STRATIGRAPHY OF LAKE MANIX BEDS IN THE MANIX SUBBASIN, MOJAVE DESERT, CA

Deposits of Lake Manix are well exposed along the bluffs of the Mojave River and Manix Wash, approximately 30 km east of Barstow, CA. George Jefferson established the sequence of members in the Manix Formation and made a huge contribution to the stratigraphy of the formation (summarized in GSA Special Paper 368, 2003). Our work has consisted of field measurements, correlations, and dating of stratigraphic sections, and sedimentological and geochronological investigations in a sediment core taken near the bluff exposures of the Manix Formation.

We have found that, except for Member A of Jefferson, which consists of alluvial fan deposits, and Member D, which consists of fluvial-deltaic sands, most sediments in the Manix Formation were deposited in shallow lacustrine to mudflat environments punctuated by short episodes of deeper water. Although some units can be traced for kilometers and are relatively uniform in character, some units undergo facies changes over short distances. For example, unit Upper B of Jefferson is a sandy gravel unit along the Manix Wash bluffs, a mud unit in the core several hundred meters away, and a widespread sand bed for several km to the west of the core site. Tufa-coated gravel beds mark episodes of lake deepening or lake transgression, and are found capping both lacustrine and alluvial gravels. Buried soil profiles or soil horizons within the lacustrine section can be identified in the core and outcrops, and suggest multiple periods of non-deposition on the low-gradient floor of the Manix subbasin during the Pleistocene.

Recently obtained radiocarbon ages of Anodonta shells in the uppermost stratigraphic unit range from 31.6 to 44.2 cal ka. The Manix tephra bed (approximately 185 ka) serves as a valuable marker in the lower part of unit Upper C (Jefferson, 2003) in the Manix subbasin. We have evidence for seven paleomagnetic excursions and estimate the bottom of the Manix core to be ~450 ka. Additional studies of geochronology, including amino acid racemization, OSL, and U-Th, are in progress. Our studies suggest some refinements to Jefferson's interpretations, including shorter lake cycles and somewhat different timing for parts of the section. The data suggest that the sediment record is reflecting Mojave River discharge and proximity of the Mojave River deltaic system.