ABSTRACT: Cretaceous Rocks of the Piceance Basin Area, Northwestern Colorado

Ronald C. Johnson

Cretaceous rocks of the Piceance basin area, northwestern Colorado, are about 1825-3350 m thick and were deposited in the foreland basin 200-360 km east of the Sevier orogenic belt. These strata are being studied as part of the Western Interior Cretaceous (WIK) Project of the Global Sedimentary Geology Program. The Cedar Mountain Formation, a thin fluvial unit of Aptian-Albian age, is the oldest Cretaceous formation in the area. Overlying the Cedar Mountain with possible disconformity are the Albian-Cenomanian Dakota Sandstone, Mowry Shale, and Frontier Formation, deposited during the initial invasion of the Cretaceous epeiric seaway. The Frontier Formation is a complex Cenomanian to upper Turonian, marine and marginal-marine

unit that contains a hiatus of early and middle Turonian age. Total thickness of the Cedar Mountain through Frontier sequence is about 120-210 m. The marine Mancos Shale, about 1220-1825 m thick, was deposited conformably over the Frontier Formation in a broad, U-shaped marine embayment during Coniacian through Campanian time. The Coniacian-Santonian part of the Mancos thins and grades eastward into calcareous shale equivalent to the Niobrara Formation. Sandy offshore-marine strata of the informal Mancos B interval, which exhibit large-scale, northward-prograding clinoforms, occur in the lower Campanian part of the Mancos. The upper Campanian part of the Mancos intertongues with the overlying marginal-marine part of the Mesaverde Group. This zone of intertonguing was deposited while t e Cretaceous shoreline regressed, as many as seven times, southeastward and eastward across the area. The upper part of the Mesaverde Group is late Campanian to Maastrichtian in age and nonmarine, except for the marginal-marine Lion Canyon Sandstone in the northeast corner of the area. A regional unconformity occurs at the top of the Mesaverde; the amount of truncation apparently increases toward the west and southwest. The Mesaverde Group thins from about 1450 to 760 m toward the west and southwest, and Maastrichtian rocks are mostly very thin or absent along the western margin of the area, although variations in the rate of subsidence may be a partial cause for this thinning.

AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990