DEPOSITIONAL ENVIRONMENT AND RADIOCARBON CHRONOLOGY OF THE UPPER LAKE MANIX DEPOSITS, MOJAVE DESERT, CALIFORNIA

At the type section of the Manix Formation in the central Mojave Desert, CA, extensive outcrops were used by George Jefferson and colleagues during the 1990s to establish the chronology and paleontology of middle to late Pleistocene Lake Manix. The upper unit of the Formation, Unit D, is present only in this area, whereas underlying units mostly consisting of lacustrine mud and sand are more widely exposed in the former lake basin. We have studied the outcrops of Unit D and analyzed 6 Anodonta shells by AMS radiocarbon methods to establish its depositional environment and age. Unit D ranges from 8.5 to 10 m thick, and its surface is nearly planar at an altitude of 535 m, about 8 m below Lake Manix highstand levels. The unit is composed of reddish brown grus-derived sand and fine gravel, with less common mud beds and laminae; the clast composition is strikingly similar to that of modern and latest Pleistocene Mojave River deposits. Sedimentary facies and structures also mimic those of the Mojave River, including cross-bedded channel-fill deposits, crevasse-splay deposits, marsh, and floodplain deposits. Foreset beds expected for a delta environment are largely absent. However, thin intervals of very well sorted silt or sand beds may represent brief lacustrine deposition, and a few micro-delta beds are present. We conclude that Unit D represents progradation of fluvial deposits over lacustrine units, with only brief lake inundations. Radiocarbon results range from 40.0 ± 0.6 14C ka from a shell 0.5 m above the base to 26.3 ± 0.2 14C ka 1.2 m from the top. The mean depositional rate from the dates is ~0.6 m/ka, and when extrapolated to the top and bottom, suggests an age range of 40 to 25 ka for Unit D. This time period spans 2 or 3 highstand beach-forming events elsewhere in the basin. We interpret the lake stands as brief to account for the lack of lacustrine facies in Unit D, which apparently represents a very active Mojave River fluvial plain during marine oxygen isotope Stage 3, with brief deep lakes fed by sustained winter storms and/or higher groundwater.