Sedimentary Facies from the Modern Coral Reefs,. Jordan Gulf of Aqaba, Red Sea. C. Gabri61 and L. Montaggioni 2. 1 Laboratoire de G6ologie, Universit6 ...
Coral Reefs
Coral Reefs (1982) 1: 115-124
9 Springer-Verlag 1982
Sedimentary Facies from the Modern Coral Reefs, Jordan Gulf of Aqaba, Red Sea C. Gabri61 and L. Montaggioni 2 1 Laboratoirede G6ologie,Universit6 Franeaise de l'Oc6an Indien, B.P. 5, F-97490 Sainte-Clotilde,France D.O.M., and 2 Laboratoirede G~ologieMarine, Universit~d'Aix-Marseille2, Luminy,F-13288 Marseille, France Received 28 December 1981; accepted 1 February 1982
Summary. Sixty-three sediment samples collected from the modern fringing reefs off the Jordan coast (Gulf of Aqaba, Red Sea) have been analysed in order to determine variations of composition and texture by using correspondence factor analysis. From the shore seawards, the following physiographic zones are recognized: beach; shallower backreef zone; reef flat zone; forereef zone including sandy or coral-built slopes. Eight sediment facies and subfacies can be recognized on the basis of total component composition and foraminiferal assemblages and four sediment facies can be recognized using gra!n-size data. Wellsorted, fine to medium, quartzofeldspathic sands (terrigenousfacies) occur on beaches and outer sandy slopes close to wadi mouths. Backreef areas exhibit relatively well-sorted fine sands of terrigenous-coral and MiliolidaeSoritidae facies. Poorly sorted, coarse sands of coral-coralline algal and Homotremid facies characterize reef flats and the upper parts of coral-built forereef areas, which respectively display an Amphistegina-Spirolina subfacies and an Acervulina one. Poorly sorted, medium sands of eoral-
molluscan-foraminiferal (Amphistegina-Acervulina) facies are restricted to the lower parts of the forereef zone. Lateral limits of the various biofacies coincide with the distribution of the related organic communities.
Introduction The Gulf of Aqaba, which forms the northern extention of the Red Sea, is part of the Syrian-African rift system (Freund et al. 1970), extending further North to the Jordan and the Dead Sea valley (Fig. 1A); its length is about 180kin, width 15 to 25kin and depth up to 1,800m. Living coral reefs are extensively developed along the coasts of the Gulf, where the predominant type is the fringing reef. The morphology and/or ecology of these modern reefs has been studied first by Walther (1888), and more recently by Por and Lerner-Seggev (1866), Friedman (1968), Loya and Slobodkin (1971), Fishelson (1970, 1971), Mergner (1971), Gvirtzman et al. (1977), Gvirtzman and Buch-
binder (1978), Guilcher (1979) and Sneh and Friedman (1980). Information about foraminiferal populations of these reefs can be found in Said (1950), Reiss (1959) and particularly in Reiss et al. (1977). Investigations of the modern reef sediments in the Gulf were reported by Emery (1963), Friedman (1968) and Erez and Gill (1977). However, all these studies were carried out on the western (Israeli) shore of the Gulf only, while the eastern reefs were long neglected by research workers. The first morphological and ecological investigations were undertaken in 1974 (Mergner and Schuhmacher 1974). Much more recently, Bouchon (1980) and Bouchon et al. (1981) published papers on zonation of hermatypic corals and on evolution of the Jordan reef tracts respectively. The purpose of the present study is to investigate possible correlations between sedimentary component types, particle sizes and depositional environments, and to define statistically recognizable sediment facies on the eastern reef area of the Gulf of Aqaba.
General Physical Conditions The climate of the Gulf of Aqaba is extremely arid. Average annual rainfall is estimated at as low as 25 mm and average annual evaporation is about 2,000 ram. The deficit is balanced by inflow of water from the Red Sea. The mean annual temperature of the water is 23.0 ~ and the water is hypersaline (41.5%o43%o) (data from Friedman 1968 and Gvirtzman et al. 1977). According to Hulings (1979), the generalized pattern of sea water circulation for the Jordan area of the Gulf is as follows: for the north coast, the predominating direction of wind-induced currents is an eastern trend along the littoral as well as offshore to a depth of 10 m. At depths of 20 and 50 m off the north coast, the prevailing direction offshore from the 1 to 50 m depths is southward. There is a general decrease in mean velocity with depth (10.1 to 7 cm/s). As a result, sediments move southward and eventually dump into the Gulf. The sea is usually calm; only rarely do wave heights exceed half a meter. Nevertheless,
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Zonation of Depositional Environments The Jordan coast is fringed by discontinuous and narrow living reefs (total length: 13 km; maximum width: 50 m). They are principally developed around headlands and are separated by predominantly sandy embayments that correspond to the mouths of river beds with a torrential r6gi-
Fig. 1. A Northern part of the Red sea and the Gulf of Aqaba; B distribution of the modern fringingreefs and related depositional zones along the Jordan coast; location of sampling lines
me (wadis) (Fig. 1B). The following natural environments can be distinguished on the well-developed reef areas, from the shore seawards (Fig. 2):
(1) The Beach Zone. Quartzofeldspathic material collected by nearby wadis from surrounding mountains are partly trapped onshore; they made up poorly developed beaches, about 20 m wide. Fauna includes mollusks (Littorinidae, Ostreidae), crustaceans (crabs, cirripeds). (2) The BackreefZone. Only present in the widest central parts of reefs, the backreef zone is a sandy channel, with a maximum width of 40 m and 1.5 to 2 m deep below mean sea level. Bottom sediment is colonized by scattered coral heads (Stylophora, Seriatopora, Platygyra, Mille-
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Fig.2A, B. Typical cross-sections of depositional zones along the Jordan coast, Gulf of Aqaba, A well-developed fringing reefs; B outer sandy slopes in the axis or at the vicinity of embayments. LT, mean low tides
Porites, v a r i o u s Faviidae). A series o f s a n d y p o o l s generally r e m a i n s c o n s p i c u o u s between 5 a n d l0 m deep; at d e p t h s o f 15 to 50 m, only small p o c k e t s o f sediment accum u l a t e at the surface o f coral p a v e m e n t s .
