lower miocene coastal lagoon carbonates and evaporites of … · 2012-12-30 · coastal lagoon. the...

JKAU: Mar. Sci., Vol. 23, No. 2, pp: 131-164 (2012 A.D. / 1433 A.H.)

DOI : 10.4197/Mar. 23-2.8

131

Lower Miocene Coastal Lagoon Carbonates and

Evaporites of Rabigh Area, Red Sea Coast,

Saudi Arabia

Rushdi J. A. Taj

Department of Petroleum Geology and Sedimentology, Faculty of Earth

Sciences, King Abdulaziz University, Jeddah, Saudi Arabia.

E-mail: [email protected]

Abstract. The carbonate-evaporite sequence in Rabigh area forms Al

Jahfah Formation that is conformably overlying the siliciclastic

sequence of Al Haqqaq Formation. Field examination of Al Jahfah

Formation indicates that the thickness of the carbonate rocks is limited

(< 15m) in contrast to > 50 m thick for the evaporite rocks (mainly in

the quarries). The carbonate rocks are well exposed due south, east

and north of the evaporite rocks.

Microscopic examination of the studied rocks indicates the

existence of the following carbonate microfacies types; (a) dolomitic

foraminiferal packstone, (b) dolomitic oolitic wackestone, (c)

dolomitic intraclastic wackestone, (d) dolomitic mudstone, and (e)

boundstone. The recorded evaporite microfacies types are: (a)

porphyroblastic gypsum, (b) granoblastic gypsum, (c) alabastrine

gypsum, (d) satin spar gypsum veins, (e) secondary anhydrite, and (f)

micritized microbial laminae.

The diagenetic processes that affected the carbonate rocks

during early diagenesis are: Micritization, aggrading neomorphism,

compaction, dissolution, early cementation and dolomitization.

Displacive growth of gypsum nodules are assumed to be formed

during early diagenesis. The alteration effect of burial stage of

diagenesis is more pronounced on the evaporite rocks. This is

attributed mainly to their solubility, where gypsum is converted to

anhydrite. The uplift stage of diagenesis is characterized by late

cementation of the carbonates and hydration of burial anhydrite to

secondary gypsum rocks. Due to solar heating, the secondary gypsum

dehydrates to felted anhydrite on outcrop.

132 Rushdi J.A. Taj

Field, sedimentary and petrographic criteria point to the

formation of the studied carbonate and evaporite rocks in shallow

coastal lagoon. The carbonate rocks were formed at the periphery of

the lagoon, whereas the evaporite rocks were formed in the central,

continuously subsiding part of the lagoon. Therefore, a bull's eye

distribution pattern of the carbonate-evaporite rocks is inferred.

Keywords: Coastal lagoon, carbonates, evaporites, Rabigh, Saudi

Arabia.

Introduction

Early reconnaissance mapping of the Red Sea coastal plain of Saudi

Arabia was started in the second half of the last century by Brown et al.

(1963), followed by Brown (1970). After wards, the importance of the

Saudi Arabian Red Sea was increased rapidly due to oil exploration in

Midyan peninsula (Hughes and Johnson, 2005). Various informal

lithostratigraphic schemes have been applied to the Saudi Arabian Red

Sea succession by numerous authors (see Hughes and Johnson, 2005 for

details). Filatoff and Hughes (1996) integrated micropaleontological,

palynological and lithological analyses of the Saudi Arabian Red Sea

sediments. They stated that supratidal, freshwater conditions prevailed

during the Late Cretaceous, Oligocene, Early and Late Miocene to

Recent. Marginal marine conditions prevailed in the Paleocene to Lower

Eocene successions. Marginal marine conditions involving periodic

hypersaline sabkha and hypersaline lake development existed during the

Early and Late Miocene. Deep water conditions prevailed in late Early

Miocene to early Middle Miocene that culminated with episodes of

hypersalinity in the late Middle Miocene.

Due to increase of the economic aspect of the Neogene succession

and the greater accessibility to Neogene subsurface samples, Hughes and

Johnson (2005) revised the Neogene lithostratigraphy of the Saudi Red

Sea region. They reported that the sedimentary succession was deposited

during the Cretaceous to Pleistocene times on the Proterozoic basement

rocks. Some of the Neogene formations display significant lateral and

vertical facies variations. For example, the siliciclastics of Al-Wajh

Formation is overlain by carbonate of the Musayr Formation, or by

anhydrite of the Yanbu Formation (Hughes and Johnson, 2005). This

facies variation of the Miocene formations extends also from the north to

Jeddah at south.

Lower Miocene Coastal Lagoon Carbonates and Evaporites of … 133

Most studies carried out north of Jeddah area were concerned with

the Quaternary coral reefs and conglomerate (e.g. Behairy, 1980; El-

Sarbouti, 1983; Behairy and El-Sayed, 1984; Dullo and Jado, 1984;

Dullo, 1986; Durgaprasada Rao and Behairy, 1986; Durgaprasada Rao et

al. , 1987; Gheith and Abou Ouf, 1997; Basaham and El-Shater, 1994,

Basaham, 1998; 2004; Bantan, 2006; and Basaham et al. , 2006). On the

other hand, recent supratidal sabkhas are studied by several authors (e.g.

Sabtan and Shehata, 2003; Basyoni, 2004; Basyoni and Aref, 2007; 2009;

2010 and 2011; and Taj and Aref, 2009 and 2011). However some works

were done regarding the Miocene formations (e.g. Abou Ouf and Gheith,

1997; Abou Ouf, 1998; Taj et al. , 2001; 2002 and 2004; Taj and

Hegab, 2005; Mandurah and Aref, 2010 and 2011; Aref and Mandurah,

2011; Ghandour and Al-Washmi, 2011; and Taj, 2011).

The purpose of the present work is to study the distribution and

petrographic characteristics of the carbonate and evaporite rocks in

Rabigh area on the Red Sea coastal plain of Saudi Arabia (Fig. 1). The

main diagenetic processes that modify the primary texture and

mineralogy of the carbonates and evaporites were discussed during

shallow and deep burial and at uplift. A model for the depositional

environment of the studied carbonate and evaporite rocks were

constructed.

