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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 41 (1957)

Issue: 10. (October)

First Page: 2284

Last Page: 2297

Title: Franciscan Group in Coast Ranges and Its Equivalents in Sacramento Valley, California

Author(s): William P. Irwin (2)


The Franciscan group is an assemblage of detrital and chemical sedimentary and volcanic rocks that crops out discontinuously in a structurally complex, northwesterly trending belt along the Coast Ranges of California. On the east, along the west side of the Sacramento Valley, a thick section of detrital sedimentary rocks has been subdivided into the Knoxville, Paskenta, and Horsetown formations of Late Jurassic and Early Cretaceous age, and several units of early Late Cretaceous age. The Sacramento Valley section represents deposition during most of the time from Late Jurassic to early Late Cretaceous.

The Franciscan group is widely held to be restricted to Late Jurassic age, the Knoxville formation to be an upper shaly phase of the Franciscan group, and the two to be overlain unconformably by detrital strata of Cretaceous age. Sparse paleontologic evidence indicates this view to be incorrect. Rather, the Franciscan group seems mainly to have been deposited contemporaneously with the Knoxville, Paskenta, Horsetown, and lower Upper Cretaceous strata, as fossils ranging from Late Jurassic to early Late Cretaceous in age have been found in the Franciscan group. The Franciscan group and strata of the Sacramento Valley section therefore may represent two facies of the same stratigraphic section.

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