph assemblages typical of the Eday Flags in the upper part of well 13/22-1 suggests that although the Orcadian Lake in Caithness had begun to be filled with sandstones by the middle Givetian, a lacustrine environment was maintained in the offshore Moray Firth area. The lake in which the Eday Flags of well 13/22-1 were deposited must have extended into Orkney where similar beds were laid down, and it is probable that both represent the deposits of the diminished Orcadian Lake.

Upper Old Red Sandstone

By the end of Middle Devonian times the lacustrine Orcadian Basin had become largely intake', to be replaced by fluvial, wadi and playa-lake environments in a desert-belt setting. Upper ORS rocks crop out along both coasts of the Moray Firth as well as at Dunnet Head in Caithness, on Hoy in Orkney and also in Shetland. Many of these sections are assigned to the Upper ORS on the basis of fossil fish, although additional evidence of an Upper Devonian age is available from Hoy (Halliday et al., 1982), where the sandstone overlies lava radiometrically dated as 379 ± 10) Ma and 366 ± 8 Ma (Middle/Late Devonian).

Although Upper ORS rocks occur in the coastal belt bordering the Moray Firth, they have nor been proved in the inner Moray Firth. possibly because they were removed during a phase of post-Middle Devonian uplift and erosion which preceded Permo-Triassic deposition. Deegan and Scull (1077) nevertheless suggested that, by comparison with the Hoy succession, a 685 m thick redbed sequence overlying a volcanic unit in well 12/23-1 may be of Late Devonian age. However, more recent dating of lavas underlying these redbeds has shown that the sandstone is more likely to be of Permian age (see Chapter 6).

Upper ORS sediments have been recorded in the outer parts of the Moray Firth, for example at the Buchan Oilfield where they have been assigned Late Devonian to Early Carboniferous ages (Hill and Smith. 1979; Richards, 1985b). The Upper ORS section in Buchan Oilfield well 21/1-6 is 675 m thick and generally fines upwards (Figure 17). Within this overall upward-fining sequence are numerous small-scale fining-upward cycles composed of conglomerate, sandstone and siltstone, sometimes with cornstones representing soil horizons. The number of siltstone and cornstone beds increases upwards, and they become common in the uppermost parts of the succession. Fining-upward cycles can be up to 10 m thick, but are commonly thinner than 1 m. The thicker cycles have been interpreted as the deposits of mobile, fluvial-channel belts, and the thin cycles as waning-flow sheetflood deposits (Richards. 1985b).

The Buchan Oilfield sediments may have been derived from a source which includes igneous rocks, since they commonly contain zircon and hornblende, only sporadic garnets are derived from metamorphic basement (Anderson et al., 1979). The Caledonian South Halibut Granite pluton to the north-west of the oilfield (Figure 12) may have provided this source (Richards 1985b). because pebbles of granite have been found in wells adjacent to the pluton, indicating that it was being eroded during the Devonian. Indeed, the granite coasts occur in well 13/24-1 below thin Middle ORS siltstone beds, showing that the pluton was already being eroded during the Middle Devonian.

Gradual erosion through Late Devonian time may have progressively reduced topographic slopes, resulting it the upward fining of the fluvial sediments. and the development of soils as more stable flood basins formed over lower-angled depositional surfaces (Richards. 1985b). A similar process of highland retreat has been invoked by Bluck (1980) to explain upward-fining Upper ORS sediments in the Midland Valley of Scotland, where it has been noted that an upward change from sandstones to sandstones with cornstones occurs near the Devonian-Carboniferous boundary (Paterson and Hall, 1986) By analogy, it is possible that the cornstone-bearing beds in the Buchan Oilfield also occur near the base of the Carboniferous.

Chapter 5 Carboniferous

In the Outer Moray Firth Basin, Old Red Sandstone rocks are overlain at depth by a distinctive coal-bearing facies of Carboniferous age that has been informally termed the Forth formation (Maher and Harker, 1987). Carboniferous rocks are generally absent from the Inner Moray Firth Basin (Figure 18): to dare only a single occurrence of volcanic rocks his been identified.

Deposition of the Upper Old Red Sandstone red bed facies in the Outer Moray Firth Basin continued, at least locally, into Early Carboniferous time. The upper part of the fining-upward sheetflood and channel deposits at the Buchan Oilfield contains numerous cornstones: these are pedogenic layers whose appearance may not only reflect topographic levelling, but also a change to a more pluvial climate near the Devonian–Carboniferous boundary. In a few wells in this region, palynological dating of redbed sequences indicates a Tournaisian age, and the Old Red Sandstone reservoir at the Buchan Oilfield contains microflora that range from Fammenian to as young as Viséan (Hill and Smith. 1979).

The tectonic regime in which the Carboniferous rocks of the Moray Firth were deposited is not clear, and two differing models based upon nearby analogues have been proposed. In north-eastern England, the Early Carboniferous was dominated by fluviodeltaic sedimentation during a phase of active crustal extension (Leeder, 1982), while in the Midland Valley of Scotland there is no clear evidence for this rifting phase, and thermal subsidence with superimposed strike-slip tectonics is considered to have predominated (Read. 1987).

Coal-bearing succession

Coal-bearing sediments of Viséan and Namurian age occur beneath the Permian in parts of the Outer Moray Firth Basin. Deposition took place in an extensive, nonmarine, fluviodeltaic setting with lenticular delta-lobe sands and fluvial-channel sands interfingering with overbank and prodelta shaly mudstones and coals (Maher and Harker, 1987). The formation of numerous coal seams indicates that the climate at this time was hot and wet, stimulating abundant growth of vegetation. The occurrence of coal-bearing strata is restricted to the Witch Ground Graben and the Peterhead Ridge, although they may also be present over much of the Aberdeen Platform (Figure 18). The Highland Boundary Fault had a strong control on sediment distribution in the Midland Valley of Scotland; there was a source area to the north-west, and no Carboniferous sediments are found at present to the north of the fault. However, in the Moray Firth sediments are preserved north and west of the extrapolated line of the Highland Boundary Fault, which runs into the south-eastern corner of the report area (Figure 4).

Most information on the Carboniferous comes from wells in the Claymore Oilfield. the Piper Shelf and other highs in the Witch Ground Graben. Although the Carboniferous rocks are probably also present within the deeper parts of the graben. they have not been penetrated and cannot be resolved on seismic profiles. They are however absent from the Galley Oilfield within the Witch Ground Graben and may also be absent from other prominent highs in that graben. It is probable that Carboniferous sediments were deposited over a wider area than that in which they are preserved, for a reworked Carboniferous microflora is widespread in Jurassic rocks as far west as the onshore outcrops in the Inner Moray Firth Basin (Windle, 1979; Linsley et al., 1980).

The coal-bearing facies can be dated on the basis of rich miospore assemblages, and ranges in age from Viséan to Namurian (Figure 19). Diagnostic forms from the Brigantian Stage of the uppermost Viséan are identified in most wells, including 14/19-1. The preceding Asbian Stage is recognised in wells such as 15/19-2 on the Piper Shelf, and Namurian stages A-B in well 20/4-2 farther south.

No wells penetrate the base of the coal-bearing succession. The maximum thickness drilled is 715 m in the Claymore region (Maher and Harker, 1987), where the entire succession is younger than the 598 m found in well 15/19-2 but predates the 204 m in well 20/4-2. Thus a combined minimum thickness in the region may be 1500 m. Even thicker sequences may be expected within the Witch Ground Graben, although seismic control is insufficient at these depths to show whether faults were active at the time of deposition. Since the succession has been truncated by post-Carboniferous erosion, present thicknesses cannot with reliance be related to depositional thickness.

The coal-hearing facies consists of cycles of thinly bedded sandstone. Siltstone, shaly mudstone and coal, with correspondingly variable geophysical log responses (Figure 20). In a typical cyclical development, the sandstone fines upwards into siltstone and dark grey to blackish, highly carbonaceous shaly mudstone, and the top is characterised by a coal band that is usually overlain by carbonaceous shaly mudstone. The low-velocity coal seams are particularly evident on sonic logs, and low gamma-ray values define the sand units. The sandstones are white to grey, quartzose, fine to coarse-grained, often micaceous, carbonaceous in part, and show some haematite-stained mottles. Poor sorting is characteristic, and ferroan calcite cement is extensive and often replacive, comprising up to 43 per cent of the total rock (Maher and Harker, 1987). The sandstone units are up to 20 m thick and are developed both as components of fining-upward and coarsening-upward cycles that are usually less than 30 m thick, but can be far more complex than the typical cycle described. Some wells record the presence of volcanic rocks that include agglomerate, basalt and tuff. In other wells there is limestone and dolomite with sphaerosiderite; these may be of either freshwater or marine origin.

Vitrinite reflectance studies show the coal rank to be in the range of high-volatile bituminous coal. but it is likely to be of higher rank in the deeper, unsampled parts of the basin. Coals are thickest and most abundant in the lower (Asbian) part of the sequence, as may be seen by comparing wells 15/19-2 and 14/19-1 in (Figure 20). In well 15/19-2, the Asbian coals average 1.2 m in thickness and constitute 17 per cent of the succession, with one seam 4.3 m thick.

Volcanic rocks

The only proven Carboniferous strata in the Inner Moray Firth Basin are the volcanic rocks in well 12/23-1 (see (Figure 1) for location), although interpretation of seismic profiles suggests that they occur widely on the Smith Bank High. The 58 m-thick volcanic section in well 12/23-1 is found between two undated redbed sequences that are possibly of Devonian and Early Permian ages respectively. The succession comprises olivine basalt and devitrified basalt overlain by volcanic ash with only minor interbedded basalt.

A core sample of analcite-rich, alkali, olivine basalt from near the base is fine grained, containing phenocrysts of clinopyroxene and olivine together with tabular plagioclase. There is evidence of secondary alteration including serpentinite-filled shears. The overlying ash comprises welded fragments of volcanic glass, and is succeeded by sediments containing volcanic detritus derived from the weathered top of the lava. The age of this volcanic unit is unclear. Three K-Ar age determinations in the range 308-329 Ma suggest a Viséan to Westphalian age (Linsley et al., 1980), but a ⁴⁰Ar-³⁹Ar age determination of 340 Ma indicates an older minimum age near the beginning of the Viséan O G Mitchell, written communication. 1984). Comparison of rock types and K-Ar ages with those of the Middle to Upper Devonian Hoy Lava on Orkney. which also give K-Ar ages at c.300– 330 Ma, suggests that there could be a correlation between the two lavas. However, this is not supported by the ⁴⁰Ar–³⁹Ar age of 366-379 Ma assigned to the Hoy Lava (Halliday et al., 1977; 1982).

The occurrence of Viséan volcanics both in the Inner Moray Firth Basin and interbedded in the coal-bearing sequence of the Outer Moray Firth Basin is comparable with a widespread Permo-Carboniferous volcanic suite found in Northumberland and the Midland Valley of Scotland.

Chapter 6 Permian

The Early Permian was the time of initiation of the present forms of the Inner and Outer Moray Firth basins: sediment accumulation took place in simple, subsidence-dominated basins, with no evidence of Permian crustal extension. Strata of strictly Permian age have been proven at outcrop or close to the sea bed both in the West Fair Isle Basin and immediately north of the Buchan coast, but elsewhere, possible Permian deposits have been mapped as Permo-Triassic undivided (BGS Solid Geology sheets). Permian rocks are present at depth in wells over a large part of the report area, and their distribution is much more extensive than that of the Carboniferous. This emphasises the effect of intervening Hercynian uplift and erosion, which can also be deduced by the absence of Upper Carboniferous rocks from the area.

During Permian time, most of the Moray Firth area lay in an embayment on the north-western margin of the Northern Permian Basin, which was the main site of sedimentation in the North Sea region north of the Mid North Sea High (Glennie, 1984). Both the Early and Late Permian facies of this embayment are generally more marginal, or shelf-like. than those found elsewhere in the Northern Permian Basin to which the deposits in the extreme south-east of the study area belong. This platform– basin configuration was controlled during the Permian by the north-easterly trending Highland Boundary Fault (Threlfall, 1981). Relatively little is known of the Northern Permian Basin compared with the Southern Permian Basin of the southern North Sea, and much doubt remains as to the actual age of supposed Early Permian deposits in the Moray Firth area because of their lack of fossils.

Lower Permian (Rotliegend)

Numerous undated, reddened, elastic sequences have been drilled beneath the Upper Permian in the Moray Firth area. The identification of supposed Rotliegend deposits is consequently based upon their lithology, and their stratigraphic position between Zechstein deposits above and either ORS sediments or, in one case, Lower Carboniferous lavas below. In the Inner Moray Firth Basin, stratigraphic position and lithology of Rotliegend deposits could suggest a possible correlation with the Late Devonian Hoy Sandstone (Deegan and Scull, 1977), but an Early Permian age is preferred.

Rotliegend beds are not generally considered to be present in the onshore Moray Firth outcrops (Peacock et al., 1968). However, the presence of deformed bedding within the Hopeman Sandstone Formation has prompted a comparison with the Weissliegend sands of the southern North Sea Rotliegend. This is a deposit in which there are similar deformed structures thought by Glennie and Buller (1983) to be the result of the escape of air through the wet surface of aeolian sand dunes prior to the deposition of the basal Zechstein Kupferschiefer. However, other authors have noted the random occurrence of deformation structures in the Hopeman Sandstone Formation, interpreting them as slumps on the flanks of large dunes following heavy rain. They also refer to the probable Late Permian or earliest Triassic vertebrate fauna found in the formation (Peacock, 1966: Benton and Walker, 1985; Clemmensen, 1987).

Two distinct zones of Rotliegend deposition are recognisable: the marginal zone of the Inner Moray Firth Basin including the East Orkney Basin, and the basinal succession around the Peterhead Ridge that was part of the Northern Permian Basin (Figure 21). These occurrences are now separated by the Halibut Horst and the South Halibut Granite, although Permian sediments may well have been deposited across this intervening area.

In the central part of the Inner Moray Firth Basin, the Rotliegend is up to 700 m thick, but some thinning is indicated both to the south-west and to the east, where there is also a change in facies. A 1000 m-thick sequence in the East Orkney Basin is also thought to be Rotliegend, but has not been penetrated in boreholes. To the south-east, on the margins of the Northern Permian Basin, up to 120 m of Rotliegend deposits are present. Three main facies associations with clear geographic distributions can be recognised (Figure 21): sandstone, interbedded sandstone and shaly mudstones, and anhydritic shaly mudstone.

The Inner Moray Firth Basin succession is characterised by very fine to fine-grained, well-sorted, reddish brown sandstones, commonly with dark reddish brown, non-calcareous mudstone beds up to 2 m thick and sporadic traces of anhydritic shaly mudstone ((Figure 22), well 12/23-1). The top of the unit is either a clean Weissliegend-like sandy facies or more argillaceous. These fine-grained and well-sorted sandstones are interpreted, at least in part, as aeolian dune sands.

On the Halibut Platform the sandstone/shaly mudstone facies has a tripartite division in well 13/18-1 (Figure 22). A basal fine-grained sandstone of 63 m thickness is overlain by a siltstone/mudstone unit 65 m thick, which is in turn succeeded by 154 m of sandstone with interbedded siltstones and mudstones. No core is available from this facies but fluvial and aeolian deposition can be postulated.

The anhydritic shaly^, mudstone succession has been defined as the Fraserburgh Formation in the Type Well (21/11-1), just east of the report area (Deegan and Scull, 1977). The facies is limited to the south-eastern part of the report area (Figure 21) and (Figure 22), well 20/9-2). The rocks consist of grey to reddish brown. dolomitic shaly mudstone with thin dolomitic and micaceous sandstone stringers containing anhydrite nodules (Deegan and Scull, 1977). The overall facies, including the presence of anhydrite and adhesion ripples, is indicative of a sabkha environment.

Upper Permian (Zechstein)

Zechstein strata rest unconformably on Carboniferous or Devonian rocks over much of the Outer Moray Firth Basin, and even where they rest on Rotliegend beds in the Inner Moray Firth Basin, there is local dipmeter evidence of an angular contact. The beginning of the Late Permian was marked by a rapid, widespread marine transgression which led to the burial of the sub-Zechstein landscape by the Kupferschiefer Formation. During Late Permian times, most of the Moray Firth area was a very wide, shallow, marine shelf forming an embayment in which carbonate and sulphate deposition dominated. The embayment lay to the north-west of the Northern Permian Basin, which was a deeper. hypersaline basin surrounded by a landward rim of elastic sedimentation. The transition between shelf and deep basin lies in the extreme south-east of the report area, and might be expected to be the site of an as yet undiscovered, dolomitic, high-energy barrier zone (Taylor. 1981).

Five main lithofacies are recognised in the Zechstein of the Moray Firth: carbonate, anhydrite, halite, redbed elastics and the basal sapropelic shaly mudstone (Figure 23). The Zechstein shelf sequence in well 15/26-1 (Figure 24) is divided into a basal Kupferschiefer Formation of sapropelic shaly mudstone, overlain by a lower, dolomitic and shaly Halibut Bank Formation and an upper. predominantly anhydritic, Turbot Bank Formation (Deegan and Scull, 1977). The carbonate facies and the thicker anhydrite facies are more commonly found in separate wells such as in wells 12/23-1 and 8/27a-1. Neither the elastic nor the halite facies have been formally defined. Dating of the offshore successions, particularly the carbonate and mudstone facies, is facilitated by the presence of a characteristic striate. bisaccate pollen flora. Although the marginal elastics are typically red beds, their sparse microflora supports their identification as Late Permian rather than Triassic in age.

The thickness of Zechstein strata varies considerably, largely reflecting the facies present. In general. the sequence is 50 to 150 m thick, with carbonate sequences thinner than those dominated by evaporites. The thickest preserved Zechstein shelf sequences occur in the Dutch Bank Basin where they are over 230 m thick. However, where salts are present on the Aberdeen Platform, the Zcchstein locally reaches 1000 m thickness in pillows as a result of halokinesis. Although the Zechstein is now absent from some intrabasin highs such as the Halibut Horst and Fladen Ground Spur. it is likely that it was deposited and subsequently eroded from these areas. The presence of anhydrite clasts in Upper Jurassic conglomerates from the south Viking Graben is taken to indicate that deposition extended over the Fladen Ground Spur.

The Kupferschiefer Formation occurs almost everywhere in the area, but is not shown on (Figure 23) where it underlies other facies. As in 13/18-1, it is typically represented by a 1 m-thick, dark grey, laminated, silty, sapropelic shaly mudstone deposited under anoxic conditions below wave-base. It has a strong gamma-ray log response of up to 200 API units as a result of a concentration of uranium and a finely disseminated, brass-yellow sulphide which may be chalcopyrite.

The Halibut Bank Formation consists of hard, pale brown to grey dolomite having a microcrystalline, sucrosic, vuggy, laminated or brecciated texture. Carbonate bound-stones, packestones and grainstones have been described, as well as an isolated occurrence of oolite in well 12/23-1. The carbonate facies is dominant in the Witch Ground Graben region of the Outer Moray Firth Basin and much of the central Inner Moray Firth Basin (Figure 23). At the Ettrick Oilfield. there is a dolomite composed of layers of tubular structures; these are considered to be of algal origin comparable to Calcinema permiana thatis common in the southern North Sea, and interpreted as an organic buildup capable of forming patch mounds (Amiri-Garroussi and Taylor. 1987). Core from the Claymore Oilfield includes a 10 m-thick, mud-matrix, dolomite breccia that was deposited as a non-turbid slurry or debris flow; the chaotic matrix-supported nature of the breccia, the presence of various lithologies, and the preservation of fragile clam and some sharp clast corners are all consistent with this interpretation. Similar debris flows described in north-east England were possibly initiated by synsedimentary fault reactivation (Kaldi, 1980). This flow origin contrasts with the more widespread Zechstein breccia formed by solution collapse (Clark, 1986).

The Zechstein in the Dutch Bank Basin and south-east of the Halibut Horst is dominated by the anhydride Turbot Bank Formation (Figure 23) and (Figure 24), well 8/27a-1). The anhydrite has a displacive, chickenwire texture and is common interbedded with mudstone, dolomite and sandstone, any of which can form moderately thick and locally correlatable beds. The number of anhydrite beds appears to increase with total thickness, and there is evidence locally for mudstone-dolomite-anhydrite shallowing-upward cycles.

Sequences dominated by elastics are found in the west and north (Figure 23) and (Figure 24), wells 11/30-1 and 13/18-1), and thicknesses of up to 300 m of sandstone occur elsewhere ((Figure 24). wells 8/27a-1 and 20/2-1). At the Beatrice Oilfield, a grey to reddish brown shaly mudstone with dolomite and limestone stringers is thought to represent the Zechstein. South of the Wick Fault, white to reddish orange sandstones and subordinate reddish brown mudstones and siltstones pass up into siltstones; these are assigned to the Zechstein due to the presence of the highly radioactive basal Kupferschiefer shaly mudstone and characteristic microflora. The microflora has also been recorded in grey sandstone and mudstone from BGS borehole BH71/23 and in a fine sandstone from the West Fair Isle Basin. Since dated Late Permian rocks are present in the West Fair Isle Basin, it is possible that they occur widely at the base of the Permo-Triassic throughout that basin, in the adjacent East Orkney Basin, and in the East Fair Isle Basin. These elastic sequences probably represent aeolian and fluviatile deposition on the margins of the Zechstein Sea, with occasional elastic influxes extending into the areas of anhydrite (sabkha) and carbonate deposition on the shelf. Onshore, the Hopeman Sandstone Formation of Late Permian or earliest Triassic age is an aeolian unit containing evidence for complex star dunes and north-northeast trade winds (Clemmensen. 1987). There are more clastics in the Moray Firth area than in the Southern Permian Basin. perhaps suggesting increased elevation of the surrounding areas.

Over the Aberdeen Platform, the thin basal Kupferschiefer unit with dolomite is overlain by thick basinal evaporites preserved in a series of salt pillows, but diapirs have not been recognised. Over 1000 m of sediment are present, comprising four evaporitic cycles that, in addition to the dominant halite, contain potassium and magnesium salts, and anhydrite.

Studies of porosity in Zechstein dolomites have shown the importance of local leaching. This process is apparently restricted to sites of later structures formed during the Jurassic, such as at the Claymore and Ettrick oilfields; this suggests the presence of pre-existing highs at these locations. At the Claymore Oilfield, good porosity results largely from freshwater leaching of replacive anhydrite in the crustal location prior to the deposition of Triassic sediments. In the downthrown areas, carbonates have escaped exposure and the interbedded evaporites are largely preserved (Clark, 1986; Maher and Harker, 1987). At the &trick Oilfield there have been at least two periods of leaching, a vadose episode followed by a deeper episode (Amiri-Garroussi and Taylor, 1987).

Correlation of the Moray Firth succession with the classic five-cycle division of the Zechstein elsewhere in the North Sea region has proved difficult. Most, if not all. of the succession probably represents a marginal facies of the first cycle (Z1) of the Southern Permian Basin. However, one or more of the later cycles could be represented in the sabkha or lagoonal facies of the thick Turbot Bank Formation (Taylor, 1981). In the south-east, the thick basinal sequences that include halite may represent cycles Z1 to Z4 (Deegan and Scull, 1977).

Chapter 7 Triassic

During the Triassic Period, the Moray Firth area formed the western margin of an extensive continental plain that lay at a latitude of 10°N and experienced a hot, semiarid, continental climate. The Moray Firth forms part of a Triassic basin that extends eastwards to Denmark and occupies much of the central and northern North Sea, but is partially separated from the Southern North Sea Basin by the Mid North Sea High (Lervik et al., 1989). Within this North Sea basin, a central belt of mudstone separates two regions of differing sandstone facies: thick, coarse-grained, often conglomeratic sandstones of the Skaggerak Formation occur in the east, and finer-grained elastics are found in the west, which includes the Moray Firth.

Triassic rocks occur extensively within the report area, and have been sampled offshore in many commercial wells and BGS boreholes. The sequence has been removed or truncated both on major basement highs and on pronounced tilt blocks (Figure 25). The rocks generally overlie similarly distributed Permian strata, probably unconformably, although this relationship is not clearly seen on seismic profiles. In the west, the Triassic strata are conformably overlain by Lower Jurassic sediments, but Lias rocks are absent elsewhere in the Moray Firth.

The structural development of the Triassic basin in the Moray Firth was dominated by subsidence accommodated by steep. planar faults (Roberts et al., 1990) with no evidence for active extension as suggested by Frostick et al., (1988). Unlike some areas of the North Sea (Jakobsson et al., 1980), synsedimentary faulting had a minimal control on thickness and facies in the Inner Moray Firth Basin; seismic sections show the Triassic to be a blanket of relatively uniform thickness. Evidence from the West Fair Isle Basin, where the seismic resolution of the outcropping Triassic is greatest, suggests that where gross Triassic thickness increases towards the bounding faults, internal bed thickness remains constant. This, together with lack of facies changes in the vicinity of faults, shows that post-Triassic faulting has preserved downfaulted pockets of a formerly more extensive Triassic cover.

The total thickness of Triassic rocks is greatest in the Inner Moray Firth Basin and the marginal basins, where over 500 m are common. A drilled maximum of 771 m has been recorded south of the Wick Fault in an area where a maximum thickness of 1200 m is estimated from seismic data ((Figure 25); Roberts et al., 1990). Around the basin margin, Triassic rocks crop out on the sea bed (BGS Moray-Buchan, Caithness and Fair Isle Solid Geology sheets) with only small occurrences on land. Near Elgin, 108 m of Triassic elastics divided into the Hopeman, Burghhead and Lossiemouth sandstone formations, rest on Old Red Sandstone (Warrington et al., 1980). There is also a small outcrop at Golspie on the western shore of the Moray Firth where unfossiliferous Triassic sandstone is overlain by the latest Rhaetian to Hettangian Dunrobin Pier Conglomerate (Batten et al., 1986).

Three broad facies associations have been identified in the Moray Firth area (Figure 25) and (Figure 26): a westerly rim of arenaceous deposits, capped by the Stotfield Cherry Rock, is probably laterally equivalent to red mudstones of the Smith Bank Formation in the Outer Moray Firth Basin. The third facies is a micaceous sandstone, informally termed the Skaggerak formation, that overlies the Smith Bank Formation in a small area of the Outer Moray Firth Basin.

Dating offshore is limited to palynological studies, and data from the Moray Firth are sparse. Elsewhere in the North Sea Basin, the Smith Bank Formation ranges in age from Early Triassic to Middle Triassic (Anisian) or younger (Brennand, 1975; Lervik et al., 1989). The dating of the Lossiemouth Sandstone Formation onshore as Late Triassic is based upon the age of reptiles and their footprints. On this same basis, the basal Hopeman Sandstone Formation is considered to be Late Permian or perhaps earliest Triassic in age (Benton and Walker, 1985).

The Triassic–Jurassic boundary is recorded in the Inner Moray Firth Basin, but elsewhere the Lower Jurassic is absent. The dolomitisation of the Stotfield Cherry Rock, which implies an influx of brackish water, provides the first evidence of the end of continental deposition at the close of the Triassic Period (Frostick et al., 1988), in the coastal outcrop, the latest Rhaetian to Henangian Dunrobin Pier Conglomerate represents an alluvial-fan deposit laid down above an unconformity during an interval when the climate changed from hot and semiarid to one of increased rainfall, and a cover of vegetation developed on contemporaneous land areas (Batten et al., 1986).

Arenaceous facies

The thick arenaceous successions in the Inner Moray Firth Basin arid the marginal basins form part of a rim of aeolian and fluviatile sedimentation on the margin of the Triassic North Sea Basin close to an upland region. The arenaceous sediments comprise a lower, predominantly sandy unit, and an upper unit with interbedded, fine-grained sandstones, siltstones and mudstones ((Figure 26), well 12/28-1). The sandstones vary in colour from grey or brown to orange-red, and are fine to coarse grained. Thin beds of reddish brown mudstone with traces of anhydrite are particularly numerous in the upper unit. Correlation with the onshore sequence is problematic; the lower unit may be the lateral equivalent of the Burghead Sandstone Formation, and the upper unit the distal equivalent of the Lossiemouth Sandstone Formation (Frostick et al., 1988). At the depocentre south of the Wick Fault, fine-grained sandstones and siltstones are interbedded with coarser beds, and the upper half is interpreted from the decreasing gamma-ray values in well 13/18-1 (Figure 26) as a large-scale, upward-coarsening unit.

Throughout the Inner Moray Firth Basin, the top of the Triassic is marked by the Stotfield Cherry Rock, a 10 to 12 m-thick unit that varies from a calcrete to a silcrete. Locally it consists of microcrystalline silica that includes scattered sand grains, although the deposit is more typically a tan-coloured, micritic, partly dolomitised limestone with chert inclusions and relics of concretionary layering. The Stotfield Cherty Rock is believed to have originated as a calcrete during a period of tectonic quiescence; it formed as an extensive soil cover on a semiarid flood plain in a landscape with subdued relief (Peacock, 1966). The cherry lithology produces an extremely strong, easily identified seismic reflection that is very useful for seismic correlation.

Smith Bank Formation

The Smith Bank Formation, for which the type well is 15/26-1 (Deegan and Scull, 1977), is a reddish brown, argillaceous sequence that is laterally equivalent to the arenaceous facies. It is the most widespread Triassic deposit in the Outer Moray Firth Basin (Figure 25). Although generally uniform, it is in part silty and contains some thin sandstones and traces of anhydrite and dolomite. A reddish brown colour is characteristic, but this becomes pale greenish grey in spots or bands. Sedimentary structures are not common, but small-scale trough cross-bedding and ripple cross-lamination are present in the fine-grained sandstones. Thin fining-upward cycles also occur. These red mudstones have been equated lithologically with the Bunter Shale Formation in other parts of the North Sea (Brennand, 1975).

During the Triassic Period, the outer Moray Firth area formed part of an extensive continental plain of low relief. Sedimentation was dominated by the red mudstone facies of the Smith Bank Formation, which also developed in the central North Sea area. Some of the fine-grained sediments are thought to be distal deposits of ephemeral streams flowing from the surrounding uplands. However, the majority, with their common traces of anhydrite, are thought to be the deposits of extensive, shallow, playa-lakes, In the south-east, movement of Permian halite was initiated during the Triassic, and the mudstones accumulated mainly in lows between developing salt pillows.

