CO-OCCURRENCE OF TRYPANITES AND GLOSSIFUNGITES SUBSTRATE CONTROLLED TRACE FOSSIL ASSEMBLAGES AT SIGNIFICANT STRATIGRAPHIC SURFACES IN A MIXED SILICICLASTIC-CARBONATE ENVIRONMENT, WILLISTON LAKE, BRITISH COLUMBIA, CANADA

Substrate-controlled trace fossil assemblages commonly demarcate significant stratigraphic surfaces. Firmground assemblages, included within the Glossifungites ichnofacies, are common in siliciclastic successions whereas Trypanites assemblages have typically been reported from carbonate successions. Substrate-controlled trace fossil assemblages are common in mixed siliciclastic-carbonate strata of the Upper Triassic Baldonnel Formation at Williston Lake in British Columbia. Previous publications of mixed siliciclastic-carbonate environments focus on the sedimentology of the systems, but frequently overlook or underemphasize the ichnological attributes. The Baldonnel records multiple transgressive-regressive cycles with key surfaces demarcated by substrate-controlled trace fossil assemblages. This study focuses on several well-exposed surfaces to identify the lithological, ichnological and physical characteristics (including spectral gamma radiation) of key surfaces. Trypanites suites are common at coplanar lowstand surfaces of erosion/transgressive surfaces of erosion (LSE/TSE). Traces at these contacts include Trypanites, Rogerella, Entobia and large, sack-like traces attributable to Gastrochaenolites. Traces at coplanar LSE/TSE surfaces are commonly infilled with granular clastic detritus and show strong Thorium spikes indicating the presence of abundant heavy minerals. Glossifungites surfaces are common at regressive surfaces of erosion and are characterized by Diplocraterion, Rhizocorallium, Skolithos, and large Thalassinoides. These surfaces alternate in the stratigraphic succession, with Trypanites suites occurring on dolomitic limestone beds and Glossifungites suites occurring on top of calcareous siltstone units and infilled with poorly sorted bioclastic calcareous sandstone. Identification of these surfaces are integral in local and regional stratigraphic correlation and their co-occurrence is a hallmark of mixed siliciclastic-carbonate successions.