Eruptive origins of a lacustrine pyroclastic succession: insights from the middle Huka Falls Formation, Taupo Volcanic Zone, New Zealand

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1 New Zealand Journal of Geology and Geophysics ISSN: (Print) (Online) Journal homepage: Eruptive origins of a lacustrine pyroclastic succession: insights from the middle Huka Falls Formation, Taupo Volcanic Zone, New Zealand HJ Cattell, JW Cole, C Oze & SR Allen To cite this article: HJ Cattell, JW Cole, C Oze & SR Allen (2014) Eruptive origins of a lacustrine pyroclastic succession: insights from the middle Huka Falls Formation, Taupo Volcanic Zone, New Zealand, New Zealand Journal of Geology and Geophysics, 57:3, , DOI: / To link to this article: Published online: 16 May Submit your article to this journal Article views: 259 View Crossmark data Citing articles: 5 View citing articles Full Terms & Conditions of access and use can be found at

2 New Zealand Journal of Geology and Geophysics, 2014 Vol. 57, No. 3, , RESEARCH ARTICLE Eruptive origins of a lacustrine pyroclastic succession: insights from the middle Huka Falls Formation, Taupo Volcanic Zone, New Zealand HJ Cattell a *, JW Cole a, C Oze a and SR Allen b a Department of Geological Sciences, University of Canterbury, Christchurch 8140, New Zealand; b ARC centre of Excellence in Ore Deposits and School of Earth Sciences, University of Tasmania, Hobart, Australia (Received 16 January 2014; accepted 6 March 2014) Current and ancestral lakes within the central Taupo Volcanic Zone (TVZ) provide depocentres for pyroclastic deposits, providing a reliable record of eruption history. These lakes can also be the source of explosive eruptions that directly feed pyroclast-rich density currents. The lithofacies characteristics of pyroclastic deposits allow discrimination between eruption-fed and resedimented facies. The most frequently recognised styles of subaqueous eruptions in the TVZ are shallow-water phreatomagmatic and phreatoplinian eruptions that form subaerial eruption columns. However, deeper source conditions (>150 m water depth) could generate subaqueous explosive eruptions that feed water-supported pyroclast-rich density currents, similar to neptunian eruptions. Such deep-water eruptions have not previously been recognised in the TVZ. Here we study a subsurface deposit, the middle Huka Falls Formation (MHFF), in the Wairakei Tauhara Geothermal Fields (Wairakei Tauhara), TVZ, which we interpret to be the product of a relatively deep-water pyroclastic eruption ( m). The largely subsurface Huka Falls Formation records past sedimentary and volcaniclastic deposition in ancient Lake Huka. Deposits examined from eight drill cores reveal a lithic-rich lower unit, a middle volumetrically dominant pumice lapilli-tuff and an upper thinly bedded suspension-settled tuff unit. A coarse lithic lapilli-tuff within the lower unit is locally thick and coarse near well THM12, suggesting proximity to a source located beneath Lake Huka. This research provides an understanding of the origin of the MHFF deposit and offers insights for evaluating and interpreting the diversity of subaqueous volcanic lake deposits elsewhere. Keywords: subaqueous explosive eruption; lacustrine; Huka Falls Formation; Taupo Volcanic Zone; pumice; neptunian eruption; Lake Huka Introduction Extensional volcanic arc settings commonly host long-lived or ephemeral lakes that are formed by either structurally controlled subsidence, subsidence following explosive eruptions or by volcanic eruptions blocking water outflows (Manville et al. 2007). These lakes are depocentres for extra- or intra-basinal pyroclastic deposits (e.g. Cas et al. 1990, 2001; Nelson & Lister 1995; Manville 2001). The central Taupo Volcanic Zone (TVZ) hosts large and deep lakes (<1 45 km long; < m deep). Explosive caldera-forming eruptions in the TVZ generated current Lake Taupo and Lake Rotorua and volcanic damming formed Lake Rotoiti, Lake Tarawera and Lake Okataina (Manville et al. 2007). The position, thickness and orientation of lacustrine deposits assigned to the Huka Falls Formation (HFF; Grindley 1965) define the distribution of ancient Lake Huka, a precursor of Lake Taupo (Smith et al. 1993; Manville & Wilson 2004; Rosenberg et al. 2009a; Bignall et al. 2010). Lakes and marine basins within or close to volcanic centres are depocentres for pyroclastic deposits and serve as a record of eruptive activity (e.g. Manville 2001). In some cases, the pyroclastic deposits that punctuate the thin fine-grained background sedimentation are thick, massive to graded, pumicerich, density current deposits fed directly from volcanic eruptions. The source of these eruptions can be from either relatively deep ( 150 m) subaqueous vents or hot pyroclastic flows traversing the shoreline from a subaerial vent (Cas & Wright 1991; White 2000; Allen & McPhie 2009; Allen et al. 2012). Identifying the vent setting for thick, pumice-rich density current deposits can be problematic as the material from both sources mixes turbulently with water and is transported in water-supported mass flows producing similar deposits. Additionally, deposits may also be poorly