Sedimentology of the upper Dinantian Charlestown Main Limestone: implications for the controls on cyclothem deposition, eastern Midland Valley of Scotland

Abstract

Synopsis The Charlestown Main Limestone of the upper Dinantian (Brigantian) Lower Limestone Group was deposited in an intracratonic basin undergoing differential subsidence. In the eastern Midland Valley two areas of increased susidence are recognized, the Fife-Midlothian Low and the Central Low, separated by an area of reduced subsidence, the Burntisland High, centred on West Lothian and Central Fife. The Charlestown Main Limestone forms the base of a typical Yoredale cyclothem and the area of deposition was flanked to the NW and NE by fluvio-deltaic systems. Five distinct lithofacies are recognized within the Charlestown Main Limestone (CML); bioclastic wackestones, argillaceous lime mudstones, mottled limestones, bioclastic grain-stone/packstones and carbonate buildups. Bioclastic wackestones are present at the base of the CML throughout the eastern Midland Valley. However, distinct regional differentiation of lithologies followed: buildups and bioclastic grainstones and packstones developed on, or near, the Burntisland High whereas fine terrigenous material (introduced from the flanking fluvio-deltaic systems) preferentially accumulated in the bounding lows where the CML is dominated by more argillaceous lithologies (the argillaceous lime mudstone and mottled limestone lithofacies). Progradation of prodeltaic muds from the NE curtailed carbonate deposition earlier in East Fife and accounts for the thinning of the CML towards this area. In distal areas, such as Midlothian, this progradation was heralded by an influx of fine terrigeneous sediment reflected in an increasingly argillaceous upper part of the limestone. Eustasy appears to have been the primary control on the deposition of the Charlestown Main Cyclothem: sea level rise resulted in transgression and initiation of carbonate sedimentation while delta progradation, which curtailed carbonate production, was probably triggered by a sea-level fall. Differential subsidence and autocyclic processes (such as major channel avulsion) were secondary factors and controlled the distribution and thickness of lithofacies within the limestone and the nature of the overlying deltaic sediments.