Mississippian reef development in the Cracoe Limestone Formation of the southern Askrigg Block, North Yorkshire, UK

Abstract

The southern margin of the Askrigg Block around Cracoe, North Yorkshire, shows a transition from carbonate ramp to reef-rimmed shelf margin, which, based on new foraminiferal/algal data, is now constrained to have initiated during the late Asbian. A late Holkerian to early Asbian ramp facies that included small mudmounds developed in comparatively deeper waters, in a transition zone between the proximal ramp, mudmound-free carbonates of the Scaleber Quarry Limestone Member (Kilnsey Formation) and the distal Hodderense Limestone and lower Pendleside Limestone formations of the adjacent Craven Basin. The ramp is envisaged as structurally fragmented, associated with sudden thickness and facies changes. The late Asbian to early Brigantian apron reefs and isolated reef knolls of the Cracoe Limestone Formation include massive reef core and marginal reef flank facies, the latter also including development of small mudmounds on the deeper water toes of back-reef flanks. The position of the apron/knoll reefs is constrained to the south (hangingwall) of the North Craven Fault, but it is syn-depositional displacement on the Middle Craven Fault that accounts for the thick reefal development. Subsequent inversion of this structure during the early Brigantian caused uplift and abandonment of the reefs and subsequent burial by the Bowland Shale Formation.