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Correlation, dispersal, and preservation of theKawakawa Tephra and other late Quaternary tephralayers in the Southwest Pacific OceanLionel Carter a , Campbell S. Nelson b , Helen L. Neil b & Paul C. Froggatt ca NIWA‐Marine , New Zealand Oceanographic Institute , P.O. Box 14–901, Kilbirnie,Wellington, New Zealandb Department of Earth Sciences , University of Waikato , Private Bag 3105, Hamilton, NewZealandc Department of Geology , Victoria University of Wellington , P.O. Box 600, Wellington,New ZealandPublished online: 23 Mar 2010.

To cite this article: Lionel Carter , Campbell S. Nelson , Helen L. Neil & Paul C. Froggatt (1995) Correlation, dispersal, andpreservation of the Kawakawa Tephra and other late Quaternary tephra layers in the Southwest Pacific Ocean, New ZealandJournal of Geology and Geophysics, 38:1, 29-46, DOI: 10.1080/00288306.1995.9514637

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New Zealand Journal of Geology and Geophysics, 1995, Vol. 38: 29-460028-8306/95/3801-0029 $2.50/0 © The Royal Society of New Zealand 1995

29

Correlation, dispersal, and preservation of the Kawakawa Tephra and other lateQuaternary tephra layers in the Southwest Pacific Ocean

LIONEL CARTER

New Zealand Oceanographic InstituteNIWA-MarineP.O. Box 14-901Kilbirnie, Wellington, New Zealand

CAMPBELL S. NELSON

HELEN L. NEILDepartment of Earth SciencesUniversity of WaikatoPrivate Bag 3105Hamilton, New Zealand

PAUL C. FROGGATTDepartment of GeologyVictoria University of WellingtonP.O. Box 600Wellington, New Zealand

Abstract Voluminous rhyolitic eruptions and prevailingwesterly winds have dispersed late Quaternary ash from theTaupo Volcanic Zone (TVZ) of the North Island, NewZealand, across the Southwest Pacific Ocean. We identifythe Taupo (1850 14C years), Waimihia (3280 yr),Rerewhakaaitu (14 700 yr), and Kawakawa (22 590 yr)Tephra layers in deep ocean cores, mainly on the basis oftheir stratigraphic position, radiometric age, and glass shardchemistry.

Approximately 25 km3 of Taupo Tephra were dispersedENfE at least 650 km from the TVZ whereas c. 22 km3 ofWaimihia Tephra and c. 14 km3 of Rerewhakaaitu Tephratravelled over 500 km to the east. In contrast, at least 400km3 of Kawakawa Tephra occur out to 1400 km southeastof the TVZ. Such widespread dispersal is not only a functionof the size of the Kawakawa eruption, but is also influencedby the strong wind regime during the last glaciation asmanifest by high aeolian quartz contents of sedimentsencasing the tephra. More ash appears to have depositedoffshore than is predicted by exponential thinning models.Taupo Tephra, in particular, has a conspicuous secondthickness maximum, 660 km from the eruption centre.

Dispersal has extended over different depositionalsettings that have affected the tephra layers. The bestpreserved deposits are in zones of high sedimentationincluding channel levees, submarine fans, and boundarycurrent drifts. In contrast, preservation is poor in regions ofactive currents including the continental shelf, the crest ofChatham Rise, and the foot of Chatham Rise - Hikurangi

C94004Received 2 February 1994; accepted 21 September 1994

Plateau where a deep western boundary current is intensified.Primary tephra deposits are also at risk in regions of frequentgravitational mass movement such as offshore Hawke Bayand eastern Bay of Plenty. Further postdepositionalmodification is by bioturbation, especially where tephra arec. <1 cm thick; thicker deposits tend to survive, whichimplies a smothering of the benthic fauna.

Keywords marine tephra; late Quaternary; SouthwestPacific Ocean; deep-sea cores; correlation; Taupo;Kawakawa; dispersal; volumes

INTRODUCTION

Throughout the Quaternary, the Taupo Volcanic Zone (TVZ)of New Zealand has been the site of major explosive silicicvolcanism associated with the convergence and westwardsubduction of the Pacific plate beneath the North Island(Fig. 1) (Ballance 1976). Onshore, individual events haveemplaced extensive ignimbrite and airfall deposits withvolumes of many exceeding 100 km3 and reaching amaximum of at least 1000 km3 (e.g., Wilson et al. 1984). AsNew Zealand straddles the "Roaring Forties" wind belt,volcanic ash has been dispersed widely offshore.

Ninkovich (1968) was first to record five rhyolitic ashlayers in deep-sea cores as far as 1800 km northeast of NewZealand. These tephra accumulated 0.9-0.3 m.y. ago andwere tentatively correlated with ignimbrite eruptionsonshore. Watkins & Huang (1977) extended the ashstratigraphy to 4 Ma for cores sited out to 2000 km east ofthe TVZ, and Nelson et al. (1986) identified middle Mioceneto Pleistocene tephra in DSDP Leg 90 cores in the TasmanSea and Southwest Pacific Ocean. The largest Quaternaryeruption so far identified is the 320 ka Rangitawa/Whakamaru event, which is estimated to have involved atleast 500-1000 km3 of ignimbrite and +700 km3 of airfallash dispersed widely across the Pacific region (Froggatt etal. 1986; Nelson 1988).

The late Quaternary phase (last c. 50 000 yr) ofvolcanicity in the TVZ is marked onshore by at least 40formally named silicic tephra units (e.g., Froggatt & Lowe1990). The majority of late Quaternary eruptions wererelatively small volume or low explosion events whoseejecta either did not reach the sea or was readily dispersedby hydraulic and biologic processes. Only a few of thedeposits have been recorded offshore (Table 1): from thenear-source region of Bay of Plenty, north of the TVZ(Kohn & Glasby 1978; Pillans & Wright 1992); fromoffshore Hawke Bay, downpath of the prevailing westerlywind (Lewis & Kohn 1973; Stewart & Neall 1984); and onChatham Island, which lies 900 km southeast of the TVZ(Mildenhall 1976).

Since those studies, late Quaternary tephra layers havebeen identified in cores further offshore from the TVZ. This

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30 New Zealand Journal of Geology and Geophysics, 1995, Vol. 38

KermadecI? TrenchAUSTRALIAN

PLATE

HikurangiFan-drift

S. W. Pac/fiic Basin

Hikurangi TroughU950U95V

Chatham Rise

• U938• Q215

Bounty Trough H552• 0217

Campbell Plateau

Fig. 1 Location of cores together with major sediment depocentres and path of the deep western boundary current (DWBC) fromCarter & McCave (1994). TVZ, Taupo Volcanic Zone; TVC, Taupo Volcanic Centre; OVC, Okataina Volcanic Centre; HB, HawkeBay; BP, Bay of Plenty.

new information is presented here to provide a betterappreciation of volumes of erupted material, its dispersaland deposition, as well as identifying datum planes that willprovide correlation between Southwest Pacific basinalsediments and terrestrial sequences.

Depositional frameworkAsh has accumulated within highly variable submarinesettings. East of the South Island, the continental shelfyields to a canyon-dissected slope that descends to a vastplateau complex composed of the Chatham Rise andCampbell Plateau (Fig. 1). Depths on the complex typicallyrange over 500-1500 m, and the prevailing sediment type iscalcareous pelagic ooze (Mitchell et al. 1989). Separatingthe rise and plateau is the Bounty Trough with its 900 kmlong axial channel, which descends gradually eastwardsfrom the continental slope to the 4500 m deep abyssal floorof the Southwest Pacific Basin. There the channel feeds theBounty Fan with sediment derived mainly from the alpineregions of South Island (e.g., Carter & Mitchell 1987). Partof the fan is eroded by a deep western boundary current

(DWBC) that flows northeastwards at depths of 2500-5000 m (e.g., Carter et al. 1990).

