Holocene evolution of an estuary on a tectonically rising coast: the Pakarae River locality, eastern North Island, New Zealand

Sedimentary Geology, 80 (1992) 151-165 Elsevier Science Publishers B.V., Amsterdam

151

Holocene evolution of an estuary on a tectonically rising coast: the Pakarae River locality, eastern North Island, New Zealand K e l v i n R . B e r r y m a n a, Y o k o O t a b a n d A l a n G . H u l l a a D.S.LI~ Geology and Geophysics, P.O. Box 30368, Lower Hutt, New Zealand b Geography Department, Yokohama National University, Hodogaya-Ku, Yokohama 240, Japan

(Received November 15, 1991; revised version accepted January 1, 1992)

ABSTRACT Berryman, K.R., Ota, Y. and Hull, A.G., 1992. Holocene evolution of an estuary on a tectonically rising coast: the Pakarae River locality, eastern North Island, New Zealand. In: J.F. Donoghue, R.A. Davis, C.H. Fletcher and J.R. Suter (Editors), Quaternary Coastal Evolution. Sediment. Geol., 80: 151-165. Estuarine and beach deposits in the vicinity of the present coastline at Pakarae River record the infilling of an estuary and subsequent development of a sequence of seven marine terraces during Holocene time. At the maximum of the last glaciation about 18,000 years ago the shoreline at the ancestral Pakarae River was approximately 20 km east of the present shoreline. By about 9000 years BP the sea had transgressed across most of that coastal plain to lie within a few hundred metres of the base of the present coastal hills. Seventeen radiocarbon ages from estuarine deposits record the overall rise in post-glacial sea level, but in the period c. 9500-7000 yrs BP there are reversals to the overall rising trend. Between 9500 and 8500 yrs BP there appears to have been a eustatic fall in sea level of at least 4 m. This observation is supported by data from several other localities around New Zealand. Maximum transgression occurred about 6500-7000 yrs BP when the sea reached the base of hiUslopes and an extensive estuary existed behind a barrier bar. Since that time the barrier bar disappeared, probably due to stranding in an uplift event, and the coastline advanced progressively outward toward its present position. Coastal progradation (sea level regression) and subsequent erosion have occurred in association with episodic large earthquakes at about 6700, 5400, 3910, 2450, 1570, 1000 and 600 yrs BP. The present distribution of terraces has been influenced by coastal erosion, which has removed all trace of some terraces from some areas, and river erosion has modified the marine terraces near the river.

Introduction

ferred to be in association with major earthquakes (Ota et al., 1991).

A t P a k a r a e River, a b o u t 25 km n o r t h e a s t of G i s b o r n e (Fig. 1), a s e q u e n c e of seven uplifted H o l o c e n e m a r i n e terraces provides a record of

a h i n t e r l a n d c o m p o s e d of N e o g e n e m a r i n e sedim e n t a r y rocks. A l o n g the lower reaches of the

coastal e v o l u t i o n within the rapidly uplifting zone of the H i k u r a n g i s u b d u c t i o n margin. T h e highest a n d oldest terrace is u n d e r l a i n by e s t u a r i n e de-

river, fluvial terraces occur at m a n y levels recording both d o w n c u t t i n g w h e n sea level was at lower levels t h a n present, a n d the response to coastal

posits that record the t r a n s g r e s s i o n following the last glaciation ( O t a et al., 1988). Lower a n d y o u n g e r terraces were f o r m e d episodically, in-

uplift. T e c t o n i c analyses of the coastal terrace s e q u e n c e have b e e n m a d e by O t a et al. (1983, 1991) a n d B e r r y m a n et al. (1989). I n this p a p e r we emphasise g e o m o r p h i c evolution of the coastal area e x t e n d i n g a b o u t 1 kilometre n o r t h a n d south of the river m o u t h a n d a b o u t 1 kilometre u p s t r e a m to the limit of m a r i n e influ-

Correspondence to: K.R. Berryman, D.S.I.R. Geology and Geophysics, P.O. Box 30368, Lower Hutt, New Zealand.

P a k a r a e River is a m e d i u m - s i z e d river d r a i n i n g

0037-0738/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved

152

K.R |}|.Rl~'t %l\X t l

ence in the former estuary. We use data from natural exposures along the present river channel, supplemented by shallow augering. We have obtained 28 radiocarbon ages from wood and shell samples (all noted in terms of Libby haiflife of 5568 yrs with 2 standard deviations of counting errors) from these exposures to constrain the timing of coastal evolution. The Pakarae River locality provides a valuable area for study of coastal evolution in uplifting regions because of extensive natural exposures, and abundance of dateable material. The uplift rate at Pakarae River is about 3 m m / y r (Ota et al., 1991) and has been sufficient to raise almost

all of the Holocene transgression deposits rdatcd to the present sea level. Examination ol thesc exposed sediments provides a rarely available opportunity to assess the relative significance of sea level movement and tectonic uplift in coastal evolution through the Holocene. Local tectonic and geologic setting

The Pakarae River locality is situated within the complexly deformed area of eastern North Island known as the East Coast Deformed Belt (ECDB) of Sp6rli (1980) and Pettinga (1982). To the east, approximately 150 km offshore, the Pa-

I

178"E North Island

' ~

40 ~ ,~

Fig 6

>

~~,._.. dary ol ~i)"~e"Region C of Ota et al (1983] Reverse fault inferred by Ota el al (1991} // 38"

60~ram/y, / / --

,n,.'~.
-

~/

Fig

South Island

/

t

ARIEL BANK c~ /

% i/

J,q

39" m

l

J

°E Fig. 1. M a p

',

~

'I ,]

0 ~

+0

J km

Cb

showing location o f s t u d y a r e a a n d active, o f f s h o r e s t r u c t u r e s . B a t h y m e t r y in m.