Skaggerak Formation

In a limited area around the Claymore Oilfield, a 100 to 300 m-thick, micaceous sandstone unit overlies the red mudstones of the Smith Bank Formation (Figure 25) and (Figure 26), well 14/18-1). The unit contains cycles commonly 0.25 to 4 m thick with mud-flake conglomerates at the base that grade upwards to fine- or very fine-grained, planar-laminated, cross-bedded and rippled sandstones. The red haematite staining in the sandstones is commonly reduced to a greyish green colour. Detrital mica and authigenic clay (smectite and kaolinite) constitute up to 37 per cent of the rock. In the top 5 m below the base Jurassic unconformity, any haematite staining has been completely removed and changes in clay mineralogy reflect the subsequent incoming of marine water in the Early Jurassic (Spark and Trewin, 1986). Porosities are good at 9 to 22 per cent, but the reservoir potential of the sandstone is restricted by the authigenic clay which typically blocks the pore throats.

On the basis of broad lithological similarities, these sandstones have been informally referred to the Skaggerak formation of the eastern central North Sea (Maher and Harker, 1987). They are not however formally attributed because they lack the conglomeratic character of those rocks to the east, and are geographically separate. The fining-upward cycles in the report area have a fluvial origin from a semiarid environment (Spark and Trewin. 1986), and they may be the deposits of a braided-river system draining the flanks of a proto-Halibut Horst in latest Triassic time. The unit thins away from the Halibut Platform and there is possibly some control on sedimentation by faulting. A similiar but isolated succession in the Dutch Bank Basin, represented by the upper sandstone unit in well 8/27a-1 (Figure 26), may have a comparable origin. Tectonically, the fault-controlled Skaggerak sandstones herald the more widespread faulting that was characteristic of the Jurassic Period.

Chapter 8 Jurassic

Jurassic rocks are widely distributed in the offshore basins of the Moray Firth area. They lie for the most part beneath a Cretaceous cover, but crop out at sea bed in the extreme west as a narrow, coast-parallel strip (Figure 2) extending from Helmsdale in the north round to Banff in the south (BGS Moray-Buchan and Caithness Solid Geology sheets). Jurassic strata are exposed onshore at a number of locations between Helmsdale and Eathie on the northern shore of the Moray Firth (Lee, 1925), providing a valuable 'window' on the subsurface geology offshore. Lower Jurassic strata are also present on the southern shore around Lossiemouth, where small outliers are concealed beneath superficial deposits.

The various Jurassic sub-basins of the Moray Firth area together form the western arm of a tripartite system of major elongate basins in the central and northern North Sea. The two other arms are the north-westerly trending Central Graben and the northerly to north-north-easterly trending Viking Graben. The three converge in a zone to the east of the present study area, south-east of the Fladen Ground Spur. Across this zone there was probably continuity of depositional environments throughout much of the Jurassic. There is also a measure of structural continuity at Jurassic level, with the north-westerly trending Witch Ground Graben in the Outer Moray Finh Basin passing south-eastwards into the Central Graben.

The Jurassic is the single most important geological system in the Moray Firth from an economic viewpoint. Reservoirs for oil are developed principally in Middle and Upper Jurassic sandstones that were deposited in environments ranging from alluvial plain to submarine fans. The principal hydrocarbon reservoir units are the Orrin Formation, the Beatrice Formation, the Piper Formation, and the Claymore and Galley sands (Figure 27). In the inner Moray Firth, the main Jurassic source of hydrocarbons is the paralic shaly mudstone of the Brora Coal Formation, whereas in the outer basin it is the marine shaly mudstone of the Kimmeridge Clay Formation.

Jurassic sedimentation took place against the background of an overall, but pulsed. rise in eustatic sea level until the Volgian (Hallam. 1984; Hag et al., 1987). In addition. the Jurassic was a period of crustal stretching and graben formation (or re-activation) in the North Sea. This is manifested both by the tilted fault-block style of structure influencing the Jurassic strata (that are so critical for the formation of many of the hydrocarbon traps), and by the thick wedges of coarse clastic sediment found on the downthrown sides of many faults active during the Jurassic.

The largely arid, continental conditions typical of the Triassic Period ended during the Rhaetian. Sedimentation from the Rhaetian until the early Callovian is recorded in the Inner Moray Firth Basin as a broadly transgressive-regressive-transgressive sequence. An open-marine environment was first established in the late Sinemurian to Pliensbachian when the Lady's Walk Shale Member was deposited. Following the regressive phase, during which the Orin and Brora Coal formations were laid down, marine conditions were re-established in the early Callovian. A similar trend is evident in Lower to Middle Jurassic strata elsewhere in the North Sea (Brown, 1986), notably in the north Viking Graben (Brown et al., 1987) where the predominantly arenaceous, deltaic deposits of the largely Bajocian to Bathonian Brent Group overlie marine mud-stones of the Dunlin Group and are succeeded in turn by marine mudstones of the Heather Formation. However, the precise age and nature of the major facies changes vary with location.

The area surrounding the confluence of the Central, Witch Ground and Viking grabens was the site of a Middle Jurassic volcanic centre. The subaerial eruption of basaltic lavas occurred here at a time of broad upwarping in the central North Sea region. Away from the uplift, contemporaneous sedimentation occurred in alluvial plain, barrier coast, and deltaic environments in which coals accumulated. During Middle Jurassic time the Moray Firth Basin try at about latitude 40°N and had a generally humid climate associated with the widespread development of coal-forming swamps.

Marine conditions were re-established throughout the central and northern North Sea area by, at the latest, Oxfordian time. Transgressive marine sands occur locally, and although much of the Upper Jurassic is of marine argillaceous facies it contains laterally discontinuous wedges of marine sands, many of which were deposited from sediment gravity flows (Turner et al., 1984).

In the UK sector of the North Sea, Jurassic strata are thickest within the major grabens, although deposition did encroach locally on to marginal platforms, most notably to the west of the Central Graben. Although some deposition of Jurassic sediment is likely to have occurred across tie present structurally higher platforms and ridges which define the Moray Firth basins, little evidence has been preserved. The loci of maximum Jurassic thicknesses are commonly adjacent to contemporaneously active faults. In the Inner Moray Firth Basin, sequences of Jurassic siliciclastic deposits are up to 3.6 km thick, and in the outer basin there are at least 1.5 to 2 km of Middle Jurassic lavas and associated volcaniclastic material.

The present north-western limit of Jurassic rocks coincides with the Helmsdale Fault and the southern edge of the Caithness Ridge (Figure 2). Jurassic strata extend into the Dutch Bank Basin and thin gradually to the east against the Fladen Ground Spur. The Fladen Ground Spur is a markedly asymmetric feature; its eastern edge forms the faulted margin of the south Viking Graben where some 3 km of sediment accumulated in an apron of overlapping submarine fans during the Late Jurassic (Stoker and Brown, 1986). The southern limits of Jurassic deposits in the Moray Firth are formed by the Banff Fault and Peterhead Ridge.

The Jurassic succession is most complete in the Great Glen Sub-Basin, for pre-Callovian strata are progressively cut out eastwards beneath an unconformity across the Inner Moray Firth Basin (Figure 28). There is a central belt, which includes the Halibut Platform and an area to the south of the Halibut Hoist, where mid to late Oxfordian strata rest directly on pre-Jurassic rocks. The Halibut Horst itself has no Jurassic cover. There is no reliable evidence of Lower Jurassic deposits in the Outer Moray Firth Basin; Middle Jurassic volcanics and sediments for the most part rest unconformably on Triassic strata. This is probably the results of erosion of a once more widespread Lower Jurassic sequence over the central North Sea upwarp. The Lower to Middle Jurassic has therefore a discontinuous distribution in the Moray Firth, and is entirely missing from a central area. The oldest strata presently found throughout the Moray Firth are mid to late Oxfordian in age.

A summary of Jurassic lithostratigraphy is shown on (Figure 27). Some terms defined from onshore sections have been used offshore, notably in the Lower to Middle Jurassic. The chart displays the extent of the principal unconformities in the Jurassic sequence and the basin-wide distribution of mid-Oxfordian and younger strata.

In addition to using lithostratigraphy or biostratigraphy as a basis for stratigraphic subdivision, it is possible to devise a stratigraphic subdivision of a sedimentary succession in the subsurface based on seismic reflection profiles (Payton, 1977). The seismic section shown in (Figure 29) has been interpreted to give a first-order, two-fold division of the Jurassic succession based on both the termination of individual reflectors and on a change in the geometry of sediment packages. When the seismic data are calibrated with geological information from exploration wells, the lower unit (Unit 1) proves to be Rhaetian to mid-Oxfordian in age. The pattern of thickness variation in the Rhaetian to mid-Oxfordian strata of the Inner Moray Firth Basin is one of relatively simple thickening towards the Helmsdale Fault (Figure 30). The overlying wedge-shaped Unit 2 is ?mid-Oxfordian to Ryazanian (earliest Cretaceous) in age, and corresponds in this area to the Kimmeridge Clay Formation (Figure 31). The boundary between the units, referred to as the intra-Oxfordian event, marks a shift to greater contemporaneous fault control on sedimentation after mid-Oxfordian times.

Lower Jurassic

The Lower Jurassic is restricted so the Inner Moray Firth Basin, principally to the Great Glen Sub-Basin and Smith Bank Graben, and crops out on the shores of the Moray Firth (Figure 2). Offshore it thickens towards the Helmsdale Fault, attaining a thickness of about 300 m, but is absent to the east beneath the basal Callovian unconformity (Figure 28). The occurrence of preserved remnants of Lower Jurassic in structural lows untested by drilling cannot be completely toted out in the Outer Moray Firth Basin.

In the more complete Lower Jurassic sections, for example at Golspie and in wells in the Beatrice Oilfield (Figure 28), there is an overall upward decrease in grain size and an upward increase in the number of marine fossils in the Hettangian to Pliensbachian. This section is referred to the Dunrobin Bay Formation which forms the larger part of the Lower Jurassic. At Golspie, Batten et al., (1986) recognise three lithological units within the Dunrobin Bay Formation:

1. A basal conglomerate and pebbly sandstone, the Dunrobin Pier Conglomerate Member (7.32 m thick), dated as ?Rhaetian by Neves and Selley (1975), but as ?latest Rhaetian to Hettangian by Batten CE al. (1986).
2. Carbonaceous siltstones and shaly mudstones passing up to medium- to coarse-grained, cross-bedded sandstones of the Dunrobin Castle Member (7.73 m thick).
3. Dark marine shaly mudstones of the Lady's Walk Shale Member (7.48 m thick).

Batten et al. (1986) postulate an initial conglomeratic alluvial fan development triggered by movement on the Helmsdale Fault. followed by a gradual change from estuarine to fully marine conditions through the Dunrobin Castle and Lady's Walk Shale members. The Dunrobin Bay Formation rests unconformably on the Stotfield Cherty Rock, a silcrete which represents a Triassic soil profile: fragments of Cherty Rock are found in the Dunrobin Pier Conglomerate.

The Stotfield Cherty Rock is widely distributed across the Inner Moray Firth Basin and is probably responsible for the strong seismic signal from the 'base Jurassic' horizon, Offshore, in the Beatrice Oilfield, the Stotfield Cherty Rock is overlain by interbedded sandstones. siltstones and mottled mudstones of the Varicoloured unit (Figure 27). This is of uncertain. Rhaetian to earliest Jurassic age, and is interpreted as an alluvial floodplain and levee-crevasse splay deposit (Linsley et al., 1980). Batten et al. (1986) have suggested that it represents the distal deposits of Early Jurassic fluvial systems which deposited the Dunrobin Pier Conglomerate at Golspie. The overlying strata in the Beatrice wells consist of white sandstone overlain in turn by grey and blackish shaly mudstones, with indications of gradually increasing marine influence on deposition (Figure 28). Linsley et al. (1980) report ammonites of early Pliensbachian age from the marine mudstones at Beatrice that are similar in age to the Lady's Walk Shale Member onshore.

The Lower to Middle Jurassic boundary onshore at Golspie is faulted out, but in Beatrice wells the Lady's Walk Shale unit is overlain, probably disconformably, by ?Toarcian siltstone, The Toarcian strata coarsen upward to Bajocian sands, and together form the Orrin Formation (Andrews and Brown, 1987).

Near the southern margin of the Jurassic in the western Moray Firth, a 69 m-thick sequence of Lower Jurassic strata, probably of Sinemurian age, was encountered in the Lossiemouth borehole (Berridge and Ivimey-Cook, 1967). It consists of interbedded sandstone and shaly mudstone deposited in lagoonal or estuarine environments which became more truly marine. This transgressive trend is observed elsewhere in the Inner Moray Firth Basin, but a reversion to sandstone deposition indicative of freshwater conditions appears to have taken place at an earlier stage in the Lossiemouth section than elsewhere.

Middle Jurassic

Middle Jurassic strata are more widely distributed than the Lower Jurassic. Bajocian to Bathonian strata in the Inner Moray Firth Basin are restricted to the Great Glen Sub-Basin and Smith Bank Graben, and are cut our to the east beneath the basal Callovian unconformity. However, Callovian strata extend farther eastward in the Smith Bank Graben and also into part of the Wick Sub-Basin. Middle Jurassic strata are absent from a central belt stretching from the Halibut Platform to south of the Halibut Horst. They are present in the Outer Moray Firth Basin, but precise correlation between Middle Jurassic strata in the inner and outer basins has not been achieved.

The upward-coarsening, regressive sequence of the Orrin Formation overlies the Lady's Walk Shale unit in the Beatrice Oilield, and spans the Lower to Middle Jurassic boundary (Figure 28). The Orrin Formation consists of muddy siltstones passing upwards to fine- or medium-grained, bioturbated, cross-bedded sandstones. It is up to 60 m thick and was deposited during progradation of a shoreline, possibly in a deltaic environment. A prominent mudstone layer towards the top of the formation contains an assemblage of brackish-water algae, with marine acritarchs and dinoflagellate cysts. This layer represents lagoonal or estuarine sedimentation, and is overlain by sandstones of fluvial origin. Orrin Formation strata are assigned a Toarcian to early Bajocian age by Linsley et al. (1980), and are succeeded by the Brora Coal Formation in and around the Beatrice Oilfield. Onshore, strata equivalent to the Orrin Formation are not exposed, the section being cut out by faulting.

At Brora, the Brora Coal Formation is considered by both Neves and Selley (1975) and Lam and Porter (1977) to be Bathonian in age. Its onshore outcrop has been subdivided into a lower Doll Member of sideritic-cemented mudstones with fine- to medium-grained, trough cross-bedded sandstones, and an upper Inverbrora Member of grey to blackish, carbonaceous shaly mudstone (Hurst, 1981). The Inverbrora Member has at its top two thin coals, the upper referred to as the Brora Coal by Hurst (1981). The formation is interpreted as a largely fluvial sequence, with the finer-grained sediment as overbank deposits, but there is some evidence of a marine or estuarine influence. The Brora Coal is considered to be a back-barrier deposit that accumulated in a lagoon, whereas sands near the top of the Brora Coal Formation onshore at Cadh'-an-Righ are interpreted as washover deposits transported landward across a barrier island during storms (Sykes, 1975a).

The Brora Coal Formation is up to 140 m thick in Beatrice Oilfield wells. Here it consists of interbedded, very fine- to medium-grained, cross-bedded and ripple-laminated sandstone, as well as siltstone with rootlets and sphaerosiderite, and mudstone with rootlets and associated coaly zones. The sediments occur in stacked, upward-fining cycles. The formation is capped by a coal which is probably the lateral equivalent of the Brora Coal onshore

There is a broad similarity between the facies of contemporary Lower to Middle Jurassic sedimentary units of the Beatrice Oilfield and onshore outcrops, at least below the Callovian. MacLennan and Trewin (in press) however demonstrate a contrast between lagoonal deposits below the Brora Coal at outcrop near Brora, and alluvial plain deposit; beneath the coal in the Beatrice Oilfield. These authors date the Brora Coal at Brora as latest Bathonian to earliest Callovian.

Callovian and younger Jurassic strata show a greater degree of lateral facies variation. Immediately above the Brora Coal at Brora is a bioturbared, medium-grained sandstone with a diverse marine fauna. It is known as the Brora Roof Bed and is dated as early Callovian by Sykes (1975b; it is believed by Neves and Selley, (1985) to record a Callovian transgression. The succeeding Callovian section onshore is 116.5 m thick (Sykes. 1975b) and consists of marine sandstones, siltstones and mudstones. These, together with the Brora Roof Bed, represent the Brora Argillaceous Formation and the Brora Arenaceous Formation. In these. sandstones probably represent migrating bars on a shallow-marine shelf.

Callovian strata extend from the onshore outcrops into the offshore basins and farther eastwards towards the Halibut Horst, They are not proven in the Witch Ground and Buchan grabens, although Callovian radiometric ages have been reported from Middle Jurassic lavers in the eastern part of the Buchan Graben (see below).

In the Callovian section at the Beatrice Oilfield, sandstones are restricted w the Lambeni Zone or older. These predominantly sandy Callovian strata include marine mudstones and bioturbated siltstones. They belong to the Beatrice Formation of Andrews and Brown (1987), so called because the sandstones provide the major reservoir in the oilfield. The sandstones are fine to medium grained. and are developed in stacked, upward-coarsening sequences. The overlying dark, marine, shah mudstones, termed the Upper Formation, represent latest Callovian and Oxfordian sedimentation. These act as the seal to the main Beatrice reservoir. In onshore outcrop, strata equivalent in age to the Upper Formation are represented by sandstones of the Brora Arenaceous Formation.

In the Outer Moray Firth Basin, the Rartray Formation consists of tuffacenus beds with subordinate siliciclastic deposits and intercalated basaltic lavas that are the product of subaerial volcanic activity. These pass north and west into the Pend and Formation, which comprises siltstones and shaly mudstones with some coals and subordinate volcanic rocks. Such Middle Jurassic strata are found in the Witch Ground and Buchan grabens, on the Renee Ridge and Piper Shelf, and extend east and south-east beyond the report arca. The lavas consist of alkali-olivine basalts, ankaramites, and hawaiite types (Gibb and Kanaris Sotiriou. 1976; Fall et al., 1982). Fall et al. (1982) also report intrusive mugearites and hawaiites from well 21/3-1A, just outside the report area. Maximum thickness estimates for the Rattray Formation range from 1.5 to 4 km (Howitt et al., 1975: Woodhall and Knox. 1979; Dixon et al., 1981). The age of the volcanics was given as Bajocian to Bathonian by Howitt et al. (1975) based on radiometric dating and limited biostratigraphic control. More recently, Ritchie et al. (1988), dating radiometrically both intrusive and extrusive igneous rocks from well 21/3b-3 (again just outside the present area), postulate a Callovian to Oxfordian age for both rock types. The lava analysed by these authors was sampled from near the base of the 21/3b-3 sequence, suggesting that at least part of the volcanic activity occurred after the phase of erosion represented by the basal Callovian unconformity in the Inner Moray Firth Basin.

Upper Jurassic

In the Inner Moray Firth Basin, Upper Jurassic strata rest with little or no stratigraphic break on the Middle Jurassic. In the other basins, middle or upper Oxfordian strata have been reported to lie unconformably on the Middle Jurassic, usually on ?Bathonian strata of the Pentland or Rattray formations. However the Callovian to Oxfordian dates for the Rattray Formation given by Ritchie et al. (1988) place in some doubt the time span represented by this unconformity.

Onshore outcrops along the north-western shore of the Moray Firth provide exposures of Oxfordian to Volgian strata (Lam and Porter, 1977). The Oxfordian is represented by the upper part of the shallow-marine bar sands of the Brora Arenaceous Formation. This is in turn overlain in the Brora region by the Ardassie Limestone Member of the Balintore Formation. The mid-Oxfordian Ardassie Limestone is strictly a unit of muddy calcareous sandstones interbedded with Rhaxella spiculites in which the opaline sponge spicules have been replaced by calcite. It is equivalent in age to the muddy. glauconitic sandstones and limestones of the Port-an-Righ Ironstone Member of the Balintore Formation (Sykes. 1975b) that is exposed near Balintore farther to the south-west. At Brora there is a gap in exposure, and the oldest sediments above the Ardassie Limestone have been dated as early Kimmeridgian by Lam and Porter (1977). At Balintore, Sykes (1975b) has described dark, bituminous siltstone and mudstone (Port-an-Righ Siltstone) above the ironstone, and assigned them a mid-Oxfordian age.

In the inner Moray Firth. the early to mid-Oxfordian strata are predominantly shaly mudstones, and are a continuation of the late Callovian facies that overlies the Beatrice reservoir; these are referred to the Uppat Formation. They have a slight upward decrease in gamma-ray response due to an upward-coarsening grain size from mudstone to siltstone containing Rhaxella spicules. In the Smith Bank Graben, the Uppat Formation has at its top a fine- to very fine-grained, bioturbated, spiculitic sandstone termed the Alness unit ((Figure 28); Andrews and Brown, 1987). This sandstone consists in places of up to 50 per cent sponge spicules and is a shallow-marine deposit which passes westwards into mudstone towards the Beatrice Oilfield.

The top of the Uppat Formation offshore in the Inner Moray Firth Basin corresponds to the intra-Oxfordian reflector recognised on seismic sections (Figure 29), and to a shift to higher values on gamma-ray and sonic logs within shaly sequences. Faunally there is an upward change across the boundary from a microfauna dominated by benthonic foraminifera to one dominated by agglutinating forms. The seismic reflector marks a change in the gross geometry of packages of Jurassic strata; the Rhaetian to mid-Oxfordian package shows relatively minor thickening towards faults, whereas the later Jurassic succession thickens markedly towards major faults (Figure 29) and (Figure 31). Normal fault movement contemporaneous with deposition is characteristic of the late Oxfordian to Volgian sequences in many parts of the North Sea (Brown, 1986).

The Outer Moray Firth Basin has mid to late Oxfordian heterolithic strata that were deposited in coastal plain to lagoonal environments. These rest unconformably on poorly dated Middle Jurassic volcanics and volcaniclastics, or on Triassic strata (Figure 32). These sediments, the Sgiath Formation of Harker et al. (1987), record a brackish-water transgression across the area which culminated in the late Oxfordian with the widespread deposition of a thin marine shaly mudstone named the I Shale unit by Maher (1981). and termed the basal deposit of the Piper Formation by Harker et al. (1987). The Piper Formation is predominantly arenaceous, and acts as a major hydrocarbon reservoir in the Piper and Tartan oilfields; it consists of fine- to very fine-grained. quartzose, well-sorted sandstones interbedded with minor dark marine mudstones. it has been interpreted both as an alternating sequence of transgressive and regressive strata deposited as shallow marine bars or in littoral environments (Deegan and Scull, 1977), and as a deltaic deposit (Turner et al., 1984; Harker et al., 1987). The Piper Formation is late Oxfordian to Kimmeridgian in age and is overlain, in places unconformably, by the Kimmeridge Clay Formation.

The Piper Formation passes laterally into dark marine mudstones both to the north of the Halibut Horst and south of the Renee Ridge. These mudstones, together with the Piper sands, are equivalent to marine mudstone in the Lower Warm Shale unit of the Kimmeridge Clay Formation (Figure 27) in the Beatrice area (Andrews and Brown, 1987).

Above the Piper Formation, Kimmeridgian to late Ryazanian successions in the Moray Firth are broadly similar in character in all the sub-basins. Dark grey to blackish, organic-rich, marine shaly mudstones, commonly with a high gamma-ray response ('hot shale), are found throughout the area. The interval was one of active normal faulting, and the mudstones commonly pass laterally into coarse elastic sediments on the downthrown sides of major faults. This is most dramatically seen onshorc, where wedges of conglomerate, sedimentary breccia and sandstone forming the Helmsdalc Boulder Beds and associated units were shed southwards from the active Helmsdale Fault (Linsley, 1972). Pickering (1984) has interpreted the Helmsdale Boulder Beds as submarine sediment gravity flow deposits which accumulated both in base-of-fault screes and in submarine channels.

Wedges of coarse elastics are also found associated with intrabasinal faults, most notably at the Claymore Oilfield where fine- to medium-grained turbidite sands of Kimmeridgian to early Volgian age, probably derived from the Halibut Horst to the south, act as the principal oil reservoir. Localiscd wedges or cones of Late Jurassic sand, such as the Claymore Sand Member and Galley sands (Figure 32), are enveloped by mudstones of the Kimmeridge Clay Formation, the most important source of North Sea oil. Given sufficient burial of the mudstones to promote oil generation, this relationship with the fault-controlled sands is highly favourable for migration from source rock to reservoir rock.

The Late Jurassic to Ryazanian organic-rich mudstone of the Kimmeridge Clay Formation reach thicknesses in excess of 2.5 km in the Inner Moray Firth Basin (Figure 31). The base of the formation is mid- to late Oxfordian in age, and is marked by an upward increase in gamma-ray and sonic log response. Over much of the Inner Moray Firth Basin, the mudstones can be subdivided into four 'warm’ and 'hot' units on the basis of variations in gamma-ray log values (Figure 33). The base of the Kimmeridge Clay Formation in the Inner Moray Firth Basin corresponds closely in age to the lowest Upper Jurassic strata resting unconformably on the Middle Jurassic in the Outer Moray Firth Basin.

The top of the Kimmeridge Clay Formation throughout theMoray Firth area is taken at a sharp upward decrease in gamma-ray and sonic log response. It is commonly overlain by calcareous mudstones of low radioactivity assigned to the Early Cretaceous Cromer Knoll Group (Figure 33). In stratigraphically complete sections, the boundary lies within the Stenomphalus Zone of the upper Ryazanian, and has been interpreted as the result of a regional, isochronous, basin-flushing event (Rawson and Riley, 1982). This destroyed an anoxic layer which is thought to have existed in the stratified watermass of the Late Jurassic sea. On some intrabasinal highs and basin margins, the Cromer Knoll Group rests unconformably on the Kimmeridge Clay Formation.

Chapter 9 Cretaceous

Lower Cretaceous

Lower Cretaceous strata occur widely offshore in the Moray Firth, where they are predominantly calcareous argillites with local sandstones. These were deposited in an aerobic, marine environment similiar to that in other North Sea areas at that time. The change from the Late Jurassic anaerobic environment was possibly in response to an increase in water circulation caused by a global sea level rise and lot the effects of thermal subsidence of the graben following the main phase of extension (Rawson and Riley, 1982; Hesjedal and Hamar. 1983).

The Lower Cretaceous is represented only by glacial erratics on the adjacent land; these include a large mass of quarried sandstone of Valanginian to Elauterivian age at Leavad in Caithness (Crampton and Carruthers. 1914). Both the thickness and distribution of the offshore Lower Cretaceous deposits have been greatly influenced by contemporaneous faulting and subsequent erosion (Figure 34). The sediments are more than 900 m thick in the Witch Ground Graben and the &trick Basin, but are thickest in the Inner Moray Firth Basin where they exceed 1500 m adjacent to the Little Halibut Fault. The deposits thin significantly over intrabasinal highs, and over the Dutch Bank Basin and East Shetland Platform to the north where thin limestones and marls predominate. In the Witch Ground Graben. fault-block rotation led to uplift and erosion of hoists, and the deposition of coarse elastic sediment in the graben (Boole and Gustay. 1987).

The present distribution of Lower Cretaceous strata has also been greatly influenced by Tertiary uplift: this affected the Inner Moray Firth Basin, the western margins of the East Orkney and Dutch Bank basins, and the eastern margin of the Grampian High (McQuillin et al., 1982). In these areas, Lower Cretaceous strata crop out at sea bed or are overlain only by thin Quaternary sands and clays. Seismic profiles here show no thinning of Lower Cretaceous units as they approach sea bed (Roberts et al., 1990), indicating that the present geometry is the result of post-depositional erosion. However, a change to a sandier facies indicates a contemporaneous shoreline in the west during the Early Cretaceous. In the Outer Moray Firth Basin, Lower Cretaceous strata are in general conformably overlain by Upper Cretaceous deposits.

The Lower Cretaceous in the Moray Firth Basin is largely synonymous with the Cromer Knoll Group as defined by Rhys (1974) for the southern North Sea. The name is now also commonly applied to deposits farther north (Deegan and Scull. 1977). The Cromer Knoll Group ranges in age from late Ryazanian to late Albian (Figure 35), the older Ryazanian being represented by the Kimmeridge Clay Formation in the Outer Moray Firth Basin. The Cromer Knoll Group is divided by Deegan and Scull (1977) into the Valhall, Sola and Rødby formations, all of which can be identified in the Outer Moray Firth Basin. However such a subdivision is not practicable in the Inner Moray Firth Basin, so the strata there are described separately.

Valhall Formation

The Valhall Formation, which ranges in age from late Ryazanian to early Aptian (Figure 35), is in general a transgressive unit. It consists predominantly of calcareous mudstones and mark with local sandstones, and is widely distributed throughout the outer Moray Firth where it is up to 800 m thick. Its base is generally unconformable in the shallower parts of the basin where the sediments appear to lap on to the underlying surface; in deeper parts of the basin the formation conformably overlies older units. The Valhall Formation sediments onlapped, and ultimately buried, topographic highs as sea level rose (Casey and Rawson. 1974). On the East Shetland Platform, Dutch Bank Basin and Halibut Platform, thinner marls and limestones were deposited.

The change from Kimmeridge Clay Formation 'black shale' deposition to calcareous mudstone and limestone deposition during the late Ryazanian reflects a major environmental change. Anaerobic conditions were present at the sediment–water interface in a basin with restricted water circulation during the Late Jurassic and early Ryazanian. The subsequent increase in oxygen levels was associated with the Stenomphalus Zone transgression and the consequent 'flushing out' of the stratified water column to give an open-marine environment (Rawson and Riley. 1982). There was an associated change in the foraminifera from a predominantly agglutinated assemblage in the 'black shale' facies to one dominated by benthonic hyaline forms. This was accompanied by change to a predominantly planktonic palynological assemblage in which terrestrially derived palynomorphs are consistently subordinate. The underlying cause for this major environmental change may have been the demise of active rifting and the subsequent linking of partially isolated basins.

In a restricted area to the north-east of the Halibut Horst, up to 700 m of upper Ryazanian to Barremian sandstones were deposited: these are referred to informally as the Scapa sands (Figure 34). The Scapa Sand Member (Figure 36) as defined by Harker et al., (1987) is of very restricted occurrence, but was probably coeval with the Scapa sands. A unit of early to middle Ryazanian age in the Kimmeridge Clay Formation beneath the Scapa Sand Member consists of burrowed and bioturbated sandstone and sandy limestone' interbedded with marl and pale grey mudstone. This unit increases in thickness towards a fault. suggesting an earlier period of limited active erosion and clastic deposition.

