Holocene coastal evolution under the influence of episodic tectonic uplift: Examples from New Zealand and Japan

Quaternary International, Vol. 15/16, pp. 31-45, 1992.

1040-6182/92 $15,00 © 1992 INQUA/Pergamon Press Ltd

Printed in Great Britain. All rights reserved.

HOLOCENE COASTAL EVOLUTION UNDER THE INFLUENCE OF EPISODIC TECTONIC UPLIFT: EXAMPLES FROM NEW ZEALAND AND JAPAN Kelvin R. Berryman,* Yoko O t a t and Alan G. Hull*

*Institute of Geological and Nuclear Sciences, PO Box 30368, Lower Hutt, New Zealand fGeography Department, Yokohama National University, Tokiwadai, Hodogaya-ku, Yokohama, Japan 240

Coastal geomorphology in many parts of New Zealand and Japan has been strongly influenced by tectonic uplift since the culmination of the Holocene transgression at 5.5-7 ka ago. Examples of historic coastal uplift in Japan and New Zealand, accompanied by major earthquakes provide analogues for interpreting coasts where Holocene marine terraces are arranged in staircase fashion. Despite complexities in their origin, there is commonly only one shore platform for each relative stillstand of sea level. Shore platforms are subhorizontal, and are usually graded to low tide level. The width of platforms depends primarily on the erodibility of bedrock and the length of time available for formation. The simplest setting for assessing the influence of uplift on coastal evolution requires readily erodible, homogeneous bedrock, and a microtidal environment so that wide shore platforms of limited surface relief can be cut within + 1.0 m of mean sea level. Earthquake-related uplift (where uplift is greater than the tidal range) is an instantaneous and catastrophic event that overwhelms on-going processes of shore platform formation. Determining the degree of tectonic control on coastal evolution is more difficult where hard or layered, dipping, bedrock exists or where the coast is subject to spatial variation in wave energy. Temporal and spatial variation in wave energy and variation in sediment supply make beach ridges more difficult to relate to former sea-level positions than either hard or soft bedrock platforms. More precise measurement of past uplift events are possible in coral reef environments than at higher latitude sites because coral reef colonies grow within _+ 0.10 m of the mean low water spring tide and are sensitive to sea-level changes.

INTRODUCTION 100 ° E

In many parts of Japan and New Zealand (Fig. 1) coastline changes have occurred catastrophically during large to great earthquakes (M, magnitude 7 or greater). In these two countries the land has usually been uplifted, in response to contractional strains at convergent plate boundaries. At Boso Peninsula on the southeast coast of Honshu, Japan (Fig. 2) two historic uplift events, one in 1703 and the second in 1923, have been described (e.g. Matsuda et al., 1978). By analogy, higher elevated shorelines were ascribed by Sugimara and Naruse (1954, 1955) to uplift during prehistoric earthquakes. Similarly, Wellman (1967) documented 2.7 m of uplift accompanying the -M8.2, 1855 earthquake at Cape Turakirae on the southern North Island, New Zealand (Fig. 3) coastline and concluded that higher shorelines were uplifted during earlier earthquakes. In this paper we describe several sequences of Holocene marine terraces in Japan and New Zealand to determine the principal factors controlling Holocene coastal evolution in uplifting areas of the two countries. We show that although episodic tectonic uplift is a major factor influencing coastal evolution, it is the interplay among tectonic processes and sea-level, tidal range, rock type and hardness, sediment supply, and erosion that produces sequences of terraces arranged in staircase fashion. 31

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FIG. 2. Map of Japan with the axis of principal subduction zones shown by barbed lines. Arrows indicate direction and rate of convergence on subducting plate. Inset A (lower right) shows localities of Boso Peninsula discussed in text or other figures. The Holocene coastal plain is indicated by shading. Note the Nojimazaki promontory at the southern tip of Boso Peninsula which was an island prior to uplift in 1703. Inset B (upper left) shows Sado Island and location of Ogi Peninsula at the southern tip.

SHORE P L A T F O R M M O R P H O L O G Y AND

DEVELOPMENT This section reviews the process responsible for shore platform development, to provide a basis for interpretation of marine terrace sequences where the prehistoric period is not well researched. Shore platforms are prominent components of coastal plains but display a wide variety of forms: from steep bedrock ramps with boulder ridges, to narrow rocky headlands at multiple levels, to wide and smooth subhorizontal rock benches

devoid of sediment cover. Shore platform development results from the interplay of many factors including bedrock characteristics, wave energy, tidal range, sealevel fluctuations, and tectonic uplift and subsidence. These factors vary in importance from site to site. Bedrock characteristics such as lithology, hardness, chemical composition, jointing, bedding and attitude strongly influence many aspects of shore platform morphology. Massive lithologies such as mudstone tend to form extensive platforms because of its easy erodibility, and uniform grain size. Interbedded lithologies,

