Ages on Weathered Plio-Pleistocene Tephra Sequences, Western North Island, New Zealand

Home , Mangakino, Ohinewai, Tokoroa

riwtioll: Lowe. D. ~.; TiP.I>CU. J. M.: Kamp. P. J. J.; Liddell, I. J.; Briggs, R. M.: Horrocks, 1. L. 2001. Ages 011 weathered Pho-~Je.stocene tephra sequences, western North Island. New Zealand. Ill: Juviglle. E.T.: Raina!. J·P. (Eds). '"Tephras: Chronology, Archaeology', CDERAD editeur, GoudeL us Dossiers de f'ArcMo-Logis I: 45-60.

Ages on weathered Plio-Pleistocene tephra sequences, western North Island, New Zealand

Ages de sequences de tephras Plio-Pleistocenes alteres, fie du Nord-Ouest, Nouvelle lelande

David J. Lowe·, J, Mark Tippett!, Peter J. J, Kamp·, Ivan J. LiddeD·, Roger M. Briggs· & Joanna L. Horrocks·

Abstract: using the zircon fISsion-track method, we have obtainedfive ages 011 members oftwo strongly-...-eathered. silicic, Pliocene·Pleislocelle tephra seql/ences, Ihe KOIIIUQ and Hamilton Ashformalions, in weslern North !sland, New Zealand. These are Ihe jirst numerical ages 10 be oblained directly on these deposils. Ofthe Kauroa Ash sequence, member KI (basal unit) was dated at 2,24 ± 0.19 Ma, confirming a previous age ofc. 1.25 Ma obtained (via tephrochronology)from KlAr ages on associatedbasalt lava. Members K1 and X3 gave indistinguishable ages between 1,68.±0,/1 and 1.43 ± 0./7 Ma. Member K11, a correlQlilV! ojOparau Tephra andprobably also Ongatiti Ignimbrite. was dated at 1.18:i: 0.11 Ma, consistent with an age of 1.23 ± 0.02 Ma obtained by various methodr on Ongaiiti Ignimbrite. Palaeomagnetic measurements indicated that members XI3 to XIJ (top unit, Waiterimu Ash) are aged between c. 1.2 Ma and O. 78 Mo. Possible sources of/he Kauroa Ash Formation include younger \!Oleanic centres in the sOllthern Coromandel Volcanic Zone orolder volcanic cenlres in the Taupo Volcanic Zone, or both. Ofthe Hamillon Ash sequence, Ihe basal member Ohinewai Ash (HI) was dQ/ed at 0.38: 0.04 Mo. This age malches those oblained by variollS methods on Rangilowa Tephra of0.34-0.35 Ma, supporting correlalion wilh Ihis Whakamaru-coldera derived deposit. The origin of the other Hamilton Ash beds is unknown but various younger \!Olcanic centres in the Tal/po /lolcanic ame an:possiblesources. The topmQ1t memlMr, 7ikotiko Ash (H6-H7), is estimaled to be agedbetween c. 0.18 and 0.08 Ma. /larious silicic pyroclastic deposits documented in North Island and in marine cores may be co-emf with members ofthe Kauroo Ash and Hamillon Ash sequences on the basis oftheir age.

Keywords: tephra, [usion-track ages. zircon. geochronology. brown ashes, Kauroa Ash, Hamilton Ash, Oparall Tephra, Ongatiti Ignimbrite, Ohinewai Ash, Rangitowa Tephra, Plio-Pleisrocene, Waikoto region. New Zealand.

Resume: en utilisanr la methode des traces de[ISS ion sur zircon. nouS avons obtenll cinq ages sur des membres de deux , s~uences de tiphras siliciquesjortement alteres du Pliocene final et du Pleistocene: les formalions de Kauroa Ash el de Hamilton Ash du North Island occidental, Nouloelle·Zilande. Cf! sont les premie~ ciges numeriques oblenus sur de teis dipiJts. ~ 10 siquencf! de Kauroo Ash. Ie membre KI (unite inferieure) a ete date de 2,14:l 0,29 Mo, conjinnant un age de 1,15 Ma obtenu amerieurement (par tephrochronologie) a partir d'ciges KlAr de COI11ees de basalte cwociees. Us membres K1 et K3 ant donne des ages compris entre 1.68: 0.11 and 1.43:l 0,17 Ma. u membre Kll, carrilati/dll OparolJ Tephra el proboblemenl allssi de l'Ongatiti Ignimbrite. a ete dati de 1,28 ± 0, II Ma, ce qui est consistant a\'ec IIfJ age de 1,13 :l 0.02 Mo, obleml par differelltes methodes Sill' I'Ongalili Ignimbrite. Des meSJll-es p<1leomagnetiques indiql/aienl que les membreJ' K13 to K 15 (Ullile supel'ieuI'e, Waitel'imu Ash) ont un age compris efllre environ 1,1 Mo and 0,78 Mo. Les sources possibles de laformation KawYJO Ash comprelllleni des cenlres volcaniques recents dollS 10 partie meridionole de la Zone /lo/conique CoromOlrdel, au des centres plus anciellS de 10 Zone Volcanique Tal/po. allies deux. Le membre basal de 10 sequence de Hamilton Ash (HJ) a ete date de 0.38 ± 0,04 Ma, ce qui corresponda 1'age de 0,34 0,35 Ma obtenu par differ-entes methodes sur le Rangitowa Tephra, Sllpportant ainsi la correlarion avec les depOts diri'llis de la caldera Whakamaru. L'origine des autres /its de 10 Hamilton Ash est inconnue, mois divers centres WJleaniques r«ents de 10 Zone Volcanique Taupo som possibles. L 'age du membre sommiral de 10 Tikotiko Ash (H6 H7), est €Stime entre 0.18 and 0,08 Ma. Divers depOts pyrodastiql/es siliciqllf!S signales dons 10 North Island el dons des caroues oceaniques devraient etre en correlation avec des membres des seqrlem:es de 10 Kauroa Ash el de to Hamilton Ash, sur la base de leur age.

