A new, Middle Cambrian, Burgess Shale-type biota, Bolaspidella Zone, Chancellor Basin, southeastern British Columbia

Abstract

A newly discovered Burgess Shale-type (BST) biota occurs in southeastern British Columbia on Haiduk and Tangle peaks. The fossiliferous rocks of the informally named Vermilion sub-unit and Duchesnay unit occur in the Bolaspidella Zone, one trilobite biozone younger than the Burgess Shale Formation. The younger rocks abut the Eldon Escarpment in a stratigraphic and depositional relationship that mirrors that of the Burgess Shale Formation and Cathedral Escarpment. The biota, concentrated at the Eldon Escarpment, is hosted predominantly in the Duchesnay unit, and includes Burgess Shale genera of sponges, worms, some arthropods and brachiopods. New taxa include an arborescent, benthic graptolite with preserved fuselli. Microbial fossils, including vendotaenid bacteria, are locally abundant and range up to 750 m basinward of the Escarpment in both the Vermilion sub-unit and Duchesnay unit, while concentrations of animal fossils are restricted to within 100 m. In the Vermilion sub-unit, large autochthonous carbonate mud mounds, previously interpreted as platform-derived olistoliths, originated at the base of the Escarpment, probably over fluid conduits, and then slid downslope, bulldozing and entraining sediments in their paths. Local magnesian- and barian-rich brine pool deposits occur up to 750 m distal of the Escarpment; fractionation of the fluids may explain the differing geochemistries. At least one MgO-rich brine pool deposit on Haiduk Peak is fringed by abundant animal fossils, including concentrations of priapulid worms. This assemblage, together with high-density, low-diversity assemblages on Tangle Peak likely represent “fringing fauna facies” around brine seeps, a model erected for similar fossil concentrations and distributions in the Burgess Shale. Intrabasinal comparison of the Vermilion sub-unit and Duchesnay unit with the Burgess Shale Formation reveals an identical stratigraphic ordering of carbonates and shales, brine pool deposits, major debris flow deposits, carbonate mud mound growth and fossiliferous deposits. The Eldon and Cathedral platform formations are known to share complimentary brine-derived characteristics as well. The stratigraphic patterns signify that the depositional processes producing the suite of older lithofacies recurred during deposition of the younger units. Interbasinal correlation with the Wheeler and Marjum formations, Utah, identifies the Duchesnay unit deposit as the youngest basinal Burgess Shale-type deposit in North America. Decreasing abundance of animal fossils away from the Eldon and Cathedral escarpments is generally attributed to increasing development of tectonic fabrics. However, on Haiduk Peak, rocks hundreds of metres basinward of the Escarpment contact are essentially undeformed and contain rare Planolites, brachiopods and trilobites as the only macrofauna; locally abundant vendotaenids and Morania are evidence that BST preservation occurred at these distances. The concentration of animal fossils at the Escarpment is therefore a primary ecological feature, not due to a preservational or rheological bias. Communities were clustered at the face of the Escarpment where brines were seeping.