THROUGH THICK AND THIN: INDICATIONS OF CLIMATE CHANGE AND TECTONIC ACTIVITY IN THE DEVONIAN RIDGEWAY CONGLOMERATE FORMATION OF SW WALES, UK

The Devonian Old Red Sandstone Ridgeway Conglomerate Formation (RCF) in Pembrokeshire, Southwest Wales was deposited as part of a dryland alluvial fan/axial fluvial valley complex. Regionally structural blocks and basins are defined by a series of extensional faults, with intermittent movement on the Benton and Ritec faults resulting in alluvial fan sedimentation and condensed sequences. The RCF was deposited in a half-graben as a hanging-wall alluvial fan that thickens into the Ritec fault, indicating the control this fault had on its development. The RCF is heterolithic, comprising conglomerates, sandstones and gritty mudstones that reflect differences in processes, suggesting sheetfloods, low relief lateral accretion and cohesive debris flows. The alluvial fans appear to have prograded northward, interfingering with deposits from an axial high-sinuosity fluvial system. The later contain thinly laminated interbedded mudstones and sandstones that were deposited in ephemeral axial valley lakes at the toe of the fan. The lakes possibly developed under a sub-humid climate, or were sustained by baseflow through permeable fan deposits. Sedimentary structures in this lacustrine association include pinstripe lamination, matgrounds, algal ‘roll-up' structures, calcretized teepees and root traces. Periods of semi-arid climate and/or active tectonic conditions are represented by fan gravels prograding into the axial system and the presence of calcretes of various stages of development.