preserved due to reworking events, hydrothermal alteration and segmented uplift and exposure. Detailed lithological examination is a key method for determining transport and depositional processes as well as for inferring eruption conditions of pyroclastic deposits (e.g. Cas & Wright 1987; McPhie et al. 1993). Pumice rounding, lithic clast type and clast distribution are important attributes that enable the two different origins to be identified (Allen & McPhie 2009; Allen et al. 2012). Drilling in Wairakei Tauhara Geothermal Fields (Wairakei Tauhara) in the TVZ, New Zealand, intersected over 300 vertical metres of the HFF (THM13). The middle unit of the Huka Falls Formation (MHFF; Rosenberg et al. 2009a) is a thick pyroclastic deposit enclosed by lacustrine *Corresponding author. hamishcattell@hotmail.com 2014 The Royal Society of New Zealand

3 332 HJ Cattell et al. sediments. The MHFF is entirely subsurface and has been intersected by coring in eight wells (THM12 14, THM17 19, WKM14 and WKM15; Rosenberg et al. 2009b). Here, detailed facies analysis and broad quantitative parameters are used to determine the origin and infer transport and depositional processes responsible for the MHFF. The results provide insight into past interactions between lakes and explosive volcanism within the TVZ. Terminology The term volcaniclastic is used to describe undifferentiated accumulations of volcanic particles (Fisher 1961). Pyroclastic refers to consolidated or unconsolidated volcaniclastic particle accumulations deposited by a primary pyroclastic process (fall, flow or surge; McPhie et al. 1993). The MHFF uses unconsolidated terminology to describe the individual size of particles (e.g. ash, lapillus) sourced from a consolidated pyroclastic units (e.g. tuff, lapilli-tuff), according to White and Houghton (2006; Table A1). Geological setting and stratigraphy The central Taupo Volcanic Zone The TVZ is an active volcano-tectonic intracontinental rift system in the central North Island, New Zealand (Fig. 1), where volcanic activity began c. 2 Ma (Wilson et al. 1995). Offshore subduction resulting in continental rifting and crustal thinning in the TVZ has concentrated silicic-dominated explosive calderaforming eruptions and smaller effusive dome eruptions to this zone. The period between 2 Ma and 0.34 Ma is defined by activity at the Kapenga and Mangakino calderas (Fig. 1; Wilson et al. 1995). Ignimbrites erupted from Whakamaru caldera (0.34 Ma) commence the young TVZ period which includes up to the present day (Wilson et al. 1995). Deposition of the HFF occurred in the recent period ka (Rosenberg et al. 2009a; Downs et al. 2014). The present-day TVZ contains numerous volcanic centres and lake-filled basins (Manville & Wilson 2004; Manville et al. 2007) providing an environment where pyroclastic deposits are commonly deposited. Distributions of the HFF intersected by drilling and in surface outcrops between southern Lake Taupo and Reporoa caldera in the Taupo Reporoa depression preserve the location of the ancient Lake Huka (Fig. 1; Smith et al. 1993; Manville & Wilson 2004; Manville et al. 2007; Rosenberg et al. 2009a). The extent and number of lakes that may have comprised Lake Huka remain unclear; however, repeated regional faulting overprinting caldera structures is suggested as the main structural control (Manville & Wilson 2004; Rosenberg et al. 2009a). Volcanic vents located within lakes in the TVZ have been the source of phreatomagmatic eruptions (e.g. early Ohakuri fall deposits; Gravley 2004; Gravley et al. 2007) and high-intensity phreatoplinian eruptions (e.g. Oruanui, Hatepe and Rotongaio deposits; Self & Sparks 1978; Walker 1981; Wilson 1993). Vents sourced in deep water Figure 1 The TVZ boundary and TVZ eruptive centres. Ok, Okataina; Ro, Rotorua; Ka, Kapenga; Oh, Ohakuri; Rp, Reporoa; Ma, Mangakino; Wh, Whakamaru; Tp, Taupo (Houghton et al. 1995; Wilson et al. 1995; Gravley et al. 2007) relative to Wairakei Tauhara Geothermal Fields locality (box shows location of Fig. 2A) and the northeast inferred extent of HFF deposits (bordered by dashed line) after Manville et al. (2007). Insert shows location of the TVZ, central North Island, New Zealand. Figure modified from Deering et al. (2012). where the eruption column is largely subaqueous have not previously been documented for the TVZ. The Wairakei Tauhara Geothermal Fields and Huka Falls Formation The Wairakei Tauhara Geothermal Fields lie northeast of Lake Taupo in the TVZ (Figs 2A, 2B). Extensive well drilling in 2009 by Contact Energy Limited confirms that most major stratigraphic units are laterally continuous between the two adjacent fields (Rosenberg et al. 2009a, b; Bignall et al. 2010). The HFF is a relatively thin shallow lacustrine succession (between elevations +400 and +100 mrl, elevation relative to mean sea level) above the Waiora Formation and beneath the Oruanui Formation (Grindley 1965). Within most of the Wairakei Tauhara area, the HFF contains three lithologically distinct units: lower (LHFF) and upper (UHFF) units; mudstones interbedded with volcaniclastic detritus; and the middle pumice-rich MHFF (Fig. 2B; Rosenberg et al. 2009a).

