Jurassic, Palaeogene volcanic districts of Scotland

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Emeleus, C H, and Bell, B R. 2005. British regional geology: The Palaeogene volcanic districts of Scotland. Fourth edition. Keyworth, Nottingham: British Geological Survey.

Introduction

Mesozoic basins of the Inner Hebrides and onshore outcrops. P914123
Upper Triassic to Jurassic lithofacies of the Inner Hebrides. P914124
Honeycomb weathering in the Elgol Sandstone, Elgol, Skye. P580459
Concretions in the Valtos Sandstone Formation, Bay of Laig, Isle of Eigg. P580455
Landslips at The Quiraing,Trotternish, Skye. P580491

Rocks ranging in age from the Triassic to the Late Cretaceous occur throughout the Inner Hebrides and on Arran (P914123). They are part of the extensive basinal deposits found mainly offshore, and in many instances represent the feather edges of these successions. The basins are important structural elements in the Hebridean Igneous Province. In addition to their dominant control on Mesozoic sedi­mentation, the basins and associated structures appear to have influenced the emplacement of the Paleocene central complexes and possibly also the accumulation of the lava successions.


Jurassic

Thick sequences of Jurassic strata are important components of the Mesozoic offshore basins and parts of these successions extend onshore at several localities (P914123). Following the conti­nental conditions that were prevalent during the Triassic Period, marine sedimentation predominated in Early Jurassic time. Shallow-water and brackish water, deltaic deposition characterised much of Mid Jurassic time, although there was a return to marine sedimentation in the Callovian, which continued into the Late Jurassic. With the exception of the Great Estuarine Group (uppermost Bajocian–Bathonian), the ammonite faunas permit correlation with successions in England. The lithological variation in time and space through the district is summarised in P914124.

The Jurassic rocks are commonly immature and incompletely lithified; even the earliest deposits of this age were probably buried to a depth no greater than about 1 km prior to eruption of the Paleocene lavas. A maximum of about 2 km thickness of lavas may have covered some of the Mesozoic successions but it is unlikely that the sedimentary rocks were greatly affected by heat from that source (England, 1992a). By contrast, significant thermal metamorphism has occurred in the vicinity of the central complexes. The formation of hornfels is limited to the country rocks immediately adjacent to the central complexes (less than 500 m), although hydrothermal alteration extends for several kilometres from the margins of the large complexes, for example on Skye and Mull (Taylor and Forester, 1971). Elsewhere, there is generally little more than slight induration of the sedimentary rocks, and alteration of their organic content extending from dykes and other minor intrusions for a distance of up to half their width (e.g. Hudson and Andrews, 1987; Bishop and Abbott, 1993). However, given the great abundance of intrusions in some dyke swarms and sheet complexes, the aggregated effects may have been significant for hydrocarbon maturation.

In addition to the references cited in the text, there is much detailed information about the Jurassic rocks of Skye and Raasay in Morton and Hudson (1995).


Lower Jurassic

In the southern part of the Hebridean area, the lower part of the Lias Group is Hettangian to lower Sinemurian (bucklandi Zone;Table 4), and consists mainly of limestones and fissile calcareous mudstones that are lithologically similar to beds of equivalent age in southern England. They have consequently been termed the Blue Lias Formation (Oates, 1978). In Ardnamurchan, these lithologies interdigitate with more arenaceous rocks (sandstone, sandy limestone, siltstone), which, in turn, become predominant at similar stratigraphical levels to the north on Skye and Raasay, where the beds are now termed the Breakish Formation (formerly the ‘Lower Broadford Beds’). The Breakish Formation is overlain by non-calcareous sandstones, siltstones and mudstones of the Ardnish Formation (formerly the ‘Upper Broadford Beds’; Morton, 1999a, b). The overlying Pabay Shales, Scalpay Sandstone, Portree Shale and Raasay Ironstone formations are recognised throughout the district.

