DISTRIBUTION, CORRELATION, AND MAPPING OF OLIGOCENE ASH-FLOW TUFFS IN THE WESTERN NEVADA VOLCANIC FIELD

Detailed mapping at 1:6000 currently in progress in the Sinkavata Hills (SH) and Monte Cristo Mtns (MC), Mineral, Churchill, and Nye counties, Nevada, and reconnaissance mapping in surrounding areas shows the discontinuous distribution of at least 14 separate, regionally extensive Oligocene ash-flow tuffs. Individual tuffs are mapped with high precision and are broken into sub-units to show internal stratigraphy. The ash flows were deposited predominately within WNW-trending paleovalleys and not all flows from each eruptive sequence made it to far western Nevada and the Sierra Nevada.

The tuff of Axehandle Canyon is mapped as a single cooling unit in far western Nevada but new mapping in the SH and MC has revealed that it is a composite unit with a cooling break between upper and lower sub-units. In some locations the contact is marked by a thin, coarse-grained sandstone and/or a paleoweathering surface, indicating at least a minor time gap separated the deposition of the two sub-units. A biotite-rich sample from the lower sub-unit yielded a new ⁴⁰Ar/³⁹Ar sanidine age of 31.447 ± 0.022 Ma, which is identical to that of the upper sub-unit.

The multiple-flow tuff of Return Mine (Trm) was not previously dated; a sample from the SH has a new ⁴⁰Ar/³⁹Ar sanidine age of 29.172 ± 0.020 Ma, which is equivalent to the age of tuff E (Tte). In some areas Trm overlies Tte and a sharp contact separates the tuffs. Though it is interpreted as part of Tte based on age, the name “tuff of Return Mine” is still used here because Trm differs petrographically from Tte and the two tuffs are not everywhere present together.

In the Gabbs Valley area, Ekren and Byers (1986a, b) mapped the “tuffs of Gabbs Valley” as Units 1, 2, and 3. Unit 3 correlates with the 24.9 Ma Underdown Tuff and Unit 2 correlates with the 25.1 Ma tuff of Elevenmile Canyon (Tec). Hence, “tuff of Gabbs Valley” only applies to Unit 1 (Tgv), which is 25.1 Ma.

The tuff of Arc Dome (Tad) overlies Tgv in the Paradise Range and Tec overlies Tgv in the SH and MC. Based on the same age of 25.1 Ma, stratigraphic relations, phenocryst content and petrographic texture, a distinctive and consistent contact with the underlying Tgv, and published trace element, REE, and isotopic data, Tad correlates with Tec. This correlation rules out the “inferred” caldera source for Tad at the south end of Reese River Valley (Brem et al., 1991).