HOLOCENE EVOLUTION OF AN ESTUARY ON A TEC'TONICALLY RISING COAST

cific plate is being subducted at a rate of about 50 mm/yr beneath the leading edge of the Australian plate at the Hikurangi Trench. In the terminology of Cole and Lewis (1981) the study area at Pakarae River is part of the highest accretionary ridge at the inner-most part of the subduction complex. There is a pronounced northeast-southwest structural grain to the region and within the highest accretionary ridge in southern Hawkes Bay, Pettinga (1982) demonstrated strong contractional deformation. This deformational style extends offshore (Lewis, 1971; Lewis and Bennett, 1985) to the east of Pakarae River, but onshore recent faulting is a mixture of both reverse and normal (Francis, 1984; Mazengarb, 1984). Ota et al. (1991) argue that the formation of the Holocene marine terrace se66

153

quence at Pakarae River, and in a region extending 15 km northeastward, is in response to episodic movement on an active reverse fault located immediately offshore (Fig. 1). No other localities with comparable high uplift occur to the north along the coast until just west of East Cape (Ota et al., 1983). Marine siltstones and mudstones of Oligocene and Miocene age (Kingma, 1964) occur at the Pakarae River locality, separated by the northstriking Pakarae fault.

Present-day geomorphology The dominant geomorphic elements at the Pakarae River locality are deeply dissected coastal 67

68

82

82

81

81

80

80

66

67

68

Fig. 2. Geomorphic m a p of coastal Pakarae River area. The sequence of marine terraces is labelled T1-T7. Letters refer to locations where samples have been obtained for radiocarbon dating (Table 1). N u m b e r s are elevations of terrace surfaces amsl. T h e position of the profile shown in Fig. 4 is shown. Terrace sequence after Ota et al. (1991). Grid on figure is N Z M S 260 m a p grid.

l -4 4

hills, which are disrupted by surficial slumping, and a coastal plain comprising a sequence of fluvial and marine terraces. A dune field and beach ridge complex are prominent components of the geomorphology in the southern part of the area. Near the mouth of the Pakarae River, on both the east and west banks, the coastal plain comprises a sequence of Holocene marine terraces (Ota et al., 1983, 1991). Four terraces on the west bank are partially obscured by a sand dune belt and beach ridge system. The terrace sequence is also offset by active traces of the Pakarae fault (Fig. 2). The highest terrace (T1) is 27 m amsl on the upthrown side of the fault and extends up-valley to merge with a prominent fluvial terrace. In the coastal area, T1 is underlain by estuarine silt containing shells and fluvial beds with tree stumps in growth position. Lower terraces on the west bank of the river (T2 at 12.7 m amsl, T4 at 7.0 m and T5 at 3.4 m) are regressive and composed of thin beach deposits which unconformably overlie eroded estuarine sediments related to the transgressive phase. Terraces have progressively younger ages at each lower level and record a progressive fall in relative sea level. At one T1 locality, to the west of the Pakarae fault, shells from beach deposits resting on mudstone basement have very similar ages as shells from estuarine silt. On the east bank of the Pakarae River, five marine terraces are composed of thin beach deposits resting on wave-cut shore platforms cut in Miocene mudstone. The morphology of the terraces on both the west and east banks is of subhorizontal surfaces separated from higher and lower terraces by steep terrace risers. Riser heights vary between 2 and 11 m. Radiocarbon ages of terrace deposits show that not all terraces occur on both banks of the river. Terraces 1, 2, 4 and 5 occur on the west bank of the Pakarae River and Terraces 3, 4, 5, 6 and 7 occur on the east bank. Terrace ages are assigned (Table 1) from the youngest date obtained from each terrace since that is inferred to be close to the time at which uplift stranded one terrace tread and fell, relatively, and began to form a terrace at about mean sea level (Hull, 1987; Berryman et al., 1992).

k t ~, B I RI.Pi '<-It ~, I ; ,

Description of Holocene sediments

Sediments of the Holocene transgression Several sedimentary facies occur within the deposits of the transgressive phase. Truly estuarine sediments are dominated by silt, sometimes with distinct shell horizons usually composed of dual valve molluscs of intertidal estuarine habitat, and often with finely disseminated redeposited wood and forest litter material. Estuarine silt often interfingers with coarse sand and fine gravel, perhaps in estuary margin or bay head situations where the fluvial influence was stronger. Fluctuations between estuarine, fluvial and beach environments are inferred from shelly sand and silt units, erosional unconformities, tree stumps in growth position, and beach gravel with shell fauna of rocky shore habitat. Ota et al. (1983) presented the first description of the stratigraphy of estuarine deposits beneath T1. Since that time several further radiocarbon ages especially from detrital wood and organic silt from near the base of the sequence have been obtained. Ota et al. (1991) note the stratigraphy but include only sufficient ages to show the generally upward younging sequence that terminates at about 6700 yrs BP (Fig. 3). Previous descriptions of Terrace 1 deposits have only presented a section at Loc. A (e.g. Ota et al., 1983, 1991). At I_~c. Z (Fig. 2), about 500 m upstream from Loc. A there is another section comprising three parts, each separated by a few metres, has been dated at each level (Fig. 2, 3). There are significant apparent age reversals in this composite section. Trees in growth position, from three horizons, apparently growing within and overlain by laminated silt, indicate fluctuations in base level at the probable inner margin of the estuary. At about 2.0 m above mean sea level (amsl) at Loc. Z, a coarse, iron-stained sand may indicate an erosion break in the section. At about 8 m amsl charred remnants of a shrub forest, probably of manuka (Leptospermum scoparium), suggest that it was destroyed by the fall of an overlying tephra. This part of the former estuary, must, therefore have been emergent at this time. Radiocarbon ages on wood from Loc. Z are all

