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Cretaceous Paleogeography of the Southern Western Interior Region
A complex paleogeographic history for the southern Western Interior region can be reconstructed from the widely distributed Cretaceous deposits that remain after dissection by Tertiary uplift and erosion. These deposits largely reflect migrating fluvial, deltaic, beach, and shelf-depth marine sedimentation that was intermittently interrupted by minor to major breaks in deposition. Gross depositional patterns reflect the interplay between tectonism, primarily associated with thrusting in the Sevier orogenic belt to the west, and eustatic sea level rise and fall, producing marine incursions and disconformities in the Western Interior basin.
The Early Cretaceous was primarily a time of erosion or non-deposition in non-marine environments. Age control on the few preserved deposits of that epoch is poor up to the upper Albian, which records the first incursions of the sea from both the north and the south.
The bulk of Cretaceous sedimentary rocks in the southern interior basin were deposited during the Cenomanian through middle Campanian interval, when peak eustatic sea level, high rates of thrusting in the Sevier orogenic belt, and rapid subsidence of the associated foreland basin provided extensive sediment accommodation along the western margin. Four major transgressive-regressive marine cycles are evident in the southern interior region, reflecting third-order eustatic cycles of Haq and others (1987). Maximum transgression of the earliest cycle, preserved near the Cenomanian-Turonian Stage boundary, resulted in the most widespread marine sedimentation, covering the entire Colorado Plateau and extending eastward across Iowa and Missouri. During subsequent high-stands in the Coniacian to Santonian, early Campanian, and middle to late Campanian intervals, marine sedimentation was primarily confined to the northeastern half of the Colorado Plateau and areas to the east; location of the eastern margin of the seaway is poorly constrained because of subsequent erosion. Widespread erosive surfaces and conglomerates were developed during the regressive periods that occurred between these broad transgressive events.
During the latest Campanian, rates of thrusting and subsidence declined, and Laramide style movement on deep-seated fault blocks commenced. This tectonic regime produced widespread erosion and non-deposition on the western side of the southern interior basin, where intermontane basins were developed and filled with alluvial fan, alluvial-plain, and lacustrine deposits. Marine deposition continued to the east, as shorelines progressively prograded from the southwest until the late Maastrichtian, when the sea receded from the area entirely.
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