THE ALLUVIAL DEPOSITS OF LOWER WAHWEAP CREEK: LINKING CLIMATE AND INCISION RECORDS IN THE CENTRAL COLORADO PLATEAU

Alluvial terrace deposits in the Colorado River basin record evidence for past hydroclimate change in the Rocky Mountain headwaters and regional river incision following cutting of the Grand Canyon. Wahweap Creek in southern Utah contains a long terrace record that includes a unique fine-grained alluvial fill containing Middle Pleistocene megafauna and botanical remains dating to Marine Isotope Stage 6 (MIS 6, 190-130 ka). Research presented here uses geomorphic mapping and optically stimulated luminescence (OSL) dating to document the alluvial stratigraphy of lower Wahweap Creek and assess the relative influences of climate and base-level fall on the alluvial records of this tributary to the Colorado River.

Field mapping and stratigraphic descriptions have identified five alluvial terraces (Qat1-Qat5) deposited in the last ~0.5 Mya. Major periods of aggradation occurred at >250 ka (age control to be determined), 160-130 ka, ~100 ka, 60- 40 ka, and 20-10 ka. The megafauna-bearing fine-grained alluvial facies of the fourth-highest terrace on-laps and fills paleo-bedrock channels and topography, suggesting at least 30-50m of aggradation during MIS 6. The terrace chronology along Wahweap Creek is similar to the terrace record of the Paria and Colorado Rivers downstream at Lee’s Ferry. Linkages to regional fluvial records and spring deposits of the Las Vegas Formation suggest the Wahweap alluvial deposits coincide with regionally wetter periods and rates of incision are linked to base-level fall along the Colorado River.