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The Hebridean Basins and Adjacent Areas |
1 Department of Mineral Resources Engineering, Imperial College, London SW7 2BP, UK
2 Department of Geology, Polytechnic South West, Plymouth PL4 8AA, UK
3 Unocal UK Ltd., 32 Cadbury Road, Sunbury-on-Thames TW16 7LU, UK
4 Department of Petroleum Engineering, Heriot-Watt University, Edinburgh EH14 4AS, UK
Sediments of Cretaceous age in northwest Scotland outcrop in small, often isolated exposures throughout the Inner Hebrides and Morvern. Despite limited exposure, a range of sedimentary facies are recognized which potentially influence the interpretation of offshore data, in particular with respect to the hydrocarbon potential of offshore Cretaceous sections.
The Mid-Late Cretaceous development of the Inner Hebrides basin includes two periods of marine transgression. The first began in the latest Albian and continued through the early Cenomanian with the deposition of the marginal clastic facies of the Morvern Formation (Fig. 1). In the Late Cenomanian, a minor period of regression occurred during which the Lochaline Sandstone Member (the White Sandstone of Humphries 1961) was deposited.
In and around the Lochaline sand mine (Grid Ref. NM 680450) extensive fresh exposures afford considerable potential for sediment-ological and petrophysical studies of a large siliciclastic sand body (Lewis et al. 1990), further detail of which is reported here. Regional sedimentological and palaeoenvironmental data, including description and interpretation of the late Cenomanian and Turonian strata, are reported in Braley (1990).
Within the mine, between 3 and 8 metres of the Lochaline Sandstone Member are exposed, the workings generally lying between a lower silty horizon and an upper quartz-cemented horizon. Along the east shore of Loch Aline the sandstone is approximately 12 metres thick. Thicker, cleaner sandstone units are often located in fault footwalls. Small-scale syndepo-sitional faulting is also seen with evidence of sand draping. Slump structures provide additional evidence of sediment failure.
An idealized vertical sequence through the mined
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