Arran

A remanié mass of Lias fissile mudstone and limestone that crops out within the Central Arran Ring-complex has yielded a large bivalve fauna belonging to the angulata Zone (Hettangian) as well as ammonite remains (Schlotheimia). This is the only occurrence of Jurassic rocks on Arran. When taken in conjunction with nearby remanié masses of Cretaceous rocks, it provides a strong indication that Mesozoic strata were formerly much more widely distributed over western Scotland, as they are now in Antrim, a relatively short distance to the south-west (e.g. Mitchell, 2004).

Mull

With the exception of small outcrops of limestone, calcareous mudstone and sandstone on the west coast at Aird na h-Iolaire, and beds of the Blue Lias and Pabay Shale formations at and near Tobermory, Jurassic rocks are restricted to the east and south of the island. They crop out in the cores of anticlines marginal to the Mull Central Complex, from Scallastle Bay almost to Loch Spelve, in many places on the east coast from Loch Don to Loch Buie, and also at Carsaig Bay. The beds range in age from Hettangian through to Bajocian (Mid Jurassic; garantiana Zone) and contain a moderately abundant marine fauna. Near Port nam Marbh, there is a fairly con­tinuous succession from the uppermost 30 m of the Pabay Shale Formation, represented by slightly calcareous siltstones with abundant Gryphaea cymbium and Pecten aequivalvis, to the Bajocian. At Torosay, the Pabay Shale Formation contains the Torosay Sandstone Member, which is over 20 m thick and is cross-bedded in places (Hesselbo et al., 1998).

Morvern

The most complete exposures are at Loch Aline, where the Blue Lias and Pabay Shale forma­tions are each about 30 m thick. The sequence thins eastwards towards Inninmore; north-westwards, the Lias crops out below Cretaceous rocks on the shores of Loch Teacuis but is largely absent beneath the Cretaceous and Paleocene outliers north of the loch. An early Sinemurian hiatus is recognised in Morvern, which appears to be a local phenomenon (Hesselbo et al., 1998) and is attributed to sediment starvation. The existence of a widespread, diachronous unconformity, supposedly early Sinemurian in the south and late Sinemurian in the north, is now discounted (compare Fyfe et al., 1993).

Ardnamurchan

Despite being much disturbed by faulting and folding, and having undergone pervasive and locally severe thermal metamorphism, a fairly complete succession of Lower and Middle Jurassic rocks is preserved on the south side of the peninsula. Indurated, fossiliferous Lias mudstones, thin limestones, calcareous sandstones and thin ironstones are intruded by numerous inclined sheets; they crop out along the southern margin of the central complex from the vicinity of Port Mìn to Kilchoan, and form scattered outcrops eastwards as far as Loch Mudle. On the north coast, Lias beds crop out at several localities for about 4 km west of Fascadale Bay, and to the east more than 70 m of mudstone and limestone (interdigitating Blue Lias and Breakish formations) overlie Triassic rocks at Swordle. Fossils from the angulata and bucklandi zones have been obtained from this area (Oates, 1978).

A succession of beds ranging from the Pabay Shale Formation to the Middle Jurassic Bearreraig Sandstone Formation has been recognised amongst the dense cone-sheet swarms around Kilchoan and Sròn Bheag (Richey and Thomas, 1930). Rocks belonging to the Portree Shale and Scalpay Sandstone formations occur among the isolated outcrops of Jurassic strata on the north coast of the peninsula.

Rum

Thin, fault-bound slivers of indurated to highly metamophosed limestone, sandy limestone, sandstone, mudstone and rare ironstone are preserved within the Main Ring Fault of the central complex on the eastern slopes of Beinn nan Stac. The limited fauna of these beds, and their lithological similarities to Lower Jurassic rocks east of Broadford, Skye, indicate that they probably correspond in the main to the Breakish and Ardnish formations, although the thin bed of altered ironstone at Dibidil may be from near the base of the Pabay Shale Formation (compare Smith, 1985).

Skye

A wide outcrop of Lower Jurassic strata extends south from Broadford Bay through Strath to Loch Slapin. The type localities for the Breakish and Ardnish formations are to the east of Broadford, and for the Pabay Shale Formation on the island of Pabay. Lias rocks crop out on the south of Scalpay and on the northern slopes of Glamaig where they have been much disturbed by granites of the Western Red Hills Centre. Other outcrops occur to the west of the Camasunary Fault, where the limestones and silty beds are generally thermally altered, and on the north side of Soay Sound.

