Palaeomagnetic location of the Jaramillo Subchron and the Matuyama-Brunhes transition in the Castlecliffian stratotype section, Wanganui Basin, New Zealand

42

Earth and Planetary Science Letters, 100 (1990) 42-50 Elsevier Science Publishers B.V., Amsterdam

[DT]

Palaeomagnetic location of the Jaramillo Subchron and the Matuyama-Brunhes transition in the Castlecliffian stratotype section, Wanganui Basin, New Zealand Gillian M. Turner

a a n d P e t e r J.J. K a m p b

Institute of Geophysics, Research School of Earth Sciences, Victoria University of Wellington, P.O. Box 600, Wellington (New Zealand) b Geochronology Research Unit, Department of Earth Sciences, University of Waikato, Private Bag 3105, Hamilton 2001 (New Zealand)

Received December 12, 1989; revised version accepted April 16, 1990 ABSTRACT Magnetostratigraphy is described for the lower part of the Castlecliff section in Wanganui Basin, New Zealand, based on analysis of multiple samples from 37 horizons. The Castlecliff section is the stratotype of the middle Pleistocene Castlecliffian Stage and comprises eleven unconformity-bound sequences which accumulated at inner shelf depths, during interglacials. A range of rock magnetic experiments undertaken on representative lithologies within the section show that the bulk of the remanence in all specimens is carried by ferrimagnetic mineral close to magnetite in composition, in the stable single domain or pseudo single domain size range. However, at some sites post depositional chemical changes appear to have produced small amounts of another ferrimagnetic mineral, chiefly within the superparamagnetic-single domain size range, resulting in hard secondary components of magnetization. Nevertheless, at most sites the primary component of remanence was clearly identified by stepwise demagnetization. Four magnetozones are identified and these can be correlated to the magnetic polarity time scale using existing biostratigraphic constraints. The section sampled spans the interval from below the Jaramillo Subchron to above the Matuyama-Brunhes boundary. The Matuyama-Brunhes transition extends over 10.5 m of section within the Upper Westmere Siltstone. Virtual geomagnetic pole positions describe an equatorial excursion in an otherwise far sided transition path. These represent the first published data on the occurrence of the Matuyama-Brunhes boundary on-land in New Zealand.

1. Introduction W a n g a n u i Basin is o n e of m a n y N e o g e n e conv e r g e n t m a r g i n basins in the east a n d southwest of the N o r t h Island of N e w Z e a l a n d that have subsided rapidly an d b e e n infilled with thick a c c u m u lations of m a i n l y terrigenous m a r i n e sediments, w hic h are n o w ex p o s e d on-land. T h e Pleistocene strata in the n o r t h w e s t e r n part of the W a n g a n u i Basin c o m p r i s e a series of u n c o n f o r m i t y b o u n d cycles or sequences each typically c o m p o s e d of a basal shellbed an d overlying siltstone unit [1]. T he se cycles are c o n s i d e r e d to have a glacioeustatic origin, m a r i n e d e p o s i t i o n having o c c u r r e d d u r i n g successive interglacials, which n o n d e p o s i tion and erosion d u r i n g the i n t e r v e n i n g gtacials [2]. T h e m o d e r n coastal cliff n o r t h w e s t of W a n g a n u i (Castlecliff section, Fig. 1) c o n t a i n s exposures of m a n y of the interglacial shelf sequences. A c o r r e l a t i o n of these sequences with the 0012-821x/90/$03.50

© 1990 - Elsevier Science Publishers B.V.

o d d - n u m b e r e d global o x y g e n i s o t o p e stages, based chiefly on b i o s t r a t i g r a p h i c d a t u m s , has b e e n suggested previously [2,3]. H o w e v e r , as the section is the s t r a t o t y p e of the N e w Z e a l a n d m i d d l e Pleistocene Castlecliffian Stage, i n d e p e n d e n t age c o n t r o l is particularly i m p o r t a n t . T h e r e are no volcanic horizons suitable for r a d i o m e t r i c d a t i n g in the section, so g e o m a g n e t i c p o l a r i t y stratigrap h y was chosen. T h e m a g n e t o s t r a t i g r a p h y rep o r t e d here indicates that significant m o d i f i c a tions of the p r e v i o u s l y suggested c o r r e l a t i o n s are necessary. T h e i m p l i c a t i o n s of these new correlations will be p r e s e n t e d separately [4].

