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

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.