TESTING THE MONTEREY HYPOTHESIS: A NEW AGE AND STRATIGRAPHIC MODEL FOR THE MIOCENE MONTEREY FORMATION ALONG THE CENTRAL COAST OF CALIFORNIA

The Monterey Formation has long served as a natural laboratory for investigating Miocene climatic change and organic matter production and preservation, with strata spanning much of the Miocene Epoch and outcropping extensively along the Central Coast of California. Hypotheses linking the deposition of organic matter and phosphorites in the Monterey Formation to global cooling of the Middle Miocene Climate Transition (MCCT) have been proposed on the basis of limited biostratigraphic and chemostratigraphic correlations to global C and O isotope records, and invoke a variety of climatic and oceanographic teleconnections in relation to both organic burial and phosphogenesis. However, the timing and synchronicity of organic preservation and phosphogenesis in the Monterey Formation has remained uncertain due to a paucity of radiometric geochronological constraints. Here, we present over 24 new U-Pb LA-ICPMS zircon ages from volcanic ash horizons throughout the Monterey Formation. We couple these ages with measured stratigraphic sections, biostratigraphy, magnetostratigraphy, and new C and O chemostratigraphy to build a new age model for deposition of the Monterey Formation along the Central Coast. The new age model demonstrates that the deposition of the most organic-rich members of the Monterey Formation postdates the MCCT, and that preservation of organic-rich and phosphatic strata is diachronous in the basin. The new stratigraphic model highlights the importance of local paleotopography for effectively depositing and preserving organic matter and concentrating phosphorous within a tectonically-active borderland basin. We conclude that from both a timing and mass-balance perspective, organic burial in the Monterey Formation and correlative units in the eastern Pacific cannot be responsible for the MCCT. These data provide a temporal framework to more directly compare the Monterey Formation to other records globally and further test potential drivers of Miocene climate change.