Case Study: Lykins Formation

Small but significant areas of Colorado are underlain by bedrock that is composed of evaporative minerals. These are salts and sulfates that precipitate out of salt-concentrated surface waters. In the geologic past these minerals were deposited in shallow seas within closed or restricted basins where the seawater evaporation rate exceeded the replenishing supply. Current environments that are similar include the Great Salt Lake in Utah and the Dead Sea in the Middle East. These minerals are predominantly anhydrite (CaSO4) and halite (rock salt – NaCl) at depth, and gypsum (CaSO4*H2O) near the surface. Over geologic time, the evaporative minerals filled the sea basins and were subsequently buried beneath younger sediments. Through burial diagenesis, these deposits become evaporite bedrock. After the Rocky Mountains rose, millions of years of subsequent erosion and downcutting of rivers has now exposed some of these evaporite rocks at the surface.

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.

One example of evaporative bedrock in Colorado is the Permo-Triassic Lykins Formation redbeds that contain massive gypsum deposits, up to 50 feet (15 m) thick. Dissolution of those beds and some of the thin algal limestone within the unit is responsible for many sinkholes and ground subsidence features inside the main Dakota Sandstone hogback that marks the boundary of the Eastern Plains and the Front Range.

On May 1, 1999 a sinkhole spontaneously opened on the westbound shoulder of U.S. Highway 34 within the Colorado Department of Transportation (CDOT) right-of-way, about 3 miles west of Loveland. Just north of the sinkhole the beds of gypsum in the Lykins Formation were historically mined by the United States Gypsum Company. The mine has closed and the property is being redeveloped as a gated community. The roadside sinkhole was approximately 25 feet in diameter and 20 feet deep. Eye-witness reports that the material simply disappeared down a hole that angled slightly to the north. CDOT drilling revealed large voids within the Forelle Limestone member of the Lykins formation. The Forelle can be in an advanced state of karstic dissolution and brecciation because of the proximity of more soluble gypsum beds below.

Topographic lowering and closed depression are common within the Lykins valley along the Front Range. Where streams exit the foothills, the adjacent narrow water gaps in the tilted hard sandstone of the Dakota hogback are favorable location for dams. Two prominent reservoirs are situated on the Lykins: Carter Reservoir and Horse Tooth Reservoir. The U.S. Bureau of Reclamation has had to mitigate seepage problems directly related to gypsum dissolution at both sites. Sinkholes open frequently at the south end of Horse Tooth during seasonal drawdown of lake levels.

The thickest and most exposed gypsum of the Lykins Formation is found in the region around Table Mountain in Larimer County near the border with Wyoming. Thick gypsum beds are exposed at the surface and there are several historic gypsum mines in the area.

Karst features—several sinkholes and a small creek that is captured into a gypsum cave—also exist in that locale. This beautiful area 25 mi (40 km) north of Fort Collins is accessible as part of Larimer County’s Red Mountain Open Space and is lightly travelled.

Our colleague at the Denver Museum of Nature and Science, Dr. James Hagadorn recently released a detailed report on the Lykins Formation in the central and northern reaches of the Front Range. This from the standpoint of exploring public access to this interesting geologic feature. Many outcrop locations are accessible within the open space parks of Jefferson, Boulder, and Larimer counties.