The BGS Lexicon of Named Rock Units — Result Details

Lealt Shale Formation

Computer Code: LASH Preferred Map Code: LaSh
Status Code: Full
Age range: Bathonian Age (JN) — Bathonian Age (JN)
Lithological Description: The formation typically comprises fossiliferous silty fissile mudstones with subordinate thin limestones and septarian nodules. A gradual coarsening-upward trend is discernible.
Definition of Lower Boundary: The boundary of the Lealt Shale Formation with the underlying Elgol Sandstone Formation is abrupt. The uppermost bed of the Elgol Sandstone is a very coarse-grained sandstone with sporadic granules and pebbles, and is overlain by the fissile mudstones of the Lealt Shale (Harris and Hudson, 1980, fig.5).
Definition of Upper Boundary: The Valtos Sandstone Formation-Lealt Shale Formation transition is a gradational boundary characterized by interbedded fissile mudstones, silty fissile mudstones and siltstones. The top of the Lealt Shale is marked by the range top of the conchostracan Cyzicus. The base of the Valtos Sandstone is marked by the lowermost occurrence of silty mudstone with monotypic Neomiodon beds. The formation is unconformably overlain by extrusive rocks of the Paleocene Skye Lava Group in places.
Thickness: Typically between 45m and 50m. However, thicknesses quoted by authors for the same localities may be at considerable variance, due to the nature of the topography (e.g. sea cliffs) and of the laterally variable sequences, and to the difficulties of aggregating thickness ranges given for components. All thicknesses below are from Harris and Hudson (1980), except where indicated. In Trotternish, north Skye, including the Lonfearn Member type section, the formation ranges from 39 (British Geological Survey, 2007) to 43m thick, and on Raasay from 50 to 62m (British Geological Survey, 2006). On Strathaird it is about 46m thick, on Eigg between 45m (Emeleus, 1997) and 52m, and on Ardnamurchan, 2 to 3m (Richey and Thomas, 1930).
Geographical Limits: Hebrides Basin (Inner Hebrides and the Sea of the Hebrides sub-basins/troughs), northwest Scotland: onshore outcrops on Skye (Strathaird and Trotternish districts), Raasay, Eigg and Ardnamurchan. Outcrops are separated/interrupted by Palaeogene igneous intrusions. The offshore extension of the parent Great Estuarine Group within the sub-basins is inferred but uncertain (Fyfe et al., 1993).
Parent Unit: Great Estuarine Group (GEST)
Previous Name(s): Estheria Shales (-1012)
Estheria Shales Plus Mytilus Shales (-1638)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Reference Section  Bay north of Elgol, Loch Scavaig, Elgol, southern Skye. The Lonfearn Member is fully exposed and 20.5m thick; at least 8.8m of the underlying Kildonnan Member, including the well-developed stromatolitic limestone at the top, are visible. The base is not seen here but can be observed farther south. Morton and Hudson, 1995, 238-240. 
Partial Type Section  Cliffs east of Lonfearn, south of Port Earlish, east Trotternish, northern Skye. Type section of the Lonfearn Member is a composite succession, between 26 and 30m thick. Harris and Hudson, 1980, fig.6; Morton and Hudson, 1995. 
Partial Type Section  Coastal exposure, 23m thick, located 2.5km north of Kildonnan, Eigg: the type section of the Kildonnan Member. Includes "Hugh Miller's Reptile Bed", but the boundary with overlying Lonfearn Member is not seen. Harris and Hudson, 1980, fig. 6; Emeleus, 1997 
Reference Section  Outcrops at the Lealt River and cliffs north of the Lealt River mouth, Trotternish provide an accessible reference section for the formation which is 48m thick. The Lonfearn Member is especially well developed here. The different exposures in this area can be readily correlated using marker beds. Composite section in Hudson and Harris, 1979, fig.2. 
Reference(s):
British Geological Survey. 2007. Portree. Scotland Sheet 80E. Bedrock and Superficial Deposits. 1:50 000 Geology Series. (Keyworth, Nottingham: British Geological Survey.) 
Anderson, F W and Dunham, K C. 1966. The geology of northern Skye. Memoirs of the Geological Survey of Great Britain, Scotland. Sheet 80 and parts of sheets 81, 90 and 91. Edinburgh, HMSO. 
Hudson, J D. 1970. Algal limestones with pseudomorphs after gypsum from the Middle Jurassic of Scotland. Lethaia, Vol.3, 11-40. 
Andrew, J E and Hudson, J D. 1984. First Jurassic dinosaur footprint from Scotland. Scottish Journal of Geology, Vol.20, Part 2, 129-134. 
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.) 
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. 
Emeleus, C H. 1997. Geology of Rum and the adjacent islands. Memoir (sheet) of the Geological Survey of Great Britain (Scotland) (60). (The Stationery Office for the British Geological Survey.) 171pp. 
British Geological Survey. 2006. Raasay. Scotland Sheet 81W. Bedrock and Superficial Deposits. 1:50 000 Geology Series. (Keyworth, Nottingham: British Geological Survey.) 
Anderson, F W. 1948. Algal beds in the Great Estuarine Series of Skye. Proceedings of the Royal Physical Society of Edinburgh, Vol.23, 123-141. 
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. 1966. Hugh Miller's Reptile Bed and the Mytilus Shales, Middle Jurassic, Isle of Eigg, Scotland. Scottish Journal of Geology, Vol.2, 265-281. 
Richey, J E and Thomas, H H. 1930. The geology of Ardnamurchan, north-west Mull and Coll. Memoir of the Geological Survey of Great Britain. Sheet 51 and part of sheet 52 (Scotland). 
Morton, N and Hudson, J D, 1995. Field Guide to the Jurassic of the Isles of Raasay and Skye, Inner Hebrides, north-west Scotland. In: Taylor, P D (editor), Field Geology of the British Jurassic. Geological Society of London, 209-280. 
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. 
Fyfe, J A, Long, D and Evans, D, 1993. United Kingdom offshore regional report: the geology of the Malin - Hebrides sea area (London: HMSO for the British Geological Survey). 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
S060 S071 S080 S080 S081