Depositional characteristics and volcanological interpretations of Miocene – Pliocene age marine ash deposits in the forearc East Coast Basin, North Island, New Zealand.
The purpose of this study is to use features of Miocene-Pliocene ash layers, superbly exposed in the East Coast basin to -
- Investigate the details of deep-marine ash distribution and deposit characteristics in order to determine the physical processes by which they were deposited.
- Extract from the ash beds information about the eruptions that formed the ash
- Examine depositional characteristics of the enclosing strata at discrete and well-constrained time intervals during basin evolution, to better constrain interpretations of overall basin evolution.
Large-magnitude, explosive volcanic eruptions produce abundant ash, which may be distributed widely both through aerial transport and by distribution within the ocean by suspension currents (turbidity currents) or thermohaline currents. Wide distribution of rhyolitic ash from such eruptions in subaerial settings is well known, and explosive large-magnitude eruptions are increasingly recognised on submerged continental crust (e.g. Kano, 2003), with ash dispersal dominated by marine transport processes.
Marine tephra deposits have been used to infer the size and dynamics of individual eruptions (e.g. Ledbetter and Sparks, 1979; Shane, 2000). Observations from the 1991 Mt Pinatubo eruption, and recent experimental work (Manville and Wilson, 2004), show that the longstanding view that primary sedimentation of marine tephra is by simple suspension settling in the ocean requires modification. Over-rapid sedimentation has been suggested to result from particle aggregation or biological effects, but Manville and Wilson (2004) demonstrate that loading the ocean surface with ash from an eruption plume can cause development of a type of vertical density current, or downward-directed plume. It is not known what happens to such a plume when it reaches the ocean floor, but lateral transport along the seafloor is highly likely. One object of the current study is to assess whether the turbidity current origin previously inferred for the East Coast tephra deposits is correct in all instances, or whether some or all of the deposits might result from lateral currents fed directly by ash deposited on the ocean from eruption plumes.
Deposition of tephra in deep-marine settings may also follow subaerial deposition with subsequent riverine transport to the marine environment. In this circumstance, the depositional features of a deep-marine tephra will reflect sedimentary processes of the same sort that form ordinary marine turbidite sequences. These deposits contain no information regarding original eruptive transport processes or rates, though they may be useful as chronostratigraphic horizons.
One outcome of the study will be a comprehensive dataset to be extracted from the more than 800 marine tephra beds (Gosson, 1986) of Miocene to Pliocene (≈ 24 - 3.5 million years ago) age that are exposed in the forearc deposits of the East Coast Basin in the North Island of New Zealand. A second outcome will be actualistic models for development of deep-marine tephra beds following explosive subaerial and submarine eruptions; these models will have predictive value for distribution of ash from future eruptions. A third outcome will be increased knowledge of the types and magnitudes of explosive eruptions in Mio-Pliocene time in NZ. Finally this study will increase our understanding of the sedimentary regime during infilling of the East Coast Basin.
- Gosson, G.J. (1986) Miocene and Pliocene silicic tuffs in marine sediments of the East Coast Basin, New Zealand. Unpublished PhD thesis. Victoria University of Wellington 123 p.
- Kano, K. (2003) Subaqueous pumice eruptions and their products. In: Explosive Subaqueous Volcanism, (Eds J. D. L. White, J. L. Smellie and D. A. Clague) American Geophysical Union Monograph, 140, 213-230.
- Ledbetter, M. T. & Sparks, R. S. J. (1979) Duration of large-magnitude explosive eruptions deduced from graded bedding in deep-sea ash layers. Geology, 7, 240-244.
- Manville, V., Wilson, C.J.N., (2004) Vertical density currents: a review of their potential in the deposition and interpretation of deep-sea ash layers. Journal of the Geological Society of London. 161. pp. 947-958.
- Shane, P., (2000) Tephrochronology: a New Zealand case study. Earth-Science Reviews. 49. pp. 223-259.
- White, J.D.L., Smellie, J.L., Clague, D.A., (2003) Introduction: a deductive outline and topical overview of subaqueous explosive volcanism., In: Explosive Subaqueous Volcanism Eds. White, J.D.L.,
- Smellie, J.L., Clague, D.A., American Geophysical Union, Washington DC. 379 p.
The Evolution of Lake Rotorua before ≈ 26 ka
The Rotorua volcanic centre (RVC) forms a well-defined topographic depression ≈ 21 km by 22 km in diameter that is located along, and partly delineates, a structural embayment in the western margin of the central TVZ
Three littoral terraces, formed during periods of stable high water levels surround Lake Rotorua.
Three littoral terraces, formed during periods of stable high water levels surround Lake Rotorua. Deposits exposed in these terrace outcrops include those of beaches, subaqueous channels, fine-grained delta fronts and well-formed delta foresets. These can be separated by their geomorphology and field relationships, in particular by identifying the unconformities and correlative pyroclastic deposits that separate highstand deposits. Sediment from these deposits was also characterised by Electron Microprobe analysis of glass shards, granulometry, ferromagnesian mineral assemblages, and XRD and SEM analysis. Allostratigraphy provides an objective means of correlating these heterogenous lacustrine deposits, and we use the field relationships and deposit components to construct an integrated stratigraphic model in which deposits are subdivided into three alloformations based on their bounding unconformities.
Each suite of deposits is informally named for the volcanic event inferred to have initiated a high lake level. (1) Post-Mamaku alloformation (up to ≈ 415 m a.s.l.); (2) Post-Rotoiti alloformation (up to ≈ 380 m a.s.l.); (3) Post-Hauparu alloformation (up to ≈ 349 m a.s.l.). The first highstand following formation of the Rotorua depression may have ended when the caldera wall adjacent to the Pohaturoa Dome collapsed, with this breach forming the Hemo Gorge.
Subsequent highstands followed eruptions from the neighbouring Okataina Volcanic Centre (OVC) that deposited large, northwardly dispersed, tephra deposits which formed natural dams across the Rotoiti channel, the northern outlet of Lake Rotorua. Eventually these dams were breached either by headward stream erosion or by landsliding of weak damming material. At very high lake levels, the lake may have spilled across the caldera margin above Mission Bay; this water would have entered the Kaituna River, to drain into the Western Bay of Plenty. At this high level, the lake extended through the Hemo Gorge and flooded the neighbouring Kapenga depression; an alternative spillway to the south would have drained into the Waikato River and thence to the Firth of Thames. This study elucidates the history of Lake Rotorua relative to volcanism in the region, and demonstrates the utility of combining unconformities and correlative primary pyroclastic deposits to produce an allostratigraphic framework for analysis.