The Scapa sands are interpreted as having been deposited from turbidity currents which accumulated on aprons adjacent to the major tilted fault-block structures (Figure 37). Early turbid ices were coarser grained and conglomeratic, but were overlain by finer. more sand-rich turbidites (Booze and Gustay. 1987). The sandstones are fine to medium grained with coarser laminae, and are arranged as nongraded or upward-fining units interbedded with shaly mudstoncs and finely laminated siltstones. The sandstones are commonly strongly cemented, and include irregularly shaped calcite nodules. The Scapa sands were probably eroded from the Halibut Horst and adjacent tilted fault-block structures as they became exposed, possibly during local tectonic adjustments. Deposition of the Scapa sands ended abruptly with no apparent fining towards its top, which may suggest a rapid drowning of the clastic source. Other Valhall Formation sandstone bodies found to the south of the Halibut Horst and north-east of the Renee Ridge are of similiar facies (Figure 34).

Sola Formation

The Sola Formation sediments range in age from early Aptian to early Albian (Figure 35), and generally consist of nonca careous. dark grey, organic-rich mudstones with thin limestone and siltstone beds. Sandstones dominate the succession locally (Hesjedal and Hamar, 1983). The formation is up to 200 m thick, and its base is recognised as a strong peak on gamma-ray logs. This formation shows a steady upward increase in calcareous content. The micro-fauna of the mudstones is typified by agglutinating foraminifera such as Glomospira and Recurvoides which are indicative of deposition in a more restricted, less open-marine environment than that of the late Ryazanian to Barremian. This change is related by Hesjedal and Hamar (1983) to the constraining of water circulation both by a decrease in water depth and minor tectonic uplift. However, throughout the succession there are beds of mudstone containing flood proportions of planktonic foraminifera which may be the result of transgressive pulses. On the platform areas and in the Dutch Bank Basin, sediments are much thinner and consist predominantly of limestones and marls.

The drop in sea level at the start of deposition of the Sola Formation resulted in the exposure and erosion of horsts and surrounding hinterlands; this in turn led to deposition of sand bodies in the Witch Ground and Buchan grabens. As sea level rose again, the coarse-clastic source areas became drowned and the sands were superseded by mudstones of the upper part of the Sola Formation.

The most significant accumulation of sands in the Sola Formation lies just outside the report area, in the extreme south-east of the Witch Ground Graben. Different oil companies have referred to these as the Bosun, Kopervik and Aasgard formations, but the general term Bosun sands is used here for the Aptian to Albian sandstones of the Outer Moray Firth Basin. The Bosun sands typically consist of interbedded sandstones and mudstones in which the sandstone/mudstone cycles thin towards the top of the sequence, and the sand units fine upwards as a whole. The sands are poorly sorted, fine to coarse grained, massive to poorly laminated, and contain coal fragments and rip-up clasts. Fluidisation structures are common, particularly dish and pillar structures, and a common Jurassic microflora suggests much reworking of Jurassic strata. The formation is interpreted as having been deposited by turbidity currents and debris-flows in submarine fans.

Rødby Formation

The Rødby Formation is the uppermost unit of the Cromer Knoll Group. It ranges in age from mid to late Albian (Figure 35), and consists of grey to reddish brown sandy marls and calcareous mudstones. The formation is up to 150 m thick and is characterised by high gamma-ray values. Recognisable Rødby Formation strata are present throughout the basinal areas of the Outer Moray Firth Basin and, unlike the older formations, can be interpreted as extending westward into the Inner Moray Firth Basin. There is a locally transitional boundary with the underlying Sola Formation sediments, and a gradual upward decrease in grain size. There is also a progressive upward increase in carbonate content leading ultimately to carbonates of the Upper Cretaceous Chalk Group. Both the westward extension of the area of deposition of the Rødby Formation and the increase in carbonate content are associated with a relative rise in sea level. This transgression is reflected by a change in the foraminiferal population, with dominantly planktonic forms succeeding the largely agglutinated assemblages of the underlying Sola Formation.

Inner Moray Firth Basin

The Lower Cretaceous of the Inner Moray Firth Basin is predominantly sandy and is therefore markedly different from sediments in the outer basin. The strata have not been given formal names. Several BGS shallow boreholes in the region (BGS Moray-Buchan and Caithness Solid Geology sheets) have recovered blackish shaly mudstone and sandstone lithologies ranging in age from Hauterivian to Aptian. The sediments unconformably overlie more steeply dipping Jurassic sediments (Roberts et al., 1990) and thicken towards major fault zones, particularly at the contemporaneously active Wick, Little Halibut and Smith Bank faults. Well 13/11-1 in the Wick Sub-Basin penetrated 1675 m of Lower Cretaceous sandstone and mudstone in the thickest part of the succession (Figure 34).

In the south of the Inner Moray Firth Basin, the late Ryazanian to Valanginian basal section is predominantly sandy, but is overlain by more argillaceous sediments. These mudstones are blackish to dark grey, locally silty, with shell fragments; they may be equivalent to Sola Formation strata in the Outer Moray Firth Basin.

In the northern part of the basin, the sediments are fine-to coarse-grained sandstones interbedded with thin limestones, siltstones and mudstones. Towards the Halibut Platform, the sands become coarser grained and occasionally conglomeratic. These sandstones have been interpreted as turbiditic, submarine-fan deposits (Hancock, 1984), although a fault-controlled, south-westward-trending channel in the Beatrice Oilfield area has been interpreted as broadening into a fan delta. (Bird et at., 1987), Rødby Formation sediments are present in the eastern part of the Inner Moray Firth Basin as a result of the Albian transgression which caused a westward spread of the outer basin depositional regime. In this region, calcareous muds are interbedded with richly glauconitic sands which may indicate the proximity of a contemporary shoreline to the west (Ziegler, 1981).

Upper Cretaceous and Danian (Chalk Group)

Throughout Late Cretaceous and earliest Paleocene times the entire area of the Moray Firth was submerged by an incursion of the Boreal Ocean, resulting in the deposition of a thick sequence of chalk and chalk-marl. Although subsequently partially removed by Tertiary erosion, these deposits represent the longest period of relatively homogeneous sedimentation in the area. They are laterally correlatable with similar deposits found throughout the central and southern North Sea, southern Britain and over large areas of north-west Europe. Northwards, chalk deposits become progressively more argillaceous and grade into a calcareous mudstone lacks that dominates the Upper Cretaceous of the North Viking Graben and the northern United Kingdom and Norwegian continental shelves.

No in-situ occurrences of Upper Cretaceous strata are recorded onshore adjacent to the Moray Firth, but chalk blocks and flints are found in glacial deposits. BGS offshore mapping has located Upper Cretaceous beds cropping out beneath Quaternary in a broad are across the Inner Moray Firth Basin (Figure 2) and (Figure 38) and to the east of Peterhead (BGS Solid Geology sheets). At depth, the Upper Cretaceous is represented on seismic sections as an often featureless package whose top is marked by strong, continuous reflections traceable over much of the North Sea.

The main depocentres are located in the Witch Ground and Buchan grabens, with subsidiary centres located in the Wick Sub-Basin and the Dutch Bank Basin. The Upper Cretaceous is almost 1000 m thick in the east of the area (Figure i8), and thins to a few metres over major structural highs like the Fladen Ground Spur and the Halibut Horst, although it is probably entirely absent in only a few places. Westward, the limit of the package is generally erosional, but locally is defined by major faults. The orientation of the Upper Cretaceous troughs can be seen partially to reflect the structural pattern of the earlier Mesozoic basins.

The transition from Lower to Upper Cretaceous rocks reflects a change from clastic deposition to predominantly biogenic sedimentation. This coincided with a general rise in sea level that had begun in the Albian and reached its acme in the early Maastrichtian. No angular unconformity is generally detectable at the base of the Cenomanian offshore, aut in the Inner Moray Firth Basin Upper Cretaceous greensand overlies Lower Cretaceous mudstone to indicate a possible retreat of the late Albian shoreline. However, the consistently early Cenomanian age for the inundation of previously emergent nearby land, such as the Hibernian/Hebridean area (Wilson. 1972), indicates a vigorous renewed transgression leading to the onset of widespread chalk deposition.

Three fundamental controls acted in concert to shape the geological development of the Moray Firth area during the Late Cretaceous:

1. Global fluctuations in sea level that gave rise to transgression and regression and the development of regionally extensive onlaps and hiatuses.
2. Local tectonism produced by continuing crustal extension and subsidence in the North Sea region controlled the rate of differential subsidence. uplift and deformation. This affected both overall sediment thickness and the location and degree of interformational erosion.
3. The basin environment and ecology depended on factors including oceanic circulation, climatic belt movement, perturbations in the earth's orbit and catastrophic events such as intense volcanic activity or meteorite collision. These controlled the primary biogenic productivity, the input of clastic material and the stratification of the water column; these in turn determined sediment types and the character of cyclical sedimentation.

It was perhaps the dominance of the third type of control that gave the Upper Cretaceous sequence its prime and somewhat unique characteristics. The unprecedented volume and extent of oceanic water within a continental setting was able to absorb, or at least dilute, the effect of local tectonic and source-related controls on sedimentation.

Conditions in this trangressive chalk sea are interpreted as being marine, oxygenated and warm at a 25°C average temperature. The sea is thought to have been between 200 and 500 m deep, though the apparent lack of the trace fossil Thalassinoides in North Sea occurrences has been taken to indicate water depths up to 1000 m (Hancock, 1984).

The input of elastic material into the Moray Firth basins was severely limited, although it increased northwards and westwards over the UK Continental Shelf and showed cyclical variation during the Senonian. This can be interpreted as indicating an arid climate conducive to very low rates of weathering and sediment transport, and/or very low relief in the local land areas.

Recent studies have shown that gravity-flow processes have been important in building up the thick chalk sequences in the North Sea basins (Taylor and Lapre, 1987). A spectrum of deposit types from slumps and debris flows to turbidites has been described in the Ekofisk area of the central North Sea, and similar erratic log motifs are seen for the Maastrichtian in some Witch Ground Graben wells. However, log responses in the Moray Firth mainly suggest that pelagic sedimentation was dominant except in the Maastrichtian when thick sedimentary wedges built up dose to the active Wick and Banff faults.

Although sedimentation appears to have been nearly continuous in the North Sea basins during the Cretaceous, stratigraphic breaks are recorded in the Moray Firth area. These occur in the late Albian/early Cenomanian (only in the marginal areas), late Cenomanian/early Turonian, late Turonian, intra-Senonian (Santonian?), late Campanian, and late Maastrichtian (Figure 39). The expression of the individual stratigraphic breaks depends to some extent on location, for whereas only a change in sediment type may be recorded in the basins, an angular unconformity may be present in marginal areas (Figure 40).

In the Moray Firth, the complex of fault-controlled basins that developed during the Jurassic and Early Cretaceous had, by Aptian time, become stabilised and largely infilled, with sloping basin margins rather than fault scarps. However, the differentiation between platform and basin was maintained until late Campanian time, although the intrabasinal highs appear to have been transgressed periodically, especially during the Turonian and Senonian when widespread unconformities and onlap events were developed (Hancock and Kauffman, 1979).

Although the marginal fault zone in the South Viking Graben was active during the Campanian, evidence for major contemporaneous fault movement is lacking in the report area, except along the Wick and Banff faults in the upper Maastrichian (Figure 40). Nevertheless, examples of minor fault offsets with associated sediment wedges are found in the lower Senonian of the Inner Moray Firth Basin, and are presumably linked with the unconformity recorded at this level.

Relative thickening of Maastrichtian deposits across some intrabasinal highs, like the Renee Ridge, records an inversion episode related to reverse movements on some of the faults bounding the earlier Mesozoic basins. By the end of Maastrichtian times, turbiditic influxes of elastic material occurred in the Witch Ground Graben, followed by major restriction of the basin during the Danian. Subsequently, widespread fault-related uplift and mass-flow resedimentation of the chalk occurred during the early Paleocene.

Lithologies

Chalk

The term chalk is used here to denote a coccolithic limestone with minimal clay content as indicated by petrographic description or wireline log response; it has no implications as to the degree of induration, which varies considerably. Chalk is composed largely of a pure form of biogenic, crystalline calcite derived from the disintegration of coccoliths (Jeans and Rawson. 1980; Hsü and Jenkyns, 1974). The biogenic fraction may comprise up to 99 per cent of 'clean' chalk intervals. but may be admixed with varying proportions of clay to form a range of marl lithologies through to calcareous mudstone. Bioturbation is extensive, with near total destruction of primary sedimentary structures.

Marl

Marl is more commonly encountered in the offshore Upper Cretaceous than onshore, and is generally assumed to have formed from hemipelagic sedimentation, with a higher clay fraction input than for the chalk; the proportion of clay required to warrant the use of the term marl has not been defined. There is some indication that marl development in the Witch Ground Graben is influenced by proximity to structural highs. but it is difficult to differentiate between local factors and the northward regional trend to marly facies. Alternating chalk –marl cycles are a common phenomenon; they have been related to changes in biological productivity and carbonate preservation in response to orbitally induced climatic cycles (Einsele and Seilacher, 1982). Such cycles are too thin to be resolved on well logs, but thicker clay-marl-chalk cycles are recorded in the Cenomanian, Turonian and Senonian. These are not synchronous with locally developed stratigraphic breaks and are more likely to be a reflection of the regional tectonic or transgression/regression cycles.

Mudstone

Mudstones are thin (1 to 5 m) and occur, not very commonly, either as interbeds with chalk as part of sedimentary cycles, or as single beds rich in organic material that may be regionally persistent and mark a complete change in depositional environment. The basal, radioactive 'hot shale' of the Plenus Marl Formation, that can be traced over thousands of square kilometres, is a good example of the latter.

The mode of formation of the Chalk Group mudstone can be attributed to an influx of clastic material caused by instability and uplift of local source areas. Alternatively, it may relate to phases of maximum transgression and pelagic accumulation below the carbonate compensation depth over a long time interval when biological input had terminated due to anoxic conditions. The latter explanation is reasonable in the case of the Plenus Marl Formation; the former is more applicable to Maastrichtian beds of turbiditic aspect.

Greensand

Greensand, or glauconitic sandstone, occurs in the Inner Moray Firth Basin. It is typical of the greensand facies developed in the European Cretaceous and is an indicator of marine conditions close to a contemporary shoreline.

Stratigraphy

Few cored sections are available for study in the Moray Firth. and stratigraphic studies are based on wireline log response and micropalaeontological dating. Seismic resolution of internal unconformities within the chalk is generally very poor because of low impedance contrasts. A lithostratigraphic scheme for the Upper Cretaceous was initiated by Rhys (1975) for the chalk-dominated southern North Sea sequence: this was refined and extended by Deegan and Scull (1977) to the more argillaceous sequences of the central North Sea. Deegan and Scull (1977) divided the Chalk Group of Rhys (1975) into six formations: the Hidra, Plenus Marl, Herring, Flounder, Tor and Ekofisk formations, the latter being of early Paleocene (Danian) age (Figure 39). A refinement by Burnhill and Ramsay (1981) informally subdivides the Hidra and Herring formations, while the Flounder Formation may also be subdivided on the basis of chalk-marl cycles. This stratigraphic scheme can, despite changes in facies, be closely correlated with other areas of the North Sea and — farther afield (Hancock. 1984), it reflects the regional synchroneity of the major stratigraphic breaks, and lends support to the generally accepted view that many geological events during the Late Cretaceous were subject to global controls.

Hidra Formation

Thc Hidra Formation (Figure 42) (Figure 43) (Figure 44)(Figure 45 Note:- Symbols are diagrammatic; facies changes may be graditional or interbedded on a fine scale." data-name="images/P944802.jpg">(Figure 41) is mainly of Cenomanian age. although its base is often assigned a late Albian age due to unbroken sedimentation across the Lower/Upper Cretaceous boundary in basinal areas. Over much of the Moray Firth Basin, the basal Upper Cretaceous seismic reflector is weak and masked by closely spaced top Cenomanian or top Turonian reflectors. The base of the Hidra Formation is taken at the top Lower Cretaceous seismic event, marked in wells as a relatively undistinguished log break of variable amplitude; it is associated with a thin chalk bed that interrupts an otherwise progressively decreasing gamma-ray log response and increasing velocity from the underlying argillaceous Rødby Formation.

The lithology varies from thin beds of chalk interbedded with mudstone and marl (H1 and H3 units) to thick, massive chalk with a uniformly high velocity (H2 and H4 units). The H1 unit is notable for the presence of green-sand beds in the Smith Bank Basin and the common occurrence of glauconite in the Claymore Oilfield region.

The formation is thickest in the basinal areas of the southern and eastern pans of the area where it is some 120 m :hick. It is present close to the erosional limit of the Upper Cretaceous in the inner Moray Firth and is often identifiable in condensed form over structural highs. Onlap of the formation is not easily demonstrated, but in some sections the HI unit is very thin or possibly absent; Burnhill and Ramsay (1981) have ascribed the absence of the uppermost unit or units over some highs to post-depositional erosion.

Plenum Marl Formation

The Plenus Marl Formation (Figure 42) (Figure 43) (Figure 44)(Figure 45 Note:- Symbols are diagrammatic; facies changes may be graditional or interbedded on a fine scale." data-name="images/P944802.jpg">(Figure 41) offshore is consistently taken to mark the base of the Turanian Stage, despite the fact that onshore the Plenus Marl, and some of the strata overlying it, are assigned to the Sciponoceras gracile Zone in the Cenomanian (Rawson et al., 1978). Nevertheless, it is generally assumed to be isochronous. The characteristic log motif is a sharp, high gamma-ray, low velocity log spike corresponding to the basal carbonaceous mudstone. This grades up into argillaceous chalk with interbedded mudstone.

The Plenus Marl Formation reaches a maximum development of about 35 m in the Witch Ground Graben, but is in many places useful as a marker horizon less than a metre thick. The formation is remarkably persistent and can be recognised over most of the North Sea area: it has a correlative in sections in England and the South-West Approaches.

The Plenus Marl Formation is similar to the Hidra Formative in pattern of distribution and thickness, though there is some evidence of onlap of the basal high-gamma mudstone. The top of the Plenus Marl Formation is a variably eroded unconformity surface, and the unit may have been completely stripped from some structural highs.

Herring Formation

The Herring Formation (Figure 42) is generally considered to be of Turanian age, although the age of its top is disputed. The formation includes two units with high velocities (G1 and G3/4) that are composed of microcrystalline, thin-bedded, often hard, chalk. Both are topped by a thin 'hot shale' band (although this is not seen in G1 in (Figure 42)). Separating these units is an interval dominated by calcareous mudstone and thin, interbedded chalk (G2).

The Herring formation, particularly the high-velocity chalk facies, is most thickly developed in the Peterhead Sub-basins and in the south-east Witch Ground Graben (250 m). It thins rapidly northwards within that graben and on to the Halibut Platform, where a more marly variant dominates. Unlike the Hidra and Plenus Marl formations, the Herring Formation prominently oversteps the platforms and intrabasin highs to the extent that only the uppermost unit is locally represented. The formation is virtually absent in the Inner Moray Firth Basin due both to nondeposition and erosion.

Flounder Formation

The Flounder Formation (Figure 43) was largely deposited during Senonian times, although the basal Fl section is considered to be of late Turonian age (Figure 39). No high-gamma mudstone development is present in the Senonian part of the section, but marl-chalk cycles are recorded that are apparently not influenced by any major break or onlap episode during the latest Coniacian early Santonian. The formation is up to 500 m thick, much thicker than earlier Chalk Group formations, and displays much less lithological variety. Each cycle is essentially a continuous gradation between chalk and marl, with very subdued changes in log response. A cyclic variation is apparent at the [00 m scale in the basinal sections, and gives the basis for a subdivision into the four units (F1–F4) illustrated in (Figure 43). The topmost F4 unit has a particularly well-defined marly section that marks the top of the Flounder Formation over the whole of the Moray Firth and central North Sea areas.

The Flounder Formation is widely distributed and covers many of the major intrabasinal highs. Onlap is evident in peripheral areas such as in the Inner Moray Firth Basin and over the Renee Ridge where only the uppermost part of the F4 unit is present. Thickness variations occur within individual units that are apparently not related to the underlying highs and basins. An intraformational unconformity or hiatus is recognised between the F2 and F3 units, and the amount of erosion tends to control the overall thickness of the formation. A manly interval at the top of the formation commonly shows evidence of having been eroded before deposition of the overlying Tor Formation.

Tor Formation

The major cyclical pattern established during deposition of the Flounder Formation was disrupted at the end of Campanian times by a period of erosion. This was followed by the last and most extensive transgression of the Late Cretaceous. This led to the deposition of the Tor Formation (Figure 44), probably over the whole of the North Sea region, for its present limits are determined by later erosion. The Tor Formation forms a virtually continuous blanket deposit about 200m thick over the report area, and seismic data show that it reaches its thickest development of 500 m in the Wick Sub-Basin.

The Tor Formation is almost entirely of Maastrichtian age, although its lowermost section may be late Campanian. The top of the formation forms a strong, regionally developed seismic reflector, and marks the top of the Cretaceous System.

Over much of the report area the Tor Formation is characterised by its almost featureless log response and high, uniform velocity (Deegan and Scull, 1977); these reflect a chalk lithology with a few thin marl)^, or mudstone bands. The Tor Formation chalk is generally structureless, but thin turbidite units near the top give rise to the variable sonic log response at this level shown in (Figure 44).

Thin sequences of chalk found on platforms and major structural highs are normally assigned to this formation, and as such it represents the most widespread Chalk Group deposit. It is present at the western limit of the Upper Cretaceous in the Inner Moray Firth Basin where it directly overlies the Hidra Formation greensand in a setting similiar to that seen in Northern Ireland (Wilson, 1972). BGS Borehole BH74/19, located very close to the western limit of the Upper Cretaceous, drilled 78.4 m of white chalk with flints before penetrating probable Lower Cretaceous sandstone; the chalk has been dated as Conia. cian to Maastrichtian but probably belongs to the Tor Formation.

The formation exhibits thickness variations and locally thins into the basin centres. It is not yet possible to determine whether thickness variation is due to onlap, the presence of condensed sequences, or postdepositional erosion, although seismic evidence suggests that onlap occurs elsewhere in the North Sea.

Ekofisk Formation

The Ekofisk Formation (Figure 45) is assigned to the Danian, the lowest stage of the Paleocene. Its base marks the Cretaceous-Tertiary boundary. It is the only Chalk Group formation to display rapid lateral facies variation, and so cannot be characterised by a standard wireline log response. In all sections, the velocity is markedly lower than in the underlying Tor Formation, and is commonly erratic. The formation has a fairly uniform thickness of 50 to 80 m, but is not as extensive as older chalk formations.

Knox et al., (1981) subdivided the formation in the Outer Moray Firth Basin into a lower manly unit, a middle chalky unit and an upper manly or sandy unit. These are not developed regionally, although some form of basal marl or shaly mudstone unit is generally present. Over much of the southern pan of the Moray Firth the formation comprises a single argillaceous chalk unit, in contrast to sand-dominated parts in the Witch Ground Graben and Halibut Horst (Figure 40).

Chapter 10 Tertiary

Palaeogene

Palaeogene rocks are present in the eastern Moray Firth, but are absent in the west and on the surrounding landmass. The sediments extend from the Moray Firth into the central and northern North Sea and thicken eastwards. Paleocene and Eocene rocks reach a combined maximum thickness of over 1100 m north-east of the Halibut Horst (Figure 46), and the Oligocene sediments exceed 400 m. The westward thinning towards the platform area appears to reflect both the depositional regime and later erosion; the sediments thin towards their erosive limit and often exhibit facies changes which suggest that there was a shoreline to the west during their deposition.

Palaeogene strata are part of the North Sea post-rift sedimentary sequence deposited in a basin that was subsiding as a result of lithospheric cooling (Donato and Tully, 1981). There was little fault activity during the Palaeogene other than on normal faults bounding the Halibut Horst, and movement resulting from salt withdrawal in the south-east of the area. This quiescence was in marked contrast to the contemporaneous igneous activity on the west coast of Scotland and other areas around the north-east Atlantic, which was a consequence of the late Paleocene to early Eocene opening of that ocean. However, the Paleocene and Eocene sedimentary sequences of the Moray Firth do include many ash layers associated with this volcanic activity (Knox and Morton, 1988).

A marked eustatic sea-level drop at the end of Maastrichtian times (Haq et al., 1987; Stewart, 1987) may have temporarily exposed the landmasses of the Orkney/Shetland Platform and Scottish Highlands. It led to local sand deposition in the Chalk Group Ekofisk Formation of the inner Moray Firth area. Sedimentation of the Chalk Group extended into mid-Danian times, but is not described here as it is included in the Cretaceous section. A more substantial drop in relative sea level took place towards the end of Danian times, resulting in a marked change in facies; coarse elastics of submarine-fan origin were deposited on the hemipelagic sediments of the Chalk Group. Later in the Paleocene and much of the Eocene, delta systems developed which built out eastwards; beyond these, hemipelagic sedimentation remained dominant. The Oligocene epoch was chiefly an interval of global marine regression (Vail et al., 1977; Haq et al., 1987), and local thermal subsidence centred on the graben areas of the North Sea Basin continued. The Scottish Highlands and uplifted Orkney–Shetland Platform remained important areas of sediment supply for the Moray Firth area throughout the Palaeogene.

Post-Chalk Group Paleocene

The first lithostratigraphic subdivision of the upper Paleocene succession was by Deegan and Scull (1977), who divided it into three groups (Figure 47). The Montrose Group at the base comprises mainly submarine-fan sediments, the overlying Moray Group is formed of deltaic sediments, and the Rogaland Group comprises distal argillaceous rocks whose deposition continued into the

Eocene. Although the Rogaland Group was originally believed to be laterally equivalent to the Moray Group by Deegan and Scull (1977), a modification of their scheme forms the basis of description here. Reference is also made to the genetic subdivision by Knox et al. (1981) and the seismostratigraphic scheme of Stewart (1987). (Figure 47) attempts to show the relationships of the subdivisions made in the three publications.

Montrose Group

Deposition of the Montrose Group was initiated by a major late Danian regression that was accompanied by uplift and erosion of land areas to the north and west. Parker (1975) was the first to recognise that the generally sandy Montrose Group includes submarine-fan sediments which combine to form a wedge-shaped deposit that thickens eastwards, particularly around the Halibut Horst where over 700 m of sediment are present (Figure 48). The sediment-flow processes may however have been concentrated in the axial zones of the basin (Rochow, 1981), for deposits are thinner and more argillaceous along the basin margins and over the Halibut Horst.

The Montrose Group is divided into three units; the Maureen Formation, the Andrew Formation, and an upper Undifferentiated unit (Deegan and Scull, 1977). The Maureen and Andrew formations are predominantly composed of sandstone with argillaceous interbeds, and are of late Danian to early Thanetian age (Figure 47).

The Danian Maureen Formation is commonly conglomeratic, and in its lower part includes reworked Cretaceous and Danian chalk clasts that were presumably derived from the intrabasinal highs and the platform area to the west. The upper boundary of the Maureen Formation was described by Deegan and Scull (1977) as marking the start of submarine-fan and turbidity deposition. The overlying Andrew Formation is finer grained and fines upwards; it consists predominantly of fine- to medium-grained sandstone with some mudstone interbeds which have distinctively lower seismic velocities.

Many of the sandstone beds in the Maureen and Andrew formations are massive, commonly with fluidisation structures, and were interpreted by Knox et al. (1981) as being the products of mass-flow processes. The interbedded mudstones and shaly mudstones are typically pale green to grey, micaceous and silty, and were probably deposited by low-density turbidity currents or hemipelagic processes. The biota are characterised by a moderately diverse dinoflagellate cyst flora and by agglutinating foraminifera; their presence suggests deposition in a normal marine environment, albeit one with a high elastic input. On the evidence of heavy mineral composition and sediment distribution patterns, Knox et al. (1981) proposed that these sediments originated from the Orkney/Shetland Platform. They were deposited in a lowstand setting in which much of the sedimentation process bypassed the relatively marginal area of the Moray Firth (Stewart, 1987), so that maximum deposition occurred in the Central Graben area to the east.

The early Thanetian Andrew Formation is overlain by hemipelagic mudstones of the Undifferentiated unit, which is interpreted by Stewart (1987) as having been deposited during a highstand of sea level following a transgressive phase. The main locus of deposition shifted westwards from the Central Graben into the Moray Firth area, with maximum deposition in the outer Moray Firth. The lower part of the unit (equivalent to Stewart's sequence 4) occurs extensively in the outer Moray Firth area where it is a tuffaceous sandstone adjacent to the Halibut Horst. Elsewhere it is a volcaniclastic mudstone (Figure 49). The sandstone is green, fine to medium grained and tuffaceous with palagonitised glass shards and detrital feldspar (Knox et al., 1981; Knox and Morton, 1983; Stewart, 1987). The volcaniclastic mudstone is the 'basal Thanetian tuff' of Jacqué and Thouvenin (1975).

The middle part of the unit (Stewart's sequence 5) marks a return to large-scale, submarine-fan sandstone deposition (Knox et al., 1981). The heavy mineral assemblages again indicate that much of the sand is derived from the Scottish highland areas. The succession is thickest adjacent to the Halibut Horst and thins rapidly eastwards; the sands are developed in units up to 100 m thick and separated by tufaceous shales.

The uppermost sediments (Stewart's sequence 6) occur throughout the outer area, and are predominantly greyish brown, non-tuffaceous mudstones characterised by a microfauna of low-diversity agglutinating foraminifera. Deposition was in an open-marine environment during the period of greatest transgression associated with the Undifferentated unit (Stewart. 1987).

Moray Group

The late Thanetian Moray Group occurs extensively in the outer Moray Firth area (Figure 50) where it is up to 700 m thick. In general, the sequence comprises shelf or deltaic arenaceous sediments which on seismic sections are clearly seen to thicken and prograde eastwards towards a delta front (Rochow, 1981). To the east of the delta front, the Moray Group sediments attenuate and become shaly. They and their westward lateral equivalents are believed to have been derived from the Scottish Highlands (Knox et al., 1981). and were laid down during a regressive period. The Moray Group was divided by Deegan and Scull (1977) into the Dornoch and Bendy formations (Figure 47). Comparison of the Montrose and Moray group thickness maps (Figure 48) and (Figure 50) suggests that by the time of Moray Group deposition the Halibut Horst was no longer a positive structural feature.

