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Northwest El Fashn Recent Continental Sabkha: A Possible Source for Generating
Hydrocarbon, Western Desert, Egypt
* Wali, A.M.A, ** Zaki, R. and ** Mosa, M.
*Dept. Geology- Faculty of Science- Cairo University
Currently: Dept. Geology and Geophysics, College of Science, King Saudi University
** Geology Department, Faculty of Science, Minia University
Abstract
A continental sabkha, northwest El Fashn area, Minia governorate was identified,
occupying the bottom of closed basin fringed with sand dunes and Middle Eocene rocks.
The sit receives the excess of discharging irrigation water sourced from a well within the
limestone rocks (>40 m deep) in addition meteoric water from sand dune aquifers. The
capillary rise powered by evaporative pumping mechanism is responsible for continually
feeding the area. The sabkha is subdivided into four sedimentological zones; elevated
temporary dry marginal zone, temporary wet saline mudflat broad zone, hypersaline pool
zone and permanent saline pan zone. The predominate minerals in most samples with minor
gypsum, anhydrite, leonite, bloedite, bischofite and sulfohalite. The non-evaporative
minerals are quartz, calcite, plagioclase, chlorite, hematite and clay minerals (kaolinite, illite
and montmorillonite).
The upward increase in salinity and Na and Cl ions support the evaporative pumping
mechanism plays a significant role in the upward movement of brines and formation of the
evaporative salts under arid conditions. The repeated cycles of salt precipitation reflects the
complexity of the diagenetic history in both mineralogy and sedimentary textures. The bioproductivity of the sabkha site including bio-masses and its rapid degradation supports its
possibility to act as a source for hydrocarbon generation.
Keywords: sabkha complex sequence, brines, organic matter, evaporative pumping.
Introduction and Geologic Setting:
The study area is a part of the western Desert, north Upper Egypt. It lies in the arid
belt of the North Africa. It located at the northewst of El-Fashn City (Lat. 280 50 - 28
0
56-
N and Long. 300 30- - 300 34- E), Beni Suef Governorate and can be reached through the
road of road toward Wadi Mwuilih (Fig.1). The study area is a plain to hilly region. The
elevation varies from 22 to 44 m above sea level. The exposed rocks from the entrance of
Wadi El Mwuilih to the study sabkha represented by Middle Eocene rocks, Pleistocene and
Holocene sediments. It includes, 1-Samalut Formation, consists of hard Nummulitic
limestone (Early Lutetian). 2- Muweilih Formation, consists of limestone and claystone
(Middle Lutetian).
3- Midawara Formation, gypsiferous marl and limestone (Late
Lutetian). 4- Sath El Hadid Formation, consists of chalky limestone (Bartonian). 5- The
Pleistocene deposits composed of unconsolidated sands and gravels. 6- Holocene deposits
composed of Nile silt and sand dunes. The sand dunes form two elongate ridges of Seif and
Longitudinal types and bounded the sabkha from the western and eastern sides by a
continuous strip extend to about 16 km long and 0.5 km wide. The main trends of dunes are
NW- SE and NE-SW directions. These dunes have dense halophytes cover, i.e. Phyragmites
australis, Tamarix nilotica and Phragmites communis. The biostratigraphy and
paleontological studies of these rocks were discussed by many authors, e.g. Said (1962),
Abel Gahny (1990) and Abd El-Aziz (2002).
The rate of evaporation in the continental sabkhas is relatively higher than that in the
coastal ones due to the more arid conditions. Consequently, the groundwater table plays a
substantial role in the development of continental sabkhas, which are less developed than
coastal sabkhas, and are predominantly tectonically and/or topographically controlled. The
sediments of these sabkhas are consists predominantly of gypsum, quartz and calcite with
halite always existing as the crust (Kinsman, 1969).
