Burial diagenetic evolution of the Lower Carboniferous (Dinantian) of the southern margin of the Askrigg Platform and a comparison with the Derbyshire Platform

In order to predict the style and impact of post-depositional modification of carbonate successions, well-studied and accessible outcrop analogues are invaluable. The Lower Carboniferous (Dinantian) carbonate platforms of the Pennine Basin of northern England have a long history of investigation. As such, they offer the potential to evaluate the mechanisms and timing of fluid flux during extensional tectonism, post-rift basinal subsidence and inversion. This study concentrates upon the diagenetic evolution of the late Dinantian of the southern margin of the Askrigg Platform of North Yorkshire and a comparison with published data from the age-equivalent Derbyshire Platform. A pattern of consistent, diagenetic modification during early diagenesis is evident, but key differences occur in the burial realm. On both the southern margin of the Askrigg Platform and the Derbyshire Platform, patterns of dolomitization, hydrocarbon emplacement and mineralization can be determined on the platform that reflect the diagenetic evolution of the adjacent basins. However, within the study area of the Askrigg Platform, there is only local evidence for a fault/fracture control on the migration of Mg-enriched, hydrocarbon-bearing fluids. In contrast, on the Derbyshire Platform, burial diagenesis is intimately associated with NW–SE- and NE–SW-trending faults and fractures. Data suggest that pervasive cementation in the marine and meteoric realm occluded matrix porosity in both areas, such that fluid migration was almost entirely fracture controlled. With the localization of structural deformation along the Craven Fault Zone, and a low abundance and density of open fault/fracture networks, circulation of fluids on to the southern margin of the Askrigg Platform was inhibited, however. Furthermore, the presence of local aquifers in the Craven Basin may have led to fluid expulsion from the basin during early burial.