33

Holocene Coastal Evolution

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F I G . 3. M a p of N e w Z e a l a n d with s u b d u c t i o n z o n e s and localities m e n t i o n e d in text. A r r o w s i n d i c a t e d i r e c t i o n and rate of c o n v e r g e n c e of the Pacific p l a t e with respect to the A u s t r a l i a n plate. Inset shows localities of M a h i a P e n i n s u l a i n c l u d i n g Figs 6 a n d 7.

especially those in steeply dipping sequences, often form ramps, ramparts and caves, depending on the dip and strike of the interbedded sequence in relation to the coastline and the contrast in hardness of the alternating beds. Complex chemical reactions between sea water and several carbonate lithologies shape some shore platforms. Along high energy coasts, fracturing and jointing of bedrock may be the principal factor in shore platform development. Kirk (1977) reviewed the literature concerning shore platform development and presented results of a quantitative shore platform erosion study at Kaikoura Peninsula, New Zealand (Fig. 3). Bedrock at Kaikoura Peninsula consists of Paleocene limestone and Oligocene mudstone. The limestone is relatively harder than the mudstone, and platforms developed on the limestone display more irregularity than those developed on the mudstone. The literature records considerable debate as to whether mechanical or chemical processes are most effective in abrasion of shore platforms (see for

example Bartrum 1938; Edwards, 1951; Mii, 1962; Healy, 1968a,b; Hills, 1971; Takahashi, 1975). Either process may dominate, but mechanical erosion is particularly effective in removing jointed and fractured bedrock above mean tide levels on high energy coastlines. Chemical erosion is more effective on poorly indurated sedimentary rocks in the intertidal zone where wetting and drying cycles are most pronounced. Biological erosion, whether by rock-boring shell species or the plucking of jointed blocks of rock by the roots of kelp are also very effective in some locations. Tidal range also plays an important part in shore platform development. Along coasts with a small tidal range the sea surface will remain longer near mean sealevel, focusing abrasion, whether mechanical, chemical or biological, within a narrow elevation range. Kirk (1977) along with So (1965), Hills (1972), Trenhaille (1972) and Trenhaille and Layzell (1981) have all noted a tendency toward a flattening of shore platform profiles at about mean sea level. Gill (1972) and

34

K.R. Berryman et al.

Trenhaille (1972) observed that in macrotidal (tidal range > 4 m) regions, multiple shore platforms may correspond to a single sea level position. Exposure to storm wave energy also has an effect on the elevation of shore platform development as observed by So (1965), Trenhaille (1972, 1974) and Kirk (1977). These authors note that platforms are frequently at a higher elevation on headlands than in adjacent bays; the elevation difference is dependent on the height of storm tides. In terms of a unified model of shore platform development, there is general agreement that platforms are graded at their outer margin to mean low water, and that a low water cliff often occurs seaward of this point. Davis (1896) initially proposed that shore platforms widened progressively by erosion at a high-tide cliffiine and by platform surface lowering. The concept was extended by Johnson (1919) and by Challinor (1949) who noted that a dynamic equilibrium is reached when both high-tide and low-tide cliffs retreat at the same rate. Sunamura (1973) demonstrated such a parallel retreat of shore platforms in Japan, as did Trenhaille (1974) for macrotidal storm wave platforms in Wales. At a site with few complicating factors in shore platform development, Kirk (1977) found that in soft bedrock in the microtidal environment at Kaikoura Peninsula, New Zealand, that 90% of all platforms occurred within _+ 1.0 m of mean sea level. Thus, in developing a model of geomorphic evolution for tectonically uplifting coasts it is important to gauge the relative importance of all processes modifying that coast. At coasts with readily erodible, homogeneous rocks, and a microtidal range, shore platforms will be wide and of limited surface relief with the entire shore platform generally within + 1.0 m of mean sea level. In these environments resolution of vertical tectonic movements of about the same amplitude as the tidal range may be achieved. Less precise resolution can be expected in more complex coastal settings.

EXAMPLES OF UPLIFTED SHORE PLATFORMS IN SOFT ROCKS Having presented a review of the processes of shore platform development, we now interpret several sequences of elevated marine terraces in New Zealand and Japan, in relation to the foregoing discussion. New Zealand Sequences of elevated Holocene marine terraces are common features along the east coast of the North Island, New Zealand. In this region the maximum tidal range does not exceed 2.5 m, the bedrock lithology is most often Neogene marine sandstone and siltstone, and abundant organic material within beach deposits overlying shore platforms provides good dating control. The coastline is 100-150 km west of the Hikurangi Trench (Fig. 3), at the landward margin o f an imbricated accretionary prism. At Cape Kidnappers (Fig. 3) a single marine terrace, up to 6 m above present mean sea-level, was uplifted at ca. 2.3 ka (Hull, 1987). The shore platform beneath the terrace was cut in easily eroded Pliocene sandstone and is smooth and subhorizontal where exposed along the seaward margin of the terrace. Hull (1987) obtained 10 radiocarbon ages from intertidal marine mollusc shells from along 15 km of coastline where the terrace is preserved. The shells were collected from distances up to 160 m seaward of the uplifted shoreline angle. A moderate correlation (r = 0.79) exists between sample age (cal BP) and distance from the shoreline angle (Fig. 4), and Hull (1987) presented two alternative models of shore platform development to account for the correlation. Hull (1987) argued that if relative sealevel was stable, the combination of backwasting and surface lowering of the shore platform within the intertidal zone would result in a change in environments at any one position on the shore platform. For example from storm beach to high tide beach, midtide

