BURGESS SHALE-TYPE BIOTA AND DEPOSITIONAL ENVIRONMENT OF THE VERMILION/DUCHESNAY UNITS, UPPER MIDDLE CAMBRIAN, SE BRITISH COLUMBIA

A new Burgess Shale-type (BST) biota of Marjum age occurs in the Chancellor basin, SE British Columbia, exposed on Haiduk Peak. The fossiliferous rocks of the informally-named Vermilion and Duchesnay units are one trilobite biozone younger than the Burgess Shale. They abut platformal rocks of the Eldon Formation in a stratigraphic and depositional relationship that mirrors the Burgess Shale and Cathedral Escarpment. In the Vermilion, repeated cycles of increasing carbonate deposition were coincident with episodes of carbonate mud mound growth. Many mounds (previously interpreted as platformal olistoliths) moved downslope, bulldozing and entraining sediments in their paths. MgO- and Ba/K₂O-enriched brines issued probably from the base of the escarpment and pooled in depressions at distances of 0- ~300 m from the escarpment. A Mg-rich brine pool in the lower Duchesnay at the escarpment contact preserves a soft-bodied assemblage on its periphery.

The Duchesnay unit lacks mud mounds, is less chaotically-bedded and hosts more animal fossils than the underlying Vermilion. Like the Burgess Shale, abundant animal fossils in these units are found only in a ~50m zone next to the escarpment. Decreasing frequency of fauna away from the Eldon and Cathedral escarpments is generally attributed to increasing development of tectonic fabrics. However, on Haiduk Peak, rocks hundreds of metres outboard are essentially undeformed, and yield rare brachiopods and trilobites, but no BST animals.

BST fossils include: Ottoia and thinner priapulids; sponges such as Wapkia, Protospongia, Choia and Vauxia; a chancelloriid; the alga Margaretia; Naraoia and lightly-skeletonized arthropods reminiscent of Sidneyia, Canadaspis and Alalcomenaeus; hyolithids; and the phosphatic tubes of Byronia. Locally abundant alleged algal fragments of Marpolia and Morania described from the Utah basin are also known from Haiduk Peak but at greater distances basinward than other BST fossils and dominantly in the Vermilion rather than Duchesnay.

The concentration of animal fossils at the escarpment is therefore ecological, and not due to a rheologic bias. BST preservation of only "algal" fragments outboard of the escarpment confirms that the BST animals were not there at all, but clustered at the escarpment, where fluids were seeping.