NEW GEOLOGIC MAPPING AND DATING OF VALLEY SPRINGS FORMATION TUFFS, CALAVERAS COUNTY, CALIFORNIA

New geologic mapping as part of the California Geological Survey’s geologic investigation of 7.5-minute quadrangles within the San Andreas 30 x 60-Minute Quadrangle identified samples for ⁴⁰Ar/³⁹Ar dating of tuffs within the Oligocene and Miocene Valley Springs Formation in Calaveras County, California. The results provide insights into the age, stratigraphy and paleogeomorphology of the Cenozoic sedimentary and volcanic units in the Central Sierra Nevada. Our detailed mapping has identified several tuffs previously not recognized that supports further stratigraphic differentiation and allows us to better constrain and define Oligocene and Miocene paleochannel cross-sectional geometry and channel gradient.

Five tuff samples collected for ⁴⁰Ar/³⁹Ar (sanidine) dating within the Columbia 7.5-Minute Quadrangle near Vallecito, CA indicate ages ranging from approximately 28.9 Ma to 23.7 Ma. This age range represents the oldest and youngest tuffs dated in the Valley Springs Formation within the greater Murphys and Columbia 7.5-minute quadrangle area. One tuff sample collected near the intersection of State Highway 4 and Red Hill Road yielded an age of 23.96 Ma, which is similar in age to two samples collected just west of Ironstone Vineyard having reported ages of 23.94 Ma and 23.75 Ma. Two additional tuff samples collected along Camp Nine Road approximately 2.0 km east of the town of Vallecito have a reported age of 28.89 Ma. These new data support ongoing work to correlate these tuffs on a regional basis, with Oligocene and Miocene tuffs that occur in the Sierra Nevada and Great Basin provinces to the east.