Geology of the Haunui valley, central North Island

Author:

Murphy, Linda (Linda Susan)

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Abstract:

The mapped region encompassing the Haunui Valley covers 32 km2 and is situated on the western margin of the Taupo Volcanic Zone, 12 km southwest of Rotorua. Six formations are identified. The oldest formation is the densely welded, quartz poor Waiotapu Ignimbrite which is stratigraphically overlain by phreatoplinian, pyroclastic flow and plinian eruptives of the non-welded, crystal poor Onuku Breccia. The oldest phase of Haparangi Rhyolite extrusion follows these, and is represented by the Horohoro Rhyolite Dome. Onlapping and overlying this, is the Korokoro Ignimbrite, an ignimbrite formation newly proposed in this thesis. The youngest pyroclastic flow formation is the voluminous quartz-rich Mamaku Ignimbrite forming the Mamaku Plateau. A period of lake sedimentation produced the Huka Group in the Guthrie Graben from 42 000 yrs B.P. until 20 000 yrs B.P. after which a series of plinian tephra formations were deposited.
Laboratory work involves several aspects. Firstly, correlations of ignimbrite formations to areas outside the Haunui Valley are presented, based on mineralogy and stratigraphy. Secondly, grainsize analyses and scanning-electron-microscope data are used to investigate the phreatoplinian nature of the Onuku Breccia. Thirdly, previously unestablished characterisation of ignimbrite formations in terms of recrystallization zones are presented. Finally, petrographic modal analyses are compared to XRF major element chemical analyses in vertical sequences of ignimbrite formations. The results are inferred to reflect compositional zonation of the magma chambers involved, by convection-driven thermogravitational diffusion.
The Haunui Valley is delineated by a tectonic rim, inferred to be associated with Holocene anticlinal warping of the Mamaku Plateau. Two other major faults are present. The oldest is the northerly trending Pokaitu Fault, which represents post-Korokoro Ignimbrite tectonism. The youngest fault is the probably recent active, northeasterly trending Horohoro Fault and its subparallel, discontinuous derivatives.

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104 leaves : ill. (some col.), maps ; 30 cm.

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1983Murphy

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POLYGON ((176.637176364569257 -38.06589282132169,176.620366085807575 -38.06736589198146,176.619419395834797 -38.05428587136263,176.636527691491267 -38.052803761732413,176.637176364569257 -38.06589282132169))

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http://download.otagogeology.org.nz/temp/Abstracts/1983Murphy.pdf

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Citation

Murphy, Linda (Linda Susan), “Geology of the Haunui valley, central North Island,” Otago Geology Theses, accessed February 13, 2025, https://theses.otagogeology.org.nz/items/show/167.

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