Structural Control on the Complex Platform to Basin Transition of a Mississippian Carbonate Platform in the Southern East Irish Sea Basin, UK

Mississippian‐aged (Lower Carboniferous) syn‐rift carbonate platforms in the UK have been extensively studied in outcrop. They have been interpreted to grow principally on the footwall of faults, with deeper marine sedimentation in the adjacent hanging wall basins. However, the transition from the shelf margin to the basin is often poorly constrained due to a lack of exposure and the scarcity of high‐quality seismic data. With renewed interest in Mississippian carbonate strata as potential geothermal reservoirs in northern Europe, a better understanding of the detailed geometry of these carbonate platforms, and the controls on their growth and demise, is crucial as it provides insights into their occurrence, size and thickness and burial/exposure history. This study uses high‐resolution 3D seismic data from the southern part of the offshore East Irish Sea Basin (EISB), western UK, to identify, characterise and map the platform to basin transition of the North Wales carbonate platform, exposed on the North Wales coastline. The results indicate that there is not a simple platform to basin transition, as has previously been mapped, but that the North Wales platform gives way offshore to numerous small carbonate platforms, the presence of which is predominantly controlled by N‐S‐oriented extensional faults. The fault orientation is not consistent with the regionally interpreted N‐S stress direction during the Mississippian, but fault growth analysis suggests that their orientation most likely reflects the precursor structural grain. These faults facilitated the development of horst‐graben structures, promoting carbonate growth on footwalls within the EISB. Six areas of potential carbonate platform development (A1–A6) were mapped and evaluated. Thicknesses range from ~1 to 2 km. The platforms prograded during the Tournaisian, characterised by low‐angle slopes, followed by a backstepping phase in the Visean, marked by steeper slopes. The platforms significantly shrank in size from the early Tournaisian to the Visean, resulting in the formation of complex, patchy carbonate platforms with diverse shapes and sizes. The results demonstrate that numerous small carbonate platforms grew in the EISB on structural highs but were susceptible to environmental change at the end of the Mississippian, causing them to become increasingly isolated and to eventually drown.