Reconstructing pyroclastic flow dynamics and landscape evolution using the Upper Bandelier Tuff, Puye quadrangle, New Mexico
David P. Dethier and Stephanie K. Kampe
Exposures of the upper unit (Tshirege Member) of the Bandelier Tuff on the northeastern Pajarito Plateau, preserve information about the dynamics of distal pyroclastic flows, the landscape buried by the tuff, and post-Bandelier geomorphic evolution. Our analysis reflects mapping of upper Bandelier units on the Pajarito Plateau in the Puye 7.5-minute quadrangle, 15 to 20 km from their eruptive source, and synthesis of research from adjacent areas of Los Alamos National Laboratory (LANL). Understanding the present distribution and internal boundaries within the upper Bandelier Tuff aids in paleoseismic studies and analysis of groundwater and contaminant transport in the Los Alamos area. Tshirege tuff crops out beneath narrow mesas in the western and central portion of the Puye quadrangle. Tuff classification systems developed in the LANL area apply reasonably well in the study area. Unit 1g, the basal Tshirege tuff, is mainly covered and protected by unit 1v. Unit 2 is thinner than the underlying tuff units and forms only a thin, resistant cap on some mesas. At a few locations, thin remnants of unit 3 cap the Tshirege section. In the LANL area to the west, Tshirege units are slightly thicker, less deeply eroded and more continuous. Isopach maps of unit 1g display thickness variations that highlight paleotopography, delineate an easterly flow path from the rim of the Valles caldera into the Puye quadrangle and indicate lobate flow morphology. Where lobes overlap, unit 1 contains abundant pumice concentrations and surge deposits. Exposures of units 2 to 4 indicate deposition as pyroclastic fans that spilled out across the western Pajarito Plateau little affected by topography, leaving a relatively smooth surface. Structural contours on the base of unit 1g show that two broad, Tshirege-filled paleodrainages underlie the study area; the largest can be considered a “paleo-Guaje Canyon”. South of modern Guaje Canyon, buried canyons trend southeast, bending south in the southern LANL area. Flow thickness and the degree of welding in Tshirege units, base-level changes along the Rio Grande and pre-Bandelier stratigraphy influenced Pleistocene landscape evolution. Tshirege outcrops overlying Pliocene lacustrine deposits failed in massive slumps as headward incision by tributaries removed lateral support. Canyon cutting initiated erosion of the Tshirege, but in most areas upstream from the slumps, weathering processes, block-failure and surface erosion drove cliff retreat.
- Dethier, David P.; Kampe, Stephanie K., 2007, Reconstructing pyroclastic flow dynamics and landscape evolution using the Upper Bandelier Tuff, Puye quadrangle, New Mexico, in: Geology of the Jemez Region II, Kues, Barry S.; Kelley, Shari A.; Lueth, Virgil W., New Mexico Geological Society, Guidebook, 58th Field Conference, pp. 344-353.