LOWSTAND EVAPORITES ONLAPPING HIGHSTAND CARBONATES IN MIXED SILICICLASTIC/CARBONATE SEQUENCES OF THE PENNSYLVANIAN HERMOSA GROUP, EASTERN PARADOX BASIN, COLORADO

This study documents for the first time basin-centered evaporites onlapping shelf carbonates, in the middle Pennsylvanian Hermosa Group outcrops of the eastern margin of the Paradox basin. Distinctive facies associations (interbedded laminated carbonates, and black mudstones) in a new measured section on the cliffs south of Goulding Creek (37o 30.282” N; 107o 49.607” W) can be correlated eight kilometers down dip (to the south) to evaporite outcrops in the type section of the Hermosa Group, near Hermosa, Colorado (Franczyk et al., 1995). The Goulding Creek South section also provides a stratigraphic linkage to the correlative up-dip, mixed carbonate –siliciclastic sequence stratigraphic framework of Gianniny and Miskell-Gerhardt (2007). The evaporites are inferred to be mid-Desmoinesian, equivalent to Hite's cycle 6 of the Akah oil and gas interval. These strata occur on the western wall of the Animas Valley north of Durango, Colorado, where there is a nearly fifteen kilometers long, dip-oblique outcrop exposure. The evaporite-associated facies onlap the shelf carbonates forming a lowstand wedge near the distal portion of the southwesterly prograding upper Paradox Formation (Gianniny and Miskell-Gerhardt, 2007). This is in agreement with previous work using subsurface data on the western margin of the basin that show lowstand evaporites onlapping highstand carbonates (e.g. Weber et al., 1995, Grammer et al., 1996).

Clearly documenting the evolution of carbonate-evaporite-siliciclastic systems, in well- exposed surface outcrop examples, such as this one; provide a basis for building predictive models for the evolution of this basin and other, less understood basins.