Geologic map of the Hartsel Quadrangle, Park County, Colorado

The Hartsel quadrangle sits nearly in the center of the complex South Park Laramide structural basin. Generally, the basin can be described as an asymmetrical down-faulted feature, dipping to the east. It is bounded by two northwest-trending uplifts: the Sawatch uplift to the west and the Front Range uplift to the east. The west-verging Elkhorn thrust, which places Proterozoic intrusive and metamorphic rocks within the Front Range uplift over Phanerozoic sediments in the basin, passes just east of the quadrangle. Seismic data and deep oil and gas well logs indicate that a series of imbricate thrust faults extend west, and in front of, the Elk horn thrust fault. The Hartsel uplift is a westward-jutting structural salient of the Front Range uplift that brings Proterozoic rocks farther into the basin south of the town of Hartsel. The quadrangle also spans the late Paleozoic boundary between the central Colorado trough (DeVoto, 1972) to the west and Frontrangia (Mallory, 1958) to the east. The Neogene Rio Grande rift system lies to the west of South Park Basin in the upper Arkansas River valley. Examples of Neogene extension can be found throughout South Park, as described by Stark and others (1949), De Voto (1971), and Ruleman and others (2011). In addition, there is evidence of ongoing local deformation related to dissolution and possible collapse of Paleozoic evaporite deposits across much of the west side of the basin (Kirkham and others, 2012).