North of Chatham Rise and east of the North Island, acomplex setting has formed at the boundary between thecolliding Pacific and Australian plates. Here the continentalmargin is part of the Hikurangi subduction systemcomprising an accretionary prism of anticlinal ridges andslope basins (shelf and slope), and the 2000-3000 m deepHikurangi Trough, with a thick fill of terrigenous sediment(Lewis 1980). The trough axis is occupied by HikurangiChannel, which meanders north to Hawke Bay beforeswinging east across Hikurangi Plateau to eventuallydebouch onto the abyssal Pacific floor, where a large fanhas developed (Carter & McCave 1994). The plateau itselfcovers a large triangle of ocean floor (40 000 km2) and hasa sediment cover of turbidites overspilt from HikurangiChannel, hemipelagites and sediment drifts, as well aswidespread layers of tephra.

The Kermadec Trench section of the subduction zonepasses westward into a back-arc basin complex of activeQuaternary volcanism that incorporates the TVZ and itsoffshore extension, the Havre Trough (Wright et al. 1990).

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Carter et al.—Kawakawa Tephra in the SW Pacific 31

Tablel Data for cores with tephra layers. References : 1, Pillans & Wright (1992); 2, this study; 3, Kohn & Glasby (1983); 4,Fenneret al. (1992); 5, Lewis & Kohn (1973); 6, Stewart & Neall (1984); 7, Mildenhall (1976); 8, Barnes et al. (1991). P = present.

Stn

SH04S'94S')36R646SS03H214H213H209H210S')32R539S')31Q85809390941S?83S785S775S778S78008590929S938F67108610860F690F681P69F595S « 9F592F591F593F594F59OS9241950Q29819510311RllH34719390219193802150200021202200581W20217W7WllW30585

Area

S Havre TroughS Havre TroughKermadec TrenchHikurangi DriftS Havre TroughBay of PlentyBay of PlentyBay of PlentyBay of PlentyHikurangi FanHikurangi LeveeRekohu DriftHikurangi PlateauHawke Bay SlopeHawke Bay SlopeHawke Bay SlopeHawke Bay SlopeHawke Bay SlopeHawke Bay SlopeHawke Bay SlopeHikurangi PlateauHawke Bay SlopeHikurangi PlateauS Hawke Bay SlopeHikurangi PlateauHikurangi PlateauS Hawke Bay SlopeS Hawke Bay SlopeS Hawke Bay SlopeS Hawke Bay SlopeN Chatham SlopeS Hawke Bay SlopeS Hawke Bay SlopeS Hawke Bay SlopeS Hawke Bay SlopeS Hawke Bay SlopeN Chatham SlopeN Chatham RiseCanterbury SlopeN Chatham SlopeCanterbury SlopeChatham IslandCanterbury SlopeS Chatham RiseS Chatham RiseS Chatham RiseBounty TroughBounty TroughBounty FanBounty TroughBounty FanBounty FanBounty TroughBounty FanBounty FanBounty FanSW Pacific Basin

Latitude (

35 51.1636 18.636 22.736 38.0936 41.836 55.537 02.537 09.537 13.337 15.0838 26.539 27.439 49.639 50.639 51.939 52.239 54.439 5739 57.539 57.939 57.939 58.640 01.9740 0240 14.540 16.340 1940 2340 23.840 3640 4740 5040 5340 5440 5640 5941 3542 40.742 42.642 42.942 44.844 04.144 31.944 29.745 00.945 04.545 23.245 59.746 0746 17.546 23.246 25.446 27.946 36.747 11.847 1249 42.2

°S) Longitude

177 17.12E176 48.7 E178 57 W177 30 W176 36.4 E177 26.5 E177 10.5 E177 44.4 E177 32.7 E176 11.9W175 44.4 W176 25.1 W178 03.5 E177 20.2 E177 25.2 E177 25.2 E177 30.9 E177 11.6 E177 12.8 E177 14 E178 30.3 W177 08.2 E179 59.7 E177 50 E179 24.1 W179 03.9 W177 26 E177 33 E177 59.8 E177 29 E171 32.9 W177 42 E177 53 E177 28 E177 14 E177 59 E171 30W176 54.8 E173 35.2 E176 54.8 E17401.6E176 31.3 W173 25 E179 30.1 E174 59.2 E179 29.9 E177 59.6 E172 01.5 E178 01.3 W174 58.5 E178 27 W177 48.3 W175 04 E178 03.4 W177 43.2 W177 14 W177 55.5 W

Depth (m)

2585240649905821168820452065167516585235463040973735

292413407548274433524

3660200

3003116930103216172619362195181442402432240021762063746935561000520850

1960_

585130011222700263213704390

5804163429619364475459146834354

Tp

_-2-60-510--16-198-195-20?7-15--182522311906354_120—10—-14-18?80-33--__—52-57---_-—---------------

Depth in

Wm

_-———-------11.5-13--15590260-132-4014.5-16—25—17-1821-2235-27

----208?1231___-_-—-—_----------_-

core (cm)

Rk

_-———205-206166-170-130-140---97__——---100_—137-140___-230-__———157-159—----—-—_----------—-

Kk

185185-18855-62/66

—149-153-243-248----70-75---_—----_284-287__——123-125550112-11446-51.5121-12350-53

—150—5827-33P58-60415810-82350076-8342127-13131-3755195-199P52-55307-31342176-18226-30160-16324

Ref.

11221333322248888888482544556525555

222227222222222222222

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32 New Zealand Journal of Geology and Geophysics, 1995, Vol. <

Fig. 2 Examples of core strati-graphies from Hikurangi Plateauand Bounty Trough for isotopestages 1 and 2, including mega-scopic tephra layers. Core Q858,modified from data of Fenner etal. (1992), also shows aeolianquartz accumulation rates, whichidentify variations in paleowindstrength. 818O is based on thebenthic foraminifera Uvigerina.

Core Q858 CaCO3%

Hikurangi PlateauQz Ace. Rate

g/cmVka 5180%o

Wm

Rk

— _

mod

0.5-

1.0-

1.5-

metres

Core W2

0

0.5-

1.0-

1.5-

2.0"

2.5-

3.0Kk

metres

1 0 2 0 3 0 4 0 1 2 3 4 5 3 4 . 5Age

yr B.P.

10,860

16,110

18,650

Bounty FanCaCO3% 8180%o Age10 20 30 40 2 3 4 yr B.P.

11,400

15,660

fljJI

HiWm

Rk

Kk

| Calc. biopelagite1 Hemipelagite3 Mud turbidite

1 Sand/silt turbidite

Tephra

Waimihia

Rerewhakaaitu

Kawakawa

MethodsThe study consists of the identification and correlation ofmacroscopic tephra in 58 piston and gravity cores collectedmainly east and north of New Zealand (Fig. 1; Table 1). Thelarge number of cores from eastern localities necessitatedsome evaluation of additional cores from the sparselysampled western (Tasman Sea) side of the country. Werelied upon analysis of new core material (S. D. Nodderpers. comm. 1993) combined with published informationfrom Ninkovich (1968), Eade & van der Linden (1970),Monastero (1972), and Norris & Grant-Taylor (1989).

Individual tephra layers were identified on the basis i»ftheir stratigraphic position within cores, the chemicalcomposition of their glass shards, and, in some instances,their heavy mineral component. Stratigraphic sequenceswere determined from sediment lithology, including colour,texture, composition of sand components, and calciumcarbonate contents (Fig. 2) (Griggs et al. 1983; Neil 1991;Fenner et al. 1992). These data usually allow a preliminarydivision into isotope stages, which have been corroboratedby microfaunal studies (Griggs et al. 1983; Cooke 1988;Fenner et al. 1992) and oxygen isotope data (Nelson et al.

