New Zealand Journal of Geology and Geophysics abstracts
The 26.5 ka Oruanui eruption, New Zealand: a review of the roles of volcanism
and climate in the post-eruptive sedimentary response
Vern Manville
Institute of Geological & Nuclear Sciences
Wairakei Research Centre
Private Bag 2000
Taupo 2730, New Zealand
email: v.manville@gns.cri.nz
Colin J. N. Wilson
Institute of Geological & Nuclear Sciences
Gracefield Research Centre
P.O. Box 30 368
Lower Hutt 6315, New Zealand
Abstract The landscape response to large explosive pyroclastic
volcanic eruptions is one of the most dramatic processes in sedimentology
and geomorphology. Processes of post-eruptive erosion and resedimentation
are maximised by large erupted volumes, abundant unconsolidated ash-sized
material, destruction of the vegetation cover (particularly by burial by ignimbrite),
and inhibition of vegetation regrowth (e.g., by harsh climatic conditions).
The 26.5 ka Oruanui eruption from Taupo volcano in the central North Island
of New Zealand created optimal conditions for a large-scale sedimentary response
that was influenced and prolonged by the succeeding climatic nadir of the
Last Glacial Maximum. About 530 km3 of rhyolitic magma was erupted
as 420 km3 of fall deposits, 320 km3 of pyroclastic
density current deposits (mostly non-welded ignimbrite), and 430 km3
of primary intracaldera fill. The eruption, and formation of the Oruanui
caldera, destroyed one major lake but created the forerunner to modern Lake
Taupo. This lake initially stably overflowed to the northwest before breaking
out in a catastrophic flood during establishment of a northeasterly outlet
along the line of the modern Waikato River. Suppression of revegetation by
the contemporaneous harsh periglacial climate contributed to intense erosion
and remobilisation of Oruanui pyroclastic units, triggering massive downstream
fluvial aggradation in impacted catchments. In particular, aggradation caused
the lower 180 km of the Waikato River to avulse from its long-established
route via the Hauraki Plains into the Hamilton Basin where it was subsequently
trapped. Aeolian reworking created localised dune fields, while generation
of tephric loess formed deposits over much of the central North Island. The
initial perturbation to fluvial sedimentary systems created by the eruption
was generally sustained by climatic conditions until c. 17 ka. Climatic amelioration
eventually stabilised primary sediment sources through the re-establishment
of forest vegetation, but incision and recycling of alluvial material stored
in volcaniclastic fans continued until as late as c. 14 ka. Overall, the
scale of the post-Oruanui sedimentary response was primarily driven by characteristics
of the eruption, whereas its duration was controlled by the Last Glacial
Maximum climate.
Keywords Oruanui eruption; Taupo Volcanic Zone; Hinuera
Formation; volcaniclastic sediments; Waikato River; Last Glacial Maximum;
Mokai Sand
G03053; Received 28 May 2003; accepted 21 October 2003; Online publication
date 7 September 2004
New Zealand Journal of Geology & Geophysics, 2004, Vol. 47: 525-547
0028-8306/04/4703-0525 © The Royal Society of New Zealand 2004
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