GRAVEL ON THE STAIRCASE: MIDDLE TO LATE PLEISTOCENE PIEDMONT DEPOSITS AND ENVIRONMENTAL CHANGE, GRAND STAIRCASE, SOUTHERN UTAH

Middle to Late Pleistocene alluvial deposits drape erosional bedrock benches in the Grand Staircase region of southern Utah. These piedmont gravels form dominant landforms within Colorado River tributaries upstream of the Grand Canyon. Deposits record climate-driven changes in sediment supply, largely during intermediate climates between full glacial and interglacial conditions. Abandonment of piedmont surfaces and bedrock incision occurred episodically, driven by local changes in hillslope and cliff-face sediment supply and ultimately in response to base-level lowering due to the cutting of Grand Canyon.

Geomorphic mapping, soil stratigraphy and luminescence dating indicate three dominant piedmont gravel surfaces with underlying alluvium dating to ~50ka, 70-90 ka, 100-120 ka, 160-190 ka and >250 ka. Piedmont gravels dating to the last glacial maximum (LGM) are conspicuously absent. Regional records from pluvial lakes indicate wetter conditions during the late Pleistocene and glacial advances indicate cooler conditions during the LGM, suggesting a link to sediment transport and supply processes in these mid-elevation piedmont settings (1.6-2.2 km asl). The geomorphic records from five tributaries indicate pulsed episodes rapid incision between longer periods of piedmont gravel deposition. Linkages to regional terrace chronologies and climate records are discussed to gain a broader understanding of mid to late Pleistocene geomorphic change in the central Colorado Plateau.