The morphology and surface geology of the Islands of Tongatapu

CCOP/SOPAC Technical Report 62
May 1990
THE MORPHOLOGY AND SURFACE GEOLOGY
OF THE ISLANDS OF TONGATAPU AND VAVA'U,
KINGDOM OF TONGA
by
Peter S. Roy*
Department of Mineral Resources
New South Wales Government
GPO Box 5288, Sydney
New South Wales 2001
Prepared for: South Pacific Applied Geoscience Commission (SOPAC) formerly Committee for
Co-ordination of Joint Prospecting for Mineral Resources in South Pacific Offshore Areas
(CCOP/SOPAC) Tonga Project: TG.12
*Contributed by: ESCAP/UNDP Project RAS/81/102: Investigation of Mineral Potential of the
South Pacific
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TABLE OF CONTENTS
Page
ABSTRACT ................................................................................................................... 5
INTRODUCTION
........................................................................................................
6
PHYSICAL, SETTING .................................................................................................
8
TONGATAPU
Morphology ................................................................................................................ 9
Late Quaternary Geology ..................................................................................... 12
Pleistocene Deposits
...................................................................................... 13
Holocene Deposits
.......................................................................................
Evolution of Tongatapu
16
................................................................................... 21
The Tilting Model - a Reconstruction .......................................................... 22
The Interior Lagoons and the 5 m Scarp ....................................................... 25
Beaches and Spits - Estimated Accretion Rates ................................................ 26
VAVA'U
Morphology ............................................................................................................. 27
Surface Geology
................................................................................................
34
Geomorphological Development of Vava'u .......................................................... 36
Solution Cliffing
..........................................................................................
37
Terraces, Rims and Depressions on Vava'u ..................................................... 41
CONCLUSIONS
REFERENCES
........................................................................................................
43
............................................................................................................. 47
APPENDIX
Programme Element CCSP/TG.12 .................................................................... 51
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LIST OF FIGURES AND TABLE
Figure
1
Page
Location map showing the central Tonga Ridge and the main
Tongan Islands
.................................................................................
7
2
Morphology of Tongatapu Island and adjacent lagoon/reef complex ..... 10
3
Tongatapu Island showing main morphological features........................... 11
4
Morphology of central Tongatapu showing depressions occupied
by the interior lagoons ...........................................................................
14
East-west cross section through Kolovai village and the twin
Kolovai ridges ......................................................................................
15
Tongatapu Island, showing distribution of main Holocene
depositional environments ....................................................................
17
5
6
7
Schematic diagram illustrating Late Quaternary evolution
of the slowly emerging open ocean coast of Tongatapu .................... 18
8
Tilting reconstruction for Tongatapu island and lagoon ........................... 23
9
Pre-tilting configuration of Tongatapu
.................................................. 24
10
Vava'u group of Islands showing simplified bathymetry .......................... 28
11
Vava'u Islands; the cross-sections show typical relief ............................ 29
12
Sketches of the main morphological features on Vava'u ........................ 30
13
Vava'u's main islands showing the distribution of cliffed
shorelines
........................................................................................
31
14
Vava'u showing distribution of various morphological features
......... 33
15
Emergent reefs provide evidence of substantial marine erosion ........ 38
16
Channel excavation by solution cliffing ............................................ 40
Table
1
Late Quaternary stratigraphy on Tongatapu ........................................... 13
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ABSTRACT
Reconnaissance geological mapping from aerial photographs supported by limited field
observations of a number of Tongan Islands has provided insights into their modes of formation
and the occurrence of resources such as construction materials.
The islands of Tongatapu and Vava'u are composed of emerged and tilted limestones of
Pliocene and Quaternary age with a volcanic soil mantle. Their morphologies and surface geology
are mainly the result of subaerial and marine erosion. A marine dissolution process, termed
solution cliffing, is thought to be responsible for excavating depressions and channel-ways below
present sea level in the interiors of the islands. Factors that promote solution cliffing include (1)
tilting of the atoll surface which provided connections between lagoon and open sea at virtually all
eustatic sea levels; (2) tidal dispersal of the dissolved limestone products from the interior of the
atoll; (3) a rate of biogenic sedimentation in the interior waterways that is slower than the rate of
erosion.
Erosion, and especially solution cliffing, has reached a more advanced stage of development
in Vava'u than Tongatapu, due probably to Vava'u's greater rate (or duration) of uplift. In
Tongatapu, many primary depositional features - reef rim, patch reefs and lagoon bed - are still
evident and some may be associated with relict deposits of construction material. This is not the
case in Vava'u where primary depositional features are absent. Here new sources of sand and
gravel are likely to be found in large overwash lobes on the modern reef flat.
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INTRODUCTION
Coastal and nearshore mapping, the results of which are documented in this report,
constitutes a new programme element (CCSP/TG12) for the Kingdom of Tonga CCOP/SOPAC
work programme. The initial objective was to evaluate the usefulness of regional geological
mapping from aerial photographs to identify resources such as construction materials, and for
planning future developments (see Appendix 1 for description of Programme Element, TG.12).
The work carried out in March/April 1986 involved air photo interpretation and reconnaissance
field checking in two areas: Tongatapu Island, including the lagoonal area on its northern side,
and Vava'u Island in the Vava'u Group (Figure 1).
All of Tonga is covered by aerial photographs and 1:25,000 orthophoto maps. The air photos
used in this project were flown in 1968 and are at a scale of about 1:20,000.
The work was carried out by Peter Roy and Bruce Richmond (Marine Geologists with
CCOP/SOPAC) and David Tappin (Government Geologist in the Ministry of Lands, Surveys and
Natural Resources, Kingdom of Tonga). The geological experience of David Tappin, especially in
Tongatapu, and the local knowledge of Ikani Prescott in Vava'u were invaluable; both people
contributed their Easter holidays to the project.
Mapping in Tongatapu was carried out in conjunction with a lagoonal sediment survey (see
Technical Report - TR 63) which forms part of an ongoing programme element: CCSP/TG.6,
Sand Inventory of Tongatapu. It also coincided with mapping by David Tappin of a low level
scarp/terrace feature around the central part of the island, carried out as part of an
archaeological investigation. Four and a half days were spent interpreting the air photos using a
stereoscope and three days on field inspection of Tongatapu and of a number of nearby offshore
small islands (Fafa, Velitoa, Onevai, Motutapa, Nuku, Ata and Malinoa) shown but not named on
Figure 1.
