Rates of sedimentation during the Late Cretaceous in the Rocky Mountain-Great Plains region compare favorably with rates of sedimentation shown by Quaternary sediments of the Atlantic Ocean basin margins. An average rate of about 1 in./1,000 yr is reported for Quaternary deep-water sedimentation in the Atlantic Ocean by Ericson et al. (1964). This rate is prevalent in regions where sediments are largely biogenetic, consisting mainly of coccoliths and foraminifers with few land-derived minerals. In contrast, rates 20-40 times greater are reported in the shoreline zones of the Atlantic. Similar rates are inferred from radiometric dates for Late Cretaceous sedimentation in the region of the Rocky Mountains-Great Plains. Slow rates approximating 1 in./1,000 yr were prevalent ear the geographic center of the basin during the time of deposition of rock units such as the Greenhorn, Niobrara, and Mobridge. These units consist largely of coccoliths and foraminifers with small quantities of land-derived minerals. In the equivalent shoreline zone for each time-stratigraphic unit, rates of sedimentation are at least 20 times greater than depositional rates in the basin.

The differential rates of sedimentation greatly influenced the rates of subsidence within the Cretaceous basin. Tectonic downwarping of the basin was accentuated in some areas by sediment loading resulting from deposition in the general shoreline zone, especially in the deltaic areas. Large-scale intrabasin warping developed with structural dips as great as 50 ft/mi. Early migration of petroleum was influenced by these early intrabasin tectonic elements before destruction of the large depositional basin by Laramide folding.

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