Rocky Mountain (56th Annual) and Cordilleran (100th Annual) Joint Meeting (May 3–5, 2004)

Paper No. 4
Presentation Time: 2:00 PM


CASHMAN, Patricia and TREXLER, James, Department of Geological Sciences, Univ of Nevada, Reno, Mail stop #172, Reno, NV 89557,

Sedimentary rocks of the Neogene Verdi Basin record deposition with no evidence of active faulting from about 10 Ma to < 2-2.5 Ma along the east edge of the Sierra Nevada near Reno, NV. These rocks are deposited on Mesozoic crystalline and Cenozoic volcanic rocks. Local boulder fanglomerates near the base of the section record faulting associated with basin initiation. Basaltic andesite flows 10.0 – 10.3 Ma are intercalated with the oldest basin sediments; lava flows do not occur elsewhere in the Verdi Basin section. Basal fluvial conglomerate is overlain by a continuous section of deltaic and lacustrine deposits that range up to < 3.04 Ma. Renewed fluvial deposition overlying the lacustrine section indicates filling of the basin; these youngest rocks contain horse teeth that are Blancan V, interpreted by different workers as no older than 2.6 Ma or no older than 1.9 Ma. Dramatic incision of the fluvial rocks accompanied by a change in clast provenance and depocenter location documents renewed tectonism at the end of the basin history. There is no sedimentary evidence of faulting between basin initiation and the incision near the top of the section.

The post-depositional deformation of the Verdi Basin deposits demonstrates significant strike-slip, as well as extensional, motion since 2.6 (or 1.9?) Ma. Tilting (predominantly eastward) of the Neogene section pre-dates most of the faulting, and suggests that much of the initial deformation was extension accomplished by a west-dipping normal fault system farther east, probably along the west flank of the Virginia Range. Several sets of faults post-date most of the tilting; these include: NW-striking dextral, NE-striking sinistral, and N-striking normal-dextral oblique-slip faults. These faults are widely distributed throughout the Verdi Basin, and some (particularly the dextral and oblique-slip faults) have tens or hundreds of meters of slip. They accommodate both E-W extension and significant NW-directed dextral slip.

The sedimentary rocks of the Verdi Basin record faulting and basin initiation around 10.5 Ma, and a dramatic tectonic and paleogeographic change < 2–2.5 Ma. The latter included the initiation of extension as well as NW-striking dextral strike-slip motion west of the “Walker Lane” sensu stricto, in rocks that are part of the eastern edge of the Sierra Nevada block.