The Beni Suef Governorate is characterized by the arid climate and high rate of
evaporation. It is a marked by warm winter and hot summer. During the spring season hot
winds and dust storm occur during a period of about 50 days and locally known as El
khamasin winds. The average maximum temperature during the summer months is 360 C;
while the average minimum temperature during the winter months is 6.90 C. Monthly the
relative humidity ranges between 50 % and 86 % with an average 53 %. Mean annual
rainfall is 6 mm. the evaporation rate reaches 13 mm/day in summer (Egyptian
Meteorological Authority, Cairo).
The study area is affected by NE asymmetrical fold and NW-SE oriented normal
faults with minor NE-SW joints, faults and gypsum veins. These faults deep- seated back to
Precambrian time (Youssef, 1968). Wadi El Muweilih runs along the fault plane. It has 15
km long and taking trend N600 W. Faults in the study area are the main agents that have
determined the relief and bounded the scarp of Gabal Al Qalamun of Midawara Formation.
These faults and fractures are playing an important process to raise the ground water to the
surface. The wells of Deir (= monastery) Samweil, north of the study area are aligned along
the fault trend (Khalifa and Youssef, 1978). Koral (1983) mentioned that the en-echelon
folds associated with major strike-slip faults flatten upward and twist away from the strike
of fault. Gypsum veins north of the study area at Rayan Formation have NW-SE orientation.
The average normal to such veins indicate the local minimum principal stress axis
(Letouzey, 1986).
The hydrologic framework affecting the study sabkha is by surface and subsurface
waters. The rainfall represents the surface source, occurs as ephemeral, brackish to
hypersaline water and trapped by the high permeable surrounding sand dunes which
accumulate rainwater and then act as unconfined aquifers supplying water to the salinas
when the water level of the salina falls below the level of the water table in the surrounding
dunes. Subsurface saline water is derived to the sabkha by the seepage from the subsurface
saline aquifers (Setto et al., 1991) by capillary forces as a result of high evaporation
attributed to the arid climate and/or subsurface fractures. However, the irrigation wastes
water from the adjacent high cultivated lands in the neighborhood area of Deir Samwil farm
is another subsurface source.
The purpose of this work is to discuss the composition and origin of modern
continental sabkha northwest El Fashn, Western Desert in Egypt and its possibilities as a
source of hydrocarbons.
2. Material and Methods
A total of 120 hand specimens representing different surface salts including the surrounding
rocks. Core samples were collected from a depth range of 10-12 cm and 16 trenches were
raised by digging (March, 2008 till May, 2009. About twenty samples of surface pan water
were collected to identify the animal and algae and ten samples of halophytes were collected
to identify the bio-species.
Six representative samples were analyzed by using Cu-Kά radiation, X-ray diffeactometer of
the JEOL type of the Central Laboratory, El Minia University. Fourteen samples from
groundwater, trench water (water extracted from core sediment samples by using a suction
instrument), surface pool water and surface pans were analyzed to determine TDS (total
dissolved salts using standard techniques, Rainwater and Thatcher, 1960), Na+, K+, Ca+2,
Mg+2,
HCO3,
Cl
and
SO4-2
by
Flame
Photometer,
Spectorophotometer
and
complexomterically in the Geology Department, Faculty of Science, El Minia University.
Seven samples were analyzed to define the total carbon chemical and organic carbon) and
the organic carbon (percentage of carbon formed by the organisms). The analysis is
investigated by using LECOCS-300 instrument, Institute of Earth Science, Graz University.
However, another 6 samples were analyzed to identify the organic matter content in the
Faculty of Agriculture, El Minia University (Table .. illustrate the collected samples).
3. Sabkha characteristics and Basin Zonations
The studied recent sabkha shape is as elongate irregular (Fig..) occupying depression
areas. It is about 19 km long, 4.5 km wide and covers an area of about 90 km2. The sabkha
can be subdivided into four main zones based on variation in lithology, sedimentary
structures, elevation differences, biological activity and depth of the water table.
3.1. Temporary dry elevated marginal zone
This zone covers about 20 km2 and located on the southern outer marginal flank of
the sabkha area. The surface is flat, soil color is pale yellow to yellowish brown and
characterized by the complete dryness in summer and temporary wet in winter. The surface
is characterized by the abundance of halophytes, i.e., Tamarix Nilotica and Phragmtes
Communis and desiccation features such as mud cracks; shrinkage and wind blown sands.
