Magnetic polarity stratigraphy of a Plio-Pleistocene marine sequence of North Island, New Zealand

Earth and Planetary Scwnce Letters, 80 (1986) 353-360 Elsexaer Science Pubhshers B V, Amsterdam - Pnnted m The Netherlands

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Magnetic polarity stratigraphy of a Plio-Pleistocene marine sequence of North Island, New Zealand Diane Seward

1, D.A.

Christoffel 2 and B. Lienert 3

t Institute of Nuclear Scwnces, D S I R (New Zealand) 2 Research School of Earth Sciences, Vwtorta Umverstty, Welhngton (New Zealand) 3 Hawau lnstuute of Geophysics, Unwerslty ofHawan, Hawau, HI 96822 (U S A )

Received July 29, 1985, revised version received August 8, 1986 Thick sequences of relatively undisturbed Pho-Plelstocene sechments in the Wanganul Basin, North Island, New Zealand consist of well exposed silts, clayey silts, sandstones, rare limestones, and several tephras Oriented specimens were collected from a section more than 2500 m thick and palaeomagnetic measurements were made using A C demagnetisations in fields up to 35 mT With the aad of tephrochronology the age of the upper sequence is now well established and falls within the Matuyama epoch The lower two-thirds of the section except for the basal 500 m is predominantly normally magnetised and is interpreted as a very extended sequence of the Gauss epoch The lowest 500 m then represents the Gilbert epoch The Pho-Plelstocene boundary, as defined at Vnca, Italy, falls wathm the upper part of the section studied, in the Upper Nukumaruan stage For the first time a reliable correlation is made with the international boundary, using as mtermedmnes the palaeomagnetlc and tephrostratigraphy of deep-sea cores from the southwest Pacific As a result of the high deposmon rate (of the order of 1 2 m/ky) and the apparent lack of unconformities, the temporal resolution is high, short-hved magnetic events are detected, especially m the lower Matuyama and upper Gauss epochs These generally correlate well with events reported from other extended sections

1. Introduction F e w s e d i m e n t a r y sequences exist through the U p p e r P h o c e n e / P l e i s t o c e n e that are thick, complete a n d tectonically u n c o m p h c a t e d . Such a n ext e n d e d sequence is exposed in N o r t h Island, N e w Z e a l a n d a n d is theoretically ideally suitable for p a l e o m a g n e t l c studies. The longest complete sedim e n t a r y sequences of this time range so far studied are those of Liddlcoat et al. [1] with a 930 m thick core of lacustrine a n d alluvial material t h r o u g h Searles Lake, U.S.A a n d of Cooke et al [2] a n d R o h a t [3] with two boreholes, 1200 m thick, i n the n o n - m a n n e sequence of the G r e a t H u n g a r i a n Plain. The Searles Lake core was interpreted as e x t e n d i n g t h r o u g h the Brushes epoch almost to the base of the Gauss. Five zones of i n c l i n a t i o n were recognlsed that are n o t o n the s t a n d a r d polarity time scales. I n a n y extended sequence this m u s t be expected as most p a l e o m a g n e t l c stratigrap h y has b e e n b a s e d o n t e m p o r a l l y c o n c e n t r a t e d sediments m the deep sea or o n lava sequences w h e r e t i m e is e r r a t i c a l l y r e p r e s e n t e d The 0012-821X/86/$03 50

© 1986 Elsevier Scaence Pubhshers B V

H u n g a r i a n sequences also c o n f i r m the generally accepted time scale with the presence of fine detail n o t usually found. T h e section studied m N e w Z e a l a n d has the a d d e d advantage of the presence of tephra horizons which either have b e e n dated radiometrlcally or c a n be correlated to deep sea equivalents with age i n t e r p o l a t i o n s from p a l e o m a g n e t i c stratigraphy. 2. Geological setting Tluck deposits of Phocene a n d Pleistocene m a n n e sediments are exposed in the W a n g a n u i Basin of N o r t h Island, N e w Z e a l a n d (Fig. 1). This m a n n e sequence is thickest (approximately 4360 m) i n the geographical centre of the basin, a n d is generally c o n t i n u o u s with few structural c o m p h c a tlons. T h e sediments lie u n c o n f o r m a b l y o n preCretaceous greywackes. I n the b a s i n centre, i.e. in the region of the Rangltikei River (Fig. 1), the beds dip generally to the south a n d southwest between 6 a n d 10 °. The

