The Brigantian transition from the Askrigg Block to Craven Basin between Grassington and Greenhow, North Yorkshire, UK

A lateral variation in the heterolithic platform facies is present on the southern flank of the Askrigg Block where it transitions to mudrock-dominated basinal facies of the Craven Basin during the Brigantian substage. Revised geological mapping and new micropalaeontological assessment is used to correlate Transition Zone strata with the mixed carbonate and deltaic (Yoredale) cyclic facies of the Alston Formation of the Askrigg Block. It shows the Hawes Limestone to be entirely early Brigantian in age, with a consistent thickness across the study area. The overlying Gayle and Hardraw Scar cycles show marked variations consistent with displacement on the North Craven Fault, whereas the subsequent Simonstone and Middle Limestone cycles are absent within the Transition Zone. The Transition Zone includes younger Brigantian strata than found on the southern Askrigg Block. The Transition Zone is demonstrated to be: (1) affected by early Brigantian regional extension leading to transtensional oblique-slip displacement on the North Craven Fault and northward tilting of the Askrigg Block; (2) development of an erosive palaeo-slope in the Transition Zone during the early to late Brigantian extending across the North Craven Fault in response to transpressional structural inversion, uplift and erosion; (3) onlap of the late Brigantian Alston Formation and subsequently Bowland Shale Formation onto the incised Alston Formation during post-rift regional subsidence; and (4) ultimate northward tilting of the Askrigg Block during the early Pendleian in response to transpressional oblique-slip on the North Craven Fault, prior to deposition of the Millstone Grit Group.