STRATIGRAPHIC CONTEXT OF LATE CAMBRIAN TRILOBITE EXTINCTIONS: EVIDENCE FROM EASTERN NEVADA

Late Cambrian (Steptoean-Sunwaptan) strata of east-central Nevada comprise two sedimentary grand cycles whose boundaries are associated with trilobite extinction events. The bases of both the Steptoean and Sunwaptan stages involve deepening and drowning of carbonate banks. We have focused on the lower boundary of the Sunwaptan in the Windfall Formation, where deep subtidal, thin-bedded lime mudstones, calcisiltites, packstones and shales (Catlin Member) appear above cliff-forming, shallow subtidal lime mudstones and wackestones of the Barton Canyon Member. The upper bed of the Barton Canyon is the Irvingella major coquina, a bioclastic rudstone that marks both the base of the Sunwaptan and the onset of extinctions over much of North America. Biostratigraphic correlation of the well-defined flooding surface at the top of the Barton Canyon with other regions suggests that it is not a sequence boundary. Rather, it appears to correlate into the transgressive systems tract of a depositional sequence with a lower boundary of mid- to Late Steptoean age.

Superposition of deep water and shallow water lithofacies and biofacies in the basal Sunwaptan of Nevada contribute to the apparent abruptness of the local signature of the extinction, which is characterized by immigration of pandemic olenids and agnostoids and the disappearance of a largely endemic shallow water fauna. Physical evidence for a major break is lacking, and biostratigraphic evidence indicates at most a minor hiatus. Moreover, the fact that the extinction is recorded in monofacial shallow water successions in other parts of North America indicates that it is not simply a stratigraphic artifact. Although the extinction is related to onlap and deepening in at least parts of the outer shelf, the forcing mechanism remains uncertain.