ABSTRACT

The objective of this study is to reconstruct the depositional environments of Permian sediments that formed the Phosphoria Formation in the southeastern Bighorn Basin, Wyoming. Data of stratigraphy, petrology, and sedimentation are synthesized to achieve this objective.

Upper and lower members of the Phosphoria are defined. Each is characterized by distinctive facies patterns, separate sources of sedimentary materials, and differences in depositional environments. Extensive eastward transgressions and lesser regressions produced the facies change in the upper member from dolomite in the southwest to anhydrite and redbeds eastward. Sand grains derived from the underlying Tensleep Sandstone characterize the lower member; this detritus was supplied from topographic highs of the Tensleep erosion surface and was deposited in shallow bays bordering the transgressing lower Phosphoria sea.

Five types of dolomite are described: (1) dolomite mudstone, (2) detrital dolomite, (3) skeletal dolomite, (4) pelletal dolomite, and (5) oölitic dolomite. These types are distinguished on the basis of limestone megatextures that persisted through dolomitization. The dolomite is associated with claystone, anhydrite, intraformational breccia, sandstone, and conglomerate.

Dolomitization and redistribution of anhydrite are the most important diagenetic processes that have modified or destroyed porosity.

The study area lay in an embayment extending eastward across the stable shelf bordering the Permian miogeosyncline. This bay occupied the present western half of Wyoming, and the study area is a part of the eastern shoreline. The floor of this bay had a relief of somewhat more than 100 feet as Phosphoria deposition began, but it became a nearly plane surface dipping seaward at a few feet per mile by upper Phosphoria time. Landward topography during both phases was an extension of the submarine topography. Surface sea water flowed shoreward across this embayment from the area of upwelling along the margin of the miogeosyncline; and the temperature, salinity, and pH increased as the water approached shore. Highly saline water returned seaward along the bottom, and this water may have been the agent that dolomitized the calcium carbonate sediments. The climate was arid and either tropical or subtropical.

Depositional conditions varied within the embayment where the different rock types were deposited. The dolomites are dolomitized counterparts of the types of sediments currently forming on the Bahama Banks; they were deposited under conditions similar to those now existing in this area of present sedimentation, where diverse megatextural types form in response to currents of different energies. The claystone is a tidal flat deposit. Dynamic rather than physical restrictions during deposition of the upper member aided the concentration of sea water to the point where calcium sulfate precipitated; however, physical barriers isolated bays where calcium sulfate accumulated in the lower member. Intraformational breccias were dumped in channels crossing the tidal flats. Sandstones formed as nearshore deposits or as parts of deltas, and the conglomerates are intraformational deposits or residual gravels.

Most Phosphoria oil is produced from dolomite within 100 feet of the top of the formation. Eastward migration of oil is limited by the facies change from dolomite to anhydrite and claystone, and reservoir rocks are types of dolomite consisting predominantly of relic grains. Dolomitization modified the original porosity, and anhydrite further plugged porosity locally. Phosphoria rocks contain both source and reservoir beds.

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