03-Late Neogene unconformity bounded tuffaceous sequences Northwestern Chatham Rise New Zealand - Enfermagem (2024)

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Full Terms & Conditions of access and use can be found athttps://www.tandfonline.com/action/journalInformation?journalCode=tnzg20New Zealand Journal of Geology and GeophysicsISSN: 0028-8306 (Print) 1175-8791 (Online) Journal homepage: www.tandfonline.com/journals/tnzg20Late Neogene unconformity‐bounded tuffaceoussequences: Northwestern Chatham Rise, NewZealandPhilip M. Barnes & Philip A. R. ShaneTo cite this article: Philip M. Barnes & Philip A. R. Shane (1992) Late Neogeneunconformity‐bounded tuffaceous sequences: Northwestern Chatham Rise, NewZealand, New Zealand Journal of Geology and Geophysics, 35:4, 421-435, DOI:10.1080/00288306.1992.9514537To link to this article: https://doi.org/10.1080/00288306.1992.9514537Published online: 23 Mar 2010.Submit your article to this journal Article views: 232View related articles https://www.tandfonline.com/action/journalInformation?journalCode=tnzg20https://www.tandfonline.com/journals/tnzg20?src=pdfhttps://www.tandfonline.com/action/showCitFormats?doi=10.1080/00288306.1992.9514537https://doi.org/10.1080/00288306.1992.9514537https://www.tandfonline.com/action/authorSubmission?journalCode=tnzg20&show=instructions&src=pdfhttps://www.tandfonline.com/action/authorSubmission?journalCode=tnzg20&show=instructions&src=pdfhttps://www.tandfonline.com/doi/mlt/10.1080/00288306.1992.9514537?src=pdfhttps://www.tandfonline.com/doi/mlt/10.1080/00288306.1992.9514537?src=pdfNew Zealand Journal of Geology and Geophysics, 1992, Vol. 35: 421-4350028-8306/92/3504-0421 $2.50/0 © The Royal Society of New Zealand 1992421Late Neogene unconformity-bounded tuffaceous sequences: northwesternChatham Rise, New ZealandPHILIP M. BARNESNew Zealand Oceanographic InstituteNational Institute of Water and Atmospheric Research Ltd.P. O. Box 14 901, KilbirnieWellington, New ZealandandDepartment of GeologyUniversity of CanterburyPrivate BagChristchurch, New ZealandPHILIP A. R. SHANEResearch School of Earth SciencesVictoria University of WellingtonP. O. Box 600Wellington, New ZealandAbstract Unconformity-bounded, late Miocene to Recentsedimentary sequences produced by fluctuating paleoceano-graphic conditions on the northwestern Chatham Rise havebeen sampled in a series of piston-cores from exposures atwide, mid-bathyal, oblique to slope, current-scour channelsand from submarine canyons at the head of the adjacentHikurangi Trough. The biostratigraphic framework for thesemostly hemipelagic sequences is based on foraminiferan andnannofossil dating of the cores. Volcanic glass-rich horizons(tephra), with glass shards of calc-alkaline rhyolitic composi-tion, occur commonly in late Opoitian to Haweran (latePliocene - late Pleistocene) sediments. Although the physicaloceanography and sedimentary processes of the region areunfavourable for preserving megascopic tephra, five chemi-cally and stratigraphically distinct tuffaceous horizons arerecorded in Pleistocene cores, implying at least five discreteeruptions. Two late Pleistocene tuffaceous horizons arecorrelated tentatively with Layer E (c. 0.27 Ma) and Layer D(= Mt Curl Tephra; c. 0.35 Ma) in several southwestern Pacificdeep-sea cores. Other tuffaceous horizons contain two or morechemical populations of glass shards that have been mixed andreworked by extensive bioturbation and current winnowing,during periods of very slow sedimentation. Distances of 460-600 km between the core sites and source vents in theCoromandel Volcanic Zone (early Pliocene) and TaupoVolcanic Zone (Pleistocene) imply very explosive eruptions.Keywords late Neogene; sedimentary sequences; seismicreflection; currents; cores; biostratigraphy; glass chemistry;rhyolitic tephra; Chatham RiseG92019Received 27 April 1992; accepted 17 July 1992INTRODUCTIONThe gently dipping (1-6°) northwest slope of the ChathamRise extends for 80 km from Mernoo Bank on the rise crest tothe southern part of the Hikurangi Channel about 60 km off thenortheastern South Island coast (Fig. 1). The region lies withinthe broad zone of deformation associated with the NewZealand plate boundary, and is undergoing extensionalfaulting, which resumed in the late Neogene after a period ofPaleogene and early Neogene quiescence (Lewis et al. 1986;Wood et al. 1989). The slope lies north of Mernoo Saddle—the 580 m deep depression between the Chatham Rise and theSouth Island—and has been the site of a complex, Pliocene-Pleistocene sedimentation history, involving the waxing andwaning of regional-scale, mid-bathyal currents sweepingobliquely across the slope (Barnes 1992). High-resolutionseismic profiles and piston cores reveal that numerous,unconformity-bounded, Pliocene-Pleistocene sequences onthe slope are the result of alternating, climatically influencedepisodes of current erosion, with sediment drift aggradationand hemipelagic sedimentation. The cores were obtained froma number of stratigraphically different sequences exposed atthe seabed in areas of late Quaternary current scour.Volcanic ash (tephra) is common in Pliocene-Pleistocenedeep-sea sediments elsewhere in the Southwest Pacific, atsites up to 1100 km east and southeast of volcanic centres incentral New Zealand (Ninkovich 1968; Watkins & Huang1977; Froggatt et al. 1986). Tephras have been correlatedbetween cores, and with tephras exposed onshore in NewZealand, by integrating magnetostratigraphy, fission-trackages, and chemical finger-printing (Watkins & Huang 1977;Froggatt 1983; Froggatt et al. 1986; Shane & Froggatt 1991).The cores of Pliocene-Pleistocene sediments from thenorthwest Chatham Rise slope also contain a significantamount of volcanic glass in tuffaceous horizons.This paper briefly outlines the late Neogene sequencearchitecture of the northwestern Chatham Rise slope, andpresents the biostratigraphy and sedimentology of the cores.We also present here the chemistry of various glass-rich(tuffaceous) horizons and identify the likely source region ofthe eruptions. We discuss the sedimentary aspects of volcanicglass accumulation and reworking in this deep-sea, current-swept environment, and attempt to correlate tuffaceoushorizons (within the biostratigraphic framework) with tephrain other deep-sea cores and with onland sequences nearer thesource.DEEP-SEA PLIOCENE-PLEISTOCENESEQUENCES: PRODUCTS OF ALTERNATINGCURRENT SCOUR AND DEPOSITIONThe regional, late Quaternary sedimentary processes of thenorthwestern Chatham Rise slope and southern HikurangiTrough have been examined by 3.5 kHz echocharactermapping (Barnes 1992). The Hikurangi Trough is a sink for422 New Zealand Journal of Geology and Geophysics, 1992, Vol. 35-40°STASMANSEAChannelCoromandel , r x / 7Peninsula (UVZ-PACIFICOCEANMernoo Chatham Rise _. _M e r n o o B a n k '"'""' "'"' '"' •iui«"i««" -™™™» >»• -mm Chatham Is.Saddl* X ^DSDP 594*1, •?175°E 18O°Fig. 1 Regional physiographyand location of the study area.TVZ, Taupo Volcanic Zone; CVZ,Coromandel Volcanic Zone;STCZ, Subtropical ConvergenceZone. The bold line with teethmarks the plate-boundary deform-ation front of the offshoreHikurangi margin and onshoreAlpine Fault.voluminous turbidite deposits. The turbidites have beenchannelled through canyons on the north Canterbury con-tinental slope and in eastern Cook Strait, and fed into theHikurangi Channel in the axis of the trough (Fig. 2) (Lewis1980; Herzer 1981; Carter et al. 1982; Barnes 1992).On the North Chatham Slope north of Mernoo Saddle,there are four associations of echotypes representing differentlate Quaternary sedimentary processes (Fig. 2). These include:(1) large, elongate areas of current-scoured and winnowedseafloor; (2) several coalescing sediment drifts between anddownslope of current-scoured