Progradational Sequence Sets on the Tectonically Active Eastern Margin of the Pennsylvanian Paradox Basin, Southwestern Colorado

Gianniny, Gary L.¹, Kimberlee J. Miskell-Gerhardt² (1) Fort Lewis College, Durango, CO (2) Consulting Geologist, Durango, CO

A nearly continuous 15 kilometer long dip-oblique outcrop exposure of the middle Pennsylvanian Hermosa Group north of Durango, Colorado provides a unique opportunity to relate updip stratal architecture to downdip production in the Paradox and San Juan basins. Continuous high resolution photographic panoramas and measured sections were used to analyze facies relationships and sequence stratigraphic packaging.

Within the Hermosa Group, a 250 meter-thick package of middle to upper Desmoinesian mixed carbonate-siliciclastic sediments form a progradational sequence set building to the south and southwest. A preliminary sequence stratigraphic interpretation suggests that sequences are formed by lowstand coarsening-upwards siliciclastic facies successions from prodelta through outer stream mouth bar. A paucity of trace fossils or other fauna in these siliciclastics is consistent with increased salinity in the basin during lowstands. The deltaic sequences are interrupted and capped by transgressive to highstand shallow-water carbonates including pylloid algal biostromes. Sequence boundaries are most typically placed at the top of shallowing-upwards carbonates where karst, coalified roots, or rhyzoliths are observed. In some cases, sequence boundaries show evidence of significant siliciclastic sediment bypass.

In updip locations reservoir quality in this interval may be compromised by immature clastic sediments which contain abundant mica and feldspar in all grain size categories. Increased sediment maturity is predicted with increased sediment transport distance (i.e. downdip locations). Abundant phylloid algal biostromes may provide reservoir quality moldic and shelter porosity.