Sedimentary environments and unconsolidated carbonate sediments of the fringing coral reefs of Mahé, Seychelles

Marine Geology - Elsevier Publishing Company, Amsterdam

SEDIMENTARY ATE

ENVIRONMENTS

SEDlMENTS

MAHe,

OF

THE

AND

- Printed in The Netherlands

UNCONSOLIDATED

FRINGING

CORAL

REEFS

CARBONOF

SEYCHELLES

MICHAEL

SAMUEL

LEWIS’

Department of Geology, The University, Glasgow (Great Britain)

(Received February 13, 1968) (Resubmitted July 3, 1968)

SUMMARY Extensive

Seychelles

fringing

Archipelago.

coral reefs occur around MahC: the largest island in the Several sedimentary environments, which are best

developed on reefs exposed to the Southeast Monsoon (the dominant wind), are associated with the reefs. These are: the beach; rippled sand zone; marine grass bed: radial zone; algal ridge; reef edge; reef slope; “plateaux”; mangrove and former mangrove swamps; deltas; channels; the fore-reef environment. Skeletal carbonate sands form a veneer on the reef flats and are widespread in the fore-reef environment and locally they pass into quartz sands. Skeletal debris from sessile organisms in the reef flat deposits is only abundant landward of the areas in which they live: indicating that current transportation is dominantly landward, with little material being carried from the reef flats into the fore-reef and channel environments. It is often difficult to relate the grain-size measures of the sediments to their environment of deposition. The turbulent conditions in the beach, algal ridge and reef edge environments are reflected in the absence of fine material and the low dispersion high dispersion

values

values

and

of the associated are relatively

Radial zone samples show much variation of the conditions

of deposition.

sediments.

Grass

bed samples

rich in fines trapped

have

by the grasses.

in grain-size measures due to the diversity

The skewness

in mangrove

swamp

sediments

is

positive, indicating normal current deposition. Sheltered fore-reef deposits tend to be rich in fines and to have positive skewness values due to the quiet conditions, whereas fine material is winnowed from windward fore-reef sediments. INTRODUCTION

The Seychelles Archipelago Seychelles Bank-which seldom

rises from a shallow submarine platform--the exceeds 65 m in depth. The archipelago lies

1 Present address: Science Department,

The British Council, 59 New Oxford Street, London

W.C.l, Great Britain. Marine Geol., 7 (1969)

95-127

M. S. LEWIS

96

I

N

0

I

A

0

N

C

E

A

N

/

I

Fig. 1. Map of Mahk, showing the distribution (shaded).

I

of fringing coral reefs and of the “plateaux”

just south of the equator in the western Indian Ocean (Fig.l), and it is largely composed of a late Precambrian granite which is cut by basic dykes (MILLER and MUDIE, 1961; BAKER and MILLER, 1963; MATTHEWS and DAVIES, 1966). The Seychelles Bank is covered by a veneer of skeletal carbonate sediments (LEWIS and TAYLOR, 1966). Fringing coral reefs occur along the coasts of most of the islands of the archipelago.

Those along the east coast of MahC, the largest island in the group, Marine

Geol., 7 (1969) 95-127

SEDIMENTS OF THE CORAL REEFS OF MAHi,

are very extensive larger

bays.

(Fig.]).

97

On the west coast of Mahe, reefs are restricted

Unconsolidated

skeletal

and they occur widely in the fore-reef the deposits

SEYCHELLES

of the Seychelles

sediments environment,

Bank. Locally,

to the

cover the reef flats superficially where they pass laterally

quartz

grains (derived

into

from the Sey-

chelles Granite) are abundant in the sediments. This paper gives the results of two visits to Mahe in 1963 and 1964, which were made

in conjunction

with the International

Indian

Ocean

Expedition.

It

gives the first detailed account of the reef sediments. The nature and the environments of deposition of the reef flat and fore-reef deposits are described and the relationships

between

these environments

and the properties

of the sediments

are

examined. CLIMATE AND HYDROLOGY

The climate

is dominated

by the Southeast

Monsoon,

which blows from late

April until early November, and which produces a heavy southeasterly swell throughout most of the year. The Northwest Monsoon, which occurs from December to March, is a season of alternating squalls and calms. The annual rainfall, most of which falls during the Northwest Monsoon, is about 2.5 m. Mean monthly temperatures vary little, with minima averaging 24.5”C and maxima averaging 28.5 “C. The salinity

of Indian

Ocean surface water around

Mahe ranges from 34.0x,,

to 35.5x,, being highest during the Southeast Monsoon. Very low salinities were, however, recorded near the mouths of streams which flow on to the reef flats. The surface water is alkaline, with pH values ranging from 7.9 to 8.4, while the water temperature varies between 27.O”C and 31 .O”C: the highest temperatures being recorded during the day at the beginning of the Southeast Monsoon and the lowest ones at the onset of the Northwest Monsoon. Tidal

currents

tidal regime

cause continual

being mixed,

semi-diurnal.

the neap-tide range is less than These water and climatic

water

circulation

across

The spring-tide

1 m. conditions

preclude

the reef flats: the

range is about

the formation

2 m and

of carbonate

deposits, such as oolitic sediments, which are characteristic of regions of high water temperature and excess evaporation rates, such as the Bahamas (BLACK, 1933; TLLING, 1954; NEWELL et al., 1959; CLOUD, 1961) and the Persian Gulf (EMERY, of

wholly

1956). The carbonate skeletal

grains in the sediments

around

MahC are, therefore,

origin.

TYPES OF REEF

Along coasts which are exposed to the Southeast Monsoon, the coral reefs are similar to the windward reefs of the Indo-Pacific region (compare EMERY et al., Marine

Geol., 7 (1969) 95-127

* 81

-80

q

q q El

Reef

edge

& algal

Grass

bed

Beach,

deltas,

Plateaux

&

r,dqt

etc

mangraves

Granite

-241

Fig.2. Map of the windward reef flat off Anse aux Pins, with the distribution sediment sample stations.

