SURFICIAL GEOLOGIC MAP OF THE QUATERNARY DEPOSITS OF MOHAWK VALLEY, NORTHEASTERN CALIFORNIA

Mohawk Valley is an inter montane basin located at the northernmost end of the Sierra Nevada Mountains in northeastern California. Surficial geologic mapping was undertaken as part of a Ph.D. dissertation study of the Quaternary history and basin evolution of Mohawk Valley. We conducted field mapping at a scale of 1:24,000, based on initial interpretations from a combination of LiDAR imagery, 10 m DEM and derivatives, color aerial photographs, 7.5 minute quadrangles, and Google Earth. We delineated glacial, lacustrine, fluvial, alluvial fan, and colluvial deposits and landforms. Map units were grouped into relative ages based on stratigraphic and geomorphic position, cross-cutting relations, degree of preservation, relative weathering, and relation to numerically dated sections primarily from 26 different tephras that span 740 ky.

From our mapping, we interpreted a complex and dynamic Quaternary history. Interpretations using the sedimentologic and geomorphic record were possible because of the uncommon geologic setting, where geomorphology is both well preserved and well exposed. Before 740 ka, Mohawk Valley became an internally-drained basin. The valley began to fill with water and throughout the next several hundred thousand years, Mohawk Lake fluctuated in depth, but overall rose. During that time, multiple generations of glacial advances extended towards and into Mohawk Lake. Each subsequent glacial advance graded to an ever increasing base level and deposited deltaic sediments. After ~200 ka, Mohawk Lake reached the sill elevation (~1525 m) and began to spill to the west through the Middle Fork of the Feather River (MFFR). This discharge led to slow incision of the outlet channel and incremental lowering of Mohawk Lake which emptied by about 7 ka. Tributaries to the MFFR are now actively incising and this provides exposures of the glacial and lacustrine deposits on the edges of Mohawk Valley. The Mohawk Valley Fault Zone cuts through Mohawk Valley. We did not include a complete inventory of active faults on our map. However, through our study we located wide fault zones, not always clearly expressed on the surface, that disrupt middle through late Pleistocene-aged deposits. This allowed some observations regarding the style of faulting, timing of faulting, and distribution of faults within Mohawk Valley.