Sevier Orogenic Belt in Nevada and Utah
In Nevada and Utah, sedimentation in the Cordilleran miogeosyncline began before the appearance of Cambrian fossils and continued without orogenic interruption through the Triassic. During the Jurassic, deformation and regional metamorphism occurred in the western part of the miogeosyncline, and the area of sediment accumulation shifted onto the Colorado Plateau. A major source of clastic material appeared along the eastern margin of the Cordilleran miogeosyncline in Early Cretaceous time; this source supplied the sediments that filled the Cretaceous to Paleocene Rocky Mountain geosyncline. Clasts in the Cretaceous conglomerates show an inverted stratigraphy, reflecting successive exposure of older and older rocks in an evolving orogenic belt along the eastern side of the Cordilleran miogeosyncline. This source area was the Sevier orogenic belt, which had a history of deformation through most of the Cretaceous (Sevier orogeny). Decollement thrusts with displacements of tens of miles are the characteristic structures of the belt, but several large folds are also known. The largest thrusts are overlain unconformably by uppermost Cretaceous conglomerates. Thrusting in the Sevier orogenic belt had virtually ceased by the time the Laramide orogeny began east of the Sevier belt in latest Cretaceous time. Laramide mountains were the result of uplift of great blocks of crystalline basement along nearly vertical, reverse, and steep thrust faults. The Uinta arch, which intersects the Sevier orogenic belt almost at a right angle, is the only one of these basement uplifts closely involved with the deformation of the Cordilleran miogeosyncline. North-south-trending regional normal faulting of post-Oligocene age has broken up the orogenic belt so that it is not immediately recognizable on geologic maps. Arch ranges, intrusive domes, and gravity slides are additional complications of the Tertiary geology, but widespread Tertiary deposits, particularly Oligocene ignimbrites, make a paleogeologic reconstruction possible; thus, the Sevier orogenic belt can be viewed as it existed before the normal faulting.