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Abstract: Magnetic Stratigraphy of the Refugian-Zemorrian (Upper Eocene-Lower Oligocene) San Lorenzo Formation, Santa Cruz County, California


The San Lorenzo Formation in the Santa Cruz Mountains is the only place in the Pacific Coast which provides an apparently continuous record of the middle Eocene to late Oligocene Narizian, Refugian, and Zemorrian benthic foraminiferal stages. It is also the only place that yields planktonic microfossils that can be used to correlate these strata with the global time scale. Magnetic samples were taken from the Refugian-Zemorrian portion of the Rices Mudstone Member of the type San Lorenzo Formation in San Lorenzo River, north of Riverside Grove. After overprinting was removed, a stable remanence was obtained which passed a reversal test. The latest Narizian, early Refugian, and early late Refugian strata were all of reversed polarity. Based on the presence of P16 planktonic foraminifera in the latest Narizian, we correlate this interval with late Eocene Chron C13r (33.5-34.6 Ma). Latest Refugian strata were of normal polarity, correlated with early Oligocene Chron C13n (33.0-33.5 Ma). The earliest Zemorrian strata are of reversed polarity. Based on the presence of P19-20 planktonic foraminifera and CP18 calcareous nannofossils, these strata represent late Chron C12r (30.8-31.5 Ma). Apparently, a major unconformity of at least 1.2 million years in duration is present at the Refugian/Zemorrian boundary in the San Lorenzo Formation. These data clearly show that the Refugian begins in Chron C13r (34.2 Ma), about 2 million years later than it begins in the type section in the Santa Ynez Range. The Refugian is at least as young as late Chron C13n (33.0 Ma), about a million years younger than the Refugian/Zemorrian boundary in the Santa Ynez Range.

Search and Discovery Article #90945©1997 AAPG Pacific Section Meeting, Bakersfield, California