Magnetic polarity stratigraphy and possible differential tectonic rotation of the Miocene-Pliocene mammal-bearing San Timoteo Badlands, southern California

The San Timoteo Badlands, consisting of the Mt. Eden beds conformably overlain by the San Timoteo beds, is a 3000 m thick sequence of nonmarine sediments deposited in a tectonically active region in southern California. Paleomagnetic samples were collected from 132 sites within these badlands. A composite magnetostratigraphic section for this sequence correlates to the MPTS from the late part of chron C3An.1n to the early part of chron C2r.2r, between about 6 and 2.3 Ma. The classic late Hemphillian (late Miocene) Mt. Eden fossil mammal quarry occurs near the base of the composite section within chron C3r, between 5.8 and 5.3 Ma. Although the overall unit-mean declinations indicate negligible or slight counterclockwise (CCW) vertical-axis rotation (−8.8° ± 4.9°), analysis of declination subsets from different parts of the section suggest possible differential small-block rotation within the San Timoteo Badlands: Prior to 3.5 Ma (represented by the Mt. Eden beds) there is negligible rotation; the overlying portion of the San Timoteo Badlands (represented by the lower San Timoteo beds) dated between 3.5 and 2.3 Ma (chrons C2An.3n to C2r.2r), indicate significant apparent CCW rotation (R ± ΔR) of −20.7° ± 5.2°. These data suggest decoupling of the upper part of the San Timoteo Badlands and independent CCW vertical-axis rotation relative to the lower part of this sequence.