ABSTRACT: Cretaceous Stratigraphy, West-Central Denver Basin, Colorado

Robert J. Weimer

Cretaceous strata exposed in the Golden-Morrison area are 3000 m thick and consist of four dominant lithofacies related to depositional environments. These are black shales and limestones (facies 1, deeper neritic); gray shales, siltstones, and thin sandstones (facies 2, shallow-neritic); sandstone (facies 3, shoreface and shoreline); shale, coal, sandstone, and conglomerate (facies 4, fluvial and coastal plain). The strata are characterized generally by slow rates of sedimentation in a stable tectonic setting in the axial region of the basin, 700 km from both the east and west margins.

The lower 90 m of the Cretaceous consists of intertonguing marine and nonmarine sandstones and shales, and conglomerates (facies 1, 2, 3, and 4, Dakota group--Albian), overlain by 210 m of cyclic marine

shale and limestone (facies 1, Graneros, Greenhorn, Carlile and Niobrara formations--Cenomanian through Santonian). The next younger unit dominates the section, 2200 m of marine shale and siltstone with minor sandstone (facies 2, Pierre Shale--Campanian and Maastrichtian). The uppermost 500 m are synorogenic shale, sandstone, coal, and conglomerates which record the Laramide uplift of the Front Range (facies 3 and 4, Fox Hills, Laramie, Arapahoe, and lower Denver formations--Maastrichtian). The upper 1000 m of the marine Pierre Shale may also be synorogenic. The Laramide orogeny takes its name from the deformation of the coal-bearing Laramie Formation in this area.

Unconformities and condensed sections are used in a sequence stratigraphic analysis to evaluate sea-level changes and/or regional tectonic events. Presently, nine unconformities (sequence boundaries) have been observed within the Cretaceous strata in the Denver basin. Three are in the Albian, and one occurs in each of the Cenomanian, Turonian, Coniacian, Santonian, Campanian(?), and Maastrichtian stages.

Organic-rich black shale zones (condensed sections), possibly representing sea-level highstands, occur between several of the unconformities interpreted as caused by sea-level lowstands.

Both regional and local tectonics influenced the distribution and thicknesses of depositional facies. Sequence boundaries, if they occur within unconformable sections, need to be more clearly related to unconformities elsewhere in the Cretaceous basin.

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