The Dornoch Formation thickens to approximately 500m adjacent to the delta front, where it thins rapidly eastwards. The formation has been described by Rochow (1981) as a composite deposit whose character results from shifting deltaic environments; it is considered to represent the eastward progradation of shelf facies during a high-stand of sea level. Its most westerly portion is the landward equivalent of the submarine-fan sediments of the Forties Formation of the Central Graben (Stewart, 1987) which extend into the extreme south-east of the study area. The Dornoch Formation is seen on seismic sections as easterly prograding clinoforms, and comprises an upward-coarsening unit of grey silty mudstone interbedded with fine- to coarse-grained sandstone. These sediments may be laterally equivalent to the mudstones and siltstones of the Sete Formation (Knox et al., 1981), although Stewart (1987) considered the Sele Formation strata to be younger than those of the Moray Group (Figure 51).

The Beauly Formation, which overlies the Dornoch Formation. is distinguished from it by the presence of lignite beds interbedded with fine- to coarse-grained. poorly sorted sandstones and greyish brown mudstones. It rarely exceeds 120m in thickness. The distribution of lignite beds (Figure 50) is readily apparent on seismic sections, for they return a strong high-amplitude reflection. Their eastern limit marks the Moray Group delta front. Beauly Formation sediments are interpreted by Knox et al., (1981) as having been deposited in a fluviolacustrine environment behind the advancing delta.

Upper Paleocene to Eocene

Rogaland Group

The Rogaland Group of Deegan and Scull (1977) includes the Sele and Balder formations, whose sedimentation extended from the late Paleocene into Eocene times.

Sele Formation

The argillaceous Sele Formation occurs over much of the North Sea Basin. principally to the east of the Moray Group delta front. However, the precise relationship between the Moray Group and the Sele Formation at the delta front remains to be fully resolved (Figure 51). The Sele Formation is a tuffaceous shaly mudstone and is some 40 m thick in wells described by Deegan and Scull (1977). They thought it to be the distal equivalent of the Moray Group, but Stewart (1987) considered his Sequence 9 to overlie the deltaic sediments of the Moray Group. Fish and plant remains are common within the Sele Formation, but foraminifera are uncommon.

Balder Formation

The Balder Formation (Figure 47) is of early Eocene age and consists of greenish blue, tuffaceous, silty mudstones interbedded with significant volcanic ash bands. The unit is typically 15 to 25 m thick and overlies the Sele Formation strata in the eastern part of the Outer Moray Firth Basin (Figure 51), but locally overlies Beauly Formation sediment west of the delta front (Knox et al., 1981; Stewart, 1987), The Balder Formation ash-fall deposits occur over much of the North Sea Basin and north-west Europe, and their deposition coincided with the main phase of rifting in the north-east Atlantic (Knox and Morton. 1988).

Hordaland Group

The remainder of the Eocene strata above the Balder Formation are included in the Hordaland Group of Deegan and Scull (1977), although these authors did not formally define lithostratigraphic units in the Moray Firth area. As a result of late Tertiary erosion, the group generally thickens eastwards to a maximum approaching 800 m in the report area (Figure 52). Seismic sections indicate that patterns of sedimentation during Eocene times were similiar to those during the Paleocene; phases of eastward progradation of shelf and deltaic sediments were punctuated by intervals of predominantly basinal, hemipelagic sedimentation.

The basinal sediments are generally mudstones, or in places siltstones, commonly carbonaceous and interbedded with thin limestones. Some coarse-grained elastic deposits are also associated with these basinal deposits.

The basal unit is characterised by reddish brown and pale grey mudstones of early Ypresian age with higher gamma-ray response and lower velocity than the overlying sediments. The unit thickens eastwards to over 60 m and may locally contain thin siltstones and sandstones. It is described as sequence 10 by Stewart (1987), who proposed that it is disconformably overlain by the remaining Hordaland Group sediments. The base of sequence 10 contains a predominantly planktonic foraminiferal assemblage suggestive of open-marine depositional conditions, whereas the agglutinated assemblage that predominates towards the top of the sequence is indicative of a return to a more restricted marine environment (Stewart, 1987).

The basinal sediments overlying sequence 10 are in places over 300 m thick, and are characterised by siltstones and mudstones with a low, and reasonably constant, gamma-ray response. Sandstone units are present locally, and are typically 20 m in thickness, but can be up to 100 m thick; these are believed to have been deposited by gravity-flow processes. Farther west, the shelf and deltaic deposits above sequence 10 are characterised by upward-coarsening units. These grade from clays to medium-grained sand-stories, and include a few lignites and thin limestone beds. More than one episode of progradation is inferred both by onlap of seismic reflectors on to inclined beds believed to be delta fronts, and by the presence in some wells of stacked upward-coarsening cycles.

Oligocene

At the end of Eocene time there was a sharp climatic change, with a 12°C drop in North Sea bottom temperatures (Buchardt, 1978) which caused increased oxygenation and a fall in the carbonate compensation depth. Vail et al. (1977) postulated a major global eustatic fall in sea level in the mid-Oligocene, and there were several fluctuations until the middle Miocene.

Oligocene sediments are restricted to the Outer Moray Firth Basin east of 1°W, where they comprise a mixed assemblage of lithologies ranging from clays to sandstones with closely spaced, thin interbeds of sand and mud.

There is an overall upward increase in sand content that reflects a regressive trend, although the earlier pattern of finer-grained deposition in the east continued. The foraminifera are generally typical of those in a marine, temperate, shelf environment.

The Eocene/Oligocene boundary is placed at the base of calcareous nannoplankton zone NP 21 (Martini, 1971), but this is difficult to identify in the North Sea because of the absence of diagnostic species (King. 1983). In the North Sea Basin as a whole, there is generally a lack of significant lithological change across the boundary (Sutter, 1980). The Oligocene sediments of the North Sea were not differentiated by Deegan and Scull (1977), but are commonly placed within their Hordaland Group. However, Sutter (1980) described them as the Thurso Formation, which is defined in well 15/17-6, near the Piper Oilfield; he recognised the formational base at a sharp increase in gamma-ray values, usually accompanied by changes in resistivity and sonic responses. The Oligocene has a maximum thickness of over 400 m near the centre of the Witch Ground Graben (Figure 53).

The lower part of the Oligocene has a variable lithology that comprises bluish green to greyish brown, silty, slightly calcareous clays with brown to greenish brown, argillaceous siltstones and thin (3 to 6 m), glauconitic, very friable. silty sandstones. These sediments contain a sparse foraminiferal assemblage of little diversity; some forms are indicative of the maximum marine transgression of the early Oligocene, with water depths of about 150 m, although planktonic foraminifera are only locally abundant. Bivalves, echinoid spines and ostracods are often small and delicate in form, and their well-preserved nature suggests a low-energy environment of deposition.

The upper part of the Oligocene is lithologically similiar, but generally coarser grained than below. The upward-increasing grain size is accompanied by increasing numbers of large shell fragments. There is very abundant shell debris, subrounded to subangular quartz, glauconite, muscovite and minor lignite. Although some sands are clean and well sorted, most are argillaceous shell sands with fine bedding. Within the Witch Ground Graben. the upper part of the unit does not coarsen, but consists of mudstones or siltstones, with thin limestone bands towards the central and southern parts of the basin. Foraminiferal assemblages are similiar to those in the lower part, but some changes reflect decreasing water depths. Bivalves, gastropods, scaphopods, echinoid fragments and bryozoans are common, and fish teeth, otoliths and vertebrae also occur. Many larger shells are rounded and broken, indicating transport, possibly from beach to deeper water. The uppermost shell sands are thought to include high-energy lag deposits caused by strong tidal or wave action when the winnowing of the sediments resulted in a high concentration of organic remains.

Well 8/27-1 (Figure 53) contains a sequence of sands in which several periods of non deposition may be represented; these sands are thought to be offshore sand shoals or bars with high-energy reworking in places. In the Piper Oilfield region, which was probably not so close to the contemporary shoreline, Oligocene sediments in well 15/17-7 appear to have been deposited farther from the coast than those in well 15/11-3 to the west. Farther south around latitude 58°N, near the axial zone of the Outer Moray Firth Basin. the Oligocene is predominantly argillaceous and contains only minor sands of possible delta-front or marine-shelf type. These facts, together with the presence of thin limestone horizons, suggest that the Witch Ground Graben remained an area of deeper water, possibly in an outer-shelf environment.

Neogene

From mid-Eocene time onwards, Tertiary sedimentation in the central areas of the North Sea Basin was dominated by monotonous sequences of marine muds and silts. although sands were deposited at the periphery. The Palaeogene/Neogene boundary is represented in the North Sea sequence by an unconformity which Bjørslev Nielsen et al, (1986) used to distinguish the Hordaland Group below from the Nordland Group above, the latter extending to the present sea bed. However, Deegan and Scull (1977), who originally defined the groups, placed their boundary at a marked wireline log change at an unconformity dated as Middle Miocene. In the North Sea, the base of the Neogene is commonly taken at a sharp change from Oligocene shell sands to Miocene muds and silts; this coincided with an interval of low sea level. At the end of the Palaeogene, water depths had decreased such that the North Sea was at its smallest extent during the Tertiary (Lovell, 1986). It has probably widened steadily since, other than during glacial intervals. Neogene thicknesses increase up to 650 m in the south-east of the report area, although the main depocentre was farther south-east in the Central Graben (Björslev Nielsen et al., 1986). Estimates of subsidence for the North Sea Basin as a whole suggest that rates increased from 3 to 7 cm/ka during the Miocene. and to 12 cm/ka by the end of the Pliocene (Clarke, 1973). The increase in subsidence rate is reflected by a greater thickness of Pliocene (up to 400 m) than Miocene sediments (up to 300 m), despite the latter being deposited over a longer interval.

The Neogene was a time when extensive, deep weathering under temperate conditions occurred on land (Hall, 1985), The only known Neogene sediments onshore around the Moray Firth are the isolated bodies of probably Pliocene gravels in Aberdeenshire commonly known as the 'Buchan Gravels' (Hall, 1984), and a 7.8 m thick glacially transported mass of Miocene clay at Leavad in Caithness thought to be derived from the Moray Firth (Crampton and Currurhers, 1914).

Offshore, Neogene sediments are generally restricted to the region east of 1°W (Figure 2), and it is unlikely they were deposited very much farther west because the Inner Moray Firth Basin was uplifted at that time. The sediments occur beneath a commonly thick cover of Quaternary sediments, except where they crop out in the Fladen Deeps. Seismic profiles show a sequence of eastward-dipping wedges of sediment resting unconformably upon the Palaeogene. They are overlain conformably in the east by lower Pleistocene deposits, but in the west, upper Pleistocene glacial sediments overlie them unconformably.

Examination of Neogene sediments has been largely limited to cuttings from the few released wells. but processed shallow seismic records reveal low-angle, southeasterly dipping reflectors that suggest a sequence of alternating lithologies. Some reflectors with high amplitudes have been attributed to sand or lignite-rich sand layers, and bright spots suggestive of gas-rich horizons have been identified. No regional unconformities have been detected within the Neogene sequence.

Erosion of the Scottish Mainland, the East Shetland Platform, and parts of Scandinavia led, in marginal areas like parts of the Moray Firth, to the deposition of deltaic and coastal barrier sands from mid-Eocene time onwards. Several marine transgressions truncated these regressive units, and are represented by thin, sublittoral beds rich in glauconite or phosphatised faecal pellets (King, 1983). Micropalaeontological studies have revealed two biofacies in the Neogene: an inner. sublittoral biofacies around the edge of the basin in water depths up to 50m, and an outer, sublittoral, epibathyal biofacies over much of the basin that indicates water depths between 50 and 200 m.

Clay mineralogy in the North Sea Basin shows a gradual decrease in smectite and a concomitant increase in illite through the Neogene (Karlsson et al., 1979; Berstad and Dypvik, 1982). This probably corresponds to a reduction in the amount of volcanic glass available for alteration within the fines, and to an increase in elastic input. The upper Miocene shows increases in illite and the appearance of chlorite, together with a reduction in kaolinite that corresponds with the change to a colder climate and a reduction in weathering intensity. The ratios of day minerals remained relatively stable throughout the Pliocene; some anomalously rich occurrences of kaolinite are thought to be caused by the reworking of older material.

Miocene

Miocene sediments only occur east of about 1°W (Figure 54), and are generally overlain by Pliocene deposits (Figure 2). The sediments increase in thickness to the south-east where they are up to 300 m thick. (Figure 54) shows that the predominant sand and sandstone lithologies in the north and west change south-eastwards through siltstones to mudstones in the Witch Ground Graben. There appear to be two tongues of coarser material trending east-south-east that correspond with the ridges outlined by the contour on the top of the Miocene; these may indicate the position and direction of sediment input into the basin from the north-west. The sands, which may be littoral to sublittoral are mostly fine grained. but locally medium to coarse-grained. They are often glauconite-rich, and common, contain abundant shell fragments including bivalves echinoids and gastropods. Lignite is a common component of Miocene sands, particularly in the south-west Glauconitic nodules occur in the clay facies of the Witch Ground Graben, which was probably deposited in an outer-shelf environment.

The only sea-bed occurrences of Tertiary sediment in the Moray Firth area are on the flanks of the Fladen Deeps where fragments of limestone and lignite have beer recovered; the Fladen Deeps are a series of open channel that extend down to 100 m below the level of the surrounding sea floor (Figure 54). A trawled specimen of limestone with silicified shells and minor interstitial wood material was examined by Newton (1916); he suggested that while it is lithologically similiar to the Pliocene Coralline Crag of East Anglia, it is probably of Miocene age or the basis of its molluscan fauna which indicates a littoral or sublittoral environment in water less than 30 m deep.

Pliocene

Onshore, the 'Buchan Gravels' are probably of Pliocene age (Flett and Read, 1921). They are made up of the Windyhills and Buchan Ridge gravels, which are thought to be of fluviatile and littoral origin respectively (McMillan and Merrit, 1980). They have been subjected to, or are derived from, deep Tertiary weathering that produced an abundant kaolinitic matrix and a restricted clast lithology of highly stable metaquartzite, vein quartz and flint (Hall, 1984; 1985). Offshore, Pliocene sediments are up to 150 m thick in the southern part of the Witch Ground Graben, but thin westwards where they extend only slightly farther than 1° W. Near their western limit, the deposits occur either as a thin layer or as small pockets beneath a Quaternary cover (Figure 55). The Pleistocene lies conformably on the Pliocene in the centre of the North Sea Basin, but unconformably nearer the coast.

The lower Pliocene is locally absent, usually where Miocene deposits are also absent, implying mid-Pliocene erosion. During the Pliocene there was a significant westward expansion of the Miocene clay facies; only west of 0° are significant amounts of sand recorded within the muds and mudstones. Shells and shell fragments are uncommon or absent within the clay facies, and there are only a few occurrences of the glauconite and lignite which are more common in the sandier facies. The peripheral, predominantly sandy, facies is recorded only in well 8/27-1.

Defining the Pliocene/Pleistocene boundary is complicated by inconsistencies in definition. In the southern North Sea, the boundary is placed at about 2.3 Ma, when the first indications of a cold climate occur in Dutch sequences (Zagvijn. 1985). In the northern and central parts of the North Sea, a boundary has been taken close to the top of the Olduvai palaeomagnetic event (Bowen. 1978), although there is often no strong seismic reflector near its assigned position in boreholes. However, a crenulate reflector has been identified close to this level on several processed seismic records: it is often characterised by bright spots, and hyperbolic reflections which may suggest the presence of boulders. These imply erosion either prior to, or during a marine transgression thought to have occurred close to the Pliocene/Pleistocene boundary.

BGS borehole BH81/19 encountered 60 m of clay, sand and lignite beneath Quaternary sediment (Figure 55). Within the lignitic clay are several layers of kaolin which have been shown to be well ordered; these are possibly comparable with the kaolinitic layers in the Buchan Gravels. The dinoflagellate cyst assemblage obtained from the lower part of this borehole sequence is similiar to that of the early Pleistocene Winterton Shoal Formation of the southern North Sea (R Harland. written communication, 1983). However, pollen from the lignitic layers shows an overwhelming predominance of hickory, a species thought to have been virtually absent from Europe since the end of the Tertiary, and which is only a very minor constituent of the earliest Pleistocene (M C Boulter, written communication. 1986). Such evidence suggests at least a late Pliocene age for the sequence. and indicates differences in temperature and stratigraphy between the Moray Firth and the southern North Sea.

The Pliocene Pleistocene boundary zone was encountered at the base of BGS borehole BH77/2 (Figure 55) in which there is a uniform presence of Pliocene foraminifera, and low numbers of the dinoflagellate cyst Amiculosphaera umbracula Harland from 192 m to 217 m (TD) below sea bed. Pliocene sediments in the borehole comprise greenish grey clay with shell fragments, carbonaceous matter and small burrows. A prominent seismic reflector at about 200 m depth on nearby geophysical profiles has been taken as the top of the Pliocene in the Fladen area (BGS Fladen Quaternary Geology sheet).

Chapter 11 Quaternary

The Quaternary comprises the Pleistocene and Holocene epochs and has been a time of dramatic fluctuation in climate. There were periods of severe erosion by glacial processes, rapid changes in sea level and very high sedimentation rates. Seismic profiling in the outer Moray Firth shows a Quaternary sequence of westward-thinning wedges of subparallel reflectors with numerous levels of erosion, particularly in the upper part. The Quaternary deposits are over 400 m thick in the Witch Ground Basin (Figure 56), but thin westwards to become generally less than 20m thick or absent west of 2°W, except in five small Quaternary basins off the southern shore of the firth (Chestier and Lawson, 1983).

Early workers, most notably Jamieson (1866; 1906), found evidence that ice had moved from the Moray Firth on to the coast, and although up to three major periods of glaciation are reported to have affected the area (Bremner. 1934; Sutherland, 1984), no irrefutable evidence of pre-Weichselian glaciations has been presented. Recent summaries of the onshore Quaternary sequences around the Moray Firth are provided by Hall (1984) and Hall and Connell (1990).

There is controversy over the age of the last major glaciation in Buchan, Caithness and the Orkney Islands. Several workers have suggested that these areas were predominantly ice-free during the late Weichselian (Synge. 1956: Sutherland. 1984; Rae, 1976). They propose that the last major ice advance that deposited the Shelly boulder clay of Caithness was of early Weichselian age or older. This has been refuted by Clapperton and Sugden (1975) and Hall and Whittington (1989), who consider that the late Weichselian glaciation was more extensive. On Fair Isle, immediately north of the study area, two glaciations are presumed to have occurred during the Weichselian; the island was covered initially by ice moving from the east, then by ice crossing from north-west to south-east and transporting erratics derived from Shetland (Flinn, 1978).

Almost all offshore Quaternary sediments were initially attributed by BGS to the last glaciation (Holmes, 1977; Chesher and Lawson, 1983). However, more recent work has suggested that much of the succession in the North Sea is of early Pleistocene age and that Weichselian deposits rarely exceed 50 m in thickness. This therefore contrasts with the situation on land, where the Quaternary deposits are thought to be almost entirely of Weichselian age, and rarely exceed 25 m in thickness.

Quaternary sedimentation in the Moray Firth was controlled by the same tectonic influences that had been active throughout the Tertiary, resulting in continued subsidence. Subsidence rates during late Quaternary times in the outer Moray Firth have been estimated at between 0.6 and 0.9 m/ka (Sejrup et al., 1987). This agrees well with an estimate of more than 0.5 m/ka during the whole of the Quaternary in the central North Sea (Clarke, 1973). However, superimposed on the overall subsidence of the North Sea Basin were glacioisostatic and eustatic effects which resulted in the development of a complex sequence of sediment packages bounded by erosion surfaces.

The outer Moray Firth succession can be broadly subdivided into a lower sequence of predominantly early Pleistocene marine and deltaic sediments, and an upper sequence of largely glacial and glaciomarine deposits of middle Pleistocene to Holocene age. Within the middle Pleistocene sequence there is an extensive erosion surface of Elsterian age that forms a very prominent regional seismic reflector. The sequence in the outer Moray Firth has been divided into formations (Figure 57) whose distribution is shown in (Figure 58). The sequence in the inner Moray Firth is described separately.

Aberdeen Ground Formation

The oldest Quaternary sediments are of early to middle Pleistocene age (Eburonian to Elsterian) and are termed the Aberdeen Ground Formation (Figure 57). They occur in much of the eastern and central parts of the area where they are up to 200m thick (Stoker et al., 1985). The charactcristic appearance of the Aberdeen Ground Formation on sparker records is as a series of strong, parallel reflectors that onlap westwards (Figure 59). These are seen most clearly in the east where the formation is least affected by later phases of erosion. In the central part of the area, where the formation is thinner, the internal structure is less distinct and locally chaotic (Stoker et al., 1985). The acoustic character is consistent with deposition having occurred in a gently subsiding shelf environment.

The lower part of the formation is a thick sequence of monotonous muds deposited as sublittoral sediments in a broad marine basin (Bent, 1986). The sediment comprises dark greenish grey, stiff mud or sandy mud with scattered shells and shell fragments. Although predominantly massive, laminae and lenses of sandy silt are present, as are monosulphide bands and discrete bioturbated layers. Carbonaceous and sulphide material is disseminated throughout, and nodular ironstone is developed locally. There is a 16.5 m-thick layer of poorly sorted, structureless, very fine-grained muddy sand in BGS borehole BH75/33 that has an aerated appearance caused by secondary gas (Bent, 1986). The clay mineral assemblage, which changes little throughout this lower part of the formation, is dominated by illite with subordinate chlorite and kaolinite. A 30 mm-thick phosphate-rich (>10 per cent P₂O₅) bed occurs within the Aberdeen Ground Formation in borehole BH75/33 (Stoker and Bent, 1987). Thinly interlaminated muds and sands recorded in borehole BH82/16 are representative of the area between 1°W and 1°30′W; these sublittoral sediments are interrupted by sandy beds representing regressive periods. The evidence suggests that nearshore marine processes were predominant, and implies that the early Pleistocene coastline was considerably farther east than at present, similar to its position in late Pliocene times.

Both foraminiferal and dinoflagellate cyst assemblages in the lower part of the formation are consistent with deposition in a temperate marine environment, with some minor fluctuations in temperature. The lower Pleistocene dinoflagellate cysts Operculodinium israelianum (Rossignol) Wall and Tectatodinium pellitum Wall suggest south-temperate to almost subtropical conditions in a neritic environment (Harland. 1988a). Temperature fluctuation is implied by the sporadic presence of Achornosphaera andalousiensis Jandu Chene and Bitectatodinium tepthiense Wilson which suggest north-temperate to arctic conditions, although the low proportions of Proloperidinium spp, cysts preclude the presence of much sea ice (Harland, 1988a). The foraminiferal assemblages include Cassdulina feretis Tappan, a species which became extinct from the North Sea region about 400 000 years ago. Reworking of late Pliocene sediments is suggested by the identification of a few individuals of Cibicides grossa Ten Dam and Reinhold (Sejrup et al., 1987).

The sediments are generally reversely magnetised in the lower part of the Aberdeen Ground Formation (Figure 57), although a few normally magnetised samples have been attributed to the Jaramillo Event (Stoker et al., 1983). The early to middle Pleistocene boundary coincides with thy; Bruhnes-Matuyama magnetic polarity reversal, which is recorded within the Aberdeen Ground Formation. Magnetic reversals attributed to this event have been identified in several boreholes in the central North Sea (Stoker et al., 1983) and locally coincide with a break in sedimentation.

Although the lower part of the Aberdeen Ground Formation was deposited in a largely nonglacial environment, a glaciamarine facies occurs in the upper portion. However, two glacial stades recorded in the Moray Firth area and the central North Sea were of limited climatic intensity, and the deposits which represent them are very restricted in distribution (Bent, 1986). Sediments representing a glacial/interglacial/glacial cycle between 158 and 146m depth in borehole BH81/26 have been correlated with oxygen isotope stages 20-21-22 (Sejrup et al., 1987). The sediments at the base of this cycle are the oldest North Sea Quaternary deposits that suggest glacial conditions.

Above the Bruhnes-Matuyama reversal level is the 'Cromerian Complex' Stage. This is the earliest extensively identified period of glacigenic deposition in the area, and was a period during which the North Sea was cut off from the warmer waters of the North Atlantic Drift. 'Cromerian' deposits signify a marked change in the pattern of sedimentation in the area and show the increased importance of glacioisostatic influences on the depositional environment. This was primarily reflected by the establishment of shallow, hyposaline, arctic, marine conditions and the transition from marine to glaciomarine and glacial facies. An extensive ice sheet, ending as a tidewater glacier, covered the central and north-eastern pan of the Moray Firth during early 'Cromerian' time. Clans in the sediments are of Scottish origin. suggesting that the ice sheet extended from the British Isles rather than from Scandinavia. Beyond the ice front, proximal and distal glaciomarine sediments accumulated in an arctic sea, and subsequent westward retreat of the ice front was accompanied by limited westward migration of the glaciomarine environment. During the final retreat, up to 50 m of ice-marginal sediments were deposited in the south-eastern Moray Firth area, and minor re-advances of the ice front are shown by the interdigitation of the glacial and glaciomarine sediments (Bent, 1986).

A U-Th date on an Arctica islandica Linne shell from the top of the Aberdeen Ground Formation in BGS borehole BH81/26 gave a possible minimum age of 243.1 + 36.3/-27.1 ka (Sejrup et al., 1987): this shows that the formation extends ir to the Elsterian (Figure 57). Where early Elsterian deposits are preserved at the top of the Aberdeen Ground Formation, they indicate shallow, glaciomarine conditions. The presence of A. islandica and Cerastoderma sp. in these Elsterian deposits in borehole BH81/26 suggests boreal–arctic conditions with water depths less than 10 m (Seirup et al., 1987).

Post-Aberdeen ground formation erosion

The most regionally extensive seismic reflector in the North Sea Quaternary succession has been attributed to erosion associated with the Elsterian glaciation (Stoker et al., 1985: Cameron. et al., 1987). This erosion surface is the boundary between the Aberdeen Ground Formation and the overlying Fisher or Ling Bank formations (Figure 57) and is characterised by large channels which are locally in excess of 100 m deep (Figure 59). This major unconformity is commonly a level where gases accumulate, causing the formation of either bright spots or acoustic blanking on seismic records. Locally, the gas has percolated up into the sands of the overlying Ling Bank Formation (Figure 59). These gases were sampled in BGS borehole BH77/33 and proved to be hydrocarbons that are assumed to be petrogenic.

The extensive occurrence of the reflector and the ubiquitous presence of large channels suggest that the whole area was subjected to subglacial or proglacial erosive processes. The base lines of the channels dip towards the centre of the Witch Ground Basin. The method of channel formation has been the subject of much debate; the two processes usually considered are subglacial meltwater erosion and proglacial fluvioglacial modification of existing fluvial systems. Sudden meltwater discharges at a tidewater ice front is another possible method of formation (Long and Stoker, 1986). If channel formation was by subglacial meltwater erosion or direct glacial erosion, then there was an extensive Elsterian ice sheet with probable amalgamation of Scandinavian and British ice. If channel erosion was proglacial and subaerial, then a large expanse of the sea bed would have been exposed and subjected to denudation and channelling during the late Elsterian, with uplift and consequent downcutting apparently greatest towards the centre of the Witch Ground Basin. Given the latter situation, ice is likely to have been present in the inner Moray Firth to provide a source of meltwater.

Ling Bank Formation

The large channels cut into the Aberdeen Ground Formation contain up to three stages of fill representing a late glacial/interglacial/early glacial cycle. This infill has been termed the Ling Bank Formation (Figure 57), which was deposited from late Elsterian to early Saalian times (Cameron et al., 1987). On sparker records, the internal reflector pattern of the Ling Bank Formation varies from chaotic at the base to subhorizontal or oblique asymmetric higher up, and there is a distinct planar erosion surface at the top which marks the base of the overlying Fisher Formation (Stoker et al., 1985). Within the Moray Firth area. no Holsteinian interglacial Ling Bank Formation sediments have been sampled, although there is evidence for a weak amelioration at that time in borehole BH84/13 between 47 and 49 m. Outside the study area, the interglacial sediments are usually only a few metres thick and indicate temperate conditions, possibly warmer than at present (Stoker et al., 1985); they contain remains of the freshwater plant Azolla filiculoides Lambert (Griffin. 1984), which is absent from sediments younger than Holsteinian in Britain and north-west Europe (Godwin. 1975).

The type location for the Ling Bank Formation is BGS borehole BH81/34 which lies outside the study area (56º07.68′N 1°35.21′E). It has a rich dinoflagellate flora with only a low level of reworking. The assemblage in much of the formation reflects Holsteinian temperate conditions. for the lower part is dominated by Operculodinium centrocarpum (Deflandre and Cookson) Wall with Spiniferites spp. (Harland. 1988a). This indicates the marked influence of the North Atlantic Current (Harland. 1983). However, at the base of the Ling Bank Formation and through much of its upper part, there is a significant reduction in specimen numbers, with a marked decrease in the proportion of O. centrocarpum and a reciprocal increase in B. tepikiense and Spinifirites spp. while A. andalousiensis has also been recorded (Harland. 1988a). These changes imply less favourable conditions. The basal foraminiferal assemblage is dominated by Bulimina marginata d'Orbigny, but is succeeded by an interglacial fauna dominated by B. vickburghensis Cushman and Ellison, while Elphidium clavatum Cushman and Cassidulina reniforme Norvang dominate a deeper-water glaciomarine assemblage at the top (Bent. 1986).

Deposition of the upper part of the Ling Bank Formation took place in Saalian time, a period of climatic cooling that is evident in both the dinoflagellate cyst and foraminiferal assemblages. The change is also reflected in the transition from marine to glaciomarine sedimentation. Bent (1986) considered that at this time there was a tidewater ice front north and east of the Witch Ground Basin with subaqueous sedimentation. The ice front retreated rapidly westwards after the deposition of glaciomarine sediments at the top of the Ling Bank Formation; rapid isostaric recovery and sea bed emergence may have prevented any migration of the glaciomarine environment in the direction of ice retreat. However, this interpretation is uncertain due to poor borehole control and the likelihood of subsequent erosion.

