FALLING DUNES ON THE NAVAJO NATION, NORTHEASTERN ARIZONA:

Throughout the Quaternary history of the American Southwest, particularly in northeastern Arizona, periods of aeolian sediment mobility have occurred in response to changes in climate. The Quaternary history of aeolian sediment transportation and deposition is poorly documented in this region, especially with regard to landscape response to changing climate conditions on glacial timescales. Much of the aeolian sand currently mantling northeastern Arizona is believed to have been derived under glacial conditions although previous regional aeolian chronologic studies have not yet confirmed this hypothesis. In sediment transport applications, the optically stimulated luminescence (OSL) dating technique measures the length of time since a grain was buried. OSL dates in periodically active dune forms often reflect only the most recent episodes of deposition. Dating of older periods of activity becomes challenging because the repeated exposure to sunlight during periods of dune activity reset the OSL ages. Approximately one third of northeastern Arizona’s Navajo Nation is mantled by a wide variety of stable and active aeolian dune forms including linear, transverse, parabolic, coppice, sheet, ramp, climbing, and falling deposits. Falling sand dunes form in canyons and in the lee of other topographic obstructions such as mesas and buttes. These dunes may retain the history of aeolian activity because they are less vulnerable to reworking during periods of increased dune activity inherent to periodically active dune systems. Significant regional work by John T. Hack from the early 1940’s proposed that linear and falling dunes presently stabilized by piñon and juniper forests were deposited under glacial conditions. This study presents ten new OSL ages from several falling dunes in northeastern Arizona that confirm significant aeolian deposition occurred in northeastern Arizona approximately 50 – 70 Ka B.P., during marine isotope stage 4.