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Volcanology of the Owharoa and Waikino ignimbrites, Waihi, Coromandel Volcanic Zone
No full textThe Waihi Caldera (Coromandel Volcanic Zone) is defined by a 15 km diameter gravity anomaly, which was active during the Pliocene and has been infilled by a 1.5 km-thick succession of volcanic deposits and lake sediments. The Owharoa and Waikino ignimbrites are both suspected to have been a product of the Waihi Caldera (Brathwaite & Christie, 1996). The aim of this study was to determine and compare the eruption and emplacement processes of the Owharoa and Waikino ignimbrites.
The ignimbrites were characterised in the field by facies analysis and dated by U-Pb zircon dating using laser ablation inductively coupled plasma mass spectrometry. Their petrographic characteristics were described by optical microscopy, scanning electron microscopy and X-ray powder diffraction. Geochemical characteristics were identified by electron microprobe analysis (on minerals and glass shards) and X-ray fluorescence spectrometry (on pumice and bulk ignimbrite).
The Owharoa Ignimbrite (3.76 ± 0.05 Ma, Vincent, 2012) is poorly sorted with variable degrees of welding degrees. Facies identified include pumice-rich and lithic-rich facies (O1), flattened pumice rich facies (O2), lithic rich, pumice poor facies (O3), dark grey, densely welded, fiamme rich facies (O4) and pumice rich facies (O5). Juvenile clasts include creamy rounded woody-textured pumice in the east, and dark to black, lensoidal, glassy fiamme in the west with quartz and plagioclase phenocrysts. Lithic clasts include volcanic lithics (rhyolite, andesite, and ignimbrite), and occasional sedimentary lithics (sandstone/siltstone).
The Waikino Ignimbrite (3.48 +/- 0.19 Ma) is a finer-grained, relatively well sorted, massive, glass shard matrix-rich (~93%) ignimbrite that is separated into two facies, W1, a softer, massive yellow basal facies; and W2 a grey, well welded, massive facies. The pumice within the Waikino Ignimbrite (1%) was no larger than coarse ash-sized, white fragments. The Waikino Ignimbrite had plagioclase, quartz, biotite (larger than the Owharoa Ignimbrite) and opaque minerals. Geochemically the Waikino Ignimbrite was rhyolitic with higher alkali content than the Owharoa Ignimbrite.
The Owharoa Ignimbrite represents the deposit of an intra-caldera, pulsating depositional pyroclastic flow that shows subtle variations in pumice and lithic abundance. The massive nature of the Waikino Ignimbrite indicates a consistent, steady pyroclastic flow, derived from an intensely fragmented magma to form a glass shard-rich, pumice-poor deposit. Both ignimbrites were sourced from the Waihi Caldera due to their proximity to the caldera, the lithic characteristics and their similar geochemistry and mineralogy. The closeness in age of the ignimbrites signifies the relationship between the deposits, and relationship they have with the Waihi Caldera. It is possible that the Waikino Ignimbrite was the last major eruption from the Waihi Caldera, therefore the end of the duration of the caldera can be identified as after the Waikino Ignimbrite eruption at 3.48 +/- 0.19 Ma
Volcanic history of the Mount Misery rhyolite domes, Tauranga Volcanic Centre
Get PDFThe Mount Misery rhyolites are a group of three complex lava domes, which are part of the Minden Rhyolites within the Tauranga Volcanic Centre. The lava domes are a previously unstudied volcanic area and have in recent years been partially exposed by a series of quarries, creating an opportunity to understand the volcanic history. A pyroclastic succession flanking the side of the Pukunui dome was studied to compare it to the Otawera ignimbrite and potentially correlate it to the Pukunui succession. This study aims to determine the volcanic emplacement and post-emplacement processes of the Mount Misery series domes and pyroclastics, achieved through field work, thin section microscopy, X-ray diffraction, scanning electron microscopy, and X-ray fluorescence.
