MAGNETIC STRATIGRAPHY OF THE OLIGOCENE-MIOCENE VAQUEROS FORMATION AND MOLLUSCAN STAGE, CALIFORNIA

The Vaqueros Formation was the basis for the Vaqueros molluscan stage in California, which has long been considered early Miocene, although its precise age has been controversial. Several sections of the Vaqueros Formation with precise biostratigraphic control were sampled for paleomagnetic analysis to determine the age limits of the Vaqueros Stage. The type section of the Soda Lake Shale Member in the northern Caliente Range was sampled, and yielded a stable remanence held in magnetite. Based on its early Miocene planktonic microfossils, we correlate it with Chrons C6C2n to C6B2n (22.8-23.8 Ma). In the Simi Valley, Vaqueros strata which conformably overlie the upper Oligocene (based on late Arikareean mammals) Sespe Formation are dated at 28.0 Ma. And in Orange County, Vaqueros strata which conformably overlie lower Miocene Sespe beds (containing an early Hemingfordian mammalian fauna) are dated at 17.0 Ma in age. Thus, the Vaqueros molluscan stage varies widely in age from place to place, and spans almost 11 million years of the late Oligocene and early Miocene.