o f foraminifers i n h a b i t sea grasses (Soritidae).
(3) The Reef-Flat Zone. This zone, a b o u t 20 m wide, is a d e a d c o r a l p a v e m e n t , with a 0.5-1 m r e a r - r e e f step. C o r a l c o m m u n i t i e s are c o m p o s e d o f small-sized colonies (StyIophora, Seriatopora, Acropora, m a i n l y ) a s s o c i a t e d with hyd r o c o r a l s (Millepora). The reef p l a t f o r m is also covered with dense a l c y o n a r i a n colonies a n d coralline algae. M o l lusks (Tridacna, Vermetus, Trochus) a n d encrusting foraminifers ( H o m o t r e m a t i d a e ) are also fairly a b u n d a n t . Scattered p o o l s t r a p sands which h o u s e a v a r i e d e p i b i o t a (echinoderms, crabs) a n d e n d o b i o t i c mollusks. The reef f r o n t f o r m s a n e a r l y vertical d r o p - o f f , 2 - 4 m high, c h a r a c terized b y the occurrence o f Millepora a n d b r a n c h i n g red algae. (4) The ForereefZone. T w o m a i n types o f o u t e r reef slopes can be defined on the basis o f the n a t u r e o f substrates: loose s e d i m e n t a r y slopes, c o r a l - b u i l t slopes. The first type o f f o r e - r e e f is a s a n d y talus f o u n d in the vicinity o f e m b a y m e n t s . D e n s e m a t s o f scattered patches o f Halophila p l a n t s cover the b o t t o m d o w n to 50 m; they are i n h a b i t e d b y diverse c o m m u n i t i e s o f s e d i m e n t - f o r m i n g o r g a n i s m s such as mollusks, b r y o z o a n s a n d foraminifers ( A m p h i s t e g i n i d a e , Miliolidae, Soritidae). N o coral f o r m a tion has d e v e l o p e d between the d e p t h s o f 1 a n d 50 m. The c o r a l - b u i l t fore-reef is best d e v e l o p e d close to h e a d l a n d s . Its m a x i m u m d e v e l o p m e n t is achieved when the p a v e m e n t b e c o m e s c o n t i n u o u s with the reef front. T h e u p p e r p a r t o f the slope (0-20 m) is d o m i n a t e d b y b r a n c h ing g r o w t h forms (Stylophora, Seriatopora, Acropora, Echinopora). The lower f o r e - r e e f zone (20-50 m) exhibits prolific massive g r o w t h forms (Montipora, Astreopora,
Methods and Material Sixty-three surface sediment samples were collected along predetermined transect lines show in Fig. 1B from the shore to the depth of about 45 m. Grain-size analyses were made using the French AFNOR sieves. The results were expressed as weight percentage in each of the 22 size classes of the ~ / ~ millimetric scale used (from 5 to 0.04 mm). The values of three textural parameters (mean size, sorting, skewness) defined by Folk and Ward (1957) were obtained by means of the moment method. For the component analysis, in each sample, 1,200 to more than 2,000 grains were examined either under a binocular microscope or in thin sections. The grains were catalogued in 12 component categories (e.g. corals, coralline red algae, Halimeda,mollusks, benthonic Foraminifers, echinoderms, bryozoans, alcyonarians, sponges, crustaceans, serpulids and terrigeneous particles). In addition to that, the main foraminiferal forms were recognized from representative samples of the various depositional zones. Finally grain-size and compositional zones data were analyzed using correspondence factor analysis, which is Benz6cri's generalization of factor analysis (Benz6cri 1973). As shown in the pertinent literature (Lebart and F6nelon 1973; Jor~skog et al. 1976), in factor analysis, a data matrix consisting of Nv variables which describe No observations (or samples) may be imagined to define either M o points in a Euclidean space of Nv dimensions (variablespace)or to define My points in such a space of M o dimensions (observationspace). Factor analysis permits the projection of a large set of points into a very reduced space. Each point takes a part in defining axes (this property is termed absolutecontributionof point to axis); reciprocally, each factor axis contributes to define the position of a given point (relativecontributionof axis to point) (for more detailed explanations, see Lebart and F6nelon 1973); the values of these contributions vary within the range 0 to 1. In correspondence analysis, observations and variables play a symmetrical role; possible proximities between observation points and variable points can be measured simultaneously. Consequently, this method is particularlyuseful for the type of problem with which this paper is concerned because it permits a characterization of facies that takes numerous relationships into account simultaneously.
