Coastal morphology of Rarotonga, Cook Islands

CCOP/SOPAC Technical Report 65
October 1990
COASTAL MORPHOLOGY OF RAROTONGA,
COOK ISLANDS
by
Bruce M Richmond
Techsec
Prepared for: South Pacific Applied Geoscience Commission (SOPAC)
Training Programme; Coastal and Nearshore Programme, Cook Islands Project: CK.4
[3]
TABLE OF CONTENTS
Page
SUMMARY .....................................................................................................................
5
INTRODUCTION .........................................................................................................
ISLAND GEOLOGY
..........................................................................................
6
6
COASTAL MAPPING WORKSHOP - RESULTS
Coastal Morphology Map of Rarotonga ...........................................................
6
Coastal Cross Sections and Beach Profiles .......................................................
9
COASTAL MORPHOLOGY - DISCUSSION
Coastal Plain ................................................................................................
Beaches
Storm
.............................................................................................
Ridge
.............................................................................
9
13
13
Inland Depression (Swamps) .............................................................
30
Stream Channels ..............................................................................
Fringing Reef
..........................................................................................
30
30
Reef Flat
....................................................................................
31
Reef
Crest
.....................................................................................
31
Reef
Passages
Motu (Islets)
..........................................................................
....................................................................................
31
32
OBSERVATIONS OF COASTAL PROCESSES - DISCUSSION ............................
PRELIMINARY GEOHAZARD ASSESSMENT .............................................
32
CONCLUSIONS
.........................................................................................................
36
............................................................................................................
37
Student-prepared descriptions for sediment samples ......................
39
REFERENCES
APPENDIX
[TR65 - Richmond]
33
[4]
LIST OF FIGURES
Figure
1
Coastal morphology map of Rarotonga .........................................................
Page
7-8
2
Student-prepared coastal cross sections of Rarotonga ...................................
10-11
3
Student-prepared Rarotonga beach profiles ..................................................
12
4
Location of photographs shown in Plates ...............................................
14
5
Zones of potential geohazards of the Rarotonga coast ..............................
34
LIST OF PLATES
Plate
la
Steep cobble beach face of the north coast near Pue .............................
15
1b
Eastern shoreline north of Ngatangiia ..................................................
16
1c
Sand beach with small erosional backbeach scarp .......................................
17
1d
2a
2b
Wide sandy beach of west coast near Kavera .....................................
Sand and gravel mining operation at the mouth of Avana Stream ...............
Southern coast east of Turoa Stream ........................................................
18
3a
Unconsolidated sandy gravel of storm ridge exposed at a construction site .
21
3b
Unconsolidated sands of storm ridge near Arorangi on the west coast ......
22
4
Remnants of a small flood delta on the reef flat ....................................
23
5a
5b
Narrow reef flat north of Matavera Stream on the east coast ..............
Wide reef flat near Aroa along the southwest coast ................................
24
25
5c
Storm-derived gravel bank near the International Airport
.......................
26
6a
High-level reef limestone exposed along the north coast of Motutapu .....
27
6b
Close-up of coral colony in growth position .............................................
28
6c
Gravel and sand shoreline of southeast Motutapu ...................................
29
[TR65 - Richmond]
19
20
[5]
SUMMARY
Rarotonga is a steep-sided volcanic island with maximum elevation of about 650 m and encircled
by Quaternary coastal landforms and deposits. The coastal zone can be divided into: (1) a coastal
plain composed of storm ridges, beaches, and swampy inland depressions which are dissected by
stream channels and flood plains, and (2) a fringing reef made up of a reef flat, reef crest, and six
deep (as much as 20 m) passages; several small islets (motu) are developed on the reef flats.
Storm ridge morphology, sediments, and elevation above the reef flat can be related to exposure.
The highest storm ridges (as much as 7 m above sea level) occur along the north coast, the area most
exposed to direct storm surges associated with tropical cyclones. The coarsest sediment (gravel) is
distributed along the northeast coast where the narrowest reef flats exist. Except near the mouths of
the larger streams, where terriginous detritus may predominate, most of the sediments of the beaches
and reef flats are derived from adjacent reefs.
