TRILOBITE PALEOECOLOGY AND TAPHONOMY OF THE STEPTOEAN (UPPER CAMBRIAN) DUNDERBERG FORMATION, CHERRY CREEK RANGE, NEVADA

The upper Dunderberg Formation. in the Cherry Creek Range, east central Nevada, comprises 15 repeated, shallowing-upward cycles, each with a thickness of about 10 m. Each cycle is composed of shales with thin (cm-scale) carbonate interbeds and lenses, and is capped by a thick bioclastic packstone or grainstone. The frequency of carbonate interbeds increases upward through the shaly part of each cycle. Many of these interbeds are bioclastic horizons with abundant trilobite sclerites and echinoderm debris. Features such as starved megaripples and intraclastic rudstone interbeds suggest a storm-influenced shelf setting.

Over 100 trilobite individuals were counted at each of 29 bioclastic pack- and grainstone horizons. Trilobites are disarticulated and have been subjected to taphonomic sorting throughout the section, with cranidia accounting about 70% of the sclerites present. Representative species of the Dicanthopyge, Prehousia, Dunderbergia, and Elvinia zones of the Steptoean stage are present. Biofacies analysis using hierarchical cluster analysis supports the hypothesis that distinct trilobite assemblages successively occupied the region, with assemblage turnover becoming more gradual higher in the section. Turnover between polymeroid trilobite assemblages at zonal boundaries does not appear to correlate with lithologic changes. Ranges of agnostids cross polymeroid zonal boundaries, indicating that the agnostids were less sensitive to factors controlling assemblage turnover.