EVAPORITE KARST SUBSIDENCE HAZARDS IN COLORADO

Significant areas of Colorado are underlain by near-surface bedrock that is composed of thick beds of evaporite minerals. These strata, mostly halite and anhydrite/gypsum, were deposited during the cyclic evaporation of Pennsylvanian-Permian and Permo-Triassic shallow seas and lakes. In these areas of evaporite rocks, karst features such as caverns, open fissures, subsidence depression, and sinkholes are potential hazards. The Colorado Geological Survey has inventoried subsidence features and mapped those areas where evaporite rocks are at, or near surface.

Localized subsidence can be destructive to facilities and potentially life threatening when spontaneous. Other important risks include seepage susceptibility where reservoir dams are located and water-quality concerns with salt loading in streams. Most cataloged sinkholes of Colorado occur in surficial deposits such as flat-lying river terraces, recent valleyfill sediments, or on pediment slopes that all overlie and obscure the underlying evaporite bedrock. While these surfaces are relatively flat, the underlying bedrock surface can be more irregular. The highest densities of sinkholes that are manifested at the surface in Colorado occur in the Roaring Fork River corridor in Garfield Co., the Eagle River corridor in Eagle Co., and south of Fairplay in Park Co.

Important identification and mitigation strategies are necessary in engineering geology and geotechnical investigations within the evaporite terrain mapped in this study. Avoidance is generally the best mitigation solution where subsidence features are exposed at the surface and properly identified. Many older sinkholes may be hidden. Only subsurface inspections, either by investigative trenching, a series of investigative borings, geophysical means, and/or observations made during overlot grading or utility installation, can ascertain whether sinkholes exist within a development area. Ground-modification and structural solutions can help mitigate the threat of localized subsidence. Drainage issues and proper water management are also important. In Colorado’s semi-arid climate, additional increases of fresh water may accelerate dissolution and further destabilize certain subsidence areas.