areas; (3) several steep,irregular areas of the lower slope east of the sediment drifts,resulting from combined alongslope current activity anddownslope mass-wasting processes; and (4) areasof pre-dominantly hemipelagic sedimentation on the mid and upperslope. The mid-bathyal currents are inferred to be pre-dominantly Antarctic Intermediate Water (AIW) that risessouth of the Chatham Rise, becomes entrained within theSubtropical Convergence Zone, flows northward throughMernoo Saddle, and then diverges down and across the NorthChatham Slope. The flows and associated seabed erosion arethought to have been intensified during periods of lowerglacio-eustatic sea level (Barnes 1992).From high-resolution seismic-reflection profiles, 12mappable sedimentary sequences (c. 10-40 m thick) above aregional early Pliocene onlap surface (L) are recognised(Fig. 2, 3). The sequence boundaries are represented byerosional unconformities and onlapping seismic reflectors.Sequences are referred to as numbers 1 to 13, and theirboundaries by letters A-L and LM. Most of the sequences areexposed at the seafloor in areas of late Quaternary currentscour (Fig. 2). It is this favourable exposure that enabledseveral sequences to be sampled using conventional piston-coring equipment.Stratigraphically below surface L there is a downslope-thickening sequence of strong, parallel to downslopediverging reflectors overlying a mid-lower slope wedge ofweaker reflectors (Fig. 3A). The boundary (LM) betweenthese two sequences, shown below to be of late Miocene age,is exposed in the axis of Pukaki Canyon close to the position ofcore S871 (Fig. 2). A packet of strong, parallel reflectorsbelow this lower wedge has been traced in airgun profiles tothe Canterbury shelf, where it represents the Oligocene AmuriLimestone (Lewis et al. 1986; Wood et al. 1989).The Cenozoic sequences on the slope are characteristic ofthe Chatham Rise, thickening downslope and being erosion-ally truncated on the rise crest, which has experienced a longhistory of erosion and nondeposition (Cullen 1980; Wood etal. 1989).CORING AND LABORATORY METHODSFifteen cores from the northwest Chatham Rise slope werecollected in 1988 and 1989 using a modified KullenbergBarnes & Shane—Pliocene-Pleistocene sediments, Chatham Rise 423Canyon/channel-levee &overbank turbidite associationCurrent scoured andwinnowed seafloorLate Quaternary current flow• Sediment driftwmim Combined current-controlledH sedimentation and massE8S8S8S2a wasting processesI I Hemipelagic & minorI I mass-flow depositsismic Uncon- N.Z.Sequence formity Stage?Late Wc-WqLate Wn-?early WeFault with active trace(bathymetric expression)_ Y Exposed Late Neogene Fault•" without bathymetric expressionFig. 2 Regional late Quaternary sedimentary processes of the northwestern Chatham Slope and southern Hikurangi Trough, with geologicalmap of exposed seismic sequences and active faults in areas of seafloor erosion. Positions of piston cores examined in this study are indicated.New Zealand stage symbols: Wo, Opoitian; Wp, Waipipian; Wn, Nukumaruan; We, Castlecliffian; Wq, Haweran.piston corer on R.V. Rapuhia. Sampling sites were chosenafter shipboard interpretation of seismic profiles, and they liein water depths ranging from 411 to 2830 m (Fig. 2). The coresare 69 mm in diameter and they range from 1.1 to 3.2 m long.A Shipek Grab sample was collected also from each site. Atsite S871, only a sample from the core head was recovered.In the laboratory, cores were split, photographed, anddescribed wet. One-half was retained for reference. Slices ofrepresentative lithologies were X-ray radiographed forexamination of fine-scale sedimentary structure, and 38samples (2-4 representative samples from each core) weretaken for grainsize and carbonate analyses. Grainsize was424 New Zealand Journal of Geology and Geophysics, 1992, Vol. 35Fig. 3 Examples of seismic pro-files of the northwest ChathamSlope showing seismic sequences,faults, and major sedimentary fea-tures. Positions of profiles areplotted in Fig. 2, and stratigraphyis summarised in Fig. 7. A, Airgunprofile 1; B, Interpreted 3.5 kHzprofile 2. C, Interpreted 3.5 kHzprofile 3. D, Interpreted 3.5 kHzprofile 4.HIKURANGITROUGHB0 m-|approx 0.1 km approx 1 km75 m-I3 kmdetermined by the pipette method, and CaCO3 by vacuumgasometric analysis. In addition, sand fractions were extractedfrom all cores at 100-300 mm intervals and examined so thatmicroscopic tuffaceous horizons could be identified foranalysis. For comparisons with relevant tephras in other deep-sea cores and nearer the source, the glass chemistry of shardsfrom 25 tuffaceous horizons in 8 cores was analysed by a Jeol733 electron microprobe. Methods and standards for the Jeol733 are described by Froggatt (1983). For microprobeoperating conditions we used an 8 nA current at 15 kV and a20 pm beam diameter. To determine ages, the foraminiferalfauna of 12 cores were analysed, along with the calcareousBarnes & Shane—Pliocene-Pleistocene sediments, Chatham Rise 425nannofossil flora of 6 cores. The foraminifera microfossil dataare archived on New Zealand Fossil Record File (SE42174/f 1,f3-8; SE42175/f3-6; SE42176/G-3). An attempt was made toestablish a magnetostratigraphy of the cores using a MolspinSpinner Magnetometer. However, the natural remnantmagnetisation intensities of pilot samples from five cores weretoo low to provide reliable data after stepwise cleaning bystandard alternating field methods.SEQUENCE LTTHOLOGIESLithological characteristicsCores are shown in Fig. 4-6, and their sequence stratigraphyappears in Fig. 7. Cores were recovered from the crests oflower slope sediment drifts (U645, U651, U653); from areasof mid-bathyal current erosion (U640-U643, U646, U650,U652, U654, U661) and turbidity current erosion (U649,S871); and from winnowed seafloor on the upper slope ofMernoo Bank (U647).The cores typically consist of unconsolidated to wellcompacted, greyish olive to greyish green mud with minoramounts of sand (Fig. 4, 5). Different cores have differentproportions of terrigenous detritus and planktic microfossils.Sand fractions consist of variable amounts of glauconite, clearglass shards, foraminifers, radiolarians, iron sulphides, andterrigenous detritals.For ease of description, we recognise four lithofacies onthe basis of texture and sedimentary structure, that areindependent of age: (1) hom*ogeneous mud; (2) laminatedsandy mud; (3) mottled silts and muddy sands; and (4) car-bonate sandy muds and muddy sands. In addition, cementedhorizons characterise some buried erosion surfaces. Contactsbetween facies are mostly gradational but some are sharp.hom*ogeneous mudsThese are the finest grained and most abundant sediments,representing 73% of total core recovered. Even in carefullycleaned core splits, they appear to be almost completelystructureless, greyish olive, light olive grey or dusky yellowgreen muds (Fig. 5A-B, 5D, 5F). However, X-ray radiographsshow that they are not always hom*ogeneous (Fig. 6A). Thetexture is predominantly clay with slightly less silt and veryminor sand (1-6% sand), although a few samples analysed arealmost pure silt (>80% silt), and one is a muddy sand (61%sand). Carbonate content is moderately low (5-20%).Bioturbation is thorough, destroying primary sedimentarystructure. Pyrite-filled polychaete worm tubes may be abun-dant (Fig. 6A), and some consolidated samples are boredwhere they are exposed at the seabed by erosion (Fig. 5D).Laminated sandy mudsGreyish olive or light olive grey sandy muds occur in threecores from eroded sequences (U642, sequence 6; U643,sequence 7; U661), forming 8% of total core recovered. Thetexture and structure is gradational with hom*ogeneous mudand with stratified sections of mottled silts and muddy sandfacies. The laminations are