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The Los Pinos Formation, generally considered Miocene in age, has been remapped in the Tusas Mountains of north-central New Mexico. The lower part contains igneous breccias and volcaniclastic facies equivalent in petrology and age to the Oligocene Conejos Formation. The upper part of the Los Pinos is composed of volcaniclastic sediments from three source areas: (1) the San Juan Mountains, (2) the Questa area, and (3) a possible volcanic center southeast of Tres Piedras. This newly determined possible center southeast of Tres Piedras was apparently the source of several rhyolitic ash-flow tuffs. Ash-flow tuffs, including some from the Treasure Mountain Tuff, the Masonic Park Tuff, and two unnamed tuffs, underlie or interfinger with the upper Los Pinos Formation. Clasts of sh-flow tuffs from the source area southeast of Tres Piedras are similar to those in the Abiquiu Tuff.
The Los Pinos Formation and its probable stratigraphic equivalents (Picuris Tuff and Abiquiu Tuff) were deposited from at least Chama to Taos and from the San Juan Mountains to the Jemez Mountains. This pattern of deposition was disrupted by the formation of the Jemez volcanic center, the Rio Grande rift, and the excavation of the San Juan basin.
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