The present work is based on the following: (1) Four field trips to

the Miocene carbonate and evaporite rocks of Rabigh area were carried

out. (2) Four stratigraphic sections of the Miocene evaporite and

carbonate rocks were measured and sampled (Fig. 1). (3) A total of 40

standard thin sections of the carbonate and evaporite rocks were prepared

by using epoxy cement under dry, cool condition. In order to differentiate

between calcite and dolomite, half part of each thin section of the

carbonate samples was stained with Alizarin Red-S according to the

method described by Adams et al. (1984). (4) The mineralogical

compositions of 11 carbonate and evaporite samples were confirmed by

XRD technique (Fig. 1) at the laboratory of Faculty of Earth Sciences,

King Abdulaziz University.

134 Rushdi J.A. Taj

Geologic Setting

North of Jeddah area, the Precambrian basement rocks form the

eastern shoulder of the Red Sea coastal plain of Saudi Arabia (Fig. 1).

Fig. 1. Geologic map (A) and lithologic sections (B) in Rabigh area, Red Sea, Saudi

Arabia.

A

RED

SEA

Wadi

Wadi Al Jarba

0 10 km

22° 45’

22° 43’

22° 41’

22° 39’

39° 07 39° 09’ 39° 11’ 39° 13’

4 2

1

3Miqat Al Jahfah

Miocene Precambrian Recent sabkha

Quaternary sand Neogene volcanics Quaternary coral

Quaternary

B

d,

d

d, q

c

d, c, q

1

2

3 4

d

c

g

g

g

Mineralogy

(XRD)

d: dolomite

c: calcite

q: quartz

g: gypsum

Shale

Fossil. limestone

Gypsu

Sandstone

Non-fossil.

Coral

0

3

A


They are unconformably overlain by the Cretaceous to Miocene

sedimentary rocks (Haddat Ash Sham, Usfan, Shumaysi, Khulays, Dafin

and Ubhur formations) in the west and by the Miocene to Pliocene lavas in

the north (Ramsey, 1986; Moore and Al-Rehaili, 1989). Extensive areas

of the coastal plain and the major wadies are surficially covered by the

Quaternary deposits (sand and gravel).

In Rabigh area, the Miocene Dafin Formation of Ramsey (1986) is

composed of three lithofacies; siliciclastics, carbonates and evaporites.

Taj and Hegab (2005) assigned Al Haqqaq Member for the lower

siliciclastic sequence, Al Jarba Member for the middle carbonates

sequence, and Al Jahfah Member for the upper evaporite sequence. The

siliciclastics lithofacies is widely exposed in most of the eastern part of

Rabigh area at Wadi Al Haqqaq, Wadi Al Hajar and Wadi Al Jarba. The

carbonate facies crops out at the southeastern and eastern parts of the

sedimentary cover (Wadi Al Jarba and Wadi Al Haqqaq). The evaporite

facies crops out at the most northwestern part of the sedimentary cover

(Miqat Al Jahfah) and increases in thickness towards north and

northwest.

In the present work, the siliciclastics of Al Haqqaq Member is

conformably overlain at one time by the carbonate facies in the south and

east and by evaporite facies in the north. Also, in the same hill, the

evaporites crop out at one side and the carbonates crop out on the other

side at the same stratigraphic level, in which their lateral facies change is

obscured by a mantle of weathered materials and sand dunes. These

observations indicate the lateral facies variation of the carbonate-

evaporite rocks and their difficulty for being different members.

Therefore, the carbonate-evaporite facies is considered as one unit as

they are stratigraphically equivalent.

Hughes and Johnson (2005) mentioned that the Lower Miocene Al

Wajh and Yanbu formations have regional distribution from Al Wajh and

Yanbu basins at north to Jeddah at south. By comparison of the results of

Hughes and Johnson (op.cit.) and the distribution and facies

characteristics of the rock units in Rabigh area, the siliciclastics of Al

Haqqaq Member can be raised to the formation rank and they are

equivalent to the siliciclastics of Al Wajh Formation. The carbonates-

evaporites facies of Al Jarba and Al Jahfah members are also raised to

the formation rank and they are equivalent to the evaporite of Yanbu

136 Rushdi J.A. Taj

Formation. Accordingly, it is suggested that the Dafin Formation is

obsolete and Al Haqqaq Formation is used for the siliciclastic succession,

and Al Jahfah Formation is used for the carbonate-evaporite succession.

In the present section, both carbonate and evaporite rocks are

discussed in details.

Carbonate rocks

The Miocene carbonate rocks have small stratigraphic exposures in

Wadi Al Jarba and Wadi Al Haqqaq relative to the evaporite rocks. The

carbonate rocks are generally mantled with powdery weathered carbonate

soil and/or basaltic boulders (Fig. 2). They form conical hills and buttes,

less than 15 m in height (Fig. 3).

Field investigation of two stratigraphic sections (‘1’ and ‘2’ in Fig.

1B) of the carbonate rocks indicates that they are composed generally,

from bottom to top, of dirty white, pale yellow dolomitic limestone, with

variable amounts of foraminifers, gastropod and bivalve shells embedded

in fine bioclastic matrix (Fig. 4 and 5). Sand sized quartz grains with

pebbles and cobble-sized volcanic rock fragments are recorded

sporadically in some dolomitic limestone layers. Millimetric moldic vugs

of bivalve and gastropod shells are common (Fig. 4), where the internal

and external molds of the shells are filled with bioclasts and fine sand

sediments. Local high concentration of shells and shell fragments (e.g.

Clypeaster and Echinolampas sp. or bivalves) are recorded (Fig. 4).

Fig. 2. Basalt boulders overlying the

weathered carbonate rocks at Wadi

Al Jarba.

Fig. 3. A conical hill consists of carbonate

rocks that conformably overlying Al

Haqqaq sandstone.

3 2


Fig. 4. Molds of bivalve shells set in a

matrix composed of fine bioclasts

and detrital quartz and volcanic

grains.

Fig. 5. Internal mold of a gastropod shell

sets in a matrix composed of fine

bioclasts and detrital grains.

Near the top of the carbonate section, a varicolored yellow, pink to

white, unfossiliferous lime mudstone is recorded. Colonial corals in

upright position are recorded below a thick section of Quaternary

conglomerate (Fig. 6). When the carbonate section is not covered by

Quaternary conglomerate, coral debris are recorded at the top of the

carbonate hills.

