BEDROCK AND SURFICIAL GEOLOGY OF THE NORTH HALF OF THE SPANISH SPRINGS PEAK 7.5 MINUTE QUADRANGLE, NORTHWEST NEVADA

New geologic mapping (1:24,000 scale) of the north half of the Spanish Springs Peak 7.5’ quadrangle in south-central Washoe County, northwest Nevada, provides insight into Oligocene-Miocene depositional processes, the geometry of the southern end of the Warm Springs Valley fault system, and the distribution of Oligocene ash-flow tuffs that serve as key structural markers for Walker Lane deformation. Mapping was conducted using a tablet computer and traditional field methods, supplemented by aerial imagery and newly acquired LiDAR data.

Rock units in the map area include Mesozoic granitic plutonic rocks and associated metasedimentary and metavolcanic rocks, multiple yet undifferentiated Oligocene ash-flow tuff units, intercalated Miocene basalt/basaltic andesite flows and clastic deposits including coarse tuffaceous breccia and megabreccia, and unconsolidated Quaternary sediments.

A prominent Oligocene ash-flow tuff several hundred meters thick with no exposed base crops out in the map area. This and the other newly mapped Oligocene ash-flow tuffs support the previously proposed approximate location of an east-west trending mid-Oligocene paleochannel several hundred meters deep that extends through the center of the north half of the quadrangle. Basalt/basaltic andesite flows intercalated with laterally discontinuous, coarse breccia and megabreccia deposits with a maximum thickness of approximately 200 m overlie the undifferentiated ash-flow tuffs. The megabreccia deposits contain predominantly tuff clasts, including clasts as large as 10s of meters in diameter, and share similarities to other megabreccia deposits in northwest Nevada interpreted as originating from landslides or “dam-burst” type floods.

Several previously unmapped fault strands observed in newly available LiDAR data displace Quaternary sediments and trend parallel to strands of the Warm Springs Valley fault system previously mapped to the north.

Six spatially separated ash-flow tuff units yielded abundant sanidine grains from which 40Ar/39Ar ages will be acquired. Precise ages will aid in differentiation of the tuff units within the map area, facilitate regional correlations, and potentially provide structural markers for characterizing fault slip history within the Walker Lane shear zone.