BIOSTRATIGRAPHY AND PALEOECOLOGY OF LATE CAMBRIAN (MARJUMAN) TRILOBITES FROM THE EMIGRANT SPRINGS FORMATION, SOUTHERN SCHELL CREEK RANGE, NEVADA

Member B of the Emigrant Springs Formation (Late Marjuman) in eastern Nevada is a shallowing-upward shelf sequence containing interbedded calcisiltites, oolitic/bioclastic grainstones, intraclastic rudstones, and thrombolitic buildups. It is almost 100 meters thick in a section at Patterson Pass in the southern Schell Creek Range, where it is overlain by massive, cliff-forming thrombolites of Member C. Ooids, wave ripples and extensive bioturbation all point to shallow water deposition, and intraclastic rudstones indicate storm reworking of lithified layers near the sediment-water interface.

Seventeen collections from Member B yielded more than 1000 trilobites, representing 11 genera that indicate a correlation with the Crepicephalus Zone. Species of Crepicephalus, Coosina,and Blountiaare most common, and are associated with Kingstonia, Terranovella, Tricrepicephalus,and Komaspidella,among others. Genera show substantial variation in relative abundances between lithofacies, which is interpreted as recording original environmental distribution patterns. Collections from thrombolitic buildups are dominated by Blountia,whereas Crepicephalusand Coosinaare most abundant in oolitic/bioclastic grainstones that probably record subtidal sand shoals.