DIFFERENTIATING HYDROCLIMATES USING ALLUVIAL FAN GEOCHRONOLOGY IN THE VIZCAINO DESERT, CENTRAL BAJA CALIFORNIA, MEXICO

Abstract

Alluvial fans are abundant geomorphic features in arid regions and can provide important information about past hydroclimatic changes. The large alluvial fans of the Vizcaino Desert in central Baja California, Mexico, are of particular interest because they can be used to infer paleoclimatic conditions in this sensitive, yet understudied region. Four main Quaternary alluvial fan units (Q1 through Q4) were identified based on their morphostratigraphic relationships and relative soil development. Optically stimulated luminescence, 14C, and 10Be geochronology were used to constrain the depositional ages of the alluvial units. Two main pulses of alluvial aggradation were identified in the Vizcaino Desert, one around 125 ka (Q2) and another around 20 ka (Q3). The fans display low-gradient surfaces and sedimentary features indicative of upper flow regime conditions, such as horizontal-planar bedding and low-angle cross bedding. These sedimentary features suggest that their formation was the result of extreme runoff under intense storm activity related to either tropical cyclones or long-lived atmospheric rivers that dominated the hydroclimate of the region. The lack of precessional cyclicity in fan aggradation observed elsewhere in the peninsula may indicate a nonlinear geomorphic response. The timing between main aggradational pulses may reflect the time needed to accumulate sediments on rocky hillslopes in this arid region. This latter possibility is consistent with Holocene fan activity (Q4), when incision of the fan surfaces dominated, and deposition occurred in small volumes within channels.