Two profiles were taken in this zone (Fig.2) and composed mainly of 45-40 %
calcareous sand, 30-20 % lime with calcareous limy sands, 15-5 % sands and 15-10 % halite
crust. The calcareous sand is the erosion products from the surrounding Middle Eocene
sediments.
3.2. The temporary wet saline mudflat broad zone
The temporary wet saline mudflat broad zone is a continuous extension of the
temporary dry zone and cover area about 60 km2, permanent wet pan zone and sand dunes
bound the zone from the south, east and west, respectively (Fig. 3). Topographically flat and
water table varies from less than 10cm to more than 50 cm below sabkha surface with
respect to sea level. The main characteristic features and sedimentary structures of this zone
are:
1- The occurrence of dense halophyte vegetation, i.e., Zygophylum Album (1m
height) and Phyrogmites Australis (3- 5 m height).
2- The presence of extensive saline mudflats with halite crusts forming rhythmic
layers and/or bands reflects the seasonal changes in the brine composition.
3- The surface features are distinguished by the occurrence of the tepee and peetee
structures (Fig. 4) which can be subdivided into; a- Immature type, consists of a crusts
ranges from less than 3 cm up to 10 cm, in diameter it ranges between 30 cm and …. bMature type that consists of thick crusts (10 cm in average), average height is 50 cm. The
origin and mechanism of formation of tepees were discussed by many authors, i.e., Sherman
(1978) and Warren (1982). The peetee structures (Fig. 5) describes the biogenic structures
that are primarily due to the cohesive character of filamentous cyanobacteria (Gerdes and
Krumbien, 1981 ?) or mats subsurface gas accumulation (Reineck et al., 1990). The
recorded types of the biogenic structures are; (1) Alpha-petees, which are dome shaped
buckles and folds in soft microbial mats including no encrusted and encrusted rounded
crests. (2) Beta-peetees, which are encrusted alpha petees with ruptured crests. The
sediments forming the stromatolites are composed of fine calcareous sediments and often
lithologically different than the surrounded sediments where the structure is embedded. The
organic filaments active in the formation of most algal stromatolites are most probably a
complex of filamentous and unicellular green (chorophyta) and blue- green (cyanophyta)
algae (Logan et al., 1964).
The lithologic sequences of eight sabkha profiles were sketched in Figure 2 and
composed mainly of: (1) 50-20 % highly calcareous sand ranging in color from yellow,
yellowish brown, greenish yellow and pale yellow, (2) 40-20 % greenish calcareous mud,
(3) 20-10 % muddy sands, and (4) 20-10 % halite aggregates and mixed with sands.
Halite is the dominating evaporite minerals with minor gypsum. The halite is snow
white to pinkish white occurring as crusts. The crusts vary from a few cm to 10 cm thick.
Gypsum as interstitial selenite crystals and aggregates occur within the calcareous
sediments. The organic matter (algal mats, plant remains, plant roots) are occurring within
the sediments.
3.3. The hypersaline pool zone
The hypersaline pool zone is represented by two saline ponds. It has a nearly circular
shape and covers 35 m2. It is surrounded by dense vegetations (Phyragmites Australis and
sand dunes). The depth of water in the pool is seasonally fluctuated between 30 up to 65 cm.
The floor is covered by black muddy sand layer mixed with plant roots and dense algal mats.
The second pool is an ephemeral shallow wet pond covers 250 m2. The depth of the water is
about 50 cm and completely dries in the summer and dominated by the Crustacean Artimia
salina. The floor is composed of reddish clays, sands; plant roots microbial mats and
desiccated salt crust. In summer, the presence of unpleasant odor and gas outburst reflect the
high organic activity and the decay of microbial mats and algae.
3.4. The permanent saline pan zone
The permanent saline pan zone occupies the lowest topographic level and covers 10
Km2. It is bounded from the south by the temporary dry zone and from the other sides by the
temporary wet zone. The basin is man made ??; oval shaped with elevations 23 m above sea
level. The dominated minerals in the saline pans are dominated by halite with minor gypsum
and dense biological activity.
