Beiling Liu, Fred M. Phillips, Molly M. Pohl, P. Sharma

Quaternary Research 45 (1996) 30-37

A chronology of alluvial surfaces on piedmont slopes below the western Ajo Mountains, southern Arizona, has been obtained using cosmogenic ³⁶Cl accumulation and AMS radiocarbon dating. The apparent ³⁶Cl ages of individual boulders range from 520,000 to 13,000 yr, and the ¹⁴C ages of organic material in the two young terraces are 2750-2350 and 17,800 cal yr B.P. The sequence of ³⁶Cl ages is consistent with the apparent stratigraphic order, but groupings of similar ages for different surfaces appear to result from repeated reworking of older surfaces associated with the deposition of younger ones. The youngest surface gave a distribution of ³⁶Cl ages about 30,000 yr older than the ¹⁴C and soil ages; however, this distribution had ³⁶Cl ages that overlapped with ³⁶Cl ages from active channels and hillslopes. We attribute the older-than-expected exposure ages of sampled boulders to inheritance of ³⁶Cl while residing near the surface during very slow erosion on the mountain front. Our results show that although cosmogenic nuclide accumulation can help establish chronologies for surfaces in piedmont settings, care must be used in evaluating the effects of complex exposure histories.