reconstructing eruption processes of a miocene monogenetic volcanic field from vent remnants:...

Reconstructing eruption processes ofa Miocene monogenetic volcanic ¢eld from vent remnants:Waipiata Volcanic Field, South Island, New Zealand

Ka¤roly Ne¤meth a;b;�, James D.L. White a

a University of Otago, Geology Department, P.O. Box 56, Dunedin, New Zealandb Geological Institute of Hungary (Magyar AŁ llami Fo«ldtani Inte¤zet), Stefa¤nia u¤t 14, Budapest, H-1143, Hungary

Received 10 June 2002; accepted 20 January 2003

Abstract

The Miocene Waipiata Volcanic Field, New Zealand, is an eroded phreatomagmatic intracontinental volcanicfield formed during a period of weak lithospheric extension. The field includes remnants of at least 55 volcanoes in anarea of V5000 km2. Vent-filling deposits comprising predominantly lava (e.g. plugs, necks, lava flows, or dykes),often associated with thin basal phreatomagmatic pyroclastic deposits, were classified as type 1 vents and are inferredto be the remnants of scoria cones. Vents represented by predominantly pyroclastic infill are classified as type 2 ventsand are inferred to have been the substructures of phreatomagmatic tuff ring and/or maar volcanoes. Type 3 ventcomplexes are groups of closely spaced or overlapping vents, with voluminous preserved lava flows; they are inferredto be the remnants of volcanoes comprising adjoining to coalescing maars and tuff rings with magmatic explosive andeffusive products. Pyroclastic rocks of most of the Waipiata vents record initial phreatomagmatic explosive activityfuelled by groundwater, followed by strombolian-style eruptions. Aligned and clustered vents are accommodated tostructural features of the regional basement rock (Otago Schist).9 2003 Elsevier Science B.V. All rights reserved.

Keywords: phreatomagmatic; scoria; tu¡ ring; basanite; intracontinental; erosion

1. Introduction

Young monogenetic volcanic ¢elds are commonworldwide, and their products are well character-ised (Basaltic Volcanism Study Project, 1981; Ta-naka et al., 1986; Hasenaka, 1994; Connor andConway, 2000). Less is known from geological

remnants of old ¢elds, although the deposits with-in vents and vent complexes are rich in informa-tion concerning the eruption styles of ancient¢elds (e.g. Schmincke, 1977; Lorenz, 1979, 1984,1986; Lorenz and Bu«chel, 1980; White, 1990,1991a,b; Valentine and Groves, 1996). In this pa-per we present results from a study of the rem-nants of a Miocene volcanic ¢eld in New Zealand,which is located along the active plate boundarybetween the Paci¢c and Australian plates. NewZealand’s North Island is a well-known site ofconvergent margin volcanism (e.g. Wilson, 1996).

0377-0273 / 03 / $ ^ see front matter 9 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S0377-0273(03)00042-8

* Corresponding author.E-mail addresses: [email protected]

(K. Ne¤meth), [email protected] (J.D.L. White).

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Journal of Volcanology and Geothermal Research 124 (2003) 1^21

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However, widespread Cainozoic volcanism hasalso taken place on the South Island (Fig. 1;Johnson, 1989), in a region adjacent to a trans-form segment of the plate boundary; magmas ofthe ¢eld are of intraplate origin. The sites of mostvoluminous South Island magmatism in the Cai-nozoic are two large Miocene shield volcanoes:Banks Peninsula and the Dunedin Volcano (Fig.1). The earliest known South Island intraplatevolcanism occurred o¡shore during the Palaeo-cene. A second period of volcanism commencedin the Late Eocene and ended by Early Oligo-cene, producing submarine to emergent volcanicmounds and tu¡ cones (Coombs et al., 1986; Caset al., 1989; Martin and White, 2000; Martin,

2002). In a third period of volcanism, the locusof activity was farther south, and the DunedinVolcanic Group was formed. It includes the Al-pine Dyke Swarm, the Dunedin Volcanic Com-plex (DVC) and the Waipiata Volcanic Field(WVF). The DVC is located at the coast andnearby inland a few tens of kilometres from theshoreline, adjoining a half-circular area contain-ing the WVF volcanic rocks. The WVF is a sev-eral hundred metres thick accumulation of alkalicvolcanic rocks erupted subaerially in the middleMiocene over a period of at least 3 million years(Benson, 1969; Coombs et al., 1986; Reay et al.,1991). Older rocks of the southern part of theDVC on the Otago Peninsula formed as explosive

Fig. 1. (A) Overview map of the Waipiata Volcanic Field; BR=Black Rock; TC= ‘The Crater’; LB= ‘Little Brothers’ ; LP=Lit-tle Puketapu; PHVC=Pigroot Hill Volcanic Complex; FHVC=Flat Hill Volcanic Complex; SVC=Swinburn Volcanic Complex.(B) Inset New Zealand’s South Island map: WVF=Waipiata Volcanic Field; DVC=Dunedin Volcanic Complex; ADS=AlpineDyke Swarm; BP=Banks Peninsula.

