Abstract

Southwestern Utah contains many areas subject to landslide hazards, mainly on steep slopes underlain by slide-prone geologic units. The most frequently occurring landslides in the region include rock falls, slumps, debris slides, and debris flows. Geologic formations commonly involved in slump, slide, and flow failures include the Sevier River Formation, Tertiary volcanic rocks (Bullion Canyon volcanics, Mt. Dutton Formation, and Mt. Belknap volcanics), Claron Formation, Tropic Shale, Carmel Formation, Chinle Formation, Moenkopi Formation, Wheeler Shale, and Chisholm Shale. The distribution of landslides and of the most slide-prone geologic formations generally indicate where landslide hazards in southwestern Utah are greatest.

In southwestern Utah, landslides are most common in the High Plateaus subdivision of the Colorado Plateau, and least common in the Basin and Range physiographic province. Higher precipitation in the High Plateaus, and an abundance of slide-resistant rocks in the Basin and Range explain the disparity between the two regions.

All counties in southwestern Utah contain prehistoric landslides and damaging historical failures have been documented in all but Millard County. Landslides in Millard County are concentrated in shale in the House Range, and in Tertiary basalts near Cove Fort. Landslides in Beaver, Piute, western Sevier, and western Garfield Counties are predominantly in clay-rich Tertiary volcanic tuffs in and near the Tushar Mountains. In the plateaus of eastern Iron County, landslides have formed mainly in Tertiary volcanic rocks, the Tropic Shale, and the Moenkopi Formation. Throughout Washington County, landslides are common in the Petrified Forest Member of the Chinle Formation. In western Kane County, landslides occur predominantly along cliffs where the Dakota Formation, Tropic Shale, and Carmel Formation crop out.