Dating of Pliocene Colorado River sediments: Implications for cosmogenic burial dating and the evolution of the lower Colorado River

Ari Matmon*, Greg M. Stock, Darryl E. Granger, Keith A. Howard

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

22 Scopus citations

Abstract

We applied cosmogenic 26Al/10Be burial dating to sedimentary deposits of the ancestral Colorado River. We compared cosmogenic burial ages of sediments to the age of an independently well-dated overlying basalt flow at one site, and also applied cosmo genic burial dating to sediments with less precise independent age constraints. All dated gravels yielded old ages that suggest several episodes of sediment burial over the past ~5.3 m.y. Comparison of burial ages to the overlying 4.4 Ma basalt yielded good agreement and suggests that under the most favorable conditions, cosmogenic burial dating can extend back 4-5 m.y. In contrast, results from other sites with more broadly independent age constraints highlight the complexities inherent in burial dating; these complexities arise from unknown and complicated burial histories, insufficient shielding, postburial production of cosmogenic isotopes by muons, and unknown initial 26Al/10Be ratios. Never the less, and in spite of the large range of burial ages and large uncertainties, we identify samples that provide reasonable burial age constraints on the depo si tional history of sediment along the lower ancestral Colorado River. These samples suggest pos sible sediment deposition and burial at ca. 5.3, 4.7, and 3.6 Ma. Our calculated basinwide erosion rate for sediment transported by the modern Colorado River (~187 mm k.y.-1) is higher than the modern erosion rates inferred from the historic sediment load (80-100 mm k.y.-1). In contrast, basinwide paleo-erosion rates calculated from Pliocene sediments are all under 40 mm k.y.-1 The comparatively lower denudation rates calculated for the Pliocene sediment samples are surprising given that the sampled time intervals include significant Pliocene aggradation and may include much incision of the Grand Canyon and its tributaries. This conflict may arise from extensive storage of sediment along the route of the Colorado River, slower paleobedrock erosion, or the inclusion of sediments that were derived preferentially from higher elevations in the watershed.

Original languageAmerican English
Pages (from-to)626-640
Number of pages15
JournalBulletin of the Geological Society of America
Volume124
Issue number3-4
DOIs
StatePublished - Mar 2012

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