Mors-clb: tephra. doratlollS, trace defission. ;:il1:0ll, gi!ochl'rmologie, cendre bl1l11, KOllroa Ash, Hamilton Asl1, Oparau Tephra, Ongatiti Ignimbrile, Ohinewai Ash, Rongiw",o Tephra. Plio-Pleistoce,Ie, region Waikalo, NOIll'elle Zilande.

1. Dep:utmenl of Eanh Scieoc". Uni~"ersity of Waikato. Pnvalc &ag J t05. H3ffilltoo. New Z(31:md TEPHRAS: chronology. archeology

1. Introduction 2, Stratigraphy of Kauroa and Hamilton Ash sequences Tephra deposits resulting from large-scale volcanic eruptions provide important stratigraphic markers that assist in correlating sequences in both terrestrial and ma 2.1, Kallroa Ash Formation rine environments, and in linking such sequences (e.g. Pillans et al., 1993; Carter el al., 1995; Naish et al., 1996; The Kauroa Ash Fonnation comprises a sequence, up to Shane el 01., 1996a). They also provide juvenile c. 12 Tn thick, of extremely weathered, clay-rich (c. 70 mineralogical components for numerical dating and hence 90% <2-J.trn clay), multiple tephra deposits and associated a means for independently testing the veracity of dates paleosols, together possibly with interbedded loessic based on biostratigraphy, magnetostratigraphy or other materials in places (Kirkman, 1980; Kamp & Lowe, 1981; methods (Naish et al., 1996; Newnham et al., I999a; Shane, Selby & Lowe, 1992; Briggs elal., J,994; Horrocks, 2000). 2000). In addition, tephras are useful as recorders of The formation was initially recognised and defined in the volcanism, particularly in distal environments (e.g. Kyle Waikato region by Ward (1967), with the type locality & Seward, 1984; Lowe, I988a; Shane et al., 1996a, 1998; recorded at 'Woodstock', near Raglan (Fig. I; Waterhouse Shane, 2000), because proximal, near-vent deposits are & White, 1994). (The name 'Woodstock' derives from the complex and typically 'incomplete' with successive original name of the adjacent farm.) The sequence is eruptive events tending to destroy or bury antecedent underlain by basaltic deposits of the Okete Volcanic For deposits (e.g. Krippner et al., 1998). mation inc.luding lava dated at c. 2.25 Ma by Briggs el 01. (1989), and overlain by the Hamilton Ash Formation, of Cn North Island, New Zealand, there is a record ofexplo which the lowermost member (H I) is dated indirectly at sive, rhyolite-dominated volcanism dating from c. 10 Ma c. 0.35 Ma by tephrochronology (see below). in the Coromandel Volcanic Zone (CVZ) (Adams el al., 1994; Shane et al., 1998) and from at least c. 1.6 Ma in the Two members of the Kauroa Ash sequence have been Taupo Volcanic Zone (TVZ) (Houghton el al., 1995; Wil individually defined. The uppermost is Waiterirnu Ash, son et al., 1995a; Krippner et al., 1998) (Fig. I). Despite named from the type locality at grid reference S 131095096 considerable research in recent years on the stratigraphy, near Waiterimu (Fig. 1; Ward, 1967). (Grid references here age, and composition oferuptives from CVZ and especially and elsewhere in the text are based on the metric I: 50,000 TVZ, there remain extensive, thick sequences of topographical map senes NZMS 260.) The other is Oparau Quatel11ary or older pyroclastic deposits (induding both Tephra, an ignimbrite named from the type locality at R15/ fallout tephras and ignimbrites) in the North Island with 797477 at Papakura Creek (Fig. IA; Pain, 1975; Fergusson, no known source and little or no age control. Many of 1986). Salter (1979) subsequently divided the Kauroa Ash these deposits, commonly weathered (sometimes very Fonnation at Woodstock into fifteen major members, Kl strongly), have been broadly mapped as portmanteau units (base) to KI5 (top), and this sequence, slightly modified termed 'undifferentiated brown ruffs' or 'brown ashes' following further work (Briggs el 01., 1994), is shown in (Fig. I; Pullar el 01., 1973). Detailed regional studies have Fig. 2. Distinctive sand-sized 'micaceous' or platy minerals commenced on such deposits in an attempt to document in beds K3 and Kl2 have been identified as kaolinite books their stratigraphic inter-relationships and to correlate them and stacks (Salter, 1979). with defined units, to determine their ages, and to identify their modes of emplacement and sources (e.g. Briggs el No direct radiometric ages have been obtained on any al., 1996; Manning, 1996; Newnham el aI., I999b). We members ofthe Kauroa Ash Formation. However, Briggs have begun similar work on two extended, Pliocene el 01. (1989) used tephrochronology to show that member Pleistocene sequences ofstrongly-weathered, silicic, tephra KI is likely to be aged c. 2.25 Ma because it occurs deposits in western North Island in the Waikato region, intercalated with co-eruptives of KJAr-dated alkali basalts namely the Kauroa Ash and Hamilton Ash beds (Fig. I). (basanite) of the Okete Volcanics at nearby Ohiapopoko In this paper we review the stratigraphy ofthese two long cone (2.26 ± 0.08 Ma) and Cleaves cone (2.25 ± 0.10 Ma) sequences. We then present new zircon fission-track (ZFT) of the Maungatawhiri centre (Fig. IA). The stratigraphic inter-relationships used to obtain this aoe for KI at age detenninations for four members of the Kauroa Ash . 0 sequence and for one member of the Hamilton Ash Oh13popoko are shown in Fig. 3. An age of< 1.81 Ma was sequence, the first radiometric ages to be obtained directly assigned to members K I2 (Oparall Tephra) and K 15 on these deposits. Our results show that the members KI (Waiterimu Ash) because these members ul1COnfOllnably (basal unit) to K 12 (Oparau Tephra) of the Kauroa Ash overlie co-eruptives or KJAr-dated alkali basalts (hawaiite) sequence were erupted between c. 2.25 and c. I 2 Ma and of the Ngatutura Volcanics at Foxs centre (1.81 ± 0.07 that member HI (Ohinewai Ash) of the Ha~ilton 'Ash Ma) in the northern Waikato region (Briggs et a/.. 1989; sequence was erupted c. 0.38 Ma. Some implications Fig. I). regarding possible eruptive sources and correlations arising from these new ZFT age determinations are discussed. 2,2, Hamilton Ash Formalioll

