The Role of Overconsolidated Shales In Triggering Megalandslides In the Grand Canyon Region

Prehistoric megalandslides are common throughout the Colorado Plateau, where stream incision or cliff retreat has exposed overconsolidated shales. The exposed shales undergo stress relief, dilation, and creep, which tend to promote strain softening. Saturation and strain softening lead to significant loss of shear strength. These processes contribute to large scale landsliding within incised canyons and regressing escarpments in the Grand Canyon region of Arizona, New Mexico, and Utah. At least five shale formations appear to influence landslippage in this region. These include the Cretaceous age Mancos Shale, which contributes to widespread landsliding around Black Mesa and Hopi Mesas in northeastern Arizona. Bedrock slumps are also present along the exposed escarpments of the Vermillion and Echo Cliffs, which extend from Tuba City to House Rock, AZ. The Triassic Chinle Formation, mainly the montmorillonite rich Petrified Forest Member, is responsible for landslippage along these escarpments. A lesser number of slides are present in the Triassic Moenkopi Formation where it is exposed along these same escarpments. The Uinkaret Plateau north of the western Grand Canyon exhibits numerous slides in escarpments floored in Moenkopi Shale which are capped by late Tertiary age lava flows. The largest landslides occur in the Grand Canyon proper, which have volumes ranging from several hundred to more than 2 billion cubic meters. All of these features have basal ruptures that floor in the Cambrian age Bright Angel Shale. The Bright Angel Shale exhibits strain softening, losing about 2/3 of its cohesion when saturated. The Precambrian Galeros Formation in the eastern Grand Canyon has also spawned several large landslides where it is uplifted along the Butte Fault. Some smaller slides occur in the upper Precambrian shales, including the Dox Formation. There appears to be is a correlation between the overconsolidated shales and all of the mapped landslides in this region.