HISTORY AND DISTRIBUTION OF PENNSYLVANIAN SILICICLASTIC WEDGES IN THE UNCOMPAHGRE TROUGH, CENTRAL ANCESTRAL ROCKY MOUNTAINS, COLORADO AND UTAH

On the western margin of the Uncompahgre uplands in the central Ancestral Rocky Mountains extensive erosion during the earliest Morrowan removed any Chesterian and Meramecian strata thereby exposing the Kinderhookian-Osagian Leadville carbonates and leaving a karsted surface with low-relief hills and valleys. This area became the floor of the Uncompahgre Trough on the western margin of the Uncompahgre Uplift and the deepest part of the Pennsylvanian-Permian Paradox Basin. Initial deposition in the trough in later Morrowan included local reddish non-marine strata and widespread cyclic marine organic-rich shales, fossiliferous limestones, anhydrites and fine-grained siliciclastics that on-lapped the Leadville. Cyclicity continued into the Permian Wolfcampian, and siliciclastic wedges occur in most of the 75 recognized cycles, including many of the 35 cycles with halite. Most of the wedges occur above the evaporites within each cycle and were deposited during the latter part of lowstand and prior to the next transgression. The shelf margin along the Uncompahgre uplift was probably always narrow, the floor of the adjacent basin was deep, accommodation space was high, and turbidites built many of the siliciclastic wedges until the late Desmoinesian when multiple thick prograding cliniforms of coarse-grained fluvial sediments overwhelmed the accommodation space, advanced out into the basin, and overloaded and deformed the underlying beds of halite.