Abstract

Permian rocks in the west central United States are a complexly intertonguing sequence dominated by shallow water clastic, carbonate and evaporite sediments that were deposited in at least six main depositional cycles. Distribution of facies was influenced strongly by paleogeographic-paleostructural positive and negative features that persisted during most of late Paleozoic time. The most active of these were associated with the building of the “Ancestral Rocky Mountains” in the southern Rocky Mountain and southwestern Midcontinent areas.

Transgressive-regressive sedimentary cycles are bounded by regional disconformities in the main shallow water shelf areas, but deposition was essentially continuous in most of the actively subsiding basins. Wolfcampian sedimentary facies are dominated by clastics, except for the southern Midcontinent and the Permian basin of west Texas - New Mexico, where cyclic carbonate and clastic deposits prevail. Coarse arkosic beds as much as 1,000 m. or more thick were deposited in steep-sided basins along the margins of the major elements of the Ancestral Rocky Mountains during Wolfcampian and Early Leonardian time. However, during Late Leonardian time, organic carbonate sedimentation became much more prevalent and continued through the Guadalupian. Carbonate buildups several hundred meters thick accumulated at this time along sharply defined shelf margins in the western part of the Permian basin in west Texas and southeastern New Mexico. Organic carbonate mounds also formed in parts of the Cordilleran Miogeosyncline and adjacent cratonic shelf margin and in the northwestern Rocky Mountain region. During Late Leonardian and Guadalupian time, extensive deposits of high-organic phosphatic shale and phosphorite formed in the Sublett basin of southeastern Idaho and on the western margin of the Wyoming shelf. Gypsum or anhydrite, commonly associated with redbeds, was deposited in much of the Midcontinent, northern Great Plains, and northeastern Rocky Mountain areas at this time, and halite and potash formed in the centers of some of the main basins. The Permian Period closed with complete emergence of the Rocky Mountain and Midcontinent regions and the deposition of thick halite, potash and calcium sulfate beds in the central part of the Permian basin.