ZIRCON U-PB DATING OF MONTEREY FORMATION TUFF IN THE PALOS VERDE PENINSULA OF LOS ANGELES

The middle to upper Miocene Monterey Formation and stratigraphic equivalents are widespread in coastal California and around the North Pacific. Monterey-like facies are exceedingly heterogeneous and pose fundamental questions with respect to depositional environments, local versus global controls of lithology and stratigraphy, and correlation. Variably altered siliceous tuff including bentonite beds are a widespread component of the Monterey and relatively unaltered tuff has been radiometrically dated in prior work by K-Ar and ⁴⁰Ar/³⁹Ar methods. Here we report a new zircon U-Pb age from altered Monterey Formation tuff of the Palos Verde Peninsula which demonstrates the usefulness of zircon dating in refining the chronostratigraphy of the Monterey Formation. Laser Ablation Inductively Coupled Plasma Mass Spectrometry analysis of zircon from a diagenetically altered tuff near the boundary of the tuffaceous and cherty lithofacies on Palos Verde yields a well determined age of 14.00±0.15 Ma with a mean square weighted deviation of 1.06. Slightly older zircons in the 15-16 Ma range are interpreted as older recycled grains. This result is consistent with a previously determined plagioclase K-Ar age of 14.0±1.1 Ma from the Portuguese tuff bed deeper in the section and thus demonstrates the utility of zircon dating in the Monterey Formation, especially in the altered tuffs that are so prevalent.