Several potential geohazards of the Rarotonga coastal zone were identified during the study.
These include overwash events of the coastal plain during storm surges associated with tropical
cyclones, flooding of low-lying coastal areas during high intensity rainfalls, and shoreline erosion both
natural and man-induced.
[TR65 - Richmond]
[6]
INTRODUCTION
This report presents (1) data collected during the fourth CCOP/SOPAC-USGS Coastal Mapping
Workshop, (2) additional information based on subsequent visits by the author and, (3) other
published data. This study provides information pertinent to part of SOPAC Work Programme project
CK.4: "Study of sediments and sedimentary processes of beach, lagoon and adjacent offshore areas of
Rarotonga, and other islands to assist with coastal management problems."
The Workshop was held in Rarotonga, Cook Islands, from 15 to 17 July 1986. Twenty-one
students representing eight member countries attended the workshop. Student exercises during the
workshop included
(1)
mapping geological features of the coastal plain and fringing reef using aerial photographs,
existing maps, and field inspections;
(2)
surveying cross-sections from the reef to the base of the alluvial fans and coastal terraces;
(3)
sediment sampling and description, interpretation of the sediment characteristics;
(4)
measuring beach profiles and probing of the beach to determine sediment thickness.
ISLAND GEOLOGY
Rarotonga is a relatively young volcanic island surrounded by a fringing reef. Basalts and
phonolites from uppermost cone lavas have K-Ar ages between 1.8 and 1.2 m.y. (Dalrymple et al,
1975). The island rises 4000 m from the seabed and has a maximum elevation above sea level of
about 650 m. The oval-shaped island is about 12 km long by 6.5 km wide with nearly 36 km of
shoreline. Three major landform types occur: (1) a highly dissected and densely forested rugged
volcanic interior; (2) a surrounding apron of terraces and alluvial fans; (3) a coastal plain and
fringing reef of Quaternary deposits forming a near-continuous girdle around the island.
COASTAL MAPPING WORKSHOP - RESULTS
Coastal Morphology Map of Rarotonga
A map (two sheets) showing the coastal morphology of Rarotonga is presented in Figure 1. The
map (two sheets) is also available at a scale of 1:10,000 from the SOPAC Technical Secretariat, Suva,
Fiji. This map is based on previously published information combined with the results of the Coastal
[TR65 - Richmond]
[7]
Figure la. Coastal morphology of Rarotonga, sheet 1 of 2.
[TR65 - Richmond]
[8]
Figure 1b. Coastal morphology of Rarotonga, sheet 2 of 2.
[TR65 - Richmond]
[9]
Mapping Workshop. Features of the alluvial terraces, coastal plain, fringing reef, and offshore
bathymetry are displayed on the map.
The geology and morphology of the coastal zone are related to the soil type. The Muri soils are
formed on the Holocene storm and beach ridges which encircle the island; the Vaikai and Ngatangiia
soils form
in swamps and depressions; the Avana, Takuvaine, Rutaki, and Matavera soils are
developed from flood plain materials; and the Pouara and Nikao soils are terrace products (Leslie,
1980). Offshore bathymetry was taken from "Rarotonga Nearshore Bathymetry" - a
CCOP/SOPAC-NZOI Miscellaneous Series Chart (Utanga and Lewis, 1981). Features of the reef flats
and shorelines were interpreted from New Zealand Lands and Survey Department 1962 vertical aerial
photographs. Additional shoreline and reef flat features are based on student field observations.
Coastal Cross Sections and Beach Profiles
The students, as members of teams, were required to produce a coastal cross section from the
reef crest or as far seaward as was practicable, through the reef flat and coastal plain to the base of
the alluvial deposits. The cross sections (Figures 2a and 2b) display the surficial geology and sample
locations. The samples are described in the Appendix.
Associated with the cross sections are beach profiles. The thickness of the beach sediment was
determined by probing with an iron bar until bedrock (beachrock or limestone) was reached. The
profiles are shown in Figure 3 (no probe data were collected along the E series cross sections).
