Sequential evolution and diagenesis of Pleistocene coral reefs (South Sinai, Egypt)

Sedimentary Geology, 78 (1992) 59-79 Elsevier Science Publishers B.V., Amsterdam

5 t)

Sequential evolution and diagenesis of Pleistocene coral reefs (South Sinai, Egypt) A n d r ~ S t r a s s e r ~, Christian S t r o h m e n g e r b . , E r i c D a v a u d b and A n d r e a s B a c h c " lnstitut de G~ologie, Pdrolles, 1700 Fribourg, Switzerland b D@artement de Gdologie et Paldontologie, 13 rue des Marafchers, 1211 Genet,a, Switzerland ' lnstitut fiir Umweltphysik, Irn Neuenheimer Feld 366, 6900 Heidelberg, Germany (Received November 12, 1991; revised version accepted March 24, 1992)

ABSTRACT Strasser, A., Strohmenger,,Ch., Davaud, E. and Bach, A., 1992. Sequential evolution and diagenesis of Pleistocene coral reefs (South Sinai, Egypt). Sediment. Geol., 78: 59-79. The uplifted Pleistocene coral reefs in South Sinai represent two major depositional sequences. They generally form two morphological terraces, which locally are tilted and offset by still active faulting, The major reef sequences are composed of several small-scale sequences. Corals grew rapidly when accommodation space was created by eustatic sea-level rise or fault-block subsidence. As the accommodation decreased, coral rubble and siliciclastic sands prograded over reefal and lagoonal sediments, thus creating a shallowing-upward sequence. Terrigenous input was further stimulated by a more humid climate during interglacial times. Z~°Th/234U dating places the older reef cycle between 350,000 and 270,000 years B.P., which corresponds to the interglacial period of isotope stage 9. The younger reef cycle has been dated between 140,000 and 60,000 years B.P. (isotope stage 5). An intermediate, relic sequence found in only one outcrop can be attributed to isotope stage 7. Correlation of the small-scale sequences is difficult: age-dating is not precise enough, and local tectonic activity in many cases overruled the smaller eustatic fluctuations. The younger reef sequence commonly exhibits marine cements, whereas the older sequence was exposed to freshwater dissolution and cementation. An important feature is the locally pervasive dolomitization especially of the older reef. Oxygen and carbon isotope values suggest that the dolomite formed in a seawater-dominated mixing zone, Multiple phases of dissolution, cementation, and dolomitization point to a very complex diagenetic history.

Introduction Recent coral reefs and elevated Pleistocene reef terraces are well developed along the coasts of the Red Sea. Ever since 1888, when Johannes Walther first described the reefs of the Sinai Peninsula, extensive studies on morphology, taxonomy, and ecology of the Recent fringing reefs have been carried out (for an overview see Dullo, 1990), and several authors have discussed ages

* Present address: BEB Erdgas und Erd61 GmbH, Riethorst 12, 3000 Hannover 51, Germany. Correspondence to: A. Strasser, Institut de G~ologie, P~rolles, 1700 Fribourg, Switzerland.

and diagenesis of the Pleistocene reef terraces (e.g., Gvirtzman et al., 1973; Gvirtzman and Friedman, 1977; Gvirtzman and Buchbinder, 1978; Dullo, 1984, 1986, 1990; M'Rabet et al., 1989). Our study focuses on the sedimentological and diagenetic evolution of two Pleistocene reef sequences in South Sinai (Egypt), which are remarkably well exposed in an area ranging from Shark Bay to Ras Muhammad (Fig. 1). Nine stratigraphic sections have been measured, where the geometric relationships between the sequences themselves as well as with the prePleistocene substrate can be observed. 140 samples have been taken in order to document facies

0037-0738/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved

6()

,k b l I~. "~,SSI) R Ii i' ,'\l

f M Red Sea

f \

\

~'X"~~sMuhammad

I 5 km

I

Fig. I. Location of the studied sections in South Sinai. and diagenetic features; 20 more samples were used for age-dating. The aims of this paper are twofold. First, we present an examination of a coral reef system whose evolution was affected not only by rapid changes of climate and eustatic sea level, but also by rapid tectonic movements on the border of an active rift basin. Second, we document the complex diagenetic history of superposed reef sequences which recorded the influences of marine and meteoric pore waters.

Tectonic, stratigraphic, and climatic setting The southern tip of the Sinai Peninsula is located at the triple junction between the African

plate, the Arabian plate, and the Sinai subplatc. Rifting of the Red Sea system began in the Late Oligocene, reactivating Late Precambrian zones of structural weakness (Makris and Rihm, 1991). The Gulf of Suez began forming by extension and basin subsidence in the Early Miocene. By Middle Miocene times, the extension in the Gulf o! Suez had slowed down. The plate motions were accommodated by a left-lateral transform zone. along which the Dead Sea and the Gulf of Aqaba were initiated as pull-apart basins (Freund et al.. 1970; Makris and Rihm, 1991). Since the Pliocene. however, E - W extensional tectonics predominate (Lyberis, 1988). Intense uplift of the graben shoulders and block-faulting started in the Middle to Late Miocene and continue until today

SE(..)UENTIAL EVOI.UTI()N AND DIAGENESIS OF PLEISTOCENE UORAL RE[:FS

(Briem,

1989; P u r s e r et al., 1990). T h e

total

The

6]

developing rift filled with M i o c e n e to

a m o u n t of crustal uplift in s o u t h e r n Sinai is esti-

Pliocene sediments, which were periodically de-

m a t e d to have b e e n nearly 5 kin, causing almost

formed a n d tilted ( P u r s e r et al., 1990). T h e over-

c o m p l e t e removal of the pre-rifting s e d i m e n t cover a n d deep erosion of the P r e c a m b r i a n crys-

lying Q u a t e r n a r y s e d i m e n t s are s u b h o r i z o n t a l and relatively u n d i s t u r b e d . O u t c r o p s consist of ma-

talline b a s e m e n t ( G v i r t z m a n a n d 1978; G a r f u n k e l , 1987).

rine sandy limestones a n d reef limestones, as well

Buchbinder,

as of c o n t i n e n t a l , mainly alluvial sediments. Duc

Fig. 2. (a) The two reef terraces between sections B and C. The Flandrian terrace is visible to the left of the boat. (b) Faulted and tilted older reef terrace to the south of section G. (c) Section D, where the younger reef sequence (light in colour, foreground) onlaps the older reef (darker in colour, background) in an almost vertical contact. (d) Neptunian dyke in the older reef sequence of section D, filled with material of the younger sequence.

to tectonic uplift, the oldest Pleistocene reef terraces in southern Sinai are situated up to 120 m above present sea level. A second group of reefs reaches 50 to 80 m above sea level (Garfunkel, 1987). A third group of Pleistocene reefs (which are the subject of this paper) is uplifted to 30 m at the most and extends inland for a few hundred metres only, where the reef facies passes into alluvial deposits.

