Datapages, Inc.Print this page

Comparison of Paleogene Paleogeography: Southern Coast Ranges and Western Transverse Ranges

SCHUSSLER, SHERRYL A., Unocal Corporation, Bakersfield, CA

The Paleogene stratigraphic sequence exposed in the southwestern San Rafael Mountains at the southern terminus of the Coast Ranges, consists of up to 850 m (2800 ft) of marine limestone, sandstone, and mudstone that lies with a unique depositional contact upon Franciscan Complex rocks. Lithofacies identified represent four sedimentary environments: (1) foreslope talus deposits of a neritic algal bank (Sierra Blanca Limestone), (2) bathyal basin plain and outer submarine fan deposits (Juncal/Anita Formation and Cozy Dell Shale), (3) suprafan lobe deposits of a bathyal submarine fan (lower Mitilija Sandstone), and (4) sublittoral shelf deposits (upper Matilija Sandstone).

To the east of the southwestern San Rafael Mountains and north of the Santa Ynez fault, Paleogene strata thicken markedly, represent more proximal-to-source environments, and depositionally overlie Cretaceous "Great Valley-type" and silici plutonic rocks. This is inferred to represent an east-west transect of a northwest-trending Paleogene forearc basin. The Paleogene sequence of the southwestern San Rafael Mountains represents sediments deposited upon a structural high composed of Franciscan rocks at the seaward edge of this forearc basin.

Similarities between Paleogene rocks in the southwest San Rafael Mountains and the western Santa Ynez Mountains of the Transverse Ranges, approximately 60 km (40 mi) to the west, suggest deposition in a similar paleogeographic setting. Paleomagnetic data suggests post-Paleogene clockwise rotations of the western Transverse Ranges of 90 degrees +. Counterclockwise rotation of the western Transverse Ranges by this amount aligns the similar depositional sequences of the western Transverse Ranges with the northwest-trending Paleogene forearc basin of the southern Coast Ranges and eliminates the necessity for an east-west-oriented Paleogene basin at the site of the present western Transverse Ranges.


AAPG Search and Discovery Article #91009©1991 AAPG-SEPM-SEG-SPWLA Pacific Section Annual Meeting, Bakersfield, California, March 6-8, 1991 (2009)