M. T. Whalen
The Late Permian witnessed a second order sea-level lowstand approximately 210 m lower than the preceding Leonardian second order highstand. This pronounced drop in global sea level is overprinted on third and possibly fourth order sea-level changes that are incompletely preserved along the western conti- nental margin of Pangea. The Permian Park City Fm. and associated units in Utah and Wyoming were deposited on the continental shelf and in marginal basins of western Pangea as three depositional sequences bounded by platformal unconformities and basinal conformities. Carbonates of the Park City Fm. were particularly sensitive to changes in sea-level and climate and record progressively lower third-order sea-level highstands. These facies were deposited as transgressive, highstand, and initial lowstand systems tracts.
Prior to deposition of the Park City Fm. eolian and shallow marine clastic sediments covered the Utah/Wyoming continental margin. A late Leonardian third order sea-level highstand permitted progradation of a mixed carbonate-siliciclastic platform over the shelf. A basal transgressive unit represents reworking of previously deposited lowstand clastics. In situ carbonate production progressively diluted the clastic component and carbonates dominate the upper portion of the sequence. Within the carbonate portion of the sequence clastic units and conglomerates may indicate fourth order sea-level lowstands.
A subsequent third order sea-level lowstand exposed the eastern portion of the shelf forming a platformal unconformity and the base of the second depositional sequence. Carbonate conglomerates were deposited along the shelf margin and upper slope and clastic sediments once again were deposited on the shelf. Transgression reworked shelf clastics and a marine flooding surface is marked by the onlap of restricted basinal facies. During initial transgression bottom water anoxia and high surface productivity, induced by meridional upwelling, prevented benthic carbonate production. A condensed section consisting of reworked and phosphatized sands, pelletal and nodular phosphorite, and organic-rich shales was deposited. With continued transgression and the re-initiation of normal marine circulation carbonate production resumed and a second cycle of carbonate platform progradation ensued atop the Utah/Wyoming shelf. Thin clastic units intercalated with platformal carbonates indicate forth order sea-level lowstands.
During the next third order sea-level lowstand the third sequence boundary was created as the platform was exposed to subaerial erosion. Carbonate conglomerates were once again deposited as a lowstand wedge and clastic sediments prograded across the shelf. In northern Utah sabkha and intercalated paleosol facies represent this interval. Subsequent transgression caused the onlap of restricted basinal and high productivity facies in Wyoming and southwestern Montana producing a marine flooding surface and condensed section. These facies, however, are absent in northern Utah. Further transgression re-initiated carbonate production over the Utah/Wyoming shelf but Utah facies indicate shallow water, restricted marine conditions. This implies an overall lower third order sea-level highstand reflecting the long-term second order drop in eustatic sea level.