Cambrian–Ordovician boundary interval extinctions: implications of revised trilobite and brachiopod data from Mount Wilson, Alberta, Canada

Abstract

The type section of the Basal Silty Member of the Survey Peak Formation spans the Cambrian–Ordovician boundary (North American usage) at Mount Wilson in the southern Canadian Rocky Mountains. The zonal and subzonal terminology through the boundary interval developed in Texas and Oklahoma is applicable to the trilobite faunas recovered from the section. The oldest trilobites recovered in this study occur in the top of the underlying Mistaya Formation and are assigned to the Saukiella serotina Subzone of the Upper Cambrian Saukia Zone. Trilobites and brachiopods of the S. serotina and Eurekia apopsis Subzones of the Saukia Zone occur in the lower half of the Basal Silty Member; trilobites and brachiopods assigned to the Lower Ordovician Missisquoia Zone and the Symphysurina brevispicata Subzone of the Symphysurina Zone occur in the upper half of the Basal Silty Member. The S. brevispicata Subzone extends an unknown distance into the Putty Shale Member of the Survey Peak Formation.The extinction horizons at the base of the Eurekia apopsis Subzone and at the base of the Missisquoia depressa Subzone (the Cambrian–Ordovician boundary) occur within the Basal Silty Member of the Survey Peak Formation, not at the formational contact with the underlying Mistaya Formation. This leaves hypotheses linking immigration of the replacement trilobite faunas to major lithofacies changes through the boundary interval as untenable. Critical review of the evidence for the extinctions at the end of the Ptychaspid Biomere suggests that they were caused by an invasion of the shelf region by cold, anoxic water.Forty-seven taxa are illustrated and 18 of those which provide new taxonomic information are discussed. One new genus, Rampartaspis Loch, is described in addition to four new species: Eurekia plectocanthus Loch, Highgatella wilsoni Derby, Macronoda punctata Derby, and Rampartaspis dissimulosulcus Loch. The identifications of trilobites and brachiopods in this paper revise those of Aitken and Norford (1967) and Derby et al. (1972) and result in minor changes in the reported positions of the bases of the Missisquoia and Symphysurina Zones. Revision of the identification of some trilobites in Dean (1989) changes the biostratigraphic interpretation of the Basal Silty Member at Wilcox Pass, Albert, Canada.