Magnetostratigraphy of the Mercia Mudstone Group (Devon, UK): implications for regional relationships and chronostratigraphy in the Middle to Late Triassic of Western Europe

Abstract

Global synchronization of environmental change in terrestrial successions in deep time is challenging owing to the paucity of dating methods, a case also applicable to the Middle to Upper Triassic Mercia Mudstone Group in Britain. Using coastal cliff sections, magnetostratigraphy was evaluated at 263 horizons, defining 53 magnetozones. Magnetozones from the lower 140 m of the group demonstrate correspondence to those from the mid-Ladinian to early Carnian polarity timescale, dating that is compatible with magnetostratigraphy from the underlying Sherwood Sandstone Group. Magnetostratigraphy of the Dunscombe Mudstone Formation, and associated palynological data, suggest a late Carnian to earliest Norian age, and a dramatically lower accumulation rate than for adjacent formations. The polarity record demonstrates coeval flooding events, evaporite deposits and intervals of sand supply between the Wessex Basin and the Central European Basin in the Carnian. This is the result of linked climatic and eustatic changes between these separate basins, related to aeolian dust supply and the shrinkage of hyper-arid source regions for the fine material. Magnetostratigraphy from the Branscombe Mudstone and Blue Anchor formations demonstrates their Norian and early Rhaetian age. These and other data suggest an alternative synchronization of marine and non-marine polarity records for the Norian polarity timescale. Supplementary material : Section details and detailed logs of the sampled sections and inferred sequence boundaries, magnetic mineralogy data, demagnetization behaviour and mean directions, summary of virtual geomagnetic pole data and a comparison with other European poles, construction of other composite reference sections and revised polarity scales, an Excel sheet of magnetic data, statistically evaluated correlation models and age models are available at https://doi.org/10.6084/m9.figshare.c.6613788