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

Abstract

ABSTRACT: The Ordovician-Silurian succession of the Llandovery area, mid-Wales, preserves shal-low marine sediments deposited during the Hirnantian (end-Ordovician) extinction event. Ichno-logical analysis of the rocks shows that the degree and depth of bioturbation in this part of the WelshBasin was often low across this interval, both during glacioeustatic sea level fall and the early part ofthe subsequent transgression. This pattern suggests that stratigraphic dilution was not the primarycontrol on trace fossil abundance: an increase in sediment supply during the regression could explainlow abundance in early Hirnantian units, but not in the post-glacial transgressive strata. A significantdecrease in oxygen levels during sea level rise might be invoked instead, but this cannot have beenthe sole cause, as the occurrence of burrows up to 20 mm in diameter in the Bronydd Formation (lateHirnantian–Rhuddanian) shows that seafloor oxygen levels were at least intermittently high. Anabsence 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 benthicinfauna during the extinction event, but the 2 pulses of extinction described previously in body fossils are not evident ichnologically in this part of the Welsh Basin.KEY WORDS: Ichnofabrics · Benthos · Infauna · Mass extinctions · Trace fossils