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Journal of Applied Sciences Research, 9(8): 5344-5369, 2013
ISSN 1819-544X
This is a refereed journal and all articles are professionally screened and reviewed
ORIGINAL ARTICLES
Contribution to the Mineralogy, Geochemistry, and Provenance of the Lower Eocene
Esna Shale in the Farafra Oasis, Western Desert, Egypt
1
1
2
Abou El-Anwar, E.A.; 1EL-Wekeil, S.S. and 2Gaafar, S.Sh.
Geological Sciences Department, National Research Center, Cairo
Egyptian Mineral Resources Authority
ABSTRACT
The Esna Shale in El-Quss Abu Said Depression, Farafra Oasis is unconformably underlain by Tarawan
Formation and is conformably overlain by Farafra Formation with a gradational contact in - between. Two well
exposed Esna Shale successions were selected for the present study. Lithologically, the Esna Shale Formation
has similar lithological characteristics. It is mainly consists of vari-colored shales intercalated with argillaceous
limestone in parts. Their detailed petrographic, mineralogic and geochemical characteristics were determined
using several techniques including thin-section examination, X-Ray diffractometry, thermal analyses, scanning
electron microscopy and X-Ray Fluorescence. The Esna Shale Formation is texturally classified as mudstones.
Their clay fractions consist entirely of moderately-crystalline smectite with subordinate kaolinite and trace
concentration of illite. Generally, the studied shales consist mainly of smectite specially at the southern area
and deposited in the deeper part of the basin. While the relative concentrations of kaolinite increase at the upper
part of the studied sections specially at the northern area and deposited in shallower water setting. The contents
of SiO2 Al2O3, Na2O and K2O are lower while those of CaO and P2O5 are higher than the values reported for the
Upper Continental Crust (UCC) and Post Archaean Australian Shale (PAAS). In contrast, Fe2O3 and TiO2
content higher than (UCC) and lower than (PAAS). The predominantly mafic characters of the source rocks for
the northern section shales are being most related to the PAAS. The more felsic nature of the source rocks of the
southern section shales being mainly related to the UCC. The more enriched in Zr and, to a much lesser extent
Th in shales of the southern section revealed that it subjected to intensive chemical weathering than the northern
shales section. The shales at El–Quss Abu Said plateau are entirely detrital and relatively immature. They are
product of intensive chemical weathering of crystalline igneous and metamorphic rocks (granitic and basaltic)
(especially at the southern area) to the south of Egypt. The provenance constituted a part of passive margin and
was characterized in semiarid climate. The developed soils were carried out by fluvial action to the basin of
deposition, which finally interfered and admixed with marine environment. The abundances of smectite and, to
a lesser extent kaolinite in the clays in these shales may be related to changes in the nature of source rocks,
climatic conditions and/ or the transgression and regression of the sea level during the Early- Eocene.
Geochemical analyses data are reflected the depositional environment occurred in Phanerozoic-Proterozoic
under oxic to anoxic or dyoxic marine conditions coupled with the effect hydrothermal solutions. The
environmental parameters reveal that the locality of Farafa Oasis is unpolluted to very strong pollute.
Key words: Esna Shale, Mineralogy, Geochemistry, Provenance, Farafra Oasis, Western Desert.
Introduction
The Farafra Oasis forms a depression that lies in the central part of the Western Desert of Egypt.
Geomorphologically, this depression is roughly triangular in shape having a maximum length of ~120 km and
an east- west axis extending for ~90 Km. Its eastern, western and northern sides are bounded by scarps and
cliffs. The eastern cliff of the El- Quss Abu Said plateau (Farafra Limestone) represents the western scarp of the
depression (Said, 1990). Structurally, the tectonic pattern of the Farafra Oasis is a result of the impact of the
Syrian Arc Folding System on lithologies of vastly different competence. During the Late Cretaceous-Eocene,
several tensional forces led to the development of many faults dissecting the area (Neev and Hall, 1982 and ElEraqi and Atwa, 1999). Omara et al., 1970 and Sherif, (2006) emphasized that the sedimentary record of the
Farafra Oasis reflects the occurrence of a number of tectonic movements at different times. These movements
produced four gently- folded structures represented by two anticlines (Farafra main or “central” and Ain Dalla
anticlines) and two synclines (faulted El- Quss Abu Said and El Ghord synclines). Faults are rather difficult to
detect in the Farafra Oasis due to the presence of sand cover. On the other hand, joints are well marked in the
chalk forming the floor of the Farafra Depression. These joints are usually mineralized being filled with calcite,
Corresponding Author: Dr. EL-Wekeil, S.S., Geological Sciences Department, National Research Center, Cairo
E-mail: [email protected]
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J. Appl. Sci. Res., 9(8): 5344-5369, 2013
iron oxides and pyrite (Sherif, 2006). El-Ramly (1964) concluded that these joints are partly contemporaneous
with the folding affecting the Farafra Oasis.
