Sedimentation and tectonics: the marine Silurian–basal Lower Old Red Sandstone transition in southwest Wales

Both regional and localized tectonic events controlled deposition within the Wenlock and early Ludlow of SW Wales. Estuarine deposits within north–south-tending incised valleys dominate the youngest (Homerian) Gray Sandstone Group, valley incision being probably related to changing base-levels associated with Avalonia/Laurentian collision. Available accommodation space was outpaced by sediment supply, with the Red Cliff Formation (Late Ludfordian) defining a conformable transition from marine to Old Red Sandstone (ORS) deposition within the Marloes Peninsula. Sedimentation was dominated by fine-grained pedified siliciclastics, with subordinate fine-grained ephemeral sheet-flood sandstones. Local palaeocurrents indicate sediment transport from the south and west, though long-distance transport from a distant Laurentian provenance is assumed. A probable tectonically generated sequence boundary marks the base of petrographically distinctive, multi-storey pebbly sandstones of the Albion Sands Formation, deposited within the hangingwall valley of the active east–west-trending Wenall Fault. Sediment accommodation space was controlled by proximity to the tip-point of this important growth fault within the Lower ORS. Debris-flow-dominated fans, shed from both the hangingwall and footwall of the Wenall Fault, deposited the Lindsway Bay Formation, an exotic-clast conglomerate unit sourced predominantly from the south and west. It is uncertain as to whether movement along the Wenall Fault was caused by collision-related transtension, or rifting associated with the southern margin of Avalonia. Copyright © 2004 John Wiley & Sons, Ltd.