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Karoly Nemeth (PhD completed 2001)

This page was last updated by Karoly while he conducted his PhD research at the Department of Geology, University of Otago. Find out what Karoly is doing now on our Alumni page

Project description

The Dunedin Volcanic Group comprises only one part of the extensive volcanism in the Otago region during the Cenozoic. Cenozoic volcanism in North, East and Central Otago can be subdivided into three groups, based on age and geographical location. In the third period, the most extensive volcanism commenced with a southward migrating locus of volcanism produced the Dunedin Volcanic Group.

Three types of vents have been identified at the WVF. Dashed lines indicate stages of erosion.
Three types of vents have been identified at the WVF. Dashed lines indicate stages of erosion.

Overview of 'The Crater' volcano. Note the ridge in the background formed by Otago Schist. Cenozoic units are exposed only in the valley.
Overview of "The Crater" volcano. Note the ridge in the background formed by Otago Schist. Cenozoic units are exposed only in the valley.

Dunedin Volcanic Group includes the Alpine Dike Swarm (ADS), the Waipiata Volcanic Field (WVF) and the Dunedin Volcanic Complex (DVC). This volcanism is a continental alkaline basaltic province centered on the Dunedin area. The earliest lava flows of the WVF were erupted at 21 Ma during a period of mild crustal extension related to the opening of the Tasman Sea and the separation of New Zealand from Gondwana. The major volcanism associated with the crustal extension occurs about 120 km to the east of the Alpine Fault, with minor alkaline volcanism immediately to the east of the Alpine Fault, and with no volcanism between the two areas. All volcanic activity in the Otago area ceased at about 10 Ma as a result of the change to compressional tectonics. The volcanic rocks are frequently in close proximity to faults.

The WVF and DVC volcanics are classified as intraplate volcanics by their geochemistry and tectonical position. Intraplate volcanics would imply that volcanism is not generated near a plate boundary. The WVF and DVC occur within the Pacific plate, but they are close to the Indo-Australian and Pacific plate boundary, which is located to the west along the Alpine Fault.

The pre-volcanic Cenozoic stratigraphy at WVF can be characterized by a series of non-marine and marine clastic sediments deposited on an early Cretaceous erosional surface of a schist basement (Otago Schist). The oldest terrestrial clastic sediments deposited on the schist surface form the Hogburn Formation. Widespread marine transgression followed Late Cretaceous extension and separation of New Zealand from Gondwana causing widespread marine clastic sedimentation in the area (Oligocene marine sequence). The area probably reemerged in the early Miocene in response to transpressional tectonics with the inception of the Alpine Fault, with renewed terrestrial clastic deposition (fluvio-lacustrine) taking place (Dunstan Formation). A late Miocene - Pliocene rise of ranges on the north side of the schist belt initiated a deposition of extensive fanglomerates and braidplain deposits to the south of these newly emerged ranges (Wedderburn Formation). These deposits grade upward into a more voluminous, immature, greywacke-dominated conglomerate (e.g. Maniototo Conglomerate). Most of the Cenozoic sedimentary sequence has eroded away especially in uplifted areas and their syn-volcanic presence can be supported by the occurrence of fragments derived from these units in the pyroclastic units of eroded volcanic diatremes.

Phreatomagmatic pyroclastic sequence at the Pigroot Hill western cliffs. Note the steepening of the bedding due to microrelief (dashed line), and the impact sag (arrow). Capping units are spatter deposits and agglutinates with interbedded clastogenic lava flows.
Phreatomagmatic pyroclastic sequence at the Pigroot Hill western cliffs. Note the steepening of the bedding due to microrelief (dashed line), and the impact sag (arrow). Capping units are spatter deposits and agglutinates with interbedded clastogenic lava flows.

Volcanic eruptions took place in late Miocene area with strong relief (valleys and ridges) along hydrologically active zones, producing widespread phreatomagmatic maar/diatreme volcanism with extensive Strombolian scoria cone and lava flow forming eruptions. Just only at WVF at least 52 volcanic zone can be identified few of them with multiple vent systems which would imply that at least approximately 100 vent was active during volcanism.