A deep, oil-exploration borehole through the Paleocene lavas and the Mesozoic rocks was drilled at the southern end of the Waternish peninsula, about 5 km north-east of Dunvegan, and provides some information about the likely Lower Jurassic succession (Hesselbo et al., 1998, fig. 23).

The Breakish Formation in its type locality at Ob Lusa consists mainly of sandy limestones, calcareous sandstones and thin fissile mudstones with an 8 m-thick, massive sandstone near the top of the succession. Bivalves are abundant in certain beds, although diagnostic ammonites are not common. The coral Isastrea is found in the Ob Lusa Coral Bed, near the base of the succession, and Thecosmilia occurs at a higher level at Ob Breakish. The lower part of the Ardnish Formation (formerly the Upper Broadford Beds) is well exposed on the Ardnish peninsula, where mudstone, siltstone and sandstone occur in coarsening-up cycles. The rocks are commonly micaceous and in places also ferruginous; they yield abundant Gryphaea arctuata and ammonites, especially Coroniceras and Arnioceras. An oolite containing chamosite oxidised to goethite forms the Ardnish Ironstone in the lower part of the succession (Morton and Hudson, 1995).

On the coast of Loch Slapin, south of Camas Malag, sandstones, siltstones and limestones of the Ardnish Formation rest directly on dolostones of the Ordovician Strath Suardal Formation, the Breakish Formation being absent. Here, fossils from the semicostatum Zone are common and the basal Jurassic layer is crowded with Gryphaea and other bivalves attached to the under­lying Ordovician dolostone. In sea cliffs a short distance to the north-west, the overall lithofa­cies illustrates one of the few unequivocal exposures of a Jurassic shoreline in Britain. Clefts and hollows in a palaeokarst topography eroded in the Ordovician dolostone are infilled with a thin sequence of sedimentary breccia, conglomerate and rhythmically bedded, silty sandstone and sandstone. These rocks comprise the Camas Malag Formation (Nicholson, 1978); they are of Early Jurassic age (Amiri-Garoussi, 1982) and were laid down under marine conditions (Farris et al., 1999). On the Strathaird peninsula, part of a tibia belonging to a carnivorous dinosaur has been found in beds of probable Sinemurian age (Benton et al., 1995).

On the island of Pabay, only the upper part of the Pabay Shale Formation is exposed, from the Suishnish Sandstone Member, upwards. The beds, which are over 75 m in thickness, are of dark, micaceous fissile mudstone with red-weathering (iron-bearing) carbonate concretions, from which Murchison collected the type specimen of Platypleuroceras (Ammonites) brevispina in 1826. The whole formation is excellently exposed on either side of Rubha Suishnish on the shores of Loch Slapin and Loch Eishort, where a detailed succession has been established (Hesselbo et al., 1998). Soft, grey, micaceous mudstones with calcareous concretions become sandier up the succession, passing into thick grey micaceous sandstones of the Suishnish Sandstone Member. Ammonites diagnostic of the semicostatum Zone are present in the lower part of the succession, as well as bivalves and brachiopods, and towards the top, ammonites from the obtusum Zone include Asteroceras stellare, Promicroceras planicosta and Xipheroceras sp. (Hesselbo et al. 1998). Sinuous, lobate contacts between Paleocene basalt dykes and silty sandstones exposed in a roadstone quarry north of Suishnish (and elsewhere) indicate that the sedimentary rocks were poorly consolidated and probably wet when intruded. Sandstones of the Hallaig Sandstone Member (Hesselbo et al., 1998) are also present at Loch Eishort but these are now considered to belong to the Ardnish Formation (Morton, 1999a). The Pabay Shale Formation also crops out on south-east Scalpay and on the west side of Beinn Dearg Mhòr in Strath.

