A SALT ANTICLINE-CONTROLLED FLUVIAL SYSTEM: A PRELIMINARY STUDY OF THE ALI BABA MEMBER OF THE TRIASSIC MOENKOPI FORMATION, EASTERN UTAH AND WESTERN COLORADO

The Ali Baba Member of the Moenkopi Formation was deposited in a braided fluvial system in the Salt Anticline region of eastern Utah and western Colorado. Contacts between the underlying Permian Cutler Formation, Tenderfoot Member of the Moenkopi, and overlying Chinle Formation are all angular unconformities indicating that salt uplift occurred before, during, and after Ali Baba deposition. Preliminary analyses of the member along the flanks of four salt valley anticlines (Paradox, Sinbad, Fisher, and Castle valleys) indicate that coarse-grained river systems originated on the southwest flanks of the Uncompahgre highlands. Upon emitting from the highlands, however, these west-flowing rivers were deflected northwestward by the first line of northwest-trending salt anticlines. Proximal drainage was focused into a trough bounded by the remnant Uncompahgre highlands to the northeast and the actively rising Paradox, Sinbad, and Fisher salt anticlines to the southwest. The Ali Baba Member on the northeast flank of Castle Valley suggests that similar northwest-directed rivers developed between the salt anticlines farther west. Here the member was deposited in a salt withdrawal syncline between the the Fisher Valley anticline on the northeast and the Castle Valley anticline to the southwest. Petrology of sandstone in the basal part of the Sinbad and Paradox sections show a relatively high amount of carbonate lithic fragments and, coupled with fossiliferous limestone pebbles from a basal conglomerate in Sinbad Valley, suggest an early sediment source in Late Paleozoic carbonates, probably uplifted in the adjacent salt anticlines immediately to the southwest. The previously unstudied fluvial Ali Baba Member records a complex river system controlled by remnant Uncompahgre highlands and actively rising salt anticlines in the foreland, before continuing westward to the Early Triassic shoreline.