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The Big Horn basin of northwest Wyoming is primarily a Laramide structural basin. The area has been a portion of larger sedimentary basins throughout most of geologic history.
The basin is located on the eastern shelf of the Cordilleran geosyncline, east of the hinge line separating the shelf from the former deep parts of the syncline.
Local structural deformation on the sites of several Laramide anticlines in the basin is suggested by slight thinning noticeable in strata of Ordovician age. Local structural influence upon the present-day basin, however, is not evident until at least as late as the beginning of Upper Cretaceous.
During the pre-Laramide eras, periods of regional movements indicate a "see-saw" action with repeated northerly tilting, deposition, emergence and erosion which resulted in truncation of the Ordovician, Devonian and Mississippian sediments from north to south. There is a complete absence of Silurian sediments.
During Pennsylvanian, Permian and Triassic, the area of the present-day basin underwent southerly tilting, deposition, erosion and truncation that resulted in the formations thinning from south to north.
Jurassic and Lower Cretaceous formations show an increase in thickness from south to north. They also show the development of a low-relief structural arch that appears to be the buried, northwest-plunging nose of the Casper arch and Laramide Range in south and central Wyoming.
By the beginning of Upper Cretaceous time the embryo of a structural basin may have been present which affected some of the basal sands of the Frontier Formation.
Later transgressions and regressions of the Upper Cretaceous seas continued until the Laramide Orogeny came into strong evidence at the beginning of Fort Union time. The period of intense movement continued into Eocene with thrust faulting followed by deposition and partial erosion of volcanics on the western margin of the basin.
This movement resulted in peripheral mountain building, pronounced unconformities at the margins of the basin, the development of conglomerates in the Tertiary beds, and the development of the intense anticlinal folds preserved today.
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