Our resulLs are pall ofan ongoing programme of research The Hamilton Ash Formation comprises a sequence, tip to 011 the Knuroa and Hamilton deposits including a revision c. 6 m thick. ofstrongly weathered, clay-textured (c. 60 of their stratigraphy, measurements of palaeomagnetic 85% <2-jJm clay) multiple rhyolitic tephra beds and polarity and magnetic susceplibility, and analyses of associated paleosols (Tonkin. 1970; Kamp & Lowe, 1981; various physical. mineralogical and geochemical propcl1ies Selby & Lowe 1992; Lowe & Percival, 1993; Briggs et together with further 2FT dating. Findings from these al.. 1994). The sequence may also contain interc(llaled studies will be reported later. loessic beds at some sites (e.g.. Bin'ell el al.. 1981). The fonllation. widespread throllghout the Waikato and Cora- Ages on lNf'at!1ered Plio-Pleistocene tephra sequences. \lVf"srern Norch Island, New Zealand

N t Coromandet Pacific Ocean / Volcanic Zone o sO 100 km \ I I I

Tasman

Sea :':-"="-KAPENGA (0.9 Me-60 ka)

~ ODP Site 1124 (600 km 9asl) New Plymouth

, Napier ISLAND

Rangiflkei GRrver c; Yianganul- ~. Ml Curl t::JJ f-KE-y------>--'0Rangitawa

('.:.,;....:] Urdff. weathered tephra beds· "...-";:: ':. ('brown ashes') aged c. 2.3-0.1 Me o Caldera and period of activity Inferred caldera(s) aged C? ~c.2.5Ma " Sampling/re/erence site , Pirongia Ie Muna 0;> ellington SOUTH I. Qtorohange OT

Figure 1. Generalised distribution of weathered Plio-Pleistocene tephra deposits ill North Is/and. New Zealand. and locations of possible ~'ollrce caldera volcanoes in Tal/po and Coromandel volcanic zones. Caldera age ranges after HOl/gh/oll et 01. (1995) and Wilson et 01. (19950): inferred calderas: a, 'Kalikati'; b, 'Te Puna '; c, 'Kaimai '; d, 'offshore BOP' (see text). Insets A and B sho\ll locations of2FTsampling sites at Woodstock and '0 Block', ,mel reference sec/iOIlS at B,:vant Home (BH). Te Uk/{ (TU). Ohiapopoko cone (OH), Cleaves cOile (eL), Papakura Creek (PC), and O/orohanga (OT). Locations ofother sections referred to in the lext are shown on rhe main map. ·AfterPul/ar et al. (1973) and Kohli el 01. (1992): nameddeposils (general OCCl/rrence. approximate age ranges) include HamUtoll Ash beds (Waikato-Coromalldel-Bay ofPlellty. c. 0.1-0.35 Maj. Rangilalt'a Tephra (Sal/them Norlll Island, c. 0.35 Maj. Kauroa Ash beds (Waiknlo-W Bay ofPlenty. c. 2.3-0.35 Maj, Pahoia Tephra beds (W Bay ofPlenty. c. 2.2-0.35 Ma). and Toblelands Tephra beds (E. Bay ofPlenty.

Depth (m) F.:=====-~011·'~'!"~'~' ~. J'8' k'- Post Hamilton Ash tephras ("'64"'4 ka) -- gE Tikotiko Ash :;:u.. Member E.c ",m 1 -irr.+.",-{ J:« HlIH2- Ohinewai Ash Mb. (Rangitawa T. 0.35 Ma)

Clay beds/paleosols K14b 24

4 :.:K:..:1~4=a~ffiTI''I¥i''ITr~;:--prominent 22 nodules K13 21 5 20 19 Very dark yell. red-brown clay, firm ::> ". 18 Orange clay, firm c: ",'" o ~ i: K12 Q. Q. 6 17 Creamy clay, halloysite nodules w ~ ~t±-==~~gg~~~~~~~~~~~~19202-051throughout. pink mottles at top :E f----+Trrrr'i'T'Ti\0 16 o K11 u.. -71----1,,; K10 15 .c ---t'f;1'r'i1"'if,TI: 14 m « Kg 13 --8-hocoLTor",,,-!nor-rJ' =12 K8b '"o 11 Clay beds/paleosols 10 9 8

~~T_L{g92~OQ2~-0Q§J8 Dark red-brown clay with '!! grey veins

Dark red-brown weathered scoria Ibasalt 14 (2.25 Ma)

