Laramide-Age Structures of the northern Colorado National Monument-Fruita area of the Uncompahgre Plateau, western Colorado

William Walsh, Michael Dunlop, Richard Livaccari, Roderick Maclean, Max Schultz, Aaron Tofsrud, Alexander Asay, Benjamin Haveman, and Tyrell Kipp

Laramide-age structures of the northern Uncompahgre Plateau include a complex pattern of monoclines and oblique-slip faults linked together by a series of left-lateral strike-slip faults. We have recognized five Laramide-age structures in the northern Colorado National Monument – Fruita, CO area. From south to north these are: (1) the Lizard Canyon normal fault system (new name), (2) the Bull Canyon-Redlands monocline and reverse fault system, (3) the Devils Canyon monocline (new name), (4) the Flume Creek monocline and (5) the Colorado River monocline (new name). The Lizard Canyon fault system consists of a series of steeply north-dipping, normal faults with a minor right-lateral (E-W striking faults) or a left-lateral (WNW-ESE striking faults) strike-slip component. The Bull Canyon-Redlands monocline and reverse fault system is a regionally developed structure that consists of a WNW-ESE striking, steeply south-dipping, left-lateral reverse fault and associated monocline. This structure has 500 ft of reverse dip-slip and is inferred to have even more left-lateral strike-slip displacement. The existence of left-lateral strike-slip is based on the presence of oblique, west-raking slickenside striations. The Devils Canyon monocline is a locally developed, WNW-ESE striking structure that has 350 ft of vertical structural displacement. The NW-SE striking the Flume Creek monocline is a regionally developed structure that terminates in this area. The NW-SE striking Colorado River monocline is a regionally developed structure that is found in the Dinosaur Hill area. This structure is inferred to be the largest structure in this area with up to 2,500 ft of vertical structural displacement. The Colorado River Monocline has little topographic expression because most exposed strata defining this structure are the slope-forming Jurassic Morrison and Cretaceous Mancos Formations. The cliff-forming strata that are found along this structure (Cretaceous Burro Canyon and Dakota Formations), locally form small cliffs along the southern bank of the Colorado River. All of these structures are strain compatible and indicate a Laramide-age NE-SW shortening direction.

AAPG Search and Discovery Article #90156©2012 AAPG Rocky Mountain Section Meeting, Grand Junction, Colorado, 9-12 September 2012