SEDIMENTOLOGY AND STRATIGRAPHY OF TERTIARY CONTINENTAL DEPOSITS OF THE PONCHA PASS TRANSFER ZONE AND ADJACENT AREA, COLORADO

Transfer zones have been recognized during recent years as being fundamental features of all extended terrains, yet they are poorly understood. Important hydrocarbon reservoirs have been found in these zones. The Poncha Pass area is located between the west-dipping San Luis basin and east-dipping Upper Arkansas valley. According to the transfer zone classification of Morley, Poncha Pass transfer zone would be considered a conjugate, convergent transfer zone.

This study is field oriented. Sedimentary sections were measured using tape and Jacob staff. The Tertiary sedimentary rocks in the transfer zone form a structural trough, lower than the surrounding Sawatch and Sangre de Cristo Ranges, and consist of conglomerate, sandstone, siltstone, mudstone, limestone, shale, and volcanic ash. Two types of volcanic ash are recognized: gray vitric tuff from the Yellowstone hotspot track, and white biotite-bearing dacitic tuff likely from a source in the Basin and Range. Microprobe analyses show no match for the gray vitric tuff, but it has a composition typical of ash beds from the Twin Falls volcanic field, and has a likely age between 10.5 and 8.5 Ma. Conglomerates are predominately igneous-clast conglomerates and diamictities. Sandstones are immature in composition, but submature or immature in texture.

Sedimentary facies in the Poncha Pass transfer zone include matrix-supported conglomerate, cross-bedded pebbly coarse sandstone, massive sandstone, siltstone or mudstone, limestone, calcareous shale, and tuff. Facies assemblages form three depositional systems: alluvial fan, braided-river, and lacustrine systems. The three systems overlap or interfinger with each other in places, but braided-river systems dominate both vertically and spatially. Provenance study shows that Tertiary sediments have two different source areas and controlled by axial and transverse depositional systems.