Marine Jurassic Exposed in Bighorn Basin, Pryor Mountains, and Northern Bighorn Mountains, Wyoming and Montana

ABSTRACT The marine Jurassic rocks exposed in the Bighorn Basin in north-central Wyoming and in adjoining parts of Montana are, for purposes of this paper, divided into three major units, of which those in Wyoming are the approximate lithic and time equivalents of those in Montana. The lowest is of Middle Jurassic age, the middle of early Callovian age, and the upper of Oxfordian age. These units, according to the writer’s interpretation, represent three distinct marine invasions that are separated by intervals of erosion during late Bathonian and late Callovian time. Evidence for erosion is particularly well shown in exposures along the east side of the Bighorn Basin. An upwarp in the vicinity of Tensleep influenced sedimentation during Callovian and Oxfordian time and apparently rose above the sea as a low island near the beginning of Callovian time.