Magnetostratigraphy and biochronology of the San Timoteo Badlands, southern California, with implications for local Pliocene-Pleistocene tectonic and depositional patterns

摘要

The San Timoteo Badlands expose nearly 2000 m of nonmarine sedimentary rocks ideally situated temporally and spatially to address questions regarding major tectonic events that occurred in southern California from latest Miocene to medial Pleistocene time. These events include development of the San Andreas–San Jacinto fault system and uplift of the Transverse Ranges. The badlands sequence also spans three of the four most recent North American Land Mammal "ages" and therefore coincides with important trans-Beringian and Neotropical mammal dispersal events. No other terrestrial sequence in North America records relatively continuous deposition over this interval of time. Magnetostratigraphy and mammalian biochronology provide a temporal framework that allows greater resolution of the geologic and biotic events recorded within the sequence than previous studies on this region. Data show that the sequence extends from chron C3An.2n, ca. 6.3 Ma, at the lowest part of the section in the study area, to within the Brunhes chron, or younger than 0.78 Ma, near the top. Refined age estimates for geologic and biotic events recorded within the sequence indicate that (1) a Peninsular Ranges basement source south of the badlands region dominated deposition from before 6.3 Ma to about 4.6 Ma, after which time the provenance shifted to a San Gabriel Mountains basement source from the north; (2) deposition of San Gabriel Mountains–type material continued until as recently as ca. 0.70 Ma, although sediments from the San Bernardino Mountains appeared in the northwesternmost area of the badlands ca. 1.5 Ma; (3) the late Hemphillian Mount Eden Local Fauna is dated as 5.6 Ma, compared with previous estimates of 5.0–5.4 Ma; (4) the early Irvingtonian El Casco Local Fauna is 1.3–1.4 Ma; and (5) the Shutt Ranch Local Fauna is ca. 0.7–0.99 Ma. Paleomagnetic data show that the badlands region may be slightly rotated in a counterclockwise direction, contrary to previous models predicting clockwise rotation in right-lateral shear zones.