PANCHO'S RADICAL RUNUP: EVENT SCENARIO

Main remnants of Pancho’s Event are five similar rock slabs distributed along the south slope. Each has three layers, indicating they were originally part of a continuous 1.3-km-wide sheet: (1) a basal polymict cushion, as much as 3 m thick, plastered on the south slope; (2) a middle brittle brecciated dolostone, as much as 6 m thick, comprised of two members that were originally separate within the BAS; (3) overlying shattered remnants of the original slide mass. The combined middle dolostone interval extends across and above its equivalent south slope stratal positions, thus it must have originated from the north slope.

The working scenario accounts for known characteristics of the Runup: (1) slide was initiated in the BAS by north slope listric detachment and rotation; (2) detached ~600 m rock column drove landslide across canyon; (3) dolostone members knifed through the BAS, and conjoined into a thin sheet; (4) brittle dolostone sheet was brecciated in early slide phases; (5) dolostone clasts vibrated and evolved into a granular fluidized flow (sturzstrom), with little clast rotation or dispersal; (6) slide mass translated across paleoriver and grazed top of Tapeats Sandstone near base of gorge, leaving slickensides; (7) fluidized dolostone flowed up south slope and carried main slide volume; (8) vibrating dolostone was partially powdered, heated, and dehydrated to quicklime; (9) released CO² added bouyancy to flow; (10) breccia sheet stalled and stepped down by 1 to 3-m-thick, gravity-induced, vertical relaxation faults, injected by sedimentary dikes from cushion; (11) quicklime rapidly rehydrated into carbonate cement that fixed the cushion and the stepped slabs in place. Slabs today remain as the most resistant rock along the south slope Runup zone.