The BGS Lexicon of Named Rock Units — Result Details

Elgol Sandstone Formation

Computer Code: ESA Preferred Map Code: ESa
Status Code: Full
Age range: Bajocian Age (JB) — Bathonian Age (JN)
Lithological Description: The Elgol Sandstone Formation comprises bioturbated, clay-rich sandstone intercalated with silty fissile mudstone, overlain by coarsening-upwards white, pure, non-calcareous sandstone.
Definition of Lower Boundary: The Elgol Sandstone Formation overlies the Cullaidh Shale Formation with a gradational boundary. The base of the Elgol Sandstone Formation is marked by the first appearance of relatively intensely bioturbated, pro-deltaic clay-rich sandstones intercalated with the dark fissile mudstones 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 Elgol Sandstone Formation is significantly less fossiliferous than the underlying Cullaidh Shale Formation.
Definition of Upper Boundary: The boundary of the Elgol Sandstone Formation with the overlying Lealt Shale Formation is sharply-defined. The uppermost bed of the Elgol Sandstone is a very coarse sandstone with local developments of granules and pebbles, and this is abruptly overlain by the silty or bituminous fissile mudstones of the Lealt Shale (Harris and Hudson, 1980, fig.5). The formation is unconformably overlain by extrusive rocks of the Paleocene Skye Lava Group in places.
Thickness: Thicknesses from Harris and Hudson (1980) except where indicated. At the type section in Strathaird, the Elgol Sandstone Formation is 22m thick. Northwards, on the Trotternish Peninsula, its thickness varies between 9m at Rigg, at least 12m at Invertote and 17 to 32m (BGS, 2007) in south Trotternish and Raasay. Farther south, the formation is discontinuous with thin (<1m) brown silty fissile shales present on Eigg, although Emeleus (1997) disputes its presence.
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 and possibly Eigg. 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)
Alternative Name(s): none recorded or not applicable
Type Section  Fully exposed in cliffs at the north side of Port Na Cullaidh, Elgol, Strathaird, southern Skye. 22m thick. 
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. 
Donovan, D T and Hemingway, J E. (eds.). 1963. Jurassic of England, Wales and Scotland. Lexique Stratigraphique Internationale, 1, 3 a, x. 
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.) 
Emeleus, C H and Bell, B R. 2005. British Regional Geology: the Palaeogene volcanic districts of Scotland (Fourth Edition). (Nottingham: British Geological Survey.) x + 214pp. 
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 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. 
Morton, N and Hudson, J D. 1995. Field guide to the Jurassic of the Isles of Raasay and Skye, Inner Hebrides, NW Scotland. 209-280 in Taylor, P D (Ed.), Field Geology of the British Jurassic. (London: Geological Society.) 
Murchison, R I. 1828. Supplementary remarks on the strata of the Oolitic Series, and the rocks associated with them in the counties of Sutherland and Ross, and in the Hebrides. Transactions of the Geological Society of London, Vol.2, 353-368. 
Riding, J B, Walton, W and Shaw, D. 1991. Toarcian to Bathonian (Jurassic) palynology of the Inner Hebrides, northwest Scotland. Palynology, Vol.15, 115-179. 
Emeleus, C H, 1997. Geology of Rum and the adjacent islands. Memoir of the British Geological Survey, Sheet 60 (Scotland). 
Fyfe, J A, Long, D and Evans, D. 1993. The geology of the Malin - Hebrides sea area. United Kingdom offshore regional report. (London: HMSO for the British Geological Survey.) 
British Geological Survey. 2007. Portree. Scotland Sheet 80E. Solid and Drift Geology. 1:50 000 Geology Series. (Keyworth, Nottingham: British Geological Survey.) 
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.) 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
S071 S080 S081