DIVERSE ICHNOFOSSIL ASSEMBLAGES FROM THE LOWER TRIASSIC OF NORTHEASTERN BRITISH COLUMBIA, CANADA: EVIDENCE FOR A SHALLOW MARINE REFUGIUM ON THE NORTHWESTERN COAST OF PANGAEA

The subsurface Lower Triassic Montney Formation in the area of the Ring-Border / Pedigree and Kahntah River fields of northern British Columbia consists of two successive shallowing upwards trends. Detailed biostratigraphic analyses has shown that both trends were deposited during the Griesbachian (i.e. earliest Triassic).

Trace fossil assemblages in the older trend are characterized by extremely low population densities but high ichnotaxonomic diversity. Lithologies interpreted to be of proximal offshore to offshore-transition origin contain rare, isolated specimens of Halopoa, Helminthopis, Planolites and Treptichnus in heterolithic interbedded shale and silty sandstone and isolated specimens of Cruziana, Diplichnites, Monomorphichnus, Rhizocorallium, Rusophycus, Spongeliomorpha and Thalassinoides on the soles of thin, sharp-based sandstone tempestites. Trace fossils in lower shoreface very fine-grained sandstone beds consist of isolated Rhizocorallium, Thalassinoides, Planolites, Spongeliomorpha & Protovirgularia. Upper shoreface sandstone beds are apparently barren.

Trace fossil assemblages in the younger trend are characterized by high population densities but moderate ichnotaxonomic diversity. Offshore-transition to lower-shoreface successions are characterized by gradation from a Cruziana assemblage (Cruziana, Palaeophycus, Planolites, Teichichnus and Thalassinoides) to a mixed Cruziana-Skolithos assemblage (Cruziana, Cylindrichnus, Diplocraterion, Planolites, Phycodes, Phycosiphon, Rosselia, Palaeophycus, Skolithos and Thalassinoides). Upper-shoreface sandstone beds are characterized by rare, isolated Skolithos.

The presence of robust and diverse trace fossil assemblages in the Lower Montney Formation in the study area indicate that adverse environmental conditions were not a limiting factor on infaunal communities on this part of the northwestern Pangean coastline during the earliest Triassic. High trace fossil diversity, as well as an abundance of trace fossil forms normally considered characteristic of the Paleozoic, supports the hypothesis that the study area provided a shallow marine refugium during the Permian-Triassic biotic crisis.