4 Eruptive origins of a lacustrine pyroclastic succession 333 Figure 2 A, Wairakei Tauhara Geothermal Fields defined by a resistivity boundary (Risk 1984) and geographically divided by the intersection of the Waikato River. Dots show the well locations of core samples examined and the locations of wells (italicised) mentioned in the text. Solid lines represent known intersected locations of the MHFF (GNS Science: 3D geological model of Wairakei Tauhara; S. Alcaraz, pers. comm. 2013) and grey areas are the inferred extents of the lower, middle and upper units defined in the text. B, Conceptual HFF architecture (vertically exaggerated) through Fig. 2A (A A ) above a summary of stratigraphic formations (Grindley 1965), HFF units (Rosenberg et al. 2009a) and MHFF facies units (this paper). The LHFF comprises planar-bedded, cross-bedded and laminated volcanogenic sandstone and lacustrine mudstone (Rosenberg et al. 2009b). Drilling suggests it is partially discontinuous between Wairakei and Tauhara (Fig. 2B; Rosenberg et al. 2009a). In contrast, the UHFF is less lithologically variable and similarly consists largely of bedded lacustrine mudstone and fine-grained volcaniclastic detritus. The MHFF is a thick (c. 100 m), graded, vitric tuff with angular pumice clasts and dense lithic clasts. The abundance of angular pumice clasts and lack of evidence for erosional or reworked boundaries supports emplacement by a single eruption-fed density current that terminated deposition of the LHFF. The MHFF is inferred to be largely confined to the Wairakei Tauhara area (Fig. 2A) reflecting structural and alluvial aspects of the original depositional lake basin (Manville et al. 2007; Rosenberg et al. 2009a; Bignall et al. 2010). However, eastern and northern extents of the MHFF are not well constrained where little drilling has taken place. Near the Wairakei Power Station (Figs 2A, 2B), the MHFF is reportedly 200 m thick from where it quickly pinches out towards the north, west and gradually to the

5 334 HJ Cattell et al. southeast in Tauhara (Fig. 2A) leaving a single unit of HFF lake sediments (Fig. 2B; Rosenberg et al. 2009a; Bignall et al. 2010). Drilling across Wairakei Tauhara intersects the MHFF as a series of discontinuous patches. Geological models calculate that these patches (Fig. 2A) have a (minimum) volume of c. 6 km 3 within Wairakei Tauhara (GNS Science: 3D geological model of Wairakei Tauhara; S. Alcaraz, pers. comm. 2013). A conservative estimate of the MHFF volume is 10 km 3 over an inferred area up to 100 km 2 (Fig. 2A) characterising it as a small TVZ eruption. Characteristic larger TVZ explosive eruptions (30 >300 km 3 ) form thick, widespread ignimbrites with associated calderas (Wilson et al. 1995). Materials and methods A drill core for assessing the HFF was provided by Contact Energy Limited at the Wairakei Steam Field core shed. The cores are 6 cm in diameter and have a calculated recovery rate of 94% in the HFF. All available core samples intersecting the MHFF were examined in the study. Core samples were used over cuttings to assess in situ textures, grain sizes and lithological relationships of the Huka Group by physical observations and high-resolution (8 megapixels) digital photographs. All cores were drilled within the Wairakei Tauhara geothermal resistivity boundary (Figs 2, 3; Risk 1984) and intersect a hydrothermally altered zone characterised by an intermediate argillic alteration assemblage (illite-smectite, illite, chlorite, pyrite, calcite and quartz vein fill; Browne & Ellis 1970; Rosenberg et al. 2009b). Alteration replaces mafic minerals and volcanic glass and weakly cements the rock, precluding detailed grain size analysis. Qualitative visual estimations of clast sorting, rounding and volume (Fig. 3) were made with comparisons to respective standards charts. Where clasts were present, sizes of the ten largest were measured in situ (two-dimensional) within 5 m sections (5 1 m core lengths) to characterise the transport and source conditions. Measurements were made using a ruler along the long axis; however, larger clasts (>6 cm) were often bisected by drilling, providing a minimum size. Small clasts (<5 mm) were common and difficult to distinguish from the matrix (<2 mm), particularly in core photographs. The volume and distribution of larger clasts (1 >6 cm) were the focus of the assessment. Measurement error was negligible relative to the 2D in situ and bisected sample types. A washed sample of rotary-drilled pumice cuttings from the MHFF in well WK308 (Fig. 2A) and a sample of phenocrysts from the hydrothermally altered pumice (clay) clasts from THM12 was provided by Michael Rosenberg, GNS Science, Wairakei Research Centre. Cuttings are loose, small, mixed chips ( 1 cm diameter) that were collected at 10 m drilling intervals over 75 m. Unlike the core samples, the pumice chips have no obvious hydrothermal alteration and retain most primary volcanic features, allowing the composition and microvesicularity of the MHFF pumice to be assessed. Fine-grained units, dense lithic clasts and altered pumice phenocrysts (THM12) and