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Results
Recognition and Distribution of Facies Component Facies. The component distribution pattern was investigated in the following way. The variables (12 components) and observations (63 samples) were projected onto the same plan of factor axes; their positions relative to one another are shown on Fig. 3. Only axes 1 and 2, accounting for 77.2% and 10.1% of the total variance, are considered because the third axis has a low percentage (6.5% of variance). Factor 1 weights mineral particles (absolute contribution AC = 0.60) against skeletal components, particularly, coral (AC = 0.23) and mollusks. Factor 2 is largely concerned with an opposition between corals (AC = 0.13) associated with foraminifers and coralline algae (AC=0.68) in association with alcyonarians and echinoderms. Four groupings of samples can be recognized within the factor space [1, 2] (Fig. 3): Group A chiefly includes samples from the sandy outer slopes and beaches. Factor 1 has a moderate to heavy influence on this group samples (relative contribution RC varying between 0.45 to 0.98), while Factor 2 has a very weak to moderate influence on it (RC=0.02-0.48); Group B comprises certain samples from coral-built forereef areas and from backreef zones. The respective contribution of the first two factors to the spatial arrangement of these samples is as follows: moderate to heavy for factor 1 (RC=0.30 to 0.90), weak for factor 2 (RC, less than 0.01 to 0.15); Group C concerns most samples from the lower and intermediate parts of coral-built forereef areas. Factor 1 contributes moderately to heavily to the distribution of these samples (RC = 0.30-0.90), while factor 2 contributes
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Fig. 3. Plot of the first two factor axes of correspondence analysis. Both variables (12 component types) and observations (63 sediment samples) are plotted simultaneously. COR, corals; CAL, coralline algae; MOL, mollusks; FOR, foraminifers; ALC, alcyonariarts; BRY, bryozoans; SPO, sponges; ECH, echinoderms; SER, serpulids, HAL, Halimeda; CRU, crustaceans; MIN, mineral particles
very weakly to moderately to it (RC, less than 0.01 to O.6O); Group D relates to samples from reef flats and the upper part of coral-built forereef areas. The contribution of factor 1 to samples is moderate to heavy (0.48-0.98), that of factor 2 is weak (less than 0.01 to 0.30). Examining associations of variables and samples indiCates that terrigeneous grains control the groupings corresponding to reef areas receiving alluvium from neighbouring wadis (Group A and B). Within Group C, corals, foraminifers and mollusks permit coral-built slopes to be discriminated. Likewise, coralline algae mainly govern the sample grouping concerning the reef flat zone (Group D). The other components, displaying values of absolute contribution less than 0.02, are less informative; however, alcyonarians and echinoderms allow a valid discrimination for certain reef flat and front reef areas. Thus, it appears that components types (which represent facies) display a consistent relationship to the depositional environmets. Sediments occurring along the sandy outer talus and outer buttress system close to embayments and on beaches, are of terrigenous facies. The terrigenous-coralfacies is restricted to the backreef zone and forereef areas studded with coral patches. The lower and intermediate forereef areas belong to coral-molluscan-foraminiferalfacies; the reef fiats and adjacent outer slopes are characterized by coral-coralline algal facies (Table 1).
