GSA QUATERNARY GEOLOGY AND GEOMORPHOLOGY DIVISION FAROUK EL-BAZ AWARD FOR DESERT RESEARCH 2023: REGIONAL CORRELATION OF LATE QUATERNARY ALLUVIAL FANS FROM BAJA CALIFORNIA SUR TO PANAMINT VALLEY, CALIFORNIA

Alluvial fans provide a fundamental record of Quaternary climate and tectonic histories in desert and dryland regions. A continuing question regarding desert regions of the U.S. and Mexico is the linkage between variations in climate and regional alluvial fan stratigraphy. Most current models are based on the model proposed by Bull (1991, Geomorphic Response to Climate Change). Bull’s model largely suggests that regionally synchronous pulses of fan deposition during the Pleistocene-Holocene transition could be attributed to increased sediment flux driven by a combination of decreasing vegetation cover and an increase in monsoonal moisture. Revisions of Bull’s model consider that fan propagation is driven predominantly by either (1) decreases in vegetative cover on basin slopes related to the onset of aridity or (2) pronounced increases in regional rainfall related to an increased frequency of large extreme winter frontal storms and/or East-Pacific tropical cyclones.

Examination of regional alluvial fan records across widely varying geomorphic settings (i.e., elevation, vegetation, precipitation regimes) provides evidence for how climate change related to large storm events may drive fan propagation. Over the past three decades, colleagues and I have constructed a regional framework of alluvial fan stratigraphy across eight fan complexes that extend from the subtropical deserts of southern tip of Baja California Sur (Mexico) to the northern most parts of the Mojave Desert. Local age control is derived from terrestrial cosmogenic nuclide, radiocarbon, and luminescence dating of fan surfaces and deposits. Soil stratigraphy facilitates the local subdivision of fan deposits, supports correlation of fan complexes between contrasting mountain basins, and allows assessment of long-term surface stability. Regional correlations indicate that major periods of alluvial fan deposition occurred approximately at: 0.5-1 ka, 2-4 ka, 4-8 ka, 10-15 ka, 30-40 ka, 60-70 ka, and 100-110 ka. Correlation among contrasting basin geomorphology and vegetation cover implies that (1) regional precipitation patterns drove pronounced periods of fan deposition, and (2) large-scale precipitation events are most likely associated with a combination of extreme winter frontal storms (increasing northward) and East-Pacific tropical cyclones (increasing southward).