Subdivision of the San Lorenzo Formation (Eocene and Oligocene) west-central California

AAPG Bulletin


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The San Lorenzo Formation was for many years considered synonymous with Oligocene Series. The formation name was extended, incorrectly in most cases, to rocks as far north as British Columbia and as far south as southern California. The formation in its type area was never adequately studied, resulting in fallacious concepts of its faunas and erroneous correlations.

The San Lorenzo Formation has been subdivided into the Twobar Shale Member of Narizian (late Eocene) age and Rices Mudstone Member of Rufugian and Zemorrian (Eocene and Oligocene) age. Faunas and lithology suggest that the older member was deposited slowly in a bathyal, open-sea environment, whereas the Rices Mudstone Member seems to have been deposited rapidly in a restricted basin. Glauconite at the contact of the members suggests a stratigraphic break in the depositional sequence.

Most, perhaps all, of the so-called characteristic San Lorenzo mollusks are from the upper (Zemorrian) part of the Rices Mudstone Member, or from the overlying Vaqueros Sandstone. The "transitional sandstone" of Arnold is Vaqueros, not San Lorenzo.

Subdivision of the San Lorenzo Formation and the discovery of glauconitic marker beds provide additional stratigraphic control in an oil province characterized by poor exposures.

Publication type Article
Publication Subtype Journal Article
Title Subdivision of the San Lorenzo Formation (Eocene and Oligocene) west-central California
Series title AAPG Bulletin
Volume 48
Issue 5
Year Published 1964
Language English
Publisher American Association of Petroleum Geologists
Publisher location Tulsa, OK
Contributing office(s) Geology, Minerals, Energy, and Geophysics Science Center
Description 10 p.
First page 670
Last page 679
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