A second mapping exercise was arranged to test the feasibility of air photo mapping on an
island that is geologically less well known than Tongatapu. At the same time, it was hoped to
make a very preliminary assessment of insular phosphate in Tonga (Programme Element TG.2)
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and for this reason Nomuka, an island with a closed lagoon, was originally suggested as a target.
However, Vava'u Island was chosen instead because of ease of access (not the case in Nomuka)
and the availability of local assistance with the field work. The field work was planned for the
Easter holidays (28/3 -1/4) since the marine survey off Tongatapu was shut-down at this time.
Like Tongatapu, Vava'u is composed of limestone but is much higher and more rugged and thus
provides different geological factors to study.
Four days were spent examining the air photos of Vava'u followed by two and a half days
inspecting road cuttings and shoreline exposures on the island.
This report contains separate sections on Tongatapu and Vava'u describing their morphology
and surface geology and discussing various interpretations of these observations. In the final
section, certain general conclusions are drawn from the geological interpretations.
PHYSICAL SETTING
The Kingdom of Tonga is located in the tropics (latitude 18'-22'S) and temperatures range
from 20 to 27'C. The dominant winds are the southeast trades; those from the south and
northeast are less frequent. Rain falls mainly from December to June and is in the order of
1,600-2,000 mm/year, increasing towards the north. Tides are diurnal and their spring range is
about 1.2 m. Occasional tropical cyclones occur in summer months and generally travel in a
southerly direction. Woodroffe (1983) describes the impact of cyclone Isaac in 1982 which caused
severe damage and was accompanied by a storm surge of several metres in some places.
The Tongan Islands are located on the crest of the Tonga Ridge, an active fore-arc bordering
the Tonga Trench at the Pacific Plate boundary, to the west lies the volcanically active Tofua arc
(Figure 1). The ridge is divided into separate fault blocks (Cunningham and Anscombe, 1985).
The surfaces of individual blocks are less than 200 m deep and have an irregular limestone cap,
the emergent portions of which form islands. The southern (Tongatapu) block has the islands of
Tongatapu and 'Eua at its southern end and a lagoon to the north. The reverse is true for the
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Vava'u block, located on the northern part of the ridge, which has islands on its northern side and
is tilted down to the south (Taylor, 1978).
All the Tongan Islands have well-developed soils rich in volcanic material. Mapping studies
by Wilde and Hewitt (1983) and Orbell and others (1985) suggest that these are principally
composed of volcanic ash (tephra) hut are now extensively degraded.
TONGATAPU
Morphology
Tongatapu is made up of Pliocene and Pleistocene limestone 130-250 m thick overlying lower
Pliocene and older volcaniclastics (Cunningham and Anscombe, 1985). The limestone is elevated
above present sea level and reaches a maximum height of 65-70 m at the southern end of the
island (Figure 2). This forms the high point of a narrow and irregular ridge (0.5-1.25 km wide
and mostly rising more than 20 m above sea level) that extends to the northeast and northwest
along the windward coasts. The ridge encompasses a broad, low area in the central and northern
part of the island that rises gently to the south. The sea bed on the island’s windward coast
slopes steeply to depths of 200 m but the northern part of the Tongatapu block comprises a
shallow lagoon (mostly <50 m water depth) about 600 km in area (Figure 2).
The most rugged topography on the island is associated with near-vertical sea cliffs up to
30 m high on the windward coasts. In most areas these are fringed by a narrow reef flat with a
well-developed algal rim. Elsewhere, the morphology of the island is very subdued. Typically the
land surface is flat to gently undulating with occasional steep-sided hills 10-25 m high and linear,
scarp-like features. The dimensions of the hills range from a few hundred metres to 1 km in
diameter and their distribution is apparently random. Except along exposed coasts, the scarps
typically have gentle inflexions (1:20 - 1:100) and have been mapped over distances of tens of
kilometres (Figure 3). They occur in two settings or associations:
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(i)
Scarps and slope zones delineate both sides of the main ridge. Except for near-vertical sea
cliffs at the coast, the scarps are gently sloping, somewhat irregular features. They merge
and bifurcate and occasionally rise in a series of steps (Figure 3); they generally follow
the regional trend of the land surface. which rises toward the south. Scarp faces range in
elevation up to 20 m and are soil covered.
(ii)
A low-level, near-horizontal scarp, termed the ‘5 m scarp’, is found around the northern
coastline of Tongatapu and borders Fanga'uta Lagoon (Figure 3). It is in the order of 3
to 8 m high and typically occurs at the contact of older limestone with unconsolidated
Holocene sediments at its base (Figure 4). Its upper surface usually has a well-developed
soil cover but on its face soils are thin or absent.
Fanga'utu and Fanga Kakau Lagoons are shallow water bodies in the interior of the island
and occupy an irregular depression bounded by the 5 m scarp (see Sections 1-4 in Figure 4).
Holocene muddy sediments occur on the lagoon beds and also form intertidal flats colonised by
mangroves. A flood-tidal delta composed of fine calcareous sand and coral rubble has
accumulated in the lagoon mouth (Figure 6). Its outer part is colonised by coral patches but
elsewhere in the lagoon, coral growth is very limited.
Late Quaternary Geology
The Pliocene/Pleistocene limestones that form the core of Tongatapu are veneered around its
present shoreline by a number of mappable units of late Pleistocene and Holocene age. Taylor
(1978) recognises two Pleistocene reef limestones with insitu coral heads and a Holocene coral
pavement, all of which are emergent above present sea level. Their ages, maximum elevations
and informal names are listed in Table 1.
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Table 1. Late Quaternary Stratigraphy on Tongatapu (after Taylor, 1978)
Informal Formation
name
Niutoua
Utulau
Nuku’alofa
Maximum Elevation
above low tide (m)
Age based on radiometric dating (yrs BP)
c. 205,000-240,000
4
6-7 (6.6)
140,000-120,000
2
< 6,500
Pleistocene Deposits
The upper surfaces of the limestones listed in Table 1 are assumed to correspond to
maximum sea levels during the last 3 interglacials although this ignores possible lowering by karst
weathering. The two oldest limestones occur at sites around the open coast where the hinterland
is everywhere higher than the reefs (i.e. > 7 m) and thus show a fringing and onlapping
relationship with the older limestone core of the island. Holocene sediments here are quite
limited; they mainly occur on the islands northern side.