Foraminiferal Facies. The most common benthonic foraminifers belong to the following families and/or species: Homotrematidae [Homotrema rubrum (Lamarck), Miniacina miniacea Pallas, Carpenteria monticularis Carter], Acervulinidae [Acervulina inhaerens Schultze, Gypsina globula (Reuss), Gypsina vesicularis (Parker and
119 Table 1. Statistics of four component facies (mean values are given; numbers in parentheses refer to those of samples regrouped, values in italics denote representative component categories) Terrigeneous facies (12) Corals Coralline algae Halimeda
Mollusks Foraminifers Echinoderms Alcyonarians Bryozoans Serpulids Crustaceans Sponges Mineral grains
Terrigeneous-coral facies (8)
Coral-molluscanforaminiferal facies (16)
Coral-coralline algal facies (21)
3.24 1.18 0.02 3.28 5.06 0.55 1.53 0.18 0.08 0. 24 0.01
34.62
51.63
7.53 0.08 15.90 12.28 1.66 3.30 0.35 0.22 0.40 0.08
6.33 0.00
42.35 20.61
84.60
23.55
20.24 t3.04
1.46 2.02 0.38 0.57 0.30 0.07 3.94
Jones)], Amphisteginidae [Amphistegina lessonii (d'Orbigny), A. radiata (Fitchell and Moll), A. lobifera (Larsen)], Soritidae [Sorites orbicules Ehrenberg, Amphisorus hemprichii Ehrenberg, Spirolina arietina (Batsch)], diverse Miliolidae and Nubeculariidae (Quinqueloculina, Triloculina, Massilingt, Pyrgo, Spiroloculina) Planorbulinidae [Ptanorbulina acervalis (Brady), P. mediterranensis (d'Orbigny), Planorbulinella larvata (Parker and Jones)], Nummulitidae [Heterostegina depressa d'Orbigny, Opercutina ammonoTdes Gronovius)], Textulariidae, Alveolinidae [Borelis schlumbergeri (Reichel)] and Elphidiidae [Elphidium crispum (Linn6)]. Correspondence analysis was applied to 16 variables (significantly contributing foraminifers) and 31 sediment samples. The first three factors are sufficient to explain 83.2% of the total variance. The spatial arrangement of variable points and observation points in the planes [1, 2] and [1, 3] suggest the following comments (Figs. 4 and 5): Factor axis 1, accounting for 46% of the variance, opposes Homotrematidae (AC=0.30) associated with Spirolina (RC = 0.34) and Amphisorus (RC = 0.30) against Acervulina (AC=0.47) associated with Planorbulinella (RC = 0.42) and Gyspina (RC = 0.39); the distribution of Amphistegina, Borelis and Nummulitidae variables results also from factor 1, but more moderately (RC varying between 0.21 to 0.30). the Amphistegina point is located near the barycentre, which is due to the high ubiquity of this foraminifer within reef sediments in question. This axis makes it possible to distinguish samples from lower forereef areas (25 to 40 m deep) rich in Acervulina and Amphistegina from sediments deposited on upper forereef and topreefareas rich in Homotrematidae; it contributes moderately to heavily to the distribution of all the reef sediments (RC = 0.40-0.80). Factor axis 2, accounting for 30.2% of the variance, is based on the opposition between Homotrematidae (AC = 0.30) and Spirolina (AC = 0.21) associated with Triloculina (RC = 0.75), Quinqueloculina (RC = 0.58); other Miliolidae or Nubeculariidae (RC=0.37), Sorites (RC=0.52) and Amphisorus (RC = 0.49). Axis 2 separates two distinct
0.00 17.87 6.38 3.13 3.80 0.14 0.18 0.37 0.02 5.13
areas: one rich in Miliolidae, Nubeculariidae and Soritidae (bachreefzone), one rich in Homotrematidae (upper forereef zone and reef front). The arrangement of the corresponding sediment samples is moderately to heavily influenced by factor 2 (RC > 0.40). Factor axis 3, accounting for 7% of the variance, weighs the variable group including Amphistegina (AC = 0.42), Borelis (AC = 0.12) and Nummulitidae (AC = 0.12) against Acervulina (AC=0.20) associated with Planorbulina (RC = 0.15). This axis tends to isolate, among forereef samples, those displaying an Amphistegina enrichment from those with an Acervulina enrichment; its contributes weakly to moderately to the arrangement of these samples (RC = 0.10-0.34). Elphidiidae and Textulariidae are badly defined by the three factors used (total relative contributions less than 0.2). These microorganisms do not allow a valid discrimination for sedimentary facies in the Gulf of Aqaba. From the foregoing analysis, four foraminiferal facies and/or snbfacies can be distinguished in relation to the depositional environments and physiographical zones (Table 2). The Acervulina - Amphistegina facies is related to the lower parts of the forereef zone. A Homotrematidae facies must be defined on the outer margins of reef tracts; it involves two subfacies: Homotrematidae - Acervulina subfacies, typical of the upper parts of the forereef zone, and Homotrematidae - Amphistegina- Spirolina subfacies occurring on the reef flat zone, As for the backreef zone, it is characterized by a Miliolidae - S0ritidae facies. Textural Facies. Sediments consist of coarse to very fine sands (mean sizes Mz varying between 0.71 to 0.11 ram) except for a few reef flat samples ranging in the granule class (Mz=2.30 to 3.55 mm). Generally, sediments from outer reef flagstones and reef flats are coarser than those from the other forereef areas and backreef zones (M~ = 0.71-0.43 and 0.49-0.11 mm, respectively). Sorting is moderate to very good (So = 0.49 - 0.09) except for poorly sorted sediments from the outer parts of reef flats (S o > 0.60). The mean-size and sorting thus display an appreciably parallel evolution.
