UPPER NEOGENE TEPHROCHRONOLOGIC CORRELATIONS IN THE NORTHERN RIO GRANDE RIFT, NEW MEXICO AND SOUTHERN COLORADO

We have developed a high-resolution Neogene chronostratigraphy using electron-microprobe analyses on volcanic glass separated from 166 pumice-fall, ash-fall, and ash-flow tephra units in the northern Rio Grande rift, New Mexico and southern Colorado. These tephra layers provide time-and-space horizons for stratigraphic studies in the Jemez Mountains, Española basin, Albuquerque basin, and San Luis Valley regions. Tephra layers derived from the Jemez Mountains volcanic source are mainly rhyolites and dacites; from youngest to oldest, the units are: El Cajete Member of Valles Rhyolite (~50-60 ka); Tshirege Member including basal Tsankawi Pumice Bed of the Bandelier Tuff (both ~1.21-1.25 Ma); Cerro Toledo Rhyolite (~1.22-1.59 Ma); Otowi Member including basal Guaje Pumice Bed of the Bandelier Tuff (both ~1.61-1.68 Ma); the informal San Diego Canyon ignimbrites (~1.84-1.87 Ma); Puye Formation tephra layers (~1.75-5.3 Ma); upper Keres Group—Peralta Tuff Member of the Bearhead Rhyolite (~6.76-6.96 Ma); and lower Keres Group—Paliza Canyon Formation and Canovas Canyon Rhyolite (~7.4-<12.4 Ma). Widespread tephra layers derived from other sources augment those of the Jemez Mountains; they include: the Lava Creek B ash bed (~640 ka) derived from the Yellowstone caldera of northwestern Wyoming, ~1000 km to the north; the Nomlaki Tuff (~3.3 Ma) ~1300 km from its eruptive source in the Southern Cascades of California; the tuff of McMullen Creek and/or tuff of Wooden Shoe Butte (8.6-8.9 Ma), the Sheep Dip ash (9.46 Ma), the tuff of Ibex Hollow (11.93 Ma), and the CPT XIII, CPT XII, and CPT XI tuffs (10.94, 11.19, and 11.31 Ma, respectively) all derived from the Snake River Plain–Yellowstone Hot Spot track in Idaho and northwestern Wyoming; and the Rainier Mesa Tuff (11.72 Ma), the tuff of Quartet Dome (13.90 Ma), and the Peach Springs Tuff (~18.5 Ma) all derived from the southern Nevada volcanic field (Timber Mountain, Black Mountain, and Oasis calderas). Data obtained on these tephra layers are integrated with isotopic and magnetostratigraphic data and geologic mapping to provide a spatial and temporal framework for studies of surface and subsurface chronostratigraphy, structure, and hydrogeology in the Española and Albuquerque basins of New Mexico and San Luis Valley of southern Colorado and northern New Mexico.