sedimentological significance and brine chemistry of ... · crystallization starts at the brine...

JKAU: Mar. Sci., Vol. 22, No. 2, pp: 135-158 (2011 A.D. / 1432 A.H.)

DOI : 10.4197/Mar. 22-2.8

135

Sedimentological Significance and Brine Chemistry of Recent Coastal Sabkha, Northwest Libya

Mohamed Abdel Galil and Esmail El-Fergany Faculty of Science at El-Khums, Misratah University, Libya

[email protected]

Abstract. Sabkha deposits occupy the relatively low topographic areas and are separated from the sea by coastal sand dunes. The sabkha sediments are relatively finer compared to those of the coastal sand dunes and beach. Grain size grading with improvement in sorting occurs in the direction of sediment drift landward.

The brines of the saline pans are of recent marine water origin. During spring and summer months, due to evaporation, the water level in the saline basin is lowered to a level below or nearly equal to that of the Mediterranean Sea from which the waters seep into the Salina. The brackish waters are partially or completely evaporated which lead to deposition of evaporite minerals in the saline basins and the surrounding sabkha plains. In autumn and winter months, the Salina is filled with water and the surrounding sabkha plain is moistened with seawater seepage and sporadic rainfall. These waters led to partial dissolution of the former summer deposited halite and/or gypsum. The halite crusts in the coastal saline pans are subjected to dissolution during seawater and/or meteoric water flood stage, and to cementation during the desiccation stage. The resulting dissolution and re-precipitation features are diagnostics of the ephemeral saline pan halite.

The salinities increase from the sea landward (46.9 g/l, up to 180.4 g/l and up to 323.8 g/l for the seawater, coastal sand dunes pans, and the sabkha brines respectively). Accompanying the increase in salinities is the very high concentration of Na+ and Cl- ions. The brines are highly saturated with NaCl, which favors a dominant halite precipitation (65.02- 78.12%), while bicarbonate salts are traces (0.13-0.79 %).

Introduction

Sabkha is an Arabic word for salt flat area. The study coastal sabkha is

situated in an extensive sabkha plain about 6 Km to the east of Zuwarah

136 Mohamed Abdel Galil and Esmail El-Fergany

city and about 50 Km to the east of Tunisia border (Fig. 1). Deposition

and dissolution of the evaporite minerals in the recent deposits are

interpreted using the saline pan cycle (Lowenstein and Hardie, 1985),

which consists of a flood stage (brackish lake), an evaporative

concentration stage (saline lake), a desiccation stage (dry saline pan) and

return to a flood stage (brackish lake). As evaporation and halite

crystallization continue, the saline lake shrinks, ultimately drying out

(Fig. 2).

According to Meteorological Authority data of Zuwarah station,

the study area has a Mediterranean climate where arid to semi-arid

conditions are predominating. The average temperature rises to 500C

during summer months, while it drops to 200C during the winter.

December and January are the wettest months and rain is often

concentrated in a few heavy showers. Wind speed increases in November

until April causing dust storms.

The aim of the present work is to study the field relationships,

textural characteristics of Zuwarah sabkha sediments and to delineate the

water origin from the brine chemistry.

Materials and Methods

Twelve water samples were collected from sabkha brines, from

sand dune pans and from the seawater. The collected samples were

analyzed for the major ions (Na+, K

+, Ca

++, Mg

++, Cl

- , SO4

- -, HCO3

- and

CO3- -

). All concentrations are expressed as equivalent per million (epm=

ppm/ equivalent weight), whereas the salinity is expressed as gram per

liter (g/l). Results of the chemical analyses were recalculated to e % of

major cations and major anions (epm of specific cation or anion/sum of

epm of cations or anions) to interpret the origin of brines. Moreover,

fifteen sediment samples were collected from beach, coastal sand dunes

and sabkha plain. The collected sediment samples were washed by

distilled water several times, dried and mechanically analyzed using a

Ro-Tap shaker at half phi interval according to Folk and Ward (1957).

