Regional Paleotopographic Trends and Production, Chesterian and Morrowan Strata, Western Interior

DOLSON, J., C. ADAMS, and K. VAN ZANT, Amoco Production Company, Denver, CO

Late Chesteran (Mississippian) and Morrowan (Pennsylvanian) unconformity-bounded packages in the Western Interior have traditionally been viewed as sequences separated by a regional "basal Pennsylvanian" unconformity. Analysis of alluvial valley trends and paleontological data suggests that these sequences are related and formed in response to multiple relative sea level fluctuations initiated in the Chesterian which continued throughout the Pennsylvanian.

The Darwin, Tyler, Morrowan, and Humbug formations of the Western Interior appear to have formed in retrogradational parasequence sets of alluvial valley fills and marine transgressive systems tracts repeatedly developed around the flanks of the transcontinental arch. Relative sea level rises following each major incisement created a complex intertonguing of marine strata responsible for fluvial aggradation of each valley system. These sequences progressively backstep and grow younger toward the transcontinental arch, where Atokan and Desmoinesan strata cap the oldest preserved surfaces.

At least four major paleodrainage basins have been identified, and valley-fill production has been established in three of these. The previously unrecognized age equivalency of these formations provides a new regional sequence stratigraphic interpretation which has hydrocarbon exploration implications in frontier areas of Utah and Colorado. Seismic resolution and an understanding of hydrocarbon migration paths hold the key to developing new exploration trends.

AAPG Search and Discovery Article #91010©1991 AAPG Rocky Mountain Section Meeting, Billings, Montana, July 28-31, 1991 (2009)