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and e¡usive products of a number of small sub-marine, Surtseyan volcanoes (Martin and White,2001; Martin, 2002). In contrast to the DVC, theWVF consists of the remains of numerous non-contiguous individual monogenetic volcanoes, in-ferred to have erupted subaerially (Coombs et al.,1986). The earliest eruptions of the WVF tookplace at V21 Ma during a period of mild litho-spheric extension related to the opening of theTasman Sea and the separation of New Zealandfrom Gondwana (Adams, 1981). All volcanic ac-tivity in the Otago area ceased at about 10 Ma,perhaps as a result of a change to compressionaltectonics (Koons et al., 1999; King, 2000). Mono-genetic volcanic ¢elds such as the WVF comprisenumerous individual vents each of which charac-teristically forms during a single eruption lastingfrom hours to years (Condit and Connor, 1996;Conway et al., 1998; Connor and Conway, 2000).Such volcanic ¢elds as a whole, however, are typ-ically active for millions of years. Fundamentalphysical characteristics of volcanic ¢elds that arethe focus of ongoing research include (1) the num-ber, type and eruption history of individual vents;(2) the timing and recurrence rates of the volcaniceruptions in a given volcanic ¢eld; (3) the spatialand temporal distribution of vents and volcaniccomplexes ; and (4) the relationship of volcanic¢elds and the volcanoes within them to tectonicfeatures such as basins, faults, and rift zones.Analysis of the WVF may thus provide informa-tion on Miocene magma generation and the con-trols on its ascent beneath southern New Zealand(Johnson, 1989).

2. Geological setting

The basement rock of the WVF is the OtagoSchist consisting of early Mesozoic greywackesthat were metamorphosed in the Cretaceous dur-ing time- and space-pervasive deformation intogreenschist facies (Fig. 1; LeMasurier and Landis,1996). A regional erosion surface of low relief cutinto the basement rocks is diachronous and inpart overlain by £uvial quartz pebble conglomer-ates that young from Late Cretaceous near thePaci¢c coast to Eocene inland (Bishop, 1994).

Marine transgression from the east began at c.70 Ma in the Cretaceous (Molnar et al., 1975;Carter, 1988) and resulted in accumulation of ma-rine strata (glauconitic sandstone and limestoneunits) over the quartz pebble conglomerates (Hog-burn Formation) or directly on the schist (Fig. 1).At peak transgression, in the Oligocene, most ofthe New Zealand region may have been sub-merged (Carter, 1988; LeMasurier and Landis,1996). Uplift in response to development of theAustralian/Paci¢c plate boundary through NewZealand (Carter and Norris, 1976; Cooper etal., 1987) initiated regional regression and re-emergence of the Otago area beginning in theearly Miocene. Distributed deformation sincethen has produced a series of NE-trending andNW-trending folds and reverse faults in Otago(Norris and Carter, 1982). NW-trending grey-wacke and semischist ranges on the northeastside of the schist belt started rising during thelate Miocene. Pre-marine and marine sedimentsin northwest and central Otago were recycledinto the Dunstan Formation during the earlystages of uplift (Youngson et al., 1998). Disrup-tion of early Miocene drainage systems and rela-tive subsidence resulted in deposition of severalhundred metres of lacustrine silts and muds (theBannockburn Formation) in an extensive Mio-cene lake complex in central Otago. In the lateMiocene, the Dunstan and Bannockburn Forma-tions and, locally, older sedimentary rocks, wererecycled into the Wedderburn Formation duringdeformation along a broad NW-trending zone offaults (Youngson et al., 1998). As ranges contin-ued to rise, the Wedderburn Formation graduallygave way to deposits rich in newly eroded semi-schist and greywacke basement detritus (the Ma-niototo Conglomerate) (Youngson et al., 1998).

3. Vent characteristics

At least 55 individual vent sites have been iden-ti¢ed in a V5000-km2 area in Otago (Fig. 1).Vents form alignments mimicking the structuralpattern (Norris and Carter, 1982) of the basementOtago Schist. The longest alignments reach 30 kmin length and parallel a major fault zone (Bishop,

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K. Ne¤meth, J.D.L. White / Journal of Volcanology and Geothermal Research 124 (2003) 1^21 3

1974; Bishop and Laird, 1976; Norris et al., 1990;Youngson et al., 1998) of the region (WaihemoFault) (Fig. 1). Vent clustering is apparent (Fig.1), probably re£ecting the intersection of faultscutting the subjacent basement rocks. Perhaps,vent clustering may also be related to the length

and depth of ¢ssures functioning as pathways ofuprising melt (Connor, 1990; Connor et al.,1992). Pyroclastic rocks of the vent sites lie aboveor adjacent to di¡erent pre-volcanic rock units innon-conformable to conformable or angularly un-conformable relationships (Figs. 2 and 3). The

Fig. 2. Simpli¢ed geological maps of small-volume volcanic erosional remnants of type 1 (a^c) and type 2 (d and e) vents. Abbre-viations: s =Otago Schist; hg=Hogburn Formation; m=Oligocene marine sediments; gl =Green Valley Limestone Formation;d=Dunstan Formation; L1^L5=volcanic pyroclastic lithofacies associations. Note in b how the post-volcanic tilting may a¡ectthe calculation of erosion and missing sedimentary units (see text) in relationship to type 1 vents. In c a simpli¢ed landscape evo-lution model of a type 1 vent is given, illustrating representative land surface pro¢les through time. ‘1’ represents an erosionalsurface on the pre-marine units; ‘2’ represents an erosional surface on the marine deposits where in valleys Dunstan £uvio-lacus-trine sedimentation occurred; ‘3’ represent the syn-volcanic palaeosurface. In d a map view of a type 2 vent is shown. In e thesame type 2 vent cross-sectional view is given.

Fig. 3. Simpli¢ed geological map and cross-sections showing the internal architecture of the Pigroot Hill Volcanic Complex (type3 vent complex). Abbreviations: s =Otago Schist; hg=Hogburn Formation; m=Oligocene siliciclastic deposits ; gl =Green ValleyLimestone; d=Dunstan Formation; L1^L3=pyroclastic lithofacies associations; l = lava £ow. Thin black arrows (in cross-sec-tion) indicate lava £ow directions.