Fisher Formation

Widespread erosion of the top of the Ling Bank Formation is defined by a major planar to subplanar unconformity formed during a marine transgression in mid-Saalian time. The marine deposits associated with the transgression are overlain by glacial and glaciomarine facies sediments attributed to a major glacial episode during the late Saalian when most of the inner Moray Firth was covered by ice. The unit above the unconformity (Figure 57) and (Figure 58) is known as the Fisher Formation (Stoker et al., 1985). On sparker records, the reflection pattern of the formation varies from parallel to chaotic, with occasional intraformational channelling up to 25 m deep: zones of ice-push deposits have also been noted.

An ice-proximal environment is suggested for much of the outer Moray Firth area during the deposition of the Fisher Formation; a large subaqueous fan built out from a tidewater glacier forming a series of overlapping fans (Bent, 1986). The lithologies include massive and stratified sands, diamicts and slump deposits; a range which reflects the variety of processes active in that environment. Subglacial deposition took place north of 58°30′N. and possibly also in the inner Moray Firth. During northward retreat of the ice front, a series of stillstands or re-advances occurred when large subaqueous moraines were deposited. These continue to form significant topographic highs at the sea bed today. They were first identified by Jansen (1976) who attributed them to Saalian ice, but later stated that the ridges contain interglacial material of Eemian age (Jansen et al., 1979). Analysis of core from BGS borehole BH84/13 has reaffirmed Jansen's original Saalian interpretation.

A 40 m-thick diamict in borehole BH81/26 has been interpreted as a basal till attributed to a Saalian glacial episode (Sejrup et al., 1987). However, regional studies place this diamict at the top of the Aberdeen Ground Formation. and indicate the absence of the Fisher Formation at this site (Bosies Bank Quaternary Geology sheer). Amino acid dating of the core has been inconclusive, for a wide range of ratios has been obtained, suggesting incorporation of shells of different ages. The clasts, often with glacial striae, are dominantly of granitic, schistose, basic igneous and red sandstone types consistent with derivation from the Scottish mainland. The diamict may have been deposited during a single glaciation, but minor changes in grain size distribution and carbonate content could reflect deposition during several advances of Saalian ice between 130 and 200 ka.

Coal Pit Formation

The Coal Pit Formation was deposited from late Saalian to early Weichselian times, and includes the end of a glacial period, the Eemian interglacial and the start of the last major glacial episode (Stoker et al., 1985). An Eemian age for the interglacial within the Coal Pit Formation is suggested by the final occurrence of Elphidium ustulatum Todd, which had become extinct in the North Sea by early Weichselian time (Gregory and Bridge. 1979). Interglacial deposits that include the uppermost in-situ occurrence of E. ustulatum have been identified in BGS boreholes BH75/33, BH77/2, and BH81/26 as well as at the Tartan Oilfield (Gregory and Bridge. 1979; Jansen and Hensey. 1981; Sejrup et al., 1987; Fladen Quaternary Geology sheet).

Foraminiferal assemblages show significant numbers of B. marginata, and dinoflagellate cyst assemblages are dominated by O. centrocarpum, indicating the warming influence of the North Atlantic Current. Reversed palaeomagnetic sections identified in boreholes BH75/33, BH77/2 and BH81/26 during the amelioration have been tentatively correlated with the Blake Event of about 105 to 115 ka. (Stoker et al., 1985; Sejrup et al., 1987; Fladen Quaternary Geology sheet).

The Coal Pit Formation fills channels that are rarely more than 120 m deep. in a similar manner to the Ling Bank Formation. It also occurs as a blanket deposit up to 40 m thick, particularly in the east of the area within the Witch Ground Basin. On sparker profiles the internal reflection pattern varies from chaotic to subparallel, with abundant discontinuity surfaces. in many cases, reflectors drape the basal surface (Stoker et al., 1985).

Swatchway Formation

Early Weichselian time is represented in the outer Moray Firth succession by the upper part of the Coal Pit Formation and the lower part of the overlying Swatchway Formation (Figure 57). The seismic reflection pattern of both formations is essentially structureless with uncommon subhorizontal reflectors, one of which is taken as their formational boundary.

The sediments in the lower part of the Swatchway Formation vary from muddy sand to slightly sandy mud with a few small lithic clasts. They are generally slightly over-consolidated. The foraminiferal assemblage is dominated by northern and Arctic forms, and is suggestive of high-arctic, shallow-water conditions (Jansen and Hensey. 1981).

The upper part of the Swatchway Formation is of late Weichselian age and is a reworked proximal glaciomarine deposit up to 25m thick. The poor dinoflagellate cyst assemblage contains high proportions of B. tepikienie together with specimens of Spiniferites elongates Reid, suggesting harsh climatic conditions associated with the Weichselian glacial maximum. A reversed shear strength profile is often recorded, and may be related either to loading by ice or to sediment freezing, perhaps due to submarine permafrost.

Post-Swatchway Formation sediments

Late Wcichselian sediments, which include the upper part of the Swatchway Formation, occur near the sea bed over most of the area. Vibrocores and boreholes, together with high resolution boomer records, provide more information for these deposits than is available lower in the sequence. However, seismic mapping has not been possible in the inner Moray Firth, where boreholes indicate differing environments of deposition. The interval from the maximum glaciation to the Younger Dryas/Holocene boundary at 10 ka is recorded in sediments of the Swatchway and Witch Ground formations in the outer Moray Firth area, and in the Forth Formation in the central portion of the study area. The sediments of the Witch Ground and Forth formations were initially ascribed a Flandrian age (McCave et al., 1977), but a late Weichselian age is now accepted (Jansen et al., 1979; Stoker et al., 1985).

Witch Ground Formation

The boundary between the Swatchway and Witch Ground formations is an irregular erosion surface marked on boomer or pinger records by a change from the lower unit with no significant internal reflectors to multilayered reflectors above. The layering in the Witch Ground Formation becomes more distinct both with depth and towards the centre of the Witch Ground Basin (Stoker et al., 1985), but the multi-layered appearance is locally disturbed by pockmarks both at the sea bed and buried within she sediment (Figure 59). The Witch Ground Formation is divided into three members; the Fladen and Witch members are of Weichselian age, and the Glenn Member is Holocene (Figure 57). The boundary between the Fladen and Witch members is identified by a change in seismic signature from well-defined, multilayered reflectors to more transparent and less continuous reflectors above. The Glenn Member is generally too thin to be identified seismically.

At the base of the Witch Ground Formation, seismic records; how depressions up to 25 to 300 m wide and 2 to 3 m deep; these decrease in size and become less common towards the basin centre, such that beneath 165 m below sea level the erosion surface is relatively smooth. This surface crops out locally to reveal the depressions as linear features trending subparallel to the bathymetric contours. This led Stoker and Long (1984) to interpret them as iceberg ploughrnarks or sea-ice keelmarks. There is often little lithotogical or geotechnical difference between the Witch Ground and Swatchway formations, and the erosion surface probably represents merely the last disturbance of the palaeo-seabed by sea-ice keels (Long et al., 1986; Paul and Jobson, 1989).

Fladen Member

The Fladen Member comprises soft, greyish brown, interbedded muds and silty to sandy muds that show gradational contacts and have abundant monosulphide layers. It is a glaciomarine sediment that was deposited distally in a shallow arctic sea affected by sea ire. There was no contact with temperate Nonh Atlantic waters, and the sea was probably low in nutrients and oxygen (Long et al., 1986). Shells from the sediments have been dated at 13.5 to 14 ka BP (Hedges et a]., 1988). The dinoflagellate cyst assemblage is characterised by a significant degree of reworking and is dominated by B. tepikiense, with specimens of Spiniferites frigidus Harland and Reid and A. andalousiensis indicating cold water. Round brown Protoperidinium cysts produced by non-photosynthesising dinoflagellates suggest periods of sea-ice cover (Harland. 1988b). Small variations in the shallow-water, arctic foraminiferal assemblage appear to relate to lithological changes, with the coarser-grained sandy muds and muddy sands containing a richer and more diverse assemblage than the finer-grained lithologies. The macrofauna also has a restricted diversity, and is dominated by the bivalves Portlandia artica Gray and Portlanda (Yoldiella) lenticula Moller (Long et al., 1986).

Witch Member

The Witch Member is formed of soft, brown, homogeneous muds without monosulphides. In most of the member, the dominance of the dinoflageilate cyst O.centrocarpum over B. tepikiense and the presence of the more temperate Spiniferites spp. suggests an amelioration. However, the benthonic foraminiferal assemblage is still dominantly of cold-water species, which may imply that the water mass was stratified. Macrofossils including P.(Y.) lenticula and Nuculana pernula Muller suggest that the water was warmer than during the deposition of the Fladen Member, but colder than at present. There was a decrease or absence of sea-ice cover concomitant with a greater influx of warmer Atlantic waters which has been correlated to the Bølling and/or Allerød interstadial.

Near the top of the Witch Member there is evidence for a brief period of harsh, cold climate. An increase in reworked cysts and the reappearance of the heterotrophic round brown Protoperidinium cysts suggest the return of sea-ice, but there is no apparent change in lithology. The continuing dominance of O. centrocarpum implies continued contact with the North Atlantic Current (Harland, 1988b). The foraminifera are all temperate, and imply fully saline deep water.

Shards of volcanic glass found in the Witch Member have been geochemically correlated to the Vedde Ash Bed in Norway. which is dated as 10 600 years BP (Long and Morton, 1987). The identification of this important chronostratigraphic marker places this cold phase of Witch Member deposition within the Younger Dryas of northwest Europe, which is the Loch Lomond Stadial of onshore Britain.

Glenn Member

The Glenn Member of the Witch Ground Formation is a thin layer of very well-sorted silt formed by pockmark reworking of underlying sediments. This a process during which gas escaping from pockmarks throws argillaceous sediment into the water column where currents carry away the finest fraction, but allow the silts to fall quickly on to the sea bed. predominantly back into the pockmarks. The Glenn Member is usually less than 0.3 m thick but is up to 2.5 m thick in the centre of some larger pockmarks and is absent in areas that lack sea-bed pockmarks. Very low Holocene sedimentation rates are indicated by both the identification of the Vedde Ash Bed at the top of the Witch Member, and detailed radiocarbon dating of two cores (Erlenkeuser. 1979).

Forth Formation

To the west of the Witch Ground Basin in the central Moray Firth, the upper Weichselian is termed the Forth Formation. This sequence is acoustically similar to the Witch Ground Formation, but tends to have less extensive internal reflectors and occurs in small basins and elongate channels. The base of the formation locally appears conformable, but is commonly a significant unconformity with channels up to 30m deep. The silt content of the muds increases westwards, where layers of muddy sand also occur; there are more clasts and fewer whole shells than to the east. Within the area of the Forth Formation are several north–south-trending ridges and hummocks approximately along longitude 1°40′W: these comprise pebbly muddy sands, and are interpreted as flow tills. The clasts from these deposits can be striated, and are derived from Scotland. The ridges are commonly exposed as bathymetric highs, and elsewhere they can be traced as buried features. Sparker records from this region show deformation structures interpreted as the result of ice-pushing of ridges or moraines by a grounded tidewater glacier that marked the maximum eastern extent (Figure 60) of the late Weichselian ice sheet (Bent, 1986).

Some zones may have been subaerially exposed beyond the glacier margin (Figure 60). They were subjected to desiccation by permafrost to produce the overconsolidation noted in the Coal Pit Formation which crops out in The southern part of the area (Peterhead Quaternary Geology sheet), Likewise, some areas on the flanks of Bressay Bank in the north-east may also have been exposed; this area is dissected by deep. partially infilled channels where the in-filling sediment is thought to be late-glacial to Holocene in age. Due to lack of reworking by currents, sedimentation rates are above average in the channels: they can therefore be used as modern analogues of the many infilled channels evident on seismic records from the North Sea.

The Inner Moray Firth

Examination of 28 shallow BGS boreholes from the inner Moray Firth has revealed seven lithological units which may range in age from mid-Weichselian to Holocene (Chesher and Lawson, 1983). There is little or no geophysical correlation between the boreholes, many of which are in separate basins, and no correlation has been proposed between these sediments and those described to the east.

The basal lithology (unit I) is a diamict comprising grey. pebbly, compact sandy clay and muddy sand interpreted as basal till. Associated with it are local developments of sand termed. unit II. The till occurs east and north of a line from Macduff to Dunbeath, and is locally at least 50 m thick. Radiocarbon assays from derived shell material in borehole BH74/17 (Figure 56) have been interpreted differently as yielding mid-Weichselian or younger ages (Chesher and Lawson. 1983). or pre-mid-Weichselian ages (Lawson in Harkness and Wilson. 1979).

Overlying the till are probable glaciomarine sequences (units III and IV) comprising grey, laminated, silty day with occasional clasts. Four radiocarbon dates from derived wood fragments recovered in borehole BH71/35 indicate an age of about 16 ka BP (Harkness and Wilson, 1979). A second diamict (unit V) above these glaciomarine deposits occurs south-west of the area of unit I and has been described as an ablation till (Chesher and Lawson, 1983). Unit VI is in part the lateral equivalent of this till horizon, and comprises up to 13 m of red, pink or brown mud which becomes sandy to the north-west. These sediments have been dated at the top of the sequence in borehole BH74/18 as 12 398 ± 100 years BP. The identification of two till-like units (units I and V) offshore mirrors results from Caithness and Buchan where two tills have been attributed to different ice streams during the same late Weichselian glaciation (Hall and Whittington, 1989).

Unit VII comprises pale olive-grey to olive-green. calcareous mud with few scattered pebbles, and was taken as Holocene in age by Chesher and Lawson (1983). However, part of unit VII in borehole BH72/20 has been subsequently recorded as Coal Pit Formation and is hence much older (Peterhead Quaternary Geology sheet). The fauna within this unit tends to suggest climatic conditions less favourable than today, and may indicate that the lower part of unit VII is at least late-glacial in age. Holocene sediment thicknesses are generally much greater in the inner Moray Firth than to the east, and a thickness of up to 47 m is recorded in borehole BH71/34.

The innermost firths

Sediments recovered from boreholes in the Cromarty Firth have been related to the Windermere Interstadial and the Loch Lomond Stadial (Peacock et al., 1980). Late-glacial sediments with a high boreal marine fauna also occur at Inverness below deltaic deposits of the Loch Lomond Stadial (Peacock, 1977). These sequences, together with evidence of former shorelines, show that marine incursion was very extensive soon after the late Weichselian glacier retreat (Synge, 1977). Onshore, there are locally thick. early postglacial marine sequences deposited during periods of marine transgression (Synge, 1977). The absence of deposits similar to the Errol Beds of south-east Scotland suggests that the innermost firths were probably under ice until 13.5 to 13 ka (Peacock et al., 1980).

Chapter 12 Sea-bed sediments

The sea floor of the Moray Firth mainly consists of Holocene sediments whose distribution (Figure 61) reflects both the glacial history of the area and the present hydrodynamic regime. Sediment input from the land is negligible at present, but biological and geological processes are local agents of sediment production. Bioturbation, and more particularly hydrodynamic processes, are responsible for reworking older deposits, particularly in areas of high tidal velocities.

The reworking of offshore Pleistocene deposits is. and has been, the principal source of lithic material. Rising sea level related to the main late-glacial melting eventually overtook isostatic uplift and culminated in the rapid rise in sea level at the start of the Holocene, 10 000 years ago. This resulted in an extensive lag of palimpsest sediments over much of the area (Bent, 1986), which continues to be reworked.

Coastal erosion is not an important sediment source, and fluvial sediment supply is limited to the southern and south-western shores of the inner Moray Firth. The fluvial sediment input into the Moray Firth has been calculated to be a minimum of 140 000 tonnes/year, and may reach as much as 460 000 tonnes/year (Reid and McManus. 1987). However. at least 50 per cent of this material is peat which does not contribute to the mineral matter of the sea bed sediments. Much of the fluvial input contributes to major coasta, sediment accumulation in such areas as the Crorrnsty, Dornoch and Beauly firths. For example, Holocene silts and silty sands are up to 23 m thick in the Crommy Firth (Peacock et al., 1980).

In the north-eastern part of the study area, input is dominated by calcareous shell debris from the sea floor around Orkney. This area is probably a site of high carbonate production by bivalves and encrusting organisms. The carbonate sediment is produced by maceration and bioerosion of shells and skeletal material, and provides an average sediment accumulation on the entire North Orkney Platform and East Orkney Shelf of 125 g/m²/year (Allen, 1983; Farrow et al., 1984).

There are very few radiometric dating results for the surface sediments in the Moray Firth area. In the east of the area. low Holocene sedimentation rates are implied on the Haden Grund (Erlenkeuser, 1979; H. Erlenkeuser, written communication, 1988). and carbonate sand at the sea bed east of Orkney revealed a range of ages from 3270 ± 110 to 4570 ± 70 years BP for different shell fractions (Allen, 1983).

Beaches

The most accessible form of sea-bed deposits are on the beaches around the report area, but these are not fully representative of marine material since nearshore processes have a major influence on beach composition. The pattern of accretion can also be complicated by the influence of large aeolian deposits, such as at Culbin Sands (Reid and McManus, 19S7).

The eastern beaches of Orkney are formed predominantly of well- to moderately-sorted, medium- to fine-grained sands with high carbonate contents (> 50%); media i diameters and sorting coefficients generally decrease northwards (Mather et al., 1974). Within Scapa Flow, the beaches contain much less carbonate (<5%), perhaps due to reworking of offshore glacial deposits. There are few beaches along the north-western coast of the Moray Firth. In Caithness, they are formed of medium-grained, moderately sorted sand with a carbonate content that decreases southwards from more than 80% in the Pentland Firth to 60% at Sinclair's Bay (Ritchie and Mather, 1970). Farther south, the beaches of Easter Ross and Sutherland comprise sands that change from moderately well-sorted. coarse-grained sand in the north to very well-sorted, fine-grained sand in the south. There is also a southward reduction in the carbonate content from about 40% north of Brora to about 2% at Rosemarkie (Smith and Mather, 1973). The extensive sands of Morrich More at the mouth of the Dornoch Firth are actively extending seawards (Smith and Mather, 1973).

The beaches along the southern shore of the Moray Fitch comprise fine- to medium-grained, well-sorted sand with a westward decrease in carbonate content from 40% around Fraserburgh to trace amounts near Nairn (Ritchie et al., 1978). Spit and bar developments show both westward and shoreward migration, a process that is not always accumulative since distal growth appears to be at the expense of proximal erosion (Reid and McManus, 1987).

The circulation of water masses in the northern North Sea has been summarised by Lee (1980). He showed that water enters the north-west of the study area through both the Pentland Firth and the Fair Isle Channel, and generally moves in a south-easterly direction, although local variations occur along the coast. The bottom-layer water masses have salinities of 34.5% to 35.3%, with temperatures between 6°C and 12°C in summer and 5°C to 7°C in winter. Near sea bed, tidal current velocities at mean spring tides are below 0.5 m/s over most of the area. although they increase to between 1 and 1.5 m/s off Rattray Head, and are in excess of 1.5 m/s around the Orkney Islands (Belderson et al., 1971). In the Pentland Firth, velocities of up to 5.25 m/s have been recorded (Allen. 1983). The Moray Firth lies between a zone of bedload convergence in the south off Rattray Head, and a zone of bedload parting in the Pentland Firth (Kenyon and Stride. 1970). although the existence of the latter has been questioned by Allen (1983).

Bathymetry and hydrography

The bathymetric map in (Figure 62) shows that in the coastal regions between Peterhead and Buckie, between Helmsdale and Dunnet Head, and along the east coast of Orkney, the sea floor slopes away from the coast to a depth of about 60 m in a distance of some 5 to 10 km. The rugged coastline in these areas is accompanied by an irregular nearshore sea-bed topography. In the innermost parts of the Moray Firth, a smoother sea floor slopes more gently from the coast to about 50m depth some 15 km offshore. except for a narrow channel trending north-east to southwest which is believed to be the continuation of the Beauly and Ness valleys from Inverness [Chesher and Lawson. 1983). Farther out, the sea floor generally deepens eastwards to a maximum of more than 150 m in the Witch Ground.

The most conspicuous features of the area are a series of deeps that occur in an arcuate band from the Buchan coast to about 59°15′ N 0°50′ E. These vary in orientation from east-west in the south to north-south in the north. The deeps are cut to more than 200 m below sea level, are generally 2 to 3 km in width, and vary from I0 km to over 75 km in length. The longest, known as the Southern Trench, has a maximum width of 9 km. The origin of these features is uncertain, but may be associated with the release of glacial meltwater, perhaps subglacially (Long and Stoker. 1986). However. the magnitude of the Southern Trench is in part due to its association with a major bedrock fault.

There are several banks in the area which rise 30 to 40 m above the surrounding sea bed. They include Smith Bank, West Bank. Bosies Bank, Little Halibut Bank and Halibut Bank. as well as other unnamed banks. The sea bed also shallows in the north-eastern part of the area on to Bressay Bank. Other topographic rises are large sand waves, the most notable of which is Sandy Riddle (also known as Muckle Skerry Bank) which extends 10 km east-south-east from the Pentland Skerries. and rises 60 m above the surrounding sea floor.

Distribution of sediments

The sea-bed sediments have been formed in the Holocene sedimentary environment, and rest either on glacial sediment, postglacial deposits, or on solid rock. (Figure 61) indicates the range of sediment types using a modified Folk (1954) classification; it shows a general trend of coarser sediment in the west and finer sediment in the east, with local variations often related to topography. Detailed analyses of grain-size distribution have been undertaken in the inner Moray Firth by Reid (1988), and off Peterhead by Owens (1980).

Areas of rock outcrop extend locally from the coast, as well as occurring in isolated zones south of Fair Isle, southeast of Orkney and off Tarbat Ness and Lossiemouth. Pleistocene and older sediments that crop out on the sides of some enclosed deeps include Neogene limestone trawled from the Fladen Deeps (Newton, 1916).

Gravel is mainly restricted to small patches close to rock outcrops off Orkney and near the north-western coast of the Moray Firth; this implies that high sea-bed currents prevent sedimentation of all but the coarsest material. There is also a large area (>100 km²) of gravel off the mouth of the River Spey at Lossiemouth which fines eastwards. This was penetrated in BGS borehole BH71/15 (Chesher and Lawson, 1983) where 2 m of subrounded to well-rounded pebbles of granite. syenite, vein quartz and sandstone were recovered.

Sandy gravel occurs in a coastal zone up to 15 km wide, although it is absent in the innermost parr of the Moray Firth and is found more extensively around the northern islands of Orkney and south of Fair Isle. The well-sorted sandy gravel off Orkney is predominantly biogenic, with a carbonate content of 80 to 100% (Allen. 1983). The carbonate content decreases southwards to about 30% off Helmsdale, where the sorting is poorer. The sandy gravels off Buchan are predominantly titbit:, with only a 10 to 20% biogenic component (Chesher and Lawson, 1983): the lithic clasts have a wide range of lithologies, some with glacial striae. and may be derived either from the erosion of morainic banks (Owens. 1980) or from the River Spey (Reid and McManus, 1987). A tongue of well-sorted sandy gravel extends 30 km north-eastwards from Rattray Head (Owens, 1981) where strong currents have removed the finer sediment. Sandy gravel also occurs on Smith Bank, where the gravel is predominantly biogenic, although the lithic content increases to more than 50% on its southern flank.

Seaward of the areas of gravel and sandy gravel, sands with more than 1% gravel occur north of 58°N out to 50 km from the coast. These gravelly sands are rich in biogenic carbonate, typically with values greater than 50% . Gravelly sands are present up to 45 km east and north-east of Rattray Head, but the carbonate content here is much lower at 10 to 20%. Sediments upon several of the banks are also gravelly sands; these have very low carbonate contents (< 10%) because they are mainly derived from the winnowing of underlying morainic material which has a carbonate content of less than 10%.

A broad, irregular swath of sand extends from 50 km east of Fair Isle to 50 km east of Peterhead. Sand also forms the sea bed on the flanks of several banks and in much of the inner part of the Moray Firth. The sand usually has a low carbonate content of less than 20%, and is moderately well sorted.

Muddy sands are restricted to the following zones: the eastern half of the report area in water depths of more than 110 m, the southern part of the inner Moray Firth where water depths are greater than 70 m, and the approaches to the innermost firths. In the inner Moray Firth, the muddy sands are moderately well sorted and contain about 20% carbonate. Visual surveys of the sea bed reveal it to be fairly soft, often burrowed and rippled, with uncommon gravel-size shells (Chestier and Lawson. 1983). The muddy sands n the eastern part of the report area are generally poorly sorted, with very low carbonate contents of less than 10% north of 59N.

The finest-grained sea-bed sediments, the sandy muds and muds, occur chiefly in the outer Moray Firth where water depths emceed 120 m. They are also found in isolated deeps nearer shore. In the east, the sediments are usually poorly to very poorly sorted (Johnson and Elkins, 1979), and in areas of pockmark activity there is a thin (5 mm) layer of moderately sorted silt which can thicken to over 2 m within some pockmarks. The clay mineralogy is dominantly illite (45-60%), with lesser amounts of chlorite (5-25%), montmorillonite (15%) and kaolinite (10-25%) (Elkins, 1977),

Bedforms

Sand ribbons, sand waves and lineations

Provided there is sufficient sediment supply. bedforms are indicative of the hydrographic regime. Sand ribbons characteristically occur where tidal currents have a near-bottom velocity in excess of 1 m/s, and have been described north-east of North Ronaldsay (Belderson et al., 1971). Sand waves occur in areas of lesser velocities (>0.5 m/s), and have been mapped off Rattray Head, to the east of Orkney (Belderson et al., 1971; BGS Sea Bed Sediments sheets), and in the inner Moray Firth (Reid. 1988). The orientation of the sand waves off Rattray Head supports the notion of north-west to south-east sediment transport (Owens, 1977).

Allen et al. (1979) reported starved lunate sand waves east of Stronsay that are 30 m in width, at least 0.5 m high, with wavelengths of 50 m and a north-easterly alignment. The sand waves are south-easterly facing and the largest are 18m high. Farther east. the sand waves are smaller and south facing. Sidescan records show megaripples running either parallel or slightly oblique (20°) to the strike of the sand waves. Shallow seismic data show the large sand waves superimposed on strongly reflecting bedrock ridges that tray have caused flow separation at their crests and low currents in their lees, resulting in sand-wave formation (Farrow et al., 1984).

The largest sand wave in the area, called Sandy Riddle. occurs at the eastern entrance to the Pentland Firth; it may be considered as either a sandbank or a shoal. it is 10 km in length, 1 to 2 km wide, up to 60 m high, and is composed predominantly of sandy gravel (Figure 63). Very strong tidal currents of up to 5.25 m/s occur within the Pentland Firth, but decrease rapidly south-eastwards in the open water to the east of the Pentland Skerries. Over Sandy Riddle, there is a complex pattern of eddies during periods of south-going tidal stream. The sandbank is generally asymmetric in cross-section (Figure 63), with its steep side faring south-west. Sand waves with heights of 10 m and wavelengths of 80 to 200 m are superimposed on the north-eastern flank of the sandbank; these are orientated north-south with their steeper slopes facing west. Megaripples on their backs run parallel or slightly oblique (up to 20°) to their strike, and the steep south-western face of the bank has large megaripples about 0.5 m high. The bank's profile becomes less distinct south-eastwards, and the northern end is piled against the rocky ridge of the Pentland Skerries. Seismic profiles suggest that the bulk of the shoal comprises Holocene sediments. although it is possible that there is a core of glacial material (Allen. 1983).

Lineations such as furrows, troughs and sand patches are found in the inner Moray Firth aligned parallel to the Buchan coast. They suggest both eastward and westward sediment movement (Reid, 1988). A variety of lineations has also been observed running into the innermost part of the firth. where the asymmetry of sand waves suggests net movement to the south-west (Reid. 1988).

Pockmarks

In deep water in the east of the report area there areshallow, commonly ovoid depressions at the sea floor. These are known as pockmarks. They range up to 200 in in diameter and 10 m in depth, but are typically 50m wide and 2 m deep with internal slopes of 1°. The largest pockmarks known in the area are 700m in diameter and 17 m deep; these are in the south-east of the study area where several large and probably active pockmarks have been noted (Hovland and Sommerville, 1985). Pockmarks occur mainly in areas of soft. fine-grained sediments such as those found in the Witch Ground. where up to 40 per km² have been recorded using sidescan sonar (Figure 64). As the sea-bed sediment becomes coarser, the pockmarks decrease in size and abundance, and become too small to be identified acoustically.

The preferred theory for the formation of pockmarks is that the escape from the sea bed of gases or liquids, particularly petrogenic gases, is accompanied by the venting of fine sediments into suspension. These are then redeposited, partly away from the site of emission, to leave a depression on the sea floor (Hovland and Judd, 1988). Pockmark activity may take place either as a continuous seep or as an explosive event. It should be noted that the venting of gas or liquid may occur at any sea floor, but pockmarks are not formed if the sediment is too coarse. Gas seepages in the North Sea have been recorded on echo-sounder traces, indicating that these processes are active at present. Some pockmarks have been shown to be sites of carbonate formation (Hovland et al., 1987). which may explain the high carbonate content of the mud-size fraction observed in the Witch Ground region (Farrow and Fyfe. 1988).

Carbonate deposits

There are many extensive carbonate deposits around the Scottish coast, and those around Orkney are among the purest, with potential for commercial extraction. Biogenic production of calcareous material is a highly significant contributor to the sedimentary regime of the report area, and carbonate sediments dominate the north-eastern region. The main sites of production are around the Orkney Islands. From Fair Isle to the Pentland Firth, the carbonate sediment is swept eastwards over the edge of the rock platform, and is progressively dumped to form extensive sand-wave fields. Farther down the transport path. in deeper water, the sand becomes finer grained and is augmented by considerable quantities of locally produced infaunal debris. The carbonate deposits have been examined in detail by Allen (1983), on whose work the following account is largely based.

Accumulation rates quoted for the deposits have been determined by Allen (1983) assuming accumulation over at least the last 6000 years, a period of relative sea-level stability following the postglacial submergence of the shelf. The maximum period of accumulation is probably 10 000 years, which may be nearer the correct figure for deeper-water deposits.