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Carter et al.—Kawakawa Tephra in the SW Pacific 33

1985, 1993a, b; Cuthbertson 1988). Further control isprovided by a number of new and published radiocarbondates from key stratigraphic horizons within sedimentsassigned to isotope stages 1 and 2 (e.g., Griggs et al. 1983;Fenner et al. 1992; Pillans & Wright 1992).

The chemical composition of glass shards coarser than63 (im was determined with a JEOL JXA-733 electronmicroprobe using a 10 (im diameter beam and 8.0 nA beamcurrent. The full analytical procedure follows that reportedby Froggatt (1983). Further glass characterisation involvedgrain-size analyses using pipette and hand-shaken sievetechniques, the latter to minimise shard breakage. X-rayradiographs were made of 1 cm thick slices of some tephra/sediment sections in order to assess the effects ofbioturbation.

NomenclatureThe stratigraphic nomenclature associated with onshoretephra deposits is highly variable. For example, depositsfrom a c. 22 590 yr old event from Taupo have been variouslyascribed to the Oruanui Ash and Breccia, WairakeiFormation, Tiromoana Ash, Aokautere Ash, Scinde IslandAsh, Rekohu Ash, and Kawakawa Tephra (e.g., Vucetich &Howorth 1976; Kohn 1979; Campbell 1986; Self & Healy1987). The nomenclatural maze has been specificallyaddressed by Froggatt & Lowe (1990) in their review oflate Quaternary silicic tephras, and we follow theirrecommendations.

Thus, deposits discussed in this paper are formallyaccorded the following formational names with abbrev-iations in parentheses: Taupo Tephra (Tp); Waimihia Tephra(Wm); Rerewhakaaitu Tephra (Rk); and Kawakawa Tephra(Kk).

CORE STRATIGRAPHY

Two of the tephra layers are confined to sediments ofisotope stage 1, the remainder occupying the upper part ofstage 2. Identification of these stages was made initially on1 ithological criteria, particularly in the Bounty Trough andsouthern Chatham Rise, where there have been markedchanges in the terrigenous input between glacial andinterglacial periods (Fig. 2). The Last Glacial (isotope stage2) is characterised by greenish grey, hemipelagic mudsinterspersed with terrigenous fine sand and silt turbidites,whereas sediments from the Last Interglacial period (stageI) are light grey, pelagic nannoplanktic/foraminiferal oozes(Griggs et al. 1983; Nelson et al. 1985, 1993b; Neil 1991).

In contrast, north of Chatham Rise, the terrigenous supplyto the ocean has persisted irrespective of climatic state,although sediment supply was higher in glacial episodes(Kohn & Glasby 1978; Stewart & Neall 1984; Fenner et al.992). Consequently, lithological distinctions between stages

•. and 2 are less obvious although they are still evident fromthe down-core distribution of calcium carbonate, biogenic•dlica, and aeolian quartz (e.g., Fig. 2). In the western Bayof Plenty, oceanic sediments again have a strong pelagic/hemipelagic differentiation of interglacial and glacialsediments (Pillans & Wright 1992), similar to that in BountyTrough.

Broad corroboration of the lithologically identified stages1 and 2 is provided by the biostratigraphy, particularly thatbased on key planktic foraminifera such as Neoglobo-

quadrina pachyderma (left coiled) and Globigerinabulloides, which become more abundant into stage 2,accompanied by a corresponding decline in Globorotaliainflata (e.g., Griggs et al. 1983; Cooke 1988; Fenner et al.1992; Nelson et al. 1993b).

Final confirmation of the stage 1-2 stratigraphy comesfrom a suite of oxygen-isotope profiles presented byCuthbertson (1988), Neil (1991), Fenner et al. (1992), andNelson et al. (1993a). For oceanic sediments, glacial benthic818O values are c. 4-5%c whereas interglacial values droprapidly to c. 3%c (Fig. 2). This 2/1 transition, Termination 1of Broecker & van Donk (1970), commenced c. 15 ka agoand ended near 10 ka ago for the Bounty and Hikurangiregions, with the distinct pelagic to hemipelagic sedimentchange being dated at 12 ka in the Bounty Trough and Bayof Plenty (Griggs et al. 1983; Neil 1991; Pillans & Wright1992).

TEPHRA CHARACTERISTICS ANDIDENTIFICATION

General descriptionFresh, wet tephra layers have colours which range fromyellowish grey (5Y 8/1) to the more common light brownishgrey (5YR 6/1) (Geological Society of America 1963).Discrete airfall layers are usually from 1-6 cm thick to anobserved maximum of 15 cm. Some tephra layers,redeposited from turbidity currents, are as much as 34 cm(Lewis & Kohn 1973). Layers <1 cm thick are uncommon.Instead, thin deposits tend to be preserved as small lensesand burrow infillings that imply disruption of the originallayer by currents and/or bioturbation (e.g., Kennett 1981;Nelson et al. 1986).

Tephra layers usually have sharp contacts with the hostsediment (Fig. 3) although, in some instances, the uppercontact is gradational on account of bioturbation or currentreworking (e.g., Kohn & Glasby 1978). Structurally, thedistal oceanic deposits described here are massive, whereastephra layers that mantle the continental shelf and slopemay have textural and structural attributes consistent withash showering (Kohn & Glasby 1978) or redeposition byturbidity currents (Lewis & Kohn 1973).

TextureThe admixture of ash and sediment in the thinner anddisrupted tephra layers precludes their detailed texturalanalysis. Consequently, analysis is restricted to the thicker(>1 cm) and purer layers, which are mainly very fine sandysilt and occasionally silty very fine sand (Table 2; Fig. 4).The exception is proximal deposits of Taupo and WaimihiaTephra (Fig. 4), which may also include gravel- and sand-sized pumice clasts in various stages of degradation (e.g.,Barnes et al. 1991).

Most analyses have been carried out on the KawakawaTephra because of its extent and thickness. These depositsare mainly sandy silt or silt with a small clay component(Fig. 4). Modal grain size is typically in the very fine sandclass but can extend to coarse to medium silt. The maximumsize is medium sand; the mean grain size is coarse tomedium silt; sorting is poor and size distributions are mainlyfine skewed. Textural fining with distance from the sourceis not clear; the most remote samples analysed—W3 andW7 some 1100 km from the TVZ—are also the coarsest

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New Zealand Journal of Geology and Geophysics, 1995, Vol. 38

Core W2 - Bounty Fan

3.1 -

CD

0 3.2 -

3.3 -

BioturbatedhemipelagicmudSed. rate26 cm/ky

KawakawaTephra

Silt/mudturbidites

Grain sizeweight %

20 40 60

<2 n% 2 - 4% 4 - 10

10-20% 20 - 30% 30 - 63% 63 - 125

>125 tim

Fig. 3 X-radiograph of Kav-1 -kawa Tephra from the Bonn yFan, highlighting reverse gradii :.well-defined tephra base (i.e. obioturbation), and a sharp top th atis only slightly bioturbated by afew burrows.

Sand

with almost 50% sand. These textural properties areconsistent with those reported by Campbell (1986) for distaloccurrences of Kawakawa Tephra on the South Island,where deposits have a loamy sand to silty loam field texture.In addition, distal Kawakawa deposits on Bounty Fan coarsenupsection (i.e. reverse grading), with sand contents increasingfrom 10 to 50% from the base to top of the deposit (Fig. 3).

CompositionMost fine sandy tephra consist predominantly of translucentglass shards with minor amounts of primary ferromagnesianminerals. Small amounts of "foreign" terrigenous minerals,foraminifera, and radiolarians may also occur, especiallywithin disrupted tephra layers. In contrast, coarser tephrasuch as proximal Taupo and Waimihia deposits have a>125 [im sand fraction that is dominated by pumicefragments.

