| Investigations of the Holocene Malpais landslide, located on the northeastern end of the Shoshone Mountains in north-central Nevada, show that very weakly clay-cemented Tertiary fluviolacustrine sedimentary rocks controlled the failure. The sedimentary rocks overlie competent deep-ocean Paleozoic basement rocks and underlie Miocene dacite flow units. The landslide, which originated at a local high point along the fault-controlled Malpais Rim, flowed north into Whirlwind Valley. It has a surface area of ∼2.2 km2 and volume of ∼0.032 km3. The landslide mass is composed of the Tertiary sedimentary and volcanic rocks. The only Paleozoic rocks in the landslide are remobilized clasts in the Miocene sedimentary rocks, thus the slide apparently did not cut into the Paleozoic basement itself.;Field and laboratory testing show that the sedimentary rocks are extremely weak, and that the dacite has high intact rock strength, but also has pervasive columnar joints. Modeling results suggest that the initial landslide originated in the weak sedimentary rocks and followed the joints in the dacite to the surface. The over-steepened rear scarp subsequently failed in a retrogressive manner, allowing rock falls, topples, intact block rotations, and slides to occur. Continued retrogressive failure and scarp formation progressed farther up slope until the overall slope equilibrated at the present head-scarp location near the crest of the Malpais Rim.;Possible failure triggering mechanisms include an increase in the water table level and/or seismic loading, given the proximity to the Malpais and other faults that have documented Holocene movement. The Malpais landslide failure depended on the presence of very weak lacustrine sedimentary units, and similar weak units are present at other landslides in the region. Those units may have been contributing factors, along with the destabilizing influence of water pressure and seismic loading, in the formation of those landslides. |
