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In late Precambrian time, sedimentary rocks were deposited in a developing geosyncline in the Cordilleran region. Eastward extensions of this geosynclinal sea occupied parts of the Rocky Mountain Region. Following gentle deformation and erosion the sea spread eastward during the Cambrian and Ordovician Periods.
Discontinuous Ordovician, Silurian, and Early Devonian rocks indicate short intervals of marine invasion interrupted by periods of erosion. A major invasion of the sea over the craton is recorded by the onlap of Devonian and Mississippian carbonates and Devonian evaporites which rest on rocks ranging in age from Precambrian to Early Devonian.
The pattern of widespread shallow seas of the Mississippian Period was interrupted in the Pennsylvanian and Permian Periods by significant tectonic activity (Ancestral Rockies). Portions of the uplifts remained positive until Triassic or Jurassic Periods and supplied coarse clastics to adjacent areas within late Paleozoic basins. At greater distances from land areas, sandstones, redbeds, evaporites, and carbonates accumulated.
Marine Triassic sediments were deposited in southeastern Idaho and adjacent areas. Triassic and Early Jurassic continental deposits accumulated throughout much of the region.
A series of Jurassic marine invasions from the Arctic initiated another major sequence of events. The boreal sea moved southward into the northwestern and western parts of the region in Middle Jurassic and successive transgressions reached as far southeast as northern Colorado by Late Jurassic. After withdrawal of the Jurassic sea, the pattern of overlap was continued by deposition of nonmarine Jurassic and Cretaceous sediments.
In the Early Cretaceous a sea again invaded from the north and in late Early Cretaceous joined a southern sea forming a seaway which persisted during the remainder of the Period. During Early Cretaceous, clastic sediments were derived from the craton to the east and from the Cordilleran region to the west. In Late Cretaceous, the western source area predominated.
The present tectonic framework was initiated during the Late Cretaceous and early Tertiary with the development of uplifts and intermontane basins (Laramide Orogeny) accompanied by the emplacement of the Idaho batholith and associated intrusions. Extensive thrust faulting occurred in the western part of the region. Lacustrine and fluviatile sediments, derived from surrounding uplifts, were deposited within the intermontane basins.
Volcanic activity was moderately important to the west during the Cretaceous Period, but igneous intrusion and volcanic activity became widespread throughout the Rockies in the Tertiary.
The present drainage system was largely developed as the intermontane basins filled. Subsequent stream erosion, accompanied by Pleistocene glaciation and regional uplift, resulted in the shaping of the present topography.
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