~ > v

Figure 2. Slratigraphy oflhe Kauroa Ash and Marked paleosol otherforma(ions 01 Woodslock site (Fig. IA) al o 0 0 0 Q Halloysite nodules I1II11 (Bt honzons) RI4/783734. and sampling positions/or 2FT 'Micaceous' flakes ____ Boundary of intra- daring ofKauroa beds K/-K3 and K/2 bed units (ages obtained aloe listed in Table 3). After Briggs et 01. (/994, p. 32). ZFT sample Yell.= yellowish Ages on 'Neathered PlIO-Pleistocene cephra sequences. ~scem Norm Island. New Zealand 49

mandel regions. was initially defined by Ward (1967), marine cores. DSDP 594 and 50-36-61; Pillans el ClI., following Taylor (1933). with the type localily at RI41 1996). The validity of these ages was displlted by Vella & 813729nearTeUku(Fig.IA; Waterhouse & White. 1994). Vucetich (1995. 1996) and Vella (1997) (see also VlIcelich The sequence is underlain, usually with a marked erosional l1l1conformity, by the Kauroa Ash Formation. ofwhich the Depth 1m) - O',TII!lBTI.H"'~""·'--'P"'~-'-_H-.-m-,,,-~-c.-,,,-,-...-.-.,,,,,,,.,.,-~-c'-o-c••-C'-'-'M-..-.-..-c'-H uppermost member has an age of < 1.81 Ma as described ~ above. It is overlain by a composite group of younger. Tlli.olil.oAsl'lMb multi·sourced tephra fonnations. the basal one being the 1\lJ: I' H5 Roto~hu Ash Member of the Rotoiti Tephra Formation (Pain. 1975; Lowe. 1986. 1988b; Froggatl & Lowe. 1990). ~'fl ± Rotoehu Ash has an age of64 4 ka based on KlAr-dating ,0;J.~ . Clay becbfpaleosols ofencapsulating lavas (Wilson el ,iI.. 1992; Lowe & Hogg. 1 'h,_iLl 1995), but could be younger (c. 45 ka) according to Lian , H4 & Shane (2000). "7.""'~' \\\;ti::r Ward (1967) divided the Hamilton Ash Formation into il;UUi members numbered from HI (basal unit) to H9. However, a members H8 and H9 (fonnerly defined as the 'Mairoa Ash Member'. but now known to comprise the Rotoehu Ash and younger tephras) are redundant, and so the uppermost beds of the Hamilton sequence are H6 and H7 (top unit). These two beds together define the Tikotiko Ash Member, wilh the type locality recorded at SI6/030313 near Otorohanga (Fig. IA ; Ward. 1967). Commonly. the Tikotiko Ash and overlying composite tephra beds provide lllultisequal soil parent materials throughout the Waikato southern Auckland region (Selby & Lowe. 1992; Bakker el al.. 1996; Lowe. 2000).

The oldest member in the Hmnilton Ash Formation. numbered HI or H2 depending on locality. was defined as the Ohinewai Ash Member by Ward (1967) with the type section at SI3/018104 ncar Ohinewai (Fig. I). This member was renamed Ohinewai Tephra Formation by _'- 4 L __-'-"""·"·"~.!!·"·~"""·'- _ Vucetich eI ai, (1978) at the same locality. A possible correlative. Aratora Tephra Formation. was defined by • lFT sample VlIcetich el al. ( 1981) al Ahllroa Road (S 16/061138) near Te Klliti (Fig. I; see also Birrell el 01.. 1981). The Ohinewai Ash is characteristically pinkish- to brownish-grey in Figure 4, SI/'(/ligrap/~l' Q!",he HUllli1!o11 A,\·II amI ollIe" .j(muClfiu/l.\· £If u-S/rlck .I'ite Clf 5/4//43767, and .Wlmpling colour. silty. halloysitic. and contains distinctive sand-sized P().~ilicms.liJ/· ZFT dating o./Hal1lilron A,\II lied HI golden 'platy' minerals identified as 2: I: I inlerstratilied (age.\· obtained CII'l! li.\·feef ill u,ble 3). Smup'" 9]{)]-OJ/O!/O) micaceous-kaolinite intergrades (Shepherd. 1984; Lowe is (/ ('(JlJlpositt:. Ah/lllt!. : "hum/m". & Percival. 1993). A thin (C. 5 em) yellowish layer containing coarse sand-sized quartz crystals marks the base of the tephra (Fig. 4). el 01.. 1(96). but these criticisms were refuted by Kohn el al. (1996) and Pillans & Kahn (1998). No direct radiometric ages have been obt

Rangilawa Tephra Ohinewai Ash MtCurl (HI)' Tephra Anal. No. Z 2 1 4 5 6

SiO, 7R.20 (0.32) 77.34 (0.2R) 77.95 (1l.15) 7M.IS (1l.32) 77.lJl (0.22) 7R.1l9 (IUO) AI~6J 12.42 (0.13) 12.21 (1l.O

I Means and standard de.... iations (in parentheses) orn analyses (indi.... idual g.lass shards) nonnalized tu IIX)O/O loss-free (wt.%). Analys\.'S by Jeol JXA-733 electron microprobe at Analytical Facility. Vicloria Uni ....ersity or Wellington. using u IO..-20-~lm bl.:am. ~O-nA beam current. and accelerating vollage of 15 kV. with standards and other eondition$ as in Froggalt ( 19K3). na. nut analysed. ! All material from type locations (Fig. I): 1-3. Rangit

Method and ag~ Rcti::rCllce' (Ma ± 10')

Fbis;on trod.,) (1.35 ± 0.02 (11-4) lsothcnnal plateau (glass) 1.2.3 0.35 ± O.!)4 (II ;; 5) External detector (zin.::on) 2.3