During the last glacial period, the level of the Red Sea dropped by about 130 m (Gvirtzman et al., 1977). The coral population died. and deep canyons were cut into the pre-existing reef. Submerged terraces occur at several levels below the present water surface (Gvirtzman and Buch-binder, 1978; Reiss and Hottinger, 1984). A new fringing reef formed when sea level rose again during the Holocene, and the canyons were

(A) older reef

y o u n g e r reef

(B)

----HT

aeolian dune

(C)

........~ ¢

1//5

2a/6 2a/5

' 5m

~

.....

2a/11

~:o

Fig. 3. Sketches of studied sections (for location refer to Fig. 1). Sections A and H have not been measured in detail. Section E is situated just south of the jetty of N a a m a Bay, section G at the diving locality called Tower. Section I at Ras M u h a m m a d has been measured along the footpath leading up to the observation point. The western part of the section is downset by a fault. Numbers correspond to dated samples (see Table 1).

SEQUENTIAL

EVOLUTION

AND

D1AGENESIS

OF PLEISTOCENE

CORAL

flooded to form embayments (Gvirtzman et al., 1977). The sedimentary record in the Red Sea system was controlled not only by tectonic movements related to rifting, but also by fluctuations of climate and eustatic sea level. The resulting changes in circulation patterns and increased salinities due to intense evaporation induced faunal' and mineralogical changes (Locke and Thunell, 1988).

63

REEFS

In the deep-water sediments, the vertical distribution of planktonic organisms and the variations in oxygen and carbon isotopes indicate fluctuations of salinity and temperature (e.g., Deuser et al., 1976; Reiss and Hottinger, 1984). These fluctuations correlate well with the generalized oxygen-isotope curve (Shackleton and Opdyke, 1973; Reiss et al., 1980). This agreement implies that the Red Sea area followed the world-wide pat-

(D) y o u n g e r reef

recent reef

/21 -HT

77 (E)

younger reef older reef corals red algae

v

bivalves

T

borings

vermetid

<22~

gastropods

coral rubble siliciclastic pebbles

°. m ~

-rr'r

- - H T

siliciclastic s a n d s beachrock substrate

(F) 3/13

5m 5m I

Fig. 3 (continued).

/k

root traces

HT

high tide

04

\ % i I{ \ S N I : R I I AI

tern of climatic changes, which were themselves based on orbitally controlled variations of insolation on the Earth's surface (Milankovitch, 1941; Hays et al., 1976). Insolation changes also induced shifting of the atmospheric cells and of monsoonal rain belts. In the case of the Red Sea area, rainy periods with abundant terrigenous

run-off generally correspond to warm, interglacial times with a high sea level (Deuser et al., 1976; Rossignol-Strick, 1985; Klein et al., 1990). Consequently, the studied Pleistocene reef sequences formed under the influence of several inter-related factors: tectonic movcments, custatic sea-level fluctuations, variations in salinity.

(G)

\ \X

\

f

5m

4,!, ~

~ - ~_/~~--~

,_nte_rme_d,at e

- -

older

6/4

- HT recent reef

(H)

keystone

vugs

•k e y s t o n e

~ ~

"

vugs

eolian

dune

younger reef rubble

Fig. 3 (continued).

reef

SEQUENTIAL EVOLUTION AND DIAGENESISOF PI_EISTO(?ENECORAL REEFS

65

Fig. 4. (a) Blocks of the older reef sequence incorporated in the younger one (section B). (b) Heavily bored surface of a block of the older reef sequence (section B). (c) Tabular and slightly inclined lamination and fining-up in siliciclastic sands in the intermediate sequence. The overlying carbonates belong to the younger reef cycle (section G). (d) Lateral equivalent of outcrop shown in (c), where the siliciclastic sands are preserved only as lenses and blocks (section G}.

66

temperature and sediment load of the ocean water, and varying input of freshwater and terrigenous sediment. Reef sequences

In the study area of southern Sinai, two major reef sequences can be recognized. They generally form two morphological terraces (Fig. 2a), but can locally be disrupted and tilted by~ still active faults (Fig. 2b). Therefore, terraces do not always correspond to reef sequences. The higher (older) terrace commonly levels off at about 30 m above present sea level, whereas the lower (younger) terrace generally is situated at 10 to 15 m. Both terraces can form cliffs overhanging the shoreline (Fig. 2c). In contrast to our observations, Gvirtzman and Buchbinder (1978) mention in the same area three elevated reef terraces which they dated at 108,000-140,000, 238,000, and older than 250,000 years B.P. Reiss and Hottinger (1984) speak of three terraces on the Sinai side of the Gulf of Aqaba, which are situated between 15 and 30 m above present sea level and give ages between 81,000 and 306,000 years B.P. Locally, a terrace or wave notches are cut into the uplifted reef sequences at 1 to 2 m above present sea level (Fig. 2a), They have, at other localities on the Red Sea, been dated between 4,200 and 6,500 years B.P. (Reiss and Hottinger, 1984; Dullo, 1990) and correspond to the Flandrian sea-level highstand, which has also been monitored on Mediterranean shorelines (Paskoff and Sanlavitle, 1983). The Recent fringing reef forms a thin veneer on a terrace cut into the Pleistocene sequences, slightly below today,s sea level. Active reef growth is seaward to form reef crest and reef slope (Dullo, 1990).