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Holocene Coastal Evolution platform, and finally to a low tide or subtidal platform. The present-day vertical tidal range at Cape Kidnappers is 1.9 m so according to this model, environmental changes would occur within this tidal range and marine sediments should, therefore, not exceed 1.9 m in thickness. Ages of shell from the shoreline angle would date the cessation of landward cutting of the shore platform and would closely approximate the timing of uplift of the shore platform above marine conditions (Fig. 5). Ages on shells from basal marine gravel preserved further seaward on the platform would relate to an earlier position of the shoreline angle. Alternatively, marine sediments overlying the shore platform could have been deposited during a period of relative sea-level rise induced by gradual interseismic tectonic subsidence or as a result of regional sea-level fluctuation. In this situation deposition of sediment on a shore platform occurs progressively in a landward

direction until relative sea level stabilizes or falls. The in situ molluscs fauna would be progressively younger to landward, matching the timing of the marine transgression. Gibb (1986) has shown that for the New Zealand region eustatic changes in sea-level during the past 4 ka were not greater than + 0.5 m. Thus, if the thickness of marine sediments overlying the shore platform at the shoreline angle exceeds 0.5 m, then the accumulation of these sediments might be evidence for downwarping of the land. Coastal subsidence between major coseismic uplift events occurs in some circumPacific areas (e.g. Plafker, 1969; Matsuda et al., 1978; Savage and Plafker, 1991; Plafker et al., 1991), and might also be expected in New Zealand. Thus the apparent sea-level rise on the Kidnappers coast deduced from the up to 1.5 m thick marine deposits might be an indication of interseismic subsidence but it might also represent the normal strati-

A. STABLE SEALEVEL, EROSION AND

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at maximum development of shore platform it is graded to MLWS, slopes gently seaward, and is mantled by marine deposits with storm beach deposits at the inner margin

UPLIFTEDSHOREPLATFORM .AND iANTLING DEPOSITS tectonic uplift terminates the development of the shore platform and, if uplift is greater than the tidal range, strands the former shore platform

ANDSLIDE DEBRIS SAND DUNE terrestrial deposits, including airfall tephra, dune sand, and landslide deposits from the backedge of the terrace, are deposited on the marine and beach sediments

FIG. 5. Schematic diagram of sequential development of uplifted shore platforms with stable sea level, coastal erosion, and episodic tectonic uplift. Model applies to microtidaienvironmentswith readily erodible bedrock and uplift events larger than the tidal range. M.H.W.S. and M.L.W.S. refer to mean high water, and mean low water spring tide.

36

K.R. Berryman et al.

graphic succession under stable relative sea-level with a tidal range of 1.9 m. In considering the mechanism of uplift, Hull (1987) noted that the shore platform is subhorizontal and that the intertidal fauna in the marine deposit show little evidence of transport, suggesting that the terrace was uplifted suddenly at the time of a major earthquake. Furthermore, the presence of the youngest radiocarbon ages at the shoreline angle and their similarity in age over the 15 km extent of the preserved terrace, indicates that dated samples from the shoreline angle provide the best estimate of the time of sudden uplift. Across Hawke Bay from Cape Kidnappers, similar uplifted marine terraces are preserved at Mahia Peninsula (Fig. 3). A maximum of five uplifted shore platforms in addition to the modern platform exist along the north coast of the peninsula (Berryman, 1988). These shore platforms are overlain by marine sediments, which often contain shell material, sea rafted pumice and terrestrial sediments including airfall tephra. Individual terraces can be mapped almost continuously along the northern coast of the peninsula and correlated using radiocarbon ages of shell material in the marine sediments. Terraces decrease in elevation to the west, and differences in elevation between adjacent terraces generally diminish in proportion to the total amount of uplift (Fig. 6). Bedrock is largely of moderately NW-dipping (10-30 °) Miocene mudstone and turbidite sequences, that is generally planed to extensive and subhorizontal shore platforms (Fig. 7). Hard tufts within the sedimentary sequence, especially along the eastern margin of the peninsula, frequently

form ramparts on some shore platforms, usually at their outer margins and immediately landward of the lowwater cliff. Suzuki et al. (1970) described shore platforms from Miura Peninsula of eastern Honshu, Japan (Fig. 2) where the tuffs also formed ramparts in rhythmically bedded Miocene tuffs and mudstone. Shell ages from the Mahia terraces are generally oldest at the base of sections and decrease upward. The youngest age from each terrace is often similar to the oldest age obtained from the next younger terrace. This suggests that uplift occurred in a very short interval of time compared with the time taken to accumulate the marine sediments on a shore platform. A coseismic uplift origin is thus the most likely mechanism for the formation of risers between shore platforms of the terrace sequence. It is not possible at Mahia Peninsula to test further the alternative models presented by Hull (1987): cliff retreat and shore platform lowering versus interseismic subsidence for accumulation of marine sediments on shore platforms with the data from Mahia. This is because marine sediments reach a maximum thickness of ca. 1.25 m on the uplifted shore platforms at Mahia Peninsula, compared with the 2.0 m maximum tidal range. Detailed sedimentological study of the marine sediments might more accurately establish the position and trend of sea-level during the period of deposition of marine shore platform sediments. Japan Boso Peninsula is situated on the southeast coast of Honshu (Fig. 2), about 30 km north of the Sagami

