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Facies and diagenesis of the Park City Formation: Sheep Mountain Anticline, Wind River Basin, Fremont County, Wyoming
Hallau, Daniel Griffin
Hallau, Daniel Griffin
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2014
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2014
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Abstract
The Phosphoria Rock Complex (PRC) includes: cherts, organic-rich mudrocks, and phosphorites of the Phosphoria Fm.; carbonates of the Park City Fm.; sandstones of the Shedhorn Fm.; and redbeds and evaporites of the Goose Egg Fm. The Permian succession at Sheep Mountain anticline near the southern margin of the Wind River basin in Fremont County, WY, is dominated by carbonates of the Park City Fm., but also includes phosphorites and mudrocks of the Phosphoria Fm. All three members of the Park City Fm. (i.e., Grandeur, Franson, and Ervay) crop out at Sheep Mountain. With potential source rocks to the west and potential sealing facies to the east, understanding the facies and facies changes in the carbonate members is crucial to exploring for stratigraphic traps in the Wind River basin. Carbonate facies and facies associations in the Ervay and Franson members indicate that deposition occurred along a very gently-dipping, westward-facing, carbonate ramp. Facies distribution between these two members is relatively similar, with the Ervay having a higher proportion of peloid packstones and ooid grainstones, and the underlying Franson having a higher proportion of bioclast packstones and intraclast packstones and wackestones. The position of Sheep Mountain along the carbonate ramp during Ervay time, therefore, was shallower than during Franson time, which may reflect the synchronous northwestward shift of the basin depocenter. The basal Grandeur Member is significantly more complicated, and a carbonate ramp may not be the most appropriate model for describing its depositional system. It is dominated by calcareous sandstones, silicified microbialites, and sandy carbonates. Sequence stratigraphic studies of the Park City Fm. to date have generally concluded that periods of dominant carbonate growth coincided with highstand systems tract deposition. Three main "cycles", separated by regional sequence boundaries, have been described by previous workers in terms of their respective carbonate members (i.e., Ervay Cycle, Franson Cycle, and Grandeur Cycle). The findings of this study support the hypothesis of carbonate growth dominating during highstands but also show that significant carbonate growth occurred during transgressive systems tract deposition. Indeed, this interpretation is supported by facies association stacking patterns in outcrop, increase in phosphate content upward and basinward, hardground development as a result of transgression, and well log character in the surrounding subsurface wells. As such, most of the Franson carbonate at Sheep Mountain likely represents the transgressive systems tract deposition (and possibly lowstand systems tract deposition) of the Ervay Cycle. Based on the integration of X-ray fluorescence (XRF) data, phosphate accumulation generally occurred during transgressive and early highstand deposition. In this study, XRF-derived chemical signatures were only moderately successful at differentiating between depositional facies and between lithologies. The diagenetic history of the Park City Fm. at Sheep Mountain is complex. In outcrop, the Park City Fm. shows significant variability in dolomitization, silicification, and phosphogenesis. Phosphogenesis and dolomitization were likely the earliest events in the paragenetic sequence, where dolomitization may have helped to drive the Mg[superscript 2+]/Ca[superscript 2+] ratio of porewaters low enough to encourage phosphate precipitation. As a feedback, precipitation of phosphate may have helped drive up the Mg[superscript 2+]/Ca[superscript 2+] ratio. The presence of dolomite rhombs within collophane, however, indicates that phosphogenesis occurred prior to dolomitization. Marine cementation probably occurred next, followed by or synchronous with some silicification events. Later silicification, de-dolomitization, and calcite precipitation also occurred.
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