BLOCKAGE OF BROAD SANDSTONE CANYONS BY EOLIAN SAND AND SUBSEQUENT CUTTING OF SLOT CANYONS IN SOUTH-CENTRAL UTAH FROM ~ 55 TO 42 KA

Runoff from modern convective storms easily sweeps eolian sand accumulations from the floors of Colorado Plateau canyons. New evidence from an area within Grand Staircase-Escalante National Monument indicates that, during OIS 3, dune sand blocked at least four bedrock canyons. Seven slot canyons are cut into the Jurassic Navajo SS along a 7 km-long stretch of the Dry Fork of Coyote Gulch. Two of the slots (“Peekaboo” and “Spooky”) are at the distal ends of south-flowing tributary drainages. These tributary slots lie parallel to and just east of much wider bedrock canyons that contain fills of eolian sand. Based on OSL dating, this sand accumulated 42-48 Ka. A third “blind” slot canyon lies just east of a third tributary (Brimstone Canyon) that lacks a sand fill.

Thick, sandy alluvium dated at 50 ka is widespread in Dry Fork upstream from the three tributaries. Downstream, the main stem of Dry Fork has cut two slots that each lie north of wide, eolian-sand-blocked canyons. Two samples of sand blocking one of these canyons have been dated at 35 and 55 ka.

The geomorphic history was as follows: 1) During an extended arid interval beginning before 55 ka, southerly winds deposited falling dunes that blocked the main stem of Dry Fork. 2) As alluvial sand started to accumulate behind the blockages, wind moved much of this sediment up the three side canyons, eventually blocking them. 3) When runoff eventually increased (after 42 ka), Peekaboo, Spooky, and the main stem of Dry Fork spilled over low points of their bedrock walls upstream of the sand dams, and rapidly cut slot canyons that bypassed the blockages. Brimstone also spilled over its wall and started cutting a slot, but then spilled over its sand fill, abandoned the incipient slot canyon, and swept away the blocking eolian sand.

North American speleothem records indicate rapid cooling at 55 ka. Cooling likely diminished convective storms and allowed blockage of these canyons by dune sand. A subsequent increase in the frequency of intense runoff events after 42 ka, combined with the diversion of flow over steep but unchannelized bedrock surfaces, led to a brief and unusual episode of rapid canyon cutting. This work illustrates a specific mechanism by which climate change can induce river incision, and conversely how information on climate may be recorded in the morphology of erosional landscapes.