Older reef sequence The older reef sequence disconformably overlies the Miocene-Plioeene substrate, which generally exhibits an accentuated palaeotopography. The contact is clearly visible in section A at Shark Bay (Fig. 3), where it is accentuated by abundant

A. S ' F R A S S E R L ! : \ I .

root traces and siliciclastic pebbles. In section 1~ at the jetty of Naama Bay (Marsa el At, Fig. 3), the contact locally is subvertical. The older reef sequence can be subdivided into several smaller sequences which are separated from each other by siliciclastic sands, bored surfaces, or simple bedding planes (Fig. 3). These small sequences are primarily composed of reef carbonates with most corals remaining in growth position, or of slightly reddish, muddy or sandy carbonates dominated by red algae (which weather white in the outcrop), but also containing corals. These two major facies pass vertically or laterally into each other. The massive coral facies corresponds to a reef-crest to upper reef-slope environment, whereas the red-algae facies implies a back-reef to lagoonal setting. A similar facies, rich in rhodoids can be observed in the small modern lagoons at Ras Muhammad. The siliciclastic sands commonly are immature and contain abundant quartz, feldspars and lithic fragments. They exhibit in many cases low-angle planar stratification and can be assigned to a beach environment. Keystone rugs occur in section I. Beachrock blocks are found in several sections (A, C and D in Fig. 3); in some cases, they have been strongly tilted (section A, Fig. 3). Pebbles of coral rock may be incorporated. In sections C, D and I, bidirectional foresets and festoon cross-bedding indicate a subtidal deposition controlled by tidal currents. Aragonitic ooids with a tangential cortical texture have been found in this facies in section I. Very fine, well sorted quartz sands with a reddish tint suggest an aeolian environment (section I). Locally, large, metre-scale blocks of the older reef sequence (limestone and siliciclastics) cover the flanks of the corresponding terrace (sections A, B and H in Fig. 3). Up to the level of the lower terrace, they are incorporated into the younger reef sequence (Fig. 4a). Their surfaces display in most cases perforations by boring molluscs (Fig. 4b) and may be encrusted by red algae and serpulids of the younger reef cycle. In higher topographic positions, no sediment filling the spaces between the blocks could be found. This suggests that the blocks formed when the older reef was subaerially exposed, and that they were

S E Q U E N T I A l . E V O L U T I O N AND D I A G E N E S I S OF P L E I S T O C E N E C O R A L REEFS

covered by water and sediment only up to the level to which the younger reef cycle reached. The older reef sequence was uplifted by 10 to 20 m before the younger reef was constructed. At Ras M u h a m m a d (section I), however, the strata are now vertically offset by some 20 m along a northwest-southeast-trending fault. A neptunian dyke filled with sediment from the younger reef cycle (section D, Fig. 2d) also indicates that the area was tectonically active between the deposition of the two sequences.

Intermediate sequence In section G, the older reef sequence is overlain by channelled siliciclastic conglomerates. They are succeeded by coral blocks and a (up to 1.5 m thick) constructed coral facies, which is replaced laterally by a conglomerate with siliciclastic matrix (Fig. 3). The top of this sequence is formed by siliciclastic beach sands and beachrock blocks (Figs. 4c and 4d). The younger reef sequence then starts with encrusting vermetid gastropods. The important erosional features as well

67

as age-dating (see below) place this sequence between the older and the younger reef cycles.

Younger reef sequence The younger reef onlaps the previously uplifted and partly eroded older sequence. Contacts are subvertical (sections B and D, Fig. 3) to subhorizontal (sections C and G). In some cases, an organic veneer of serpulids, red algae and vermetid gastropods encrusts the eroded and commonly bored surface of the older reef (section D). The younger r e e f sequence is composed of at least two sub-sequences, separated by siliciclastic sands and reworked limestone pebbles (sections A, D and G, Fig. 3), or even by large blocks of reef limestone of the older sequence (sections B and D, Fig. 4a). Facies are mostly in-situ reef limestones (Fig. 5a), but coral rubble is common in the upper part of the sub-sequences (Fig. 5b). This rubble probably accumulated on the lower foreshore, as it can be observed on the Recent beach. It then passes into siliciclastic beach facies

Fig. 5. (a) Coral facies of the younger reef sequence, displaying well preserved coral heads and a Tridacna shell in living position (section D). (b) Coral rubble in the upper part of the younger reef sequence (section B).

'-.. ' , I R . \ % S t

,Sg

Fe. I I .',J

Older Reef

B A

H

,

/

/

.........

,

'\

E

.27o \\-

.270 "340

I \\

I

~>ar,

~

G

///

~ ,

o

I~tr[-

./"/7

"

,302 '\ D \

F j-~--~284

~.~'" I ~ t

1

I \

I~1

i

[

_,~' _, "

\,",.

O314

"~

,,~ c. . . . .

L.. . . . . . a

[? m t

p~

Fig. 6. Tentative correlation of small sequences and dating of the older reef sequence. U,/Th ages are given m k y (seu l a n e I h~ error margins). Symbols are as in Fig. 3. Circled numbers correspond to boundaries of small-scale sequeJ~cc,~

(sections B and H). An aeolian facies is present in section B, where it partly covers fallen blocks of the older sequence. The younger reef sequence may entirely mask the older sequence up to the level of the lower terrace, but may also be preserved only as relics (sections F and I).

Formation of sequences Many of the small, metre-scale sequences composing the older and younger reef sequcnces display a shallowing-upward trend: reef or lagoonal facies are overlain by coral rubble a n d / o r silicb clastic beach sands, which in turn may pass into

Younger Reef

A

B %:.

~

F

G

H

. :... ::£..

122

:~_~<2["59

R:::-'-'.~ I

gr/7.

D

/

~-~e86

I

,

//',

121

118

f

I Z

t,, ,, 10140

• /

x~ ~89~7

I

,

i

'~ I

I---i ,,

/

:

,,

t___d@

i

r

]

[2 rn

J

I

intermediate

!

q

H[

reef

Fig. 7. Tentative correlation of small sequences and dating of the younger reef sequence. Ages are in ky (for error margins relev h, Table 1). Symbols are as in Fig. 3. Circled numbers correspond to boundaries of small-scale sequence,

SEQUENTIAL EVOLUTION AND DIAGENESIS OF PLEISTOCENE CORAL REEFS

aeolian deposits. This trend indicates diminishing water depth, which was controlled by a combination of changes in sea level, subsidence rate, and sediment accumulation. In the studied sections, the older reef sequence contains up to seven small-scale sequences. They are best developed in Ras M u h a m m a d (section I, Fig. 3). Three major siliciclastic punctuations can be distinguished, but their detailed correlation between the sections is difficult (Fig. 6). In most sections, the uppermost part of the older reef sequence is characterized by a bored surface which is overlain by two distinct beds of reef limestone. These features have been tentatively correlated (Fig. 6). The younger reef generally exhibits two small-scale sequences (Fig. 7). The vertical stacking of the small sequences indicates that enough space was created by subsidence a n d / o r rising eustatic sea level to accommodate the sediment. Even if South Sinai was submitted to a general, long-term uplift,

6 t)

blocks of the graben shoulders must have periodically subsided. This made the recording of the reef sequences possible. The observed lateral and vertical variability of facies in the small-scale sequences reflects the different, closely juxtaposed depositional environments of a narrow fringing-reef system (the sketch in Fig. 8 summarizes various facies as they occur today along the Sinai Peninsula). Reef growth was favoured when relative sea level rose and accommodation space was created (transgressive deposits). Between the reef bodies, lagoonal sediments accumulated. When the rate of relative sea-level rise decreased and accommodation was reduced, coral rubble and siliciclastic sands prograded over the reefs and the lagoonal facies (highstand deposits). During interglacial times, siliciclastic input was further stimulated by rainfall in the hinterland (Klein et al., 1990). If progradation did not reach very far, coral growth could continue on the reef crest. A drop of rela-

w~di beach

~.