Other than cemented coral/algal rubble associated with the ancient reefs, few detrital deposits
of late Pleistocene age have been recognised in Tongatapu. The main deposits are at Kolovai on
the islands northwestern peninsula (Duphorn, 1981) (see Figures 3 and 5). Here twin
north-south trending sand ridges, formerly thought to be a single feature, rise to elevations of
12-21 m above sea level. The highest point at the southern end of the western ridge has been
augered over a small area to assess its sand resources (Tiffin 1981, 1982) (Figure 5). Here, 1-4 m
of unconsolidated fine, well-sorted sand overlies a cemented sand layer; a soil 1-4 m thick
blankets the whole area. The fine sand is almost certainly aeolian and presumably overlies beach
sand (Duphorn, 1981). From analyses reported in Tiffin (1982), medium-grained sand - possibly a
beach deposit - occurs in auger holes at elevation of + 9 m. Nearby, an unofficial quarry on the
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crest of the eastern ridge at about +9 to +10 m, exposed a shell coquina (lagoonal shell species)
interbedded with medium to coarse sand (Figure 5). These are interpreted as supratidal beach
deposits washed from the adjacent lagoon during easterly and northeasterly storms.
Duphorn (1971) suggests a Last Interglacial age of formation for the Kolovai ridge deposits
and invokes 4 to 6 m of uplift as well as eustatism to explain their elevation. I believe that storm
surge and set-up effects in the shallow lagoon can account for wave deposition 3-4 m above the
Last Interglacial sea-level minimum (+5 to 6 m). It would therefore seem likely that the
sediments up to an elevation of about 9 m comprise beach deposits with dune sand and volcanic
soil forming ridges above this level (Figure 5).
If the inferred Last Interglacial age of formation for the Kolovai sand ridges (c.120-140,000
yrs BP) is correct, they provide a benchmark to assess rates of tephra deposition during the late
Pleistocene. The average thickness of soils blanketing the sands is 2.5 m (Tiffin, 1982) which, if
they accumulated gradually as Orbell (1971) suspects, indicates an average rate of accretion in this
part of Tonga of 2 mm/century.
Holocene Deposits
Holocene deposits in Tongatapu include reef limestones and detrital sediments that have
accumulated in beach, tidal flat and lagoonal environments (Figure 6).
The mid-Holocene corals described by Taylor (1978) - the Nuku’alofa formation (Table 1)
form a surficial pavement characterised by Porities labata microatolls and Acropora fragments.
They occur principally around the northern coastline beneath low lying parts of Nuku’alofa
township and at Kolonga, but are occasionally found in small re-entrants, such as Monotapu
Beach, on the open coast (Figure 6). They grew in the mid-Holocene when sea level was to up
2 m higher than today and were undoubtedly quite widespread at this time especially on the north
coast. The subsequent slow recession that exposed them caused considerable erosion and
degradation of reef flat surfaces, although coral growth presumably continued unabated on reef
fronts. It is virtually impossible (without drilling and age dating) to delineate the extent of
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Holocene reef progradation and to distinguish it from planation surfaces cut into Pleistocene reefs
at present sea level (Figure 7). The most extensive areas of active coral growth today appear to
be off the northwestern coast of the island and in the main lagoon, centred on the offshore
islands (Figure 2).
Modem fringing reef flats in Tongatapu are up to 400 m in width. The widest occur on the
northern coast and commonly have associated with them beaches and spits composed of
reef-derived sand and coral rubble (Figure 6). Reefs on the exposed SE, S and SW facing coasts
are usually less than 100 m wide and average 50 m. The coast here is cliffed and, in places,
notches related to present sea level attest to contemporary erosion. Elsewhere, narrow, coarse
sand beaches front a now-inactive cliffline, presumably an old sea cliff.
In the main lagoon, most of the islands are sand cays with varying amounts of beachrock
(Stoddart, 1975;Woodroffe, 1983). From their elevations (up to +4 m), most are of mid to late
Holocene age. The islands of Velitoa, Atata, Polola and Alikibeau also contain emergent
Pleistocene reef limestone that probably corresponds to the Utulau formation of Taylor (1978).
Of the Holocene detrital deposits the most extensive and least well known are the broad
intertidal flats on the northwestern side of the island and the bottom sediments in Fanga'uta
Lagoon (Figure 6). The former consist of a veneer, generally less than 50 cm thick, of silty sand
overlying a rubble layer or cemented pavement (Gauss and Carter, 1982) of mid-Holocene or late
Pleistocene age.
Sediments in Fanga'uta Lagoon are very muddy except in its mouth where sand banks form a
tidal delta shoal. Sedimentation rates in the lagoon are presumably very slow and it is possible
that sediments and other features, are relict. For example, the low area around Tokomololo on
the western side of the lagoon rises imperceptibly from the lagoon shoreline; here the 5 m
contour is located 1.5 km inland (Figure 6). Within about 0.5 km of the present shoreline, the
sediments are sandy and show traces of ancient settlements (middens) that seem to show a
chronology reflecting the retreat of the lagoon foreshore as sea level fell in the late Holocene
from its +2 maximum (D. Tappin, pers. comm.). It is likely that wave reworking of pre-existing
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deposits was responsible for winnowing the sands and, in this sense, they are Holocene. The
original sediments, however, may be considerably older.
The distribution of Holocene beach deposits in Tongatapu is shown in Figure 6. They occur
discontinuously at the base of the cliffs along the open ocean coast and form prograded spits at
the end of littoral drift systems on the island’s northern coast.
Open coast beaches are composed of coarse sand and rubble. They occur at two levels with
berm crests at 2-3 m and 5-6m above mean sea level; rarely do they exceed 25 m in width. The
lower beach represents accretion under present-day conditions with sediments supplied from the
adjacent reef; its surface fluctuates in response to episodes of storm erosion and recovery. The
upper beach, locally termed the ‘Pumice Terrace’ (Duphorn, 1981), is vegetated and has a poorly
developed soil with abundant pumice. It is a relict feature related to the same mid-Holocene
phase of high sea level that formed the Nuku’alofa coral pavements. Sand from the ‘Pumice
Terrace’ is quarried for construction purposes but the resource is limited because conditions have
changed (viz. sea level has fallen) since it was deposited and it is not now being naturally
replenished (Duphorn, 1981).
Two major spit complexes and some minor ones are shown in Figure 6, together with the
inferred directions of spit progradation. Beach deposits making up the spits rarely rise more than
2 m above mean sea level. On their landward sides they usually contain mangrove and freshwater
swamps (Thaman, 1976). The spit to the west of Nuku’alofa faces the lagoon and is relatively
protected compared to the eastern spit between Kolonga, Manuka and Nukuleka, which is acted
on by refracted open ocean waves from the northeast. Both major spits have prograded from east
to west, the dominant direction of littoral drift.
