THE SURPRISE VALLEY LANDSLIDE COMPLEX: 3 MILLION YEARS OF ROTATIONAL BEDROCK LANDSLIDING AND RIVER DIVERSIONS IN THE CENTRAL GRAND CANYON

The Colorado River drainage contains numerous examples of rearrangement by cutoff meanders, but few are the result of landslide damming. In the Surprise Valley area of Grand Canyon, we document a sequence of 8 intact, back-rotated bedrock landslides (Toreva blocks), the largest of which dammed the Colorado River, cut off a meander loop through Surprise Valley, and re-routed the river ~1.5 mi south to near its present course. Subsequent slides blocked and diverted the Colorado River and tributaries. Slides likely formed short-lived lakes, evidenced by gravels on top of landslide debris. Speculative triggering mechanisms include wetting of detachment horizons via increased local spring discharge or inundation by water from downstream landslide or lava dams.

We reconstructed the landslide sequence from detailed geologic mapping and restored cross-sections, and inferred ages by measuring landslide base height above the modern strath, coupled to an assumed ~100 m/Ma semi-steady bedrock incision rate derived from local incision studies and calibrated by cosmogenic burial ages on two of the slides. 1) Surprise Valley slide (266 m, ~2.9 Ma) filled an old meander loop and rerouted the river ~1.5 miles to its modern course south of Cogswell Butte. 2) Cogswell West slide (110 m, ~1.3 Ma) dammed an ancient Deer Creek course, diverting it ~0.3 mi west. 3) 133 Mile slide (75 m, ~0.98 Ma), sourced from the south canyon wall, dammed and diverted the river to the north; landslide-mantling gravels here suggest a 770 m spillway elev. 4) Piano slide (70 m, 0.88 ± 0.44 Ma) dammed and diverted the river near the Granite Narrows; paleo-canyon wall outcrops suggest a 715 m minimum spillway elev. 5) Poncho’s slide (61 m, 0.98 ± 0.42 Ma) drove a northerly derived, bedrock runup across and up the south canyon wall to 823 m elev. and dammed the river to 720 m elev. 6) The Tapeats slides (73 m, ~0.73 Ma) fill tributary terraces at 63 and 83 m above Tapeats Creek and diverted the tributary to the east. 7) Patio slide (32 m, ~0.55 ka) was a smaller reactivation of Poncho’s that dammed and diverted both the Colorado and Deer Creek, and resulted in the Deer Creek Falls knickpoint. 8) Backeddy slide (0 m, ~0.23 Ma), another small reactivation of the Poncho’s slide mass, dammed and diverted the river to the south and filled a well-preserved channel across from Backeddy river camp.