Micaceous sandstones, siltstones and calcareous sandstones of the Scalpay Sandstone Formation crop out on Scalpay and at Rubha na Sgianadin, north-west of Broadford. The formation also crops out in small areas on the west side of Loch Slapin, in shore exposures on the west side of the Sound of Raasay, north and south of Portree Harbour, and in a cliff south of Holm, in Trotternish. It belongs to the spinatum Zone (Upper Pliensbachian) and is some 30 m thick.

The principal occurrence of the Portree Shale and Raasay Ironstone formations is found in the cliffs and on the shore between Holm Island and Tianavaig Bay in eastern Skye, although the beds are almost everywhere obscured by landslips. North of Portree Harbour, more than 20 m of grey, micaceous, ferruginous siltstone, ooidal ironstone and dark-weathering micaceous siltstone belong to these formations. The abundant ammonites in the topmost of these beds indicate that they belong to the falciferum Subzone (Middle/Lower Toarcian). A thin seam of jet near Holm Islandis at a similar stratigraphical level to the Jet Rock of Yorkshire. To the north, a borehole near the mouth of the Bearreraig River proved about 1.6 m of sideritic chamosite­ oolite belonging to the Raasay Ironstone Formation, but it was not of economic grade. There are also two small exposures of the Portree Shale and Raasay Ironstone formations on the east coast of the Strathaird peninsula a short distance south of Faoilean on Loch Slapin.

Raasay

The Breakish Formation is sparsely exposed on Raasay. The greatest development occurs at the east end of the bay at Hallaig, where about 35 m of interbedded limestone and mudstone are seen. The beds contain a bivalve-dominated fauna that includes Modiolus, Cardinia and Liostrea. Ammonites (Franziceras sorlei, liasicus Zone) from the lowermost beds indicate that the marine transgression affected this area earlier than the Broadford area, and was therefore diachronous (Morton, 1999b).

East of Hallaig, a 50 m-thick cliff section exposes a succession of limestones and fissile mudstones in its lower part, with calcareous siltstones and micaceous sandstones (also well exposed in the waterfall at the seaward end of Hallaig Burn), seen at higher levels. These latter beds belong to the Hallaig Sandstone Member which marks the top of the Ardnish Formation (P914124) (Morton and Hudson, 1995; Morton 1999a; compare Hesselbo et al., 1998). Bivalve, brachiopod and crinoid remains occur in the lower part of the member, and ammonites (Arnioceras, Euagassiceras) are present in beds about 20 m above the base, but the upper half is only sparsely fossiliferous. The Pabay Shale Formation is exposed in the Allt Fearns, south-west of Beinn na’ Leac, where it consists of dark coloured, fissile mudstones that pass upwards into micaceous, silty rocks. Bivalves (including Gryphaea, Cardinia, and Hippopodium ponderosa) and a variety of ammonites (including Echioceras, Crucilobiceras and Vininodiceras simplicostata) occur in these beds.

The Scalpay Sandstone Formation is prominent on the east coast of Raasay from Screapadal to Hallaig and south-west to Inverarish. The formation is nearly 100 m thick east of Beinn na’ Leac, where micaceous siltstones and flaggy sandstones are overlain by about 30 m of massive sandstone. An abundant fauna has been obtained from black, calcareous sandstone beds with iron-rich ooids, immediately below the topmost sandstone. Bivalves are present (Pseudopecten aequivalve, Gryphaea gigantea) and several species of the ammonite Pleuroceras, indicating the apyrenum Subzone (uppermost Pliensbachian). The ooids here, and in Jurassic ironstones elsewhere in the Inner Hebrides, are generally described as chamositic, but are more likely to be composed of the mineral berthierine, which is compositionally similar but structurally different (e.g. Deer et al., 1962). A new species of glypheoid lobster (Pseudoglyphea foersteri) has been described from the Scalpay Sandstone. Examples were found near Hallaig and also south of Holm Island on Skye (Feldmann et al., 2002; Elliott and Feldmann, 2003).