Key findings from this study show similar phenocryst assemblages but the groundmass textures are complex. The rhyolite groundmasses analysed varied between glassy, crystalline and devitrified, while key phenocrysts found were plagioclase and quartz, and within Maungatūtū/Mount Misery and Greenpark domes also minor clino- and orthopyroxenes. Quartz polymorphs such as cristobalite were common in the groundmass as needle-like structures, while at Pukunui, pure quartz composed the thin bands. The thin bands found at Pukunui and Greenpark formed as a strain feature that caused segregation of quartz along preferential axial planes. Sanidine was found at all three locations as devitrification of glass within the groundmasses. Maungatūtū/Mount Misery had sanidine present as radiating spherulites, while Greenpark had a new find of sub-circular crystalline inclusions which appeared to be some variation of a spherulite.
The domes have similar geochemical and mineralogical compositions, proving they are linked to the same source magma, but would have erupted at different points in time, likely with Maungatūtū/Mount Misery as the oldest and Pukunui as the youngest. The flow banding and flow folding present at Pukunui and Greenpark, and the lack of shearing features, have been used to identify the domes as endogenous. The Pukunui pyroclastic succession included multiple flow units that were correlated to the Otawera ignimbrite elsewhere, which was likely emplaced as a proximal ignimbrite from a dome collapse event at Pukunui. This was one of the final volcanic events in the Mount Misery series area
Volcanic and sedimentary geology of the basaltic Karaka Volcano, South Auckland
Get PDFIntraplate monogenetic volcanic fields are a common volcanic feature of the upper western North Island. There are four geographically related fields, approximately 38 km apart which young northwards from Raglan to Auckland. The South Auckland Volcanic Field (SAVF; 1.56–0.51 Ma) is older and more degraded than the Auckland Volcanic Field (AVF; active since 250 ka) to the north. Located between the two fields and overlying the Late Pliocene to Mid-Pleistocene pumiceous fluvial deposits of the Puketoka Formation, is the newly discovered Karaka Volcano. Although residents had referred to a volcanic landform in the area, it was in 2018 that evidence for a volcano based on geomorphology, a magnetic anomaly, weathered surficial deposits and water bore data was reported.
This thesis will present the recent findings of the volcano-sedimentary geology of the Karaka volcano. Evidence is based on stratigraphic analysis of two new drill cores, a complementary resistivity survey, petrography, scanning electron microscopy and geochemical investigations. One drill core intersected an upper 6 m blanket of Hamilton Ash Formation and a lower 7 m succession of interbedded, laminated dark brown organic-rich silts, and distal tephras. The lower succession is consistent with lake sediments associated with a crater lake within the geomorphic tuff ring. The second drill core, on the central high point of the volcanic landform, intersected an upper 6 m of brown and red clay. Beneath this, is a lower 9.5 m succession of volcanic ash, coarsening downwards, to a basaltic lapilli ash deposit with a lithic-rich matrix. This represents a phreatomagmatic phase in the upper part of the volcanic sequence. The resistivity survey connects the two drill holes giving an indication of extent and layering of the geology beneath the surface. No recognised volcanoes are within 5 km of this site, so it is unlikely that this pyroclastic deposit came from a distal vent, despite its isolated location outside of the predefined margins of the South Auckland and Auckland volcanic fields.
Basalt lapilli within the pyroclastic succession are porphyritic with a trachytic to glassy groundmass with predominant phenocrysts of olivine, augite, opaque minerals and plagioclase. Geochemical analysis was conducted on whole rock samples of basalt lapilli by X-ray fluorescence spectroscopy (XRF) and on pyroxene, olivine, plagioclase and zeolites by electron probe microanalysis (EPMA). This data identified Karaka Basalt as a member of the Group B basalts of the SAVF. Lithics were fragments of the underlying sandstone and siltstone of the Waitemata Group. The surrounding matrix comprised small basalt fragments, lithics, quartz and plagioclase.