East Orkney

The deposits to the east of Orkney occur in water depths of 30 to 90 m, and include a large sand-wave field (Figure 61). The thickness of the deposits varies considerably in the 2180 km² area, but averages 2 m with an accumulation rate of 123 g/m²/year. The sediment comprises 91% sand and 9% gravel, of which the carbonate fraction consists of bivalves, barnacles and bryozoa, with foraminifera becoming a major constituent in deeper water. Barnacle debris is more abundant than bryozoan debris in the shallower water. The nearshore area of the deposits is very rocky, with wide, sediment-filled gullies controlled by fracture patterns in the bedrock. The gullies lead south-eastwards on to a sloping rock platform where there is an extensive sand-wave field of biogenic carbonate sediment.

To the north of the East Orkney deposits, a small sandbank with sand waves lies east of the island of North Ronaldsay (Figure 61). It has an average carbonate content of 95%, with a suggested accumulation rate of 388 g/m²/year. Farther north, the North Ronaldsay North Bank is up to 20 m thick, and may have a morainic core. It is an extremely pure deposit (99% carbonate) with a probable accumulation rate of 541 g/m²/year.

Sandy Riddle

This long, narrow bank extends south-eastwards from the Pentland Skerries between Orkney and the Scottish mainland (Figure 63). Sandy Riddle has a high carbonate content (94%) rich in barnacle fragments (37%) and bivalves (32%), with lesser amounts of serpulids (12%) and bryozoa (10%). The proportion of bryozoan debris increases southwards, inversely to the serpulid distribution. The coarsest material in the north includes a significant proportion of well-rounded pebbles of the local Devonian flagstones. The accumulation rate is 581 g/m²/year.

Inner Moray Firth

The carbonate content of the sea-bed sediment decreases steadily south-eastwards away from the Pentland Firth area. South of Wick, the deposit becomes a narrow, 5 km wide strip close to the Caithness coast and thins to less than a metre. Bivalve debris (44%) dominates the sand, with smaller quantities of barnacles (19%) and bryozoa (16%). The carbonate is probably derived from the Pentland Firth and Sandy Riddle, as well as being locally produced. Carbonate mud gradients define tongues of mud-size carbonate sediment moving into the innermost parts of the Moray Firth from the vicinity of Orkney. About 30% of the mud-size material consists of carbonate, which implies the production and movement of 80 000 tonnes/year of carbonate (Reid and McManus, 1987).

Geochemistry

Very few analytical results of sea-bed geochemistry have been published, but regional variations in chemical abundances appear to be a function of grain size. Mean values for silica abundance decrease with decreasing grain size, while other chemical components generally increase considerably in the finer-grained sediments. Studies of muds in the eastern part of the report area suggest an enrichment of iron and manganese in the surface layer attributed to a gradient of iron and manganese in the sediment pore waters caused by lower Eh conditions (Elkins, 1977).

The distribution of copper, zinc, lead, nickel and cobalt (Basford and Eleftheriou, 1988) in general reflects that of the fine-grained sediment. The concentration of cadmium is undetectable in many areas, but occurs in excess of 0.3 ppm in the surface sediment east of Orkney and south of Fair Isle.

Studies of the organic carbon content of the surface sediments have been undertaken as part of hydrocarbon exploration programmes. These show values of up to 3.5% at the sea bed in the Fladen Grund (Elkins. 1977). and up to 1% in deep-water areas of Buchan (Basford and Eleftheriou, 1988): organic carbon content increases as mean grain size decreases. A strong association between the distribution of chlorophyllous pigments and organic carbon (Basford and Eleftheriou. 1988) indicates that most detrital material is of phytoplanktonic origin. Direct measurements of interstitial hydrocarbon gas concentrations reveal high values up to 0.517 ppm ΣCn, predominantly of methane (typically 85-92%) thought to be of thermogenic origin and leaking from deep sources (Faber and Stahl, 1984).

Pesticide contamination of the area appears negligible. with values often below the detection limit, while the occurrence of PCBs at about 2 parts per billion are reported by Basford and Eleftheriou (1988). The distribution of contaminants is again closely associated with that of fine-grained sediments.

Chapter 13 Economic geology

The most important economic mineral deposits discovered in the Moray Firth area are the offshore hydrocarbon accumulations, but other deposits of potential economic importance are found. It is also noteworthy that studies of the Quaternary geology and the sea-bed sediments are of importance to all constructional aspects of economic development, including oil platforms, pipelines and possible wave-energy installations (Probert and Mitchell, 1983). Economic minerals in the adjacent land areas are summarised by Johnstone (1966) and Johnstone and Mykura (1989).

Oil and gas

Despite the presence of major oilfields offshore, there has been little drilling activity around the Moray Firth. The Sutherland No.1 well was drilled by Premier Consolidated in 1980 south of Helmsdale (Figure 65). The main exploration target was Jurassic sandstones similar to those in nearby offshore oilfields. Although the well encountered 673 m of Jurassic sediment resting on Palaeozoic and Precambrian basement, drill-stem tests did not flow hydrocarbons, and the well was plugged and abandoned.

The first well to be drilled offshore in the Moray Firth was in 1967 by Hamilton Brothers in block 12/26. Over 167 exploration and appraisal wells and 103 field development wells have been drilled in the area since 1967, with the major phase of exploration and drilling activity beginning in 1973. Following the success of this major exploration phase came the oilfield development drilling activity which shows peaks around 1977 and 1980 (Figure 66).

The first oilfield to be discovered in the Moray Firth was Piper in January 1973. Since then a total of eight fields have come into production; these had produced a total of 1237 million barrels of oil by the end of 1986 (Department of Energy, 1987). A further two fields, Ivanhoe and Rob Roy, are undergoing development drilling, and another eleven 'significant discoveries' have been made. A 'significant discovery' indicates a measure of flow rates rather than the potential commerciality of a find (Department of Energy, 1987). The ten fields in production or under development had total original recoverable reserves of 1902 million barrels (Department of Energy, 1987) and these fields alone are therefore capable of producing another 665 million barrels of oil (Figure 67). When the potential reserves of the eleven significant discoveries are added to this figure, it is clear that the Moray Firth will be an important area of oil production for a considerable time.

The fields discovered include three 'giant' fields, which are usually defined as being capable of producing 100 million barrels or more of oil, although a 500 million barrel figure is needed to qualify for 'giant' status in the Middle East, North Africa and Asiatic Russia. The Piper Oilfield had original recoverable reserves of 951.75 million barrels, and therefore qualifies as a giant even by comparison with Middle Eastern fields. The Claymore Oilfield reserve figure is smaller at 460.5 million barrels of recoverable oil, and the Beatrice operator calculated original recoverable reserves of 127.5 million barrels (Department of Energy, 1987).

A number of geological factors are required for the formation of an oilfield. A trap, capped by an impermeable barrier is required to allow the accumulation of hydrocarbons, and a porous and permeable reservoir is needed at the site of the trap. There must have been generation of hydrocarbons from deeply buried mudstones, and these hydrocarbons must have migrated up into the reservoir in the structurally highest part of the trap, which needs to have been formed before the migration. Many of the oilfields in the Moray Firth are found in tilted fault blocks sealed by Upper Jurassic or Cretaceous mudstones; for example the Piper Oilfield where the Piper sands form the reservoir. By contrast, the Old Red Sandstone reservoir of the Buchan Oilfield is a faulted horst block draped by Cretaceous argillaceous rocks (Figure 68).

Oil is found in Palaeozoic, Jurassic and Cretaceous rocks in the Moray Firth, and most traps were probably formed during Late Jurassic or Cretaceous times. The oldest reservoir is in the Old Red Sandstone at the Buchan Oilfield, at a depth of about 2650 m below sea level. This reservoir has a vertical oil column of up to 582 m in sandstones which have a low primary porosity but are extensively fractured. The field produces mostly from this fracture porosity.

Subsidiary reservoirs have been found in two Palaeozoic beds in the Claymore Oilfield, although the main reservoir is Jurassic sandstone. The Carboniferous reservoir in the Claymore Oilfield is a deltaic sandstone which has good porosity (up to 23%) but fairly low permeability (maximum of only 500 mD). This reservoir also has a low sand:shale ratio, and would not have been economic to develop if the production facilities had not already existed for the main reservoir (Maher and Harker, 1987). The second Palaeozoic reservoir is in Permian evaporites with large, secondary-solution porosity cavities caused by freshwater leaching during the Triassic. Reservoir quality of these carbonates is variable, with porosities of 2 to 19% and permeabilities of 2 to 899 mD recorded. Although oil has been recovered from these evaporites, they have not been economic to develop (Maher and Harker, 1987).

Jurassic sandstones form the main reservoir in most Moray Firth oilfields. These reservoirs range in age from Rhaetian (uppermost Triassic) to Volgian, and were formed in a variety of depositional environments. Lower to Middle Jurassic sandstones contain oil in the Beatrice Oilfield, but the Callovian Beatrice Formation sands form the main reservoir, with over 79% of the field reserves. These sands have porosities of 12 to 20% and permeabilities of 200 to 800 mD.

Jurassic reservoirs in other fields are younger than Callovian, and generally fall into two types: 1) Oxfordian to Kimmeridgian sandstones of Piper Formation affinity, and 2) Kimmeridgian to Volgian sandstones similar to the Claymore Sand Member. Piper Formation sandstones are widespread over the outer Moray Firth and form the main reservoir in the Piper, Tartan, Ivanhoe and Rob Roy oilfields where they have average porosities of 24% and permeabilities up to 10 darcies. Sand:shale ratios may be 95%, and the interval is commonly over 100m thick. The Kimmeridgian to Volgian sandstones have much the same distribution as the Piper sands, and form the main reservoirs in the Claymore and Ettrick oilfields. In the Claymore Oilfield these sandstones occur above a Piper Formation reservoir and have porosities up to 29% and permeabilities of 200 to 1300 mD (Maher and Harker, 1987).

Although not common as a reservoir. Lower Cretaceous sandstones do contain hydrocarbons, for example in the Claymore Oilfield. These sandstones contain over 90% of the reserves of the northern fault block of the Claymore Oilfield; they have porosities of 20 to 30% with permeabilities up to 4 darcies. Lower Cretaceous sandstones also locally contain hydrocarbons in the extreme eastern part of the area.

The main hydrocarbon source rock in the Moray Firth, and indeed over most of the northern North Sea, is the Late Jurassic to earliest Cretaceous Kimmeridge Clay Formatior . This formation is composed predominantly of claystones which are either silt-laminated, fissile or silty and bioturbated (Stow and Atkin. 1987). Average organic carbon content of these claystones ranges from 2% in bioturbated silty mudstones to 15% in the fissile claystones.

The kerogen (insoluble residue of organic matter) found in organic mudstones is a mixture of bacterially degraded algal remains, humic matter, and minor amounts of woody debris and oxidised plant material (Cornford, 1984). These kerogens crack on burial to produce hydrocarbons. with oil produced first, and then gas as the depth of burial, and therefore temperature and pressure, increases. The depth at which oil generation occurs is known as the 'oil maturity stage', and peak oil generation in the northern North Sea area occurs at a burial depth of about 3000 m. In much of the outer Moray Firth, the Kimmeridge Clay Formation is mature for oil generation. having been buried to depths in excess of 3000 m. especially in the structurally lower parts of the basin. In the inner Moray Firth area, the Kimmeridge Clay Formation is presently buried to between only 1000 and 3000m. However, there may have been up to 1000m of uplift of the area during the Tertiary. so some parts may have at one time teen buried to greater depths.

Oil from the Beatrice Oilfield is particularly waxy. This indicates that it was derived from a source rock rich in terrestrially derived plant material, rather thanfrom marine algae is typical of oil derived from the Kimmeridge Clay Formation. It may originate from an unusual coastal facies of the Kimmeridge Clay which had a high terrigenous organic content, or the oil may have been derived from older Jurassic or Devonian mudstones, all of which may have contained a greater amount of non-algal plant material than the Kimmeridge Clay Formation. Devonian mudstones in the area have been postulated to be potential hydrocarbon source rocks by Duncan and Hamilton (1988). Hillier and Marshall (1988) and Peters et al. (1989).

Most of the producing fields in the outer Moray Firth area deliver their crude by subsea pipeline to a large terminal at Fiona, in the Orkney Islands (Figure 65). In 1986 alone, this terminal received 115 million barrels of oil from the Piper, Claymore, Tartan, Highlander. Scapa and Petronella oilfields (Department of Energy, 1987). Oil from the Buchan Oilfield goes by pipeline to the Hound Point terminal in the Firth of Forth via Cruden Bay in Aberdeenshire, and Beatrice oil goes by pipeline to the Nigg Bay terminal on the Cromarty Firth.

Coal and lignite

The Middle Jurassic Brora Coal at Brora in Sutherland is the oily Jurassic coal seam in Scotland which has been commercially mined. It has been intermittently mined since 1598, and the most recent venture was an inclined adit opened in 1969 but abandoned in 1975. The coal is up to 1.2 m thick and burns freely, but has a high sulphide content and includes a band of pyrite. The Brora Coal crops out on the foreshore south of Brora, and more Jurassic coal is present offshore. Carboniferous coals ate known to occur in the Outer Moray Firth Basin, and a large area (5000 km²) of low quality Eocene lignite (Figure 65) of unknown thickness has been identified (Moses, 1981). Additionally, bands of lignite up to 3 m thick of probable late Pliocene age were sampled in BGS borehole BH81/19.

Metalliferous minerals

Minor amounts of metalliferous minerals have been found onshore, including uranium deposits both in the Old Red Sandstone of Caithness and Orkney, and in the Helmsdale Granite. Near Helmsdale, small quantities of alluvial gold have been found in the Helmsdale river and its tributaries for more than a century. A BGS diving survey found no evidence for gold enrichment in the sediments immediately offshore from Helmsdale, and it is thought likely that any gold entering the sea is quickly dispersed by the strong south-westerly longshore drift. In the latter half of the 18th century, galena was worked at the old mines at Manse Bay on the east coast of South Ronaldsay, Orkney (Figure 65), and the lead ore may well extend offshore (Mykura, 1976).

Sand and gravel

The local demand for sand and gravel is small, and exploitation of offshore resources has not yet been required. Shelly sand has been extracted from several of the beaches of Orkney and is mainly used as a source of agricultural lime. Noteable areas of extraction include Sandside on Mainland Orkney and Burray Links (Figure 65), where intense extraction has depleted the offshore sandbanks and is causing exposure of rock platforms just below the low-water mark (Machu et al., 1974; Mykura, 1976). Beach sand has also been extracted from Freswick Bay in Caithness (Ritchie and Mather, 1970). Offshore sources of sand and gravel include lithic gravel off the mouth of the River Spey which was sampled in BGS borehole BH71/15 (Chestier and Lawson, 1983); this is thought to be an extension of large gravel deposits identified on land (Aitken et al., 1979).

Carbonates

Biogenic carbonate deposits occur around much of the Scottish coast, including the northern and western part of the Moray Firth area. Their possible commercial use as sources of lime for either agriculture or cement have been the subject of a thesis by Allan (1983) from which the following information is abstracted. The carbonate deposits are produced by the maceration and/or bioerosion of shells and are typically made up of bivalves (46%), barnacles (18%), bryozoans (11%), calcareous worms (7%), and accessory components that include gastropods and echinoids. Only very few foraminifera normally occur, and calcareous algae are only locally important (Farrow et al., 1984). Five main areas of accumulation are recognised (see (Figure 65) for locations).

1 North Ronaldsay, North Bank

A large ovoid sandbank of 27.1 km² occurs 5 km northwest of North Ronaldsay and may be up to 20 m chick. Differing surveys indicate that it is highly mobile, and it is thought to include 110 x 10⁶/m³ of carbonate gravelly sand with an extremely pure (99%) carbonate content that occurs in a gravel:sand ratio of 1:2.

2 North Ronaldsay. East Bank

East of the island of North Ronaldsay is a small sandbank with sand waves. It has an average carbonate content of 95%, covers 11.6 km². and has a likely volume of 35 x 10⁶/m³.

3 East Orkney

This deposit occurs in water depths of 30 to 90 m up to 35 kmfrom the coast. It covers an area of 2180 km² that includes a large sand wave field, and has a likely total volume of 2140 x 10⁶/m³.

4 Sandy Riddle

This long narrow shoal extends south-eastwards from the Pentland Skerries between Orkney and the Scottish Mainland. It is 10 km long. 1 to 2 km wide and has a maximum relief of 60m. Grain size decreases towards the southern end of the feature, and it has a very high carbonate content (94%) with a possible volume of 134 x 10⁶/m³.

5 Western Moray Firth

The carbonate content of the sea-bed sediment decreases steadily southwards away from the Pentland Firth area. South of Wick, the deposit becomes a narrow, 5 km-wide strip off the Caithness coast that is probably less than a metre thick. It has a total volume of some 350 x 10⁶/m³.

Chapter 14 Guide to geological excursions

This section is included in order to lead the reader to important land sections which are relevant to the geology offshore, for the Moray Firth is the only area on the east coast of Scotland where Mesozoic rocks can be examined on land. Only an outline of locality details is provided. and it is estimated that an itinerary for a one day excursion would include up to six localities in reasonable proximity to each other. As most localities are coastal, the state of the tide should be taken into account when planning an excursion! A map showing the field localities is given in (Figure 69), and National Grid references are given in the text.

Basement

The basement rocks consist mainly of metamorphosed Moine and Dalradian sediments. Whereas the Dalradian is present throughout the Grampian and Highland regions, the Moine only occurs in the west near Inverness, in inliers at Cromarty and Rosemarkie (with Lewisian), and within the Devonian outcrop of Easter Ross. Large granitic masses intruded into the basement are thought to have been emplaced over an extended period between the Early Ordovician and the onset of Early Devonian deposition.

Moine

Locality 1 Rosemarkie [NH 744 585] to [NH 767 620] (Rathbone and Harris, 1980). On the foreshore north of Rosemarkie, metasediments are associated with older hornblendic gneiss which may be Lewisian. At least four episodes of deformation are evident.

Locality 2 Nigg (Rathbone and Harris. 1980). The Cromarty inlier consists of striped psammitic Moine rocks, probably Glenfinnan Division, which have suffered high-grade regional metamorphism and probably have been transformed locally into mobilised granite. They are well exposed in the quarry on the hillside above Nigg Bay construction yard [NH 805 689], where intense folding is seen.

Locality 3 Ardchronie Quarry, 2 km south-east of Bonar Bridge [NH 617 884]. Extensive quarries are present by the roadside showing pelitic schist and folded, metamorphosed psammite with excellent cross-bedding.

Dalradian

Locality 4 Portnoy coast section [NJ 590 665] (Gillen, 1987). Near the swimming pool at Portsoy, complex relationships may be seen involving amphibolite, gabbro, serpentinite and anorthosite bodies intruding graphitic schist, quartzite and limestone. The beds show strong deformation and there are excellent examples of kyanite pseudomorphing chiastolite in the schist.

Locality 5 Meavie Point [NJ 690 647]. Banff (Hudson, 1987) On the coast at Meavie Point by the gasworks. Dalradian greywacke with cordierite porphyroblasts may be seen.

Locality 6 Tarlair coast section [NJ 720 647], Macduff (Hudson, 1987) In the coast section by the swimming pool, Dalradian greywacke and grit of greenschist grade exhibit well-preserved sedimentary structures, medium-scale folding, and related cleavage.

Locality 7 Coast section by Kinnards Head Lighthouse, Fraserburgh [NJ 999 676] (Kneller, 1987) Below the lighthouse is tightly folded, sillimanite-grade greywacke with calcareous lenticles. A metagrit channel in-fill is also seen on the foreshore.

Helmsdale Granite

Locality 8 Kildonan [NC 912 209] and Suirgill [NC 898 251] The Helmsdale Granite may be examined in these stream sections where small amounts of gold occur in the alluvial deposits.

Locality 9 Cearn Ousdale and Ousdale Burn [ND 076 184] (Gallacher et al., 1971) The Helmsdale Granite is well exposed here, as is the associated uranium mineralisation in the overlying Devonian arkose.

Old Red Sandstone (ORS)

Devonian deposits of ORS aspect around the Moray Firth comprise as much as 5000 m of sediments resting unconformbly on the Moine Dalradian basement. They are fluvial and lacustrine deposits that are subdivided by unconformities into Lower, Middle and Upper ORS. The distribution of ORS rocks is shown on (Figure 12) and (Figure 69).

Locality 10 Sanlet Head, south of Wick [ND 350 433] (Donovan, 1970) Beds ascribed to the Lower ORS crop out in a restricted anticlinal area around Sarclet Head in eastern Caithness, where a basal conglomerate is overlain by what appears to be braided-river sandstone. This in turn is overlain by a rhythmic sequence of hard, green mudstone with some marl, siltstone and sandstone: these were probably deposited in a shallow lake that periodically retreated to leave a deltaic alluvial plain.

Locality 11 Ousdale [ND 066 205] 5 to 10 km south of Berriedale, the Lower ORS rests on Helmsdale Granite. The basal member at Ousdale is an arkosic conglomerate, but elsewhere it is made up of redeposited, unweathered granite debris not easily distinguished from the underlying granite. The overlying mudstone is well exposed both in road cuttings and in a quarry north of the Ord of Caithness [ND 055 179] where miospores and fish scales have indicated an early Emsian age.

Locality 12 Creag an Amalaidh [NH 764 980] Cuttings in the roadside at Creag an Amalaidh show excellent examples of the Lower ORS conglomerate that forms the adjacent hills.

Locality 13 Gardenstown to Pennan [NJ 806 650]–[NG 855 659] (Trewin, 1987) The Lower ORS of the Turrif Basin is well exposed on the coast at Gardenstown, Crovie, New Aberdour and Pennan; it consists of basal alluvial fan sediments passing up into floodplain and braided-stream deposits. At Pennan, the unconformity with the Middle ORS is well exposed.

Locality 14 Berriedale [ND 121 226] Exposed sections of the Middle ORS Caithness Flagstone Group occur on the northern side of the stream at Berriedale where the basal breccia is overlain by the red. arkosic Berriedale Sandstone and Berriedale Flags. The latter contain thick, red, channel sandstone and well-developed, interbanded sandstone and siltstone exhibiting synaeresis fractures and suncracks.

Locality 15 Wick, near Castle of Old Wick [NO 372 492] A series of old quarries south of the coastguard station near the Castle of Old Wick exhibit a cyclic sequence of flagstones characterised by extensive shrinkage cracks. They lie within the Middle ORS Caithness Flagstone Group.

Locality 16 Duncansby Head [ND 405 734]. Caithness Overlying the Caithness Flagstone Group is the John O'Groats Sandstone Group. This may be examined along the shore west of Sannick Bay [NO 395 737] where yellow, red and buff sandstones are exposed, or along the spectacular sea cliffs and shore 1 km south of Duncansby Head [ND 405 728], where fluvial fining-upward cycles produced by meandering streams are well exhibited.

Locality 17 Foreshore at Hilton of Cadboll [NH 873 673], near Balintore A reduced Middle ORS succession is seen on the foreshore north of the jetty at Hilton of Cadboll; it consists of beach and lacustrine deposits. Notable features are sandstone inject Ion, nodular limestone and repeated bedding convolution.

Locality 18 Old Shandwick Quarry [NH 858 745], near Balintore The quarry displays a section through several large Middle ORS dune cross-bed sets in a coset which overlies dessicated red mudstone and thin sandstone attributable to sheet flooding in a floodplain environment.

Locality 19 Dunnet [ND 214 710] Along the foreshore and low cliffs below the village of Dunnet, red and orange, medium-grained sandstone of the Upper ORS is exposed. A braided-river facies and an aeolian/placer facies may be recognised.

Locality 20 Embo [NH 820 929], 4 km north of Dornoch The Upper ORS on the foreshore consists of yellow and red, medium-grained, cross-bedded sandstone and appears to be entirely fluvial in origin. Isolated fish scales have been recorded.

Locality 21 Wilkhaven [NG 945 782], Tarbat Ness On the coast near the lighthouse, the Upper ORS largely comprises pebbly sandstone of braided-river type, with rare overbank mudstone.

Permo-Triassic

The Permo-Triassic (Peacock et al., 1968) sequence is best exhibited along the Grampian coast between Burghead [NJ 108 690] and Lossiemouth [NJ 232 704], and inland to the west and north of Elgin in a series of small outliers resting on ORS. The sediments in the southern belt are worthy of examination because they have been extensively quarried and many of the famous Permian and Triassic reptile remains were collected from these former workings.

Locality 22 Quarry Wood, 4 km west of Elgin [NJ 185 637] to [NJ 180 636] There are many old quarries at Quarry Wood, and in Cutties' Hillock Quarry [NJ 185 637] reptile-bearing Permo-Triassic sediments were formerly seen to overlie similar sandstone containing the Late Devonian fish Holoptychius nobilissimus. Unfortunately this quarry is no longer well exposed, and it is better to visit the quarry at [NJ 180 636] where the Upper ORS Rosebrae Beds are overlain above the eastern quarry by the basal beds of the Upper Permian Cutties’ Hillock Sandstone. These beds are similar in appeaiance to the Upper ORS but contain wind-rounded sand grains with local dune cross-bedding, and have a Late Permian faunal assemblage.

Locality 23 Coastal cliffs and quarry [NJ 160 703], 1 to 2 km east-north-east of Hopeman. On the coast, the unconformity between ORS and Permian rocks is not exposed, but the Late Permian Hopeman Sandstone Formation forms the 9 km stretch of much-quarried cliffs between Cummingstown and Lossiemouth. It is a yellow to buff, medium- to coarse-grained sandstone with rounded grains and dune-bedded units up to 5 m thick. The rock is comparable with the Rotliegend facies offshore, and has yielded rare traces of reptile bones. Reptile footprints have been noted in the coastal quarries, particularly at Clashach Quarry [NJ 163 702].

Locality 24 Burghead [NJ 108 690] Burghead Beds are well exposed in cliffs above the quayside at Burghead, and although they overlie the Hopeman Sandstone Formation, the junction between them is not seen. These fluvial sediments consist of cross-bedded sandstone with pebbly lenses and thin beds of siltstone. They contain no diagnostic fossils, but are clearly Triassic in age on account of their stratigraphic position between the Hopeman and Lossiemouth sandstone formations.

Locality 25 Lossiemouth [NJ 232 704] The Late Triassic, reptile-bearing Lossiemouth Sandstone Formation is exposed in old quarries on the southern side of Lossiemouth Hill. It generally consists of hard, white, fine-grained, dune-bedded aeolian sandstone with siliceous cement, and may be equated with the offshore Cormorant and Statfjord formations. The sandstone passes up into the Stotfield Cherry Rock, and is well exposed on the shore at Stotfield [NJ 246 710], where it is faulted against ORS rocks.

Jurassic

A broken succession ranging in age from Triassic to Late Jurassic occurs between Dunrobin and Helmsdale, where it forms the coastal belt that is up to 5 km wide. These Mesozoic sediments are faulted against ORS or older rocks to the north-west along the Helmsdale Fault. In the Cromarty region, a reduced Jurassic succession is faulted against both ORS and Moine rock by the Great Glen Fault. A generalised onshore Jurassic succession is given in (Figure 70).

Locality 26 South-east of Dunrobin Castle [NC 852 005] to [NC 856 008] (Neves and Selley. 1975) The progressively more marine sediments of the Lower Jurassic (Dunrobin Bay Formation) cover the continental beds of the Upper Triassic (Dunrobin Pier Conglomerate) without angular discordance in a situation that is closely comparable to that in the Viking Graben. The youngest Lower Jurassic sediments are not seen because of faulting.

Locality 27 Foreshore south of the Brora River [NC 901 029] to [NC 914 032] (Sykes. 1975b) The Bathonian, represented by the Brora Coal Formation at Sowes Nose Point, exceeds 40 m in thickness and forms the lowest part of the Middle Jurassic sequence. These rocks were deposited in a deltaic/estuarine environment during a regressive marine phase that culminated with the formation of delta-swamp coals; this is directly comparable with the conditions of deposition in the upper part of the Middle Jurassic in the Moray Firth Basin. The start of the Callovian was marked by a period of renewed marine transgression that is represented by the 88 m-thick Brora Argillaceous Formation; this is well exposed on the foreshore between Sown Nose Point. and the sandstone reefs formed by the overlying Brora Arenaceous Formation at Brora Point. The formation may in part be correlated with the Heather Formation in the northern North Sea.

Locality 28 Brora River [NC 898 038] to [NC 903 040] (Sykes. 1975b) The upper Callovian to lower Oxfordian Brora Arenaceous Formation was deposited in a shallowing sea with extensive development of coastal sand bars. These 56 m-thick deposits are well exposed at the Brora River. and have the best reservoir characteristics seen in rocks of the Moray Firth coastal region. They are broadly equivalent to the upper part of the Heather Formation in the northern North Sea, and may be directly compared to the sands in the lower part of the Piper Formation in the Outer Moray Firth Basin.

Locality 29 Ardassie [NC 914 040] (Sykes. 1975b) The highest Oxfordian beds, the Ardassie Limestone, may be seen overlying the Brora Sandstone at Ardassie Point.

Locality 30 Lothbeg (Pickering, 1984) The Kimmeridgian was a period of marine transgression throughout the northern North Sea. The earliest Kimmeridgian Allt-na-Cuile Sandstone can be seen between Lothbeg Point and the railway bridge [950 099]; it has good reservoir potential, and may in part be equivalent to the sand in the upper section of the Piper Formation offshore.

Locality 31 Portgower [ND 008 124] to [ND 010 133] (Bailey and Weir, 1932) As the Kimmeridgian transgression progressed, sedimentation became more argillaceous, but penecontemporaneous faulting along what is now the coastal margins of the Moray Firth resulted in the formation of a distinctive series of boulder beds that include the Fallen Stack. These are best examined 1 km south of Portgower. Occurrences of similar fault-associated sediments have been recorded in the North Sea.

Locality 32 Balintore [NH 865 757] (Sykes, 1975b) On the foreshore south of Balintore at Port an Righ [NH 853 733], a reduced Jurassic succession is present in which the lowest exposed Jurassic beds are of Bathonian age. They comprise clay and shaly mudstone overlying white sandstone. The Brora Coal is well exposed and is overlain by Callovian and Oxfordian siltstone and shaly mudstone with ironstone bands. The line of the Great Glen Fault clearly defines the coastline where the Jurassic sediments are faulted against both ORS and Moine rocks.

Locality 33 Ethie [NH 778 635] (Waterston, 1951) The uppermost Jurassic beds comprise carbonaceous shaly mudstone, sandstone and thin limestone in a reduced Kimmeridgian succession.

