The Laramide-Age Structural Pattern of the northern Uncompahgre Plateau, western Colorado

Richard Livaccari

Laramide-age structures of the northern Uncompahgre Plateau include a complex pattern of monoclines and reverse faults linked by a series of left-lateral strike-slip faults. The NW-SE striking Redlands monocline and reverse fault is found in the Colorado National Monument. The Redlands reverse fault is a steeply SW-dipping structure (60 to 85 degrees SW). This fault places Proterozoic basement rocks over Triassic and Jurassic-age strata and has up to 1,600 ft of vertical structural displacement. Strata along this structure typically dip 80 to 30 degrees NE. Along the central part of the Redlands monocline and reverse fault structure (Gold Star Canyon area), the width of the strain zone is very narrow and the strata are overturned (dips of 50 to 85 degrees SW). Topography along the Redlands monocline and reverse fault is spectacular due to the cliff-forming nature of the deformed Proterozoic basement rocks and the Jurassic strata (Wingate, Kayenta, and Entrada Formations). The Colorado River Monocline (new name) is a second NW-SE striking monocline. This structure is found NE of the Redlands monocline and reverse fault. The Colorado River Monocline is inferred to have up to 2,500 ft of vertical structural displacement, but 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. The NW-SE striking monoclines and reverse faults are actually restraining bends linked together by a series of right-stepping (in map view), left-lateral strike-slip faults (Bull Canyon, Glade Park, and Cactus Park ñ Bridgeport fault systems). These structures are WNW-ESE striking, subvertical, oblique-slip faults with a predominance of left-lateral strike-slip and lesser amounts of dip-slip. Both the dip direction and the sense of dip-slip change along the strike of these faults from steeply N-dipping, normal dip-slip, to steeply S-dipping, reverse dip-slip. The presence of oblique slickenside striations suggests left-lateral strike-slip deformation occurred along these structures. The pattern of WNW-ESE striking left-lateral faulting and NW-SE striking reverse faulting is strain compatible and indicates 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