subtle, but clear in X-rayradiographs, and are defined by discontinous concentra-tions of silt and sand with intervening mud (Fig. 5C, 6B).Pyrite-filled worm tubes may be common, but bioturbationhas been insufficient to destroyprimary sedimentarystructures.Mottled silts and muddy sandsThese occur in six cores (U640, U642, U643, U650, U652,U654) from various, eroded, Pliocene-Pleistocene sequencesin gradational contact with both hom*ogeneous and laminatedmuds (Fig. 4). The texture is very heterogeneous and com-prises pockets, lenses, and irregular layers of muddy sand, silt,and mud, commonly producing a crude horizontal strati-fication (Fig. 5A-C, 6C). In 11 samples analysed, sandfractions range 1-62%, silt 19-38%, and clay 10-58%.Carbonate content is low (c. 5%). The high glauconite contentof sandy horizons produces a greyish olive green or darkgreenish grey colour.Carbonate sandy mud and muddy sandCarbonate sediments characterise one core (U647) from411 m water depth upslope from the strongly eroded seafloornorth of Mernoo Saddle (Fig. 2, 4). The core is from anirregular seabed morphology close to an area of exposed rockswhich are thought to be volcanic (Barnes 1992). The highcarbonate content of 40-59% results from an abundance ofdispersed shelf and slope shell fragments, typically 1-5 mmlong, including the gastropods Bathypoma parengonius,Cominella alertae, Scaphander otagoensis, and Uberellavitrea, the bivalve Sacella bellula, the scaphopod Dentaliumzelandicum, and fragments of echinoderm plates. The greyisholive sediment is relatively coarse grained (37-63% sand) andfaintly stratified, although fine-scale lamination is absent.Cemented horizonsCemented horizons up to 70 mm thick occur within two cores(U652, U653) at sequence boundaries (Fig. 4, 5F). In bothcores, the cemented horizons are gradational with underlying,compacted or well-consolidated Pliocene mud and muddysand. They represent late Quaternary erosion surfaces nowburied by soft, hom*ogeneous mud of the topmost sequence.The horizons are interpreted to represent in-situ submarinecementation and hardground formation on a current-sweptseafloor.SEDIMENTARY PROCESSESThe sedimentary processes of the region are discussed byBarnes (1992), who integrated both the sediment lithologiesand their high-resolution seismic characteristics. hom*o-geneous mud characterises cores of seismic sequence 1 as wellas large intervals of other Pliocene-Pleistocene cores fromeroded sequences further upslope (Fig. 2,4).The cores from older, Pliocene-Pleistocene sequencesfrom upslope of the drifts contain hom*ogeneous muds inassociation with other less common deposits. These sequencesexhibit the characteristics of hemipelagic deposition includingbioturbation, mixed terrigenous and biogenic compositions,fine grainsize, and inferred slow sedimentation (e.g., Doyle etal. 1979). They are inferred to have accumulated duringperiods of diminished and localised current activity. Thepresence and absence of lamination in the muds is largely afunction of the degree of bioturbation. The glauconitic sandyhorizons in the mottled silts and muddy sands are inferred tohave been redeposited in mass flows from the upper slopes ofMernoo Bank where carbonate and authigenic aprons exist(McDougall 1982), although textures of these deposits are nottypical of turbidites (Walker & Mutti 1973). Reworking of theSediment DriftU645 105 1Upper SlopeApronIJG47Eroded SeabedU842 U643 IJ6501.0-2 . 0 -ddbbb——__—.Z-Z-z-z-zGlass shard0 20 40%bbb\ b' b-z>z-z-z--z-Glass shardo 20 40%Glass shard0 20 40%Glass shard0 20 40%Glass shardo 20 40%Wq< «JZ> I•0 bMr Zi^ z^ zeaaaaIK—"—--~ ~(TVim\0***(411m)?Wq(1885m)WqWqEroded SeabedI «19Glass shard0 20 40%(828m)Wqfaaaaa~Z—- z -- z --Z-~z-Z-***(2284m)Wc-Wq(1300m)Wqu1.0 -2 .0 -aaaa* -?-I-—Z~(1332m)Wc-WqSMilIGlass shardo 20 40%ara-z~—_—t#*IJ64GGlass shard0 20 40%Kill)(1707m)Wn-i"" 'IJG61Glass shardWe(1408m)WnLEGEND'hom*ogeneous' mudsLaminated sandy muds•r Bioturtjation-~— Sharp contact- - - Gradational contact0 20 40%IJG54Glass shard0 20 40%IJG52Glass shard0 20 40%Edge ofDriftU653Glass shard0 20 40%(1801m)Wo(2830m)Wo-Wp(1260m)WoMottled silts & muddy sandsCarbonate sandy muds& muddy sandsCemented horizonsMicrofossil analysis, Glass chemistryanalysis(2830 m) Water depthGlass Shard Chemical Characteristics1. Single population2. Single population + anomalous shards3. Two populations4. Multiple populationsSS.55 l a!S" CJ d! *(1713m)Wo & Wqg8,oFig. 4 Sedimentary lithofacies, relative volcanic glass distribution, and glass shard chemical population characteristics in cores from the north Chatham Slope. Cores aregrouped according to New Zealand Stage ages (Wo, Opoitian; Wp, Waipipian; Wn, Nukumaruan; We, Castlecliffian; Wq, Haweran; see summary in Fig. 7). Majorsedimentary environments are indicated, along with sample positions analysed for biostratigraphy and glass chemistry. Glass shard percentages are concentrations in thesand fractions. Sediment colour codes: a, 10Y4/2 greyish olive; b, 5Y5/2 light olive grey; c, 5Y7/2 yellowish grey; d, 5GY5/2 dusky yellow green; e, 5GY3/2 greyish olivegreen; f, 5GY4/1 dark greenish grey.£§Barnes & Shane—Pliocene-Pleistocene sediments, Chatham RiseU650 U652 U643 U 640427B50-60-70-80-90-100-cmU643 U65308170-180-190-200-210-cmtFig. 5 Examples of lithofacies in cores. Positions of illustrated sections are shown on core logs in Fig. 4 by dotted lines. HM, hom*ogeneousmud; Mz & mS, mottled silt and muddy sand; LsM, laminated sandy mud; CsM & mS, carbonate sandy mud and muddy sand; CH, cementedhorizons.Fig. 6 X-ray radiographs of10 mm thick slices of cores illus-trating the three main lithofacies.Large, irregular white areas arefractures in the core slices. A,Early Pleistocene, bioturbated,hom*ogeneous mud with abundantsulphide-filled worm tubes. B,Late Pleistocene, laminated sandymud with discontinuous stratifi-cation. C, Early Pliocene, mottledsilts and sandy muds. See Fig. 4 forsection positions marked by dottedlines.U646 U643 U654428 New Zealand Journal of Geology and Geophysics, 1992, Vol. 35Fig. 7 Summary of late Neogenestratigraphy on the northwestChatham Rise. Seismic sequencesand reflection termination char-acteristics are indicated, alongwith the piston core stratigraphicpositions, key microfossil datums,and tentative tephra correlations.Dashed vertical lines from coresindicate uncertainty in seismiccorrelation resulting from faultingnear the core site. O, onlappingreflectors; T, truncation followedby onlap; C, conformable. Coreshave sampled only small parts ofsequences, hence, the ages ofsequence boundaries are not wellconstrained.PLEIST-OCENEPLIOCENEL. MIOCENEM.MIOCENEKapitianTongaporutuan1WaiauanIMai0 r5.06.010.512.0UJrocEt>WUJa.UJOCEN-in.,ANEWZEALANDSTAGEHaweran(Wq)Castlecliffian(We)Nukumaruan(Wn)Mangapanian(Wm)Waipipian(Wp)Opoitian(Wo)A.Ma0.41.23. 