of zones and

Marine Geol., I (1969) 95-l 27

Marine Geol., 7 (1969) 95-127

100

M. S.

LEWIS

1954). The reef off Anse aux Pins, between Pointe la Rue and Pointe au Sel, may be taken

as a typical

example

(Fig.1,

2). The seaward

margin

is almost

continuous,

being broken only locally by reef passes. On the reef slope buttresses and grooves occur (MUNK and SARGENT, 1954; LEWIS, 1968). In addition, well-defined ecological zones (WELLS, 1954, p.396), which parallel the seaward margin, occur on the reef flats (Fig.2) (LEWIS and TAYLOR, 1966, p.283). Windward

reefs also occur between

Pointe au Sel and Anse Marie Louise and along the east and south shores of the ile aux Cerfs group of islands (Fig.l). In contrast, along coasts sheltered from the Southeast Monsoon, notably between Anse itoile and Cascade (Fig. 1, 9), the seaward margins of the reefs are very irregular (compare GUILCHER et al., 1958, plate XXVI), being cut by numerous channels. Furthermore, the bottom topography of Victoria Harbour is very complex (LEWIS, 1968), with linear reefs and reef knolls, some of which have been drowned due to Late Pleistocene sea level changes. On the sheltered reef flats, the ecological zones are poorly defined and they grade into one another. SEDIMENTARYENVIRONMENTS GINSBURG and LOWENSTAM(1958) showed that biological communities have a profound effect upon sedimentary environments. The zones on the reef flats have been considered, therefore, as separate depositional environments. From the shore to the reef front the zones are (Fig.2,3): (1) beach; (2) rippled sand zone; (3) marine grass bed; (4) radial zone; (5) algal ridge; (6) reef edge; (7) reef slope (or reef front). Even on the windward reef flats some of these zones may be absent. In addition, a number of depositional environments occur which do not belong to the series of zones paralleling the reef margins. These are :(I) “plateaux”; (2) mangrove swamps and former mangrove swamps; (3) deltas; (4) channels and deep depressions in the reef flats; (5) the fore-reef environment. The beach Well developed beaches, 15-25 m wide, occur behind most windward reef flats, but on sheltered coasts they are usually replaced by mangrove swamps or the sites of former mangrove swamps. The beaches are normally gentle, but near the berm slopes as steep as 30” occur. The crest of the berm lies between

2.5 and 3.5 m

Fig.4. Sedimentary environments on the reef flats. A. Coral/algal cobbles and skeletal sands, beach, Pointe au Sel. B. Rippled sands, reworked by worms. St. Joseph’s, AnseauxPins; the shore is towards the left (coin ==30 mm). C. Dense 7’halassiu, marine grass bed, Anse aux Pins reef flat. D. Algal cobbles, radial zone, Anse aux Pins reef flat (test = 90 mm). E. Cobbles, skeletal sands, Sargussum and holothurian in radial zone, Anse aux Pins reef flat. F. Acroporoid coral roofing over surge channel, reef edge, Anse Faure. Note the colony of Tuhipora (centre). G. Mangrove swamp, Anse Boileau. H. Sand mound produced by a crustacean, former mangrove swamp, Montfleuri. Marine Geol., 7 (1969) 95-127

0 i_-.

L-B

5mm

Marine Cd.,

7 (1969) 95-127

M. S. LEWIS

102 above

mean

beachrock granite

low water.

are sometimes

headlands,

Although

storm

beaches

are not found,

large blocks

of

cast upon the beach, as at Anse Cachee (Fig. 1), while near

as at Pointe

au Sel (Fig.l),

coral/coralline

algal cobbles

and

pebbles abound (Fig.4A). Depositional structures in the upper part of the beach are commonly destroyed by crabs which produce numerous burrows and small piles of sand and at the base of the beach the sediments

are continually

reworked

by burrowing bivalves. Sand-spits extend from the beaches of some of the islands in the ile aux Cerfs group. They form where conflicting currents cross the surrounding reef flats (compare GUILCHER et al., 1958, Fig. 16). Radical changes in the morphology of these spits occur in the two monsoons: similar to the seasonal changes described in other regions (UMBGROVE 1947, pp.735-739; FAIRBRIDGE and TEICHERT, 1948, pp.74-75). The profile of some of the more exposed beaches, as at Anse Nord-Est, also changes markedly with the change of season. Rippled sand zone

At the base of most beaches is a narrow zone, 5-10 m wide, in which the sediments are strongly ripple marked. The ripples parallel the beach and are asymmetrical, with their steeper slopes facing the shore. Their wavelengths range from 20 to 60 cm and their amplitudes from 4 to I5 cm. This zone is normally covered by a few centimetres of water, even during low water. Deltas

At the extend up to ln common deposits are while below

mouths of several streams which flow on to the reef flat small deltas 100 m from the shore. They are normally exposed during low tide. with the spits, their direction of growth changes seasonally. The continually reworked by crustaceans, worms and waves (Fig.4B). the surface they are blackened due to the anaerobic and reducing

conditions. Marine grass bed

Seaward of the rippled zone is a wide strip of the reef flat which is colonised by dense growths of marine grasses (Fig.4C); chiefly Thalassia and Cymodocea. On windward reef flats, the zone varies in width from 100 to 300 m, except near headlands, where it is much narrower (Fig.2). Extensive grass beds occur at Anse aux Pins, on the MahC side of ile Sud-Est and ile Anonyme and on the fle aux Cerfs reef flats. Narrow grass beds occur on the reef flats on the west coast of MahC and at Anse Royale. On the sheltered reef flats, the grass beds are patchy and they grade into the adjacent zones. On the windward reef flats, both the seaward and landward margins of the zone are irregular (Fig.2): the pattern of indentations on the seaward edge being controlled by the ridges in the radial zone. Both margins are between 15 and 60 cm Marine Geol.. 7 (1969) 95-127

SEDIMENTS OF THECORALREEFSOF MAH&SEYCHELLES above the adjacent

zones and on the landward

from wave-erosion

of the sediments

(MOULINIER

water,

beneath

103

side a pronounced the matted

overhang

and PICARD, 1952, p.185). Much of the zone is exposed

especially at spring tides. The function of the grasses in stabilising

results

surface of the grass bed

the sedimentary

during

environment

low has

MOULINIER and PICARD,1952, pp.l70-171; GINSBURGand LOWENSTAM, 1958, p.312; DAETWYLER and KIDWELL, 1959, p.13; GUILCHER, 1965, p.27; SWINCHATT,1965, p.79). Skeletal material is bound by the plants and in certain localities, as at Anse Souillac, sand-grade sedibeen discussed

by a number

of workers (for example,

ments on the reef flat are virtually confined to the grass zone. At Anse aux Pins grass plants were binding ripples on the landward edge of the zone. As noted by SWINCHATT (1965, p.80), the grass bed offers protection to an indigenous fauna of bivalves, gastropods, worms, crustaceans and holothurians. The role which these animals play in reworking the deposits is difficult to assess, but it is evident from the abundant mounds and burrows in the grass beds that the sediments are being continually reworked. Furthermore, skeletal debris from these organisms forms part of the grass bed deposits, seaward of the zone.

