Abstract

In the Rocky Mountain region between central Montana and central New Mexico, sedimentologically isolated nonmarine basins were produced by basement deformation during the Laramide orogeny within the area formerly occupied by a broad Late Cretaceous foreland basin in which laterally continuous marine facies had accumulated previously. Laramide structures of varying trend and scale reflect heterogeneity of crustal strain caused by shear between the continental lithosphere and an underlying subhorizontal slab of subducted oceanic lithosphere. Laramide basins include perimeter basins along the cratonic periphery of the arcuate Laramide province, axial basins along a north-south intramontane trend, and ponded basins located farther west closer to the overthrust belt.

Twelve specific stratigraphic and sedimentologic criteria for the onset, duration, and termination of Laramide deformation allow the chronology of basin development to be inferred independently for each basin. Maastrichtian initiation of Laramide deformation was approximately synchronous throughout the Laramide province, but termination of Laramide deformation was systematically diachronous from north to south between early and late Eocene time. Widespread Eocene erosion surfaces truncate syntectonic Laramide sequences and are overlain by largely volcanic and volcaniclastic post-Laramide strata of Eocene age in the north and Oligocene age in the south.

Fluvial depositional systems draining toward the Great Plains were dominant in perimeter and axial basins, but ponded basins were occupied at times by large lakes that served as regional sediment traps. Paleocene drainages from ponded basins also led eastward toward the continental interior, but partly interconnected lakes that developed within several ponded basins by mid-Eocene time were either closed hydrologically, or else they drained westward into the "Tyee" paleoriver of the Pacific Northwest.

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