Abstract

In the proximal Paradox basin near Gateway, Colorado, previous mapping demonstrates that the Permo-Pennsylvanian Cutler Group depositionally onlaps Precambrian basement of the late Paleozoic Uncompahgre uplift. Nearby well and seismic data, however, suggest the presence of a large subsurface thrust; thus the deposition of >4 km of Cutler Group strata into the Paradox basin has traditionally been attributed to syndepositional thrusting during the late Paleozoic. Detailed facies and structural mapping of the Cutler strata within 2 km of the onlap contact confirm that the Precambrian-Cutler contact is a depositional onlap rather than a fault. Bedding orientations measured within the most proximal Cutler Formation here exhibit a semi-radial pattern of dip steepening in a small area adjacent to the nonconformity. This pattern, previously unrecognized, is here inferred to record primary depositional dips associated with a Gilbert-type delta system. This interpretation is consistent with 1) new work proposing a lacustrine setting for the proximal Cutler Formation and 2) the presence of an inferred subaqueous, syndepositional slide. The absence of syndepositional tectonic structures in the Cutler system exposed here implies that thrusting on the subsurface Uncompahgre fault had ceased before deposition of the youngest Cutler strata exposed at the surface. This cessation of movement signals the end of the Ancestral Rocky Mountain (ARM) orogeny in the proximal Paradox-Uncompahgre system.