• No results found

Before any eruptions occurred, the area of the Bombay Quarry consisted of Waitemata and Tauranga group country rock that had been eroded into a hill-and-valley landscape (Fig. 6.1A).

Effusive volcanism first occurred with the emplacement of the older (lower basalt) lava flow into the palaeovalley where it ponded (Fig. 6.1A). The eruption must have had a stable outpouring of lava, fire fountaining, or a mixture of both to deposit a relative small, but thick deposit. Multiple layers of horizontal

‘sheeted’ joints were formed where the basalt had formed lava-tubes (Figs. 3.5, 3.6).

A period of volcanic quiescence occurred where Quaternary sediments of alluvial pumiceous sand, silt/mud and/or Kauroa ash formation were deposited above the lower basalt flow (Fig. 6.1B) likely to have been derived locally from floodplains and/or from distal volcanism from the TVZ. An effusive eruption occurred depositing the massive and thick middle basalt lava over the Quaternary and Kauroa ash sediments (Fig. 6.1C) and produced multiple horizontal-sheeted joints. Entablature irregular curved joints occurred where the columns deviate

91

Figure 6.1: Illustration of the volcanic eruption of the Auckland pit. (A) Waitemata and Tauranga group exposed as basement rocks within the palaeovalley and deposition of the lower basalt. (B) Quaternary alluvium/Kauroa ash sediment deposition. (C) Deposition of middle basalt. (D) Deposition of intercalated scoria and tephra within the Quaternary

alluvium/Kauroa ash sediments. (E) Deposition on the upper basalt. (F) Deposition of Quaternary alluvium/Kauroa ash sediments and tuff ring.

Figure 6.1: Continued.

93

Another period of volcanic quiescence followed the emplacement of the middle basalt, where further Quaternary sediments of alluvial pumiceous sand, silt/mud and/or Kauroa ash formation were deposited above the middle basalt flow (Fig. 6.1D). A tephra deposit is intercalated with the Kauroa ash formation where it is dated at 1 - 1.2 Ma (Alloway et al., 2004) and is associated with the post-Ongatiti tephra layer.

During this time, explosive strombolian eruptions also occurred locally, within the area of the Auckland pit, producing at least two or more scoria cones (Fig. 6.1D; e.g., Cas, 1989). Strombolian eruptions are characteristic of basaltic volcanoes which consist of prolonged sequences of intermittent, short-lived explosions, usually lasting a few seconds to several minutes. Strombolian clasts of ejecting ash to bombs, typically reach heights of < 100 m above the vent and the plume generated by the explosion generally rise to < 200 m (Parfitt, 2004;

Houghton & Gonnermann, 2008). Scoria cones would have been erupted at shorter trajectories and higher explosivity (Sumner et al., 2005), forming massive deposits within the quarry walls.

Another effusive eruption occurred depositing a younger thick unit of lava (upper basalt), overlying the scoria and Quaternary alluvium and/or Kauroa ash sediments (Fig. 6.1E). The eruption must have had a steady supply of lava to produce columnar basalt that extended across the centre of the valley (Figs. 3.4, 3.6). Within the basalt, multiple layers of sheeted basalt are also present where the basalt had formed lava tubes, or where the basalt was overriding the base of the cooled lava surface.

Volcanic quiescence again allowed the deposition of further Quaternary alluvial sediments and/or Kauroa ash above the youngest basalt flow (Fig. 6.1F).

A scoria cone erupted by a strombolian eruption from a nearby vent on the eastern side of the pit depositing basaltic lapilli and blocks/bombs above the uppermost Quaternary alluvium and/or Kauroa ash sediments (Fig. 6.1F).

The final stage of volcanism, which followed after another period of volcanic quiescence, involved a phreatomagmatic eruption that ensued as rising magma interacted with a water source and produced a tuff ring deposit above the

underlying volcanic succession (Fig. 6.1G). In the later stages, a dike intrusion occurred on the western side of the Auckland pit, intruding the Quaternary alluvium and/or Kauroa ash sediments which then reached the surface and released a small flow within a small depression.

In this study, the volcanic deposit of the tuff ring and the dike intrusion were not studied in the Auckland pit, and therefore the volcanic eruption history is from observations only.

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Chapter Seven

Conclusions

This study focuses on the Auckland pit in the Bombay Quarry which lies on the western side of the South Auckland Volcanic Field that was active 1.59 – 0.51 Ma. The SAVF comprises at least 82 known volcanic centres that are exposed and easily accessible. The Bombay Volcanic Complex is a large basaltic volcano that comprises a succession of multiple volcanic and sedimentary deposits which are exposed within the Bombay Quarry and, in particular for this study, the recently opened Auckland pit.

Stratigraphic logs of drill core show that within the Auckland pit there are three individual basalt lava flows with intercalated scoria and sedimentary deposits that were emplaced within a palaeovalley.

Basalt facies show a range of vesicularities and textural characteristics which are generally arranged within each lava flow into an upper and lower vesicular zone and a dense coherent non-vesicular zone; this facies architecture is typical of pahoehoe to transitional lava flows. Intercalated sediment overlies each of the three basalt lava flows, representing periods of volcanic quiescence. Scoria deposits within the basalt indicate a change in magmatic eruption style to strombolian.

Petrography of the three basalts reveal that each lava flow has similar mineral characteristics of olivine, clinopyroxene, plagioclase, opaques, mafic minerals and vesicle texture, however, there are slight differences in their proportions for each lava flow.

From geochemical analysis, there is a slight difference in mineral compositions between the three lava flows. The upper and lower basalt flows have higher Mg-rich and calcic-rich compositions than the middle basalt with lower Mg-rich and calcic-rich mineral compositions. The three basalts, however, are all within the broad group B basalt type which is recognised with the SAVF lavas.

The Auckland pit has revealed that the succession of volcanic and sedimentary units, deposited within the palaeovalley is characteristic of a polygenetic eruption history. From this interpretation, the Waikato pit should also have a polygenetic eruption history, as it lies adjacent to the Auckland pit.

Considering the Auckland pit lavas are related to the larger, broad Bombay Volcanic Complex nearby, it should be considered that the Bombay Volcanic Complex has multiple volcanic/sediment deposits and a polygenetic eruption history.

Future work on the Bombay Volcanic Complex could include a more detailed study of the palaeo-environment and origin of the sedimentary units to better focus on the history of the Bombay Volcanic Complex.

The evolution of the volcanic and sedimentary units in this thesis offers insight into the importance of polygenetic volcanoes within monogenetic fields and may be an ideal analogue for understanding the future of shield volcanism in both the South Auckland and Auckland Volcanic Fields (i.e. Rangitoto Island).

97

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Appendices

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