The ferromagnesian mineral assemblages can helpidentify individual tephra units (e.g., Kohn 1973; Froggatt& Lowe 1990). The Waimihia Tephra contains anassemblage dominated by hypersthene with occasional tracesof augite, which concurs with the offshore data of Lewis &Kohn (1973) and onshore assemblages from the Tauporegion (Ewart 1963). The Rerewhakaaitu mineralogy is

distinguished by hypersthene, hornblende, and abundantbiotite (Cole 1970; Kohn & Glasby 1978; Stewart & Neall1984). Ferromagnesian minerals from the Kawakawa Tephraare dominated by hypersthene and hornblende, with rareaugite (Howorth et al. 1980). Such an assemblage confirmsthe presence of the Kawakawa off Hawke Bay (Lewis &Kohn 1973), Bay of Plenty (Kohn & Glasby 1978; Pillans& Wright 1992), and in Bounty Trough (Neil 1991).

Class shard chemistryThe major element chemistry of sand-sized glass shardsfrom 17 tephra layers in 16 cores, including the ChathamIsland occurrence (Rll), revealed all glasses were silicic(Table 3). Low standard deviations for element concen-trations indicate all samples comprise homogeneous andunmixed populations of glass shards, consistent withderivation from a particular eruptive event. Comparisons ofanalyses to type material (Table 3) produce similaritycoefficients (SC) close to 100 (Borchardt et al. 1971), therebyconfirming our identifications.

When plotted on a ternary diagram of CaO-FeO-0.3K2<(Fig. 5), the shards fall into two clusters related by Pillans etal. (1993) to (1) a Taupo Volcanic Centre source that eruptedbetween 0 and 10 ka (i.e. Taupo and Waimihia Tephra), and

34

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Carter et al.—Kawakawa Tephra in the SW Pacific 35

S = sandcS = clayey sandmS = muddy sandzS = silty sandsC = sandy claysM = sandy mudsZ = sandy siltC = clayM = mudZ= silt

SAND

• Taupo+ WaimihiaA Kawakawa

CLAY SILT

Fig. 4 Textural classification of tephra (extended from Neil1991).

12) the Taupo source of the Kawakawa Tephra. The lattercluster is not unique to Kawakawa deposits; it alsoencompasses shards from Okataina Volcanic Centre (OVC)(Pillans et al. 1993). Most late Quaternary OVC eruptionsappear to have been low explosive events and have not beenidentified in the deep ocean beyond a c. 500 km radius ofthe TVZ (i.e. the limit of the Rerewhakaaitu Tephra, whichis the only deposit found in this study that is derived fromOVC).

Age

Four new radiocarbon dates (Table 4) are compatible withI he age of the terrestrial Kawakawa Tephra, which is widelyaccepted as 22 590 + 230 yr B.P. (Wilson et al. 1988;Froggatt & Lowe 1990). Sediments immediately below theitsh on Hikurangi Fan-drift yielded an age of 21 680 ± 160yr B.P. Similarly, peat deposits underlying KawakawaTephra on Chatham Island provided a date of 23 150 + 150yr B.P. However, the ages of the overlying peat and theroots within the deposit were c. 10 000 years younger,suggesting that the blanketing effect of this ash, coupledwith the glacial climate, profoundly inhibited vegetationalgrowth.

\SH DEPOSITION

The offshore distribution of tephra is influenced by thedynamics of the eruption and transport by winds and water

• Marine Taupo & Waimihia

T Terrestrial Taupo30

A Marine Kawakawa

• Chatham Island Kawakawa

K Terrestrial Kawakawa

%FeO 4040

%CaO

3050

30 ' ' ' 4'0 '0.3(%K20)

30

Fig. 5 Compositional classification of the marine tephra and ofrepresentatives of terrestrial counterparts recorded by Froggatt(1983) and Pillans et al. (1993).

currents. Secondary, but significant controls are the processesoperating in the depositional environment. Such controlsare suggested by irregularities in the offshore distributionof tephra thickness and grain size. Kawakawa Tephra, forexample, locally thickens and coarsens away from the TVZ,in contrast to the usual thinning and fining trend evident interrestrial deposits (e.g., Walker 1981). The following sectionexamines some factors affecting tephra deposits.

Rate of sediment supplyThe tephra layers of this study are best preserved in areas ofrapid sedimentation and minimal current winnowing. Thus,any risk of postdepositional modification is reduced.Waimihia Tephra, for example, is well preserved in leveesand overbank deposits of Hikurangi Channel, where averagesedimentation rates are 8.4 cm/1000 yr for isotope stage 1,and 12 cm/1000 yr for stage 2 (Fenner et al. 1992). Similarly,up to 7 cm of Kawakawa Tephra is buried in the leveedeposits of the Bounty Fan system, which has rates of 5.6cm/1000 yr for stage 1 and 18.3 cm/1000 yr for stage 2(Neil 1991). In these settings, the ash would be subject torapid burial by terrigenous turbidites which had a maximumfrequency of occurrence of 1 per c. 440 years for glacialperiod sediments in Bounty Trough (determined from dataof Neil 1991) and 1 per c. 380 years for glacial/interglacial

Table 2(1990).

General characteristics of the offshore tephra, including their ages from Froggatt & Lowe

Tephra

Taupo

WaimihiaRerewhakaaituKawakawa

Age(yrB.P.)

1850± 10

3280± 2014 700 ±11022 590 ± 230

Distancefrom TVZ(max. km)

660

560500

1370

Tephrathickness(max. cm)

11

8107

Tephra texture

Variable, from gravel-sized pumiceclasts to very fine sandy silt

Silty fine sand to fine sandy siltVery fine sandy mudFine sandy silt to silt

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36 New Zealand Journal of Geology and Geophysics, 1995, Vol. 3

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Hikurangi sediments (from data of Lewis & Kolm1973; Lewis 1980, 1985). Preservation has beenaided by enhanced hemipelagic mud depositionduring the Last Glacial period, although the prec hecontribution of mud from hemipelagic versi sturbidity current sources has not been resolved.

Outside the fan-levee complexes of the trough •-,marine tephra can be well preserved in other region sof high deposition, including the hemipelagic andturbiditic depocentres of the slope basins off Hawk eBay, and the sediment drifts formed by the deepwestern boundary current along the eastern marginof the Chatham Rise - Hikurangi Plateau (Fig. 1)

The protective influence of high sedimentationis well shown in the Bay of Plenty. In the westernbay, sedimentation rates for isotope stage 1 average5.8 cm/1000 yr, and contemporaneous macroscopictephra are infrequent, and most ash is dispersedwithin the host sediment (cores S794 and S8O3 ofPillans & Wright 1992). By comparison, thecorresponding sedimentation rate in eastern Bay ofPlenty, just 75 km away, is 14.4 cm/1000 yr (basedon data of Kohn & Glasby 1978), and macroscopictephra are more frequent and thicker than theirwestern counterparts.

RedepositionAlthough tephra are commonly interbedded withturbidites in the various slope basin and fan/leve,systems, the ash is probably airfall in origin. Manstephra are either massive deposits, lacking theinternal sedimentary stuctures and characteristicsof turbidites or exhibit reverse grading (Fig. 3). Todate, only a few possibly redeposited tephra havebeen recorded. Lewis & Kohn (1973) documenteda 34 cm thick bed of Waimihia Tephra in the 2000 mdeep Pauanui Basin off southern Hawke Bay. Thebed appears to have been deposited from severalturbidity currents, each identified by graded sand tosilt units, some of which include detrital minerals.The ash-charged flows either entered the basin viaknown sediment conduits in the north and south orcascaded from marginal banks and ridges, whichhave only a thin, discontinuous mantle of bioturbatedash.

