Anomalous facies of the Kinnesswood Formation in the Cumbraes Basin: a late Famennian playa complex in southwest Scotland

Abstract

Among the important geological attributes of the small Cumbraes Basin in the southwestern off-shore Midland Valley of Scotland, the 4 km-long, fragmented but well-exposed section through the Devonian-Carboniferous boundary sequence in southern Great Cumbrae island ranks highly. In a continuous short stretch of the south and west coasts, large portions of this sequence are superbly preserved in tide-washed strandline surfaces and in old post-glacial sea cliff. This enhanced exposure leaves no doubt that the outcrops belong to two, immediately successive, members of the Kinnesswood Formation -- the lower the Doughend Sandstone, the upper the Foul Port Mudstone – and that they provide some of the best available exposures on which to base description and interpretation. The facies of the fluviatile Doughend Sandstone shares many characteristics with the underlying Kelly Burn Sandstone of the Upper Old Red Sandstone, whereas that of the Foul Port Mudstone is distinct and unusual, attributed to local tectonics, influenced by movements on older and more major fractures such as the Highland Boundary Fault, with sediment accumulation taking place in a playa-lake complex of considerable extent.