NEOGENE THROUGH QUATERNARY STRATIGRAPHY AND STRUCTURE IN A PORTION OF THE TRUCKEE MEADOWS BASIN: A RECORD OF RECENT TECTONIC HISTORY

Tilted and faulted sedimentary rocks in a portion of the Truckee Meadows basin document active tectonism since about 2.5 Ma. The oldest sediments are (~11 Ma) alluvial fan and fluvial deposits upon and around Sierran arc volcanic edifices. Sandy deltaic and diatomaceous rocks accumulated in local topographic lows with no evidence of tectonism until extensional faulting and eastward tilting of the section began at about 2.5 Ma. This formed the Carson Range, Verdi Range, and Peavine Peak to the west, and the Truckee Meadows basin. Soon afterward, the modern Truckee River drainage connected the glaciated Sierra interior with the Truckee Meadows basin, depositing outwash floodplain deposits now preserved as terraces in the west Reno foothills. Older terraces are tilted east more steeply than younger ones, attesting to syn-depositional motion on a fault farther east.

Our gravity studies allow projecting surface exposure data beneath the basin-fill. Gravity data suggest that a major fault system bounds the east side of a Quaternary sub-basin within the Truckee Meadows, here called the Reno sub-basin. The fault system, here named the Virginia Lake fault zone (VLFZ), appears to be a north-striking system of west-side-down normal faults located at the longitude of the Reno city center. Eastward tilting of the Neogene section is accommodated on these faults. Tilt-fanning of outwash terraces indicates that the VLFZ was active through much of the Quaternary. The VLFZ is partly buried by Holocene outwash fan sediments. We are studying ways to date the principal outwash units (terraces) in order to determine rate of Quaternary tilting and motion on the VLFZ. East of the VLFZ, gravity models suggest that the Truckee Meadows basin is filled by relatively young Neogene strata, and that this fill is thin relative to the Reno sub-basin.

Groundwater quality and elevation data reflect geologic features identified in cross-sections based on gravity, surface mapping, and borehole data. The VLFZ is a hydraulic flow barrier defined by offsets in groundwater elevations and by boundary effects during pumping tests. Data indicate that the Quaternary outwash (a principal aquifer in the Truckee Meadows) is thickest in the west Reno area. These features have a significant influence on groundwater movement and will be used to support a groundwater flow model.