The Farafra Oasis comprises a number of rock units ranging in age from Late Cretaceous to Early Eocene
(Fig. 1). They display distinct lateral and vertical variations in thickness and lithology which includes chalks,
chalky limestones, argillaceous limestones, shales, mudstones, dolostones, sandstones and evaporites. The
geology, lithostratigrphy and micropaleontology of these rock units were the subject of numerous investigations
since the last century (e.g. Beadnell,1901; Le Roy,1953; Said and Kerdany, 1961; Said,1962; El-Naggar, 1963;
Youssef and Abd El-Aziz, 1971; Issawi,1972; Barakat and Abd El-Hamid 1974; Omara et al., 1976;Barthel and
Herrmann-Degen,1981; Zaghloul, 1983; Khalifa and Zaghloul, 1985; Hermina, 1990; Samir, 1995 and 1999;
Abd El-Kireem and Samir, 1995; El- Azabi and El-Arabi, 2000; Abdel Mohsen, 2002; Khalil and El-Younsy,
2003; Temraz,2005; Sherif, 2006; Wanas, 2012; and El-Ayyat, 2013).The lithostratigrphic units of the Upper
Cretaceous - Lower Eocene succession in the Farafra Oasis (Fig.1) are represented by the formations (arranged
from older to younger): El- Hefhuf Formation (Santonian- Campanian), the Khoman Chalk and chalky
limestone (Maastrichtian), the Dakhla Formation (Early-Late Paleocen), the Tarawan Formation (LatPaleocene), the Esna Shale (Early –Eocene) and the Farafra Formation ( Early-Eocene). The informally mapped
Quaternary clastic –carbonate unit assigned by Wanas (2012) crops out as scattered small hills above the
Maastrichtian Khoman Chalk near the eastern escarpment of the Farafra Depression at Bir-Karawein area.
The term “Esna Shale” was used for the first time by Beadnell (1905) to describe a succession of laminated
green and grey shale that are exposed at Gebel Oweina (type section) located 22 km southeast of Esna, Nile
Valley. Said (1962) amended the term Esna Shale restricting its usage for the shale beds existing between the
Tarawan Formation and the Thebes Formation.
In the Farafra Oasis, the Esna Formation has the same stratigraphic position as in the southern occurrences.
It shows lateral changes in thickness and facies. The distribution of the Esna Formation is controlled, to great
extent by the structurally- folded areas over which deposition took place. In the low synclinal areas, the
formation is represented by shales with thin intercalations of carbonates and has a maximum thickness of 130 –
150 m (El-Quss Abu Said and Ain Maqfi areas), whereas in the structurally high anticlinal areas, the formation
is entirely composed of carbonates and its thickness is reduced to ~ 20 m (northwest Ain Maqfi area). Wilson
(1975) and El-Ayyat (2013) mentioned that the rhythmic bedding might have been caused by fluctuations in the
terrigenous input and characteristics of depositional environment such as surface productivity, current velocities
and water depth. Khalifa and Zaghloul (1985), Sherif (2006) and El-Ayyat (2013) interpreted this distribution
being the result of the transgression of the Paleocene sea over the folded areas, depositing shale and thin
limestone beds in low areas, contemporaneous with the accumulation of carbonate over the crests of the
structural highs.
Based on the presence larger foraminifers, Khalifa and Zaghloul (1985) concluded that the Esna Shale was
deposited in a shallow subtidal environment. It is inferred to have been accumulated mostly in inner- to middleshelf environments (Said, 1990). Samir (1995) emphasized that the basal part of the Esna Shale was deposited
in a lower- neritic (100 – 200 m) environment, its middle part is bathyal (600 – 1000 m), whereas the upper part
of the formation deposited witnessed the prevalence of shallower conditions. Khalil and El-Younsy (2003)
stated that the Esna Formation was deposited in open deep marine environment (deep subtidal) with intermittent
regressive pulses. On the other hand, Temraz (2005) stated that the abundance of planktonic foraminifera in the
Esna Shale strongly suggests deposition in an oxidizing environment (middle to inner- shelf). Based on the
faunal and lithologic characteristics of the Esna Shale in the Farafra Oases, Sherif (2006) emphasized that in the
paleo- low areas (synclinal) the basal part of the Esna Formation indicates deposition in a deep, middle-to outershelf environments, the middle and upper parts of the formation were accumulated under shallower conditions;
mainly shallow subtidal to upper deep subtidal setting (shallow inner- shelf). El-Ayyat (2013) stated that the
predominance of planktonic forams over the benthonic indicates to deposition in an open marine environment.
