NEW COSMOGENIC ³⁶CL AGES FOR RELICT SOILS ON ALLUVIAL FANS IN DEATH VALLEY, CALIFORNIA

We used cosmogenic ³⁶Cl nuclides to date the classic, well-varnished alluvial fans on the west side of Death Valley in the National Park. This technique differs from cosmogenic exposure dating of boulders in that it measures the total cosmogenic ³⁶Cl nuclide accumulation as a function of depth in relict soils developed on the alluvial fans. The Pleistocene chronologic framework in Death Valley is based mostly on geomorphic criteria (relative position, surface morphology, and development of desert varnish, pavement, and soils). Prior numerical ages by others come from U/Th dating of tufas related to ancient shorelines of Lake Manley (best preserved on the east side of Death Valley), and from AMS varnish and surface-exposure dates of the west-side fans.

Our previous ³⁶Cl dating north of Furnace Creek had shown that surficial materials typically contain clasts with the equivalent of 20-130 k.y. of inherited ³⁶Cl. By sampling soils older than 50 ka in profile, we can distinguish the inherited component from the in situ component. Nine 2- to 3-m-deep soil pits were excavated along roads or from steep arroyo exposures. We described the soils, then obtained five 0.5- to 1-kg samples of 1- to 2-cm clasts from a depth profile. In addition, four samples of modern alluvium were analyzed for comparison with soil parent materials. New Mexico Tech prepared 47 samples for AMS ³⁶Cl analysis by PRIME Laboratory (Purdue University). Using the program CHLOE, the total ³⁶Cl was calculated from ³⁶Cl nuclide concentrations, bulk density, grain size, and sample depth. Further assessments will model soil changes (mainly bulk density and silt content) since deposition of the fans.

Determining the age of these alluvial units enables us to examine the role of paleoclimate during deposition. Preliminary interpretation of the ³⁶Cl data shows that most of the intermediate-age alluvial fans (our unit Qai; Q2 of Hunt and Mabey, 1966, USGS Prof. Paper 494-A) are 60-110 ka, whereas older remnants (our units Qaio and Qao) are >150 to 250 ka. A lacustrine bar complex at 30 m above sea level on the north side of Hanaupah Canyon is 128-145 ka and correlates with a deep lake at marine OIS VI. This bar predates units mapped as Qai and postdates units mapped as Qaio and Qao, and thus provides an important stratigraphic datum.