Case Study: Colorado Mountain College, Spring Valley

2003-02-04 | CGS Admin

In early February of 2003, a 24-foot-wide (7.5 m) sinkhole spontaneously opened on a soccer field at the Colorado Mountain College (CMC) – Spring Valley campus, about 10 miles (16 km) southeast of Glenwood Springs in Garfield County. The CMC physical plant maintenance staff back-filled the hole with road pavement waste, but the sinkhole subsequently reopened in 2004 and enlarged to about 35 feet (10.5 m) in diameter.

The site is located near the southern end of Spring Valley. This interesting subsidence feature lies within the Carbondale Collapse Center. Spring Valley, originally a lake, has been interpreted as a drag feature that opened when a large block of basalt-capped redbeds (Maroon Formation) moved laterally, rafting on flowing salt that moved from high-pressure environments under Little Grand Mesa to lower pressure terrain at the axis of the Cattle Creek anticline at the floor of the Roaring Fork River valley. At the southern end of Spring Valley, evaporite dissolution and subsidence has created a complex and chaotic mixing of broken and intact basalt cap rocks, variable bedrock, and depression in-fill of sediments that have been mapped as “collapse debris” in the geologic maps completed by CGS in the late 1990’s.

During the construction of the campus, rolling hills were graded flat to create the soccer field. While basalt blocks occur on the hill top above the field, a large spine of gypsum is exposed in the 25- to 30-foot cut slope that was required to make the field flat. An investigative drill boring at the sinkhole revealed gypsum of the Eagle Valley Evaporite at a depth of 65 feet. This was buried by sediments washed into a natural depression and 30 feet of fairly uniform, mostly reworked windblown sediments (loess) that can be seen in the walls of the sinkhole.

The subsurface void chimneyed up through these sediments from a cavern in the evaporite bedrock below. Just a few months earlier in the summer and fall of 2002, this location was repeatedly driven over by a tractor pulling overlapping gang mowers to cut the grass of the sports fields.

The citation section below includes links to a number of the CGS research publications and journal articles related to evaporitic collapse in the region and statewide.