Halite occurs as white crystals displaying hopper structures forming crust (20 cm
thick). Halite is clear, euhedral, void filling cements and displacive crystals with in
interlayer mud. Competitive overgrowth of growing crystals at the pan bottom produces
millimeter to centimeters long vertically oriented as chevron halite crystals. Gypsum
displays different shapes, i.e. tooth like, needles and flat circles. The algal mat form thin
films of green and red color raft and compared to those described by Freidman et al. (1973)
and Friedman (1978). Also, polygonal mats are often cross-cut by nearly polygonal
desiccation cracks (Fig.6). These polygonal mats are similar to those reported from the
Persian Gulf by Kendall and Skipuith (1969).
4. Petrography and Mineralogy
Mineralogical composition of the bulk samples representing the different zones of
sabkha were determined by using X-ray diffraction analysis. Halite is the predominate
mineral in most samples with minor gypsum, leonite {K2Mg(SO4)2(H2O)4, anhydrite,
bloedite (Na2Mg(SO4)2(H2O)4, schoenite {MgK2(SO4)H2O)6], bischofite (MgCl2.6H2O),
sinjarite (CaCl2.2H2O) and sulfohalite [Na6( SO4)2Cl F]. The non-evaporite minerals are
quartz, calcite, plagioclase, chlorite [Mg2 Al3(Si3Al)O10], afwillite [Ca3(SiO3OH)2.2H2O],
stishvite (SiO2), wavellite {Al3(PO4)2(OH)3.5H2O}, hematite and clay minerals (kaolinite,
illite and montmorillonite).
Halite occurs as crust and aggregates of cubic and mosaic crystals. Sometimes, it is
contaminated by clay minerals, microcrystalline and coarse spary calcite, angular quartz
grains, and gypsum crystals. The surface of cubic halite crystals (Fig.7) are partially
dissolved and trapped muds, calcite (Fig.8) with dense fluid inclusions. This feature
coincides with the replacement feature of Faulds et al. (1997). This texture is interpreted as
syndepositional in origin. Sometimes, the halite crystals show vertical oriented and elongate
chevron crystals (Fig.9), probably due to depth variation (Cooke, 1966 TOO OLD
REFERENCE). Displacive growth of halite was formed within relatively fine sediments
when the brine reached saturation with respect to halite. Inclusive growth of halite could be
attributed to the rapid growth of halite (Rouchy et al., 1994).
Then, periods of non-deposition or micro no-conformity suggest by deposition of
calcareous sand and sandy lime of the profiles represented by detrital aggregates of
calcareous minerals of calcite and minor dolomite with clastic minerals of quartz, minor
plagioclase and gypsum, microbial mats and shell fossil fragments within halite crust. These
materials are washed out during the rainfall or recycled from the surrounding Middle Eocene
sediments.
Halite sometimes crystallize as skeletal, pyramidal hopper crystals, square to
rectangular shaped plates and coalesced aggregates of hopers and plates forming rafts
(Fig.10). The continuous growth of halite cement under dry surface of the salt crust as well
as the primary growth of the chevrons halite causes lateral expansive growth of the surface
crust. This leads to the disruption of the crust into meter scale polygons surrounded by
pressure ridges. These ridges are overriding each other or ruptured. The compressional force
(pressure growth) resulted from cement growth may be a dominant cause for the formation
of ruptured polygons as explained by Lowensten and Hardie, 1985. Evaporative pumping of
subsurface brine through the polygonal cracks leads to a spongy efflorescent halite (Fig.11).
Gypsum mainly forms a prismatic and as rosettes of lenticular twined crystals
(Fig.12) growing within the calcareous sediments at different depths in the profiles and
mainly exhibit displacive characters. It consists of coarse crystalline, grey and/or transparent
to translucent crystals. Sometimes, it is occasionally contaminated by aggregates of
microcrystalline calcite, clay minerals and quartz and root plants. Reworked gypsum crystals
are observed within sabkha profiles. The presence of gypsum with abundant NaCl rich
sediments containing abundant specific types of dissolved organic material (Cody, 1979).
