The group was first defined by Griggs in 1964 his study of groundwater resources near the town of Los Alamos, New Mexico. The group was named for the Tewa Mountains, an obsolete name for the Jemez Mountains. Bailey, Smith, and Ross adopted Grigg's nomenclature in 1969 as part of their work establishing the stratigraphy of the Jemez Mountains, but proposing some further subdivisions of formations and members.
Geology
The Jemez Mountains lie on the intersection of the western margin of the Rio Grande Rift and the Jemez Lineament. Here magma produced from the fertile rock of an ancient subduction zone has repeatedly found its way to the surface along faults produced by rifting. This has produced a long-lived volcanic field, with the earliest eruptions beginning at least 13 million years ago in both the northern and southern portions of the volcanic field. High-silica eruptions of the Tewa Group began about 1.85 million years ago and continued almost to the present day The Tewa Group is a sequence of mostly rhyolitepyroclastic flows and domes that underlies much of the Valles Caldera and forms extensive outflow sheets surrounding the caldera. It has a maximum thickness of. It overlies the Santa Fe, Keres, and Polvadera Groups to the east and south of the caldera and older rocks ranging in age from Paleoproterozoic to Permian to the west and north. It forms the upper surfaces of the Pajarito Plateau east of the caldera and the Jemez Plateau west of the caldera. There is geochemical evidence that the Tewa Group originated through rejuvenation of intrusions associated with the Paliza Canyon Formation of the Keres Group.
Formations
From oldest to youngest, the formations recognized within the Polvadera Group are the Bandelier Tuff, the Cerro Toledo Formation, and the Valles Rhyolite. However, the Cerro Toledo Formation was emplaced between the Otowi Member and Tsherige Member of the Bandelier Tuff. The Bandelier Tuff is a sequence of rhyolitic pyroclastic flows erupted in three caldera eruptions, at 1.85 Ma, 1.62 Ma, and 1.25 Ma. These form a vast outflow sheet surrounding the Jemez Mountains. The Cerro Toledo Formation consists of rhyolitic domes erupted in the Toledo Embayment, a structural feature in the northeast caldera rim, between the Toledo and Valles events. It also includes ring fracture dome remnants of the Toledo event and mixed pumice and sediment beds separating the Otowi and Tsherige Members. Ages range from 1.54 Ma for the west Los Posos dome to 1.20 Ma at Pinnacle Peak. The Valles Rhyolite consists of rhyolite domes and flows associated with caldera resurgence and with the ring fracture of the Valles event. It includes the most recent eruptions in the Jemez Mountains, ending with the Banco Bonito flow. The earliest members, the Deer Canyon Member and Redondo Creek Member, are associated with resurgence, with argon-argon ages of 1.23 to 1.28 Ma. The Cerro del Medio Member, Cerros del Abrigo Member, Cerro Santa Rosa Member, Cerro San Luis Member, Cerro Seco Member, San Antonio Mountain Member, and South Mountain Member are individual ring fracture dome complexes that show a progression in age counterclockwise along the presumed ring fracture of the Valles caldera. The El Cajete, Battleship Rock, and Banco Bonito flows are the youngest flows in the Jemez volcanic field, erupting from 77.4 to 68.3 ka. The quartz latite of Cerro Rubio, originally included in the Tewa Group, has been reassigned to the Tschicoma Formation of the Polvadera Group based on radiometric dating and geochemisry.
