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An Alluvial Surface Chronology Based on Cosmogenic36Cl Dating, Ajo Mountains (Organ Pipe Cactus National Monument), Southern Arizona

Published online by Cambridge University Press:  20 January 2017

Beiling Liu ,
Fred M. Phillips ,
Molly M. Pohl  and
Pankaj Sharma
Beiling Liu
Affiliation:
Department of Geoscience, New Mexico Tech, Socorro, New Mexico, 87801
Fred M. Phillips
Affiliation:
Department of Geoscience, New Mexico Tech, Socorro, New Mexico, 87801
Molly M. Pohl
Affiliation:
Department of Geography, Arizona State University, Tempe, Arizona, 85287
Pankaj Sharma
Affiliation:
PRIME Lab. Physics Department, Purdue University, West Lafayette, Indiana, 47907
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Abstract

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 of36Cl 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 had36Cl 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.

Type
Research Article
Information
Quaternary Research , Volume 45 , Issue 1 , January 1996 , pp. 30 - 37
Copyright
University of Washington

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