Five salt samples were examined by using X-ray diffraction analysis.

Also, Scanning Electron Microscope (SEM) photographs were carried

out for selected seven salt samples.

Sedimentological Significance and Brine Chemistry of Recent Coastal Sabkha, … 137

Fig. 1. Location map of the study area and schematic cross-section showing the general

distribution of Quaternary deposits (Anketell and Ghellali, 1991).

Lithostratigraphy

Ephemeral saline pans occupy the lowest topographic depressions

in the sabkha plain. According to Anketell and Ghellali (1991) Jeffara,

Gargaresh and the upper member of Qasr Al-Haj formations cap the Plio-

Pleistocene deposits (Fig. 1). These deposits are capped by a veneer of

Holocene superficial deposits comprising recent sand dunes, wadi

deposits and sabkha (El-Hinnawy and Cheshitev, 1975).

Sabkha deposits occupy the relatively low topographic areas and

are separated from the sea by coastal sand dunes (Fig. 2). The dry sabkha

is frequent in the interdune areas and characterized by halophytes.

Landward, the sabkha deposits are underlain by silt of Jeffara Formation

(Fig. 3).


Fig. 2. Coastal sand dune separated a relatively low area from the sea. Note, as evaporation

continues, the saline lake shrinks.

Fig. 3. A trench in the coastal sabkha showing salt crust contains crystals of evaporite

minerals overlain Jeffara Formation.

Coastal sand dune

Salt crust

Salt crust

Jeffara Formation


Lowenstein and Hardie (1985) grouped the layered evaporites in

three depositional settings: (1) deep perennial (density stratified) basins,

(2) shallow perennial lakes or lagoons, and ephemeral saline pans.

Deposition of the evaporite minerals in Zuwarah sabkha took place in

shallow and flat basins that are normally dry in summer and flooded with

water in winter. These basins represent the ephemeral saline lake sub-

environment of Hardie et al. (1978), and the ephemeral saline pans of

Lowenstein and Hardie (1985).

The saline pans range in size from a few square meters to hundreds

of square meters depending on the amount of ground water seepage, the

slope, and the surface area of the evaporite basin. The saline pans usually

occupy the center of the evaporite basin, but may be shifted towards the

margin depending on the topographic location of small depressions

within the basin. The saline pans are filled with water in winter and

floored with layered salt in summer (Fig. 4). As a result of continued

evaporation, the saline pans are encrusted with halite crusts, hence they

can be termed halite pans, similar to that described by Lowenstein and

Hardie (1985) and Smoot and Lowenstein (1991).

The saline pan zones are surrounded with brine saturated mudflats

that are covered with scattered halophytes surround the saline pan (Fig.

5). Near the coastal saline basin, the water table is close to the surface of

the sabkha sediments. The main supply to the coastal saline basin and the

surrounding sabkha sediments is either through storm flooding of sea

water, seawater seepage or high tide seawater spray, in addition to minor

input from groundwater inflow through the highly permeable fluvial and

dune sands after torrential rains. The inflow of both marine and

nonmarine waters into the saline pans causes the level of groundwater to

rise (Basyoni and Mousa, 2009).

During spring and summer months, due to evaporation, the water

level in the saline basin is lowered to a level below or nearly equal to that

of the Mediterranean Sea from which the waters seep into the saline pans.

The brackish waters are partially or completely evaporated which lead to

deposition of evaporite minerals in the saline basins and the surrounding

sabkha plains (Fig. 4 and 5). X-ray examinations show a dominant halite

while bicarbonate salts are traces. In autumn and winter months, the

saline pans are partially filled with water and the surrounding sabkha

plain is moistened with seawater seepage and sporadic rainfall. These


waters led to partial dissolution of the former summer deposited halite

and/or gypsum (Fig. 6).

Fig. 4. Saline pan floored with layered salt in summer. This pan will be partially filled with

water in winter.

Fig. 5. Scattered halophytes surround the saline pan.