HOLOCENE EVOLUTION OF AN ESTUARY ON A TECTONICALLY RISING COAST

from tree stumps, apparently in growth position (Fig. 3). The oldest (9380 + 220 yrs BP; NZ 7084), is from 8.0 m amsl from trees growing in estuarine silt. The growth of small trees in the laminarbedded estuarine silts indicate a relative fall in base level prior to the age of the shrub forest. The second oldest is from 6.2 m amsl and the youngest is from 1.8 m amsl. Laminar-bedded silt overlies each horizon in which the trees grew. We therefore infer a relatively rising sea level to inundate the forest at or just younger than ages obtained from the outer parts of the trees. Growth-position tree stumps and detrital wood are also prominent in the stratigraphic section exposed at [x~c. A (Fig. 3). At about 7 m amsl detrital wood has an age of 8453 + 110 yrs BP (NZ 7807). Further wood occurs at 5.1 m amsl with an age of 8650 + 240 yrs BP (NZA 1209) and at 3.6 m amsl detrital wood has an age of 9160 + 360 yrs BP (GaK 10460).

Sediments of the post-transgression period The stratigraphy of younger regressional terraces (T2-T7) is described in detail by Ota et al. (1991). Terraces are composed of beach deposits that rest either on wave-cut shore platforms developed on Tertiary-age mudstone, or wave-cut platforms cut in older transgressive deposits. The stratigraphy of terraces on the east bank of the river is shown in Fig. 4. Ages of terrace deposits and correlations are listed in Table 1. In most cases there are several ages from each terrace. The exception is Terrace 3, which has only a single age determination of 3910 + 140 yrs BP (NZ 7130) from Loc. G. Several ages fall well outside the cluster typical for each terrace. At Loc. F, a date of 2920 + 440 yrs BP (GaK 10461) on shells from beach and dune sands is substantially younger than the c. 5500 yrs BP ages of shells from beach deposits of the terrace and may date the younger accumulation of dune sand. At Loc. C, an age on shells of 760 + 200 yrs BP (GaK 10465) was obtained from Terrace 4. This is clearly incompatible with other ages and may be from a Polynesian occupation horizon. These ages that appear unreasonable for

155

their stratigraphic position are noted in Table 1, but are not included further in the interpretation. The height and areal distribution of Terraces 2-7 are shown in Fig. 2 and their time and mechanism of formation have been discussed in detail by Ota et al. (1983, 1991). Each terrace is considered to have been formed at the time of coastal uplift associated with movement on a fault located offshore. The height of risers between each of the terraces is a measure of the amount of uplift in each area, except near the Pakarae fault. Here, terrace risers on the upthrown side of the fault represent both regional uplift and differential movement on the Pakarae fault, which Ota et al. (1991) have shown ruptures at the same time that regional uplift occurs. Chronology of sea level changes at Pakarae River A detailed evaluation of the fluctuating baselevel in the estuary is hampered by problems in comparing radiocarbon ages determined by Gakushuin University, Japan (GaK ages in Table 1) and Institute of Nuclear Sciences, DSIR, New Zealand (NZ ages in Table 1). Gakushuin ages are not corrected for the ocean reservoir effect that is about 336 yrs in the New Zealand coastal environment (Jansen, 1984). However, we cannot confidently subtract 336 yrs from the Gakushuin ages to make them directly comparable with the NZ ages because the ocean reservoir effect is dependent on the degree of carbon isotope fractionation, t$13C//14C ratio data are not available for the Gakushuin samples and so we cannot judge whether there is variation in the carbon isotope fractionation. There also appears to be a difference between wood and shell ages obtained from the same horizon (compare GaK 10459 and 10460 at Loc. C; Table 1). A relative sea level curve for the period c. 10 ka BP to present, during the period when rapid infilling of the Pakarae River estuary took place in association with the Holocene transgression and subsequently when sea level has been relatively stable, is compared with the eustatic sea level curve developed by Gibb (1986) in Fig. 5. We use only NZ radiocarbon ages to overcome incompatibility of ages between NZ and GaK

151~

K . I < l';t~Rl,~'l?;~,\ ~, [ i \ I

TABL[{ I Radiocarbon ages and details of samples from Pakarae River area Sample

Species and ecology ~'

Location A, grid ref. 671816 d. shell As, estuary shell As, estuary shell As, estuary shell shell As, estuary shell branch twigs branch shell As, estuary shell organic silt Location Z, grid ref. 672821: small tree tree tree

Elevalion I m amsl)

Terrace correlation

Radiocarbon age (yrs BP) t~

I.ab no. ~

Reference

15.5 15.5 14.1 14.1 12.2 111.4 7.0 5.1 3.6 3.5 1.9 - 1.6

TI T1 TI TI T1 TI TI TI TI T1 T1 T1

69211 ± 240 74411 ± 480 6740 ± 300 7380+_ 460 69411 +_ 11211 8579 +_ 17(1 8453 +_ 110 8650 + 2411 9160 +_ 3611 9960 ± 500 96011 + 184 10360 + 2611

NZ 7088 GaK 10456 NZ 5572 G a K 10457 G a K 10458 N Z A 1438 NZ 7807 N Z A 1209 G a K 10460 G a K 111459 N Z A 1437 N Z A 1357

Ota et al., 1991 Ota et al., 1983 Ota et al., 1983 O t a e t a l . 1983 Ota et al., 1983 This study This study This study Ota et al., 1983 Ota et al.. 1983 This study This study