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many outcropping vent remnants have beengrouped into di¡erent types de¢ned on the basisof their preserved lava to pyroclastic rock ratios,their volume, and the geometry of the deposits.Three representative localities are presented insimpli¢ed geological maps and cross-sections,and their features represent those of similar ventremnants on a ¢eld-wide basis (Figs. 2 and 3).In some northern outcrops of the WVF there

are a few preserved sur¢cial pyroclastic depositsrepresenting the tu¡ ring rim beds of the vents

(proximal extra-crater deposits). Pyroclastic rocksof these outcrops appear to represent two basictypes: (1) basal units, which are characterised ei-ther by abundant lithic clasts derived from coun-try rocks and/or by abundant chilled juvenile py-roclasts, and (2) capping pyroclastic units that areenriched in red or brown scoria and/or carryabundant rugged, spindle-shaped lava fragments.There is a wide range in vesicularity among theglassy pyroclasts in the basal pyroclastic rocks,consistent with phreatomagmatic disruption of a

Table 1Summary of identi¢ed pyroclastic lithofacies from the WVF on the basis of grain size (tu¡, lapilli tu¡ and tu¡ breccia), sedimen-tary features (bedding characteristics listed in the column) and dominance of juvenile fragments (a= scoriaceous, b= siderome-lane)

Lithofacies have been grouped into ¢ve lithofacies associations on the basis of their common interrelationships, occurrences, bed-ding and compositional features.

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vesiculating magma (Lorenz, 1973; Houghtonand Wilson, 1989). The capping pyroclastic unitsare inferred to result from fragmentation by ex-panding juvenile gases, but also contain chilledjuvenile fragments suggestive of local interactionwith water in the vents. Deposit terminology andgrain size classi¢cation follow Schmid (1981), withterms for mixed deposits (e.g. tu¡, lapilli tu¡, tu¡breccia) based on the criteria of Fisher (1966).Here the pyroclastic terms follow the broad usagegiven by Fisher and Schmincke (1984), in whichtu¡, lapilli tu¡, and tu¡ breccia comprise all frag-ments ‘‘generated by disruption during volcaniceruptions’’ (Fisher and Schmincke, 1984). Individ-ual pyroclastic beds are classi¢ed on the basis oftheir grain size (e.g. tu¡ breccia, lapilli tu¡), com-position (e.g. juvenile- or accidental lithic-rich),texture (e.g. matrix- or clast-supported) and bed-ding (e.g. dune-bedded, di¡usely strati¢ed) char-acteristics (Table 1), and have been grouped intodi¡erent facies. Those facies commonly occurringtogether, and often characteristic of certain strati-graphic positions, are in turn grouped into litho-facies associations. Interrelationships among in-dividual facies and their typical architecture inoutcrop are summarised in graphic logs from se-lected sites (Figs. 4^6).

3.1. L1 lithofacies association: accidentallithic-rich, structureless tu¡ breccia and lapilli tu¡

3.1.1. DescriptionThese pyroclastic rocks are identi¢ed from the

lower to medial position in most of the vent rem-nants (Figs. 2 and 3). They are typically con¢nedto semicircular areas in map view, and have steep-ly dipping contacts against pre-volcanic rocks.The L1 association is characterised by well-ce-mented, massive, matrix-to-clast-supported lithic-rich tu¡ breccia and lapilli tu¡ (Fig. 7a; Table 1),which are commonly intercalated with clast-sup-ported beds of angular to sub-rounded accidentallithic blocks and lapilli of heterogeneous compo-sition. The association also includes medium- tothick-bedded, unsorted beds of structureless lapillitu¡. A few bodies of matrix-supported lapilli tu¡are preserved between columns of columnarjointed dykes, and contain large (6 50 cm) irreg-

ular clasts of mudstone. Juvenile clasts of the as-sociation are dominantly (s 70% of total shards)light brown to yellow, variably palagonitised,angular, microvesicular (5^20%), and slightly mi-crocrystalline sideromelane shards (6 2^3 cm).Vesicles appear round or slightly elongate inthin section. Fresh sideromelane shards are pre-served in places (Pigroot Hill west), but in others,e.g. on the northern £ank of Pigroot Hill and thenorthern margin of the Swinburn area (along theWaihemo fault), are highly altered, with shardcolours of light yellow to deep red, the latter hav-ing a tachylite-like appearance. Microcrystallineglass shards often have trachytic texture, and con-tain idiomorphic pyroxene and olivine crystals orcalcite pseudomorphs. Small cauli£ower bombs(6 15 cm) are present in a matrix of ¢ne-grainedash comprising palagonitised shards and acciden-tal lithic grains. Large, angular accidental lithicclasts of schist are present in tu¡ breccias (Fig.7a). Well-rounded quartz pebbles are sparselypresent. Microcrystalline lava fragments (6 2cm) are present in most localities. At vent sitessuch as Swinburn and Pigroot Hill, pyroclasticrocks of this association contain many roundedto sub-rounded blocks to boulders of poikiliticlava. Numerous glaucony grains dispersed in thetu¡ matrices often give a smooth, greenish-yellow-ish tan to the lapilli tu¡. Rare pyroxene mega-crysts and peridotite lherzolite xenoliths also oc-cur.

3.1.2. InterpretationThe presence of dispersed ¢ne-grained glass py-

roclasts in the matrix and the general angularityof most clasts are consistent with a primary pyro-clastic origin. The predominance of sideromelaneor its palagonitised remains, together with theabundance of country rock fragments and thevariation in particle vesicularity, suggests thathydromagmatic processes played an importantrole in formation of these deposits (Fisher andSchmincke, 1984). The poorly sorted and weaklystrati¢ed, thick-bedded nature of the deposit sug-gests deposition by rapid fallout from phreato-magmatic eruption clouds, possibly from high-concentration base surges (Sohn and Chough,1989) into which large volumes of tephra were

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Fig. 4. Measured stratigraphic log demonstrating L4 and L3 lithofacies associations, part of a phreatomagmatic sequence subse-quently subsided into a phreatomagmatic vent at The Crater.

