NEW GEOLOGIC MAPPING OF THE NORTH BLOOMFIELD 7.5 MINUTE QUADRANGLE, NEVADA COUNTY, CALIFORNIA

Recent geologic mapping of the North Bloomfield 7.5’ quadrangle shows significant improvement from previous mapping in the area and indicates that additional mapping is needed for adjacent quadrangles. The Sierra Nevada foothills northeast of Sacramento is an area of unique geological significance ranging from complex, deformed Paleozoic terranes, metamorphosed marine sequences from island arc assemblages, to crustal obduction. Also, the presence of paleovalleys and associated deposits across the region have been the subject of studies addressing the timing and nature of Cenozoic uplift of the Sierra Nevada. The area around North Bloomfield in Nevada County, California, contains examples of these features. The California Geological Survey (CGS) recently completed a 1:24,000-scale geologic map of the North Bloomfield Quadrangle under the U.S. Geological Survey’s STATEMAP program. This is the first 7.5’ quadrangle mapped as part of CGS’s effort to perform detailed mapping of the Truckee 30' x 60’ quadrangle, which will be mapped over the next several years.

Field mapping occurred over the course of approximately one calendar year from 2023-2024. Due to abundant forest cover, the detailed field investigations benefited greatly from analysis of lidar-based topography. During the field investigation, several rock samples were analyzed for whole rock geochemistry and age dating. The new zircon U-Pb date for the previously undated Spring Creek Pluton indicates an age of 173.4 Ma +/- 3.5 Ma. This is older than the adjacent Yuba Rivers Pluton which was previously dated at approximately 159 Ma.

One of the main improvements to the map was the detail of the landslide mapping. We identified over 200 individual landslides and classified them into five failure/age types. In particular, many of the young landslides are shallow earthflows that appear to be a result of the anthropogenic erosion of hillsides by hydraulic mining where shallow soil slides downhill into the pits depositing an apron of debris at the base. The continuing risk of landslides from hydraulically disturbed areas, age relationships of plutons and emplaced terranes, and distribution of deposits in paleovalleys show that there is much additional work that can be performed in adjacent quadrangles for applied geology and geologic hazard purposes.