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

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

Magnetostratigraphy of Plio-Pleistocene Lake Sediments in the Confidence Hills of southern Death Valley, California

Over 200 meters of continuous playa and lacustrine sediments and volcanic ashes are exposed in the Confidence Hills of southern Death Valley. Oriented samples from two stream canyons which cut through the sediments possess stable characteristic components of Natural Remanent Magnetization (NRM). Progressive demagnetization experiments yield several normal and reversed polarity zones which are stratigraphically distinct, and the characteristic components pass the reversal test. The presence of the Huckleberry Ridge volcanic ash (c.a. 2 Ma) in one of the reversed polarity zones provides a dated stratigraphic marker for correlation of the magnetostratigraphy to the magnetic polarity timescale. The correlation indicates that deposition began before the Reunion magnetic event (2.14-2.15 Ma) in the early Matuyama reversed chron (late Pliocene), and continued through the Olduvai normal subchron (c.a., 1.79 Ma) into the early Pleistocene. Deposition rates for the portion of the formation studied average about 26 cm/ky. Despite the close proximity of the sequence to strands of the Death Valley Fault, no net vertical-axis tectonic rotation could be detected

XRF, tephra chemistry, and paleomagnetic data from the Rulo site, southeastern Washington, USA

Worldwide, loess-paleosol sequences are archives of terrestrial Quaternary paleoenvironment data. The Rulo site is a newly described site in the Palouse loess of southeastern Washington State, USA. The site reveals a 19 m thick deposit of loess and flood sediment, preserving a total of 30 m of reconstructed stratigraphic section. Five unconformity-bound sequences of sediment are exposed at the site, the lowest two of which have yielded fossil remains, including the first reported occurrence of flat-headed peccary (Platygonus compressus) in northwestern North America. The unconformities, two diamicts bearing extrabasinal clasts, and a number of clastic dikes provide evidence for pre-Wisconsin glacial outburst floods in southeastern Washington. More than a dozen paleosols are preserved at the site. These soils are relatively unweathered and therefore primarily record periods of slow sedimentation accumulation in the Palouse, rather than changes in weathering rates. A paleosol near the base of the sequence is unusually weathered for Palouse paleosols, and records a period of increased weathering during the Middle Pleistocene. All units above the sand are normal paleomagnetic polarity; additional age control is provided by a tephra in the upper third of the sequence that matches a ca. 220 ka tephra from Newberry Volcano, and by stratigraphic correlation of later units to other sites in the Palouse. The Rulo site adds to the record of Irvingtonian fauna in North America, and supports accumulating evidence for pre-Late Wisconsin glacial outburst floods in the Pacific Northwest