MIDDLE TO LATE PLEISTOCENE FLUCTUATIONS OF LAKE MANIX, MOJAVE DESERT, CALIFORNIA

From about 500 ka until 25 ka, Lake Manix was the terminus for the Mojave River during periods of enhanced runoff from the Transverse Ranges of southern California. We obtained a 45-m core from the thickest remaining section of lake sediments, near the focus of previous outcrop studies conducted by George Jefferson at the confluence of the Mojave River and Manix wash. Analyses include particle size, ostracode assemblages, stable isotopes (δ¹⁸O and δ¹³C) on ostracodes, inorganic carbonate, and magnetic properties. Mapping and dating of freshwater Anodonta shells from exposures supplemented these analyses.

Chronology of the Manix core is based on one tephra, correlation of the top 8 m to dated outcrops nearby, and paleomagnetic excursions. The core contains the ~185-ka Manix tephra, identified by chemical correlation, at a depth of ~19 m. The upper 8 m is correlated on the basis of characteristic bedding, grain size, and buried soils to outcrops of fluvial and deltaic sand ranging in age from 30 cal ka at the top to >45 cal ka at the base. The magnetic record is of good quality and our preliminary assessments indicate the presence of 7 or 8 directional excursions, all within the Brunhes Chron. A conservative interpretation of the ages of these excursions that is consistent with the dated tephra and with paleointensity determinations yields an estimated age of ~490 ka for the base of lake sediments at the core site, just before the beginning of marine oxygen-isotope stage (MIS) 12. Prominent buried soils, sometimes overlain by beach sands, generally correspond with MIS boundaries. However, if the age model is correct, then sedimentologic, ostracodes, and isotope data suggest that some episodes of moderately deep water occurred during interglacial as well as glacial periods.