Tectonic environment of the Devonian Gramscatho basin, south Cornwall: framework mode and geochemical evidence from turbiditic sandstones

Journal of the Geological Society - Tập 144 Số 4 - Trang 531-542 - 1987
P.A. Floyd1, Brian E. Leveridge2
1Department of Geology, University of Keele, Staffordshire, ST5 5BG, UK
2British Geological Survey, St Just, 30 Pennsylvania Road, Exeter EX4 6BX, UK

Tóm tắt

The Portscatho Formation, within the allochthonous unit of the Middle and Upper Devonian Gramscatho Group, is a thick sequence of deep-water sandstones and interbedded slates deposited by southerly-derived turbidity currents into the Gramscatho basin of south Cornwall. Throughout an approximately 3.5 km thick sequence, the Portscatho Formation is petrographically and chemically coherent, except that the upper section shows a higher proportion of metamorphic clasts, high, but variable Cr, and low, uniform Zr abundances. Complementary framework mode and bulk geochemistry indicate that the sandstones were derived from a dissected continental magmatic arc of predominantly acidic composition, similar to average upper continental crust, but with an admixture of minor intermediate/basic material. Flysch deposition took place in a fore-arc setting. The presence of an arc to the south of Cornwall during the Devonian implies that there was subduction at the margin of the Gramscatho basin, whose ultimate closure was accommodated by the northward stacking of flysch–ophiolite nappes.

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Journal of the Geological Society

Tập 144 Số 4

531-542

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