EFFECTS OF LARGE-SCALE LANDSLIDING IN CENTRAL GRAND CANYON NATIONAL PARK, ARIZONA

At least six rotational landslides of widely differing ages have blocked the Colorado River and its tributaries in the Grand Canyon, forcing them to reroute around the debris and excavate new channels. The landslides are commonly 2000 feet or more in thickness and are comprised of the Cambrian Bright Angel Shale through the Permian Esplanade Sandstone. The extreme topographic relief of this area and exhumation of the buttressing Bright Angel Shale by river incision causes the walls of the inner gorge to fail. Huge blocks fill the drainages by detaching from the walls of the inner gorge and simultaneously rotating backwards while traveling along listric normal faults. The sequence of landslide mobilization, damming of drainages, and the drainage reorganizations that have altered both the planimetric and cross sectional morphology of the Grand Canyon between Colorado River miles 132 and 138 are delineated in this study.

The wide variations in age are revealed by differing elevations of floors of the buried channels. Examining relationships between elevations of buried channel floors reveals the sequence of landsliding. Any abandoned channel placed in elevation above another channel is recognized as part of the older drainage system. And, since the occurrence of these slides and consequent reorganization of the drainages are synchronous, each landslide event can be placed in proper temporal order.

Landsliding caused a succession of six channel reorganizations. Some blocked the Colorado River, others blocked specific tributaries and some caused blockages of both. The oldest buried channel, located at Colorado River mile 134, is 940 feet above the modern Colorado River floor. A slide detached from Surprise Valley, translated .5 miles, filled the oldest observed Tapeats drainage, and forced it to excavate a new drainage 1 mile to the east. The youngest channels are west of Deer Creek canyon. Here, huge blocks detached from the north wall, and filled the Colorado River and Deer Creek channels forcing the streams to reroute. The Colorado River excavated a parallel channel 250 feet to the south and Deer Creek 30 feet to the east. The bottoms of the blocked channels are nearly at the same elevation as the bottom of the modern Colorado River and Deer Creek canyons, attesting to the youth of the slide.