Magnetic stratigraphy of the Morales Formation: Late Neogene clockwise rotation and compression in the Cuyama Basin, California Coast Ranges

Abstract

The Cuyama basin in the southern Coast Ranges of California underwent a transition from strike‐slip to compressional faulting, synchronous with the deposition of the Pliocene‐Pleistocene Morales Formation. Paleomagnetic stratigraphy was used to date the Morales Formation and, by inference, the beginning of compressional tectonics. Sections sampled below the Whiterock and Morales thrusts in the western Cuyama basin are predominantly normal and are correlated with the Gauss chron (3.57–2.60 Ma). An abrupt appearance of clasts derived from the overlying thrust sheet in the section below the Whiterock thrust suggests uplift of the Caliente Range during the middle to late Gauss chron. Seismic reflection data indicate that the eastern sections were deposited earlier. In conjunction with fossil evidence, the eastern sections are correlated to a time between the late Gilbert and early Matuyama chrons. The presence of a crystalline boulder bed midway in an eastern Cuyama basin section indicates uplift of the Mount Pinos‐Frazier Mountain highlands during the Matuyama chron between 2.60 and 0.78 Ma. Paleomagnetic directions of the Morales Formation document approximately 23° clockwise rotation of the Cuyama basin.