0.345 ± 0.012 (II ;; lJ) Mean FT ag~ 3

Marilll! O.\:1'geI1 ;SO(Opl! .I'1l'aligraphy (aslrtJl/OII1ic'al t;IIJI!~'('(/II!J" 0.340 ± 0.007 (n "" 2) 3,4

¥/Arl JVAr (.I'illgle-('ryslallaserflls;onr O.3112± O.IXIS ('I: 4) 3

I Hendy el al. (19XO) reported 8 corrected U·Th disequilibria age on Fe-Ti oxide phuses (magnetite. ilmenite) extrdcted from HI (Ohinewai Ash) ofO.2(X) + 0.120 - 0.1)45 Ma (11 ;; 2). Using SHRIMP ~.l~Ur-""Pb. Brown & Fletcher (199lJ) derived

Table 2. Summary ojpl1!viollS radiomf!tr;c agf!.5 Oil Rallgihlll't/ Tl!phra ""d COI'I"f!/,,'il·e.\·I. surfaces. and polished lIsing alumina slurry followed by the HIFAR reactor. New South Wales. ALlstralia. Nominal I-J.tm diamond powder. Etching of spontaneous tracks in fluences of 3 x. IOl~ neutrons Cl"'~ for zircons were used. the zircon crystals took place in mohen KOH-NaOH eutectic solution at c. 2050 C for about 50 hours until all After irradiation, Ihe mica detectors were elched in 40% spontaneous tracks intersecting the polished surface were HF for 24 minutes at 240 C. FT density detenninations revealed. After etching, zircon mounts were cleaned by were undertaken using a Zeiss Axioplan"M optical micros immersion in dilute HF for 10 minutes. cope (x 1250. dry). Tracks were counted independently by two operators (JMT. IJL) and the zeta calibration method Low-uranium mica external detectors were sealed in applied throughout (Hurford & Green. 1983: Green, 14.)85: contiguous contact with the mounts using envelopes of Hurford & Watkins, 1987; see also Tippen & Kamp. I(93). heat-shrink plastic. The mounts, together with uranium Weighted mean zeta values (eN I) on zircon determined dosimeter glass eN I. were irradiated in the X-7 facility of by JMT and IJL are reponed in Table 3. Ages on weat~red Plio·Pleiscocene tephra sequences, \.Vf"stern North Island New Zealand 51

Because of anisotropic etching, only zircons with well age on the Gauss-Matuyama transition: Naish elo/., 1998) etched tracks in all orientations were counted. Seward & (Horrocks, 2000). The error-weighted mean ofall ages now Kahn (1997) recommended that, ideally, 15-20 crystals available for KI is 2.26 ± 0.06 Ma (n ~ 3). Although should be counted when dating Quaternary-aged tephras members K2 and K3 are not separable by ZFT ages alone, using the ZIT method. These limits were met in six ofthe KJ stratigraphically overlies K2 with an intervening paleosol nine individual assessments that were made (Table 3), and and therefore must be younger by possibly some tens-of so they are likely to provide realistic 'stable' ages, Ages thousands of years. The older end (c. 1.7 Ma) of the ZFT calculated for the remaining three assessments (in which age-range for members K2 and K3 suggests that one orother 9-13 crystals were counted) are statistically identical to might fall within the Olduvai (or Gilsa) Subchron, which those ofthe replicates, The final error-weighted mean ages has normal polarities within the Matuyama reversal. The (pooled data) are all based on >20 crystals (Table 3). age determinations are thus testable by palaeomagnetic measurement. 3.2. Results Member K12 of the Kauroa Ash Fonnation is considered Analytical data are reported in Table 3. FT ages were . equivalent to the Oparau Tephra, which in tum is probably calculated using the standard FT-age equation (Hurford & a correlative ofthe Ongatiti Ignimbrite (Pain, 1975; Salter, Green, 1983) with conventional errors (Green, 1981) 1979; Fergusson, 1986; Horrocks el al., 1999). Ongatiti quoted at ± Icr, as used by Kohn el al. (1992). The Ignimbrite. derived from the Mangakino Volcanic Centre probability of grains counted in a sample belonging to a (Fig. I; Briggs elal., 1993; Wilson el al., I995a), has been single population of ages is assessed by a chi-square well dated at 1.21-1.25 Ma using several independent tech statistic (Galbraith, 1981). A probability of less than 5% niques (Table 4). Shane el al. (I996a) suggested 'hat may indicate that the grains represent a mixed population, Mangatewaiiti tephra (dated at 1.23 ± 0.09 and 1.24 ± 0.07 Where P (X') <5%, a central age (Galbraith & Green, 1990) Ma) may be a distal tephra-fall correlative of Ongatiti is quoted in Table 3, Ignimbrite. The ZFT age of 1.28 ± 0.11 Ma obtained for KI2 is consistent with all these ages (they are statistically Samples 9202-03, 9202·01/02/03, 9202-05, 9202-06, and indistinguishable) and hence with the proposed 9202·07 all yielded track-density data with high chi-square correlations, Assuming that member K12 is a correlative values, indicating the probability of unimodal populations of the Ongatiti Ignimbrite (and Maungatewaiiti tephra), of ages. The duplicate age assessments for each sample then the error-weighted mean of all available ages all this are statistically indistinguishable from each other and hence deposit is 1.231 ±0.016Ma(n~8). were able to be pooled to provide error-weighted mean ages (Table 3), (All assessments ofage homogeneity using the test statistic T' are at the 5% level, P 0.05.) Sample 9202-08 'failed' the chi-square test suggesting that there may be two populations ofzircon grains, The single grain ages ranged from 0.6 ± 0.5 to 4.9 ± 1.4 Ma, There is no legitimate reason to exclude either the low- or high-age crystals, and therefore the mean age is reported, The source of the larger-than-expected single-grain age variation could originate from geological mixing processes either in the sec tion or during the eruption process, or (less likely) from con tamination in the laboratory. The exclusion ofthe apparently 'too young' or