C~

old reef terrace

":: :.- ~...~.~:.~oo~'"

~ TSD

~N~'~

~-~

~

TSD

SB/TS--

SB/TS--

MFS -

MF--

..'.:"

HSD MF-

TSD

TSD SB/TS--

~.~.~..-.. HSD ..~

;. HSD

[TSD

HSD MFS -

gT~',~v~,

TSD

TSD

Fig. 8. Sketch of c o m m o n l y o b s e r v e d d e p o s i t i o n a l e n v i r o n m e n t s a l o n g the m o d e r n Sinai coast, w h e r e various facies types are j u x t a p o s e d . P l e i s t o c e n e small-scale s e q u e n c e s such as i l l u s t r a t e d in Figs. 6 and 7 may have f o r m e d due to f l u c t u a t i o n s of relative sea level which led to a transgressive, t h e n regressive vertical facies evolution. For f u r t h e r discussion refer to text. boundary;

TS = t r a n s g r e s s i v e surface; TSD

facies c o r r e s p o n d i n g to m a x i m u m flooding; t r a n s g r e s s i v e deposits; H S D = h i g h s t a n d deposits.

MF= -

MFS

SB -

sequence

= m a x i m u m flooding surface:

B

I/25

B

1/22

:> 350

F 3/1 (1)

Age (ky)

Section: Sample:

2.14 0.034 + 0.001 0.448 + 0.025 1,278 + 0.097 0.475 ± 0.027 0.447 + 0.008 1.002 + 0,058

303 (242- 401)

Weight (g) 232Th (ppm) 23~Th ( d p m / g ) AU :3SU (ppm) -':~4U (drop/g) >~Th/234U

Age (kwl

Older reef (continued)

2.92 0.048 ± 0.010 (1.592 ± 0.018 1,147 + 0.056 0.658 ± 0.023 0.556 ± 0.009 1.065 ± 0,030

Weight (g) e32Th (ppm) z3~Th ( d p m / g ) AU >SU (ppm) 234U( d p m / g ) 23~ITh/234U

350

6.62 0.007 + 0.003 0.336 +_0,012 1.368 ± 0.095 0.295 + 0.016 11.298 + 0.006 l, 127 + 0.048 ~5!~

2.18 0,110 ± 0.022 1.460 ± 0,041 1.369 ± 0.076 1.290 + 0.055 1,303 ± 0.030 1.120 ± 1/.040

F

3 / 1 3 (11

3/1 (2)

302 (270-350)

3.15 11.035 + 0.012 l. 100 _+_1t.034 I. 166 ± 0.055 0.963 ± 0.023 I. 123 ± 0.037 0.979 + 1t.026

2a/5 ( I)

C

F

274 (257 294)

3.22 0,568 + 0.01 (I 1.130 ± 0.022 ~.295 ± 0.038 1.20(1 + 0.027 I. 150 + 0.1tl 5 0,982 + 0.1117

Older reef (all samples from Tridacna shells)

Section: Sample:

284 ~255 323)

3.30 0.051 + 0.014 1.428 + 0,037 1.437 ± 0.065 1.325 + 0.046 1,404 + 0.024 1.011 + 0.024

3/13 (2)

F

> 350

4./) 0.096 ± 0.074 2.300 ± 0.073 1.190 ± 0.031) 2.478 _+0.046 2.175 _+0.027 1.057 + 0.036

2 a / 5 (2)

C

U / T h ages of 7))dacha shells and corals (sample positions are indicated in Fig. 3)

TABLE 1

~.

)

314 1270--370)

4.0 11.022 + 0.006 0.660 _+0.01 ? 1.376 + 0.077 0.633 + 11.027 0.643 + (7.013 1.026 ± 0.035

6 / 4 (I)

G

340 (2911

4.0 0,048 + 0.(~(t9 0.715 + 0,017 1. }00 ± 0,034 I).886 + 0.021t 0,726 + 0.011 0.985 + 0,027

2 a / 6 ( 1)

C

35I~

4.54 0.025 + 0.010 0.715 + 0.028 t.356 + 0.081 0.640 _+0.029 11.641 + 0.Ill 5 I. 115 + 0.052

6 / 4 12)

G

270 124{)-3211)

4,0 11,042 + 1/.011 11,716 ± 0.034 1,361i + 0.047 0.863 + 0.026 11.724 + 0.013 0.990 + 0.037

2 a / 6 (2)

C

> 350

11.166 ± {7.033 6.211 ± 0.100 !).984 :~ 0,018 7.710 ± 0.0 t17 5.600 + 0.122 I. 109 + 0,020

2,96

2a/7 ( I)

(7

270 1230-3161

2.46 (7.821 ± 0.076 1.460 ± 0.050 l .I)63 + 0.032 2.010 + (7.043 1.573 + 0.026 0,928 ± 0.035

2 a / 7 (2)

C

G 6 / 7 (1)

G 6 / 7 (2)

B I / 5 (1) T

228 (208-250)

C 2a/ll c

Age (ky)

Section: Sample:

3.54 0.058 _+0.017 1.170 +_0,037 1.146 + 0.043 2.020 + 0.056 1.708 _+0.040 0.685 -+ 0.27

121 (111-130)

Weight (g) 233Th (ppm) 23°Th ( d p m / g ) AU 23SU (ppm) 2WU ( d p m / g ) 23°Th/234U

Age (ky)

Younger reef (continued)

4.49 0.122 ± 0.019 0.887+0.025 1.321 + 0.044 0.977 + 0.024 0.952 + 0.015 0.932 + 0.030

Weight (g) 232Th (ppm) 23°Th ( d p m / g ) AU 23SU (ppm) 234U ( d p m / g ) 23°Th/234 U

86 (79 94)

4.0 0.156 ± 0.290 + 1,074 + 0.665 + 0.527 + 0.550 + 0.01)7 0.015 0.078 0.034 0.015 0.032

C 2a/ll T

170 (160 180)

4.0 0.036 ± 0.021 0.805_+0.020 1.300 + 0.037 1.010 + 0.022 0.967 _+0.013 0.832 ± 0.023

65 (63 68)

