T164 Magnetostratigraphy of Critical Intervals in Earth History: Contributions to Geochronology, Geobiology, Paleogeography, and Global Change

Tuesday, 24 October 2006
1:30 PM - 5:30 PM

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

Paper number: T164-13
Program: Technical Sessions
Time: 4:45 PM – 5:00 PM

Authors

Abstract

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.