GEOMORPHOLOGY AND COSMOGENIC EXPOSURE AGES OF VOLCANIC BOULDER DEPOSITS CAPPING MESAS HIGH ABOVE THE NORTHERN TRIBUTARIES OF THE ESCALANTE RIVER, SOUTH-CENTRAL UTAH

Coarse volcanic boulder deposits cap high mesas above the northern tributaries of the Escalante River in south-central Utah. These deposits are composed of 99+% volcanic clasts (trachyandesites) derived from the southern margins of Boulder Mountain and Aquarius Plateau. The large boulder sizes (2+m b-axis clasts are common), dominant volcanic lithology, and exposed stratigraphy of these deposits suggest they are debris-flow run-out (hyperconcentrated flow) deposits that have been fluvially reworked. Soils forming in the deposits have thin Av horizons and significant pedogenic carbonate accumulations ranging from stage II-III. The surfaces of the deposits are covered with Pinyon-Juniper vegetation, locally derived eolian sands, and a mixture of volcanic clasts and broken and overturned pedogenic carbonate rinds which likely formed in a soil that has since been stripped away. The highest of these bouldery deposits sit 140–230 m above the modern drainages, indicating considerable incision since deposition. Cosmogenic ³He exposure ages of boulders exposed on the modern tread of four of these deposits range from 400–1200 ka. On most of the surfaces the highest (tallest) boulder we sampled yielded the oldest exposure age. The soil and exposure age data suggest that these deposits have been significantly eroded and possibly aggraded (eolian, pedogenic) at times in the past and that the exposure ages are minimum age estimates for deposition. Using the oldest exposure age from each surface we estimate long-term incision rates of 170−330 m Ma^-1 for the northern tributaries of the Escalante River.