This is in agreement with the presence of microbial mats and halophyte plants predominate,
where they have the ability to excrete-amylase enzyme to the water and soil promoting the
formation of gypsum (Cody and Cody, 1889) or denotes their formation in warm NaCl rich
brine, which causes a decrease in a nucleation density, producing slower growing, larger
crystals (Cody, 1979). Moreover, during the rise in the underground water level to the
ground surface, this water dissolves part of the transported old gypsum and re-deposited
again at the ground surface under arid climate. The crystals causes penetration features,
where crystal boundaries are corroded and curved causing disturbance in the cleavage
pattern and presence few smaller halite crystals into coarser one.
The sabkha is highly enriched in sodium chlorite, which allows the precipitation of
halite by evaporation. This causes sodium to decline, but has a relative minor effect on
chlorites (Sandford and Wood, 1991). The precipitation of sulfohalite probably took place
during the precipitation of halite by addition of sulphate and fluorine. Afwillite is recorded
in few samples and may be formed primary or secondary during evaporative concentration
stage of the brine.
The studied water pans indicate that is dominated with different kinds of algae, i.e.
Chamydomonas sp. belonging to division of clorophyta. Also, diatoms predominate, i.e.
Naviuls sp. However, pool water is dominated by Artimia sp. belonging to class of
Crustacean and subclass Branchiopod (Hussaini, 1992).
5. Hydrogeochemistry
The relationship between brine solution and the associated minerals renders the
method of formation and the diagenetic processes to be closely delimited groundwater
associated with sabkha sediments. The fourteen collected water are classified into four
categories:
1-two groundwater samples from wells (40 m depth).
2- four trench water (water extracted from core sediment samples by using a suction
instrument).
3- two surface water collected from pools and 4- six surface water collected from
pans.
Complete chemical analyses of the saline water samples including major cations,
anions and total dissolved salts (TDS) were undertaken using standard techniques (Papova
and Stylorva, 1972). Results of these analyses of the different water categories are given in
Table (1). Their chemical composition are expressed in ppm (part per million) and epm
(equivalent per million) and epm%.
The average total dissolved salts (TDS) is 4100, 149465, 224165 and 341136 ppm in
the groundwater, pools, trenches and pan water samples respectively. The salinity averages
indicate that there is a gradual upward increase in salinity, suggesting that capillary
movement of water and evaporative pumping mechanism plays a significant role in the
upward movement of brines as proposed by Hsu and Schneider (1973). The source of
salinity are mainly from; 1- the water recharge from the upward leakage of groundwater
through the exist faults in the region. 2- Rainwater that dissolves and transports the
sediments from the surrounding Middle Eocene highs and also dissolves the salt crust
formed in the arid periods.
The average calcium content is 74, 37500, 29375 and 17600 ppm in the
groundwater, pools, trenches and pan water samples respectively. The averages of calcium
concentration in the different zones of sabkha indicate that the calcium content increases in
pans, trenches and pools than wells. This attributed to the effect of nearest Middle Eocene
sediments containing carbonate and gypsum veinlts that partially dissolved and transported
by rainfall; the gypsum formed in the sabkha area during arid periods which is partially
dissolved by both rainfall and groundwater (capillary upward movement of groundwater).
The average magnesia content is 15, 2550, 2872 and18878 ppm in the water wells,
trenches water, pool water and pan water samples respectively. The high concentration of
magnesia in water pan samples may be due to occurrence of clay minerals from the
surrounding calcareous shale. Also, the percolating rain waters may be enriched with
dissolved materials rich in magnesium and/or dominance algae.
The average sulfate content is 20, 5800, 6150 and 7500 ppm in the water wells, pan
water, pool water and trenches water samples respectively. The high concentration of sulfate
in trench water attributed to the presence of gypsum crystals formed by evaporation in arid
periods and partially dissolved by water table.
