Stratigraphic relations and lithologic variations in the Jemez Volcanic Field, New Mexico

Abstract

Over 100 radiometric dates and recent detailed geologic mapping allow some refinements of the stratigraphic relations of major units and generalization of temporal lithologic variations in the Jemez volcanic field. Volcanism had begun in the area by about 16.5 Ma with episodic eruptions of alkaline basalts. By 13 Ma, alkaline volcanism had been replaced with eruptions of more voluminous olivine tholeiite. High‐silica rhyolite, derived from melts of lower crust, also was erupting by about 13 Ma. Basalt and high‐silica rhyolite continued to be erupted until about 7 and 6 Ma, respectively, but effusions of dominantly andesitic differentiates of basalt that began as early as about 12 Ma volumetrically overshadowed all other eruptive products between 10 and 7 Ma. From 7 to 3 Ma the dominant erupted lithology was dacite, which appears to have been generated by mixing of magmas whose compositions are approximated by earlier andesites and high‐silica rhyolites. Less than 4–3 Ma volcanism was dominated by eruption of rhyolitic tuffs. Field relations, geochemistry, and dates specifically indicate the following with regards to stratigraphie relations: (1) distinctions among basalt of Chamisa Mesa, Paliza Canyon Formation basalts, and Lobato Basalt for other than geographic reasons are artificial; basaltic volcanism was continuous in volcanic field from >13 to 7 Ma, (2) Canovas Canyon and Bearhead rhyolites form a continuum of high‐silica rhyolite volcanism from >13 to 6 Ma, (3) hypabyssal and volcanic rocks of the Cochiti mining district probably represent the exhumed interior of a Keres Group volcano(s), (4) temporal overlaps exist among the major stratigraphie groups which may imply some genetic relations, and (5) the Tewa Group formation Cerro Rubio Quartz Latite may more appropriately be considered part of the Tschicoma Formation of the Polvadera Group. Preliminary analysis of hydrothermal alteration in the context of the volcanic stratigraphy suggests at least three distinct hydrothermal events have occurred in the volcanic field's history.

Evaluation of the potential for surface faulting at TA-63. Final report

“…although eruption continued until about 130 ka with the eruption of the El Cajete pumice (Gardner et al, 1986). The poorly defined eastern margin of the rift is marked by a transition from Tertiary-Quaternary rift sediments to Pre-Cambrian to Oligocene rocks that form the western front of the Sangre de Cristo Mountains.…”

“…1984; Gardner, 1985;Aldrich, 1986;Gardner et al, 1986;Gardner and House, 1987;Aldrich and Dethier, 1990;Gonzalez and Dethier, 1991). These investigations provide stratigraphic and structural information that have been used by WCFS to assess Quaternary faulting and seismic hazards in the LANL area.…”

Geology of the Western Part of Los Alamos National Laboratory (TA-3 to TA-16), Rio Grande Rift, New Mexico

“…50 to 60 ka [age from Toyoda et al (1995) and Reneau et al (1996b)]. In the formal stratigraphy of the Jemez volcanic field, this unit is the El Cajete Member of the Valles Rhyolite (Bailey et al 1969;Gardner et al 1986). Commonly, Qec consists of primary fallout deposits of dominantly pumice lapilli that directly overlie the pre-eruption soil, but some locations on the main escarpment of the Pajarito fault provide evidence for local reworking.…”

“…The younger member of the Bandelier Tuff, the Tshirege Member, is exposed in the study area, whereas the older member, the Otowi, has been encountered in one relatively deep hydrologic monitoring well in the study area (well R-25, south side of Cañon de Valle; Plate 1). Beneath the Otowi Member in well R-25 are sands and coarse gravels of the Puye Formation (Broxton et al 2002), which ranges in age from about 7 Ma to about 1.6 Ma (Bailey et al 1969;Gardner et al 1986). …”

Continental Scientific Drilling Program: Valles Caldera, New Mexico

“…Two eruptions, dated at 1.5 and 1.1 Ma, deposited approximately 300 to 400 km3 of ash and pumice (the Bandelier Tuff) and created the 22-km-diam Toledo and Valles calderas (Heiken et al, 1986 for the Valles caldera and the Bandelier Tuff, the volcanic field also contains a diverse suite of basaltic to rhyolitic rocks that were erupted from more than 13 Ma to 0.13 Ma (Goff et al, 1992). These are divided into three groups: the Keres, the Polvadera, and the Tewa (oldest to youngest) (Gardner et al, 1986).…”