STRATIGRAPHY AND AGE OF CENOZOIC SYN-EXTENSIONAL SEDIMENTARY BASINS, EAST CENTRAL NEVADA

Despite extensive geologic mapping and numerous studies that address the timing of Cenozoic events in east central Nevada, consensus has not been reached on the age of metamorphic core complex detachment faulting and the onset of Basin and Range faulting. This study analyzes the stratigraphy and age of sedimentary basins that flank the northern Snake Range (NSR), from the Kern Mountains south to Sacramento Pass.

The Cenozoic unconformity, defined by the contact between unmetamorphosed Devonian-Permian carbonates and sandstones with the overlying Eocene (~35 Ma) Kalamazoo Tuff and younger volcanic units, is exposed in tilted fault blocks to the north and south of the ~E-W trending Kern Mountains. Minor faults and erosional channels cut Eocene volcanic rocks and are overlain by a 1 km+ coarsening upward sequence of alluvial fan deposits that contain clasts of metamorphosed limestone, the Eureka Quartzite, granite boulders from the Late Cretaceous Tungstonia pluton and other Cenozoic igneous rocks now exposed in the Kern Mountains. The base of this sequence consists of a ~20 m thick lacustrine limestone and siltstone unit, ~30 m thick conglomerate of volcanic (Tcv) and Paleozoic (Tcpz) clasts and ~120 m thick lacustrine limestones, siltstones and sandstones. There is little angular discordance between the Eocene volcanic rocks and the basal sedimentary units. Preliminary U-Pb detrital zircon (DZ) geochronology of the basal sandstones suggests the maximum depositional age for these strata is 13.5 Ma north of the Kern Mountains and 21 Ma to the south. These relations are similar to those documented in the Sacramento Pass Basin, where the onset of faulting has been dated by a tuff horizon intercalated with avalanche deposits (~20-21 Ma, bi N=5). Here, conglomerates and sandstones directly above the Ely limestone yield DZ spectra compatible with little structural relief in the region prior to Miocene faulting.

These relationships imply: 1) minor Eocene faulting prior to the onset of Basin and Range extensional faulting in the Miocene, which was responsible for unroofing deep-seated strata in both the Kern Mountains and NSR and 2) a transition from predominantly locally derived volcanic and upper Paleozoic lithologies from ~35 Ma to ~21 Ma to the subsequent rapid uplift and erosion of granitic and metamorphic lithologies.