DEEP-SEATED BEDROCK LANDSLIDING IN GRAND CANYON: IMPLICATIONS FOR GRAND CANYON EVOLUTION

Large-scale, deep-seated bedrock landsliding is an incompletely understood erosional process that operates in canyons incised into horizontally-stratified rocks. In Grand Canyon (GC), the most significant of such slides occur in central GC between river miles (RM) 130-140. We combine field relationships, U-series, cosmogenic burial age dating, and channel profile analysis to explore the sequence, chronology, and failure mechanisms of landslides, as well as the duration and effect of landslide dams on the Colorado River (CR) and incision of GC. Our results show landsliding in this reach progressed downstream over the past 2 Ma. 1) At ~2 Ma, the 133-Mile landslide (RM 132-133.5L) filled the CR paleochannel when it was ~135m above the modern river level, diverted the river to the north, and created a dam that was mantled by river gravels as high as 165m. 2) At RM 134-136, the main Surprise Valley/Cogswell Butte landslides involved detachment and riverward-translation of strata which filled and pushed the river southwards at ~1 Ma. 3) Surprise Valley’s Piano Slide (RM 135-135.5R) filled the CR paleochannel when it was 65-70m above modern level and gives a cosmogenic burial age date on buried CR gravels of 0.88 ± 0.22 Ma (1σ); Pancho’s Radical Runup (RM 136-137L) was an intact bedrock slide that crossed from N to S across the CR paleochannel (at 61m) and gives a cosmogenic burial age of 0.98 +/- 0.21 Ma (1σ). 4) Two younger Deer Creek slides (RM 136.3-136.6R & 136.7-137.2R) separately diverted Deer Creek and CR paleochannels when they were a few meters above their current levels; this event may be associated with fine-grained lake deposits dated at ~21 ka by OSL/IFSL that are capped or inset by far-traveled CR gravels upstream (RM 134.5). Inset detachment relationships and decreasing paleo-channel heights all suggest downstream younging of landslide events. Fine-grained deposits and a lack of thick gravel packages above landslide debris suggest rapid fluvial-lacustrine aggradation followed by dam dismantling with the arrival of river-transported gravels. Rivers re-incised into bedrock, having circumvented debris dams. Explanations for these multiple landslides include high-volume N-Rim spring discharge, weak horizons, and micro-seismicity, but likely also involved continued reactivation of unstable landslide debris.