The Quaternary History of Mohawk Valley, Northeastern California
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Mohawk Valley is an inter montane basin with a rich Quaternary record, located at the northernmost end of the Sierra Nevada Mountains in northeastern California. Geologic mapping of surficial deposits, stratigraphy, tephrochronology, geomorphology, and soil development were used to interpret the past 740 ky of Quaternary history of Mohawk Valley. The robust tephrochronologic record within Mohawk Valley includes twenty-six different tephras and sixty-seven tephra beds that range in age from 740 to 7 ka. Geochemical analyses and correlations with previously identified volcanic tephras have resulted in revised age estimates for tephra beds distributed within, and beyond, Mohawk Valley. The tephra beds were deposited in lacustrine deposits of Mohawk Lake. Elevations of shorelines and minimum lake-levels based on elevations of waterlain tephra beds were used to reconstruct the history of Mohawk Lake. Mohawk Lake began to fill prior to 740 ka and continued to fluctuate, but overall rise, until after 175-235 ka when the lake reached the sill elevation, began to spill to the west, then incrementally lower and empty by ~7 ka. Throughout this period, there were at least five, and up to nine, different generations of glacial deposits that extended towards Mohawk Lake. These glacial deposits have been mapped, their soil development and weathering properties characterized, and ages estimated based on stratigraphic relations with tephra beds deposited within Mohawk Lake deposits. This mostly continuous, 740 ky record of sedimentation has enormous potential to examine paleoclimate in this area from any of a number of paleoclimate proxies. The interpretation that a deep lake existed in Mohawk Valley requires a mechanism to allow for deposition and preservation of organic-rich deposits in deep water. Mohawk Lake was likely a meromictic lake, a setting that leads to an anoxic environment that can preserve organic-rich sediments such as those found in Mohawk Valley. In addition, shorelines around Mohawk Valley and across much of the Mohawk Valley Fault Zone are at consistent elevations suggesting there is not a significant vertical component of faulting since 175-235 ka, and maybe since 570-610 ka. This indicates a change from the history of subsidence since the early Pliocene.