Silurian marginal marine sedimentation and the anatomy of the marine-Old Red Sandstone transition in Pembrokeshire, SW Wales

Abstract

Abstract Deposition of the Silurian (Wenlock) siliciclastic Gray Sandstone Group of southwest Pembrokeshire took place within littoral environments close to the palaeogeographical shelf-margin of the Welsh Basin. Sedimentation described a northerly, basinward progradation across an earlier Late Ordovician? to Aeronian rift basin. Changes in relative sea level (rsl) had profound effects on depositional environments, and five depositional sequences are recognized. During highstands of rsl, the area was influenced by wave-dominated, shallow-marine conditions. During lowstands of rsl, shelf incision and sediment bypass occurred. Associated valley fills vary in nature from high-sinuosity estuarine channels, tidal flats and tidally-influenced, high-sinuosity fluvial channels. The last of these predominate within the youngest sequence, with abundant subaerial emergence indicators heralding the onset of true continental deposition, and the conformable transition into the overlying Old Red Sandstone Red Cliff Formation within the Marloes Peninsula. A renewal of tectonic activity ensued within the Lower Old Red Sandstone, with pebbly low-sinuosity alluvium of the Albion Sands Formation, and fanglomerates of the Lindsway Bay Formation refecting reactivation of earlier rift-margin faults, probably within a transtensional tectonic regime.