Magnetic stratigraphy of the lower middle Eocene (type Ulatsian) Vacaville Shale, Solano County, California

Mallory (1959) designated exposures of the Vacaville Shale on Ulatis Creek, northwest o 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. The section showed two normal zones and three reversed magnetozones. Based on calcareous nannofossils and planktonic foraminifera, we correlate the type Ulatisian with Chron C20r-C22r (45.0-50.5 Ma), which is latest early Eocene to early middle Eocene on the global time scale. Our data calibrate what has been previously 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.