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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 67 (1983)

Issue: 3. (March)

First Page: 513

Last Page: 513

Title: Mid-Cretaceous Biostratigraphic Units, Unconformities, and Diastrophism in Wyoming, Colorado, and Adjacent Areas: ABSTRACT

Author(s): E. A. Merewether, W. A. Cobban

Article Type: Meeting abstract


In the central Rocky Mountains and adjoining Great Plains, lithologies and hiatuses within sequences of mid-Cretaceous formations reflect widespread fluctuations of sea level and intermittent tectonism during Cenomanian, Turonian, and Coniacian time (88 to 96 m.y.B.P.). Siliciclastic and carbonate strata of marine origin in the Graneros, Belle Fourche, Greenhorn, Carlile, and Niobrara Formations grade laterally into marine and nonmarine siliciclastic beds of the Frontier Formation. The clastic strata are products of uplift and erosion in both the Sevier orogenic belt of Utah, Idaho, and western Montana, and a contiguous region within Utah, Colorado, Wyoming, Montana, and Nebraska. The ages of these rocks, the durations of intervening hiatuses, and the times of diastrophis were determined mainly from a detailed succession of marine molluscan index fossils.

Outcrops at scattered localities in this region, in a western part of the Cretaceous seaway, as well as outcrops in eastern South Dakota and northwestern Iowa, near the eastern shore of the seaway, indicate a marine transgression in the Cenomanian and early Turonian (Belle Fourche and Greenhorn time), a marine regression in the middle Turonian (early Carlile time), and a marine transgression in the late Turonian and Coniacian (late Carlile and early Niobrara time). However, the stratigraphic record of these widespread events has been obscured in most of Wyoming and Colorado by submarine and subaerial erosion and attendant sedimentation associated with episodic orogenic activity during the Turonian and Coniacian (88 to 91 m.y.B.P.).

In central and northwestern Wyoming, strata as young as late Cenomanian (early Greenhorn age) are disconformably overlain by beds of early middle Turonian (latest Greenhorn) age. Uplift and erosion in these areas probably occurred during early to earliest middle Turonian time. In western Colorado, early middle Turonian strata (Fairport Member of the Carlile) and older rocks are disconformably overlain by late middle Turonian strata (Blue Hill Member of the Carlile), indicating deformation and truncation in the middle Turonian. Moreover, truncated beds as young as late middle Turonian (Codell Sandstone Member of the Carlile) are overlain by early late Turonian strata (Juana Lopez Member of the Carlile) in Colorado and Wyoming, reflecting earliest late Turonian orogenic activity in the icinity of the Front Range, Laramie Range, and Bighorn Mountains. Some of the rocks of early late Turonian age (a lower part of the Wall Creek Member of the Frontier) were, in turn, uplifted and eroded during later late Turonian time in an area that extends from Yellowstone Park to central Wyoming. Furthermore, at outcrops near the Laramie Range, truncated beds of late Turonian age (Wall Creek Member of the Frontier) and the overlying Coniacian strata (basal Niobrara) indicate earliest Coniacian tectonism and erosion in southeastern Wyoming.

Elongate areas of uplift and erosion and of discrete sedimentary facies of mid-cretaceous age commonly trend approximately southeastward and northeastward in the central Rocky Mountains-western Great Plains region. From Yellowstone Park southeast to the vicinity of Casper, Wyoming, intermittent truncation and deposition during the Turonian are especially evident.

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