SAND DUNES EXERT CONTROL ON ALLUVIAL BASE LEVEL AND SOIL FORMATION, EASTERN MOJAVE DESERT, USA

The formation of the Kelso dunes in the eastern Mojave Desert, CA, was a landscape changing event. Geochemical data and luminescence ages support the hypothesis that the Kelso dunes were derived from increased sediment supply following the incision of Afton Canyon and the formation of Intermittent Lake Mojave I (~22 ka), reinforced with local sources of sand. Migration of the sand dunes to the east occurred along a well-defined sand transport corridor. Rapid accumulation of the sand created temporary blockages of Kelso Wash and other large washes draining the Providence and Granite Mountains resulting in substantial aggradation of mixed eolian and fluvial sediments. This aggradation event corresponds to a regional pulse of alluvial fan sedimentation that occurred about 14-8 ka. Geochronology and stratigraphic evidence suggests that breaching of the dune dams and incision through aggraded sediments occurred when another major pulse of alluvial fan sedimentation occurred about 6-2 ka. A dune dam along Kelso Wash resulted in landscape stability downstream allowing formation of a moderately developed soil (7.5 YR hue, Btk horizons, stage 2 carbonate morphology) on abandoned fluvial deposits. This soil is not present in areas where the dune dams fostered rapid sediment accumulation or in areas mantled by dunes. The presence of this buried soil and its formation between 15 to 6 ka also indicates that a part of the sand transport corridor was relatively sand free during this time – no pulse of aeolian sand from Lake Mojave II migrated to this portion of the landscape. Aeolian system state theory has suggested that aeolian activity in the Mojave Desert is closely tied to enhanced sediment supply, primarily related to the Mojave River/Lake Mojave system. Our data suggests that the sand supply was largest when Intermittent Lake Mojave I formed. A large increase in sediment supply and availability impacted the geomorphology on a regional scale by providing sand that blocked drainages and altered the regional and local base level. Dune dams withstood a major episode of alluvial fan sedimentation and likely persisted for 10 kyr until the next major episode of alluvial fan activity (<6ka), as drainages readjusted to the new base level. Successive lake forming events had a smaller impact on the regional geomorphology.