Palaeosols as indicators of geomorphic stability in two Old Red Sandstone alluvial suites, South Wales

Palaeosols (fossil soils) are abundant in the alluvial late Silurian–early Devonian Moor Cliffs Formation and Rat Island Mudstone (Freshwater West Formation) of south Dyfed, South Wales. The palaeosols occur in thick mudstones deposited on floodplains under a seasonal, semi-arid climate, and represent Vertisol-type soils. Simple, single profiles are rare and most of the mudstone intervals exhibit complex overprinted profiles. However, a major difference exists between the palaeo-Vertisols in each of the two units studied. The Moor Cliffs Formation contains thick floodplain intervals with evidence of frequent erosion and reactivation of earlier soil horizons. The associated coarser channel deposits mainly represent ephemeral streams. The alluvial system was unstable, flashy and prone to extensive stripping of stored floodplain sediment. Arroyos (unstable ephemeral stream channels) probably developed during this interval. Evidence for any marine influence on soil development is missing and previous speculations that the formation may represent deposition in an upper-intertidal or supratidal system are not supported. The Rat Island Mudstone represents a complete contrast in that there was little or no stripping of floodplain sediments. The channel deposits are typically of the fining-upwards type representing more stable, low sinuosity channels and with ephemeral-stream floods. By integrating the palaeosols, a more detailed picture of the geomorphic history of these units can be formulated. Comparison with contemporary drainage basins indicates that a change in climate was a possible cause for the differences between the two units, less frequent and more severe storms occurring during the deposition of the Rat Island Mudstone.