Simms, M.J., Chidlaw, N., Morton, N. & Page, K.N. 2004. British Lower Jurassic Stratigraphy. Geological Conservation Review Series, No. 30, Joint Nature Conservation Committee, Peterborough, ISBN 1 86 107 495 6.
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N. Morton
The Cardha Carnach GCR site contains one of the best-documented and finest exposures through the Pliensbachian to basal Toarcian Scalpay Sandstone Formation. The site has also furnished important data on higher parts of the Lower Jurassic succession; the Lower Toarcian Portree Shale and Raasay Ironstone formations, and the uppermost Toarcian Dun Caan Shale Member of the Bearreraig Sandstone Formation. The 300 m-high cliff Druim an Aonaich, towards the north in the GCR site, extends as an almost linear north–south feature from Dun Caan to Screapadal and dominates the east coast of the Isle of Raasay
The southern part of Druim an Aonaich, north of Cadha Carnach, shows a clear tripartite division. The main vertical part of the cliff comprises the (latest Toarcian to Upper Bajocian) Bearreraig Sandstone Formation. Below there is a distinct sloping ledge developed on the Dun Caan Shale Member (Aalensis Zone), the Raasay Ironstone Formation (Serpentinum Zone) and the Portree Shale Formation (also Serpentinum Zone). These are exposed only occasionally, being largely obscured by fallen blocks and scree from the massive units above, but a trench was excavated by the [British] Geological Survey, during the mapping of Raasay, to document the 'Upper Lias' succession. The steep slope below these units is formed by the Scalpay Sandstone Formation (here Davoei Zone to Tenuicostatum Zone). Although partly covered by vegetation, stream gullies expose a complete succession down to the foreshore.
Above the main cliff a ledge marks the position of the Garantiana Clay Member and the Cullaidh Shale Formation, both exposed at Cadha Carnach. To the west, gentler scarps and slopes are underlain by the various units of the Great Estuarine Group, cut by sills and capped by Palaeocene plateau basalts forming, within the GCR site, the summit of Dun Caan. On the western slopes of Dun Caan fragments of presumed Cenomanian greensand indicate the presence of a thin Upper Cretaceous sequence between the Jurassic and Palaeocene successions.
Useful sections through the Lower Jurassic succession can be seen at various points, notably towards the northern end of Druim an Aonaich (south of Screapadal) and towards the southern end. This southern section forms the Lower Jurassic part of the Cadha Carnach GCR site, named after the break in the cliff immediately east of Dun Caan. The Cadha Carnach GCR site was designated as a Lower Jurassic site largely because of the section through the (Pliensbachian to basal Toarcian) Scalpay Sandstone Formation, described by Howarth (1956, 1958) and Phelps (1985). However, important data can also be obtained (Lee, 1920) about the Portree Shale and Raasay Ironstone formations (both Lower Toarcian) and the (topmost Toarcian) Dun Caan Shale Member of the Bearreraig Sandstone Formation.
The eastern boundary of the Cadha Carnach GCR site lies between the in-situ strata of Druim an Aonaich and the Hallaig landslip. The western boundary is the top of the cliff with a westwards extension at the southern end to include Dun Caan. The GCR site extends north to the edge of another landslip, which forms a shallow recess in the cliff and has buried the Lower Jurassic succession beneath large fallen blocks of the Bearreraig Sandstone Formation. Access to the site is difficult, involving a long walk (c. 7 km) from the end of the road at North Fearns.
Structurally the area is simple, comprising part of a large westerly tilted fault-block bounded to the west by the Screapadal Fault (see Morton and Hudson, 1995). Within the GCR site the strike of the beds is almost north–south, with dips between 13° and 21° to just north of west. The only fault within the site is an arcuate structure near the southern boundary, south of Cadha Carnach. Major dolerite and granophyre sills have been intruded into the Middle Jurassic sediments. The small granophyre sills represent the north-eastern edges of a large granophyre sheet which covers much of southern Raasay, while the dolerite sills are part of the extensive suite of sills which are prominent features in Trotternish, Isle of Skye. Minor dolerite dykes are confined to Cadha Carnach itself.
