SURFACE ANALYSIS USING BOTH COSMOGENIC NUCLIDE DATING AND AIRBORNE LASER SWATH MAPPING (ALSM) ON OFFSET ALLUVIAL FANS ON THE CALICO FAULT, MOJAVE DESERT, CALIFORNIA

Both relative and absolute surface dating techniques are imperative to understanding the dynamics of depositional environments. Cosmogenic nuclide dating techniques require an understanding of post-depositional evolution. Our goal is to compare the different errors associated with sampling different targets. Boulders likely offer the best age estimate because transportation is minimal compared to the desert pavement sediments. On aging alluvial fan surfaces, where bar-swale smoothing is apparent, the sediment moves from topographic highs, exhuming sediment less exposed to cosmic radiation, to topographic lows, where more exposed sediment is buried. The desert pavement age may be under-estimated due to erosion and exposure history, but may be skewed towards the actual age because of near-surface ingrowth. We are able to get better error estimates by comparing boulder ages to those of the desert pavement. This problem can be attacked from two different directions: (1) direct age measurements come from careful sampling of quartz monzonite and basalt boulders and desert pavement on one of our chronosurfaces. We used cosmogenic nuclide dating techniques, sampling both large boulders (up to 1.5 meters) and desert pavement with large pebbles/small cobbles (4-8cm). Boulders from the modern wash were also sampled to constrain inheritance from exposure to pre-depositional cosmic radiation. Ages from the quartz monzonite are determined by measuring the ¹⁰Be concentration, and from the basalt by measuring ³He. (2) The smoothing rate of the surfaces may be extracted from surface topography analysis of Airborne Laser Swath Mapping (ALSM, also called airborne LIDAR) data. Initial spectral analysis gives a fairly fast smoothing rate of bar-swale topography, which is predicted to move the desert pavement ages closer to the true age. The bar-swale topography has an amplitude of approximately one meter and a wavelength of 5-7 meters. A comparison between ALSM data and differential GPS, with sub-centimeter resolution, was made to constrain the reliability of the ALSM data. Given our results, future work would be to create a bar-swale diffusion numerical model and develop theoretical errors/offset associated with desert pavement ages so we can develop better sampling techniques for desert pavements.