Although many workers dealt with paleontology, the lithostratigraphy and classification of the Upper
Cretaceous -Lower Eocene succession in the Farafra Oasis, however, a relatively limited number of studies
were concerned with the mineralogy and geochemistry of these rocks. Therefore, the main target of the present
work is to study in detail the mineral and geochemical characteristics of the Lower Eocene Esna Shale in order
to shed more light on their sedimentary and tectonic history.
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Fig. 1: Geological map of the study area showing the locations of the selected stratigraphic sections (modified
after Sherif, 2006).
Lithostratigraphy of Esna Formation:
On the slopes of the El-Quss Abu Said plateau, sediments of the Esna Formation have similar lithologic
characteristics. It consists mainly of mudstones intercalated in parts with argillaceous limestone. The mudstones
show varying degrees of fissilty which may reach the shale characteristics.
Two stratigraphic sections representing the Esna Shale Formation at El-Quss Abu Said depression were
measured and sampled (Fig. 1). The first section (1) represents the northern slope of El-Quss Abu Said
depression. It is ~ 130 m thick (Fig. 2A). While, the second section (2) was measured in the southern slope of
El-Quss Abu Said depression. It is ~ 100 m thick (Fig. 2B).The bases of both sections are unexposed.
Generally, the shales of the Esna Formation are vari-colored (olive, green, grey, yellow, yellowish grey and
greenish grey), hard to moderately- hard, massive, partly fissile, locally calcareous, ferruginous and contain
gypsum veinlets, and black manganese patches. They are characterized by the presence of larger planktonic
forams at the top and dwarfed faunal and plant remains at its base (Sherif, 2006 and El- Ayyat, 2013). The
argillaceous limestones are greyish white to yellowish grey, moderately- hard, massive contain minor gypsum
veinlets and iron oxides and fossiliferous with larger foraminifers. The Esna Formation is unconfromably
underlain by the Tarawan Formation which has more or less the same lithology in the study area. It is composed
mainly of chalky limestones which are white, hard, massive, compact, ferruginous in parts, fossiliferous and
contain plant remains. These limestones are intercalated with olive to green, mainly compact, calcareous shale.
The boundary between the Tarawan chalk and the overlying Esna Formation is distinct. The Esna Shale is
conformably overlain by the Farafra Formation where the contact between the two formations is gradational and
defined by the presence of fragmented limestone with larger foraminifera. The Farafra Formation consists
mainly of Alveolinid limestone which has a great extension, forming the plateau surface of El-Quss Abu Said. It
is greyish white, yellowish white to creamy, moderately hard, massive, intercalated with shale and compact
argillaceous limestone and fossiliferous containing larger foraminifers.
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Fig. 2: (A) Lithostratigraphy of section (1) measured in the northern slope of El-Quss Abu Said plateau.
Fig. 2: (B) Lithostratigraphy of section (2) measured in the southern slope of El-Quss Abu Said plateau.
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Materials and methodology:
Two stratigraphic sections representing the Esna Formation at El-Quss Abu Said depression were measured
and sampled (Fig.2 A and B). Fifty four representative samples were collected from the shale beds encountered
in section (1) and section (2) measured at the northern and southern parts of the El-Quss Abu Said depression.
The petrographic characteristics of twenty mudstone samples were determined by examining their thin sections
under polarizing microscope. The clay fractions of these samples were subjected to X-Ray diffraction analysis
(XRD) in order to identify their clay mineral composition following the method described by Carroll (1970).
Three oriented mounts were prepared for each sample: untreated, glycolated and heated at 550oC for two hours.
Semi- quantitative determination of the identified clay minerals was undertaken based on the method adopted
by Pierce and Siegel (1969). The X-Ray diffractgrams were obtained using Philips X–Ray diffractometer model
PW/1710 with monochromator, Cu k -α radiation (‫=ג‬1.542 A°) at 40 kV, 35 mA and scanning speed of 0.02o
/sec. Also differential thermal analysis (DTA), differential thermogravimetry (DTG) and thermogravimetry
(TG) for two selected samples were conducted in order to check the results of XRD and determine the thermal
behavior of the clays. The powdered sample was heated from the room temperature up to 1100 oC using a
heating rate of 10 oC/min and α – Al2O3 as a reference material. The analysis was conducted using Schimadzu
DTA- 50H, and TAG-50H. Also, scanning electron microscopy (SEM) was utilized for four selected samples in
order to determine the composition, morphology and crystallinity characteristics of the recorded clay minerals
and to clarify some petrographic details. In addition, X–Ray fluorescence technique (XRF) was used for fiftyfour samples and twenty- seven selected samples to determine their major and trace elements contents. The
analysis was carried out for their powder >74 μm fractions using X-Ray fluorescence equipment model
PW2404 having six analyzing crystals. The chemical analyses, XRD analysis, the thermal analysis, and SEM
were carried out in the Central Laboratories of the Geological Survey in Cairo.
