PATTERNS OF BASIN FILL IN THE UPPER PALEOZOIC OF THE DENVER EMBAYMENT, SOUTHEAST COLORADO, USA

Our study of subsurface geology of Morrowan (Pennsylvanian) through Wolfcampian (Permian) strata in the Paleozoic Denver Embayment, southeast Colorado indicates that basin fill proceeded in a series of punctuated events characterized by initial aggradation vs. subsidence (A:S) rates that favored more complete preservation of Transgressive/Regressive glacioeustatic cycles, transitioning to A:S rates that favored incomplete and thinner depositional cycles. These basin fill events correspond to time and lithostratigraphic units recognized in Midcontinent stratigraphy, and are demarcated by second order flooding surfaces. These second order flooding surfaces include the basal Morrow, Atoka, Missouri, Virgil, Council Grove and Wolfcamp formation boundaries. Sedimentation during the early stages of the basin fill events consists of sand/shale or carbonate/shale couplets, with shales becoming thinner during subsequent depositional cycles, resulting in the stacking of multiple sandstone or carbonate beds with thin or absent intervening shales.

Basin fill patterns also show systematic variation in isopach and lithofacies distribution within each of the time-stratigraphic intervals. For example, isopach values are thickest in areas proximal to the Ancestral Front Range and Apishapa uplifts, and decrease in easterly and northerly directions respectively, with increasing distance from the uplifts. With each successive time-stratigraphic unit, overall isopach values decrease and relative proportions of shale decrease, indicating a progressive decrease in available accommodation space as basin fill proceeded. Sedimentation patterns indicate coarse clastic fill along proximal basin margins on the west and south, with distal areas being filled by finer-grained clastic and carbonate sediment. Proximal sediments were deposited in fluvial and coastal marine depositional environments, whereas distal sediments were deposited in coastal, shoal, shelf and basinal marine environments.