Barnes et al. (1991) noted a strongly graded, 10cm thick deposit of Waimihia Tephra in a slopebasin off Hawke Bay (Fig. 6). The presence ofdetrital grains favours the hypothesis of turbidileemplacement rather than an airfall origin.

Erosion by currentsWithin the regions of ash fallout there has beenlocalised modification of tephra deposits by strongcurrents. On the continental shelf, tephra depositionhas been controlled by the wave and current regimeand changes superimposed upon this regime byfluctuations of sea level and climate. At the time ofthe Kawakawa eruption, 22 590 yr B.P., thecontinental shelf was an exposed, wind-swept plain(Stewart & Neall 1984; Fenner et al. 1992). Anyash that survived on the shelf was liable to beeroded by a postglacial transgressing sea that

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Fig. 6 X-radiograph of gradedWaimihia Tephra from the HawkeBay slope, with grain sizes ofupper and lower sections. Thebase of the deposit may beerosional, but there are too fewlaminae in the underlying mud topositively identify truncation. Thepresence of detrital grains in theash suggests a turbiditic origin.

1 5 —

Core S780 - Hawke Bay slope

WaimihiaTephra

Hemipelagicmud

Weight %

Sand

supported a wave/current regime more intense than presenton account of strong, glacial period winds (Thiede 1979;Stewart & Neall 1984). Similar conditions presumablyaffected the Rerewhakaaitu Tephra, which accumulatedwhen sea level was about 75 m lower than present (e.g.,Carter et al. 1986).

The Waimihia Tephra was laid down when sea levelattained its modern position and meteorological conditionswere similar to the present. In such a situation, this andt ither late Holocene tephra settled where the hydraulic regimepermitted, namely the mud zones of the middle to outershelf off Hawke Bay (Pantin 1966) and Bay of Plenty(Kohn & Glasby 1978). In these two areas, the combinationof semi-sheltered waters and a high terrigenous supplyfavoured tephra preservation. By comparison, the inner tomiddle shelf (<50 m depth) is frequently stirred and scouredby storm-induced waves and currents, so that preservationis minimal. Outside of Hawke Bay and Bay of Plenty, no-helf tephra deposits have been found, although areas likethe East Cape shelf have not been adequately sampled.

Few tephra layers have been found at the Hawke Bayshelf edge and uppermost continental slope down to c. 500 mdepth (Lewis 1973; Barnes et al. 1991). These are mainly ofcoarse-grained pumiceous Taupo Tephra layers, whereas

Table 4 Radiocarbon ages for a marine tephra layer on the*iekohu Drift (S931) and samples associated with the KawakawaTephra layer in core Rl 1 from Chatham Island.

Sample no. Core depth (m) Age (yr B.P)

Rekohu DriftS931 (NZA373)1

Chatham IslandWk 15912

Wk 15912

Wk l5932

0.76-0.78 (below tephra) 21680±160

0.800-0.8090.814-0.8230.849

(above tephra)(within tephra)(below tephra)

12 600 + 7011 150121023 150 +150

'NZOI core with Institute of Geological & Nuclear Sciencesradiocarbon reference in brackets.

-University of Waikato Radiocarbon Dating Laboratory numbers.

the Waimihia Tephra is found only as bioturbated ash "clots",which suggest the original ash was thin (<c. 1 cm, see nextsection). By contrast, Waimihia Tephra is thicker and morecontinuous downslope (Lewis & Kohn 1973). It is unlikelythat thinning is a consequence of gravitational massmovement, as the trend is apparent in disturbed andundisturbed sediments (Barnes et al. 1991). More likely,the dearth of Waimihia Tephra at the shelf edge is aconsequence of winnowing and dispersal by currents thatresemble the modern alongshelf and offshelf circulation(Ridgway & Stanton 1969), possibly aided by internal waveactivity.

Seaward of the continental margin, evidence for erosion,or at least nondeposition, is confined to the KawakawaTephra. Sediments from the <400 m deep crest of theChatham Rise, for example, have no recorded macroscopictephra (Cullen 1978), even though an average of 11 cm ofKawakawa Tephra accumulated on Chatham Island(Mildenhall 1976). This absence is probably related tocurrent erosion, as attested by the reworked appearance andthin, patchy distribution of the postglacial sediment cover(Cullen 1978). Modern currents, with high mean speeds ofc. 20 cm/s (Heath 1981), seem sufficiently energetic tomove sediment. At the time of the Kawakawa eruption,conditions for deposition and preservation would have beeneven more unfavourable, because currents over the risecrest were probably enhanced in response to lowered sealevel and a more intense wind regime (e.g., Stewart & Neall1984). Once below 400 m water depth on the flanks of theChatham Rise, however, the Kawakawa Tephra layer iswell preserved.

Another example of erosion or nondeposition is in thevicinity of the deep western boundary current (DWBC; Fig.1). On the Bounty Fan, the Kawakawa Tephra layer initiallycoarsens eastward into the path of the current (Neil 1991)but is missing, presumably eroded, further east in a fast-flowing zone of the DWBC at c. 4700 m depth (Carter &Carter in press). We suspect that the Kawakawa Tephranever settled, or the deposit was eroded along most of the2000 km course of the DWBC where it is intensified againstprominent bathymetric relief. In contrast, tephra are

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38 New Zealand Journal of Geology and Geophysics, 1995, Vol.:' S

preserved in large sediment drifts along the margins of thefast-flowing zone of the current (Fig. 1).

The reverse grading of Kawakawa deposits on BountyFan (Fig. 3) could also be a function of winnowing by theDWBC. However, an upwards coarsening of deposits onSouth Island (Campbell 1986) suggests that this texturalchange is related more to the dynamics of the eruptionitself. Certainly, the textural data reported by Self (1983)indicate a coarsening of terrestrial airfall deposits in thelater phases of the Kawakawa eruption.

BioturbationBiological reworking of tephra layers is influenced by severalvariables that include rate of ash accumulation, depositthickness, benthic abundance, and types of genera (Huanget al. 1975; Watkins & Huang 1977; Kennett 1981; Carter& Grange 1992). The tephra layers off New Zealand typicallyhave sharp bases even in regions where the host sediment isbioturbated. This lack of tephra disruption suggests that theash inhibits benthic activity. For example, observation ofthe benthos off White Island, immediately after aneruption, showed a marked reduction in the abundance,percentage cover, and diversity of organisms (Carter &Grange 1992).

Bioturbation may also disrupt tephra from the top (Fig.3) (Watkins & Huang 1977). Such modification willultimately depend upon the type of sedimentation proceedingash deposition. If a pelagic or hemipelagic phase follows,then sufficient time may be available for establishment of abenthic community and subsequent bioturbation of thetephra. If, however, the ash is followed by a sand/silt turbidite,as for some examples of Kawakawa Tephra on the BountyFan, then the ash is buried and bioturbation is minimised.

Another consideration is the spatial and temporalvariability of bioturbation. Preliminary investigations ofthe Chatham Rise indicate differences in benthic abundance(and by association degree of bioturbation) either side ofthe Rise (Probert in press). In such a situation, it is arguedthat ash falling in a region of lower benthic abundance has abetter chance of survival than in an area of higher abundance.The situation is further complicated by probable changes inbenthic abundance with time. Rowe (1981) suggested thatabundance is directly related to primary productivity ofsurface waters, which Fenner et al. (1992) has shown to besignificantly higher in glacial periods due to more intense,wind-induced upwelling. Thus, during the last glaciationwhen Kawakawa ash was accumulating, benthic activitywas probably higher than today, as was the potential forbioturbation.

