THE SOLDIER MEADOW TUFF OF THE HIGH ROCK CALDERA, NORTHWESTERN NEVADA

The Soldier Meadow Tuff (SMT) was defined and described at its type section west of Soldier Meadow (Noble et al. 1970). We define a Soldier Meadow rock type that forms several petrographically similar, quartz-sanidine phyric, peralkaline rhyolitic tuffs and lavas that are associated with the High Rock caldera (HRC), source of the 16.33±0.03 Ma Summit Lake Tuff (Noble et al. 2009). Throughout most of its outcrop area near the eastern flank of the HRC, the SMT forms a single cooling unit, at least 85 m thick in upper Warm Springs Canyon, of high SiO₂, peralkaline, welded tuff that also contains minor Na-ferrorichterite and Fe-rich clinopyroxene phenocrysts. This “massive” SMT 1 km west of Soldier Meadow Ranch consists of numerous flow units 0.5-3 m thick and yields an Ar/Ar age of 16.12±0.05 Ma.

The type locality of the SMT is a distinctively different, thinly bedded to laminated single cooling unit that extends to the south at least as far as Fly Canyon and yields a younger date of 15.86±0.04 Ma. This “layered” SMT, deposited by surges flowing to the SE, varies in thickness and exceeds 119 m at the type locality. Abundant lithic blocks of massive SMT up to 1 m diameter suggest a near-vent facies, with vents to the W or NW. Although the massive and layered SMTs have not been stratigraphically connected and may be lateral facies variations, the Ar/Ar dates and lithic inclusions of massive tuff suggest the thinly bedded facies is a younger unit that incorporated fragments of the older tuff during eruption. This strongly supports a major hiatus between eruptions.

Voluminous petrographically and chemically similar lavas and minor tuffs north of Soldier Meadow Ranch (Korringa, 1973) erupted at 15.89±0.06 Ma from fissures, probably along HRC ring fractures. These fissures may also have been vents for the early, massive SMT. Additional SMT-type lavas crop out around the ring fracture at Grassy Rock (15.93±0.07 Ma), south of Grassy Canyon, north of Soldier Meadow, and at the base of the type locality (16.04±0.03 Ma).

The tuffs of Alkali Flat and Badger Mountain near the northeast ring fracture of the HRC are similar to and once mapped as SMT but are younger and compositionally distinct, confirming published data (Park et al., 1973) that demonstrate different accessory phenocrysts, chemical composition, and TRM direction.