The Scalpay Sandstone Formation forms the steep slope in the lower part of the cliff and the foreshore but is accessible in steep parallel stream gullies
Exposures of the Portree Shale and Raasay Ironstone formations are more limited along the narrow steep ledge above the Scalpay Sandstone Formation, but have been recorded at several locations
The succession in the Scalpay Sandstone Formation of the Cadha Carnach area was first described in the [British] Geological Survey memoir (Lee, 1920). A more detailed description of the section was given by Howarth (1956) and this has served as the basis for more recent accounts (Phelps, 1985; Hesselbo et al., 1998). The succession reproduced here
The more sandy upper part of the Scalpay Sandstone Formation forms a steeper slope, up to the ledge formed by the Toarcian shales. The lowest massive sandstone, Bed 26, is the exception. It is mottled by pervasive bioturbation and is softer and more poorly cemented, forming a recessed ledge and giving rise to distinctive honeycombe weathering. These features can be recognized in other sections on Raasay, notably south of Rubha na' Leac (see GCR site report). Beds 28 and 30 are more calcareous, with nodular bands of sandy limestone, and more fossiliferous with frequent Gryphaea gigantea, Pseudopecten equivalvis, ammonites and nests of brachiopods (especially rhynchonellids). Other beds contain more scattered calcareous doggers, and fossils are less common. The thick massive-weathering sandstone (Bed 36) that forms the main upper part of the Scalpay Sandstone Formation is thinner here than south of Rubha na' Leac. It is capped by thinner beds of ferruginous and calcareous, though usually decalcified, sandstone which appears to correlate with the higher of the two calcareous beds at Gualann na Leac (so here also numbered Bed 39).
There is a sharp lithological boundary between the top of the Scalpay Sandstone Formation and the shales of the Portree Shale Formation. The shales are dark-grey to black, fine grained with very small mica flakes in the lower part. Towards the top the mica flakes become larger and scattered ooliths appear so that, In the southern part of the Cadha Carnach GCR site at least (see below), there is an upward transition into the Raasay Ironstone Formation. Howarth (1956) recorded shales with Dactylioceras spp. from the top of his section 7
The succeeding Raasay Ironstone Formation also is rarely exposed along the ledge at the foot of the high cliff. Data obtained by the [British] Geological Survey (Lee, 1920, and original Geological Survey fieldslip) is available for three localities:
There is no evidence as to whether the northward thinning (from 2.4 m at the main opencast mine
The Raasay Ironstone Formation is succeeded by the Dun Caan Shale Member. The latter was proposed by S.S. Buckman, on account of the conspicuous feature these shales make below Dun Caan, and subsequently accepted by the [British] Geological Survey (Lee, 1920). Natural exposures of this part of the succession are rare and occur in small disconnected patches. Hence Tait's trench section (at
Within the Cardha Carnach GCR site a virtually complete succession can be seen from near the base of the Scalpay Sandstone Formation to near the top of the Bearreraig Sandstone Formation. However, the section is steep and parts are almost inaccessible (the southern end of the vertical cliff of the Bearreraig Sandstone Formation can be climbed by a steep gully on the northern corner of Cadha Carnach). Natural exposures of the Toarcian succession are rare, but the rocks can be exposed with a little effort and have been documented in the past (Lee, 1920).
Dating of the Scalpay Sandstone Formation in the Cadha Carnach section conies largely from the work of Howarth (1956, 1958), with some supplementary information about the lowermost part from Phelps (1985). The lowest beds (i and ii) have been placed in the Maculatum Subzone, but comparison with the Hallaig Shore GCR site suggests that they may be Ibex Zone. Androgynoceras maculatum and Arieticeras sp. in Bed v indicate the Maculatum Subzone, and Aegoceras capricornus in Bed 19 indicates the Capricornus Subzone, but the position of the subzonal boundary is uncertain. From beds 21 to 26, Howarth (1956) recorded a good succession of amaltheid faunas, summarized on
The [British] Geological Survey (Lee, 1920) recorded specimens of Dactylioceras, indicating the Tenuicostatum Zone, from the top 1.8 m (6 ft) of the Scalpay Sandstone Formation. These records have been confirmed from elsewhere on Raasay, but the preservation is too poor for specific identification, so that subzonal attribution is not possible. The abruptness of the facies change to the overlying Portree Shale Formation suggests a hiatus representing part of the Tenuicostatum Zone, but this cannot be confirmed from the ammonite evidence.
