Distribution, geochemistry, and correlation of Pliocene tephra in the Pajarito Plateau
Giday Woldegabriel, Amanda P. Naranjo, and Melissa M. Fittipaldo
The Pajarito Plateau, a deeply dissected east-tilted tableland capped by Quaternary Bandelier Tuff, occupies the western part of the Española Basin. Lava flows of variable compositions dominated the earliest volcanic eruptions in the area currently occupied by the plateau; however, small-volume pyroclastic deposits erupted in the southern and northeastern parts of the Jemez volcanic field during the late Miocene and Pliocene. A dozen proximal primary and reworked tephra units are interbedded within the Puye Formation in Ancho, Bayo, Rendija, Guaje, Sawyer, and Santa Clara Canyons. In Ancho Canyon at the southern part of the plateau, two primary tephra units are interbedded within pumice-bearing volcaniclastic sandstone below conglomerate of the Totavi Lentil at the base of the Pliocene Puye Formation that is capped by Pliocene basalt (3.04±0.5 Ma). In Bayo Canyon, primary pumice and vitric ash units occur at the base of the Puye Formation about 2 m above upper Miocene basalt (8.86±0.05 Ma). The vitric ash yielded an age of 5.3 Ma. None of the tephra units in Rendija, Guaje, and Santa Clara Canyons were dated but they mostly occur in the upper half of the Puye Formation above the conglomerate of the Totavi Lentil. Major element compositions of glass shards determined by electron microprobe analysis indicate that most tephra are low- and high-silica rhyolites except for a pumice unit that is rhyodacite to trachydacite in composition. The upper tephra units in Ancho and Bayo Canyons are chemically identical. Four of the tephra samples from Rendija and Guaje Canyons are chemically correlative and represent low-silica rhyolite, whereas three other samples have distinctive compositions of low- to high-silica rhyolite. The lower tephra in Ancho Canyon is chemically correlative to a bedded tuff in Guaje Canyon. Chemical correlation was also noted between Rendija and Santa Clara Canyon tephra units. Based on chemical similarities, most of the Pliocene tephra erupted from Tschicoma Formation centers in the northeastern part of the Jemez volcanic field. Stratigraphic relations and correlation suggest that the ages of tephra and sedimentary deposits of the Puye Formation generally increase southward. Preexisting structures and contemporaneous felsic and basaltic volcanism on the west and east sides of the plateau, respectively, dictated Pliocene sedimentation in the area currently defined by the plateau. Fluvial pumiceous sandstone and fanglomerate of dacite and basaltic rocks hosting the tephra represent at least three source areas that are dominated by these lithologic types.
- Woldegabriel, Giday; Naranjo, Amanda P.; Fittipaldo, Melissa M., 2007, Distribution, geochemistry, and correlation of Pliocene tephra in the Pajarito Plateau, 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. 275-283.