THE TSHIREGE MEMBER OF THE BANDELIER TUFF: A NEW ATTEMPT AT A COMPREHENSIVE STRATIGRAPHY

The 1.25 Ma Tshirege (upper) Member of the Bandelier Tuff (Qbt) was first described by Smith and Bailey (1966). Since then, most studies of the unit’s stratigraphy, mineralogy, petrology and chemistry have focused on its eastern lobe, which forms the Pajarito plateau. This study extends the litho- and chemical stratigraphy previously developed for the Qbt on the Pajarito plateau to the less documented western lobe on the Jemez plateau, based on 34 sections 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 entire unit including both the Jemez and Pajarito Plateau lobes. There is considerably less chemical variation in the western lobe than the eastern lobe; the late-erupted, chemically less evolved compositions are missing from the Jemez Plateau. The existence of a more complete eruption stratigraphy in the east is consistent with the deepest collapse of the asymmetric Valles caldera, and hence more complete evacuation of the magma body, 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 subunits 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.

Reference: Smith, R.L., Bailey, R.A., 1966. Bull. Volc. 29, 83-104.