Sedimentary and faunal events revealed by a revised correlation of post-glacial Hirnantian (Late Ordovician) strata in the Welsh Basin, UK

The discovery of a previously unrecognized unconformity and of new faunas in the type Llandovery area underpins a revised correlation of Hirnantian strata in mid Wales. This has revealed the sedimentary and faunal events which affected the Lower Palaeozoic Welsh Basin during the global rise in sea level that followed the end-Ordovician glacial maximum and has allowed their interpretation in the context of local and global influences. In peri-basinal shelfal settings the onset of post-glacial deepening is recorded by an unfossiliferous, transgressive shoreface sequence (Cwm Clyd Sandstone and Garth House formations) which rests unconformably on Rawtheyan rocks, deformed during an episode of pre-Hirnantian tectonism. In the deep water facies of the basin centre, this same sequence boundary is now recognized as the contact between fine-grained, re-sedimented mudstones and an underlying regressive sequence of turbidite sandstones and conglomerates; it is at a level lower than previously cited and calls into question the established lithostratigraphy. In younger Hirnantian strata, graptolites associated with the newly recognized Ystradwalter Member (Chwefri Formation) demonstrate that this distal shelf unit correlates with the persculptus graptolite-bearing Mottled Mudstone Member of the basinal succession. Together these members record an important macrofaunal recolonization of the Welsh Basin and mark a key event in the post-glacial transgression. Further deepening saw the establishment of a stratified water column and the imposition of anoxic bottom water conditions across the basin floor. These post-glacial Hirnantian events are consistent with the re-establishment of connections between a silled Welsh Basin and the open Iapetus Ocean. However, a comparison with other areas suggests that each event records a separate deepening episode within a pulsed glacio-eustatic transgression, while also reflecting changes in post-glacial climate and patterns of oceanic circulation and associated biotic flux. British Geological Survey (C) NERC 2009. All rights reserved