Glass compositions of tephra by EPMA provide information that has been used to produce a tentative minimum age on the crater lake and by proxy the volcano itself. Tephra TW gives a possible correlation to the Kidnappers Ignimbrite which has been dated previously at ~0.99 Ma. The age of this tephra along with belonging to the SAVF, and a covering of Kauroa and Hamilton Ashes, gives the Karaka Volcano an approximate age range of 1.56 – 0.99 Ma
Deposit characteristics and dynamic processes of large volume pyroclastic density currents of the Taupo Volcanic Zone approaching and entering the sea
Get PDFPyroclastic density currents (PDCs) entering the sea are a rare but dynamic phenomenon that cause significant change to the topography and bathymetry of coastal and island arc settings. Very little is known about their complex behaviour and depositional processes, as observation of such an event is uncommon and conducting experiments that simulate PDCs is challenging. A better understanding of this phenomena can aid in future volcanic monitoring and hazard management around coastal and oceanic settings.
This study combines both field observations and laboratory experiments of PDCs entering the sea. Case studies around North Island, New Zealand of pre-historic pyroclastic deposits found near the coast focused on, the Rotoiti Ignimbrite that originated from the Okataina Volcanic Centre (Pacific Coast Highway between Matata and Maketu; Mimiha Road near Matata), and the Ongatiti Ignimbrite (Glenbrook Beach, Waiuku; Kihi Road, inland of Kawhia Harbour) from the Mangakino Volcanic Centre. At each location, field observations were recorded, and samples collected, followed by grainsize and texture analysis in the laboratory (scanning electron microscopy, laser diffraction particle size analysis, dry sieving). Laboratory experiments that simulated small-scale PDCs generated by a column collapse mechanism and a dam-break mechanism into a flume were conducted.
The Ongatiti Ignimbrite at Glenbrook Beach presented flow-water interaction features through soft sediment deformations such as dewatering structures, flame structures, and thin beds of convoluted laminae. The Rotoiti Ignimbrite at Mimiha Road showed distinctive crossbedding. Both examples highlighted PDC processes after entry into the water. The Rotoiti and Ongatiti ignimbrites along the Pacific Coast Highway and Kihi Road, respectively presented typical subaerial ignimbrites that had travelled long distance overland and were still undergoing significant deposition near the coastline. The flume experiment demonstrated that PDCs segregate into two parts when entering the water, (1) a dilute overflow cloud, and (2) a dense underwater current. The entrance of pyroclastic ash material into water also generated a wave. Underwater obstructions will affect a submergent current by significantly decreasing its height and runout distance
Subsurface andesite geology and hydrothermal alteration at an exploration prospect north of Waihi
Get PDFThe Coromandel Volcanic Zone hosts the Hauraki Goldfield in the North Island of New Zealand. The goldfield consists of approximately 50 low sulphidation Au-Ag deposits. Situated in Coromandel Group rocks, which host 95% of the gold production in the region, the area of focus of this study is a prospect located 4 km north of the world class Martha Hill Mine. Such epithermal deposits are characterised by zones of altered rock called halos. Seven angled exploratory drill holes from the area were examined in this study. These cores were logged using high resolution photographs from a geological perspective for the first time. Samples were taken for petrographic and mineralogical analysis including X-ray diffraction analysis and for X-ray fluorescence spectroscopy. Leapfrog was used to display the spatial relationship of these results. The aim of this study was to determine the volcanic geology and alteration origins of this project. This was achieved by identifying facies as well as their spatial distribution, determining the mineralogical properties of the facies and their variations down-hole, as well as identifying the style and spatial extent of alteration.
