Age and eruptive center of the Paeroa Subgroup ignimbrites (Whakamaru Group) within the Taupo Volcanic Zone of New Zealand

Downs, Drew T.; Wilson, C. J. N.; Cole, Jim; Rowland, J. V.; Calvert, Andrew T.; Leonard, Graham S.; Keall, Jon

doi:10.1130/b30891.1

Cited by 35 publications

(33 citation statements)

References 40 publications

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“…A lake deposit sequence that includes intercalated rhyolite lavas, rhyolite breccias, and minor basaltic breccias overlies the volcanic‐dominated Tahorakuri Formation succession. This unit is separated from a similarly complex, near‐surface (<1 km) sequence of tuffs, lavas, and sediments, by the regionally significant Whakamaru Group ignimbrites (Downs et al, ). Smectite clay alteration crosscuts the stratigraphic sequence (Figure ) and forms a low‐permeability cap over the deep reservoir.…”

Section: Geological Setting and Sample Descriptionmentioning

confidence: 99%

Mineral Alteration and Fracture Influence on the Elastic Properties of Volcaniclastic Rocks

et al. 2019

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In geothermal environments, the physical properties of rocks, such as porosity and alteration, are highly variable and control the reservoir's elastic and hydraulic properties. Elastic wave velocity in volcaniclastic rocks and their relation to fractures, pore shapes, and mineral alteration is mostly unknown. We measure ultrasonic P and S wave speeds on volcanic rocks from the Ngatamariki Geothermal Reservoir, New Zealand. Data clustering of wave speed versus porosity and density allow us to classify lithotypes of variable propylitic and phyllic mineral alteration. Wave speeds increase first due to porosity reduction as a result of mineral alteration and welding and, second, due to the high elasticity of alteration minerals (epidote, chlorite, and carbonates). We model the rock porosity as composed of equant pores and oblate-spheroidal microfractures following an elastic effective medium model. For our samples, microfracture porosities range from 4% in the volcaniclastic tuffs to less than 1% in the ignimbrites, which we validate with pycnometer porosity data under effective pressure. We quantify that within one geological volcaniclastic formation, wave speeds vary up to 47% due to rock alteration and welding, while the effect of microfractures and changes in effective stress on wave speeds is secondary (up to 11%) but not negligible. From the experimental and numerical results we show that equant pores remain open at reservoir conditions (2,000 m) and can retain considerable porosity (10%). Our analysis has implications for microfracture and pore network characterization in geothermal reservoirs, in particular, in vapor-dominated systems where matrix porosity and permeability play a role.

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Section: Geological Setting and Sample Descriptionmentioning

confidence: 99%

Mineral Alteration and Fracture Influence on the Elastic Properties of Volcaniclastic Rocks

et al. 2019

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“…Deposition of the Waiora Formation took place after the emplacement of the Whakamaru Group. Several volcanic centres were active during this period, such as Taupo, Okataina, Kapenga, and Rotorua (Downs et al, 2014b). This formation consists of pyroclastic and volcaniclastic rocks with interlayered sedimentary rocks.…”

Section: Volcanic Framework Of Wairakei Geothermal Systemmentioning

confidence: 98%

“…In the Wairakei system, the weakly to densely welded ignimbrite is the product of the Whakamaru Caldera-forming eruption (Grindley, 1965;Houghton et al, 1995). Age dating shows that the whole Whakamaru sequence was emplaced at 349 ± 4 ka to 339 ± 5 ka (Middle Pleistocene) (Downs et al, 2014a(Downs et al, , 2014b. The member of this group represented in the wells at Wairakei is the earliest member of a crystal-rich unit locally named Wairakei Ignimbrite (~350 ka) (Grindley, 1965;Rosenberg et al, 2009;Bignall et al, 2010).…”

Section: Volcanic Framework Of Wairakei Geothermal Systemmentioning

confidence: 99%

Geology and Geothermal Setting of Tompaso Geothermal System, Indonesia, with Comparisons of Andesite Alteration Patterns with Wairakei, New Zealand

