Stratigraphic and Structural Relations in the Permo-Pennsylvanian Cutler Formation of the Proximal Paradox Basin, Colorado

Moore, Katherine¹, G.S. Soreghan² (1) OU, Norman, OK (2) University of Oklahoma, Norman, OK

Where exposed immediately adjacent to the Uncompahgre Plateau (western Colorado), the Pennsylvanian/Permian Cutler Group consists of coarse-grained detritus that was shed from the Precambrian-cored Uncompahgre uplift into the proximal Paradox basin during the Ancestral Rocky Mountains (ARM) orogeny. In the area surrounding Gateway, Colorado, previous mapping demonstrates that the Cutler Group depositionally onlaps Precambrian basement; however, well data nearby and seismic data from elsewhere along the southwestern Uncompahgre front suggests the presence of a large overthrust in the subsurface. The deposition of >4 km of Cutler Group sediment into the Paradox basin has been attributed to syndepositional thrusting in the late Paleozoic. We conducted detailed facies and structural mapping of the Cutler strata within 2 km of the onlap contact in order to assess the depositional and structural history of this system. These data confirm that the exposed contact between the Uncompahgre uplift and the Cutler Group is a depositional onlap rather than a fault contact. We hypothesize that steepening of dips in a localized area adjacent to the nonconformity onlap records primary depositional dips associated with a Gilbert-type lacustrine delta system. This interpretation is also consistent with the presence of an inferred syndepositional slump that juxtaposes proximal (subaqueous) boulder conglomerate atop more distal deltaic facies. The apparent absence of syndepositional tectonic structures in the Cutler system here implies that thrusting on the Uncompahgre frontal system had ceased before deposition of the youngest Cutler strata exposed at the surface, signaling the end of the ARM orogeny here.