The Ohakuri pyroclastic deposits and the evolution of the Rotorua-Ohakuri volcanotectonic depression. (2004)
AuthorsGravley, Darren MClurgshow all
The caldera-forming Ohakuri pyroclastic deposits (~ 100 km3 magma) were erupted at ~ 240 ka from the newly defined Ohakuri caldera, which is located within the central Taupo Volcanic Zone (TVZ) of New Zealand. The Ohakuri pyroclastic deposits are remarkable for their widespread lateral and vertical lithofacies variation that is attributed to phreatomagmatic eruption dynamics and a variable depositional environment. The Ohakuri pyroclastic deposits overlie 3 precursor airfall units erupted from a source within what was to become the Ohakuri caldera. The third of these fall units, which directly underlies the Ohakuri deposits, is a plinian-style deposit (unit 3) that is interbedded with distal Mamaku ignimbrite (>145 km3 magma) from the Rotorua caldera, ~ 25 km north of the Ohakuri caldera. It is thus inferred that these two major eruptions must have overlapped such that the Ohakuri pyroclastic deposits and Mamaku ignimbrite were erupted at most weeks to months apart. The complex Ohakuri deposits, previously documented in part as sediments, are here described in terms of pyroclastic lithofacies that are grouped into 5 geographically distinct lithofacies associations. Each Ohakuri lithofacies association thus represents a distinctive style of deposition in an eruption that was highly variable in time and space. One lithofacies association consists of giant-dune-bedforms with wavelengths up to 42 m that are characterised by an anomalously high fraction of fine ash. Two other lithofacies associations are related to the interaction between primary pyroclastic density currents and a wet depositional environment that triggered several episodes of secondary hydroeruptions. These hydroeruptions and their deposits provide evidence for time breaks in the Ohakuri eruption sequence that, otherwise, could (the time breaks) not be discerned from thick accumulations of structureless ignimbrite. Geochemically, the Ohakuri pumice compositions range from silicic type 1 and 2 compositions to a dacitic type 3 composition. The distribution of the 3 pumice types, matched with the distribution of Ohakuri lithofacies associations, reveals an eruption sequence that can be divided into two main events. The Ohakuri and Mamaku eruptions, together with the syn-volcanic subsidence of the central Kapenga area (> 100 km2), formed what is defined here as the Rotorua-Ohakuri volcanotectonic depression. With respect to the central Kapenga area, paleogeographic reconstruction from fieldwork and age data shows that > 250 m of vertical displacement occurred on its western margin (the Horohoro Fault scarp) in one large 'superfault' event. The subsidence of this region was induced by lateral withdrawal of magma, via a NE-SW trending conduit system, which was then erupted from the Ohakuri caldera.