T1 TI TI ~

9380 +_ 2211 8730 +_ 220 8200 ± 211/

N Z 7094 N Z 7093 N Z 7012

This study This study This study

TI T2

8120 +_ 240 5490 +_ 120

NZ 7126 NZ 7113

This study Ota et al., 1991

3.4 4.6 4.1

T1 ~ T4 g T5

7710 +_ 300 23611 +_ 260 1680 _+ 120

G a K 10464 G a K 10465 N Z 7101

Ota et al., 1983 Ota et al., 1983 Ota et al., 1991

11.0

T1 h

6880 _+ 200

N Z 7124

Ota et al., 1991

10.0

T1 h

6820 +

180

N Z 7131

Ota et al., 1991

12.8

T3

3910 +

140

N Z 7130

Ota et al., 1991

Location I, grid ref. 678814: shell exposed rocky shore

11.0

T4

2450 _+ 100

N Z 7104

Ota et al., 1991

Location C, grid ref. 671813: shell exposed rocky shore shell

7.0 7.4

T4 T4 ~

2700 +- 90 760 5 : 2 0 0

N Z 7047 G a K 10463

Ota et al., 1991 Ota et al., 1983

Location F, grid ref. 671807: shell Ps, rocky shore

1.9

T5

1570 +

130

N Z 7078

Ota et al., 1991

Location J, grid ref. 678814: shell rocky shore

7.5

T5

1625 +-

70

N Z 6466

Ota et al., 1991

Location K, grid ref. 678813: shell rocky shore shell rocky shore

4.2 3.2

T6 T6

1030 -+ 1000 +

80 70

N Z 6489 N Z 6490

Ota et al., 1991 Ota et al., 1991

Location B, grid ref. 670815: shell As, estuary shell As, estuary with some rocky shore species Location E, grid ref. 672812: shell As, estuary shell shell Ps, rocky shore Location G, grid ref. 664813: shell As, estuary with some

8.6 6.4 2.2

9.1 12.7

rocky shore species shell

As, estuary with some rocky shore species

Location H, grid ref. 673813: shell Ts, intertidal exposed rocky shore species

HOLOCENE

EVOLUTION OF AN ESTUARY ON A TECTONICALLY

RISING

157

COAST

LOC A sand

24 silt ! ......... .0,, .....

LOC

3.2ka tephra

il

[ - ~

20

~

shell

' ~

detrital wood

~ - ~

tree stump

(shell)

beach gravel & sand 6920-+240 -

0 ~ ° . ......... ~.-~..~vv~'~

16

(shell)

6740-+300-

--

14

18

LOC

Z

~'¢'~,~'~'~, 14

- 14 degradation terrace

beach sand

,..:.:::....::.:.::;

(shelO 5490+-120

~' ~ o ~

water laid tephra

vvvvvv .........

'~

~

":'---------" I

estuarine silt

12

i 10

-..--. -~.=~.-..--_-. -_-_- - -_- .-..-.

)shell) 8120+-240 (wood) estuarine slit (woodl

4

organic (si,t )

aeolian sand & silt

10

estusrine silt & sand

8650_+240 estuarlne sand

-

~

'L~-';' - 12

- 10 - S/-~7 S/-~7 -v- " 8

",ANVVVVVVV VVV v v~

> ~ " "'"Z'Z'Z

-

6

stump

4

-----~---------

beach sand

10360±260 -

( t r9e3e8 0p- )+ 2 2 0 s t u m

1

,~---Jk-

-

estuarino silt

0 --

12

8

8453+-110 tephra -

(shell1 9600±184

2 mean sea level

unconformity

gravel

22

"" o ~:, <:> o o

-

~

lephra

....

uppermost marine deposits - -

B 16

,

V v v v

I

6 4

tephra iron stained sand

f

" .oo.to

\

P/' river level

':' ° °°':":',-F

-2

fluvial gravel

Fig. 3. Stratigraphic sections of transgressive deposits at Pakarae River. Locations Z, A and B are shown in Fig. 2. All radiocarbon ages are from the Institute of Nuclear Sciences Laboratory and are in relation to Libby halflife of 5568 yrs with 2 sd errors.

ages. The curve is based on ages of wood samples from the base of sections at Loc. A and Z, shell ages from the upper part of the sections A and B, and shell ages from younger terraces. The contrast between the relative sea level curve devel-

oped for the Pakarae River area with the New Zealand eustatic curve is striking. For every interval of time except the present-day, the palaeo sea level position at Pakarae River is substantially above the inferred eustatic position, as one would

Notes to table 1: a A s = Austrovenus stutchburyi; Ts = Turbo smaragdus; C s = C o o k i a sulcata; Ps = P h o l a d i d e a suteri. b Radiocarbon ages in terms of Libby halflife of 5568 yrs, 2 sd errors. c N Z = Institute of Nuclear Sciences Laboratory; N Z A = age calculated by Accelerator Mass Spectrometry method; GaK = Gakushuin University Laboratory. d Grid references in terms of New Zealand Map Grid of N Z M S 260 Sheet Y17. Grid is shown on Fig. 2. e The elevation of this tree is considered suspect for its age, it may have slumped downslope to its present position. See text for further discussion.

f The elevation of this sample is inconsistent for its age. The shell sample is thus considered to have been eroded upstream and redeposited at its present site. Although the age of this sample is consistent with other ages for T4, the elevation is several metres lower than expected. The sample may have been redeposited. h Location is on the downthrown side of the Pakarae fault (see Fig. 2). i A g e is substantially younger than expected for T4. The sample may come from a midden deposit.

