EARLY TRIASSIC TRACE FOSSILS IN NW CANADA: RECOVERY OF INFAUNA AFTER THE PERMIAN-TRIASSIC EXTINCTION

            Early Triassic ichnofaunas in NW Canada record the recovery of benthic infauna after the end-Permian extinction. Outcrops in SE Yukon Territory preserve Griesbachian trace fossils, whereas drill cores from the Ring Border and Kahntah River gas fields, NE British Columbia, preserve Griesbachian to Dienerian traces. Shale, siltstone, and sandstone of wave-dominated, distal-shelf to shoreface origin make up both successions.

            Ring Border and Kahntah River strata record increasing trace-fossil diversity and abundance during the earliest Triassic. Early Griesbachian strata contain a low-diversity assemblage (six ichnogenera) of isolated trace fossils, whereas Late Griesbachian-early Dienerian strata are more pervasively bioturbated, with a more diverse ichnofauna (eleven ichnogenera). Similar trends occur globally in strata of this age.

            Both successions record ichnogeneric diversity and abundance gradients across the shelf in the Griesbachian. Diversity is low in distal-shelf facies and moderate to high in offshore-transition to lower-shoreface strata. The most common ichnofossils are arthropod-produced burrow networks (Spongeliomorpha, Thalassinoides) that locally occur in compound forms with other ichnotaxa (Cruziana, Teichnichnus). Simple infaunal burrows (Planolites, Palaeophycus) are also common. Bioturbation is commonly absent or weakly developed, but some beds are strongly bioturbated by Spongeliomorpha.

            Griesbachian upper-shoreface strata are unburrowed. Late Griesbachian-early Dienerian upper-shoreface beds contain only scattered, small Skolithos. The paucity of burrows in Griesbachian shoreface strata may record extinction-related decimation or reflect basinward migration of burrowers in response to reduced competition on the shelf. Shoreface recolonization may have occurred only when competition on the shelf reached levels similar to those before the extinction. This may be comparable to landward and basinward displacements of infaunal behaviours recorded during other adaptive radiations.