ANALYSIS OF COMPOSITE LANDSLIDES IN FLAT-LYING SEDIMENTARY STRATA

The Colorado Plateau (CP) region of the United States contains numerous examples of composite megalandslides with basal sliding zones developed in the overconsolidated shales. The largest identified slides displace over 850 m of strata while slides exceeding 600 m are common in the Grand Canyon (GC). Although these large slides are mapped and noted in literature, past analysis has lacked detail to explain their likely genesis.

Similar landslides have been identified by the authors in the flat-lying Paleozoic rocks of Midwestern states using topographic expression and field verification. Their smaller scale and a thick vegetative cover have precluded recognition of most of these features. Although relief is more subdued than that of the CP, the landslides share many commonalities. They usually floor in shales or shale lenses and occur where watercourses have incised the toe of slopes. Both regions experienced a much wetter climate during the Pleistocene and landslides in both locations show an older geomorphic expression. Finite element models (FEM) have been employed by others to study landsliding in the CP and have provided insights into some of the likely variables promoting landslippage. Conventional limit equilibrium analyses have proven inappropriate in terrain of extreme topographic relief in jointed rock. The megalandslides in the CP appear to evolve through progressive failure, likely due to strain softening initiating at the toe of the slope and progressing upslope. FEM, Finite Difference, and DDA analyses are being used to examine the triggering mechanisms and mechanics of the resulting deformations. An incipient megalandslide flooring in the Bright Angel Shale has been mapped in Peach Springs Canyon, a prominent tributary of the GC. This block has dropped 15-20 m and a circular headscarp has developed (mapped as a fault), but no significant back rotation has occurred. Evaluating this incipient landslide may aid in understanding the triggering mechanisms responsible for megalandslides in the CP and other areas with similar conditions. The authors are using palynology in an attempt to determine the paleoeclimate present at the time of landslippage. Recovered palynomorphs indicate a full glacial climate at the time of the oldest recognized landslide complex in the central GC.