EVIDENCE FOR DUNE DAMMING AND LAKE FORMATION ALONG THE EASTERN MARGIN OF THE KELSO DUNES, MOJAVE DESERT, USA

Ephemeral washes blocked by dunes play a major role in modifying local hydrology. In the Mojave Desert, ephemeral washes that drain sediment from the Providence and Granite Mountains are intercepted by the eastern margin of the Kelso Dunes. Temporary blockages of these washes by the dunes ultimately formed a lacustrine environment. Multiple exposures were described along an incised channel and samples were collected for grain size, elemental composition, carbonate and salt content. The exposures revealed a fining-upward sequence composed of dune sand, rhythmically bedded silts ranging from 10 cm to 30 cm thick, a clay bed about 20 cm thick, and capped by alluvial fan gravel. The clay bed contains ~60% clay-sized particles, suggesting standing water for a long enough time to result in the suspension deposition of clay. The fan gravels capping the silt and clay beds are correlative with an alluvial fan terrace from the Providence Mountains. The terrace underlies ca. 8 ka dunes suggesting the lake sediment and fan gravels were deposited at the Pleistocene-Holocene transition, consistent with the timing of regional sedimentation. Evidence suggests that the only reasonable way to form a lake in this geomorphic setting is by the formation of dune dams. Dam failure and incision occurred sometime before the mid-Holocene based on the presence of additional inset alluvial fan gravels. These sediments may be related to additional thick accumulations of sediment downstream along Kelso Wash that are interpreted to be related to dam failure.