Savitri¹

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Tompaso geothermal system is a typical volcanic arc geothermal system in North Sulawesi, Indonesia. Although situated close to the Tondano caldera, subsurface lithologies and structures do not show any evidence for caldera-related features and the system is inferred to be related to the andesitic Soputan volcano. The subsurface geology of Tompaso consists of Tuff B unit, Rhyolite unit, Andesite B unit, Pitchstone unit, Pyroclastic Breccia unit,Andesite A unit, Pumice unit, and Tuff A unit, respectively, from the oldest penetrated unit. The silicic Pitchstone and Rhyolite units are presumed to be sourced from the same magma chamber. Petrological and mineralogical observations using binocular and petrographic microscopy, short-wave infrared (SWIR) analysis, and back-scattered electron (BSE) imaging combined with energy dispersive X-ray spectroscopy (EDS) have been applied to cuttings and limited core material from three boreholes: LHD-26, LHD-27, and LHD-32. Age dating has not been undertaken and, thus, conclusions on correlations between subsurface geology inferred here with surface formation groupings from previous works cannot be drawn. Tompaso geothermal system is characterised primarily by variations in the fracturing within the reservoir. Secondary mineralogy and the structure of present-day temperature of the system suggest that the movement of hydrothermal fluids at Tompaso is controlled by faults: the Soputan, Tempang, and A-A’ faults, the last defined for the first time in this thesis. Soputan Fault controls the outflow of the system. On the other hand, the influence of Tempang and A-A’ faults is dominant only in the upper portion of the system. The A-A’ fault likely acts as a channel for cooler meteoric surface water, while the Tempang Fault is inferred to have experienced self-sealing and appears to be an impermeable structure in the system. The self-sealing process of the Tempang Fault and/or the introduction of meteoric water through the A-A’ fault may be related to the cooling of the northern and western part of the system. The challenges in identifying protoliths in active geothermal areas is addressed here through studies of the influence of andesite textures on the preferences of hydrothermal alteration processes. Wairakei andesites were chosen for comparison to Tompaso andesites, especially because of its different geological setting and geothermal reservoir structure. The results suggest that mineral composition and arrangement affect the preference of hydrothermal alteration on andesites.

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“…An active period of volcanism in central TVZ, referred to "young TVZ", is delimited by the ~350 ka Whakamaru Group eruptions and the 45-61 ka Rotoiti eruption (Wilson et al, 2007;Danišík et al, 2012;Downs et al, 2014b). The majority of large (~3000 km 3 ) and caldera-forming eruptions occurred in the first ~100 ka of this period, which is generally referred to as the TVZ flare-up event (Gravley et al, 2016).…”

Section: 4mentioning

confidence: 99%

Volcanic and tectonic perspectives on the age and evolution of the Wairakei-Tauhara geothermal system