The average sodium content is 850, 16750, 17625 and 47250 ppm in the water wells,
trenches water, pool water and pan water samples. The higher concentration of sodium in
the water pan samples is attributed to the high rates of evaporation processes, dissolution of
preexisting halite crust by rainwater and groundwater as well as the transported feldspars
and clay minerals of Middle Eocene sediments that confirmed by X-ray diffraction analysis.
The average chloride content is 1462, 62636, 98575 and 102490 ppm in the
water
wells, water pans, pools water and trenches water. The high content of Cl- in the water
trench samples indicate the upward increasing of chloride ion due to evaporation and may be
attributed to the dissolution of preexisting halite crust in the sabkha area.
The average content of bicarbonate is 196, 317 and 344, 395 ppm in the water wells;
pools water, trench water and pans water samples respectively. The high bicarbonate content
increase in trenches and pans indicate the effect of surrounding Middle Eocene limestone.
The average potassium content is 20, 1600, 3500 and 3650 in the water well, pool
water, water trenches and water pan samples. (Reduce the descriptive values and much
better to illustrate the values in a table and/or graphs)
5.1. Ion dominance, water type and salt combination
Ion dominance, water type and salt combination for the collected water samples are
listed in Table (1). From the arrangements of anions and cations in the water wells have
Na>Ca>Mg and Cl>HCO3>SO4 arrangement. The trenches water has Mg>Na>Ca and
Ca>Na>Mg and Cl>SO4>HCO3 arrangement. The pool water is Ca>Na>Mg and
Cl>>SO4>HCO3 arrangement. The pan water has Na>Mg>Ca and Cl>SO4>HCO3
arrangement.
The average highest anion concentration in the analyzed water samples of wells and
pans is chloride (91 to 98 epm %), while the average highest cation concentration is sodium
(26 to 86 epm %). So the type of water in the investigated area is sodium chloride. However,
the water samples of saline pans are magnesium chloride may be due to the accumulation of
clay minerals and/or dominance algae in these pans. The surface pools is concentrated by Ca
2+
cations (74 epm %) and Cl anion (96 epm %), and the type of water is calcium chloride.
The trench water is concentrated by Ca 2+ & Mg 2+ cations (79 and 45 epm %) respectively,
while the concentrated anion is chloride 95 epm %. The water types are calcium chloride
and magnesium chloride. The main salts are sodium chloride, magnesium chloride, and
calcium chloride. The evolution pathways of brines are containing less sulfate producing
sulfo-chloride after Hardie and Eugster (1970).
5.2. Geochemical classification of water
Figure 13 illustrate the results of the grid system classification (Collins, 1923, Atwa,
1979). This method based on the reacting values of the three major cations and the major
anions expressed in epm %. From this application, it clear that the water types of the
investigated samples from water wells is sodium chloride with an excess of chloride. Pool
water is calcium chloride, water pans are sodium chloride and magnesium chloride and
water trench is magnesium chloride and calcium chloride.
The geochemical trilinear diagram introduced by Piper (1944)was used to define the
character of water through the relation among the alkalis (Na+, K), the alkaline earth, (Ca2+,
Mg2+), the alkalinity (CO3 + HCO3) and the salinity (Cl- +SO42-) is one of the most
important and useful graphs for representation and comparing water type. The investigated
water samples are mixed water (marine with meteoric waters) and characterized by strong
acids exceeds weak acids (SO4 +Cl) > ( CO3+HCO3), except sample (No. 1) is lie in subarea (2) characterizing by alkalis exceeds alkaline earth 's (Na+K) > ( Ca+ Mg).
5.3. Hydrochemical coefficient
This parameter serve to know the ratios of ions over each other to understanding
what salt can be form first and also to illustrate mechanism of salt formation to great extent.
