Intrabasin tectonics and sea level changes influenced patterns of deposition and geographic distribution of major unconformities within the Cretaceous of the Western Interior. Nine major regional to near regional unconformities have been identified. Previous workers have related five of these unconformities to sea level changes and to well known regressive-transgressive cycles. The origin of the other four unconformities may be related either to tectonic movement or sea level changes.

The approximate dates for unconformities are estimated as follows (formations involved are in parentheses): (1) late Neocomian to early Aptian, 112 m.y. (base lower Mannville, Lakota, Lytle); (2) late Aptian-early Albian, ~100 m.y. (upper Mannville, Fall River, Plainview); (3) Albian, ~97 m.y. (Viking, Muddy, Newcastle, or J Sandstone); (4) early Cenomanian, ~95 m.y. (lower Frontier-Peay, and D); (5) Turonian, ~90 m.y. (base upper Frontier or upper Carlile); (6) Coniacian, ~89 m.y. (base Niobrara or equivalents); (7) early Santonian, ~80 m.y. (Eagle, lower Pierre and upper Niobrara); (8) late Campanian, ~73 m.y. (mid-Mesaverde Ericson, base Teapot); (9) late Maestrichtian, ~66 m.y. (top Lance or equivalents). Variations in the accuracy of the dating are probably within 1 m.y. because f problems in accurately defining the biostratigraphic level of the breaks and in the precision of radiometric dates. The unconformities are grouped into three types: those completely within nonmarine strata such as at the base and top of the Cretaceous, those involving both marine and nonmarine strata, and those within marine strata, as currently mapped.

Three examples are described as typical of the unconformities, all thought to be related primarily to drops in sea level, but with minor influence by tectonic movement. One is the ~97 m.y. unconformity, with which the petroleum-producing J and Muddy Sandstone is related. A second is ~90 m.y. unconformity which is recognized by relationships within the shelf, slope, and basin deposits of the Greenhorn, Carlile, and Frontier formations. The third is the ~80 m.y. unconformity within the basin and shoreline regression associated with the upper Niobrara, lower Pierre, Eagle, and Shannon formations.

Several billion barrels of oil were found in sandstones associated with unconformities in the Cretaceous of the Rocky Mountain region. Future stratigraphic trap exploration is guided by a knowledge of tectonic influence on sedimentation during sea level changes and how these factors control distribution of source rock, migration patterns, reservoir rock, and seal.