LANDSLIDES ALONG THE MOGOLLON RIM, GILA COUNTY, ARIZONA

The Mogollon Rim is one of the most prominent topographic features in Arizona and defines the southern boundary of the Colorado Plateau. Despite high relief, abundant rainfall, and diverse stratigraphy, few landslides have been mapped along the Rim. In March 2019, a small landslide near Buckhead Mesa threatened SR 87, emphasizing the need for a renewed landslide assessment of the area. We used LiDAR data from 2017 and 2019 to analyze bare-earth morphology beneath the dense forest cover and reveal a plethora of previously unrecognized landslides.

Landslides have been recognized within steep canyons carved by Fossil and Hardscrabble Creek and on the flanks of Buckhead and Cedar Mesa. The geologic setting of these slides is Neogene basalt overlying Paleogene conglomerate and siltstone atop Pennsylvanian sedimentary rocks of the lower Supai and Naco Formations, which both contain mudstones that are likely sources of failure. Several additional landslides in this geologic setting were identified in this study.

Three other geologic settings along the Rim are also prone to landslides. On the slopes of Buckhead Mesa, small slides likely ruptured along the clay-rich baked zone/paleosol at the contact between Neogene basalt and Paleogene conglomerate, including the March 2019 slide. Along the lower slopes of the Rim north of Payson numerous south-flowing streams slice into the lower Supai Formation. We identified over twenty landslides along these streams. We also identified landslides in the Permian Schnebly Hill Formation at the base of the steep slopes of the Rim along Pine Canyon, Moore Creek, and Big Canyon. A ~9000 m² slide along Moore Creek is historic, with aerial imagery constraining the failure from 1992 to 1997.

Landslides are abundant along the Rim south of Willow Springs Lake. Here, the geologic setting consists of Paleogene Mogollon Rim Formation overlying Pennsylvanian lower Supai and Naco Formations. The sharp margins and blocky morphology of many of these slides suggest that they are relatively young. We estimate most of these slides likely formed in the Holocene and some may be historic or still active. Field verification of an earthflow in this area revealed tension cracks in the upper portion of the slide, saturated muddy zones in the middle, and water oozing from the toe, indicating that the slide is still moving.