GEOGRAPHICAL CHANGE IN BRITAIN DURING THE CARBONIFEROUS PERIOD

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Abstract
SUMMARY: The Gulf Coast plain of S.E. U.S.A. is suggested as a modern analogue of the Carboniferous shelf across Britain. The coastal plain has developed over a comparable period to the shelf, they are of similar size and the gross environment of deposition, structural setting and broad evolution are much the same. Deposition on the British Carboniferous shelf began with fluviatile sedimentation following the Middle Devonian compressional event that produced widespread uplift and erosion across Britain south of the Scottish Highlands. Marine deposition commenced at the beginning of the Carboniferous in the south and is diachronous across Britain arriving in the Scottish Borders in the Asbian and with the Brigantian Hurlet transgression in South Scotland. In part this can be ascribed to progressive northward spreading epeirogenic downwarping which was first recognised in Devonian marine deposits and called the ‘bathyal lull’. Concurrently tensional stress in the upper crust, owing to slab pull subduction to the south, caused differential subsidence and block and basin extensional tectonics over the shelf. A combination of differential subsidence of sedimentary basins with regional subsidence caused deep Dinantian downwarping in the north and the formation of thick sequences of shallow water deposits. By the Namurian, differential movement was slackening in the north with change to more regional epeirogenic sinking; the centre of deposition or depocentre migrated southward to the Central basin at this time. This process of developing seaward stability continued in the Silesian with a depocentre migrating further south to Cheshire and the Midlands in the earlier Westphalian and to the south-west of Britain, south of the Wales-Brabant ridge, in the late Westphalian. Differential subsidence persisted in the south into the Westphalian and stabilisation of the shelf had not evolved to completion by the time it was terminated by the late Carboniferous Hercynian collision orogeny. From early in Westphalian B times a new arid land surface developed with red-bed deposition across Britain, spreading from the north southwards. Red beds were laid down in some basins in the Midlands and South Scotland apparently without significant break in deposition from the Carboniferous through into the Permian. The sequence of structural change with tensional stress, compressional stress and epeirogeny, together with independent factors such as eustatic changes in sea-level, combined to provide an evolving environment of deposition which is described and briefly compared with similar processes associated with the Gulf Coast plain of U.S.A.

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