The end Triassic mass extinction record of Williston Lake, British Columbia

Abstract

Triassic–Jurassic boundary sections on the shore of Lake Williston, British Columbia record a spectrum of depositional environments from the western Canadian continental margin. A primarily deep-water, anoxic record of deposition, indicated by V/V + Ni, V/Cr, Th/U indices and pyrite framboid and facies data, is seen throughout the Late Triassic–Early Jurassic (Norian-Hettangian) interval in a section at Black Bear Ridge. Abundant Monotis bivalve populations in Norian strata are interpreted to have lived on the seabed during transient oxygenation events that punctuated the background anoxic conditions. Monotis went extinct around the Norian/Rhaetian boundary and the deep-water assemblages are subsequently dominated by thin-shelled pectinids. An intra-Rhaetian sea-level fall generated a major hiatus in the proximal Black Bear Ridge section whilst distinctive lowstand strata were developed in the more distal and more complete Ne Parle Pas Point and Pardonet Creek sections. These consist of shallow-water, phosphatic ooid sand facies that contain diverse bivalves. The majority of the infaunal bivalves (but not the epifaunal ones) disappeared at the acme of the regression. This is the end-Triassic mass extinction event and it occurred shortly before a terminal Triassic flooding event. This transgressive event is associated with a rapid negative shift of δ 13C org values interpreted to be the Initial Isotope Excursion of Hesselbo et al. [Hesselbo, S.P., Robinson, S.A., Surlyk, F., Piasecki, S., 2002. Terrestrial and marine extinction at the Triassic-Jurassic boundary synchronized with major carbon-cycle perturbation: a link to initiation of massive volcanism? Geology 30, 251–254]. The Rhaetian record of Williston Lake thus records an extinction of shallow-water infaunal bivalves during a lowstand prior to the development of a transgressive record that is associated with a major, negative C isotope.