Fig. 6. Colonial corals in a growth

position below Quaternary

gravels set in lime mud matrix.

Evaporite rocks

Field investigation of two stratigraphic sections of the evaporite

rocks (‘3’ and ‘4’, Fig. 1B) at outcrop and in active quarries of Al-Arabia

and Al-Janobia Cement companies indicates that the evaporite sequence

is composed of two distinctive horizons separated by a 1.5 meter thick

mudstone layer. The outcropping evaporite sequence is conformably

overlying green, brown sandstone, siltstone and mudstone layers (Fig. 7).

Near the top part of the clastic sequence, angular intraclasts (rip-up

6

54

138 Rushdi J.A. Taj

breccias) of mudstone is recorded at the middle of a siltstone layer

(Fig. 8). The lower evaporite sequence is represented (from bottom to

top) by displacive nodular anhydrite structure within brown mudstone

(Fig. 9). Near the topmost of the mudstone layer, numerous gypsified

rootlets (rhizocretions) are recorded (Fig. 10). They are followed by

several evaporite layers that show stromatolitic, grass-like, microbial

laminated and clastic gypsum.

Fig. 7. Brown sandstone of Al Haqqaq

Formation is conformably overlain

by the evaporites of Al Jafa Formation.

Fig. 8. Rip-up breccia consists of angular

mudstone clasts that dispersed in a

siltstone layer.

Fig. 9. Displacive nodular anhydrite in

brown mudstone.

Fig. 10. Gypsified rootlets at the base of the

evaporite sequence.

The stromatolitic gypsum layers range from 20 to 50 cm thick and

are composed of wavy regular and irregular microbial laminae that form

laterally linkage head of stromatolite type (Fig. 11). The stromatolites are

formed of thin (3-5 cm) gypsum layers that interlayered with greenish to

7 8

9 10


brownish micritized microbial laminae (< 1 cm thick), (Fig. 5), or thicker

(5 cm thick) greenish carbonate layer. On the bedding surface, the

stromatolitic gypsum forms ripple like morphology of irregular, non-

bifurcated or bifurcated crests (Fig. 12).

Fig. 11. Stromatolite structure consists of

dark microbial laminae and white

gypsum laminae.

Fig. 12. Rippled appearance of the

stromatolites on the bedding

surface.

The grass-like gypsum layer is composed of stacked single,

twinned or rosette gypsum crystals that form 3-5 cm thick layering, with

greenish carbonate mud in-between (Fig. 13). Some of the gypsum

crystals are turned to white due to climatic dehydration into anhydrite.

The clastic gypsum layers, 15-20 cm thick, are composed of

fragments of prismatic and twinned white gypsum crystals (< 3 cm long)

that disperse in brownish carbonate mud (Fig. 14). They represent

reworking of grass-like gypsum by slight agitated water. The microbial

laminated gypsum layers, 70-120 cm thick, form of slightly irregular

thin, dark green to brown microbial carbonate laminae and thicker white

to pale grey or yellow gypsum laminae. The last facies has a significant

thickness and forms most of the upper part of the lower evaporite

sequence at the quarry faces.

The upper evaporite sequence is composed of black, regular

microbial laminated gypsum layers, highly enriched in horizontal satin

spar gypsum veins (Fig. 15), and yellow, brown or black massive

gypsum layers.

11 12

140 Rushdi J.A. Taj

Fig. 13. Grass-like gypsum consists of

stacks of vertically oriented crys-tals

with dark carbonate mud in-

between. Note satin spar gypsum

vein near the top.

Fig. 14. Reworked prismatic and swallow-

tail gypsum crystals within brown

carbonate mud.

Fig. 15. Regular interlamination of dark

gypsum laminae and lighter car-

bonate laminae.

Petrography

(1) Petrography Of The Carbonate Rocks

Petrographic examination of 16 thin sections for the carbonate

rocks were carried out. Thin sections were stained with Alizarine Red-S.

Furthermore, 8 samples were analyzed by XRD technique in order to

confirm their mineralogy. The above investigations led to identification

of the following microfacies types (according to the petrographic terms

of Dunham (1962): (a) dolomitic foraminiferal packstone, (b) dolomitic

oolitic wackestone, (c) dolomitic intraclastic wackestone, (d) dolomitic

mudstone, and (e) boundstone.

13 14

15


(a) Dolomitic Foraminiferal Packstone Microfacies

This microfacies type is similar to "Foraminiferal Limestone"

facies of Taj and Hegab (2005). It is recorded near the bottom of the

carbonate section. The skeletal materials are represented dominantly by

miliolid, alveolinid and/or soritid foraminiferal tests (Fig. 16), with

subordinate amounts of bivalves, gastropods, echinoid spines and ooids.

The matrix is composed of dolomicrite and dolomicrosparite. The

foraminifera, echinoid spines and ooids are mimically replaced by

dolomite (Sibley and Gregg, 1987), whereas the bivalve and gastropod

shells are dissolved and filled with drusy calcite spar (Fig. 17). The

bivalve and gastropod shells are surrounded by micrite envelopes that

preserve their primary morphology from being totally dissolved. Also,

the ooids are suffered from intense micritization and dolomitization that

transformed them to peloid grains

It is important to note that some parts of this microfacies type

contains fine-sand sized quartz grains and granule sized volcanic

fragments set in-between the skeletal components (Fig. 16).

Fig. 16. Aveolinid foraminiferal test with

radial wall structure and detrital

quartz set in dolmicrite and dolo-

microspar, Polars Crossed.

Fig. 17. A gastropod shell dissolved and

filled with sparite (now dolosparite),

and the champers are filled with

micrite (now dolomicrite), Plane

Light.

(b) Dolomitic Oolitic Wackestone Microfacies

This microfacies is equivalent to "Ooidal Limestone" facies of Taj

and Hegab (2005). It consists dominantly of ooids with subordinate

amounts of gastropods and bivalves (Fig. 18) set in dense dolomicrite

and dolomicrosparite matrix. Most parts of the ooids are micritized with

relics of the original concentric structure. However, all ooids and the

250 µm 100 µm

16 17

142 Rushdi J.A. Taj

skeletal grains, as well as the matrix are completely and mimically

replaced with dolomite. The bivalve and gastropod shells are dissolved

and later filled with mosaics of drusy calcite spar and blocky calcite

crystals.

