Geology of the Frijoles 7.5-minute Quadrangle, Los Alamos and Sandoval Counties, New Mexico

Goff, Fraser; Gardner, Jamie; Reneau, Steven

The Frijoles 7.5 minute quadrangle covers an area within Los Alamos and Sandoval counties and includes parts of the southern Pajarito Plateau and southeastern Jemez Mountains, New Mexico (Figure 1). Topographically, the Frijoles quadrangle is bounded by Los Alamos Canyon on the north, the Sierra de los Valles on the west, the San Miguel Mountains on the southwest, and White Rock Canyon of the Rio Grande on the extreme southeast. Much of the area of the quadrangle is covered by the Pajarito Plateau, formed by thick pyroclastic deposits of Bandelier Tuff emplaced at 1.2 to 1.6 Ma. The plateau of pyroclastic deposits has been deeply incised by streams, yielding a distinctive and rugged finger-mesa and canyon topography. The quadrangle includes portions of the Pajarito fault system, which forms the local active western boundary of the Rio Grande rift. Generally speaking, this fault zone separates the Jemez Mountains volcanic field on the west from the Pajarito Plateau and the modern Española Basin of the Rio Grande rift on the east. Sedimentary deposits beneath the Pajarito Plateau contain deep aquifers with potable waters that supply the needs of Los Alamos and White Rock, and several U.S. government installations. The objectives of the Frijoles quadrangle map are to provide structural and stratigraphic boundary conditions for environmental and hydrologic programs of Los Alamos National Laboratory, to contribute to the geologic quadrangle mapping program of the New Mexico Bureau of Mines and Mineral Resources and U.S. Geological Survey, and to furnish archeological and volcanic information for the Bandelier National Monument and the Valles Caldera National Preserve. The geology of the quadrangle was mapped primarily during the spring and summer months of 2001, supplemented by published and unpublished mapping of the authors. Regional geology and stratigraphy have been previously published by Griggs (1964), Bailey et al. (1969), Smith et al. (1970) and Kelley (1978). Detailed mapping of the St. Peter's Dome area was compiled from the geology of Goff et al. (1990). Gardner and Goff (1996) published a geologic map of the northern half of the Valle Toledo 7.5 minute quadrangle northwest of the Frijoles quadrangle. Detailed mapping of western Los Alamos National Laboratory was compiled from Gardner et al. (1999, 2001) and Reneau (unpub.). Detailed mapping of DP Mesa in the extreme northeast corner of the quadrangle was compiled from Goff (1995) and detailed mapping of the Chaquehui and Ancho canyon areas in the southeastern part of the quadrangle was compiled from Reneau et al. (1995a, and unpub.). Dethier (1997) published the geology of the White Rock Canyon 7.5 minute quadrangle, immediately east of Frijoles quadrangle. Additionally, it should be noted that most of the area of Los Alamos National Laboratory was previously mapped by Rogers (1995).

Geology of the Frijoles 7.5-minute Quadrangle, Los Alamos and Sandoval Counties, New Mexico

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