] 55

KR

expect on a tectonically rising coast. Former relative sea level reaches a maximum of c. 18 m ab¢we present sea level at the culmination of the post-glacial transgression c. 6.7 ka BP. The culmination of eustatic sea level rise also marks the change from a rising to falling trend in relative sea levels at Pakarae River. This change in trend can also be restated in terms of principal factors affecting coastal evolution. Prior to c. 6.7 ka BP, sea level rise was more significant than tectonics in shaping the coastal environment but the opposite is true after the c. 6.7 ka BP inflection point. The shape of the relative sea level curve indicates that tectonics was a significant factor because the curve is much flatter than the eustatic curve for the same period. If we assume that the thickness of sediment infilling the Pakarae estuary is a first approximation to the amount of sea level rise between c. 10.0 and 6.7 ka BP, while sea level rose from - 3 4 m to the present only 12 m of sediment was deposited in the Pakarae River estuary in the same time span. The 22 m difference ( 3 4 - 12 m) is a crude measure of the amount of tectonic uplift in the 3.5 ka period. Twenty-two metres must be a maximum uplift because erosion of estuary material after successive uplift events would further reduce net sediment accumulation. The occurrence of unconformities and weathering horizons within the

EAST

BANK

I~I-RR~ MAN

t ! \!

stratigraphic sections exposed are indicative ot that loss of sediment. Of the data displayed in Fig. 5 we suspect that the c. 8.2 ka BP age on the apparent growth position tree stump is unreliable. Good reproducibility has, in general, been a feature of radiocarbon age determinations in our study so we suspect that the observed elevation of this sample, rather than the age, is unreliable. Although the tree stump appears to be in growth position it now seems likely that it has fallen down the steep terrace edge and come to rest fortuitously in an upright position. It does not seem possible that sea level could inundate forest at the 6 - 7 m level at a c. 8.7-8.4 ka BP, fall by at least 6 m and still allow time for trees up to 1 m in diameter to grow prior to their inundation at c. 8.2 ka BP. The remaining data suggest there were marked fluctuations in sea level in the period c. 9.5 ka to c. 8.5 ka BP. The apparent offset in sea level trend at about 8.5 ka BP (Fig. 5B) coincides with a change from ages on wood to shell samples, a dating problem noted above. The sea level fluctuation coincides approximately with a marked change in the sea level trend observed by Gibb (1986) (Fig. 5A). Additionally, Ota et al. (1988) describe eight other sites along the east coast of the North Island where tree stumps in growth position, growing in and overwhelmed by estuar-

PAKARAE

RIVER

SOUTH

NORTH T7

T6

T5

T4

T3 /.

39,o_+1,o,sho,,,

' ~ -

,,,

-~ lo

o

_

~

L

o

.

c

//////Y2~5o~_~o'o~

_~/ / / /

V/I

shell)i/j

I / / / / / / / / / / / / / / / A

lo

o

Fig. 4. Profile of marine terraces on east bank of Pakarae River. Location of profile is shown on Fig. 2. Terraces are composed of terrestrial deposits, sometimes containing tephra, over beach deposits containing shell which, in turn, overlie wave-cut abrasion platforms cut in Tertiary mudstone.

159

HOLOCENE EVOLUTION OF AN ESTUARY ON A TECTONICALLY RISING COAST

times be identified by their field characteristics but otherwise by their mafic mineral assemblage (Froggatt and Lowe, 1990) in conjunction with radiocarbon ages on shell or wood in stratigraphic proximity. The oldest tephra immediately overlies a wood sample from Loc. Z that has an age of 9380 + 220 yrs BP. This tephra has a mafic mineral assemblage of hypersthene and augite, suggesting a Taupo Volcanic Centre source. Froggatt and

ine deposits have ages of c. 9-8 ka BP. We therefore believe that the fluctuation in sea level that exposed some of the Pakarae estuary between 9.5 and 8.5 ka BP came about by eustatic sea level fall rather than tectonic uplift. Tephra stratigraphy Several tephras occur within the terrace deposits at Pakarae River (Fig. 3). They can some-

xlO3radiocarbon years B.P.

A PRESENT SEA LEVEL

/" . . . . . . . . . . . .

, , , /,/~

/../" . -

. . . . . =---__"S_. . . . - . . . . . ~ . . . . . . .

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-5

-5

-10

-10

//

-15

/ ,y/

-20

j j'

/~-'~

-25

// if /

/ / //

-30 -35

0

~ .f~"'v

/ /

/

,/ ,'

NEW ZEALAND EUSTATIC CURVE OF GIBB 119861

-

-15

dashed lines form an

-

-20

envelope defined by error bars on ages used tO conStruCt the Curve

,/ /

~ -

-25 -30

I1~1/1 I ¢'¢11

/[i

-35 -40

-40

I

I

I

t

I

I

t

[3 20

woo,

2O

T1 12

,o

15

T4 ~ ' ~

70

/'

~p

T5

E

s ÷s--T'~22.

5 0

I

PAKARAE RIVER note offset in curve at the transition from

15

I

I'

+,

10 5

PRESE SEA 0 inferred timing of uplift events

/ -5

-5

shell & detrital wood sample ages ages of tree stumps in growth position (Outer parr)

+

-10

-10

indicate trend of sea level inundating trees in growth position

error

2sd

I

1

I

I

I

10

9

8

7

6

bars L 5

I

I

4

3

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Lx)we (1990) report an error weighted mean age of 9050 _+ 40 yrs BP (2 sd) for the O p e p e Tephra (which has a hypersthene + augite heavy mineral assemblage) and it is probable that O p e p e Tephra is the oldest Holocene tephra exposed at Pakarae River. At Loc. Z, a second tephra occurs about 2 m above the lowermost wood sample, which has an age of 8200 _+ 210 yrs BP. However, there is doubt about the stratigraphic position of this tree in relation to the estuarine deposits (see earlier discussion). Further study of this tephra, including its mafic mineral assemblage is warranted. Judging by the tephra's stratigraphic position (Fig.

