Stratigraphy and dispersal of the Mangaone Subgroup pyroclastic deposits, Okataina Volcanic Centre, New Zealand

The Mangaone Subgroup from the Okataina Volcanic Centre consists of silicic plinian pyroclastic units that lie between the Oruanui deposit from the Taupo Volcanic Centre (corrected 14C calendar age=26.5 ka) and the Rotoiti (flow)/Rotoehu (fall) deposits from Okataina Volcanic Centre (K–Ar age=64 ka). In this study we present a new stratigraphy for the subgroup that is significantly revised from the earlier work and was developed after extensive field mapping and laboratory study. We now recognize and describe a total of 12 rhyolitic units (as opposed to the 8 that had been recognized previously), with a total bulk eruptive volume of approximately 81 km3. The vents for all the Mangaone Subgroup units were within the Haroharo caldera and possibly migrated through time, first within a roughly north–south trending zone in the central part of the caldera and then toward the eastern part of the caldera. We present new radiometric ages suggesting that the whole sequence was erupted in a relatively short time span (possibly 10–15 ka). Our findings of more eruptions in a shorter time span than in previous work underscores the utility of detailed field studies, particularly when volcanic hazards are being assessed. All the fall units have plinian dispersals but show various degrees of magma-water interaction. The majority of the eruptions were large and intense enough to generate pyroclastic flows, yet flows were observed in only 2 units and is volumetrically small.