SURFICIAL, BEDROCK AND GEOHAZARD MAP OF THE MESA LAKES QUADRANGLE, GRAND MESA COLORADO

The geology of the Mesa Lakes 7.5’ Quadrangle was mapped in 2017 as part of the USGS EDMAP program. The study area lies primarily on the northern slope and top of the Colorado’s Grand Mesa, among the world’s largest and highest erosional landforms. The mesa is a striking example of inverted topography, with its basalt cap initially emplaced within a late Miocene valley. The mesa formed in response to late Cenozoic incision of the Colorado and Gunnison Rivers, was exposed to at least three glacial periods, and is currently undergoing extensive mass wasting.

The middle Miocene basalt cap consists of 26 lava flows. Beneath the basalt cap are weak Paleocene to Miocene sedimentary rocks that are exposed above Upper Cretaceous sandstones of the Mesa Verde Group. The unit directly below the basalt is the variegated claystone informally named the Goodenough formation, which rests on the Uinta and Green River formations, respectively. Although outcrops of the sandstone and siltstones of the Uinta are exceedingly rare in the quadrangle, the marlstone and shales of the Green River are more widely exposed. The mudstone of the Wasatch formation constitutes the lowest rock unit and figures prominently in the northern section of the quadrangle.

Previous mapping of Grand Mesa identified the terminal moraine of what may be the Pinedale glaciation. This moraine extends across the top, north, and south sides of the quadrangle with a prospective section of the Bull Lake moraine near the western border atop the mesa. Of note, a possible pre-Bull Lake moraine exists on the western part of the mesa outside the quadrangle. Five sediment core samples were retrieved from closed depressions within these moraines for ¹⁴C dating, which may establish which moraines are Pinedale and which are older.

The quadrangle also hosts ubiquitous landslides, earthflows, rockslides, and rockfalls. Recent (2015–2017) lidar surveys revealed extensive earth flows, recent and ancient voluminous landslides as well as previously-documented retrogressive coherent slump blocks. In addition to the massive slump blocks, the basalt rim is also fracturing on smaller scales and “calving” off the rim of the mesa, creating a major hazard. Lastly, geologic conditions of the Mesa Lakes area are similar to those of the deadly 2014 West Salt Creek rock avalanche that occurred 10 km east.