Palaeosols from the Upper Triassic of South Wales

Temporary exposures recording the Upper Triassic–Lower Jurassic transition near Bridgend, South Wales provided a rare opportunity to assess how this major stratigraphical interval is manifested in terrestrial deposits in close proximity to the intensively studied marine and marginal marine successions in SW Britain. Following the progradation and exposure of a shoreline sandbody, up to 8 m of coastal plain deposits formed during a subsequent punctuated transgression. These deposits host a series of palaeosols with unusual carbonate textures, complex mottling styles and marked lateral variability, with evidence of several changes in hydrological conditions. Initially calcrete-bearing soils capped the shoreline sandstones but were overprinted by ferroan-carbonate-bearing gleys likely marking the influence of marine waters. Subsequently, a transition to well-drained soils with calcrete horizons occurred. Continued mud grade deposition was marked by the development of pseudo-gley conditions culminating in gleyed soils, overlain by marine sediments. This provides an example of the often complex polyphase and polygenetic nature of palaeosols encountered in aggrading sedimentary systems.