AN INVENTORY AND ANALYSIS OF THE GRAND MESA LANDSLIDES, COLORADO, USA: INSIGHTS TO HIGH-VOLUME, HYPERMOBILE MASS-WASTING EVENTS

Typically, the two hazardous components of a landslide are volume and mobility. The deadly, massive (41 Mm³), and highly mobile (mobility index [MI] = 7.14) 2014 West Salt Creek Landslide (WSCL) on Colorado's Grand Mesa (southern Piceance Basin in the Colorado Plateau) prompted an investigation into the late Quaternary recurrence interval of such events. The Grand Mesa formed in response to late Cenozoic incision of the upper Colorado River system after ~11 Ma emplacement of basalt flows that cap and diminish the erosion of underlying sedimentary rocks. The Mesa now stands ~1600 m above local base level. Mass wasting on the flanks of Grand Mesa can be subdivided into higher-elevation Toreva-block slumps involving the basalt cap and lower-elevation deep-seated landslides.

We inventoried 781 >1,400 m² landslides using lidar topography, GIS techniques, and field verification. The landslides were characterized for up to 33 geologic and topographic parameters and analyzed using correlation and regression-tree analysis. Thirty-one landslides in the inventory are similar to or larger than the WSCL. Landslide age estimation utilized geomorphic techniques, augmented by surface roughness analysis and six radiocarbon samples.

Over the last ~30 ka, WSCL-scale events (>20 Mm³) occur, on average, about once every 1000 . The late Holocene (last ~5 ka) recurrence interval averages one landslide every ~17 years for the complete inventory , while the late Quaternary, or "background" (last ~30 ka) recurrence interval averages one landslide every ~39 years. The flanks of the Mesa generated 52 hypermobile (MI ≥ 9) and 163 highly mobile (MI ≥ 6 < 8.99) landslides during the late Quaternary. In all, 122 landslides have higher MIs than the WSCL. Failure of the Green River/interfingered Uinta Formations accounts for 90.5% and 33.8% of the high-volume and the hypermobile landslides, respectively, while exposed along only 28% of the Grand Mesa. The combination of stratigraphic elevation and varied and clay-rich lithology is likely responsible for this stratigraphic unit's proclivity in landslide production. Overall, while significant landsliding on the Grand Mesa is a common hillslope geomorphic response to the ongoing incision of the Colorado and Gunnison Rivers, highly mobile or hypermobile megaslides are rare occurrences.