2. Section description and sampling T h e Castlecliffian section is a 12 k m long, nearly c o n t i n u o u s e x p o s u r e of 3 0 - 5 0 m high sea cliffs n o r t h west of the city of W a n g a n u i (Figs. 1

43

J A R A M I L L O S U B C H R O N A N D M A T U Y A M A - B R U N H E S T R A N S I T I O N IN T H E C A S T L E C L I F F 1 A N S T R A T O T Y P E S E C T I O N

•

Formation from the overlying Butlers Shell Conglomerate (Fig. 2). The top of the type Castlecliffian has recently been redefined as the top of the Putiki Shell Bed [3], which occurs a few kilometres inland, at the south east end of the coastal section, where the Wanganui River enters the sea, and in the Languard Bluff drillhole. Cores were drilled from 37 sites between the top of the Upper Maxwell Formation and the top of the Upper Westmere Siltstone. In view of the cliffed nature of the section we have shown the sample sites on a reproduction of Fleming's sketch of the cliff stratigraphy (Fig. 2), rather than in map form, which would make relocation more difficult. Elevations were recorded from the base of the Butlers Shell Conglomerate (Fig. 3). An average stratigraphic interval of about 3 m was maintained between sites: in places the interval was increased to a maximum of 5 m due to a combination of little exposure and poor lithology for drilling. Our preliminary results had indicated a polarity reversal close to the Kaikokopu Shell Grit, so more detailed sampling was carried out in this part of the section. At all sites the surface material was scraped away and firm, unweathered material was sampled. Between 4 and 8 cores, 2.5 cm in diameter, and ranging from 5 to 16 cm in length were drilled from each site. In most cases fine-grained, blue-grey siltstone was sampled, but within the shell beds and Kaimatira Pumice Sand we were forced to drill coarser material; conse-

j -¢ '~Wanganui section ~"~..Landguard

o,1,,=,o

-

__.__~llington

.}

km

Fig. 1. Location map of North Island, New Zealand, with enlargement including most of Wanganui Basin and showing localities referred to in the text. and 2). The section was described in detail by Fleming [1], who named the lithostratigraphic units. An angular uncorformity occurs about twothirds of the way up the cliff face and above this lie the Rapanui Terrace Deposits of oxygen isotope stage 5 age and younger. Below the unconformity are the Castlecliffian beds, which dip between 2 ° and 4 ° to the southeast. The stratigraphic base of the type Castlecliffian is another unconformity that separates the Upper Maxwell

NORTHWEST A [

Trig JJ ...,,, .... S a n d

Ul~per Maxwell Formation

37

d u n e s

33

E

..: i

:::

A'

E

3b

13

2 1

14 O m a p u Shellbed

The Pinnacles

i KaikOkOpU Shell Grit

...... r 0 t

i

i _1

i

500 t i

10(30 m

Upl3er Caslleclilf S h e l l b e d

L i ~

Fig. 2. Stratigraphic sketch of the Castlecliff section occurnng as sea cliffs north west of Wanganui City, showing positions of sampling sites. Redrawn from sketch by C.A. Fleming [1].

44

G.M. TURNER AND P.J.J. KAMP MARINE SEQUENCES M 3 - -'Mr' k ~

GRAPHIC LOG Castlecliff Landguard Section Bluff

LITHOSTRATIGRAPHIC NOMENCLATURE

BIOSTRATIGRAPHY

Sherwood Sand Kaiwhara Allu',aum Waipuna Conglomerate Land~luard Sand

" Sea level 0

~.-30 $~:~

--~

FAD Fmlliania huxleyi

N.Z. STAGES I Haweran Stage

I

Pulik! Shellbed _

K g

! cl

Karaka Sillstone

a

I

o

I

m •

I P

x:

I

Upper Casteclilf Shellbed Shakespeare Cliff Sand Shakespeare Cliff Siltslone Tainui Shellbed

