Magnetostratigraphy of the Eocene-Oligocene San Lorenzo and Vaqueros formations, Santa Cruz Mountains, California; implications for California biostratigraphic zonations

Abstract

The San Lorenzo and Vaqueros formations in the Santa Cruz Mountains are the only place in the Pacific Coast that provide a superposed record of the middle Eocene to late Oligocene Narizian, Refugian, and Zemorrian benthic foraminiferal stages, and also yields planktonic microfossils that can be used to correlate these strata with the global time scale. Magnetic samples were taken from three stratigraphic sections spanning the entire formation: the type section of the Twobar Shale and Rices Mudstone members of the San Lorenzo Formation at Kings Creek, the thickest section along the San Lorenzo River, and the upper part of the section along Zayante Creek. After overprinting was removed, a stable remanence was obtained which passed a reversal test and fold test. Our results suggest an age of 36.5 Ma for the early/late Narizian boundary, 35.5 Ma for the Narizian/Refugian boundary, 32.5 Ma for the Refugian/Zemorrian boundary, and 30.0 Ma for the early/late Zemorrian boundary, consistent with results reported from other regions. However, the beginning of the Vaqueros Stage is demonstrably time transgressive, with typical Vaqueros molluscs (e.g., Turritella inezana) first occurring as early as 29.5 Ma in the Santa Cruz Mountains, while they first occur at 27.5 Ma in Ventura County, and 17.5 Ma in Orange County, California.