fresh pumice cuttings (WK308) were thinsectioned to identify common constituents and textures. Whole-rock major and trace element analysis of the pumice chips (WK308) and excavated granodiorite lithic clasts (THM12) were undertaken on a Philips PW2400 XRF using procedures outlined by Norrish & Chappell (1967). Minor mineral phases within the pumice phenocrysts samples (THM12) were identified by scanning electron microscopy (SEM) and energy-dispersive x-ray spectroscopy (SEM-EDS) using a JEOL 7000F (15 kv, 0.59 na at 4742 cps). Microvesicularity and crystallinity of 12 randomly selected thinsectioned pumice chips were assessed from photomicrographs at 4 magnification and SEM backscattered images (BSI) and backscattered electron images (BSE) at 95, 100 and 250 magnifications at 15 kv accelerating voltage. Pumice buoyancy was assessed by placing 100 randomly selected 6 8 mm pumice chips (sieved 3φ to 2.5φ) in water and examining their settling behaviour over a 24 hour period. All analyses were carried out using equipment at the University of Canterbury. Results The MHFF can be divided into three stratigraphic units (Fig. 3): the lower unit is rich in coarse lithics and is m thick; the middle unit consists of pumice lapilli-tuff and is 60 m thick; and the upper unit is 3 10 m and consists of fine tuff. The contact between the LHFF and MHFF is sharp in all of the examined wells; however, drilling was terminated before the contact was intersected in THM12 (Fig. 3; elevation +55 mrl). The upper MHFF contact between the thinly bedded tuff (upper unit) and UHFF mudstone is sharp in all wells with the exception of THM12 and THM19, where it is gradational and possibly reworked (Fig. 3). Pumice crystallinity and micro-vesicularity Fresh pumice chips are weakly porphyritic (3 vol.%) and contain plagioclase feldspar, quartz, amphibole and accessory pyrite. The altered pumice clasts phenocryst population includes dominantly large (>500 µm), normally zoned, euhedral plagioclase (c. 60%), quartz (c. 30%), a circular Ca Ta silicate identified as leucoxene under SEM-EDS (c. 7% and 250 µm in size) and accessory pyrite (3%). No mafic mineralogy was preserved in the altered pumice samples. Thin-sections identify that the matrix of the core samples consists of silicified and devitrified formerly glassy bubble wall and pumice shards and subordinate lithic fragments. Medium pumice lapilli clasts (1 3 cm) comprise 5 15 vol.% (up to 40 vol.%) of the MHFF middle unit. Cored pumice clasts are angular, blocky and lenticular shapes with curviplanar or ragged edges while hydrothermal alteration destroyed the internal textures. The largest pumice clasts (up to 9 cm) are blocky and scattered (<2 vol.%) within the lower coarse lithic lapilli-tuff in THM12.

6 Eruptive origins of a lacustrine pyroclastic succession 335 Figure 3 Graphic logs summarising unit relationships and estimated clast proportions in the examined Tauhara and Wairakei core samples. Z, silt size; S, sand size; G, granule size; P, pebble size; Undiff. matrix, undifferentiated matrix particles <2 mm. Structurally intact pumice chips are highly vesicular with spherical and dominantly elongate cylindrical (or tube) domains (Figs 4A 4C). Vesicles in the chips are uniformly small (20 75 µm) and elongate (aspect ratios of >8:1). Medium-sized ( µm) vesicles are less common and are more spherical or weakly elongate (ratio 2:1). Some show coalescence. Coarse vesicles (>150 µm) are rare and elongate (ratios of 4:1). Pumice samples are devoid of vesicle sizes >0.5 mm. Larger vesicles were likely preferentially lost during mechanical fragmentation during drilling of the chip samples. Most chips (63%) floated in water indicating specific gravities of <1 g cm 3 and vesicularities of 54% (DRE estimated 2.2 g cm 3, Allen & McPhie 2003; equation in Houghton & Wilson 1989). Of those that sank, most were the smaller sample fraction (6 8 mm) dominated by small vesicles. Lithic clast componentry Lithic clasts in the MHFF are dominantly restricted to the lower unit. They are typically medium lapilli-size (e.g. THM13), but range from coarse ash to block (e.g. THM12 and THM14). Lithic clasts in the overlying middle unit are rare ( 5 vol.%), subrounded, coarse ash to medium lapilli. Common lithologies include undifferentiated rhyolite, basalt, consolidated tuff, siltstone and rare granodiorite. Large mudstone clasts (10 cm) in the MHFF appear to be rip-up clasts derived directly from the underlying LHFF mudstone substrate. Rare granodiorite lithic clasts in the lithic-rich unit were largest in THM12 (typically 4 cm and up to 13 cm) and smaller (<2 cm) and less abundant in THM13 and THM14. Clasts are highly evolved consisting of plagioclase feldspar (c. 50%), quartz (c. 30%), alkali-feldspar (c. 15%), biotite (c. 5%) and accessory hornblende, chlorite, magnetite, zircon, apatite, sericite and calcite. Similar mineralogy and enriched whole-rock compositions between the pumice chips and granodiorite clasts (Fig. 5) support a co-magmatic relationship. Stratigraphy and lithofacies characteristics Lower unit: medium lithic-pumice lapilli-tuff and coarse lithic lapilli-tuff The MHFF sequence begins with a sharp, planar contact with the underlying LHFF where mudstone rip-up clasts are concentrated. The MHFF lower unit can commonly be divided into two further units (Fig. 2B). The basal unit comprises a 15 m thick, medium lithic-pumice lapilli-tuff unit that represents c. 10% of the total MHFF thickness. This medium lithic-pumice lapilli-tuff is moderately sorted and contains 5 vol.