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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
Lower Miocene Coastal Lagoon Carbonates and Evaporites of … 135
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
Lower Miocene Coastal Lagoon Carbonates and Evaporites of … 137
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
Lower Miocene Coastal Lagoon Carbonates and Evaporites of … 139
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
Lower Miocene Coastal Lagoon Carbonates and Evaporites of … 141
(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
Lower Miocene Coastal Lagoon Carbonates and Evaporites of … 143
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
Lower Miocene Coastal Lagoon Carbonates and Evaporites of … 145
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
Lower Miocene Coastal Lagoon Carbonates and Evaporites of … 147
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
Lower Miocene Coastal Lagoon Carbonates and Evaporites of … 149
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.
Lower Miocene Coastal Lagoon Carbonates and Evaporites of … 151
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.
Lower Miocene Coastal Lagoon Carbonates and Evaporites of … 153
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.
Lower Miocene Coastal Lagoon Carbonates and Evaporites of … 155
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.
Lower Miocene Coastal Lagoon Carbonates and Evaporites of … 157
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.
Lower Miocene Coastal Lagoon Carbonates and Evaporites of … 159
Fig. 32. Schematic model for the depositional environment of the carbonate and evaporite
rocks in marginal marine lagoon.
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