Bioturbation levels during the end-Ordovician extinction event: a case study of shallow marine strata from the Welsh Basin

The Ordovician-Silurian succession of the Llandovery area, mid-Wales, preserves shallow marine sediments deposited during the Hirnantian (end-Ordovician) extinction event. Ichnological analysis of the rocks shows that the degree and depth of bioturbation in this part of the Welsh Basin was often low across this interval, both during glacioeustatic sea level fall and the early part of the subsequent transgression. This pattern suggests that stratigraphic dilution was not the primary control on trace fossil abundance: an increase in sediment supply during the regression could explain low abundance in early Hirnantian units, but not in the post-glacial transgressive strata. A significant decrease in oxygen levels during sea level rise might be invoked instead, but this cannot have been the sole cause, as the occurrence of burrows up to 20 mm in diameter in the Bronydd Formation (late Hirnantian-Rhuddanian) shows that seafloor oxygen levels were at least intermittently high. An absence of vertical bioturbation through much of the succession indicates that sessile, suspension-feeding organisms were generally scarce. The overall pattern probably reflects a decline in benthic infauna during the extinction event, but the pulses of extinction described previously in body fossils are not evident ichnologically in this part of the Welsh Basin.