The Anatomy of a Mississippian (Viséan) carbonate platform interior, UK: Depositional cycles, glacioeustasy and facies mosaics

Abstract Late Visean platform interior carbonate strata of northern England and Wales have traditionally been interpreted to comprise shallowing-up cyclothems, the upper boundaries of which are marked by unconformities that record subaerial exposure events. These strata were deposited during what is understood to be a greenhouse-to-icehouse transition, and the cyclothems are assumed to reflect glacioeustatic sea-level oscillations associated with this climatic change. Importantly, these interpretations reflect a qualitative model which assumes that facies respond in a simple way to water depth variations and so migrate and stack in a predictable pattern as eustatic sea-level fluctuates. Exposure surfaces and most facies are, however, laterally discontinuous across the platform tops. Quantitative statistical analysis and simple 1D stratigraphic forward modelling suggest that the observed vertical facies stacking patterns on the Derbyshire and North Wales Platforms are not cyclical. The most ordered arrangement of facies that could be interpreted as “ideal cycles” in each section are variable within and across the platforms. This observational and analytical evidence suggests that instead of a simple, dominant external glacioeustatic forcing on sedimentation, a model commonly proposed for the Late Visean carbonate strata of northern England and Wales, strata were deposited as a facies mosaic, under variable hydrodynamic conditions influenced by autogenic processes as well as local environmental fluctuations in salinity and temperature, and perhaps in conjunction with glacioeustasy and syn-rift tectonic processes which occurred from metre-scale to kilometre-scale, as suggested by the ideal cycles.