A NEW LOOK AT THE DEPOSITS AND ERUPTION SEQUENCE OF THE OTOWI MEMBER, BANDELIER TUFF FORMATION, JEMEZ MOUNTAINS, NEW MEXICO

The Otowi Member (1.6 Ma) produced Plinian and co-ignimbrite fall deposits, outflow and intra-caldera ignimbrite, and the first collapse event of Valles caldera (24 km diameter). Previous work showed the eruption withdrew magma from a high-silica-rhyolite chamber possessing strong zonation in trace elements. Recent work reveals new insights on the products and eruption sequence: 1) The first phase produced Guaje fall deposit unit A, after which column collapse produced coeval pyroclastic flows and fall deposits (units B-F) in a complex depositional pattern. In places the ignimbrite is intra-Plinian and in others fall units could accumulate while pyroclastic flows bypassed the area. Combined physical and pumice compositional data from deposits suggests that Plinian (vent-derived) fall deposition persisted late into the eruption. 2) The previous idea of switching from an initial central vent to vents on caldera ring-fractures at the time of deposition of co-ignimbrite lag-breccias is revised to reveal that the central vent persisted longer into the eruption on grounds of pumice composition combined with lithic abundances in ignimbrite. Lag-breccia deposition was thus not proximal to vent. 3) 50 km from vent, near Truchas, a preserved Otowi sequence shows fall units A-E and thin ignimbrite veneer; reworked fine ash at top may have co-ignimbrite component. This shows that at least some of the pyroclastic flows were widespread and energetic; compositional data identifies these as some of the late-erupted flows. Moreover, Otowi ignimbrite filling the Rio Grande valley were rapidly remobilized and are found as lahar-like deposits downriver near Socorro. 4) A distal fall deposit extends 100s km from source over Texas and consists of both Plinian column-derived and co-ignimbrite ash; ash composition is similar to lower units of Guaje fall deposit. 6) A revised minimum volume (DRE) estimate for Otowi deposits is: fall deposits 80 km³; outflow ignimbrite 140 km³; intra-caldera ignimbrite 150 km³; total ~ 370 km³. Co-ignimbrite component of distal ash is most probably underestimated here; the likely original total is possibly 450-500 km³, placing the eruption at the low end of the super-eruption spectrum (Magnitude 8.1).

[Statements herein are those of the authors and do not reflect the view or regulatory position of US NRC.]