14.0PISTONCORES HZo>cm- U j J S -J651 U653U642U650 H^~t-U649 *U641U646 9U640IIII4U654J653U65210111213U§1 usX'-- KoO/TO/TO/TO/TO/TRELEVANTBIOSTRATIGRAPHICDATUMS- LAD Gephyrocapsa carribeanica- FAD Globorotalia hirsuta- Layer E tephra- Mt. Curl Tephra tephra- LAD Pseudoemiliania lacunosa- LAD Globorotalia tosaensisFAD Globorotalia truncatulinoidesFAD Gephyrocapsa sinuosa- ? FAD Globorotalia crassula- LAD Cibicides finlayl- FAD Globorotalia inflataS87113LM- FAD Globorotalia crassaconica- FAD Globorotalia puncticulata- FAD Globorotalia crassaformis- LAD Globoquadrina dehiscens• FAD Bolivinita quadrilateraB. pohanaBarnes & Shane—Pliocene-Pleistocene sediments, Chatham Rise 429deposits by weak bottom currents, incapableof seismicallyexpressed seabed scour and drift sedimentation, is evident bythe association of sandy mottles, discontinous lamination, andhom*ogeneous texture (e.g., Gonthier et al. 1984). Reworkingand dissolution of calcareous microfaunaand volcanic glass inthese deposits is described below.The hom*ogeneous cores from the crests of sediment driftsmay be comparable to the muddy contourite facies describedfrom Northern Hemisphere deep-water sediment drifts (e.g.,Stow & Holbrook 1984; Gonthier et al. 1984).BIOSTRATIGRAPHYThe biostratigraphic ages of cores are compatible with theyounging demonstrated by seismic stratigraphy (Fig. 7).Sequences are discussed below in order of decreasing age.Reflector LM(S871)Core S871 from the axis of Pukaki Canyon (Fig. 2) waspositioned very close to reflector LM, the base of the stronglyreflective sequence 13 on airgun profiles (Fig. 3). The samplecontains Bolivinita quadrilatera and B. pohana (first occur-rence (FO) at the base of, and in, the early TongaporutuanStage, respectively; Edwards 1987; Hornibrook et al. 1989). Italso contains Globoquadrina dehiscens (last occurrence (LO)at 9.2 Ma; Wright et al. 1985) and Globorotalia miotumida(sinistral), which also indicate an early Tongaporutuan (lateMiocene) age of 10.5-9.2 Ma.Sequence 13 (U653, U652, U654)Three cores recovered samples from stratigraphic positionsclose to the top of sequence 13. Each core containsGloborotalia puncticulata s.s., G. crassaformis, and G.crassaconica, taxa which first appear in the early OpoitianStage (early Pliocene, 5.0-3.6Ma; Hornibrook 1982; Edwards1987; Edwards et al. 1988), together with early Pliocene,Opoitian-Waipipian Stage G. subconomiozea (Scott et al.1990). Core U654 also contains more advanced Globorotaliainflata morphotypes and G. puncticuloides'1., and may be aslightly younger sample than U653 and U652. However, italso contains Cibicides finlayi so it is not younger thanOpoitian (Hornibrook et al. 1989). Thus, sequence 13 isinferred to extend (Fig. 7) from early Tongaporutuan (10.5-9.2 Ma) up to the late Opoitian - early Waipipian (c. 4.0-3.5 Ma, late Miocene - late early Pliocene).Although the seismic stratigraphy of the easternmost coresite (U661) is not established, the sample contains Globor-otalia crassaconica, G. subconomiozea, sinistral forms of G.crassaformis and advanced G. puncticulata, along with G.inflata. It is inferred to be late Opoitian-Waipipian in age.Sequences 12-11No cores were recovered from sequences 12 and 11, so theprecise ages of the sequences and their boundaries K and J areuncertain. The sequences fall within the period between earlyWaipipian and late Nukumaruan (c. 3.6-1.4 Ma).Sequence 10 (U640)Core U640 is inferred to be mid-late Nukumaruan (latePliocene - early Pleistocene, 2.4-1.2 Ma) age (Fig. 7). Itcontains well-developed specimens of Globorotaliacrassacarina and G. puncticuloides, atypical specimens of G.crassula, and specimens of the nannofossils Gephyrocapsasinuosa (FO in mid Nukumaruan; Edwards 1987) andGephyrocapsa oceanica group. Globorotalia crassaformis(thought to terminate in the Nukumaruan; Scott et al. 1990)does not occur in the sample. Reworked late Eocene-Miocenenannofossils are present in small numbers.Sequence 9 (U646)Core U646, positioned to sample sequence 9, containsGloborotalia crassula, G. puncticuloides, G. crassacarina,and G. truncatulinoides tosaensis (well developed in the mid-late Nukumaruan; Scott et al. 1990), along with specimens ofGephyrocapsa oceanica group, G. sinuosa, and Pseudo-emiliania lacunosa. Its age is inferred to be mid-lateNukumaruan Stage. Small numbers of reworked late Eocene-Miocene nannofossils occur.Sequence 8 (U641, ?U649)Specimens of both Globorotalia truncatulinoides and G.crassacarina occur in U641, which was positioned to samplesequence 8, indicating a late Castlecliffian age for thesequence. Approximately 90% of G. truncatulinoides in coreU649 have a keel developed on the last chamber, whichsuggests that the sample lies close to the transition to G.truncatulinoides truncatulinoides, thought to occur in theCastlecliffian Stage (1.2-0.4 Ma) in the New TjesbsnA region(Scott et al. 1990). The samples do not contain Pseudoe-miliania lacunosa (LO at 0.46 Ma, late Castlecliffian;Edwards 1987) or Gephyrocapsa carribeanica, although theabsence of the latter may be due to postdepositional dis-solution of the species rather than deposition occurring after itsextinction at 0.2 Ma (Edwards 1987). Thus, sequence 8 isinferred to be late Casdecliffian or earliest Haweran (mid-latePleistocene) age. Very minor reworking of Miocene nanno-fossils is recorded.Sequence 7 (U650, U643)Over 80% of specimens of Globorotalia truncatulinoides arekeeled in core U650, and Pseudoemiliania lacunosa is absent.These indicators, together with the seismic stratigraphicposition of the sequence and the possible presence of Mt CurlTephra in core U650 (see below), suggest a Haweran (latePleistocene, <0.4 Ma) age for sequence 7 (Fig. 7). It is inferredthat the overlying unconformity F is close to c. 0.3 Ma. If so,Gephyrocapsa carribeanica is unexpectedly absent fromU650, but this is inferred to be due to the substantial degree ofcorrosion resulting in reduced nannofossil content and lowdiversity in the sample. Minor amounts of reworked, mainlyOligocene nannofossils occur.Sequence 6 (U642)The presence of Globorotalia truncatulinoides (nearly allkeeled), and the absence of Pseudoemiliania lacunosa, G.crassacarina, and the late Quaternary entrant Globorotaliahirsuta, together with the seismic stratigraphic position of thesequence and tentative correlation with a 0.27 Ma deep-seatephra, layer E (see below), suggests an early-mid Haweranage for core U642. The absence of Gephyrocapsacarribeanica may not be reliable due to sample corrosion.Sequences 5-2No cores were recovered from sequences 5-2; however, theyare constrained by enveloping sequences to a late Haweran(late Pleistocene) age.430 New Zealand Journal of Geology and Geophysics, 1992, Vol. 351175 77SiO2 wt%79Fig. 8 Compositional range displayed by 230 glass shards fromChatham Rise cores.Sequence 1 (U651, U645, U653)One of three cores from sequence 1 was examined formicrofossil content. A clear late Quaternary age for core U651is indicated by the presence of Emiliania huxleyi andGloborotalia hirsuta (FO 0.27 Ma and 0.23 Ma, respectively;Edwards 1987), and the absence oi Pseudoemiliania lacunosaand Gephyrocapsa carribeanica. This sequence is inferred tobe largely post-last glacial in age, based on an interpretationthat it represents a recent phase of reduced current activity.Reworked Oligocene nannofossils include excellentlypreserved specimens of Chiasmolithus altus, suggesting anearby source.VOLCANIC GLASSConcentration and mineralogyMegascopic tephra layers were not recognised in any core,but dispersed, clear, volcanic glass shards represent adominant component of the sand fraction in many samples(Fig. 4). In some tuffaceous horizons, we also found sparse,euhedral, ferromagnesian crystals, some of which haveadhering glass indicating a volcanogenic origin. Hyperstheneand green hornblende are identified, consistent with therhyolitic composition for the glasses from electron microprobeanalysis.Considerable amounts of glass occur in eight cores of lateOpoitian to mid-late Haweran (late early Pliocene - latePleistocene) age (Fig. 4). We have analysed those horizons inTable 1 Electron microprobe analyses of glass shards in cores from the Chatham Rise. Analyses are recalculated to 100% on a volatile freebasis and presented as a mean and standard deviation (in parentheses). Water by difference, n = number of shards analysed.SiO2A12O3TiO2FeOMgOCaONa2OK2OaH2OnSiO2A12O3TiO2FeOMgOCaONa2OK2OaH2OItSiO2A12O3TiO2FeOMgOCaONa2OK2OaH2On642/31A77.7412.340.171.430.141.113.953.000.185.858(.21)(.12)(.05)(.10)(.02)(.11)(.08)(.12)(.04)(.99)646/9176.1912.750.241.760.161.044.393.230.24(.20)(.12)(.06)(.07)(.02)(.07)(.09)(.07)(.09)6.19(1.56)7652/2374.9912.910.181.950.100.974.104.560.24(.44)(.28)(.05)(.21)(.02)(.14)(.19)(.38)(.03)6.13(1.28)10642/3 