LOWER CARBONIFEROUS PALAEOGEOGRAPHY OF THE BRITISH ISLES

Abstract

Summary Palaeogeography rests in a well-founded stratigraphy. Rigidly defined palaeontological zones are of very limited use in neritic rocks, and their criteria must be relaxed and adapted to changing fossil associations to meet the needs of wide-ranging regional studies: in particular, Vaughan’s scheme, though the most convenient, is applicable in only a general sense to the several Lower Carboniferous provinces. The earliest known Carboniferous fossils of much of midland and northern England, though hitherto considered to characterize the Zaphrentis and Lower Caninia zones, do not give evidence of any rocks older than Viséan; and except perhaps in the Craven Lowlands the Tournaisian stage is not certainly known to occur outside the South-Western Province (and its continuation in southern Ireland). Viséan transgression was a major diastrophic episode in Dinantian times. The residual Tournaisian sediments are generally uniform in lithology (though not in thickness) and provide little direct evidence of contemporary shorelines. The Lower Limestone Shales mark a northward overlap of marine Devonian rock-types continued into Carboniferous times. The succeeding Main Limestone was deposited in an east-and-west facies belt, but now gives few clues to contemporary land-masses. The Tournaisian Culm is Piltonian— a mud-zone sediment that accumulated under conditions not unlike those of the Lower Limestone Shales. St. George's Land is well defined both by facies changes and by marginal unconformities in the Viséan rocks of the South-Western Province, and its shorelines are locally precisely recognizable. Its northern margin is even more clearly defined in the Dibunophyllum Zone of North Wales, and can be followed eastwards as a virtually continuous shore. The Viséan rocks of Ireland provide a complete range of depositional zones from Culm in the south to coarse pebble beds bordering the mainland in the north. Marginal delta beds are strongly developed from Mayo to Antrim, showing progressive diachronism to higher horizons as they are followed north-eastwards. Southwards they pass laterally into shelf lime-stones of a great variety of kinds deposited in clearer waters off-shore, the transition in early Viséan times approximately coinciding with the structural line of the Highland Boundary fault-belt and the Ox Anticline. The lime-stones include thick Waulsortian sheet reefs in the lower part, probably covering intermittently several thousand square miles in area. At higher horizons shoal and patch reefs are common and widespread in host sediments of thick bioclastic limestones. The Viséan cuvette in Ireland extended unbrokenly eastwards into the Central Province of England, where thick limestones and shales provide clear indications of southern and northern margins; and, divided by the Manx–Cumbrian–Pennine barrier, north-eastwards into the Northumbrian trough of estuarine and deltaic sediments. The drowning of a northern mainland coast to allow extension of the Irish calciferous sandstones into Scotland was not complete probably until mid-Viséan times. In the Midland Valley the deposition of a variety of kinds of semi-marine and “ continental “ deposits was controlled by locally powerful vulcanicity, by minor upwarps, and by the persistently revived major island or peninsula of the Southern Uplands—a barrier that continued to be effective until the close of Lower Carboniferous times even after the regional subsidence marked by the Iate-Viséan Hurlet transgression.