COASTAL MORPHOLOGY - DISCUSSION
Coastal Plain
The coastal plain of Rarotonga is composed of beaches, a prominent storm ridge and a
swampy inland depression, all of which are dissected by occasional stream channels. It is bounded on
its landward margin by alluvial fans and terraces and its seaward margin by a fringing reef. When
viewed from the air or by sea it forms a conspicuous low-relief landform bordering the island; vertical
relief is less than 10 m.
[TR65 - Richmond]
[13]
Beaches
The beaches of Rarotonga, which encircle almost the entire island, are composed predominantly
of detrital carbonate sand and gravel with smaller amounts of terriginous sediment which may be
locally important. In general the beaches are sandier along the south and west coasts, and contain
more gravel along the north and east coasts (Appendix). Plate 1 (see Figure 4 for location of plates of
photographs) illustrates beach variations around Rarotonga. Beach slope and grain sue tend to be
inversely related to the width of the adjacent fringing reef - steep, gravel beaches where the reef is
narrow (for example, the northeast coast) and flatter, sandier beaches next to the wider reef areas (for
example, the south and west coasts). The majority of the beach sediments are composed of bioclastic
carbonates primarily from corals, Halimeda, molluscs, echinoderms and calcareous algae. Usually only
minor amounts of volcanic material occurs, but when present in larger proportions the sediment has a
distinct "salt-and-pepper" appearance. Volcanic sediment comprises most of the material at the month
of the Avana Stream (Plate 2a).
Beachrock occurs at numerous localities around the island and typically its texture and
composition mimics the nearby unconsolidated beach sediments. Beachrock forms intertidally at depth
within the sediment column - its occurrence at the surface indicates overlying sediment has been
removed. Beach sand mining over the last 20 years or so may have contributed to an increase in
beachrock exposures. Much of the shoreline is armoured by artificial structures to prevent further
erosion (Plate 2b).
Storm Ridge
A large ridge is a dominant feature of the coastal plain. It is composed mainly of unconsolidated
carbonate sand and gravel (Plate 3; Aroa Sands of Wood and Hay, 1970). It varies in height from
several metres on the south coast to about 7 m along the northeast and northwest coasts. The
overall form is one of a single large ridge with occasional superimposed smaller ridges, although much
of the original surface topography has been modified by human activity. The unconsolidated sediments
of the ridge are indicative of their recent deposition, presumably under storm conditions. The
variation in ridge elevation gives an indication of wave run-up height during storms and tropical
cyclones. The higher ridges occur in areas most prone to tropical cyclone associated storm surge.
Clast size of the ridge sediments are related to width of the adjacent reef flat. Gravel is the dominant
material along the north and northeast coasts where the reef flat is narrow (< 100 m) and sand
predominates elsewhere.
[TR65 - Richmond]
[30]
Wood and Hay (1970) report a raised reef underlying portions of the Aroa Sands (storm ridge)
along the north coast, however, this was not observed during the present study. Underlying in-situ reef
implies either island uplift or reef formation under a relative higher sea level position.
Inland Depression (Swamps)
Between the storm ridge and the base of the alluvial fans is a flat-bottomed depression as much
as several hundred metres wide and generally less than 1 metre deep. Mostly swamp-like in character,
the depressions contain finer sediment than the adjacent ridges and are the sites of much taro
cultivation (hence the common name of taro swamp). Many of the smaller streams draining the
interior terminate at the depressions without a direct channel continning to the ocean. The storm ridge
has created a barrier which impounds stream discharge resulting in inland swamp areas. Freshwater in
the swamps may be responsible for dissolution of carbonate material thereby increasing swamp size.
Stream Channels
The larger streams, such as the Takuvaine, Avana, and Avatiu, maintain direct channels to the
sea. They are typically bordered by narrow flood plains. Extensive, well-developed estuaries arc absent,
however near the mouth of the Avana Stream some estuarine-like muds are present. The streams are
ephemeral in character; most of the time exhibiting low flows which are punctuated by high
discharge/rapid flooding events. Considering the island is mostly volcanic it is somewhat surprising
volcanic sediment does not make up a larger proportion of beach material. This is due in part to
stream discharge bypassing the fringing reef during floods - terriginous material is transported directly
to deeper water via the reef passages.
