Stratigraphic relationships across the Triassic-Jurassic boundary; Big Horn County, Wyoming
The stratigraphic relationships across the Triassic-Jurassic boundary have not been clearly defined due to poor biostratigraphic control. Establishing these relationships is important to develop an understanding of the paleogeographic and stratigraphic framework of the Late Triassic Chugwater Group and the Middle Jurassic Gypsum Spring Formation. The deposition of the Late Triassic strata on the eastern side of the Bighorn Basin was influenced by plate tectonics and sea-level fluctuations. The Red Peak Formation of the Triassic Chugwater Group records seven depositional cycles of regressive and transgressive units which are identifiable by evidence of subaerial exposure in outcrop or by alternating peaks and troughs in geophysical log signatures. Following the deposition of the Triassic sediment, a series of uplift and erosion contribute in the truncation and thinning of these sedimentary groups. Jurassic seas formed and resulted in the deposition of the Middle Jurassic Gypsum Spring Formation which lies unconformably above the Chugwater Group. This extensive unconformity is recognized throughout Big Horn County. Previous researchers have recognized several Triassic and Jurassic unconformities throughout the western interior. One of these unconformities, the J-1 surface, is recognized as the result of an episode of uplift and erosion during the early Jurassic. These unconformities have been used to identify the Triassic-Jurassic boundary on the eastern section of the Bighorn Basin. The combination of tectonics and eustatic changes have contributed to the stratigraphy of the Triassic and Jurassic in Big Horn County, Wyoming. Correlation of the Triassic and Jurassic strata within the study area aid in the illustration of the degree of erosion and thinning that developed following the deposition of the Chugwater Group.