Fewer details of facies and depositional environment are available than for the other Raasay GCR sites, but in general they appear similar. The Scalpay Sandstone Formation varies in grain size from silty shales to fine-grained sandstones. Three coarsening-up cycles can be identified here, as at Gualann na Leac, and are represented by beds i–20, beds 21–29, and beds 30–36. Both early- and late-diagenetic calcareous nodules and doggers occur. Fossils are scattered except for some nodules which are crowded with various types, especially brachiopods (see Ager, 1956a), and some layers with Gryphaea or Pseudopecten. Extensive bioturbation is evident throughout, though identifiable trace fossils are rare.
The Cadha Carnach GCR site includes some of the few localities on Raasay that expose the Portree Shale Formation, their value being enhanced by information gained from the exposure in Tait's trench. The thickness recorded here, 3.96 m, is greater than the 3 m at Gualann na Leac (see Rubha na' Leac GCR site report) or the 2.5 m at the opencast ironstone workings (Morton and Hudson, 1995), but is significantly less than on the Trotternish Peninsula of Skye (see Prince Charles' Cave to Holm GCR site report). At all these localities there is an abrupt facies change from sandstones to dark-grey shales, which become slightly coarser and more micaceous upwards. Towards the top, scattered chamositic ooliths are present, indicating an upward transition to the Raasay Ironstone Formation. No benthic fossils have been recorded, only ammonites and belemnites, suggesting deposition in an anoxic environment during an episode of sea-level rise. This correlates with the Exaratum Subzone global sea-level event identified by Jenkyns (1988; see Morton, 1989).
Buckman (in Lee, 1920) identified species of Dactylioceras, Harpoceras and Peronoceras in the material from Tait's trench, assigning the Portree Shale Formation to the Serpentinum Zone, with both the Falciferum and Exaratum subzones present. More recently these age assignments have been corrected on the basis of new collections made by Andrew B. Smith (of the Natural History Museum, London) at Bearreraig Bay and identified by Howarth (1992). This material suggests that only the Exaratum Subzone and possibly part of the Tenuicostatum Zone are represented in the Portree Shale Formation.
Above the Portree Shale Formation the Cadha Carnach GCR site exposes only a thinner more marginal facies of the Raasay Ironstone Formation than that seen at the type section in southern Raasay. Furthermore, the formation disappears completely towards the northern edge of the GCR site, although whether this is due to lateral thinning/facies change or to subsequent erosion before deposition of the Dun Caan Shale Member is unclear. Ammonites collected from the Raasay Ironstone Formation in Tait's trench were identified by Buckman (in Lee, 1920) as Hildoceras bifrons d'Orbigny (non Bruguiere), Dactylioceras cf. crassiusculosum and, in the upper part, Coeloceras dayi. They were interpreted as indicating a Bifrons Zone, Commune Subzone, age but with the Dactylioceras possibly remanié, an age assignment accepted by Arkell (1933), Hallam (1967a) and Howarth (in Cope et al., 1980a). However, material collected by Andrew Smith and identified by Howarth (1992) included Harpoceras falciferum, Hildoceras laticosta and Dactylioceras toxopborum, indicating the Falciferum Subzone, and Cleviceras elegans indicating the top of the underlying Exaratum Subzone. There is no evidence on Raasay of younger ammonites within the Raasay Ironstone Formation, which contrasts with the presence of Thouarsense Zone ammonites in Ardnamurchan (Howarth, 1992).
