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Magnetostratigraphy: An Excellent Tool for Much-Needed Numerical Age-Dating in the Monterey Formation of California

OMARZAI, SHERAZ KHAN, and ROBERT COE, University of California, Santa Cruz, CA, and JOHN BARRON, U.S. Geological Survey, Menlo Park, CA

Numerical age-dating and long-range intra- and interbasin correlation have been hampered in the petroleum-bearing Miocene Monterey Formation of California. This stems mainly from the poor preservation of age-diagnostic siliceous microfossils, the majority of which were totally destroyed by wholesale diagenesis during burial. Paleomagnetism offers a powerful stratigraphic tool of great promise for absolute age-dating in the Monterey and, for this reason, we are conducting a detailed study of the Monterey in five widely separated basins in coastal California. The results of our study of the Monterey at Shell Beach (Pismo basin) and Point Ano Nuevo (outer Santa Cruz basin), which is the only study that provides precise numerical ages for the Monterey rocks, clearly confirm the potential hat paleomagnetism and magnetostratigraphy hold toward improving our basic understanding of the Monterey Formation.

Our high-resolution magnetic polarity stratigraphic data from the Shell Beach section, which are constrained by limited diatom biochronologic control, and which pass both fold and reversal tests, show that the 290-m-thick Monterey section at Shell Beach was deposited between approximately 15.20 Ma and 11.0 Ma at an average post-compaction sediment accumulation rate of 96 m/m.y. These data result in the recognition of nine normal and eight reversed polarity zones, two of which are defined with an average resolution of about 35 Ky. This level of resolution, which is critical, for example, for accurate sediment accumulation rates and duration of sedimentary cycles in the Monterey, is certainly not offered by any other method of age-dating in the Monterey. Our data also provide rather int iguing but tentative links between lithologic changes within the Monterey and global Miocene oceanographic-climatic events that seem to have left their memory in the Monterey rocks at Shell Beach.

The results of our recently completed study of the 140-m-thick Monterey section at Ano Nuevo in northern California show that this section of the Monterey was deposited between 14.87 Ma and 13.73 Ma at a constant post-compaction sediment accumulation rate of 123 m/m.y. We are currently analyzing some 2200 oriented Monterey samples from the 1000-m section in Horse Canyon (Salinas basin). The data derived from these samples will also be presented.

 

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)