(c) Dolomitic Intraclastic Wackestone Microfacies

This microfacies type consists of angular and subangular intraclasts

that are composed of different skeletal and non-skeletal components set

in dolomicrite and dolosparite (Fig. 19). In addition, broken gastropods,

bivalves, foraminifers and echinoid spines are dispersed in dolomicrite

matrix. It is important to note that all components of this microfacies

(ortho-and allochemical components) are completely dolomitized, where

their original fabric and microstructures are still preserved.

Fig. 18. Micritized ooids set in dolomicrite,

Plane Light.

Fig. 19. Intraclasts and peloids set in dolo-

micrite and dolomicrospar, Plane

Light.

(d) Dolomitic Mudstone Microfacies

This microfacies is recorded near the top of the carbonate section.

It is equivalent to "Fine-grained (micrite) Limestone" facies of Taj and

Hegab (2005). It consists dominantly of very fine, dense dolomicrite,

dolomicrosparite and fine silt sized quartz grains (Fig. 20). Skeletal

and/or non-skeletal carbonate grains or their ghosts are not observed in

this microfacies type.

(e) Boundstone Microfacies

This microfacies type is similar to "Coralline Limestone" of Taj

and Hegab (2005). It is recorded at the top most part of the carbonate

section. It consists of scleractinian colonial coral, whereas the septa and

dissepiments consist of fibrous and mosaic of calcite crystals.

Sometimes, the septa and dissepiments are dissolved and filled with

19

250 µm

18

250 µm


elongated and blocky calcite crystals that preserve the radial structures of

the corallites (Fig. 21). No dolomitization process is recorded in this

microfacies, in spite of the severe dolomitization of the previously

mentioned microfacies types.

Fig. 20. Silt-sized quartz grains scattered in

dense dolomicrite and dolomicro-

spar, Polars Crossed.

Fig. 21. Scleractinian coral filled with

radial calcite crystals, stained

section with Alizarin red-S, Polars

Crossed.

(2) Petrography Of The Evaporite Rocks

Petrographic investigations of 24 thin sections of the evaporite

rocks indicate that they are composed dominantly of secondary gypsum

and anhydrite. The mineralogical composition is confirmed by analyzing

3 selected samples by XRD technique (Fig. 1B). The secondary evaporite

textures partially to completely mask the primary morphology of the

evaporite rocks, unless if micritized microbial laminae are present. The

latter preserves their original morphology (e.g. radial and random

prismatic, lenticular, clastic and swallow-tail crystals) and fabrics (e.g.

lamination, grass-like and stromatolitic structures) of the deposited

primary gypsum. The secondary diagenetic gypsum and anhydrite

crystals allowed the interpretation of the diagenetic history of the

evaporite rocks, whereas the ghosts of the primary gypsum crystals and

fabrics and the morphology of the micritized microbial laminae allowed

the interpretation of the depositional environment. The recorded

microfacies types of the studied evaporites are subdivided into three

categories: (1) Porphyroblastic, granoblastic, alabastrine, and satin spar

secondary gypsum, (2) prismatic, stair-step and felted anhydrite crystals,

and (3) micritized microbial laminae and scalenohedral calcite crystals.

The porphyroblastic gypsum is the most abundant type in the quarries as

21

250 µm

20

100 µm

144 Rushdi J.A. Taj

well as the outcropping evaporite rocks, whereas the granoblastic and

alabastrine gypsum are dominant near the top of the quarries. Satin-spar

gypsum veins are dominant and associated usually with the micritized

microbial laminae. The micritized microbial laminae and scalenohedral

calcite intersect all secondary gypsum crystals that form persistent

laminated and stromatolitic structures. The following is a description of

the microfacies types of the studied evaporites:

(a) Porphyroblastic Gypsum Microfacies

This type is recorded as coarse (> 1000 µm) interlocking crystals

near the base of the gypsum sequence in the quarries. It is also recorded

as floating crystals within granoblastic and alabastrine gypsum near the

upper part of the gypsum sequence. The porphyroblastic gypsum crystals

are characterized by smooth irregular or interpenetrating crystal

boundaries with the surrounding crystals (Fig. 22). Near the bottom of

the sequence, some of the gypsum crystals enclose relics of prismatic and

stair-step bassanite and anhydrite crystals (Fig. 23). On the other hand,

towards the top, some of the porphyroblastic gypsum crystals are

replaced with granoblastic and/or alabastrine gypsum (Fig. 24).

Whenever micritized microbial laminae exist intersecting the

porphyroblastic gypsum, they preserve the primary morphology of

prismatic and lenticular gypsum crystals (Fig. 25).

Fig. 22. Coarse porphryroplastic gypsum

with irregular crystal boundaries,

Polars Crossed.

Fig. 23. Relics of stair-step anhydrite

surrounded with fibrous bassanite

within porphyroblastic gypsum,

Polars Crossed.

23

100 µm

22

250 µm


Fig. 24. Porphyroblastic gypsum

surrounded with finer granoblastic

gypsum, Polars Crossed.

Fig. 25. Crust of euhedral gypsum crystals

surrounded with micritized

microbial filaments, Plane Light.

(b) Granoblastic Gypsum Microfacies

This microfacies type is recorded near the middle and top parts of

the evaporite sequence. It increases in abundance (together with

alabastrine gypsum) on the expense of the porphyroblastic gypsum

towards the top of the evaporite sequence. The granoblastic gypsum

crystals are composed of coarse (~ 400 µm), clear, subhedral gypsum

crystals that form patches within and at the boundaries of porphyroblastic

gypsum (Fig. 26). They have euhedral crystal faces toward the

porphyroblastic gypsum indicating their replacive origin. The gypsum

crystals of this facies do not enclose relics of former secondary anhydrite

crystals, on the contrary to the fact that most of the porphyroblastic

gypsum crystals have variable amounts of precursor corroded anhydrite

(Fig. 23). Therefore, the granoblastic gypsum crystals are formed on the

expense of porphyroblastic gypsum that also formed on the expense of

precursor anhydrite.

(c) Alabastrine Gypsum Microfacies

Alabastrine gypsum is recorded near the top part of the evaporite

sequence, and it is less abundant than the porphyroblastic and

granoblastic gypsum. The alabastrine gypsum consists of aggregates of

microcrystalline gypsum with sizes less than 100 µm (Fig. 27).

