The BGS Lexicon of Named Rock Units — Result Details

Staffin Shale Formation

Computer Code: SFSH Preferred Map Code: SfSh
Status Code: Full
Age range: Callovian Age (JC) — Kimmeridgian Age (JD)
Lithological Description: The Staffin Shale Formation is a succession of fissile and massive mudstone, siltstone, sandy siltstone and sandstone, with minor calcareous, phosphatic and sideritic nodules. A coarsening-upward trend is discernible in Trotternish, north Skye, where five members are recognized, and in Strathaird, where a further four members are recognized. The sequence on Eigg is recognized as another member.
Definition of Lower Boundary: The base of the formation is sharply-defined. In Trotternish, it is represented by the base of bituminous fissile mudstones with thin glauconitic siltstones with Chondrites (Dunans Clay Member) abruptly overlying the fine- to medium-grained sandstone of the Belemnite Sands Member, Staffin Bay Formation. In Strathaird, the lowermost surface of the formation is also sharp. There is a significant upward facies change from the medium- to coarse-grained sandstone of the Carn Mor Sandstone Member (Staffin Bay Formation) to the silty mudstone of the Tobar Ceann Siltstone Member (Sykes, 1975, fig.6).
Definition of Upper Boundary: The top of the formation is not well exposed but is thought to be unconformably overlain widely by extrusive rocks of the Paleocene Skye Lava Group. At Kildorais, Trotternish, the uppermost, dark grey mudstone bed of the Flodigarry Shale Member (SS45 of Morton and Hudson, 1995) is seen to be overlain by Palaeocene palagonite tuff. The grey siltstone of the Camasunary Siltstone Member in Strathaird and the mudstone of the formation at Strollamus and Scalpay are overlain unconformably locally by pebbly limestone and sandstone of the Upper Cretaceous Strathaird Limestone Formation (Sykes, 1975, p. 71).
Thickness: At the type section at Staffin Bay in Trotternish, the formation is at least 116.8 m thick and in the wider district attains 151 m (British Geological Survey, 2006). In Strathaird, the formation is seen to at least 150.5 m, on Scalpay it is about 100 m thick and on Eigg, at least 33 m thick (Sykes, 1975). The thickness present on Mull is unknown.
Geographical Limits: Hebrides Basin (Inner Hebrides and the Sea of the Hebrides sub-basins/troughs), north-west Scotland: onshore outcrops on Skye (Strathaird and Trotternish districts and Strollamus [NG 601 266]), Scalpay, Eigg and Mull [NM 7244 3457]; not present on Raasay or Muck. Outcrops are separated/interrupted by Palaeogene igneous intrusions. The offshore extension of the formation within the sub-basins is inferred but uncertain (Fyfe et al., 1993, p.43).
Parent Unit: Not Applicable (-)
Previous Name(s): Oxford Clay, Corallian And Kimmeridge Clay (-3668)
Oxford Clay (-1175)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Section  Composite of coastal outcrops on the foreshore of Staffin Bay, Trotternish, northern Skye: NG 472 708 (Dunans Shale Member - Point 5); NG 473 699 (Dunans Clay Member - midway between Points 3 and 4); NG 472 708 and NG 473 698 (Glashvin Silt Member - south of Point 5 and Point 3); NG 474 694 (Digg Siltstone Member - south of Point 1); and NG 474 694 and NG 473 696 (Flodigarry Shale Member - Points 1 and 2). Points on fig.38 of Morton and Hudson (1995). Sykes (1975, p.64-67). See also Morton and Hudson (1995, table 4), Riding and Thomas (1997, figs. 2, 3). 
Reference Section  Foreshore at Rudha na h'Airigh Baine; Scaladal Sandstone, Camasunary Sandstone and Camasunary Siltstone members, unconformably overlain by Upper Cretaceous Strathaird Limestone Formation (Sykes, 1975). Presence/exposure of basal Tobar Ceann Siltstone Member uncertain. (Sykes, 1975) 
Reference(s):
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. 
Wright, J K. 1964. The so-called Oxfordian shales of Eigg, Inner Hebrides. Geological Magazine, Vol.101, 185-186. 
Arkell, W J and Callomon, J H. 1963. Lower Kimmeridgian ammonites from the drift of Lincolnshire. Palaeontology, Vol.6, 219-245. 
Turner, J A. 1966. The Oxford Clay of Skye, Scalpay and Eigg. Scottish Journal of Geology, Vol.2, 243-252. 
Cope, J C W (Editor) 1980b. A correlation of Jurassic rocks in the British Isles. Part Two: Middle and Upper Jurassic. Geological Society of London Special Report, No.15 
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.) 
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. 
Riding, J B. 1984. The palynology of the Tobar Ceann Siltstone Member, Staffin Shale Formation (Jurassic: Callovian/Oxfordian), Strathaird, southern Skye. British Geological Survey Short Communications, 16/10, 1-5. 
Riding, J B and Thomas, J E. 1997. Marine palynomorphs from the Staffin Bay and Staffin Shale formations (Middle-Upper Jurassic) of the Trotternish Peninsula, NW Skye. Scottish Journal of Geology, Vol.33, 59-74. 
Sykes, R M. 1975. The stratigraphy of the Callovian and Oxfordian stages (Middle-Upper Jurassic) in northern Scotland. Scottish Journal of Geology, Vol.11, 51-78. 
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. 
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. 
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). 
British Geological Survey. 2006. Staffin. Scotland Sheet 90. Bedrock and Superficial Deposits. 1:50 000 Geology Series. (Keyworth, Nottingham: British Geological Survey.) 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
S060 S071 S080 S090