EVAPORITE KARST HAZARDS OF THE LOWER FORK RIVER VALLEY, WEST-CENTRAL COLORADO

The lower Roaring Fork River valley in west-central Colorado is underlain by Pennsylvanian Eagle Valley Evaporite. Karst features in the valley include sinkholes, ground fissures, and subsidence troughs. Related phenomena are groundwater and thermal springs with high TDS concentrations. Most sinkholes are located on Pleistocene terraces but also occur in upland areas. Three general types of sinkholes have been recognized in the corridor. Type A are deep rubble-pipe sinkholes, which involve the entire soil column above an evaporite void. Type B are shallow fissure piping sinkholes only affecting the upper fine-grained soils, apparently piping through fissured outwash gravels to evaporite voids below. Type C sinkholes are roof collapses of solution cavities in evaporite bedrock. Five stages of sinkhole development are present.

The valley is centered in the Carbondale Collapse Center, a 1,165 km2 structural depression that has collapsed 1,200 meters in response to late Cenozoic plastic flow and dissolution of the evaporite. Deeply buried evaporite, under very high overburden pressures, flows towards the river valley where overburden pressures are markedly reduced. This salt tectonism has formed structural basins and sags, deformed terrace treads, large-scale collapse debris, and a major river-centered anticline. Exposures of evaporite rock along the valley walls reveal highly contorted fine-grained clastic beds, massive gypsum, and evidence of diapiric piercement. Sinkhole spatial frequency increases where geologically recent diapiric upwelling has occurred.

Damage or distress from sinkhole subsidence has been rare, but risk and probability of damage has risen due to increased development density. Ground openings and sinkhole subsidence have impacted utilities, irrigation ditches, development layouts, and both county and state highways. Dry, low-density gypsic soils, surficial deposits derived from evaporite rocks, are prone to hydrocompaction and create further settlement problems. High natural salinity that is introduced to the Colorado River is a by-product of regional dissolution. While geologically recent, it is currently unknown whether regional subsidence continues today and whether it poses a significant hazard.