Evaporite Karst Subsidence
2015-05-05 | CGS Admin
[ED: Originally written by Jonathan White, Senior Engineering Geologist (emeritus staff) for a 2001 volume of our paper RockTalk bulletin, this short introduction reveals some special geological areas of the state worthy of notice for a variety of reasons.]
Many areas of Colorado are underlain by bedrock that is composed of evaporite minerals (Figure 01). Indicative of the word evaporite, these minerals were deposited during the cyclic evaporation of shallow seas that existed in central Colorado millions of years ago. As the water continued to evaporate, the remaining solution became hyperconcentrated with salts: minerals such as gypsum, anhydrite, and halite (rock salt). These minerals precipitate out of solution and accumulate in shallow nearshore basins on the bottom of the sea floor. Depending on the paleoevironment, thinly interbedded fine sandstone, mudstone, and black shales can also occur in the evaporite. Mostly Late Paleozoic and Mesozoic rock formations contain evaporite beds in Colorado. Some are thin and discontinuous — only minor beds within a rock formation. Others are massive, with evaporitic minerals many hundreds of feet thick.
Millions of years of burial, plastic deformation, mountain building, and erosion have forced the evaporite beds to the shallow subsurface and/or ground surface today. Evaporite minerals in Colorado are a valuable mining resource. Historic mining occurred throughout the state where thin gypsum beds were exposed. Active mining continues in the massive deposits near the town of Gypsum.
Two characteristics of evaporite bedrock are important. One is that evaporite minerals can flow, like a hot plastic, when certain pressures and temperatures are exceeded. The second, and most important to land use and development, is that evaporite minerals dissolve in the presence of fresh water. It is this dissolution of the rock that creates caverns, open fissures, streams outletting from bedrock, breccia pipes, subsidence sags and depressions, and sinkholes. These landforms are described collectively as karst morphology. Karst morphology originally referred to limestone areas known for characteristic closed depressions, sinkholes, caverns, and subterranean drainage. Evaporite karst comprises similar morphology where these features develop as a result of dissolution of the evaporite minerals.
Exciting scientific work is ongoing in areas of evaporite karst in Colorado. The evaporite terrains of the Roaring Fork and Eagle River areas are centered in areas of Neogene deformation and regional subsidence, related to flowage, diapiric upwelling, and dissolution of evaporite minerals. Precise geologic mapping, river and hot springs water chemistry, and inconsistencies in superposition (elevations) of various volcanic flows compared to their determined ages has brought about new theories, and definable and defensible limits to the area of regional collapse. Highly contorted strata (Figure 03), collapse debris, structural sags, deformation of river terrace gravel, piercement structures, and river-centered anticlines are all geomorphic evidence of the subsidence and deformation.
While regional collapse related to evaporite flowage and dissolution is fascinating to geoscientists, it is the associated risk from localized and potentially spontaneous subsidence that can damage or destroy infrastructure and potentially be life threatening. Secondary considerations include seepage susceptibility and potential failure where reservoir dams are located, along with water-quality concerns with dissolved-salt loading of rivers.
Most catalogued sinkholes in the state lie on surficial deposits such as flat-lying glacial outwash terraces, recent valley side sediments, or older deposits on pediment slopes overlying the evaporite bedrock (Figure 02). Some sinkholes, fissures, and caverns are exposed in the actual bedrock (Figure 04). In surficial-soil mantles, subsurface borings in the vicinity of sinkholes show wide irregularities of bedrock depths, as do exposures along road cuts. While the surface of a river terrace is relatively flat, the underlying bedrock surfaces is likely more indicative of karst topography. The highest densities of sinkholes that are manifested at the surface in Colorado occur in the Roaring Fork River-Carbondale area in Garfield County, the Eagle River around Gypsum and Edwards in Eagle County, the Buford-North Fork White River area in Rio Blanco County, and Park County south of Fairplay.
Where evaporite bedrock is exposed at the surface or underlies thin surficial soils, there is some risk that subsidence could occur. The associated risk rises where the frequency of sinkholes increases. Dangerous and life-threatening, spontaneous collapse and opening of subsurface voids are rare, but do occur (Figure 05). More commonly, differential strains and settlement from localized ground subsidence or piping (removal of fine soils into subsurface voids) will damage facilities that are unknowingly constructed over a sinkhole or subsidence trough. Where subsidence features are exposed at the surface and properly identified, they should be avoided if possible. Many older sinkholes have been covered with recent soil infilling and are completely concealed at the surface. While some concealed sinkhole locations can be seen in aerial photography by vegetation changes, only subsurface inspections, either by investigative trenching, a series of investigative borings, geophysical means, and/or observations made during overlot grading or utility installation, will decisively ascertain whether sinkholes exist within a development area.
If such hidden sinkholes are located, an experienced geotechnical firm should be retained to evaluate the hazard and potential for future subsidence on the development. There are both ground-modification and structural solutions that can help mitigate the threat of subsidence if avoidance is not an option. Drainage issues and proper water management are as important as they are for collapsible soils. Additional increases of fresh water may accelerate dissolution, destabilize certain subsidence areas, re-open or establish new soil pipes to bedrock voids, and rejuvenate older sinkholes.