Characterizing a landslide hazard along the Wasatch Mountain Front (Utah)

Construction in mountainous areas of northern Utah is a concern due to landslide potential. As a result of dense vegetation and poor outcrops (from advanced weathering), the study of landslides along the Wasatch Mountain "Front" benefits from integrating geological and geophysical methods. We present a study of a Paleogene volcanic landslide along a portion of the Traverse Mountains. An integrated approach distinguishes an actual landslide from the weathered in situ volcanic rocks that mantle the area. We incorporate surface mapping, seismic methods, trenching, boreholes, and Light Detection And Ranging (LiDAR) to characterize the landslide. Our study reveals a faulted, chaotic body of variably altered block-and-ash flow tuffs. Multiple clay-lined shear planes and open fissures, now filled with soil, show that prehistoric landslides were focused within the altered block-and-ash flows. A major reflector lies at a depth of 80-100 m (262-328 ft) in seismic profiles, has an apparent dip of 4 degrees to the south, and is approximately parallel to surface topography. The depth and character of this boundary indicate a contact between Paleogene volcanic rocks and underlying Paleozoic mixed siliciclastic and minor carbonate rocks. This unconformity may have acted as the slip surface for subsequent landsliding. The combination of weak block-and-ash flow tuffs and abundant clay alteration produced a body highly susceptible to multiple landslides. Using LiDAR and surface geology, discrete margins of the mapped landslide have been inferred in several areas. An integrated interpretation provides the range and resolution needed to assess the complex geology of such landslides.