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Fig. 5. Measured stratigraphic log across an angular unconformity between L4 and L3 lithofacies associations from the westernescarpment (inner part of the diatreme) of the Pigroot Hill Volcanic Complex. Facies codes adjacent to column are as in text.

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Fig. 6. Measured stratigraphic log illustrating complex facies interrelationships characteristic of near-vent to medial crater rim se-quences of the western outcrops of Pigroot Hill Volcanic Complex. Facies codes adjacent to column are as in text.

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being added, either by column collapse or by bal-listic fallout, nearest the vent margin (Lowe,1988; White, 1991a,b; White and Schmincke,1999). However, in cases where sideromelane glassshards are strongly altered and overall the pri-mary origin is poorly constrained, a syn-eruptive,intra-crater debris £ow (e.g. lahar) deposition isalso plausible (Lorenz, 1986; Smith and Lowe,1991). The local crude strati¢cation can be relatedto shearing within a high-concentration suspen-sion or £ow unsteadiness that leads to pulsatorydeposition from alternating suspension and trac-tion (Chough and Sohn, 1990) and/or from a tem-poral variation in the fallout rate. Because of thedeposits’ localised occurrence and steeply dippingcontacts against pre-volcanic rocks a vent-¢llingorigin seems most likely. The presence of juvenileglass fragments of di¡erent sizes, shapes and ve-sicularities suggests fragmentation of inhomoge-neous magma and/or recycling of clasts as a resultof repeated explosions (Houghton and Smith,1993). The presence of di¡erent types of acciden-tal lithic clasts in these pyroclastic rocks suggestssub-surface explosions and/or unstable conduitconditions during eruptions (Lorenz, 1986;White, 1996; Ort et al., 1998; Doubik and Hill,1999). The large amount of quartzofeldspathicdebris suggests the presence of poorly con-solidated sands (e.g. in the marine strata) in thewalls of the erupting vent. The often clot-likedistribution of quartzofeldspathic fragmentsthrough the matrix may indicate that the matrixcomprises depositional aggregates formed by dis-ruption of ¢ne-grained, muddy pre-volcanic strata(White, 1990, 1991a). The large, irregularlyshaped mudstone blocks were similarly incom-pletely consolidated at the time of their depositionwithin this association. In summary, this litho-facies association represents the variably alteredvent-¢lling products of phreatomagmatic erup-tions.

3.2. L2 lithofacies association: juvenile-rich,non- to weakly bedded tu¡ breccia and lapilli tu¡

3.2.1. DescriptionThese rocks have been identi¢ed at lower to

intermediate stratigraphic levels of individual

vent remnants, and tend to form semicircularareas in plan view (Fig. 2). There are siderome-lane- and tachylite-rich versions of L2. Siderome-lane shards are fresh, angular or somewhat £uid-form clasts (Fig. 7b). Vesicles (10^30 vol%) aresub-spherical to elongate (sub-millimetre size);glass shards with microcrystalline zones are lessvesicular than other glass shards. Some grainshave whole or partial ¢ne ash coatings (armouredlapilli). Tachylite clasts are dark brown to red orblack (6 3^5 mm), but otherwise tend to be tex-turally similar to sideromelane ones. Accidentallithic clasts consist mostly of glaucony grains(Fig. 7b), sandstone, quartz grains or mica aggre-gates. Schist fragments and well-rounded schistpebbles are present in some samples.

3.2.2. InterpretationThe angular and £uid-form shapes of the glassy

clasts, poor sorting and abundance of glassy vol-canic grains suggest an inhomogeneous, hydro-magmatic fragmentation process (Lorenz, 1973;Houghton and Schmincke, 1986; Houghton etal., 1996). The areal (circular) and the inferredcross-sectional (funnel-¢lling) distribution, pres-ence of country rock fragments, and geometricalrelationships with other rocks (see Fig. 1, cross-sections) suggest explosions rooted within theschist basement, with preservation of the rocksin vent or crater settings. Because the dominanttype of volcanic glass is variably vesicular side-romelane, it is inferred that sudden chilling andfragmentation of magma upon contact withgroundwater in the schist was the major causeof fragmentation of the magma (Heiken, 1972,1974; Heiken and Wohletz, 1986). The relativelysmall amount of accidental lithic fragmentsin these pyroclastic rocks (6 30 vol%) suggestslimited fragmentation energy, at least relative torock strength at the fragmentation sites. The lackof apparent bedding and occurrence of thisassemblage in sub-cylindrical outcrops suggestsin¢lling of funnel-shaped vent or crater sites,with deposition by captured density currentsand/or fallback (e.g. White and Schmincke,1999). Coated clasts suggest some recycling of py-roclasts (Houghton and Smith, 1993) within thevent zone.

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a

g

0.5

mm

b

C d

Fig. 7. (a) L1 tu¡ breccia (The Crater) with large, angular, schist fragments. (b) Photomicrograph showing lapilli tu¡ of L2 (TheCrater). Note the angular sideromelane glass shards (dashed lines), and the glaucony grain (g). (c) Overview of a tu¡ breccia se-quence of L3 (Pigroot Hill Volcanic Complex). A large irregularly shaped sandstone block is indicated by the dashed outline. (d)Cored bombs from L3 beds high in the stratigraphy at Pigroot Hill Volcanic Complex. Cores are doleritic lava.