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Skewness values present strong changes in the various reef deposits. However a general tendency can be drawn: coral-built forereef and reef flat areas are characterized by slightly negative to symmetrical skewness (So = - 0.16 to +0.07), whereas sediments from sandy outer talus and backreef areas are symmetrically to positively skewed ( S o - ~ - 0 . 0 3 to +0.11). Correspondence analysis was applied to 22 variables (weight percentages of sediment within each of the 22 sieves used) and 61 sediment samples; two reef flat samples (poorly sorted granules) were left out because they distort-
Fig. 5. Plot of the factor axes 1 and 3. Both variables (16 foraminiferal types) and observations (31 sediment samples) are plotted simultaneously. See Fig. 4 for the significanceof symbols used
ed factor axes and consequently altered the results. Sorting index was computed as additional variables, i.e. noncontributors to variance in factors; this was done in order to estimate the relationship between size classes and this parameter 9 Only axes 1 and 2 were considered because they account for 86.8% of the total variance and the third axis has a low percentage (5.4%)9 Within the factor space (Fig. 6), variables and observations are distributed as a continuous series passing from larger (5 mm) to smaller sizes (0.04 mm). Factor axis
121 Table 2. Statistics of foraminiferal facies (mean values are given; numbers in parentheses refer to number of samples regrouped, values in italics denote representative foraminiferal types)
AcervulinaAmphistegina facies
Homotrematidae Planorbulina P lanorbuline lla Gypsina Acervulina Sori~es Amphisorus Spirolina Elphidium Amphistegina 7?'iloculina Quinqueloculina Other Miliolidae and Nubeculariidae N ummulitidae Borelis Textulariidae
Homotrcmatidae facies
(13)
Acervulina subfacies (5)
AmphisteginaSpirolina subfacies (9)
6.29 1.33 1.47 0.73 39.84 6.27 2.82 1.16 0.89 22.73 2.79 5.20 3.09
59.45 2.18 0.55 0.20 13.64 2.72 2.59 3.05 0.00 8.80 1.70 1.71 1.56
45.I0 0.60 0.26 0.04 0.74 3.10 6.02 109 0.09 15.83 3.37 8.90 1.65
12.21 0.46 0.00 0.00 0.94 10.15 11.16 23.65 0.63 7.60 I2.32 14.74 4.94
1.45 1.51 2.93
0.13 0.00 1.70
0.00 0.00 3.41
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1, explaining 63.5% of variance, is based on the opposition between the coarse-sand range (5 m m to 0.5 m m variables) and the fine-very fine sand r a n g e (0.20 m m to 0.04 m m variables). Factor axis 2, explaining 23.5% of variance, mirrors opposition between the coarse-sand range and the medium-sand range (0.40 m m and 0.31 m m variables). Four groupings of samples (which represent different textural facies) can be distinguished within the factor space considered (Table 3): Group 1 is defined by the variables corresponding to size classes larger than 0.5 mm. It results from weak to
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heavy contributions of axes (RC values ranging between 0.03 to 0.87). Sediments are moderately to well sorted, very coarse to coarse sands; they chiefly occur on the reef flats and outer slopes with coral flagstones. Group 2 is defined by the 0.5-0.25 mm size range. It results from weak to heavy contributions of axes (RC = 0,006-0.84). Sediments are well-sorted, medium sands; they are, for the most part, deposited on'the outer buttress system and beaches. Group 3 is defined by the size classes smaller than 0.25 ram. It results from a moderate to heavy contribution of axis 1 (RC = 0.20-0.94). Sediments are very well to well-
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MiliolidaeSoritidae facies
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reef f i a t s [] backreef -~ beach O
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factor1 63.5%
Fig. 6. Plot of the first two axes of correspondence analysis. Both variables (22 grain-size classes) and observations (63 samples) are plotted simultaneously; 2 parameters (2 degrees of sorting) are plotted as additional variables. The numbers (5, 4, 3.1...0.063, 0.05, 0.04) refer to the 22 size classes of the millimetric scale used; gS, good sorting; pS, poor sorting
122 Table 3. Grain size statistics: values of grain size parameters in the four textural facies and mean percentage by mass retained at each class used (numbers in parentheses refer to the number of samples regrouped; values in italics correspond to dominant fractions) Group 1 (14)
Group 2 (10)
Group 3 (14)
Group 4 (22)
Mean size (mm) Mean Range
0.54 0.44 - 0.71
0.35 0.34 - 0.55
0.20 0.11 - 0.26
0.34 0.26 - 0.48
Sorting Mean Range
0.36 0.25 -0.58
0.20 0.09 -0.3
0.29 0.17-0.35
0.68 0.61-0.76
Size classes used (mm) Pebble (> 10) Gravel (10 - 5) Granule (5 - 2.5) Very coarse sand (2.5- 1) Coarse sand (1-0.5) Medium sand (0.5-0.25) Fine sand (0.25- 0.05) Very fine sand ( < 0.05)
0 0.97 2.60 17.20 36.70 31.78 10.45 0.26
0 0.83 1.18 4.85 21.99 63.20 7.92 0.03
sorted, fine to very fine sands; they are restricted to the sandy outer talus and backreef zones. G r o u p 4 includes samples exhibiting average granulometric characteristics; they comprise appreciably equal proportions of fractions from 2.5 to 0.25 m m and, as a consequence, belong to an intermediate type sediment. These sediments are moderately sorted, medium sands, which occur on the various depositional zones described.