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FIG. 6. Height distribution of Holocene terraces along north coast of Mahia Peninsula. Shells from transgressive cstuarine deposits at Whangawehi Stream (truncated by erosion) and Mahia Beach provide the oldest ages. Error bars on terrace height calculated from uncertainty in elevation measurement and relationship to mean sea-level at the time of formation of the terrace. The height distribution reflects episodic uplift and tilt on the western flank of the Lachlan Anticline whose axis is offshore, east of the peninsula. Radiocarbon ages (14C BP) of shell samples from each locality are listed and the inferred time of uplift events is shown on each correlation line. From Berryman (1988).

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Holocene Coastal Evolution

37

FIG. 7. Oblique, westward view across northern Mahia Peninsula. A sequence of four uplifted Holocene terraces occur at Table Cape (foreground). Ramparts composed of Miocene tuff mark the outer part of the modern shore platform in the surf zone. Photo: D.L. Homer, Institute of Geological and Nuclear Sciences.

Trough where the Philippine Sea plate is subducted beneath the Eurasia plate. G r e a t (M t> 8) earthquakes that struck the region in 1703 and 1923 each produced coastal uplift. Matsuda et al. (1978) discuss the amount and causes of uplift during each event: a maximum of ca. 6 m in 1703 and 1.5 m in 1923 (Fig. 8). Uplift in 1703 formed the lowest well-developed terrace of a sequence of four m a j o r terraces comprising the Holocene coastal plain; the 1923 event raised further the pre-existing 1703 terrace rather than forming a distinct separate terrace. Radiocarbon ages from marine sediments from the prehistoric terraces suggest earlier uplift events occurred at ca. 6.3 ka, ca. 4.4 ka and ca. 2.9 ka (Yonekura, 1975; Nakata et al., 1980). At Nojimazaki p r o m o n t o r y at the southern tip of Boso Peninsula (Fig. 2), a sea stack about 200 m offshore prior to 1703 was uplifted and connected to the mainland by the formation of a subhorizontal rocky terrace at the time of the 1703 earthquake. The 1923 earthquake produced an additional 1.5 m of uplift but increased the areal extent of the terraces only slightly. Historical records indicate that there had been no uplift

prior to 1703 for at least 1000 years, as suggested by the radiocarbon ages obtained from deposits overlying the next higher platform. The uplifted terrace sequence at Boso Peninsula may contain a record of uplift due to both reverse faults (as in the North Island, New Zealand examples - B e r r y m a n et al., 1989) and to rupture of the subduction zone (Kayanne and Yoshikawa, 1986). The resultant terrace sequence therefore contains a record of uplift events of contrasting type, and contrasting uplift pattern. Further factors complicating the interpretation of tectonic and non-tectonic processes at Boso Peninsula are the less readily eroded bedrock c o m p a r e d to the Neogene sediments of eastern North Island, New Zealand, the lack of exposure along the densely populated Boso Peninsula coastline, and few shells for dating in the gravel typically found on shore platforms. Most workers have described a sequence of four m a j o r uplifted terraces around Boso Peninsula (e.g. Sugimura and Naruse, 1954; Yonekura, 1975; Matsuda et al., 1978; Nakata et al., 1980) with the ' G e n r o k u ' terrace uplifted in 1703. Matsuda et al. (1978) suggest the

38

K.R. Berryman et al. 139°E

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FIG. 8. Uplift contours (metres) of 1703 and 1923 events at Boso Peninsula. Contours show net uplift after as much as 2 m of interseismic subsidence since the 1703 event. From Matsuda et al. (1978).

interval between uplift events is 0.8-1.5 kyr and relate uplift to rupture of the subduction zone along the Sagami Trough, south of Boso Peninsula (Fig. 2). Yokota (1978) and more recently, Kayanne and Yoshikawa (1986) presented findings of microtopographical studies at Boso Peninsula and while they agreed that there are four principal terraces separated by risers ca. 3-5 m high, they have proposed that the shore platform underlying each principal terrace is made up of multiple levels separated by smaller risers 1-2 m high (Fig. 9). Kayanne and Yoshikawa (1986) suggest Boso Peninsula is uplifted during earthquakes like those in 1703 and 1923. Thus in addition to only four major earthquakes in ca. 6 ka, this latest analysis suggests

