SYNCHRONOUS MAXIMUM FLOODING ACROSS THE PARADOX BASIN, PENNSYLVANIAN HERMOSA GROUP, SOUTHEASTERN UTAH AND SOUTHWESTERN COLORADO

While the idea of relative sea level being controlled by a combination of eustasy, tectonism and sediment supply is not new, the ability to demonstrate this in Pennsylvanian sediments of the Hermosa Group on opposing sides of the Paradox basin in SW Colorado and SE Utah has only recently become possible. New studies on the eastern shelf of the basin facilitate this by combining sequence stratigraphy (Gianniny and Miskell-Gerhardt, 2009), conodont and fusulinid biostratigraphy (Ritter et al., 2002, and 2010) as well as new outcrop to basin well log correlations (Miskell-Gerhardt et al., 2010).

On the more tectonically quiescent western shelf, the maximum flooding surface of the entire Hermosa Group is marked by the open marine conodont-bearing black dolomudstones, of the Chimney Rock Shale in the Desert Creek oil and gas interval. Ritter et al., 2002 demonstrated that this shale shares a distinctive conodont and fusulinid fauna with the eustatic highstand of the Midcontinent Verdigris cyclothem. Also on the western basin margin, the anomalously thick, stacked Desert Creek carbonate mound parasequences in the Aneth field, SE Utah (Weber et al., 1995), occur 20-40km basinward of the older and younger carbonate shelf buildups exposed in outcrops of the Goosenecks area. However, in contrast to the eastern margin, subsurface data indicate that the Aneth Field carbonate mound complex developed as an isolated platform on a fault block that was insulated from sediment supply dynamics on the western shelf during the deposition of the Desert Creek interval.

On the tectonically active, eastern margin of the basin north of Durango, Colorado, thick carbonate phylloid biostromes cap coarse delta deposits that are coeval to Chimney Rock Shale/early Desert Creek interval sediments. In the latest part of the Desert Creek interval the marine portion of the sequence was terminated by a 9km basinward shift in facies. Even though sediment supply (coarse clastics and shallow water carbonate) during the deposition of previous sequences moved the shoreline towards the basin, the Verdigris-equivalent eustatic high in the lower Desert Creek interval created enough accommodation space to produce backstepping and the thickest carbonate unit of the eastern shelf of the basin.