although

most of the skeletal

content

originated

Radial zone

This zone occurs only on windward reef flats, as at Anse aux Pins (Fig.2, 3) and to the east of ile aux Cerfs. Within the zone are numerous ridges, 2-3 m high, which run perpendicularly to the reef edge, except near reef passes. The ridges widen seawards, eventually merging into the algal ridge. They are separated from each other by sand-filled troughs, l-2 m deep, which widen towards the shore until the ridges disappear to leave a relatively deep, sand-filled depression, 1-2 m deep at low tide, immediately seaward of the grass beds. The reef flat rises gently seaward from this depression and the larger ridges are partially exposed during low water springs. The zone is analogous to the “trickle” or “radial” zone on reef flats in the Great Barrier Reef complex (FAIRBRIDGE,1950, p.338). The ridges are composed of algal cobbles (Fig.4D), which may be more or less cemented by coralline algae. Locally, the ridges are composed of cavernous reef platform, and this becomes more important towards the algal ridge. Both

GARDINER(1906, p.456) and FOSLIE(1907, p. 180) stated that coralline algae were absent from the reefs of MahC, yet they abound in the radial zone and elsewhere. Locally, the ridges and troughs are replaced by large colonies of Heliopora. These are particularly extensive at Anse Royale, where they form “micro-atolls” similar to those of Bikini Atoll (LADD et al., 1950, p.415). Large areas of the more elevated parts of the radial zone are covered by dense growths of brown with the grasses of the previous zone, algae, chiefly Sargassum. In common these plants protect sand-grade sediments from being winnowed by currents. Both the green alga Halimeda and the articulated coralline alga Amphiroa Marine

Geol., 7 ( 1969) 95-l 27

104 are abundant:

M. S. the former is most common

LEWIS

near the grass beds, where the sediment

surface may be littered with its leaves, while the latter is chiefly found on the ridges, particularly near the algal ridge. Although lithophylloid algae are the main reef building organisms in the radial zone, corals are common. Small colonies occur on the ridges and in the deeper areas large branching forms of Actqmtz and Poritrs are common. In the troughs, the sands are usually rippled: the ripples being asymmetrical with their steeper slopes facing the shore. Their wavelengths are about 15 cm and they have amplitudes of around 5 cm. Sands are also found in hollows on the ridges. Even in the troughs, these deposits form only a veneer and the underlying reef platform may be exposed as a smooth surface in which “fossil” corals have been truncated by CIosion. The sands are associated with algal cobbles (Fig.4E) and with coarse debris from acroporoid corals. In general, the amount of coarse debris increases towards the reef edge and on some of the west coast reef flats this material forms the bulk of the unconsolidated sediment between the reef edge and the grass bed. The sands are subject to continual reworking. Burrowing gastropods and holothurians are very common (Fig.4E) and small mounds and burrows abound. On the sheltered reef flats, the radial zone is represented by large, irregular sandy areas and patches of slightly elevated reef platform. The former predominate near the grass beds and the latter are most important near the reef edge. The elevated platform consists of dead, cavernous coral encrusted by coralline algae and covered by Sargassutn. Live coral is restricted to the margins of these patches and consists chiefly of massive forms. Locally, colonies of massive Porites, l-2 m high, occur. The sands generally form a superficial cover upon an eroded platform, and their surface may be bound by filamentous algae. Halitnrda may be so abundant that the surface of the sediments is covered with debris from this alga. The algal ridge This feature forms the highest part of the reef flat, being partially

exposed at

low water. It is formed by papillate, nodular and encrusting lithophylloid algae and seldom exceeds 30 m in width. This zone grades into the radial zone and reef edge environments. It only occurs on windward reefs, such as Anse aux Pins, and even on these it may be absent. Few of the spectacular features of the algal ridges of Pacific reefs are seen around Maht and in many respects the ridge at Anse aux Pins is similar to the inner part of the alga1 ridge on the windward reefs of Bikini Atoll (JOHNSON, 1961, p.25). At Anse Faure (Fig.2) surge channels which cut the reef edge become roofed over by coralline algae as they pass into the alga1 ridge: only to reappear on the landward side of the ridge at the head of troughs in the radial zone. This feature has been termed “pillar and room structure” (WELLS, 1957, p.615). In addition, dead coral colonies, encrusted by coralline algae, which are of the same Marine Geol., 7

(1969) 95-127

105

SEDIMENTSOF THE CORAL REEFSOF MAHB,SEYCHELLES forms as those now living a few metres seaward this zone very irregular. Large blocks of reef limestone

of the ridge, make the surface of

are occasionally

found

resting

on the algal

ridge. They are composed of massive and branching corals and skeletal sands which are bound and encrusted by coralline algae. These blocks are themselves being bound

into the algal ridge by coralline

and algal cobbles are locally abundant the reef platform by coralline algae.

algae. Debris from branching

and this material

is also rapidly

corals

bound

into

Over much of the algal ridge, dense growths of brown algae obscure the substratum and protect any sand, which is restricted to crevices. Elsewhere, small corals and alcyonarians, of the same forms as those in the adjcent reef edge environment, are common. Anzphiroa occurs widely, but Halinwda is virtually absent. The red, encrusting Foraminiferum, Homotrema ruhrum, is abundant and other common animals include gastropods, small echinoids, crustaceans and holothurians, although none is very important. On the sheltered reef flats, the algal ridge is represented by a luxuriant growth of coralline algae, especially on those reefs which receive some southeasterly swell. There is no development, however, of a topographical feature. The rwf edge This environment has been described as one in which detritus is almost absent and one in which “... algal encrustation is a dominant feature and has resulted in the formation of a strong, wave-resistant front of the reef”. (MAXWELL et al., 1961, p.219). The reef edge is best developed on windward reefs, as at Anse Faure, where it has a hummocky surface. The reef platform is composed of dead corals which are encrusted by algae (compare CROSSLAND, 1902, p.497), which produce a multitude of overhanging surfaces and a brittle framework. Surge channels, which are probably connected to the grooves on the reef front (MUNK and SARGENT, 1954) cut the reef edge. Near the algal ridge these are often roofed over by calcareous

algae

or branching

corals

(Fig.4F),

but they widen

seaward

until

the

growing edge of the reef is limited to irregular patches of coral. In many places, small branching forms of Acropora, Pocillopora and Stylophora abound, while along the margins of surge channels, and in other protected places, Tubipora musica (Fig.4F) is abundant. Heliopora is also found growing laterally into the channels or forming micro-atolls. The higher parts of the reef edge are exposed during low water springs. Elsewhere. such as Pointe au Sel, corals are less common, Tubipora is absent and surge channels do not occur. Nevertheless, the seaward margin is very irregular and lined by colonies of Millepora. The irregularities are the surface expression of spurs and grooves on the reef slope (LEWIS, 1968). Amphiroa abounds in hollows on the reef edge and both bryozoans and Howtotretna rubrum are common beneath overhanging surfaces. Much of the reef Marine Geol., 7 (1969) 95-127

106

M. S. LEWIS

edge is covered

by alcyonarians.

