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Magnetic Stratigraphy of the Lower-Middle Eocene Vacaville Shale (Type Ulatisian Stage), Solano County, California

E. E. Brabb1 and D. R. Prothero2
1U.S. Geol Survey, Menlo Park, CA
2Geology, Occidental College, Los Angeles, CA

Mallory (1959) designated exposures of the Vacaville Shale on Ulatis Creek, northwest of Vacaville, California, as the type section of his lower middle Eocene Ulatisian benthic foraminiferal stage. Magnetic samples were taken from the 75 m of exposed section and yielded a stable remanence held mainly in magnetite which passed a reversal test. Based on calcareous nannofossils and planktonic foraminifera, we correlate the type Ulatisian with Chrons C20r-C22r (45.0–50.5 Ma), which is latest early Eocene to earliest middle Eocene in age. Our data calibrate what has previously been suggested by micropaleontologists: the base of the Ulatisian in the Vacaville Shale is 2 million years older (50.5 Ma) than it is in Media Agua Creek in the Temblor Range (48.5 Ma), and that the top of the Ulatisian is a million years younger (46.5 Ma) in the Vacaville section then it is at Media Agua Creek (47.5 Ma). This is another example of the problems with time-transgressive benthic foraminiferal stages.

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