9706046 Prothero The Eocene-Oligocene transition was critical period in Earth history, when the "greenhouse" conditions of the early Eocene were replaced by the "icehouse" conditions of the Oligocene. The mass extinctions and other biotic responses caused by this climatic change have been studied in many places, but the excellent fossil record of molluscs and foraminifera found in California marine strata has been relatively little studied because the chronostratigraphy has been so poorly resolved and calibrated. The middle Eocene to Oligocene marine strata of the Santa Cruz Mountains provide the best superposed record in California of the Narizian, Refugian, and Zemorrian benthic foraminiferal stages that are also calibrated by planktonic microfossils. Preliminary magnetic analysis of the late Narizian-Refugian-early Zemorrian interval in San Lorenzo River yielded results which pass both a fold test and reversal test, and show that the latest Narizian-Refugian correlate with Chrons C13r-C13n (34.5-33.0 Ma), and the early Zemorrian with late Cron C12r (31.5-31.0Ma). Three additional sections in the area will be sampled: one spans the early and late Narizian, the second the early and late Zemorrian, and the third is the type section of the Twobar Shale and Rices Mudstone members of the San Lorenzo Formation. Magnetostratigraphy of the entire Narizian through Zemorrian interval in the Santa Cruz Mountains will greatly help in the correlation and calibration of these long-controversial benthic foraminiferal stages. Such high-resolution chronostratigraphy, in turn, will allow us to correlate events during the Eocene-Oligocene transition in the Pacific Coast with the excellent records in the Gulf and Atlantic Coasts, with faunal and floral changes recorded in terrestrial sections, and with the global record/
