PALEOMAGNETICS AND TEPHROCHRONOLOGY OF PLEISTOCENE AND PLIOCENE LAKES, DEATH VALLEY, CALIFORNIA

The age and extent of ancient lakes in Death Valley, California, are key to paleoclimatic, geobiologic, and tectonic reconstructions of the western Basin and Range. Fine-grained, laminated deposits of late Pleistocene and Pliocene age are found at several locations in Death Valley. Using a tephrochronologic framework, we have collected paleomagnetic data that refine the ages of these deposits. At Mormon Point, the 0.756 Ma Bishop Ash Bed (normal polarity) is interbedded with alluvial-fan conglomerates. Beginning 10 m below the Bishop Ash Bed are 60 m of fine-grained, laminated, lacustrine deposits that have reverse polarity, clearly indicating that the lower section is within the Matuyama Reverse Chron (0.78-2.58 Ma). A narrow zone of transitional paleomagnetic directions is recorded about 40 m below the Bishop Ash Bed and above the upper tuffs of Glass Mountain (0.8-1.2 Ma). This transitional zone would be stratigraphically consistent with the Jaramillo Normal Subchron (0.9-1.07 Ma); however, additional data are required to confirm this as a normal polarity interval. The base of the section is not exposed. At Zabriskie Wash in central Death Valley, a Pliocene section contains four tephra beds interbedded with lacustrine silts and clays that are overlain and underlain by alluvial-fan sediments. The lower two tephra beds are within a normal polarity interval, whereas the upper two have reverse polarity. The uppermost tephra layer, at the top of the lacustrine section, correlates with the 3.28 Ma Nomlaki Tuff; thus, the reverse/normal boundary within this section is the base of the Mammoth Reverse Subchron (3.33 Ma). These same relations are found to the northwest and southeast, which show that a narrow playa to shallow lake occupied Death Valley during the middle Pliocene.