T258 Records of Quaternary Climate, Hydrology and Landscape Evolution in the Great Basin, USA

Wednesday, 18 October 2023
1:30 PM - 5:30 PM
321

NEW COSMOGENIC 36CL PROFILES AGES CONFIRM AN LGM AGE FOR BLACKHAWK LANDSLIDE, CA, USA

Paper number: T258-4
Program: Technical Sessions
Time: 2:10 PM – 2:25 PM

Authors

Abstract

The Blackhawk landslide (Lucerne Valley, CA) is one of the best known long-runout rock avalanches in the world, with a constraining radiocarbon date of 21±1 Cal ka reported in a GSA abstract (Stout, 1975) and a field trip report (Stout, 1976). More recent attempts to corroborate this age using cosmogenic surface dating have resulted in inconsistent and scattered results. We collected two new 3-m depth profiles in the Blackhawk landslide deposit and measured 36Cl in amalgamated clast samples. Both a first-order approximate analysis of the depth-concentration profiles, and a new Markov-Chain Monte Carlo (MCMC) inversion model, support an age identical to the older radiocarbon determination. Our best independent model estimates for the two profiles are 19 ka (95%CI 14-22 ka) and 24 ka (95%CI 19-28 ka), respectively, with the difference explained by ~10-20 cm of differential postdepositional erosion. We demonstrate how MCMC inversion of whole cosmogenic depth profiles can illuminate the history of difficult to date Pleistocene deposits, even where rock type prohibits using 10Be.

Geological Society of America Abstracts with Programs. Vol. 55, No. 6

doi: 10.1130/abs/2023AM-394246

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