Small flood deltas may form on inner reef flats at the mouths of small streams (Plate 4). Flood
deltas form by the rapid deposition of sediments during floods. Flow expansion as streams reach the
reef flats causes them to deposit their sediment load. This material is later reworked commonly
leaving behind a gravel lag deposit. Sand-size sediment is transported alongshore and added to the
beaches.
Fringing Reef
The fringing reef varies from less than 30 m wide in the northeast to nearly 900 m in the vicinity
of Taakoka Motu. Composed of a reef flat, reef crest, and reef front terrace, the fringing reef is
continuous except for a few narrow passages (Avatiu, Avarua, Ngatangiia, Avaavaroa, Papua and
[TR65 - Richmond]
[31]
Rutaki). Except near the passages, the fringing reef is shallow, ranging in depth between a few tens of
centimetres to about 1.5 m at low water and is therefore a true fringing reef.
Reef Flat
The reef flat is a low-relief surface extending from the beach to the reef crest (Plate 5). It
usually has a broad, shallow, shore-parallel channel or moat. Sediment cover is sporadic, generally
thin, and tends to be greater on the inner reef flat areas. Most of the sediment is skeletal carbonates
from corals, Halimeda, calcareous red algae, molluscs, and echinoderms of sand and gravel size. Fine
sediment is quickly removed from the reef flat by waves and currents. There is sparse coverage by live
corals and occasional Porites microatolls. At Muri Lagoon near Ngatangiia Passage there are dense
colonies of free-living, spherical, massive, and branching rhodoliths (calcareous red algae) and
coralliths (corals, Scoffin et al, 1985).
The reef flat can be broadly subdivided into outer and inner zones. The outer reef flat is
composed mostly of coral rubble, scattered small coral colonies and coralline algae typically overlying
a pavement-like surface. The inner reef flat typically contains more sand-size sediment and in deeper
areas seagrass and other marine flora occur.
Reef Crest
The reef crest is slightly elevated above the reef flat and typically is a substrate of encrusting
corals and bioclasts bound primarily by encrusting coralline algae. Because of its position as the
highest part of the reef it receives most of the incoming wave energy. Sediment cover is very sparse
and is restricted to small pockets of poorly sorted coarse material. Shallow borings (less than 3 m)
through the reef crest on the north coast (Yonekura et al, 1984) show a vertical stratigraphy
composed of interbedded algal pavement, rigid framework of coral heads, and reef rubble. Seaward
of the reef crest depth increases rapidly to a depth between 17 and 20 m to the reef front terrace
which extends out to sea for 200 m to 600 m, (Lewis et al, 1980).
Reef Passages
The reef passages around the island are narrow (3-60 m) steep-sided, flat-bottomed channels.
Their position along the coast can be related to seaward extensions of antecedant terrestrial drainage
systems. Depths range from a few metres near the shore to 10-20 m opposite the reef crest. Outside
the reef the single channel branches into numerous intertwining grooves developed within a broad
trough. According to Lewis et al, (1980) sediment type and channel configuration vary between the
[TR65 - Richmond]
[32]
south and north coasts. The Avarua and Avatiu Passages (north coast) are floored by sand rich in
rounded coral and basalt pebbles, and are somewhat wider and shallower than the southern passages,
which are floored by a more immature Halimeda-rich sand plus coral pebbles. Whereas all the other
channels essentially flow directly across the reef flat, the channel at Ngatangiia meanders from its
origins near the mouth of Avana Stream. Sediment, consisting of basaltic sand and gravel and reefal
carbonates, is perched near the rim of the channel. Infilling of the channel has been dramatic since
the turn of the century (Kirk, 1980), primarily due to the clearing of land within the Avana watershed.
Motu (Islets)
Several motu are developed on the reef flats, most notably along the southeast comer of the
island. Three of the motu (Motutapu, Oneroa, and Koromiri) are composed of detrital reefal
carbonates while the other motu (Taakoka) is volcanic in origin. High-level reefal limestone (Plate 6)
is exposed on the northern portion of Motutapu and across Ngatangiia Harbour. Rising as much as
3 m above sea level the limestone was probably formed during the last interglacial (Stoddart, 1972).
