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RESEARCH PRODUCT

New sedimentological and stratigraphical data on the Permian and Triassic in northeastern Utah, USA

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description

International audience; We present the Permian to Triassic sedimentological evolution in the Vernal area, northeastern Utah, todetermine: (1) the magnitude of the Permian-Triassic (P-T) unconformity, (2) the depositional environmentsabove the unconformity, as well as (3) their evolution in space and time. Within this area, Permian depositsconsists of the Weber Sandstone, which represents aeolian deposits, overlain by the Park City Formation,composed of silty and sandy marine limestones, dolomites and phosphatic shales. In some places, anunconformity is observed at the top of the Weber Sandstone, which is directly overlain by the MoenkopiGroup. This formation is mostly composed of siltstone, with gypsiferous levels and ripple marks, depositedin nearshore continental to marine environments. Within the studied area, this formation is assumed to beEarly Triassic. A second unconformity at the base of the Chinle Group truncates the Moenkopi Group. Thebase of the Chinle Group is characterized by the Gartra Formation composed of coarse-grained and conglomeraticsandstones that are stream and river deposits overlain by red beds that are stream (primarilyfloodplain) and lake deposits.The P-T succession in northeastern Utah encompasses a profound unconformity and is not a completesuccession that could be used to analyze P-T boundary extinctions. Detailed sedimentological analyses allowthe characterization of the depositional environments across the P-T unconformity. The upper Permiandeposits preserved below the unconformity show erosion or non-deposition in the Vernal area. Above theunconformity, sandstone facies including coiled and uncoiled nautiloids, as well as stromatolite mounds,which suggest an open marine environment and attributed to the Permian based on its faunal content. Thesemarine deposits are abruptly overlain, above a breccia surface, by more classical Moenkopi facies attributedhere to delta and turbidite facies overlain by sabkha deposits with gypsum and microbialites. The unconformityabove the Moenkopi Group is overlain by either fluvial deposits of the Gartra Formation or red bedfacies of the Chinle Group. This study also helps to define the relationships between the Gartra Formation,the red beds and the paleosols within the Petrified Forest Formation of the Chinle Group.