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The Big Horn basin of northwestern Wyoming is primarily a Laramide structural basin, but the area has been a part of larger sedimentary basins through most of geologic history. The basin is on the eastern shelf area of the Cordilleran geosyncline just east of the hinge line separating the shelf from the former deeply subsiding parts of the geosyncline.
Local structural deformation on the sites of some Laramide anticlines in the basin is suggested by slight thinning as early as Ordovician time, but local influence of the present basin is not expressed until at least as late as the beginning of the Late Cretaceous.
During pre-Laramide time, periods of regional movements indicate a see-saw action with repeated northerly tilting, deposition, emergence, and erosion. This 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 time, the area of the present basin underwent southerly tilting, deposition, erosion, and truncation that resulted in thinning of the formations from south to north.
Jurassic and Lower Cretaceous formations 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 Laramie Range in south-central Wyoming.
In earliest Late Cretaceous time a newly forming structural basin may have affected the distribution of some of the basal sandstones of the Frontier Formation. Later transgressions and regressions of the Late Cretaceous seas continued until the Laramide orogeny at the beginning of Fort Union time. This period of intense movement continued into Eocene time with thrust faulting followed by deposition and partial erosion of volcanic rocks on the western margin of the basin.
This movement resulted in peripheral mountain building, pronounced unconformities at the margins of the basin, the deposition of Tertiary sediments, and the development of the intense anticlinal folds preserved today.
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