AN 11.8 ± 0.2 MA SILICIC VOLCANIC ASH IN THE MIOCENE PAYETTE FORMATION: BOISE BASIN, IDAHO

A 12-meter section of fluvial-lacustrine sediments is well exposed on a south-facing slope 0.3 miles east of Idaho City in the Boise Basin of Idaho. Near the base of the exposure, a single 1-meter-thick gray vitric ash bed is intercalated with moderately to nonindurated layers of arkosic sands, silts and tuffaceous clays. The ash layer, informally referred to as the Idaho City ash, contains aphyric glass shards with bubble-wall morphologies. The lower 38-cm has a median grain size of 0.0625-mm, the upper 56-cm has a median grain size of 0.15-mm. The strata that contains the Idaho City ash are associated with sediments belonging to the Miocene Payette Formation mapped by T. Kilsgaard (1997, Idaho Geological Survey, Map 7). These sediments stratigraphically overlie a normal-polarity basalt flow Kilsgaard correlates with the 14-17 Ma. Miocene Columbia River Basalt Group. A previous upper Miocene age estimate for the five discontinuous Payette Formation outcrops in the Boise Basin vicinity is based on fossil flora preserved in the Thorn Creek area. However, a more precise age for these sediments is established here.

Glass from the basal 10-cm of the layer was analyzed by electron microprobe for Si, Ti, Al, Fe, Mn, Mg, Ca, Ba, Na, K, Cl, F and O. The Idaho City ash composition was statistically compared to the composition of 450 other Neogene fallout or ash-flow tuffs in the University of Utah tephrochronologic database (Perkins, M.E., et al., 1998, GSA Bulletin, 110; p. 344-360). The glass shard composition and the pattern of shard-to-shard variation of the Idaho City ash most closely matches the Logan Ranch ash of Perkins et al., 1998. This ash unit is exposed in stratigraphic sections at Huntington Creek and Ibapah Badlands in central and western Nevada. The Logan Ranch ash lies between two well-dated ashes, the Rainer Mesa white metaluminous ash (11.72 ± 0.03 Ma) and the Ibex Hollow gray metaluminous ash (11.93 ± 0.03 Ma). Based on the chemical analysis and correlation, the Idaho City ash has a middle Miocene estimated age of 11.8 ± 0.20 Ma. The Logan Ranch ash (i.e., the Idaho City ash equivalent) has strong chemical affinities with ash-flow tuffs erupted from the southwest Idaho Bruneau-Jarbridge volcanic center, active from 12.7 to 10.5 Ma.