Environmental Significance of Factor Axes The factor axes related to correspondence analysis can be explained in terms of physiographical or hydrodynamical gradients. For the total component analysis, the first factor axis can be interpreted as being the outcome of bipolar origin of deposits; it separates two areas which have fundamentally different sediment sources: the one, on its positive end, is fed by allochthonous quartzofeldspathic material derived from the surrounding mountains; the other, on its negative end, is directly fed by autochthonous organic communities. As a result, this axis discrimates terrigeneous sandy outer slopes from all organism - produced reef units. The second axis sets deposits from coral-built forereef areas against the top-reef bodies; it seems to be concerned with a physiographical gradient which controls the primary production of skeletal particles. In fact the composition of biogenic sediments changes along the transverse axis of the reef tract; the amount of molluscan, foraminiferal, bryozoan and sponge detritus diminishes from the outer reef zone to the backreef zone while the frequency of coralline algal, alcyonarian and echinoid particles increases. F r o m the analysis of foraminiferal populations, it appears that the distribution of sediment samples within the three-dimensional space roughly reflects their effective po-
0 0.01 0.2 5.41 7.75 25.98 58.89 1.76
0 0.32 2.02 10.16 22.44 30.4 32.6 2.06
sition across the reef tract. Axis 1 is considered to be a reflection of a physiographical gradient, indicating a consistent opposition between the outer reef deposits and those from the reef flat and backreef zone. The proportions of Acervulina, Amphistegina, Planorbulinidae, Nummulitidae, Alveolinidae tend to decrease from the forereef to the topreef zones as Homotrematidae, Miliolidae, Nubeculariidae and Soritidae become increasingly abundant. Axis 2 can be interpreted in terms of sediment-producing organisms; it clearly separates two different microfaunal populations according to their ecological requirements: shade-loving encrusting forms (Homotrematidae) colonizing vuggy hard substrates and epiphytic and free benthonic forms (Soritidae, Miliolidae) mainly inhabiting dense sea grass beds or soft-surfaced deposits. Furthermore, it is obvious that this axis is also a virtual partition line (i.e. reef front line) between two main depositional environments (Forereef and topreef zones, respectively). Axis 3 can be explained in a similar manner as the second factor; it seems to isolate two ecologically different types of foraminiferal communities: on the one hand, encrusting Acervulinidae and Planorbulinidae; on the other hand, free benthonic Alveolinidae and Nummulitidae. Concerning grain-size distribution, factor axis 1 can be regarded as a hydrodynamical gradient, controlling the sediment grain-size from its negative side (higher energy areas) to its positive one (lower energy areas). The sequence of textural types along axis 1 expresses the decrease of mean size and the transition from negative to symmetrical skewness. Axis 2 chiefly controls the sorting index from its negative end (moderately sorted sediments) to its positive one (well sorted sediments); in addition, this axis seems to represent a virtual boundary (i.e. reef front line) between two discrete zones of sedimentation: a preferentially sediment-producing zone characterized by moderately to well sorted, coarse to medium sands, and a preferentially sediment-trapping zone characterized by well
123 sorted, medium to very fine sands. Thus, axes 1 and 2 reflect the gradual decrease in the available energy of currents producing sediment transport from the reef front to shoreward or downward. Discussion and Conclusions
Statistical classification of sediment samples from the Jordan reef areas of the Gulf of Aqaba allows several combinations of facies to be identified into the various depositional environments.
(1) The Beach Zone It is characterized by the terrigenous isometric-mediumsand facies. Sediments contain less than 0.1% material finer than 125 gm; this size distribution agrees with those of beach sands in other areas of the Gulf of Aqaba (Friedman 1968) where the wave-washed beaches are virtually free of fine fractions that were taken in suspension and removed seaward into deep water. The scarcity of reef-derived carbonate particles emphasizes the prevailing seaward removing of grains and their eventual dumping into the Gulf.
(2) The Backreef Zone In this zone, terrigenous-coral isometric-fine-sand facies occurs. The high percentage content of quartz and feldspar particles has been contributed by adjacent subtidal deposits, whereas the most part of coral fragments has been derived probably from the breakdown in situ of fragile, finely branched colonies such as Seriatopora and Stylophora. The richness in coral detritus thus appears irrespective of the percent cover of grain-producing colonies (less than 1% of the substrate surface, according to Bouchon 1980); this emphasizes the complex relation between the degree of resistance to disintegration of individual skeletons and the productivity of corresponding organic communities in skeletal elements. A facies can be recognized here on the basis of foraminiferal composition: the Miliolidae-Soritidaefacies, governed by high amount of Triloculina, Quinqueloculina, Sorites, Amphisorus and Spirolina. These results are in harmony with Erez and Gill's (1977) reporting the relative abundance of Soritidae on the backreef channel at Ras Burka. These microfaunal assemblages are of autochthonous origin. The smaller Miliolids are almost exclusively living as free forms on soft bottoms whereas, as pointed out by Hottinger (1977), the larger soritids are living as epiphytes on the leaves of marine Phanerogams.
