PERMIAN-TRIASSIC TRANSITION EXISTS IN THE PHOSPHORIA BASIN, EASTERN IDAHO

Sedimentation at the Permian-Triassic transition, including upper Guadalupian in the Western US on the margin of the craton, is traditionally associated with the large time gap of at least 10 Ma. There were some controversial suggestions (Kozur, 1995) of a potentially more complete succession due to the occurrence of conodont Merrillina divergens in the upper Phosphoria Formation, but this fact was basically ignored. The index of the base of the Triassic, the conodont Hindeodus parus, is found at the base of Dinwoody Formation in Utah, Idaho, Wyoming and Nevada (Paull & Paull, 1986). Recently, continuous sedimentation at the Permian-Triassic transition was suggested based on δ13C and 87Sr/86Sr trends across the Gerster and Thaynes formations in the western United States (Saltzman and Sedlacek, 2013), although the 87Sr/86Sr data reveal significant alteration. We report a U-Pb zircon age from a Wuchiapingian volcanic ash bed, the first time found within the Western US craton. The ash is near the top of Meade Peak Formation in Trapper Creek Canyon, Cassia Mountains, eastern Idaho. The succession above it is medium dark grey chert and cherty carbonate of the Rex Chert Member that is at least 100 meters thick. In Cassia Mountains, the Rex Member is up to 275 m thick and overlain by the Dinwoody Formation. This volcanic ash is solid evidence of continuous sedimentation at the Permian-Triassic transition in the western US. Further isotopes studies may precisely constrain the boundary and find the source of volcanic centers at this time on the passive margin of western US craton.