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Case Histories |
Department of Geology, Leicester University, Leicester LE1 7RH, UK
High frequency glacial-eustatic allocycles have been identified in the upper part of the early Pendleian Limestone Coal Formation in the north-central Midland Valley of Scotland. Repeated eustatic oscillations caused extensive lateral migrations of a wide spectrum of depositional environments, ranging from quasi-marine to fluvial. This throws doubt on the assumptions that underlie some existing depositional models of similar late Carboniferous cyclic coal-bearing successions.
The study interval can be divided into three facies associations. The distal association was dominantly controlled by glacial-eustatic oscillations, which resulted in a remarkably regular ‘layer cake’ succession with laterally persistent coals. In the more sandy and variable transitional facies association, local episodic fluvial processes tended to obscure the underlying allocycles. Here coals may split and also amalgamate to form thick composite seams. The highly variable proximal association was dominantly controlled by irregular autocyclic fluvial sedimentary processes which largely obscured the background changes in relative sea level. Sequence stratigraphy is easy to apply to the distal facies association, harder to apply to the transitional association and very difficult to apply to the variable proximal association.
