Abstract

Between the Lower Triassic Montney Formation and the Middle Triassic Doig Formation, there is an unconformity-bound interval recently identified as the Sunset Prairie Formation which occurs on a regional scale within eastern British Columbia. The interval is Middle Triassic in age and is characterized by interbedded light grey, pervasively bioturbated sandstone and dark grey, minimally bioturbated siltstone. Trace fossils present within the interval include Phycosiphon, Rosselia, Cylindrichnus, Teichichnus, Asterosoma, Scolicia, Helminthopsis, Palaeophycus, Chondrites, Planolites, Diplocraterion, Rhizocorallium, Thalassinoides, Skolithos and Zoophycos, which are indicative of the Cruziana Ichnofacies.

Seven facies are identified, based on sedimentological, ichnological and paleontological analysis of core. They are interpreted to have been deposited in offshore, offshore transition and lower shoreface environments, which were weakly storm affected. The unconformity bound unit exhibits flooding surface sequence boundaries at its top and base, and internal geometries of the facies stacking suggest retrogradational parasequence sets. The Sunset Prairie Formation overall thickens to the west, but is spatially complex due to the deposition and preservation of the interval being influenced by structural features within the basin. The Sunset Prairie Formation is thickest (80 m) within the Fort St John Graben system and Hudson Hope Low.

The Sunset Prairie Formation provides the first regionally expansive, highly bioturbated assemblage of diverse trace fossils within the Middle Triassic strata of the Western Canada Sedimentary Basin. Bioturbation is limited to the proximal offshore to lower shoreface settings, which is similar to the few examples of diverse ichnological assemblages within the Montney Formation. However, those earlier published examples are associated with refugia environments and are not laterally extensive. The lateral distribution of the Sunset Prairie Formation suggests that favorable environmental conditions were widespread during the earliest Middle Triassic (Anisian) and could play a key role in understanding the biotic recovery following the end-Permian mass extinction.