Eight new facies were identified which are hosted in andesitic rock. Three of the facies were based on visible crystal concentration which ranged from, 3% – 65% and occurred throughout the core. Fresh samples were dominated by plagioclase, with relatively minor amounts of quartz and pyroxenes and altered samples comprised of calcite+quartz assemblages with various suits of sulfides and oxides, predominantly pyrite and magnetite. The mineral assemblages and their abundances are comparable to the Waipupu Formation andesite with vast similarities to active andesite volcanoes and Late Archean examples. The rocks exhibit seriate porphyritic textures and glomeroporphyritic clots that are consistent with polybaric fractionation. The textures resulted in the host rocks innate high porosity and permeability which increases with each propagation of hydrothermal fluid. While the common resorption of quartz indicates magma mingling occurring in the magma chamber, these same resorption features as well as observed swallowtail plagioclase microlites also reflect degassing processes within the magma during ascent. Four breccia facies were described based on the concentration and orientation of breccia clasts. These facies occurred dominantly around the edge of alteration zones, which can be attributed to their relatively higher porosity. While some breccia display properties of auto breccia, hydrothermal breccia’s are dominant with all breccia likely altered to some degree. Finally, an irregular facies was described and relates to hydrothermal alteration processes exclusive to moderate and high alteration zones. Using the mineralogy data, in conjunction with stratigraphic logs, zones of alteration could be identified and designated based on intensity, describing two zones of high intensity alteration separated by a zone of moderate alteration and low alteration occurring closer to the surface. These zones give an insight into the flow of paleo fluid during alteration and can hint at the location of Au bearing veins
Volcanic geology of the early Pleistocene ignimbrite succession in the western Papamoa Region, Bay of Plenty
Get PDFVolcanic activity in the Tauranga Volcanic Center (TgaVC) occurred between 2.95 to 1.9 Ma, some of the eruptions were explosive and led to the distribution of ignimbrites of varying volumes across the western Bay of Plenty Region (Tauranga Basin), North Island. The Papamoa Formation forms part of the landscape in this area. This study was aimed at determining the volcanic history and processes involved in the distribution of the Papamoa Formation within the eastern Tauranga Basin.
Stratigraphic, petrographic and geochemical investigations were undertaken in the field and from samples within the study area. Field observations of the ignimbrites involved stratigraphic logging and lateral relationships, component measurements and lithological descriptions. Ignimbrite petrography was undertaken by optical microscopy. To determine the geochemical characteristics of the source magmas for the ignimbrites, the following analyses were conducted: electron microprobe analysis (EMPA) on glass shards extracted from the ignimbrite matrices, glass from pumice, and on minerals in the pumice, x-ray fluorescence spectroscopy (XRF) on whole pumice clasts and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) on the same glass samples used for EMPA.
Five individual eruptions that led to the distribution of five ignimbrites have been identified within the ignimbrite succession of the Papamoa Formation: an early unnamed ignimbrite, followed by the Welcome Bay, Wharo, Arateka, and Otawera ignimbrites. The beige-brown, non-welded, pumice-rich, Welcome Bay Ignimbrite (2.4 Ma) is the most voluminous in the study area and also has an underlying pumice fall deposit. The dark brown to black, pumice-/fiamme-rich Wharo Ignimbrite (2.26 Ma, M. Prentice unpublished data, 2021) is the only moderate to densely welded ignimbrite. Both the Welcome Bay and Otawera ignimbrite contained black (andesitic-dacitic) and white (dacitic) pumice. The beige, non-welded Otawera Ignimbrite (2.21 Ma) is the least voluminous ignimbrite in the study area and comprises only white pumice. The beige-yellow brown Arateka ignimbrite is comprises of both the black and white pumice clasts. The unnamed Unit A ignimbrite, at the base of the succession has only one pumice population – grey pumice, but it is poorly exposed. All the ignimbrites are generally lithic poor. Petrographic analysis found that the main minerals occurring in the ignimbrites were: plagioclase, pyroxenes, hornblende and opaques. The main mineral in the black pumice was plagioclase and pyroxene. The white pumice, however, is mainly composed of plagioclase and hornblende. In terms of rock textures, the Welcome Bay Ignimbrite, Arateka Ignimbrite, Otawera Ignimbrite and Unit A-Unknown Ignimbrite had either a porphyritic or vitrophyric or a combination of both textures. The
Wharo Ignimbrite possessed a porphyritic and/or eutaxitic texture. Geochemical data showed that although the glass shards from each ignimbrite has a unique chemical composition, they were all rhyolitic. However, in terms of the two pumice types, the white pumice was dacitic while the black pumice was andesitic.
