Magnetostratigraphy and Clockwise Rotation of the Plio-Pleistocene Mojave River Formation, Central Mojave Desert, California

Abstract

Oriented samples collected for paleomagnetic analysis from sediments of the newly-named Mojave River Formation (Nagy & Murray, 1991, this volume) possess stable characteristic components of Natural Remanent Magnetization (NRM). Progressive demagnetization reveals characteristic components of both normal and reversed polarity which are stratigraphically distinct. The oldest sediments exposed within the field area are reversely magnetized and were probably deposited during the early portion of the Matuyama reversed Chron. Stratigraphically higher units contain what appears to be the Olduvai normal Subchron, as well as a shorter normal zone which probably is either the Cobb Mountain or Jaramillo Event. The location of the Brunhes/Matuyama boundary at one site is within an alluvial fanglomerate which grades upward conformably into the lowest unit of the overlying Manix Formation, possibly accounting for the absence of the Bishop ash in the section. Demagnetization data from 143 samples yielding acceptable least-squares lines suggest a net clockwise rotation of 8 ± 2.7° over the past two million years, perhaps with some of the rotation during deposition. This rate of rotation could account easily for larger rotations reported elsewhere in the Mojave Desert on units of Miocene age.