Evidence of Pleistocene Climate-Driven Alluvial Fan Activity in the Mojave Desert and Southern Great Basin, USA

Cyr, Andrew; Miller, David; Menges, Christopher; Schmidt, Kevin; Mahan, Shannon; Maher, Katherine; and Liu, Tanzhou
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It has been suggested that alluvial fans are good archives of climate information, especially in semi-arid landscapes where even minor climate events may precipitate large changes in geomorphic processes and/or process rates. Indeed, a large body of soils-geomorphic correlations across the Mojave Desert and southern Great Basin, supported by a growing population of numerical ages, indicates that (i) past alluvial fan aggradation may have been synchronous across several climate zones and tectonic environments and (ii) this synchroneity is likely to have been triggered by large climate drivers. A comparison of the timing of Holocene alluvial fan activity has proved moderately successful at identifying these driving forces. However, data that demonstrate similar mechanisms in the middle and late Pleistocene have proved elusive. Were there periods of regionally synchronous alluvial fan activity in the Mojave Desert and southern Great Basin, and what might have been the primary trigger of that activity?

In order to address this question, we compiled nearly 200 Pleistocene alluvial fan ages determined by a variety of chronometric methods, including over 30 sets of re-scaled cosmogenic 10Be exposure ages, across the Mojave Desert and southern Great Basin. This compilation provides an opportunity to examine the timing and nature of regional drivers of alluvial fan activity. We find at least three major periods of alluvial fan activity, ~930-500 ka, ~235-125 ka, and ~105-20 ka, with as many as seven shorter-term pulses during the latter two major phases. Some of these periods of alluvial fan activity correspond to glacial stages, however, our ability to make causal links between specific pulses of fan activity and major climate cycles is limited by the resolution of the data set. This is likely the result of several factors, including an increasingly sparse occurrence of alluvial fan units, and decreasingly precise age control of those units, in the older part of the stratigraphic record. Despite these limitations, these data are broadly consistent with conceptual models of how alluvial fans in semi-arid landscapes respond to changes in the amount of surface water available to do drive geomorphic processes.

AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013