PALEOCLIMATIC CONTROLS ON PLEISTOCENE ALLUVIAL FAN DEPOSITION IN THE MOJAVE AND COLORADO DESERTS AND SOUTHERN GREAT BASIN, SOUTHWESTERN USA (Invited Presentation)

The apparent connection of deposition and incision of alluvial fans to regional climate variability stems from observed regional similarities among surface characteristics and the level of pedogenic development. Earlier work placed similar deposits into a relative chronology and generated hypotheses about long timescale climate drivers (glacial-interglacial cyclicity). More recent work has used numeric ages to examine more specific climate controls (e.g., seasonality or intensity of precipitation). This work has significantly improved our understanding of alluvial fan-climate connections during the Holocene; yet the influence of climate on the deposition of older alluvial fan units, which have significantly greater volumes, remains unexplored. Are there inter-regional climate variations that result in alluvial fans that persist for several tens of thousands of years? Were older fans, especially those ≥0.5 My, built by different processes?

 We have compiled >200 numeric ages, determined by several Quaternary dating methods, from the Mojave and Colorado Deserts and southern Great Basin. These ages form broad clusters, regardless of location or dating method, between 20-85 ka, 100-235 ka, and ~500-600 ka. In younger deposits, which have more precise age control, shorter duration clusters of 20-40 ka, 55-85 ka, and 100-140 ka are evident. Although simple conclusions are precluded by the complexity of the dataset, ages more frequently cluster prior to peak glacial conditions as inferred from proxy δ¹⁸O records. Both the fine structure of the age distribution in younger deposits, and the dearth of numeric ages in older deposits indicate that more work is needed, including improved cross-dating, to more directly address climate drivers of fan deposition. The data also highlight the importance of geologic context, in that many surface and soils-stratigraphic units appear to be composites of a few sub-pulses of fan aggradation. This is evident in areas that have been mapped in detail. However, more areas need to be described in detail to be able to use these sub-pulses to explore ties between climate and fan deposition. Finally, although our compilation shows that fan aggradation is likely connected to periods of wetter climate, the specifics, e.g., seasonality or the role of temperature, remain open questions.