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3.3. L3 lithofacies association: scoriaceous non- toweakly bedded tu¡ breccia and lapilli tu¡

3.3.1. DescriptionAll L3 pyroclastic rocks consist predominantly

of tightly packed £uidally shaped black to redvolcanic bombs and lapilli, with a small propor-tion of ¢ne ash as matrix, forming capping tu¡units at each locality (Figs. 2^6). Bedding, wheredeveloped, is subtle but laterally continuous overat least tens of metres. Large irregularly shapedlava clasts tend to be £attened parallel to the pa-laeosurface. Interbedded coarse- and ¢ne-grainedbeds often give a layered appearance to the out-crops. These deposits are only crudely bedded,but have a planar fabric de¢ned by £attenedclasts. Tabular bodies of tu¡ breccia are interlay-ered with, or ¢ll incisions cut into, bedded lapillitu¡ layers; tu¡ breccia forms funnel-shaped in¢ll-ings in places (e.g. Pigroot Hill ; Figs. 3 and 5).The proportion of accidental lithic clasts in tu¡breccias is locally high (s 50 vol%). Large (s 10cm) lava fragments tend to have chilled crusts afew millimetres thick and irregularly vesicular in-teriors. Small (6 10 cm) rugged lava fragmentsare very irregular in shape and slightly to ex-tremely vesicular. In the stratigraphically lowerlevels of deposits, the tu¡ breccias have few orno spindle-shaped bombs; these become moreabundant up-section. The tu¡ breccia has abrown matrix containing accidental lithic clasts,particularly glaucony grains and quartz aggre-gates. Sideromelane and tachylite shards are an-gular and microvesicular, with palagonite rims;microlites are commonly present. Clinopyroxeneand olivine occur as broken, angular, sub-milli-metre size crystals. Tu¡ breccia often containslarge chunks of sandstone or lapilli tu¡, tu¡ frag-ments of irregular shape (6 3 m) (Fig. 7c). Mostsandstone fragments have margins with fusedgrains, suggesting thermal alteration. In places li-monitic, highly irregularly shaped sandstone blobsare present that have cores of coarse white sandthat closely resembles some marine sandstones ofthe pre-volcanic sequence. Tu¡ breccia also con-tains large numbers of irregularly shaped clasts(6 50 cm) of bedded lapilli tu¡ or tu¡ that closely

resembles bedded tu¡ and lapilli tu¡ that underliethe tu¡ breccia units (e.g. at Pigroot Hill, FlatHill). In the upper parts of L3 tu¡ breccia unitsof this lithofacies association, ellipsoid coredbombs are common (Fig. 7d), with the coresmostly comprising thermally recrystallised quartzsandstone. The cores of the bombs are rimmed byglassy basalt, which tends to be more vesicularaway from the core and to have a rugged outersurface. In places of the L3 pyroclastic rocks,highly vesicular lava is intercalated as layers 6 5 mthick. These highly vesicular zones of the lavahave ellipsoid vesicles £attened parallel to lapillitu¡ bedding. Peridotite lherzolite xenoliths, pyro-xene megacrysts (6 1 cm), and angular crystal-line lava fragments (doleritic cognate lithics) arepresent, mainly in the lower beds of this associ-ation. The most abundant accidental lithic clastsare small (6 5 cm), angular, platy schist clasts.

3.3.2. InterpretationEvidence supporting a primary, spatter-fall ori-

gin of L3 includes the following. (1) Elongate,£attened to teardrop-shaped clasts indicate ballis-tic transportation and £uid deformation upon im-pact (Head and Wilson, 1989), and larger lavablocks with rugged surfaces, £ow banding and/or small-scale £ow ridges indicate possible creepdeformation of still hot and plastic blocks afteremplacement (Wol¡ and Sumner, 2000). (2) Fineinterconnections between clasts would have bro-ken apart if there had been reworking by sub-sequent mass £ow movements. (3) Irregularlyshaped accidental lithic clasts show no signi¢cantabrasion. The vesicular lava £ows are interpretedto be of clastogenic origin (Wolfe et al., 1988;Wol¡ and Sumner, 2000). Such lavas form thicklayers associated in northern areas of the WVFwith vent remnants, which are interpreted as rem-nants of Hawaiian-type spatter cones. The tu¡breccias are eruption products of predominantlymagmatic eruptions, which at some times mayhave had simultaneous phreatomagmatic andmagmatic activity at di¡erent vents or parts ofsingle vents (Macdonald, 1962; Richter et al.,1970; Houghton and Schmincke, 1986; Houghtonet al., 1996; Vespermann and Schmincke, 2000).

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c

Fig. 8. (a) Overview of a phreatomagmatic bedded lapilli tu¡ and tu¡ sequence (L4) from the Pigroot Hill Volcanic Complexwestern escarpment. Black thick arrow points to a doleritic lava bomb with accompanying impact sag structure. (b) Overview ofa phreatomagmatic bedded lapilli tu¡ and tu¡ sequence (L4) from the Pigroot Hill Volcanic Complex exhibiting dune-bedded, ac-cretionary lapilli-rich tu¡s overlain by lapilli tu¡ with abundant impact sags (top middle of photo). (c) Overview of the top ofThe Crater type 2 vent. Note the dish-like structure formed by smoothly inward-dipping chilled juvenile-rich bedded lapilli tu¡and tu¡ (L5) beds.

