Upper Triassic and Lower Jurassic stratigraphy from exploration well L134/5-1, offshore Inner Hebrides, west Scotland

A thick (c.1368 m) Upper Triassic to Lower Jurassic sedimentary sequence from exploration well L134/5-1, offshore Inner Hebrides, has yielded a rich and diverse foraminiferal and ostracod microfauna. Many of the taxa have been previously described throughout northwest Europe. Poor preservation (often due to crushing and/or overgrowth) and low numbers preclude a complete taxonomic review of this material, although changes in the faunal composition, rates of faunal turnover and palaeoenvironmental analyses are discussed. This is the first published account of ostracod and foraminiferal assemblages from the Sea of Hebrides and they indicate that the sediments (argillaceous, arenaceous and carbonates) were deposited in a shelf-marine setting with environmental fluctuations that are possibly the result of local relative sea-level changes.


INTRODUCTION
Exploration activity in the Hebrides Basin has remained very low compared to other regions within the United Kingdom continental shelf. To date (April, 2001) only three exploration wells have been drilled, one in the North Minch Basin and two within the Sea of Hebrides. A large number (75) of shallow boreholes have, however, been drilled by the British Geological Survey throughout the Hebrides Basin. British Petroleum drilled the first of the three exploration wells (156/17-1) in the North Minch Basin in 1989 (Fig. 1). This was followed by the first of the two Sea of Hebrides wells, the onshore Upper Glen-1 well drilled by Pentex Oil in 1989 on the island of Skye. Chevron UK drilled well L134/5-1 in the Sea of Hebrides, south of Skye and west of Rhum, within the Inner Hebrides, off the west coast of mainland Scotland. Chevron L134/5-1 was spudded in February 1991 and plugged and abandoned in May 1991. It reached a total depth of 2472 m, terminating in Stornoway Formation sandstones of Triassic age. This paper describes the Upper Triassic and Lower Jurassic sediments and microfaunas (foraminifera and ostracods) from well L134/5-1. Particular emphasis has been placed on the microfossil taxa, because it is the first detailed published study of such faunas from this region. The paper describes the microfaunas and their associated biostratigraphical and palaeoenvironmental implications. Poor preservation due to compaction and heating of the surrounding sediments by igneous intrusions and/or low abundance preclude a complete taxonomic review of this material. Although the palynological dataset for well L134/ 5-1 has not been used in this study, the authors have had access to the company biostratigraphic report, and have noted that much of the palynomorph biostratigraphic data complements the microfaunal age assignments for the well.
Wireline log characteristics. The upper boundary is moderately sharp; defined by an increase in gamma ray response and an associated increase in sonic velocity values, reflecting a lithological change to claystones. The lower boundary of the Pabay Shale Formation is sharp; defined by a decrease in gamma ray response and an associated increase in sonic velocity values, reflecting a lithological change to interbedded limestone and calcareous claystone. The Pabay Shale Formation is characterized by slightly serrated wireline log motifs with moderately low gamma ray responses and moderately high sonic velocities.

The Hallaig Sandstone Member equivalent: (1005-1097.5 m, log)
The sediments occurring within this interval are considered to be a lateral equivalent to the Hallaig Sandstone Member of Triassic/Jurassic micropalaeontology, Inner Hebrides Hesselbo et al. (1998). The sediments in well L134/5-1 are finer grained than their onshore counterparts, comprising mainly siltstones/silty claystones, with rare fine-grained sandstones. They are considered to represent a more distal equivalent to the onshore Hallaig Sandstone Member sensu stricto.
Lithology. This member is dominated by dark grey-brownish black, non-to slightly calcareous silty claystone and siltstone, while subsidiary off-white to light olive grey, very fine-to fine-grained, micaceous, argillaceous sandstone occurs mid-way through the member. Disseminated pyrite occurs throughout.