4.0 0.048 + 0.373 ± 1.134 + 11.979 + 0.820 ± 0.455 + 0.007 0.010 0.031 0.020 0.010 0.013

C 2a/12 (1) T

87 (85-90)

4.0 0.043 + 0.005 0,280_+0,006 1.234 _+0.031 0.573 + 0.010 0.522 ± 0.005 0.563 -+ 0,012

Intermediate reef (Tridacna) Younger reef" (T: Tridacna samples; c: coral samples)

Section: Sample:

TABLE 1 (continued)

65 (62-68)

4.0 0.046 _+0.010 0.377 -+ 0.014 1.125 ± 0.041 1.000 + 0.027 0.834 _+0.015 0.452 ± 0.018

C 2a/12 (2) T

96 (87 106)

5.0 0.045 _+0.012 0.322 ± 0.016 1.171 + 0.107 0.624 ± 0.020 0.540 ± 0.020 0,596 + 0,038

B 1/5 (2) T

0.011 0.030 0.030 0.048 0.045 0.020 113 (11}7-119)

4.5 0.469 ± 1.410 + 1.170 + 2.480 + 2.141 ± 0.659 +

D 2/21 c

102 (93-112)

4.0 0.393 +_0.021 0.180+0.009 1.160 + 0.062 0.742 + 0.029 0.636 _+0.015 0.618 -+ 0.036

B 1/9 T

118 (108-130)

3.29 0.025 + 0.008 0.265 ± 0.013 1.169 _+0.67 0.454 + 0.019 0.392 + 0.006 0.677 ± 0.034

D 2/21 T

59 156-61)

4.0 0.110 + 0.015 0.613_+0.018 1.184 _+0.003 1.670 _+0.003 1.456 + 0.022 0.421 + 0.014

B 1/12 (1) T

0.003 0.006 0,066 0.014 0.005 0.024 86 1811-92)

4.65 1/.076 ± 0.141 ± 1.087 + 0.3211 ± 0.257 ± 0.550 ±

D 2/26 T

59 (57-61)

3.0 0.124 + 0.022 1,460_+0.038 1.176 ± 0,024 4.000 + 0.058 3.469 + 0.060 0.421 + 0.011

B 1/12 (2) T

140 1132-148)

4.0 (}.026 _+0.005 0.331 + 0.009 1.222 + 0.051 0.487 ± 0.015 0.404 i 0.006 0.745 ± 0.023

O 6/11 T

122 (110-139)

4.89 0.013 + 0.006 0.233_+0.012 1.214 _+0.095 0.375 ± 0.021 0.363 ± 0.008 0.693 _+0.040

B 1/26 T

m

© z

©

<

>

O c

72

tire sea level led to subaerial exposure of the reef and to erosion of non-consolidated sediment, thus defining a sequence boundary (e.g., Vail, 1987: Van Wagoner et al., 1990). Low sea level during glacial times coincided with arid climatic conditions in the Sinai, which reduced terrigenous input. Lowstand sediments largely bypassed the shallow depositional environment and accumulated in deeper water (Locke and Thunell, 1988). Aeolian dunes, however, may represent late highstand or lowstand deposits. Renewed flooding resulted in bored and organically encrusted surfaces of the previously exposed reef, in reworking of siliciclastic sands in a marine environment, and finally in a new phase of reef growth (Fig. 8).

U / T h dating and re&tion to eustatic sea-level changes Dating based on 23°Wh/234Uratios has been performed on some well preserved corals, but mostly on thick shells of the bivalve Tridacna, which show less diagenetic alterations than corals. Sampling technique and measuring methods are described in Bach (1990). The limit of the dating method is 350 ky. In some cases, two measurements have been performed on different fractions of the same sample (]'able 1). For the younger reef sequence, the results are quite consistent. However, differences occur between ages of a coral and a Tridacna shell collected nearby (samples 2 a / l l c and 2 a / l l T of section C: Table I). In the older reef, ages obtained from the same sample may diverge significantly, suggesting diagenetic alteration, even though X-ray analyses still indicate the original aragonitic mineralogy. The ages do not always confirm the correlations based on the interpretation of the smallscale sequences (Figs. 6 and 7), which may be due to wrong correlations, or to diagenetic alteration of the dated material. The Tridacna shell of the beachrock in section B (younger reef) yields an age which is much too old for its stratigraphic position (Fig. 7: sample 1 / 2 6 T in Table 1). Either the shell was reworked from the underlying small-scale sequence, or the exposure in the beach environment modified the original isotopic composition. Nevertheless, the dating places the older

x

N I't~,.'~bS~:l{ ~t: I /\1

reef cycle between somewhat over 35/!,1100 and 270,000 years B.P., and the younger one between 140,000 and 60,000 years B.P. The older reef cycle thus corresponds to lhc interglacial period of isotope stage 9 {Hays el al., 1976) when climate was warm and sea level high. The temperature curves based on oxygen isotopes and calculation of the orbital paramcter~ of tilt.7 Milankovitch cycles place the warmest period (and probably the highest sea teve[) around 330,00(t v B.P. (Hays et al., 1976; Berger, 1980). The youngel reef cycle formed during the last interglacial (iso tope stage 5), which reached its climatic maximum at about 125,000 y B.P. (Hays c t a l . , 197e~: Berger, 198[); Kaufman, 1986: Shackleton, 1987: Vacher and Hearty, 1989). Furthermurc, Johnson (199l) shows evidence for an carlicl (~cold"} sea-level highstand between 135 and 130 ky B.P., which was caused by deglaciation in the Northern Hemisphere due to moisture deficit. Reefs which formed during the interglacia~ period of isotope stage 7 (250,000-190J)Ot! years B.P.; Hays et al., 1976) have been identified bs, Gvirtzman and Buchbinder (1~78) in Sinai, b~ AI-Rifaiy and Cherif (1988) in ,Iordan, and h~ Dullo (1990) on the Saudi-Arabian side ~H the Gulf of Aqaba. In the outcrops studied here, 1he only evidence for this intermediate reel cycle was found in section G (Figs. 3 and 71. A small and partly broken reef body is containcd between siliciclastic sands, pebbles and beachrock Nooks, U / T h ages of a Tridacna shell place il between 23(1 and 170 ky B.P. It is possible that local tectonic activity forestalled the tormation o! this reef cycle in the study area, or that most ol il was eroded prior to the deposition of the ~,ounge~ sequence. Age-dating of the studied reel sequences thus permits to assign them to the world-wide sca-levcJ highstands of the last three interglaciaI periods, which were controlled mainly by the 100,(t00-ycar eccentricity cycles of the Earth's orbit (Milankovitch, 194t; Hays et al., 1976: Berger, 1980). blustatic sea-level fluctuations related to the 21,000year precession cycles certainly influenced the formation of the small sequences, but short-term accommodation was also strongly controlled by tectonic movements. Dating of the small-scale

S E Q U E N T I A L EVOI~UTION AND I ) I AGENESI S OF P I J S I S T O ( ' [ ' N E C O R A L REEFS

sequences is not accurate enough to correlate them with isotope substages, or to propose a timing of the tectonic movements.

