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

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