ASH DISPERSAL

Offshore dispersal of ash is presented in a series of isopachcharts which also incorporate dispersal patterns recordedonshore by various authors (Fig. 7A-D). Isopachs havebeen drawn for uncompacted layers as marine tephra tendto be about half the thickness of their terrestrial counterparts(Watkins et al. 1978;McCoy 1980). Thus, layer thicknessesrecorded from cores have been doubled and contouredaccordingly. Such a correction has some validity when thethickness of the Kawakawa Tephra on Chatham Island(mean 11 cm) is compared to ash layers (3-7 cm) on theadjacent ocean floor (Fig. 7D).

In spite of these corrections, the isopachs must be con-sidered approximations in light of the wide scatter of da apoints and the potential for modification of deposits by themarine processes outlined in the previous section of this pape .The isopachs are a best estimate with the available data

The distribution patterns of terrestrial and offshore teph] aoutline a generally eastward dispersal. By way of con -parison, late Quaternary macroscopic tephra are absent : ncores from the western continental margin of the NorthIsland (Norris & Grant-Taylor 1989; S. D. Nodder per .comm. 1993), Lord Howe Rise (Eade & van der Linden1970), and Tasman Basin (Monastero 1972).

Taupo Tephra is believed to have been ejected from avent within Lake Taupo, close to the northeastern shore(Walker 1980). Onshore, isopachs reveal an ENE dispersal,which continues at least 500 km offshore (Fig. 7A). At thatdistance there appears to be a thickening of tephra in contrastto the frequently observed exponential thinning of depositsaway from the eruptive centre (e.g., Fierstein & Nathenson1992). Although sample control is poor along the axis ;>fdispersal, thickening is inferred by the reappearance of the20 cm isopach at the distal part of the mapped airfall zone.The other 20 cm isopach, as derived from the data ofWalker (1980), is completely constrained onshore. Anotii; raspect is the distribution of the offshore isopachs close I •>the source. These indicate a more widespread dispersal thantheir onshore counterparts; for example, the 5 cm isopacliwas originally mapped by Walker (1980) as being entirelyonshore out to longitude 178°E, whereas the marine dataindicate that this isopach occurs up to 110 km in the Bay ofPlenty (Fig. 7A).

The eruptive centre for the Waimihia Tephra was onthe eastern side of Lake Taupo. Isopachs of terrestrial pumicedeposits from this eruption show that early in the eruptiondispersal was due east, but later was ESE, presumably inresponse to a wind change (Walker 1981). This swing iseven more pronounced for ash transported offshore. Ejectatravelled southeastward out to a minimum of 500 km fromLake Taupo (Fig. 7B). The change in dispersal directionmay be attributed to ejection of the bulk of the ash late inthe eruption, when the northwesterly wind had fullydeveloped as inferred from the terrestrial pumice deposits.This hypothesis has some support from onshore sections ofWaimihia Tephra, which display a fining of ejecta towardsthe end of the eruption (Walker 1981). Alternatively, theremay have been differences in the high- and low-level windregimes that affected ash and pumice dispersal, respectively(see section on Paleowinds).

The onshore dispersal of Rerewhakaaitu Tephra hasnot been fully mapped. Vucetich & Pullar (1964) producedisopachs of the thickest deposits that indicated a restrictednortheastward dispersal from the OVC to the Bay of Plentycoast (Fig. 7C). Later, Topping & Kohn (1973) and Lowe etal. (1980) extended the range of distal tephra south andnorth of Okataina, respectively, without altering thenortheastward trend. Bearing in mind the poor core control,offshore transport appears to have been mainly eastward forat least 500 km from Okataina (Fig. 7C).

The Kawakawa event was one of the largest eruptionsin late Quaternary times. From the eruption centre in thevicinity of northern Lake Taupo, ash was dispersed overmuch of New Zealand and the Chatham Islands (Mildenhall1976; Self 1983; Campbell 1986). The isopachs of terrestrialashfall deposits show a pronounced dispersal southeast of

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Lake Taupo with a much reduced westward and northwardtransport (Fig. 7D). Isopachs on Chatham Islands reveal asimilar southeastward trend although this has been locallyinfluenced by pluvial runoff into peat bogs where the thickestdeposits are located (Mildenhall pers. comm. 1993). Thus,there is some variation in thickness of 2-38 cm, and wehave arbitrarily taken a mean value of 11 cm (33measurements).

Marine occurrences of Kawakawa Tephra extend thesoutheastward dispersal to at least 1370 km from Lake7 aupo (Fig. 7D). Campbell (1986) has suggested that coarse-grained glass in peat deposits on Campbell Island may beKawakawa, in which case transport would have been 1600km from the source, but positive identification of the glasswas not made. Kyle & Seward (1984) suggested thatKawakawa ash may have reached Antarctic waters, butmore recent work by Shane & Froggatt (1992) indicatesthat is unlikely.

As dispersal occurred mainly over the deep ocean, it islikely that ash distribution was influenced by currents asparticles settled through the water column. Once belowwind-affected surface waters, particles would be entrainedI orthwards, especially within the DWBC whose depth rangeis 2200-5500 m (Warren 1973; Carter & McCave 1994).Transport would be greatest where the flow is strongest,which is approximately along the 4000-5500 m isobaths(Fig. 1). Thus, northward transport would be enhanced intie more easterly regions such as the distal reaches ofKawakawa and Taupo Tephra, resulting in skewed dispersalpatterns.

PaleowindsModern winds over New Zealand are mainly from therorthwest to southwest, with a dominant westerlycomponent. Upper-level wind frequencies compiled by Reid<k Penney (1982) reveal a prevalence of winds from thewest to northwest below 3 km altitude over the centralNorth Island. At greater heights, westerlies becomei icreasingly dominant and winds from the northwest andsouthwest have a slightly reduced, but roughly equivalent,presence (Table 5).

The eastward distribution of tephra offshore is consistentwith a general westerly wind regime. Specifically,northwesterly winds prevailed at the time of the Kawakawa,Waimihia, and possibly the Rerewhakaaitu eruptions,whereas a WSW component affected dispersal of TaupoTephra. It may be coincidental that the three older tephralayers accumulated under northwesterly winds given thatthis direction is subordinate under the modern wind regime.Alternatively, northwesterly winds may have prevailed at

1 able 5 Percent occurrence and variation in direction of high-level winds with altitude at two North Island sites located on Fig.1. (Data from Reid & Penney 1984.)

Pressure(inbar)

100300500700900

Altitude(km)

16.09.06.03.00.9

NW

1721191611

Auckland

W

5634282114

SW

2122242326

NW

1921261930

Ohakea

W

5432282927

SW

192323196

39

some time in the past. In the southern North Island, forexample, the Kawakawa Tephra is interbedded with loesstransported from the northwest (Palmer 1982). A thirdalternative, that ash was only ejected to heights <3 kmwhere northwesterly winds may have prevailed, is unlikely.We estimate that the clouds from all four eruptions readilyexceeded c. 6 km altitude; this value is derived by applyingash grain size and distance from the source to the model ofShaw etal. (1974).

The extent of wind-induced transport was probablygreatest for the Kawakawa event, which occurred near thepeak of the Last Glacial period when westerly windsintensified in response to enhanced thermal gradientsbetween the poles and equator. Last Glacial sediments eastof New Zealand contain significant concentrations of aeolianquartz, which declined sharply to modern levels between16.1 and 14.7 ka (Fig. 2) (Stewart & Neall 1984; Fenner etal. 1992). Onshore, many southern North Island occurrencesof Kawakawa Tephra are preserved within loess (Palmer1982; Pillans et al. 1993).

