New Mexico Geological Society Annual Spring Meeting — Abstracts

The Cerro Toledo interval (Broxton and Renau, 1995) consists of high-silica rhyolite tephras and volcaniclastic sediments that were deposited primarily to the east and southeast of the Valles Caldera. in the areas of the Pajarito Plateau and St. Peter's Dome, Jemez Mountains. between 1.22 and 1.61 Ma. Beneath the Pajarito Plateau, Cerro Toledo Rhyolite tephras and interbedded volcaniclastic sediments lie stratigraphically between the Otowi and Tshirege Members of the Bandelier Tuff. Stratigraphy of the Cerro Toledo interval has been examined at several locations to the nonh and south of Los Alamos National Laboratory (LANL), however the distribution of individual eruptive units and volcaniclastic sediments below LANL remains unclear. Previous studies of the Cerro Toledo Rhyolite have focused primarily on eruption processes, and on the geochemical evolution of the magma chamber between eruptions of the Otowi and Tshirege Members of the Bandelier Tuff. Characteristic ratios of Zr/Nb:Nb, K/Cs, Rb/Sr, La/Yb. and Th/Nb for Cerro Toledo tephra units (Stix et al.. 1988 and Spell et al. 1996)) allow for correlation of tephras using trace-element geochemistry. Examination of the Cerro Toledo interval in several cores drilled for the Environmental Restoration Project at Los Alamos National Laboratory allows for a detailed examination of the distribution and thickness of Cerro Toledo tephras and sediments. Because it contains numerous pumice-fall deposits and coarse gravels. as well as fine-grained hydrovolcanic units, the Cerro Toledo interval could be the tirst signiticant perching layer encountered by infiltrating water. The subsurface stratigraphy of the Cerro Toledo interval is imponant for understanding groundwater flow beneath the Pajarito Plateau and determining paleodrainage patterns and landscape evolution between deposition of the Otowi and Tshirege Members of the Bandelier Tuff.