Halophytes

Saline pan


Fig. 6. The halite crusts are subjected to dissolution during meteoric water flood stage. Note

the isolated remnant salt.

Halite continues to precipitate from the groundwater brine as clear,

void filling cement and displacive crystals within mud (Lowenstein and

Hardie, 1985; Casas and Lowenstein, 1989; Smoot and Lowenstein,

1991). During the flood stage, dilute floodwater is pounded in the saline

pan and dissolves the underlying halite crusts (Fig. 7 to 10). The textural

features produced during the flood stage include: (1) Horizontal

truncation surface, (2) cavities formed by dissolution, and (3) mud

partings between and within evaporite crystals (Casas and Lowenstein,

1989).

Fig. 7. SEM photomicrograph cavities

formed by dissolution (arrows).

Fig. 8. SEM photomicrograph of partial

dissolution of cubic halite crystals.


Fig. 9. SEM photomicrograph of the cleavage

planes in halite showing dissolution.

Fig. 10. SEM photomicrograph of cavities

and partial surface dissolution.

During the evaporative concentration stage, the ephemeral pans

reach saturation with respect to halite and turns into saline pans.

Crystallization starts at the brine surface as small plates and hopper

crystals, which sink to the bottom (Fig. 11), and as bottom growth of

chevrons and cornets (Arthurton, 1973).

When the brine reaches saturation with respect to halite, halite

crystallizes at the brine-air interface as millimeter-sized rectangular and

square-shaped plates and pyramidal hoppers (Arthurton, 1973). The

crystals are suspended horizontally by surface tension. With continuous

growth of halite at the brine-air interface, the growth sequence

commences with chains, which form nets. This is similar to that

described by Shearman (1970), Arthurton (1973), Aref et al. (1999),

Sanford and Wood (2001), Tyler et al. (2006) and Basyoni et al. (2008).

When the weight of the suspended mat overcomes the surface tension,

the rafts sink to the bottom under the effects of gravity (Handford, 1991).

The halite rafts may be later reworked by small currents (Warren, 1982

and Last, 1984) to form clastic halite, or may form nucleation sites for

bottom growth of chevrons and comets. When the brine is slightly

agitated at the early stage of nucleation of the halite crystals, waves

disturb the surface tension, and the individual halite crystals settle to the

bottom as aggregates of cumulus crystals (Smoot and Lowenstein, 1991).


Continuous concentration of the brine by evaporation produces

supersaturated brine from which halite crystals grow on the earlier settled

rafts and cumulus crystals (Fig. 11 to 16). The growth on the earlier

formed crystals and the competitive growth of halite produce vertically

oriented crystals that resemble the halite teeth. The morphology of the

upward growing crystals depends on the attitude of the parent crystals.

When syntaxial overgrowth begins on a halite cube lying on the edge, the

resulting overgrowth will be chevron-shaped with an upward pointing

coin (Arthurton, 1973; Lowenstein and Hardie, 1985).

Zoning probably results from varying crystal growth rates where

the cloudy bands are formed rapidly during periods of intense

evaporation while clearer bands have crystallized more slowly during

periods of lower evaporation rates (Shearman, 1970; Holser, 1979; and

Roedder, 1984).

Fig. 11. Crystallization of small plates.

Fig. 12. The plates become massive.

Fig. 13. Crystals of teeth shapes.

Fig. 14. Vertically oriented crystals.


Fig. 15. Aggregates of halite and gypsum

crystals.

Fig. 16. Crystals grow as rosette pattern;

salt rose.

The coastal sabkha pans under study receive flood and seeping

seawater from the Mediterranean Sea, in addition to sporadic torrential

rains in the surrounding fluvial and dune sands. During winter the lowest

topographic depressions in the sabkha plains are filled with brackish

water, whereas their margins are covered with microbial mats. The mats

form multicolored layers (Fig. 17 to 20), similar to that described by

Noffke et al. (1997) and Basyoni (2004). The multicolored zonation of

the microbial laminae is due to the presence of diatoms (yellow),

cyanobacteria (blue to dark green), phototrophic sulfur bacteria (purple)

and sulfate-reducing bacteria inducing black iron sulfide coatings on

sediment grains (Gerdes et al., 1985 and Noffke et al., 1997).

