COSMOGENIC NUCLIDES APPLIED TO DESERT PIEDMONTS: A TECHNIQUE TO DETERMINE NUMERIC AGES FOR CROSS PIEDMONT CORRELATION

Understanding the chronology and magnitude of Basin and Range piedmont deposits is important for unraveling the relative influences of tectonics, climate and local environmental conditions during their deposition and in addressing regional landscape development questions. Cosmogenic radio nuclides (CRNs) can improve on the existing relative chronologies by providing numeric ages for specific deposits, but application of this technique to piedmont deposits is challenging as they require explicit treatment of all three variables in the system: age, local erosion rate and inherited nuclides from previous exposure. New data analysis techniques were developed to determine the age and local erosion rate (allowing inheritance to vary for each sample) of two piedmont deposits in the White Tank Mountains, AZ using 26Al and 10Be CRN depth profiles. The amounts of isotope measured in each sample were modeled as the sum of the spallation, negative muon, and fast muon production pathways, and inheritance. Inheritance was calculated both as two individual variables representing the inherited amount of 26Al and 10Be and in terms of a single variable representing the source area erosion rate. For each sample, all combinations of erosion rate, age and inheritance that produced amounts of isotope within the error range of the sample were identified. These suites of possible solutions for samples within the same deposit were then intersected to identify the most probable age and erosion rate for that piedmont deposit. The most probable solutions are three packages of sediment in the older deposit (1.2 Ma, 2.0 Ma and 2.7 Ma) and two packages of sediment in the younger deposit (0.7 Ma and 1-1.5 Ma). The CRN analysis not only provided numeric ages of deposits that allow for cross-piedmont correlation, but also suggested sedimentary contacts not seen in the field providing a more robust geomorphic understanding of the unit relationships.