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3.4. L4 lithofacies association: accidentallithic-rich, bedded lapilli tu¡s and tu¡s

3.4.1. DescriptionThese pyroclastic rocks occur in the upper

stratigraphic level in most of the vent remnantsof the northern WVF (Figs. 2^6). Within smallvent remnants they commonly form the bulk oflarge (10-m scale) blocks tilted (6 35‡) toward theapparent vent centres (Fig. 2). In contrast, wherethey are part of larger-volume volcanic remnantssuch as those of vent complexes in the northernWVF, they occur as sheet-like sequences inferredto have been deposited around an active vent on asyn-volcanic palaeosurface (e.g. tu¡ ring) and/ormantling crater interiors within the complexes(Fig. 3). The L4 association is characterised byscour ¢ll-bedded lapilli tu¡ and tu¡, lapilli tu¡with accretionary lapilli, di¡usely strati¢ed lapillitu¡, thinly bedded lapilli tu¡ and tu¡, cross-strati-¢ed, undulatory and dune-bedded lapilli tu¡, andtu¡ (Fig. 8a,b). Beds in the association have sharpand laterally persistent (on a 100-m scale) beddingplanes (Fig. 8a). Individual beds are unsorted,locally normally graded, and rich in very fresh,weakly vesicular, angular, microlite-poor sidero-melane glass shards. Scours ¢lled by scoriaceouslapilli or, less commonly, by accidental lithicclasts are cut into ¢ne to coarse tu¡; in¢lledbomb sags generally occur between small topo-graphic barriers such as dunes. Deformed, rim-type (Schumacher and Schmincke, 1995) accre-tionary lapilli (6 1 cm) are common, and oftenconcentrated near the tops of beds. In zones withfew accretionary lapilli, clot-like features of sandash aggregates, or larger bodies of coarse ash or¢ne lapilli particles, are present in random orien-tations. The cores of most of the accretionarylapilli comprise non-volcanic accidental lithicgrains. Accretionary lapilli-rich beds tend to occuroverlying dune-bedded, cross-bedded tu¡ and lap-illi tu¡ beds. The pyroclastic rocks of L4 are richin accidental lithic clasts or mineral phases de-rived from country rocks, mostly broken quartzgrain aggregates, schist and quartz pebbles, andbeds in these rocks have a yellowish grey colourdue to the presence of ¢nely dispersed siderome-lane and rounded glaucony grains. Oval-shaped

cavities (6 1 mm) are common in the tu¡ matrixof the lapilli tu¡ beds (vesiculated tu¡ after Lo-renz, 1974). Dunes in dune-bedded tu¡ and lapillitu¡ have wavelengths of 50 cm to 1 m, and am-plitudes of 10^40 cm; these are in beds interca-lated with thinly and/or undulatory bedded lapillitu¡ and tu¡ (Fig. 8b). The dune shapes tend to besymmetrical to moderately asymmetrical, similarto the Type III and Type IV dunes of Schminckeet al. (1973). Bedding sags are non-systematicallydistributed in relation to dune-bedded lapilli tu¡and tu¡ (Fig. 8a,b). Di¡usely strati¢ed lapilli tu¡forms the bulk of pyroclastic rocks in intermedi-ate structural and stratigraphic positions, justbelow rocks such as L3 scoriaceous tu¡ brecciaand lapilli tu¡. Throughout L4 there are large(s 25 cm), sub-spherical bombs cored with vari-ous pre-volcanic fragments, and there are numer-ous clasts of peridotite lherzolite (6 5 cm), pyrox-ene megacrysts (6 5 cm), crystalline doleritic lava(6 25 cm), and fossilised remnants of plants.Platy schist fragments form laterally persistentclast trains, giving a crude strati¢cation to thebeds. Poorly sorted lapilli tu¡ that is crudelycross-strati¢ed (moderately low-angle: 6 10‡) ismore common from the higher parts of the asso-ciation within vent complexes in the northern partof the ¢eld.

3.4.2. InterpretationThe abundance of angular sideromelane glass

shards along with clasts of shattered basementrock, the unsorted, weakly bedded character ofrocks of this association and their positions withinapparent in¢llings of vent complexes suggestphreatomagmatic fragmentation and a primarypyroclastic depositional origin. The thinly beddedlapilli tu¡s and tu¡s were deposited by turbulentand low-concentration pyroclastic density cur-rents (Chough and Sohn, 1990). Small-scale un-dulations of bed thickness are typical for low- totransitional-regime bed forms produced by rela-tively low-concentration density currents withpossible high shear stress (Lowe, 1979). Scouringis thought to result from turbulence at the surgehead (Sohn and Chough, 1989). The thicker beds(decimetre scale) with weakly developed strati¢ca-tion, in association with the thin beds described

VOLGEO 2591 18-4-03


above, are probably products of traction deposi-tion from high-concentration (pyroclastic) densitycurrents (Chough and Sohn, 1990). Many of thethin beds (centimetre scale) are rich in accretion-ary lapilli, indicating moisture during transporta-tion that probably resulted from condensation ofsteam. Rim-type accretionary lapilli, comprisingthin coatings on substantial cores, characterisepyroclastic beds throughout the WVF, suggestingnear-source origins (Schumacher and Schmincke,1995). In summary, pyroclastic units of L4 areinferred to be products of variably concentratedand damp pyroclastic density currents (Vbasesurges). The resultant deposits formed parts oftu¡ rings, which were locally broken apart andpartly preserved as subsided blocks within thephreatomagmatic vent complexes.

3.5. L5 lithofacies association: chilled juvenile-richbedded lapilli tu¡ and tu¡

3.5.1. DescriptionThis association is characteristic of pyroclastic

deposits high in the central parts of vent remnants(Figs. 2 and 8c); it consists of thinly to thicklybedded, di¡usely strati¢ed, lapilli tu¡ with com-mon scour ¢ll structures. Characteristically rocksof L5 are unsorted, consist of angular, slightlyvesicular sideromelane glass shards, and formnon-graded beds ranging from a few cm to 15cm thick in which large clasts commonly de¢nea di¡use layering. Bedding contacts are sharpand often slightly undulatory. In this associationthese thicker beds are commonly intercalated withthin- to very thin-bedded or laminated ¢ne lapillitu¡ and medium to ¢ne tu¡. Minor accidentallithic clasts comprise round grains of glaucony,angular quartz aggregates, and platy schist frag-ments that are commonly imbricated with respectto bedding. Large pyroxene xenocrysts and smallperidotite lherzolite nodules (centimetre scale) arecommon. Scour surfaces of centimetre scale areubiquitous.