% fine to medium lithic lapilli clasts and 15 vol.% fine to medium pumice lapilli in an ash matrix. Lithic and pumice clasts in the basal unit are the largest ( 16 mm) and most common (40 vol. %) in the lower 2 m of THM19 than in other samples. Sorting is very poor (THM19), poor to moderate (THM12 14) or well sorted (THM17 and THM18). In WKM14 and WKM15 no lower unit was recognised. Distinguished by a change in clast size and sorting, the underlying medium lithic-pumice lapilli-tuff unit grades over 1 2 m into the coarse lithic lapilli-tuff unit. The overlying unit is m thick and comprises c. 20% of the MHFF. Clasts are usually matrix-supported (30 50 vol.%), poorly sorted, subangular angular, medium to coarse lapilli. Lithic

7 336 HJ Cattell et al. Figure 4 SEM images of a single fresh pumice chip sample from the MHFF in well WK308. A, Oblique BSI image of cylindrical vesicles at 95 and 250. B, Top-down perspective BSI image of circular vesicles (looking down cylindrical vesicles) at 95 and 250. C, BSE binary images of microvesicularity at 100 magnification. Vesicles are black, glass and phenocrysts are white. clasts are rhyolite (c. 25 vol.%), basalt (5 vol.%), undifferentiated silicified volcaniclastics (c. 15 vol.%), mudstone (c. 10 vol. %) and granodiorite (<<1 vol.%). A thick, exceptionally coarse ( 30 cm) lithologically diverse coarse lithic lapilli-tuff occurs at THM12 where the average coarsest clast size (ten largest) is highest at 9 cm. In both THM12 and THM14 (6.2 cm average large clast) the coarse lapilli-tuff unit consists of a coarse, 10 m thick, clast-supported lower section overlaid by a m thick finer-grained normally graded upper section. Rare, angular and blocky, white pumice clasts (<2 vol.%) occur among the lithics in these samples. In other samples, the unit is moderately coarse in THM18 (4.6 cm) and is finer grained and better sorted in THM13 (2.7 cm), THM19 (2.2 cm) and THM17 (1.2 cm). Middle unit: pumice lapilli-tuff The middle unit is up to 68 m thick, comprising c. 60% of the MHFF. The dominant unit comprises thickly bedded to massive pumice lapilli-tuff. Components are pumice lapilli (c. 15 vol.% and up to c. 40 vol.%) and lithic lapilli ( 5 vol.%) supported in the vitric tuff matrix (c. 80 vol.%) (Fig. 6B). Pumice grading is weakly defined normal or is absent. Pumice lapilli clasts (typically 1 2 cm) are angular, ragged and lenticular forms that are moderately to poorly sorted. The proportion of pumice clasts in the middle unit varies between well locations. The upper 30 m of the pumice-rich units in THM17 and lower 65 m of THM12 and WKM15 appear devoid of pumice clasts. In WKM14 the middle unit is absent.

8 Eruptive origins of a lacustrine pyroclastic succession 337 Figure 5 Fresh pumice chips (WK308) and granodiorite lithic clast (THM12) whole-rock major geochemistry summary table (*recalculated LOI totals) and trace element spider diagram results support a co-magmatic relationship. In THM13, THM18, THM19 and the upper 20 m of THM12, there are up to eight (THM13), 5 m thick, matrix-supported pumice lapilli (10 vol.%) beds with a lower sharp (sheared?) contact. Scattered lithic clasts (3 5 vol.%), typically concentrated and normally graded at the bed bases, are subrounded medium lapilli (0.5 2 cm). In THM14, pumice lapilli are largest (2.3 cm), abundant ( 40 vol.%) and clearly defined in the upper 25 m of ragged and lenticular clasts (Fig. 6B). In contrast, the lower 20 m consists of block-, wedge-shaped and lenticular angular clasts. Upper unit: thinly bedded fine tuff The upper unit is 5 10 m thick and comprises only 10% of the MHFF (Fig. 3). The unit is extremely fine to fine grained tuff and commonly has a sharp upper contact with mudstones of the UHFF (with the exception of THM12 and THM19 where it is gradational) (Fig. 3). The tuff is thinly bedded to laminated (Fig. 6A; average bed thickness 0.4 cm) and may contain flame structures or other soft sediment deformation features (e.g. THM14 and THM18). Well THM12 is unusual in that it contains two, 3 m thick, moderately sorted beds of concentrated (35 vol.%), subrounded and rounded, matrix-supported, fine to coarse pumice lapilli ( 1.9 cm), the same size as those in the underlying middle unit (Fig. 3; rounded pumice lapilli-tuff facies). The two beds grade into fine tuff and sharply overlie one another. Above the unit in the UHFF are 2 30 cm thick beds of extremely fine tuff interbedded with the dominant mudstone. Figure 6 Core samples from well THM14 showing the three MHFF units: A, an upper unit of fine vitric tuff; B, a middle unit of angular pumice lapilli and vitric tuff; and C, a lower unit of coarse lithic lapilli-tuff.