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draanage pattern across the basm is alSO to the south, the rivers flowing down dip from the greywacke basement through the complete sedimentary sequence of Pleistocene and Upper Phocene sediments. The complete sequence m the Rangttlkel Valley was not sampled because (a) the lowermost formation, the Waaouru Sandstone is maccesslble, and (b) the upper portton ts composed of generally coarser material, often volcaniclastlc and frequently iron stmned. Thus the 2600 m studied here represents the large freddie sequence of PlioPleistocene sedtments. The lower half of the sampled sequence (Fig. 2) is predominantly mudstones wtth occasional shell limestones, conglomerates and upwards increasing numbers of concretionary horizons. A major limestone, the Hautapu Shell Limestone, hes between the

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Mungaweka Mudstone and the Utlku Sandstone (Fig. 1). The upper section studied ts composed predominantly of sandy mudstones wbach are fosslhferous in many places. Three tephra beds, Ohingaitl, Walpuru and Mangahou Ashes, which are critical to the palaeomagnetlc mterpretattons, outcrop m this sequence. Above the Maxwell Group (Figs. 1 and 2) there ts a dramatic hthologtcal change as a massive reflux of volcanic detritus from the Central Volcanic Dtstrtct to the north of the basra is recorded in the marine sediments. The ash sequence has been dated by the fission track method [4] and provides a baseline for palaeomagnetlc studies. The age at the top of the sequence in the current study is 1.1 Ma [5], a fission track age on the Palotukura Purmce (Fig 2). The Brunhes/Matuyama boundary has been tentatwely ldenUfied by Seward [6] 400 m above

355

the top of the section studmd here. The biostratigraphy of the sequence is discussed by Superior [7], Te Punga [8] and Collen [9]. Stage boundaries in the Rangitlkel sequence are apparently difficult to assign as m a n y important species are not present (N. de B. Hornlbrook, personal commumcation, 1983) so all estimates must be considered broad. Hornibrook (personal commumcatlon, 1979) places the Opoltlan/Waipipian boundary at approxamately the top of the T a i h a p e M u d s t o n e , and the W a l p i p m n / M a n g a p a n i a n boundary in the lower third of the Mangaweka Mudstone (Fig. 2). The M a n g a p a n i a n / N u k u m a r u a n boundary is that defined by Fleming [10]. Because of the paucity of planktonic foraminifera, the Pho-Pleistocene boundary may still be controversial but according to Beu and Edwards [11] and Edwards [12], the best biostratlgraphlcal esttmate of the boundary as defined at Vrlca is now in the upper part of the N u k u m a r u a n Stage. One of the aims of the present study was to more closely position the internationally defined Pho-Pleistocene boundary within the New Zealand stages. 3. S a m p l i n g and laboratory procedures

The river was accessed by jet boat, and the samples were cored from river cliffs. Samples were generally taken only m blue grey mudstones. At least three specimens were drilled per site with less than 10 cm stratigraphic difference between cores. Imtial samples were taken at stratigraphic intervals of 50 m or less; a second collection was made later ( i n sequences that were difficult to interpret) with sites as close as 10 m apart. In previous work on se&ments in an adjoining basin, Lienert et al. [13] used thermal cleaning throughout. It was reasonably satisfactory except that chemical alteration fairly clearly appeared at temperatures between 250 ° and 350 ° C Allowing for this restriction with chemical cleaning, there appears httle difference between results achieved with A C or thermal cleamng. A.C. cleaning was more convenient in the present study and was therefore used. The point of stability for each core was individually assessed, by cleaning in stepwlse increments to 70 m T Because of the generally low lntenmty of magnetisation of the sediment, measurements at higher demagnetisation fields were

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approaching the noise level of the system. In most cases this point was less than 35 m T (Fig. 3). Directions of magnetisation were measured in a cryogenic magnetometer 4. Results

4 1 General

The N R M values for all measurements (Fig. 4A) show a broad distribution of directions for the lower hermsphere (reversed). Their mean dechnatlon is 062 °, lnchnation 79 ° with a 95% cone of confidence of + 19 °. A low Fisher k value of 2 means that little confidence can be placed on these statistics. The cores with upper hemisphere (normal) inclination have magnetisation directions clustering around the present magnetic field vector. The mean &rection has declination 022 °, inclination - 6 0 ° with a 95% cone of confidence of + 5 °. The present magnetic field direction at the collecting sites is 022 ° declination, - 5 9 ° inchnatlon.

