RECENT MAPPING OF TERTIARY VOLCANICLASTIC ROCKS IN THE TUSAS MOUNTAINS, NORTHERN NEW MEXICO

Recent mapping by the author and colleagues under the New Mexico STATEMAP program of the New Mexico Bureau of Geology and Mineral Resources has added detail to the Tertiary stratigraphic history of the northern part of the Tusas Mountains in north central New Mexico. The Las Tablas and Cañon Plaza Quadrangles have been mapped at 1:12,000 scale over the last four years (2009-12). These quadrangles are adjacent to each other and are located southwest of Tres Piedras and north of Ojo Caliente, NM. The bulk of the Tertiary record is represented by the Los Pinos Formation. The oldest Tertiary unit is the Las Tablas Tuff (formerly Tuff of Las Tablas) which is a ~28 Ma tuffaceous unit that is interpreted as a combination of primary ignimbrite and large-scale, probably syn-eruptive, mass wasting deposits. This unit is age-correlative with two named ignimbrites in northern New Mexico/Southern Colorado (the Tuff of Tetilla Peak in the Latir Field and The Fish Canyon Tuff of the San Juan Field), but is geochemically dissimilar to both in some respects. If the Las Tablas Tuff is a distinct ignimbrite then at least three caldera forming(?) eruptions may have occurred in this area in a limited time. If the Las Tablas Tuff is equivalent to the Fish Canyon Tuff it is the southernmost deposit of that unit which is already thought to represent one of the largest eruptions ever recorded. Mapping reveals field relations indicating that the Abiquiu Formation is a fine-grained equivalent of the Cordito Member of the Los Pinos Fm., confirming litholgic correlations. Tertiary units in the mapped area onlapped a topography developed in Proterozoic rocks that has at least 100 m of relief. The area was therefore uplifted between the end of the Laramide orogeny and ~28 Ma or represents a highland that existed during the transition from Laramide to Rio Grand Rift tectonics, possibly generated during the widespread volcanic episode that preceded rifting.