Alabastrine gypsum crystals are usually recorded as rounded, irregular or

elongated patches adjacent to, or within the porphyroblastic gypsum

crystals. The boundaries between the finer alabastrine gypsum and the

coarser porphyroblastic gypsum are usually gradational interpenetrating

contacts, indicating the replacement of the coarser gypsum crystals by

finer gypsum. The process of replacement of the coarse gypsum crystals

25

250 µm

24

250 µm

146 Rushdi J.A. Taj

by finer gypsum has been described before from the Messinian

evaporites of Italy (Testa and Lugli, 2000), from pedogenic gypsum

crusts (Watson, 1988; Aref, 2003), and from the Miocene evaporites of

the Red Sea (Aref et al. , 2003; Mandurah and Aref, 2010).

Fig. 26. Granoblastic gypsum crystals

replacing porphyroblastic gypsum,

Polars Crossed.

Fig. 27. Alabastrine gypsum with

granoblastic gypsum, Polars

Crossed.

(d) Satin Spar Gypsum Veins Microfacies

Discontinuous satin spar gypsum veins are recorded between

micritized microbial laminae and gypsum laminae, i.e. oriented parallel

to the depositional surface (Fig. 13). The veins are composed of fibrous

gypsum crystals, with lengths up to 3 cm, and widths < 2 mm. The

gypsum fibers of the veins are usually aligned vertically to the wall of the

vein and appear to grow from one wall of the micrite laminae to the

other, with micrite ships aligned between fibers, or crossing them (i.e.

filling origin, Fig. 28). The gypsum veins may be composed of straight

fibers, with unit extinction or slightly twisted fibers showing shadowy

optical extinction. The latter may represent original growth under the

influence of syngrowth shear during fracture dilation (El Tabakh et al. ,

1998; Aref et al. , 2003). The existence of several micritic materials

parallel to the fracture wall indicates an episodic opening of the fracture,

contemporaneous with filling of the fracture with gypsum crystals,

similar to the observation and description by El Tabakh et al. (1998). The

morphology of the satin spar gypsum reflects crystal growth from very

pure, supersaturated fluids, which favored extreme elongation parallel to

the c-axis (Magee, 1991).

27

250 µm

26

250 µm


Fig. 28. Stacks of fibrous gypsum crystals growing normal to porphyroblastic gypsum that

enclose irregular microbial filaments, Polars Crossed.

(e) Secondary Anhydrite Microfacies

This microfacies is recorded in two types: The first is prismatic and

stair-step anhydrite crystals and the second is felted anhydrite crystals.

The first type (prismatic and stair-step anhydrite crystals) is recorded as

relatively coarse crystals (~ 500 µm) corroded and engulfed by the

porphyroblastic gypsum (Fig. 23) near the bottom of the evaporite

sequence. The boundaries between anhydrite and gypsum is a direct

replacement boundaries, or through hemihydrate or bassanite as the

intermediate replacement stage between them. This type of anhydrite was

formed in the deep burial setting during the increase in temperature.

These anhydrite crystals are not of primary origin because of the

existence of ghosts of the primary morphology of lenticular and prismatic

gypsum within the porphyroblastic gypsum (Fig. 25).

The second type, felted anhydrite crystals, is replacing all

secondary gypsum types and forms white powdery crust that mantle the

evaporite sequence. The felted anhydrite crystals are smaller in size (< 50

µm), and are recorded as aggregates of fine laths that grow parallel to the

elongation of the porphyroblastic and granoblastic gypsum crystals (Fig.

29). The restriction of the felted anhydrite in the evaporite crust indicates

their formation during exhumation and solar dehydration of the

secondary gypsum into epigenetic felted anhydrite crystals.

28

250 µm

148 Rushdi J.A. Taj

Fig. 29. Fine laths of felted

anhydrite replacing

prismatic gypsum that

surrounded with

micritized microbial

filaments, Polars Crossed.

(f) Micritized Microbial Laminae Microfacies

The existence of the micritized microbial laminae preserve the

texture of the evaporite sequence (such as laminites, stromatolite and

grass-like gypsum), and the morphology of the entrapped primary

gypsum crystals (such as prismatic, lenticular and swallow-tail crystals).

Therefore, the existence of the micritized microbial laminae is of great

environmental significance because they help in interpretation of the

depositional environment of the evaporite sequence.

Petrographic examination of the evaporite rocks showed the

presence of dense micrite grains (< 10 µm in size) that are arranged in

continuous or discontinuous, highly irregular laminated structure

indicating their microbial origin (Fig. 25). Sometimes, scalenohedral or

lenticular calcite crystals (< 200 µm in size) are randomly associated

with the micritized microbial laminae. These large calcite crystals usually

contain black organic matter arranged in zonal pattern within the calcite

crystals (Fig. 30).

The micritized microbial laminae are relatively thin (< 200 µm), in

contrast to the thicker (> 500 µm) gypsum laminae (Fig. 25). The growth

of the porphyroblastic and granoblastic gypsum crystals usually intersect

and enclose several areas of the former gypsum-micritized microbial

lamination. The micritized microbial laminae may preserve the

morphology of the entrapped lenticular and prismatic primary gypsum

crystals (Fig. 31).

Diagenesis

The diagenetic processes that affected both the carbonate and

evaporite rocks of Al Jahfah Formation took place during early, burial

29

250 µm


and uplift diagenetic stages (Table 1). It is important to note that the

susceptibility of the carbonate and evaporite rocks to each diagenetic

stage is varied. The following is a description of the diagenetic processes

for the studied carbonate and evaporite rocks:

Fig. 30. Scalenohedral calcite crystals

enclose concentric organic matter

that are randomly arranged within

micritized microbial filaments,

Polars Crossed.

Fig. 31. Lenticular gypsum crystals

entrapped within micritized

microbial laminae, Polars Crossed.

Table 1. Hypothetical paragenetic sequence for the studied carbonate and evaporite rocks in

Rabigh area.

