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The Laramide orogeny (c. 80 to 40 m.y.B.P.), which culminated during early Eocene time, resulted in the development of numerous uplifts and basins in the foreland of the western United States. Uplifts are assignable to three general classes: (1) Cordilleran thrust belt uplifts, (2) basement-cored, fault-bounded uplifts of the classic Laramide Rocky Mountains, and (3) monocline-bounded uplifts of the Colorado Plateau. Basins were also of three types: (1) Green River type--large equidimensional to elliptical basins bounded on three or more sides by uplifts and commonly containing lake deposits, (2) Denver type--asymmetrical, synclinal downwarps with a related uplift along one side, and (3) Echo Park type--narrow, highly elongate basins with through drainage and of strike-slip origin. Gr en River-type basins exhibit quasiconcentric zonation of facies, in contrast to the unidirectional, proximal-to-distal facies tract of Denver-type basins. Facies distribution in Echo Park-type basins is complex and often difficult to reconstruct due to faulting, erosional truncation, and cover.
The prevalence of en echelon structures in the deformed zone east of the Colorado Plateau, and evidence for significant crustal shortening north of the plateau, suggest that the major structural features of the Laramide foreland were produced by large-scale, north-northeastward translation of the relatively rigid Colorado Plateau block. The magnitude of this motion, as indicated by dextral offset of lineaments which cross the eastern margin of the plateau and by the amount of crustal shortening in the Wyoming province, may be as great as 65 to 120 km (40 to 75 mi). This translation probably resulted from the interaction of relatively competent Colorado Plateau lithosphere with the underlying, gently dipping Farallon plate, which was being overridden by the western United States in Lar mide time.
Evidence for increased strain rates in early Eocene time includes: (1) markedly higher rates of deposition and sand/shale ratios in the Gulf Coast geosyncline (Wilcox Group), (2) formation of several new basins in the southern Rocky Mountains in which Eocene deposits rest unconformably on pre-Cenozoic rocks, and (3) the generally coarser and more arkosic nature of Eocene sediments, as compared to older Laramide deposits, in many areas throughout the foreland. The early Eocene culmination of Laramide tectonism appears to result from two factors. First, the subducted Farallon plate achieved its shallowest dip at about 55 m.y.B.P., resulting in increased viscous coupling with the overriding continental lithosphere. Second, changing spreading-center geometries in the Labrador Sea, Norwegi n Sea, and Arctic Ocean caused the maximum horizontal stress direction to shift to a northeasterly orientation, causing the Colorado Plateau block to increasingly decouple from the craton along north-trending wrench faults in the southern Rocky Mountains. Translation of the Colorado Plateau to the north-northeast during Laramide time resulted in a series of transpressive uplifts and basins along its eastern margin and large-scale crustal shortening in the Wyoming province to the north.
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