The BGS Lexicon of Named Rock Units — Result Details

Great Estuarine Group

Computer Code: GEST Preferred Map Code: GEst
Status Code: Full
Age range: Bajocian Age (JB) — Bathonian Age (JN)
Lithological Description: Succession of heterolithic, dominantly siliciclastic, sedimentary rocks dominated by mudstone and sandstone with subordinate variously shelly, algal and dolomitic limestone beds representing a variety of fluvial and lagoonal depositional settings. There are no fully marine fossils, and no zonal faunas. Seven formations are recognized.
Definition of Lower Boundary: The base of the Group is coincident with the base of the Cullaidh Shale Formation. The latter unit overlies the Garantiana Mudstone Member of the Bearreraig Sandstone Formation. At the type section of the Cullaidh Shale, this boundary is gradational and hence is conformable. The fossiliferous blocky grey or brown mudstone of the Garantiana Mudstone passes up gradationally into the black, bituminous, fissile mudstone of the Cullaidh Shale. The base of the Cullaidh Shale is taken at the first upsection occurrence of carbonaceous, fissile mudstones with fish scales. There are marked facies variations to the north and south, which make the boundary between the Cullaidh Shale and the Garantiana Mudstone locally difficult to identify (Harris and Hudson, 1980, pp. 234, 235).
Definition of Upper Boundary: The top of the Group is coincident with the top of the Skudiburgh Formation. The top of the Skudiburgh Formation is sharply-defined, and is marked by the change from mottled red and grey-green mudstones to dark, shelly fissile mudstones (Upper Ostrea Member, Staffin Bay Formation) in Trotternish and a non-sequence overlain by sandstone (Carn Mor Sandstone Member, Staffin Bay Formation) in Strathaird. The Group is unconformably overlain by extrusive rocks of the Paleocene Skye Lava Group in places.
Thickness: The thickest composite section is on the Trotternish Peninsula, where Harris and Hudson (1980, fig.3), give a thickness of 264m for the group, but aggregating the quoted formations' thicknesses gives up to 289m. It thins dramatically southwards from Trotternish via Raasay, estimated 137-139m on Strathaird (British Geological Survey, 2002), about 120m on Eigg (Emeleus, 1997), seen to 33m on Muck (Harris and Hudson, 1980, fig.3).
Geographical Limits: Hebrides Basin (Inner Hebrides and the Sea of the Hebrides sub-basins/troughs), northwest Scotland: onshore outcrops on Skye (Strathaird, Trotternish and Waternish districts), Raasay, Eigg and Muck. Outcrops are separated/interrupted by Palaeogene igneous intrusions. The offshore extension of the Great Estuarine Group within the sub-basins is inferred but uncertain (Fyfe et al., 1993).
Parent Unit: Not Applicable (-)
Previous Name(s): Great Estuarine Series (-3426)
Loch Staffin Beds (-1557)
Alternative Name(s): none recorded or not applicable
Type Area  Trotternish to Strathaird districts, Skye, Inner Hebrides. Morton and Hudson, 1995, p.216. 
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). 
Hudson, J D. 1962. The Great Estuarine Series (Middle Jurassic) of the Inner Hebrides. Unpublished PhD thesis, University of Cambridge, Cambridge, UK, 350pp. 
Murchison, R I. 1829b. 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. 
Judd, J W. 1878. The secondary rocks of Scotland. Third paper. The strata of the Western Coast and Islands. With a note on the foraminifera and other organisms in the Chalk of the Hebrides by T R Jones. Quarterly Journal of the Geological Society of London, Vol.34, 600-743. 
British Geological Survey. 2002. Broadford. Scotland Sheet 71W. Solid and Drift Geology. 1:50 000 Geology Series. (Keyworth, Nottingham: British Geological Survey.) 
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. 
Judd, J W. 1873. The secondary rocks of Scotland. Quarterly Journal of the Geological Society, London, Vol.29, 97-195. 
Lee, G W and Pringle, J. 1932. A synopsis of the Mesozoic rocks of Scotland. Transactions of the Geological Society of Glasgow, Vol.19, 158-224. 
Murchison, R I. 1829a. On the coalfield of Brora. Transactions of the Geological Society of London, Vol.2, 293-326. 
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. 
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. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
S060 S061 S071 S080 S080 S090