SEEING THE FOREST AMIDST THE TREES; ALTERNATING CARBONATE/SILICICLASTIC FACIES FEEDBACK IN PROGRADATIONAL SEQUENCES, DESMOINESIAN OF THE EASTERN PARADOX BASIN, SW COLORADO

Too often carbonate and silicilastic depositional components of a basin margin are studied as nearly unrelated phenomena, as isolated depositional segments by separate researchers. This study demonstrates that an integrated sequence stratigraphic analysis of the interactions between the carbonate and siliciclastic systems provides insights to facies prediction and shelf evolution during the Desmoinesian (Pennsylvanian) of the eastern flank of the Paradox basin.

North of Durango, Colorado, a nearly continuous 15 kilometer long, dip-oblique outcrop exposure of Hermosa Group reveals prograding fluvio-deltaic successions that controlled the locus of shallowing upward phylloid algal carbonate bank deposits during the subsequent marine transgressions and highstands. Similarly, inherited topography from thick, delta-top carbontes created accommodation lows and forced siliciclastic bypass farther into the basin, driving the progadational system as the basin began to overfill. As the delicate balance of sedimentation, subsidence, and ice-house eustasy interacted on this shelf, thicker units of either siliciclastic or carbonates appear to have limited overlying deposition to thinner, more up dip facies, creating patterns of facies and sequence hierarchy.

These facies feedback relationships and the sequence stratigraphic framework of the depositional shelf margin are documented by continuous, high-resolution photographic panoramas and measured stratigraphic sections. Additional outcrop correlations allow us to extend this framework to a total of 40km, from Molas Pass to Hermosa, Colorado. The general stratigraphic position of these exposures with respect to producing zones in the Paradox Basin may range from the Akah to Ismay oil and gas producing intervals, and the Honaker Trail Formation (Franczyk et al., 1995; Brown, 2000, and Wahlman, 2007 pers. com.).