The mobile

fauna

includes

Foraminifera,

mol-

lusts, echinoids, holothurians and crustaceans; all of which are common but rarely abundant. Brown algae are virtually absent from the reef edge, except locally, where they may be so prolific as to obscure the substratum and restrict active coral growth to the reef front. There are far fewer corals in the reef edge environment reefs than are found on the windward Millepora

of the sheltered

reef edges. Tubiporu has not been found and

is rare.

The reqfslope (or recffiont) Coral growth is extremely active on most reef slopes, especially on the upper parts close to low tide level, where branching forms, dominated by Acropora, are prolific. Massive corals, however, are also common on the fronts of the sheltered reefs. The corals grow upon a cavernous framework of dead coral branches which are encrusted by coralline algae. Alcyonarian corals are also abundant on this part of the reef slope. Below - 10 m, corals become fewer. In parts of Victoria Harbour the reefs support little active coral growth and they are composed ofdead branching corals which have been encrusted by coralline algae. The lower part of the reef front is littered with debris from branching corals. This material is rapidly bound into the reef framework by the calcareous algae. The ,fore-reef environment Seaward of the windward reefs the bottom topography is fairly uniform with few undulations. The reef slope has a concave profile and it descends to about -27 m (Fig.3). The sea floor in Victoria Harbour and Cerf Channel, however, is extremely uneven (LEWIS, 1968), with prominent reef knolls, linear reefs which are elongated parallel to the reef edges off Mahe, drowned linear reefs which form bare rock platforms at about - 10 m, and deep troughs which separate the different linear features. This topography results in a complex depositional environment leading to rapid lateral changes from skeletal to terrigenous sediments, Channels and deep depressions

in the reqfflats

The channels which dissect the sheltered reef flats and the enclosed depressions in the reef flats (LEWIS, 1968) provide abnormally quiet conditions of deposition. The margins of the channels and depressions are lined by large colonies of massive corals which grow slightly above the general level of the adjacent reef flat (compare GUILCHER et al., 1958, Fig.28) and which act as a barrier to the migration of sands from the reef flats into the channels. Seawards, the channels may open out into a complex of reef knolls which are composed of coralline algae and branching and massive corals: well seen at Cascade (Fig.9). In contrast, the passes of the windward reefs have turbulent water conditions and they are lined by both massive and branching corals. At both Anse aux Pins Marine Geol..7 (1969) 95-127

SEDIMENTSOF THE CORAL REEFSOF MAHE, SEYCHELLES and

Anse

margins

Royale,

alcyonarians

and

bryozoans

abound

along

the

of the reef passes.

Mangrove

swamps and,fbrmer

Extensive Glaud,

Millepora,

107

mangrove

mangrove swamps

swamps

occur on the west coast,

Anse Boileau and Baie Police (Fig.])

common Mangroves

between

and narrow

particularly

Anse Etoile and Anse la Rue (Fig.9) on the sheltered

also line some of the streams

at Port

fringes of mangroves

which cross the “plateaux”

are

reef flats. (Fig.2, 3).

Early accounts suggest that mangrove swamps were formerly much more widespread (FAUVEL, 1909, pp.1355139). The mangroves form very dense thickets (Fig.4G) which not only trap sediment (NEWELL et al., 1959) but which also produce such acid conditions that carbonate grains are dissolved: so much so that the columella of many of the Cerithium shells which cover the surface of the deposits may be exposed, even in live specimens (compare REVELLEand FAIRBRIDGE, 1957, p.28 I). The large deltaic spreads of terrigenous sediments found opposite stream mouths between Anse ctoile and Cascade are believed to have been formerly mangrove swamps. Beneath a surface oxidized layer, the deposits are black due to reducing conditions and the high humus content. The deposits become paler seaward as the carbonate content increases. In places, the surface of these deltaic spreads is littered with the remains of Cerithium, while the sediments are intensively reworked by burrowing bivalves and crustaceans. some of which produce large mounds of sand (Fig.4H). Thr “plateaux” These form the only extensive areas of low ground on Mahe. They are about 2 m above mean low water (Fig.3): slightly below the berms of the beaches. They are best developed behind the larger bays, as at Anse Royale and Anse aux Pins (Fig.2), although they occur all round Maht (Fig. I). The water table, which fluctuates slightly with the tide and after periods of heavy rainfall, lies 2-3 m below the “plateau” surface. The “plateaux” consist largely of skeletal sands, which may be slightly indurated near the water table. Small alluvial fans of terrigenous gravels occur on the landward side of most “plateaux” (BAKER, 1963, p.23). DESCRIPTlONOF THE SEDIMENTS Unconsolidated elastic, skeletal, carbonate sediments occur in all the reef environments around Maht: forming a veneer on the reef flats (Fig.3), but attaining unknown thicknesses in the fore-reef and channel environments. Locally, they pass into terrigenous deposits.

Marine

Geol., 7 (1969) 95-127

LINIDEhTIFIED GRAINS FINES. MISCELLANEOUS FRAGMENTS

C’;IQACOI! PFVAINt

GENTHONIC ‘i

FORAMINIFFRAL

‘,i’l

CRUSTACEAN AND

ECHINOID

FRAGMENTS

MOLLUSCAN

QEMAINS

OF

CHIEFLY

ALCYON

EIZI HALIMEDA DEBRIS

m FRAGMENT; P’EF CORALS

OF

@UlLDlNG

OYGAhISMS

AND ALGAE

c7

0

50

100

TERRIGENOJS GRAINS MAINLY OUARTL

METRES

Fig.5. Composition of reef flat sediments from Anse aux Pins in relation to the environment of deposition. Note, especially, the occurrence of Halimedu, Amphiroa and quartz grains.

