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Abstract: Magnetic Stratigraphy of the Upper Miocene "Santa Margarita" and Chanac Formations, Tejon Hills, Kern County, California


Magnetostratigraphic sampling was conducted on two key sections of the middle-upper Miocene "Santa Margarita" and Chanac formations to correlate them to the magnetic polarity time scale, and to determine if they had undergone any tectonic rotation. The Tejon Hills, on the southeastern rim of the San Joaquin Basin in Kern County, exposes a 200-m-thick sequence of marine "Santa Margarita" and fluvial Chanac formations. It is the type section of Savage's (1955) early Clarendonian "Cerrotejonian Stage." Magnetic analysis yielded a stable remanence that passes a reversal test, and shows no tectonic rotation. These strata are correlated with Chrons C5n to C5An (10.4-12.2 Ma), based on comparisons with the Clarendonian sequence in the Ricardo Group in the northeastern Mojave Desert. However, several taxa which define the late Clarendonian in the Ricardo Group (Hipparion forcei, Osteoborus diabloensis) occur earlier in the Tejon Hills (Chron C5r, 11.5 Ma) than they do in the Ricardo Group (Chron C5n, about 10.5 Ma). This calls into question the criteria for defining the early and late Clarendonian stages.

A second section of about 150 m of the "Santa Margarita" and ?Chanac Formations was taken in Chorro Grande Creek, in the upper Sespe Creek drainage, Ventura County, California. This section was entirely reversed in polarity, and probably correlates with Chron C5r (10.5-11.5 Ma). It is rotated 83 degrees clockwise, which is consistent with other rotations reported for Eocene rocks in this same tectonic block between the Santa Ynez and Pine Mountain faults.

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