The Twin Creek Limestone of Middle Jurassic age was studied in the Tunp, Salt River, and Wyoming Ranges in southwestern Wyoming. Modern carbonate environments and their ancient analogs were compared with information obtained from field study and petrographic analysis of samples of the Twin Creek Limestone in order to delineate environments of deposition, paleogeography, and diagenetic history. Six major lithofacies were recognized: (1) carbonate mudstone; (2) carbonate mudstone breccia; (3) fossiliferous wackestone; (4) packstone-grainstone; (5) terrigenous mudstone; and (6) sandstone. These lithofacies were deposited in a variety of environments, including outer shelf platforms (carbonate mudstone and fossiliferous wackestone), oolitic sand belts (packstone-grainstone), open to restricted lagoons (carbonate mudstone, fossiliferous wackestone, and terrigenous mudstone), tidal flats, and supratidal environments (terrigenous mudstone, sandstone and carbonate mudstone breccia). The Twin Creek epeiric seaway experienced two major transgressions (early Bajocian and late Bathonian-early Cal lovian) and two regressions (early Bathonian and middle Callovian). Lateral migration of the adjacent facies occurred in response to these changes in sea level. Eogenetic features include minor compaction, micritization, coarse fibrous rim cementation, granular cementation, syntaxial rim cementation and silicification of carbonates. These features were produced in environments ranging from freshwater phreatic to marine phreatic. Mesogenetic diagnesis was characterized by pressure-solution features and neomorphism. Telogenetic features are limited to calcite vein-fillings and oxidation coatings on carbonate and detrital gains and cements.

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