EVIDENCE FOR A LARGE LAKE IN THE MONO BASIN AT 3.5 MA

The Mono Basin is a structural depression bounded by the Sierra Nevada on the west, Bodie Hills on the North, Cowtrack Mountains on the east and Long Valley caldera on the south. Mono Lake is a terminal lake that is known to have existed for at least 767 kyr based on a well drilled to the Bishop Tuff. The structural development and volcanism are known to have begun in the 3 to 5 Ma time frame. This presentation concerns the ⁴⁰Ar/³⁹Ar age of a pillowed volcanic unit in the Cowtrack Mountains. The outcrop was discovered while searching for datable carbonate deposits from the last high shoreline of Mono Lake, 2155 m, near the current spill point for the lake into the Adobe Valley. Coincidentally, at about this elevation, the texture of the volcanic is pillowed below and blocky above, suggesting the approximate lake level at that time. Of course, the elevation may be different than at the time of the eruption, but the presence of the pillowed lava suggests a significant body of standing water at the time.

Fine groundmass was selected from a sample of the volcanic rock and co-irradiated with Fish Canyon sanidine monitor standard. Step-heating of the sample yielded an isochron age of 3.5+/-0.5 Ma and initial ⁴⁰Ar/³⁶Ar of 303.0+/-1.8. All 16 steps give a plateau age of 3.54+/-0.08 Ma using the trapped initial. These data provide direct evidence for a significant lake in the Mono Basin at ~3.5 Ma, and suggest that a record of hydroclimate in the Mono Basin for the entire Pleistocene could be achievable with continental drilling.