Rosenberg¹

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This thesis presents a revision and update of the geology of the Wairakei-Tauhara (WKTH) geothermal system. The research is focussed around the use of U-Pb dating of zircons in key stratigraphic units, coupled with presentation of a robust stratigraphic architecture to establish the timing of major volcanic and volcano-tectonic events, including those that help to constrain the broader scale evolution of the Whakamaru Caldera and Taupo-Reporoa Basin (TRB) and those that initiated or rejuvenated the geothermal system. The present day Wairakei-Tauhara geothermal system is much younger than previously estimated and is now proposed to be a <60 ka manifestation of the previously identified North East (NE) dome magma system. This system is represented by volcanic outputs spanning from contributions to the Mt Tauhara mixed-magma dacite at ~60 ka, to eruption of Puketarata rhyolite at ~16 ka. The earliest TVZ volcanism is represented locally by silicic pyroclastic units that were emplaced prior to 1.8 Ma on a basement now at >3 km depth (beyond the limits of drilled wells). At the Wairakei and Tauhara fields, these pyroclastics are intercalated with greywacke sediment, demonstrating that the area was within a long-lived basin westwards of the North Island axial ranges. At ~1.8 Ma, the earliest dated volcanism at Wairakei began with eruption of andesitic lavas at the eastern margin of TVZ. These lavas were later buried by sediments and rhyolitic pyroclastics, including the locally named Stockyard ignimbrite that is correlated by its U-Pb zircon age spectrum with 1.0 Ma Raepahu Formation ignimbrites erupted from the Mangakino Volcanic Centre. Magmatic activity in the following ~0.3- 0.4 Ma is represented locally by only two WK-TH volcanic episodes (at 0.95 Ma and 0.73 Ma). Wairakei well stratigraphy also highlights a major hiatus in local volcanism and deposition until the ~0.35 Ma Whakamaru eruptions. These and associated structural collapse were the events that may have initiated frequent volcanic and volcanotectonic episodes in the Wairakei-Tauhara area. Syn-eruptive piecemeal collapse of the caldera is inferred to have created the fault pathways for small batches of crystal-poor rhyolite, probably derived from a Whakamaru-like magma system, to intrude and erupt in much the same locations in the WK-TH area. This post-Whakamaru period (0.31 Ma until 0.22 Ma), overlaps with the age span of the majority of surficial rhyolitic volcanism in the Maroa Volcanic Centre, north and northwest of the WK-TH area. The Taupo-Reporoa Basin evolved in major volcano-tectonic episodes beginning around 0.35 Ma. In the south, displacement of Whakamaru Group ignimbrite across an elongate, central graben-like structure between the Wairakei and Tauhara fields is explained by 0.9-1.7 km subsidence of large NE-SW trending fault blocks, during piecemeal collapse of the Whakamaru Caldera. Further local collapse is proposed, which was nearly contemporaneous with 0.31 Ma eruptions that formed a ~10 x 12 km block faulted, newly recognised Waiora caldera and generated >30 km³ of Waiora Formation ignimbrites that syn-eruptively filled the new caldera. Rhyolite lava eruptions also starting at ~0.31 Ma on the eastern margin of the Tauhara Field and in the Wairakei Field, were the first of many spatially overlapping lavas and intercalated local pyroclastic units collectively mapped as the Waiora Formation. U-Pb and U-Th dating of zircons, zircon growth textures revealed by cathodoluminescence, and whole rock REE geochemistry are integrated with petrography to outline overlapping compositions of the lavas, several of which are also analytically indistinguishable in age. Correlations between Waiora Formation rhyolitic lavas and ignimbrites are equivocal with the available data, but the geochemistry suggests that early Waiora Formation ignimbrites may have been derived from a Whakamaru-like magma source that was distinct from contemporaneous Maroa rhyolite lavas. Infilling and subsidence of the Waiora caldera continued at varied rates until ~0.22 Ma. After 0.22 Ma there was a hiatus in volcanism in the local WK-TH area, during which lacustrine sediments accumulated in the local part of Lake Huka at ~1 mm/y. These fine grained Huka Falls Formation deposits became the impermeable cap rock of the modern WK-TH geothermal system. Volcanism did not wholly cease in the local area, or elsewhere in the TRB, however. There were shallow intrusions and subaerial dome extrusions of rhyolite at ~0.14-0.12 Ma, and at ~0.1 Ma there was the subaqueous eruption of middle Huka Falls Formation rhyolite. The new U-Pb ages reflect magma systems active in the local area at least at 1.8 Ma, 0.95 Ma, and 0.73 Ma, but there is no evidence of any associated geothermal system during one of these episodes. The Whakamaru eruptions and collapse episodes may have initiated the first geothermal system in the WK-TH area. The overlapping ages of Waiora Formation rhyolites, and spatial clustering beneath the Wairakei Field, show that underlying magmatism might have supported a geothermal system in more or less the same location at numerous times during the past ~0.3 Ma. Previously, the geothermal system was inferred to have been active since ~0.5 Ma, but in its modern state, the system is now interpreted to have begun at ~60 ka and has been sustained since then by the presence of an underlying NE dome magma system.

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Age and eruptive center of the Paeroa Subgroup ignimbrites (Whakamaru Group) within the Taupo Volcanic Zone of New Zealand

Cited by 35 publications

References 40 publications

Mineral Alteration and Fracture Influence on the Elastic Properties of Volcaniclastic Rocks

Mineral Alteration and Fracture Influence on the Elastic Properties of Volcaniclastic Rocks

Geology and Geothermal Setting of Tompaso Geothermal System, Indonesia, with Comparisons of Andesite Alteration Patterns with Wairakei, New Zealand

Volcanic and tectonic perspectives on the age and evolution of the Wairakei-Tauhara geothermal system

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