Stratigraphy and Tectonic Implications of the Tensleep Sandstone, NE Bighorn Basin, WY



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This study combines stratigraphic and structural evidence to provide new insights into previously unrecognized Ancestral Rocky Mountain deformation occurring within the northeast Bighorn Basin. Although previous studies in the area have suggested Late Paleozoic Ancestral Rocky Mountain deformation in the Bighorn Basin, little work has been done to determine the extent and timing of this possible Late Paleozoic deformation in the region. The focus of this study is John Blue Canyon, the largest canyon cut into the westernmost monocline of a series of three basement-involved monoclines near the Bighorn Canyon National Recreational Area, in north-central Wyoming. Exposures of Mississippian Madison Limestone through Triassic Chugwater Formation crop out along the walls of the canyon. At this site, the Pennsylvanian Tensleep Sandstone, which is more than 100 m thick elsewhere in the Bighorn Basin, appears to pinch-out and is truncated by the overlying Goose Egg Formation. Detailed fieldwork, involving geologic mapping and measuring sections, was undertaken to characterize the pinch-out as well as facies variations and thickness changes within the Late Paleozoic sediments of the Amsden, Tensleep, and Goose Egg Formations in the northeast Bighorn Basin. Additional areas of study at Little Sheep Mountain, Sheep Mountain and Medicine Lodge Creek provide a broader regional context for the observations in John Blue Canyon. This study recognizes and explores several lines of evidence for deformation occurring before, during, and after deposition of the Tensleep Sandstone. This study shows that the Tensleep overlies the Amsden Formation with angular unconformity in John Blue Canyon. The Tensleep was subsequently uplifted, eroded, and overlain by the Goose Egg Formation, resulting in the observed pinchout. The patterns of pre- and post-Tensleep uplift and erosion approximately coincide geographically with the Laramide deformation that formed the present-day monocline. The pinchout of the Tensleep Sandstone and previously unrecognized underlying and overlying angular unconformities discovered at Little Sheep Mountain provide compelling evidence that Ancestral Rocky Mountain deformation was an important control on the deposition of Late Paleozoic stratigraphy in the northeast Bighorn Basin.



Big Horn Basin, Tensleep Sandstone, Pinchout, John Blue Canyon, Ancestral Rocky Mountains