Variable alluvial sandstone architecture within the Lower Old Red Sandstone, southwest Wales

Abstract

AbstractSandstone bodies within the Lower Old Red Sandstone (ORS) in southern Pembrokeshire exhibit variability in architecture, sediment grade and composition both spatially and temporally. Four architectural styles are observed, namely decimetre‐ to metre‐thick sheets, metre‐thick multi‐storey amalgamations, inclined‐heterolithic units and ribbon geometries. Sandstone bodies in the Freshwater East Formation are sheet‐like, heterolithic units several metres thick. An association with lingulids and wave ripples alludes to a marine influence, possibly estuarine tidal flats or storm washovers. Within the Moor Cliffs Formation, the most common sandstone bodies are centimetre‐ to metre‐thick sheets with high width‐to‐depth ratios. Fine‐grained sandstones represent sheet‐flood deposition on unconfined, planar surfaces, whereas coarser‐grained sandstones constitute distinctive amalgamations of discrete flood events, reflecting either a change in provenance or tectonic influence. Clear incision of coarse‐grained, multi‐storey units within the Inter‐Tuff Moor Cliffs Formation reflects a change in relative sea‐level, possibly tectonically induced.The base of the Conigar Pit Sandstone Member (CPSM) is marked by a distinctive, exotic‐clast conglomerate defining the base to heterolithic, lateral‐accretion bedsets and sandstone sheets. This association defines a significant influx of coarse‐grained sediment post‐Chapel Point Calcrete formation, an interval of presumed topographic stability across the Anglo‐Welsh Basin. This influx must reflect rejuvenation of source regions, with changes in base‐level reflecting either eustatic or tectonic influences. Commonly observed in the CPSM are fine‐grained, inclined‐heterolithic bedsets recording deposition by highly sinuous rivers with flashy discharge. Up‐sequence within the CPSM are metre‐thick, multi‐storey amalgamations of predominatly trough cross‐stratified medium‐ to coarse‐grained sandstone. It is likely that these units are genetically related to contemporaneous decimetre‐thick sandstone sheets, the latter being ‘splay’ events marginal to the main channel axis. The interbedding of multi‐storey sandstones and fine‐grained laterally accreted units reflects changes in provenance, slope and/or climate.Thickness variations within the Lower ORS detail significant thickening of all units northward into the Benton Fault. It seems likely that this thickening reflects variable accommodation space development associated with active growth along this and other WNW–ESE‐trending faults, and migration of channel belts toward the footwall. Copyright © 2004 John Wiley & Sons, Ltd.