The presence of two pumice types (black and white) in the Welcome Bay, Wharo and Arateka Ignimbrites, supports the idea that the magma from which the ignimbrites were sourced, had a mixed andesitic to dacitic composition. The distribution and internal stratigraphy of the Papamoa Formation record a history of multiple ignimbrite-forming eruptions that were sourced locally, likely south of the study area; at least one ignimbrite (Welcome Bay Ignimbrite) was preceded by a sustained Plinian eruption column
Volcanology and secondary alteration of the 1.6 Ma Ngaroma Ignimbrite, Upper Waipari Valley
Get PDFThe 1.6 Ma Ngaroma eruption from the Mangakino Volcanic Centre (MVC) is the first known rhyolitic event of the Taupo Volcanic Zone (TVZ), and variations of primary components recorded throughout the Ngaroma Ignimbrite provide insight(s) into pre-eruptive processes and post-emplacement mechanisms.
A field study of the Ngaroma Ignimbrite was undertaken proximal to the source caldera (~25 km) around the township of Ngaroma. Two primary facies (a lithic- rich and lithic-poor facies) and four subfacies have been identified and are presented in four stratigraphic logs, which vary both vertically and laterally throughout the field area. The facies reflect syn- and post-deposition process that are identifiable from one another based on macroscopic observations including changes in primary componentry, the degree of welding and nature of secondary alteration. Variations in depositional structures and textures highlight the intense welding, limited incorporation of vent-derived lithics, and the absence of a pumice/ash fall deposit underlying the ignimbrite, which suggest that the parent pyroclastic flow formed from the immediate collapse of a highly explosive, caldera-forming eruption column.
Petrographic observations and mineralogic analyses involving optical microscopy, scanning electron microscopy (SEM), and X-ray diffraction (XRD) record high concentrations of sanidine and cristobalite, followed by clay minerals (kaolinite), which suggest the deposit endured significant post-emplacement processes such as vapour-phase alteration and devitrification. The ignimbrite matrix varies from a eutaxitic texture comprising (a) visible, well-formed, but devitrified, glass shards typically 300 to 800 μm, to (b) a fine-grained, homogenous, optically unresolvable medium; the latter can be resolved under SEM and comprises intergrown <3 μm- sized K-feldspar (sanidine), amorphous/nano-crystalline silica (cristobalite ± tridymite), and pore cavities.
Geochemical analyses of major and trace elements on bulk ignimbrite and pumice were determined using x-ray fluorescence (XRF) spectrometry and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), and on individual minerals by electron microprobe analysis (EPMA). Samples used in these geochemical analyses have been pervasively altered, and that due to the lack or absence of primary material (fresh glass and pumice), some of the patterns observed here are presumed to in part reflect post alteration processes (vapour-phase alteration and devitrification) and therefore, are of limited use for understanding primary magmatic processes. The major and selective trace elements of bulk rock ignimbrite and pumice confirm a rhyolitic composition and are consistent with the effect of syn- and post-emplacement alteration processes. Geochemical signatures of rare earth elements (REE) observed on multielement diagrams are associated with subduction related magmas and indicative of crustal contamination. Major element compositions of both feldspar and pyroxene phenocrysts range from An₁₅₋ An₂₈ and Or₅₋Or₁₂ and from En₃₂ to En₄₁, respectively, and are typical composition of rhyolitic magmas
Felsic volcanism in the eastern Waihi area; process origins of the Corbett and Ratarua ignimbrites and the Hikurangi Rhyolite
No full textVolcanic activity began in the north Coromandel Volcanic Zone (CVZ) at 18 Ma, migrated southward during the Miocene to Pliocene, and continued until the Pleistocene (1.9 Ma). The Waihi Caldera was active during the Pliocene and has been infilled with lake sediments (Romanga Formation) and three rhyolitic ignimbrites of the Whitianga Group, the Corbett, Waikino and Owharoa ignimbrites. This study constrains the volcanic history and processes involved in the formation of three volcanic units in eastern Waihi; the Corbett Ignimbrite, Ratarua Ignimbrite and Hikurangi Rhyolite. Detailed stratigraphic logs, petrographic studies, geochemical analyses and U-Pb dating of zircons are presented for each unit.
