SE UTAH CONGLOMERATE INDICATES UNCOMPAHGRE UPLIFT WAS UP AND BEING UNROOFED DURING MEDIAL PENNSYLVANIAN (DESMOINESIAN)

Dane (1935) described an unusual conglomerate in Salt Valley with large limestone boulders containing Mississippian fossils. Dane noted the complicated structure, the absence of boulders belonging to later rocks, and no comparable conglomerate in the region (except in Molas Fm outcrops 130 mi southeast in SW Colorado), and suggested an E. Pennsylvanian age. Elston, Shoemaker and Landis (1962) mapped and, based on correlations to limestone-bearing conglomerates in two nearby wells, assigned the conglomerate to the “Permian” Cutler Fm (conglomerates in wells are now considered debris of E. Triassic namakiers). Re-examination of the outcrops shows the conglomerate is interbedded with the Pennsylvanian strata in the core of the Salt Valley diapir. At one exposure, the single conglomerate (10 ft thick) is vertical, recumbent and faulted. Limestone boulders are up to 30 inches in diameter with numerous smaller quartzite boulders and chert pebbles. Arkose and igneous and metamorphic rocks are absent, but there are a few mica flakes in the associated sandstone lenses and in strata directly beneath the conglomerate. Underlying siltstones and fine sandstones have trace fossils (shallow water deltaic suggested) and the underlying shales (deeper water pro-deltaic suggested) have macrofossils (brachiopods and mollusks) and microfossils (abundant conodonts). The conodonts are similar to Hermosa Group Desmoinesian forms described by Ritter, Barrick and Skinner (2002) 100 miles south in Utah. Borehole and geophysical data preclude a nearby buried source for the Mississippian limestones (Leadville Fm), but suggest the boulders were eroded from adjacent Desmoinesian Uncompahgre highlands (closest logical source area 15 mi to the northeast), and deposited in a distributary channel of a fan delta. Mississippian, Devonian and Cambrian strata were being eroded in the nearby highlands during the Desmoinesian with minor incision by rivers into the Precambrian basement.