Abstract: Paleomagnetism Applied to the Miocene Monterey Formation of California

Sheraz Khan Omarzai, R. S. Coe, J. A. Barron

Monterey Formation of California is a dominantly biosiliceous unit that extends for over 600 km along the California coast. Because of its complex lithologic architecture and proven economic importance, it has been intensively studied. In contrast with an impressive body of geologic data, high-resolution absolute ages of the Monterey rocks are completely unknown. It has long been recognized that highly resolved temporal control for the Monterey is vital to the advancement of our basic understanding of this economically important unit of California and its link(s) with neogene climatic-oceanographic changes of global significance. The rarity of suitably preserved age-diagnostic siliceous microfossils (diatoms) in the majority of the Monterey points to paleomagnetism/magnetostratigraphy as the only reliable means for high-resolution age-dating and long-range intra- and interbasin correlation in the Monterey. Accordingly, we have been conducting a detailed paleomagnetic study of the Monterey sections in five widely separated Neogene basins in coastal California. The results of our study, first of its kind in the Monterey, of the Monterey sections at Ano Nuevo (outer Santa Cruz basin), Point Reyes (Bodega basin), Shell Beach (Pismo basin), Lion's Head (Santa Maria basin), and in Horse Canyon (Salinas basin), clearly demonstrate how paleomagnetism can furnish precise sedimentation rates, ages of unconformities, and high-resolution absolute age data with which to probe the origin of the Monterey and to perceive possible links between its lithologies and two-fold facies divi ion and major global paleoclimatic-paleoceanographic changes and regional tectonic factors that controlled Monterey deposition and its lithofacies evolution. Our data from Shell Beach and Horse Canyon sections marshall quantitative evidence for a link between Monterey facies and middle Miocene global glacio-eustatic changes.

Apart from discussing data from the Lion's Head section (Santa Maria basin) of the Monterey and highlighting the role of paleomagnetism as a powerful age-dating tool providing ages at a level

of resolution well beyond that achieved hereto, this presentation shall also focus on paleomagnetic properties of the Monterey rocks and basin-to-basin difference of these properties.

AAPG Search and Discovery Article #90981©1994 AAPG Pacific Section Meeting, Ventura, California, April 27-29, 1994