IB78.19 (.25)12.35 (.17)0.11 (.01)1.14 (.06)0.11 (.03)1.02 (.13)3.88 (.16)3.07 (.09)0.17 (.04)5.70 (.97)3646/11176.20 (.46)12.81 (.17)0.23 (.06)1.67 (.20)0.16 (.04)1.09 (.11)4.29 (.15)3.34 (.13)0.20 (.05)5.45 (.95)10661/2177.57 (.12)12.06 (.11)0.12 (.03)1.23 (.09)0.10 (.01)0.96 (.03)3.91 (.12)3.83 (.04)0.23 (.02)3.91 (.95)10642/10377.42 (.97)12.44 (.42)0.17 (.09)1.41 (.35)0.16 (.05)1.15 (.12)3.93 (.12)3.16 (.21)0.19 (.06)6.29 (.98)8640/8277.19 (.90)12.31 (.35)0.15 (.05)1.24 (.28)0.12 (.04)0.98 (.19)4.06 (.28)3.75 (.26)0.21 (.04)5.53(1.18)10661/3376.65(1.16)12.44 (.49)0.17 (.07)1.47 (.38)0.13 (.06)1.08 (.25)4.08 (.24)3.79 (.38)0.22 (.05)3.99 (.67)8642/13178.0412.170.151.260.121.073.923.080.204.7010(.25)(.16)(.03)(.14)(.02)(.05)(.08)(.09)(.04)(•99)654/0377.6012.270.141.240.131.083.953.440.21(.19)(.17)(.02)(.18)(.04)(.11)(.20)(.52)(.03)5.79(1.84)10661/4177.03(1.25)12.310.151.530.131.093.983.580.20(.43)(.08)(.52)(-07)(.23)(.21)(.25)(.03)4.64(1.01)8642/16177.9812.200.131.410.111.073.823.090.195.958(.55)(.14)(.05)(.42)(.03)(.06)(.07)(.13)(.05)(2.06)654/2176.6912.570.161.430.130.984.233.620.216.1410(.72)(.26)(.05)(.25)(.03)(.13)(.23)(.10)(.05)(1.43)661/7676.3612.620.201.700.211.214.133.500.184.0510(1.50)(.50)(.09)(.54)(.12)(.37)(.23)(.37)(.05)(.54)642/18178.0212.210.131.110.121.083.873.240.225.61110(.02)(.11)(.03)(.10)(.04)(.08)(09)(.11)(.06);i.l4)654/7777.1012.680.141.350.151.364.043.020.187.909(.38)(.19)(.03)(19)(.03)(.08)(.15)(.09)(.06)(.88)661/10375.74(1.24)12.900.231.790.201.574.153.270.15(.47)(.08)(.35)(.09)(.25)(.23)(.34)(.05)5.73(1.48)12650/9177.6411.920.151.030.120.843.604.470.244.0410(.32)(.11)(.04)(.12)(.03)(.10)(.08)(.22)(.08)(•94)652/0475.7112.880.151.240.121.014.204.070.288.189(1.18)(.52)(.04)(.52)(.03)(.12)(.34)(.36)(.04)(1.15)661/13175.3512.940.271.930.271.584.233.300.166.039(1.32)(.44)(.09)(.33)(.06)(.28)(.15)(.17)(.03)(.03)650/18178.01 (.33)11.92 (.06)0.13 (.04)0.93 (.18)0.10 (.03)0.79 (.23)3.56 (.09)4.32 (.34)0.24 (.04)6.10(2.45)6652/1175.22 (.49)13.02 (.25)0.15 (.04)1.84 (.25)0.09 (.02)0.92 (.13)4.22 (.22)4.37 (.40)0.23 (.05)6.15(1.12)10661/13376.65(1.16)12.44 (.49)0.17 (.07)1.47 (.38)0.13 (.06)1.08 (.25)4.08 (.24)3.79 (.38)0.22 (.05)3.99 (.67)8650/21077.3112.030.150.980.131.113.554.470.274.367(-79)(.25)(.05)(.05)(.03)(.61)(.15)(.11)(.02)(1.36)652/2075.0212.980.171.850.110.924.074.620.276.1510(.38)(.21)(.03)(.13)(.02)(.06)(.14)(.23)(.04)(.88)Sample names = U core number/depth from top in centimetres.Analyses with large standard deviations (e.g., FeO >0.15 wt%, TiO2 and MgO >0.04 wt%) represent the means of two or more compositionalpopulations within the sample (e.g., Fig. 9).Sample 642/31 has been separated into two glass populations (A and B).Barnes & Shane—Pliocene-Pleistocene sediments, Chatham Rise 431o1.2.1.1 -1.0-0 .9 .0 .8 .0.7-0.6-A•++• +• • •° BDD+ 642/180o 646/91• 650/912.00.5 0.7 0.9 1.1 1.3 1.5 1.7 1.9FeO wt%CaOI .O "1.6-1.4-1.2-1.0-0.8-B* ***X661/1031.0 1.5 2.0FeO wt%2.5 3.01.8-1 1.6-OaO 1.4 -1.2-1.01.81.6-3^ 1.4-O 1.2-O1.0-1.0-0.6xX X661/1031.0 1.2 1.4 1.6 1.8 2.0FeO wt%2.2 2.4661/0761.00 1.50 2.00FeO wt%2.50Fig. 9 Characteristics of tuffaceous horizons in Chatham Rise cores as shown by individual shard analyses. A, Three horizons, eachconsisting of a single, discrete, compositional population. B, Horizon consisting of a large population and containing a chemicallyanomalous shard. C, Horizon consisting of two compositional populations. D, Multiple populations or a near-continuum within atuffaceous horizon.which glass shards are most abundant. These horizons are notevident on X-ray radiographs of core slices, which only revealtextural and structural features of the sediments (e.g., cf. Fig.4,6).Glass chemistryOver 230 shards were analysed by electron microprobe from26 tuffaceous horizons. All shards have calc-alkaline rhyoliticcompositions, with SiO2 content in the range 73.5-79.3 wt%,and total alkalis in the range 7.2-8.5 wt% (Table 1). Most haveNa/K > 1. On Harker variation diagrams (Fig. 8), the shardsform a unimodal grouping, indicating a single or closelyrelated source provenance with similar petrogenetic origins.Their calc-alkaline composition and stratigraphic age indicatetwo possible source regions within New Zealand: TaupoVolcanic Zone (TVZ) (Cole 1979; Froggatt 1983; Wilson etal. 1984; Shane & Froggatt 1991) and the now-extinctCoromandel Volcanic Zone (CVZ) (Skinner 1986; Briggs &Fulton 1990). No evidence is seen for eruptive products fromcontemporary, intraplate basaltic volcanic centres such as atTimaru (Duggan & Reay 1986) and the Chatham Islands(Grindley et al. 1977). We cannot differentiate a TVZ sourcefrom a CVZ source on the basis of glass composition.Only seven of the tuffaceous horizons examined havehom*ogeneous chemical populations of shards indicative ofsingle eruptive events. The rest are heterogeneous with two ormore populations of glass mixed together (Fig. 9). The glassshards in the tuffaceous horizons can be grouped into fourbroad classes (Shane 1991): (1) single, hom*ogeneous glasspopulations representing single eruptive events (Fig. 9A); (2) amajor glass population with a few anomalous shards, probablyreworked (Fig. 9B); (3) two discrete populations of glassshards inferred to represent separate eruptive events (Fig. 9C);and (4) multiple and indistinguishable glass populations thatindicate mixing of two or more eruptive events (Fig. 9D). Thedistribution of these classes in cores is shown on Fig. 4, andimplications of mixed glass populations are discussed in thesection on Pliocene-Pleistocene tephra deposition on acurrent-sweet seafloor (below).Because of the degree of mixing of different eruptiveproducts in the cores, it is uncertain how many eruptive eventsare recorded. For Pleistocene-aged sediments, we haverecognised five, chemically distinct horizons or zones, basedon a dominant glass population within the samples. Threechemically distinct horizons occur in core U642 at (1) 0.31 mdepth; (2) 1.31 m depth; and (3) 1.81 m depth. A fourth432 New Zealand Journal of Geology and Geophysics, 1992, Vol. 35chemically distinct horizon occurs at depths of 0.91 m and1.11 m in core U646, and a fifth horizon occurs at 0.91 m incore U650. Because each horizon is chemically and strati-graphically different, five major eruptive events are recorded.In addition, a number of smaller eruptive events are recordedas anomalous shards within and between these horizons.CorrelationsSeveral authors have attempted to correlate Pleistocene tephraexposed in onshore sequences in New Zealand with deep-seacores up to about 1000 km away in the Southwest Pacific,using magnetostratigraphy, biostratigraphy, fission-trackages, and more recently by chemical fingerprinting of glassshards (Watkins & Huang 1977; Froggatt 1983; Froggatt et al.1986; Nelson et al. 1986; Shane & Froggatt 1991). Using thesequence stratigraphy established for the North ChathamSlope (Fig. 7), and comparing the glass chemistry oftuffaceous horizons in our cores with tephra in other deep-seacores and in onshore sequences in New Zealand, we can maketwo tentative correlations. These are (1) the late Pleistocenecore U650, 0.91 m depth, can be correlated with the wide-spread Mt Curl Tephra (referred to as Rangitawa Tephra byKohn et al. in press), variously dated at 0.23-0.38 Ma(Froggatt et al. 1986) but which is probably close to 0.35 Maold (Kohn et al. in press); and (2) the late Pleistocene coreU642, 1.80 m depth, with the c. 0.27 Ma Layer E tephra ofNinkovich (1968) and Watkins & Huang (1977).We compare glass compositions for correlation purposesusing similarity coefficients (SC) (Borchardt et al. 1971),based on all oxides except Cl, which is often invariant. Highvalues of SC (>0.92) are obtained for correlative samples. Thecompositions of tephras from