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Magnetic Stratigraphy and Tectonic Rotation of the Lower Miocene Rincon Formation, Santa Barbara County, California: Implications for the Saucesian Stage

S. G. Rapp and D. R. Prothero
Geology, Occidental College, Los Angeles, CA

The foraminifera from the Rincon Shale in Ventura and Santa Barbara Counties, California, were the original basis for the Saucesian Stage of Kleinpell (1938). This stage was originally considered early Miocene, but it was impossible to tell how much of the early Miocene it represented. Paleomagnetic samples were collected from the lower 270 m of the Rincon Shale in Tajiguas Landfill, west of Santa Barbara. The samples yielded a stable remanence held mainly in magnetite, which passes a reversal test and shows about 130 degrees of clockwise tectonic rotation. This rotation is consistent with others reported from nearby rock units in the Transverse Ranges. Based on calibration from Zone CN1 calcareous nannofossils and N4 planktonic foraminifera, the best correlation of this section is with magnetic Chrons C6Br-C6AAr (22.2–23.2 Ma) or with Chrons C6An- C6r (20.0–21.5 Ma). No matter which correlation is adopted, however, it is clear that the Rincon Shale spans at most 1.5 million years of the 8 million years of the early Miocene. As has been demonstrated repeatedly, the stratotypes of most biostratigraphic stages actually span only a small portion of the time they were once thought to represent.

AAPG Search and Discovery Article #90904©2001 AAPG Pacific Section Meeting, Universal City, California