The Portree Shale Formation comprises 2 to 3 m of dark, micaceous siltstone exposed in the Inverarish Burn and in nearby, abandoned opencast workings, where a thickness of 2.4 m of the overlying Raasay Ironstone Formation is found. The base of the Raasay Ironstone Formation consists of a thin chamosite ooid-bearing black fissile mudstone with abundant fossils (e.g. Dactylioceras sp.). A thin-bedded, ooidal chamosite ironstone (about 2 m thick) was the principal source of iron ore (P532649). The ironstone is cross-bedded in places, and was deposited in shallow water. Randomly orientated belemnites are a striking feature on bedding planes exposed on the floor of the former opencast workings. Comprehensive suites of fossils have been obtained from old iron ore dumps near Suishnish Pier, including the ammonites Dactylioceras taxophorum, Hildoceras laticostata, Harpoceras falciferum and Cleviceras elegans, all from the falciferum Subzone (Lower Toarcian).

Shiant Isles

About 10 m of indurated fissile mudstone is preserved between Paleocene dolerite sills. The mudstone has yielded species of Dactylioceras characteristic of the falciferum Zone. The topmost beds may be of Bajocian age.


Middle Jurassic

Onshore deposits of this age are largely confined to the northern part of the district, where the majority of the type sections occur on Skye. The marine, ammonite-bearing Bearreraig Sandstone Formation (Aalenian–Bajocian) includes the thickest sandstones of the onshore British Jurassic. The overlying deposits of the Great Estuarine Group (top Bajocian–Bathonian) (Table 5) were laid down in lagoons, lagoonal deltas and mudflats, which varied from freshwater through brackish to marine (e.g. Harris, 1992). A major hiatus and, locally, an unconformity separate the Raasay Ironstone Formation from the overlying Middle Jurassic strata in the north of the district.

Skye and Raasay

Extensive outcrops of Middle Jurassic rocks occur on the Strathaird peninsula, in Trotternish where a complete succession from the Aalenian to the Oxfordian is preserved, and in southern Raasay.

Bearreraig Sandstone Formation

The Bearreraig Sandstone Formation outcrop extends for several kilometres in near-vertical cliffs in eastern Trotternish, where it comprises massive sandstones, commonly calcareous, with rich ammonite faunas at several localities. The succession across the Aalenian–Bajocian boundary is exposed in the pipeline cutting above the hydroelectric powerhouse at the south side of Bearreraig Bay. The lithology and ammonite fauna at this locality have been described in detail by Morton (e. g., in Morton and Hudson, 1995) who proposed the section as the boundary stratotype for the basal boundary of the Bajocian Stage. Remains of a thyreophoran dinosaur have been recovered from the Bearreraig Sandstone at Bearreraig Bay (Clark, 2001). Below the sandstones south of Bearreraig Bay, about 21m of micaceous mudstone of the Dun Caan Shale Member (ranging from late Toarcian, but mainly Aalenian in age), rest disconformably on the Raasay Ironstone Formation. On Raasay, the Dun Caan Shale Member crops out above the old opencast workings, but the thickest development (about 30 m) is at Gualann na’ Leac, on the north-east side of Beinn na’ Leac. Farther south, the Druim na Fhuarain Sandstone Member of the Bearreraig Sandstone Formation forms a broad bench on the eastern side of the Strathaird peninsula. These sandstones are predominantly cross-bedded and contain only rare ammonites. Cross-bedding indicates two flow directions, one southerly and a more common northerly one. Both may occur at the same locality and are attributed to tidal flow action redistributing the sediments. In keeping with this vigorous depositional environment, a large proportion of the fossils are preserved only as shell fragments. In southern Strathaird, up to 470 m of sandstone is present, capped by 12 m of pyritiferous fissile mudstone representing the Garantiana Shale Member. The abrupt change from sandstone to mudstone at the base of the Garantiana Shale Member marks the onset of deeper water conditions, and corresponds to a widely recognised transgression elsewhere. The sandstones thin northwards and cross-bedding appears at progressively higher horizons such that, on Raasay, the cross-bedded sandstones do not occur until well above the Aalenian–Bajocian boundary. There, the sedimentary structures indicate a north-north-east-directed water flow. One of the more spectacular outcrops of the Bearreraig Sandstone on Raasay is the deeply fissured (and possibly landslipped) mass that forms the summit of Beinn na’ Leac.