Marine Geol., 7 (1969) 95-127

SEDIMENTS

OF THE CORAL

Sedimenfs

of the windward reef fiats

Composition.

microscope and coralline

REEFS OF MAH&

SEYCHELLES

IQ9

The composition of the sediments was determined examination. The remains of reef-building organisms, algae,

form between

45 and 85% of the deposits

by binocular chiefly corals (Fig.5).

from Halimeda, Amphiroa and alcyonarians occurs in most samples, varies considerably in abundance. For instance, Hulimedu fragments

Debris

although it form over

30 2; of some sediments (sample M.194, FigS), but they are virtually absent from other samples. In general, only sediments around and landward of prolific growths of Halimedu are rich in the remains of this alga. Detritus from Amphiroa is most abundant which it may amount to 10% of the sediment

in sediments on the algal ridge, in (sample M.186, Fig.5). Grains of

Tuhipora are never abundant in the sediments, but they are distinctive when they occur. The sediments from some traverses contain almost none of these grains (for instance, samples M.187-M.24, Fig.S), while in those areas in which Tubipora is common in the deposits, its abundance decreases shorewards. Alcyonarian spicules are common in many reef flat samples, particularly near and on the reef edge: the source area for most of the spicules (Fig.5). Of other sessile organisms only the remains of bryozoans and serpulids are locally important in the sediments. Molluscan debris is ubiquitous in all reef flat sediments, although samples from the grass beds are slightly richer in this material than those from elsewhere. Even in this zone, molluscan remains do not amount to more than 10% of any sample (Fig.5). Echinoid and crustacean fragments also occur widely in the sediments: only rarely forming a major constituent as, for instance, in sample M.187

(Fig.5). Tests of benthonic Foraminifera are the only other major skeletal component. amounting to about 5% of some samples. The skeletal content of most samples is very fresh, especially in the radial zone, algal ridge and reef edge environments. In the grass bed deposits, however, many of the larger grains appear to be “corroded” (sample M.236, Fig.6D), with only Hulimeda fragments, which are very local in origin, being fresh. In the beach sediments, many of the skeletal grains are pale orange rather than white. In most reef flat environments the skeletal grains are angular: their shape partly resulting from the cell-structure of the organisms (Fig.6). Fragments of a number of organisms, such as alcyonarians (Fig.6B, C), Amphiroa (Fig.6C), Halimedu (Fig.6D) and Foraminifera (Fig.6C), may retain their external shape and ornament. ln beach deposits, however, grains in all size-fractions may be very well rounded and highly polished and they may attain a high sphericity (sample M.165, Fig.7). Quartz grains are only abundant in the deposits of the beach, delta and rippled sand environments, being most important in the sediments at the foot of the beach (Fig.5). Grain-,yize characteristics.

Despite the variety of skeletal constituents, Marine

many of the

Geol., 7 (1969) 95-127

111

SEDIMENTS OF THE CORAL REEFS OF MAHk, SEYCHELLES

samples

display

unimodal

grain-size

frequency

curves (plotted

at 0.5 0 intervals,

Fig.8). Samples from the foot of the beach, the rippled sand zone and the radial zone commonly display bimodal and even polymodal curves, while polymodal curves also occur in some deposits near the margins of the grass beds. In general, are absent

significant

amounts

of material

from reef edge and beach sediments

finer than (Fig.8).

+ 3.0 0 (0.125 mm)

The highest

proportion

of fine material is found in some grass bed sediments. Radial zone deposits contain variable amounts of fine material, while both grass bed and radial zone sediments vary considerably in the amount of coarse debris which they contain (Fig.8). The mean size (INMAN, 1952) of all reef flat samples from Anse aux Pins falls within the sand grades (WENTWORTH, 1922),ranging from -0.6 0 (1.510 mm) to + 2.4 0 (0.189 mm). The beach deposits are coarsest at the foot of the beach (Fig.8), while the rippled sands exhibit considerable variation in mean size values. Mean size values for grass bed sediments, however, vary little. The greatest variation in values occurs in radial zone sediments and the least variation is found in reef edge and algal ridge sediments. The smallest range in dispersion values (INMAN, 1952) occurs in reef edge and algal ridge sediments, with values falling between 0.2 and I. 1. Very low values are also found for samples from the top of the beach Fig.8). Dispersion values for grass bed sediments vary between

(see histograms: 0.5 and 2.2, but

they tend to be nearer the higher figure. Radial zone deposits exhibit almost as great a range as the grass bed samples. The skewness of the grain-size frequency curves of radial zone sediments varies from being positive to being negative. Most beach and rippled zone samples have negatively skewed curves, but those from the top of the beach have positively skewed curves.

Grass bed deposits

normally

display

negatively

skewed frequency

curves.

S&men

ts _fiom the sheltered reef flats

Composition. In common with the windward reef flat sediments, these deposits are largely composed of coral and coralline algal grains and they contain little or no terrigenous material (samples M.99, M.100, M.134-M.136, Fig.9, 10). In areas where

Halirneda

is prolific,

as at Anse fitoile,

its remains

are abundant

in the

deposits (samples M.134 and M.135, Fig. IO). Molluscan debris may also be abundant, especially in the sediments near the shore. In other respects, the skeletal

Fig.6. Reef flat sediments. A, B, C. Sample M.215, radial zone, Anse aux Pins. D. Sample M. 236, grass bed, Anse auxPins. E. Sample M.99, grass bed, Ansela Rue(sheltered reef). (COY= coral; Hal = Halimeda; Ale = alcyonarian spicule; Ech = echinoid spine; Crust = crustacean grain; For c foraminiferal test (Cakarina); A/g = algal grain.) A. -0.75 0 (1.680 mm) to 0 0 (1.000 mm). B. 0 B to $0.75 0 (0.595 mm). C. $0.75 0 to -1 1.50 0 (0.353 mm). D. -0.75 0 to 0 0. E. --1.50 0 (2.820 mm) to -0.75 8. Marine

Geol., 7 (1969) 95-127

0

1I

I

1

5mm

Marine

Ged.,

7 (1969) 95-127

113

SEDIMENTS OF THE CORAL REEFS OF MAW& SEYCHELLES

content

is similar

to that in the windward

deposits

grade

addition,

the skeletal grains become pitted on their surfaces (Fig.6E)

into

the black

of the grains becomes

sediments

reef flat sediments. of the former

Near the shore the

mangrove

swamps.

In

and the shape

very irregular.

Grain-size characteristics. Sediments

on the sheltered reef flats are richer in material

reef flat deposits. This fine material occurs in all environments (compare sample M.99 (grass bed) and sample M. 136 (reef edge), Fig.10). Mean size values fall in the sand-grades (Fig.10) and dispersion values are normally greater than 1.0: rather higher than those for many finer than

samples skewed

+ 3.0 LJ(0.125 mm) than are most windward

from

the windward

grain-size

frequency

reef flats. The deposits

usually

exhibit

negatively

curves.

