New Mexico Geological Society Annual Spring Meeting — Abstracts

New 1:24,000 scale mapping in the Jemez Mountains outside the Valles caldera is revealing previously undescribed volcanic units and/or stratigraphic relationships. We have identified several major Tschicoma flows and centers in the northern Jemez Mountains, including the massive Rendija lava flow in northern Los Alamos County. We recognize a general trend in the age of volcanism across the northern Jemez volcanic field. Lobato basalt, andesite, and dacite are 9-10 Ma in the eastern part of the field and 7-8 Ma toward the west. Tschicoma andesite and dacite are7-8 Ma in the west and 3-5 Ma towards the east. We also observe Tschicoma-type dacite lava and pumice interbedded with or underlying 9-10 Ma Lobato Formation basalt. Miocene (9-10 Ma) basalt is present in both the northern (Lobato) and southern (Paliza Canyon) parts of the field. Basaltic centers (Lobato) with few interbedded sendiments prevail to the north, while 1 to 5 m thick basalt flows (Paliza Canyon) with approximately 10-15 m of interbedded volcaniclastic sediments are common to the south, suggesting proximal and distal portions of the same volcanic system. We have identified a unit that covers much of the northeastern quadrant of the Jemez Mountains that consists of pumice and a <0.1 m-thick lithic-rich tuff resting on the Puye Formation gravels. This unit may be related to the Otowi member of the Bandelier Tuff or San Diego Canyon Tuff. Shards of obsidian are also common in this area. The observations listed here are representative of the exciting work to be discussed in this special section.