~ m 3' ~ n Stratigraphic Sample No. No. crystals Spontaneous Induced Px.2,% pd Nd Operator Age (Ma) Weighted mean ,i§ 0 ~ unit ps Ns pi Ni ±Ia age (Ma) ±Ia • "~ •0 Hamilton Ash Formationl ~

HI 9202·03 25 0.063 35 10.903 6060 89.8 1.054 2503 JMT 0.41 ± 0.07 9202·01/02103 18 0.045 25 7.865 4387 99.0 0.907 2240 IJL 0.35< 0.07 0.38± 0.04

Kauraa Ash Formation

KI2l 9202·05 20 0.108 122 5.464 6171 10.8 1.068 2537 JMT 1.41 ±0.13 9202·05 9 0.063 25 3.974 15gg 32.0 0.928 2294 IJL 0.99 ± 0.20 1.28± 0.11

K3 9202·06 24 0.265 124 11.246 5261 29.4 1.075 2564 JMT 1.70±0.16 9202-06 13 0.22g 61 8.716 2336 37.8 0.935 2312 IlL 1.65< 0.22 1.68 ± 0.12

K.2 9202·07 18 0.125 51 6.045 2457 83.6 1.082 2581 JMT 1.50 ± 0.22 9202-07 10 0.127 22 6.067 1050 96.8 0.942 2329 IJL 1.33± 0.29 1.43± 0.17

KI 9202-08 17 0.311 137 10.830 4777 0.4 1.089 2598 JMT 2.24 ± 0.29

w2 I Track densities (p) are x 106 tracks cm , All analyses arc by the extemal detector method using 0.5 for the 47t12n: geometry correction factor. Zircon ages are calculated using dosimeter glass eNl; zetawCNI "" 133.9 ± 1.4 for JMT, 135.1 ± 2.8 for IJL. P(X2) is the probability ofobtaining x. 2 value for v degrees of freedom (where v is the number ofcrystals -I). Central age (Galbraith & Green, 1990) is reported for 9202-08 as P(X 1) value is <5%. ~ Ohinewai Ash Member (Ward, 1967), equivalenllo Rangitawa Tephra (Kohn et 01., 1992). J Probable correlative or Oparau Tephra (Pain, 1975) and Ongatiti Ignimbrite (Horrocks et 01.. 1999; Horrocks. 2000).

Table J. Zircon fission-track data' for selected beds within Kauroa and Hamilton Ash formations. Ages on we.1thered Plio-Pleistocene tephril sequences, western North Is/and New Zealand 53

Method and age Material Reference2 (Ma ± Icr)

Fission-track 1.25±0.12 Isothermal plateau on glass I 1.23 ± 0.09 Isothermal plateau on glassJ 2

KlAr 1.25 ± 0.09 Hornblende separate 3

"IIAr/ J9Ar (single-crystal laser fusion) 1.251 ± 0.060 Feldspar separate 4 1.23 ± 0.02 Feldspar separate 5 1.21 ± 0.04 Feldspar separate 6 1.24 ±O.07 Feldspar separate' 2

Palaeomagnetic timescale >0.780 1,3 (Matuyama Chron)

I Error-weighted mean radiometric age for all samples, including ZFT age on Kl2 (sample 9202-05, Table 3) is 1.231 ± 0.016 Ma (n = 8). , I, Black el al. (1996); 2, Shane el al. (l996a); 3, Soengkono el al. (1992); 4, Pringle el al. (1992); 5, Briggs el al. (1993); 6, Houghton el al. (1995). J Maungatewaiiti tephra (Hawke's Bay) - provisionally correlated with Ongatiti Ignimbrite by Shane et at. (1996a).

Table 4. Summary ojpreviOlls radiome/l"ic ages on Ongatiti Ignimbrite and possible correlatives'. near Raglan (Fig. IA; Briggs el al., 1994; Goles el al., The oldest caldera centres of the TVZ are the Mangakino 1996). At one such section (Fig. 5), the age of c. 1.2 Ma and Kapenga centres, dating ostensibly from c. 1.6 Ma on K12 provides a minimum age for the underlying laharic (Fig. 1~ Houghton et al., 1995). However, Mangakino deposits, which represent the late cone-building stage of activity is probably older than i.6 Ma because Krippner el the volcano's history (Goles el al., 1996). Similarly, the al. (1998) demonstrated that there are earlier inaccessible newly-dated members of the Kauroa Ash sequence help (buried) rhyolitic eruptive products. Moreover, if the provide age constraints where they occur interbedded with earliest member of the Kauroa Ash sequence is TVZ defined units of the Tauranga Group or Kaihu Group in derived, then the age we obtained here on Kl (2.25 Ma) the Waikato-South Auckland regions (e.g. Kear & suggests that explosive rhyolitic TVZ volcanism may date Schofield, 1978; Kamp & Lowe, 1981; Nelson elal., 1988, from around this time. We note that the 'blurred' distinc* 1989; Stokes, 1988; Waterhouse & White, 1994). tion between the cessation of CVZ volcanism and the ini tiation of TVZ volcanism may be ultimately clarified by 4.2. Possible sources and correlatives of analysis of aDP Leg 181 deep-sea cores, including that Kauroa Ash beds from Site 1124 (Fig. I) in which at least c. 140 macroscopic layers of tephra up to 0.9 m thick and dating back to c. 11 The source(s) of the Kauroa Ash Formation is unknown Ma have been preserved (Hayward, 1998; Carter et aI., 1999). but various options have been considered (e.g., Briggs et at., 1989, 1994). On the basis ofthe age range established here, from c. 2.25 Ma to 0.78 Ma, and assuming that the Notwithstanding the question of timing of initial rhyolite volcanism in TVZ, numerous ignimbrites and associated beds are predominantly rhyolitic, the fonnation may re fall deposits have empted from Mangakino since c. 1.6 late to either 'late' CVZ or 'early' TVZ emptions, or both. Ma, including Ongatiti Ignimbrite (1.23 Ma) which we correlated with member KI2 of the Kauroa Ash Fonna The youngest caldera centres of southern CVZ (Skinner, tion. In addition to Ongatiti, the most voluminous 1986) are not well defined and are poorly dated but possi Mangakino-derived eruptives include Ngaroma (1.6 Ma), ble calderas

