GYPSUM KARST OF THE LYKINS FORMATION AND EFFECTS FOR COLORADO FRONT RANGE WATER PROJECTS; HORSETOOTH AND CARTER LAKE RESERVOIRS

The Lykins Formation is a Permo-Triassic sedimentary formation consisting largely of interbedded siltstones, claystones, limestones and gypsum-anhydrite evaporite deposits that outcrops along the Colorado Front Range of the Southern Rocky Mountains. Previous geologic work on the Lykins Formation is very limited. Investigations over the past 12 years by Reclamation geologists have revealed that the Lykins contains structures and stratigraphy typical of gypsum karst throughout the entire thickness of the Lykins Formation. Some of these features include extensive intrastratal breccias, and large circular or near-circular paleo collapse chimneys that cross-cut the entire Formation as well as a paleo-karst network of solution cavities and conduits in highly pervious limestone beds and discontinuous bodies of massive gypsum and anhydrite at relatively shallow depths. The Lykins Formation was uplifted and tilted to the east during the Laramide. Because the Lykins Formation consists of weak sedimentary lithologies, it is typically found in long strike-valleys that have been eroded along the extent of the Front Range. Two major federal water projects; Horsetooth Reservoir and Carter Lake Reservoir were constructed in these valleys in the late 1940's by the U.S. Bureau of Reclamation as part of the Colorado Big-Thompson Project. These reservoirs are key to the infrastructure that supply water for irrigation and municipal uses as well as power generation to Colorado Front-Range communities. During the late 1980's sinkholes formed at the south end of Horsetooth Reservoir and through the 1990's seepage increased dramatically at Horsetooth Dam. In late 2000, a sinkhole was discovered on the upstream toe area of the dam. At Carter Lake Dam No. 2 over 4 cubic ft. per second of seepage exits downstream of the dam. A portion of this seepage is nearly saturated with dissolved gypsum. These changing seepage conditions are attributed to karst dissolution and erosional processes induced by reservoir seepage. The investigations at these reservoirs and consequent dam-safety concerns have led to major modifications at Horsetooth Dam.