Dismembering the Lava Creek Tuff, Yellowstone volcano

The youngest large explosive eruption of Yellowstone volcano generated the 0.63 Ma Lava Creek Tuff (LCT) ignimbrite and a voluminous, widespread fall deposit accompanying formation of Yellowstone Caldera. The tuff is conventionally divided into members A (LCT-A) and B (LCT-B), defined by a reversal in density/welding profiles and the presence of phenocrystic amphibole in LCT-A. Remapping within the northeastern caldera sector has, however, documented multiple ignimbrites that yield LCT eruption ages but are distinct from documented members A and B. Thus, the framework by which the LCT was originally subdivided is re-examined here using field observations, petrologic analyses, and sanidine geochemistry from LCT-A and LCT-B ignimbrite (as previously mapped) at reference localities around the caldera. Mineral modal abundances in both LCT-A and LCT-B outcrops are dominated by quartz, followed by sanidine, with sizes and abundances that generally increase up-section, as previously documented. Amphibole is found by us, however, only in western and southern sectors, in deposits mapped as both LCT-A and LCT-B, and only as a trace phase. Further, a locality reported as LCT-A with amphibole present yields a sanidine ⁴⁰Ar/³⁹Ar date of 0.775 ± 0.002 Ma, precluding any LCT correlation. Ignimbrite samples from mapped LCT-A and LCT-B exhibit variable crystal sizes, sanidine cathodoluminescence image populations and Ba concentrations, with no clear distinction between the mapped members. Collectively this research found no evidence for a definable LCT-A versus LCT-B, meaning that the traditional A and B subdivision does not accurately represent the eruptive architecture of the Lava Creek Tuff and warrants revision.