Wireline log characteristics. The upper boundary is sharp; defined by a slight decrease in gamma ray response and a marked increase in sonic velocity values, reflecting a lithological change to siltstone. The lower boundary is sharp; defined by a slight increase in gamma ray response and a marked decrease in sonic velocity, reflecting a lithological change to claystones. This member is characterized by a slightly bowed log profile, with the lowest gamma ray response and highest sonic velocity below the midpoint reflecting the most arenaceous part of the section.

The Blue Lias Formation: (1582-1785 m, log)
The Blue Lias Formation interdigitates with the Broadford Formation in many of its more northerly outcrops of west Scotland (e.g. Ardnamurchan), while along its more southerly outcrops (e.g. Mull) it occurs as a lateral equivalent of the Broadford Formation (Hesselbo et al., 1998). The Blue Lias Formation is considered to be a deeper-water equivalent of the Broadford Formation.
Lithology. This formation is dominated by an interbedded sequence of claystones and limestones. The former comprise medium dark grey to dark grey, olive grey, locally silty, slightly to very calcareous claystones, while the latter comprise yellowish grey, light olive grey, medium dark grey, microcrystalline and sucrosic limestones. The claystones are increasingly lighter in colour and also more calcareous towards the base of the section. Rare beds of off-white to light olive grey, very fine-to finegrained, micaceous, argillaceous sandstone are also noted.
Wireline log characteristics. The upper boundary is sharp; defined by a decrease in gamma ray response and an associated increase in sonic velocity, reflecting a lithological change to interbedded limestone and calcareous claystone. The lower boundary is moderately sharp; defined by a decrease in gamma ray response and an associated increase in sonic velocity, reflecting a lithological change to limestone. The Blue Lias Formation is characterized by a regular, serrated 'spiky' log motif, reflecting the limestone-claystone interbeds. The gamma ray response for the Blue Lias Formation is slightly subdued compared to the overlying Pabay Shale Formation.
The Broadford Formation: (1785-1845.5 m, log) Onshore, this formation has been described as a shallow water equivalent of the Blue Lias Formation. It comprises a succession of littoral carbonate and arenaceous sediments, locally represented by reefs and beach environments (Hesselbo et al., 1998). The lower parts of the sequence may yield reduced diversity marine faunas (Hallam, 1959).
Lithology. This formation is dominated by an interbedded sequence of claystones, limestones and sandstones. The claystones are medium dark grey to dark grey, olive grey, locally light brownish grey, in part silty, slightly to very calcareous, while the limestones comprise yellowish grey, light olive grey, medium brown grey, locally sandy, microcrystalline to sucrosic, mudstones. Sandstone beds occur towards the base of the section, comprising off-white to greenish grey, very fine-to fine-grained, slightly argillaceous, calcareous sandstones.
Wireline log characteristics. The upper boundary is moderately sharp; defined by a decrease in gamma ray response and an associated increase in sonic velocity, reflecting a lithological change to limestone. The lower boundary is sharp; defined by a decrease in gamma ray response and an associated decrease in sonic velocity, reflecting a lithological change to non-calcareous claystones. The Broadford Formation is characterized by an irregular, highly serrated log motif, reflecting the limestone, claystone, sandstone interbedding. Lithology. This interval solely comprises medium light grey, dark grey/olive black, waxy, non-calcareous, claystones. These sediments conformably overlie reddened limestones and calcareous claystones of the Stornoway Formation at 1861 m (log).
Wireline log characteristics. The upper boundary is sharp; defined by a decrease in gamma ray response and an associated decrease in sonic velocity, reflecting a lithological change to non-calcareous claystones. This interval is characterized by serrated wireline log motifs with moderately high gamma ray responses and moderately low sonic velocities.

Intrusives
Palaeogene. A number of thin (<10 m), dusky yellowish green to dark greenish grey, crystalline, fine-to medium-grained dolerite intrusions are noted within the interval between the Penarth Group and the lower section of the Pabay Shale Formation. On wireline log criteria, they are identified by their low gamma N. R. Ainsworth & I. Boomer ray responses and high sonic velocities, forming blocky log motifs.

