Yet another atempt to date the Banco Bonito rhyolite, the youngest volcanic flow in the Valles caldera, New Mexico

Abstract

The Banco Bonito rhyolite flow is the youngest member of the El Cajete Series of the Valles Rhyolite Formation and represents the latest stages of extrusive volcanism associated with the Jemez Mountains volcanic field and the Valles caldera . The age of the Banco Bonito is important to understand the eruptive history and to evaluate volcanic hazards in the region . In this study we have applied optically stimulated luminescence (OSL) dating to a baked colluvial sediment layer immediately below the lava flow . Three distinct sets of ages emerged . OSL SAR ages from quartz sand averaged 140 ± 22 ka, corresponding to fission track ages for the Banco Bonito of ca . 130–140 ka (Marvin and Dobson 1979; Miyaji et al . 1985) . Measured IRSL MAAD ages from polymineral fine silts averaged 20 .7 ± 2 .5 ka, corresponding to a sizable group of electron spin resonance (ESR) ages from members of the El Cajete Series (Ogoh et al . 1993) . IRSL MAAD ages incorporating a regionally inferred data correction averaged 34 .0 ± 4 .2 ka, corresponding with cosmogenic 21Ne ages for the Banco Bonito (Phillips et al . 1997) . These correspondences may be linked to recurring thermal events in the caldera, as has been interpreted by past researchers but, ultimately, the OSL ages do not resolve the eruption age of the Banco Bonito flow .