COMPREHENSIVE STRATIGRAPHY OF THE TSHIREGE MEMBER OF THE BANDELIER TUFF, NEW MEXICO

The 1.25 Ma high-silica rhyolite Tshirege Member of the Bandelier Tuff, erupted from Valles caldera, New Mexico, consists of the basal Plinian Tsankawi Pumice deposit and an overlying compositionally-zoned, ignimbrite flow unit sequence. Previous studies of the Tshirege Member stratigraphy, chemistry, mineralogy, and petrology have focused on its eastern lobe, which forms the Pajarito Plateau. This study extends the litho-chemical stratigraphy previously developed for the Tshirege Member on the Pajarito Plateau to the less documented western portion on the Jemez Plateau. The extended stratigraphy is based on 36 stratigraphic sections and 14 individual sites which are distributed around the caldera, and 182 whole-rock and pumice samples analyzed for major and trace elements, and provides a new attempt at a definitive, comprehensive stratigraphy for the Tshirege Member including both the Jemez and Pajarito Plateaus. There is considerably less chemical variation in the western lobe than the eastern lobe; the late-erupted, chemically less evolved compositions are for the most part missing from the Jemez Plateau. The existence of a more complete eruption stratigraphy to the east is consistent with the deepest collapse of the asymmetric Valles caldera having occurred in its southeastern portion. Nonetheless, there is a tendency for compositional gaps that appear in the western lobe data to be filled in the eastern lobe data, and vice-versa, suggesting switching of pyroclastic flow emplacement between the two major depocenters as the eruption proceeded. Discrimination and correlation of the units of the Tshirege Member of the Bandelier Tuff (Qbt) is important because it can provide structural control, event stratigraphy, a means to evaluate ignimbrite facies, and a regional stratigraphic framework from which an eruptive sequence can be developed, giving a perspective on the eruption and pre-eruption evolution of a zoned magmatic system.