Cretaceous and Tertiary

Locality 34 Leavad [ND 173 460] Lower Cretaceous sediments are present on the land around the Moray Firth only as glacial erratics derived from offshore. The most significant is a mass of sandstone at Leavad by a stream just south of Tacker in Caithness. It is sufficiently large to have been quarried, and contains Valanginian to Hauterivian faunas. Due to quarrying, little remains of this erratic. Beneath the Cretaceous erratic lies dark green, foraminiferal clay up to 7.8 m thick that is of Miocene age and also thought to be derived from the Moray Firth.

References

Note references to BGS maps are not given here, but an index map showing their distribution is presented inside the back cover.

AITKEN, A M, MERRITT J. W, and SHAW. A J. 1979. The sand ard gravel resources of the country around Garmouth, Grampian Region: description of 1:25 000 resource sheet NJ 36. Mineral Assessment Report, Institute of Geological Sciences, No. 41.

ALLEN, N H. 1983. Recent temperate carbonate deposits on the continental shelf north and west of Scotland: distribution and reserves. Unpublished PhD thesis, University of Strathclyde,

ALLEN, N H. FANNIN, N G T, and FARROW, G E. 1979. Resin peels of vibrocores used in the study of some shelly sediments on the Scottish Shelf. Marine Geology, Vol. 33. M57-M65.

AMIRI-GARROUSSI, K, and TAYLOR, J C M. 1987. Complex diagenesis in Zechstein dolomites of the Ettrick Oil Field. 577–539 in Petroleum geology of North West Europe. BROOKS, J, and GLENNIE, K W, (editors). (London: Graham and Trotman.)

ANDERTON, R, BRIDGES, P H, LEEDER, M R, and SELLWOOD, B 1979. A dynamic stratigraphy of the British Isles. (London: Allen and Unwin.)

ANDREWS, I J, and BROWN, S. 1987. Stratigraphic evolution of the Jurassic, Moray Firth. 785–795 in Petroleum geology of North West Europe. BROOKS, J. and GLENNIE, K W (editors). (London: Graham and Trotman.)

ARKELL, W J. 1933. The Jurassic system in Great Britain. (Oxford: Oxford University Press.)

ASTIN, T R. 1985. The palaeogeography of the Middle Devon..an Lower Eday Sandstone, Orkney. Scottish Journal of Geology, Vol. 26, 353–375.

BAILEY, E B, and WEIR, J. 1932. Submarine faulting in Kimmeridgian times: east Sutherland. Transactions of the Royal Society of Edinburgh, Vol. 57, 431–67.

BAMFORD, D. 1979. Seismic constraints on the deep geology of the Caledonides of northern Britain. 93–96 in The Caledonides of the British Isles reviewed. HARRIS, A L, HOLLAND, C H. and LEAKE, B E (editors). Special Publication of the Geological Society. No. 8.

BAMFORD, D., NUNN, K, PRODEHL, C, and JACOB, B. 1978. LISPB-IV. Crustal structure in northern Britain. Geophysical Journal of the Royal Astronomical Society, Vol. 54, 43–60.

BARR, D. 1985. 3-D Palinspastic restoration of normal faults in the Inner Moray Firth: implications for extensional basin development. Earth and Planetary Science Letters, Vol. 75. 191–203.

BARR, D. I., MCQUILLIN, R, and DONATO, J A. 1985. Footwall uplift in the Loner Moray Firth Basin, offshore Scotland. Journal of Structural Geology, Vol. 7, 267–268.

BARTON, P, and WOOD, R. 1984. Tectonic evolution of the North Sea basin: crustal stretching and subsidence. Geophysical Journal of the Royal Astronomical Society, Vol. 79, 987–1022.

BASFORD, D, and ELEFTHERIOU, A. 1988. The benthic environment of the North Sea. Journal of the United Kingdom Marine Biological Association, Vol. 68, 125–141.

BATTES, D J, TREWIN, N H, and TUDHOPE, A W. 1986. The Triassic-Jurassic junction at Golspie, inner Moray Firth Basin. ScottishJournal of Geology, Vol. 22, 85–98.

BEACH, A. 1984. Structural evolution of the Witch Ground Graben. Journal of the Geological Society, Vol. 141. 621- 628.

BEACH, A. 1985. Some comments on sedimentary basin development in the Northern North Sea. Scottish journal of Geology. Vol. 21, 493–512.

BELDERSON, R H, KENYON, N H, and STRIDE, A H. 1971. Holocene sediments on the continental shelf west of the British Isles. 157–170 in ICSU/SCOR Working Party 31 Symposium. Cambridge 1970: The geology of the East Atlantic continental margin. 2. Europe. DELANY, F M (editor). Report of the Institute of Geological Sciences, No. 70/14.

BENT, A J A. 1986. Aspects of Pleistocene glaciomarine sequences in the North Sea. Unpublished PhD thesis, University of Edinburgh.

BENTON, M J, and WALKER, A D. 1985. Palaeoecology. taphonomy and dating of Permo-Triassic reptiles from Elgin, North-east Scotland, Palaeontology, Vol. 28, 207–234.

BERRIDGE, N G. and IVIMEY-COOK, H C. 1967. The geology of a Geological Survey borehole at Lossiemouth, Morayshire. Bulletin of the Geological Survey of Great Britain, Vol_ 27. 155–169.

BERSTAD, S, and DYPVIK, H. 1982. Sedimentological evolution and natural radioactivity of Tertiary sediments from the central North Sea. Journal of Petroleum Geology, Vol. 5, 77- 88.

BIRD, T J, BELL, A, GIBBS, A D. and NICOLSON, J. 1987. Aspects of strike-slip tectonics in the Inner Moray Firth Basin, offshore Scotland. Norsk Geologic Tichskrift. Vol. 67, 353–369.

BJØRSLEV NIELSEN, O, SØRENSEN, S. THIEDE, J, and SKARBØ, O. 1986. Cenozoic differential subsidence of the North Sea. Bulletin of the American Association of Petroleum Geologists, Vol. 70, 276–298.

BLUCK, B.J. 1980. Evolution of a strike slip fault controlled basin, upper Old Red Sandstone, Scotland. 63–78 in Sedimentation in oblique slip mobile zones. READING, H G, and BALLANCE., P F (editors). Special Publication of the International Arsociation of Sedimentologists. Vol. 4.

BOOTE, D R D. and GUSTAV. S H. 1987. Evolving depositional systems within an active rift, Witch Ground Graben, North Sea. 819–833 in Petroleum geology of North West Europe. BROOKS, J, and GLENNIE, K W (editors). (London: Graham and Trotman.)

BOWEN, D Q. 1978. Quaternary geology: a stratigraphic framework for multidisciplinary work. (Oxford: Pergamon Press.)

BREMNER, A. 1934. The glaciation of Moray and ice movements in the north of Scotland. Transactions of the Edinburgh Geological Society, Vol. 13, 17–56.

BRENNAND, T P. 1975. The Triassic of the North Sea. 295–311 in Petroleum and the continental shelf of North-West Europe. WOODLAND, A W (editor). (London: Applied Science Publishers.)

BROWN, S S, 1986. Jurassic. 133–160 in Introduction to the petroleum geology of the North Sea (3rd edition). GLENNIE, K W (editor). (Oxford: Blackwell Scientific Publications.)

BROWN, S S, RICHARDS. P C, and THOMSON. A R. 1987. Patterns in the deposition of the Brent Group (Middle Jurassic) UK North Sea. 899–914 in Petroleum geology of North West Europe. BROOKS. J and GLENNIE, K W (editors). (London: Graham and Trotman.)

BUCKHARDT, B. 1978. Oxygen isotope palaeotemperatures from the Tertiary period in the North Sea area. Nature, London. Vol. 275, 121–123.

BURNHILL, T J, and RAMSAY, W V. 1981. Mid-Cretaceous palaeontology and stratigraphy, Central North Sea. 245–254 in Petroleum Geology of the Continental Shelf of North-West Europe. ILLING, L V, AND HOBSON. G D (editors). (London: Heyden and Son.)

CAMERON. T D J. STOKET M S, and LONG. D. 1987. The history of Quaternary sedimentation in the UK sector of the North Sea Basin. Journal of the Geological Society, Vol. 144. 43–58.

CASEY. R, and LAWSON. P F. 1974. A view of the boreal Lower Cretaceous. 415–430 in The Boreal Lower Cretaceous. CASEY. R. and RAWSON, P F (editors). Special Issue of the Geological Journal, No. 5.

CHESHER, J A, and BACON, M. 1975. A deep seismic survey in the Moray Firth. Report of the institute of Geological Sciences, No. 75/11.

CHESHER, J A, DEEGAN. C E. ARDUS, D A, BINNS, P E. and FANNIN. N G T. 1972. IGS drilling with m.v. Whitethorn in Scottish waters 1970–1971. Report of the Institute of Geological Sciences, No. 72/10.

CHESHER, J A, and LAWSON. D. 1983. The geology of the Moray Firth. Report of the Institute of Geological Sciences. No. 83/5.

CHRISTIE. P A F, and SCLATER, J G. 1980. An extensional origin for the Buchan and Witchground Graben in the North Sea. .Nature, London, Vol. 283, 729–732.

CLAPPERTON, C M, and SUGDEN, D E. 1975. The glaciation of Buchan: a reappraisal. 9–22 in Quaternary studies in north east Scotland. GEMMELL, A M D (editor). (Aberdeen: Aberdeen University.)

CLARK. D N. 1986. The distribution of porosity in Zechstein carbonates. 121–149 in Habitat of Palaeozoic gas in North West Europe. BROOKS, J, SOFF. J, and VAN HOORN, B (editors) Special Publication of the Geological Society. No. 23.

CLARKE. R H. 1973. Cainozoic subsidence in the North Sea. Earth and Planetary Science Letters, Vol. 18, 329–332.

CLEMMENSEN, L B. 1987. Complex star dunes and associated aeolian bedforms, Hopeman Sandstone (Permo-Triassic). Moray Firth Basin, Scotland. 213–231 in Desert sediments: Ancient and Modern. FROSTICK, L E, and REID, 1 (editors). Special Publication of the Geological Society. No. 35.

COLLETTE. B J. 1960. The gravity field of the North Sea. 47–96 in Gravity expeditions 1948–58, Vol. 5. BRUINS, G J (editor). (Delft: Netherlands Geodetic Commission.)

CORNFORD, C. 1984. Source rocks and hydrocarbons of the North Sea. 171–204 in Introduction to the petroleum geology of the North Sea (1st edition). GLENNIE. K W (editor). (Oxford: Blackwell Scientific Publications.)

CRAMPTON. C B. and CARRUTHERS. R E. 1914. The geology of Caithness (sheets 110 and 116 with parts of 109, 115 and 117). Memoir of the Geological Survey. Scotland.

DEEGAN. C E, and SCULL. B J. 1977. A proposed standard lithostratigraphic nomenclature for the central and northern North Sea. Report of the Institute of Geological Sciences. No. 77/25: Bulletin of the Norwegian Petroleum Directorate, No. 1.

DEPARTMENT OF ENERGY, 1987. Development of the oil and gas resources of the United Kingdom. (London: HMSO.)

DIMITROPOULOS, K. and DONATO. J A. 1981, The Inner Moray Finh Central Ridge, a geophysical interpretation, Scottish Journal of Geology, Vol. 17, 27–38.

DIXON, J E, FITTON, J G. and FROST. R J C. 1981. The tectonic significance of post-Carboniferous igneous activity in the North Sea basin. 121–37 in Petroleum geology of the continental shelf of North West Europe. ILLING,L V. and HOBSON, G D (editors). (London: Heyden and Son.)

DONATO. J A. and TULLY. M C. 1981. A regional interpretation of North Sea gravity data. 65–75 in Petroleum geology of the continental shelf of North-West Europe. L V, and HOBSON, G D (editors). (London: Heyden and Son.)

DONOVAN. R N. 1970. The geology of the coastal tract near Wick. Caithness. Unpublished PhD thesis, University of Newcastle-upon-Tyne.

DONOVAN. R N. 1980. Lacustrine cycles, fish ecology and stratigraphic zonation in the Middle Devonian of Caithness. Scottish Journal of Geology, Vol. 16, 35–60.

DONOVAN. R N. ARCHER, A. TURNER. P, and TABLING, D H. 1976. Devonian palaeogeography of the Orcadian Basin and the Great Glen Fault. Nature, London, Vol. 259, 550–551.

DUNCAN. A D, and HAMILTON, F R M. 1988. Paiaeolimnology and organic geochemistry of the Middle Devonian in the Orcadian Basin. 173–201 in Lacustrine petroleum source rocks. FLEET, A .J, KELTS, K, and TALBOT, M R (editors). Special Publication of the Geological Society. No. 40.

EINSELE and SELLACHER. A. 1982. Cyclic and event stratification. (Berlin: Springer-Verlag.)

ELKINS, S R. 1977. Recent sediments of. the northern North Sea: factors controlling their composition and distribution. Unpublished MSc thesis, University of Minnesota.

ERLENKEUSER, H. 1979. Environmental effects on radiocarbon in coastal marine sediments. 453–469 in Radiocarbon dating. BERGER. R, and SUESS, H E (editors). Proceedings of the Ninth International Conference. (Los Angeles and la Jolla: University of California Press.)

FABER. E. and STAHL, W. 1984. Grochernical surface exploration for hydrocarbons in the North Sea. Bulletin of the Amerman Association of Petroleum Geologists. Vol. 68. 363- 386.

FALL, H G. GIBB F G F. and KANARIS-SOTIRIOU, R. 1982. Jurassic volcanic rocks of the northern North Sea. journal of the Geological Society, Vol. 139, 277–292.

FARROW, G E. ALLEN, N H. and AKPAN. E B, 1984. Bioclastic carbonate sedimentation on a high-latitude, tide-dominated shelf: northeast Orkney Islands. Scotland. Journal of Sedimentary Petrology, Vol. 54. 373–393.

FARROW, G E. and FYFE, J A. 1988. Bioerosion and carbonate mud production on high-latitude shelves. 281–297 in Non-tropical shelf carbonates- modern and ancient. NELSON. C S (editor). Sedimentary Geology. Vol. 60.

FLETT, J S, and READ, H H. 1921. Tertiary gravels of the Buchan district of Aberdeenshire. Geological Magazine, Vol. 58. 215–225.

FLINN, D. 1978. The most recent glaciation of the Orkney- Shetland Channel and adjacent regions. Scottish Journal of Geology. Vol. 14, 109–123.

FOLK, R L. 1954. The distinction between grain size and mineral composition in sedimentary-rock nomenclature. Journal of Geology. Vol. 62, 344–359.

FROST, R T C, FITCH, F J, and MILLER, J A. 1981. The age and nature of crystalline basement of the North Sea Basin. 43- 57 in Petroleum geology of the continental shelf of North-West Europe, ILLING, L V. and HOBSON. G D (editors). (London: Heyden and Son.)

FROSTICK. L. REID. I. JARVIS. J. and EARDLEY, H. 1988. Triassic sediments of the Inner Moray Firth, Scotland: early rift deposits. Journal of the Geological Society, Vol. 145, 235–248.

GALL/MINER, M J MICHIE. U McL, SMITH. R T. and HAYNES, L. 1971. New evidence of uranium and other mineralisation in Scotland. Transactions of the Institute of Mining and Metallurgy. Vol. 680, 150–173.

GARSON, M S. and PLANT, J. 1972. Possible dextral movements on the Great Glen and Minch Faults in Scotland. Nature Physical Science, London, Vol. 240, 31–35.

GIBB, F G F, and KANARIS-SOTRIOU, R. 1976. Jurassic igneous rocks of the Forties Field. Nature. London. Vol. 260, 23–25.

GILLEN, C. 1987. Dalradian of Portsoy district. 71–78 in Excursion guide to the geology of the Aberdeen area. TREWIN H, KNELLER, B C. and GILLEN, C (editors). (Edinburgh: Scottish Academic Press.)

GLENNIE, K W (editor). 1984. Introduction to the petroleum geology of the North Sea (1st edition). (Oxford: Blackwell Scientific Publications.)

GLENNIE, K W and BULLER, A T. 1983. The Permian Weissliegend of NW EUrope: the partial deformation of aeolian dune sands caused by the Zechstein transgression. Sedimentary Geology. Vol, 35, 43–81.

GODWIN, H. 1975. History of British flora: a factual basis for phytogeography. (Cambridge: Cambridge University Press.)

GREGORY, D M. and BRIDGE, V A. 1979. On the Quaternary foraminiferal species Elphidium ?ustulatum Todd 1957: Its stratigraphic and palaeoecological implications. Journal of Foraminifiral Research, Vol. 9, 70–75.

GRIFFIN, K. 1984. Plant macrofossils from a Quaternary deposit in the North Sea. 33 in Quaternary stratigraphy of the North Sea. Symposium abstract Volume. AARSETH, I, and SEJRUP. H P (editors). (Bergen: University of Bergen.)

HALL, A M. 1984. Buchan field guide. (Cambridge: Quaternary Research Association.)

HALL, A M. 1985. Cenozoic weathering covers in Buchan. Scotland and their significance. Nature, London, Vol. 315, 392–395.

HALL, A M. and WHITTINGTON, G. 1989. Late Devensian glaciation of southern Caithness. Scottish Journal of Geology, Vol. 25, 307- 324.

HALL, A M. and CONNELL, E R. 1990. The lowlands of north-east Scotland. 129–137 in Glacial deposits of the British Isles. GIRBARD, P., ROSE, J. EHLERS, J (editors). (Rotterdam: Balltema.)

HALLAM, A. 1965. Jurassic. 343–356 in The geology of Scotland (2nd edition). CRAIG, G Y (editor). (Edinburgh: Oliver And Boyd.)

HALLAM, A.1984, Pre-Quaternary changes in sea-level. Annual Review of Earth and Planetary Science. Vol. 12, 205–243.

HALLIDAY, A N, McALPINE, A, and MITCHELL J G. 1977. The age of the Hoy Lavas, Orkney. Scottish Journal of Geology, Vol. 13, 43–52.

HALLIDAY, A N, 1982. ⁴⁰Ar/³⁹Ar age of the Hoy Lavas, Orkney. 928–931 in Numerical dating in stratigraphy. ODIN, G S (editor). (Chichester: Wiley.)

HANCOCK, J M. 1984. Cretaceous. 133–150 in Introduction to the petroleum geology of the North Sea (1st edition). GLENNIE, K W (editor). (Oxford: Blackwell Scientific Publications.)

HANCOCK, J M. and KAUFFMAN, E G. 1979. The Great Transgressions of the Late Cretaceous. Journal of the Geological Society, Vol. 136, 1:5–186.

HAQ, B U. HARDENROL, J, and VAIL. P R. 1987. The chronology of fluctuating sea-level since the Triassic. Science, New York, Vol. 235, 1136–1167.

HARKER, S D, GUSTAV, S H, and RILEY, L A. 1987. Triassic to Cenamanian stratigraphy of the Witch Ground Graben. 809–818 in Petroleum geology of North West Europe. BROOKS, J. and GLENNIE, K W (editors). (London: Graham and Trotman.)

HARKNESS, D D, and WILSON. H W. 1979. Scottish Universities Research and Reactor Centre radiocarbon measurements III. Radiocarbon, Vol. 21, 203–256.

HARLAND, R, 1983, Distribution maps of Recent dinofisgellate cysts in bottom sediments from the North Atlantic Ocean and adjacent seas. Palaeontology, Vol. 26. 321–387.

HARLAND, R, 1988a. Quaternary dineflagellate cyst biostratigraphy of the North Sea. Palaeontology, Vol. 31, 877–903.

HARLAND, R, 1988b. Dinoflagellates, their cysts and Quaternary stratigraphy. New Phytology. Vol. 108, 111–120.

HARMS, J C. TACKENRERG, P. PICKLES, E, and POLLOCK, R E. 1981. The Brae oilfield area. 352–357 in Petroleum geology of the continental shelf of North-West Europe. ILLING, L V, and HOBSON. G D (editors). (London: Heyden and Son.)

HEDGES. R E M. HOUSLEY, R A, LAW, I A, PERRY, C, and HENRY, E. 1988. Radiocarbon dates from the Oxford AMS system, Archacornctry datclist 8. Archaeometry, Vol. 30. 291 - 305.

HESIEDAL, A. and HAMAR, G P. 1983. Lower Cretaceous stratigraphy and tectonics of the south-south-eastern Norwegian offshore. 135–144 in Petroleum geology of the south-eastern North Sea and the adjacent onshore areas. KAASSCHIETER, J P H. and REIJERS. T J A (editors). Geologie en Mjinbouw. Vol. 62.

HILL, P J, and SMITH, G. 1979. Geological aspects of the drilling of Buchan Field. Offshore Europe 79 Conference Society of Petroleum Engineers, Paper 8153.1.

HILLIER, S J, and MARSHALL J E A. 1988. Hydrocarbon source rocks, thermal maturity and burial history of the Orcadian Basin, Scotland, 203–204 in Lacustrine petroleum source rocks. FLEET, A J, KELTS, K, and TALBOT, M R (editors). Special Publication of the Geological Society, No, 40.

HOLGATE, N. 1969. Palaeozoic and Tertiary transcurrent movement on the Great Glen Fault. Scottish Journal of Geology, Vol. 5, 97–139.

HOLMES, R. 1977. Quaternary deposits of the central North Sea, 5. The Quaternary geology of the UK sector of the North Sea between 56° and 58°N. Report of the Institute of Geological Sciences, No. 77/14.

HOVLAND, M, and JUDD, A G. 1988, Seabed pockmarks and seepages; Impact on geology, biology and the marine enviroment. (London: Graham and Trotman.)

HOVLAND, M, and SOMMERVILLE, J H. 1985. Characteristics of two natural gas seepages in the North Sea. Marine and Petroleum Geology, Vol. 2, 319–326.

HOVLAND, M, TALBOT, M R, QVALE, H, OLAUSSEN, S, and ASHERG. L. 1987. Methane-related carbonate cements in pockmarks of the North Sea. Journal of Sedimentary Petrology, Vol. 57, 881–892.

HOWITT, F, ASTON. E R, and JACQUE M. 1975. The occurrence of Jurassic volcanics in the North Sea. 379–388 in Petroleum and the continental shelf of North West Europe. WOODLAND, A W (editor). (London: Applied Science Publishers.)

HSU, K J, and JENKYNS, H C (editors). 1974. Pelagic Sediments: on land and under the sea. Special Publications of the International Association of Sedimentologiris, No. I.

HUDSON, N F C. 1987. Macduff to Whitehills, Buchan-type regional metamorphic zones. 89–98 in Excursion guide to the geology of the Aberdeen area. TREWIN, N H, KNELLER, B C. and GILLEN, C (editors). (Edinburgh: Scottish Academic Press.)

HURST A R. 1981. Mid Jurassic stratigraphy and facies at Brora. Sutherland. Scottish Journal of Geology. Vol. 17. 169- 178.

INSTITUTE of GEOLOGICAL SCIENCES. 1972. Aeromagnetic map of Great Britain, Sheet 1. England. Scotland and Northern Ireland (north of National Grid line 500 km N.) 1:625 000. (Chessington. Surrey: Ordnance Survey for Institute of Geological Sciences.)

JAKORSSON, K H, HAMAR, G P, ORMAASEN, D E, and SKARPNES, O. 1980. Triassic facies in the North Sea north of the Central Highs. 1–10 in The sedimentation of the North Sea reservoir rock. (Oslo: Norsk Petroleumsforening.)

JAMIESON T F. 1866. The glacial phenomena of Caithness. Quarterly Journal of the Geological Society of London, Vol. 22. 261–281.

JAMIESON T F. 1906. The glacial period in Aberdeenshire and the southern border of the Moray Firth. Quarterly journal of the Geological Society of London, Vol_ 62. 13–39.

JANSEN, J H F. 1976. Late Pleistocene and Holocene history of the northern North Sea, based on acoustic reflection records. Netherlands Journal of Sea Research. Vol. 10, 1–43.

JANSEN, J H F., DOPPPERT, J W C. HOOGENDOORN-TOERING, K. JONG, J D E, and SPAINK. G. 1979. Late Pleistocene and Holocene deposits in the Witch and Fladen Ground area, northern Norh Sca. Netherlands Journal of Sea Research. Vol. 13. 1–39

JANSEN, J H F. and HENSEY. A M. 1981. Interglacial and Holocene sedimentation in the northern North Sea: an example of Ecmian deposits in the Tartan Field. 323–334 in Holocene Marine Sedimentation in the North Sea Basin. NIO, S.D. SHÜTTENHELM, R P E, and VAN WEERING, Tj C E (editors). Special Publication of the International Association of Sedimentologiot. Vol. 5.

JAQUE, M. and THOUVENIN, J. (1975). Lower Tertiary tuffs and volcanic activity in the Nonh Sea. 455–465 in Petroleum and the continental shelf of North West Europe. WOODLAND, A W (editor). (London: Applied Science Publishcts.)

JEANS, C V. and RAWSON. P F (editors). 1980. Andros Island. chalk and oceanic oozes, unpublished work of Maurice Black. Occasional Publication of the Yorkshire Geological Society. No. 5.

JOHNSON. R J. and DINGWALL, R G. 1981. The Caledonides: their influence on the stratigraphy of the Northwest European continental shelf. 85–87 in Petroleum geology at the continental shelf of North-West Europe. ILLING, L V. and HOBSON. G D (editors). (London: Hcyden and Son.)

JOHNSON. T C. and ELKINS. S R. 1979. Holocene deposits of the northern North Sea; evidence for dynamic control of their mineral and chemical composition. Geologie en Mijnbouw, Vol. 58. 353- 366.

JOHNSTONE, G S. 1966. British regional geology: the Grampians Highlands (3rd edition). (Edinburgh: HMSO for Institute of Geological Sciences.)

JOHNSTONE, G S. and MYKURA, W. 1989. British regional geology: the Northern Highlands of Scotland (4th edition). (London: HMSO for British Geological Survey.)

KALDI. J. 1980. The origin of nodular structures in the Lower Magnesian Limestone (Permian) of Yorkshire. England. 45–60 in The Zechstein Basin with emphasis on carbonate sequences. FÜCHTAUBER, H, and PEav^-r. T (editors). Contributions to sedimentology. Vol. 9. (Stuttgart: E. Schweizerbareshe Verlagsbuchhandlung.)

KARLSSON, W, VOLLST, J. BØRLYKKE, K. and JØRGENSEN, P. 1979. Changes in mineralogical composition of Tertiary sediments from North Sea wells. 281–289 in Developments to sedimentology. Vol. 27. MORTLAND, M M. and FARMER, V C (editors). International Clay Conference. 1978. (Amsterdam: Elsevier.)

KENNEDY. W Q. 1946. The Great Glen Fault. Quarterly Journal of the Geological Society of London. Vol. 102, 41–72.

KENYON. N H, and STRIDE. A H. 1970. The tide-swept continental shelf sediments between the Shetland Isles and France. Sedimentology. Vol. 14. 159–173.

KING, C. 1983. Cainozoic micropalacontological biostratigraphy of the North Sea. Report of the Institute of Geological Sciences, No. 82/7.

KIRTON, S R, and HITCHEN, K. 1987. Timing and style of crustal extension north of the Scottish mainland. 501- 510 in Continental extensional tectonics. COWARD, M P, DEWEY, J F, and HANCOCK, P L. (editors). Special Publication of the Geological Society, No. 28.

KNELLER, B C. 1987. Dalradian of Fraserburgh. 99–106 in Excursion guide to the geology of the Aberdeen area. TREWIN, H. KNELLER, B C. and GILLEN. C (editors). (Edinburgh: Scottish Academic Press.)

KNOX, R W O'B. and MORTON. A C. 1983. Stratigraphical distribution of early Palaeocene pyroclastic deposits in the North Sea Basin. Proceedings of the Yorkshire Geological Society. Vol. 44, 355–363.

KNOX, R W O'B. and MORTON. A C.  1988. The record of early Tertiary North Atlantic volcanism in sediments of the North Sea Basin. 407–420 in Early Tertiary volcanism and the opening of the NE Atlantic. MORTON, A C. and PARSON, L M (editors). Special Publications of the Geological Society, No. 39.

KNOX, R W O'B. and MORTON. A C.  and HARLAND, R. 1981. Stratigraphical relationships of Palaeocene sands in the UK sector of the central North Sea. 267–281 in Petroleum geology of the continental shelf of North-West Europe. ILLING, L V. and HOBSON, G D (editors). (London: Heyden and Son.)

LAM, K. and PORTER, R. 1977. The distribution of palynomorphs in the Jurassic rocks of the Brora Outlier. NE Scotland. Journal of the Geological Society, Vol. 134. 45–55.

LEE. A J. 1980. North Sea: physical oceanography. 467–493 in The North- West European shelf seas: the sea bed and the sea in motion. II. physical and chemical oceanography, and physical resources. BANNER F T. COLLINS. M B. and MASSIF.. K S (editors). (Amsterdam: Elsevier.)

LEE, G W. 1925. The geology of the country around Golspie, Sutherlandshire: Mesozoic rocks of East Sutherland and Ross. Memoir of the Geological Survey of Great Britain.

LEEDER, M R. 1982. Upper Palaeozoic basins of the British Isles Caledonian inheritance versus Hcrcynian plate margin processes. Journal of the Geological Society, Vol. 139, 497–491.

LERVIK, K S, SPENCER, A M, and WARRINGTON, G. 1989. Outline of Triassic stratigraphy and structure in the central and northern North Sea. 173–189 in Correlation in hydrocarbon exploration. COLLINSON, J D (editor). (Oslo: Norsk Petroleumsforening.)

LINSLEY, P N. 1972. The stratigraphy and sedimentology of the Kimmeridgian deposits of Sutherland. Unpublished PhD thesis, University of London.

LINSLEY, P N. POTTER, H C, McNAB, G, and RACHER, D. 1980. The Beatrice Field, Inner Moray Firth, U.K. North Sea. 117–129 in Giant oil and gas fields of the decade 1986–1878. HALBOUTY, M T (editor). Memoir of the American Association of Petroleum Geologists, No. 30.

LONG, D, and five others. 1986. Late Quaternary palaeontology, sedimentology and geochemistry of a vibrocore from the Witch Ground Basin, central North Sea. Marine Geology, Vol. 73, 109–123.

LONG, D, and MORTON, A C. 1987. An ash fall within the Loch Lomond Stadial. Journal of Quaternary Science, Vol. 2, 97 - 101.