The investigated water samples have r(Na +K)/ rCl. This indicate the excess of chloride in
the studied samples and the ability of water to form sodium chloride salt (halite) by
evaporation. The ratio of rCa /r Mg indicate that the samples No. 1 , 2 ,4 , 6 , 9 , 11 , 13 and
14 is meteoric water contamination (rainfall). This attributed to leaching effect of some
terrestrial salts rich in calcium and ion exchange process. The samples No. 3, 5, 7, 8, 10 and
12 have ratios rCa/ rMg attributed to the groundwater moves upward and the last gypsum
dissolves in sabkha. The ratios of rSO4/rCl in all samples refer to the ability of the water to
precipitated sulfate salts (gypsum) before the precipitation of sodium chloride (halite). This
illustrates the presence of the gypsum crystals scattered within the sediments beneath the
halite crust.
6. Organic Matter Content and Hydrocarbon Possibility
It is quiet known now that the closed basin continental sediments are important
hydrocarbon source rocks and reservoirs in many parts of the world (Renaut et al, 1994). A
considerable number of oil and gas fields are more or less directly related to evaporative
bearing sequences. From a sedimentological view point, the parallic environment could be
responsible for the far greatest part of evaporitic deposits and is also produces endogenous
organic matter with flourishing into great amounts. Parallic settings produce at least ten
times more of organic matter than marine environments (Basson et al., 1977; Allen et al.,
1979). Consequently, the studied wet and dry pan zones with extensive growth of microbial
mats may represent recent model for the hydrocarbon accumulation in evaporate salt due to
the intimate association between extensive evaporite precipitation and microbial mats
(Warren, 1986; Busson, 1988). The soil coloration ranges from yellow to dark brown color,
which represents thermal alteration index values ranging between -2 and -3 ( according to
Robinson scale, 1989). So, the organic material reaches a mature state.
Determination of organic matter as total carbon and sulfur and organic carbon is
given in the following Table (3). Seven samples were analyzed to determine the total
carbon, organic carbon and sulphur. The total carbon ranges from 0.12864 to 2.78082 %
with an average of 1.421%. The sulphur ranges from 0.49245 and 2.77666% with an
average 1.354%. The organic matter is ranges from 0.81 % to 8.45 % with an average of
3.786%. Also, another six samples were analyzed to determine the organic matter and total
carbon percentages by other method (describe the method or add reference). The carbon
in soil samples is oxidized at a temperature of approximately 120◦C by adding a potassium
dichromate solution and concentrated sulphuric acid. Excess of potassium dichromate, not
reduced by the organic matter of the soil is determined by titration with ferrous sulphate
using diphenylamine. Sulphuric acid Ba-salt is used as indicator. Phosphoric acid is added to
form a complex with ferric-ion providing a sharper color change of the indicator (Blac et al.,
1965)
The organic matter accumulation is due to the lateral and vertical extensive growth
of the microbial mats (Gerdes et al., 1991 and Cornee et al., 1992). There are two origins for
the organic matter in the evaporites are generally proposed, algae and bacteria growing in
the upper water body of salt lakes and higher plant debris from terrigenous influx (Warren,
1986 and Javor, 1985). The sediments that laid down under anoxic conditions are
considerably rich in organic carbon than the sediments deposits under oxygen-bearing
waters (Richards, 1985). Hydrogen sulfide is produced in the anoxic environment by sulfate
reducing bacteria according the this equation;
Organic matter + SO4 → Bacteria → H2S + CO2 +H2O
It is a fact that anoxic burial conditions plays important role in enriching these sediments at
rapid rates due to the following facts:
1- Prevalence of non-oxygenated conditions.
2- No light penetration.
3- Flourishing anaerobic bacteria which accelerates the degradation of the organic
matter into its primitive components (i.e. Cn Hn).
4- Continuous preservation of the degraded organic matter in reducing condition
leading to formation of sapropel.