The motu typically are fronted by coral rubble on their seaward borders and broad sandy calcareous
beaches on their lagoon margins. A small motu (Motutoa) occurs on the north coast reef flat near the
Rarotonga International Airport.
OBSERVATIONS OF COASTAL PROCESSES - DISCUSSION
Wave processes dominate sediment transport within the fringing reef. Wave set-up at the reef
crest increases water level in the lagoon. Wave-generated currents flow alongshore until they reach
topographic lows (passages) and develop into a seaward return flow. The passages therefore are ebb
dominant during most tidal stages. Lineations of coral and sediment on the reef flat define transport
pathways during storms. The lineations trend normal to the reef crest for most of their distance,
veering more parallel to the coast near the shoreline. Under these transport conditions, sediment
(primarily reef-derived) can be expected to move landward and then be redistributed alongshore by
short-period waves generated within the shallow lagoons. Once sediment reaches the head of a passage
it is transported offshore and lost from the nearshore reef system. The direction and strength of the
nearshore currents varies with changes in the wave climate.
Nearshore current studies near Ngatangiia passage (Kirk, 1980) under "typical" southeast
(non-storm) tradewind conditions indicated a dominant northward flow with velocities sufficient to
regularly transport sand and intermittently transport gravel. The extensive colonies of coralliths and
rhodoliths in this area indicate reasonably strong currents.
[TR65 - Richmond]
[33]
Sediment movement during tropical cyclones is often significant, both in terms of volume and
transport distance (Cowan and Utanga, 1987). The gravel-covered storm ridge along the north and
east shore demonstrate the ability of these storms to move boulder-size coral blocks several hundred
metres to heights as great as 7 m above present sea level. Tropical cyclones with sustained winds of
80 knots can be expected to occur every 20 years (Carter, 1984), with the strongest winds usually
attacking the north coast.
The coastal plain storm ridges are constructional features built by successive storm overwash.
Schofield (1970) described them as prograding with the oldest deposits along their landward margin.
Field evidence (for example, buried multiple landward-dipping soil horizons; e.g. Plate 3b) suggests a
substantial component of vertical accretion occurs during overwash events. Sediments of the ridges are
similar to the sediment type of the adjacent beach - the probable source in addition to the reef flats.
In summary, the ridges have grown laterally and vertically during the Holocene primarily during
low-frequency 'catastrophic' events in which reef and beach sediments are deposited inland.
Deposits formed entirely by wind action are rare or absent - perhaps the only aeolian deposits
are on the broad low-lying coastal plain near the Rarotongan Hotel on the southwest corner of the
island. The area is a cuspate headland probably built by sediment transported from east to west along
the south coast and from sediment moving south along the west coast. Fine sand inland could be
wind-blown.
In summary sediment transport in the coastal zone is influenced by several parameters including:
(1) southeast tradewind conditions which impart a general east to west transport direction, (2) wave
set-up at the reef crest promotes a landward gravity-driven circulation, (3) seaward return flow at reef
passages which can lead to significant local reversals of the 'normal' east to west transport and (4)
variations from tradewind conditions, especially during westerlies and cyclones, when transport
reversals and sediment redistribution occur.
PRELIMINARY GEOHAZARD ASSESSMENT
Geohazards in the Rarotonga coastal zone include shoreline erosion, overwash events of the
storm ridge during cyclones, and flooding of low-lying areas during intense rains. The distribution of
potential geohazards are shown in Figure 5. This preliminary map is based on relationships between
the local geology and coastal processes involved in the formation of the deposits.
[TR65 - Richmond]
[35]
Based on observations made in 1926, Marshall (1930) noted "...for almost the complete circuit of
Rarotonga the coast was a dazzling white sandy beach...". He also mentioned a short section of coast
near Matavera where the beach was formed of cobbles and coral rock. Erosion of the shoreline and
subsequent engineering attempts to protect the shore has modified the coast since Marshall's
observations. Sand mining has occurred intermittently along the coastline of Rarotonga, particularly at
stream months along the south coast. Much of the present southern shoreline is armoured to protect
roads and buildings (Figure 1). Poorly designed engineering structures (for example, vertical seawalls)
may be further contributing to coastal erosion (Byrne, 1984) by increasing scour and preventing
deposition. Shorelines mapped as unstable or eroding in Figure 5 exhibited beachrock, undermined
coastal vegetation, beach scarps, and coastal protection works. Moderately stable shorelines are fronted
by wide "healthy" beaches or shorelines not showing signs of erosion. Beaches act as a buffer between
the land and sea - retreating during storms and building out in times of fair weather. An interruption
of the local sediment budget reduces the effectiveness of beaches to protect the coast.
