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During Cretaceous time, the Western Interior of the United States was the site of part of an elongated epicontinental seaway that extended from the Arctic Ocean to the Gulf of Mexico (Fig. 1). On the west, the seaway was separated from the Pacific Ocean by the tectonically active Cordilleran highland, which was the main provenance for terrigenous sediment shed into the seaway. In contrast, the east side of the seaway was flanked by the low-lying, stable platform of the central and eastern United States and Canadian Shield, which, except during late Early to early Late Cretaceous time, provided only minor amounts of detritus to the seaway.

The sedimentary fill of the Western Interior Basin is dominantly Cretaceous in age, ranging from Aptian through Maastrichtian. The basin fill was asymmetric, with thicknesses of as much as 5,000 m along the west flank. The asymmetry of the basin resulted from varying rates of subsidence relative to sedimentation, sediment loading adjacent to the major bounding orogenic belts, and tectonic loading by thrust plates in the orogenic belt (Jordan, 1981).

Much of the Cretaceous section is preserved in the many early Tertiary or Laramide structural basins within the Western Interior of the U.S. Because of vast amounts of gas, oil, and coal contained in them, Cretaceous rocks of the Western Interior have been widely studied throughout much of the area.

Although many of the areas of preserved Cretaceous rocks are now separated, fairly accurate biostratigraphic correlations are made on the basis of good to

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