The Raasay Ironstone Formation at Cadha Carnach contains carbonate and siderite ooliths scattered in a fine-grained matrix. This contrasts with the more ferruginous, coarser-grained and sometimes cross-bedded ironstone seen at the main outcrops in southern Raasay (see also Rubha na' Leac GCR site report). Interpretation of the depth of the depositional environment of the Raasay Ironstone Formation regionally is controversial — above wave-base in places (with cross-bedding, stromatolites) or below wave-base in a deep-water environment. The latter seems more likely at Cadha Carnach. Deposition occurred during an interval of sediment starvation caused by sea-level rise (Hesselbo and Jenkyns, 1998) and/or tectonic stability (Morton, 1989). Supporting evidence includes the occurrence of a belemnite lag, with Dactyloteuthis, at the top of the ironstone.
Most of the fossils collected by Tait from the Dun Caan Shale Member were ammonites, which were submitted to S.S. Buckman (in Lee, 1920). Buckman's identifications were summarized in his table I (In Lee, 1920), together with the collector's detailed stratigraphy given in feet below a thin limestone bed crowded with ammonites (Leioceras 'spp.' which would now be classified as of the L. comptum group) belonging to the lower Aalenian Opalinum Zone. This work has never been superseded or independently verified, but is probably reliable because Buckman had recently monographed (1887–1907) the ammonites of this age in southern England and hence was very familiar with the faunas even though those from Raasay are crushed. Selected species are recorded in
Within the Dun Caan Shale Member Buckman (in Lee, 1920) distinguished an upper Venustula Zone 4.9 m (16 ft) thick, and a lower Aalensis Zone proper 16.8 m (49 ft — should be 55 ft, corrected from table I) thick. In the appendix Buckman gave further details (table I and p. 67) and, comparing the Raasay succession with that in the Bridport Sand at Chideock, Dorset (Buckman, 1910), was able to identify three hemerae. Based on his identifications and comments on the faunas, the following characteristic morphological features can be recognized:
(iii) an upper Venustula Hemera (4.90 m) characterized by very finely ribbed ammonites in some of which 'bundling' of the ribs occurs, including Pleydellia (Canavarina)venustula and Pleurolytoceras leckenbyi.
(ii) A middle Digna Hemera (5.80 m) characterized by well-ribbed ammonites with clear separate ribs which curve onto the venter, including Pleydellia (Canavarina)digna, forms transitional from this to P. (C.) steinmanni, and Pleurolytoceras leckenbyi.
(i) A lower Cotteswoldia Hemera (11.00 m) characterized by ammonites with strong distant ribs which fade before the venter and also towards the aperture, including Pleydellia (Cotteswoldia) subcandida, P. (C.) costulata, P. aalensis, P. (Walkericeras) cf burtonense, P. subcompta.
These faunal subdivisions of the Aalensis Zone on Raasay appear to be comparable with the subzones recognized in Spain by Goy and Ureta (1991), as shown here in
The lithology of the Dun Caan Shale Member is apparently uniform except for slight upwards coarsening and occasional shelly layers which suggest episodes of relative sediment starvation. It marks a return to deposition in a strongly subsiding basin, which at Cadha Carnach possibly lay below wave-base in a dysaerobic environment. This follows a long interval of stability which resulted in the Toarcian hiatus (see Morton, 1989).
The Cadha Carnach GCR site makes available steep, but largely accessible, Lower and Middle Jurassic sections (excluding parts of the vertical cliff of the Bearreraig Sandstone Formation). The section in the Scalpay Sandstone Formation complements those farther south on Raasay, in the Hallaig Shore and Rubha na' Leac GCR sites. It is particularly important for the succession of Upper Pliensbachian ammonite faunas in the Margaritatus and lower Spinatum zones.
The GCR site is also of importance for understanding the Toarcian history of the Hebrides, especially the section exposed by the [British] Geological Survey in the trench excavated by Tait. The ammonites found there enabled S.S. Buckman to identify a major hiatus representing much of the Toarcian Stage, since confirmed as including the Bifrons to Pseudoradiosa (excluding Aalensis) zones. The ammonites collected from the Dun Caan Shale Member in the trench, the type section of this lithostratigraphical unit, were also identified and reported by Buckman. This collection, which has not yet been replicated, provides important evidence about the detailed succession of ammonite faunal horizons in Britain. It has also enabled comparisons to be made with the Iberian subzonal scheme.