~

Pinnacle Sand Lower Castlecliff Shellloed

Seafield Sand

~etic tes

a_

~

Upper Kai-iwi Si~lstone

LAD

em#iania lacunosa Pseudo

Kupe Formation Upper Westmere Siltslone

'7 22 21

Kaikokopu Shell Grit Lower ~stmere Sdtstone Omapu Shellbed

=

I

o L

~ I

-

i

(3

I

FAD Pecten

Lower Kai-iwi Siltstone

Kaimatira Pumice Sand L

Upper Okehu Siltstone Okehu Shell Grit Lower Okehu Siltstone Butlers Shell Conglomerate Upper Maxwell Formation

Maxwell Group

Nukumaruan Stage

Fig. 3. Stratigraphic column of the Castlecliff section and Landguard Bluff drillhole, biostratigraphic datums and elevations of sampling sites. quently the results from these latter sites were not always usable. Distinct flazer bedding in the Butlers Shell Conglomerate made retrieval and preservation of the cores very difficult, despite the fine grain size. After orientation, the cores were removed from the exposure and immediately wrapped to avoid contamination from the modern magnetic beach sand. Each day the cores collected were transferred to mu-metal shields. After sawing into 2.4 cm specimens, they were returned to the mu-metal shields and allowed to air-dry. The drying was carried out in zero magnetic field to

prevent gain alignment with the laboratory field during the dewatering and drying process, which may lead to spurious components of magnetization [5,6]. 3. Magnetic measurements

3.1. Stepwise demagnetization of natural remanence Initially four specimens were selected from each site for stepwise thermal demagnetization. Experience with similar mudstones from N e w Zealand

J A R A M I L L O S U B C H R O N A N D M A T U Y A M A - B R U N H E S T R A N S I T I O N IN T H E C A S T L E C L I F F I A N S T R A T O T Y P E S E C T I O N

[7,8] and overseas [9] has shown that thermal demagnetization is generally more effective than alternating field demagnetization in isolating the primary component of remanence. However, thermally induced changes to the magnetic mineralogy are also found to occur above about 350 o C. Consequently the magnetic susceptibility of each specimen was measured after each heating step, and the demagnetization sequence stopped when W/UP 1.2-

W/UP 6-

/ ~N 4

~~

°

2 i) U. Westmere Siltstone Site 28

N 0.4

ii) U. Westmere Siltstone

Site 24

,o.o [1 W/UP

~

I

V/UP -1 i o,.°'~,°"-o~ . ''U°'az, -N

~"'~-°" °

4

/

'

3

iii)L. Westmere Siltstone Site 16

W/UP 06

iv) U Maxwell Fro. Site 6 H 9

°

8~ tI ~

0.2

t

°

\

\x,

( /

N

Iv Kai-iwi Siltstone Site 30

'V vi) L Kai-iwi Siltstone Site 30

0,3

Fig. 4. Orthogonal c o m p o n e n t plots for the stepwise thermal demagnetization of typical specimens at N R M and 50 o intervals from 1 0 0 ° C to 3 5 0 ° C . Specimens are identified by site n u m b e r and lithoiogical unit. Circles = projection on horizontal plane: W - E vs. N - S component; d o t s = projection on vertical N - S plane: vertical vs. N - S component.

45

the first changes were noted. Demagnetization temperatures of 100, 150, 200, 250, 300, 350, and 375 ° C were used routinely, with additional intermediate steps in some cases. Remanence measurements were carried out using a two-axis cryogenic magnetometer. The stability of remanence and the ease with which the primary component of magnetization could be isolated varied widely between the different lithological units. We believe that this is mainly due to the presence of secondary components of magnetization in different parts of the magnetic grain spectrum (see below). Some typical stepwise demagnetization results are shown in Fig. 4. Three types of behaviour can be distinguished: (a) Excellent results were obtained from the Upper Maxwell Formation and from most sites in the Lower and Upper Westmere Siltstones. All three sites in the Upper Maxwell carried a strong, stable natural remanent magnetization (NRM) of reversed polarity, with negligible secondary overprints. A sequence of reversed, transitional and normal N R M directions was obtained from the Lower and Upper Westmere, again with negligible secondary components, confirming our earlier indication of a polarity reversal. The directions of the primary components of magnetization of these specimens were estimated from the best-fit straight lines through the linear parts of the vector component plots. (b) The majority of sites in the Lower and Upper Okehu and Lower Kai-iwi Siltstones gave results which were less readily interpreted. We attribute this to one or both of two causes. Firstly the primary and low-blocking temperature secondary components occupy overlapping parts of the blocking temperature spectrum, and secondly, in some specimens there seems to be another secondary component, occupying the upper part of the blocking temperature spectrum. This latter effect results in orthogonal component plots which do not terminate towards the origin, even after the removal of a low blocking temperature secondary component. It is difficult to investigate further by thermal demagnetization, because of the mineralogical alteration which occurs above 350 ° C. At many sites however we have been able to recognize a common remanence component in the mid blocking temperature range, and it is this which we interpret as the primary magnetization. We

