The 26.5 ka Oruanui eruption, New Zealand: an introduction and overview

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Abstract

The 26.5 ka Oruanui eruption, from Taupo volcano in the central North Island of New Zealand, is the largest known ‘wet’ eruption, generating 430 km3 of fall deposits, 320 km3 of pyroclastic density–current (PDC) deposits (mostly ignimbrite) and ∼420 km3 of primary intracaldera material, equivalent to ∼530 km3 of magma. Erupted magma is >99% rhyolite and <1% relatively mafic compositions (52.3–63.3% SiO2). The latter vary in abundance at different stratigraphic levels from 0.1 to 4 wt%, defining three ‘spikes’ that are used to correlate fall and coeval PDC activity. The eruption is divided into 10 phases on the basis of nine mappable fall units and a tenth, poorly preserved but volumetrically dominant fall unit. Fall units 1–9 individually range from 0.8 to 85 km3 and unit 10, by subtraction, is 265 km3; all fall deposits are of wide (plinian) to extremely wide dispersal. Fall deposits show a wide range of depositional states, from dry to water saturated, reflecting varied pyroclast:water ratios. Multiple bedding and normal grading in the fall deposits show the first third of the eruption was very spasmodic; short-lived but intense bursts of activity were separated by time breaks from zero up to several weeks to months. PDC activity occurred throughout the eruption. Both dilute and concentrated currents are inferred to have been present from deposit characteristics, with the latter being volumetrically dominant (>90%). PDC deposits range from mm- to cm-thick ultra-thin veneers enclosed within fall material to >200 m-thick ignimbrite in proximal areas. The farthest travelled (∼90 km), most energetic PDCs (velocities >100 m s−1) occurred during phase 8, but the most voluminous PDC deposits were emplaced during phase 10. Grain size variations in the PDC deposits are complex, with changes seen vertically in thick, proximal accumulations being greater than those seen laterally from near-source to most-distal deposits. Modern Lake Taupo partly infills the caldera generated during this eruption; a ∼140 km2 structural collapse area is concealed beneath the lake, while the lake outline reflects coeval peripheral and volcano–tectonic collapse. Early eruption phases saw shifting vent positions; development of the caldera to its maximum extent (indicated by lithic lag breccias) occurred during phase 10. The Oruanui eruption shows many unusual features; its episodic nature, wide range of depositional conditions in fall deposits of very wide dispersal, and complex interplay of fall and PDC activity.

Introduction

Several deposits of young explosive volcanism at Taupo volcano in New Zealand have become archetypes for studies of large-scale ‘wet’ explosive interaction between silicic magma and external water (Self and Sparks, 1978, Walker, 1981, Self, 1983). Of these, the deposits generated in an eruption at 22,590±23014Cyearsbp (Wilson et al., 1988; equivalent to ∼26.5 calendar ka, using the second-order polynomial U/Th- to radiocarbon-age calibration of Bard, 1998) represent the largest and most widespread products of wet volcanism documented. Many names have been applied to this eruption and its deposits (Self and Healy, 1987, for summary). Here and in previous papers I use the term ‘Oruanui’ because (a) it was the first name applied (by Vucetich and Pullar, 1969) that recognised the essence of the deposits (viz., an extremely widespread ash-fall deposit accompanied by a voluminous ignimbrite), and (b) the alternative term ‘Wairakei’ used by Self, 1983, Self and Healy, 1987 had previously been and is applied to a wholly different, older welded ignimbrite (Grindley, 1965).

In this paper, I present an overview of the Oruanui eruption and its deposits to provide a framework within which subsequent papers can be placed in context. I discuss the stratigraphy and lateral correlations of the fall deposits in some detail here as these differ significantly from those previously proposed. In addition, the wide range of Oruanui pyroclastic density current (PDC) deposits are described in outline as a contribution to the recognition of a broader-than-usual spectrum of pyroclastic ‘flow’ deposits. Further papers will present the detailed data for individual units within the fall and PDC deposits and consider the dynamics of their parental eruptive phases.

The Oruanui eruption represents one of the youngest caldera-forming events in the rhyolite-dominated central Taupo Volcanic Zone (TVZ) in New Zealand (Houghton et al., 1995, Wilson et al., 1995a; Fig. 1). Taupo volcano and its neighbour Maroa to the north (Fig. 1) are built on the remnants of an older large caldera associated with the 340–320 ka Whakamaru-group welded ignimbrites (Wilson et al., 1986, Houghton et al., 1995). At Taupo the modern activity of overwhelmingly pyroclastic (>95%) volcanism, from vents mostly now concealed beneath Lake Taupo, began around 65 ka and has waxed coevally with the waning of lava-dominated activity at Maroa. Pyroclastic deposits exposed in the Maroa–Taupo area represent >9 eruptions from ∼64 to ∼27 ka (CJNW, work in progress; cf. Vucetich and Howorth, 1976a), the Oruanui eruption, then a sequence of 28 eruptions, all but three of them in the last ca. 12,000 years (Wilson, 1993).