Diagenesis Marine diagenesis Most carbonate grains in the studied samples show superficial or penetrative micritization which was caused by dissolution and reprecipitation of carbonate on the micron-scale along perforations of endolithic organisms (Bathurst, 1971, p. 384 if). The most common cement types indicating a marine-phreatic diagenetic environment are scalenohedral crystals of ,high-magnesian calcite and pointed or blade-shaped needles of aragonite. They are well preserved in the younger reef sequence, and also occur locally in the older reef. In most cases, high-magnesian calcite precipitated first, either as scalenohedral crystals on the surfaces of particles, or as peloidal cement. Aragonite then grew on top (Fig. 9a). Locally, however, aragonite and high-magnesian calcite coprecipitated, or high-magnesian calcite peloids grew on top of aragonite needles (Fig. 9b). It is probable that cementation was stimulated and partly controlled by microbial coatings on the grain surfaces (Mitterer and Cunningham, 1985; Strasser and Davaud, 1986), and that the peloidal cement formed around microbial nuclei (Chafetz, 1986). In some of the studied samples there is evidence that marine cementation took place in several phases. Each generation is separated from the following one by impurities such as micrite and pyrite, both produced probably by microbiological activity (Fig. 9c).

Freshwater diagenesis In some samples of the younger reef, aragonitic skeletons of corals or gastropods show beginning dissolution, and aragonite cement needles are corroded. In the older reef, dissolution of unstable carbonate phases (aragonite and, to a lesser extent, high-magnesian calcite) destroyed much of the original framework and created im-

73

portant secondary porosity. In many cases, only the micritic envelopes are preserved (Figs. 10a and 10c). Dissolution was selective according to the microstructure of the organisms (Gvirtzman and Friedman, 1977; Constantz, 1986; Dullo, 1986). Furthermore, aragonite particles surrounded by a micritic matrix are generally better preserved than those in highly permeable grainstones, rudstones or framestones. Where highmagnesian calcite is preserved in the older reef, it still contains 12 to 17% of MgCO 3 (microprobe analyses of selected samples). Isopachous fringes of low-magnesian calcite crystals commonly occur in the older reef sequence and indicate cementation in the phreatic freshwater zone. The crystals either grew into pore space (Fig. 10a), or replaced earlier marine cements (Fig. 9c). Strontium contents of up to 80(1 ppm suggest aragonite precursors (microprobe analyses). Meniscus cements indicating a vadose zone were found only on the top of sections B and C (older reef; Fig. 3), where they are associated with pedogenetic features such as glaebules and desiccation cracks. The predominance of features suggesting freshwater diagenesis in the older reef sequence can be easily explained: tectonic uplift raised the top of the older reef out of reach of the two subsequent sea-level highstands (corresponding to isotope stages 7 and 5). The more humid climate of these warmer periods (Klein et al., 1990) then allowed freshwater lenses and rainwater to invade the exposed reef. The younger reef sequence, however, was permanently exposed only since the end of the last interglacial period, and climate since that time was rather arid.

Dolornitization Much of the older reef sequence and patches of the younger one are dolomitized. In samples of the younger reef, dolomite is commonly associated with high-magnesian calcite (Fig. 9d), but also grew between aragonite needles. In the older reef, dolomite crystals tended to grow on highmagnesian calcite (Figs. 10a and 10b). Many corals of the older reef, after complete dissolution of their aragonite skeletons, have been cemented by

74

.,\. S I R A S S | : P ,

I! 1 ~,i

Fig, 9. (a) Peloidal and aragonitic cements: aragonite needles grew on a dark micritic layer enveloping shell fragment and on peloids composed of high-magnesian calcite; P = pore space (section D, younger reef, thin-section photomicrograph). (b) l~cally, high-magnesian calcite peloids formed after the precipitation of aragonite needles (same sample as in (a), SEMI. (c) Multiple phases of cement growth, separated by impurities of micrite and pyrite (underlined at left of photograph); the original aragonite has been replaced by calcite which preserved the needle-like habitus of aragonite, but shows crystal terminations (arrows) typical of calcite; P = pore space (section F, older reef, thin-section photomicrograph). (d) Organic fragment is overgrown by small crystals of high-magnesian calcite, pore space is filled by large aragonite crystals. Dolomite rhombs precipitated mainly on high-magnesian calcite, but also invaded the aragonite needles (section D, younger reef, SEM).

d o l o m i t e (Fig. 10c). M o s t d o l o m i t e r h o m b s have a p r i s t i n e a s p e c t (Fig. 10b). Locally, however, d o l o m i t e crystals m a y be heavily c o r r o d e d (Fig. 10d). In a l a t e r stage, the d o l o m i t e m a y again be c o v e r e d by m a r i n e c e m e n t s . X - r a y analyses o f d o l o m i t i z e d corals, r e d algae, a n d total s e d i m e n t show t h a t d o l o m i t e contents vary from a few p e r c e n t to up to 100 percent. T h e d o l o m i t e s are all rich in calcium (53 to

65 m o l e % C a C O 3) a n d p o o r l y o r d e r e d (int e g r a t e d intensity 35.30/37.3 ° from 0.10 to 0.84), which is typical o f an early d i a g e n e t i c origin ( B a t h u r s t , 1971, p. 238). S t a b l e - i s o t o p e values of c o m p l e t e l y d o l o m i t i z e d corals a n d a l g a e N o t bet w e e n + 3 a n d + 4 % 0 ~13C PDB, a n d b e t w e e n + 2 a n d + 4 % o 61~O P D B (Fig. 11). T h e s e values a r e c o m p a r a b l e to t h o s e of H o t o c e n e s a b k h a d o l o m i t e s ( M e K e n z i e , 1981) a n d o f s o m e