46

G.M. TURNER AND P.J.J. KAMP

conclude that the high blocking temperature component is of secondary, chemical origin (see below). Attempts were made to investigate further the remanence of these specimens firstly by alternating field demagnetization of specimens already thermally demagnetized to 375 ° C, and secondly by alternating field demagnetization of fresh specimens from the same sites. Unfortunately no coherent results were obtained at the level of demagnetization required, ( > 35 mT), as has previously been found with similar mudstones [7,8], so no additional information was obtained. (c) In five sites, from Kaimatira Pumice Sand and Butlers Shell Conglomerate, which generally consist of coarser-grained, less cohesive material it was impossible to identify confidently a primary component of magnetization. N o results are presented from these sites.

3.2. Rock magnetic experiments A series of rock magnetic measurements was made to identify differences in magnetic mineralogy, grain size or magnetic domain state between the different lithological units which might cause

the variation in palaeomagnetic reliability observed above. The measurements, made on representative specimens from each of the units are summarized in Table 1. They include low field reversible m a g n e t i c susceptibility (X), the frequency dependence of susceptibility (Xf~), the growth and stepwise alternating field (a.f.) demagnetization of isothermal remanent magnetization (IRM) and anhysteretic remanent magnetization (ARM). In Fig. 5 we show, for two of the specimens tested, the growth of I R M to saturation, the a.f. demagnetization of saturated I R M (SIRM) and A R M (grown in a peak alternating field of 99 m T and a direct biassing field of 0.05 mT), and the reduction of SIRM by the application of a direct field in the reverse direction. The coercivity of remanence, BcR, the median destructive fields (MDF's) of A R M and S I R M and the direct field at which 98% of the saturation I R M is reached, B98, are all obtained from these plots, and are also listed in Table 1. For a general discussion of the interpretation of such data and derived parameters see, for example, reference [10]. The values of BCR, of around 50 mT, and the fact that 98% of the SIRM is acquired between

TABL E 1 Data from rock magnetic experiments Site

Strat. height (m)

25

77.3

23

72.6

29

58.8

30

55.5

1

36.8

4

24.2

10

7.6

8

1.25

6

- 8.0

Formation

U. Westmere siltstone U. Westmere siltstone Omap u shellbed L. Kai-iwi siltstone Kaimatira pumice sand U. Okehu siltstone L. Okehu siltstone Butler Shell conglomerate Maxwell Fm.

Xsp 1 (10 -8 m 3 k g - 1)

Xfd 2 (%)

SIRM 3 (10 4AmZ k g - 1)

BCR 4 (mT)

B98 5 (mT)

MDF 6 IRM (roT)

MDF ARM 7 (mT)

SIRM Xsp (103 A m

SIRM ARM 1

9.5

0.0

1.68

54

300

25

27

1.76

46.6

9.6

1.0

1.86

52

500

24

26

1.95

51.6

7.9

1.4

1.55

44

300

18

24

1.97

45.6

8.7

1.8

2.26

37

300

14

22

2.59

41.8

9.'2

2.0

1.83

51

500

23

23

1.99

43.8

8.9

0.6

0.82

46

300

22

26

0.92

31.4

8.8

0.2

1.31

49

300

22

25

1.49

34.5

8.4

0.0

2.31

65

500

33

28

2.76

46.5

15.7

2.5

3.03

58

400

27

20

1.93

49.0

i Magnetic s u s c e p t i b i l i t y / u n i t mass; 2 frequency dependent susceptibility; 3 saturated isothermal remanent magnetization; 4 coercivity of remanence; s field at which 98% of saturated isothermal remanence is reached; 6 median destructive field, where remanence is subjected to stepwise alternating field demagnetization; 7 anhysteretic remanent magnetization.

