MIDDLE TO LATE PLEISTOCENE ALLUVIAL DEPOSITS AND ENVIRONMENTAL CHANGE, GRAND STAIRCASE, SOUTHERN UTAH

Middle to Late Pleistocene alluvial deposits drape erosional benches in the Grand Staircase region of southern Utah. These piedmont gravels, terrace deposits and alluvial fills form dominant landforms within Colorado River tributaries upstream of Grand Canyon. Piedmont gravels and terrace deposits record base-level driven changes in channel gradients and climate-driven changes in sediment supply. In at least one location, alluvium preserves one of the oldest mammoth and plant macro-fossil sites in Utah (130-160 ka). Geomorphic mapping indicates three dominant piedmont and terrace levels in the region. Underlying deposits from these landforms include alluvium dating to ~ 50ka, 70-90 ka, 100-120 ka, 160-190 ka and >250 ka. These alluvial records are discussed in relationship to regional terrace chronologies and climate records to gain a broader understanding of incision rates and geomorphic change in the central Colorado Plateau.