The BGS Lexicon of Named Rock Units — Result Details

Cullaidh Shale Formation

Computer Code: CUD Preferred Map Code: Cud
Status Code: Full
Age range: Bajocian Age (JB) — Bajocian Age (JB)
Lithological Description: The Cullaidh Shale Formation comprises up to 6m of black, bituminous, fossiliferous fissile mudstone. Oil shale is developed towards the lowermost 2m of the unit. The uppermost unit is sandy fissile mudstone; locally black fine-grained sandstones are developed. The fauna is restricted and is largely fish scales and bivalves, together with sparse crinoids, fish, small gastropods, echinoids, trace fossils (Planolites) and conchostracans (Chen and Hudson, 1991).
Definition of Lower Boundary: The Cullaidh Shale Formation overlies the Garantiana Mudstone Member of the Bearreraig Sandstone Formation. At the type section the boundary is gradational and hence conformable. The blocky grey or brown mudstone with ammonites of the Garantiana Mudstone Member passes gradationally into the black, bituminous, fissile mudstone of the Cullaidh Shale Formation. The base of the Cullaidh Shale Formation is taken at the first upsection occurrence of carbonaceous, fissile mudstone with fish scales. There are marked facies variations to the north and south, which make the boundary between the Cullaidh Shale Formation and the Garantiana Clay Member difficult to identify (Harris and Hudson, 1980, p.234, 235).
Definition of Upper Boundary: The boundary of the Cullaidh Shale Formation with the overlying Elgol Sandstone Formation is gradational, and thus conformable. The base of the Elgol Sandstone Formation is marked by the first appearance of relatively intensely bioturbated, pro-deltaic clay-rich sandstone intercalated with the dark fissile mudstone typical of the Cullaidh Shale Formation (Hudson and Harris, 1979, figs 2-4; Harris and Hudson, 1980, fig.5; Harris, 1989, figs 3, 6, 8, 10; Morton and Hudson, 1995, fig.19). The Cullaidh Shale Formation is more fossiliferous than the Elgol Sandstone Formation.
Thickness: On the Trotternish Peninsula, Skye, the formation is 6m thick. Farther south it is 3m on Raasay and at the type section at Port Na Cullaidh, Strathaird, Skye it is 4 to 6m and it may be about 6m thick on Eigg (all thicknesses from Harris and Hudson, 1980), although Emeleus (1997) suggests it is not present on Eigg.
Geographical Limits: Hebrides Basin (Inner Hebrides and the Sea of the Hebrides sub-basins/troughs), northwest Scotland: onshore outcrops on Skye (Strathaird and Trotternish), Raasay and Eigg. Outcrops are separated/interrupted by Palaeogene igneous intrusions. The extent of the formation offshore within the sub-basins is inferred but uncertain (Fyfe et al., 1993).
Parent Unit: Great Estuarine Group (GEST)
Previous Name(s): BASAL OIL SHALE
Alternative Name(s): none recorded or not applicable
Type Section  Foreshore at Port Na Cullaidh, Elgol, Strathaird, southern Skye. Discontinuous exposures on storm beach that vary with beach state, and typically expose 1 to 2m of dark mudstone with fish scales and a sparse shelly fauna, intruded by basaltic sills. Base not seen, top exposed at foot of cliff. 
Cox, B M, Page, K N and Morton, N, 2002. The Middle Jurassic stratigraphy of Scotland. In Cox, B M and Sumbler, M G (editors), British Middle Jurassic Stratigraphy. Geological Conservation Review Series, Vol.26. (Peterborough: Joint Nature Conservation Committee/Chapman and Hall.) 
Morton, N and Hudson, J D. 1995. Field Guide to the Jurassic of the Isles of Raasay and Skye, Inner Hebrides, north-west Scotland. 209-280 in Taylor, P D (editor), Field Geology of the British Jurassic. (Geological Society of London.) 
Harris, J P and Hudson, J D. 1980. Lithostratigraphy of the Great Estuarine Group (Middle Jurassic), Inner Hebrides. Scottish Journal of Geology, Vol.16(2/3), 231-250. 
Chen, P-J and Hudson, J D. 1991. The conchostracan fauna of the Great Estuarine Group, Middle Jurassic, Scotland. Palaeontology, Vol.34, 515-545. 
Harris, J P. 1989. The sedimentology of a Middle Jurassic lagoonal delta system: Elgol Formation (Great Estuarine Group), NW Scotland. 147-166 in Whateley, M K G and Pickering, K T (eds), Deltas: Sites and Traps for Fossil Fuels. Geological Society Special Publication, No.41. 
Hudson, J D. 1962. The stratigraphy of the Great Estuarine Series (Middle Jurassic) of the Inner Hebrides. Transactions of the Edinburgh Geological Society, Vol.19, 135-165. 
Hudson, J D and Harris, J P. 1979. Sedimentology of the Great Estuarine Group (Middle Jurassic) of north-west Scotland. Symposium sur la Sedimentation de Jurassique Ouest Europeen, Paris, 9-10 May 1977, Association des Sedimentologists Francais, Publication Speciale, No.1, 1-13. 
Peach, B N, Horne, J, Woodward, H B, Clough, C T, Harker, A and Wedd, C B. 1910. The geology of Glenelg, Lochalsh and the south-east part of Skye (Explanation of One-Inch Map 71). Memoir (Sheet) of the Geological Survey of Great Britain (Scotland). (Edinburgh: His Majesty's Stationery Office.) 206pp. 
Emeleus, C H, 1997. Geology of Rum and the adjacent islands. Memoir of the British Geological Survey, Sheet 60 (Scotland). 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
S071 S080 S081