Great Estuarine Group

The Cullaidh Shale Formation consists of dark bituminous fissile mudstones and black sandstones; it varies in thickness from 12 m in the south to about 2 m in the north. The formation rests, with gradational boundary, on the Garantiana Shale Member, and may include the Bajocian–Bathonian boundary. It is overlain by the Elgol Sandstone Formation (30 m thick), which comprises white, pure, non-calcareous sandstone of deltaic origin (Morton and Hudson, 1995). At the type locality, it consists of a classic coarsening-upwards deltaic cycle (Harris, 1989), and forms a prominent cliff with spectacular honeycomb weathering (P580459).

The Lealt Shale Formation is generally divided into two members. The top of the lower, Kildonnan Member is marked by a thin algal stromatolite band on Eigg (the type locality) and elsewhere throughout the district; it is overlain by the Lonfern Member. The mudstones and thin limestones of the formation, together with the algal band, show little variation from Eigg to northern Trotternish, indicating uniform, low-energy conditions of sedimentation. The beds are highly fossiliferous, although the diversity of gastropods, brackish water bivalves, ostracods and concostrachans is limited. Fish and other vertebrate remains are common (see under Eigg and Muck). The formation is well exposed in the coast section north of Elgol. Re-alignment of the road north of Portree has somewhat obscured the original type section at Lealt, but sections are available at the base of the steep cliff between Inver Tote and Lonfearn, one of which yielded the first dinosaur footprint discovered in Scotland (Andrews and Hudson, 1984).

The Valtos Sandstone Formation forms distinctive exposures of pale yellow sandstone that commonly contains abundant large, near-spherical, carbonate concretions or doggers over much of the area (P580455). In Trotternish, the formation is about 120m thick, with two cross-bedded upward-coarsening sandstones of similar thickness, separated by about 27 m of fine-grained sandstones, silty mudstones and limestones containing Neomiodon shells. There is evidence of cyclic sedimentation; in places five cycles of mudstone and limestone passing up to sandstone with concretions have been recognised. The Valtos Sandstone Formation near the type locality has yielded diverse bones and fossil footprints, including a bone from a sauropod dinosaur (Clark et al., 1995). On Raasay, exposures of concretionary Valtos Sandstone occur between Dun Caan and the Screapadal Fault. The equivalent beds on Strathaird are thinner (about 24 m) and consist largely of sandy limestone, limestone and mudstone; the characteristic concretions are absent. Despite induration and alteration by the adjacent granite, concretions are still visible in the Valtos Sandstone Formation on the eastern slopes of Glas Bheinn Dearg in Strath, where it dips steeply eastwards. The Valtos Sandstone Formation is also present in north-west Skye, at Loch Bay and Waterstein. The formation has been interpreted as a system of delta lobes built out into the lagoons in which the Lealt Shale Formation had been deposited (Hudson and Harris, 1979; Harris, 1992).

The base of the Duntulm Formation is marked everywhere by the presence of limestone beds crowded with remains of the oyster Praeexogyra hebridica. Fossil shell banks, up to 2 m thick, were developed in marine to brackish water lagoons (Andrews and Walton, 1990). At Duntulm, a thickness of about 55 m of strata consists of interbedded mudstone and limestone with a marine to brackish water fauna. Several of the limestones contain nodular algal masses. Dinosaur tracks occur in the Duntulm Formation at Staffin Bay (Clark et al., 2004; Plate 8). Towards the top of the section there are indications of terrigenous influence and freshwater incursions as the beds are more sandy and contain the bivalves Unio and Neomiodon. There are a few poor exposures of the Duntulm Formation on Raasay. The formation is also present in the Strathaird succession, in which there is no indication of a freshwater influence or that land lay nearby. There are fossiliferous outcrops at Loch Mòr, Waternish and Waterstein Head, Duirinish.