LONG, D, and STOKER, M S. 1986. Channels in the North Sea: the nature of a hazard. 339–351 in Proceedings of Oceanology International '86, Brighton. (London: Graham and Trotman.)

LOVELL, J P B. 1986. Cenozoic. 151–169 in Introduction to the petroleum geology of the North Sea (2nd edition). GLENNIE, K W (editor). (London: Scientific Publications.)

MACLENNAN, A M, and TREWIN, N H. In press. Palaeoenvironments of the late Bathonian-mid Callovian in the Inner Moray Firth. In North-west European Micropalaeontology and Palynology. (British Micropalaeontological Society.)

MAHER, C E. 1980. Piper Oil Field. 131–172 in Giant oil and gas fields of the decade 1968–1978. HALBOUTY, M T (editor). Memoir of the American Association of Petroleum Geologists, No. 30.

MAHER, C E. and HARKER, S D. 1987. Claymore Oil Field. 835–845 in Petroleum geology of North West Europe. BROOKS, J, and GLENNIE, K W (editors). (London: Graham and Trotman.)

MARTINI, E. 1971. Standard Tertiary and Quaternary calcareous nannoplankton zonation. 739–785 in Proceedings of the 2nd Planktonic Conference, Rome. FARINACCI, G (editor). Tecnosclenza, Vol. 2.

MATHER, A S, SMITH, J S, and RITCHIE, W. 1974. Beaches of Orkney. (Aberdeen: University of Aberdeen.)

MCCAVE, I N, CASTON, V N D, and FANNIN, N G T. 1977. The Quaternary of the North Sea. 187–204 in British Quaternary Studies, recent advances. SHOTTON, F W. (editor). (Oxford: Clarendon Press.)

MCCLAY, K R, NORTON, M G, CONEY, P, and DAVIS, G H. 1986. Collapse of the Caledonian orogen and the Old Red Sandstone. Nature, London, Vol. 323, 147–149.

MCKENZIE, D. 1978. Some remarks on the development of sedimentary basins. Earth and Planetary Science Letters, Vol. 40, 25–32.

MCMILLAN, A A, and MERRITT, J W. 1980. A reappraisal of the 'Tertiary' deposits of Buchan, Grampian Region. Report of the Institute of Geological Sciences, No. 80/1.

MCQUILLIN, R, DONATO, J A, and TULSTRUP, J. 1982. Development of basins in the Inner Moray Firth and the North Sea by crustal extension and dextral displacement of the Great Glen Fault. Earth and Planetary Science Letters, Vol. 60, 127–139.

MOSES, K. 1981. Britain's coal resources and reserves-the current position. 40–49 in Assessment of energy resources. The Watt Committee on Energy, Report No. 9. (London: Watt Committee on Energy.)

MURCHISON, R I. 1859. On the succession of the older rocks in the northernmost countries of Scotland. Quarterly Journal of the Geological Society of London, Vol. 17, 145–151.

MYKURA, W. 1976. British regional geology: Orkney and Shetland. (Edinburgh: HMSO for Institute of Geological Sciences.)

MYKURA, W.  1983. Old Red Sandstone. 205–251 in The geology of Scotland (2nd edition). CRAIG, G Y (editor). (Edinburgh: Scottish Academic Press.)

MYKURA, W. and OWENS, B. 1983. The Old Red Sandstone of the Mealfuarvonie Outlier, West of Loch Ness, Inverness-shire. Report of the Institute of Geological Sciences, No. 83/7.

NEVES, R, and SELLEY, R C. 1975. A review of the Jurassic rocks of north-east Scotland. 5/1–5/19 in Jurassic Northern North Sea Symposium. FINSTAD, K G, and SELLEY, R C (editors). (Norwegian Petroleum Society.)

NEWTON, R B. 1916. On a fossiliferous limestone from the North Sea. Quarterly Journal of the Geological Society of London, Vol. 72, 7–22.

NORTON, M G, MCCLAY, K R, and WAY, N A. 1987. Tectonic evolution of Devonian basins in northern Scotland and southern Norway. Norsk Geofisk Tidsskrift, Vol. 67, 323–338.

OWENS, R. 1977. Quaternary deposits of the central North sea, 4. Preliminary report on the superficial sediments of the central North Sea. Report of the Institute of Geological Sciences, No. 77/13.

OWENS, R. 1980. Holocene sedimentation in the central North Sea. Unpublished PhD thesis, University of Edinburgh.

OWENS, R. 1981. Holocene sedimentation in the north-western North Sea. 303–322 in Holocene marine sedimentation in the North Sea Basin. NIO, S-D, SHÜTTENHELM, R T E, and VAN WEERING, T C E (editors). Special Publication of the International Association of Sedimentologists, No. 5.

PARER, J R. 1975. Lower Tertiary sand developed in the Central North Sea. 447–453 in Petroleum and the continental shelf of North West Europe. WOODLAND, A W (editor). (London: Applied Science Publishers.)

PATERSON, I B, and HALL, I H S. 1986. Lithostratigraphy of the Late Devonian and Early Carboniferous rocks in the Midland Valley of Scotland. Report of the British Geological Survey, Vol. 18, No. 3.

PAUL, M, and JOBSON, L. 1989. Geotechnical properties of soft clays from the Witch Ground, Central North Sea. 66–79 in Quaternary Engineering Geology. Pre-prints of papers from the 25th annual conference of the Engineering Group of the Geological Society, 10–14 Sept 1989, Heriot-Watt University.

PAYTON, C E (editor). 1977. Seismic stratigraphy applications to hydrocarbon exploration. Memoir of the American Association of Petroleum Geologists, No. 26.

PEACOCK, J D. 1966. Contorted beds in the Permo-Triassic aeolian sandstones of Morayshire. Bulletin of the Geological Survey of Great Britain, No. 24, 157–162.

PEACOCK, J D. 1977. Subsurface deposits of Inverness and the inner Cromarty Firth. 103–105 in The Moray Firth area: geological studies. GILL, G (editor). (Inverness: Inverness Field Club.)

PEACOCK, J D. BERRIDGE, N G, HARRIS, A L, and MAY, F. 1968. The geology of the Elgin district. Memoir of the Geological Survey of Scotland.

PEACOCK, J D. GRAHAM, D K, and GREGORY, D M. 1980. Late and post-glacial marine environments in part of the inner Cromarty Firth, Scotland. Report of the Institute of Geological Sciences, No. 80/7.

PETERS, K E, MOLDOWAN, J M, DRISCOLE, A R, and DEMAISON, G J. 1989. Origin of Beatrice oil by co-sourcing from Devonian and Middle Jurassic source rocks, Inner Moray Firth, United Kingdom. Bulletin of the American Association of Petroleum Geologist, Vol. 73, 454–471.

PICKERING, K T. 1984. The Upper Jurassic Boulder Beds and related deposits: a fault-controlled submarine slope, NE Scotland. Journal of the Geological Society, Vol. 141, 357–374.

PROBERT, P K, and MITCHELL, R. 1983. Environmental implications of wave energy proposals for the Outer Hebrides and Moray Firth. Ocean Engineering, Vol. 10, 459–469.

RAE, D A. 1976. Aspects of glaciation in Orkney. Unpublished PhD thesis, University of Liverpool.

RATHBONE, P A, and HARRIS, A L. 1980. Moine and Lewisian near the Great Glen Fault in Easter Ross. Scottish Journal of Geology, Vol. 16, 51–64.

RAWSON, P F, and seven others. 1978. A Correlation of the Cretaceous rocks in the British Isles. Special Report of the Geological Society, No. 9.

RAWSON, P F, and RILEY, L A. 1982. Latest Jurassic-Early Cretaceous events and the "Late Cimmerian Unconformity" in the North Sea area. Bulletin of the American Association of Petroleum Geologists, Vol. 66, 2628–2648.

READ, H H, and MACGREGOR, A G. 1948. British regional geology: the Grampian Highlands (2nd edition). (Edinburgh: HMSO.)

READ, W A. 1987. The interplay of sedimentation, volcanity and tectonics in the Passage Group (Arnsbergian E2 to Westphalian A) in the Midland Valley of Scotland. 143–152 in The role of tectonics in Devonian and Carboniferous sedimentation in the British Isles. ARTHURTON, R S, GUTTERIDGE, P, and NOLAN, S C (editors). Occasional Publication of the Yorkshire Geological Society, No. 6.

REID, G S. 1988. Sediment distribution in the Moray Firth, north-east Scotland: transport pathways, sources and sinks. Unpublished PhD thesis, University of Dundee.

REID, G S. and McMANUS, J. 1987. Sediment exchanges along the coastal margin of the Moray Firth, eastern Scotland. Journal of the Geological Society, Vol. 144, 179–185.

RHYS, G H. 1974. A proposed standard lithostratigraphic nomenclature for the Southern North Sea and an outline structural nomenclature for the whole of the (UK) North Sea. A report of the joint Oil Industry-Institute of Geological Sciences Committee on North Sea nomenclature. Report of the Institute of Geological Sciences, No. 74/8.

RHYS, G H. 1975. A proposed standard lithostratigraphic nomenclature for the southern North Sea. 151–164 in Petroleum and the continental shelf of North-West Europe, WOODLAND, A W (editor). (London: Applied Science Publishers.)

RICHARDS, P C. 1985a. A Lower Old Red Sandstone lake in the offshore Orcadian Basin. Scottish Journal of Geology, Vol. 21, 381–83.

RICHARDS, P C. 1985b. Upper Old Red Sandstone sedimentation in the Buchan oilfield, North Sea. Scottish Journal of Geology, Vol. 21, 227–37.

RICHARDSON, J B. 1964. Middle Old Red Sandstone spore assemblages from the Orcadian Basin north east Scotland. Palaeontology, Vol. 7, 559–605.

RITCHIE, J D, SWALLOW, J L, MITCHELL, J G, and MORTON, A C. 1988. Jurassic ages from intrusives and extrusives within the Forties igneous province. Scottish Journal of Geology. Vol. 24, 81–88.

RITCHIE, W, and MATHER, A. 1970. The beaches of Caithness. (Aberdeen: University of Aberdeen.)

RITCHIE, W, SMITH, J S, and ROSE, N. 1978. The beaches of North East Scotland. (Aberdeen: University of Aberdeen.)

ROBERTS, A M, BADLEY, M E, PRICE, J D, and HUCK, I W. 1990. The structural history of a transtensional basin: Inner Moray Firth, NE Scotland. Journal of the Geological Society, Vol. 147, 87–103.

ROBERTS, A M, PRICE, J D, and BADLEY, M E. 1989. Discussion on Triassic sediments of the Inner Moray Firth, Scotland: early rift deposits. Journal of the Geological Society, Vol. 146, 361–362.

ROCHOW, K A. 1981. Seismic stratigraphy of the North Sea "Palaeocene" deposits. 255–266 in Petroleum geology of the continental shelf of North-West Europe. ILLING, L V, and HOBSON, G D (editors). (London: Heyden and Son.)

ROGERS, G, DEMPSTER, T J, BLUCK, B J, and TANNER, P W G. 1989. A high precision U-Pb age for the Ben Vuirich granite: implications for the evolution of the Scottish Dalradian supergroup. Journal of the Geological Society, Vol. 146, 789–798.

RUSSELL, M J, and SMYTHE, D K. 1983. Origin of the Oslo Graben in relation to the Hercynian-Alleghenian orogeny and lithospheric rifting in the North Atlantic. Tectonophysics, Vol. 94, 457–472.

SEJRUP, H P, and nine others. 1987. Quaternary stratigraphy of the Fladen area, Central North Sea: a multidisciplinary study. Journal of Quaternary Science, Vol. 2, 35–58.

SMITH, J S, and MATHER, A S. 1973. The beaches of east Sutherland and Easter Ross. (Aberdeen: University of Aberdeen.)

SMYTHE, D K. 1989. Rockall Trough-Cretaceous or Late Palaeozoic? Scottish Journal of Geology, Vol. 25, 5–43.

SMYTHE, D K. SKUCE, A G, and DONATO, J A. 1980. Geological objections to an extensional origin for the Buchan and Witchground Graben in the North Sea. Nature, London, Vol. 287, 467–468.

SPARK, I S C, and TREWIN, N H. 1986. Facies-related diagenesis in the main Claymore Oilfield sandstones. Clay Minerals, Vol. 21, 479–496.

SPEIGHT, J M, and MITCHELL, J G. 1979. The Permo­Carboniferous dyke-swarm of northern Argyll and its bearing on dextral displacement of the Great Glen Fault. Journal of the Geological Society, Vol. 136, 3–11.

STEWART, I J. 1987. A revised stratigraphic interpretation of the Early Palaeogene of the Central North Sea. 557–576 in Petroleum geology of North West Europe. BROOKS, J, and GLENNIE, K W (editors). (London: Graham and Trotman.)

STOKER, M S, and BENT, A J A. 1987. Lower Pleistocene deltaic and marine sediments in boreholes from the central North Sea. Journal of Quaternary Geology, Vol. 2, 87–96.

STOKER, M S, and LONG, D. 1984. A relict ice-scoured erosion surface in the central North Sea. Marine Geology, Vol. 61, 85–93.

STOKER, M S, LONG, D, and FYFE, J A. 1985. A revised Quaternary stratigraphy for the central North Sea. Report of the British Geological Survey, Vol. 17, No. 2.

STOKER, M S, SKINNER, A C, FYFE, J A, and LONG, D. 1983. Palaeomagnetic evidence for early Pleistocene in the central and northern North sea. Nature, London, Vol. 304, 332–334.

STOKER, S J, and BROWN, S. 1986. Coarse clastic sediments of the Brae Field and adjacent areas, North Sea: a core workshop. British Geological Survey Marine Geology Internal Report, No. 86/8.

STOW, D, and AITKEN, B. 1987. Sediment facies and geochemistry of Upper Jurassic mudrocks in the central North Sea area. 797–808 in Petroleum geology of North West Europe. BROOKS, J, and GLENNIE, K (editors). (London: Graham and Trotman.)

SUNDERLAND, J. 1972. Deep sedimentary basin in the Moray Firth. Nature, London, Vol. 236, 24–25.

SUTHERLAND, D G. 1984. The Quaternary deposits and landforms of Scotland and the neighbouring shelves: a review. Quaternary Science Review, Vol. 3, 157–254.

SUTTER, A A. 1980. Palaeogene sediments from the United Kingdom sector of the central North Sea. Unpublished PhD thesis, University of Aberdeen.

SYKES, R M. 1975a. Facies and faunal analysis of the Callovian and Oxfordian in northern Scotland and east Greenland. Unpublished PhD thesis, University of Oxford.

SYKES, R M. 1975b. The stratigraphy of the Callovian and Oxfordian stages (Middle-Upper Jurassic) in northern Scotland. Scottish Journal of Geology, Vol. 11, 51–78.

SYNGE, F M. 1956. The glaciation of north-east Scotland. Scottish Geographical Magazine, Vol. 72, 131–143.

SYNGE, F M. 1977. Land and sea level changes during the waning of the last regional ice sheet in the vicinity of Inverness. 83–102 in The Moray Firth Area: geological studies. GILL, G (editor). (Inverness: Inverness Field Club.)

TARLING, D H. 1985. Palaeomagnetic studies of the Orcadian Basin. Scottish Journal of Geology, Vol. 21, 261–273.

TAYLOR, J C M. 1981. Zechstein facies and petroleum prospects in the central and northern North Sea. 176–185 in Petroleum geology of the continental shelf of North-West Europe. ILLING, L V, and HOBSON, G D (editors). (London: Heyden and Son.)

TAYLOR, S R, and LAPRE, J F. 1987. North Sea chalk diagenesis: its effect on reservoir location and properties. 483–495 in Petroleum geology of North West Europe. BROOKS, J, and GLENNIE, K W (editors). (London: Graham and Trotman.)

THRELFALL, W F. 1981. Structural framework of the central and northern North Sea. 98–103 in Petroleum geology of the continental shelf of North-West Europe. ILLING, L V, and HOBSON, G D (editors). (London: Heyden and Son.)

TREWIN, N H. 1987. Pennan unconformity within the Old Red Sandstone. 127–130 in Excursion guide to the geology of the Aberdeen area. TREWIN, N H, KNELLER, B C, and GILLEN, C (editors). (Edinburgh: Scottish Academic Press.)

TURNER, C C, RICHARDS, P C, SWALLOW, J L, and GRIMSHAW, S P. 1984. Upper Jurassic stratigraphy and sedimentary facies in the Central Outer Moray Firth Basin, North Sea. Marine and Petroleum Geology, Vol, 1, 105–17.

VAIL, P R, MITCHUM, R M Jr, and THOMSON, S III. 1977. Seismic stratigraphy and global changes of sea level, Part. 4, Global cycles of relative changes in sea level. Memoir of the American Association of Petroleum Geologists, No. 26.

VOLLSET, J, and DORÉ, A G (editors). 1984. A revised Triassic and Jurassic lithostratigraphic nomenclature for the Norwegian North Sea. Bulletin of the Norwegian Petroleum Directorate, No. 3.

WARRINGTON, G, and eight others. 1980. A correlation of Triassic rocks in the British Isles. Special Report of the Geological Society, No. 13.

WATERSTON, C D. 1951. The stratigraphy and palaeontology of the Jurassic rocks of Eathie (Cromarty). Transactions of the Royal Society of Edinburgh, Vol. 62, 33–51.

WATSON, J. 1985. Northern Scotland as an Atlantic-North Sea divide. Journal of the Geological Society, Vol. 142, 221– 243.

WERNIKE, B. 1981. Low-angle normal faults in the Basin and Range Province: tectonics in an extending orogen. Nature, London, Vol. 291, 645–648.

WHITE, N, and MCKENZIE, D. 1988. Formation of the 'steer's head' geometry of sedimentary basins by differential stretching of the crust and mantle. Geology, Vol. 16, 250 – 253.

WILSON, H E. 1972. British regional geology: Northern Ireland. (Belfast: HMSO for Geological Survey of Northern Ireland.)

WINDLE, T M F. 1979. Reworked Carboniferous spores: an example from the Lower Jurassic of northeast Scotland. Review of Palaeobotany and Palynology, Vol. 27, 173–184.

WOOD, R J. 1981. The subsidence history of Conoco well 15/30-1, central North Sea. Earth and Planetary Science Letters, Vol. 54, 306–312.

WOODHALL, D, and KNOX, R W O'B. 1979. Mesozoic volcanism in the northern North Sea and adjacent areas. Bulletin of the Geological Survey of Great Britain, No. 70, 34–56.

ZAGVIJN, W H. 1985. An outline of the Quaternary stratigraphy of the Netherlands. Geologie en Mijnbouw, Vol. 64, 17–24.

ZERVOS, F. 1987. A compilation and regional interpretation of the northern North Sea gravity map. 477–493 in Continental extensional tectonics. COWARD, M P, DEWEY, J F, and HANCOCK, P L (editors). Special Publication of the Geological Society, No. 28.

ZIEGLER, P A. 1975. North Sea Basin History in the Tectonic Framework of North-West Europe. 131–149 in Petroleum and the Continental Shelf of North-West Europe. WOODLAND, A W (editor). (Barking: Applied Science Publishers.)

ZIEGLER, P A. 1981. Evolution of sedimentary basins on North-West Europe. 3–39 in Petroleum geology of the continental shelf of North-West Europe. ILLING, L V, and HOBSON, G D (editors). (London: Heyden and Son.)

ZIEGLER, P A. 1982. Geological atlas of Western and Central Europe. (Amsterdam: Elsevier for Shell International Petroleum, Maatschappij BV.)

Information sources

1:250 000 map series

All maps have been given names (see index map above), but can also be referred to by the latitude and longitude of their southwestern corner. Thus the Bosies Bank sheet may also be referred to as 58°N 02°W. The list below shows the types of maps available for each sheet in the report area using the following key:

• BA Bouguer Gravity Anomaly
• A Aeromagnetic Anomaly
• S Solid Geology
• Q Quaternary Geology
• SSB Sea Bed Sediments
• SBS & Q Sea Bed Sediments and Quaternary
• GREAT GLEN (57°N 06°W) BA, A, S
• MORAY-BUCHAN (57°N 04°W) BA, A, S, SBS & Q
• PETERHEAD (57°N 02°W) BA, A, S, Q, SBS
• CAITHNESS (58°N 04°W) BA, A, S, SBS & Q
• BOSIES BANK (58°N 02°W) BA, A, S, Q, SBS
• FLADEN (58°N 0°) BA, A, S, Q, SBS
• ORKNEY (59°N 04°W) BA, A, S, SBS & Q
• FAIR ISLE (59°N 02°W) BA, A, S, SBS & Q
• BRESSAY BANK (59°N 0°) BA, A, S, Q, SBS

Land geology maps and memoirs

The index map shows sheet numbers of published 1:250 000, 1:63 360 and 1:50 000 scale maps. An asterisk indicates that a memoir is available for the sheet.

Other BGS maps

Other smaller-scale maps produced by BGS that cover all or part of the region shown include:

• Sea bed sediments around the United Kingdom (North Sheet) Scale 1:1 000 000
• Sub-Pleistocene geology of the British Isles and the adjacent Continental Shelf 1:1 000 000 (in press)
• Geological map of the United Kingdom, North. Scale 1:625 000
• Aeromagnetic map of Great Britain, sheet 1. Scale 1:625 000

Figures

(Front cover) The view from Port an Righ looking northwards towards Tarbat Ness. The Great Glen Fault separates the Jurassic sediments in the foreground from Old Red Sandstone and Moine rocks forming the cliffs in the distance. Photo. J A Chesher.

(Rear cover) Index map — United Kingdom Offshore Regional Reports

(Figure 1) Location map of the report area showing licence blocks, released commercial boreholes (at January 1989), oilfields, BGS shallow boreholes and simplified bathymetry.

(Figure 2) Solid geology of the Moray Firth.

(Figure 3) Distribution of basement rocks recorded in the Moray Firth.

(Figure 4) Simplified map of the major structural elements of the northern North Sea.

(Figure 5) Structural framework of the Moray Firth.

(Figure 6) Bouguer gravity anomaly map of the Moray Firth.

(Figure 7) Thickness of the crust beneath the Viking and Central grabens as calculated from gravity modelling.

(Figure 8) Crustal structure beneath the Outer Moray Firth Basin as calculated from a seismic refraction profile.

(Figure 9) Cross-sections across the Moray Firth.

(Figure 10) Seismic profile showing a tilted wedge, or flower structure, within Lower Cretaceous sediments above the Great Glen Fault. From McQuillin et al. (1982).

(Figure 11) Generalised map showing the Great Glen Fault in the context of north-west European crustal-block movements. Modified from McQuillin et al. (1982).

(Figure 12) Distribution of Old Red Sandstone rocks in the Moray Firth.

(Figure 13) Simplified stratigraphy of the Old Red Sandstone in Caithness and Orkney. After Mykura (1983).

(Figure 14) Correlation of Lower Old Red Sandstone and associated undifferentiated Old Red Sandstone sections from wells in the inner Moray Firth. Locations of wells are shown in (Figure 12).

(Figure 15) The Middle Old Red Sandstone in well 13/22-1.

(Figure 16) Early Givetian (Middle Devonian) palaeogeographic reconstruction of the report area, in part after Mykura (1983). A displacement of approximately 30km has been made along the Great Glen Fault to restore the present coastline to its inferred position during the Middle Devonian. Wells are shown relative to the south coast of the Moray Firth.

(Figure 17) The upward-fining Upper Old Red Sandstone section in the Buchan Oilfield. Note the upward increase in the number of silistone beds and the appearance of cornstones in unit D. The three derailed core logs illustrate representatiave parrs of units 13, C and D.

(Figure 20) Representative Visean sections with generalised lithologies.

(Figure 18) Distribution of coal-bearing Carboniferous strata in the Moray Firth

(Figure 19) The age of Carboniferous rocks in the Moray Firth.

(Figure 20) Representative Viséan sections with generalised lithologies

(Figure 21) Facies distribution of Lower Permian (Rotliegend) sediments in the Moray Firth.

(Figure 22) Selected Lower Permian well sections. For locations see (Figure 21).

(Figure 23) Facies and thickness of Upper Permian (Zechstein) deposits in the Moray Firth.

(Figure 24) Selected Upper Permian well sections. For locations see (Figure 23).

(Figure 25) Distribution and thickness of Triassic rocks in the Moray Firth

(Figure 26) Representative Triassic well sections

(Figure 27) Summary of the main lithostratigraphic units in the Jurassic of the Moray Firth. The stippled units are the principal hydrocarbon reservoirs.

(Figure 28) Correlation of Rhaetian to Oxfordian strata in the Inner Moray Firth Basin. with location map. This succession corresponds to unit I on the seismic cross-section (Figure 29); its thickness is mapped in (Figure 30). After Andrews and Brown (1987).

(Figure 29) Seismic: section across the inner Moray Firth Basin. with interpretation. For location see (Figure 31).

(Figure 30) Generalised distribution and thickness of the Rhaetian to mid-Oxfordian (part) succession.

(Figure 31) Generalised distribution and thickness of the mid-Oxfordian (part) to Upper Rvazanian succession.

(Figure 32) Correlation of Oxfordian to Ryazanian strata in the outer Moray Finh area. For location of wells see (Figure 31).

(Figure 33) Subdivision, based on changes in gamma-ray response, of late Oxfordian to Kyazanian strata in well 11/30-2 from the Inner Moray Firth Basin. For location see (Figure 31).

(Figure 34) Distribution. thickness and lithology of the Early Cretaceous Cromer Knoll Group in the Moray Firth.

(Figure 35) Lower Cretaceous strangraphy in the Moray Firth.

(Figure 36) The Scapa Sand Member in type well 14/19-15. For location see (Figure 34). After Harker et al. (1987).

(Figure 37) Cross-section through the Lower Cretaceous Scapa sands. For location see (Figure 34). After Boote and Gustav (1987).

(Figure 38) Distribution and thickness of the Chalk Group in the Moray Firth.

(Figure 39) Stratigraphy of the Chalk Group, based on Deegan and Scull (1977) and Bumhill and Ramsay (1981)

(Figure 40) Diagrammatic cross-sections through the Chalk Group, illustrating the relationships of the formations across the area.

(Figure 42) (Figure 43) (Figure 44)(Figure 45 Note:- Symbols are diagrammatic; facies changes may be graditional or interbedded on a fine scale." data-name="images/P944802.jpg">(Figure 41) Thc Hidra and Plenus Marl formations in well 15/22-4 from the central Witch Ground Graben. Most facies boundaries are gradational. Key also used in (Figure 42), (Figure 43), (Figure 44), (Figure 45) Note:- Symbols are diagrammatic; facies changes may be graditional or interbedded on a fine scale.

(Figure 42) The Herring Formation in well 15/28-3 from the south-east Witch Ground Graben. For key to lithology see (Figure 42) (Figure 43) (Figure 44)(Figure 45 Note:- Symbols are diagrammatic; facies changes may be graditional or interbedded on a fine scale." data-name="images/P944802.jpg">(Figure 41).

(Figure 43) The Flounder Formation in well 15/17-8 from the central Witch Ground Graben

(Figure 44) The Tor Formation in well 15/28-3 on the north-eastern flank of the Renee Ridge. The well illustrates the influx of locally derived elastic material in the late Maastrichtian. For key to lithology see (Figure 42) (Figure 43) (Figure 44)(Figure 45 Note:- Symbols are diagrammatic; facies changes may be graditional or interbedded on a fine scale." data-name="images/P944802.jpg">(Figure 41)

(Figure 45) The Ekofisk Formation in well 15/22-4 from the central Witch Ground Graben. The basal mudstone grades to sandstone in some areas. For key to lithology see (Figure 42) (Figure 43) (Figure 44)(Figure 45 Note:- Symbols are diagrammatic; facies changes may be graditional or interbedded on a fine scale." data-name="images/P944802.jpg">(Figure 41).

(Figure 46) Distribution and generalised thickness of Paleocene and Eocene strata in the Morn Firth. based on well data only. Adapted from Knox et al. (1981)

(Figure 47) Stratigraphic chart of the Paleocene and Eocene showing the relationship between the seismic packages defined by Stewart (1987) and the lithostratigraphic units of Deegan and Scull (1977) and Knox cc al. (1981).

(Figure 48) Thickness and facies of the Montrose Group in the Moray Firth. The landward limits of the group are uncertain.

(Figure 49) Well 14/25-1, exhibiting a representative Paleocene sequence from the Moray Firth.

(Figure 50) Thickness and facies of the Moray Group in the Moray Firth. The landward limits of the group are uncertain.

(Figure 51) Differing interpretations of the relationships of Palacogene units. A Deegan and Scull (1977) and Knox and Morton (1981)

(Figure 52) Thickness and facies of Eocene (post-Balder Formation) sediments in the Moray Firth. The western limit of the unit is uncertain.

(Figure 53) Thickness and facies of Oligocene strata in the Moray Firth.

(Figure 54) Lirhology and depth to the top of Miocene sediments in the Moray Firth. Based on released well data.

(Figure 55) Distribution of Pliocene sediments in the Moray Firth, with a lithological log of BGS borehole 81/19.

(Figure 56) Thickness of Quaternary sediments in the Moray Firth.

(Figure 57) Quaternary stratigraphy of the Moray Firth.

(Figure 58) Distribution of Quaternary formations in the Moray Firth, with a cross-section to illustrate subsurface relationships.

(Figure 59) Sparker section illustrating the character of Quaternary formations. For location see (Figure 56).

(Figure 60) Environmental conditions in the report area at the maximum extent of the late Weichselian glaciation.

(Figure 61) Simplified sea-bed sediment map of the Moray Firth.

(Figure 62) Simplified bathymetry of the Moray Firth, in metres.

(Figure 63) Sparkcr profile through Sandy Riddle. The feature consists of superficial sediments approximately 30 m thick at this location lying directly upon bedrock. Asymmetric sandwaves can be seen on she back of the main sandbank.

(Figure 64) Mosaic of sidescan sonar records showing pockmarks in the Witch Ground. The pockmarks show as the white dots against the grey background; the predominantly north-south lines are the edges of separate records.

(Figure 65) Economic resources of the Moray Firth.

(Figure 66) Drilling history in the Moray Firth.

(Figure 67) Moray Firth oil reserves and production. Based on Department of Energy (1987).

(Figure 68) Schematic cross-sections of the Piper and Buchan oilfields.

(Figure 69) Field localities around the Moray Firth.

(Figure 70) The Jurassic succession along the north-western coast of the Moray Firth.