DEPOSITIONAL AND DEFORMATIONAL HISTORY IN NEOGENE SEDIMENTARY ROCKS OF BOCA BASIN, NEVADA AND SIERRA COUNTIES, CALIFORNIA

The Neogene Boca sedimentary basin, located within the Sierra Nevada north of Lake Tahoe and west of Reno, records the depositional and deformational history for late Miocene and Pliocene time in this part of the northern Sierra Nevada. The Neogene sediments are exposed along the > 10 km long, N-S trending Little Truckee River valley, including wave-cut cliffs bordering Boca Reservoir.

The Neogene sedimentary section is > 1 km thick and generally W-dipping. At the base, it is interbedded with intermediate volcanic rocks (consisting of mudflows, pyroclastic flows, and coarse volcaniclastic rocks) that unconformably overlie Mesozoic granite. The sedimentary section fines upward, and consists of fluvial conglomerate and sandstone with locally interbedded basalt at the bottom of the section. This is overlain by deltaic sandstone and siltstone, which gradually grades upward into thick, locally silty, diatomaceous lacustrine deposits. A tephra low in the diatomite section is 3.3 Ma; other tephras occur throughout the diatomite. Overlying the diatomite along a dramatic unconformity, medium to coarse sandstone and cobble conglomerate with rip-up clasts of the underlying diatomite indicate rapid downcutting. Trough cross-bedding in the volcaniclastic sandstone records generally NW transport of the overlying coarse clastic deposits. These strata record the development of a relatively quiescent diatomaceous lake basin overlying the intermediate Miocene volcanics, followed by tectonism that produced a change in base level and depocenter location. This tectonic event occurred well after 3.3 Ma; dating of tephras higher in the section should allow us to improve this age control.

Faulting orientation, slip data, and kinematic indicators were analyzed for over 200 faults to determine evolution of deformation after sedimentation ceased. There are 2 major faulting orientations: (1) steeply dipping, NW- to WNW-striking faults with dextral slip, and (2) steeply dipping, NE- to ENE-striking faults with sinistral slip. Most faults indicate < 1 m of stratigraphic separation, but several faults have > 10 m. Kinematic indicators indicate both strike-slip and oblique-slip (with a normal component) on these faults. Regional deformation is consistent with the Sierra Nevada block moving NW relative to the North American continent.