JARAMILLO

SUBCHRON

U Westmere Siltstone Site 22

U Maxwell Fm. Site 6

100

~

AND MATUYAMA-BRUNHES

TRANSITION

M/Mo

10o

500

100

500

-/M/Mo /~\

0

Fig. 5. Plots of rock magnetic data for two specimens. Dots = acquisition of saturated IRM, back field IRM and coercivity of IRM; circles, c r o s s e s = alternating field demagnetization of I R M and A R M respectively. See text for further details.

300 and 500 mT, suggests that the bulk of the remanence in all specimens is carried by a ferrimagnetic mineral, probably close to magnetite in composition. Pure magnetite would be expected to saturate closer to 100 mT, however if more than a trace of an imperfect antiferromagnet, such as haematite or goethite, was present, BCR would be much higher than observed. Furthermore, the remanence carrying mineral must be in the stable single domain (ssd) or pseudo single domain (psd) state, as multi-domain (md) grains would lead to much lower BCR values. The modified LowrieFuller test [11] which compares the a.f. demagnetization characteristics of an A R M and an I R M supports the above suggestion of ssd or psd cartiers in all but the two lowermost sites, those from the Butlers Shell Conglomerate and U p p e r Maxwell Formation, in which the M D F of the I R M is greater than that of the ARM. The ratio of SIRM to X is particularly sensitive to grain size, peaking at about 100 kA m -1 for grains on the superparamagnetic ( s p m ) - s s d boundary. The low values of between 1 and 3 kA m - 1 recorded here are most likely due to a mixture of spm and s s d - p s d grains, rather than to md grains, already ruled out on the above grounds. If

IN THE CASTLECLIFFIAN

STRATOTYPE

SECTION

47

significant haematite was present S I R M / x would be much higher than observed. The frequency dependence of magnetic susceptibility is also indicative of magnetically viscous grains near the s p m - s s d boundary, but is insensitive to md grains [12]. Xfd is calculated as the difference between susceptibility measured at 0.5 k H z and 5 kHz, expressed as a percentage of the low-frequency measurement. The values obtained here are relatively low, and it was initially found surprising that the maximum value of 2.5% was for the site in the U. Maxwell Formation, which gave excellent palaemagnetic stability. Very low values, of less than 1% were obtained in the Butlers Shell Conglomerate and L. and U. Okehu Siltstones, suggesting more ssd and fewer spm grains. The low values of S I R M / A R M for the two Okehu sites (and to a lesser extent the L. Kai-iwi site) also indicate more grains just above the spm-ssd boundary than are present at the other sites. It is suggested that this difference causes the observed differences in palaeomagnetic reliability: post depositional chemical changes at m a n y sites are thought to have produced small amounts of a ferrimagnetic mineral in the superparamagnetic to small single domain grain size range. These latter grains would have short relaxation times, resulting in viscous, secondary components of magnetization. However, at some of the Okehu and L. Kai-iwi sites the grains appear to have grown to stable single domain size, resulting in a hard secondary component which is difficult to remove by standard demagnetization techniques. Chemical demagnetization methods may have been more successful, but were not attempted.

4. Magnetostratigraphic interpretation Site means have been calculated from the estimated directions of primary remanence for each specimen. The declinations and inclinations of these site means are shown in Fig. 6, for 30 of the sites sampled. N o data are presented from the remaining sites, because we could not confidently identify the primary direction. At eight sites the precision of the mean was significantly improved by the omission of one specimen from the calculation. These specimens were accordingly rejected as anomalous. No more than one specimen was rejected from any one site. Values of the precision

48

G.M. TURNER

Formations

Sample sites

I

28 -26 27 -25

80 -

60

Omapu Shellbed Lower Kai-iwi Siltstone

.E

._O3 (l) ¢-

-

U. Okehu Siltstone Okehv Shellbed

20-

Lower Okehu Siltslone

_

0

-

Butlers Shell Upper Maxwell Formation

-10

I

f5 29 -30

+90 ° i

Correlation

I

it

3"

I

,runhes • 0.73 Ma

I \e I %.