BROAD-FORD
To date (April, 2001), only Copestake & Johnson (1989) have published records of the Lower Jurassic foraminifera from the Hebrides Basin, from the Loch Aline area of Morvern, Mull (Gribun) and Raasay. The Lower Jurassic ostracod faunas from the west coast of Scotland have been studied by Clark (1969), although, these data remain unpublished. Currently, this material is being re-examined by one of the present authors (NRA).

Late Pliensbachian (494-664 m)
An age no younger than Late Pliensbachian (margaritatus Zone) is indicated on the highest downhole occurrence of the calcareous benthonic foraminifera Dentalina matutina at (506 m). Confirmation of this age is indicated by the ostracod Ogmoconcha contractula Triebel at 506 m and Haplophragmoides lincolnensis Copestake at 549 m. The former taxon is restricted to the Late Pliensbachian in the Mochras Borehole (Ainsworth et al., 1989;Boomer, 1990), while the latter taxon ranges no higher than the margaritatus Zone (Copestake & Johnson, 1989 Although many of these ostracod species are known to range from the Early through to the Late Pliensbachian, none are thought to range into the Toarcian. Ostracod data from the Fastnet Basin, North Celtic Sea and the Mochras Borehole suggested that only two of these species (Ogomoconcha contractula Triebel and Pseudohealdia cf. etaulensis Apostolescu sensu Ainsworth) are stratigraphically restricted to the Late Pliensbachian (Ainsworth, 1987;Ainsworth et al., 1989;Boomer, 1990). In well L134/5-1, however, six taxa are restricted to this interval. The Late Pliensbachian ostracod assemblages are not as abundant as the foraminiferal assemblages. Similar to the foraminifera, a marked decrease in abundance occurs below mid-interval, with the earliest part of the Late Pliensbachian barren of ostracods. The assemblages within this interval are dominated by the Healdiidae, notably the genera Ogmoconcha and Ogmoconchella, with subsidiary Saipanettidae (Cardobairdia posteroprolata Ainsworth). No samples contained the latest Pliensbachian to earliest Toarcian 'vallate forms' of Ogmoconcha (of Malz, 1975)  One taxon is stratigraphically restricted to the Early Pliensbachian (Verneuilinoides mauritii (Terquem), while two taxa do not occur below the Early Pliensbachian (Glomospirella spp., Brizalina liasica (Terquem)). Throughout this interval, foraminifera are generally rare, of low diversity and are comprized almost exclusively of calcareous benthonic taxa, notably the Nodosariidae (Lenticulina varians (Bornemann)) and the Polymorphinidae. This marked decrease in foraminiferal diversity within the Early Pliensbachian has also been described from a number of localities throughout England, including Lincolnshire (Brouwer, 1969), Dorset (Barnard, 1950) and the Mochras Borehole (Copestake & Johnson, 1989 (Gründel) and Pseudomacrocypris subtriangularis Michelsen.
The occurrence of Gammacythere ubiquita Malz & Lord indicates an Early Pliensbachian (davoei Zone) age at 664 m. The presence of a tentatively identified specimen of Gammacythere klingleri Boomer suggests an Early Pliensbachian (ibex Zone) age at 747 m, while the occurrence of a caved specimen (884 m) of Ektyphocythere foveolata (Michelsen) indicates the presence of lowermost Pliensbachian (?lower ibex-jamesoni Zones) sediments within the well section. Ogmoconcha amalthei ?form A Michelsen is also a marker taxon, restricted to the Early Pliensbachian (Michelsen, 1975). From the 14 taxa recorded in this interval, 8 are stratigraphically restricted, while another 4 species do not occur below the Early Pliensbachian. The assemblages are again dominated by the Healdiidae, notably the genera Ogmoconcha and Ogmoconchella, in association with subsidiary Cytheracea such as Gammacythere. The faunas within this section are, however, generally sparser than those occurring in the overlying Late Pliensbachian.

