EVALUATION OF COSMOGENIC 21NE SURFACE EXPOSURE AGES FROM A MID-LATE PLEISTOCENE ALLUVIAL FAN AND HOLOCENE DEBRIS FLOW, SANTA BARBARA, CA

Detailed neon isotopic analyses were applied to detrital quartz grains from boulders in mid-late Pleistocene Rattlesnake Creek alluvial fan deposits, and in a Holocene debris flow within Rocky Nook and Skofield Parks, Santa Barbara, CA. Terrestrial Cosmogenic Nuclide (TCN) production of ²¹Ne yields atmosphere- and crush- corrected ages that range from 91-390 Ka and 3-341 Ka respectively. Helium analyses for all samples were at or near procedural blank levels, inferring a near lack of radiogenic/nucleogenic production sources (U-Th); thus only cosmogenic and inherited sources of ²¹Ne are indicated. Significant limitations on use of exposure ages result from inheritance of TCN neon from fluvial transport to fan deposition, exhumation prior to final in-situ exposure, and later erosion. To evaluate inheritance, a single oriented boulder was sampled at 2.5cm intervals from 10-36cm in order to obtain a vertical TCN production attenuation depth profile. Preliminary results identify three (and possibly four) apparent exponential attenuation profiles in the rock of 6-10cm thickness each. We infer that each profile represents a deposition-exposure event on the aggrading fan surface. Excess ²¹Ne ranges 2.8e⁶ to 2.3e⁷ atoms/g, with corrected production rates yielding ages from 455 to 55 Ka. Modeled inheritance yields corrected ages for each fan surface exposure to about 90 Ka. Present day exhumation of the fan suggests a 56-139 Ka exposure age. The prior fluvial transport production and multiple depositional exposure events on the fan surface underscore the need for many analyses to decipher the stochastic nature of sediment transport and surface residence times. The youngest age at a location is therefore a maximum age for the surface. The Holocene debris flow exposure ages (3-30 Ka) are not easily resolvable within the analysis of procedural errors.