The prevalence of macroscopic tephra to the east ofNew Zealand does not preclude dispersal in the oppositedirection. Nelson et al. (1986) recorded late Cenozoic tephrain Deep Sea Drilling Project (DSDP) cores up to 1300 kmnorthwest of the source in the TVZ. These deposits areregarded as the fallout of particularly large eruptions thatejected ash above an altitude of c. 20 km into a zone ofsoutheasterly stratospheric winds (Nelson et al. 1986).Although late Quaternary tephra were not recorded at theDSDP sites, it is likely that some ash was carried westward.The cloud height of the Taupo eruption is estimated byWalker (1980) to have reached c. 50 km altitude, whereasthe Kawakawa cloud may have risen to c. 30 km, as deducedby applying the median grain size (88 ^m) and distancefrom the source (1100 km for tephra on Bounty Fan) to themodel of Shaw et al. (1974).

Onshore versus offshore dispersalIn broad terms, the dispersal trends exhibited by offshoretephra deposits resemble trends of their terrestrialequivalents. Some differences are evident and may reflectchanges in wind direction during the course of an eruption(e.g., Waimihia eruption; Walker 1981), variations in winddirection with altitude (e.g., Reid & Penney 1982), sparsesample control, or some combination of these factors.

Terrestrial tephra appear to thin exponentially awayfrom their sources (Froggatt 1982). However, for theWaimihia Tephra, there is a decrease in the rate of thinningin eastern Hawke Bay (Walker 1981). The offshore data areinconclusive about thinning variability, except for the TaupoTephra, which exhibits a marked thickening 500 km fromthe shore (Fig. 7A). A similar trend was documented bySarna-Wojcicki et al. (1981) for the Mount St Helenseruption of 1980. The resultant airfall deposit had a secondthickness maximum located 350 km from the volcano. Carey& Sigurdsson (1982), supported by Hopkins & Bridgman(1985), concluded the Mount St Helens thickening resultedfrom the aerial aggregation of fine ash particles (<63 |im)followed by the premature fallout of the aggregates. Theresultant terrestrial deposit tended to be fine grained,polymodal, and poorly sorted. Samples of tephra from the"bulge" (R646, S932) are likewise fine grained (mean 18-24 ^m) and very poorly sorted (a 2.3-2.4) but are only

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40New Zealand Journal of Geology and Geophysics, 1995, Vol. !8

* Tephra absent

4 • Sample thickness (cm)for compacted tephra

• # - Isopach (cm) foruncompacted tephra

D Dispersed tephra

LL

Taupo Tephra1850yrB.P.

170°E 175° :i8Q° 175° 170°W

Fig. 7A Estimated isopachs :>funcompacted Taupo Tepfit .Onshore data are modified fromWalker (1980) to include all aidu IIproducts of the Taupo eruption(1850 yr B.P.). Dispersed teplna(D) includes clots of ash and ash-rich sediments mixed by hydraul icand biologic processes. Isopachsare drawn for uncompactedtephra, which are about twice thethickness of layers identified incores (e.g., McCoy 1980).

-35 '

40'

CHATHAM RISE

BOUNTY TROUGH

Waimihia Tephra3280 yr B.P.170°E °175°

A Tephra absent

4 • Sample thickness (cm)for compacted tephra

--©•• Isopach (cm) foruncompactedtephra

D Dispersed tephra

- Probable redepositedtephra

170°W

Fig. 7B Estimated isopachs ofuncompacted Waimihia Tephra.Onshore isopachs are modifiedfrom Walker (1981).

occasionally polymodal with a sand mode (125 (xm or64 p.m) and silt mode (8 (J,m or 16 |J.m). Thus, the bulge mayhave resulted from particle aggregation with possiblemodification by ocean currents—it is directly in the path ofthe DWBC. Alternatively, it may be caused by one or acombination of the following:

(1) injection of ash into a zone of relatively slowerstratospheric winds above the mean jet stream at c. 14 kmabove northern New Zealand (Maunder 1971). Walker

(1980) noted that the Taupo plume must have reachedc. 50 km altitude, where it obviously entered the slowerwind zone of the lower stratosphere but also extendedinto a region of more vigorous flow in the upperstratosphere;

(2) decrease in wind velocity during horizontal transportof the plume. This is a possibility in light of modernmeasurements of upper winds, which show markedchanges of speed and even direction (Maunder 1971);

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Carter et al.—Kawakawa Tephra in the SW Pacific 41

Fig. 7C Estimated isopachs ofuncompacted RerewhakaaituTephra. Onshore isopachs arederived from the data of Healy(1964) and Topping & Kohn(1973).

1 , \ 1 1 ,

-35° ^ ^ V V - 1

x v.HB* 7j

-40° N ; < D

}/ /^l ....

\

?

yo .5

y £ "' CHATHAM RISE „,r' / -

,•> (

: f / ' BOUNTY TROUGH

;> ^ ^ ^ ,

Rerewhakaaitu Tephra14,700 yrB.P.

- ^TooE™ " " I 75 ° r } - ;i80°! 1 I / - %l i

i

4 •

- # •

D

175°

I; i

Sample thickness (cm)for compactedtephra

Isopach (cm) foruncompacted tephra

Dispersed tephra

170°W

Fig. 7D Estimated isopachs ofuncompacted Kawakawa Tephra.Onshore data are modified fromCampbell (1986) and Self &Fealy (1987).

170

A Tephra absent+ Core too short to

reach tephra

4« Sample thickness (cm)for compacted tephra

-<5>- Isopach in cm foruncompactedtephra

D Dispersed tephra

.' i

°E \ , 170°E 180? 175° 170°W;\>* ••-. ____ ^ 'V 35°"

si \ : / . . . • : -.....

A i X;D-- '%2 ~ ;[ y^V* CHATHAM RISE ^^^ I,.---''V\^'° 7. '• /A \ XD 6. 4. / 45°-* K. • D \BOUNTY TROUGH 4

\ x - < ^ ••'"""!i ""Ai*3" [Kawakawa--:>x* {W ! Tephra

" - - -H- . . .^>. .^ - r—- 1 22,590 yr B.P. ji / '-•--..-' y' «0.5 •. , |

(3) pronounced polymodality in ash grain size. It islikely that Taupo ejecta was at least bimodal with agravel-coarse sand mode of pumice fragments and afiner mode (<125 im) composed mainly of glass shardsand constituting c. 80% of the erupted mass (Walker1980). These two modes separated close to the sourcewith coarse fractions accumulating onshore and on theadjacent continental shelf/slope. Ash forming the bulgeis only weakly bimodal with a prevailing fine sandfraction and a much less prominent medium silt fraction.Such evidence is inconclusive with respect to the bulge,

being a consequence of preferential sorting of the coarserfraction because the grain size of the deposited tephrawas probably affected by the DWBC, which would havewinnowed the fine grades, thereby emphasising the sandfraction.

TEPHRA VOLUMES

Up till now, volumes of late Quaternary airfall tephra havebeen estimated from terrestrial deposits. However, as Walker(1980) and Froggatt (1982) point out, such estimations can

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42 New Zealand Journal of Geology and Geophysics, 1995, Vol.

1000 T

100-

a>E

"5

o

• Marine tephraTerrestrial tephra

10-

0.5 1.0 ^.5

Isopach thickness (m)

Fig. 8 Plot of isopach thickness versus log cumulative volumesbounded by each isopach. Extrapolation of curves to the y-axisyields an estimate of total volume.

have significant inaccuracies because of the largeextrapolations required to account for the distal parts of theairfall deposits. In theory, the offshore data presented hereshould improve the quality of tephra volume determinations,although they still remain estimations in view of (1) therandom scatter of offshore control points, (2) postdepo-sitional modification of tephra thickness, and (3) theremaining need to extrapolate to the theoretical edge of adeposit; our data control extends only to between the 1 and10 cm isopachs.

