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Abstract: Magnetic Stratigraphy of the Upper Middle Eocene-Upper Oligocene Sespe Formation, Ventura County, California


Although the Sespe redbeds have been a familiar marker unit in the Transverse Ranges for almost a century, the precise age of these beds has long been controversial. Magnetostratigraphic and heavy mineral analyses, combined with mammalian faunas, have greatly improved the chronostratigraphy of the Sespe.

Sections were sampled in Brea and Alamos Canyons (north of Simi Valley), South Mountain, and at Kew Quarry in the Las Posas Hills. A stable magnetic component was isolated which passed a reversal test, and showed about 85 degrees of clockwise rotation (consistent with other pre-Miocene rocks in the Transverse Ranges). In Simi Valley, the lower and middle members, and part of the upper member span late middle Eocene Chrons C{19}n-C{16}r (41.2-36.5 Ma), based on the Uintan and Duchesnean fossil mammals. A change in heavy minerals reveals a major unconformity in the upper member. Above this unconformity, the section spans late Oligocene Chrons C{10}r-C{9}r (28.0-29.5 Ma), based on early Arikareean fossil mammals. The early Arikareean sections at South Mountain and Kew Quarry were also correlated with Chrons C{10}r-C{9}r, based on a {40}Ar/{39}Ar date of 28.2 plus or minus 0.2 Ma at South Mountain. Thus, the Sespe Formation consists of two distinct depositional packages: a middle Eocene sequence overlain by a late Oligocene sequence, with a 8-million-year hiatus in between. These results indicate that the Sespe was deposited much more rapidly and discontinuously than previously thought.

Search and Discovery Article #90945©1997 AAPG Pacific Section Meeting, Bakersfield, California