Deep-seated bedrock landsliding in Grand Canyon: unraveling the mechanics of a series of ancient landslides

Large-scale, deep-seated bedrock landsliding is an incompletely understood erosional process that operates in canyons incised into alternating competent-incompetent, horizontally-stratified rocks. The Surprise Valley landslides in central Grand Canyon between river miles 130-140 provide several ancient examples for understanding the mechanics of such landslides influenced by climatic and/or tectonic factors, including local hydrology, river and tributary incision, volcanism, and seismicity. By combining field mapping and channel profile analysis with U-series and cosmogenic burial age dating, we attempt to understand the sequence, chronology, and failure mechanisms of such landslides, as well as the potential impact of landslide dams on continental-scale rivers and canyon incision. Landslides most commonly form bedrock slumps and translational slides, i.e. large, discernible masses of minimally to moderately disrupted yet stratigraphically recognizable material back-rotated during sliding along listric detachment surfaces. In several cases, landslide debris inundated river and tributary paleochannels, diverted watercourses and initiated re-incision into adjacent bedrock, and triggered fluvial/lacustrine aggradation upstream. Detachment surfaces include shallowly dipping (<5^o), inter-fingered shales and carbonates as well as shallow-to-moderately dipping (~15^o) ash horizons. One bedrock runup event involved emplacement of a 1.5km+ wide, 10+ m-thick carbonate slab riding on a 1-3m cushion of slope debris, river cobbles, and brecciated material translated from north to south, across the river, and 200+ m up the south side where it overrides its equivalent stratigraphic units. Dating results show landsliding in this reach progressed downstream over the past ~2 Ma. Colorado River paleochannels exist at heights of 135m, 61m, 65-70m, and a few meters above river level. The 61m strath is covered by the carbonate slab runup while the 65-70 m straths represent a Colorado River paleochannel filled by a large bedrock slump. The latter two straths have been dated using the cosmogenic burial age technique giving ages of 0.98 +/- 0.21 Ma (1σ) for 61m and 0.88 × 0.22 Ma (1σ) for 65-70m. Approximately 4 miles upstream of the few-meter high landslide debris, fine-grained silt and clay interpreted as lacustrine deposits are capped or inset by far-traveled Colorado River gravels were previously dated at ~21 ka by OSL/IFSL.