Discussion and Conclusion
The modern continental sabkha located at El-Fashn area represent a closed basin
fringed with sand dunes and Middle Eocene rocks. The sabkha is subdivided into four
zones;1-temporary dry elevated marginal zone characterized by the abundance of halophytes
and desiccation features such as mud cracks; shrinkage and wind blown sands.2- temporary
wet saline mudflat broad zone characterized by dense halophyte vegetation, extensive saline
mudflats with halite crusts forms rhythmic layers; surface features are distinguished by the
occurrence of the tepee and peetee structures. Surface structures of sabkha can be
differentiated by biological surface structures (petees) of the interdunal basins from the
biogenic structures (tepees). The formation of these surface structures is completely
different, where the former, resulting from the biological activities is found in the
continental ephemeral salt lakes and the latter resulting from thermal expansion and
contraction where minor expansion is caused by fluctuation of pore pressures in porous
sediments below the tepee affected crust. The existence of tepee indicates periodic
groundwater resurgence along the basin margins. 3- hypersaline pool zone is represented by
two saline ponds and surrounded by dense vegetations and sand dunes. The depth of water
in this pool is seasonally fluctuated between 30 and 65 cm. The floor is covered by black
muddy sand layer mixed with plant roots and algal mats, microbial mats and desiccated salt
crust and dominated by the Crustacean Artimia salina and permanent saline pan zone are
common by halite with minor gypsum and biological activity. 4- Permanent pan zone are
characterized by dominated halite with minor gypsum, extensive growth of microbial mats.
Following Warren (1982b) the factors exist in permanent wet and pan zones that required
growth of microbial mats are; 1- presence of growing filaments cyanophytes, 2- continental
water saturation of the microbial sediments, 3- water with suitable salinity range, 4- the
absence of grazing gastropods.
The low topographic depression of sabkha receive brines from two different sources;
1-metoric water from the surrounding sand dune aquifers and rainfall, and 2- subsurface
marine seepage from nearby wells (>40 m deep) and irrigation discharge and subsurface
fractures in basin may be directly connected with Qarun and Rayan lakes north of the study
area by subsurface structures and they are subjected to rapid changes in salinity after the
flooding periods and/ or after heavy rainfall.
The mechanisms proposed for the explanation of the sabkha formation;1- seepage
reflux mechanism postulated by Aams and Rhodes (1960), indicated by downward flow of
irrigation water moving through the sediments enriched in residual Mg. 2- Mechanism of
capillary upward movement of brines from the groundwater as proposed by Ameil and
Friedman (1971). 3-Mechanism of evaporative pumping proposed by Hsu and Siegenthaler
(1969), where the upward movement of subsurface water is induced by a vertical hydraulic
gradient, due to evaporation within the evaporative area.
The mineral assemblage comprising halite with gypsum and sulfohalite, quartz,
calcite, chlorite, afwillite, stishvite and clay minerals (kaolinite and montmorillonite). The
type of mineral forming in non-marine evaporite deposits depends on the chemical
composition of inflow water and the mechanisms whereby these waters become brine. The
processes that modify parent waters during their evolution into brine include evaporative
concentration, mineral precipitation, syndepositional recycling, diagenetic mineral reactions
and exchange reaction with pore fluids. Halite crusts have undergone repeated episodes of
flooding, evaporative concentration and desiccation. Halite crystals are characterized by
dissolution texture formed during flood stages and cementation textures formed during latest
stages.
The chemical analysis revealed gradual upward increasing in salinity, high
concentration of Na and Cl ions and fairly high concentration of Mg, CaSO4 and HCO3,
when compared to their values in wells aquifer. This is explained are due to extreme arid
climate, high evaporation rate and reactions between the aquifer water with host rocks and
preexisting salts and mixing with irrigation waters. The studied evaporites in non-marine
setting, mineral and chemical compositions, textures, fabrics and geographic distribution of
different zones indicate that the source water must have resembled non-marine in
composition. The salt assemblages established from ions concentrations reflect the presence
of sodium chloride, magnesium chloride, and calcium chloride. The upward increase in
salinity and Na and Cl ions indicate that the evaporative pumping mechanism of the
groundwater plays a significant role in the upward movement of brines and formation of the
evaporite minerals under arid conditions.
The reached values of analyzed samples for C, O and S support that the generation of
sapropel is achieved as verified from field investigations directing the attention that with
continuity of the reducing conditions versus time will be a good recent example for
generating hydrocarbons. Sabkha sites in many places did prove to display this important
role and view in generating hydrocarbons all over the world.
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