Previous workers (Vucetich and Pullar, 1969, Vucetich and Howorth, 1976b, Self and Sparks, 1978, Self, 1983, Self and Healy, 1987) had recognised that the Oruanui eruption was large, involved large-scale magma:water interaction and was unusually complex. My work shows that the Oruanui deposits are significantly larger than previously thought, representing one of the largest documented eruptions in the last 250,000 years, only exceeded by the 74 ka youngest Toba eruption (Rose and Chesner, 1987). Previous stratigraphies of the Oruanui deposits had culminated in a 6-member division of the proximal deposits, comprising three fall members overlain by finer-grained ignimbrite (Member 4) a fall deposit (Member 5) then coarser-grained ignimbrite (Member 6: Self and Sparks, 1978, Self, 1983, Self and Healy, 1987). In distal areas, the six members were also recognised, with Members 4 and 6 being fall material inferred to be coeval with, and derived from, the corresponding proximal ignimbrite. This 6-member subdivision is not followed here because of the recognition by me of a more-detailed stratigraphic sequence (illustrated schematically in Fig. 2 and summarised in Table 1), significant miscorrelations in the fall deposits, and the presence of largely coeval generation of PDC and fall deposits throughout the eruption.

The chemistry of the Oruanui eruption products is detailed elsewhere (Sutton, 1995, Sutton et al., 1995; Blake et al., manuscript in prep.) and only specifically relevant in one respect to the eruptive stratigraphy. Of the extra-caldera Oruanui eruptives >99% are rhyolite (SiO2=71.8–76.7%, calculated volatile-free, but 70% of all analyses are 74.5–76.0% SiO2). However, the remaining <1% juvenile products are more-mafic material (52.3–63.3% SiO2) that occurs from ash-grade to 10–15 cm fragments. Hand-specimen mafic clasts are inferred to be juvenile from their often-cauliform shape, ubiquitous chilled margins and the presence of white pumiceous rhyolite material coating the surface and ingested into the clasts. Although juvenile mafic material is ubiquitous in most of the deposits (typically 0.1–0.5 wt%), it is notably abundant (1–4 wt%) in fall units 3+4, 7 and 9 and parts of the ignimbrite, forming ‘spikes’ used in stratigraphic correlation (Fig. 3; see below).

Section snippets

Oruanui stratigraphic framework

The Oruanui stratigraphic framework is founded on recognition of 10 fall units (Table 1). Nine of these can be correlated for ≥150 km as distinct units; the tenth is a ‘catch-all’ representing the poorly-preserved (but volumetrically dominant) remainder of the fall deposit. The eruption is thus divided into 10 phases, each of which corresponds to the respective fall unit. Oruanui PDC deposits are referred to by the relevant phase of the eruption that they were generated in, and are correlated

Overview of the fall deposits

The Oruanui fall units are all of wide to extremely wide dispersal (e.g. Self, 1983, Wilson, 1994). The deposits also vary in grading, sorting, and evidence for degrees of ‘wetness’ on deposition that vary quasi-systematically both within individual units and also over specific geographic areas. Here I discuss these features insofar as they reflect the evolution of the eruption. Detailed field and grain size data for individual units will be presented elsewhere.

Eruption timing and episodicity

A characteristic feature of wet volcanism in general is its non-uniform nature, seen in observed eruptions (e.g. White and Houghton, 2000) and inferred from prehistoric deposits (e.g. Houghton et al., 1999) to occur on two time scales. The first, of seconds to minutes, reflects fluctuations in the degree of magma:water interaction, leading to individual observed explosions, and/or inferred formation of individual laminae in deposits. The second, of hours to tens of hours, is driven by changes

Conclusions and comparisons

The Oruanui eruption in total represents a quite extraordinary event, both in terms of size and complexity, and represents the largest extreme of ‘wet’ volcanism yet documented (Self, 1983). Total bulk volumes inferred are ∼750 km3 for extracaldera material, and ∼420 km3 for primary intracaldera deposits, equivalent to ∼530 km3 of magma (230 km3 from intracaldera material and ∼300 km3 from extracaldera material, the latter assuming a mean dry bulk density of 1100 kg m−3 and a mean lithic content of 15 

Acknowledgements

I thank Steve Self, George Walker, Bruce Houghton and Kate Wilson for their encouragement and support in this work, Brent Alloway, Iain Campbell, Bruce Houghton, Zinzuni Jurado-Chichay, Ted Lloyd, Ian Nairn, Brad Pillans, Steve Self, and George Walker for locality information, and Mike Rosenberg, Brad Scott and Jason Britten for technical assistance. I also acknowledge many landowners for access, in particular the forestry companies of the central North Island and especially Colin Dunstan (New

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