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FIG. 9. Correlation of shore platforms and interpreted history of uplift events (arrows) on the south coast of Boso Peninsula. Narrow arrows represent events typified by the 1923 uplift while wide arrows represent events typified by the 1703 uplift. Localities are: I: Sunosaki, F: Nojimazaki, Sr: Shiramazu, Hr: Hiraiso, G: uplift in 1703 (Genroku) earthquake, T: uplift in 1923 earthquake. See Fig. 2 for Boso Peninsula localities. From Kayanne and Yoshikawa (1986).

minimum uplift of shore platforms during at least thirteen separate earthquakes. E X A M P L E S OF U P L I F T E D S H O R E P L A T F O R M S IN HARD ROCKS

In marked contrast to the extensive, subhorizontal shore platforms that characterize soft bedrock coastal environments, shore platforms formed on hard bedrock have a much less regular topography. In many hard bedrock settings there is little erosive debris and uplifted shore platforms are frequently devoid of marine cover strata. At other locations marine deposits change rapidly from gravel to sand depending on wave energy and sediment sources. Stacks are a characteristic feature of shore platforms formed on hard bedrock. Where the coastal environment is exposed to storm conditions, platforms may be formed at several levels between low tide and storm wave levels above high tide (Gill, 1972; Trenhaille, 1974; Takahashi, 1973).

Japan Ogi Peninsula, on the southern tip of Sado Island (Fig. 2), was uplifted by as much as 2 m during a destructive earthquake in 1802 (Ota et al., 1976). The exposed bare rocky platform, composed of Neogene volcanic and volcaniclastic rock, is subhorizontal, with some stacks and a rampart as much as 0.5 m above the general level at its outer margin (Fig. 10a). The earthquake produced maximum uplift at the south coast of the peninsula, and this uplift decreased steadily to the north (Fig. 10b). The elevation of the ca. 6 ka shoreline also decreases to the north. The Holocene coastal plain comprises only the ca. 6 ka terrace, formed at the culmination of the Postglacial transgres-

Holocene Coastal Evolution

39

FIG. 10a. Shore platform at Ogi Peninsula, southern Sado Island uplifted at the time of the 1802earthquake. Person at right edge of photo shows scale. See Fig. 2 for location.

sion, and the 1802 terrace. The tilt of each terrace is similar, and the tilts and the lack of prehistoric Late Holocene terraces suggests the average recurrence interval of uplift is more than 6 kyr. N e w Zealand Along the south Wairarapa coast of eastern North Island, New Zealand (Fig. 3) sequences of as many as seven uplifted terraces, cut in Mesozoic sandstone and argillite, comprise the Holocene coastal plain (Ota et al., 1990). Shore-normal topographic profiles (Fig. 11) illustrate the rough, irregular topography, and the thickness of marine cover deposit is quite variable both on one platform at many sites and compared with other platforms in the same sequence. Ramparts similar to those described by Gill (1972) in Victoria, Australia are common features. Ramparts at the outer edges of uplifted shore platforms are frequently enlarged by a storm beach deposit of marine gravel thrown up from the back of the next lower platform (for example see Te Kau Kau Point profile in Fig. 11). An exposure of the cover deposits (Fig. 11) of an uplifted shore platform at Manurewa Point (Fig. 3) provides stratigraphic and age data useful in developing a model of terrace formation along the south Wairarapa coast. The shore platform is at 2.3 m above mean sea level and is overlain by thin beach gravel, sandy

peat, coarse shelly sand with wood, and finally by coarse gravelly sand. No dateable material was found in the basal beach gravel, but ages obtained from the peat and from wood and shell in the overlying unit were indistinguishable within the error limits of dating (Fig. 11). The stratigraphy suggests the following history for this site: (i) cutting of a shore platform; (ii) deposition of marine gravel, probably as a storm beach deposit at the base of a retreating shoreline angle; (iii) uplift of the shore platform above high tide halting shore platform development; (iv) formation of a storm beach ridge at the outer edge of the uplifted shore platform coinciding with the initiation of shore platform development at the next lower level; (v) formation of a storm beach ridge that ponded drainage across the terrace surface resulting in localized peat formation; (vi) storm over-wash of deposits of shelly sand and wood on the peat soon after its formation; (vii) deposition of undated coarse gravelly sand of probable fluvial origin over the storm deposits. Further south at Cape Turakirae (Figs 3, 12) longshore drift patterns have resulted in a greater gravel accumulation, in the form of beach ridges, on

40

K.R. Berryman et al.

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3 km i

FIG. 10b. Uplift distribution (metres) of the 1802 earthquake. Data shows location of measured elevations. Data from Ota et al. (1976).