-31

=#

l l

.--~E

I

Kaimatira Pumice Sand

"~

0o i

,

16

13

._o 40 .=: Q.

J

•

ITLt8

i

I

_.._...~ •

1.11""

- / K a i k o k o p u Shbd ~. Lower Westmere Siltstone.

I

L

-

Upper Westmere Siltstone

-90 °

0o

180 °

Kupe Formation

P.J.J. K . A M P

Magnetic Polarity

Inclination

Declination

AND

e~

I t l

0.91 Ma aramillo

"4

0.98 Ma

36 33 9

37 7 '6

I

"h, /

latuyama

I

I;/

lo j

I

I

x

i

,'LZ.

i ~"

]

I

J

I

I

\

,

]

I

i

"1

Fig. 6. Declination and inclination of palaeomagnetic endpoint directions, averaged at each site (horizon). The arrows indicate sites at which an endpoint was probably not reached. Dotted fines represent geocentric axial dipole field direction for site latitude. Correlation with geomagnetic polarity timescale [18].

parameter K varied between 10 and 400, with a mean value of 56. Reversed and normal polarities are obtained from the U. Maxwell Fm. and L. Okehu Siltstone respectively. The U. Okehu and L. Westmere Siltstones are also reversed. Two sites, 30 and 31, in the L. Kai-iwi Siltstone also gave reversed directions after the removal of strong normal overprints. However, at sites 13, 14, 32 and 29, although a site mean has been calculated, it is suspected that the primary magnetization was not completely isolated, and that contamination by a normal secondary component is still present. From their reversed declinations and the changes in inclination during demagnetization, we therefore place the whole of the L. Kai-iwi in a period of reversed polarity, which extends from the U. Okehu through to the top of the L. Westmere Siltstone. The U. Westmere Siltstone yields stable transitional directions culminating in normal polarity at the top.

We have thus obtained a R - N - R - N polarity sequence which we interpret as the upper M a t u y a m a Chron, including the Jaramillo, extending into the lower Brunhes Chron. Indirect support for this interpretation is provided by the zircon and glass fission track ages (1.26 _+ 0.12 to 0.37 _+ 0.05 Ma) [13] of tephras interbedded within the more easterly Rangitikei River Section that contains Castlecliffian Stage fauna. Two biostratigraphic datums within the Castlecliff section (Fig. 3) are also consistent with this correlation: Firstly, the last appearance of the calcareous nannofossil Pseudoemiliania lacunosa occurs in the U. Kai-iwi Siltstone in the coastal section [2]. In the deep sea realm P. lacunosa became extinct during oxygen isotope Stage 12, at about 0.46 Ma [14]. However, our correlations lead to the correspondence of the U. Kai-iwi Siltstone with oxygen isotope Stage 17 [4]. We therefore conclude that the facies change to more sandy lithologies above the U. Kai-iwi Siltstone forced a premature disappearance of P.

JARAMILLO SUBCHRON AND MATUYAMA-BRUNHES

49

T R A N S I T I O N IN T H E C A S T L E C L I F F I A N S T R A T O T Y P E S E C T I O N

glomerate, with its spectacular flaser bedding, accumulated in an intertidal environment during the peak of an interglacial, it is likely that most of the early Jaramillo is intact in the section. However the lower Jaramillo transition and an unknown, but probably substantial part of the immediately older Matuyama Chron are not recorded due to the unconformity separating the Upper Maxwell Formation from the Butlers Shell Conglomerate. The top of the Jaramillo occurs at the boundary of oxygen isotope stages 27 and 28 [15], which we correlate with the unconformity between the Lower Okehu Siltstone and the Okehu Shell Grit [4]. Given the degree of relief on this unconformity, we suspect that several thousand years of the record, including the upper Jaramillo transition are missing. The Matuyama-Brunhes transition is known to have started during the final phase of a glacial, about 1000 years before the oxygen isotope stage 19/20 boundary [15]. Site 21, 15 cm below the base of the Kaikokopu Shell Grit is unequivocally

