Magnetic Stratigraphy and Tectonic Rotation of the Lower Miocene Rincon Shale (Saucesian Foraminiferal Stage), Santa Barbara County, California


Stacey Rapp

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The foraminifera from the Rincon Shale in Ventura and Santa Barbara counties, California, were the original basis for the Saucesian Stage of Kleinpell (1938). The Saucesian was originally correlated with the early Miocene, but it was impossible to tell precisely how much of the early Miocene was represented. Paleomagnetic samples were collected from 270 m of the Rincon Shale in the 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 other results reported from the Transverse Ranges. The Tajiguas Landfill section had three zones of reversed polarity and two zones of normal polarity. Based on the calibration of CN1 calcareous nannofossils and N4 planktonic foraminifera, we correlate this section with Chrons C6Bn (22.6-23.1 Ma). No matter what correlation we adopt, however, it is clear that this section of the Rincon Shale spans at most 1 m.y. of the early Miocene. As other authors have noted, the stratotypes of biostratigraphic stages turn out to span only a portion of the time they were thought to represent.


D. Prothero




Support provided by:NSF-REU and American Chemical Society-PRF

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