West-East Stratigraphic Cross Section of Cretaceous Rocks, Central Rocky Mountains to East-Central Great Plains, Utah, Colorado, and Kansas

FOUCH, T. D., U.S. Geological Survey, et. al.

Cretaceous sedimentary strata varies in thickness from nearly 3050 m adjacent to the western thrust belt at the westernmost end of the Uinta basin, Utah, to 1700 m near the Colorado-Utah boundary east of the basin axis. They thicken locally to more than 2800 m near local Late Cretaceous Laramide uplifts in the region between the Piceance and the western Denver basins, Colorado, and then to less than 600 m in the easten Denver basin and Kansas. Depositional environments represented by the rocks include coarse-grained alluvial facies deposited directly adjacent to the western thrust belt, and finer grained fluvial-deltaic and marine facies of the basin's depositional and structural axis (generally within 50-100 km of the thrust belt). Entirely fine-grained marine siliciclastic and carbo ate rocks were formed in more slowly subsiding distal parts of the basin that are as much as 1000 km east of the depositional axis. Some thick sections represent deposition in lows created by sediment and thrust loading adjacent to local Laramide thick-skin thrusts within the limits of the overall Western Interior Cretaceous basin. Depositional and geometric asymmetry of the preserved sedimentary system are the result of varied rates of regional and local subsidence due to tectonic and sediment loading. They also result from fluctuations in sea level and concomitant erosion associated with falls and from relative sediment starving of the parts of this depositional system that were far from sediment sources.

Major sequence boundaries correspond to regional unconformities. Of at least nine unconformities that can be demonstrated by biostratigraphic omission, paleotopography, and/or paleosols, over most of the region, six extend well east of the structural axis; of those six, four extend across the entire region. Of the nine, two occur in the Albian, one each occurs in the Cenomanian, late Turonian? to early Cenomanian?, Santonian, and Maastrichtian, and three in the Campanian rocks. Thin-skin thrust tectonics most influenced the distribution of lithofacies and the extent of unconformities west of the Cretaceous western interior basin's structural and depositional axis in the proximal part of the foreland. Thick-skin thrust-generated uplift in distal and proximal parts of the basin's forela d and resultant erosion of older Cretaceous strata created additional extensive unconformities.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)