elsewhere that were used forcorrelation purposes are shown in Table 2. An SC of 0.96 wasobtained for the match between glass at 0.91 m in core U650and the Mt Curl Tephra at its type section in the Manawatu-Wanganui area (Table 2). This tephra has a distinctive highK2O content and Na/K < 1 compared to other late Pleistocenetephras (Froggatt et al. 1986). The Haweran biostratigraphicage of core U650 supports this chemical correlation. Mt CurlTephra, the co-eruptive airfall equivalent of the WhakamaruIgnimbrite, has been recognised in Tasman Sea DSDP coreTable 2 Composition of tephras used in correlations to samplesfrom Chatham Rise cores. Analyses presented as in Table 1. Gridreferences from the metric NZMS 260 map series.SiO2A12O3TiO2FeOMgOCaONa2OK2OClH2OnLayer E178.1112.450.131.130.110.883.763.37-4.3521(.33)(.24)(.03)(.12)(.03)(.07)(.17)(-18)(69)MatahinaIgnimb'rite277.3812.540.121.070.110.883.913.830.185.04 <10(.45)(.27)(.02)(.07)(.02)(.05)(.15)(.17)(.02)(1.95)Layer D377.9312.260.120.870.120.783.534.38-(.28)(.17)(.03)(.11)(.02)(.05)(.14)(.18)4.38(1.03)18MtCurlTephra478.09 (.30)12.33 (.21)0.14 (.03)1.01 (.08)0.12 (.02)0.79 (.04)3.30 (.16)4.33 (.15)-4.55 (.13)132!Core RC12-215, western Pacific lat. 35°28', long. 167°53.5'(Froggatt unpubl. data).2State Highway 38, Murupara, VI7/305993.3Core RC12-215 (Froggatt et al. 1986).4Mt Curl Road, Manawatu, S22/195345 (Froggatt et al. 1986).591 and Southwest Pacific cores (Watkins & Huang 1977;Froggatt 1983; Froggatt et al. 1986; Nelson et al. 1986). In thesouthwestern Pacific, the correlative of the Mt Curl Tephra isknown as Layer D. We obtain an SC of 0.94 for comparison ofLayer D in core RC 12-215.Core U642 is inferred to be stratigraphically younger thanU650 (Fig. 7). Glass shards in core U642 at a depth of 1.81 mrepresent a chemically distinct, single population, similar toLayer E in several deep-sea cores in the western Pacific,including RC12-215 (Table 2; SC = 0.95). An age ofc. 0.27 Ma for Layer E (Ninkovich 1968) is consistent with thestratigraphic position of core U642. Froggatt (1983) suggestedthe correlation of Layer E to the Matahina Ignimbrite in theBay of Plenty region of North Island. We obtained an SC of0.93 for the comparison between glass at 1.81 m in core U642and this ignimbrite.DISCUSSIONA late Neogene, high-resolution, deep-sea sequencestratigraphyThe fortuitous exposure of many Pliocene-Pleistoceneseismic sequences by severe late Quaternary current erosionenabled various sequences to be sampled using conventionalpiston-coring equipment. In other parts of the world, suchhigh-resolution, deep-sea sequence analyses have beenundertaken only by correlating seismic sequences to long drillcores (e.g., Feeley et al. 1990).The stratigrapically upwards increase in seismic impe-dance defining the base of sequence 13 is inferred to representan increase in terrigenous sedimentation. A similar increase interrigenous sedimentation associated with changes in seismicimpedance or sequence architecture have been recorded on thesouthern flank of the Chatham Rise at DSDP site 594 (Lewiset al. 1986; Nelson 1986) and on the South Island continentalmargin in the late Miocene (Wood et al. 1989). The lateMiocene (c. 9-10 Ma) age of reflector LM coincides with theonset of significant tectonic shortening within the NewZealand plate-boundary zone and regional uplift (Walcott1978).Above reflector LM on the North Chatham Slope, thePliocene-Pleistocene sequences are inferred to reflectpaleoceanographic fluctuations resulting from global climatecycles, interacting with slow tectonic subsidence of theMernoo Saddle. A glacio-eustatic control on the NorthChatham Slope currents is inferred from: (1) a presentpostglacial phase of reduced current erosion, thought to belargely post-last glacial; (2) an inferred intensification of thecurrents during glaciations as a consequence of a number offactors, including increased constriction of flow through theMernoo Saddle during low sealevel; and (3) a broad cor-relation between the periodicity of late Pleistocene sequencesand established marine oxygen-isotope stages.Limitations on stratigraphic resolutionThe resolution of the stratigraphy is presently limited byseveral factors. Six of the 13 seismic sequences recognisedwere not sampled (Fig. 7). In addition, the 1-3 m long coresrepresent only small sections of the c. 10-40 m thicksequences. Therefore, although the cores can be used to placethe sequences into a broad biostratigraphic framework, theydo not constrain the absolute lengths of time represented byBarnes & Shane—Pliocene-Pleistocene sediments, Chatham Rise 433individual sequences and their intervening unconformities,nor do they constrain the short-term sedimentation rates.Furthermore, there is some uncertainity for certain cores as tothe actual sequence penetrated, due to faulting near the coresite (Fig. 2).The maximum long-term sedimentation rates from lowerslope areas of sediment drift accumulation are c. 70-110 mm/ka (270-430 m since c. 3.8 Ma). Considering (1) the lateQuaternary rates of hemipelagic deposition on the easternNorth Island lower continental slope (c. 100 mm/ka; Lewis1980), (2) the Pliocene-Pleistocene rates at DSDP site 594,300 km south of the investigated area (25-150 mm/ka;Kennett & von der Borch et al. 1986), and (3) the Pleistocenerates of abyssal pelagic sedimentation up to 1000 km east ofNorth Island (<2 mm/ka; Watkins & Huang 1977), we suspectthat hemipelagic sediments accumulating upslope of sedimentdrifts on the North Chatham Slope were deposited at short-term rates in the order of c. 50-150 mm/ka. This rate impliesthat individual cores (1.1-3.2 m long) represent between c. 7and 64 ka of continuous sedimentation.Apart from the two, tentative, late Pleistocene tephracorrelations, the ages of the cores have been assigned exclu-sively on the basis of their calcareous nannofossil andforaminiferan biostratigraphy. Thus, they are tied to the NewZealand late Neogene microfossil zonations, which have beendeveloped largely from onshore sequences and from theTasman Sea DSDP core 284 (Beuetal. 1987;Edwards 1987).We cannot discount the possibility of local anomalies infirst and last appearances of significant taxa that might be dueto unfavourable paleoenvironmental conditions in the regionof subtropical convergence because, at the western end of theChatham Rise, the modem surface-water circulation patterns,and interactions between cool subtropical and subantarcticwater, are complex and variable, and the Subtropical Con-vergence Zone is not well defined (Heath 1976, 1985). Forexample, south of the S ubtropical Convergence Zone at DSDPsite 594, the planktic foraminifer Globorotalia inflata appearsat the Gauss/Matuyama boundary in the adopted magne-tostratigraphy, c. 1 Ma earlier than it does at DSDP site 593(Homibrook pers. comm. 1992). Also, G. crassula makes itsfirst appearance at that site in the late Nukumaruan, whereas,elsewhere in the New Zealand region, it is recognised as adatum close tothe base of the Nukumaruan Stage (2.40-2.15 Ma; Edwards 1987).The effect of bottom currents, even in areas of modifiedhemipelagic sedimentation upslope of major seafloor scourand sediment drift accumulation, has been to produce low-diversity, corroded nannofloras with loss of solution-pronespecies. Bottom currents are also inferred to be responsible forreworking the small quantities of Oligocene and Miocenenannoflora into the Pliocene-Pleistocene sediments fromnearby sources, probably on the upper flanks of Mernoo Bank(Herzer& Wood 1988).Pliocene-Pleistocene rhyolitic tephra deposition on acurrent-swept seafloorThe rhyolitic tephra deposited on the North Chatham Slope isinferred to be the airfall product of eruptions from two, closelyrelated source provences within the late Neogene, NorthIsland, calc-alkaline