From their attitude, most of the beaches appear to be related to present sea level, but at
Kolonga at the updrift end of the eastern spit, higher level beach deposits have been recognised
by Taylor (1978). Here, dated mid-Holocene corals are overlain by a narrow ridge of
cross-bedded calcarenite to 3 m high that onlaps a surface of much older limestone. The ridge
represents the remains of a spit that formed at the same time as the ‘Pumice Terrace’ but was
mostly reworked in late Holocene times. As well as supplying sand to the eastern spit, finer
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sediment generated on the reef flat in this area has been washed into the mouth of Fanga'uta
Lagoon to form the tidal delta.
Evolution of Tongatapu
The emergence of Pliocene-Pleistocene reef limestones on 'Eua and Tongatapu led early
workers such as Lister (1891), Davis (1928) and Hoffmeister (1932) to propose that the Tongatapu
block had been uplifted in the south and tilted down towards the north, presumably during
Pliocene or Pleistocene times. Packham (1985), in his interpretation of well data on Tongatapu,
implies post-Miocene uplift and tilting of the island with a moment of rotation of about 1 m/km.
Taylor (1978) provided additional evidence for these tectonic trends on 'Eua but, from
observations of dated Last Interglacial age reefs on both islands, he concluded that tectonism
ceased in the late Quaternary. He envisaged that the island of Tongatapu evolved on an inclined
substrate that was slowly emerging so that its southern high point "began as a single paleo-island
which provided a nucleus for later leeward reef development". Figure 7 shows how erosion could
have created coastal exposures such as those described by Taylor (1978) and demonstrates the
interplay between constructional and erosional forces in creating the present-day landforms on the
open-coast of Tongatapu.
The question of whether or not uplift and tilting are still influencing Tongatapu is unresolved.
There is some indication in Taylor's (1978) data set that Last Interglacial reefs on 'Eua are, in
fact, slightly higher than on Tongatapu, and may thus indicate very minor tilting in the last 100,000
years or so. The present study does not, however, contribute new evidence to this debate. If it is
still occurring, tectonism is at a much slower rate than eustatic changes which, it is argued below,
have had a substantial erosional impact on the island's morphology.
The most recent studies of Tongatapu Island by Taylor (1978), Duphorn (1981) and Tappin
(pers. comm.) show that its present-day morphology comprises an emergent barrier reef/lagoon
complex. The ridge around its SE, S and SW sides corresponds to the former reef and the
central low area is part of the original lagoon bed. Relict patch reefs rise above the old lagoon
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bed and some of these hills are presently quarried for aggregate. The main lagoon to the north
of the island generally deepens towards the north where an arc of modem reef, incorporating a
number of offshore islands, extends around the lagoon’s southeastern side. Its configuration
mirrors that of the old atoll rim on Tongatapu (see inset in Figure 2) which probably formed in a
similar fashion.
Although many of the large scale landforms on the island are interpreted as primary
depositional forms, they have undoubtedly been modified by subaerial weathering. In most areas,
evidence for this is obscured by soil cover; an exception is the weakly-developed surface drainage
system identified in the southern interior of the island (Figure 3). Despite these and other
post-depositional changes, an attempt has been made to reconstruct the island’s former
morphology using a simple tilting model. This is illustrated in Figure 8 by a generalised north south cross section through Tongatapu Island and lagoon.
The Tilting Model - a Reconstruction
Figure 8A shows that the interior of the island and the bed of the main lagoon form an
inclined, planar surface over a distance of 25 km. The NE and NW limbs of the island’s
present-day rim and the tops of the relict patch reefs follow similar trends. The deep basin off
Nuku’alofa (a in Figure 8A) and the interior lagoons (b and c in the same figure) are seen to be
depressions below the lagoon bed. Figure 8C recreates the pre-tilted cross section of the island
and Figure 9 shows it in plan form at some unspecified time in the past; the rotation shown is
2m/km. The reconstruction shows a narrow, semi-continuous reef crest with scattered islets, open
to the northeast and encircling a 10-30 m deep lagoon. Low areas on the reef crest and 40 m
deep depressions below the lagoon bed may be artifacts of post-depositional weathering. Piha
Passage is probably the head of a submarine canyon following an old fault line. Of course, this is
only a rough approximation, since reef growth and decay, lagoon sedimentation, differential uplift
and subaerial erosion all have interacted with changing sea level throughout the last 2 million
years or more to produce the island’s present shape. The model simplistically implies that the
island grew on a level substrate that was later tilted to its present attitude. In fact, tilting and
reef growth occurred synchronously so that, as the southern end of the island emerged,
progressively younger limestones accumulated to the north.
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The model raises the possibility of predicting the existence of certain types of deposits that
are now covered by soil. For instance, spits and submarine banks composed of coarse sand,
suitable for construction purposes, might be found at the ends of palaeo-islands that were
exposed to high energy waves. Possible sites are arrowed in Figures 3 and 9.
The Interior Lagoons and the 5 m Scarp
The morphology of Tongatapu Island illustrated in Figures 4 and SA shows that the interior
lagoons occupy depressions excavated below the average level of the former lagoon bed. The
origin of the 5 m scarp is not known; both constructional and erosional modes of formation are
possible. The former interprets the scarp as the steep front of a fringing reef that grew in the
lagoon during a past period of high sea level. The most likely time was the Last Interglacial
maximum. However, as Figure 4 illustrates, the 5 m scarp lacks a planar terrace surface
corresponding to a relict reef flat. Furthermore, it was not recognised as a constructional feature
by Taylor (1978) who focused his research in Tonga on reefs of Pleistocene age.
The alternative explanation is that the 5 m scarp is an erosional feature cut into an older
limestone surface. Its base is onlapped by sediments of Holocene age (Figure 4), thus the scarp
must have formed prior to this time, presumably during the late Pleistocene. The horizontal
attitude of the scarp over considerable distances suggests a relationship to sea level; marine
processes, rather than subaerial ones appear to have been involved in its formation. Abrasion by
waves is most improbable in view of the protected nature of the inner lagoon. Whereas subaerial
weathering (karstification) undoubtedly operated during periods of lowered sea level, I propose
that scarping of the lagoon sides and possibly deepening of its bed also occurred during periods of
high sea level as a result of sea-corrosion (Pirazzoli, 1986) - the same process that causes cliffing
and notching in limestones on the open coast. I term this process ‘solution cliffing' and discuss it
in more detail in the section on Vava'u.