Early Sinemurian (1326-?1582 m, swc)
A latest Early Sinemurian age is indicated by the highest downhole occurrence of the calcareous benthonic foraminifera Vaginulinopsis exarata (Terquem) and the ostracod Kinkelinella ?sinemuriana (Ainsworth) at 1326 m. The former taxon is stratigraphically restricted to the late semicostatum-early turneri Zones in west Scotland (Copestake & Johnson, 1989). The latest Early Sinemurian (lower tuneri Zone) is denoted by the occurrence of Vaginulinopsis exarata (Terquem) at 1326 m. The presence of Planularia inaequistriata (Terquem) confirms an Early Sinemurian age at 1449 m (swc). This taxon has a short stratigraphical range (bucklandi Zone) in Morvern, west Scotland (Copestake & Johnson, 1989), however, in well L134/ 5-1 it is envisaged to have a less restricted range, extending into the mid-Early Sinemurian. Of the 25 taxa recovered, 5 species are stratigraphically restricted, while another 11 taxa do not occur below the Early Sinemurian. The Early Sinemurian can be divided into an upper interval (1326-1494 m, swc) characterized by sparse foraminiferal assemblages (some specimens of which are presumed cavings) and a lower interval (1509-1582m, swc) characterized by large numbers of poorly preserved Haplophragmoides spp., with subsidiary Nodosariidae (Lenticulina varians (Bornemann)) and the Polymorphinidae. No direct faunal comparisons can be made with these Lituolidaedominated assemblages; however, the calcareous benthonic foraminifera are similar to those described from other sites in northern Europe.
All of the above ostracod taxa are envisaged to range later than the Early Sinemurian, with both Bairdia sp. 1 Ainsworth and Isobythocypris tatei (Coryell) occurring often in profuse numbers throughout the UK offshore (Ainsworth, 1989a, b;Ainsworth et al., 1987Ainsworth et al., , 1989Ainsworth et al., , 1998 Similar assemblages have been described by Ainsworth (1989a) and Ainsworth et al. (1987Ainsworth et al. ( , 1989) from a number of wells situated in both the North Celtic Sea Basin and Fastnet Basin. Comparable faunas are also known to occur in the more offshore regions of the English Channel Basin and the Southern North Sea (Ainsworth, pers. obs.).

Earliest Sinemurian-Hettangian (?1582-1844 m)
An age no younger than earliest Sinemurian (bucklandi Zone) is indicated on the highest downhole occurrence of the ostracod Ogmoconchella aspinata (Drexler) at 1582 m. The subsequent downhole occurrence of Ogmoconcha hagenowi Drexler and Ogmoconchella michelseni Ainsworth confirm this age at 1594 m.
Involutina liassica (Jones) is a common constituent of the foraminiferal fauna at 1710 m (swc). In the UK Early Jurassic, it is often recorded in abundance within the latest Hettangianearliest Sinemurian (late angulata-bucklandi Zones) (Copestake & Johnson, 1989). In well L1343/5-1 its abundance is thought to represent its earliest stratigraphic occurrence (late angulata-Zone). Of these 12 species only one taxon is stratigraphically restricted (Reinholdella dreheri (Bartenstein)). The interval is characterized by low diversity assemblages, with localized peak abundance levels of Haplophragmoides spp., Reinholdella spp. and Involutina liassica (Jones). Many of the taxa occurring within this section may be the product of cavings.
