Influence of extensional faults and relay ramp on palaeotopography and lacustrine carbonate facies: Purbeck Limestone Group (Upper Jurassic - Lower Cretaceous), Wessex Basin, UK

Lacustrine carbonate facies distribution is controlled by multiple environmental parameters such as climate, hydrology, and tectonic setting, but few published models address this complexity. In this study, seismic and borehole data, coupled with outcrop logging, correlations, and facies models, are used to create a new tectono-sedimentary model demonstrating how extensional faults, linked by a relay ramp, control distribution of lacustrine carbonate facies in the Upper Jurassic to Lower Cretaceous Purbeck Limestone Group (Wessex Basin, UK). Accumulation occurred in half-graben sub-basins south of two extensional east-west faults, with subaerial emergence conditions on footwall blocks to the north. The lacustrine limestones of the lowest unit of this Group are characterised by in-situ microbial mounds within bedded inter-mound packstones-grainstones. These occur in three depositional intervals separated by paleosols. The distribution of facies indicates more brackish-water conditions shoreward to the west, and more hypersaline conditions basinward to the east. The relay ramp hosts extensive microbial carbonate buildups formed in response to carbonate-rich waters sourced from the northern limestone footwall blocks and that fed into extensive shallow-water areas on the low-angle relay ramp slope.