(3) The Reef Flat Zone It is characterized by the coral-coraIline algalheterometriccoarse sand facies. High concentrations of coral detritus directly reflect the coverage rate of sceleractinian corals on the reef flats, which varies between 30% for the inner parts
and 40% for the outer parts (Bouchon 1980). Just as coral grains, red algal debris result from the breakdown in situ of sessile branching forms wich are prolific in the outer margin of the reef flats. Molluscan fragments representing about 17% of the total sediment are mainly produced by local cryptofauna and endofauna. Dense communities of alcyonarians (percent cover more than 40% on the outermost reef-flat surfaces) originate relative high contents of spicules (about 4% of the total sediment), whereas echinoid populations composed generally of 5-10 individuals per m 2 are nonnegligible sediment contributors in this reef area (more than 3%). The Homotrematidae facies is here controlled by both autochthonous and allochthonous supply; it is the result of the sedimentation in situ of shade-loving encrusting Homotrema and Miniacina skeletons, while associated Amphistegina and Spirolina derive from epiphytic populations living on the outer slope and the backreef zone respectively.
(4) The Forereef Zone It exhibits several facies in relation to nature of substrate and water depth:
The terrigenous isometric fine-sand facies occurs in the sandy talus close to wadi mouths; on these very mobile sediments, benthonic communities are very sparse and, as a result, skeletal elements are in low proportions (less than 10% of the total sediment). The coral-coralline algal heterometric coarse-sand facies occurs at the upper parts of coral-built forereef areas (020 m). As on the reef flat zone, the frequency of coral particles in sediments from the buttress system and the coral pavements is a fair reflection of their considerable richness in coral communities, the coverage rate of which ranges from 50 to 80% between the depths of 5 and 40m (Bouchon 1980). The density of alcyonarian colonies does not display such variations; the percent cover, reaching the maximum value of 15% at 20 m deep, permits to explain lower amounts of spicules deposited (less than 3%) when compared to those from the reef flats.
Acervulina - Homotrematidae facies is exhibited in the upper parts of coral-built forereef areas. Distribution patterns of these foraminiferal detritus show close relationships with those of corresponding living populations. As pointed out by Hottinger and Levinson (1978), in the Gulf of Aqaba, along the outer slopes, entrusting Acervulina inhaerens brings about compaction of sediment particles, adding significantly to the skeletal deposits accumulating between living coral colonies. In the same way, Homotremid grains are mainly produced by the local disintegration of these organisms. The relative richness in Planorbulina tests (more than 2% of the total foraminiferal assemblages) at these depths can be ascribed to the same process; this is in accordance with Erez and Gill's (1977) observations.
124
The coral - molluscan -foraminiferal heterometric medium_sand facies is restricted to the whole lower parts of the forereef zone (20-50 m). The d i s t r i b u t i o n of this sedim e n t type is controlled b o t h by ecological a n d h y d r o d y n a mical factors. Coral a n d m o l l u s c a n debris, p r o b a b l y , originate partly from the disintegration of local c o m m u n i t i e s a n d partly represent a l l o c h t h o n o u s particles removed from upper forereef areas. F o r a m i n i f e r a l fractions including high c o n t e n t of Acervulina a n d Amphistegina with significantly s e d i m e n t - c o n t r i b u t i n g Operculina, Heterostegina a n d Borelis, proceed from local living m i c r o f a u n a l populations, according to Reiss (1977). Different e n v i r o n m e n t a l settings were identified o n the basis of certain skeletal c o m p o n e n t s alone (corals, coralline algae, mollusks, total foraminifers a n d even echinoderms a n d alcyonarians) in close relationships with the n a t u r a l zones. This suggests, in disagreement with Erez a n d Gill's (1977) conclusions, that, in the G u l f of A q a b a , biogenic particles other t h a n foraminifers only c o n t r i b u t e to the definition of reefal facies. However it is true that the maxim u m i n f o r m a t i o n for e n v i r o n m e n t a l discrimination is c o n t a i n e d in the assemblages o f foraminifers, because the response of foraminifers to e n v i r o n m e n t a l changes is distinct at generic level, according to Erez a n d Gill (1977), and even at family level. Results of studies o n foraminiferal assemblages (Reiss et al. 1977; this paper) indicate that the d i s t r i b u t i o n of foraminiferal p o p u l a t i o n s shows a distinct d e p t h - z o n a t i o n in this Gulf, in spite of the practically h o m o g e n e o u s water column, The d e p t h - z o n a t i o n observed is i n d e p e n d e n t of substrate, except for the upper deposits of e m b a y m e n t s , where foraminifers m a y be scarce. As in the case o f the other biogenic c o m p o n e n t s , n o large-scale t r a n s p o r t of foraminifers across the reef tracts takes places; it is apparent that the lateral limits of the various biofacies coincide with the limits of life of the related organic communities, a n d skeletal assemblages roughly reflect biocoenoses.
