Tshirege Member

Tshirege Member
Name	Tshirege Member
Type	Volcanic unit
Period	Pleistocene
Primary lithology	Rhyolite, tuff, welded tuff, pumice
Other lithology	Vitrophyre, obsidian, breccia
Named for	Keresan Pueblo
Region	Jemez Mountains, New Mexico
Country	United States
Unit of	Bandelier Tuff
Subunits	Tsankawi, Cerro Toledo intervals
Underlies	Pajarito Plateau deposits
Overlies	Otowi Member
Thickness	up to 100 m

Tshirege Member is the younger, thicker, and more widespread portion of the Bandelier Tuff, emplaced during large explosive eruptions of the Valles caldera in the Jemez Mountains of northern New Mexico. It crops out extensively across the Pajarito Plateau, forming cliffs, mesas, and ignimbrite sheets that dominate landscapes around Los Alamos, White Rock (New Mexico), and Frijoles Canyon. The unit records high-silica rhyolitic volcanism tied to caldera collapse and provides key stratigraphic markers used in mapping hazards by the United States Geological Survey and in archaeological studies of Puebloan occupation.

Description and Lithology

The member consists predominantly of high-silica rhyolitic ignimbrite composed of welded tuff, dense rhyolite flows, pumice-rich lithic tuff, and glassy vitrophyre. Dominant facies include massive, densely welded ash-flow sheets with eutaxitic textures, pumice clasts, and fiamme, interbedded with less-welded tuffs and breccias. Phenocrysts of quartz, sanidine, biotite, and plagioclase are common, with accessory zircon and titanite crystals used in geochronology. Vitrophyric zones and obsidian lenses are locally developed near erosion-resistant knobs such as those at Tsankawi and Bandelier National Monument. The unit exhibits columnar jointing, cooling fractures, and strong rheomorphic textures where post-depositional welding and compaction occurred.

Stratigraphy and Correlation

Stratigraphically the member overlies the older Otowi Member of the Bandelier Tuff and is conformably succeeded by alluvial and pediment deposits of the Pajarito Plateau. It is correlated regionally with ignimbrite units erupted during the late Pleistocene Valles caldera episode, and is time-equivalent to distal ash layers recognized in Rio Grande Rift basins. Tephrochronologic correlations link Tshirege deposits to widespread marker beds identified in Yellowstone National Park studies, Coso Volcanic Field comparisons, and distal marine records via glass chemistry and 40Ar/39Ar dates. Lateral facies changes reflect proximal-to-distal gradients preserved between Valles Grande, Cerro Grande, and peripheral canyons.

Geologic History and Depositional Environment

Emplacement occurred during one or more catastrophic pyroclastic eruptions associated with caldera-forming collapse at the Valles caldera complex in the late Pleistocene, producing high-velocity, high-temperature pyroclastic density currents. Depositional environments include welded ash-flow sheets emplaced across plateaus, channelized ignimbrite within paleocanyons, and ponded units in topographic lows. Post-emplacement processes involved devitrification, hydrothermal alteration linked to Valles hydrothermal system, faulting along structures related to the Rio Grande rift, and Quaternary erosion that sculpted amphitheaters at Frijoles Canyon and Pajarito Mesa. The thermal and deformation history influenced secondary mineralization and the preservation of archeological sites at Bandelier National Monument and nearby White Rock Canyon.

Paleontology and Fossil Record

Although primarily volcaniclastic, the member contains localized paleobiological evidence preserved in interbedded lacustrine and soil horizons, including pollen assemblages that inform Pleistocene vegetation correlations with Great Basin and Southern Rocky Mountains floras. Charcoal layers record fire events relevant to paleoenvironmental reconstructions used alongside studies at Bandelier National Monument and Valles Caldera National Preserve. Faunal remains are rare but include isolated vertebrate fragments and microfossil occurrences recovered from cave deposits and rock shelters occupied by Pueblo peoples. Tephra layers within archaeological stratigraphies provide chronological ties to occupations at Tsankawi and Puye Cliff Dwellings.

Economic Resources and Uses

Locally, the member supplies construction-grade rock, dimension stone, and aggregate for regional infrastructure in Los Alamos County and Santa Fe County. Obsidian and glassy vitrophyre were historically exploited by prehistoric peoples for toolstone traded across the Southwest and studied in exchange networks involving sites such as Chaco Culture National Historical Park and Mesa Verde National Park. Groundwater flow in porous tuff units influences municipal supplies for Los Alamos and constrained aquifer studies by the U.S. Department of Energy at the Los Alamos National Laboratory site. Hydrothermal alteration zones host minor mineralization that has been the focus of exploration by state geological surveys.

Research and Notable Studies

Key investigations include stratigraphic mapping by the United States Geological Survey, radiometric dating using 40Ar/39Ar and U-Pb analyses by university teams from University of New Mexico and New Mexico Institute of Mining and Technology, geochemical fingerprinting by researchers affiliated with Los Alamos National Laboratory, and tephrochronology integrated into paleoenvironmental studies at Smithsonian Institution-linked projects. Landmark studies on emplacement dynamics and welding were published in journals associated with the Geological Society of America and Journal of Volcanology and Geothermal Research, and interdisciplinary work links volcanology to archaeology via collaborations with National Park Service archaeologists. Ongoing monitoring of the Valles caldera system by the USGS Valles Observatory and academic consortia continues to refine eruptive histories and hazard assessments.

Category:Geologic formations of New Mexico Category:Ignimbrites Category:Jemez Mountains