Several methods of extrapolation and volume calculationhave been used in the past with varying success. For this

study, volumes have been derived from isopach maps 'ysummation of individual volumes contained within fa .hisopach pair. In this manner, reasonable estimates tireobtained out to the 1-10 cm isopachs (Fig. 7A-D). Howev; r,reliable determination of volumes outside these isopae ishas proved to be elusive. The various extrapolai* ntechniques based on isopach maps (see Froggatt lc'ic!!;Fierstein & Nathenson 1992) rely on an exponential decrea ein tephra thickness away from the eruption centre. Terres ct ildeposits from the TVZ usually decay exponentially—a tie idthat is expressed on log-based graphs as a single line, who-*eintercept with the appropriate graph axis provides somemeasure of tephra volume (Froggatt 1982). However, whenterrestrial and marine data are combined, decay is expressedas two lines which indicate that more tephra accumulatedoffshore than anticipated by the single line model (Fig. S).

Enhanced offshore dispersal is quite likely and was nodoubt aided by the injection of ash into high speed, upperwind zones and by the production of large quantities of fpieash, such as occurred for the Taupo and Waimihia eruptions(Walker 1980, 1981). Furthermore, marine deposits in iyalso contain additional airfall ash from the extern i epyroclastic flows associated with the Kawakawa and Ta j | oeruptions (Froggatt 1982).

Given the limitations of the marine database, the volumesof the tephra in question are presented at two levels, namel >.':(1) the volume within the outermost complete or near-complete isopach, and (2) the estimated total volume derivedfrom plots of isopach thickness against the log of the volumeconstrained by each isopach (Fig. 8). The former is themore accurate but is only a partial measurement of volume.The latter involves significant extrapolation and must beregarded as an approximation pending more core data,especially from the distal reaches of the airfall deposits.

Volume estimates

A comparison of the volumes calculated here with volumescompiled from extrapolated terrestrial data suggests thatthe latter tend to underestimate tephra volumes (Table 6).For Taupo Tephra, 33.8 km3 are constrained by the partial I yclosed 10 cm isopach if that isopach is continuous from thesource to the distal bulge (Fig. 7A). Estimation of totalvolume is hampered by few data points, but it may reachc. 50 km3, which well exceeds the 17.5-23 km3 fromterrestrial data.

Table 6 Volumes of tephra airfall. Volume I is calculated out to the last complete isopach (shown inbrackets) except for the Taupo Tephra, for which the volume of the incomplete 10 cm isopach iscalculated out to 175°W, assuming this isopach is continuous to this longitude. Volume II is estimatedby extrapolation of a plot of isopach thickness versus log cumulative tephra volumes (see Fig. 8).Previous estimates are from:'Froggatt & Lowe (1990); 2Walker(1980); 3Walker (1981); 4Self (1983).

Tephra

Taupo

Waimihia

Rerewhakaaitu

Kawakawa

Volume I(km3)

33.81

(10 cm)19.4

(2 cm)13.4

(lem)270

(10 cm)

Volume 11(km3)

c. 50

c. 22

c. 14

c. 400

Previousestimates

17.51

232

14'293

71

+ 701

3104

Ignimbrite(km3)

70

5

c.

150

Total volume(km3)

c. 120

c. 27

c. 14

c. 550

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Carter et al.—Kawakawa Tephra in the SW Pacific 43

The volume of Waimihia airfall out to the 2 cm isopachis 19.4 km3, with an estimated total volume of c. 22 km3.Both values exceed the 14 km3 of Wilson et al. (1984) andFroggatt & Lowe (1990) but is less than the 29 km3 calculatedby Walker (1981) using a crystal concentration technique.It the Walker figure is overestimated by 20%, as suggestedby Froggatt (1982), then the reduced figure closely matchesthat determined here.

Likewise, the 13.4 km3 of Rerewhakaaitu Tephra withinthe 1 cm isopach, and an overall estimate of c. 14 km3, arehigher than the 7 km3 extrapolated from terrestrial deposits.

Although major eruptions in their own right, the abovee'uptions are small compared to the Kawakawa event. Wec alculate that 270 km3 accumulated out to the 10 cm isopach,and possibly c. 400 km3 for the entire airfall deposit. Self(1983) estimated 310 km3 of airfall, which he regarded as aconservative figure.

With the exception of the Rerewhakaaitu event, theeruptions were accompanied by pyroclastic flows (Table6). For the Kawakawa and, to a lesser extent, the Taupoeruptions, these flows were extensive with volumes of 150km3 and 70 km3, respectively. When combined with theairfall deposits, the total volume of Kawakawa eruptives isc. 550 km3, making it one of the largest eruptions in lateQuaternary times, perhaps second only to the c. 2000 km3

cf material associated with the 75 000 year old Toba eruptionin Sumatra (Ninkovich et al. 1978).

SUMMARY

The combination of strong winds, proximity to the PacificOcean, and voluminous rhyolitic eruptions has encouragedtne offshore dispersal of late Quaternary airfall materialfrom the Taupo Volcanic Zone. Understandably, previousstudies have concentrated on terrestrial deposits, whichprovided the first insight into the large volumes and pre-vailing eastward dispersal of eruptives. In this paper wehave taken the work one step further by integrating terrestrial; nd marine data to provide a more comprehensive dispersalpicture for four major eruptions: Taupo (Tp), WaimihiaiWm), Rerewhakaaitu (Rk), and Kawakawa (Kk) Tephralayers.

'. With the exception of the Taupo Tephra, which spreadENE, tephra dispersal was east to southeast. The threeyounger tephra layers have been recorded 500-660 kmfrom the TVZ, whereas macroscopic layers of KawakawaTephra occur at least 1370 km from the source.

2. Eruptions (Rk, Kk) that occurred during the Last Glacialperiod were probably accompanied by enhanced windiransport due to an intensification of wind belts, as indicatedby increased quantities of aeolian quartz in sedimentsencasing the tephra.

3. The marine data increase previous estimates of tephravolumes extrapolated from terrestrial data. New estimatesof total airfall volumes are Tp c. 50 km3, Wm c. 22 km3, Rko. 14 km3, and Kk c. 400 km3. Incorporating the ignimbritedeposits for each of the Tp, Wm, and Kk events, yields totaleruptive volumes of c. 120, c. 27, and c. 550 km3,-espectively.

X. In all eruptions, more ash appears to have accumulatedoffshore than is predicted by simple exponential thinning

models, which suggests (1) the eruptions generated largequantities of very fine ash, (2) high concentrations of ashwere injected into high speed, upper-level winds, or (3)wind speeds changed during eruptions.

5. Taupo Tephra appears to have a second thicknessmaximum 660 km from the source. This deposit may resultfrom premature settling of aggregated ash particles or isrelated to other factors including production of polymodalash, reduction in horizontal wind speed, and injection into azone of slow winds above the main jet stream.

6. Macroscopic tephra are best preserved in regions ofhigh sedimentation such as channel levees, submarine fans,and sediment drifts. However, in areas of high seismicactivity, as off Hawke Bay and Bay of Plenty, tephra maybe redeposited by turbidity currents.

7. Erosion of tephra by currents is inferred from the absenceof deposits in areas of strong water motions and low sedimentsupply (e.g., crest of Chatham Rise, Hawke Bay, and westernBay of Plenty shelves). Less circumstantial evidence ofcurrent winnowing is the increase in grain size of KawakawaTephra into the path of the DWBC off eastern New Zealand.

ACKNOWLEDGMENTS

We wish to thank Dallas Mildenhall (Institute of Geological andNuclear Sciences) for supplying data on the Kawakawa Tephradeposited on Chatham Island, Peter Kamp (University of Waikato)for electron microprobe analyses of glass shards from ChathamRise, and Brad Pillans (Victoria University of Wellington) forcritically reviewing the manuscript. Special thanks to a journalreferee, Dr Sarna-Wojcicki, for a particularly penetrating critiqueof the paper. Word-processing was by Rose-Marie Thompson anddrafting by Peter Bennett. The project was in part financed by theFoundation for Research, Science and Technology (programme92 503 32 257; Obj. 4).

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