ENVIRONMENTAL AND ENGINEERING ASPECTS OF EVAPORITE TECTONISM, WEST-CENTRAL COLORADO

Widespread collapse of the ground surface in west-central Colorado is a result of dissolution and flow of Pennsylvanian evaporite within the Carbondale and Eagle collapse centers. Collapse affects an area of at least 3,600 km². Late Cenozoic volcanic rocks are downdropped as much as 1,200 m within collapsed areas. Much of the collapse occurred during the past 3 Ma, a time coincident with accelerated river incision. Diapiric flow is locally responsible for post-middle Pleistocene folding of terraces. The geology of the collapsed areas is described in the Geologic Society of America's Special Paper 366, which has an anticipated release date of December 2002.

Evaporite dissolved in the collapse area discharges into the Colorado River system, principally via thermal springs that flow into rivers and by subsurface seepage into alluvial aquifers. These dissolved salts comprise the first major salt loads to the river. About 0.8 million metric tons of dissolved evaporite enters the Colorado River from the collapse area annually (Chafin and Butler, 2002). Most thermal springs discharge sodium chloride waters, whereas calcium and sulfate are abundant in shallow groundwater. Sinkholes related to evaporite dissolution are obvious geologic hazards. Many historic sinkholes are associated with irrigation ditches, fields, and reservoirs, and they are common in topographic depressions formed in outwash terraces. The depressions are probably underlain by extensive active karst systems. Water that drains into sinkholes enhances evaporite dissolution at shallow depths. Modern rates of collapse and diapirism are poorly constrained. It presently is unknown if the rates of vertical deformation related to evaporite tectonism are sufficiently high to pose hazards to structures with anticipated life spans of 100 years or less.