Cross-section representing general problem in reconstruction of syn-volcanic landforms of the WVF if poor outcrop availability does not allow establishing correlation of pre-volcanic stratigraphy in a larger (km) scale.
Cross-section representing general problem in reconstruction of syn-volcanic landforms of the WVF if poor outcrop availability does not allow establishing correlation of pre-volcanic stratigraphy in a larger (km) scale. It is known that Cenozoic unit existed at Gladsmuir volcano, because its pyroclastic deposits are rich in fragments derived from that units, but there is no preserved Cenozoic sequence in the vicinity of Gladsmuir.. At Bald Hill no exposure allow to see what below the lava cap is. If Cenozoic units underlie the lava cap, the Bald Hill lava can be interpreted as an outflow of a Gladsmuir volcano (1st figure). If there is no Cenozoic unit under Bald Hill lava cap, Bald Hill rather represents 1) a lava filled plug (Type 1 vent on 2nd figure) or 2) Bald Hill is an outflow from the Gladsmuir volcano, flown to an area which was not covered by Cenozoic units (3rd figure).

Most of the eruptive centers are strongly eroded to their root zones where down-dropped sequences of pre-volcanic and former crater rim rocks can be identified.

Large number of erosion remnants is subsequently tilted. These entrapped blocks of pre-volcanic rocks in the conduits of eruptive centers provide important indications of the former lateral distribution of the pre-volcanic sedimentary units.

Tephritic composition sideromelane glass shards (s) from pyroclastic rocks of the Black Rock.
Tephritic composition sideromelane glass shards (s) from pyroclastic rocks of the Black Rock. Note the microvesicular texture of the glass shards. The green grain is a glauconite (g) derived from Oligocene marine sediments that originally formed the volcanic conduit wall. These sedimentary units have already been eroded away. Shorter side of the picture ~ 2 mm. Plane parallel light.

The research on WVF focuses on

  1. The eruptive mechanism each of the eruptive centers in relation to the fluvio-lacustrine basins where volcanism is occurred,
  2. Establishing a link between phreatomagmatism and ground-water sources. Evaluating possible water sources (surface and/or ground-water) and their controlling factors (water recharge, geography position, seasonality) to fuel phreatomagmatic interaction during volcanism,
  3. Calculating the possible stage of erosion and giving assumptions of the total relative erosion in Otago alongside the calculation the thickness of eroded Cenozoic units.
  4. Understanding the volcanic petrology of each eruptive center in light of the relation between magma plumbing systems and composition of eruptive products (both effusive and explosive). Finding answer to the question, "how monogenetic is a monogenetic volcanic field?"
  5. Give a generalized model for the volcanic activity of WVF as a volcanic field closely related to mild extensional tectonic regime in comparison to similar volcanic fields such as e.g. Bakony- Balaton Highland Volcanic Field, Hungary.
  6. Compare the volcanism to activity and eruptive products of the nearby shield volcano (Dunedin Volcano) and to other shield volcanic systems (e.g. Hawaii, Canary Island)

Published literature related to PhD topic

  • Nemeth, K., 2000a. Collapse structures of an eroded maar/diatreme volcanic field from Central Otago, New Zealand: The Crater as an example. Terra Nostra, 6: 364-375
  • Nemeth, K., 2000b. Long-term erosion rate calculation based on remnants of continental monogenetic volcanic landforms of the Miocene Dunedin Volcanic Group, New Zealand, 9th Australia New Zealand Geomorphology Group Conference. Victoria University, Wellington, Wanaka, New Zealand, pp. 62.
  • Nemeth, K., 2001a. Deltaic density currents and turbidity deposits related to maar crater rims and their importance for paleogeographic reconstruction of the Bakony-Balaton Highland Volcanic Field (BBHVF), Hungary. In: B. McCaffrey, B. Kneller, J. Peakall (Editors), Sediment transport and deposition by particulate gravity currents. International Association of Sedimentologist Special Publication, vol 31., Blackwell Sciences, Oxford, pp. 261-277.
  • Nemeth, K., 2001b. Long-term erosion-rate calculation from the Waipiata Volcanic Field (New Zealand) based on erosion remnants of scoria cones, tuff rings and maars. Geomorphologie: relief, processus, environnement (Paris - France). 2001/2, p. 137-152.
  • Nemeth, K., 2001c. On the calculation of long-term erosion in Central Otago, New Zealand, based on two erosion remnants of Late Miocene maar/tuff ring volcano. Zeitschrift fuer Geomorphologie. [in review]
  • Nemeth, K. and Martin, U., 1999a. Large hydrovolcanic field in the Pannonian Basin: general characteristics of the Bakony- Balaton Highland Volcanic Field, Hungary. Acta Vulcanologica, 11(2): 271-282.
  • Nemeth, K. and Martin, U., 1999b. Late Miocene paleo-geomorphology of the Bakony-Balaton Highland Volcanic Field (Hungary) using physical volcanology data. Zeitschrift fuer Geomorphologie N.F., 43(4): 417-438.