Approx. thickness 4.3. Ages ofHamilton Ash beds (m) The ZFT age obtained on the basal member ofthe Hamil -- 1.0 Dark brown clay - bed 26 ton Ash Fonnation, HI (Ohinewai Ash), is statistically indistinguishable from the fission-track and astronomical K151-----j ages obtained for its proposed correlative. Rangitawa -- 0.7 Yellowish brown clay ~ bed 25 Tephra and other deposits (Table 2; Pillans el af., 1996). E K14 t---~ Our age of 0.38 ± 0.04 Ma therefore supports this IC TTTTTT-C- 0.3 Pale yell. brown clay - bed 23 .c-- h- correlation (cf. McCraw, 1975). It is clearly inconsistent, : K13 -- 0.4 Dark red. brown clay-bed 22 however, with much younger ages of c. 0.23-0.24 Ma - 0.3 Pale yell. brown clay - bed 20 ~ --1------\ derived previously for Rangitawa Tephra and its ~ - 0.4 Dark red. brown clay - bed 19 correlatives that were considered unreliable by Kohn et ~ ~~~~~- K12 F - 0.20.1 OrangePInkiSh blotchyclay clay l -bed 18 al. (1992, 1996), Pillans et at. (1996) and Pillans & Kahn, frrTTTTTT-rl - 0.2 White clay - bed 17 (1998), but favoured by Vella & Vucetich (1995, 1996) 0.8 Very dark red. brown clay and Vella (1997). Kll (prismatic) - bed 16 0.4 Yenowish brown clay Kl0 (?tau"ed) -bed 15 Because this tephra is known to have been deposited near - 0.4 Olive clay (blocky) gracing 10 the end ofMOl Stage 10 (Kahn et at., 1992; Pillans er af. 1996; Pillans & Kahn, 1998), and from application of a Si-leaching mode! relating clay mineralogy and palaeoclimate (Stevens & Vucetich, 1985; Lowe, 1986, 1995; Lowe & Percival, 1993; Shepherd, 1994), we suggest :.....2.0 Reddish blotchy, clayey, dslal that the paleosols developed on Hamilton Ash members .... : '. '0' laharic debris HI and H3 were developed during MOl Stage 9 (c. 0.34 :> . 0.30 Ma), member H4 (comprising multiple units) was '"<6 . deposited during MOl Stage 8 (c, 0.29-0.25 Ma), the (a!lophanic) paleosol on member H5 was developed during MOl Stage 7 (c. 0.24-0.19 Ma), and members H6 and H7 (Road level) (Tikotiko Ash) relate to MOl stages 6 (c. 0.18-0.13 Ma) IIIII11 Marked paleosol IBt horizons) and 5 (c. 0.13-0.08 Ma), respectively (approximate age ranges for MOl stages are after Martinson el ai" 1987). Development of the paleosol on member H7 during Figllre 5. Stratigraphic illler-relationships ofzipper Kal/roa Ash interglacial MOl Stage 5e and the 5d-5a transition is con bedr and laharic deposits of/he Karioi FormatiOIl sistent with it being overlain unconformably by the (Gales et al., 1996) at Bryam Home sectioll. Manu Bay Rotoehu Ash (64 ± 4 ka or younger), which was deposited (RI41705737). The sequence is overlain by thin Hamilton Ash probably early in MOl Stage 3 (Berryman, 1992; Wright andyo/lnger tephra deposits. After Briggs et 01. (/994, p. 36). el at., 1995; Cronin el at., 1996; Palmer & Pillans, 1996; Yell. ~ yellowish: red. ~ reddish. Newnham el af., 1999a).