Ostracoda. Ten species of ostracod occur in the earliest Sinemurian to Hettangian. Highest downhole occurrences include Kinkelinella translucens (Blake), Ogmoconcha hagenowi Drexler, Ogmoconchella aspinata (Drexler), O. michelseni Ainsworth and O. ?serratostriata Ainsworth. The association of Ogmoconcha hagenowi Drexler, Ogmoconchella aspinata (Drexler), O. michelseni Ainsworth indicates an age no younger than earliest Sinemurian (bucklandi Zone) at 1594 m. All three taxa can often occur in profuse numbers. The presence of specimens tentatively assigned to Ogmoconchella serratostriata Ainsworth at 1844 m suggests an earliest Hettangian age. Six species are restricted to this interval. With the exception of Ogmoconchella serratostriata Ainsworth, all of the other nine species decrease in abundance below c.1770 m. This is envisaged to be a function of lithology resulting in poor recovery from the indurated limestones of the Broadford Formation. The ostracod assemblages are dominated by large numbers of the Healdiidae (notably Ogmoconcha hagenowi Drexler and Ogmoconchella aspinata (Drexler)), with subsidiary Cytheracea (Kinkelinella translucens (Blake)). The Hettangian to earliest Sinemurian ostracod assemblages recorded in well L134/ 5-1 are similar to contemporary assemblages described from Triassic/Jurassic micropalaeontology, Inner Hebrides other regions in northwest Europe, most of which are dominated by the Healdiidae. This faunal assemblage is also characteristic of contemporary horizons in the North Minch Basin (Ainsworth, pers. obs.).

PALAEOENVIRONMENTAL ANALYSIS
The foraminiferal and ostracod assemblages occurring in well L134/5-1 are very similar to contemporaneous records described from other areas in northwest Europe. The palaeoenvironmental reconstruction of the Hebridean area during the Early Jurassic (Hesselbo et al., 1998) places this exploration well at the southern end of a NNE-SSW-trending depositional basin opening seaward to the south, with major detrital sources to the west (Hebrides Platform) and east (Scottish mainland). Given the relatively fine-grained nature of many of the sediments occurring in well L134/5-1, the depositional environment is thought to have been more distal than the correlative sedimentary sequences on Skye to the northwest, typified by a higher incidence of coarser-grained sediments (siltstones and sandstones). The microfossil assemblages support this palaeogeographical interpretation since they are all indicative of fully marine conditions. There is little evidence of proximity to land or marginal marine conditions which would be indicated by the ostracod taxa Darwinula or Lutkevichinella.
Changes in the faunal composition of both foraminifera and ostracods in well L134/5-1 are illustrated in Figures 3 and 4. From these data, a number of distinct events with characteristic assemblages have been noted. None of the data, however, have taken into account the weight of the unprocessed sample.
The foraminifera display a number of distinct changes. Figure 3 (graphs C and D) illustrates species abundance and simple species diversity, and includes a 5-point moving mean line which highlights the major patterns of change. Foraminiferal abundance is generally higher than the ostracods throughout much of the studied interval, attaining a maximum of 71 specimens at 969 m. However, ostracods are more abundant within the Hettangian-earliest Sinemurian. Likewise foraminiferal diversity is generally higher than the ostracods (with a maximum 24 taxa at 549 m), with a number of distinct cycles clearly visible. Figure 4 (graphs D and E) shows the percentage of agglutinating and calcareous benthonic foraminifera present in each sample. Throughout much of the Early Jurassic, calcareous benthonic taxa are the prevalent group both in diversity and abundance, dominated by the Nodosaridae (Dentalina, Lenticulina, Lingulina, Marginulina), the Polymorphidae and, to a smaller extent, the Bolivinitidae (Brizalina liasica). Agglutinating foraminifera are less diverse and generally of moderate to rare abundance, dominated by Lituolidae (Haplophragmoides spp.). A number of localized peaks of Haplophragmoides spp. occur, however, between 1509 m and 1582 m (swc).