9 338 HJ Cattell et al. Transport and deposition The MHFF is interpreted to be the product of an eruption-fed subaqueous density current. The overall normal grading and lack of erosional or reworked horizons suggests the MHFF was emplaced by a single eruption and deposition event. Density segregation in the transporting currents formed a lower coarse dense-clast-rich section (Fig. 6C), a middle lower-density pumice-rich section (Fig. 6B) and upper fines-rich cover (Fig. 6A) in the final deposits. Density grading, weak bedding and the overall thick nature is typical of deposits from sustained high-density turbidity currents that segregate into pulses (e.g. Lowe 1982). The lower unit consists of two further units: a medium lithic-pumice lapilli-tuff at the base and an overlying coarse lithic lapilli-tuff. The widespread sharp MHFF lower contact is inferred to be result of shearing and erosion at the head of the transporting flow on the basis of the concentration of rip-up clasts from the underlying unconsolidated mudstone (LHFF) substrate. The material transported in this erosive flow deposited the medium lithic-pumice lapilli-tuff unit. Its thickness (3 32 m) and limited lateral extent (estimated 2 km diameter) suggest that early deposition by concentrated eruption-fed flows was sourced from a small, dense column that was initially restricted by the diameter of the vent. The coarser lithic-rich upper unit is interpreted to be a ventproximal facies reflecting progressive erosion of the conduit and vent-clearing following eruption onset. The increase in clast size between the medium and coarse lithic-rich units suggests a rapid increase in the eruptive energy in response to vent stabilising. The coarsest material accumulated near the vent and the supply of lapilli-sized clasts to the dispersed turbulent flows increased. The MHFF lower unit is coarsest and most lithologically diverse at THM12 and THM14. Given that the lake floor was presumably relatively flat-lying, these locations have been interpreted to have been closer to vent source (Rosenberg et al. 2009b). Normal and lateral grading of the coarse lithic lapillituff unit indicates that dense clasts quickly became density stratified within flows. Only the coarsest pumice clasts were incorporated in these dense flows. Clast size decreases rapidly (within a c. 2 km radius) from THM12, while the unit thickness between wells is variable (Figs 2B, 3). In the more distal sections to the northwest at WKM14 and WKM15, the lower lithic-rich unit is absent (Fig. 3). The middle unit represents deposition from the main density currents which were rich in pumice and coarse ash. Angular and subangular pumice clasts lack evidence for significant abrasion by subaerial pyroclastic flows, instead suggesting impactbuffering transport by water-supported flows (Allen & McPhie 2009). The pumice lapilli-tuff is weakly bedded and includes occasional thin beds of lithic clasts suggesting locally generated unsteadiness within the flow component. In the most distal examined well from the inferred source, WKM14, the middle pumice lapilli-tuff unit is absent (Fig. 3). This locality was apparently cut off from the dominant density current deposition. An absence of the MHFF middle unit may be explained by either the density current becoming extinguished by lateral runout or shallowing of the lake, essentially blocking density current deposition. We infer that the density currents were restricted to the deeper areas of the lake. The MHFF upper fine tuff unit occurs in all examined drill core localities and comprises WKM14 entirely. The thin bedding and fine grain size suggests deposition by passive settling of fine particles turbulently suspended after passage of the water-supported gravity currents. The beds of subrounded pumice lapilli-tuff within the upper unit in THM12 indicate that these pumice clasts have undergone a history of abrasion from grain collisions. Abrasion may have occurred by rolling of saturated pumice on the lake floor or in subaerial environments as pumice rafts or on the lake shore. The latter suggests that these clasts rose through the water column while still hot and cooled in air. Once these clasts were sufficiently waterlogged, they settled to the lake floor. Settling only appears to have occurred proximal to source, suggesting that they were most likely sourced from pumice rafts generated directly from the eruption column (e.g eruption of Havre volcano, Kermadec Arc). Alternatively, the pumice-rich beds may be reworked post-eruption subaqueous debris-flow deposits. The upper MHFF is followed by deposition of lacustrine mud (UHFF) during extremely low accumulation rates of lacustrine mud sediments (e.g. <0.3 mm a 1 in Lake Taupo; Nelson & Lister 1995). Interbedded tuff beds in the lowermost UHFF mudstone are likely to have occurred a considerable time after the eruption and represent either resedimented MHFF fine ash or younger eruptives. Discussion Source origin The MHFF is enveloped entirely by lacustrine mudstones and has no known subaerial equivalent. In Wairakei Tauhara the MHFF is exceptionally thick but drilling suggests it is not very widespread (Figs 2A, 2B), reflecting confined deposition within a basin setting. The MHFF upper tuff unit is more widespread and includes laminated bedding and soft sediment deformation features, but lacks any reworked facies implying the units were transported, sorted and deposited subaqueously. Widespread, thick and normally graded subaqueous pumiceous deposits can be sourced from subaerial, hot, pyroclastic flows that cross the shoreline, mix with water and become transformed into water-supported density currents (e.g. Allen et al. 2012), or directly from a collapsing, subaqueous eruption column (e.g. Allen & McPhie 2009). Characteristic deposits from a collapsed subaqueous eruption include proximal, coarse lithic breccias comprising vent- and conduit-derived lithic clasts and the presence of abundant angular pumice clasts and suspension deposits (Allen & McPhie 2009). Coarse lithic clasts drop out quickly from the eruption column and are not