The entire coastal plain of Rarotonga is potentially subject to overwash during storm surge
associated with tropical cyclones. The higher/coarser ridges are probably indicative of more intense
overwash events. Height of the ridges is related to the effects of succesive overwash; the higher ridges
are formed by more frequent and higher storm surges. The gravel ridges are a function of source
material and attest to the power of the storm surges. The overwash prone areas are subdivided into
either a sand or gravel phase in Figure 5.
Flood plain deposits are formed by stream overflow during high intensity rainfall. Short period
extreme rainstorms are very important as they are the primary cause of severe soil erosion and high
surface runoff. A 50 year return period rainfall for Rarotonga can be expected to provide 328 mm in
a 24 hour period (Thompson, 1986). During such a rainstorm inland erosion would be high, stream
channels are likely to overflow, and large volumes of sediment would be deposited on the flood plains
and transported to the coast. If a severe rainstorm coincides with high storm surges major flooding of
low-lying coastal areas could result. The flood prone areas shown in Figure 5 are based on the flood
plain and swamp deposits shown in Figure 1.
[TR65 - Richmond]
[36]
CONCLUSIONS
1. In a relatively short period of time (weeks) using aerial photographs, field observations, and
published information it is possible to prepare coastal maps that are useful to geologists, engineers,
and planners. Information mapped during the workshop included coastal morphology and
sedimentology, shoreline stability, sand and gavel deposits, and observations regarding nearshore
processes.
2. The coastal zone of Rarotonga consists of a coastal plain and fringing reef. The coastal plain is
made up of swampy inland depressions, storm ridges as much as 7 m above sea level, beaches
composed of sand with lesser amounts of gravel, and stream channels and their associated flood
plains. The fringing reef is made up of a low-relief reef flat, slightly-elevated reef crest, and reef
passages as much as 20 m deep.
3. Most of the sediments of the fringing reef, beaches, and storm ridges are carbonate material.
Volcanic derived sediments form a minor component except near the larger stream months.
4. Potential sand and gravel resources are limited to the coastal plain and reef passages. The reef
flats are covered by a patchy distribution of thin sediment veneers. However, the reef flat pavement is
a potential source of armour for shoreline revetments and may be underlain by unconsolidated
sediment.
5. Extensive areas of the Rarotonga shoreline, particularly along the southern coast, are armoured
to prevent further coastal erosion. Causes of the erosion include previous sand mining of the foreshore
and coastal engineering structures which disrupt longshore sediment transport and enhance scouring of
the foreshore.
6. Nearshore circulation is primarily a wave-driven phenomena and varies with changes in wave
climate and the passage of storm fronts. In general, wave set-up at the reef crest creates a shoreward
flowing current. Water is returned seaward mostly through the deep passages which are ebb dominiant
features.
7. Coastal plain storm ridge height and sediment size can be related to exposure; the highest ridges
occur from the northwest to the northeast where storm surge is usually greatest and the coarsest
ridges occur along the northeast coast where the fringing reef is narrowest.
8. Geohazards of the coastal zone include storm surge overwash of the coastal plain, flooding
associated with high intensity rainfalls, and coastal erosion both natural and man-induced.
[TR65 - Richmond]
[37]
REFERENCES
Byrne, R.V. 1984. Assessment of coastal management, Rarotonga, Cook Islands. CCOP/SOPAC Cruise
Report 102: 45 pages.
Carter, R. 1984. Baseline study for coastal management coastal engineering study at Ngatangiia Harbour
and Muri Bay, Rarotonga, Cook Islands. CCOP/SOPAC Cruise Report 100: 16 pages.