Results and Discussion
Petrography and Clay Mineralogy:
(i) Petrography:
Microscopically, the mudstones of the Esna Shale in the study areas have, more or less, the same
petrographic characteristics. Generally, they consist mainly of ferruginous clayey matrix in which fossils are
embedded. Iron oxides and carbonates exist in considerable variable proportions while Mn-oxides and silica are
much less abundant.
Iron oxides may reach up to 10% in some parts of the succession especially that of the northern section.
They are present commonly in the form of very fine material acting as pigments of the clay matrix, filling
fractures and veins, scattered patches or forming thin veneers rimming of fossil fragments or filling their
chambers(Plate Ι: A & D and Plate Π: A, C & D). In some cases, iron oxides co- exist with Mn- oxides (Plate1:
A and Plate Π: D). The modes of occurrence of these iron oxides indicate an authigenic origin being the result
of diagenetic dissolution of Fe- rich silicates (e.g. olivine, biotite, hornblende…etc) during chemical
weathering. Based on their modes of occurrence, precipitation of iron oxides preceded deposition of Mn-oxides
depending on differences in their solubility under various geochemical conditions (Eh and pH) at or near the
earth’s surface. Degens (1965) and El-Wekeil (1993) stated that at any given pH, iron oxides and hydroxides
precipitate at lower oxidation potentials than manganese oxides. Similarly, under fixed Eh, iron oxide starts to
precipitates at a considerably lower pH than manganese oxide.
Carbonates constitute appreciable proportions of the studied shales (average~13%).They occur either
replacing parts of the clay matrix or as cements which were later stained with iron oxides. The majority of
foraminiferal tests are partially or completely replaced with carbonate or their chambers are filled with them. A
few fine-grained dolomite rhombs was recorded in the shales (Plate Ι: B, C& D and Plate Π: B & C). These
were deposited most probably deposited in the supratidal zone of marginal marine environment (El-Ayyat,
2013).
Silica is rather rare in the studied shales and, if present found as recrystallized microcrystalline forms,
filling fractures or veins especially in the southern section (Plate Ι: C and Plate Π: A). Free silica can be
liberated by decomposition of clay and other silica- rich minerals and may be introduced during burial by the
hydrothermal solutions (Wei et al., 1995 and Fleurance et al., 2013).
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Plate Ι: (A) Photomicrograph showing calcareous bivalve shells together with other fossil fragments embedded
in a highly ferruginous argillaceous matrix, locally cemented with calcite (Sec.1, S.No.1, C.N.); (B)
Photomicrograph showing calcareous ferruginous argillaceous matrix containing a few fine
rhombohedra of dolomite and iron oxide veinlets (Sec.1, S.No.15, C.N.); (C) Photomicrograph
showing ferruginous lime mud matrix, partially cemented with calcite and containing silica veinlets
(Sec.1, S.No.22, C.N.); (D) Photomicrograph showing ferruginous argillaceous matrix containing
patches of Mn- oxides, fine dolomite rhombohedra and few calcareous foraminiferal tests some of
which have thin veneers of iron oxides (Sec.1, S.No.30, C.N.); (E) SEM Photomicrograph showing
detrital smectite particles and a vug filled with framboidal pyrite (Sec.1, S.No.3); (F) SEM
Photomicrograph showing aggregates of detrital smectite and moderately-crystallized of kaolinite
together with fine halite crystals (Sec.1, S.No.12).
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Plate Π: (A) Photomicrograph showing highly ferruginous an argillaceous matrix pigmented with iron- and
Mn-oxides and containing veinlets of microcrystalline silica, calcite and iron oxides ( Sec.2, S.No.38,
C.N.); (B) Photomicrograph showing an unidentifiable calcareous fossil fragment embedded in a
calcareous, highly ferruginous matrix, containing a few scattered fine dolomite rhombohedra (Sec.2,
S.No.41, C.N.); (C) Photomicrograph showing slightly deformed calcareous Planktonic tests
disseminated in a ferruginous argillaceous and recrystallized lime mud matrix which contain also
iron and Mn- oxides patches and a few zoned dolomite rhombohedra (Sec.2, S.No.47, C.N.); (D)
Photomicrograph showing calcareous bivalve shell fragments embedded in a highly ferruginous
argillaceous and lime mud matrix pigmented, in places with Mn-oxides ( Sec.2, S.No.54, C.N.); (E)
SEM Photomicrograph showing a relatively large detrital smectite aggregates embracing fine quartz
grains (Sec.2, S.No.39); (F) SEM Photomicrograph showing a detrital smectitic matrix having a vug
filled with framboidal pyrite crystals (Sec.2, S.No.45).