10 Eruptive origins of a lacustrine pyroclastic succession 339 Figure 7 Comparison between examples: A, a transformational deposit (Znp marine tephra, PM13; Allen et al. 2012); B, a neptunian deposit (graded pumice breccias from the Mount Read Volcanics, HP2; McPhie & Allen 2003; Allen & McPhie 2009); C, a phreatomagmatic deposit (Ohakuri eruptives; Gravley 2004); and D, the MHFF (THM13). Note variable y-axis scales. transported far from source. Pumice clasts remain angular as a result of clast impacts being buffered by water. Ash is elutriated as a result of turbulence within the eruption column and during density current transport followed by suspension settling. The MHFF has all these features. Density currents derived from pyroclastic flows that cross the shoreline produce deposits reflecting subaerial interactions. Deposits can include a basal lithic breccia containing locally derived shallow-water lithic clasts as dense vent-derived clasts drop out during turbulent mixing with water. Pumice clasts may be subrounded or reflect abrasion within the dusty gas medium during transport in subaerial hot pyroclastic flows (Fig. 7A; e.g. Znp tephra deposits, Allen et al. 2012). None of these features are present in the MHFF. Additionally, the MHFF lacks evidence for hot deposition (e.g. welding, pumice fiamme and eutaxitic texture) most common in subaerial settings. This is typical of subaqueous explosive eruptions where the column mixes with water (Cas & Wright 1991). The MHFF has characteristics consistent with being sourced from a subaqueous explosive eruption. The deposits are generated as a result of turbulent mixing and hydraulic sorting forming a vertical internal facies sequence. Dense clasts deposited at the base commonly overlie an erosive contact, followed by the main massive to weakly bedded middle pumice-rich facies, capped by upper suspension-settled fine ash (e.g. Allen & McPhie 2009). Eruption depth Facies characteristics in the MHFF also provide insight into the depositional setting and source. The coarseness and concentration of lithic clasts in the lower unit at THM12 led Rosenberg et al. (2009b) to infer it as the most proximal locality to the MHFF source vent. Dense clast concentrations in the lower and middle unit of THM14 (Fig. 6C) suggest its locality is also near source, but less so than THM12. Both of these units are completely absent c. 7 km (WKM14) northwest of the inferred source vent locality. The MHFF lower unit is analogous to a lithic lag breccia from explosive subaerial or shallow eruptions (Walker 1985) and similar to the neptunian lithic breccia from deep, highly explosive eruptions (Allen & McPhie 2009).

11 340 HJ Cattell et al. The lack of evidence for syn- and post-depositional reworking of the MHFF and contrasting accumulation rates between rapidly deposited pyroclastics (MHFF) overlaid by gradual lacustrine sediments (UHFF) suggest the 100 m thick unit (and locally up to 200 m thick) was deposited entirely below wave base in Lake Huka. The contact marking the transition from MHFF to UHFF deposition appears conformable, therefore indicating that Lake Huka was at least 100 m deep in Tauhara (Fig. 2) to accommodate emplacement of the MHFF. The maximum depth of its modern-day (although caldera-filled) equivalent, Lake Taupo, of m may be a plausible depth for the Lake Huka vent. We infer an eruption from a subaqueous vent of m depth. Eruption dynamics The MHFF internal facies arrangement (Fig. 7D) resembles that deposited from a deep, volatile-rich, high-intensity neptunian style eruption (Allen & McPhie 2009). Neptunian facies include a confined proximal basal coarse lithic lapilli-tuff, a middle widespread thick pumice density current deposit and an upper suspension deposit (Fig. 7B; e.g. Mount Read Volcanics, McPhie & Allen 2003; Allen & McPhie 2009). The MHFF can be similarly divided into multiple density graded beds suggesting fluctuations in clast supply from an unsteady column. While the MHFF internal facies arrangement resembles a deep high-intensity eruption, characteristics of the dominant middle unit support a phreatomagmatic origin. High fine ash (tuff) content is more typical of the highly efficient fragmentation of phreatomagmatic eruptions due to unconfined magma water interactions typically in relatively shallow water (<30 m; Allen & McPhie 2009). The high abundance of fines preserved in the middle unit reflects some degree of supressed turbulence within the density currents reflecting high-density cohesive flows (e.g. Lowe 1982). Explosive phreatomagmatic eruptions produce eruption columns that breach the water surface and generate a subaerial plume (e.g. Houghton et al. 2000; Wilson 2001; Gravley 2004). Resulting subaerial pyroclastic density current and fall deposits are commonly widespread and dominated by juvenile ash shards, low or variably vesicular pumice, accretionary lapilli and subordinate lithics (Houghton et al. 2000). Unsteady water magma interactions, more common in lower-intensity eruptions, may be pulsatory generating discrete episodic deposit packages (Fig. 7C; e.g. Ohakuri Phase 1 & 2, Gravley 2004) similar to the bedded middle unit. Other characteristics of the middle unit support a deeper water neptunian origin, however ( 200 m; Allen & McPhie 2009). The MHFF lacks accretionary lapilli and a known subaerial component, suggesting the eruption was confined to the relatively deep subaqueous setting. Vesicular pumice chips consist of uniformly small, cylindrical vesicles. Elongate vesicle morphologies suggest differential shearing and stretching of the vesiculating, crystal-poor, viscous MHFF melt during coupling and ascent prior to significant exsolution and quenching (Cashman et al. 2000). Textural characterisitcs of the pumice cuttings may be consistent with both origins, but the vesicularity appears most consistent with a volitile-rich, highly explosive neptunian origins (Cashman et al. 2000; Allen & McPhie 2009). The beds of rounded pumice lapilli clasts in the proximal MHFF suspension deposits may imply that the convecting eruption column was able to buoy the lake surface. Buoyant clasts may have remained sufficiently hot during transport, cooled in the air and became abraded and sufficiently waterlogged to sink after the eruption. Suspension pumice