The nature and age of Ohakuri Formation and Ohakuri Group rocks in surface exposures and geothermal drillhole sequences in the central Taupo Volcanic Zone, New Zealand

Gravley, Darren M.; Wilson, Colin; Rosenberg, M. D.; Leonard, Graham S.

doi:10.1080/00288306.2006.9515169

Cited by 28 publications

(17 citation statements)

References 5 publications

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“…Above the Rotokawa Andesite sequence lie the volcaniclastic and sedimentary deposits of the Reporoa Group (Tahorakuri and Waikora Formations) and, in places, an andesite of limited extent (Nga Awa Purua Andesite), and a stratigraphically younger ignimbrite of the Whakamaru Group, currently known as the Wairakei Ignimbrite (Browne et al, 1992;Gravley et al, 2006;Wallis et al, 2012;Wyering et al, 2014). The Waikora Formation is a rounded Fig.…”

Section: Subsurface Stratigraphymentioning

confidence: 97%

A review of the Rotokawa Geothermal Field, New Zealand

et al. 2016

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Section: Subsurface Stratigraphymentioning

confidence: 97%

A review of the Rotokawa Geothermal Field, New Zealand

et al. 2016

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“…The Ohakuri Caldera is a newly recognized structure in the Whangapoa basin, 25 km SSW of Rotorua (Figs 5b & 8), which is considered the source of the c. 240 ka Ohakuri pyroclastic deposits (Gravley et al 2006(Gravley et al , 2007. It overlies the northern margin of the older Whakamaru Caldera and lies adjacent to the axis of the modern Taupo Fault Belt.…”

Section: Ohakuri Calderamentioning

confidence: 99%

Caldera volcanism and rift structure in the Taupo Volcanic Zone, New Zealand

Cole

Spinks²

2009

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The Taupo Volcanic Zone (TVZ) is an active continental volcanic arc/back-arc basin in central North Island, New Zealand. It is the youngest area of volcanic activity that extends southwards from the Coromandel Volcanic Zone (CVZ), where andesitic volcanism began c. 18 Ma and rhyolitic volcanism c. 10 Ma. It is an extensional basin (average c. 8 mm a 21 ) with numerous, predominantly normal (dip .608) faults within the Taupo Rift, but with some strike-slip component. TVZ can be divided into three parts. In the north (Whakatane Graben -Bay of Plenty) and south (Tongariro volcanic centre) volcanism is predominantly andesitic, while in the central part it is predominantly rhyolitic. This central area comprises eight caldera centres; the oldest of which (Mangakino caldera; 1.62-0.91 Ma) may be transitional between CVZ and TVZ. Kapenga caldera (c. 700 ka) is completely buried by younger volcanics, but is probably a composite structure with most recent subsidence related to volcano-tectonic processes. Of the remaining five caldera centres, Rotorua, Ohakuri and Reporoa are all simple, sub-circular structures which collapsed c. 240 ka, and are each associated with one ignimbrirte outflow sheet (Mamaku, Ohakuri and Kaingaroa, respectively). Okataina and Taupo are caldera complexes with multiple ignimbrite eruptions and phases of collapse. The three simple calderas are extra-rift, occurring outside the main fault zone in the centre of the Taupo Rift system, while the two caldera complexes are both intra-rift. There is a close relationship between volcanism and structure in TVZ, and many of the structural caldera boundaries have rectangular geometry reflecting the fault pattern. Intrusion of high-alumina basalts as dykes, parallel to the fault trend, may have had a strong influence in causing rhyolitic eruptions in central TVZ.

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“…However, it is uncertain where in the Whakamaru sequence the Wairakei ignimbrites fit, and this sequence has never been dated to confirm this correlation. The Tahorakuri Formation is defined as all un-named volcaniclastic and sedimentary deposits below the Whakamaru Group ignimbrites within TVZ geothermal drillholes (Gravley et al 2006).…”

Section: Geological Settingmentioning

confidence: 99%

“…The stratigraphy reported by andRae et al (2009) is simplified following the naming conventions ofGravley et al (2006), in which the Tahorakuri Formation and any locally named ignimbrites or interbedded lava bodies form the Reporoa Group. Age data have been modified afterLeonard et al (2010).…”

mentioning

confidence: 99%

Application of Raman spectroscopy to distinguish adularia and sanidine in drill cuttings from the Ngatamariki Geothermal Field, New Zealand

Eastwood

Oze

Fraser

et al. 2015

New Zealand Journal of Geology and Geophysics

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Adularia and sanidine are polymorphs of potassium feldspar commonly present in felsic, hydrothermally altered volcanic deposits. Sanidine is a high-temperature volcanic mineral, whereas adularia forms post deposition by hydrothermal processes. Petrographically differentiating between these polymorphs in hydrothermally altered volcanic rocks may be utilised to distinguish geological units as well as provide insights into fluid-rock interactions. However, petrographic identification may be difficult or not possible in fine-grained drill cuttings. Here, polymorphic-sensitive, Raman spectroscopy and electron microprobe analyses are utilised to characterise adularia and potential sanidine in drill cuttings from the Ngatamariki Geothermal Field, Taupo Volcanic Zone, New Zealand. Differences in Raman spectra are capable of distinguishing between adularia and sanidine whether using peak positions or principal component analysis. All the Ngatamariki Geothermal Field potassium feldspars analysed by Raman spectroscopy were found to be adularia, as expected, with typical high K, low Ca compositions between Or 94 and Or 99 confirmed with electron microprobe analyses. This applied approach demonstrates that Raman spectroscopy is a fast and effective method for lending confidence to adularia and sanidine identification, which can be utilised in geothermal fields worldwide.

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The nature and age of Ohakuri Formation and Ohakuri Group rocks in surface exposures and geothermal drillhole sequences in the central Taupo Volcanic Zone, New Zealand

Abstract: The name "Ohakuri"

Cited by 28 publications

References 5 publications

A review of the Rotokawa Geothermal Field, New Zealand

A review of the Rotokawa Geothermal Field, New Zealand

Caldera volcanism and rift structure in the Taupo Volcanic Zone, New Zealand

Application of Raman spectroscopy to distinguish adularia and sanidine in drill cuttings from the Ngatamariki Geothermal Field, New Zealand

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