The ostracods also exhibit a number of distinct faunal changes throughout the Early Jurassic. Figure 3 (graphs A and B) illustrates species abundance and species diversity, including the 5-point moving mean. Ostracod abundance is highest within the Hettangian-earliest Sinemurian, with numbers reaching 62 specimens, whereas during the Early Sinemurian to Late Pliensbachian interval, totals do not exceed 35 specimens. Ostracod diversity remains relatively low throughout the Early Jurassic (never exceeding 8 taxa), however, again there are clearly a number of cycles. The percentage faunal composition of the ostracod assemblages is illustrated in Figure 4 , it should also be noted that the highest abundance of metacopids coincides with the ostracod diversity peaks, illustrating their importance within Early Jurassic ostracod assemblages. Of the 80 samples that yielded ostracods, 56 contained metacopids and in all but 10 of those samples, the metacopids make up more than half of all specimens, while 24 samples comprise more than 80%.
The earliest sediments examined in this study (Rhaetian, Penarth Group) are devoid of microfaunas. From both lithofacies evidence and palynological data (rare acritarchs and abundant miospores) a marginal marine environment is suggested for these uppermost Rhaetian sediments. The succeeding lowermost Jurassic sediments, the Broadford Formation of Hettangian age, yielded only small numbers of metacopid ostracods, in association with rare echinoderm debris. A shallow marine (inner shelf) environment is suggested, with deposition occurring during the continuing marine transgression initiated during the uppermost Rhaetian. The overlying Blue Lias Formation (Hettangian-earliest Sinemurian) yields low diversity foraminiferal and ostracod faunas. The former is dominated by calcareous benthonic taxa, notably the Nodosaridae (Lenticulina, Lingulina, Marginulina), with localized peaks of the Involutinidae (Involutina liassica) and Epistominidae (Reinholdella dreheri). Ostracods are dominated by Ogmoconcha and Ogmoconchella spp. (albeit in low diversity), with subsidiary Cytheracea (e.g. Kinkelinella translucens). Echinoderm debris, microgastropods and bivalves occur in large numbers throughout this interval. A shallow, slightly deeper, low energy, welloxygenated open marine (inner to middle shelf) environment is envisaged. The occurrence of common specimens of Reinholdella dreheri at 1754 m (swc) suggests either a slight shallowing or a period of marine restriction. The high abundance of metacopid ostracods within this interval and the low diversity and abundance of foraminifera (which occurs throughout northwest Europe) has been interpreted as the colonization of a transgressive sea by an opportunistic and successful group of organisms.
The Lower Sinemurian can be subdivided into two units. The lowermost part of the Pabay Shale Formation of intra-Early Sinemurian age is marked by large numbers of agglutinating foraminiferids (squashed Haplophragmoides spp.) occurring as three distinct peaks. Calcareous benthonic foraminifera, notably the Nodosaridae and the Polymorphinidae, occur in smaller numbers. This is associated with a marked decline in the ostracod faunas, especially with respect to the Metacopina. Although rare, the ostracod assemblages comprise mainly smooth-walled ostracods of the Bairdiidae and Cyprididae (Bairdia, Isobythocypris, Pseudomacrocypris and Cardobairdia). This association of faunal elements may suggest localized events of slightly reduced (dysaerobic) oxygen conditions upon the sea floor, within a dominantly shallow marine (inner to middle shelf) environment. Bottom water conditions are envisaged to have ameliorated during the latest part of the Early Sinemurian, denoted by a marked increase in the ostracod faunas, notably the ornate Cytheracea (Ektyphocythere). The foraminiferal faunas, however, remain sparse both in diversity and abundance, with no particular group dominating the assemblages. An increase in calcareous benthonic foraminiferal diversity and abundance occurs in the earliest part of the Late Sinemurian,  Microfaunal diversity and abundance dramatically decline within the earliest Pliensbachian, suggesting restricted water circulation within an inner to mid-shelf environment. This is supported by the occurrence of large numbers of microgastropods within this interval. Midway through the Early Pliensbachian (top Pabay Shale Formation), conditions ameliorated, denoted by an increase in microfaunal recovery. Similar to the Late Sinemurian interval, the foraminiferal