The Corbett Ignimbrite is a widespread deposit which is constrained to eastern Waihi. It has a maximum exposed thickness of 18 m at its type section, and is a creamy-buff, pumice-rich, crystal-rich, moderately-welded ignimbrite. The basal zone of the ignimbrite is pumice-rich (40-50%), crystal-rich (20-30%) and contains two distinct lithic concentration zones. At one locality the ignimbrite consists of a 9 m thick densely welded zone with abundant fiamme. The matrix is composed of fine ash with shard textures only visible under the scanning electron microscope. Crystals comprise plagioclase, quartz, hornblende, orthopyroxene, augite, titanomagnetite, ilmenite and zircon. Lithics are predominantly andesite with minor rhyolite, dacite and greywacke. Lithic concentration zones suggest that collapse and erosion of the vent occurred several times during the eruption, and these were emplaced by rapid depositional pulses. The upper half of the outcrop represents a more steady flow. Pumice and glass shard composition shows a transition from andesitic to rhyolitic. The source of the Corbett Ignimbrite was thought to have been from either a silicic centre in the vicinity of the Bowentown Rhyolite or from the Waihi Caldera. However, the U-Pb age 6.09 + 0.34 Ma is consistent with older source vents located elsewhere, with two other possible vent locations assessed.
The Corbett Ignimbrite pyroclastic flow was constrained by the welded, fiamme and crystal-rich, dacitic Ratarua Ignimbrite which has been dated at 6.79 + 0.42 Ma. The Ratarua Ignimbrite is exposed on hillsides as angular blocks ranging from 30 cm to several metres in size. Crystals comprise plagioclase, hornblende, orthopyroxene, augite, titanomagnetite, ilmenite and zircon. Lithics are andesite, dacite, rhyolite and rare sandstone.
The spherulitic Hikurangi Rhyolite dome belongs to the Homunga Rhyolite formation and overlies the Corbett Ignimbrite in southeastern Waihi. This rhyolite dome contains quartz, plagioclase, biotite, titanomagnetite and ilmenite phenocrysts. Petrographical analysis of granophyric intergrowth crystals indicates the presence of a granite body below this dome, which is the first evidence of granite present in southern CVZ. U-Pb dating of zircons determined an age of 4.53 + 0.13 Ma for this dome showing a younging in age of the Homunga Rhyolite formation southwards.
The units included in this study show a progression in volcanism with a transition from andesite dominant to rhyolitic. The Ratarua Ignimbrite formed during an earlier period dominated by andesite volcanism. This ignimbrite pre-dates the Corbett Ignimbrite, which formed during later andesitic volcanism in the eastern Waihi area, predating the Waihi Caldera. The youngest unit in this study, the Hikurangi Rhyolite, shows younger rhyolitic volcanism in southeastern Waihi
Volcanic Geology of Maungatautari: An Andesitic-Dacitic Composite Cone, Central Waikato, New Zealand
Get PDFMaungatautari is an extinct andesitic-dacitic composite cone volcano, situated 35 km southeast of Hamilton, that rises prominently in the Waikato region. A single radiometric age of 1.8 Ma, indicates volcanic activity here was contemporaneous with subduction-related volcanism of the nearby Alexandra Volcanics and the early Taupo Volcanic Zone. The geology and volcanism of Maungatautari was previously addressed (1950s-80s) in context of the broader volcanic geology of the Waikato region, however, only a single dedicated volcanic study, undertaken over 30 years ago, provided insight into the chemical composition and origin of the magmas. This study applied modern volcanological techniques and concepts to reconstruct the volcanic history of Maungatautari and how it may relate to volcanism elsewhere in the Waikato Region around 2 Ma. The work incorporates field and desktop geomorphic analysis, volcanic stratigraphy and facies analysis, petrographic and mineralogical studies, and elemental and isotopic geochemical analysis.
Maungatautari overlies Jurassic Manaia Hill group rocks of the Waipapa Terrane and abuts a range of the same rocks immediately to the west. It is surrounded to the north, east and south by a thick ignimbrite plateau which overlies the lowermost flanks of the mountain, originating from the Mangakino caldera to the southeast. Catastrophic failure of the northeastern flank produced a 0.28 km3 rock avalanche deposit over an area of approximately 1.6 km2. The deposits of rock falls, slides and debris flows were also commonly observed and demonstrate that numerous mass-wasting processes have continued to occur into the present.