Sediments qf’ the ,former mangrove swamps Composition. Molluscan remains, chiefly Cerithium shells, and quartz

grains

are

the chief constituents (samples M.98 and M. 133, Fig.9, 10). Between 20 and 90 % of the sediments may be composed of quartz: the amount declining seawards. In the coarser grades the grains are unworn (Fig. 1 lA), but they are more rounded in the finer grades (Fig.1 1B). Grain-size characteristics. The grain-size frequency curves may be either unimodal or bimodal (samples M.98 and M. 133, Fig. 10). In addition, there is a large variation in the amount of material finer than + 3.0 0 (0.125 mm) present, with fines forming

almost

20 % of some samples (M.98). Mean size values also vary greatly. They may be as high as - 1.56 0 (2.950 mm), although some values fall in the coarse- and medium-sand grades. Despite the high mean size values, the sediments

contain few grains larger than .- 3.0 o (8.000 mm). Dispersion values are moderate and range from I. 17 to 1.62. The grain-size curves are always positively skewed, with values ranging

from + 0.03 to + 0.43.

Sediments in the channels and deep depressions It is difficult to define precise boundaries environments. Cascade

This difficulty

is best illustrated

between

the channel

by considering

and fore-reef

samples

from the

area (Fig.9).

Composition. Sample M.lO1 (Fig.10) is a typical channel sediment. It is composed largely of unworn fragments and whole valves and tests of delicate, thin-shelled

Fig.7. Sample M.165, beach, Petite Police. Many of the grains are polished and well rounded. The proportion of the sediment composed of terrigenous grains decreases with decreasing grain-size. (Qz = quartz; Hal = Hulimedu; A/c = alcyonarian spicule.) A. -1.50 o (2.820 mm) to -0.75 0 (1.680 mm). B. -0.7s o to 0 0 (1.000 mm). C. 0 0 to +0.75 0 (0.595 mm). Marine

Geol., 7 (1969) 95-127

c

4

c” Ic

M3

0 +3

80 Wt %

‘Cm

..

Ml74 50 wt .% .......

3

0 *3

M 236

-3

0 *3

.y-.

Ml75

0 *3

M4

M235

...’

-3

-3

;;,

0 *3

..” “”

3 +3

Ml76

?

M234

-2

O +3

0 *3

Ml77

M233

-~

_ _“.

-3

-3

O ;3

-3

3 +3

Ml78

M232A

-3

-3

0 *3

Ml79

‘-1 ‘01

-3

0 *3

Ml80

-3

O *3

Ml82 EGGa=

0 *3-:5

Ml81

-3

-3

=

=

=

O *3

Ml83

O *3

Ml84 = =

-3

Zone

80

Ml85

O *3

Grass

Rippled

m

Beach

and

~7z-J

ENVIRONMENTS

-3

Ml86

O *3

Sed

Ezz

%

I,@)

Radial knainiy

F

Zone sands)

0

a Radial (mainly

Zone rldges)

DEPOSITION

zz Reef Algal

Edge and Rtdge

ihrt

%

SEDIMENTS

OF THE CORAL

115

REEFS OF MAHi‘, SEYCHELLES

I NDIAN

OCEAN

.

*Ml51

CTORIi *Ml53 lM15L ‘Ml52

t

. >u-_

l

.

GaLE

SCCHE

H A R B o U R VOYENNE

LONGUE Mont

.

dE GRAND ROCHER

. .

ERF . CHANNEL

D

.

f Kdometres

Fig.9. Map of the sheltered reefs between Anse Nord-Est and Pointe la Rue, with the distribution of sample stations. (Those numbered are discussed in this paper.)

bent honic animals

and it contains

only small amounts

of debris from reef-building

organisms. Its composition is in marked contrast to that of the adjacent samples M.lOO and M.102: the former being from the reef flat at the margin of the channel and the latter from an active reef knoll. The contrast between the composition of sample M.lO1 and that of sample M.104 is equally striking (Fig.10). The latter is typical of fore-reef sediments, with an abundance of branching coral fragments

Fig.% Histograms showing the grain-size frequency distribution curves of windward reef flat samples from Anse aux Pins, in relation to the environment of deposition. (E y cobbles excluded; C = cobbles included.) Marine

Geol., 7 (1969) 95-127

116

M. S. LEWIS

M

98

M

99

M

100

M

101

M

102

M

103

M

104

Pil 133

Fig.10. Composition in relation to grain-size and histograms of samples from the sheltered reefs. M.98-M.104: reef flat and channel complex off Cascade; M.133-M.137: traverse from former mangrove swamp to reef slope off Anse etoile; M.151-M.154: Victoria Harbour between Anse l?toile and he Ste. Anne.

Marine

Gtd.,

7 (1969) 95-127

SEDIMENTS

OF THE CORAL

(Fig. 11C), bryozoan

OF MAH&

SEYCHELLES

117

grains (Fig. 1 ID) and, in the finer grades, alcyonarian

Grain-size characteristics. finer than

REEFS

The channel

+ 3.0 u (0.125 mm) (sample

fall in the medium-silt

sediments

grade. Their grain-size

those of any other sediments

associated

are composed

M.lO1, Fig.lO),

spicules.

largely of material

and their mean size values

characteristics

are, therefore,

unlike

with the reefs off MahC.

Sediments in the fore-reef environment This environment is the least easily observed and, on present information, it is impossible to determine the distribution of the different types of deposit found. The sediments are very variable, especially in Victoria Harbour where the bottom

topography

is so uneven.

Composition. The fore-reef sediments off the Anse aux Pins windward reef (Fig.%) vary from skeletal sands to calcareous quartz sands (Fig. 12). Unworn fragments of branching corals and of coralline algae usually predominate, while sessile organisms are chiefly represented by alcyonarian Only occasional fragments of Halimeda

spicules (Fig.13B) and bryozoan and Amphivoa occur.

grains.

Molluscan remains are more abundant than in the adjacent reef flat sediments. They are concentrated in the coarser grades (Fig.12) and are chiefly of thick-shelled forms (Fig. 13A, C and Fig. 14B, C), although small gastropods occur in the finer grades. Many of these remains are unworn. Crustacean and echinoid fragments are common but not abundant: the former occurring chiefly in the coarser grades and the latter as spines in the finer grades. In contrast to the reef flat sediments, the fore-reef deposits of the windward reefs are rich in the tests of benthonic Foraminifera, particularly in the mediumand fine-sand grades (Fig.12). Genera include Amphistegina (Fig.l4A), Heterostegina (Fig. 14B) and Marginopora (Fig. 13C). The tests are often unworn (Fig. 13D and Fig.l4A, B), although chipped specimens are common (Fig.14C). Other skeletal debris is seldom abundant (Fig.12). In a number of samples, severely abraded grains are abundant (Fig.l3A, B), while in others there is a mixture of fresh and abraded

material.

