New Mexico Geological Society Annual Spring Meeting — Abstracts

Consisting of two members erupted as a series of ash rich ignimbrites during enormous, caldera-forming events at about 1.61 Ma and 1.22 Ma, the Bandelier Tuff’s exposure throughout the Jemez Mountains of northern New Mexico is extensive. The Bandelier Tuff controls much of the flow of groundwater beneath Los Alamos, and feed aquifers forthe surrounding communities. The material properties of the Bandelier Tuff are relevant to a number of regional environmental issues such as groundwater contamination and earthquake hazards.

Cooling units in the Bandelier Tuff, which may or may not coincide with contacts between flow units, have been characterized based on primary and secondary features, including welding, devitrification, vapor-phase alteration, oxidation and relative content of crystals, pumice, and lithic fragments. This investigation further characterizes the Bandelier Tuff on the basis of strength, as determined via Schmidt hammer rebound numbers, point load experiments, and triaxial compression tests. Schmidt hammer rebound data collected thus far yield distinct values ranging from 12 to 57 and may correspond to welding, as determined by the morphology of the Bandelier Tuff in thin section and geochemical zonation. Because the degree of welding (among other features such as devitrification and vapor-phase alteration) can vary laterally and can depend on several factors, including temperature at time of deposition, input of meteoric water during cooling, major and trace element data are often used to provide definitive identification of flow units. Our data is consistent with previous studies that show that the Tshirege Member is stratigraphically zoned from more high-silica rhyolite at its base to more low-silica rhyolite at its top.