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The AAPG/Datapages Combined Publications Database
Rocky Mountain Section (SEPM)
Relation of Unconformities, Tectonics, and Sea Level Changes, Cretaceous of the Denver Basin and Adjacent Area Areas
Intrabasin tectonics and sea level changes have 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. Five of these unconformities have been related by previous workers to sea level changes and to well known regressive-transgressive cycles. The origin of the other 4 unconformities may be related either to tectonic movement or sea level changes.
Uncertainty exists 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); 4) early Cenomanian, 95 ± (lower Frontier–Peay, and D); 5) Turonian, 90 ± (base upper Frontier, Codell or upper Carlile); 6) Coniacian, 89 ± (base Niobrara or equivalents); 7) early Santonian, 80 ± (Eagle, lower Pierre and upper 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 precision of radiometric dating.
The unconformities are grouped into 3 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.
Two examples are described typical of the unconformities, thought to be related primarily to drops in sea level, but with influence by tectonic movement. One is the 97 ± m.y. unconformity with which the petroleum-producing J and Muddy Sandstone is related. The second is 90 ± m.y. and is recognized by relations within the shelf, slope and basin deposits of the Greenhorn, Carlile and Frontier Formations.
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