Sediment cyclicity and the Carnian Pluvial Episode: evidence from spectral gamma-ray logging of the Mercia Mudstone Group, SW England

The Late Triassic exposed on the Devon coast (SW England) is analysed through spectral gamma-ray logging and considered in terms of two theories of palaeoenvironmental change: sediment cyclicity and the Carnian Pluvial Episode (CPE). A decrease in total count gamma-ray output occurs at the CPE and reflects a geographically extensive change in depositional styles from siltstones and isolated playa-lake, evaporitic mudstones to more geographical-ly-widespread evaporite-bearing mudstone cycles. Th/K and Th/U ought to increase (through humid weathering) in the CPE, but these ratios do not change until above the CPE-related Dunscombe Mudstone Formation. A delayed response in Th enrichment is considered likely, reacting to both the initiation of the 1.2my long CPE, but also longer-term and widespread Carnian rifting, marine inundation and volcanism in Pangaea.