OVERCONSOLIDATED SHALES AND THEIR ROLE IN TRIGGERING MEGALANDSLIDES IN THE GRAND CANYON SHALES

Prehistoric megalandslides are common throughout the Colorado Plateau, especially in localities where stream incision has exhumed overconsolidated (OC) shales. Such shales unload and undergo strain softening when lateral confinement is removed through erosional processes which leads to a significant strength loss near the unrestrained face. Evidence suggests that these processes eventually contribute to large scale landsliding adjacent to incised canyons and regressing escarpments which are common in the Grand Canyon region of Arizona, New Mexico, and Utah. At least five major formations play a role in landslippage throughout this region. The type locality of Toreva blocks, or rotational bedrock slumps, is located at Second Mesa, AZ within the Hopi Indian Reservation. The basal slip surfaces of the Toreva blocks occur in the Cretaceous age Mancos Shale, which contributes to widespread landsliding along other escarpments in the vicinity. These include Black Mesa and other Hopi Mesas. Bedrock slumps are also present along the margins of the Vermillion and Echo Cliffs, which span 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 at this location. The Uinkaret Plateau, located to the north of the Grand Canyon near Toroweap, has numerous slides where escarpments floored in Moenkopi Shale are capped by late Tertiary age lava flows. Weathered volcanics also play a role in some of these landslides. The largest landslides in the Grand Canyon proper, which have sizes ranging from several hundred to ~2 billion cubic meters, have basal ruptures occurring in the Cambrian age Bright Angel Shale. This shale exhibits strain softening and loses up to 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 exposed near the Butte Fault. Other smaller slides occur in some of the upper Precambrian shales, including the Dox Formation. We are preparing landslide maps of the Grand Canyon region, which include the outcrop patterns of recognized OC shale formations. There appears to be is a correlation between the OC shales and the mapped landslides.