MULTI-STAGE EVOLUTION OF GRAND CANYON'S SURPRISE VALLEY LANDSLIDES AND THEIR INTERACTION WITH CARVING OF GRAND CANYON

The Grand Canyon contains several examples of composite gravitational back-rotated landslides with listric detachment surfaces that sole into shale horizons and form Toreva-style slumps. Landslides in the vicinity of Surprise Valley (river mile (RM) 135) partially blocked and diverted the Colorado River, Tapeats Creek and Deer Creek in several stages. The sequence of landslides is deciphered using a) the heights of buried straths that were abandoned during Colorado River incision and b) new cosmogenic burial ages.

Landslides seem to have advanced downstream over the past ~2-3 million years: 1) The first and oldest event included the main Surprise Valley slide (RM 135), which detached on a sub-horizontal surface that outcrops near Thunder River spring and ultimately blocked a paleo-Tapeats Creek channel, preserved in modern Bonita Creek Canyon at a height of ~ 250 m above river level; 2) A second event pushed the Colorado River to the south where a filled paleo canyon is present at ~150 m above river level near RM 133; 3) A slide east of Deer Creek (RM 135) filled a Colorado River channel 70 m above current river level and has a new cosmogenic burial age of 940 ± 240 ka (1σ); 4) Poncho’s Radical Runup (RM 136.6), west of Deer Creek occurred when the Colorado River was at a height of about 60 m; a new cosmogenic burial date for gravels under the slide give an age of 978 ± 287 ka (1σ); 5) Two slides occurred in the vicinity of Deer Creek (RM 136.2), diverting it at least twice resulting in two straths: one ~100 m above river level and 6) another just above modern river level.

Future research goals include more precise mapping, further cosmogenic burial dates at several levels, and U-series and U-Pb dating of carbonates within detachment surfaces and perched paleo-river gravels. These should yield improved understanding of the evolution of landslide events, their influence on Colorado River geomorphology, and the mechanics of Surprise Valley detachments.