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Beaches and Spits
Spits in northern Tongatapu can be regarded as permanent sinks, or depocentres, for sand.
The same is probably not true for beaches on the open coast where sand can be lost to deep
water over the reef front during violent storms. Rates of spit growth are thus tied to sand
production on the adjacent and updrift reef flat. The approximate volume of sand in the main
spit complexes, assuming an average thickness of 2 m, is 3.9 X 106 m3 (eastern spit) and 1.7 X
106 m3 (western spit). The corresponding reef front lengths and reef flat areas are 9km/2.26 X
106 m2 and 7 km/2.15 X 10 m respectively. The spits are assumed to represent growth since
the beginning of the present high-stand, about 6,000 years ago (Taylor, 1978). Thus average
production of beach sand is 650 m/yr at the relatively exposed eastern site and 290 m/yr at the
more protected western site. This corresponds to about 70 m/yr/km of reef crest for the eastern
spit and 40 m/yr/km of reef crest for the western spit.
Gauss and others (1983) calculated annual CaCO production figures for the windward reefs
on Tongatapu. However they made no attempt to estimate the amount of sand being added to
the beaches other than to imply that it was much less than was being removed for construction
purposes. It can perhaps be argued that the absence of large accretionary features on the
windward coast is due to an even slower rate of sand production here than on the island’s
northern side, where sand spits have prograded. The very slow rates of sand production/
replenishment in the spits highlight the problem of exploiting beaches for construction sand at the
risk of destroying a resource that often has even greater value for other uses (see also Duphorn,
1981).
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VAVA'U
Most recently, the geology and morphology of the Vava'u group of islands has been dealt with
by Katz (1976) and Taylor (1978); the latter also provided a comprehensive review of previous
work. High limestone islands occur at the northern end of the Vava'u block (Figure 10) which
Taylor (1978) showed is down-tilted to the south. The surface of the block, especially in the area
of the islands, is highly irregular with local elevations of up to 210 m along the northern rim of
the main island (Figure 11).
According to Taylor (1978), limestones exposed at the base of the Vava'u sea cliffs represent
deep-water, forereef facies. Katz (1976) tentatively assigned them a Pliocene or younger age of
formation. They are overlain by coralline limestones which, judging from depositional slopes
ranging from horizontal to 150, were laid down in upper reef slope and reef flat environments.
Thick algal limestones in the interior of the island are typical of back reef and lagoonal
environments of deposition.
Morphology
The northern coasts of Vava'u Island face the open sea where steep sea cliffs have a scalloped
plan-form that suggests prolonged marine erosion. In contrast, the southern margin is highly
irregular with a fragmented, ria-like appearance (Figure 10). Major features of the island are
illustrated in Figure 12. They include a scalloped coastline; smoothly curved, usually water-filled
re-entrants; isolated hills and gently sloping plateau surfaces. Smaller-scale landforms
superimposed on these features include linear ridges (rims or ramparts), internal depressions,
terraces, solution notches and fluvial drainage patterns. Figure 13 illustrates the distribution of
very steep and cliffed shorelines, terraces and areas of marine sediment build-up; linear ridges,
internal depressions and fluvial drainage systems are shown in Figure 14.
The curved re-entrants have lengths of 3-7 km, widths of 0.5 to 1.5 km and, in areas not
completely tilled with fine carbonate sediment, water depths of up to 80 m. Their sides are
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typically steep, in places cliffed, and occasionally terraced (Figure 13). Along open water
shorelines, there is usually a solution notch at present sea level. In exposed areas, undercutting is
responsible for cliff retreat leaving a narrow erosion surface just below low tide level. Across this,
waves rework detrital material seawards to form a deposit of sand and mud along the sides and
bottom of the re-entrants (Figure 12b, inset).
A recent seismic survey found sediments in the bottom of the re-entrants ranging in thickness
up to 50 m (Richmond, in prep.). They are typically finely laminated and are presumably mainly
muds. Internal seismic reflectors are conformable in the deeper sections (> 100 m bsl) but weak
disconformities that may represent subaerial erosion surfaces are evident at shallower depths.
Submarine terraces were noted in the limestone sides of some re-entrants.
Semi-circular hills with diameters of 0.7-2.5 km rise to elevations in excess of 90 m and the
sides of some show well developed terraces. The tops are usually basin-shaped and surrounded by
linear ridges (somewhat similar to the rims or ramparts of Hoffmeister, 1932). On Taoa, the
central depression is a composite internal drainage feature with the lowest point 60 m below the
highest rim and containing a number of sub-basins or terraces (Figure 11). On Falevai the
central basin is shallower (<40 m) and has a rim breached on its western side (Figure 13). The
breach outlet connects to a lower basin which also has a rim that is breached. In most cases the
linear rims or ridges parallel the sides of the curved re-entrants (Figure 14). Presumably this
means that they formed after the re-entrants. Rims occur at all elevations; those near present sea
level are partially drowned to form narrow limestone promontories (eg. Utungake Island, Figures
12c and 13).
An undulating plateau surface surrounds the northern part of the island and slopes gently
inland (Figure 14). Relative relief is mostly less than 20 m. On areas of lowest relief, subdued
patterns of surface drainage are delineated, best developed in the eastern half of the island
(Figure 14).
The various features described above tend to group into associations located in different parts
of the island. Linear ridges, terraced hills, internal drainage basins and deep, curved re-entrants
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occur together in the central and western parts of the island but are absent in the east. Here, the
dominant feature is the plateau surface which has a number of fluvial drainage systems on its
surface (Figure 14). These connect with the heads of the two eastern-most embayments which
have irregular shorelines and, unlike the curved re-entrants on the other side of the island, are
infilled with tidal flats (Figure 13). The tidal flats are continuous with extensive reef flats
surrounding small islands along the eastern seaboard.
A single solution notch, related to present sea level, is a widespread feature of Vava'u
especially around the interior shoreline. In places, the depth of undercutting (c. 5 m) raises the
possibility that it is a composite feature formed during more than one period of high sea level. In
fact, the absence of a higher notch at c. 5-6 m - the expected product of the Last Interglacial
maximum - led Taylor (1978) to postulate that the island has subsided in the last few hundred
thousand years.
