ORIGIN OF LATE CENOZOIC LACUSTRINE DEPOSITS ON THE UNCOMPAHGRE PLATEAU OF WESTERN COLORADO

Geologic mapping and sedimentologic studies of newly discovered lake deposits on the Uncompahgre Plateau of western Colorado provide an important late Cenozoic record of paleoenvironmental conditions in the region. Lacustrine sediments are located in Cactus Park, a NW-trending strike valley on the NE edge of the Uncompahgre Uplift. River gravel deposits show that the Gunnison River used to flow to the NW through Cactus Park. Minimal dissection of the valley has preserved the river gravels as well as remarkable lake deposits. The lake beds are located at an altitude of 1900 m, which is ~475 m above the modern Gunnison River. Extrapolation of regional incision rates suggests that the lacustrine sediments may be as old as ~3 Ma.

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.