AGE OF THE PENULTIMATE LACUSTRINE HIGHSTAND IN PANAMINT VALLEY: A REASSESSMENT COMBINING SOIL STRATIGRAPHY AND NUMERICAL DATING

Relict paleo-shorelines decorate the basin of Panamint Valley, California, at elevations between ~575 and 700 m, attesting to a once-extensive lake that is presumed to have been connected upstream to the Owens River system (Jannik et al., 1991) and downstream to Death Valley (R. Smith, 1976). Although these features are commonly held to have developed during Marine Isotope Stage (MIS) 6, between 135-190 ka, local chronologic dating has been equivocal, with both luminescence and U-series ages yielding ages as young as 50 ka. Here we re-assess the age of highstand deposits at two locations employing a combination of soil stratigraphic determinations and direct dating using ¹⁰Be soil depth profiles and optically stimulated luminescence (OSL) techniques. Near Water Canyon, along the western valley, soil profiles developed into each of three separate remnants of a beach berm (~615 m elevation) have identical morphologic profiles consisting of an Av-Btkyz-BCky-Cky horizon sequence and show no evidence of previous histories of soil erosion or truncation. The tops of the three surfaces are level and occur at the same elevation, are smooth with a well-formed desert pavement, features that also indicate no surface or soil erosion. Numerical ages of beach gravel and lacustrine sediment range from ~70±11 to 98±9 ka. The beach gravels are truncated by adjacent alluvial fan deposits that exhibit desert pavements and soils with horizon sequences similar to the beach gravels. The alluvial fan surfaces have a ¹⁰Be depth profile age ranging from ~61±27 to 70±22 ka suggesting a valley-wide episode of fan development closely followed lake recession. Finally, we also sampled deposits of well-bedded sand at the southernmost end of Panamint Valley; these appear to represent deltaic topset deposits at the southern margin of the lake and near the inlet from upstream. Four samples yield OSL ages ranging from 56 to 67 ka. Taken together, the soil stratigraphy of the beach ridge and cross-cutting alluvial fan, and the deltaic deposits at the southern end of the valley, suggest that the best estimate for the penultimate pluvial lake in the valley is ~65-90 ka corresponding to MIS-4 glaciation. Previous estimates of the rates of late Pleistocene graben tilting and seismogenic slip in the eastern California shear zone may need to be modified based on these new ages.