3.5.2. InterpretationThe presence of angular, relatively fresh side-

romelane glass shards and the non-graded, non-or weakly bedded, unsorted textural characteristic

of this facies association are consistent with pri-mary pyroclastic deposition of these facies. TheL5 association comprises composite beds re£ect-ing repetitive erosion and deposition by pyroclas-tic density currents (Chough and Sohn, 1990) thatwere probably turbulent and high- to low-concen-tration. These are inferred to have dispersed ma-terial excavated by shallow sub-surface explosionsthat created, or took place under, relatively open-vent conditions. The lack of accretionary lapilli,or of any soft sediment deformation features, sug-gests an absence of condensed water droplets inthe transporting currents. It is inferred that thisis also a re£ection of deposition very near thevents, before eruptive water vapour had begunto condense during £ow (Wohletz and Heiken,1992). Very near-vent depositional sites for thisassociation are also supported by its common oc-currence together with lithofacies association L2in central positions within apparent vent remnants(Fig. 2).

4. Vent types

There are three types of volcano remnants inthe WVF, each with distinct lithofacies associa-tions, ratios of preserved pyroclastic versus lavaunits, and size and number of identi¢ed eruptivecentres (Fig. 1).Type 1 vents are remnants of individual mono-

genetic volcanoes that consisted predominantly offeeder dykes, lava lakes and/or lava £ows (Figs. 1and 2a). Type 1 vents either do not preserve py-roclastic units, or hold only minor amounts rela-tive to preserved dykes or lava (Fig. 2a). Type 1vents seem to be concentrated on elevated parts offold and/or fault blocks, mainly in the central partof the WVF. Two major types of type 1 ventscan be distinguished within the WVF: (1) ventremnants ¢lled completely with lava, (2) vent rem-nants with minor pyroclastic deposits (L2 and L3)which are abundantly cross-cut by feeder dykes,covered by remnants of inferred lava lakes, and/ormark the point sources for elongate, kilometre-scale lava ¢elds. It is uncertain in the caseof type 1 vent remnants whether they represent(1) root zones or lava in¢llings of explosive vol-

VOLGEO 2591 18-4-03


canic vents, (2) non-explosive feeders of lava-¢lled ¢ssures or (3) remnants of wholly sub-terranean dykes or sills. Some of the coarse-grained, doleritic lava rocks of uncertain signi¢-cance are inferred to be remnants of valley-¢lling ponded lava £ows (Fig. 9). Given thatthin scoriaceous pyroclastic deposits are oftenaccompanied by voluminous lava £ows (Wood,1979; White, 1991b), it is not unlikely thatmany type 1 eruptive centres had an initiallymagmatic explosive eruptive history, and repre-sent the mostly eroded remnants of former scoriacones.Type 2 vents are also remnants of individual

monogenetic volcanoes and consist predominantlyof pyroclastic rocks, with some also appearing tohave been point sources for lava £ows (Fig. 2).Type 2 vents seem to be located in the same areaas type 1 vents, close to or on elevated fold and/orfault blocks. Type 2 vents are less common thantype 1 vents, which may indicate either (1) thattype 2 vents have been preferentially eroded en-tirely away because their pyroclastic in¢llings areless resistant than coherent dyke rock or lava, or(2) that predominantly pyroclastic volcanoes werenever as abundant as lava £ows in the WVF. Py-roclastic rocks (L1, L2, L3, L5 and large tiltedblocks of L4) are volumetrically dominant intype 2 vents, and interpreted to be vent-¢llingdeposits (Fig. 2b). Most of the pyroclastic rockscontain abundant sideromelane fragments withonly minor tachylite, and show a large range invesicularity ; these are typical results of phreato-magmatic fragmentation (Houghton and Wilson,1989; White, 1991a). In many type 2 vents there isa progressive increase up-section in the ratio ofchilled juvenile fragments to accidental lithic frag-ments, suggesting that volcanic conduit clearingled to more e⁄cient near-surface fragmentationproducing juvenile-rich pyroclastic deposits. Mosttype 2 vent remnants are topped with spatter-rich pyroclastic units (L3), consistent with ex-haustion of available water at explosion sites, andhence cessation or weakening of phreatomagmaticfragmentation, during the eruptions. An observedsudden increase in mantle-derived xenoliths in thetopmost phreatomagmatic units may mark theonset of relatively rapid magma rise in the late

Fig. 9. Illustration of how lava £ow remnants may mimicremnants of type 1 vents. (A) Type 1 vent (e.g. scoria conewith tu¡ ring base) erupting in a river valley. Extensive lava£ow following the valley morphology. (B) A cross-section ofthe volcano developed in the valley. Dashed line representsthe landscape after erosion. (C) River reoccupies the valley,eroding away the tu¡ ring. (D) The erosional remnant of thevolcano. (E) The entire region tilted V20‡ producing a sce-nario where distinguishing true type 1 vents from valley-con-¢ned lava £ow remnants can be a challenge.