the hard, Mesozoic sandstone shore platforms than along the Wairarapa coast. Coastal topography takes the form of a sequence of beach ridges and intervening swales with bedrock stacks protruding through the gravel deposit. The youngest beach ridge at Cape Turakirae was uplifted by 2.7 m during the - M 8.2 earthquake in 1855. The 1855 ridge is the youngest of a sequence of five beach ridges on the coastal plain (Fig. 6), that reach a maximum elevation of 27 m (Wellman, 1969; Moore, 1987). If the depositional history inferred for the Wairarapa shore platforms is applicable at Cape Turakirae, then the beach ridges probably represent storm beach accumulations at the outer margins of each of five uplifted shore platforms. Several radiocarbon ages from Cape Turakirae (Moore, 1987) of ca. 5.9 ka to ca. 6.3 ka overlie beach gravel from the swale behind the highest ridge, supporting the correlation of this ridge with the maximum of the Holocene sea-level transgression at ca. 6.5 ka (Gibb, 1986). In comparison with the shore platforms from regions of soft substrate, those underlying the beach ridges at Cape Turakirae provide only moderate to poor criteria for determining past relative sea-level changes because they form at variable levels above sea-level. For example, the modern storm beach at Cape Turakirae, which began to form after the 1855 earthquake, is ca. 1.0 m above high spring-tide. Major storms probably still deposit gravel and other material on the ridge uplifted in 1855. Beach ridges may form well above mean sea level on exposed shorelines, but their elevation depends on aspect, sediment supply, and local bedrock outcrop. Along the south Wairarapa coast the modern storm beach varies from 1.3 m to 4.2 m above mean sea level. Although comparing the elevations of uplifted, ancient beach ridges to modern

beach ridges features is a much better measure of uplift than uplifted beach ridge elevation above mean sea level, both temporal and spatial variation in wave energy and sediment supply mean that uplift estimates from beach ridge sites will have larger uncertainties than for hard rock or soft rock shore platforms. EXAMPLES OF UPLIFTED CORAL REEF PLATFORMS

In southern parts of Japan mean annual sea surface temperatures are conducive to coral reef growth, and uplifted coral reef platforms are the morphological equivalent of erosional shore platforms formed outside the mid-latitudes. Coral reef terraces are formed by the upward growth of coral colonies to the MLWST datum (mean low-water spring tide + 0.10 m) from subtidal platforms cut in older sedimentary rocks or older reef limestone. If tidal ranges are well known and can be extrapolated back in time with some reliability, then more precise measurement of past uplift events may be obtained from the coral reef environment than from even the best microtidal soft-bedrock shore platform setting. The youngest age from each uplifted reef is a close approximation to the age of uplift because of the way in which coral continues to grow upward and outward to any given sea-level position. At Kikai Island (Fig. 2), four Holocene age terraces are composed of reef limestone, small patch reefs and microatolls that rest unconformably on erosional shore platforms cut in Pleistocene reef limestone or Pliocene mudstone (Ota et al., 1978; Nakata et al., 1978). Microatolls are very useful paleo-sea level indicators (McLean et al., 1978) because their uppermost limit of growth is MLWST. Facies models of reef lithologies

Holocene Coastal Evolution

~k North e a s t of Te Kau Kau Point amsl

m 16 -

III

10

terrace

II

VE=IO

~

I

~

41

NORTH ISLAND

LOCATION

number

/

C.PALUSER " ,V/ / / 2 Z~/ / / f "~h

/////,/

P°" : V~<.ee.a.m. m rU '

Te Kau Kau P o,n,

C.PALLISER

,oo~

200

East of Te Kau Ksu Point lowest marine terreee 4.Ore amsl tn

0t

10km J

30Or.

West of Msnuwerl Polnt lowest marine torreoe 3,4m emsl m

East of To Kau Kau Point lowest marine terrace 5.1m amsl

m O-

X.))~,Xi5.

_y///,

Right bank of Pukemuri

amsl 20

m

VII

Stm, Te Oroi

Vl

V

IV

III

II

I

terrace number

[]T]•

top soil sand

VE:10

beach gravel

10

~]

shells wood peat

[~ 0

100

200

300

400m

sandstone bedrock

FIG. 11. Topographicprofiles and stratigraphic sections of uplifted Holocene terraces from localities on south Wairarapacoast.

Stratigraphic sections of terrace deposits on shore platforms at ManurewaPoint illustrate the frequent occurrence of coeval peat within beach deposits of south Wairarapa coastal terraces. Radiocarbonages are ~4C BP. See Fig. 3 for localities. Data from Ota et el. (1990).

have also been developed for fringing and barrier reefs that relate biostratigraphy and sediment distribution to sea level (e.g. Chappell, 1974). At Kikai Island, radiocarbon ages of coral and mollusc samples from each of the four terraces (Fig. 13) illustrate the stepwise uplift and sequential formatfon of the Holocene coastal plain.

Similar patterns of step-wise uplift to that found at Kikai Island have also been described from several other parts of the Ryukyu Islands. For example, uplifted reefs at Kodakara and Takara islands (Fig. 2) that are some 20 km apart and about 100 km northwest of Kikai Island, give evidence for an uplift event at ca. 2.4 ka (Nakata et al., 1978; Koba et al., 1979). This

42

K.R. Berryman et al.

