Relation of upper visean sedimentology to the bowland shale overlap in Yorkshire, England

Abstract

The Upper Visean and Lower Namurian Bowland Shale Formation has long been supposed to bury an Upper Visean limestone escarpment which was formed by contemporaneous faulting or pre-Bowland Shale uplift and erosion of the Upper Visean Yoredale Formation. However, such an interpretation is incompatible with the latest view on correlation which is that the Bowland Shales and Yoredales are generally equivalent in age. The Upper Visean escarpment and associated Bowland Shale overlap approximately follow the line of a tectonic hinge which separates two contrasted sedimentological provinces. The northern province or Askrigg Block was a positive feature during Carboniferous times relative to the southern province or Craven Basin. Important stratigraphical and sedimentological changes in Upper Visean rocks are associated with this hinge. For example, contemporaneous topography and overlap of the Bowland Shales are restricted to the southeastern margin of the Askrigg Block where the Lower Yoredale Formation is developed as a massive carbonate facies. These Lower Yoredale limestones change facies abruptly at the line of the Bowland Shale overlap and pass into a much thinner shale sequence. Also, individual limestone members in the Yoredale Formation which typically consist of calcilutites over much of the Askrigg Block pass laterally into calcarenites near the line of the Bowland Shale overlap. These observations are thought to indicate that the contemporaneous Late Visean topography resulted primarily from differential subsidence and Late Visean starvation of the Craven Basin. Carbonate shoals which developed at the same time along the southeastern margin of the Askrigg Block enhanced topography induced by differential subsidence and helped to maintain starvation of the Craven Basin. During earliest Namurian times, the advance of deltas into the Yorkshire area again provided an adequate supply of sediment in the Craven Basin and at the same time lowered regional salinity, and so inhibited carbonate shoaling on the Askrigg Block. Consequently the Late Visean topography was rapidly buried.