deposits occur in neptunian eruptions, but clasts are much larger (metresized giant pumice; Allen & McPhie 2009). We speculate that the lake depth was insufficiently deep to allow the eruption column to effectively sort the ejecta and that collapse involved a poorly sorted mix of both pumice- and ash-generating relatively cohesive density currents. The lack of a preserved subaerial deposit suggests that any subaerial plume generated as a result of the eruption was weak and quickly dissipated. The explosive, hydromagmatic nature of the MHFF eruption deposit has characteristic features commonly associated with deposits from shallow phreatomagmatic and deep neptunian eruptions. Given that the major vesicle and juvenile jointing textures useful for fragmentation analysis are not preserved, the overall high fines material comprising the MHFF (and perhaps the pumice vesicularity) indicates that the water depth (inferred m) and accompanying hydrostatic confining pressure was insufficient to suppress fragmentation (and possibly vesiculation; Allen & McPhie 2009). To account for the MHFF deposit characteristics, the subaqueous eruption is inferred to have had phreatomagmaticstyle magmatic and fragmentation components (high magmatic and quench fragmentation, moderately high vesicularity and limited confining pressure) that erupted and developed under neptunian-like conditions (relatively deep, subaqueous confined, flow dynamics). The MHFF deposit reinforces the diversity of hydrovolcanic eruption styles resulting from complex interactions between variable magmatic, ambient vent and water conditions (e.g. Walker & Croasdale 1971; Walker 1973; Self & Sparks 1978; Kokelaar 1983, 1986; Allen & McPhie 2009; Cas & Van Otterloo 2011). Eruption evolution The high matrix content, lapilli clast size and distribution of the medium lithic lapilli-tuff unit indicates that the MHFF eruption commenced at a low to moderate intensity (cf. high-intensity neptunian). Cohesive flows depositing the coarse lithic lapillituff unit were capable of laterally transporting material from the source vent (near THM12) at least 2 km (all THM wells). Initial flows eroded the water-saturated, coherent, muddy lacustrine sediments lining Lake Huka, entraining mudstone clasts. Diluted flows and turbulent suspension transported and resedimented fine ash at least 7 km from the source (Fig. 2A; WKM14).

12 Eruptive origins of a lacustrine pyroclastic succession 341 In the coarse lithic lapilli-tuff unit, clast coarseness and angularity increase with proximity to the source vent, while sorting decreases. Increased eruption intensity caused a ventclearing, widening and stabilising phase. The resulting deposit is best observed in THM12 where accidental wall-rock clast accumulations occur proximal to the inferred explosive vent locality. The establishment of the vent led to the production of abundant pumice and ash. The deposit architecture suggests the eruption progressed by the collapse of a water-saturated column with similar dynamics to the subaqueous eruption models proposed by Kano et al. (1996) and Allen & McPhie (2009). Density graded bedding in some localities suggests successive pyroclastic emplacement from up to eight consecutive density currents fed by the unsteady collapsing column. Beds underlain by graded lithic lapilli suggest that the preceding flows did not significantly erode previously deposited beds and that their density progressively reduced. On eruption, moderate to highly vesicular pumice clasts were rapidly quenched, waterlogged and incorporated into and transported by water-supported density stratified currents where minimal clast abrasion occurred due to the buffering support by the interstitial water (Kano et al. 1996; Allen & McPhie 2009). Pumice clasts occur up to 6 km from the inferred source, but are most abundant in vent-proximal locations in Tauhara Geothermal Field (c. 2 km radius) where the host unit is thickest. Following turbulent transport within the column and density currents, the fine ash that was suspended in the water column settled through the water column along with any buoyant pumice, forming the upper 5 10 m of the laminated and deformed beds of fine tuff deposited throughout the lake. Conclusions Lithofacies analysis of core samples provides insight for inferring the vent setting, eruption style and transport and depositional processes for the MHFF. Deposited within the ancient Lake Huka, the MHFF records a pyroclastic succession which we interpret to have been directly fed by a subaqueous explosive eruption from a vent inferred at c m depth, the first of its kind recognised in the TVZ. Magmatic fragmentation conditions are inferred, although a phreatomagmatic component cannot be eliminated. The explosive eruption generated abundant ash, lithic and highly vesicular pumice lapilli that were transported within an almost exclusively subaqueous eruption column. The explosive eruption deposited a proximal, vent clearing, coarse lithic lapilli-tuff near central Tauhara Geothermal Field (THM12) followed by a collapsing, turbulent eruption column that fed water-supported gravity currents to progressively deposit pumice lapilli-tuff. Suspended fine grained ash and pumice clasts settled from the water column at the end of the turbulent eruption. The MHFF appears to have a relatively high matrix content (70 90 vol.%) compared to those inferred to have been deposited from highintensity deep-water subaqueous (neptunian) eruptions. Results highlight the potential complexity encountered when discriminating the provenance of subaqueous volcaniclastic deposits. Evidence stemming from the primary eruption and transport processes provides new insight into assessing interactions between lakes and volcanism in the TVZ. Acknowledgements The principal author wishes to acknowledge logistic support from the University of Canterbury (UC) Mason Trust Fund and UC-GNS Science Summer Scholarships. This research would not have been possible without the generous access to core, unpublished reports and support provided by Contact Energy Ltd. Dr Fabian Sepulveda and Sophie Milloy are thanked for useful discussions. 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