FIG. 12. Oblique northwest view of uplifted sequence of beach ridges at Cape Turakirae (see Fig. 3 for location). The beach ridge stranded during the 1855 earthquake is 2-3 m above the modern storm beach. Photo: D.L. Homer, DSIR Geology and Geophysics.

event stranded the Haebaru Surface (Fig. 14) and appears to be the only coseismic event to affect these islands in the Holocene. If all of the uplift has been coseismic then it suggests very large (ca. 7 m uplift) but infrequent earthquake events in this part of the Ryukyu island arc. The possibility that part of the uplift is a slow, post-seismic process should not be excluded. DISCUSSION AND CONCLUSIONS Examples of uplifted Holocene terraces from New Zealand and Japan presented in this paper illustrate the consistent association of staircase terraces with episodic, coseismic uplift. The morphology of shore platforms and terrace sequences varies considerably depending on the lithology and erodibility of the bedrock, tidal range, sediment supply, exposure to storm energy, as well as the frequency and magnitude of the uplift events. Although a complex interaction of processes operate to produce shore platforms and deposition of marine sediment on them, the staircase arrangement of Holocene marine terraces suggests coseismic uplift is

the primary factor in producing flights of terraces. This is because coseismic uplift is essentially instantaneous (except for possible minor pre- and post-seismic movement), and so it overwhelms other processes occurring at slower rates. Several generalizations can be made about the nature and timing of uplift from the model of shore platform development presented in this paper. Uplift events in New Zealand and Japan have typically been larger than the tidal range so that even the outer margins of developing shore platforms, with its accompanying marine sediments have been lifted bodily above the intertidal zone (Fig. 8). Whereas subsidence following uplift is well documented for subduction zone uplift events in Japan and Alaska it has not been demonstrated conclusively from New Zealand. In a simple situation with no interseismic subsidence and stable sea-level, renewed landward erosion and platform lowering after uplift would begin to form a new platform graded to the same sea-level datum. The height of the riser below the uplifted platform would then provide a good approximation to the amount of

43

Holocene Coastal Evolution

"°°:'°

3520+85 m,,,

Y"

'

3930+90

. . . . . . ,. . . . . .

' "

+~ ++++++

5(~0 m

m

SE-b 0

I KIKAI ISLAND 1 ~ Holocene reef limestone ~

-

I G'~£9

Holocene lagoonal limestone I Pleistocene limestone

~~.

Pleistocene

FIG. 13. Topographic profiles and locations and ages (in ~4C BP) of radiocarbon-dated samples from southeast coast of Kikai Island. The Holocene coastal plain comprises four terraces (I, II, III, IX), the lowest three of which have youngest ages of ca. 2100, ca. 3500 and ca. 5000 14C BP. Location shown on Fig. 2. Data from Ota et al. (1978).

Takara Island

Nagabatake OharNakabaru Surface~_~_~ Maegomori Surf~dce Surface 35m~ \ \ ' ~ 50 Surface dune .... 2 1 m ~ \\" J

Profile

E~

Holocene reef limestone Maegomori Surface Holocene dune sand Pleistocene reel limestone -Ohara, Nakabaru & Nakabatake Surface Pleistocene fan delta gravel bedrock

Maegomori Surface Sea-level platform LWL --~'-

---

300m

,~o,~,a~,~,~2325:t:115 Modern ~ /,-C~7,;N~,~ : ~ , 2420::1:120

2805:t: 120

-,~ ;, :, ;: :,~,',

31~2~- 1"~-~' . . ~ ~25::t: 125 ' ~ r l

groove i:x)n'om

L

~

L

~

_.r4 2

rn

'

FIG. 14. Hotocene terrace profiles and locations and ages (in 14C BP) of radiocarbon-dated samples from Kodakara and Takara islands, R y u k y u island arc. The Maegomori surface on Takara Island and H a e b a r u surface on Kodakara Island are the only uplifted Holocene terraces on each island. Island locations shown on Fig. 2. Data from Nakata et al. (1978), and Koba eta/. (1979).

44

K.R. Berryman et al.