arc. The upper, late Opoitian (c. 4.0-3.6 Ma) part of sequence 13 (cores U652, U653, and U654)and the late Opoitian-Waipipian core U661 (Fig. 4) probablypredate the onset of volcanism within the TVZ c. 2.3-2.0 Maago (Lowe et al. 1988; Grindley et al. 1988) and, hence, theshards are inferred to be products of now-extinct rhyoliticcentres (Fig. 1) on the Coromandel Peninsula (Nelson et al.1986; Skinner 1986). The number of discrete eruptionsrecorded in the early Pliocene cores is uncertain becausemixing of separate eruptive products is common (Fig. 9) andbecause the detailed stratigraphic relationships between thecores is not known (they may or may not be chronostrati-graphically synchronous).In contrast to the early Pliocene cores, the late Pliocene -late Pleistocene (c. 1.6-0.25 Ma) cores from sequences 10-6(Fig. 7) contain the distal airfall products of at least fiveeruptions from the TVZ. These include possible co-eruptivecorrelatives of the late Pleistocene Whakamaru and MatahinaIgnimbrites. Rare to very minor amounts of glass occurthroughout the post-last-glacial age sequence 1 cores U645,U651, andU653 (Fig. 4,7). Some of this glass in sequence 1 isreworked from older sequences, and some may be airfallsprinklings from TVZ eruptions.Distances of 460-600 km between the volcanic sourceareas and the core sites imply that very explosive eruptionshave occurred since the early Pliocene. As the presentprevailing winds in the region are westerlies, the presence ofglass shards at these distances to the south of the sourcessuggest the ejection of material high into the atmosphere (e.g.,Nelson et al. 1986). Large, explosive, silicic eruptions areoften associated with caldera formation. Such calderas areknown from the TVZ (Wilson et al. 1984) and have recentlybeen found in the CVZ (Briggs & Fulton 1990). Rockcompositions from these areas are broadly similar to glassesfound in the Chatham Rise cores. Thus, we consider the latterto be distal products of these large eruptions.An important feature of volcanic glass occurrence on theNorth Chatham Slope is the absence of megascopic tephra inall cores. This is unusual for deep-sea cores from east of NewZealand spanning the interval from late early Pliocene to lateQuaternary. For example, of the 17 tephras identified byWatkins & Huang (1977) in South Pacific cores, 8 are mega-scopic and include the 5 tephras recognised by Ninkovich(1968). Elsewhere, in DSDP Leg 90 cores from the SouthwestPacific and Tasman Sea, 15 late Cenozoic, silicic, megascopictephras have been recorded (Nelson et al. 1986).One explanation for the absence of megascopic tephra incores from the North Chatham Slope is that the combinedphysical oceanographic and sedimentary processes made thisregion unfavourable for developing thick concentrations ofash on the seafloor, irrespective of the rates of airfall transportof glass to the area. The sediments are inferred to haveaccumulated at low rates, and bioturbation is extensive,producing predominantly hom*ogeneous and mottled sedi-mentary textures. In addition, significant bottom-currentactivity is clear from the multiple, regional-scale erosionsurfaces and sediment drifts evident in seismic profiles (Fig.3), from the sedimentary textures in cores, and from thepresence of corroded and reworked microflora. Thesecombined processes have the effect of disseminating andreworking the glass shards into the terrigenous and biogeniccomponents of the sediments, enabling multiple tephradeposits to be mixed. Furthermore, the northwest ChathamRise slope lies due south of the inferred volcanic sources,which is probably not favourable for maximum fallout,considering the prevailing westerly winds. Some of theeruptions recorded may not have been of sufficient magnitudeto produce visible ash at the site, although this clearly does notaccount for the microscopic occurrence of Mt Curl Tephra,434 New Zealand Journal of Geology and Geophysics, 1992, Vol. 35which is 100 mm thick at DSDP site 594 to the south (Fig. 1)(Froggattetal. 1986).CONCLUSIONS1. The foraminiferan and nannofossil biostratigraphy of lateMiocene - late Quaternary piston cores from the northwestChatham Slope provides a framework for interpretation ofhigh-resolution seismic stratigraphy.2. A regional increase in the strength of seismic reflectorsoccurs at the base of sequence 13 (9-10 Ma). This reflectsan increase in terrigenous sedimentation accompanyingthe onset of shortening across the Pacific and Australianplate boundary in the New Zealand region.3. Pliocene-Pleistocene cores from 7 out of 13 uncon-formity-bound sequences consist predominantly of bio-turbated, hemipelagic mudstone with minor but variableamounts of volcanic glass, and resedimented glauconiticsand. They exhibit the textural characteristics of sedimentsdeposited at slow rates under the influence of variablebottom currents. The structures of cores, and inferredsedimentary processes, are consistent with seismicstratigraphic interpretations.4. No megascopic tephra occur in any cores, but the relativedown-core concentrations of glass indicate prominentfluctuations in the rate of tephra accumulation during thePliocene-Pleistocene. Individual tuffaceous horizons maycontain the airfall deposits of one or two eruptions, or ofmultiple eruptions mixed together. The physical ocean-ography, slow sedimentation, and extensive bioturbationin the region produced unfavourable conditions forpreserving megascopic, Pliocene-Pleistocene tephra.5. Early Pliocene (late Opoitian) glass shards originated fromthe now-extinct Coromandel Volcanic Zone, whereasPleistocene ash, including probable co-eruptive cor-relatives of the Whakamaru Ignimbrite (Mt Curl Tephra;c. 0.35 Ma) and Matahina Ignimbrite (Layer E; c. 0.27 Ma)originated from the Taupo Volcanic Zone.6. High-resolution, deep-sea sequence stratigraphic analysisis possible using conventional piston cores and high-frequency seismic profiles in favourable settings thatpreserve repeated episodes of current erosion anddeposition.ACKNOWLEDGMENTSWe thank the officers and crew of R.V. Rapuhia and NZOI staff whoassisted on cruises 2019 and 2030. Foraminifera analyses wereperformed by G. H. Scott of DSIR Geology and Geophysics, and thenannofossil analyses were by A. R. Edwards of StratigraphicSolutions (we take responsibility for stratigraphic interpretation ofthe data). P. C. Froggatt of Victoria University of Wellington kindlyprovided the unpublished analysis of Layer E; and K. Palmer of theVictoria University Analytical Facility assisted with microprobeanalyses. We thank N. de B. Hornibrook, G. H. Scott, K. B. Lewis,P. C. Froggatt, and J. R. Pettinga for comments on a draftmanuscript. Journal referee James Kennett and one other anony-mous reviewer also provided useful suggestions. The figures weredraughted by K. Majorhazi. Barnes was funded for this research,which also forms part of a Ph.D programme at the University ofCanterbury, Christchurch, by the Foundation for Research Scienceand Technology.REFERENCESBarnes, P. M. 1992: Mid-bathyal currentscours and sediment driftsadjacent to the Hikurangi deep-sea turbidite channel, easternNew Zealand: evidence from echocharacter mapping.Marine geology 106: 169-187.Beu, A. G.; Edwards, A. R.; Pillans, B. J. 1987: A review of NewZealand Pleistocene stratigraphy, with emphasis on themarine rocks. In: Itihara, M; Kamei, T. ed. Proceedings ofthe first international colloquium on Quaternary stratigraphyof Asia and Pacific area, Osaka, 1986. Pp. 250-269.Borchardt, G. A.; Havard, M. E.; Schmitt, R. A. 1971: Correlation ofvolcanic ash deposits by activation analysis of glassseparates. Quaternary research 1: 247-260.Briggs, R. M.; Fulton, B. W. J. 1990: Volcanism, structure andpetrology of the Whiritoa-Whangamata coastal section,Coromandel