The fore-reef sediments in Victoria Harbour are similar to those seaward of the Anse aux Pins reef, although debris from branching corals often exceeds that from coralline algae. This branching coral debris is typically unworn and it decreases in importance with increasing distance from the reef edges off Maht (samples M. 137, M. 151-M. 154, Fig.9, 10). Molluscan remains are most abundant in sediments some distance from the MahC reef edges, They consist mainly of bivalves in the coarser grades and of small gastropods in the finer grain-sizes. Tests of benthonic Foraminifera also increase in abundance in deposits away from these reef edges (sample M. 154, Fig. 10). The sediments vary laterally from pure skeletal sands (sample M. 152, Fig. IO) Marine Geol., 7 (1969) 95-127

M. S.

118 to calcareous ward

quartz

fore-reef

worn (Fig.l4A,

sands (sample

environments.

M.241, Fig.12) in both the shelteredand

Commonly,

the quartz

grains

B), but in some cases, they are rounded

are only

LEWIS

windslightly

and some may display

polished surfaces which are reminiscent of beach sands. In some samples, the larger quartz grains are partially encrusted by coralline algae and in these samples the skeletal the quartz

grains

have been severely

is normally

abraded.

In Victoria

Harbour,

however,

unworn.

Grain-size characteristics. Most samples from the fore-reef environment off Anse aux Pins and to the southeast of the Ile aux Cerfs reef complex have unimodal grain-size frequency curves (Fig.l2), although several contain a mixed population of skeletal and terrigenous grains. In sample M.82, which has a bimodal curve, the weak coarse mode is produced by the quartz content (Fig. 12). ln contrast to the windward fore-reef sediments, most samples from Victoria Harbour display bimodal or polymodal distribution curves (samples M. 137, M.151 and M.154, Fig.

10). In most sediments in the Anse aux Pins area there is little material finer than + 3.0 B (0.125 mm), but occasionally fines may amount to 70% of the sediment. Samples rich in fines are similar to some of the finer sediments on the Seychelles Bank (LEWIS and TAYLOR, 1966) and they represent a transition from the fore-reef to the bank sediments. The amount of coarse debris in the windward fore-reef sediments is also very variable. One sample (M.83, Fig.9), collected to the east of Ile Sud-Est, is almost wholly composed of coral/algal cobbles and pebbles. Although the deposits in Victoria Harbour are mainly sands, they contain either moderate amounts of coarse debris, or large amounts of fine material, or both. Mean size values of samples from the Anse aux Pins area range from + 3. I7 n (0.112 mm) to -0.07 B (1.050 mm): that is, the deposits range from very-finesands to very-coarse-sands, although, as mentioned above, spreads of cobbles also occur. In the sheltered fore-reef environment, mean size values also fall within the sand grades, ranging from - 0.13 0 (I .090 mm) to + 2.64 o (0.16 1 mm). Dispersion values for samples from the windward fore-reef environment fall between 0.39 and 1.55, but in the sediments of Victoria Harbour higher values are common, ranging from 1.22 to 2.11. In all samples from the Anse aux Pins area, except M.79, skewness values are negative, ranging from -0.03 to -0.62. In sheltered fore-reef sediments, the skewness of the frequency curve may be

Fig.11. Samples from the Cascade area. A, B. Sample M.98, former C, D. Sample M.104, reef knoll/channel complex (fore-reef). (Qz ~ quartz; Crust = crustacean; Lam = bivalve: C’W -= coral; A/g -7 coralline alga; spicule; Bvy = bryozoan.) A. -1.50 0 (2.820 mm) to -0.75 M (1.680 mm). (1.000 mm). C. -1.50 o to -0.75 0. D. --0.75 0 to 0 n.

mangrove swamp. Gasr = gastropod; ,4/c : alcyonarian B. --0.75 c1to 0 II

Marine Geol., 7 (1969)95-l 27

Marine Grol., 7 (1969) 95-127

120

M. S. LEWIS

M

samples

Fig.12. from

240

M

241

M

242

M

M

243

Composition in relation to grain-size seaward of the Anse aux Pins reef.

and

a44

M

histograms

245

of windward

fore-reef

Fig.13. Windward fore-reef samples from Anse aux Pins. A, B. Sample M.80, with many abraded grains. C, D. Sample M.81. (Qz -2 quartz; Lam = bivalve; Car = coral; A/c ~~ alcyonarian spicule; Gust = gastropod; For -= Foraminiferum.) A. -1.50 o (2.820 mm) to 0.75 0 (1.680 mm). B. 0 I (1.000 mm) to -1-0.75 B (0.595 mm). C. - I.50 o to -0.75 U. D. 0 M to ! 0.75 n. Marine

Geol., 7 (1969) 955127

0 L-L_-.

I

5mm

Marine

Geol., 7 (1969) 95-127

Marine

Geol., 7 (1969) 95-127

SEDIMENTSOF THE positive

CORAL

or negative,

fall between

-0.33

REEFS OF MAHk,

although

SEYCHELLES

123

most samples have positively

skewed curves. Values

and $0.56.

DISCUSSION

It has been suggested that most of the skeletal detritus with coral

reefs originates

the reef framework

from

and associated

the mechanical organisms

and

in sediments

biological

associated

destruction

of

(WELLS, 1957, p.609) and that it is

derived largely from the growing edge of the reef (MACNEIL, 1954, p.392). This material is subsequently transported by tidal currents through the different reef zones and environments. Not only may the skeletal content of the sediments have been transported through several environments, but there are a number of difficulties in relating the physical properties of carbonate sediments to the conditions of deposition (HAM and PR.4Y, 1962, pp.7-IO). These difficulties result from such factors as the death of organisms in situ; biological breakdown of grains: composite grains composed of coral and coralline algal remains; variation in the shape of skeletal debris; and differences in porosity and mineralogy (aragonite, high-magnesium calcite, low-magnesium calcite) between grains. The composition of the reef flat sediments provides strong evidence that the dominant direction of current transportation is towards the shore, with little movement of material parallel to the shore. For instance, fragments of H~llinzerlir are never abundant seaward of the areas in which this plant grows and debris from Atnphirou is most abundant in sediments on those parts of the reef edge and algal ridge where it lives (Fig.5). In addition, there is a virtual absence ofterrigenous material in sediments a few metres seaward of the foot of the beaches (compare MOBERLY et al., 1965). In certain reef flat environments, the composition of the sediments reflects special conditions in molluscan

of deposition.

