Dryland continental mudstones: Deciphering environmental changes in problematic mudstones from the Upper Triassic (Carnian to Norian) Mercia Mudstone Group, south‐west Britain

Abstract

The sedimentology, petrography and mineralogy of the seemingly monotonous Late Triassic Mercia Mudstone Group across the Severn Basin and Bristol Channel region reveals a bipartite division of mudrock facies above and below the Arden Sandstone Formation. Frequently cryptic sedimentary and pedogenic features reveal diverse alluvial, aeolian, playa-lacustrine and pedogenic processes operating at different times, locations and scales probably in response to climate change. The current study found no evidence of significant marine influence in either of the two mudrock facies associations that are described here. Blocky claystones dominate the lower Mercia Mudstone Group (Sidmouth Mudstone Formation), with pedogenic features such as slickensides, mottling, tubules and carbonate/sulphate nodules common and widespread. The claystones are of alluvial/lacustrine origin with subordinate sheet sandstones, themselves overprinted by pedogenic features, reflecting occasional high intensity rainfall events. These facies reflect a seasonal (wet/dry) semi-arid climate favouring development of transformed/neoformed (smectiterich) clay minerals of intrabasinal origin. Massive to weakly stratified silty mudstones dominate the upper Mercia Mudstone Group (Branscombe Mudstone Formation). Commonly conchoidally weathered, locally gypsiferous, but with distinct sedimentary structures scarce, they alternate with subordinate blue-grey laminated silty mudstones. Together they reflect fluctuating hydrological conditions within extensive saline mudflats and ephemeral playa lakes, with laminated facies deposited under subaqueous conditions during more humid phases whereas massive mudstones reflect modification through interstitial growth/dissolution of sulphates and deflation of surface sediments during drier episodes. These facies reflect increased aridity during deposition of the upper Mercia Mudstone Group compared with the lower Mercia Mudstone Group, favouring development of detrital/transformed (illite-chlorite) clay minerals of extrabasinal origin. The described facies associations and the sedimentary fabrics and structures that characterize them, occur widely in the Mercia Mudstone Group across the United Kingdom and comparable facies associations may be anticipated in other fine-grained red bed successions. Recognition of these facies may aid palaeoenvironmental interpretation of such sequences on Earth and, potentially, on Mars also.

Evolution from shallow‐water deltas to fluvial fans in lacustrine basins: A case study from the Middle Jurassic Shaximiao Formation in the central Sichuan Basin, China

“…The interbedding of lithofacies Gm and lithofacies Mn may indicate the frequent submergence and exposure (Sáez et al ., 2007). Carbonate and gypsum nodules, and root traces in the lithofacies Mm and lithofacies Pm, demonstrated the development of palaeosols, which are indicative of a long‐term exposure in the floodplain (Cleveland et al ., 2007; Milroy et al ., 2019).…”

A new exposure of the North Curry Sandstone Member (Dunscombe Mudstone Formation, Mercia Mudstone Group: Carnian, Triassic), near Taunton, Somerset (UK): The location of Charles Moore's vertebrate specimens resolved

“…The cause of this cyclicity is worthy of further characterisation and investigation. Some of the mottled, green-coloured mudstones, may have resulted from discrete wetting or lake expansion events (as described in Milroy et al, 2019). It is not possible at present to correlate these five beds with those in the Wiscombe Park borehole cores which were taken ~35 km to the south-east.…”

“…The NCSM is the lateral equivalent of the 4 Somerset Halite Formation and is superseded by the Blue Anchor Formation (Gallois, 2003;Gallois and Porter, 2006). The Dunscombe Mudstone Formation is generally considered to be the time equivalent of the Arden Sandstone Formation in the English Midlands (Warrington et al, 1980) which forms a regional correlative marker separating the Sidmouth Mudstone Formation from the Branscombe Mudstone Formation of the Mercia Mudstone Group (Gallois and Porter, 2006;Milroy et al, 2019). Warrington and Williams (1984) suggested that the NCSM was no older than early Carnian (Cordevolian), and more likely late Carnian (Julian or Tuvalian), on the basis of a palynomorph assemblage that includes Vallasporites ignacii, Patinasporites densus, Duplicisporites spp., Camerosporites secatus and Ovalipollis pseudoalatus.…”

The sedimentological expression of transgression–regression cycles upon aeolian–marine margins

“…This suggests periodic draw down of the water table, given the extensive presence of gypsum, the dominant process of draw-down is evapotranspiration. Vertic textures are produced by the high rate of evapotranspiration, leading to rapid shrink-swell contrasts (Milroy et al, 2019)…”