Acknowledgements.Thanks are expressed to the University of Amman, Jordan and more particularly to the staffofthe Marine Station of Aqaba for their logistic support, to Dr. J. Jaubert (responsiblefor the FrancoJordan programme of scientificcooperation) and Dr. C. Bouchon (formely French resident scientistat Aqaba). Financial support for this study was provided by the French Foreign Office, Paris, France. References Benz~criJP (1973) L'analyse des donn6es. II. L'analyse des correspondances. Dunod Editions, Paris, 619 p Bouchon C (1980) Quantitative study of the scleractinian coral communities of the Jordanian coast (Gulf of Aqaba, Red Sea): preliminary results. Tethys 9:243 246 Bouchon C, Jaubert J, Montaggioni L, Pichon M (1981) Morphology and evolution of the coral reefs of the Jordanian coast of the Gulf of Aqaba, Red Sea. Fourth Intern Coral Reef Symp, Manila, Philippines, May 1981, Abstracts:8
Emery KO (1963) Sedimentsof Gulf ofAqaba (Eilat). In: Miller RL (ed) Papers in marine geology, Shepard Commemorative Volurae. Macmillan Co, New York, pp 257-273 Erez J, Gill D (1977) Multivariate analysisof biogenicconstituents in Recent sediments off Ras Burka, Gulf of Elat, Red Sea. Math Geol 9:77-98 Fishelson L (1970) Littoral fauna in the Red Sea: the population of nonscleractinian anthozoans of shallow water of the Red Sea (Eilat). Mar Biol 6:106-116 Fishelson L (1971) Ecology and distribution of the benthic fauna in the shallow water of the Red Sea. Mar Biol 10:113-133 Folk RL, Ward WC (1957) Brazos fiver bar: a study of the significance of grain size parameters. J Sediment Petrol 27:3-26 Freund R, Garfunkel Z, Zak I, Goldberg M, Weissbrod T, Derin B (1970) The shear along the Dead Sea rift. Philos Trans R Soc London Ser A 267:107-130 Friedman GM (1968) Geology and geochemistryof reefs, carbonate sediments and waters, Gulf ofAqaba (Elat, Red Sea). J SedimentPetrol 38:895-913 Guilcher A (1979) Les rivagescoralliensde l'Est et du Sud de la presqu'fle du Sinai. Ann Geogr Paris 488:393-418 Gvirtzman G, Buchbinder B, Sneh A, Nir Y, Friedman GM (1977) Morphology of the Red Sea fringing reefs: a result of the erosional pattern of the last glaciallow-stand sea leveland the followingholocene recolonization. Second Int Fossil Coral and Coral Reef Symp, Paris, Memoir BRGM 89:480491 Gvirtzman G, Buchbinder B (1978) Recent and pleistocene coral reefs and coastal sediments of the Gulf of Elat. Tenth Int Congr Sedimentol Israel, Post congr excurs 4:163-191 Hottinger L (1977) Distribution of larger Peneroplidae, Borelis and Nummulitidae in Gulf of Elat. Utrecht Micropaleont Bull 15:3% 110 Hottinger L, Levinson G (1978) Cementation of reefs in the Gulf of Elat (Red Sea) by Acervulinid Foraminifera, Tenth Int Congr Sedimentol, Israel, Abstracts 1:315 Hulings NC (1979) Currents in the Jordan Gulf of Aqaba. Dirasat J Univ of Jordan 6:21 33 J6reskog KG, Klovan JE, Reyment RA (1976) Geolol~cal factor analysis. Methods in geomathematics, vol 1. Elsevier,Amsterdam, 178 p Lebart L, Fbnelon JP (t973) Statistique et informatique appliqu~es.Dunod Editions, Paris, 457 p Loya Y, Slobodkin LB (1971) The coral reefs of Eilat (Gulf of Eilat, Red Sea). Syrup Zool Soc London 28:117-139 Merg~aer H (1971) Structure, ecology and zonation of Red Sea reefs (in comparison with South Indian and Jamaica reefs). Symp Zool Soc London 28:141-161 Mergner H, Schumacher H (1974) Morphologie, Okologie und Zonierung yon Korallenriffen bei Aqaba (Golf yon Aqaba, Rotes Meer). Helgol Wiss Meeresunters 26:238-358 Pot FD, Lerner-SeggevR (1966) Preliminary data about the benthonic fauna of the Gulf of Elat (Aqaba), Red Sea. Israel J Zool 15:38-50 Reiss Z (1959) Notes on the Foraminifera from the Gulf of Eylath, Ras Muhamad and Tiran. Sea Fisher Res Stat Bull 20, Haifa, Israel Reiss Z (1977) Foraminiferal research in the Gulf of Elat - Aqaba - A Review. Utrecht Micropaleont Bull 15:7-26 Reiss Z, Leutenegger S, Hottinger L, Fermont WJJ, Meulenkamp JE, Thomas E, Hansen HJ, Buchardt B, Larsen AR, Drooger CW (1977) Depth-relations of recent larger foraminifera in the Gulf of Aqaba-Elat. Utrecht Micropa~eontolBull 15:1-244 Said R (1950) The distribution of foraminifera in the Nothern Red Sea. Contrib Cushman Found Foraminiferal Res 1:9-29 Sneh A, Friedman GM (1980) Spur and groove patterns on the reefs on the northern gulf of the Red Sea. J Sediment Petrol 50:981-986 Walther J (1888) Die Koralleuriffe der Sinaihalbinsel.Abhandlung Mathematisch-Physischer Classe, Koenigl Saechs GesellschWissensch 9:339-505
