Whalen, M.T.
A regional analysis of the Permian Park City Formation has revealed insights into the facies architecture, paleogeography, and paleoceanography of Pangea's western continental margin. Northern Utah and western Wyoming were the site of carbonate-siliciclastic ramp development during the mid-Permian. The ramp interfingered with sabkha facies to the east and organic-rich, oxygen-deficient, basinal facies to the west. Facies patterns indicate a low gradient, distally steepened ramp in the south grading into a homoclinal ramp with intra-ramp basins in the north.
The Park City and intercalated Phosphoria Formation were deposited as three depositional sequences. The sequences are bounded cratonward by unconformities but sequence boundaries are generally conformable at the ramp margin. Sequences are characterized by shoaling-upward cycles that vary lithologically depending on their paleogeographic location along the ramp. Inner ramp cycles begin with a phosphatic condensed section and/or shales deposited during transgression and marine flooding. Highstand facies are prograding, shallow subtidal to peritidal carbonates. Ramp margin cycles begin with a condensed section and sub-storm wavebase slope deposits and shoal upward to shallow subtidal and possibly peritidal deposits. The first two sequences contain at least three shoaling-upward cycles each. The third sequence, is incompletely preserved due to post-Permian erosion, and contains only one well documented shoaling cycle. Park City-Phosphoria depositional sequences are tentatively correlated with three mid-Permian third-order eustatic sea-level fluctuations, but available biostratigraphic data do not permit unequivocal correlations. Intra-sequence shoaling-upward cycles indicate the effects of higher-order sea-level changes.
Wide tracts of fine-grained carbonate and mixed carbonate-siliciclastic facies in northern Utah are associated with evaporites and indicate a broad, low-gradient ramp that was commonly occupied by hypersaline, peritidal environments. The stratigraphy along the ramp margin is characterized by carbonate mudstones and wackestones interbedded with graded packstones during highstand and carbonates and evaporites and/or conglomerates and siliciclastics during lowstand. This indicates steepening of the ramp margin during highstand progradation and shedding of ramp-derived detritus during lowstand.
Carbonate ramp termination was caused by increased clastic input and ultimately subaerial exposure during sea-level lowstand. Lowstand hypersaline ramp and oxygen-deficient basin deposits indicate oceanographic restriction. Regional Permian lithofacies patterns to the west of Park City-equivalent basinal facies indicate shallowing and a nearby sediment source. The Park City carbonate ramp was thus bordered to the west by a barred marginal basin. Foreland fold and thrust belt uplifts and island arc terranes are the most likely western sediment sources. These features produced a semi-enclosed basin that was at times occupied by stagnant, oxygen-deficient bottom waters. The transgression of toxic, oxygen-deficient, sulfidic water during sea-level rise temporarily prevented reinitiation of benthic carbonate production as a phosphatic condensed section formed.
*in Longman, M.W., and Sonnenfeld, M.D., eds.,1996, Paleozoic Systems of the Rocky Mountain Region, Rocky Mountain Section SEPM (Society for Sedimentary Geology), p. 355-378.
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