The Kilmaluag Formation consists of calcareous mudstones with alternations of indurated layers which may be nodular; some are also dolomitic. Thin sandstones are present in the Trotternish successions, where Andrews (1985) recognised a ‘clastic facies’, but these are not found in the southern localities on Skye, nor on Eigg and Muck, which instead have an ‘argillaceous limestone facies’. The beds are interpreted as having been deposited in short-lived lagoons and the fossil faunas indicate deposition in low-salinity environments. The gastropod Viviparus and the bivalve Unio are present, but they are not as abundant as the ostracods (e.g. Theriosynoecum) which gave the formation its former name, the Ostracod Limestones. The formation also occurs at Waternish and is well exposed on Strathaird, but it is not present on Raasay. A notable reptile and mammal fauna was obtained from Strathaird (Waldman and Savage, 1972; Waldman and Evans, 1994; Evans and Waldman, 1996); when discovered, this was only the second recorded occurrence of Mid Jurassic mammalian fossils in the world.

The Skudiburgh Formation consists of red and grey-green mottled silty mudstone and dark claystone, sandstone and lenses of silty sandstone. From its variegated colours it was formerly known as the ‘Mottled Clay’. At the type locality, the claystone and siltstone sequence is about 3.5 m thick and contains channel sands and small calcareous concretions, possibly caliche. The Skudiburgh Formation also marks the top of the Great Estuarine Group in the Strathaird succession. The formation is of alluvial origin; floodplain, channel and overbank deposits are present, which formed in a coastal plain, terrestrial environment during a late Bathonian regression (Andrews, 1985).

Eigg and Muck

The Middle Jurassic successions of Eigg and Muck have been described in some detail by Hudson (in Emeleus, 1997). The 150 m-thick succession on Eigg is the most complete, and comprises strata that range from the top of the Bearerraig Sandstone Formation up to the Kilmaluag Formation. The principal outcrops are in cliffs on the coast, from the Bay of Laig to near Eilean Thuilm in the north of the island, and discontinuous exposures also occur on the foreshore and at the base of the Paleocene lavas on the east coast. On Muck, the top of the Valtos Sandstone Formation, a full succession of highly fossiliferous Duntulm Formation strata, and the lower part of the Kilmaluag Formation are all well exposed on the foreshore at Camas Mòr.

The Bearreraig Sandstone Formation is limited to small exposures of calcareous sandstone with bivalve remains, at and below the high water mark in north-east Eigg. The overlying Lealt Shale Formation is well exposed on the north shore, although there the outcrops are greatly complicated by dolerite sheets and in places are obscured by landslips. Both the Kildonnan and the Lonfern members are present, separated by a distinctive bed of stromatolitic algal limestone, which occurs widely at this horizon in the Inner Hebrides. An abundant shelly fauna, including the bivalves Praemytilus, Unio and Tancredia, has been obtained from the fissile siltstones, thin limestones and sandstones of the lower, Kildonnan Member at the type section on the east coast of Eigg (e.g. Hudson and Wakefield, 1999). This section contains Hugh Miller’s ‘Reptile Bed’ which has yielded sharks’ teeth (Hybodus) and plesiosaur remains (Plesiosaurus dolichoderius) (Hudson, 1966). Vertebrate remains have also been found on the north shore.

Outcrops of the Valtos Sandstone Formation extend from the east coast near Kildonnan, around the north of Eigg to the Bay of Laig, where several cycles of sedimentation are recognised. Particularly fine examples of carbonate concretions occur at the Bay of Laig (P580455). The concretions were probably formed at depths of several hundred metres and at temperatures of 31º to 34ºC, when meteoric pore waters dissolved carbonate from shells. The concretions started to grow about 15 Ma after burial of the host sediments and the larger, metre-sized examples may have taken several million years to form (Wilkinson, 1992).

The section in Laig Gorge, south-east of the Bay of Laig, contains exposures of both the Duntulm and Kilmaluag formations, as well as Cretaceous strata. Thick limestones crowded with the oyster Praeexogyra hebridica, typical of the Duntulm Formation, give way to mudstones with only thin oyster beds. The mudstones grade upwards into limy mudstones and thin limestones of the Kilmaluag Formation, which contain an ostracod and conchostracan (Antronestheria) fauna. There are a few small exposures of both formations north of Laig Gorge, and both must be present beneath a cover of till and landslip in Cleadale, although they are progressively cut out eastwards by the unconformable base of the Upper Cretaceous strata and the Paleocene lavas.

