ORIGIN OF LATE CENOZOIC LACUSTRINE DEPOSITS ON THE UNCOMPAHGRE PLATEAU OF WESTERN COLORADO
Lake deposits in Cactus Park overlie the Gunnison River gravels, are up to 40 m thick, and covered at least 8 km2. The sediments are chiefly yellow planar-bedded silt with interbeds of gray clay. Beds range in thickness from 1 to 10 cm and vertical sequences typically show thin clay beds separating thicker silt units. The beds are horizontal or dip 3 to 5 degrees to the NE. Clay minerals are dominated by illite and mixed-layer clays similar to those found in the Cretaceous Mancos shale. Minor beds of sand show evidence of small-scale cross stratification. Authigenic minerals such as nodular calcite and fibrous gypsum occur along bedding planes. Trace fossils and evidence of bioturbation have not been observed.
The presence and distribution of the lake beds provide clear evidence that the Gunnison River was dammed near the mouth of Cactus Park, which led to the abandonment of this paleochannel course and, possibly, Unaweep Canyon. Relocation of the Gunnison River to the NE as the lake drained through an outlet probably established near present-day Little Dominguez Canyon. Formation of the Cactus Park lake and abandonment of this ancient Gunnison River course may have been caused by 1) late Quaternary uplift of the Uncompahgre Plateau (Scott et al., 2002), 2) an ice dam, or 3) a landslide. Abundant landslides associated with the Brushy Basin Member of the Jurassic Morrison Formation adjacent to Cactus Park suggest that landslides in the East Creek area created the lake, perhaps during a time of wetter climatic conditions.