Climate and Orogenic Evolution of the Sierra Nevada and Westernmost Basin and Range as Recorded in the Pliocene-Pleistocene Waucobi Lake Beds

Abstract:

The interplay between climate and orogenic evolution is archived in lacustrine basins as changes in basin geometry, sedimentary input, water level, and lacustrine chemistry. The Pliocene-Pleistocene Waucobi Lake Beds in the Owens Valley east of Big Pine, CA and Sierra Nevada Mountains are uplifted onto the western White-Inyo Mountain piedmont with a tephrochronology age of 2.2-2 Ma. We present ⁴⁰Ar/³⁹Ar and paleomagnetic chronology, isotopic analysis of clays and seismic data to evaluate the climate and tectonic controls on the Waucobi basin.

Within the 130-m-thick lake beds, we determined ages of 2.6-2 Ma on sanidine from intercalated tuff beds by ⁴⁰Ar/³⁹Ar single-crystal laser step-heating method. A paleomagnetic reversal identifies the Gauss/Matuyama boundary at 2.5-2.6 Ma. Clay mineral analysis shows phillipsite, an alkaline clay, dominating the lower section of Waucobi whereas the upper section contains montmorillonite, a fresh water clay. Deuterium isotopic analyses were performed on clay showing δD values for phillipsite increasing between 2.6-2.5 Ma from -105 ‰ to -60 ‰ indicating a wet climate, whereas δD values in montmorillonite decrease between 2.3-2.2 Ma from -70 ‰ to -90 ‰ implying a drier climate. Shallow active seismic studies suggest a basement depth of 300 m near the locality Duchess canyon.

Clay mineral and isotopic analyses indicate that Waucobi records an environment that does not reflect climate change represented by other lake systems in the eastern Sierra. This suggests that Waucobi records tectonic changes occurring between the Sierra Nevada and White-Inyo Mountains. Assuming a constant sedimentation rate calculated from the Duchess canyon exposure of 91 m and ages of 2.6-2.3 Ma, the seismic data indicates that the base of the lake section may be as old as 3.5 Ma. We suggest that lake formation coincides with extension-strike slip tectonics along the western Basin and Range, and uplift of the Sierra from the mid-Pliocene to Pleistocene.