IS THERE A LARGE COLLAPSE AREA ASSOCIATED WITH EVAPORITE KARST IN THE SOUTHWESTERN PART OF SOUTH PARK, COLORADO?

Two large areas in Colorado are known to be affected by evaporite deformation and collapse. Collapsed salt anticlines in the Paradox Basin in western Colorado were first described in detail by Cater (1970). In the 1990s 1:24,000-scale geologic mapping by the Colorado Geological Survey and U.S. Geological Survey discovered large active evaporite collapse areas in the Carbondale-Eagle area. They are described in GSA Special Paper 366. A third large collapse area associated with evaporite karst may exist in the southwest part of South Park.

South Park is a large, high-altitude, intermontane valley in central Colorado. Several major faults trend generally NNW-SSE across the park. They formed between the Front Range uplift to the east and Sawatch uplift or anticline to the west, and probably are chiefly Laramide in age. In the central and western parts of South Park they typically are high-angle faults; on the east side most faults are thrusts. Remnants of Laramide-age synorogenic basin fill crop out in the fault blocks in the middle of the park. De Voto (1964, 1971) described evidence of late Cenozoic tectonism in the southwest part of South Park involving synclines that deform middle and late Tertiary volcanic and sedimentary rocks. Late Quaternary faulting was reported in the southeast part of the park by Shaffer (1980).

Many of the same types of features found in the Carbondale-Eagle collapse area were recently recognized in southwestern South Park while mapping for the Colo. Geological Survey. They include thick deposits of Pennsylvanian evaporite at and near the land surface; abundant sinkholes; saline springs with high concentrations of dissolved sodium and chloride that likely came from dissolution of Pennsylvanian halite; large closed depressions in Pleistocene outwash terraces; tilted Pleistocene gravels; and age-equivalent volcanic flows at anomalously low altitudes relative to adjacent areas. The previously documented late Cenozoic synclines may be collapse features related to dissolution of underlying evaporite rather than buried normal faults. Layers of fine-grained, highly porous, low-density, crystalline gypsum locally cap Pleistocene outwash terraces; they may have formed by evaporation of shallow ground water that was rich in calcium and sulfate originally dissolved from Pennsylvanian gypsum.