A coarse conglomerate, known as “the Gravel of Reno,” fills a deep channel incised into a 2.6 Ma sedimentary section a few km west of Reno, Nevada. The canyon and its conglomerate fill record an abrupt shift in both provenance and paleocurrent direction compared with the underlying lake-marginal Neogene strata. Notably, the intermediate volcanic provenance of the Neogene section is supplemented in the overlying conglomerate by large plutonic clasts derived from the Sierran batholith. The syntectonic Gravel of Reno signals the initiation of Pleistocene faulting along the eastern edge of the Sierra Nevada near latitude 40° N.
Structures within the Neogene and Quaternary rocks reveal the progressive deformation of the Sierra Nevada's eastern margin. There is no discordance between the basal Gravel of Reno conglomerate and the underlying Neogene sedimentary section, and both are presently tilted 23° east. Therefore, significant tilting did not occur until after channel incision and deposition of the basal conglomerate. The dip within the Gravel of Reno decreases with stratigraphic height, documenting ongoing tilting during deposition. Several pervasive fault sets cut the Neogene rocks; one set of normal faults probably predates much of the tilting, but strike-slip faults appear to have been active synchronously with it. Fault sets include early west- and northwest-dipping normal faults, and two mutually cross-cutting sets of strike-slip faults: northwest-striking dextral faults and northeast-striking sinistral faults. The most continuous mappable fault surfaces, with probably much of the most recent movement, are north-striking faults with normal or oblique-slip motion. Overall, these faults accommodate east-west extension. In summary, the structural style at the northern termination of the Carson Range is characterized by distributed slip along many minor faults, and faulting was synchronous with tilting of the sedimentary section.
Gravity studies constrain the location and geometry of the main structures as they project eastward under the Reno basin. A negative anomaly extends eastward from the east-tilted Gravel of Reno. This gravity low (and the Gravel of Reno it represents) terminates eastward against a steep, north-striking gravity gradient under central Reno; we interpret this to mark a west-dipping normal fault, the “Virginia Street fault,” which was active throughout deposition of the coarse clastic section. A more localized and pronounced gravity low in west Reno corresponds to the eastward projection of the east-dipping Neogene diatomite that is exposed at the ground surface. The abrupt termination of this negative gravity anomaly requires that the diatomite terminates eastward at depth, therefore documenting the eastern margin of the Neogene lacustrine environment. Other gravity anomalies document both the relief on the sub-Neogene unconformity and a complex pattern of faults that offset Neogene and younger rocks in the Reno basin, consistent with the multiple fault sets seen in outcrop west of Reno.
- Received 23 January 2012.
- Revision received 1 July 2012.
- Accepted 7 July 2012.
- © Geological Society of America