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Preliminary Paleomagnetic Results from Miocene Monterey Formation, Shell Beach, Pismo Basin, Central California

Sheraz M. Khan, Robert S. Coe, John A. Barron

An excellently exposed 300-m thick section of the Monterey Formation at Shell Beach, Pismo basin, has been sampled at intervals of 2-5 m for paleomagnetic analysis. This study is part of an ongoing research aimed at providing detailed magnetostratigraphy and geochronology of the Monterey Formation sections from northern and central California. Here we report on the detailed magnetostratigraphy and diatom biostratigraphy of the upper 150 m of the Shell Beach section, which spans a 2-m.y. long interval in the upper middle Miocene (upper lower Mohnian). Our preliminary data reveal two distinct components of magnetization in about 75% of the dolomite samples and 40-60% of the samples from calcareous to siliceous shales and cherty porcelanitic rocks. The first of these compone ts (a), which is removed at T <= 200°C, is presumably recent viscous magnetization. The thermally discrete second component (b) had dual polarity and stable and consistent directions with a relatively narrow unblocking temperature range (200°-400°C). Usually this component remains blocked on heating to 450°-500°C and is then masked by unusually high intensity components of magnetochemical origin produced during heating in the lab. In our best samples, orthogonal vector diagrams show that "b" is the early characteristic remanence, and all evidence to date suggests that this component is pre-Pliocene-Pleistocene folding and most likely primary.

By using diatom data extracted from our samples, we are attempting to obtain detailed magnetostratigraphy for this section of the Monterey Formation.

AAPG Search and Discovery Article #91035©1988 AAPG-SEPM-SEG Pacific Sections and SPWLA Annual Convention, Santa Barbara, California, 17-19 April 1988.