Cowan, G. and Utanga, A. 1987. The effects of Cyclone Sally on Rarotonga. Paper presented at
CCOP/SOPAC Coastal Processes Workshop, Lae, Papua New Guinea, 1-7 October, 1987.
Dalrymple, G.B.; Jarrard, R.D. and Clague, D.A. 1975. K-Ar ages of some volcanic rocks from the Cook
and Austral Islands. Geologic Society of America, Bulletin 86: 1463-1467.
Kirk, R.M. 1980. Sedimentation in Ngatangiia Harbour and Muri Lagoon, Rarotonga, Cook Islands.
Unpublished report to South Pacific Commission and the Government of the Cook Islands.
Leslie, D.M. 1980. Soil Map of Rarotonga, Cook Islands Scale 1:10,000. New Zealand Soil Bureau, Map
163.
Lewis, K.B.; Utanga, A.T.; Hill, P.J.; and Kingan, S.G. 1980. The origin of channel-fill sands and gravels
on an algal-dominated reef terrace, Rarotonga, Cook Islands. South Pacific Marine Geological Notes
2(1): 1-23.
Marshall, P. 1930. Geology of Rarotonga and Atiu. Bernice P. Bishop Museum, Bulletin 72: 75 pages.
Schofield, J.C. 1970. Notes on Late Quaternary sea levels, Fiji and Rarotonga. New Zealand Journal
Geology & Geophysics 13: 199-206.
Sciffin, T.P.; Stoddart, D.R.; Tudhope, A.W.; and Woodroffe, C. 1985. Rhodoliths and coralliths of Muri
Lagoon, Rarotonga, Cook Islands. Coral Reefs 4: 71-80.
Stoddart, D.R. 1972. Reef Islands of Rarotonga. Atoll Research Bulletin 160: 1-7.
Thompson, C. 1986. The climate and weather of the southern Cook Islands. New Zealand Meteorological
Service, Miscellaneous Publication 188(2): 69 pages.
Utanga, A.T. and Lewis, K.B. 1981. Rarotonga Nearshore Bathymetry 1:20,000. CCOP/SOPAC
Miscellaneous Chart series 1 (also New Zealand Oceanographic Institute, Chart Miscellaneous series
56).
Wood, B.L. and Hay, R.F. 1970. Geology of the Cook Islands. New Zealand Geological Survey, Bulletin
N.S. 82: 103 pages.
Yonekura, N.; Matsushima, Y.; Maeda, Y.; and Kayanne, H. 1984. Holocene sea-level changes in the
southern Cook Islands. Report of the HIPAC Project : 113-133.
[TR65 - Richmond]
[39]
APPENDIX
Student-prepared Descriptions for Sediment Samples
Collected in the Rarotonga Coastal Zone
SAMPLE
A1-1
LOCATION
GRAIN SIZE
SORTING
REMARKS
reef crest
coarse Sand and
gravel
very poorly
Algae encrusted coral
fragments
2
reef flat
coarse sand and
granules
poorly
Carbonate sand from
broad trough
3
inner reef
flat
coarse sand and
gravel
very poorly
4
beach
coarse sand
well
Some basalt grains
5
coastal plain
coarse sand
well
25m from beach
6
coastal plain
corase sand
well
250m inland
reef crest
coarse sand
and gravel
very poorly
Coral gravel
2
reef flat
very coarse sand
poorly
Shallow trough
3
inner reef
flat
medium sand
very poorly
Mixture of volcanic
and carbonate
4
beach
medium sand
poorly
Some basalt grains
5
coastal plain
medium-fine
sand
well
25m from beach
Harbour
dredgings
fine sand
well
Dark coloured
Harbour
dredgings
coarse sand
and gravel
poorly
Volcanics/carbonate
mixture
Coastal
swamp
mud
well
Brown soil
A2-1
A3-1
3
B1-1
Poorly
Carbonate sand with
halimeda
reef crest
medium sand pebbles
2
reef flat
medium sand
Poorly
10% volcanics
3
beach
medium sand
well
20%volcanics
4
coastal plain
silt
well
[TR65 - Richmond]
Some volcanics
brown soil
[40]
SAMPLE
B2-1
LOCATION
GRAIN SIZE
SORTING
reef crest
fine coarse
sand
2
reef flat
fine sand
3
beach
