The BGS Lexicon of Named Rock Units — Result Details

Duntulm Formation

Computer Code: DTM Preferred Map Code: Dtm
Status Code: Full
Age range: Bathonian Age (JN) — Bathonian Age (JN)
Lithological Description: The formation is a heterolithic succession dominated by fissile mudstones and monospecific oyster beds, with subordinate limestone, algal limestone and calcareous sandstone beds. The oysters are Praeexogyra hebridica (Forbes), indicating strong marine influence. They are preserved in limestone or mudstone matrices and the shell beds vary from a single shell plaster to oyster beds 2m thick. Within the Great Estuarine Group, Praeexogyra hebridica is confined to the Duntulm Formation.
Definition of Lower Boundary: The base of the formation is defined by the first upsection occurrence of the oyster Praeexogyra hebridica (Forbes). Lithologically, this boundary is gradational with the fissile mudstones with marine fossils of the lowermost Duntulm Formation overlying the fissile mudstones with freshwater faunas of the uppermost Valtos Sandstone Formation (Harris and Hudson, 1980, fig.8; Cox, Page and Morton, 2002, fig.6.29).
Definition of Upper Boundary: The top of the formation is marked by the range top of Praeexogyra hebridica, and the first upsection occurrence of the ostracod-bearing limestones interbedded with shales, of the Kilmaluag Formation. This boundary is not observed at the type section at Trotternish. The formation is unconformably overlain by extrusive rocks of the Paleocene Skye Lava Group in places.
Thickness: Thicknesses are from Harris and Hudson (1980) unless otherwise indicated. They also claim Anderson and Dunham's (1966) 60 ft (18m) estimate for Trotternish is incorrect, probably repeated elsewhere (e.g. British Geological Survey, 2006a). The greatest thickness cited is at the type section in Trotternish: Harris and Hudson (1980) estimate 55m thick, Andrews and Walton (1990) give about 33m. The unit thins southwards: Raasay 0-10m (British Geological Survey, 2006b) Strathaird at least 17m (Andrews and Walton, 1990), although Harris and Hudson (1980) give 8m, Eigg at least 9m and Muck at least 11m (Andrews and Walton, 1990).
Geographical Limits: Hebrides Basin (Inner Hebrides and the Sea of the Hebrides sub-basins/troughs), northwest Scotland: onshore outcrops on Skye (Strathaird, Duirinish, Waternish and Trotternish districts), Raasay, Eigg and Muck. 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): Lower "Ostrea" Beds (-2770)
Ostrea Hebridica Beds (-4010)
Alternative Name(s): none recorded or not applicable
Reference Section  Foreshore north of Elgol. Discontinuous but shows top and base, limestones more dominant than Trotternish, 17m of strata seen with a few-metre gap near the base. Andrews and Walton, 1990; Morton and Hudson, 1995. 
Type Section  Foreshore at Cairidh Ghlumaig and stream at Lon Ostatoin, Duntulm, Trotternish, northern Skye. Composite section with uncertain correlations, gaps and obscured parts. Although Harris and Hudson (1980) estimate 55m maximum they recognize that only about 30m are seen and from mapping 40m is the minimum. Andrews and Walton (1990, figs 4, 5) shows a total of about 33m of strata. Harris and Hudson, 1980; Cox et al., 2002. 
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. 
Andrews, J E. 1986. Microfacies and geochemistry of Middle Jurassic algal limestones from Scotland. Sedimentology, Vol.33, 499-520. 
Hudson, J D. 1963a. The recognition of salinity-controlled mollusc assemblages in the Great Estuarine Series (Middle Jurassic) of the Inner Hebrides. Palaeontology, Vol.6, 318-326. 
Hudson, J D. 1963b. The ecology and stratigraphical distribution of the invertebrate fauna of the Great Estuarine Series. Palaeontology, Vol.6, 327-348. 
Hudson, J D. 1970. Algal limestones with pseudomorphs after gypsum from the Middle Jurassic of Scotland. Lethaia, Vol.3, 11-40. 
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. 
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. 
Andrews, J E and Walton, W. 1990. Depositional environments within Middle Jurassic oyster dominated lagoons: an integrated litho-, bio- and palynofacies study of the Duntulm Formation (Great Estuarine Group), Inner Hebrides. Transactions of the Royal Society of Edinburgh, Earth Sciences. Vol.81, 1-22. 
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. 
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. 
Hudson, J D and Andrews, J E. 1987. The diagenesis of the Great Estuarine Group, Middle Jurasic, Inner Hebrides, Scotland. 259-276 in Marshall, J D (Ed.), Diagenesis of Sedimentary Sequences. Geological Society Special Publication, No.36. 
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. 
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. Raasay. Scotland Sheet 81W. Bedrock and Superficial Deposits. 1:50 000 Geology Series. (Keyworth, Nottingham: British Geological Survey.) 
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. 
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 S080 S081 S090