More dating and tephrochronological work is needed to test and refine this provisional chronology for members spaced locations throughout North Island (see Fig. I), of the post-H I Hamilton Ash sequence. including Ohinewai (Nelson ef al., 1988), Port Waikato (Nelson el aI., 1989), Cape Kidnappers (Shane el af., 1996b), Rangitikei River (Naish el af., 1996), Te Muna 4.4. Possible sources and correlatives of (Shane e/ al., 1995, 1996a), and Patiki Road and elsewhere Hamilton Ash beds in Auckland (Moore, 1991; Newnham & Grant-Mackie. 1993; Alloway & Newnham, 1995; Newnham el af., The source(s) of the Hamilton Ash sequence is unknown 1999b), and in offshore cores (Carter et at., 1999). ex.cept that member HI (Ohinewai Ash) is now established Although a few ofthese distal deposits have been correlated as a correlative of Rangitawa Tephra and Whakamaru with the proximal units (e.g., Potaka Tephra and group ignimbrites, which have been ascribed a source in Kidnappers IgnimbritefUnit 'E': Wilson el al., 1995b). the Whakamaru caldera volcano (Kohn e/ al., 1992). most have yet to be finnly linked to one another. It is likely Shepherd & Gibbs (1984) suggested from geochemical that some of the widespread distal deposits are co-eval analysis of Fe-Ti oxides that the Hamilton Ash sequence with members of the Kauroa Ash Formation. Well-dated, originated from 'two or more vents in the Taupo-Maroa named distal eruptives in the target age range include vo!canic centres'. However, many ignimbrite-generating Ohingaiti (2.17 MaJ, Waipuru (1.85 Ma), Vinegar Hill (1.75 volcanic centres were active in TVZ in the post-0.35 Ma Ma), Pakihikura (1.65 Ma), Mangapipi (1.6 Ma), Rewa period. These include Kapenga. Rotorua. Okataina, (1.3 Ma), and Kaukalea (0.87 Ma) (Alloway el af., 1993; Reporoa, and Taupo in addilion to Whakamaru (Fig. I), Naish el af., 1996, 1998; Shane, 1994; Shane el af., 1996a, and all provide potential sources of Hamilton Ash beds 1996b; Seward & Kahn, 1997; Shane, 2000). that were emplaced subsequent to member H I. At least five or six major ignimbrites, some imprecisely dated, were A submarine caldera feattlre identified offthe Bay ofPlenty erupted from these centres between c. 0.34 and 0.20 Ma, coaSl (Fig. 1) may have an age similar to that of the including Matahina (0.34-0.28 Ma), Waimakariri (between Mangakino centre (Davey el ai., 1995). It is therefore c. 0.32 and 0.22 Ma). Kaingaroa (0.31-0.23 Ma), and another potential source for some ofthe Kauroa Ash beds. Mamaku (0.22 Ma) (Houghton e/ 01.. 1995; WilsOll e/ l.lf.. Ages on weiuhered Plio·Pleisfocene tephriJ sequences. vvestem North IsJiJnd. NewZe.aliJnd 55

1995a; Black ., 01., 1996; Briggs ., 01., 1996; Brown ., Tephra of 0.34--0.35 Ma (Table 2; Pillans el 01., 1996), 01., 1998; Beresford & Cole, 2000). A few studies have supporting correlation with this deposit. Member H5 is identified distal, rhyolitic tephra-fall deposits pre-dating estimated to be c. 0.24-0.19 Ma in age, and the youngest Rotoehu Ash and post-dating Rangitawa Tephra in eastern members of the sequence, H6-H7 (Tikotiko Ash), are North Island (Iso ., 01., 1982; Fcoggatt, 1983; Manning, estimated to be c. 0.18-0.08 Ma in age. 1996) and in Lake Omapere in Northland (Lowe, 1987), and some ofthese tephras may prove to be correlatives of Possible sources of the Kauroa Ash Formation include members of the Hamilton Ash sequence. younger volcanic centres in the southern Coromandel Volcanic Zone or older volcanic centres in the Taupo 5, Conclusions Volcanic Zone, or both (Fig. I). Various widespread silicic pyroclastic deposits in North Island, and in deep-sea cores, Using the zircon fission-track method, we have obtained may be co-eval with members of the Kauroa Ash Fonna five ages on members ofrwo strongly-weathered, Pliocene tion on the basis of their age. Member HI (Rangitawa Pleistocene tephra sequences, the Kauroa and Hamilton Tephra) of the Hamilton Ash Formation evidently Ash formations, in western North Island. originated from the Whakamaru caldera in the TVZ (Froggatt el al., 1986; Kohn el 01., 1992). Other members Ofthe Kauroa Ash sequence, member KI (basal unit) was have unknown origins but various younger volcanic cen dated at 2.24 ± 0.29 Ma, confirming a previous age of c. tres in TVZ provide potential sources. 2.25 Ma obtained using tephrochronology and KJAr ages on basalt, and consistent with palaeomagnetic polarity 6. Acknowledgements measurements. Members K2 and K3 gave indistinguishable ages between 1.68 ± 0.12 and 1.43 ± 0.17 Ma. Member We are grateful to Graham Shepherd (Landcare Research, K12, a correlative of Oparau Tephra and probably also Palmerston North) and Cam Nelson (University of Ongatiti Ignimbrite, was dated at 1.28 ± 0.11 Ma, consis Waikato) for helpful advice on the stratigraphy of the Ha tent with a previous KlAr-based age estimate of<1.81 Ma milton and Kauraa Ash beds, Tom Higham (University of (Briggs et al., 1989). It is also consistent ·with an age of Waikato) for assistance with age statistics, Frank Bailey 1.23 ± 0.02 Ma obtained by various methods on Ongatiti (University of Waikato) for draughting the figures, and Ignimbrite (Table 4). On the basis of palaeomagnetic especially Etienne Juvigne (Universite de Liege) for measurements, members K13 to K15 (top unit, Waiterimu inviting us to contribute this paper and for the English Ash) are aged between c. 1.2 Ma and 0.78 Ma (Horrocks, French translations. Diane Seward (Swiss Federal1nstitute 2000). of Technology, Zurich) and an anonymous reviewer are thanked for refereeing the paper, and Jean-Paul Rayoal Of the Hamilton Ash sequence, member· HI, defined as (Universite de Bordeaux) is thanked for his editorial help. Ohinewai Ash, was dated at 0.38 ± 0.04 Ma. This age DJL thanks Landcace Research (Hamilton) for kindly matches those obtained by various methods on Rangitawa providing facilities during study leave in 1999.

...... """..·=..""...""""''''''...'''''''''10'''...,."''''''...... ,,,,,...... ''''...'''''''''_ References

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