Cornee et al. (1992), found that the tight and continuous microbial

mats form a barrier for both gas and solute transfer between sediments

and brines and thus enhances reducing conditions in the sediments.

Microbial mats therefore not only generate organic matter, but may also

enhance its preservation at depth (Cornee et al., 1992). Therefore,

bacterial decomposition transformed the microbial mats into black

sediment with a high hydrocarbon potential (Fig.21).

Keine et al. (1986) believed in the enrichment of methanogenic

bacteria in the deeper buried organic matter. From there, gas diffuses

upwards through the sediments and becomes captured by surface

microbial mats to form a crenulated leathery surface (Fig. 21). With

increase in salinity, gypsum and/or halite crystallize on the mats surfaces

that evolve into petee structure (Fig. 22), due to the combination of the

physical forces of crystallization of gypsum and halite, and the biogenic

growth effect on the microbial laminae (Gavish et al., 1985).


Fig. 17. Multicolored microbial laminae.

Fig. 18. Blue to dark green color due to the

presence of cyanobacteria.

Fig. 19. At the margins of the pans,

microbial mates form multicolored

layers.

Fig. 20. Multicolored zonation of the

microbial laminae.


Fig. 21. Halite crystallizes on the black mats surface, evolves into scattered petee structure.

Fig. 22. Well developed petee structure due to the combination of the physical forces of

crystallization and the biogenic growth effect.

Petee

structure


Towards the saline pans, the highly wetted surface of the mud flats

is covered with microbial mats underlain by a black sapropelic layer a

few centimeters thick (Fig. 21). The microbial mats are produced by

cyanobacteria-dominated communities similar to those described from

various hypersaline environments (Cohen et al., 1977; Thomas and

Geisler, 1982; Gerdes and Krumbein, 1987).

The extensive growth of the microbial mats embedded in

sedimentary surfaces acts as a kind of soft tissue, which effectively

affects the properties of surface structures (Reineck et al., 1990). The

interplay between microbial stabilization of sediment surface and gas

formation within the sediments, due to bacterial activity, results in the

formation of crinkled surface (Fig. 22), which is defined as petee by

Gavish et al. (1985); Gerdes et al. (1993). The petee structures were

formed by surface gas accumulation (H2S, CH4) below the surficial

cohesive microbial mat tissue, which over thrusts the mat surface into

domes (Fig. 22). During flooding and evaporative concentration stages,

the crinkled surfaces of the microbial mats are submerged by shallow

saline water that precipitate surface halite and/or gypsum crust on top of

the microbial mats. The microbial mats act as nucleation site for growth

of halite and radial arrangement of lenticular gypsum crystals.

Sediment Characteristics

The grain size parameters of the studied sediments are given in

Table (1). The studied sediments are subjected to different energy levels,

reflected on their mean size values. The beach sediments are relatively

coarser compared to those of coastal sand dunes and sabkha, with

average mean size values of 1.38 Ø (medium sand), 1.47 Ø (medium

sand), and 2.07 Ø (fine sand) respectively (Table 1).

The average values of the graphic standard deviation are 0.66 Ø,

0.62 Ø, and 0.37 Ø for the beach sediments, the coastal sand dunes and

sabkha sediments respectively, indicating improving in sorting landward.

The average values of the inclusive graphic skewness are 0.02, 0.04

and 0.00 for the beach sediments, the coastal sand dunes and sabkha

sediments respectively, indicating near symmetrical frequency

distribution.

Generally, grain size grading with improvement in sorting occurs in

the direction of sediment drift landward and the coastal sand dunes


sediments show relatively increase in the finer fractions compared with

that of the beach sediments (Fig. 23).

Table 1. Description of the sediments according to their grain size parameters.