Lavas at Maungatautari consist of non-vesicular labradorite, pyroxene and hornblende andesites and hornblende dacites along with olivine basalt at the small cone at Kairangi 7 km to the northwest. Whole rock geochemical and strontium and neodymium analyses demonstrate these lavas were derived from at least three distinct mantle-melt sources: the fractionation of subduction-related, deep, garnet-bearing, depleted mantle (most andesites and dacites); a shallower, garnet-free, upper mantle (low SiO2 hornblende andesites) and an area of deeper enriched mantle (Kairangi olivine basalt). This study considers Maungatautari to reflect andesitic volcanism occurring along the edge of the continental tip of the Colville volcanic arc c. 2 Ma with the near-by Alexandra Volcanics being contemporaneous, behind-arc subduction-related volcanoes. The olivine basalt at Kairangi is more reflective of intraplate over subduction-related melts and is unrelated to magmatism at Maungatautari.
This study is the first to document a record of explosive volcanism at Maungatautari. Eruption styles which occurred at Maungatautari include the effusion of lava resulting in the generation of lava flows and domes, and vulcanian to sub-plinian explosive eruptions which produced both high and low particle concentration pyroclastic density currents
Geophysical characterisation of the Onewhero and Kellyville volcanic complexes, South Auckland Volcanic Field
No full textThe Quaternary-aged (1.59 – 0.51 Ma) intraplate, monogenetic, basaltic South Auckland Volcanic Field consists of at least 82 volcanic centres that span an area of 300 km². This study focusses on the volcanic and sedimentary histories of the Onewhero and Kellyville maars, interpreted from additional geological observations, which build upon previous geological studies, and gravimetric and magnetic surveys.
New geological observations of the Onewhero maar reveal the presence of diatomaceous sediment, and at least one lava flow, that probably originated from the nearby Klondyke cone to the south, and which occupies a significant area of the crater floor. A geological investigation of the Kellyville maar reveals a crater that has been partially filled by a thick accumulation of basalt and scoria and overlain by Karapiro Formation sediments and diatomite. Available borehole stratigraphy suggests that the Glass Hill basalt cone present in the Kellyville maar was an early-stage volcanic product rather than a late-stage product as previously thought.
Gravimetric surveys in the Onewhero and Kellyville maars revealed contrasting crater fill deposits. Raw gravity data was acquired in the field, corrected for drift and then reduced to a Bouguer anomaly. The Onewhero maar is characterised by an anomaly of -5.8 mGal that has a concave profile. The post-eruption geological body present at Onewhero is two-dimensionally modelled with a density of 1.3 g/cm³, and has a maximum thickness of 100 m in the northwest of the crater. The Kellyville maar is characterised by an anomaly of +2.5 mGal that has a convex profile. The dominant geological body present in the crater is a lava-lake deposit that is two-dimensionally modelled with a density of 2.9 g/cm³, and has a maximum thickness of 60 m.
The total magnetic field strength of the Onewhero and Kellyville maars was recorded with a proton magnetometer and the values were mapped in ArcMap. In the Onewhero maar, two subdued anomalies were outliers. A positive magnetic anomaly (213 nT) with a broad crescent shape was identified in the middle of the crater and interpreted to be either an extrusive feeder dyke or an accumulation of highly magnetic volcanic sediment deposited in a topographic trough. A circular negative anomaly (-1057 nT) in the northwest of the crater is an accumulation of lake sediment with a low magnetic susceptibility. In the Kellyville maar, two contrasting anomalies were observed. The strongly positive anomaly (1275 nT) corresponds to the geographical position of Glass Hill, a basaltic cone. A strong negative anomaly (-3315 nT) in the middle of the crater corresponds to a known deposit of diatomite.
Overall, the structure of the Onewhero maar, as inferred by this investigation, is similar to many other maars globally and its sedimentary fill is remarkably similar to the Dottingen maar in Germany. The Kellyville maar has a more unique structure but can be compared to the Domain maar in the nearby Auckland Volcanic Field
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