Early Permian (Rotliegendes) palaeowinds of the north sea

Abstract

The classical desert sediments of the Early Permian Upper Rotliegendes were deposited in a post-Variscan basin that extends from eastern England to the Russo-Polish border, which has been referred to as the Southern Permian Basin. These sediments comprise four main depositional facies: fluvial (wadi), aeolian, sabkha and lacustrine. The lacustrine facies includes important bedded halites. The Southern Permian Basin is separated from the smaller Northern Permian Basin by the fragmented Mid North Sea-Ringkøbing-Fyn system of highs: and beyond the Grampian Spur lies the even smaller Moray Firth Basin. These latter basins contain rock sequences similar to those of the southern basin, with the exception that bedded halite has not been recognised in the lacustrine facies. The Rotliegendes of the southern basin apparently conforms to deposition in a Northern Hemisphere tradewind desert similar to the modern southern Sahara. Apart from regional palaeomagnetic considerations and the occurrence of sediments of a non-arid nature south of the Variscan Highlands, the strongest evidence supporting this interpretation is the pattern of palaeowind directions deduced from the orientations of dune bedding. This is seen in both outcrop and core and derived from the continuous dip-meter logs of many North Sea wells by plotting the poles to the bedding attitudes on polar nets. Palaeowind directions have also been deduced from the dip-meter logs of wells drilled in the Northern and Moray Firth Basins. In general, they indicate winds that blew in a direction opposed to those of the Southern Basin. Thus an area of high barometric pressure seems to have been located over the Mid North Sea High. When the bedding attitudes in a well are roughly unidirectional, an origin on a transverse dune is normally indicated. With some wells, the indicated palaeowind direction is regionally anomalous. In many cases this can be rectified if the well is presumed to have drilled down through one flank of a seif dune. The indicated palaeowind is thus changed by up to 90° and then commonly fits the regional pattern. These early Permian desert basins were smaller and closer to the equator than the modern Sahara. Their location in the centre of a continent between the Caledonian and Variscan mountain ranges suggests the analogy of a more tropical equivalent of the Central Asian Takla Makan Desert.