Location Sample

Number MZ σ1 SK1 Description

Beach

sediments

1 2.30 0.63 -0.45 Fine

Sand

Moderately

Well Sorted

Strongly

Coarse

Skewed

3 1.07 0.76 0.31 Medium

Sand

Moderately

Sorted

Strongly Fine

Skewed

5 0.60 0.54 0.13 Coarse

Sand

Moderately

Well Sorted Fine Skewed

7 1.55 0.60 0.03 Medium

Sand

Moderately

Well Sorted

Near

Symmetrical

9 1.37 0.75 0.06 Medium

Sand

Moderately

Sorted

Near

Symmetrical

Average 1.38 0.66 0.02 Medium

Sand

Moderately

Well Sorted

Near

Symmetrical

Coastal

dunes

sediments

2 1.50 0.6 0.09 Medium

Sand

Moderately

Well Sorted

Near

Symmetrical

4 1.50 0.65 -0.01 Medium

Sand

Moderately

Well Sorted

Near

Symmetrical

6 1.40 0.62 -0.02 Medium

Sand

Moderately

Well Sorted

Near

Symmetrical

8 1.45 0.57 0.16 Medium

Sand

Moderately

Well Sorted Fine Skewed

10 1.50 0.66 0.00 Medium

Sand

Moderately

Well Sorted

Near

Symmetrical

Average 1.47 0.62 0.04 Medium

Sand

Moderately

Well Sorted

Near

Symmetrical

Sabkha

sediments

11 1.68 0.34 -0.21 Medium

Sand

Very Well

Sorted

Coarse

Skewed

12 1.73 0.31 -0.13 Medium

Sand

Very Well

Sorted

Coarse

Skewed

13 1.64 0.39 -0.05 Medium

Sand Well Sorted

Near

Symmetrical

14 1.82 0.31 0.12 Medium

Sand

Very Well

Sorted Fine Skewed

15 2.74 0.38 0.06 Fine

Sand Well Sorted

Near

Symmetrical

Average 2.07 0.37 0.00 Fine

Sand Well Sorted

Near

Symmetrical


Beach sediments Coastal dunes sediments

Coarse sand Medium sand Fine sand Very fine sand

Fig. 23. Beach and coastal dunes sediment fractions. Arrows show sediment transportation

landward.

0

10

20

30

40

50

60

%

Sample

(10)

0

5

10

15

20

25

30

35

40

45

50

%

Sample

(9)

0

10

20

30

40

50

60

70

%

Sample

(8)

0

10

20

30

40

50

60

70

%

Sample

(7)

0

10

20

30

40

50

60

70

%

Sample

(6)

0

10

20

30

40

50

60

70

80

90

%

Sample

(5)

0

10

20

30

40

50

60

%

Sample

(4)

0

10

20

30

40

50

60

%

Sample

(3)

0

10

20

30

40

50

60

70

80

%

Sample

(2)

0

10

20

30

40

50

60

%

Sample

(1)


Brine Chemistry

Water salinity varies from 46.9 g/l of the sea water to 323.8 g/l of

sabkha brines. Variations in salinity content (Fig. 24, 25 and Table 2)

indicate that, salinity increases in the direction from the sea toward sand

dune pans (up to 180.4 g/l) and sabkha brines (up to 323.8 g/l). The high

salinities in the sabkha brines may be related to the high evaporation rate.

Accompanying the increase in salinities is the very high

concentration of Na+ and Cl- ions (Fig. 24 and 25). The variation in

chloride concentration (Fig. 25) shows that, the chloride concentration

increases progressively from the seaward (22491 ppm) to landward

directions (up to 98616 ppm and 181661 ppm for the sand dune pans and

the sabkha brines respectively). Sulphate constitutes the second

predominant anion after chloride and varies in content between 8000

ppm for the Mediterranean Sea water and up to 19500 ppm and 38500

ppm for the sand dune pans and the sabkha brines respectively.

Generally, the sulphate distribution pattern is similar to that of chloride

and salinity contents indicating that, sulphate enrichment is associated

with salinity rise.

