PALEOSOLS AS INDICATORS OF GEOMORPHIC STABILITY IN 2 OLD RED SANDSTONE ALLUVIAL SUITES, SOUTH WALES

Palaeosols (fossil soils) are abundant in the alluvial late Silurian-e arly Devonian Moor Cliffs Formation and Rat Island Mudstone (Freshwate r 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 ar e rare and most of the mudstone intervals exhibit complex overprinted profiles. However, a major difference exists between the palaeo-Vertis ols in each of the two units studied. The Moor Cliffs Formation contai ns thick floodplain intervals with evidence of frequent erosion and re activation of earlier soil horizons. The associated coarser channel de posits mainly represent ephemeral streams. The alluvial system was uns table, flashy and prone to extensive stripping of stored floodplain se diment. Arroyos (unstable ephemeral stream channels) probably develope d during this interval. Evidence for any marine influence on soil deve lopment is missing and previous speculations that the formation may re present 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 cha nnel deposits are typically of the fining-upwards type representing mo re 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 contemporar y drainage basins indicates that a change in climate was a possible ca use for the differences between the two units, less frequent and more severe storms occurring during the deposition of the Rat Island Mudsto ne.