The morphology of solution notches in Vava'u's interior waterways is shown diagramatically in
Figure 12b. The notches, which are typically 0.5-1.5 m in both height and depth, occur at the
base of near-vertical rock faces. A smooth, gently-sloping rock surface forms at the base of the
notch, and usually is submerged below sea level. It is free of sediment and debris near the cliff
and is heavily grazed by spiny sea urchins. Sands and muds occur further offshore in water
depths of 2 m or more.
On exposed parts of the open coast (Figure 12a), solution notches are less well developed.
Here, ocean waves acting on stratified and perhaps less resistant rock types result in relatively
high rates of cliff retreat.
Surface Geology
There is no indication on Vava'u that any primary depositional morphologies have been
preserved on the island’s elevated surface. The limestones are covered by red-brown soils which,
according to Orbell and others (1985), were developed predominantly in heavily weathered
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andesitic tephra up to 10 m thick derived from volcanic eruptions on the Tofua Ridge to the west.
Two main periods of tephra accumulation are recognised based on the development of a palaeosol
separating an "older red tephra" from a "younger brown tephra" (Orbell et al., 1985). Verbal
reports of volcanic ash falls from older residents lead these authors to suggest that the brown
tephra is "reasonably recent in age". It occurs extensively in the western and central part of the
island but thins rapidly towards the cast where "soils are developed mainly in the underlying red
tephra" (Orbell et al., 1985). These typically overlie limestone which, from the gross morphology of
the island's surface, has experienced prolonged weathering; the contribution of weathered
limestone to the development of the 'red tephra' is unclear. On interior shorelines of the island
the soil cover extends to within 1.0-1.5 m of present sea level.
As on Tongatapu, the main areas of Quaternary sedimentation in Vava'u are extensive reef
flats and shallow subtidal banks on the eastern and southern coasts (Figure 13). As well, the
re-entrants contain considerable thicknesses of fine sediment. The former arc composed of coral
detritus, calcareous algae and mud. Their occurrence on the windward side of the island suggests
much higher rates of biogenic sedimentation here than on the central and western parts. Shallow
banks (? of foraminiferal sand) also occur in the headwaters of some re-entrants. Reentrant
formation (discussed below) is thought to involve selective dissolution and removal of large
quantities of limestone over long periods of time. By implication, insoluble residues would tend to
be concentrated on the channel floors. At one protected site near Neiafu, coarse sands were
found in water depths of 4-7 m + on the side of the re-entrant. The composition of the sand is
unknown; if it is widespread it could be of use as a construction material.
Beach deposits on Vava'u are few and generally poorly developed despite the embayed nature
of the coastline. Open ocean beaches occur mainly on the eastern side of the island. One of the
largest, Keitachi Beach in Onetale Bay (Figure 13), is less than 50 m wide and rises about 2 m
above the reef flat. It is composed of medium to coarse sand and coral rubble, much of which
has been extracted for industrial use on the island. In the interior waterways, many of the
shorelines arc steep and beaches occur at relatively few sites. Most are found in small bays
where adjacent reef flats are exposed to moderately strong wave action. The largest beach
deposits infill the central section of Ofu Island and the village is built on them (Figure 13). The
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beach at 'Utungake Island occurs at the end of a small littoral drift system. Soils formed on the
beach deposits, termed Nga'unoko by Orbell and others (1985), are thin, non-volcanic and indicate
a mid-to late Holocene age of formation for the beach deposits. Elevated beaches, possibly
related to the Last Interglacial, were not found.
Two overwash sand lobes - intertidal and shallow sub-tidal features found on exposed
windward coasts - have been identified on the SE coast of Vava'u near Faioa Island (see inset in
Figure 13). Formed by storm waves that carry detritus from the reef front across the reef flat
into the lagoon, overwash lobes can be expected to be composed of coarse sand and reef rubble.
They constitute a potentially valuable source of construction material as an alternative to the
continuing exploitation of the very limited deposits of beach sand.
Geomorphological Development of Vava'u
The complex of islands and submarine banks forming the Vava'u group originated on the
Tonga Ridge in late Miocene/early Pliocene times as a relatively deep water carbonate platform
that was later capped by a thick sequence of reef limestones. Although insufficient is known of
the island’s stratigraphy, the thickness of the reef deposits suggests accretion on a slowly subsiding
substrate. This phase was followed by uplift and tilting of the Vava'u block which terminated reef
growth and elevated its northern rim more than 200 m above present sea level. Taylor (1978)
suggested that this tectonic trend has now reversed and slow subsidence is occurring; a minimum
rate of 1 m/20,000 years is indicated.
Some of the earliest ideas of Vava'u's morphology postulated reef growth on rims and cones
of a subsiding caldera (see Taylor, 1978; Cunningham and Anscombe, 1985). However, nowhere
on the island do volcanic rocks outcrop and even a core of volcaniclastics, such as occurs on the
island of 'Eua, is not evident (Taylor, 1978). Skeletonization by deep subaerial dissolution and
fluvial erosion (including karstification) has also been invoked to explain the deep re-entrants that
intersect the island (Hoffmeister, 1932). However, Taylor (1978) recognised the problems of
creating and maintaining karst features on the scale required and suggested that at least some
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landforms (eg. isolated hills with terraces) may in fact be depositional. He suggested that the
greater elevation of Vava'u compared with the other Tongan islands is an important factor but
does not specify how this promotes skeletonization; 'Eua is as high and has extensive areas of
limestone but has none of Vavau's deeply incised features. Furthermore, the smoothly curved
lineaments that characterise Vava'u are in marked contrast to the highly irregular antecedent karst
structures described from lagoons by Shephard (1970) and attributed by him to subaerial
weathering.
Despite the apparent similarity of Vava'u's regional setting and stratigraphy to the other
Tongan Islands, its unique morphology has long presented scientists with an enigma. A case is
presented below that its large-scale features are erosional, due primarily to a marine process here
termed ‘solution cliffing', acting on an emerging limestone terrain. Emergence and erosion of the
island coincided with repeated glacio-eustatic fluctuations in sea level during the Pleistocene
period.
Solution Cliffing
The term solution cliffing is used here to describe the erosion of cliffs or scarps in limestone
shorelines and the lowering of reef flat surfaces by the same processes that form solution notches.
Pirazzoli (1986) uses the term ‘sea-corrosion’ to cover those diverse physiochemical and biological
processes involved in eroding limestone in the intertidal zone. However the specific mechanisms
(which also include bioerosion) are not nearly as well understood as those causing cementation
(Bathurst, 1971; Hopley, 1982; Pirazzoli, 1986; Pirazzoli and Montaggioni, 1986).