Fig. 24. Total Salinity distribution showing increase in salinity landward.

Fig. 25. Increase in the concentrations of chlorine (Cl-) and sodium (Na+) ions accompanying

the increase in salinities.

0

50000

100000

150000

200000

1 2 3 4 5 6 7 8 9 10 11 12

Sample number

ppm

Na+

Cl-

0

50

100

150

200

250

300

350

g/L

1 2 3 4 5 6 7 8 9 10 11 12

Sample number


Table 2. Chemical analysis of the collected water samples.

S.

No. Location

T.D.S.

g/l Unit

Cations Anions

Na+ K+ Mg++ Ca++ Cl- SO4

-- HCO3

- CO3

--

1 Seawater 46.9

ppm 12600 310 2016 1262 22491 8000 213.5 -

epm 547.8 7.9 165.8 63 634.3 166.6 3.5 -

% 69.83 1.01 21.13 8.03 78.85 20.71 0.44 -

2

Sand dune

Pans

69.4

ppm 19500 510 3864 922 38062 6000 579.5 -

epm 847.8 13 317.8 46 1073.4 124.9 9.5 -

% 69.23 1.06 25.95 3.76 88.87 10.34 0.79 -

3

101.8

ppm 27600 1020 4968 1844 53633 12500 244 -

epm 1200 26.1 408.6 92 1512.5 260.3 4.0 -

% 69.50 1.51 23.66 5.33 85.12 14.65 0.23 -

4

117.5

ppm 35500 660 3780 2525 55363 19500 213 -

epm 1543 16.9 310.9 126 1561 406 3.5 -

% 77.27 0.85 15.57 6.31 79.22 20.60 0.18 -

5

137.4

ppm 36600 1000 5520 4609 74394 15000 305 -

epm 1591.3 25.6 453.9 230 2098 312.3 5.0 -

% 69.16 1.11 19.73 10.0 86.86 12.93 0.21 -

6

178.6

ppm 48500 1416 9936 2765 98616 17000 336 -

epm 2109 36.2 817 138 2781 353.9 5.5 -

% 68.03 1.17 26.35 4.45 88.56 11.27 0.18 -

7

180.4

ppm 49500 1330 9936 3687 98616 17500 275 -

epm 2152 34 817 184 2781 364.4 4.5 -

% 67.52 1.07 25.64 5.77 88.29 11.57 0.14 -

8

Sabkha

Brines

299.5

ppm 76000 2800 21600 802 159169 38500 640 -

epm 3304 71.6 1776 40 4489 801.6 10.5 -

% 63.64 1.38 34.21 0.77 84.68 15.12 0.20 -

9

306.3

ppm 82500 1500 18768 1844 174200 27000 519 -

epm 3587 38.4 1543 92 4912.6 562.1 8.5 -

% 68.19 0.73 29.33 1.75 89.59 10.25 0.16 -

10

323.8

ppm 84000 2000 22632 922 181661 32000 610 -

epm 3652 51.2 1861 46 5123 666.3 10 -

% 65.10 0.91 33.17 0.82 88.34 11.49 0.17 -

11

314.9

ppm 88000 1800 14900 5531 178200 26000 518 -

epm 3826 46 1225 276 5025.4 541.3 8.5 -

% 71.21 0.86 22.80 5.14 90.14 9.71 0.15 -

12

311.6

ppm 87860 2000 16560 1844 176931 26000 427 -

epm 3820 51.2 1361.8 92 4989.6 541.3 7.0 -

% 71.74 0.96 25.57 1.73 90.10 9.77 0.13 -

Results of the chemical analyses were recalculated for both the

major cations and major anions and plotted on Sulin graph (1946) to

interpret the origin of brine. It is clear that, the brines of the saline pans

are of recent marine water origin and of MgCl2 composition (Fig. 26).

