ABSTRACT: Reference Sections for the Sand Wash Basin, Colorado, Part of the Western Interior Cretaceous (WIK) Project

Larry B. Kellison

Cretaceous strata in the Sand Wash basin of northwest Colorado is approximately 11,000 ft thick. Exposures of Cretaceous rocks are found only in the Irish Canyon area (T10N-R101W) in extreme northwestern Colorado, along the flanks of the Axial basin anticline and the Williams Fork Mountains in the southeastern portion of the basin and along the Park Range to the east. Reference sections for the Sand Wash basin were assembled as part of the Western Interior Cretaceous (WIK) Project of the Global Sedimentary Geology Program.

In the Sand Wash basin, Lower Cretaceous strata include the Aptian to Albian age Dakota Sandstone and Mowry Shale, which range in thickness from 215 to 450 ft. The Frontier Formation, Niobrara Formation, Mancos Shale, coal-bearing Mesaverde Group, Lewis Shale, and Lance Formation comprise over 10,500 ft of Upper Cretaceous strata in the basin ranging in age from upper Turonian to Maastrichtian.

Second- and third-order sequence boundaries are recognized within the Sand Wash basin and correspond to major unconformities in the Dakota Formation, at the base of the Frontier and in the basal Williams Fork Formation of the Mesaverde Group. Previously unrecognized second- and third-order boundaries may exist at the top and base of the Niobrara, near the base of the Iles Formation and near the base of the Fox Hills Sandstone of the Lance Formation. Tertiary Browns Park Formation truncates much of the Upper Cretaceous in the western portion of the basin.

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