- This event has passed.
Seminar Series – John Westgate, Henry Halls, Mike Gorton
September 20, 2018 @ 12:00 pm - 1:00 pm
Bandelier Tuff (1.24 Ma) from the Valles caldera in New Mexico identified in southern Saskatchewan: a career-long problem solved
Speakers: John Westgate, Henry Halls and Mike Gorton
Integrated field and laboratory methods were key to identifying the source of the tephra bed (Dt), up to 3 m thick, near Duncairn, Saskatchewan, Canada. An unusually low CaO content in its glass shards denies a source in the nearby Yellowstone and Heise volcanic fields. Recent analysis of a tephra bed (LSMt) from the La Sal Mountains in Utah, U.S.A., with a very similar glass chemistry suggests a more southerly source. Comprehensive characterization of these two distal tephra beds, including grain size, mineral assemblage, major- and trace-element composition of glass, paleomagnetism and fission-track dating, in conjunction with analyses of samples collected near the Valles caldera, New Mexico, shows that the source is in the Valles caldera of the Jemez Mountains volcanic field of New Mexico, requiring a trajectory of northward tephra dispersal of ~ 1500 km. Two glass populations exist in each of Dt and LSMt. The proximal correlative of Dt1 is the plinian Tsankawi Pumice and co-ignimbritic ash of the first ignimbrite (Qbt1g) of the 1.24 Ma Tshirege Member of the Bandelier Tuff. The correlative of Dt2 and LSMt is the co-ignimbritic ash of Qbt2. Mixing of Dt1 and Dt2 occurred during northward transport in a jet stream.