Evaporite Rocks Carbonate Rocks Diagenetic

Stage Relative time

Early Late

Relative time

Early Late

Displacive gypsum nodules

Micritization

Aggrading neomorphism

Compaction

Dissolution

Early cementation

Dolomitization

Early

Dehydration of gypsum to anhydrite

Burial

Rehydration of anhydrite to Gypsum

Solar dehydration of gypsum to anhydrite Late cementation Uplift

Early Diagenetic Stage

A. Carbonate Rocks

The diagenetic processes affecting the studied carbonate rocks in

this stage are micritization, aggrading neomorphism, compaction,

dissolution, early cementation and dolomitization (Table 1).

31

100 µm

30

100 µm

150 Rushdi J.A. Taj

1. Micritization

Micrite envelop is composed of thin, uniform or irregular rim of

fine, dark and dense micrite (now dolomicrite) grains that surround

gastropods, bivalves (Fig. 17) and foraminifers. Common partial to

complete micritization of bioclasts, lithoclasts and ooids are observed

that form peloid grains (Fig. 18 and 19). According to Flügel (2004), the

micrite envelopes and micritization originate from destructive and

constructive processes that take place at or near the sediment-water

interface. The classical model for destructive micritization of Bathurst

(1966) is a three step processes: (a) Microbolic products of

microendoliths lead to biochemical dissolution of skeletons (Ehrlich,

1999) leaving microborings (Golubic et al. , 2000). The latter are

colonized by filamentous cyanobacteria, green and red algae, and fungi.

(b) death of microborers and vacation of tubes. (c) emplacement of

micritic aragonite or high-Mg calcite cements within vacant tubes.

Multiple repetitions of boring and filling destroy the peripheral zone of

the grains and finally results in the formation of circumgranular micrite

rims (Fig. 19).

Constructive micrite envelopes are related to epilithic organisms

(Kobluk and Risk, 1977). Contrary to the first model, the micrite

envelope is formed without destruction or alteration of the grain

periphery. The process involves addition of carbonate to the exterior of

the grains (Fig. 17) in a low-energy environment or during shallow

burial. The cortoid results from the precipitation of microcrystalline

calcite around and between dense populations of exposed filaments of

endo- and epilithic algae and cyanobacteria. The precipitation occurs

predominantly upon dead filaments.

2. Aggrading Neomorphism

The enlargement of dense, dark micrite crystals created clear, fine

crystals of microspars and pseudospars (Fig. 20). These neomorphic

spars as well as the skeletal and non-skeletal materials are completely

dolomitized. However, the increase in number and size of the

neomorphic pseudospar crystals obscured the original structures of

fossils and ooids.

3. Compaction

The scattered random and floating nature of the bioclasts and ooids

in the carbonate mud and the rarity of point contacts between grains (Fig.


18) indicate that the original carbonate sediments were subjected only to

little compaction and early cementation in marine diagenetic

environment. The presence of shell concentration (Fig. 4) is not related to

the diagenetic compaction process, but to the depositional processes as

evidenced by the random arrangement of the bivalve shells and their

limited occurrence in certain patches.

4. Dissolution

Moldic vugs resulted from selective dissolution of aragonite and

high-Mg calcite of bivalve (Fig. 4) and gastropod shells are observed in

the studied carbonate rocks. These moldic vugs are lined or completely

filled with clear bladed or drusy mosaic calcite (now dolosparite) crystals

(Fig. 17). In some cases, the moldic vugs are filled with granular or

lenticular gypsum crystals which indicate the corrosive action of saline

water.

This dissolution process took place during short, intermittent

exposure period of the carbonate sediments at the margin of the lagoon.

Chemically aggressive meteoric waters led to selective dissolution of

aragonite and high-Mg calcite of the shells. These cavities were filled

later with low-Mg calcite in meteoric setting, and were subjected together

with the allochemical components to pervasive dolomitization (Table 1).

5. Early Cementation

Two cement generations are recorded in the studied carbonate

rocks. The first is an early diagenetic cement that took place before

dolomitization and consists of mosaic of dolosparite crystals that fill

intragranular and moldic pores (Fig. 17). This cement type is an

originally calcite spar crystals that was later mimically replaced with

dolosparite crystals (Fig. 17). The second late diagenetic cement took

place after dolomitization in the uplift diagenetic stage, and will be

discussed later .

6. Dolomitization

The studied carbonate rocks are almost completely dolomitized.

The dolomites show preservation of the original fabrics of the shells,

intraclasts (Fig. 16 and 19), ooids (Fig. 18 and 19), peloids, micrite

matrix, and the clear sparry calcite cement (Fig. 17). The fabrics of these

dolomite crystals are ‘mimicking’ (Sibley and Gregg, 1987) the fabrics of

the depositional textures of the original limestone and the early

152 Rushdi J.A. Taj

diagenetic textures (e.g. micrite envelope, early cement and neomorphic

spar).

The ‘seepage-reflux’ dolomitization model is the most probable for

the studied Miocene carbonate rocks as evidenced: (i) The preservation

of the primary morphology of the skeletal and non-skeletal components

of the limestone (Fig. 16-20), (ii) the preservation of the early diagenetic

textures (micrite envelope and micritization (Fig. 17 and 18), early

cements (Fig. 19 and 20), aggrading neospar and pseudospar crystals,

(iii) the presence of gypsum crystals in moldic vugs and fractures in the

dolomites, (iv) the corrosive action of gypsum crystals on bioclasts, and

(v) the presence of thick evaporite section that represents a facies

equivalent of the carbonate rocks at Wadi Al Jahfah.

In the seepage-reflux model (Hardie, 1987), brines are concentrated

in coastal lagoons or supratidal sabkha by surface evaporation of water.

These concentrated brines have a high Mg2+

/Ca2+

ratio resulting from

removal of Ca2+

through precipitation of gypsum and microbial micrite.

The higher density of these brines than that of normal seawater is causing

them to sink downward. Flushing of large volumes of Mg2+

-rich brine

downward through calcium carbonate sediments causes pervasive, early

dolomitization, with preservation of the original fabrics.

B. Evaporite Rocks

1. Displacive Gypsum Nodules

The overprint of early diagenetic processes is more pronounced in

the carbonate rocks in contrast to evaporite ones. However, the gypsum

nodules (Fig. 9) are formed during the early diagenetic stage. Similar

varieties of gypsum nodules were recorded by Magee (1991); Aref

(1997); Sanz-Rubio et al. (1999) and have different interpretations. They

may have resulted from the interplay of fluctuation of groundwater with

the development of biotubules (Magee, 1991), or a secondary

replacement of former swallow-tail gypsum crystals (Aref, 1997), or they

may have been controlled by the original pedogenic structure (Sanz-

Rubio et al. , 1999). Upward capillary movement of groundwater as

evaporation proceeds on the surface increases the salinity of the brine,

which consequently leads to displacive growth of gypsum nodules in the

mudstone layer.


