DATING DUST: USING OSL TO CONSTRAIN THE TIMING OF SEDIMENT ACCUMULATION ON HILLSLOPES IN THE MOJAVE DESERT

An east-west oriented drainage basin in the upper elevations of the eastern Mojave Desert provides the opportunity to study aspect controls on slope geomorphology and causes of sediment accumulation on hillslopes. The north-facing hillslopes of the drainage basin are marked by colluvium which supports growth of warm season perennial grasses. The south-facing slopes have discontinuous patches of colluvium that support less grass. The colluvium is derived from two separate sources: the coarser clasts from weathering bedrock (meta quartz-syenite) and the finer material deposited on coarse colluvium by wind. There is minimal chemical weathering of the coarse clasts, indicating that the majority of the fine sand and silt is eolian.

The timing of deposition of the eolian material can be dated by optically stimulated luminescence (OSL). Three sediment samples were collected from a 1.5-meter-thick section of exposed colluvium at depths of 20, 60, and 95 centimeters on the south-facing slope. OSL samples were collected at night because the metal tube could not penetrate the rock colluvium. We also sampled a mid Holocene terrace 3 meters above the modern channel, which also has a dust component to further test the reliability of method. Preliminary results indicate that colluviation on both north- and south-facing slopes occurred in the Pleistocene and that dust is filling and mixing in pore spaces of colluvium. The grain size of the fine-grained sediment is dominated by fine sand and silt with modes at 75, 200, and 500 microns. OSL samples were sieved to 50-150 micron and 150-250 micron sub samples, and OSL analysts performed for the different size fractions. The age of the 50-150 micron sediment is ~6-12 ka, and the age of the 150-250 micron sediment is ~15-20 ka. Young ages at depth show that both fine sand and silt are translocated into the B horizons. Ages of different modes suggest changes in dust source at different depositional times. The more xeric conditions on the south-facing slopes limit vegetation cover. During precipitation events, there is more overland flow and erosion. Stripping on south-facing slopes has increased the amount of bare bedrock, further enhancing overland flow and erosion.