The occurrence of scarps and notches on protected shorelines is evidence that processes of
sea-corrosion are not dependent on exposure to waves. Although notches provide the clearest
evidence of this form of erosion, it is on a much smaller scale than that indicated by the presence
of cliffs, especially where remnants of old limestone are found on the adjacent reef flat (see
Figure 15a) (Pirazzoli and Montaggioni, 1986). These observations indicate that limestone coasts
erode at times of high sea level by cliff retreat and down-cutting of reef flat surfaces.
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Undercutting leading to rock falls may also play a role but the rare occurrence of debris at the
base of sea cliffs suggests that it is not a major mechanism. Furthermore, according to erosion
rate estimates by Pirazzoli (1986), sea-corrosion to produce the morphologies illustrated in Figure
15a may have acted considerably faster than subaerial weathering (see also Guilcher and Pont,
1957). Usually, active algal/coral growth in the low- and subtidal zone limits the zone of erosion
to around the high tide level (Kawana and Pirazzoli, 1984). This is the case illustrated in Figure
15 for open coasts. However, under conditions that restrict biogenic sedimentation, it is proposed
that erosion of the limestone can occur below the inter-tidal zone and create channel-like water
ways within emergent limestone terrains; this is shown diagramatically in Figure 16.
In the case of Vava'u, an atoll or reef island with an irregular surface is initially envisaged,
gradually emerging above sea level. Its rim, patch reefs and shallower parts of the lagoon became
exposed above sea level, leaving slightly deeper areas submerged to form a highly irregular,
interior water body (Figure 16a). It was around this shoreline that sea-corrosion and solution
cliffing was initiated (Figure 16b). Furthermore, and most important to this theory, it is envisaged
that erosion not only affected the shoreline but, over the long-term, also deepened the
embayment. It has already been suggested that the shallow lagoonal areas of Fanga'uta on
Tongatapu may correspond to an early stage of this type of erosion.
Conditions favourable for submarine erosion include water properties that restrict biological
sedimentation (and perhaps promote dissolution) and water circulation patterns that allow the
products of erosion to be removed.
Regional uplift and tilting of the Vava'u Block has imposed a regional slope on its interior
waterways such that they have a connection to the open sea. In these channels, tongues of marine
water expanded and contracted during Pleistocene sea level fluctuations (Figure 16c-f).
Sea-corrosion has thus been focused over an active zone with a vertical amplitude of about 100 m
to excavate the channels. Much slower tectonic uplift of the island has progressively raised the
channel sides above sea level. The products of erosion were dispersed by tidal and wind currents
in ,dissolved form or as suspended particles. Coarser or insoluble matter was deposited on the
bed of the waterways and, at times of lowered sea levels, experienced subaerial leaching
Figure 16d and e).
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Factors restricting biological sedimentation, which otherwise would tend to infill the channels,
are undoubtedly quite complex. Today, in Vava'u, Holocene coral and algal growth varies from
active in the eastern (windward) coast to minimal in the centre and west where the re-entrants
are best developed (Figure 13). Here low nutrient levels or other water quality factors are
presumably responsible for low productivity. Channel excavation clearly represents a fine interplay
between erosional and depositional processes, which can and do change in time and space. The
occurrence of sediment in the heads of some re-entrants and land-bound relict cliff lines
(Figure 13) provide examples of the balance swinging in favour of sedimentation and away from
erosion.
In contrast, the almost complete infilling and highly irregular shape of the shallow eastern
embayments possibly reflects higher productivity and perhaps the localised accretion of old
coral/algal pavements on their sides in the past.
Terraces, Rims and Depressions on Vava'u
The same marine dissolution processes that formed the steep-sided re-entrants may also be
responsible for eroding terraces in western Vava'u. These postdate the re-entrant channels (Taylor,
1978) and presumably formed during stillstands of the sea that focused scarp retreat at one level
leaving an erosional platform or terrace just below sea level. They were later uplifted to their
present elevation. The alternative mechanism - progradation of fringing reefs to form accretionary
terraces as is thought to be the case on 'Eua (Taylor, 1978) - is perhaps less likely if coral growth
in western Vava'u in the past was as restricted as it appears to be today.
The terraces are generally best developed on hill sides away from present marine re-entrants
where solution cliffing is no longer active. Their absence on active shorelines suggests that solution
cliffing and notching may have undermined and destroyed old terrace features. Since. the Vava'u
Islands have experienced tilting and uplift, possibly followed by subsidence, it is not unexpected
that the terrace remnants now display little height continuity across the islands (Schofield, 1967;
see also comments by Taylor, 1978, p.189).
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Whether there is a genetic relationship between the linear ridges and depressions on Vava'u
and other features formed by solution cliffing is unclear. One possible mode of ridge formation
proposed by Hoffmeister and Ladd (1945) is by subaerial weathering. This involves preferential
leaching by rain of subhorizontal limestone surfaces leading eventually to the development of a
depression surrounded by an elevated rim or "rampart". The association of linear ridges,
re-entrants and internal depressions in Figure 14 lends some support to this explanation. However,
in other respects it is not entirely satisfactory; the ridges on Vava'u are more continuous and
smoother and much more symmetrical in cross-section than those described from 'Eua by
Hoffmeister and Ladd (1945) or produced by them in laboratory experiments.
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CONCLUSIONS
1.
Soil Formation
Geological interpretations based on mapping from air photos relies to a large extent on the
preservation of original constructional features on the land surface. Weathering, erosion and soil
development create secondary morphologies that obscure the primary features. For this reason, the
age of a land surface, the length of time it has been exposed to weathering and erosion, and the
nature of the soil cover need to he understood before the surface geology can be interpreted.
In Tonga, a feature of particular importance to coastal mapping is the widespread occurrence
of' a thick, volcanic soil. The soil blankets limestone deposits of Pliocene to (?) Pleistocene age
which were uplifted to form the present islands. Especially in Vava'u, the limestone surface has
undergone considerable weathering and erosion in the period preceding the creation of the soils.
The timing and duration of this above sequence of events - limestone deposition, uplift,
erosion and finally soil formation - is important. If the limestone surface of the island was forming
as living coral reefs and shallow lagoons in early Pleistocene times, as Cunningham and Anscombe
(1985) suggested, then the events that followed must have taken place in less than say 1.5 m.y.
with the volcanic ash falls occurring after a period of erosion in the late Quaternary. Despite
suitable conditions for rapid rock weathering (Orhell, 1971), Wilde and Hewitt (1983) stated that
the breakdown of limestone has not contributed to soil formation in Tonga, an assertion that is
questioned here.