faunas almost exclusively comprise calcareous benthonic taxa, dominated by the Nodosaridae (Lenticulina, Marginulina) and the Polymorphinidae. Although generally rarer than the foraminifera, a marked peak in ostracod abundance (35 specimens) occurs at 747 m. The ostracods are dominated by the Metacopina, notably Ogmoconcha, Ogmoconchella, in association with the ornate genus Gammacythere. A marked decline in microfauna, associated with large numbers of both echinoderm debris and microgastropods, occurs within the basal part of the Scalpa Sandstone equivalent of earliest Late Pliensbachian age. A shallow marine, partly restricted and/or shallow water environment is suggested. A marked increase in diversity and abundance occurs within the latest Pliensbachian, with the foraminiferal assemblages dominated by rich calcareous benthonic faunas, including the Nodosaridae (Dentalina, Frondicularia, Lenticulina, Lingulina, Marginulina, Saracenaria) and the Bolivinitidae (Brizalina liasica). Although rare, agglutinating taxa are dominated by Haplophragmoides and Tro-chammina. Late Pliensbachian ostracods are moderately common, again dominated by smooth-walled taxa, including the Cyprididae (Cardobairdia), and the Metacopina (Ogmoconcha, Ogmoconchella and Pseudohealdia). Deposition of the Scalpa Sandstone equivalent is envisaged to have occurred within shallow (inner shelf) open marine, well oxygenated environments, distal to an arenaceous source.

CONCLUSIONS
Exploration well L134/5-1 has yielded an extensive Triassic to Lower Jurassic sequence, which contains a valuable biostratigraphical record permitting correlation with contemporaneous sites throughout much of NW Europe. Although the Rhaetian sediments were barren of microfaunas, the Hettangian to Upper Pliensbachian yields a rich and diverse foraminiferal and ostracod fauna. A total of 100 taxa, comprising 11 agglutinating and 44 calcareous benthonic foraminifera, in association with 45 species of ostracod, are recorded. A number of taxa are envisaged to be new, however, poor preservation precludes a complete taxonomic review of this material. Not surprisingly, many of the microfaunal assemblages possess very strong similarities to those described from the west coast of Scotland, the Mochras Borehole, Fastnet and North Celtic Sea basins, as well as southern England and the various North Sea basins. Faunal comparisons can also be made further afield to those of France, Germany, Portugal and Sweden.
The environment of deposition was entirely marine from the earliest Hettangian through to the Late Pliensbachian, but a combination of local tectonic controls and eustatic changes in sea-level caused a number of abrupt changes in faunal diversity, turnover and assemblage composition. It has not been possible to ascertain the relative impact of each of these dynamic processes upon the microbenthos. Figure 5 indicates that ostracod diversity remains low despite increasing sample size, whereas the foraminiferal diversity increases with abundance.
The Early Jurassic Hebrides microfossil record can be summarized as follows.  Fig. 5. The number of specimens recorded is plotted against the number of species for both foraminifera and ostracods in each sample. The record for both groups indicates that the small samples investigated do not yield fully representative assemblages. It is commonly accepted that a sample of 300 individuals is required. The ostracods remain at particularly low diversity levels throughout, even in the most abundant samples.
The Hettangian to earliest Sinemurian interval is characterized by relatively high ostracod abundance dominated by the Metacopina; this is a similar pattern to elsewhere in Europe. The single Triassic sample examined is barren of ostracods and foraminifera.
The Early Sinemurian marks a switch to low abundance Cyprididae/Bairdiidae-dominated assemblages, with relatively high proportions of agglutinating foraminifera. This is followed by a period dominated by cytheracean ostracods.
The Late Sinemurian witnesses two peaks in diversity and abundance of both foraminifera and ostracods between the depths 1000 m and 1300 m. These peaks are dominated by metacopine ostracods and calcareous benthonic foraminifera.
The Early Pliensbachian yields poor foraminiferal assemblages, but with a peak in ostracod diversity and abundance at about 750 m; subsequently diversity and abundance of both groups falls sharply.
The Late Pliensbachian interval is characterized by increasing biodiversity, dominated by metacopine ostracods and calcareous benthonic foraminifera.