Volcanic Zone, New Zealand: facies modelevidence for the Tunaiti caldera. New Zealand journal ofgeology and geophysics 33: 623-633.Carter, L.; Carter, R. M.; Griggs, G. B. 1982: Sedimentation in theConway Trough, a deep near-shore marine basin at thejunction of the Alpine transform and Hikurangi subductionplate boundary, New Zealand. Sedimentology 29: 475-497.Cole, J. W. 1979: Structure, petrology, and genesis of Cenozoicvolcanism, Taupo Volcanic Zone, New Zealand—a review.New Zealand journal of geology and geophysics 22:631-657.Cullen, D. J. 1980: Distribution, composition and age of submarinephosphorite on Chatham Rise, east of New Zealand. Societyof Economic Paleontologists and Mineralogists specialpublication 29: 139-148.Doyle, L. J.; Orrin, H. P.; Woo, C. C. 1979: Sedimentation on theeastern United States continental slope. In: Doyle, L. J.;Pilkey, O. H. ed. Geology of continental slopes. Society ofEconomic Paleontologists and Mineralogists specialpublication 27: 119-129.Duggan, M. B.; Reay, A. 1986: The Timaru Basalt. In: Smith,I. E. M. ed. Late Cenozoic volcanism in New Zealand.Royal Society of New Zealand bulletin 23: 264-277.Edwards, A. R. 1987: An integrated biostratigraphy, magneto-stratigraphy and oxygen isotope stratigraphy of the lateCenozoic of New Zealand. New Zealand Geological Surveyrecord 23: 80 p.Edwards, A. R.; Hornibrook, N. de B.; Raine, J. I.; Scott, G. H.;Stevens, G. R.; Strong, C. P.; Wilson, G. J. 1988: A NewZealand Cretaceous-Cenozoic geological time scale. NewZealand Geological Survey record 35: 135-149.Feeley, M. H.; Moore, T. C.; Loutit, T. S.; Bryant, W. R. 1990:Sequence stratigraphy of Mississippi Fan related to oxygenisotope sea-level index. American Association of PetroleumGeologists bulletin 74: 407-424.Froggatt, P. C. 1983: Toward a comprehensive upper Quaternarytephra and ignimbrite stratigraphy in New Zealand usingelectron microprobe analysis of glass shards. Quaternaryresearch 19: 188-200.Froggatt, P. C.; Nelson, C. S.; Carter, L.; Griggs, G.; Black, K. P.1986: An exceptionally large late Quaternary eruption fromNew Zealand. Nature 319: 578-582.Gonthier, E. G.; Faugeres, J. C.; Stow, D. A. V. 1984: Contouritefacies of the Faro Drift, Gulf of Cadiz. In: Stow, D. A. V.;Piper, D. J. W. ed. Fine-grained sediments: deep-waterprocesses and facies. Geological Society special publication15: 275-292.Barnes & Shane—Pliocene-Pleistocene sediments, Chatham Rise 435Grindley, G. W.; Adams, C. J. D.; Lumb, J. T.; Watters, W. A. 1977:Paleomagnetism, K-Ar dating and tectonic interpretation ofupper Cretaceous and Cenozoic volcanic rocks of theChatham Islands, New Zealand. New Zealand journal ofgeology and geophysics 20: 425-468.Grindley, G. W.; Oliver, P. J.; Seward, D. 1988: Stratigraphy,geochronology and paleomagnetism of ignimbrites in theMatahina Basin, Taupo Volcanic Zone. Geological Societyof New Zealand miscellaneous publication 41a: 71.Heath, R. A. 1976: Oceanic circulation in the head of the HikurangiTrench, east coast, New Zealand. New Zealand journal ofmarine and freshwater research 10: 651-674.1985: A review of the physical oceanography of the seasaround New Zealand — 1982. New Zealand journal ofmarine and freshwater research 19: 79-124.Herzer, R. H. 1981: Late Quaternary stratigraphy and sedimentationof the Canterbury continental shelf, New Zealand. NewZealand Oceanographic Institute memoir 89: 71 p.Herzer, R. H.; Wood, R. A. 1988: The geology and structure ofMernoo Bank and surrounding area, western Chatham Rise.New Zealand Geological Survey record 29.Hornibrook, N. de B. 1982: Late Miocene to PleistoceneGloborotalia (Foraminiferida) from the DSDP leg 29, site284, southwest Pacific. New Zealand journal of geology andgeophysics 25: 83-99.Hornibrook, N. de B.; Brazier, R. C,; Strong, C. P. 1989: Manual ofNew Zealand Permian to Pleistocene foraminiferal bio-stratigraphy. New Zealand Geological Surveypaleontological bulletin 56: 175 p.Kennett, J. P.; von der Borch, C. C. et al. 1986: Site 594: ChathamRise. Initial reports of the deep-sea drilling project XC part1: 653-744.Kohn, B. P.; Pillans, B.; McGlone, M. S. in press: Zircon fissiontrack age for middle Pleistocene Rangitawa Tephra, NewZealand: stratigraphic and paleoclimatic significance.Palaeogeography, palaeoclimatology, palaeoecology.Lewis, K. B. 1980: Quaternary sedimentation on the Hikurangioblique-subduction and transform margin, New Zealand. In:Ballance, P. F.; Reading, H. G. ed. Sedimentation inoblique-slip mobile zones. International Association ofSedimentologists special publication 4: 171-189.Lewis, K. B.; Bennett, D. J.; Herzer, R. H.; von der Borch, C. C.1986: Seismic stratigraphy and structure adjacent to anevolving plate boundary, western Chatham Rise, NewZealand. In: Kennett, J. P.; von der Borch, C. C. et al. ed.Initial reports of the deep-sea drilling project XC part 2:1325-1327.Lowe, D. J.; Briggs, R. M.; Keane, A. J.; Itaya, T. 1988: Age of theKauroa Ash Formation, western North Island. GeologicalSociety of New Zealand miscellaneous publication 41a: 95.McDougall, J. C. 1982: Bounty sediments. New ZealandOceanographic Institute oceanic series 1:1 000 000.Nelson, C. S. 1986: Lithostratigraphy of deep-sea drilling project leg90 drill sites in the southwest Pacific: an overview. Initialreports of the deep-sea drilling project XC part 2: 1471-1489.Nelson, C. S.; Froggatt, P. C ; Gossan, G. J. 1986: Nature, chemistry,and origin of late Cenozoic megascopic tephras in leg 90cores from the southwest Pacific. Initial reports of the deep-sea drilling project XC part 2: 1161-1171.Ninkovich, D. 1968: Pleistocene volcanic eruptions in New Zealandrecorded in deep sea sediments. Earth and planetary scienceletters 4: 89-102.Scott, G. H.; Bishop, S.; Burt, B. J. 1990: Guide to some Neogenegloborotalids (Foraminiferida) from New Zealand. NewZealand Geological Survey paleontological bulletin 61:135 p.Shane, P. A. R. 1991: Remobilised silicic tuffs in middle Pleistocenefluvial sediments, southern North Island, New Zealand. NewZealand journal of geology and geophysics 34: 489-499.Shane, P. A. R.; Froggatt, P. C. 1991: Glass chemistry, paleo-magnetism, and correlation of middle Pleistocene tuffs insouthern North Island, New Zealand, and Western Pacific.New Zealand journal of geology and geophysics 34: 203-211.Skinner, D. N. 1986: Neogene volcanism in the Hauraki VolcanicRegion. Royal Society of New Zealand bulletin 23: 21-47.Stow, D. A. V.; Holbrook, J. A. 1984: North Atlantic contourites: anoverview. In: Stow, D. A. V.; Piper, D. J. W. ed. Fine-grained sediments: deep-water processes and facies.Geological Society special publication 15: 245-256.Walcott, R. I. 1978: Present tectonics and late Cenozoic evolution ofNew Zealand. Geophysical journal of the RoyalAstronomical Society 52: 137-164.Walker, R. G.; Mutti, E. 1973: Turbidite facies and faciesassociations. In: Middleton, G. V.; Bouma, A. H. ed.Turbidites and deep-water sedimentation. Society ofEconomic Paleontologists and Mineralogists Pacific shortcourse notes. Anaheim. Pp. 119-158.Watkins, N. D.; Huang, T. C. 1977: Tephras in abyssal sedimentseast of the North Island, New Zealand: chronology,paleowind velocity, and paleoexplosivity. New Zealandjournal of geology and geophysics 20: 179-198.Wilson, C. J. N.; Rogan, A. M.; Smith, I. E. M. 1984: Calderavolcanoes of the Taupo Volcanic Zone, New Zealand.Journal of geophysical research 89: 8463-8484.Wood, R. A.; Andrews, P. B.; Herzer, R. H. et al. 1989: Cretaceousand Cenozoic geology of the Chatham Rise region, SouthIsland, New Zealand. New Zealand Geological Survey basinstudies 3: 76 p.Wright, I. C.; Ashby, J. N.; Hoskins, R. H. 1985: An age for thesudden disappearance of Globorotalia dehiscens inMangapoike River Valley, New Zealand. New ZealandGeological Survey record 9: 102-104.