Permo-Triassic aeolian red clay of southwestern England and its palaeoenvironmental implications

Abstract

Late Permian and Triassic red mudstones are widely distributed and exposed in southwestern England. This homogenous, massive mudstone succession has been extensively studied for its palaeoenvironmental potential. However, its origin and palaeoenvironment remain under debate. The Triassic Mercia mudstone was early proposed to be aeolian loessic clay based on collapse nature when wetted or loaded and its similarities with aeolian clay in Quaternary Australia, but lacks experimental confirmation and is not generally accepted. In comparison with aeolian red clay in China, here we confirmed that the Triassic Mercia mudstone, together with the Upper Permian Aylesbeare mudstone, was dominated by aeolian clay. This was based on lines of evidence regarding sedimentary structure, particle grain size, geochemistry, micromorphology, and pedogenic features. The results showed that the Mercia mudstone and the Aylesbeare mudstone had sedimentary structures and grain size distributions that were similar to the aeolian red clay in the Chinese Loess Plateau. The major chemical elements correlated well with the aeolian red clay, with the exception of minor discrepancies in CaO and K2O. The similarity in the rare earth element (REE) distribution of the Mercia mudstone and the Aylesbeare mustone with aeolian red clay inferred their similar origin. Widespread pedogenic features indicated the universal development of palaeovertisols forming in alternating wet/dry climates. It was hypothesised that Permo-Triassic southwestern England was dominated by aeolian dust accumulation in a seasonal wet/dry climate, whereas perennial lakes or longstanding rivers across the study area might not exist.