Burial Diagenetic Stage

During this stage, it appeared that the evaporite rocks were affected

more than the carbonate rocks. This is because all rocks were most

probably subjected to shallow burial diagenesis where the effect of

minerals conversion took place under increase of heat and brine salinity.

When gypsum is buried and ambient temperature rises above 50-60

ºC, it converts to nodular anhydrite. The depth of transformation is

between a few meters to more than a kilometer, that depends on

lithostatic pressure, local geothermal gradient and pore brine salinity

(Warren, 2006). As gypsum converts to anhydrite, it loses the structural

water and original morphology of the gypsum crystals, unless entrapped

within micritized microbial laminae. The studied evaporite rocks are

believed to be affected by this stage as evidenced by the occurrence of

secondary anhydrite relics (Fig. 23) within porphyroblastic gypsum near

the bottom of the quarries.

Uplift Diagenetic Stage

During this stage, the carbonate rocks are subjected to late

cementation due to flushing of meteoric water. For gypsum, the burial of

secondary anhydrite rehydrates it back to secondary gypsum.

Furthermore, the secondary gypsum in outcrop is dehydrated to felted

anhydrite crystals.

A. Carbonate Rocks

Late Cementation

Cementation during late diagenetic stage is performed probably by

low-Mg calcite crystals. These are composed of granular and drusy

mosaics of clear calcite crystals that fill fractures, vugs, and moldic voids

of gastropods, bivalves and corals (Fig. 21). It is important to note that

the early diagenetic calcite cement is existed as dolosparite crystals,

whereas the late diagenetic calcite cement is not affected by

dolomitization..

B. Evaporite Rocks

Hydration of secondary burial anhydrite may take place by one of

the following mechanisms: (1) Direct addition of structural water to

anhydrite crystal lattice. Because of the difference in crystal lattice of

154 Rushdi J.A. Taj

anhydrite (orthorhombic) and gypsum (monoclinic), Mossop and

Shearman (1973) point to the difficulty of re-organization of the lattice

structure in solid state. (2) Hydration of anhydrite through intermediate

bassanite and hemihydrate that ultimately leads to the formation of

gypsum. (3) Dissolution of anhydrite and subsequent precipitation of

gypsum. In the present work, it is believed that the third mechanism of

anhydrite dissolution and subsequent precipitation of gypsum is the main

mechanism, as accepted by most workers. This dissolution-

reprecipitation mechanism is evidenced by the general absence of

features indicating any volume increase associated with gypsification in

the studied samples, a phenomenon that suggests that the excess sulfate is

carried away in solution to form gypsum veins (Fig. 28)

The widespread occurrence of porphyroblastic and granoblastic

gypsum near the base of the gypsum sequence indicates that they have

been formed when the hydration reaction took place slowly at near

equilibrium condition in the early exhumation history of the rock. When

the reactions were more rapid because of extreme disequilibrium, the

resulting crystals are fine grained alabastrine gypsum. This had possibly

occurred during the late exhumation history of the rock.

The water necessary for hydration of anhydrite might be supplied

from the infiltration of meteoric water during pluvial periods, when

intense rainfall over the uplifted anhydrite rock leads to their hydration to

gypsum. Accompanying the uplift of the evaporite sequence is its tilting

to the west and its exposure to exhumation. This resulted in the

expansion of the evaporite (CaSO4) deposits, which accompanied the

unloading of the Pliocene and some parts of the Pleistocene sediments.

Percolation of meteoric water to the evaporite sequence through fractures

that resulted during unloading or during rifting led to the hydration of

anhydrite to gypsum in two diagenetic environments. The first took place

below the water table, in a stagnant phreatic zone, leading to the

widespread hydration of anhydrite into porphyroblastic and granoblastic

gypsum under equilibrium conditions. The second took place in active

phreatic conditions, leading to the dissolution of the early-formed

porphyroblastic gypsum by undersaturated meteoric water with respect to

gypsum and its rapid recrystallization, in disequilibrium conditions, into

alabastrine gypsum.


Where different crystal types of secondary gypsum occur together

in the same sample, a mutual relationship exists. The porphyroblastic

secondary gypsum crystals usually have relics of anhydrite (Fig. 23),

indicating that they are of replacive origin during early exhumation of the

rock, similar to that described by Holliday (1970), Mossop and Shearman

(1973), Testa and Lugli (2000), Aref et al. (2003), and Warren (2006).

The relatively coarser size of porphyroblastic secondary gypsum suggests

that nucleation and growth were; in general, relatively slow at or near

equilibrium conditions. This is most likely to be achieved where the

water is introduced at depth, early in the exhumation history (Mossop

and Shearman, 1973).

Where porphyroblastic and alabastrine gypsum occur together, the

alabastrine gypsum are protruding into, and corroding the

porphyroblastic gypsum (Fig. 27). The process of replacement of the

relatively coarser gypsum by the finer gypsum is described before by

Testa and Lugli (2000), Aref et al. (2003), and Paz and Rossetti (2006),

and also in the process of gypcrete formation by Watson (1988) and Aref

(2003). The relatively fine size and the disordered crystal structures

indicate that the original nucleation centers were closely spaced and that

hydration was characterized by rapid growth under conditions far

removed from equilibrium (Mossop and Shearman, 1973) in a highly

saturated brine (Paz and Rossetti, 2006). The satin spar gypsum veins

usually have sharp boundaries with the enclosing porphyroblastic and

alabastrine gypsum (Fig. 28), indicating that they are late in the

diagenetic history of the secondary gypsum rock.

Depositional Environments

A. Carbonate Rocks

The litho- and biofacies characteristics of the studied carbonate

rocks point to their formation under different sub-environments; these are

platform-margin reefs (boundstone), platform interior- lagoon

(foraminiferal packstone, oolitic wackestone, foraminiferal intraclastic

wackestone and lime mudstone).

