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ABSTRACT: High-Resolution Magnetostratigraphy of the Petroleum-Bearing Miocene Monterey Formation of California in Horse Canyon, Salinas Basin, California

OMARZAI, SHERAZ KHAN, and ROBERT COE, University of California at Santa Cruz, Santa Cruz, CA, JOHN BARRON and DAVID BUKRY, U.S. Geological Survey, Menlo Park, CA, and MARY LOU COTTON THORNTON and GREGG BLAKE, Unocal Oil and Gas, Ventura, CA

A true understanding of the complex origin of the Miocene Monterey Formation of California depends largely on a comprehensive and high-resolution temporal control. High-resolution age dating has not been possible in the diagenetically altered sections (which constitute more than 95% of this important unit) of the Monterey because they contain no or inadequate age-diagnostic siliceous microfossils. Paleomagnetism is of primary importance for absolute age dating in the Monterey and for this reason, we are conducting a detailed paleomagnetic-biostratigraphic study of a 1000-m-thick, essentially continuously exposed section in and along a stream bed in Horse Canyon, Salinas basin. The age of the section is crudely constrained by limited diatom, nannofossil, and foram biochronologic data. e have collected some 2100 closely inspected, oriented samples from the Monterey dolomites, dolomitic calcareous-phosphatic shales, siliceous shales, and mudstones. Detailed thermal demagnetization of approximately 850 samples reveals the presence of a characteristic remnant component in about 70% of the dolomite and 25-30% of the samples from the other lithologies. We interpret this component as primary, acquired during deposition of the Monterey or shortly thereafter.

Although our analysis is incomplete, the data extracted from the 850 samples analyzed thus far show that distinct zones of both normal

and reversed polarity are present throughout the section. The three magnetic polarity zones (N1-R1-N2) present in the lowermost 148 m of the section can be uniquely correlated to the interval from the uppermost part of Chron 5DN to the Chron 5CN of Harland's standard geomagnetic polarity time scale. This correlation and the extrapolation of apparent average sediment-accumulation rate, calculated for the 10-148 m stratigraphic interval, to the base of the section indicate that the deposition of the Monterey Formation in the Horse Canyon section commenced at about 17.65 Ma. The longest and distinct Chron 5BR of the middle Miocene age is present above the 148 m level; however, its younger boundary remains to be tightly constrained through further analysis of closely spaced samples in the 148-450-m interval. Comparison of the mean directions obtained from our best samples to the expected direction for the field area in the middle Miocene indicate no vertical axis rotation relative to stable cratonic North America.


AAPG Search and Discovery Article #91016©1992 AAPG-SEPM-SEG-EMD Pacific Section Meeting, Sacramento, California, April 27-May 1, 1992 (2009)