The Picuris Formation: A late Eocene to Miocene sedimentary sequence in northern New Mexico
S. B. Aby, P. W. Bauer, and K. I. Kelson
New geologic mapping and 12 new 40Ar/39Ar ages from the Picuris Formation have provided new information on this noteworthy Tertiary sedimentary unit in Taos County, New Mexico. Where exposed along the northern and southern flanks of the Picuris Mountains, the formation is composed of: 1) a lower, basement- and volcanic-derived, highly weathered, conglomerate, sandstone, and siltstone member; 2) a middle, tuffaceous/pumicious sandstone member; and 3) a volcaniclastic pebble-conglomerate to mudstone member. Although compositional and textural variability exists within the members and between exposures, and some interfingering of the upper two members occurs, for simplicity we informally refer to them as the lower conglomerate member, the middle tuffaceous member, and the upper volcaniclastic member. The lower conglomerate member is derived from local Proterozoic basement and, in the northeastern Picuris Mountains, from a mixed basement/volcanic source. The volcaniclastic components of the upper two members are derived from the Latir volcanic field with additions from Proterozoic sources. In some areas the upper volcaniclastic member also contains Paleozoic clasts. The lower conglomerate member contains and/or underlies ash layers dated at ca. 34.5 Ma. The middle tuffaceous member contains a 27.9 Ma ash in the north and overlies the 28.3 Ma Llano Quemado breccia. The middle tuffaceous member in the south contains 27.7-23.0 Ma pumice clasts. The upper volcaniclastic member contains volcanic clasts that range in age from 27.3-18.6 Ma. A basalt clast from near the base of the upper volcaniclastic member in the south, dated at 19.8 Ma, provides a maximum age for the southern part of the upper volcaniclastic member. Because the Picuris Formation spans a time range of greater than 34.5 Ma to less than 18.6 Ma, sedimentation may span the time between the end of Laramide shortening and onset of Rio Grande rift extension. The upper volcaniclastic member of the Picuris Formation may be equivalent to the Miocene Chama-El Rito Member of the Tesuque Formation of the Santa Fe Group.
- Aby, S. B.; Bauer, P. W.; Kelson, K. I., 2004, The Picuris Formation: A late Eocene to Miocene sedimentary sequence in northern New Mexico, in: Geology of the Taos Region, Brister, Brian S.; Bauer, Paul W.; Read, AdamS.; Lueth, Virgil W., New Mexico Geological Society, Guidebook, 55th Field Conference, pp. 335-350.