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criteria but gave generally intermediate geomagnetic pole positions, e.g., at 400 m, just beneath the Waipuru Ash. We believe they represent periods of genuine magnetic t r a n s m o n but because of their intermediate nature they are not plotted m Fig 2 They are to be the basis of a further more detaded study. The stereograpluc projections of the consecutively consistent normal and reversed directions are shown in Fig. 4B. After tilt correction ( 6 - 1 0 °) the normal (upper hemisphere) mean declination is 3 °, mean inchnatlon - 5 3 °, with a 95% cone of confidence of + 5 °. For the reversed (lower hemisphere) the mean declination is 179 °, mean mchnation 65% with a 95% cone of confidence of + 8 ° (Fig 4B) F r o m the plots we note the following points: (1) Within our confidence hmlts normal and reversed are antipodal (2) The normal directions of the cleaned samples do not overlap those of the normal N R M directions at the 95% level; neither do they overlap the present field direction, (022 °, - 5 9 °) in&catmng little influence of current magnetic field. This result adds confidence to our cleaning technique. (3) Within the confidence limits there is no offset of the mean magnetic m e n & a n from the geographic m e n & a n over a time period of approximately 3 Ma. This contrasts with the mean direction found by Walcott et al. [14] for measurements on samples of slightly older age ( 5 - 6 Ma) some 100 k m away, but east of the N o r t h Island Shear Belt (inset, Fig. 1). These results fit m well w~th the current tectonic model for plate motions for the New Zealand region (H.W. WeUman, personal communication, 1986) F r o m plate tectonics the declination change for the sediments m the R a n g m k e i section should be only of the order of 2_4 °

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After cleaning and selection of end points from modified Zijderveld diagrams (Fig. 3) and stereoplots (Fig. 4), the data fell into tune sequentml groups showing d e a r l y normal and reversed behaviour (Fig. 2) Some sets of stratigraphically consecutive measurements satisfied the stablhty

4.2. Strattgraphtc mterpretatwn of results Interpretatxon of any paleomagnetic sequence usually depends on some other well founded age data within the section, be it palaeontological, radiometnc, etc. In this case, the top of the section has two dated tephras The Palohlkura Purmce (Fig 2) has a glass fission track date of 1 06 + 0.32 M a [4] which was confirmed by a zarcon fission track age of 1.06 __+0.36 Ma [5]. (All errors are quoted at the 2o level.) The M a n g a h o u Ash, 20 m

357

below, is dated on glass at 1.26 + 0.34 Ma. These two ages are stratigraphtcally consistent with each other as they are with younger ashes m the sequence [4,6]. These dates place the top of the sequence within the Matuyama epoch~ Further, the B r u n h e s / M a t u y a m a boundary has been tentatively located by Seward [6] at a stratlgraplucally higher point between two tephras dated at 0.74 _ 0.18 Ma and 0.61 + 0.12 Ma.

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4 3. Matuyama epoch On the basis of domanantly reversed pole positions and the fission track dates on the tephras, it is concluded that the mainly reversed sequence from 0 to 1000 m is part of the Matuyama epoch. Within it Is a normally magnetised sequence from 200 to 400 m. We interpret thts as the Olduval event with ages of 1.67-1.87 Ma assigned to it by Manlonen and Dalrymple [15]. Immediately below the normal interval, the directions from five closely spaced consecutive sites are Intermediate, and they have been rejected from our column. Data for the R6unlon event are sparse but we suggest that it occurs just below the Ohmgalti Ash (Fig. 2). A posssble normal polarity event identified as anomaly X by Heirtzler et al. [16] and Emlha and Hemrichs [17] was also tentatively recognlsed by Shuey et al. [18] and as N1 by Llddicoat et al. [1]. We suggest that the normal event at about 950 m in the Rangmkei Section is also the " X " event.

4.4 Gauss epoch From 1000 to 2200 m the sediments are predominantly normally magnetlsed The number of events do not fit precisely with the standard reversal stratigraphy but are very similar to the results of Liddicoat et al. [1], Cooke et al. [2], and Roflai [3] (Fig 5). We take the top of the Gauss at 1000 m, and suggest that the Kaena and M a m m o t h events are at 1800 and 2000 m respectively although several options are obviously avaalable. This places correlations of the event at 1250 m with the R2 of Liddicoat et al. [1] (Fig. 5), and the possible event at 1350 m may be the eqmvalent of their R1. The palaeomagnetlc stratigraphy at V6sztb, Hungary [3], also shows two events, above the K a e n a - M a m m o t h sequence. Interestingly, m Mankinen and Dalrymple's [15] lists of all the available dated material three reversed samples

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fall into this interval, one at 2.70 Ma and two at 2.84 Ma.