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3) a little above a weathered sand that may represent the same erosion interval observed at similar elevation at Loc. A, the tephra may have an age of c. 9 ka BP. At an elevation e. 6.8 m in Loc. A a water-laid tephra is bracketed by ages of 865(i ,t 240 and 84533 _+ 110 yrs BP (Fig. 3). Froggatt and Lowe (1990) report a weighted mean age of 8530 _+ 10 years BP for Rotoma Tephra and this seems a likely identification for the tephra at Pakarae River. Another tephra found at Ix)c. B occurs about 2 m above shell with an age of 8120 + 240 yrs BP (NZ 7126) (Fig. 3). The tephra is approximately 0.5 m below an unconformity which in turn is

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Fig. 6. Tentative palaeogeography at c. 18,000 yrs ago at the maximum of the last glaciation. Two possible courses of an ancestral Pakarae River to a shoreline are shown. Contours are present-day isobaths in m. Low coastal hills, comprising growth folds with a reverse fault on its eastern side are shown in the present location of Ariel Bank. The c. 18,000 yr shoreline is shown to follow the prominent break in slope east of the postulated active structures and then crosses isobaths to lie closer to the isobath associated with the eustatic position of sea level.

161

HOLOCENE EVOLUTION OF AN ESTUARY ON A TECTONICALLY RISING COAST

overlain by beach deposits dated at 4590 + 120 yrs BP (NZ 7113). It has a ferromagnesium assemblage of hornblende, hypersthene and augite and, considering its stratigraphic position and mineralogy, is correlated with the Mamaku Ash,

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dated at about 7250 + 40 yrs BP (Froggatt and Lowe, 1990). These are thought to be the first known occurrences of Opepe, Rotoma and Mamaku tephra from coastal eastern North Island. Two younger tephras found in the terrace se-

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Fig. 7. A. Palaeogeographic sketch of Pakarae River mouth area, c. 9000 yrs BP. The position of the present shoreline and the base of hillslopes above the coastal plain are shown for reference. Other symbols indicate locations where sediments of this age have been observed. B. As for A, at c. 7000 yrs BP.C. As for A, at c. 5000 yrs BP.D. As for A, at c. 3000 yrs BP.E. As for A, at c. 1000 yrs BP.

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quence occur as airfall beds in terrestrial deposits overlying beach deposits. Waimihia Tephra has an error weighted mean age of 3280 + 40 yrs BP (2 sd) (Froggatt and Lowe, 1990) and is a coarse sandy yellow-brown pumice with lapilli up to 2 mm in diameter at Pakarae River. T a u p o Pumice has an age of c. 1850 _+ 20 yrs BP (Froggatt and Lowe, 1990), and is a white, coarse, sandy ash with characteristically vesicular pumice up to about 1 mm in diameter in this area. Waimihia Tephra occurs in terrestrial deposits of T1 and T2. Airfall T a u p o Pumice is found in terrestrial deposits of T4 and rafted pumice up to c. 100 mm in diameter occurs in beach deposits of T5, which have an age of c. 1570 yrs BP.

Palaeogeographic reconstruction With good exposure along the Pakarae River and at the coastline, and information gained from shallow augering and radiocarbon dating, we have established the distribution of many of the elements of the past as well as the present features of the geomorphology (Fig. 1). This data enables us to reconstruct the sequential development of the coastal geomorphology at Pakarae River. 18,000 yr BP. At the maximum of the last glaciation, about 18,000 yrs BP, sea level was about 130 m below present sea level (Fairbanks, 1989; Bard et al., 1990). This corresponds approximately to the shelf break east of the present coastline (Fig. 6). The Pakarae River would have crossed a coastal plain about 20 km wide to reach the sea at this time. It is uncertain what the course of the Pakarae River would have been; it could have been either north or south of low hills that comprise the present Ariel Bank or may have been antecedent within the hills. The hills are probably growth folds bounded on the eastern side by a reverse fault. Immediately east of Ariel Bank the present shelf break is at a depth of about 80 m suggesting c. 50 m of uplift in the past c. 18,000 years. The growth of the Ariel Bank structure would appear to decrease tO the north and the 18,000 yr shoreline may be close to the present-day 120 m isobath. In the vicinity of the present coastline the Pakarae River would have been down-cutting try-

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ing to attain grade to a shoreline some 20 km distant. Although rock levels have risen due to subsequent uplift, a fairly deep channel in the vicinity of the present Pakarae River could reasonably be expected. 9000 yrs BP. By this time sea level had risen very rapidly across the c. 20 km wide coastal plain to about - 2 2 m (Gibb, 1986). Figure 7A shows the distribution of sediments in the Pakarae River area known to be about 9000 yrs BP. The deposits are in all cases estuarine so it is reasonable to assume that a barrier existed at the river mouth. Because eustatic sea level was still about - 2 2 m below the present-day level we assume the estuary could not have reached the base of the present-day hillslopes, and fluvial terraces must have drained across a narrow plain between the estuary and the hillslope. We have shown the river flowing into the sea south of its present-day position largely because of the distribution of later estuarine deposits. 7000 yrs BP. The period about 7000 yrs BP coincides with maximum transgression. Sea level may still have been a few metres below present but there is evidence that from c. 6700 yrs BP, tectonic uplift became more important than sea level fluctuation in controlling coastal geomorphic development (Ota et al., 1991). The estuary at c. 7000 yrs BP was widespread (Fig. 7B) and the estuary margin was close to the base of the surrounding coastal hills. At Loc. E (Fig. 2) gravelly beach sediments rest on a mudstone shore platform, suggesting that the estuary was fairly large so that wave energy was sufficient to deposit gravelly beach sediments. We have no direct data of the position of the barrier bar but it seems likely that the river mouth was at the southwest end of the bay because transgressive facies deposits occur in a wide tongue that extends southwest from the present river channel beneath Terraces 2 - 5 on the west bank. 5000 yrs BP. Eustatic sea level was within about ___0.5 m of its present level and the barrier bar enclosing the estuary had largely disappeared (Fig. 7C), probably as a result of stranding during an uplift event. We say this because T2, which

