HUGE PLEISTOCENE DEBRIS FLOWS THROUGH WATERPOCKET FOLD, UTAH

Oligocene trachyandesite flows, and underneath them weak Paleocene Flagstaff mudstone, form the tops of Boulder and Thousand Lake Mountains. Their high dark cliffs overlook eastward the pale Mesozoic sandstone and shale strata of Waterpocket Fold. Rubble off the mountains contains abundant angular 1–3-m andesite boulders but also 4–6-m ones and fragile Flagstaff clasts floating in a matrix of small very angular clasts. The rubble contains almost no clasts from belts of Mesozoic sandstone the flows crossed. Once thought glacial outwash, the coarse deposits instead seem huge rock avalanches and debris flows run down stream valleys. Blending with river water, they transform into watery floods that flush cobbles and boulders farther down. A strath of crystalline boulders resists erosion far more than bedrock sandstone and shale eroding grain by grain in every storm. Coarse andesitic gravel—former valley floors—caps terraces high above incising Fremont and Escalante Rivers and tributaries, the highest benches inverted into ridgecrests. The andesite being less porphyritic on Thousand Lake Mountain than on Boulder Mountain, the two debris streams can be distinguished where they merge in Fremont valley.

The youngest avalanches lie outside and predate fresh-looking moraines ³He exposure-age dated to about 22 ka (Pinedale). Bouldery rubble just beyond the Pinedale—once thought Bull Lake moraines and till—prove instead to be rock-avalanche levees and diamict carrying great boulders and fragile clasts. Their andesite boulders helium-date variously to 73–111 ka. Many kilometers down the Fremont, bouldery debris flow capping Johnson Mesa 100 m above the river (and once thought Bull Lake outwash) ³He dates to 193 ka. Four divide-capping black-boulder benches 200–230 m above steeply incised Escalante headwaters helium-date to 671 to 1395 ka—probably minimum ages of the debris flows. Since the oldest andesite debris streams, whole drainage basins have incised 300 m into Mesozoic sandstone, thousands of cubic kilometers of rock eroded. These inverted valley floors clearly date back to the early Pleistocene, the highest eastern ones probably to the Pliocene.