--> Abstract: Basin Margin Evolution in Mixed Carbonate Siliciclastic Evaporate Sequences, Eastern Margin of the Pennsylvanian Paradox Basin, Southwestern Colorado, by Gary L. Gianniny, Kimberlee J. Miskell-Gerhardt, and Scott M. Ritter; #90078 (2008)

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Basin Margin Evolution in Mixed Carbonate Siliciclastic Evaporate Sequences, Eastern Margin of the Pennsylvanian Paradox Basin, Southwestern Colorado

Gary L. Gianniny1, Kimberlee J. Miskell-Gerhardt2, and Scott M. Ritter3
1Geoscience, Fort Lewis College, Durango, CO
2Consulting Geologist, Durango, CO
3Brigham Young University, Provo, UT

The depositional record of mixed carbonates, clastics and evaporites along the eastern margin of the Paradox basin offer a unique window into understanding the architecture and evolution of tripartite mixed systems. Glacially scoured cliffs north of Durango, Colorado expose a 15km dip-oblique, semi-continuous exposure of the Pennsylvanian (Atokan- Missourian) Hermosa Group adjacent to the western edge of the Ancestral Rocky Mountains. The sequence stratigraphic framework, established during the previous field season, consists of progradational sequence sets of alternating lowstand fluviodeltaic facies (including bypass surfaces) and transgressive to highstand photic zone carbonates. These sequences show stacking patterns indicative of shared accommodation space throughout the development of the margin. Current fusulinid and conodont biostratigraphic results indicate upper Cherokee to lower Marmaton age for the middle of the section. This framework has now been extended 11km updip (to Engineer Mountain) and 8km downdip (towards Hermosa).

The updip facies are dominated by lowstand tidal and fluvial facies alternating with highstand restricted-circulation to photic zone carbonates. Several major, erosionally-based, coarse conglomerates may be related to tectonic events in the Ancestral Rocky Mountains.

At the southern-most outcroppings, down-dip lowstand clastic wedges interfinger with two (2-4m thick) evaporite beds composed of centimeter-scale gypsum/dolomite laminae. Both carbonate and siliciclastic facies below the evaporites contain abundant evidence of shallow water deposition (eg. rhizoliths, mudcracks, calcareous laminites, tuffas and dissolution breccias). Thick, fissile, black shales encase the entire evaporite package. These evaporites are generally believed to correlate to Hite cycle Akah 6 in the Paradox Basin.

 

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas