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Pleistocene marine depositionalenvironments from Mauritius island, Indian Ocean
PLEISTOCENE MARINE DEPOSITIONAL ENVIRONMENTS FROM MAURITIUS ISLAND, INDIAN OCEAN par LUCIEN F. M O N T A G G I O N I *
ABSTRACT
RI~SUM]~
The Pleistocene elevated marine carbonate complexes on Mauritius island correspond to three types of depositional z o n e s : ( 1 ) reef-crest zone, (2) backreef zone, (3) littoral zone. On reef crests, the primary frame-builders are s c l e r a c t i n i a n c o r a l s (Acroporidae, Faviidae, Poritidae) and crustose coralline algae. The secondary builders are encrusting Foraminifers (Homotrematidae) chiefly, Molluscs (Vermetidae), Bryozoans (Cheilostomata) and Serpulids. Associated internal sediments are coarse to fine sand-sized grainstones or packstones and wakestones. In decreasing order of abundance, skeletal elements include fragments of corals, branched and crustose red algae, Pelecypods, Gastropods, benthonic
A l'~le Maurice, les complexes marins carbonates 6merges d'~ge pleistocene correspondent ~t trois types de milieux de sOdimentation : (1) les platiers r6cifaux, (2) les zones d'arri0re-r~cif, (3) les zones littorales. Au niveau des platiers r6cifaux, les bioconstructeurs primaires sont repr~sent~s par les SclOractiniaires (Acroporidae, Faviidae, Poritidae) et les algues calcaires encrofitantes. Les bioconstructeurs secondaires correspondent essentiellement aux Foraminif0res encrofitants (Homotrematidae), aux Mollusques (Vermetidae), aux Bryozoaires (Cheilostomata) et aux Serpules. Les sOdiments intrasquelettiques qui leur sont associOs sont des calcarOnites h~tOromOtriques grossi0res ~ fines ou des calcilutites. Par ordre d'abondance d~croissante, on note la presence d'~lOments coralliens, d'algues calcaires branchues et encrofitantes, de P~l~cypodes, de Gastropodes, des tests de Foraminifores benthiques (Amphisteginidae, Peneroplidae, Calcarinidae, Nummulitidae, Alveotinidae et Homotrematidae), d'Echinodermes et des spicules d'Alcyonaires ; la fraction silteuse est surtout constitu6e de spicules de Tuniciers. Les ciments marins pr6coces sont de types fibreux, palissadique, micritique et pelo'fde. Les zones d'arri0re-r6cif sont caract6ris6es par la pr6sence de calcar6nites grossi~res h moyennes, de calcirudites et de calcilutites h6t6rom6triques. A c6t6 des
Foraminifera (Amphisteginidae, Peneroplidae, Calcarinidae, Nummulitidae, Alveolinidae and Homotrematidae), Echinoderms and Alcyonarians ; the most abundant mud-sized grains are identified as tunicate spicules. Primary marine cements are needle, palisade, micrite and pelletal ones. The backreef zone is characterized by coarse to medium grainstones, packstones and wakestones. In addition to pellets the most common components are skeletal in nature ; they broadly show the same mode of occurrence as the reef-crest unit. Primary cements locally occur in packed fibrous or micrite form.
MOTS-CLI~S : RI~CIFS CORALLIENS, BIOFACIES, PL]~ISTOCI~NE,MAURICE, OCI~AN INDIEN. KEY-WORDS • CORAL REEFS, BIOFACIES, PLEISTOCENE, MAURITIUS, INDIAN OCEAN. * Universit6 Fran¢aise de l'Oc6an Indien, B.P. 5, 97490 Ste-Clotilde, R~union, France D.O.M. L'Ile Maurice est une ancienne possession britannique et fair partie du Commonwealth ; la langue officielle du pays est donc l'anglais. L'anteur ayant b6n6fici~, ~ chacune de ses missions sur le terrain, d'un important support logistique de la part des autorit6s mauriciennes, a pris l'engagement de publier une partie des r6sultats acquis sur leur ~le, en langue anglaise. Geobios, n ° 15, fasc. 2
p. 161-179, 2 fig., 3 tabl., 4 pl.
Lyon, avril 1982
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In littoral areas, well-sorted grainstones to poorlysorted mudstones can be described. The distribution of the various categories o f biogenic particles is dependent upon the extent o f reef tracts ; ancient beaches f r o m narrow reefs are characterized by coralgal facies while consolidated m u d d y banks in large complexes display high a m o u n t of molluscan detritus. Primary cementation appears mainly to be the result of the precipitation o f fibrous or micrite calcite. The morphology, biological associations and sediments o f these Pleistocene limestones are homologous with those of the adjacent modern environments.
162
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p61oYdes, les constituants les plus abondants sont de nature bioclastique ; leur r6partition est sensiblement comparable ~ celle observ6e dans les s6diments de platier. Les ciments primaires sont dispos6s en faisceaux fibreux ou sont de texture micritique. Dans les secteurs littoraux, les s6diments correspondent h des calcar6nites isom6triques ou h des calcilutites h6t6rom6triques. La r6partition des diverses cat6gories de bioclastes est fonction de I'extension des appareils r6cifaux ; les anciennes plages en arri6re de r6cifs 6troits sont caract6ris6es par un faci6s algocorallien alors que les bancs lutitiques, localis6s en arri6re des vastes 6difices, montrent une dominance des 616ments coquilliers. La cimentation primaire est essentiellement assur6e par de la calcite fibreuse ou micritique. La morphologie, les associations biologiques et les s6diments de ces calcaires pleistoc6nes sont comparables h ceux des milieux actuels environnants.
CONTENTS
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
162
L a b o r a t o r y methods . . . . . . . . . . . . . . . . . . . . . . . .
163
2 - Internal sediments . . . . . . . . . . . . . . . . . . . 3 - Primary cements . . . . . . . . . . . . . . . . . . . . . B. Backreef zone . . . . . . . . . . . . . . . . . . . . . . . .
Outcrop distribution and type of marine depositional environments . . . . . . . . . . . . . . . . . . . . . ......... 163 A. The older limestone units . . . . . . . . . . . . . . .
163
B. The younger limestone units . . . . . . . . . . . .
163
Depositional environments . . . . . . . . . . . . . . . . . . .
1 - Allochems . . . . . . . . . . . . . . . . . . . . . . . . . . 2 - Primary cements and matrices . . . . . . . . . C. Littoral zone . . . . . . . . . . . . . . . . . . . . . . . . . 1 - Allochems . . . . . . . . . . . . . . . . . . . . . . . . . . 2 - Primary cements and matrices . . . . . . . . .
163
A. Reef-crest zone . . . . . . . . . . . . . . . . . . . . . . .
163
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1 - Frame-builders . . . . . . . . . . . . . . . . . . . . . .
165
References . . . . . . . . . . . . . . . . . . . . . . .
166 166 166 166 166 168 168 169 170
.........
170
INTRODUCTION The island of Mauritius lies in the western Indian Ocean at 57030 , East and 20020 , South. A p a r t f r o m some peripheral carbonate deposits, Mauritius is entirely composed of volcanic rocks of PliocenePleistocene age (Mc Dougall & Chamalaun, 1969). Over the last decade modern Mauritian reefs have been studied in some detail by M. Pichon (1967), L. Montaggioni (1974, 1978), G. Faure (1975), G. Faure & L. Montaggioni (1976), L. Montaggioni & J. Mah6 (1980). These studies dwell either on generalized mor-
phological descriptions, biological zonations or detailed sedimentological analyses. Two m a j o r reef-building episodes, dated respectively at 120.000 years and 200.000 - 250.000 years, are recognized along the Mauritian coasts (Montaggioni, I978). The purpose of this study is to elucidate and to interpret the depositional and ecological conditions of these Pleistocene environments in comparison with their modern counterparts.
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163
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LABORATORY METHODS Laboratory research included petrographic study of 218 thin sections and corresponding hand specimens. Point-count data from thin sections were tabulated under several categories, the first two of which are fundamental importance to the discussion herein • (a) biogenic and non-biogenic constituents ; (b) primary marine cements and matrices ; (c) secondary subaerial cements and matrices ; (d) total porosity (both primary depositional and secondary solution cavities). Some cements were considered submarine by analogy with recent reef material (Schroeder, 1973 ; Montaggioni, 1978) both with respect individual cements and to cement assemblages. Their primary mineral composition was inferred on the basis
of crystal habit and, when it was possible, of staining techniques. The point-count percentages were estimated within the 95 070 confidence interval (Tables 1, 2 and 3). Nomenclature used in this paper follows that introduced by R.J. Dunham (1962). The classification of limestone types is based on depositional texture: boundstone (components bound together during deposition);grainstone (grain supported components) ; packstone (grain supported with some mud present) ; wakestone (mud supported, with more than 10 070 framework grains).
OUTCROP DISTRIBUTION AND TYPE OF MARINE DEPOSITIONAL ENVIRONMENTS In Mauritius, the Pleistocene marine sedimentary rocks crop out preferentially at isolated localities (Fig. 1A) ; large continuous exposures are scarce. Local relief rarely exceeds two meters ; consequently, no appreciable vertical sections are available. In a few localities, outcrops are incompletely preserved or partly covered by late Pleistocene aeolianites so that their total geometry is unknown. A
-
THE
OLDER LIMESTONE
UNITS
The best exposure of these limestones is situated inside the Port Louis harbour, were it extends on 50 meters long and 20 meters wide at 6 meters above mean sea level. Morphological and ecological data have provided evidence indicating that this outcrop do represent a sheltered reef flat environment similar to the present ones which can be seen lying w i t h i n half a kilometer northward and southward of the harbour. The limestone is partly overlain by well-sorted, cross-bedded grainstones which represent a water-formed dune (P1. 1, fig. 1).
DEPOSITIONAL Three types of depositional zones are to be observed in these Pleistocene limestones : (a) reef-crest zone ; (b) backreef zone ; (c) littoral zone. A
-
REEF-CREST
ZONE
Reef crests are very narrow (less than 30 meters
B
-
THE
YOUNGER
LIMESTONE
UNITS
Contrary to the remains of the marine stage which were probably overlapped by later lava flows, the rock exposures related to the younger marine stage are very common along all the shores and outlying islets (see Fig. 1A for location and names of the corresponding outcrops). Reaching maximum elevations of 2-3 meters above mean sea level (PI. 1, fig. 2), the diverse carbonate terraces can be interpreted, following L. Montaggioni (1974), as : (a) reef platforms (or apron reefs), characterized by the lack of distinctive morphological and biological units across the reef-crest and of backreef area (Fig. 1B) ; well-zoned fringing reefs (presence of increasingly complex reef fiats and of narrow backreef channels) (Fig. 1C). Locally there is a tendency towards the development of a barrier-reef at Mahebourg (from Marianne islet to Vieux Grand Port).
ENVIRONMENTS wide) and consist of compact reef flats or reef flat with irregular coral growths according to the indopacific reef morphology proposed by R. Battistini & alii (1975). Outcrops from Flic-en-Flac, Vieux Grand Port and the islets of Benitiers, Plate and Gabriel are the pleistocene-counterparts of diverse modern reef units : outer or inner shingle spreads and boulder rid-
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164
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ges (PI. 1, fig. 3). Forereef units are poorly or not exposed because they were generally buried below the backreef deposits of holocene reef tracts, or partly
destroyed by mechanical erosion. However, at Plate islet, some topographical features of the reef terrace may be identified as ancient outer grooves.
57,'45' E
57"30' _19"50'S
--~20
'P:latt~l ~]a sb~t I/
"O0 "S
%
baie de I' arsena
03
.%
o
I
5 7 "40" E
por co O
flic enflac Fig. I - A . Location of Pleistocene emer-
vieux grand
O
pet(re riviera noirre ..* benitiers i s l e t ~
ne ~"
• " "i~let
I°°: ,°
.%
o•,
baie
du
beau champ
ged marine outcrops, Mauritius island. Localisation des affleurements de calcaires marins ~merg~s d'~lge pleistocene, ile Maurice. Fig. 1-B . Windward
reef
platform,
Plate islet. Plate-forme r~cifale Vent>>, lie Plate•
ii
. . . . . m o d e r n reef f r o n t [] pleistocene reef outcrops
10 km
<
Fig. 1-C . Leeward fringing reef, Baie
de l'Arsenal.
A
R~cif frangeant <>, Baie de rArsenal. m l t = m e a n l o w tide level.
zone
reef-crest
++++++ B
,
i
++++++q-+. VOlcanic ++++q-++-t-+++++++~-r
rock"
m l t = n i v e a u des basses m e r s
:
i
~j_~'~ ' , a S e
'
littoral zone R ;:::~ backraef :::::1 zone :::::.~-.~ C
i~f
moyennes.
' ~ . r ~ ~
++-t-~
I ~: IN
f
reef-crest zone ~ __ J , , , , ~ ,-'---7 ,-i
I
+ + _~ + +~°'~a,,i~--Yo~k ~;~,;,-ent +++ ~-+÷++++++++
i
I
i
I
2 m
= I \
+ -~ + +'+ + ~- ~-~'-~+++++++++++
reduced : Acropora dana+, Acropora sp. (28 %), Leptoria phrygia (20 070), Favites abdita and Goniastrea retiformis (9 %), Pocillopora damicornis (22 %), Porites sp. (1%). Such a composition indicates that this reef crest belongs entirely to the coral subzone in spite of the local exuberance of melobesian algae ; it has to be considered typical of high energy environments by comparison with the biological zonation of modern Mauritian reefs. Generally speaking, these coral diversity patterns are valid in any given terrace;however, on sheltered exposures (Petite Rivi~re No+re, Flic-en-Flac), Acroporidae tend to prevail strongly over Faviidae.
1 - Frame-builders. The basic facies pattern comprises two main elements differentiated on the basis of principal framebuilder distribution • (a) the inner scleractinian coral subzone ; (b) the outer coralline algal subzone, Contrary to its modern equivalent, the reef-crest is generally devoid of hydrocorals (Millepora). This absence may be ascribed to the burying or the destruction of the outermost parts of reef flats which correspond generally to their typical habitat. Determination of biotic composition from quadrat was made on the well-preserved reef flat of Plate islet ; 4 m2 quadrats were spaced horizontally about 3-5 m apart, according to G. Scheer's method (1972). The relative abundance of each contributor was established as follows. Coralline algae (Porolithon, Lithophyllum, Lithothamnium and subordinate Lithoporella, Archaeolithothamnium) locally have a high amount of cover (40 070). Coral assemblages at least represent 60 070 of the framework, but the number of species is
Point-count analyses indicate that main builders represent 22 to 70 070 of the total rock volume (Table 1 ; P I . 2, fig. 1,2). Subordinate builders may be important volumetrically (4-11 070 of the organic frame);in decreasing order of abundance, they include : encrusting foraminifera (Miniacina miniacea, Carpenteria monticularis), gastropods (Vertretidae), pelecypods (Modiolus), bryozoans (Cheilostomata) and serpulids (P1. 2, fig. 2,3,5,6).
,
!
!
!
!
! L
! v
! !
! !
L ,
! !
.iP°rt Louis !
~ Marianne ! islet
! ! vBaie
v Baie de ".I 'Arsenal
v'. P l a t e v islet
Gabriel
! .............................. v
! ........... !
! ...................... !
! .......... !
...........
T ..........
[Number
v
v
of thin sections
-i . . . . . . . . . . . . . . . . . . . . . . . . . . . v
~Number
of
points
12
.-, i ........... !
v
counted
3816
v
7368
! ...........
~Frame-builders
"i
".
i 31,851,5 + ' Corals ....................... i 22,6-1,3 ' Coralline algae .............. i 5 9~0,8 ' + ,' Secondary builders ........... ~. 3,3-0,6 !Primary
cements
! Needle ....................... ' Palisade ..................... ! Micrite ...................... ~Internal
! ! ! !
0,1i0,] 0,2+'0,1
~ 30,3-1,5
sediments
+
25,3-1,0 v"I 26 , 0~I,0 0,9+0,1 v 53,6-1,1
!Secondary ,
15,6+.I,2
I 21,7+'0,9
!
cements
and
matrices!
!
Tabl.
1 -
Mean
constituent
dence
interval).
+
v
i
i
Composition
moyenne
de confiance
~ 5 % de risque)•
2,8~0,3
22
I. . . . . . . . . . . . . L
..............
I
5598
~
6930
! ............
31
islet
17
16
.I . . . . . . . . . . . . . . . . !
T
!..........
l ..........
I
v ...............
! T
~-;
9765
5423
.........
5217 ~ ..........
+ 69,2-I ,3 22,2+-1 + ,0 42 , 5-I + ,3 4,5-0,5
2,0-+0,4 + 2,0-0,4
I
"I
2 , 9-0,4 +
i
2,8-0,4
! !
+
+
I• 3 6 , 7 -+1 ' 3 v" 3 1 , 0 5 1 , 2
2,5+-0,4 + 0,1-0,08
I
40,0-1,2
3 1 , 6 !+1,
1
5,350,5 3,1-0,4 + 0,8-0,2 +
0,2-0,1
+
•I 4 7 , 0 -+I ,4
29 , 8-I ,2 I + I 13,6-0 + ,9 , 3,6-0,5
+ 69,8-0,9 + 8 4, - 0+ 6 , 58,2-I + ,0 3,2-0,3 + 0,2-0,1 + 0,7-0,2
+ I ,7-0,4 + L ,2-0,3
! !
+ 65,351,3 4 1 , 1 - +1 , 3 21 , 3 -+1 , 1 2,9-0,4
! .I ! !
v ! !
I !
! I t I
,
!
2',7~0,4 0,5-0,2
I
11
, 6+0,8
1122,8+1,1 I 13,6+0,9
1,9+0,4
0,3+0,1 1 7 , 8 - 1 ,0
10,0+-0,8 3,2+.0,5
9,050,8 47,9-1,3 + 10,7-0,8 + 4,8-0,5
'f
42,6+.I,2
i I 2,3-0,9
+ 19 ,2-0,7 + 1,650,3 55,4-1,2
I
÷+
1 8 5, - 1+ , 0 2,750,5 36,4-1,3 12,9+.0,3 + 3,7-0,5
3,4+-0,4 + 1,2-0,3
f
1,5-0,6
++
2,4-0 + ,3 3,350,1 5,1-0,7
13,6+-0,7 + 1,5-0,2
I 1 6 , 3 -+1 , 1
! '
reef-crest
des platiers r~cifanx
zone,
Mauritius
pleistoc6nes
island
(The values
de Vile Manriee
!~
+ ' 9,3 0,8 t 1,350,3 30,1-1,2
i ~
1,3+.0,3
!
3,1+.0,4
1
I
of the Pleistocene
des s6diments
I ...............
18
!
i 12,5-I,0
composition
5796
Petite Rivi~re No+re
!
iJ + t" 7 ,8-0,9 + " 0,9-0 3 i 39,7-1,5
P e r o s i t ~ ................
!
i............
+ 0,150,07 1,4-0,2
i Mud-sized matrix ............. • Skeletons • ' .................. • Volcanic grains iTotal cements + sediments .....
iTotal
Cap
!
v 22,750,9 i 16,7-0,8 + 3,5-0,4 v' 2,4+.0,3 ! ! ! !
du
! ............ !
18
!. . . . . . . . . . . . . . . . . . . . . . i. !
! ...........
Champ
!
24
! ..............................
• Total ........................
Beau
!
l
are tabulated
(Les valeurs
u s i n g t h e 95 % c o n f i -
sont calcul6es
avec un intervalle
-
2 - Internal
sediments.
The amount of sediments occurring within the pores of framework varies largely even within a single cavity (P1. 2, fig. 1). The recognizable granule-and sandsized particles represent 2 to 27 % of the total rock (Table 1). They change broadly in nature depending on the size of catching pores : corals (25-45 % of the total skeletal elements), melobesians (7-68 %), mollusks (5-10 %), benthic foraminifera (2-8 %), alcyonarian spicules (less than 1 % ) developing localized spiculites, miscellaneous skeletons such as crustaceans, echinoids, serpulids and articulated algae (Halimeda, Corallina) (2-3 %). Foraminiferal assemblages strictly are similar to those described from the neighbouring modern reefs (Montaggioni, 1981) ; the most common types were identified as Amphistegina lessoniL A. radiata, Mar-
ginopora vertebralis, Calcarina calcar, Elphidium crispum, Carpenteria, Miniacina, Planorbulina, Alveolinella boscii, Heterostegina depressa, Textularia agglutinans, several Quinqueloculina and Triloculina species (Fig. 2A). The silt-sized fraction is mainly composed of tunicate spicules (P1. 3, fig. 2), unidentified skeletal debris, associated with faecal pellets and terrigenous elements. Intraparticle pores preferentially exhibit wakestones and mudstones while grainstones and packstones are the usual fillings of interparticle cavities (P1. 3, fig. 1,2,3,4). 3 - Primary
cements.
Several fabrics have been recognized : a) INITIALLYARAGONITECEMENTS.
Needle cements (continous or irregular linings of subhedral fibrous crystals 30-200).trn long and 2-10 ,um wide) occur preferentially in the chambers of corals and mollusks (P1. 2, fig. 3,5). Their amount ranges between 0,1 and 2.7 % of the total rock (Table 1). Spherulitic cements are strickly localized within conceptacles of coralline algae (P1. 2, fig. 4). J.H. Schroeder (1973) pointed out that their scarceness is chiefly due to the ease which they are altered. b) INITIALLYHIGH MAGNESIANCALCITE CEMENTS. Microcrystalline (micrite) cements (2-15~m in size) are ubiquitous, but they mainly occur in sheltered pores produced by reef-builders, as structureless masses or oval peloids (P1. 3, fig. 4). Optically undistinguishable from the biogenic mud-sized fraction, they
166
-
reach concentrations of 10-30 070 of the total limestones (Table 1). Palisade cements (very regular rims 30 to 70 jam thick) generally infill the intergranular voids of internal sediments and more rarely the internal cavities of organisms. Their amount is lower than 2 °10. B - BACKREEF ZONE
Backreef deposits consist of coarse to medium grainstones, packstones and wakestones (Table 2). 1 - Allochems.
Allochems which represent 27 to 69 % of the bulk rock are of two types. a) SKELETALCOMPONENTS. Their amount varies from 61 to 88 °70 of the total allochems except at Gabriel islet (2 e/0). These constituents broadly show the same mode of occurrence as the reef-crest units (P1.3, fig. 5,6,8). In general order of abundance, the grains a r e : c o r a l s (mean value : 39.4 e/0), melobesians (33.3 %), mollusks (9.9 %), foraminifera (8.2 %), Halimeda (3.0 %), alcyonarians (2 %), echinoids (1.5 %), tunicates (1.2 %), crustaceans (0.9 %), sponges (0.4 %) and articulated red algae (0.3 %). As for foraminiferal assemblages, they are similar to those from the reef-crest environment (Fig. 2B) ; Amphistegina, Calcarina, Quinqueloculina, Marginopora and Homotrematidae (Miniacina, Carpenteria) remain dominant types. b) NON-SKELETALCOMPONENTS. Three distinct categories are recognizable : lithoclasts, as fragments of adjacent boundstones (0.5 % 4.4 % of the total allochems) ; peloids (P1. 3, fig. 7), probably faecal pellets of invertebrates in origin (0-6.2 % ) ; s a n d - s i z e d volcanic particles (mean value : 6.9 %). 2 - Primary
cements
and matrices.
The percentages of these infillings show large variations (0.9-41% of the total rock). The intergranular strongly cristalline cements are often absent. This lack is probably as observed frequently in loose external sediments of Holocene reefs (Montaggioni, 1978). When they are present, these cements occur strictly as packed acicular crystals (3060,um long, 1.5-8pm wide) probably aragonite in origin and occupy generally less than 1 % of the total rock volume (PI. 4, fig. 1,2,3). In contrast, palisade fringes seem to be totally absent. This is in accordance with previous investigations on the Bahamas (Cloud, 1962, Traganza, 1967) and the Persian Gulf (Evans,
--
167
--
3.2
'~I~
7.~ ~
A
REEF-CREST ~ ZONE .~vo
~
, /
0o.2°
.•Z
~'~
oP
vO ~o
7.0 3,2
B "L,5
.oo
~z
•
Fig. 2 -
~'
ZONE
6samples ~
leeward
C
areas
6samples
windward a r e a s
Relative a b u n d a n c e o f the m a i n F o r a m i n i f e r a l types on the P l e i s t o c e n e e m e r g e d m a r i n e l i m e s t o n e s o f M a u r i t i u s . - A : r e e f - c r e s t z o n e . - B : b a c k r e e f zone. - C : littoral zone. A b o n d a n c e relative des p r i n c i p a u x t y p e s de F o r a m i n i f ~ r e s d a n s les calcaires m a r i n s ~merg~s pleistoc~nes de M a u r i c e - A : platiers r~cifaux. - B : z o n e s d'arri~re-r~cif. - C : zones littorales.
- 168 -
1966 ; Evamy, 1973) concerning the prevailing appearance of aragonite cements in backreef areas. This may be due to the inhibition of calcite nucleation by some cations present in sea water during sediment deposition (see Milliman, 1974, p. 13-14).
grainstone and packstone volume and about 33 °70 of the wakestones. The major part of the fraction finer than 40)a m is composed of amorphous allophane, montmorillonite, metahalloysite and gibbsite ; such a composition closely reflects the intense tropical weathering conditions of neighbouring volcanic source zones.
The amount of micrite cements and associated silt and clay-sized matrices averages 1.5 % of the total
!
!
I
!
!
!
!
!
!
!Port Louis!
!
Benitiers
Flic-en-Flac
islet
Baie de
!
t
I
i
I
!
! [Rock types ~ ! -~ !
A - B
!
! A - B
[
T
!
.--
C
A - B
C
A - B
5
i
l
4
2
6
of
points
1612
counted
i
!
! iPrimary
cements
and
+ 43, 8-2 + ,4 3 6 , 3 -+2• 3 0 , 2-0 + ,2 + I , 2-0 5 I ,8+- 0 , 8 5,3-1,0
matrices
.......................
~Needle
iPalisade
.....................
;Micrite
+ M a t r i x ..... ....... o
iTotal I
cements
1,2-1,1 0,5+0+ ,7 0,350, 6 2,1-1,5
+
I T a I ! !
0,7-0,5
t
0,2-0,5
................
i t
,0 i 2 3 , 4 + 4 , 4
2,5..I
0,9-0,5 +
3,2.+1,1
, 23,6+4,4
40,3.+3,1
[ 38,2-+5,0
+
7,9-1,3
8,4.*1,8 i 11,4-+3,3
Mean constituent composition of the Pleistocene nes, C = wakestones (The values are tabulated Composition moyenne A = grainstones;B 5 °70 d e r i s q u e ) .
- LITTORAL
639
......... + 38 , 4-3 + ,8 30 9-3 + ,9
2,0-0,8
0 • 9-0 + ,7 0,8-0 +, 7
1,8.*0,7 + 5,3-1,2
3,951,5 1,9-I,0
C
!
- ..............
! I
1 J 6-+0,7 + 2,1-0,8 + 55,6-2,7
1896 i .......... + 4 4 , 3 - +2 2 39,352,2 0,9-0,4 0,2-0,2 $
3,9-0,9 12,2.+1,5
+ 36 , 6 +- 3 , 0 31 , 2-2 + ,9 0,6-0,5 +0 , 3 -+0 ,3 4,5-1,3
5 4 1 3, - 3+8 , 41,3-3,8
; 1 6, - 0+6 , ]3,8-1,6
16,5.+2,9
30,4.*2,0
Z 33 , 6-2 9 + , 33,6-2,9 + 25,5-2,7
II,4~1,4
5,3+1,4
6,5..1 ,3
3,8.+1,5
!
4
9
. . . . . . . . . . . !. . . . . . . . . . . . . . . t . .
!
+ 55 , +9 - 1 , 8
+ 69,0-2,5+ t 58 , 4 =+ 2 , 7 t 2,550,9 •i 0 ,8 -+ 0 , 5 ' 5,2-I + ,2 ~ 2,1-0,8 +
l
2862
•~
! !
+ 0,5-0,4
backreef zone, Mauritius u s i n g t h e 9 5 °70 c o n f i d e n c e
des s6diments des milieux d'arri~re-r6cif pleistoc~nes = packstones, C = wakestones (Les valeurs
ZONE
1 - Allochems. Littoral facies contain between 47 and 69 °70 clastic grains (Table 3). SKELETAL
A - B
COMPONENTS.
The distribution of the various categories of bioge-
island ; Rock interval)•
types*
:'A
.I
I !
1 8 , 2 -+1 , 4 I 1,6-0,5 t !
T , " t t
0,9-0,5 T 5 1,2-0 6 32,7~1,7 + , 2,1-0,8 32,7-1,7 + + ! 25,2-2,4 9,0-I,0 + + 2,4 0,6 !t 3 , 8 - 1 , 0
= grainstones,
! ! !
34,351,7 n 0,650, 3 u 1,250, 4 u
t! I ! ! , I
v
The littoral exposures exhibit two major rock types : fine grainstones or wakestones, generally similar to the adjacent backreef environment.
a)
!
i
t
+
0 ,9.*0 +, 5 43,2-2,4
matrices! !
Porosit~
2 -
and
+ 32 , 3-2 + ,6 22,6-2,3 0 , 6-*0,4 +
-
iTotal ........................ [Secondary I
1284
356
+ + 4 8 , 1 -÷3 J2 ! 27 ,2-4 ÷ ,6 39 J 5-3 1 v 23 I-4 ÷ , , + ,4
2,150, 9 1,450, 7 0,5-0+ ,4 4,6-1,3
i
]
! 972 [ 1292 ! .............................. f
?
!Allochems !Total . . . . . . . . . . . . . . . . . . . . . . . . . . !Skeletons. . . . . . . . . . . . . . . . . . . . . . !Lithoelasts .................... lPeloids... ..................... !Volcanic grains ..... ........... !Unidentified ...................
C
957
I
!
3
, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
........................................
! !
islet
!
I
! !Number
! Gabriel
! ~
!
3
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Tabl.
C
! ! ...................................................................................................... ! !
S N u m b e r of thin s e c t i o n s
!
! I
l'Arsenal
!
B = packsto-
de Vile Maurice ; nature des facies p6trographiques sont calcul6es avec un intervalle de confiance
"
nic particles is dependent upon the extent of reef tracts. Indeed, the littoral outcrops related to narrow reefs (Baie de l'Arsenal, Gabriel and Plate islets) display a composition similar to that of adjacent backreef l i m e s t o n e s , i.e. a coralgal g r a i n s t o n e facies:corals (39.4-53.0 o70 of the total biogenic detritus) ; melobesians (22.0-35.1 o70) ; mollusks (8.015.1 o70) ;foraminifera (4.1-12.9 °70) ; alcyonarians (0.8-6.2 %) ; echinoids (1-1.8 %), Halimeda (0.9-3.3 o70) ; miscellaneous skeletons (0.2-0.5 °70) (P1. 4, fig. 7). In contrast, the Vieux Grand Port limestone is of mollusk wakestone type (40.5 °70 mollusk fragments
- li59 --
versus 26.3 % corals and 17.8 % melobesians) (P1.4, fig. 4-6) ; this suggest that the general form of Pleistocene reef complex of Mahebourg was most likely similar to the one of the modern adjacent barrier reef. In addition, foraminiferal assemblages are roughly identical with the reef-crest and backreef ones (Fig.
! ! !
2C). b) NON-SKELETAL COMPONENTS.
They are relatively minor sediment contributors ; lithoclasts (0.4-4.6 % of the total allochems), peloids (0.5-1.5 %), volcanic particles (1.3-6 %).
!
!
!
! Vieux Grand
! Baie d e ! l'Arsenal ! !
! Plate
!
!
!
!
! Number of thin sections
!
Port
5
5
!
islet
!
l
!Gabriel !
! l ............
!
!
!
4
islet! ! ----!
6
! !
!
l
t Number of points
! I ! ! I I !
counted
Allochems Total ........................ Skeletons .................... Lithoclasts .................. Peloids ...................... Volcanic grains .............. Unidentified .................
!
I ! ! ! i
!
1665
! ! 47,2 ! 36,6 ! 2,2 ! 0,3 ! 1,4 I 6,7
1640
2,4 2,3 0,7 0,3 0,6 1,3
!
Primary cements and matrices ! Needle ..... ! Palisade ..................... i 6,3 ~ 1,2 Micrite + Matrix ............. ! 12,5 ~ 1,6 Total ....................... .! 18,8 - 1,9
! Secondar~ cements and ! matrices .....................
! ! 31,6
' Total_ P'~rosity ............... !
1344 !
!
! !
!
l
! 47,9 ++-2 -.2'4 ! 37,2 +- 0,3 ! 1,2 -+ 0,5 ! 0,7 +- 0,4 ! 0,6 + 1 , 4 l 6,2 ,2
! 69,3 l 57,9 ! 0,3
2,5 2,6 0,3
l l l
0,4 4,1 6,6
0,3 1,0 1,3
l v
8,7
+ - 1,5
l ! 1
+ 1 ,2 - 0,5
l I
1,2
- 0,5
!
8,7
+
+~ 2 , 2
2,4 - 0,7
1926
!
+
! 41,7 ~ 2,4 ! 11,2-
- 1,5
1,5
l 13,7 ! 8,3
$ -
! 58,9 ! 49,0 ! 1,5 ! 0,4 ! 3,7 [ 4,3
!
2,2 2,2 0,5 0,3 0,8 0,9
!
v 21,3 - 1,8
[
t 21,3 ~- 1,8
l
!
[ ! l l
! !
+
1,8 1,5
! 14,9 ~ 1,6 ! 5,9 - 1 , 0
l !
Tabl. 3 - Mean constituent composition of the Pleistocene littoral zone, Mauritius island (The values are tabulated using the 95 o70 confidence interval). Composition moyenne des s6diments des milieux littoraux pleistoc6nes de rile Mauriee (Les valeurs sont calcul6es avec un intervalle
de confiance ~t 5 % de risque).
2 - Primary cements and matrices.
The best represented marine cements within the Pleistocene intertidal deposits consist of very regular fibrous rims (25-50 ~m thick) probably originated as high-magnesian calcite (P1. 4, fig. 5-8). They reach levels of 6°3 to 21,3 °70 of the total rock (Table 3). P r i -
mary aragonite crystals are relatively less common (1,2-8.7 °7o). Whatever their primary mineralogy might be, the strongly cristalline cements are always present inside the littoral outcrops. Consequently they are thought to be relict beach-rocks. The commonness of palisade cements suggests that environmental conditions rather favoured calcite precipitation : crystal
--
growth of calcite might be started by the partial dilution of magnesium ions dissolved in beach interface waters in consequence of seaward migration of the ground waters. This assertion seems to be in accordance with R.J. Russell's observations (1962) about the predominating occurrence of calcite cements in the majority of beach-rock outcrops f r o m the Carib-
170
--
bean sea and the Pacific Ocean. Microcrystalline fillings mixed with a detrital matrix are only found within the Vieux Grand Port limestone where they are particularly c o m m o n (12.5 °70 of the total rock).
CONCLUSIONS The reconstruction of Pleistocene marine carbonate environments at Mauritius island indicates that their spatial distribution, biotic associations and sediment composition are homologous in all respects with those of the adjacent modern counterparts. (1) Emergent buildups are mainly of platform or fringing types ; generally, they did not exceed 15 to 20 meters wide. (2) The reef-crest zone is distinguished by the prolific occurrence of massive coral growth (Acroporidae, Faviidae) and of encrusting coralline algae. Secondary builders consist chiefly of coelobiteforaminifera (Carpenteria, Miniacina) and of gastropods (Vermeti-
dae). (3) Reef-crest, backreef and adjacent littoral zones are respectively typified by coralgal grainstones, packstones and wakestones rich in benthonic foraminifera. In contrast, mollusk wakestones are a signature of distant littoral areas. (4) The other skeletal constituents (alcyonarians, echinoderms, Halimeda, bryozoans) and non skeletal
carbonate elements are minor or absent. (5) Foraminifera assemblages are characterized by the general plenty of Amphistegina tests associated with the other larger forms typical of the indo-pacific province (Marginopora, Calcarina, Alveolinella, Heterostegina, Elphidium) and with smaller Miliolids and Textulariids. In addition, Carpenteria and Miniacina play a m a j o r role in skeletal production. However these encrusting foraminifers are more abundant in sediments f r o m exposed windward reefs than in those f r o m sheltered leeward reefs. Such a disparity may be due to local water energy conditions, the desintegration of encrusting organisms being favoured in high energy areas. (6) Primary voids of reef framework were infilled by both aragonite and calcite submarine cements. In contrast, localized aragonite cementation preferentially occurred inside backreef deposits while littoral sediments were chiefly affected by an almost generalized precipitation of initially high magnesian calcite.
REFERENCES BATTISTINI R. & alii (1975) - l~16ments de terminologie r6cifale indo-pacifique. Thetys, Marseille, vol. 7, fasc. 1, p. 1-111. CLOUD P.E. (1962) - Behaviour of calcium carbonate in sea water. Geochim. Cosmochim. Acta, Northern Ireland, vol. 26, p. 867-884. DUNHAM R.J. (1962) - Classification of carbonate rocks according to depositional texture. I n : W . E . HAM (edit.). Classification of carbonate rocks. Amer. Assoc. Petrol GeoL, Mem. 1, Boulder, Colorado, p. 108-121. EVAMY B.D. (1973) - The precipitation of aragonite and its alteration to calcite on the Trucial coast of the Persian Gulf. In:PURSER B.H. (edit.), The Persian Gulf, Springer-Verlag Edit., Berlin, p. 329-342. EVANS G. (1966) - The recent sedimentary facies of the Persian Gulf. Phil. Trans. Roy. Soc. London, ser. A,
vol. 259, p. 291-298. FAURE G. (1975) - l~tude comparative des r6cifs coralliens de l'archipel des Mascareignes (Ocean Indien). Bull. Mauritius Inst., Port-Louis, vol. 8, fasc. 1, p. 1-33. FAURE G. & MONTAGGIONI L. (1976) - Les r6cifs coralliens Au-Vent de l'~le Maurice (Archipel des Mascareignes, Oc6an Indien). Mar. GeoL, Amsterdam, p. M9M16. MC DOUGALL I. & CHAMALAUN F.H. (1969) - Isotopic dating and geomagnetic polarity studies on volcanic rocks from Mauritius, Indian Ocean. GeoL Soc. Amer. Bull., Boulder, Colorado, vol. 80, p. 1419-1442. MILLIMAN J.D. (1974) - Marine carbonate : Part 1 Springer-Verlag Edit., Berlin, p. 1-375. MONTAGGIONI L. (1974) - Coral reefs and Quaternary
-
shorelines in the Mascarene archipelago (Indian Ocean). Proc. Second. Intern. Coral Reef Symp., Great Barrier Reef Commitee, Brisbane, vol. 2, p. 579-593. MONTAGGIONI L. (1978) - Recherches g6ologiques sur les complexes rdcifaux de l'archipel des Mascareignes (Ocdan Indien). D. Sc. Thesis, Universitd d'AixMarseille H, 2 vol., p. 1-524. (unpublished). MONTAGGIONI L. (1981) - Les associations de Foraminif~res dans les s6diments r6cifaux de l'archipel des Mascareignes (Oc6an Indien). Ann. Int. Oc~anogr., Paris, voI. 57, fasc. 1, p. 41-62. MONTAGGIONI L. & MAHI~ J. (1980) - Caract6risation faciologique des s6diments r6cifaux de l'~le Maurice par l'analyse factorielle des correspondances. Oceanol. Acta, Paris, vol. 3, fasc. 4, p. 409-420. PICHON M. (1967) - Caract~res g6n6raux des peuplements
171
-
benthiques des r6cifs et lagons de l'~le Maurice. Cah. O.R.S.T.O.M., Ser. Oc~anogr., Paris, vol. 5, fasc. 4, p. 32-45. RUSSELL R.J. (1962) - Origin of beach-rock. Zeitch.f. Geomorphol., Berlin, vol. 6, p. 1-16. SCHEER G. (1972) - Investigations of coral reefs in the Maldives islands with notes on lagoon patch reefs and the method of coral sociology. In : C MUKUNDAN & C.S.G. PILLAI (edit.). Proceed. First Intern. Coral Reef
Syrup., Madapam Camp, Marine Biol. Assoc. India, p. 87-120. SCHROEDER J.H. (1973) - Submarine and vadose cements in Pleistocene Bermuda reef rocks. Sedim. Geol., Amsterdam, vol. 10, fasc. 3, p. 179-204. TRAGANZA E.D. (1967) - Dynamics of the carbon dioxyde system on the Great Bahama Bank. Bull. Mar. Sci., Miami, vol. 17, p. 348-368.
M a n u s c r i t d6finitif requ le 23.12.1981
--
172
--
PLANCHE 1
Fig. 1 - Cross-section from the Older Limestone Units (Port Louis). The lower reef flat is overlain by a water-formed dune. Coupe dans I'Unit6 des Calcaires Inf6rieurs (Port Louis). Le ptatier r6cifal sous-jacent est recouvert par une dune hydraulique.
Fig.
2
-
Elevated reef platform at Plate Islet (Younger Limestone Units). The maximum thickness of this buildup is meter.
1.50
Plate-forme r~cifale ~merg~e de l'lle Plate (Unit~ des Calcaires Sup~rieurs). L'6paisseur maximale de la bioconstruction est de 1.50 m~tre.
Fig. 3 - Boulder ridges from the reef platform at Plate Islet. Levee d~tritique sur la plate-forme r~cifale de I'Ile Plate..
G~obios n ° 15, fasc. 2
PI. 1 L. Montaggioni
PLANCHE 2
Fig. 1 -
Coral-coralline algal boundstone, reef-crest zone. Calcalre construit i Coraux et Algues caicaires, platier r~cifal.
Fig. 2 -
Coralllne algal- Vermetid boundstone, reef-crest zone. Calcalre eonstruit A Algues calcalres et Verm~tid~s, platier r~cifal.
Fig. 3 -
Detail from the melobesian - Vermetid boundstone.
D~tall d'un calcaire A M61ob~si~es et Verm~tidds.
Fig. 4 -
Spherulitic cement within a coralline algal conceptacle, reef-crest zone. Ciment sph~rolitique dans un conceptacle d'algues calcalres, platier r~cifal.
Fig. 5 -
Chambers o f Vermetid shells completely infilled with epitaxial acicular cements, reef-crest zone.
Chambres de coquilles de Verm~tid~s totalement colmat6es par des ciments aciculaires h croissance dpitaxiale, platier
rdcifal.
Fig. 6 -
Encrusting foraminiferal
(Carpenteria) bryozoan boundstone, reef-crest zone. (Carpenteria) et Bryozoaires, platier
Calcalre constmit A Foraminif~res encrotitants
r~cifal.
Observations under reflected light. Observation en lumi~re naturelle. Abbreviations used : SCL = Scleratinian corals ; CO = Coralline algae ; G A = Gastropods ; FM = encrusting Foraminif e r a ; BR = Bryozoans ; IS = internal sediments, P = biologically bored cavities ; cs = secondary blocky cement ; an = needle cement. Abrdviations utilis6es : SCL = Scl6ractinialres ; CO = Algues calcalres ; G A = Gastropodes ; F M = Foraminif~res encrofitants ; BR = Bryozoaires ; IS = s~diments internes ; P = cavit6s de perforation d'origine organique ; cs = ciment granulaire primaire ; an = ciment fibreux.
G6obios n ° 15, fasc. 2
PI. 2 L. Montaggioni
PLANCHE 3
Fig. 1 -
Internal wakestone, reef fiat. S~diments (wakestone) internes, platier r~cifal.
Fig. 2 -
Internal wakestone rich in tunicate spicules, reef flat. Wakestone interne i spicules de Tuniciers, platier r~cifal.
Fig. 3 -
Coralgal-foraminiferal gralnstone, reef flat. Gralnstone A ~l~ments coralliens, algaux et Foraminif~res, platier r6cifal.
Fig. 4 -
Peloids infllling a coral intraskeletal cavity, reef fiat. Pelo'ides remplissant une cavit~ corailienne intra-squelettique , platier r~eifal.
Fig.
5
-
Backreef algal-foraminiferal grainstone. Grainstone / algues calcalres et Foraminif~res, zone d'arri~re-r~cif.
Fig. 6 - Backreef foraminiferal packstone. Packstone i Foraminif~res, zone d'arri~re-r~cif.
Fig. 7 - Backreef peloidal gralnstone. Gralnstone i pelo'ides, zone d'arri6re-r~cif.
Fig. 8 -
Backreef bioturbed wakestone. Wakestone i bioturbations, zone d'arri~re-r~cif.
Observations under reflected light. Observations en lumi~re naturelle. Abbreviations used : F M = M i n i a c i n a detritus ; F A = A m p h i s t e g i n i d a e ; CO = Coralline algae ; SCL = Sclaractinian corals ; FC = C a r p e n t e r i a detritus ; F P = Peneroplids tests ; an = needle cement ; cs = secondary blocky cement ; P = bioturbation cavities. Abr6viations utilis6es : F M = d6bris de M i n i a c i n a ; F A = A m p h i s t e g i n i d a e ; CO = algues calcalres ; SCL = Scleractiniaires ; FC = d6bris de C a r p e n t e r i a ; FP = tests de P e n e r o p l i d a e ; a n = ciment aciculalre ; cs = ciment granulaire secondaire ; P = cavit6s de bioturbation.
G6obios n ° 15, fasc. 2
PI. 3 L. Montaggioni
PLANCHE 4
Fig. 1,2,3 - I r r e g u l a r a n d p a c k e d linings o f i n t e r p a r t i c l e needles, b a c k r e e f z o n e . F r a n g e s irr6guli6res d'aiguilles, dispos6es e n f a l s c e a u x , i la p6riph6rie des particules, z o n e d ' a r r i 6 r e - r 6 c i f .
Fig. 4 -
Littoral mollusk - foraminiferal wakestone. W a k e s t o n e i m o l l u s q u e s et f o r a m i n i f 6 r e s , z o n e littorale.
Fig. 5 -
I n t e r p a r t i c l e p a l i s a d e c e m e n t , littoral z o n e . . C i m e n t p a l i s s a d i q n e i n t e r g r a n u l a i r e , z o n e littorale.
Fig. 6 -
D e t a i l o f a lithoclastic w a k e s t o n e , littoral z o n e . D6tall d ' u n w a k e s t o n e ~t lithoclastes, z o n e littorale.
Fig. 7 -
Skeletal p a c k s t o n e , littoral z o n e . P a c k s t o n e b i o d 6 t r i t i q u e , z o n e iittorale.
Fig. 8 -
P a l i s a d e c e m e n t e n v e l o p i n g t w o Amphistegina tests, littoral z o n e . C i m e n t p a l i s s a d i q u e e n v e l o p p a n t d e u x tests d'Amphistegina, z o n e littorale.
O b s e r v a t i o n s u n d e r reflecte~d light. O b s e r v a t i o n s e n lumi~re naturelle. A b b r e v i a t i o n s u s e d : S C L = S c l e r a c t i n i a n corals ; C O = C o r a l l i n e algae ; F A = Amphisteginidae ; LI = L i t h o c l a s t s ; a n needle c e m e n t ; c m = m i c r i t e c e m e n t a n d m u d - s i z e d s e d i m e n t s ; cp = p a l i s a d e c e m e n t ; cs = s e c o n d a r y b l o c k y c e m e n t . A b r ~ v i a t i o n s utilis6es : S C L = Scl6ractiniaires ; C O = algues calcaires ; F A = Amphisteginidae ; L I = L i t h o c l a s t e s ; a n c i m e n t aciculaire ; c m = c i m e n t m i c r i t i q u e et silts ; cp = c i m e n t p a l i s s a d i q u e ; cs = c i m e n t g r a n u l a i r e s e c o n d a i r e .
G6obios n ° 15, fasc. 2
PI. 4 L. Montaggioni
ABSTRACT
RI~SUM]~
The Pleistocene elevated marine carbonate complexes on Mauritius island correspond to three types of depositional z o n e s : ( 1 ) reef-crest zone, (2) backreef zone, (3) littoral zone. On reef crests, the primary frame-builders are s c l e r a c t i n i a n c o r a l s (Acroporidae, Faviidae, Poritidae) and crustose coralline algae. The secondary builders are encrusting Foraminifers (Homotrematidae) chiefly, Molluscs (Vermetidae), Bryozoans (Cheilostomata) and Serpulids. Associated internal sediments are coarse to fine sand-sized grainstones or packstones and wakestones. In decreasing order of abundance, skeletal elements include fragments of corals, branched and crustose red algae, Pelecypods, Gastropods, benthonic
A l'~le Maurice, les complexes marins carbonates 6merges d'~ge pleistocene correspondent ~t trois types de milieux de sOdimentation : (1) les platiers r6cifaux, (2) les zones d'arri0re-r~cif, (3) les zones littorales. Au niveau des platiers r6cifaux, les bioconstructeurs primaires sont repr~sent~s par les SclOractiniaires (Acroporidae, Faviidae, Poritidae) et les algues calcaires encrofitantes. Les bioconstructeurs secondaires correspondent essentiellement aux Foraminif0res encrofitants (Homotrematidae), aux Mollusques (Vermetidae), aux Bryozoaires (Cheilostomata) et aux Serpules. Les sOdiments intrasquelettiques qui leur sont associOs sont des calcarOnites h~tOromOtriques grossi0res ~ fines ou des calcilutites. Par ordre d'abondance d~croissante, on note la presence d'~lOments coralliens, d'algues calcaires branchues et encrofitantes, de P~l~cypodes, de Gastropodes, des tests de Foraminifores benthiques (Amphisteginidae, Peneroplidae, Calcarinidae, Nummulitidae, Alveotinidae et Homotrematidae), d'Echinodermes et des spicules d'Alcyonaires ; la fraction silteuse est surtout constitu6e de spicules de Tuniciers. Les ciments marins pr6coces sont de types fibreux, palissadique, micritique et pelo'fde. Les zones d'arri0re-r6cif sont caract6ris6es par la pr6sence de calcar6nites grossi~res h moyennes, de calcirudites et de calcilutites h6t6rom6triques. A c6t6 des
Foraminifera (Amphisteginidae, Peneroplidae, Calcarinidae, Nummulitidae, Alveolinidae and Homotrematidae), Echinoderms and Alcyonarians ; the most abundant mud-sized grains are identified as tunicate spicules. Primary marine cements are needle, palisade, micrite and pelletal ones. The backreef zone is characterized by coarse to medium grainstones, packstones and wakestones. In addition to pellets the most common components are skeletal in nature ; they broadly show the same mode of occurrence as the reef-crest unit. Primary cements locally occur in packed fibrous or micrite form.
MOTS-CLI~S : RI~CIFS CORALLIENS, BIOFACIES, PL]~ISTOCI~NE,MAURICE, OCI~AN INDIEN. KEY-WORDS • CORAL REEFS, BIOFACIES, PLEISTOCENE, MAURITIUS, INDIAN OCEAN. * Universit6 Fran¢aise de l'Oc6an Indien, B.P. 5, 97490 Ste-Clotilde, R~union, France D.O.M. L'Ile Maurice est une ancienne possession britannique et fair partie du Commonwealth ; la langue officielle du pays est donc l'anglais. L'anteur ayant b6n6fici~, ~ chacune de ses missions sur le terrain, d'un important support logistique de la part des autorit6s mauriciennes, a pris l'engagement de publier une partie des r6sultats acquis sur leur ~le, en langue anglaise. Geobios, n ° 15, fasc. 2
p. 161-179, 2 fig., 3 tabl., 4 pl.
Lyon, avril 1982
-
In littoral areas, well-sorted grainstones to poorlysorted mudstones can be described. The distribution of the various categories o f biogenic particles is dependent upon the extent o f reef tracts ; ancient beaches f r o m narrow reefs are characterized by coralgal facies while consolidated m u d d y banks in large complexes display high a m o u n t of molluscan detritus. Primary cementation appears mainly to be the result of the precipitation o f fibrous or micrite calcite. The morphology, biological associations and sediments o f these Pleistocene limestones are homologous with those of the adjacent modern environments.
162
-
p61oYdes, les constituants les plus abondants sont de nature bioclastique ; leur r6partition est sensiblement comparable ~ celle observ6e dans les s6diments de platier. Les ciments primaires sont dispos6s en faisceaux fibreux ou sont de texture micritique. Dans les secteurs littoraux, les s6diments correspondent h des calcar6nites isom6triques ou h des calcilutites h6t6rom6triques. La r6partition des diverses cat6gories de bioclastes est fonction de I'extension des appareils r6cifaux ; les anciennes plages en arri6re de r6cifs 6troits sont caract6ris6es par un faci6s algocorallien alors que les bancs lutitiques, localis6s en arri6re des vastes 6difices, montrent une dominance des 616ments coquilliers. La cimentation primaire est essentiellement assur6e par de la calcite fibreuse ou micritique. La morphologie, les associations biologiques et les s6diments de ces calcaires pleistoc6nes sont comparables h ceux des milieux actuels environnants.
CONTENTS
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
162
L a b o r a t o r y methods . . . . . . . . . . . . . . . . . . . . . . . .
163
2 - Internal sediments . . . . . . . . . . . . . . . . . . . 3 - Primary cements . . . . . . . . . . . . . . . . . . . . . B. Backreef zone . . . . . . . . . . . . . . . . . . . . . . . .
Outcrop distribution and type of marine depositional environments . . . . . . . . . . . . . . . . . . . . . ......... 163 A. The older limestone units . . . . . . . . . . . . . . .
163
B. The younger limestone units . . . . . . . . . . . .
163
Depositional environments . . . . . . . . . . . . . . . . . . .
1 - Allochems . . . . . . . . . . . . . . . . . . . . . . . . . . 2 - Primary cements and matrices . . . . . . . . . C. Littoral zone . . . . . . . . . . . . . . . . . . . . . . . . . 1 - Allochems . . . . . . . . . . . . . . . . . . . . . . . . . . 2 - Primary cements and matrices . . . . . . . . .
163
A. Reef-crest zone . . . . . . . . . . . . . . . . . . . . . . .
163
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1 - Frame-builders . . . . . . . . . . . . . . . . . . . . . .
165
References . . . . . . . . . . . . . . . . . . . . . . .
166 166 166 166 166 168 168 169 170
.........
170
INTRODUCTION The island of Mauritius lies in the western Indian Ocean at 57030 , East and 20020 , South. A p a r t f r o m some peripheral carbonate deposits, Mauritius is entirely composed of volcanic rocks of PliocenePleistocene age (Mc Dougall & Chamalaun, 1969). Over the last decade modern Mauritian reefs have been studied in some detail by M. Pichon (1967), L. Montaggioni (1974, 1978), G. Faure (1975), G. Faure & L. Montaggioni (1976), L. Montaggioni & J. Mah6 (1980). These studies dwell either on generalized mor-
phological descriptions, biological zonations or detailed sedimentological analyses. Two m a j o r reef-building episodes, dated respectively at 120.000 years and 200.000 - 250.000 years, are recognized along the Mauritian coasts (Montaggioni, I978). The purpose of this study is to elucidate and to interpret the depositional and ecological conditions of these Pleistocene environments in comparison with their modern counterparts.
--
163
--
LABORATORY METHODS Laboratory research included petrographic study of 218 thin sections and corresponding hand specimens. Point-count data from thin sections were tabulated under several categories, the first two of which are fundamental importance to the discussion herein • (a) biogenic and non-biogenic constituents ; (b) primary marine cements and matrices ; (c) secondary subaerial cements and matrices ; (d) total porosity (both primary depositional and secondary solution cavities). Some cements were considered submarine by analogy with recent reef material (Schroeder, 1973 ; Montaggioni, 1978) both with respect individual cements and to cement assemblages. Their primary mineral composition was inferred on the basis
of crystal habit and, when it was possible, of staining techniques. The point-count percentages were estimated within the 95 070 confidence interval (Tables 1, 2 and 3). Nomenclature used in this paper follows that introduced by R.J. Dunham (1962). The classification of limestone types is based on depositional texture: boundstone (components bound together during deposition);grainstone (grain supported components) ; packstone (grain supported with some mud present) ; wakestone (mud supported, with more than 10 070 framework grains).
OUTCROP DISTRIBUTION AND TYPE OF MARINE DEPOSITIONAL ENVIRONMENTS In Mauritius, the Pleistocene marine sedimentary rocks crop out preferentially at isolated localities (Fig. 1A) ; large continuous exposures are scarce. Local relief rarely exceeds two meters ; consequently, no appreciable vertical sections are available. In a few localities, outcrops are incompletely preserved or partly covered by late Pleistocene aeolianites so that their total geometry is unknown. A
-
THE
OLDER LIMESTONE
UNITS
The best exposure of these limestones is situated inside the Port Louis harbour, were it extends on 50 meters long and 20 meters wide at 6 meters above mean sea level. Morphological and ecological data have provided evidence indicating that this outcrop do represent a sheltered reef flat environment similar to the present ones which can be seen lying w i t h i n half a kilometer northward and southward of the harbour. The limestone is partly overlain by well-sorted, cross-bedded grainstones which represent a water-formed dune (P1. 1, fig. 1).
DEPOSITIONAL Three types of depositional zones are to be observed in these Pleistocene limestones : (a) reef-crest zone ; (b) backreef zone ; (c) littoral zone. A
-
REEF-CREST
ZONE
Reef crests are very narrow (less than 30 meters
B
-
THE
YOUNGER
LIMESTONE
UNITS
Contrary to the remains of the marine stage which were probably overlapped by later lava flows, the rock exposures related to the younger marine stage are very common along all the shores and outlying islets (see Fig. 1A for location and names of the corresponding outcrops). Reaching maximum elevations of 2-3 meters above mean sea level (PI. 1, fig. 2), the diverse carbonate terraces can be interpreted, following L. Montaggioni (1974), as : (a) reef platforms (or apron reefs), characterized by the lack of distinctive morphological and biological units across the reef-crest and of backreef area (Fig. 1B) ; well-zoned fringing reefs (presence of increasingly complex reef fiats and of narrow backreef channels) (Fig. 1C). Locally there is a tendency towards the development of a barrier-reef at Mahebourg (from Marianne islet to Vieux Grand Port).
ENVIRONMENTS wide) and consist of compact reef flats or reef flat with irregular coral growths according to the indopacific reef morphology proposed by R. Battistini & alii (1975). Outcrops from Flic-en-Flac, Vieux Grand Port and the islets of Benitiers, Plate and Gabriel are the pleistocene-counterparts of diverse modern reef units : outer or inner shingle spreads and boulder rid-
--
164
--
ges (PI. 1, fig. 3). Forereef units are poorly or not exposed because they were generally buried below the backreef deposits of holocene reef tracts, or partly
destroyed by mechanical erosion. However, at Plate islet, some topographical features of the reef terrace may be identified as ancient outer grooves.
57,'45' E
57"30' _19"50'S
--~20
'P:latt~l ~]a sb~t I/
"O0 "S
%
baie de I' arsena
03
.%
o
I
5 7 "40" E
por co O
flic enflac Fig. I - A . Location of Pleistocene emer-
vieux grand
O
pet(re riviera noirre ..* benitiers i s l e t ~
ne ~"
• " "i~let
I°°: ,°
.%
o•,
baie
du
beau champ
ged marine outcrops, Mauritius island. Localisation des affleurements de calcaires marins ~merg~s d'~lge pleistocene, ile Maurice. Fig. 1-B . Windward
reef
platform,
Plate islet. Plate-forme r~cifale Vent>>, lie Plate•
ii
. . . . . m o d e r n reef f r o n t [] pleistocene reef outcrops
10 km
<
Fig. 1-C . Leeward fringing reef, Baie
de l'Arsenal.
A
R~cif frangeant <
zone
reef-crest
++++++ B
,
i
++++++q-+. VOlcanic ++++q-++-t-+++++++~-r
rock"
m l t = n i v e a u des basses m e r s
:
i
~j_~'~ ' , a S e
'
littoral zone R ;:::~ backraef :::::1 zone :::::.~-.~ C
i~f
moyennes.
' ~ . r ~ ~
++-t-~
I ~: IN
f
reef-crest zone ~ __ J , , , , ~ ,-'---7 ,-i
I
+ + _~ + +~°'~a,,i~--Yo~k ~;~,;,-ent +++ ~-+÷++++++++
i
I
i
I
2 m
= I \
+ -~ + +'+ + ~- ~-~'-~+++++++++++
reduced : Acropora dana+, Acropora sp. (28 %), Leptoria phrygia (20 070), Favites abdita and Goniastrea retiformis (9 %), Pocillopora damicornis (22 %), Porites sp. (1%). Such a composition indicates that this reef crest belongs entirely to the coral subzone in spite of the local exuberance of melobesian algae ; it has to be considered typical of high energy environments by comparison with the biological zonation of modern Mauritian reefs. Generally speaking, these coral diversity patterns are valid in any given terrace;however, on sheltered exposures (Petite Rivi~re No+re, Flic-en-Flac), Acroporidae tend to prevail strongly over Faviidae.
1 - Frame-builders. The basic facies pattern comprises two main elements differentiated on the basis of principal framebuilder distribution • (a) the inner scleractinian coral subzone ; (b) the outer coralline algal subzone, Contrary to its modern equivalent, the reef-crest is generally devoid of hydrocorals (Millepora). This absence may be ascribed to the burying or the destruction of the outermost parts of reef flats which correspond generally to their typical habitat. Determination of biotic composition from quadrat was made on the well-preserved reef flat of Plate islet ; 4 m2 quadrats were spaced horizontally about 3-5 m apart, according to G. Scheer's method (1972). The relative abundance of each contributor was established as follows. Coralline algae (Porolithon, Lithophyllum, Lithothamnium and subordinate Lithoporella, Archaeolithothamnium) locally have a high amount of cover (40 070). Coral assemblages at least represent 60 070 of the framework, but the number of species is
Point-count analyses indicate that main builders represent 22 to 70 070 of the total rock volume (Table 1 ; P I . 2, fig. 1,2). Subordinate builders may be important volumetrically (4-11 070 of the organic frame);in decreasing order of abundance, they include : encrusting foraminifera (Miniacina miniacea, Carpenteria monticularis), gastropods (Vertretidae), pelecypods (Modiolus), bryozoans (Cheilostomata) and serpulids (P1. 2, fig. 2,3,5,6).
,
!
!
!
!
! L
! v
! !
! !
L ,
! !
.iP°rt Louis !
~ Marianne ! islet
! ! vBaie
v Baie de ".I 'Arsenal
v'. P l a t e v islet
Gabriel
! .............................. v
! ........... !
! ...................... !
! .......... !
...........
T ..........
[Number
v
v
of thin sections
-i . . . . . . . . . . . . . . . . . . . . . . . . . . . v
~Number
of
points
12
.-, i ........... !
v
counted
3816
v
7368
! ...........
~Frame-builders
"i
".
i 31,851,5 + ' Corals ....................... i 22,6-1,3 ' Coralline algae .............. i 5 9~0,8 ' + ,' Secondary builders ........... ~. 3,3-0,6 !Primary
cements
! Needle ....................... ' Palisade ..................... ! Micrite ...................... ~Internal
! ! ! !
0,1i0,] 0,2+'0,1
~ 30,3-1,5
sediments
+
25,3-1,0 v"I 26 , 0~I,0 0,9+0,1 v 53,6-1,1
!Secondary ,
15,6+.I,2
I 21,7+'0,9
!
cements
and
matrices!
!
Tabl.
1 -
Mean
constituent
dence
interval).
+
v
i
i
Composition
moyenne
de confiance
~ 5 % de risque)•
2,8~0,3
22
I. . . . . . . . . . . . . L
..............
I
5598
~
6930
! ............
31
islet
17
16
.I . . . . . . . . . . . . . . . . !
T
!..........
l ..........
I
v ...............
! T
~-;
9765
5423
.........
5217 ~ ..........
+ 69,2-I ,3 22,2+-1 + ,0 42 , 5-I + ,3 4,5-0,5
2,0-+0,4 + 2,0-0,4
I
"I
2 , 9-0,4 +
i
2,8-0,4
! !
+
+
I• 3 6 , 7 -+1 ' 3 v" 3 1 , 0 5 1 , 2
2,5+-0,4 + 0,1-0,08
I
40,0-1,2
3 1 , 6 !+1,
1
5,350,5 3,1-0,4 + 0,8-0,2 +
0,2-0,1
+
•I 4 7 , 0 -+I ,4
29 , 8-I ,2 I + I 13,6-0 + ,9 , 3,6-0,5
+ 69,8-0,9 + 8 4, - 0+ 6 , 58,2-I + ,0 3,2-0,3 + 0,2-0,1 + 0,7-0,2
+ I ,7-0,4 + L ,2-0,3
! !
+ 65,351,3 4 1 , 1 - +1 , 3 21 , 3 -+1 , 1 2,9-0,4
! .I ! !
v ! !
I !
! I t I
,
!
2',7~0,4 0,5-0,2
I
11
, 6+0,8
1122,8+1,1 I 13,6+0,9
1,9+0,4
0,3+0,1 1 7 , 8 - 1 ,0
10,0+-0,8 3,2+.0,5
9,050,8 47,9-1,3 + 10,7-0,8 + 4,8-0,5
'f
42,6+.I,2
i I 2,3-0,9
+ 19 ,2-0,7 + 1,650,3 55,4-1,2
I
÷+
1 8 5, - 1+ , 0 2,750,5 36,4-1,3 12,9+.0,3 + 3,7-0,5
3,4+-0,4 + 1,2-0,3
f
1,5-0,6
++
2,4-0 + ,3 3,350,1 5,1-0,7
13,6+-0,7 + 1,5-0,2
I 1 6 , 3 -+1 , 1
! '
reef-crest
des platiers r~cifanx
zone,
Mauritius
pleistoc6nes
island
(The values
de Vile Manriee
!~
+ ' 9,3 0,8 t 1,350,3 30,1-1,2
i ~
1,3+.0,3
!
3,1+.0,4
1
I
of the Pleistocene
des s6diments
I ...............
18
!
i 12,5-I,0
composition
5796
Petite Rivi~re No+re
!
iJ + t" 7 ,8-0,9 + " 0,9-0 3 i 39,7-1,5
P e r o s i t ~ ................
!
i............
+ 0,150,07 1,4-0,2
i Mud-sized matrix ............. • Skeletons • ' .................. • Volcanic grains iTotal cements + sediments .....
iTotal
Cap
!
v 22,750,9 i 16,7-0,8 + 3,5-0,4 v' 2,4+.0,3 ! ! ! !
du
! ............ !
18
!. . . . . . . . . . . . . . . . . . . . . . i. !
! ...........
Champ
!
24
! ..............................
• Total ........................
Beau
!
l
are tabulated
(Les valeurs
u s i n g t h e 95 % c o n f i -
sont calcul6es
avec un intervalle
-
2 - Internal
sediments.
The amount of sediments occurring within the pores of framework varies largely even within a single cavity (P1. 2, fig. 1). The recognizable granule-and sandsized particles represent 2 to 27 % of the total rock (Table 1). They change broadly in nature depending on the size of catching pores : corals (25-45 % of the total skeletal elements), melobesians (7-68 %), mollusks (5-10 %), benthic foraminifera (2-8 %), alcyonarian spicules (less than 1 % ) developing localized spiculites, miscellaneous skeletons such as crustaceans, echinoids, serpulids and articulated algae (Halimeda, Corallina) (2-3 %). Foraminiferal assemblages strictly are similar to those described from the neighbouring modern reefs (Montaggioni, 1981) ; the most common types were identified as Amphistegina lessoniL A. radiata, Mar-
ginopora vertebralis, Calcarina calcar, Elphidium crispum, Carpenteria, Miniacina, Planorbulina, Alveolinella boscii, Heterostegina depressa, Textularia agglutinans, several Quinqueloculina and Triloculina species (Fig. 2A). The silt-sized fraction is mainly composed of tunicate spicules (P1. 3, fig. 2), unidentified skeletal debris, associated with faecal pellets and terrigenous elements. Intraparticle pores preferentially exhibit wakestones and mudstones while grainstones and packstones are the usual fillings of interparticle cavities (P1. 3, fig. 1,2,3,4). 3 - Primary
cements.
Several fabrics have been recognized : a) INITIALLYARAGONITECEMENTS.
Needle cements (continous or irregular linings of subhedral fibrous crystals 30-200).trn long and 2-10 ,um wide) occur preferentially in the chambers of corals and mollusks (P1. 2, fig. 3,5). Their amount ranges between 0,1 and 2.7 % of the total rock (Table 1). Spherulitic cements are strickly localized within conceptacles of coralline algae (P1. 2, fig. 4). J.H. Schroeder (1973) pointed out that their scarceness is chiefly due to the ease which they are altered. b) INITIALLYHIGH MAGNESIANCALCITE CEMENTS. Microcrystalline (micrite) cements (2-15~m in size) are ubiquitous, but they mainly occur in sheltered pores produced by reef-builders, as structureless masses or oval peloids (P1. 3, fig. 4). Optically undistinguishable from the biogenic mud-sized fraction, they
166
-
reach concentrations of 10-30 070 of the total limestones (Table 1). Palisade cements (very regular rims 30 to 70 jam thick) generally infill the intergranular voids of internal sediments and more rarely the internal cavities of organisms. Their amount is lower than 2 °10. B - BACKREEF ZONE
Backreef deposits consist of coarse to medium grainstones, packstones and wakestones (Table 2). 1 - Allochems.
Allochems which represent 27 to 69 % of the bulk rock are of two types. a) SKELETALCOMPONENTS. Their amount varies from 61 to 88 °70 of the total allochems except at Gabriel islet (2 e/0). These constituents broadly show the same mode of occurrence as the reef-crest units (P1.3, fig. 5,6,8). In general order of abundance, the grains a r e : c o r a l s (mean value : 39.4 e/0), melobesians (33.3 %), mollusks (9.9 %), foraminifera (8.2 %), Halimeda (3.0 %), alcyonarians (2 %), echinoids (1.5 %), tunicates (1.2 %), crustaceans (0.9 %), sponges (0.4 %) and articulated red algae (0.3 %). As for foraminiferal assemblages, they are similar to those from the reef-crest environment (Fig. 2B) ; Amphistegina, Calcarina, Quinqueloculina, Marginopora and Homotrematidae (Miniacina, Carpenteria) remain dominant types. b) NON-SKELETALCOMPONENTS. Three distinct categories are recognizable : lithoclasts, as fragments of adjacent boundstones (0.5 % 4.4 % of the total allochems) ; peloids (P1. 3, fig. 7), probably faecal pellets of invertebrates in origin (0-6.2 % ) ; s a n d - s i z e d volcanic particles (mean value : 6.9 %). 2 - Primary
cements
and matrices.
The percentages of these infillings show large variations (0.9-41% of the total rock). The intergranular strongly cristalline cements are often absent. This lack is probably as observed frequently in loose external sediments of Holocene reefs (Montaggioni, 1978). When they are present, these cements occur strictly as packed acicular crystals (3060,um long, 1.5-8pm wide) probably aragonite in origin and occupy generally less than 1 % of the total rock volume (PI. 4, fig. 1,2,3). In contrast, palisade fringes seem to be totally absent. This is in accordance with previous investigations on the Bahamas (Cloud, 1962, Traganza, 1967) and the Persian Gulf (Evans,
--
167
--
3.2
'~I~
7.~ ~
A
REEF-CREST ~ ZONE .~vo
~
, /
0o.2°
.•Z
~'~
oP
vO ~o
7.0 3,2
B "L,5
.oo
~z
•
Fig. 2 -
~'
ZONE
6samples ~
leeward
C
areas
6samples
windward a r e a s
Relative a b u n d a n c e o f the m a i n F o r a m i n i f e r a l types on the P l e i s t o c e n e e m e r g e d m a r i n e l i m e s t o n e s o f M a u r i t i u s . - A : r e e f - c r e s t z o n e . - B : b a c k r e e f zone. - C : littoral zone. A b o n d a n c e relative des p r i n c i p a u x t y p e s de F o r a m i n i f ~ r e s d a n s les calcaires m a r i n s ~merg~s pleistoc~nes de M a u r i c e - A : platiers r~cifaux. - B : z o n e s d'arri~re-r~cif. - C : zones littorales.
- 168 -
1966 ; Evamy, 1973) concerning the prevailing appearance of aragonite cements in backreef areas. This may be due to the inhibition of calcite nucleation by some cations present in sea water during sediment deposition (see Milliman, 1974, p. 13-14).
grainstone and packstone volume and about 33 °70 of the wakestones. The major part of the fraction finer than 40)a m is composed of amorphous allophane, montmorillonite, metahalloysite and gibbsite ; such a composition closely reflects the intense tropical weathering conditions of neighbouring volcanic source zones.
The amount of micrite cements and associated silt and clay-sized matrices averages 1.5 % of the total
!
!
I
!
!
!
!
!
!
!Port Louis!
!
Benitiers
Flic-en-Flac
islet
Baie de
!
t
I
i
I
!
! [Rock types ~ ! -~ !
A - B
!
! A - B
[
T
!
.--
C
A - B
C
A - B
5
i
l
4
2
6
of
points
1612
counted
i
!
! iPrimary
cements
and
+ 43, 8-2 + ,4 3 6 , 3 -+2• 3 0 , 2-0 + ,2 + I , 2-0 5 I ,8+- 0 , 8 5,3-1,0
matrices
.......................
~Needle
iPalisade
.....................
;Micrite
+ M a t r i x ..... ....... o
iTotal I
cements
1,2-1,1 0,5+0+ ,7 0,350, 6 2,1-1,5
+
I T a I ! !
0,7-0,5
t
0,2-0,5
................
i t
,0 i 2 3 , 4 + 4 , 4
2,5..I
0,9-0,5 +
3,2.+1,1
, 23,6+4,4
40,3.+3,1
[ 38,2-+5,0
+
7,9-1,3
8,4.*1,8 i 11,4-+3,3
Mean constituent composition of the Pleistocene nes, C = wakestones (The values are tabulated Composition moyenne A = grainstones;B 5 °70 d e r i s q u e ) .
- LITTORAL
639
......... + 38 , 4-3 + ,8 30 9-3 + ,9
2,0-0,8
0 • 9-0 + ,7 0,8-0 +, 7
1,8.*0,7 + 5,3-1,2
3,951,5 1,9-I,0
C
!
- ..............
! I
1 J 6-+0,7 + 2,1-0,8 + 55,6-2,7
1896 i .......... + 4 4 , 3 - +2 2 39,352,2 0,9-0,4 0,2-0,2 $
3,9-0,9 12,2.+1,5
+ 36 , 6 +- 3 , 0 31 , 2-2 + ,9 0,6-0,5 +0 , 3 -+0 ,3 4,5-1,3
5 4 1 3, - 3+8 , 41,3-3,8
; 1 6, - 0+6 , ]3,8-1,6
16,5.+2,9
30,4.*2,0
Z 33 , 6-2 9 + , 33,6-2,9 + 25,5-2,7
II,4~1,4
5,3+1,4
6,5..1 ,3
3,8.+1,5
!
4
9
. . . . . . . . . . . !. . . . . . . . . . . . . . . t . .
!
+ 55 , +9 - 1 , 8
+ 69,0-2,5+ t 58 , 4 =+ 2 , 7 t 2,550,9 •i 0 ,8 -+ 0 , 5 ' 5,2-I + ,2 ~ 2,1-0,8 +
l
2862
•~
! !
+ 0,5-0,4
backreef zone, Mauritius u s i n g t h e 9 5 °70 c o n f i d e n c e
des s6diments des milieux d'arri~re-r6cif pleistoc~nes = packstones, C = wakestones (Les valeurs
ZONE
1 - Allochems. Littoral facies contain between 47 and 69 °70 clastic grains (Table 3). SKELETAL
A - B
COMPONENTS.
The distribution of the various categories of bioge-
island ; Rock interval)•
types*
:'A
.I
I !
1 8 , 2 -+1 , 4 I 1,6-0,5 t !
T , " t t
0,9-0,5 T 5 1,2-0 6 32,7~1,7 + , 2,1-0,8 32,7-1,7 + + ! 25,2-2,4 9,0-I,0 + + 2,4 0,6 !t 3 , 8 - 1 , 0
= grainstones,
! ! !
34,351,7 n 0,650, 3 u 1,250, 4 u
t! I ! ! , I
v
The littoral exposures exhibit two major rock types : fine grainstones or wakestones, generally similar to the adjacent backreef environment.
a)
!
i
t
+
0 ,9.*0 +, 5 43,2-2,4
matrices! !
Porosit~
2 -
and
+ 32 , 3-2 + ,6 22,6-2,3 0 , 6-*0,4 +
-
iTotal ........................ [Secondary I
1284
356
+ + 4 8 , 1 -÷3 J2 ! 27 ,2-4 ÷ ,6 39 J 5-3 1 v 23 I-4 ÷ , , + ,4
2,150, 9 1,450, 7 0,5-0+ ,4 4,6-1,3
i
]
! 972 [ 1292 ! .............................. f
?
!Allochems !Total . . . . . . . . . . . . . . . . . . . . . . . . . . !Skeletons. . . . . . . . . . . . . . . . . . . . . . !Lithoelasts .................... lPeloids... ..................... !Volcanic grains ..... ........... !Unidentified ...................
C
957
I
!
3
, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
........................................
! !
islet
!
I
! !Number
! Gabriel
! ~
!
3
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Tabl.
C
! ! ...................................................................................................... ! !
S N u m b e r of thin s e c t i o n s
!
! I
l'Arsenal
!
B = packsto-
de Vile Maurice ; nature des facies p6trographiques sont calcul6es avec un intervalle de confiance
"
nic particles is dependent upon the extent of reef tracts. Indeed, the littoral outcrops related to narrow reefs (Baie de l'Arsenal, Gabriel and Plate islets) display a composition similar to that of adjacent backreef l i m e s t o n e s , i.e. a coralgal g r a i n s t o n e facies:corals (39.4-53.0 o70 of the total biogenic detritus) ; melobesians (22.0-35.1 o70) ; mollusks (8.015.1 o70) ;foraminifera (4.1-12.9 °70) ; alcyonarians (0.8-6.2 %) ; echinoids (1-1.8 %), Halimeda (0.9-3.3 o70) ; miscellaneous skeletons (0.2-0.5 °70) (P1. 4, fig. 7). In contrast, the Vieux Grand Port limestone is of mollusk wakestone type (40.5 °70 mollusk fragments
- li59 --
versus 26.3 % corals and 17.8 % melobesians) (P1.4, fig. 4-6) ; this suggest that the general form of Pleistocene reef complex of Mahebourg was most likely similar to the one of the modern adjacent barrier reef. In addition, foraminiferal assemblages are roughly identical with the reef-crest and backreef ones (Fig.
! ! !
2C). b) NON-SKELETAL COMPONENTS.
They are relatively minor sediment contributors ; lithoclasts (0.4-4.6 % of the total allochems), peloids (0.5-1.5 %), volcanic particles (1.3-6 %).
!
!
!
! Vieux Grand
! Baie d e ! l'Arsenal ! !
! Plate
!
!
!
!
! Number of thin sections
!
Port
5
5
!
islet
!
l
!Gabriel !
! l ............
!
!
!
4
islet! ! ----!
6
! !
!
l
t Number of points
! I ! ! I I !
counted
Allochems Total ........................ Skeletons .................... Lithoclasts .................. Peloids ...................... Volcanic grains .............. Unidentified .................
!
I ! ! ! i
!
1665
! ! 47,2 ! 36,6 ! 2,2 ! 0,3 ! 1,4 I 6,7
1640
2,4 2,3 0,7 0,3 0,6 1,3
!
Primary cements and matrices ! Needle ..... ! Palisade ..................... i 6,3 ~ 1,2 Micrite + Matrix ............. ! 12,5 ~ 1,6 Total ....................... .! 18,8 - 1,9
! Secondar~ cements and ! matrices .....................
! ! 31,6
' Total_ P'~rosity ............... !
1344 !
!
! !
!
l
! 47,9 ++-2 -.2'4 ! 37,2 +- 0,3 ! 1,2 -+ 0,5 ! 0,7 +- 0,4 ! 0,6 + 1 , 4 l 6,2 ,2
! 69,3 l 57,9 ! 0,3
2,5 2,6 0,3
l l l
0,4 4,1 6,6
0,3 1,0 1,3
l v
8,7
+ - 1,5
l ! 1
+ 1 ,2 - 0,5
l I
1,2
- 0,5
!
8,7
+
+~ 2 , 2
2,4 - 0,7
1926
!
+
! 41,7 ~ 2,4 ! 11,2-
- 1,5
1,5
l 13,7 ! 8,3
$ -
! 58,9 ! 49,0 ! 1,5 ! 0,4 ! 3,7 [ 4,3
!
2,2 2,2 0,5 0,3 0,8 0,9
!
v 21,3 - 1,8
[
t 21,3 ~- 1,8
l
!
[ ! l l
! !
+
1,8 1,5
! 14,9 ~ 1,6 ! 5,9 - 1 , 0
l !
Tabl. 3 - Mean constituent composition of the Pleistocene littoral zone, Mauritius island (The values are tabulated using the 95 o70 confidence interval). Composition moyenne des s6diments des milieux littoraux pleistoc6nes de rile Mauriee (Les valeurs sont calcul6es avec un intervalle
de confiance ~t 5 % de risque).
2 - Primary cements and matrices.
The best represented marine cements within the Pleistocene intertidal deposits consist of very regular fibrous rims (25-50 ~m thick) probably originated as high-magnesian calcite (P1. 4, fig. 5-8). They reach levels of 6°3 to 21,3 °70 of the total rock (Table 3). P r i -
mary aragonite crystals are relatively less common (1,2-8.7 °7o). Whatever their primary mineralogy might be, the strongly cristalline cements are always present inside the littoral outcrops. Consequently they are thought to be relict beach-rocks. The commonness of palisade cements suggests that environmental conditions rather favoured calcite precipitation : crystal
--
growth of calcite might be started by the partial dilution of magnesium ions dissolved in beach interface waters in consequence of seaward migration of the ground waters. This assertion seems to be in accordance with R.J. Russell's observations (1962) about the predominating occurrence of calcite cements in the majority of beach-rock outcrops f r o m the Carib-
170
--
bean sea and the Pacific Ocean. Microcrystalline fillings mixed with a detrital matrix are only found within the Vieux Grand Port limestone where they are particularly c o m m o n (12.5 °70 of the total rock).
CONCLUSIONS The reconstruction of Pleistocene marine carbonate environments at Mauritius island indicates that their spatial distribution, biotic associations and sediment composition are homologous in all respects with those of the adjacent modern counterparts. (1) Emergent buildups are mainly of platform or fringing types ; generally, they did not exceed 15 to 20 meters wide. (2) The reef-crest zone is distinguished by the prolific occurrence of massive coral growth (Acroporidae, Faviidae) and of encrusting coralline algae. Secondary builders consist chiefly of coelobiteforaminifera (Carpenteria, Miniacina) and of gastropods (Vermeti-
dae). (3) Reef-crest, backreef and adjacent littoral zones are respectively typified by coralgal grainstones, packstones and wakestones rich in benthonic foraminifera. In contrast, mollusk wakestones are a signature of distant littoral areas. (4) The other skeletal constituents (alcyonarians, echinoderms, Halimeda, bryozoans) and non skeletal
carbonate elements are minor or absent. (5) Foraminifera assemblages are characterized by the general plenty of Amphistegina tests associated with the other larger forms typical of the indo-pacific province (Marginopora, Calcarina, Alveolinella, Heterostegina, Elphidium) and with smaller Miliolids and Textulariids. In addition, Carpenteria and Miniacina play a m a j o r role in skeletal production. However these encrusting foraminifers are more abundant in sediments f r o m exposed windward reefs than in those f r o m sheltered leeward reefs. Such a disparity may be due to local water energy conditions, the desintegration of encrusting organisms being favoured in high energy areas. (6) Primary voids of reef framework were infilled by both aragonite and calcite submarine cements. In contrast, localized aragonite cementation preferentially occurred inside backreef deposits while littoral sediments were chiefly affected by an almost generalized precipitation of initially high magnesian calcite.
