ABSTRACT

The Pterocephaliid-Ptychaspid biomere boundary is an Upper Cambrian extinction horizon that can be used to correlate mixed carbonate-siliciclastic strata in northwestern Wyoming. The biomere boundary is defined by the change between the Elvinia and Taenicephalus Zones of the standard North American trilobite zonation. It is also signaled chemostratigraphically by a positive trend in carbon-isotope ratios. Thus, both biostratigraphic horizons and carbon-isotope data are used to constrain the position of the biomere boundary in northwestern Wyoming. Study of 21 stratigraphic sections reveals the presence of distinct facies belts for the middle and upper Elvinia Zone and lower Taenicephalus Zone. These are interpreted to reflect deposition in three distinct paleoenvironments: (1) oolitic shoal, (2) intrashelf basin, and (3) inner platform-margin.

Following a major hiatus in sedimentation in Wyoming during much of Pterocephaliid biomere time, a healthy shallow-water carbonate factory was reestablished during Elvinia Zone time. This produced abundant deposits of oolitic and skeletal packstone in the shoal environment and an areally extensive thrombolite boundstone facies in the intrashelf basin setting. Approximately coincident with the biomere boundary extinction event (Irvingella major time), ooid production was terminated and the thrombolites were drowned. The correlation of this major change in the pattern of sedimentation allows for the recognition of parts of two depositional sequences in the study area. This drowning event (sequence boundary) may also be coeval with the accumulation of transported shallow-water carbonate deposits in an inner-platform-margin environment. During early Taenicephalus Zone time, sedimentation rates were apparently reduced and the entire study area is marked by thin concentrations of trilobite and brachiopod shells. The nonskeletal carbonate factory was never reestablished in the former shoal environment, where thick successions of homogeneous lime mud accumulated during the rest of Taenicephalus Zone time. Flat-pebble conglomerate and shale accumulated in the storm-influenced intrashelf basin during this time. Sea-level change and changes in seawater chemistry likely both played roles in the drowning event across the Elvinia-Taenicephalus boundary.

A positive trend in carbon-isotope values across the Pterocephaliid-Ptychaspid biomere boundary is observed in the six sections analyzed. This is consistent with previous studies that indicated that the mass extinction occurs within a regional shift towards positive values, and this may reflect the development of low-oxygen conditions on the shelf. The global significance of these isotopic trends, changes in relative sea level, and the mass extinction has not been confirmed, although recent study of the extinction event at the base of the Pterocephaliid biomere has revealed a similar trend towards positive carbon-isotope values that is worldwide.