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Controls on Carbonate Platform Growth Within the Lower Carboniferous of the Pennine Basin


Carbonate platforms within extensional basins are important targets for oil and gas exploration and appraisal, but the interplay of depositional, diagenetic and structural processes on their evolution is not well-understood. In particular, the interplay of fault propagation, bioconstruction, volcanism and siliciclastic interaction on platform architecture and facies distribution remains poorly described. Two Visean platforms are compared: the Derbyshire Platform (DP), situated in the Pennine Basin of England, and the North Wales Platform (NWP), located 130 km westwards. Both platforms formed in a back-arc extensional basin on the footwalls of normal and oblique-slip faults but had different siliciclastic and volcanic influence. Field-based sedimentology and stratigraphic forward modelling of Asbian and Brigantian (333 to 326.4 Ma) strata allows evaluation of multivariate controls on the divergent evolution of two coeval carbonate platforms which formed within the same basin.

Field mapping and petrography shows that in the Asbian, the windward margins of both platforms were dominated by bryozoan and coral carbonate mounds with core and flank subfacies. Grainstone shoals infilled intra-mound topography and also formed sheet-like sandbars. Behind these margins, subtidal crinoidal pack-grainstones record upward-shallowing packages, capped by exposure surfaces. On the NWP, these surfaces comprise unconsolidated, clay horizons and nodular, cemented limestones whilst on the DP mammillated limestones are overlain by clay wayboards (volcanic ashfalls). A platform-wide emergent surface marks top Asbian. Penecontemporaneous marine clastic deposition shifted towards the NWP, whilst the DP was protected from this influx by an intervening basin. During the Brigantian, migration of fluvio-deltaic systems from proximal landmasses increased water column turbidity. Facies became dominated by Gigantoproductus brachiopods, and beds became thinner and darker. As volcanism waned on the Derbyshire Platform, mounds became common on the platform top rather than the margin and formed preferentially at E-W jogs between oblique-slip faults. Siliciclastic poisoning and/or drowning of carbonate factories terminated platform growth at the end Brigantian. Numerical stratigraphic forward models based on parameter ranges from these platforms assess sensitivity of carbonate platform growth and stacking to changes in relative sea level, carbonate production rate, and sediment transport rates.