Ulrike Martin completed her PhD in 2000. She is currently at TU Bergakademie, Freiberg, Germany, you can contact her at Ulrike.Martin@geo.tu-freiberg.de, or check http://www.geo.tu-freiberg.de/dynamo/forschung_martin.htm or drop by the Otago Geology Department thesis library and examine Martin, U. (2000) Eruptions and deposition of volcaniclastic rocks in the Dunedin Volcanic Complex, Otago Peninsula, New Zealand.
PhD dissertation, University of Otago.
[Journal publications arising from the project are listed at the bottom of this page.]
Eruptions and deposition of volcaniclastic rocks in the Dunedin Volcanic Complex, Otago Peninsula, New Zealand
Active volcanism related to subduction is well known in the North Island of New Zealand, whereas widespread Cenozoic intra-plate volcanism is also present in New Zealand's South Island and an area extending south and east to the Sub-Antarctic and Chatham Islands (Fig. 1.1).
Figure1.1. Intraplate volcanic areas of New Zealand's South Island, Sub-Antarctica and Chatham Island.
The best known of these South Island volcanic areas are the Oamaru Volcanics and the large Miocene volcanic complexes of Banks Peninsula and Dunedin. In North Otago three
periods of Cenozoic volcanism have been recognised. One is known only from the presence of Paleocene tuffs (65-54 Ma), which were drilled off the Otago and Canterbury coast (Coombs et al., 1986). The Waiareka-Deborah Volcanics (Coombs et al., 1986) erupted over a period of 12 m.y. (40-32 Ma) mainly in submarine settings on a shallow continental shelf. A third and final period of Cenozoic volcanism occurred in the Miocene, represented by the Dunedin Volcanic Complex and the Waipiata Volcanics termed by Coombs et al. (1986) "Dunedin Volcanic Group"(Fig. 1.2). The Dunedin Volcanic Complex is deeply incised by the Otago Harbour, which evenly subdivides the volcanic accumulation into a western ridge and an eastern peninsula. The great variety of primary volcaniclastic rocks at the Otago Peninsula also provides an opportunity to study different eruption processes, including magmatic, hydromagmatic and phreatomagmatic ones as well as a variety of mass-flow depositional processes.
The primary aims of the thesis are to; (1) present an evaluation of the eruptive history of the Otago Peninsula; (2) give models and interpretations of eruption style and depositional processes for the early history of the Dunedin Volcanic Complex, focusing mainly on the volcaniclastic rocks of the Otago Peninsula; (3) present a new present a volcanological map which identifies eruptive centres and their major products. A subsequent part of the thesis focuses on geochemical analysis of glass and whole-rock samples. The petrological data provides additional support for the models of the volcano evolution and eruptive processes presented in the thesis.
Rocks of the Dunedin Volcanic Complex rest on Cretaceous and Tertiary sedimentary rocks which are underlain by pre-Cretaceous basement of quartzofeldspathic schist. The sedimentary rocks comprise marine sandstones, mudstones and limestones, which were deposited on the continental shelf, and a non-marine to marine basal unit. Miocene marine sediments on the Peninsula give evidence of a submarine setting for the early phases of the volcanic activity. Indicators for subaqueous volcanic activity are widespread on the Otago Peninsula and include pillow basalt, hyaloclastites and peperites exposed along almost the whole southern coast of the Otago Peninsula. Interbedded tuffs within marine sandstones and limestones also indicate a submarine environment for the early stages of volcanism, as do peperites at various sites.
Figure 1.2 Outcrops of volcanic rocks belonging to the Dunedin Volcanic Group. Mafic rocks are black, phonolites and trachytes are stippled. Faults active during the Kaikoura Orogeny are also indicated (after McDougall & Coombs 1972).
A wide range of pyroclastic deposits is found on the Otago Peninsula, including tephra erupted from shoaling volcanoes, and fully subaerial tephra (Strombolian deposits), which erupted commonly from small volcanoes. Most of the small volcanoes seem to have been short-lived eruption centres but locally there are also eruptive centres that were active for quite a substantial time or were reactivated during their history. The early history of the Otago Peninsula is marked by six stages of volcanic activity, of which four took place in subaqueous to emergent environments. This indicates that subaqueous volcanic activity took place over quite a long period of time during the early history of the Dunedin Volcanic Complex.
Generally the distribution of both subsurface and surface water exerts a major influence on the formation of phreatomagmatic eruption centres such as maars, tuff rings, and tuff cones, and the style of eruption. The types of volcanic centres developed on the Otago Peninsula would have been significantly affected by the availability of seawater. The seawater interacted explosively with erupting submarine magma, which led to phreatomagmatic explosive activity and growth of phreatomagmatic volcanic centres.
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The pattern of events suggested here for the development of the Otago Peninsula and also for the Dunedin Volcanic Complex in general has also been described for other alkalic volcanoes. The Dunedin Volcanic Complex began to form by basaltic phreatomagmatic eruptions on the Otago Peninsula. Large central-vent polygenetic structures growth then began to form. The final stage culminated in the formation of large domes, made up of phonolites, benmoreites, trachyandesites and mugearite. A similar sequence of events has been found for the hot spot volcanic island of Tenerife (Canary Islands). The composition of eruptive products from the hotspot volcanoes of the Canary Islands, which are situated near the boundary of oceanic and continental lithosphere on a slow-moving plate, is also similar to the range found on the Otago Peninsula, but the islands commonly consist of large shield volcanoes. The origin of the Canary Islands (and also the Eifel) is as controversial as the origin of the Dunedin Volcanic Complex. A hotspot or mantle plume, a zone of lithospheric deformation, a region of compressional block-faulting have been proposed by different authors. However, the hotspot theory is established on the basis of comparative studies with the prototypical hotspot-related island group of Hawaii and others. The origin of the Dunedin Volcanic Complex is still unknown and further investigations are essential to establish its beginning.
Volcanological Map of the Otago Peninsula
Summary from papers related to this topic
- Martin, U., & White, J.D.L., (2001): Depositional mechanisms of density current deposits from a submarine vent at the Otago Peninsula, New Zealand. In: B. Kneller, and J. Peakall, Eds. Sediment transport and deposition by particulate gravity currents. Blackwell, International Association of Sedimentologists Special Publication, Vol. 31, 245-261.
- Martin, U., & White, J.D.L., (in press): Melting and mingling of phonolitic pumice deposits with intruding dykes: An example from the Otago Peninsula, New Zealand. In: Skilling, I., McPhie, J. & White, J.D.L. (Eds). Peperites. Journal of Volcanology and Geothermal Research Special Publication.
- Martin, U. (in press): The evolution of a small Surtseyan volcano at Sandfly Bay, Otago Peninsula, New Zealand. Submitted to Neues Jahrbuch für Geologie und Paläontologie.