REFERENCES BATTISTINI R. & alii (1975) - l~16ments de terminologie r6cifale indo-pacifique. Thetys, Marseille, vol. 7, fasc. 1, p. 1-111. CLOUD P.E. (1962) - Behaviour of calcium carbonate in sea water. Geochim. Cosmochim. Acta, Northern Ireland, vol. 26, p. 867-884. DUNHAM R.J. (1962) - Classification of carbonate rocks according to depositional texture. I n : W . E . HAM (edit.). Classification of carbonate rocks. Amer. Assoc. Petrol GeoL, Mem. 1, Boulder, Colorado, p. 108-121. EVAMY B.D. (1973) - The precipitation of aragonite and its alteration to calcite on the Trucial coast of the Persian Gulf. In:PURSER B.H. (edit.), The Persian Gulf, Springer-Verlag Edit., Berlin, p. 329-342. EVANS G. (1966) - The recent sedimentary facies of the Persian Gulf. Phil. Trans. Roy. Soc. London, ser. A,
vol. 259, p. 291-298. FAURE G. (1975) - l~tude comparative des r6cifs coralliens de l'archipel des Mascareignes (Ocean Indien). Bull. Mauritius Inst., Port-Louis, vol. 8, fasc. 1, p. 1-33. FAURE G. & MONTAGGIONI L. (1976) - Les r6cifs coralliens Au-Vent de l'~le Maurice (Archipel des Mascareignes, Oc6an Indien). Mar. GeoL, Amsterdam, p. M9M16. MC DOUGALL I. & CHAMALAUN F.H. (1969) - Isotopic dating and geomagnetic polarity studies on volcanic rocks from Mauritius, Indian Ocean. GeoL Soc. Amer. Bull., Boulder, Colorado, vol. 80, p. 1419-1442. MILLIMAN J.D. (1974) - Marine carbonate : Part 1 Springer-Verlag Edit., Berlin, p. 1-375. MONTAGGIONI L. (1974) - Coral reefs and Quaternary
-
shorelines in the Mascarene archipelago (Indian Ocean). Proc. Second. Intern. Coral Reef Symp., Great Barrier Reef Commitee, Brisbane, vol. 2, p. 579-593. MONTAGGIONI L. (1978) - Recherches g6ologiques sur les complexes rdcifaux de l'archipel des Mascareignes (Ocdan Indien). D. Sc. Thesis, Universitd d'AixMarseille H, 2 vol., p. 1-524. (unpublished). MONTAGGIONI L. (1981) - Les associations de Foraminif~res dans les s6diments r6cifaux de l'archipel des Mascareignes (Oc6an Indien). Ann. Int. Oc~anogr., Paris, voI. 57, fasc. 1, p. 41-62. MONTAGGIONI L. & MAHI~ J. (1980) - Caract6risation faciologique des s6diments r6cifaux de l'~le Maurice par l'analyse factorielle des correspondances. Oceanol. Acta, Paris, vol. 3, fasc. 4, p. 409-420. PICHON M. (1967) - Caract~res g6n6raux des peuplements
171
-
benthiques des r6cifs et lagons de l'~le Maurice. Cah. O.R.S.T.O.M., Ser. Oc~anogr., Paris, vol. 5, fasc. 4, p. 32-45. RUSSELL R.J. (1962) - Origin of beach-rock. Zeitch.f. Geomorphol., Berlin, vol. 6, p. 1-16. SCHEER G. (1972) - Investigations of coral reefs in the Maldives islands with notes on lagoon patch reefs and the method of coral sociology. In : C MUKUNDAN & C.S.G. PILLAI (edit.). Proceed. First Intern. Coral Reef
Syrup., Madapam Camp, Marine Biol. Assoc. India, p. 87-120. SCHROEDER J.H. (1973) - Submarine and vadose cements in Pleistocene Bermuda reef rocks. Sedim. Geol., Amsterdam, vol. 10, fasc. 3, p. 179-204. TRAGANZA E.D. (1967) - Dynamics of the carbon dioxyde system on the Great Bahama Bank. Bull. Mar. Sci., Miami, vol. 17, p. 348-368.
M a n u s c r i t d6finitif requ le 23.12.1981
--
172
--
PLANCHE 1
Fig. 1 - Cross-section from the Older Limestone Units (Port Louis). The lower reef flat is overlain by a water-formed dune. Coupe dans I'Unit6 des Calcaires Inf6rieurs (Port Louis). Le ptatier r6cifal sous-jacent est recouvert par une dune hydraulique.
Fig.
2
-
Elevated reef platform at Plate Islet (Younger Limestone Units). The maximum thickness of this buildup is meter.
1.50
Plate-forme r~cifale ~merg~e de l'lle Plate (Unit~ des Calcaires Sup~rieurs). L'6paisseur maximale de la bioconstruction est de 1.50 m~tre.
Fig. 3 - Boulder ridges from the reef platform at Plate Islet. Levee d~tritique sur la plate-forme r~cifale de I'Ile Plate..
G~obios n ° 15, fasc. 2
PI. 1 L. Montaggioni
PLANCHE 2
Fig. 1 -
Coral-coralline algal boundstone, reef-crest zone. Calcalre construit i Coraux et Algues caicaires, platier r~cifal.
Fig. 2 -
Coralllne algal- Vermetid boundstone, reef-crest zone. Calcalre eonstruit A Algues calcalres et Verm~tid~s, platier r~cifal.
Fig. 3 -
Detail from the melobesian - Vermetid boundstone.
D~tall d'un calcaire A M61ob~si~es et Verm~tidds.
Fig. 4 -
Spherulitic cement within a coralline algal conceptacle, reef-crest zone. Ciment sph~rolitique dans un conceptacle d'algues calcalres, platier r~cifal.
Fig. 5 -
Chambers o f Vermetid shells completely infilled with epitaxial acicular cements, reef-crest zone.
Chambres de coquilles de Verm~tid~s totalement colmat6es par des ciments aciculaires h croissance dpitaxiale, platier
rdcifal.
Fig. 6 -
Encrusting foraminiferal
(Carpenteria) bryozoan boundstone, reef-crest zone. (Carpenteria) et Bryozoaires, platier
Calcalre constmit A Foraminif~res encrotitants
r~cifal.
Observations under reflected light. Observation en lumi~re naturelle. Abbreviations used : SCL = Scleratinian corals ; CO = Coralline algae ; G A = Gastropods ; FM = encrusting Foraminif e r a ; BR = Bryozoans ; IS = internal sediments, P = biologically bored cavities ; cs = secondary blocky cement ; an = needle cement. Abrdviations utilis6es : SCL = Scl6ractinialres ; CO = Algues calcalres ; G A = Gastropodes ; F M = Foraminif~res encrofitants ; BR = Bryozoaires ; IS = s~diments internes ; P = cavit6s de perforation d'origine organique ; cs = ciment granulaire primaire ; an = ciment fibreux.
G6obios n ° 15, fasc. 2
PI. 2 L. Montaggioni
PLANCHE 3
Fig. 1 -
Internal wakestone, reef fiat. S~diments (wakestone) internes, platier r~cifal.
Fig. 2 -
Internal wakestone rich in tunicate spicules, reef flat. Wakestone interne i spicules de Tuniciers, platier r~cifal.
Fig. 3 -
Coralgal-foraminiferal gralnstone, reef flat. Gralnstone A ~l~ments coralliens, algaux et Foraminif~res, platier r6cifal.
Fig. 4 -
Peloids infllling a coral intraskeletal cavity, reef fiat. Pelo'ides remplissant une cavit~ corailienne intra-squelettique , platier r~eifal.
Fig.
5
-
Backreef algal-foraminiferal grainstone. Grainstone / algues calcalres et Foraminif~res, zone d'arri~re-r~cif.
Fig. 6 - Backreef foraminiferal packstone. Packstone i Foraminif~res, zone d'arri~re-r~cif.
Fig. 7 - Backreef peloidal gralnstone. Gralnstone i pelo'ides, zone d'arri6re-r~cif.
Fig. 8 -
Backreef bioturbed wakestone. Wakestone i bioturbations, zone d'arri~re-r~cif.
Observations under reflected light. Observations en lumi~re naturelle. Abbreviations used : F M = M i n i a c i n a detritus ; F A = A m p h i s t e g i n i d a e ; CO = Coralline algae ; SCL = Sclaractinian corals ; FC = C a r p e n t e r i a detritus ; F P = Peneroplids tests ; an = needle cement ; cs = secondary blocky cement ; P = bioturbation cavities. Abr6viations utilis6es : F M = d6bris de M i n i a c i n a ; F A = A m p h i s t e g i n i d a e ; CO = algues calcalres ; SCL = Scleractiniaires ; FC = d6bris de C a r p e n t e r i a ; FP = tests de P e n e r o p l i d a e ; a n = ciment aciculalre ; cs = ciment granulaire secondaire ; P = cavit6s de bioturbation.
G6obios n ° 15, fasc. 2
PI. 3 L. Montaggioni
PLANCHE 4
Fig. 1,2,3 - I r r e g u l a r a n d p a c k e d linings o f i n t e r p a r t i c l e needles, b a c k r e e f z o n e . F r a n g e s irr6guli6res d'aiguilles, dispos6es e n f a l s c e a u x , i la p6riph6rie des particules, z o n e d ' a r r i 6 r e - r 6 c i f .
Fig. 4 -
Littoral mollusk - foraminiferal wakestone. W a k e s t o n e i m o l l u s q u e s et f o r a m i n i f 6 r e s , z o n e littorale.
Fig. 5 -
I n t e r p a r t i c l e p a l i s a d e c e m e n t , littoral z o n e . . C i m e n t p a l i s s a d i q n e i n t e r g r a n u l a i r e , z o n e littorale.
Fig. 6 -
D e t a i l o f a lithoclastic w a k e s t o n e , littoral z o n e . D6tall d ' u n w a k e s t o n e ~t lithoclastes, z o n e littorale.
Fig. 7 -
Skeletal p a c k s t o n e , littoral z o n e . P a c k s t o n e b i o d 6 t r i t i q u e , z o n e iittorale.
Fig. 8 -
P a l i s a d e c e m e n t e n v e l o p i n g t w o Amphistegina tests, littoral z o n e . C i m e n t p a l i s s a d i q u e e n v e l o p p a n t d e u x tests d'Amphistegina, z o n e littorale.
O b s e r v a t i o n s u n d e r reflecte~d light. O b s e r v a t i o n s e n lumi~re naturelle. A b b r e v i a t i o n s u s e d : S C L = S c l e r a c t i n i a n corals ; C O = C o r a l l i n e algae ; F A = Amphisteginidae ; LI = L i t h o c l a s t s ; a n needle c e m e n t ; c m = m i c r i t e c e m e n t a n d m u d - s i z e d s e d i m e n t s ; cp = p a l i s a d e c e m e n t ; cs = s e c o n d a r y b l o c k y c e m e n t . A b r ~ v i a t i o n s utilis6es : S C L = Scl6ractiniaires ; C O = algues calcaires ; F A = Amphisteginidae ; L I = L i t h o c l a s t e s ; a n c i m e n t aciculaire ; c m = c i m e n t m i c r i t i q u e et silts ; cp = c i m e n t p a l i s s a d i q u e ; cs = c i m e n t g r a n u l a i r e s e c o n d a i r e .
G6obios n ° 15, fasc. 2
PI. 4 L. Montaggioni