A chornostratigraphic reference set of tephra layers from the Jemez Mountains volcanic source, New Mexico
Janet L. Slate, Andrei M. Sarna-Wojicki, Elmira Wan, David P. Dethier, David B. Wahl, and Alexis Lavine

Abstract:

Electron-microprobe analyses on volcanic glass separated from 65 pumice-fall and ash-flow tephra units of the Jemez Mountains, New Mexico, show that they 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 – Canovas Canyon Rhyolite (~7.4-<12.4 Ma). The Tshirege and Otowi Members of the Bandelier Tuff are difficult to distinguish from each other on the basis of electron-microprobe analysis of the volcanic glass; the Tshirege Member contains on average more Fe than the Otowi Member. The Cerro Toledo tephra layers are readily distinguishable from the overlying and underlying units of the Bandelier Tuff primarily by lower Fe and Ca contents. The San Diego Canyon ignimbrites can be distinguished from all members of the overlying Bandelier Tuff on the basis of Fe and Ca. Tuffs in the Puye Formation are dacitic rather than rhyolitic in composition, and their glasses contain significantly higher Fe, Ca, Mg, and Ti, and lower contents of Si, Na, and K. The Bearhead Rhyolite is highly evolved and can be readily distinguished from the younger units. We conclude that the Puye is entirely younger than the Bearhead Rhyolite and that its minimum age is ~1.75 Ma. The Paliza Canyon volcaniclastic rocks are chemically variable; they range in composition from dacite to dacitic andesite and differ in chemical composition from the younger units.


Citation:

  1. Slate, Janet L.; Sarna-Wojicki, Andrei M.; Wan, Elmira; Dethier, David P.; Wahl, David B.; Lavine, Alexis, 2007, A chornostratigraphic reference set of tephra layers from the Jemez Mountains volcanic source, New Mexico, in: Geology of the Jemez Region II, Kues, Barry S.; Kelley, Shari A.; Lueth, Virgil W., New Mexico Geological Society, Guidebook, 58th Field Conference, pp. 239-247.

More information...