In the second setting (platform interior-lagoon), a lagoon was

protected by sand shoals or reefs of the platform margin. The lagoon is

sufficiently connected with the open sea to maintain salinity and

156 Rushdi J.A. Taj

temperature close to that of the adjacent sea. The sediments in this

environment are lime mud, sand and gravel sized siliciclastics, depending

on the grain size of local sediment production and the efficiency of

winnowing by waves and tidal currents. The biota is represented by

shallow water benthic foraminifera, bivalves and gastropods. The

concentration of bivalve shells in the studied area may be formed in open

platforms by current concentration or storm waves because of their

sporadic and random distribution of the shells. However, Flügel (2004)

mentioned that the concentration of shells may originate in various

environments from the coast to the deep sea and by various processes

including current concentration, storm waves or progressive lag and

condensation concentrations.

The ooids-rich wackestone and packstone may form within the

platform interior (Halley and Schmocker, 1983). The ooids were formed

in high-energy environments of oolitic shoals, tidal bars and beaches.

Then they are transported with other shell fragments by storm waves to

the platform interior. The pervasive micritization of the ooids by

microborers occurs in a very shallow environment.

B. Evaporite Rocks

To interpret the depositional environments of the evaporite rocks,

the results of the abovementioned diagenetic overprinting during burial

and uplift are not taken into consideration. The primary depositional

structures and textures that are preserved within microbial laminites and

stromatolites structures are important for interpretation of the

depositional environment. The geological setting and

sedimentological characteristics of the studied evaporite rocks

suggest that microbial mat growth and gypsum precipitation

occurred dominantly in a marginal marine lagoon. Growth of gypsum

nodules at the base of the evaporite sequence occurred during subaerial

exposure of the sediments to the supratidal sabkha setting.

Initial marine water inflow (storm and/or seepage), under arid

condition, to the fluvial sediments led to the formation of coastal

sabkha. When the sabkha sediments were perennially moistened by

evaporative pumping or capillary water supply. This led to

displacive growth of gypsum nodules within the clastic sediments.


Continuous supply of seawater probably accompanied by

lowering of the surface of the sabkha led to the formation of

shallow coastal lagoon. At low salinity value (60 – 150 g/l),

extensive microbial mat grew subaqueously at the sediment surface.

The decrease of water inflow with respect to evaporation resulted

into restriction of the shallow coastal lagoon and the increase in

salinity to over 150 g/l, where cyanobacteria could not survive and

ceased to grow. At this condition, subaqueous precipitation of free

falling gypsum crystals, or bottom growth of grass-like gypsum

crystals over the growing microbial mats are dominated.

Cornée et al. (1992) and Noffke et al. (1997) pointed out that the

maximum production of microbial mats occurs in extremely shallow

waters (2-12 cm depth) in the upper intertidal zone. The effects of

currents and waves led to the formation of ripples in the non-cohesive

deposited gypsum crystals. Decrease in flow velocity favors growth of

microbial mats on top of the ripples, which lead to their biostablization

from subsequent higher flow regime. Gerdes et al. (1993) found that

sediment stabilization by microbial mats starts in the upper intertidal

zone and increases towards the supratidal zone. Pope et al. (2000) found

that the lack of subaerial exposure surfaces, mud cracks, flat pebble

conglomerates and troughs filled with clastic carbonates, in addition to

evenly laminated stromatolites suggests that the deposition of

stromatolites was shallow enough to be influenced by wave-generated or

wind-generated currents. Therefore the studied stromatolite facies was

formed in the marginal marine part of very shallow lagoon (upper

intertidal and lower supratidal zone), without a prolonged period of

desiccation. Subsequently after deposition of the stromatolitic gypsum in

the marginal evaporite flat, the skeletal gypsum crystals were formed in a

brine pan characterized by deeper water and higher salinity.

DISCUSSION AND CONCLUSIONS

Criteria for carbonate sedimentation in Rabigh area point their

formation in shallow marginal part of a coastal lagoon (Fig. 32) because

of the following; (1) the existence of fauna of variable diversity such as

benthic foraminifera, bivalves, gastropods and echinoids, (2) the

dominance of reworked concentric ooids, (3) the existence of bioclasts

158 Rushdi J.A. Taj

and lithoclasts filled intragranular and intergranular voids in the

limestone, and (4) the presence of sand and gravel sized quartz and

volcanic grains between the components of the limestone. Also the

criteria for evaporite sedimentation point to their formation in a

supratidal sabkha and the central part of shallow coastal lagoon (Fig. 32)

because of the following; (1) the presence of evaporite nodules and

gypsified rootlets (rhyzocreation) in the mudstone, (2) the dominance of

stromatolitic and microbial (irregular and regular) laminated structures,

(3) the regular alternation of microbial laminae and gypsum laminae

throughout the evaporite exposure.

The stratigraphic settings of the carbonate and evaporite rocks are

similar. They are conformably overlying the Lower Miocene Al Haqqaq

Formation, and underlying the Harrat basalt, or Pliocene sands, or

Quaternary gravels.

From sedimentologic and stratigraphic criteria of the studied

carbonate and evaporite rocks, it was possible to reconstruct the

depositional model (Fig. 32) as in the following. In a shallow coastal

lagoon depositional setting, marine water from the Red Sea flow to the

lagoon via a barrier. At the shallow marginal part of the lagoon,

carbonates are deposited at a low salinity level. Decrease in marine water

inflow, coupled with evaporation of the restricted marine water favor

flourishing of microbial mats at a very shallow part of the lagoon. Further

increase in salinity, cease the microbial growth and allow precipitation of

gypsum crystals (prismatic, grass-like or swallowtail crystals). The

relatively large thickness of the evaporites sequence, and the persistent

microbial laminae and evaporite couplet for more than 50 m thick,

indicate that the rate of evaporite aggradation keeps pace with tectonic

subsidence, and the lagoon remains shallow throughout the deposition of

the evaporite sequence. This model of evaporite deposition is similar to

the shallow water – shallow basin model of Warren (2006); and Boggs

(2009). The relatively thin thickness of the carbonate sequence in

comparison to the evaporite sequence may be related to their early

erosion, or to higher salinity of the brine which allowed only deposition

of evaporite.


Fig. 32. Schematic model for the depositional environment of the carbonate and evaporite

rocks in marginal marine lagoon.

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