Faunal and facies dynamics in the Upper Silurian of the Anglo-Welsh basin

Faunal data are matched against major sedimentary facies to interpret benthic palaeoecology and palaeogeographical evolution through the Lower Leintwardine Formation (Ludlow Series, upper Silurian) of the Anglo-Welsh basin. In shelf areas, original patchiness of level-bottom benthic epifauna, opportunistic species, and wide faunal belts characterize the storm-influenced subtidal environments. Monospecific assemblages suggest high environmental stress. Breaks in shelf deposition, erosion, and hardground formation introduced the sequence, particularly in inshore areas, as carbonate shelf environments were replaced by clastic silt sedimentation. The most marked lateral faunal change, from skeletal benthos to graptolite assemblages, takes place across the offshore shelf margin, where tectonic controls are reflected in downslope slumping and submarine channelling. In low energy trough environments, scarcity of benthos other than infaunal lingulides suggests unfavourable bottom conditions. Leintwardinian faunas are subdivided broadly into three epifaunal brachiopod-dominated shelf associations, a brachiopod-ostracode offshore shelf assemblage, and a thanatocoenotic graptolite-lingulide trough association. Offshore spread of shelf environments in late Leintwardinian times was accompanied by immigration of new arthropod faunas, and shelf-wide domination by the distinctive Upper Leintwardine Formation association. Wider circulation patterns and regression heralded the final silting-up of the basin in response to late Caledonian tectonism