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.

There is uncertainty in dating many of the unconformities. However, by use of the time scale of Obradovich and Cobban (1975), with subsequent minor revisions, the approximate dates for unconformities are estimated as follows (formations involved are in parenthesis; numbers are millions of years before the present, m.y.): 1) Late Neocomian to early Aptian, 112 (base lower Mannville, Lakota, Lytle); 2) late Aptian-early Albian 100 ± (upper Mannville, Fall River, Plainview); 3) Albian 97 ± (Viking, Muddy, Newcastle, or J Sandstone); 4) early Cenomanian, 95 ± (lower Frontier--Peay, and D); 5) Turonian, 90 ± (base upper Frontier or upper Carlile); 6) Coniacian, 89 ± (base Niobrara or equivalents); 7) early Santonian, 80 ± (Eagle, lower Pierre and u per Niobrara); 8) late Campanian, 73 ± (mid-Mesaverde, Ericson, base Teapot); 9) late Maestrichtian, 66 ± (top Lance or equivalents). Variations in the accuracy of the dating are probably within 1 million years because of 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.