On the foreshore at Camas Mòr, Muck, Mesozoic strata from the upper part of the Valtos Sandstone Formation are exposed. The beds include typical concretionary sandstones, and grade up into highly fossiliferous limestones of the Duntulm Formation, in which shelly layers packed with Praeexogyra hebrideca are conspicuous. The lower part of the Kilmaluag Formation is seen in several small limestone outcrops totalling about 14 m in thickness. The higher beds are partly dolomitic, a feature first recognised from the mineralogy of the distinctive calc­silicate hornfelses adjacent to a large gabbro dyke (p. 78). The fauna includes conchostracans of the genus Pseudograpta and the gastropod Viviparus.

Mull and Ardnamurchan

About 30 m of sandy limestone and calcareous sandstone of the Bearreraig Sandstone Formation are capped by 1 to 2 m of siltstone of the Great Estuarine Group on the east coast at Port na Marbh, south-east Mull. Similar beds crop out fairly continuously beneath the lavas on the west limb of the Loch Don Anticline, and discontinuously on the east limb as far as Duart Bay. On the Ardnamurchan peninsula, Middle Jurassic rocks are limited to massive sandstones and subordinate limestones of the Bearreraig Sandstone Formation exposed in screens amongst the basalt sheets on Maol Buidhe and at Sròn Beag. Despite the effect of thermal metamorphism, typical Aalenian fossils have been obtained from these rocks (Richey and Thomas, 1930).


Upper Jurassic (and upper part of the Callovian)

The principal developments of the Oxfordian and Kimmeridgian occur in Skye but no strata younger than the Kimmeridgian have been reported from the district. The beds were deposited in offshore marine conditions. The transgression began in the Callovian with the deposition of the fully marine Carn Mor Sandstone Member of the Staffin Bay Formation, followed by the Upper Ostrea Member, which has a marine to brackish water fauna and has yielded Callovian palynomorphs (Riding, 1992). The overlying, fully marine Staffin Shale Formation contains an abundant fauna, including ammonites that permit zonal correlation with the Upper Jurassic strata of England (Table 6) and the Boreal Realm of Greenland and the Arctic.

Skye

The youngest Jurassic strata in the district occur in a shallow anticline beneath the Paleocene lavas of Trotternish, although there the beds are largely obscured beneath landslips (P580491), and in Strathaird. There are small outcrops near Uig and in the Kilmaluag River, but the most extensive occurrences are on the shoreface at the west side of Staffin Bay. These beds, which are in the toe of the Quiraing Landslip, are not metamorphosed but are much faulted, cut by minor intrusions and commonly have steep dips as a consequence of the landslipping. Lithologies present include shelly mudstone, siltstone and thin belemnite-bearing limestone of the Staffin Bay Formation (lower Callovian), and dark, silty claystone with subordinate silty sandstone and thin nodular limestone beds of the Staffin Shale Formation (Callovian to lower Kimmeridgian) (Morton and Hudson, 1995). The ammonites are mostly of Boreal provenance, but the occurrence of Tethyan forms is important in correlating the two faunal provinces of the European Jurassic. Elsewhere, Oxfordian mudstones, siltstones and sandstones are present near Strollamus, in the north and west of the Strathaird peninsula, and on southern Scalpay. These beds underlie Paleocene basaltic lavas and are commonly indurated and altered. Thin beds of altered tuff occur in the Staffin Shale Formation at Staffin Bay. They may possibly have been derived from volcanoes associated with the newly opened Rockall Trough (Knox, 1977). An isopod crustacean has been recovered from beds of Oxfordian age at Staffin Bay (Feldmann et al., 1994).

Eigg

The Staffin Shale Formation is exposed among boulders on the shoreface between Clach Alisdair and the Bay of Laig. It consists of about 35 m of soft mudstone with limestone beds, and ranges from the Callovian Quenstedtoceras lamberti Zone to the Oxfordian Cardioceras cordatum Zone.

Mull

A small outcrop of baked blue shale just west of Duart Bay has yielded ammonites and other fossils of Kimmeridgian age (mutabilis Zone).


References

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