Sediment Cyclicity and the Carnian Pluvial Episode: Evidence from Spectral Gamma ray Logging of the Mercia Mudstone Group, SW England.

Abstract

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 geographically- 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 longerterm and widespread Carnian rifting, marine inundation and volcanism in Pangaea.