4 5. Gdbert epoch From 2200 m to the base of the section the sequence has several reversed and normal intervals of comparable thickness. We interpret this as being the upper part of the Gilbert epoch, but concede that closer sampling is necessary for full confirmation. Tlus puts the age of the bottom of the section at 4.2-4.3 Ma. 5. Tephrochronology The Walpuru Ash occurs 350 m below the top of the section. The size of the zarcons ( < 54 #m) m this ash are too small to be handled technically m the fission track dating process. It occurs in the sequence of predommantly normal polarity which

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we conclude (Fig. 6) that (1) the Oinngaztl Ash is the eqmvalent of the "Eltanm" and M1 ashes, (2) the Watpuru Ash is the equwalent of ash C3 m the South Pacific cores and ash M2, (3) the Mangahou Ash is the equwalent of ashes D5 and M3, and (4) the Pakihlkura Purmce is the equivalent of E1 and M4 ashes.

we have interpreted as the Olduvaz event(s) (Fig. 2) In core RC12-216, taken 1600 km east of North Island, New Zealand, Watlons and Huang [19] also located a tephra (their M2) within the Matuyama epoch, m a normally magnetised sequence, interpreted as the Olduvai-Gdsa event. We conclude that the two tephra are correlatives (Fig. 6), contrary to Watklns and Huang's [19] suggestion that M2 is the d~stal equivalent of the Oinngam Ash. We now know that the Ohlngam Ash at its type section hes wlttun a reversed polarity sequence (Fig. 2). In a further study, Huang et al. [20] recogmsed 30 dispersed volcamc ash zones in cores collected m the South Pacific east of the Balleny Islands. The dominant ash, the "Eltanm Ash" occurred m a reversed polanty sequence which was estimated to be 1.8 Ma on the basis of the Cox time scale, but is now considered to be a httle older, on the baszs of the Manlonen and Dalrymple [15] time scale In a recent article, Kyle and Seward [21] correlated the "Eltanm Ash" wzth the M2 tephra. The polanty straUgraphy now shows this not to be s o - - t h e M2 tephra is more hkely to be the Walpuru Ash as dzscussed above, while the "Eltanm Ash" is most likely the correlatwe of the Oinngam Ash. Further, the Ohingaiti Ash is most probably the equivalent of the M1 ash (reversed) [19]. The fisszon track age of 1.78 + 0.44 (20) on the O h m g a m Ash, c o m p a r e d with the paleomagnet~c ages of its possible deep sea equivalent, is w~ttun the error hmlts quoted On the basis of the palaeomagnetlc straUgraphy

6. Plio-Pleistocene boundary According to Tauxe et al. [22] the proposed stratotype at Vrlca, Italy, occurs above the Olduval normal event at approxamately 1.6 Ma. Since 1953, the Pho-Pleistocene boundary in New Zealand has been accepted as coinciding with the base of the Nukumaruan Stage [10] because tins marks the first appearance of the sub-Antarctic bivalve Chlamys dehcatula (Hutton). But Edwards [12], using blostrattgraphical correlations from New Zealand to DSDP site 284 in the Tasman Sea, and then via core V28-239 m the equatorial Pacific to Vrlca, postulates the location of the Pho-Plelstocene boundary at an undefined posmon within the upper part of the Nukumaruan Stage. Beu and Edwards [11] estimate the boundary more preosely as lying at about the top of the Pukeklwl Shell Sand in the type Nukumaruan section, 60 km west of our Rangltlkel section. Although the Pukelowl Shell Sand does not occur m the Rangltike~ section, its esUmated eqmvalent posmon ~s 100 m below the top of our section. Tins zs just above the top of what we mterpret to be the Olduvai event. Our data thus confirm the posmon

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of the O l d u v a i event as e s t i m a t e d b y Beu a n d E d w a r d s . W e p l a c e the P h o - P l e l s t o c e n e b o u n d a r y 1 0 0 - 1 5 0 m b e l o w the top of o u r section 7. Sedimentation rates

S e d i m e n t a t i o n rates (Fig. 7) p l o t t e d on the b a sis of our l n t e r p r e t a u o n s show b r o a d l y three sections. T h e lowest section, p r e d o m m a n t l y the T a l h a p e M u d s t o n e , gwes a rate of 1.7 m / k y ; the b r o a d r m d d l e p o r t i o n c o m p r i s i n g the m a j o r i t y of the section gives a m e a n rate o f 1.2 m / k y . T h e short interval at the t o p of the section has a r e d u c e d rate consistent with field evidence of very shallow w a t e r s e d i m e n t a t i o n a n d occasional emergence at ttus level. 8. Conclusions