S E Q U E N I ' I A L E V O L U T I O N AND DIAGENESIS O F P L E I S T O C E N E C O R A l ; REEFS

75

Fig. 10. (a) Completely dissolved organic fragments, with only the micritic envelopes and early-diagenetic peloidal cemenl preserved. High-magnesian and low-magnesian calcite grew into pore space, some dolomite crystals (arrows) formed on top: P = pore space (section B, older reef, thin-section photomicrograph). (b) Scalenohedral crystals of high-magnesian calcite arc overgrown by rhombic dolomite crystals (same sample as in (a), SEM). (c) Completely dissolved coral, where the micritic envelopes have been overgrown by dolomite; P - pore space (section D, older reef, thin section photomicrograph). (d) Corroded and partly leached dolomite crystals; small needles of aragonite grew on top of them (section F, older reef. SEM).

dolomites interpreted to have formed in a mixing zone (e.g., Supko, 1977; Ward and Halley, 1985; A'/ssaoui, 1988; see also discussions in Hardie, 1987, and Coniglio et al., 1988). In the older reef, dolomitization is encountered only below the level corresponding to the top of the younger reef (Fig. 12). Isotope values of calcitized corals taken above this level are negative and indicate freshwater influence (Hudson, 1977). Samples below this level containing 10% to 50% dolomite plot between the samples

showing freshwater diagenesis and those composed entirely of dolomite (Fig. 11). Samples of the younger reef generally show isotope values of normal-marine limestones (Hudson, 1977). These observations suggest that dolomitization probably took place in a mixing zone. Marine waters flushed through the lower part of the already cemented but permeable older reef during the sea-level highs of isotope stages 7 and 5. The younger reef formed during isotope stage 5, but may have been exposed and flooded once or

70

..\ 5, I'R,,\~,Y~t:lz. t-:l k l

%o(513(:PDB

¢ dolomites

Od

•d•dAd i

-6

•d

•d

OA d

i

+2

,

,

-2

+2

,

*4 _%o~180PDB

dOOd

-2

freshwater influence

•

younger r e e f

•

older reef

d some dolomite Fig. 11. Isotope values of samples in older and younger reef sequence. For discussion refer to text.

a few times due to minor eustatic sea-level changes and tectonic movements, before sea level drastically dropped during the last glaciation. Aquifers in the pre-Pleistocene substrate could have functioned as point-sources of freshwater flowing into the older reef sequence (Fig. 12). Percolation through the vadose zone was probably less important (very few vadose diagenetic features have been found). Where and when dolomitization occurred thus depended on pri-

1

mary mineralogy, on rock texture, on marine flooding phases, and on phases and conduits of freshwater input. The positive oxygen isotope values may be due to arid climatic conditions which followed the more humid periods. The absence of vegetation precluded the formation of soil gas which would have induced negative values of carbon isotopes. A similar model of mixing-zone dolornitization has been proposed by Ward and Halley (1985) for

!

older

reef ~ ~

sea level1257 B.~P,

Fig. 12. Sketch illustrating the relationship between dolomitization and the last sea-level high. The older reef i~ strongl~ dolomitized up to this level, whereas the younger reef shows only patchy dolomitization. Arrows indicate freshwater flow. For discussion refer to text.

S E Q U E N T I A l . E V O L U T I O N AND DIAGENESIS OF P L E I S T O C E N E ( ' O R A l . REEFS

Upper Pleistocene reefs in Yucatfin. The diagenetic features resemble those found in the Sinai reefs, and the isotopic values of the dolomite ( + 1 to +3%0 613C PDB, +1 to +3%o a l s o PDB) correspond well to those of our study. Ward and Halley concluded that the dolomitizing pore waters in Yucatfin ranged from 75% to almost 100% seawater. Selective dolomitization of high-magnesian calcite cements has been documented by A'issaoui (1988) from Mururoa Atoll. The dolomitizing fluids had a composition close to that of seawater. Mixed-water dolomitization in Pleistocene reefs related to sea-level fluctuations has also been described from Barbados (Humphrey, 1988; Humphrey and Quinn, 1988). There, the carbon isotope values are very negative (around -15%) due to the influence of soil gas, and dolomitization was strongly dominated by meteoric waters. Land (1973) described Holocene dolomitization of Pleistocene reefs in Jamaica. Negative isotope values again suggest meteoricdominated dolomitizing fluids.

Conclusions The Pleistocene reefs of South Sinai presented in this study reflect the complex interactions of climatic, eustatic and tectonic factors. Two major reef sequences could be identified, which correspond to the eustatic sea-level highs of isotope stages 9 and 5 (Hays et al., 1976). Reef facies deposited during isotope stage 7 are present only as relics in the study area, but have been identified elsewhere on the shores of the Gulf of Aqaba (AI-Rifaiy and Cherif, 1988; Dullo, 1990). These sequences correlate well with the important eustatic sea-level fluctuations resulting from the 100,000-year orbital cycles of eccentricity (Hays et al., 1976). Smaller depositional sequences compose the major reef sequences. They commonly display reefal or lagoonal facies representing transgressive deposits, and coral rubble and siliciclastics corresponding to highstand deposits (Fig. 8). The formation of the small sequences was certainly influenced by sea-level fluctuations following the 21,000-year precession cycles, but sediment accommodation was equally controlled by

77

tectonic movements. Age-dating is not accurate enough to separate the smaller sequences. Therefore, the tentative correlations proposed in Figs. 6 and 7 are based mainly on the sequential evolution of the small-scale sequences. In areas of a relatively stable tectonic setting, reef sequences monitor astronomically controlled eustatic sea-level cycles quite accurately. Examples have been published from Barbados (Mesolella et al., 1969) and from New Guinea (Bloom et al., 1974; Aharon and Chappell. 1986; Chappell and Shackleton, 1986; Hearty and Aharon. 1988), where reef terraces actually correspond to ancient sea-level highstands. Irregular tectonic activity in the Red Sea area, however, greatly disturbed the sedimentary record. Notches. and terraces indicate relative sea-level stands, which may or may not correspond to eustatic levels (Faure et al., 1973; Behairy, 1983). It is rather the stratigraphic relationship between the reef units which defines the sedimentary cycles (AI-Rifaiy and Cherif, 1988; Dullo, 1990). Diagenesis of the reef sequences was closely related to the rapid climatic, eustatic and tectonic changes. Differences in primary composition and microstructure of the organisms, differences in sorting, packing and binding of the sediment, repeated marine flooding as well as variable input of meteoric waters led to a complex and selective, multi-phased diagenetic history. Dissolution of aragonite skeletons and subsequent dolomitization was common in the older reef, but only patchy in the younger one. Dolomitization probably took place in a seawater-dominated mixing zone. If the evolution of a sedimentary system can be tied to astronomically controlled fluctuations of climate and sea level, a time framework can be established which also allows the timing of diagenetic phases. Coral reefs are very sensitive to environmental changes, and the well preserved Pleistocene reefs are well suited for such research. In South Sinai, the overprint of tectonic movements blurs the eustatic signal, and precise U / T h dating is made difficult by the diagenetic modification of the original composition of the carbonate. Nevertheless, this study shows that there is a potential for a very detailed analysis of

A . S l lt!\~,~,J I~ t{ I .'\1

78

the sedimentary and diagenetic evolution of a coral reef system.