The main supply to the coastal saline basin and the surrounding sabkha

sediments is either through storm flooding of sea water, seawater seepage

or high tide seawater spray, in addition to minor input from water inflow


through the highly permeable fluvial and dune sands after sporadic

torrential rains. The inflow of both marine and non-marine waters into

the Salinas causes the groundwater level to rise.

Fig. 26. Sulin graph representing the water genesis in the study sabkha.

The Hypothetical Salt Assemblage

The hypothetical salt assemblages were determined and presented

in Table (3). NaCl is the highly dominated salt (65.02 – 78.12 %), MgCl2

is present in a relatively high amount, while Ca (HCO3)2 is relatively

trace (Table 3).

The relatively high magnesium content of the study area may be

attributed to local surface and subsurface environments related to the

lithology of water – bearing rocks. Moreover, surface evaporites

intercalated with the water – bearing sediments are a possible local

source for magnesium.

100

100

100 Mg2+

SO4

2-

Cl – (K+ + Na+)

(K+ + Na+) - Cl

Na2SO2

Deep meteoric

water

NaHCO2

Shallow meteoric

water

MgCl2

Recent

marine

CaCl2

Old marine

Saline pan

Sand dunes

Sea water


Halite is the predominant mineral. As the water volume decreases,

gypsum will be deposited first, but subsequently, with increasing

evaporation, there will be a mixture of gypsum and halite and finally

halite only (Braithwaite and Whitton, 1987).

Table 3. The hypothetical salt assemblage for the studied water samples.

Sample

No.

Location NaCl MgCl2 MgSO4 Ca SO4 Ca (HCO3)2

1 Seawater 70.84 8.01 13.12 7.59 0.44

2

Sand dunes

70.29 18.58 7.37 2.97 0.79

3 71.01 14.11 9.55 5.10 0.23

4 78.12 1.10 14.47 6.13 0.18

5 70.27 16.59 3.14 9.79 0.21

6 69.20 19.35 7.00 4.27 0.18

7 68.59 19.70 5.94 5.63 0.14

8

Saline Pan

65.02 19.58 14.63 0.57 0.20

9 68.92 20.67 8.66 1.59 0.16

10 66.01 22.33 10.84 0.65 0.17

11 72.06 18.08 4.72 4.99 0.15

12 72.70 17.40 8.17 1.60 0.13

Conclusions

Sabkha deposits occupy the relatively low topographic areas and

are separated from the sea by coastal sand dunes where dry sabkha is

frequent. The beach sediments are relatively coarser compared to those of

coastal sand dunes and sabkha. Grain size grading with improvement in

sorting occurs in the direction of sediment drift landward and the coastal

sand dunes sediments show relatively increase in the finer fractions

compared with that of the beach sediments.

The saline pan zones are surrounded with brine saturated mudflats

that are covered with scattered halophytes surround the saline pan. The

water table is close to the surface of the sabkha sediments and the inflow

of both marine and nonmarine waters into the saline pans causes the

groundwater level to rise.

The saline pans range in size from a few square meters to hundreds

of square meters depending on the amount of ground water seepage, the

slope, and the surface area of the evaporite basin. The saline pans are

filled with water in winter and floored with layered salt in summer.


X-ray examinations show a dominant halite while bicarbonate salts

are traces. SEM study shows the effect of the dilution of the brine on the

texture of the evaporite crystals.

During spring and summer months, the brackish waters are

partially or completely evaporated which lead to deposition of evaporite

minerals in the saline basins and the surrounding sabkha plains. In

autumn and winter months, the saline pans are partially filled with water

and the surrounding sabkha plain is moistened with seawater seepage and

sporadic rainfall. These waters led to partial dissolution of the former

summer deposited halite and/or gypsum.

Salinity increases in the direction from shore landward, and the

origin of the brines of the saline pans is interpreted as recent marine

water origin of MgCl2 composition.

The hypothetical salt assemblages were determined and show that

NaCl is the dominated salt and the bicarbonate salts are traces. Moreover,

surface evaporites intercalated with the water – bearing sediments are a

possible local source for magnesium.

References

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