PHOSPHORIA CHERT AND CRYPTIC SPONGES FROM A GUADALUPIAN GLASS RAMP, PARK CITY GROUP, UTAH AND NEVADA, WESTERN U.S.A

Middle Permian (Guadalupian) sediments from the Phosphoria Formation and the Park City Group in NW Utah and NE Nevada are chiefly biogenic in nature. Although commonly cited as a carbonate ramp, observations from outcrops and microfacies confirm that these rocks are primarily composed of biosiliceous materials, dominantly spiculitic chert and cryptically-preserved siliceous sponge fossils with subordinate calcium carbonate production likely formed through the action of microorganisms. We propose that the Phosphoria ramp is actually a glass ramp formed in a sedimentary regime dominated by siliceous biomineralizers, primarily dense communities of siliceous demospongesthat dominated benthic habitats in a wide range of shallow-water environments. Widespread establishment of biosiliceous sedimentation in the Phosphoria basin coincides with a decline in carbonate factory production and deposition of world-class phosphate deposits in the Phosphoria Formation during the earliest Guadalupian and persisted through the late Guadalupian. To clarify aspects of biosiliceous sedimentation and provide insight into some of the longstanding enigmas that characterize Phosphoria sediments, we discuss aspects of sponge paleoecology and sedimentation, evaluate depositional and diagenetic features of biosiliceous sediments, and present a depositional model for glass ramp establishment and persistence and the Guadalupian silica factory.