tectonic uplift (Fig. 8). In e r o s i o n a l s h o r e p l a t f o r m ACKNOWLEDGEMENTS settings a n d c o n s t r u c t i o n a l coral r e e f crests the We appreciate helpful reviews from Hamish Campbell, Brian y o u n g e s t age on coral o r m a r i n e i n - f a u n a will be o l d e r Atwater, Alan Nelson and Jeff Hsu. Irene Galuszka typed the t h a n but close to t h e age o f the p a l e o s e i s m i c event. manuscript and Jeff Lyall drew the figures. In m o r e c o m p l e x t e c t o n i c settings, w h e r e interseismic m o v e m e n t a n d / o r r e g i o n a l sea level fluctuation c o u l d be o f similar size as t h e coseismic m o v e m e n t , REFERENCES t h e n t h e genesis o f coastal l a n d f o r m s is m u c h m o r e difficult to i n t e r p r e t . In m a n y s u b d u c t i o n m a r g i n Bartrum, J.A. (1938). Shore platforms: a discussion. Journal of settings p o s t - s e i s m i c m o v e m e n t is o p p o s i t e in sense to Geomorphology, 1, 266-268, Berryman, K.R. (1988). Late Quaternary marine terrace distribution, the coseismic m o v e m e n t a n d m a y a c c o u n t for 1 0 - 8 0 % stratigraphy and deformation at Mahia Peninsula, New Zealand. of the coseismic uplift. F o r e x a m p l e , at B o s o P e n i n s u l a PhD Thesis, Victoria University of Wellington. t h e r e a p p e a r s to be a b o u t 2 m o f s u b s i d e n c e following a Berryman, K.R., Ota, Y. and Hull, A.G. (1989). Holocene paleoseismicity in the fold and thrust belts of the Hikurangi m a x i m u m o f 6 m o f uplift in 1703 ( M a t s u d a et al., subduction zone, eastern North Island, New Zealand. Tectono1978). In c o n t r a s t to e x a m p l e s w h e r e p o s t - s e i s m i c physics, 163, 185-195. m o v e m e n t is o p p o s i t e in sense to the coseismic Challinor, J. (1949). A principle in coastal geomorphology. Geography, 34, 212-215. m o v e m e n t , N e l s o n a n d M a r l e y (1992) d e s c r i b e postChappell, J. (1974). Geology of coral terraces, Huon Peninsula, New seismic m o v e m e n t at the C h i l e a n s u b d u c t i o n m a r g i n Guinea: A study of Quaternary tectonic movements and sea-level that has b e e n in t h e s a m e sense as coseismic m o v e m e n t . changes. Bulletin of Geological Society of America, 85, 553-570. l n t e r s e i s m i c uplift, g e n e r a l l y r e g a r d e d as p r e - s e i s m i c , Davis, W.M. (1896). The outline of Cape Cod. Proceedings of the American Academy of Arts and Science, 31, 303-332. has b e e n d e s c r i b e d f r o m the r e g i o n o f the 1964 N i i g a t a Edwards, A.B. (1951). Storm wave platforms. Journal of Geomore a r t h q u a k e ( R i k i t a k e , 1976) a n d r a p i d aseismic uplift phology, 4, 223-236. has b e e n d e s c r i b e d f r o m V a n u a t u ( T a y l o r et al., 1990) Gibb, J.G. (1986). 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A late Holocene uplifted shore platform on the Kidnappers Coast, North Island, New Zealand: Some implications fluctuations in s e a - l e v e l g r e a t e r t h a n a b o u t + 0 . 5 m. for shore platform development processes and uplift mechanism. T h e r e f o r e in t h e s e c o u n t r i e s it can be a s s u m e d that, Quaternary Research, 28, 183-195. e x c e p t for very d e t a i l e d studies, sea-level has b e e n Johnson, D.W. ( 1919). Shore Processes and Shoreline Development. Wiley, New York. 584 pp. stable d u r i n g the p e r i o d o f f o r m a t i o n o f s t e p p e d flights Kayanne, H. and Yoshikawa, T, (1986). Comparative study between of t e r r a c e s . H o w e v e r , this a s s u m p t i o n is not valid in present and emergent erosional landforms on the southeast coast of Boso Peninsula, central Japan. Geographical Review of Japan, o t h e r p a r t s o f the Pacific basin. F o r e x a m p l e , sea 59, 18-36. levels up to 2 m h i g h e r than p r e s e n t a r e r e c o r d e d in Fiji Kirk, R.M. (1977). Rates and forms of erosion on intertidal ( N u n n , 1990), and u p to 5 m a b o v e p r e s e n t in M a l a y s i a platforms at Kaikoura Peninsula, South Island, New Zealand. New Zealand Journal of Geology and Geophysics, 20, 571-613. ( T j i a a n d F u j i i , 1989). A g a i n , o b t a i n i n g local i n f o r m a tion is critical to the c o r r e c t i n t e r p r e t a t i o n of coastal Koba, M., Nakata, T. and Watabe, S. (1979). Late Quaternary reef caps of Takara and Kodakara Islands, Ryukyu Archipelago, and terrace sequences. sea-level changes of late Holocene. Earth Science, 33, 173-191. T h e e x a m p l e s of uplifted H o l o c e n e m a r i n e t e r r a c e Matsuda, T., Ota, Y., Ando, M. and Yonekura, N. (1978). Fault mechanism and recurrence interval of major earthquakes in s e q u e n c e s we have d e s c r i b e d f r o m N e w Z e a l a n d and southern Kanto district, Japan, as deduced from coastal terrace J a p a n , a n d the history o f d e v e l o p m e n t p r o p o s e d for data. Bulletin o] Geological Society America, 89, 1610-1618. their f o r m a t i o n will h a v e a n a l o g u e s in o t h e r tectoni- McFadgen, B.G. and Manning, M.R. (1990). Calibrating New Zealand radiocarbon dates of marine shells. Radiocarbon, 32, cally active areas. H o w e v e r , the m o d e l o f e p i s o d i c co22%232. seismic uplift s h o u l d not be a p p l i e d uncritically b e c a u s e McLean, R.F., Stoddart, D.R., Hopley, D. and Polach, H. ([978L Sea level change in the Holocene on the northern Great Barrier not all a p p a r e n t l y u p l i f t e d s e q u e n c e s o f t e r r a c e s will Reef. 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Holocene Coastal Evolution coral reefs and sea-level changes in the Ryukyu Islands. Geo-

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