Abstract

Landslides along the south flank of the Uinta Mountains are concentrated in areas adjacent to outcrops of the Oligocene Bishop Conglomerate that are underlain by weak Mesozoic shales and mudstones. The Mesozoic section includes several units that are rich in mudstone or siltstone, including the Cretaceous Mancos, Mowry, and Cedar Mountain Formations; the Jurassic Morrison, Stump, and Carmel Formations; and the Triassic Chinle, Moenkopi, and Dinwoody Formations. These clay-rich formations, in addition to being weak and not resistant to erosion and deformation, provide barriers to fluid movement. The combination of weak rocks and fluids is a perfect one for landslide development. All of the major landslide complexes within our study area have a main scarp in the Bishop Conglomerate. The Bishop Conglomerate contains some fairly competent and porous conglomerate and sandstone beds that maintain steep slopes and create a perched aquifer above the clay-rich Mesozoic units. When the slopes become steep enough and moisture/pore pressure conditions are high enough, landslides occur. The landslide detritus from the Bishop Conglomerate and the younger gravels is often more resistant to erosion than the softer sediments beneath, and so where old landslides are being dissected, the ridges between washes are armored with boulders from these units. Small secondary landslides form where the armored ridges have become high enough and the slopes around them steep enough to allow downslope movement.