Detailed studies by the New Zealand Soil Bureau show an upper, brown tephra layer
throughout the Tongan Islands overlying, in many areas, a lower, red tephra, the surface of which
is podsolised (Wilde and Hewitt, 1983; Orhell, et al., 1985). The upper tephra is clearly composed
of airborne ash and its thickness tends to decrease towards the southeast away from the Tofua
Arc, its undoubted source (Cowie, 1980). The distribution and composition of the lower unit,
however, is less well known. Although it is mainly volcanic in origin (Orhell, 1971 and pers.
comm., 1987), not all of it necessarily originated as airborne ash. The basal soil contact with the
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underlying limestone has not been studied in detail and it is possible that the importance of
limestone weathering products has been underestimated. If, at the time they were deposited, the
calcareous sediments incorporated significant amounts of pumice and ash, then surface weathering
of them at a later stage would produce a soil composed largely of degraded volcanic material.
In recent studies reported by Stephenson (1985), pumice was recovered in most dredge hauls
on the Tonga Ridge, and coring encountering a hard pumice cobble/coral rubble pavement at the
sea bed. The Tofua arc, which was active at the same time the limestones were accumulating on
the Tonga Ridge, is the main source of the volcanic detritus - both floating pumice and air-borne
ash.
Soil formation is thus linked to surface weathering and erosion processes initiated as soon as
the islands emerged from the sea at the end of the Pliocene. Tephra is seen as the most
important component, but not the sole source of the volcanic material on Tonga. Published rates
of karstic denudation are in the order of 70-100m of lowering per million years (Cunningham and
Anscombe, 1985). Thus, quite small amounts of volcanic material in the limestone could be
concentrated in a million years or less to produce a basal soil indistinguishable from the overlying
tephras.
Subaerial erosion and denudation of considerable magnitude and/or duration to are believed
to control the degree to which erosional landforms, such as the terraces, linear ridges and internal
depressions on Vava'u, are developed. Their morphology is atypical in lacking the jagged and
irregular relief of most karst limestone terrains. The thick blanket of volcanic soil above the zone
of weathering in the underlying limestone may have been instrumental in producing the generally
smooth land surface that characterises most Tongan islands. Conceivably, weathering is continuing
at the present time at the base of the soil layer.
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2. Geomorphology
Solution cliffing, a marine biochemical process, is thought to be mainly responsible for
excavating submarine channel-ways in the interior of limestone islands under certain environmental
conditions. These include (1) free tidal exchange to disperse the products of marine weathering
thus preventing water becoming supersaturated in CaCO and (2) water quality factors that reduce
biogenic productivity to levels below the rate of erosion. Both these conditions may be promoted
by the inflow of groundwaters, enriched in minerals leached from the volcanic soils surrounding
the interior water ways. Tilting of the islands is seen as important in maintaining tidal
interchange with the open sea at virtually all sea levels. This is not the case in non-tilted atolls
where a drop in sea level eventually creates an enclosed, non-circulating water body that soon
becomes supersaturated with dissolved carbonate.
In Vava'u, air photo mapping has shown that the individual features that make up the island’s
unusual morphology group together in regional associations. These are interpreted as resulting
principally from a long term imbalance between rates of marine erosion and sedimentation which
vary along an east-west gradient across the island. Depositional features in the eastern part of the
island are thought to be a response to the relatively high input of nutrients along windward coasts
that promote coral and algal growth and more than counterbalance marine erosion. This is not
the case in the western part of the island where low biogenic productivity and marine solution
cliffings have acted in concert with glacio-eustatic changes in sea level and gradual uplift of the
island to carve steep-sided shorelines, deep re-entrants and terraces. An erosional origin is also
indicated for the linear ridges and interior depressions but why such subaerially formed features
should be found in the same part of the island as the products of marine erosion is not
understood.
Clearly the surface of Vava'u has been modified by erosion far more than Tongatapu which
retains many of its original reef forms. This raises the possibility that the two islands represent
points on an evolutionary continuum, with Vava'u being more advanced in terms of denudation
than Tongatapu. Whether the continuum simply represents an age difference between the two
islands or different rates of denudation because of the greater uplift of Vava'u is uncertain.
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3.
Construction Resources
In a practical sense, the exercise of regional mapping using air photos has delineated a
number of potential resources on both islands. In Tongatapu, these include the Kolovai sand
ridge system which is now known to comprise two ridges, 2-3 km long, that are far more extensive
than previously thought. The tentative identification of shelly beach facies to c. +9 m overlain by
finer dune sands allows quarries to be sited so as to take advantage of sand with different
properties. The tilting model proposed for the island leads to the identification of a number of
potential buried spit deposits composed of coarse sand. This approach to selecting exploration
sites is particularly valuable in Tonga where resources often do not crop out at the surface but
are covered by thick volcanic soils and therefore need to be tested by expensive drilling
techniques.
On Vava'u, construction materials are in short supply. The contemporary trend of subsidence
proposed by Taylor (1978) largely accounts for the absence of Last Interglacial beach deposits on
the island, and because of its steep coastline, there is also a paucity of Holocene deposits. Some
beaches are presently being exploited despite their potential value for tourism and recreation. As
an alternative, large overwash sand lobes that have been identified on the eastern reef flat may
contain suitable material for construction purposes. However, their accessibility remains to be
assessed. Residual sediments in the interior waterways of the island, especially in the deeper
re-entrants, should also be sampled to assess their composition and possible applications.
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APPENDIX
Programme Element CCSP/TG.12:
RECONNAISSANCE COASTAL AND
NEARSHORE
MAPPING
OBJECTIVES:
1.To prepare morphological and geological maps of sedimentary deposits both onshore and in
shallow offshore waters.
2.To make preliminary assessments of the resource potential (construction material, insular
phosphate etc.) of the surficial sediments.
3. To provide a geological framework for evaluating future coastal developments, and where
possible, identify areas of particular environmental sensitivity.
4. To develop an understanding of the recent geological evolution of Tonga and to relate this to
present-day sedimentation trends.
5. To train local geologists in techniques of regional coastal mapping and resource assessment.
TASKS:
1.Undertake regional air-photo interpretation of coastal and shallow nearshore areas of Tonga.
Carry out reconnissance field inspections to check air photo interpretations. Prepare preliminary
geological/geomorphological maps and reports.
2.Undertake detailed geological surveys of areas identified in Task 1 in order to delineate
resource potential and environmental factors related to future developments.
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