Category:The Scottish 'Chalk Group' Succession
Upper Cretaceous strata are thin (maximum thickness c. 30 m) and widely scattered in Scotland. They occur exclusively in western Scotland, and predominantly in the Inner Hebrides, the main localities being Skye, Raasay, Scalpay, Eigg, Morvern, Mull and Arran (Rawson et al., 1978). The strata usually comprise clastic deposits at the base and top, with chalk (silicified and/or metamorphosed) sandwiched in between. Currently, these deposits comprise the Inner Hebrides Group (Mortimore, Wood & Gallois, 2001). Recent accounts of the stratigraphy are provided by Rawson et al. (1978), Lowden et al. (1992), Hancock (2000) and Mortimore et al. (2001).
Rawson et al. (1978)
The oldest Upper Cretaceous strata comprise the Morvern Greensand, and are dated as Cenomanian (Rawson et al., 1978). This is overlain by the Loch Aline Glass Sand, of unknown age . In western Mull, the preceeding units are overlain by 3.7 m of silicified chalk, which sparse macrofossil evidence suggests belongs to the Campanian (B. mucronata Zone), although microfossil data suggests an older, Santonian age.
The youngest few metres of Upper Cretaceous deposits locally exposed on Mull, comprising breccia, sandstone and mudstone, are named the Griburn Conglomerate Formation, and questionably assigned to the Maastrichtian.
Lowden et al. (1992)
The Cretaceous of NW Scotland is divided into four formations; in ascending stratigraphical order these are: Morvern Formation, Gribun Chalk Formation, Strathaird Limestone Formation and Beinn Iadain Mudstone Formation. No details of these units were published, although the Lochaline Sandstone and Clach Alasdair Conglomerate, classified as formations by Mortimore, Wood & Gallois (2001) (see below), are here treated as members within the Morvern and Strathaird Limestone formations respectively. Lowden et al. assigned the Morvern and Gribun formations to the Cenomanian, and the Strathaird Limestone Formation to the Turonian. A post Palaeocene age was questionably suggested for the Beinn Iadain Mudstone Formation.
Mortimore, Wood & Gallois (2001)
Up to six lithostratigraphical units are correlated between various localities on the Isle of Mull and in Argyll. In ascending stratigraphical order these are: Morvern Greensand Formation, Lochaline White Sandstone Formation, Coire Riabhach Phosphatic Formation, Griburn Chalk Formation, Clach Alasdair Conglomerate, and the Beinn Iadain Mudstone Formation. Of these intervals, only the Morvern Greensand is confidently dated (Cenomanian). Serpulids in the basal part of the Lochaline White Sandstone suggest a correlation with Jefferies' (1963) Bed 4 of the Plenus Marls Member in England, and thus a Late Cenomanian age. The top of the Lochaline Sandstone is possibly Santonian in age, and nannofossils in silicified chalk of the Griburn Chalk Formation might indicate the Santonian or Early Campanian. The ages of the overlying units are not known, although assignment to the Upper Cretaceous is presumed. Lignite beds locally capping successions beneath Palaeocene lava flows on Mull have generally been assigned to the Tertiary, but might be Late Cretaceous. Hancock's (2000) Griburn Conglomerate Formation equates with the combined Clach Alasdair Conglomerate and Beinn Iadain Mudstone formations (above).
HANCOCK, J M. 2000. The Gribun Formation: clues to the latest Cretaceous history of western Scotland. Scottish Journal of Geology, 36, 137-141.
LOWDEN, B, BRALEY, S, HURST, A & LEWIS, J. 1992. Sedimentological studies of the Cretaceous Lochaline Sandstone, NW Scotland. In PARNELL, J. (Ed.), Basins on the Atlantic Seaboard: Petroleum Geology, Sedimentology and Basin Evolution. Geological Society Special Publication No. 62, 159-162.
MORTIMORE, R N, WOOD, C J & GALLOIS, R W. 2001. British Upper Cretaceous Stratigraphy, Geological Conservation Review Series, No. 23. (Joint Nature Conservation Committee: Peterborough.)
RAWSON, P F, CURRY, D, DILLEY, F C, HANCOCK, J M, KENNEDY, W J, NEALE, J W, WOOD, C J & WORSSAM, B C. 1978. A correlation of the Cretaceous rocks in the British Isles. Geological Society of London, Special Report No. 9, 70 pp..
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