VOLGEO 2591 18-4-03


stages of the eruptions. Although most accidentallithic clasts are derived from the basement OtagoSchist, clasts of glauconitic sandstone from Oligo-cene marine strata and well-rounded quartz peb-bles inferred to have been derived from the Hog-burn Formation are also present, indicating thatthe regional Cainozoic sedimentary cover waslargely intact at the time of the eruptions. A cor-ollary of this observation is that present erosionallevels where the volcanic rocks outcrop withinschist are several tens to more than 100 m belowthe syn-eruptive ground surface.Type 3 vent complexes are remnants of com-

plex volcanoes that consisted of closely spacedmaars, tu¡ rings and scoria cones accompaniedby extensive lava lakes and/or valley-¢lling lava£ows (Fig. 3). They tend to be located in thenorthern side of the WVF, close to the Cainozoicsediment-¢lled basins (Fig. 1). Type 3 vent com-plexes are the least eroded volcanic remnants inthe ¢eld and locally preserve the uppermost de-posits of individual vents (L4), and thus representthe best preserved volcanic systems. Individualvolcanic centres within these compound volcanoesoften erupted simultaneously to produce inter¢n-gering layers of pyroclastic rock and lava. Hence,a type 3 vent complex is a group of coalesced type2V type 1 vents (Fig. 3). Pyroclastic rocks lowwithin the stratigraphy of the type 3 complexesare very similar to deposits described for type 2vents, but show more variety; they include acci-dental lithic-rich undulatory, dune or channel ¢ll,or accretionary lapilli tu¡ and tu¡ beds (L4)(Figs. 5 and 6). Di¡erent types of scoriaceoustu¡ breccia, lapilli tu¡, and tu¡ are preserved inthick piles (6 50 m) as capping units (L3) of type3 vent complexes ; they are inferred to record late-stage magmatic (strombolian) explosive phases ac-companying or preceding e¡usion of lava (Figs. 3and 5). Clastogenic lava is commonly preserved indish-like structures often associated with smalllava lobes (L3). Spatter-rich beds are intercalatedwith thinly bedded sideromelane-rich pyroclasticbeds, and are interpreted to represent simulta-neous magmatic and phreatomagmatic eruptionsfrom closely spaced vents (cf. Kienle et al., 1980;Houghton and Schmincke, 1986; Houghton et al.,1999a,b).

5. Conclusion

The WVF consists of the remnants of scoriacones, tu¡ rings, maars, ¢ssure vents and lava£ows of various volumes. Three di¡erent typesof vents or vent complexes are distinguished basedprimarily on abundance of pyroclastic rocks rela-tive to lava £ows in the present outcrops. Vent-¢lling deposits comprising predominantly basal-toid rocks, preserved in the form of plugs, necks,lava lake remnants, lava £ows, or dykes, are clas-si¢ed as type 1 vents. There are at least 38 type 1vents, and they are mostly located in the centraland eastern part of the volcanic ¢eld. Type 1vents are inferred to be the remnants of scoriacones, most of them with thin basal phreatomag-matic pyroclastic deposits. Vents represented bypredominantly pyroclastic remnants are classi¢edas type 2 vents. Type 2 vents are inferred to be thesubstructures of phreatomagmatic tu¡ rings and/or maars, many of which may have had associ-ated scoria cones and volumetrically insigni¢cantlava. Type 2 vents are located in the central partof the volcanic ¢eld. Type 3 vent complexes aregroups of closely spaced or overlapping type 1and type 2 vents, with voluminous associatedlava £ows. Type 3 vent complexes are locatedalong the northwest margin of the volcanic ¢eldin association with preserved pre-volcanic sedi-mentary units, and are inferred to be the relativelylittle eroded remnants of maars and tu¡ ringswith associated magmatic pyroclastic and e¡usiveproducts, i.e. nested maars and nested tu¡ rings.Pyroclastic rocks of most of the Waipiata vents

record initial phreatomagmatic explosive activityfollowed by strombolian-style eruptions. Highpercentages of accidental lithic fragments togetherwith the relict blocky to slightly £uidal, and non-or weakly vesicular, sideromelane shards stronglysuggest sub-surface phreatomagmatic explosionsfuelled by groundwater. Phreatomagmatic pyro-clastic units were deposited predominantly bybase surges, with deposit features indicatingdampness during transport and deposition. Cap-ping magmatic pyroclastic units are spatter-richtu¡ breccias and lapilli tu¡s that commonly con-tain large fragments of the underlying tu¡ ringbeds, and of pre-volcanic sedimentary rocks

VOLGEO 2591 18-4-03


from the conduit walls indicating active vent dy-namics during their formation. Preserved lava£ows provide evidence for signi¢cant pondingand topographic control. Study of the accidentallithic clast population in pyroclastic rocks fromdeeply eroded volcanic pipes indicates that theCainozoic sedimentary cover was widespreadand still completely preserved at the time of vol-canism, although over much of the ¢eld no Cai-nozoic sedimentary rock units remain today. Oli-gocene marine sedimentary units clearly extendedacross the ¢eld, along with underlying quartzosesands and gravels of the Hogburn Formation.Distribution of vents in the WVF is structurallycontrolled, with vent alignments largely followingNE^SW and NW^SE trends that coincide withorientations of young faults in the region. Thelongest vent alignment, traceable for V30 km,coincides with the largest fault zone in the Otagoregion, the NW^SE-trending Waihemo fault zone.

Acknowledgements

This paper presents results of the ¢rst author’sPhD study of the Waipiata Volcanic Field, whichwas supported by a University of Otago Postgrad-uate Scholarship. Suggestions, encouragementand ¢eld assistance during various stages of theresearch from U. Martin and M.K. McClintockare gratefully acknowledged. Reviews by Prof. DrVolker Lorenz (University of Wu«rzburg, Ger-many) and Dr Scott Rowland (University ofHawai’i at Manoa) helped strengthen the man-uscript. Editorial work of Prof. Dr StephenWeaver (University of Canterbury, Christchurch,New Zealand) helped to clarify the presentationstyle of the paper, many thanks for it.

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VOLGEO 2591 18-4-03


reconstructing eruption processes of a miocene monogenetic volcanic field from vent remnants:...

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