CESSATION OF A SUBTROPICAL GLASS RAMP DURING THE PERMIAN CHERT EVENT: MURDOCK MOUNTAIN FORMATION, WESTERN U.S.A.

Abstract: The Murdock Mountain Formation crops out across northern Utah and Nevada as part of the Park City Group; a unit within the greater Phosphoria Rock Complex. The Murdock Mountain is a hundreds-meters-thick section of mixed chert, dolomite, and siltstone. This unit is the southernmost expression of the widespread Permian Chert Event and offers unique views of sedimentology and paleoecology during this event. Previous researchers have described shallow-water facies dominated by biosiliceous sedimentary production as glass ramps. This framework has been speculatively applied by others to the units of the Park City Group in northern Utah and Nevada. In this study, we test whether the glass ramp depositional framework accurately represents the strata of the Murdock Mountain Formation. We use stratigraphic, hand sample, and thin section data to describe the sedimentological character of the Murdock Mountain Formation and the overlying Gerster Limestone. Four chert facies and one carbonate facies are recognized based upon the presence of silt, sponge spicules, carbonate bioclasts, and evaporites. The Murdock Mountain Formation shares many characteristics with other reported glass ramp localities. We interpret the transition from chert to carbonate-rich strata as an alternation between stable states with silica-producing fauna dominating the Murdock Mountain and carbonate-producing fauna dominating the Gerster. The takeover of biosiliceous deposition by carbonate highlights the termination of a glass ramp and the onset of a carbonate ramp regime.