Catchment inversion during the Silurian of SW Wales: Pediment plains, embryonic drainage networks and incised valley fills in a dryland range front alluvial system

Abstract

Abstract Ludlow to Přidoli age Old Red Sandstone alluvial to marginal marine deposits of the Anglo-Welsh Basin record a punctuated molasse shed from an inverted, transient catchment area. An erosional low-gradient sequence-bounding pediment surface was created basinward of a structurally controlled, transpressive piedmont junction. A thin mantle of debris flow dominated colluvium comprising the Abercyfor Formation was deposited proximal to a lowstand delta shoreline 90\u202fkm to the east and south. Temeside Mudstone Formation coastal plain deposits developed behind the delta passing westward into range front, dryland red bed alluvium of the Moor Cliffs Formation. This embryonic mudstone-dominated alluvial plain aggraded during an interval of enhanced accommodation space development. Muds were liberated from their Lower Palaeozoic bedrock provenance within and to the north of the piedmont junction. Dryland alluvial sandstone channels possibly represent stream capture events associated with hinterland drainage reorganisation. The latter promoted continued headward erosion and enlargement of the catchment area. Basinwide incision (to a depth of approximately 60\u202fm) of the alluvial plain occurred in the mid Přidoli, with valley locations being structurally controlled. Subsequent fills preserve the middle reaches of a simple incised valley system. Lowstand to early transgressive streamflow conglomerates of the Freshwater East Formation were deposited, with sediment transport to the SE. Valley drainage was defeated by neotectonic uplift of the Orielton Anticlinorium in downstream valley reaches leading to drainage reorganisation. Accommodation space was developed at structural tip points where maximum rates of marine transgression ensued preserving backstepping parasequences of a wave dominated estuary. Localised debritic alluvial fans were shed below the interval of maximum flooding. Continued dryland sedimentation led to highstand filling and subsequent basinwide alluvial regression which continued into the late Přidoli.