PANCHO'S RADICAL RUNUP: NEW INSIGHTS INTO AN UNUSUAL GRAND CANYON LANDSLIDE

Pancho’s Radical Runup is a remarkable deposit of rock that detached from a 600-m-high cliff of Early to Late Paleozoic formations on the north side of the Grand Canyon. The layered rocks disaggregated, crossed the canyon, grazed the top of the Cambrian Tapeats Sandstone that was then exposed at river level, and ran 200+m up the south slope. River gravel, now perched on top of the Tapeats cliff ~60m above the modern Colorado River, shows N-S slickensides on sheared cobbles where the slide passed and gives a new cosmogenic date of 940 ± 240 ka (1σ).

Remains of the runup are five distinctive rock slabs preserved along the south slope between river miles 136 and 137 below Lee’s Ferry. Each slab exhibits similar stratigraphy, implying they were once laterally connected: a basal 1-3-m-thick cushion of entrained river cobbles and heterolithic debris; an overlying 5-10-m-thick interval of brecciated Rampart Cave and Sanup Plateau dolostones, which are distinctive members in local outcrops of sub-horizontal Bright Angel Shale but rest on the south slope as tilted slabs that extend up to 60 m above the in situ dolostones, proving the cross-canyon runup; shattered blocks as much as 20m high of landslide debris that now protect parts of the underlying slabs from slope erosion and are the only other remains of the huge rock mass that once blocked the Canyon.

The cushion of mixed clasts and matrix is resistant to erosion and was likely cemented early by flow processes that pulverized and calcined the carbonate rock, liberated carbon dioxide that may have eased flow, and quickly hydrated to form calcite and other cements that locked in the cushion fabric.

During movement the brittle dolostone members became sandwiched together, pervasively brecciated, and formed the semi-rigid sole of the slide as it flowed across the canyon and reached a minimum of 200m, to the base of the Cambrian Muav Limestone. In transit, the brecciated sole was pressed against the south slope by the overlying slide volume so that breccia fragments were rotated, but not dispersed, and cemented in place early along with the underlying cushion to create the resistant slabs exposed today. The five nearly identical slabs represent an originally intact, probably continuous, 1.5+km-wide dolostone sheet that was brecciated, flowed across the paleo-Colorado River and up the south slope.