(1) In a 2.5 k m thick Pho-Pleistocene section the m a g n e t o s t r a t l g r a p h y enables us to identify the M a t u y a m a , G a u s s a n d p a r t of the y o u n g e r G i l b e r t epochs (2) C o m p a r i s o n of the m a g n e t o s t r a t l g r a p h y a n d t e p h r o c h r o n o l o g y with t h a t f r o m some deep sea cores enables us to relate the offshore t e p h r a a n d & s p e r s e d ash layers to e r u p t w e events within N e w Zealand. (3) The m a g n e t o s t r a U g r a p h y o f o u r assigned G a u s s e p o c h c o n t a i n s several events n o t r e c o r d e d m the s t a n d a r d c o l u m n s However, the p a t t e r n of events closely r e s e m b l e s t h a t of m e a s u r e m e n t s o n o t h e r e x p a n d e d sequences such as Searles Lake, C a h f o r n i a , a n d the P a n n o n l a n Basin, H u n g a r y . (4) T h r o u g h the m a g n e t o s t r a t i g r a p h y we are a b l e to locate the Plio-Pleistocene b o u n d a r y as

d e f i n e d at the V n c a section. Its p o s i t i o n ~s tow a r d s the t o p of the N e w Z e a l a n d N u k u m a r u a n Stage. (5) T h e r e s o l u t i o n a n d c o n t i n u i t y m this section are such t h a t m o r e c l o s d y s p a c e d m e a s u r e m e n t s c o u l d e s t a b h s h this as a t y p e section for the Pliocene a n d L o w e r Pleistocene. Acknowledgements

W e wish to t h a n k D r . J. CoUen, B r o n w y n Collen, T e r r y Seward a n d J o h n A d a m s for field assist a n c e d u r i n g the initial p h a s e o f the study, a n d to the late Peter C o l e m a n w h o p i l o t e d the j e t b o a t I n v a l u a b l e field help was later given b y J o h n H a r r 6 a n d his f a m i l y d u r i n g several re-collections. M a r t i n M a n n i n g , D.S I.R., h e l p e d wath the c o m p u t e n s e d p l o t t i n g o u t p u t , a n d Prof. P. Vella e d i t e d the m a n u s c r i p t . F i n a l l y we wish to t h a n k two a n o n y m o u s reviewers whose suggestions for r e w slon greatly i m p r o v e d the p r e s e n t a t i o n of o u r data. References

1 J C Lldchcoat, N D Opdyke and G I Srmth, Paleomagneuc polarity m a 930 m core from Seades Valley, Cahforma, Nature 286, 22-25, 1980 2 H B B Cooke, J M Hall and A Rofiaa, Paleomagnet~c, sedimentary and chmate records from boreholes at D6vavfinya and V6szt6, Hungary, Acta Geol Acad Sol Hung 22, 89-109, 1979 3 A Rofim, Magnetostratlgraphy of Pllocene-Quaternary se&ments m the Great Hunganan Plato, Earth Evolut Sol 3-4, 265-267, 1981 4 D Seward, Age of New Zealand Pleistocene substages by fission track dating of glass shards from tephra horizons, Earth Planet Scl Left 24, 242-248, 1974 5 D Seward, Comparison of zarcon and glass fission track ages from tephra horizons, Geology 7, 479-482, 1979 6 D Seward, Some aspects of sedlmentology of the Wanganm Basra, N Island, New Zealand, unpubhshed Ph D Thesis, Victoria Umverslty, Weihngton, 1974 7 Superior (hi Company, Geology of the Palmerston Wanganul Basra "West Side", North Island, New Zealand, by A E Feldmayer, B L Jones, C W Firth and J Kmght, N Z Geol Surv Pet Rep PR 171, 1943 8 M T Te Punga, The Geology of the RanglUke~ Valley, N Z Geol Surv Mem 8, 46, 1953 9 J D H Collen, Studies m the Wanganm Series, Phocene forarmmfera from Wanganm Basra, unpubhshed Ph D Thesis, Vxctona Umvermy, Wellington, 1972 10 C A Fleming, The geology of the Wanganut Subdtvtston, N Z Geol Surv Bull, N S 52, 362, 1953 11 A G Beu and A R Edwards, New Zealand Pleistocene and

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