Acknowledgements We thank C. Roberts (Marine Biology Department, Sharm el Sheikh) for valuable discussions. W.C. Dullo (Kiel) and P. Hearty (Tampa) kindly read a first version of the manuscript. D. Cao (Fribourg) carried out the SEM photography. R. Oberh~insli (Berne) performed the microprobe analyses, J.A. McKenzie (Zurich) did the isotope work. Their help is gratefully acknowledged. We also thank D. Bosence, B. Sellwood and an anonymous reviewer whose comments greatly improved the manuscript. Field expenses were partly financed by the Swiss National Science Foundation (project No. 2.512.087). References Aharon, P. and Chappell, J., 1986. Oxygen isotopes, sea level changes and the temperature history of a coral reef envir o n m e n t in New Guinea over the last 111s years. Pataeogeogr., Palaeoclimatol., Palaeoecol., 56: 337-379. Aissaoui, D.M., 1988. Magnesian calcite cements and their diagenesis: dissolution and dolomitizatiom Mururoa Atoll. Sedimentology, 35: 821-841. AI-Rifaiy, I.A. and Cherif, O.H., 1988. The fossil coral reefs of AI-Aqaba, Jordan. Facies, 18: 219-230. Bach, A., 1990. Systematische U n t e r s u c h u n g e n zur 2~°Th/ 234U-Datierung biogener Karbonate an jung/mittelpleistoziinen marinen Terrassen der Kiistenebene von Tarquinia, Mittelitalien und den Riffterrassen der Sharm el Sheik Region am Golf von Aqaba, Agypten. Doctoral thesis, Univ. H e i d e l b e r g (unpublished). Bathurst, R.G.C., 1971. Carbonate Sediments and their Diagenesis. Developments in Sedimentology, 12, Elsevier, A m s t e r d a m , 2nd ed., 658 pp. Behairy, A.K.A., 1983. Marine transgressions in the west coast of Saudi Arabia (Red Sea) between Mid-Pleistocene and Present. Mar. Geol., 52: 25-31. Berger, A., 1980. The Milankovitch astronomical theory of paleoclimates: a modern review. Vistas Astron., 24: 103122. Bloom, A.L., Broecker, W.S., Chappell, J.M.A., Matthews, R.K. and Mesolella, K.J., 1974. Quaternary sea level fluctuations on a tectonic coast: new 23°Th/Z34u dates from the H u o n Peninsula, New Guinea. Quat. Res., 4: 185-205. Briem, E., 1989. Die morphologische und tektonische Entwicklung des Roten Meer-Grabens. Z. Geomorphol., N.F., 33: 485-498.

Chafetz, H.S., 1986. Marine peloids: a product of bacterially induced precipitation of calcite. J. Sediment. Petrol., 5~i: 812-817. Chappell, J. and Shackleton, N.J.. 1986. Oxygen i~,otopcs and sea level. Nature, 324: 137- 140. Coniglio, M., James, N.P. and Aissaoui, D.M., 198b; l)olomitization of Miocene carbonates. Gulf of Suez, [:~gypL .i. Sediment. Petrol., 58:100-119. Constantz, B.R., 1986. T h e primary surface area oi ~_~rals and variations in their susceptibility to diagencsis. In: J.ll. Schroeder and B.H. Purser (Editors), Reel I)iagenesi~. Springer-Verlag, Berlin, pp. 53-76. Deuser, W.G., Ross, E.H. and Waterman, L.S.. 1!17~. Glacial and pluvial periods: their relationship revealed by Pleis tocene sediments of the Red Sea and Gulf ~,t Aden. Science, 191: 1168-11'70. Dullo, W.C., 1984. Progressive diagenetic sequence o! arago~ nite structures: Pleistocene coral reefs and their modern counterparts on the eastern Red Sea coast, Saudi Arabia. Paleontogr. Am., 54: 254-26(t. Dullo, W.C., 1986. Variation in diagenetic sequences: an example from Pleistocene coral reefs. Red Sea, Saudi Arabia. In: J.H. Schroeder and B.H. Purser (Editors), Reef Diagenesis. Springer-Verlag, Berlin, pp, 7 7 90. Dullo, W.C.. 1990. Facies, fossil record, and age- ~)f Pleistocene reet\s from the Red Sea (Saudi Arabia). Facies, 22: 1-46.

Faure, H., Hoang, C.T. and Lalou, ( . . 1973. Structure et g6ochronologie (231~Th/Z~U) des rdcifs coraJliens soulevd~, iL l'ouest du Golfe d'Aden (T.F.A.I.). Rex. (idogr. Phys Gfiol. Dyn., 15: 393-403. Freund, R., Garfunkel, Z., Zak, 1., Goldberg, M., Weissbrod. T. and Derin, B., 1970. The shear ahmg the I2,ead Sea rifL Philos. Trans. R. Soc. London, A 267: 10% 13(i Garfunkel, Z., 1987. Post-Precambrian sediments. In: Y . K Bentor and M. Eyal (Editors), Jebel Sabbagh Sheet. lsrael Academy of Sciences and Humanities, ,lerusalem, pp. 368-392. Gvirtzman, G. and Buchbinder, B., 1978. Recent and Pleistocene coral reefs and coastal sediments of the Gulf of Elat. In: Guidebook 10th Int. Congr. Sediment.. ,Jerusalem. pp. 162-191. Gvirtzman, G. and Friedman, G.M., 1977. Sequence ol ptt,gressive diagenesis in coral reefs. Am. A s s o c Pet. G e d . . Stud. Geol., 4: 357-380. Gvirtzman, G., Friedman, G.M. and Miller, I).S., I~,73. Control and distribution of uranium in coral reefs during diagenesis. J. Sediment. Petrol., 43: 985-997, Gvirtzman, G., Buchbinder, B., Sneh, A., Nir, 5~. and Friedman, G.M., 1977. Morphology of the Red Sea fringing reefs: a result of the erosional pattern of the last-glacial low-stand sea level and the following Holocene recolonization. M6m. B R G M , 89: 480-491. Hardie, L.A., 1987. Dolomitization: a critical view of some current views. J. Sediment. Petrol., 57: t66-183. Hays, J.D., Imbrie, J. and Shackleton, N.J., 1976 Variations

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