REVISIONS TO THE STRATIGRAPHY AND VOLCANOLOGY OF THE POST-0.5 MA UNITS AND THE VOLCANIC SECTION OF VC-1 CORE HOLE, VALLES CALDERA, NEW-MEXICO

Reexamination of the Continental Scientific Drilling Project VC-1 core, new field data, and new isotopic ages necessitate revision of the previously described geology of the youngest eruptive units from the Valles caldera. The Banco Bonito rhyolite lava flow and minor attendant pyroclastic, autoclastic, and reworked deposits about 0.9 km3 dense rock equivalent volume (DRE), are recognized as the product of the youngest eruption from the caldera. The best estimate of the age of this event is 170-240 ka (1-sigma). A slightly earlier eruption produced the El Cajete pumice fall deposit, followed by the Battleship Rock ignimbrite and the VC-1 Rhyolite, totalling about 1.8 km3 DRE. We suggest that the VC-1 Tuffs, ignimbrites recognized only in the VC-1 core hole, are not a separate unit as previously proposed but are, in fact, the Battleship Rock ignimbrite. A complex volcaniclastic breccia dating from about the time of the South Mountain Rhyolite lava flow (529 ka) is the lowermost volcanic unit in the VC-1 core. The young, post-500 ka eruption products from the SW part of the Valles caldera are all composed of very similar low-silica rhyolite and are thus distinct from earlier, postcaldera high-silica rhyolites included in the Valles Rhyolite Formation. This distinction marks a major change in the magmatic evolution of the Valles caldera. We propose that recognition of this change necessitates revisions to the stratigraphy of the upper part of the Tewa Group.