fine sand
well
volcanic/carbonate
mixture
4
coastal plain
silty sand
well
brown soil w/coral
gravel
reef crest
fine - very
coarse sand
poorly
some coral gravel
2
reef flat
fine sand
well
some volcanics
3
Beach
medium sand
well
20% volcanics
4
coastal plain
medium sand
well
volcanic/carbonate
mixture
mid-channel
med-coarse sand
moderately
thin sediment cover
6
channel margin
fine-coarse sand
poorly
thin sediment cover
7
reef flat
fine-very coarse
sand
poorly
thin sediment cover
beach
med-coarse sand
moderately
minor gravel of coral
and molluscs
3
coastal plain
medium sand
poorly
near beach
15
edge of swamp
silty coarse sand
poorly
brown soil
16
edge of swamp
muddy sand
moderately
soil and organics
17
coastal plain
coarse sand
poorly
near landward limit
(-1)
mid-channel
very coarse sand
poorly
some coral gravel
(-2)
reef flat
med sand
moderately
some shell gravel
(-3)
beach
gravelly sand
poorly
some shell and halimeda
(-4)
beach
gravelly sand
poorly
some coral and halimeda
(-5)
mid islet
gravelly sand
poorly
coral, gravel
(-11)
reef crest
gravelly sand
poorly
rich in coral and
urchin spines
B3-1
C1-5
C2-1,2
poorly
REMARKS
well
[TR65 - Richmond]
some coral gravel
somevolcanics
[41]
SAMPLE
C3-1,2
LOCATION
SORTING
GRAIN SIZE
REMARKS
beach
fine-coarse sand
moderately
rich in halimeda,
some coral
3
coastal plain
med-coarse sand
well
pale gray
4
coastal plain
fine-coarse sand
poorly
dark gray
5
coastal plain
fine sand
well
some coral gravel
D3-1
beach
fine sand
well
carbonate/volcanic mixture
2
beach
fine sand
well
"
3
beach
medium sand
moderately
"
"
4
beach
coarse sand
well
"
"
5
beach
fine sand-pebble
poorly
"
"
6
reef flat
fine sand-pebble
very poorly
all carbonate
7
reef crest
fine sand-pebble
very poorly
all carbonate
D2-1
beach
fine sand
very well
slight volcanics
2
beach
fine sand
very well
slight volcanics
3
beach
very fine sand
very well
trace volcanics
4
beach
medium sand
moderately
tracevolcanics
5
reef flat
coarse sand
poorly
all carbonate
coarse sand
poorly
all carbonate
6
D1-1
beach
fine sand
very well
2
beach
fine sand
well
carbonate
3
beach
fine sand
well
carbonate
[TR65 - Richmond]
"
slight volcanics
"
[42]
SAMPLE
LOCATION
E1-1
reef crest
sandy gravel
very poorly
red-algae encrusted coral
gravel
2
reef flat
gravelly coarse
sand
poorly
some coral and shells
3
beach (scarp)
very coarse sand
moderately
4
beach
coarse sand
well
some halimeda and
volcanics
5
coastal plain
fine sand
very well
near beach
6
coastal plain
fine sand
very well
dark soil
7
coastal plain
fine sand
well
dark soil
8
coastal plain
fine sand
very well
dark soil
reef crest
sandy gravel
very poorly
red-algae encrusted
coral gravel
2
reef flat
gravelly sand
very poorly
some coral gravel
3
beach
very coarse sand
poorly
some coral fragments
4
coastal plain
fine sand
well
dark soil
reef crest
sandy gravel
very poorly
mostly coral and
shell fragments
2
reef flat
sandy gravel
very poorly
coral and shell
gravel
3
reef flat
very coarse sand
poorly
minor coral and shell
material
4
beach (scarp)
very coarse sand
poorly
some halimeda
5
beach
coarse sand
moderately
carbonate sand
6
coastal plain
fine sand
well
near beach
7
coastal plain
fine sand
well
dark soil
E2-1
E3-1
GRAIN SIZE
SORTING
[TR65 - Richmond]
REMARKS
abundant halimeda