H O L O C E N E E V O L U T I O N O F AN E S T U A R Y O N A T E C T O N I C A L L Y RISING COAST

was uplifted about 5450 yrs BP, is underlain by beach deposits containing largely estuarine fauna, but with a few rocky shore species, on an erosion cut surface in the estuarine deposits (Loc. D, Fig. 3). T2 would have been much wider than at present and was probably present on the east bank of the Pakarae River as well. We believe that a combination of slumping of the hillslopes above and wave erosion has removed both T1 and T2 from the final sequence of terraces on the east bank. By about 5000 yrs BP the Pakarae River was essentially contained in its present channel between T1 and hillslopes on the east bank. We have no data to constrain the position of the shoreline east of the rivermouth or the extent of Terraces T1 and T2 which have subsequently been removed. 3000 yrs BP. On the east bank, prior to the formation and uplift of T3 Terrace at c. 3910 yrs BP and after c. 5000 yrs BP, coastal erosion removed all trace of T2 and slumping or deposition of slope wash had obscured T1 on the east bank (Fig. 7D). On the west bank T2 was being narrowed by coastal erosion. By c. 3000 yrs BP, an extensive T5 Terrace is postulated on the east bank and probably on the west bank also. The distribution of landforms at this time can also be estimated from the distribution of Waimihia tephra (c. 3280 yrs BP). 1000 yrs BP. In the interval between c. 3000 yrs BP and c. 1000 yrs BP there was considerable change. The riser that now exists between T4 and T2 on the west bank must have been in its present form by c. 2450 yrs BP. Therefore, any remnants of T3 and the shape of the riser to T2 close to the river had to have formed by the time of formation of T4 at c. 2450 yrs BP (Fig. 7E). Similarly, T3, on the east bank, must have been reduced to its present width by c. 2450 yrs BP. T5 was formed c. 1570 yrs BP and the Pakarae fault ruptured both T5 and T4 by about 2 m at this time (see Ota et al., 1991 for full discussion of the Pakarae fault history) and added that increment to the scarp height across Terraces T2 and T1. By c. 1000 yrs BP coastal erosion had trimmed the T5 Terrace back to its present distribution since the uplift event postulated at c. 1000 yrs BP (T6)

163

would have been imminent. The river mouth by this time was directed a little northward than previously, in much the same position as today. Interval 1000 yrs BP to present-day. Immediately following the geomorphic configuration shown in Fig. 7E approximately 2 m of coastal uplift occurred, forming T6. About 600 yrs BP, T7 was formed by a further c. 3 m of coastal uplift. Both of these terraces survive on the east bank of the Pakarae River but have both been removed by coastal erosion or are covered by sand on the west bank. The dune belt and beach ridges (Fig. 2) developed on open coastal sites and cover T5, indicate at least some continued development since 1500 yrs BP. Until c. 5000 yrs BP the sand and fine gravel that the dunes and beach ridges are composed of had been building the barrier bar. With the end of that phase of coastal evolution, a surplus of sediment was available and has been transported by longshore drift to form the dune belt and beach ridges. Other larger scale examples of rapid barrier growth enclosing a bay during the early Holocene followed by ridge development and extensive dune fields at the downdrift end of bays include Birdlings Flat-Lake Ellesmere (Armon, 1970) and Wairau Valley (Pickrill, 1976). Conclusions Our study at Pakarae River has shown that during the Holocene the coast has evolved primarily in response to sea level rise during the post-glacial transgression, and to episodic tectonic uplift. Other factors such as bedrock hardness and sediment supply have been of secondary importance. During the early part of the Holocene until about 6.7 ka BP sea level rose, on average, about 10 m/ka. This rate is about three times the average uplift rate at the coast and was the dominant factor in coastal evolution. Sea level stabilised about 6.5 ka BP (Gibb, 1986) with fluctuations no larger than _+0.5 m about the present level in the period 6.5 ka BP to present. In this period, tectonic movement, inferred to be episodic in association with major earthquake events, was the dominant factor in coastal evolution.

Because tectonic movement is episodic, the relative importance of coastal processes is dependent on the time interval. For example, ira any period longer than about 1600 yrs (the longest return period of uplift inferred from terrace ages) at least one uplift event will have occurred and that event will dominate other processes of coastal evolution. During shorter time periods of several hundred years when sea level was relatively stable and no major uplift occurred, then ongoing relatively slower coastal processes such as sediment redistribution by ocean currents and wind, and shore platform backwasting and down-cutting by both wave abrasion and chemical erosion, were the primary processes. The findings of our study at Pakarae River in relation to the process of formation and emergence of a former estuary and subsequent staircase flight of terraces have application to other coastal settings where the same suite of processes exist. The eustatic sea level curve must be similar, rates of tectonic movement should be less than sea level movement in the early part and greater in the later part of the period, tectonic movement should be episodic to form the characteristic staircase of terraces and, for full development of a flight of terraces, the interval between seismic uplift events needs to be of sufficient length for processes of terrace development to produce a terrace extensive enough to survive as a geomorphic feature after tectonic uplift. In soft bedrock situations, such as the Pakarae River locality, shore platforms form rapidly, but in hard bedrock locations the interseismic interval may not be long enough and the coastal geomorphology may not develop a staircase of terraces (see also Berryman et al., 1992), for further description of processes in the evolution of Holocene marine terraces in uplifting regions). Several parts of eastern North Island, New Zealand, and coastal areas near convergent plate margins in the circum-Pacific region have similarly developed coastal geomorphology. Further resolution of the timing and amplitude of tectonic movement during the period c. 10-6.5 ka B.P. may be possible by further study of the sediments of the former Pakarae River estuary. Detailed sedimentary analysis coupled with mi-

cropalaeontological studies attd further radiocarbon dating may establish a better event stratigraphy and resolve present inconsistencies between wood and shell ages.

