The Otowi Member of the Bandelier Tuff, Valles Caldera, New Mexico- a New Volume, and Evidence for Vent Site Evolution During the Eruption (Abstract)

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Conference Paper: The Otowi Member of the Bandelier Tuff, Valles Caldera, New Mexico- a New Volume, and Evidence for Vent Site Evolution During the Eruption (Abstract)

Abstract
The 1.6 Ma eruption of Otowi Member of the Bandelier Tuff, a chemically, mineralogically and isotopically zoned high-silica rhyolitic ignimbrite with subordinate fallout, was associated with the first major episode of collapse at the Valles caldera. We have re-evaluated the total erupted volume for the Otowi Member from existing map and borehole data, and find, within uncertainty, equal volumes for the caldera fill (149 km3 DRE) and reconstructed outflow (167 km3 DRE) for a total ignimbrite volume of 316 km3. Assuming approximately equal caldera fill, outflow, and distal ash volumes in large ignimbrites to the Otowi Member yields a value of 474 km3 DRE for the total erupted volume. Self et al. (1986) proposed that the Otowi eruption began at a single vent, and that activity migrated to vents located around the caldera ring fracture as collapse began. We have attempted to test this model by comparing vertical profiles in pumice chemistry and lithic abundances in outflow sheet sections around the caldera. The underlying assumption is that, given the zoned nature of the tuff, time-synchronous tephra (fallout or ignimbrite) erupted from a single vent, regardless of location, should have a unique combination of pumice chemistry and lithic assemblage, within constraints imposed by lithic friability (which determines whether or not lithics survive), and the rate of change of pumice chemistry during the eruption. Only a few combinations of non-adjacent sections, regardless of depositional mechanism, in fact fulfill this condition, despite little compositional variation in the early-erupted Otowi (ignimbrite sections from the northern Pajarito Plateau, east of the caldera, overlie monotonically zoned plinian tephra that records the entire chemical zoning of the tuff, and themselves contain a highly variable compositionally random assemblage of pumice clasts; these flow units are interpreted as remobilized ignimbrites, originally perched at higher elevations, and are excluded from the analysis). This suggests either that the eruption involved multiple vent sites at most stages after initiation, or that our assumption is invalid. Reference: Self S et al (1986) JGR 91, 1779.

Authors 
G.W. Cook, J.A. Wolff and Stephen Self






Conference 
AGU Fall Meeting; {{{ConferencePlace}}}"{{{ConferencePlace}}}" cannot be used as a page name in this wiki.; 2006


Published 
American Geophysical Union, 2006





DOI 
Not Provided
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Online 
Internet link for The Otowi Member of the Bandelier Tuff, Valles Caldera, New Mexico- a New Volume, and Evidence for Vent Site Evolution During the Eruption (Abstract)

Citation

G.W. Cook,J.A. Wolff,Stephen Self. 2006. The Otowi Member of the Bandelier Tuff, Valles Caldera, New Mexico- a New Volume, and Evidence for Vent Site Evolution During the Eruption (Abstract). In: EOS, Transactions. AGU Fall Meeting; 2006; (!) . (!) : American Geophysical Union; p. #V53C-1773