remains

from the indigenous

than

For example,

other sediments:

population.

grass bed deposits tend to be richer these remains

This is especially

where the grass bed grades into the former

mangrove

supports a prolific bivalve and gastropod fauna. Although it is difficult to relate the grain-size

being derived

so on the sheltered swamp environment

measures

largely

reef flats, which

of reef flat sediments

to specific environments of deposition, these measures are invaluable guides to the conditions of deposition. For instance, the grain-size frequency curves of samples from the foot of the beach, the rippled sand zone and the landward margin of the

Fig.14. Sample M.82, windward fore-reef off Anse aux Pins. The coarser grades are largely composed of terrigenous grains, but skeletal content increases with decreasing grain-size. L~/?I = bivalve; Car : coral; Gust -: gastropod.) A. (Qz = quartz; For ~~ Foraminiferum; ~-1.50 w (2.820 mm) to -0.75 o (1.680 mm). B. -0.75 o to 0 0 (1.000 mm). C. 0 o to + 0.75 o (0.595 mm). Marine

Ceof., 7 (1969) 95-127

124

M. S. LEWIS

grass bed indicate sources (compare

that sedimentary samples

particles

in these zones originate

from two

M. 188-M. 190, Fig.8).

The grain-size frequency curves of grass bed samples reflect the trapping of fine material by the plants and the supply of coarse material by the indigenous molluscan population, in that they are normally unimodal with a wide spread (Fig.8). Strongly unimodal, bimodal and polymodal curves in samples from the seaward

and landward

margins

of the beds indicate

that these areas have only

recently been colonized by the grasses. Bimodal and polymodal curves in samples from the radial zone may be caused in part by the abundance of skeletal debris from indigenous organisms, such as Hulimeda and in part by coral/algal pebbles and cobbles (samples M.227 and M.228, Fig.8). Equally typical of radial zone sediments are strongly unimodal curves. Very variable conditions of deposition are, therefore, reflected by variation in the nature of the distribution curves of different samples. In the sandy areas, the sediments are subject to continual current action, while elsewhere they are protected by the ridges and by elevated patches of reef platform and the growths of brown algae. The diversity in conditions of deposition in the radial zone leads to the wide range in mean size values, while the restricted range in these values for reef edge and algal ridge samples may be attributed to the uniformly turbulent conditions of deposition. The fairly limited range in mean size values for most grass bed sediments reflects the unique conditions which exist in this zone. The very low dispersion values of reef edge and algal ridge sediments are also produced by the turbulent conditions, but otherwise there is little correlation between dispersion values and the environment of deposition, although the range in values for radial zone sediments may again be taken to reflect the variable conditions of the zone. The rather higher dispersion values of samples from all parts of the sheltered reef flats results from the larger amount of fine material in the deposits: also a reflection of the quiet conditions of sedimentation. Under normal conditions of current transportation and deposition, sediments

have a “tail of fines”

and their grain-size

frequency

curves are positively

skewed. This is well seen in the former mangrove swamp deposits. If this “tail” be truncated by “currents of removal”, the curve would be negatively skewed. It seems, therefore, that the variation from positive to negative skewness of curves for radial zone samples may be attributed to diversity in the conditions. The curves of most grass bed samples are negatively skewed because they have a “tail of coarse material” which consists of the remains of indigenous molluscs. The winnowing of fine material from the foot of the beach and the rippled sands also leads to the grain-size curves of sediments in these environments being skewed negatively. Both the grain-size measures and the composition of the sediments in the channels confirm the very quiet conditions. Even in the channel complex off Marine

Geol., 7 (1969) 95-127

SEDIMENTS

Cascade,

OF THE CORAL

skewed

higher dispersion Harbour

OF MAHk,

SEYCHELLES

coarse skeletal debris is not transported

reef knolls. In the windward negatively

REEFS

fore-reef

frequency values

in the freshness

for any great distance the low dispersion

reflect the continual

and positively

and Cerf Channel

also reflected

environment,

curves

125

skewed curves

water

of both the skeletal

values

turbulence.

of samples

result from the much quieter

from the

from Victoria

conditions,

and terrigenous

and The

which are grains

in the

sediments. The difference between conditions in the windward fore-reef environment and those in the sheltered fore-reef environment is also seen in the tendency of sediments from the latter to have polymodal distribution curves. Furthermore, the severely abraded skeletal grains found off the Anse aux Pins reef do not occur in the sheltered fore-reef sediments. In conclusion, it has been the purpose of this paper, not only to give the first detailed account of the sediments associated with the reefs of MahC, but also to emphasize the problems of correlating the properties of carbonate sediments with their conditions

of deposition.

ACKNOWLEDGEMENTS

During the preparation of this paper, I learnt with great sadness of the death of Professor J. H. Taylor, F. R. S., and 1 should like to record my deep gratitude for his supervision and encouragement throughout this work. I am most grateful to the former Department of Scientific and Industrial Research

for a research

studentship

and to the Royal

Society

of London

for a

grant towards equipment. I should like to express my gratitude to E. 0. Rowland, Esq., of King’s College, London, for his technical assistance, and to Miss Margaret Baker, also of King’s College, and to Douglas Maclean, Esq., of Glasgow University, for their help with the photographs.

1 am also grateful to Dr. A. S. Laughton,

of the National Institute of Oceanography, for the loan of a salinometer. I wish to thank my wife for her assistance in the field and Dr. J. D. Taylor, of the British Museum (Natural History), for his help in the field and in the identification of the flora and fauna. Finally, I am most grateful to Professor for his helpful suggestions on the manuscript.

T. Neville George,

F.R.S.,

REFERENCES

BAKER,B. H., 1963. Geology and mineral resources of the Seychelles Archipelago. Mm. Geol. SNYV. Kenya, 3: 140 pp. BAKER,B. H. and MILLER,J. A., 1963. Geology and geochronology of the Seychelles Islands and

structure of the floor of the Arabian Sea. Nature, 199: 346-348. BLACK,M., 1933. The precipitation of calcium carbonate on the Great Bahama Bank. Geol. Mug.. 70: 4555466. CLOUD, P. E., 1961. Environment of calcium carbonate deposition west of Andros Island, Bahamas. U.S., Geol. Surv., Profess. Papers, 350: 138 pp. Marine

Geol., 7 (1969) 95-127

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