Acknowledgements This study was carried out with financial support from the Japanese Ministry of Education, Science and Culture (Grant-in-aid for Overseas Scientific Survey; project numbers 60041029 and 61043025; project leader, Yoko Ota), and DSIR Geology and Geophysics (formerly New Zealand Geological Survey). We thank Len Brown, Dave Francis, Katsuhiko lshibashi, Ken Yamashina, Nozomi Iso and Masumi Miyoshi for assistance in the field. Dave Francis, Sarah Beanland and Richard Pickrill provided insightful reviews of an early draft and we thank them for their interest. Jeff Lyall drew the figures and Pat Bratton typed the text.

References Armon, J.W., 1970. Recent shorelines between Banks Peninsula and Coopers Lagoon. M.A. thesis lodged at University of Canterbury. Bard, E., Hamelin, B. and Fairbanks, R.G., 1990. U-Th ages obtained by mass spectrometry in corals from Barbados: sea level during the past 130,000 years. Nature, 346: 456458. Berryman, K.R., Ota, Y. and Hull, A.G., 1989. Holocene paleoseismicity in the fold and thrust belt of the Hikurangi subduction zone, eastern North Island, New Zealand. Tectonophysics, 163: 185-195. Berryman, K.R., Ota, Y. and Hull, A.G., 1992. Holocene coastal evolution under the influence of episodic tectonic uplift: examples from New Zealand and Japan. Quat. Int. (in press). Cole, J.W. and Lewis, K.B., 1981. Evolution of the TaupoHikurangi subduction system. Tectonophysics, 72: 1-21. Fairbanks, R.G., 1989. A 17,000-year glacio-eustatic sea level record: influence of glacial melting rates on the Younger Dryas event and deep-ocean circulation. Nature, 342: 637642. Francis, D.A., 1984. The Marau Beach Fault, East Cape area, New Zealand (abstr.). Recent Crustal Movement Symp. Pacific Region. Wellington, p. 16. Froggatt, P.C. and Lowe, D.J., 1990. A review of late Quaternary silicic and some other tephra formations from New Zealand: their stratigraphy, nomenclature, distribution, volume and age. N.Z.J. Geol. Geophys:, 33: 89-109.

HOLOCENE EVOLUTION OF AN ESTUARY ON A TECTONICALLY RISING COAST Gibb, J.G., 1986. A New Zealand regional Holocene eustatic sea level curve and its application to vertical tectonic movements. In: W.I. Reilly (Editor), Proc. Recent Crustal Movements Symp. Pacific Region. R. Soc. N.Z. Bull., 24: 377-396. Hull, A.G., 1987. A late Holocene uplifted shore platform on the Kidnappers Coast, North Island, New Zealand: some implications for shore platform development processes and uplift mechanism. Quat. Res., 28: 183-195. Jansen, H.S., 1984. Radiocarbon dating for contributors. N.Z. Inst. Nucl. Sci. Rep., 328, 72 pp. Kingma, J.T., 1964. Sheet 9, Gisborne (lst Ed.) Geological Map of NZ 1 : 250 000 DSIR. Department of Scientific and Industrial Research, Wellington. Lewis, K.B., 1971. Growth rate of folds using tilted waveplaned surfaces: coast and continental shelf, Hawke's Bay, New Zealand. R. Soc. N.Z. Bull., 9: 225-231. Lewis, K.G. and Bennett, D.J., 1985. Structural patterns on the Hikurangi Margin: an interpretation of new seismic data. In: K.B. Lewis (Editor), New Seismic Profiles, Cores and Dated Rocks from the Hikurangi Margin, New Zealand. N.Z. Oceanogr. Inst. Oceanogr. Field Rep., 22: 3-16. Mazengarb, C., 1984. The Fernside Fault: an active normal

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fault, Raukumara Peninsula, New Zealand. N.Z. Geol. Surv. Rec., 3: 98-103. Ota, Y., Yoshikawa, T., Moriya, I., Ikeda, Y., Iso, N. and Hull, A.G., 1983. Holocene marine terraces in the northeastern coast of North Island, New Zealand. Abstr. Int. Symp. Coastal Evolution in Holocene, Tokyo, pp. 109-112. Ota, Y., Berryman, K.R., Hull, A.G., Miyauchi, T. and Iso, N., 1988. Age and height distribution of Holocene transgressive deposits, eastern North Island, New Zealand. Palaeogeogr., Palaeoclimatol., Palaeoecol., 68: 135-151. Ota, Y., Hull, A. and Berryman, K.R., 1991. Coseismic uplift of Holocene marine terraces, Pakarae River area, eastern North Island, New Zealand. Quat. Res., 35: 331-346. Pettinga, J., 1982. Upper Cenozoic structural history, coastal southern Hawke's Bay, New Zealand. N.Z.J. Geol. Geophys., 25: 149-192. Pickrill, R.A., 1976. The evolution of coastal landforms of the Wairau Valley. N Z. Geogr., 32: 17-29. Sp6rli, K.B., 1980. New Zealand and oblique-slip margins. Tectonic development up to and during the Cenozoic. In: P.R. Ballance and H.G. Reading (Editors), Sedimentation in Oblique-Slip Mobile Zones. Int. Assoc. Sedimentol. Spec. Publ., 4: 147-170.