SPONGE CHERTS AND BIOCALCIFIER BEDS IN THE PERMIAN PARK CITY GROUP, NORTHERN UTAH AND NORTHEASTERN NEVADA

The Middle Permian Park City Formation of northeastern Utah and Park City Group of northwestern Utah and northeastern Nevada is a dominantly carbonate unit with varying amounts of clastic material deposited along the margins of the epeiric Phosphoria Sea. Chert forms a significant component of the Park City Group (PCG), occurring as bedded units, nodules, and stringers, but its provenance and paleoecological significance is debated. We took lithologic samples and detailed stratigraphic logs at four localities in Northern Utah and Northeastern Nevada, running from Hanna, Utah in the Uinta Mountains to outside Montello, Nevada in the Leach Mountains. Cherts were sampled from the Grandeur and Murdock Mountain Formations of the Park City Group in NE Nevada and NW Utah, the Meade Peak Phosphatic Shale Tongue of the Phosphoria Formation in NE Nevada, and the Franson Member of the Park City Formation in NE Utah. We analyzed thin sections to highlight variation within chert microfacies and relative abundance of sponge spicules, as well as diagenetic features of biosiliceous sediments. Both the stratigraphic logs and microfacies analyses confirmed previous studies on the orientation of the shelf, with the Eastern Uinta mountains being the most proximal to the terrigenous sediment source, evidenced in microfacies and in outcrop by its sand-rich nature and thicker beds of shale. Biocalcifier frameworks and shell beds are most abundant in the central part of the study area (Wasatch Mountains). Each dense biocalcifier accumulation is typically dominated by a single group: crinoid, brachiopod, bryozoan, or gastropod. Finally, the westernmost rocks represent a persistent accumulation of spicules, accumulated from nearby sponge meadows. We present a depositional model of the transected seaway, and interpret ecological heterogeneity that persisted across the southern reaches of the Phosphoria Sea throughout the Middle to Late Permian.