lacunosa from the coastal section. Secondly, the first appearance of the coccolith Emiliania huxleyii in the coastal section occurs in the Waipuna Delta Conglomerate, 5 km southeast of the Wanganui River mouth [2]. This first occurence coincides with oxygen isotope Stage 7c and is dated at 0.27 Ma [14], implying that the formations sampled here are considerably older. The cyclic nature of deposition in the Castlecliff section has resulted in little or no sedimentation during the glacial periods, and this naturally raises questions as to the completeness of the magnetostratigraphic record. With the exception of the major unconformity between the Upper Maxwell Formation and the Butlers Shell Conglomerate, the breaks are unlikely to exceed 3040,000 years, hence none of the major polarity intervals should be completely absent. The base of the Jaramillo coincides approximately with the peak of interglacial oxygen isotope stage 31, which has a duration of about 37,000 years [15]. As the Butlers Shell Con-

75 o

45 o N .

150 N 25

:

15 o S

26

LJ 45 o S

11

75 ° S180 ° w

150 o w

120 ° W

~ 90 ° w

"' 60 ° w

'" 30 ° W

0°

+

~0 °

~ E

8G ° E

90 ° E

120 ° E

150 ° ~

180 ° E

Fig. 7. Virtual geomagnetic pole positions through the Matuyama-Brunhes transluon, calculated from the site averaged data of Fig. 6. Site numbers are shown alongside pole positions.

50

reversed, but site 22, 15 c m a b o v e the base of the shell grit gives a p a l a e o f i e l d d ir e c t io n some 75 o f r o m the geocentric axial dipole direction for the location, yielding an e q u a t o r i a l virtual g e o m a g netic pole position. T h e very b e g i n n i n g of the transition is t h erefo r e c o n t a i n e d within the glacially g e n e r a t e d u n c o n f o r m i t y , lying at the base of the K a i k o k o p u Shell Grit. As i n t e r m e d i a t e directions are r e c o r d e d in at least 10.5 m of the overlying U p p e r W e s t m e r e Siltstone, we c o n c l u d e that m o s t of the transition is actually recorded. M o s t e s t i m a t e s of the d u r a t i o n of the M a t u y a m a - B r u n h e s transition fall b e t w e e n 4000 a n d 10,000 years [16], w h i c h w o u ld i m p l y a m i n i m u m s e d i m e n t a t i o n rate in the U p p e r Westm e r e Siltstone of b e t w e e n 2.5 and 1.0 m m per year. This is b r o a d l y consistent with the average s e d i m e n t a t i o n rate of 0.6 m m p e r year e s t i m a t e d for the wh o l e Castlecliffian section [4], given that any n o n - d e p o s i t i o n and sub-aerial erosion d u r i n g the glacial stages cou l d n o t be q u a n t i f i e d and that significant v a r i a t i o n s f r o m this average are to be e x p e c t e d in the different units. Virtual g e o m a g n e t i c pole positions c a l c u l a t e d f r o m the site m e a n s of Fig. 6 define a large easterly excursion at e q u a t o r i a l latitudes superimpose d on an otherwise far-sided p a t h (Fig. 7). This is m a r k e d l y different f r o m p r e v i o u s l y p u b l i s h e d records of the M a t u y a m a Brunhes transition, m o s t l y from n o r t h e r n h e m i s p h e r e sites [e.g. 17, 9], which are p r e d o m i n a n t l y near-sided a n d c o n t a i n few l o w - l a t i t u d e poles. W e are cu r r en t l y u n d e r t a k i n g further, intensive studies to i m p r o v e the r e s o l u t i o n and a c c u r a c y of the M a t u y a m a - B r u n h e s transition record from the W a n g a n u i C o as t al section.

Acknowledgements This w o r k has b e e n s u p p o r t e d by grants f r o m the I n t e r n a l G r a n t s C o m m i t t e e of Victoria University of W e l l i n g t o n . W e also gratefully acknowledge the assistance of U J . G a y l o r in the field, a n d m a n y i n f o r m a t i v e discussions with B. Pillans a n d the late C.A. F l e m i n g .

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G.M. T U R N E R A N D P.J.J. K A M P

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