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(ii) Clay Mineralogy:
The X - Ray diffractogram pattern of the Esna Shale samples collected from the northern and southern
slopes of El-Quss Abu Said depression (sections1 and 2; respectively) revealed the presence of smectite
(Ca,Mg-montmorillonite), subordinate kaolinite and occasionally traces of illite (Fig.3A). Smectite peak have
high intensities but are relatively broad suggesting a moderate crystallinity. The average contents of smectite in
the studied samples are ~ 95% and ~ 75% of the total clay minerals at the southern and northern slopes of the
plateau; respectively. Kaolinite exists at certain stratigraphic levels of the two successions showing an increase
in proportions at their upper parts especially at the northern section. It averages ~ 4% and ~ 24% in the
southern and northern sections; respectively. Kaolinite is generally moderately – crystalline based on the shapes
of its diffractions peaks (cf. Schultz, 1964 and Carroll, 1970) and the number of reflections (cf. Keller, 1970).
Traces illite were reported in the southern and northern sections at the upper parts.
SEM of the shale samples under investigation confirmed the dominance of detrital Ca,Mg- smectite (Plat
Ι: and Plate Π: E & F). The absence of any conceivable volcanic precursor, such as tuff or glass in the studied
shales indicates that smectite is mainly of detrital origin (Moore and Reynolds, 1997). Also, detrital kaolinite
constitutes a minor portion of the clay minerals, mostly forms aggregates of moderately- crystallized of
medium- sized particles( Plate Ι: F). The recorded clay minerals are commonly moderately- crystallized and
their morphological characteristics indicate that their major part is detrital.
The results of thermal analyses of the selected samples ( S.No. 5,section-1and S.No.42, section-2) confirm
those obtained from X- Ray diffraction analyses and scanning electron microscopy. The DTA curves showed
that smectite is represented by three peaks. The first is a large endothermic peak at 140 oC peak due to loss of
adsorbed moisture, the second is a main endothermic peak at temperature 530 oC related to the loss of hydroxyl
water of smectite, and the third is an exothermic reaction at temperature 850 oC corresponding to structural
change. Dolomite exhibits a double endothermic peaks on DTA curve in sample No.5 at temperature (670 and
760 oC) corresponding to the decomposition of the carbonate ions associated with magnesium and calcium.
Temperature, weight, change in weight, and the thermal behavior of selected samples are shown on the charts
(Fig.3B).
Raucsik and Merenyi (2000) and Ghandour (2004) emphasized that detrital smectite in marine sediments
increase during sea-level highstand period, whereas kaolinite and mica increase during low stand period.
Followed the previous authors, the lower proportions of kaolinite and traces illite relative to smectite of the
Esna Shale sediments can be explained as kaolinite and traces illite tend to be concentrate in relatively near –
shore shallow water settings (during lowstand period) reflecting their coarse –grained nature and their sorting
tendency to flocculate compared to smectite, whereas smectite tends to settle as finer particles in deeper settings
(during highstand period). Tantawy et al., (2001) emphasized that the presence of abundant smectite is
generally linked to transgressive seas.
Generally, the results from X-Ray diffraction analyses carried out for the clay fractions of the studied Esna
Shale samples from El-Quss Abu Said depression show that the shale samples consist mainly of smectite
especially at the southern area and deposited in the deeper part of the basin while the relative concentrations of
kaolinite increase at the upper part of the studied sections especially at the northern area and deposited in
shallower water setting. The results of thermal analyses and SEM examination confirmed those obtained from
X- Ray diffraction analyses and indicate that they are mainly detrital in origin.
Geochemistry:
The concentrations and geochemical parameters of the major and trace elements in the studied Esna Shale
are given in Tables (1 to 4). Generally, the results of geochemical analyses revealed that these shales have more
or less similar contents of the major elements whereas the concentrations of trace elements show differences.
(i) The Major Elements:
SiO2 and Al2O3 are the most abundant elements in the studied shales averaging 41.54 and 13.45 %;
respectively (Table 1). The concentrations of SiO2 are lower than for Upper Continental Crust (UCC) and Post
Archaean Australian Shale (PAAS) (66 % and 62.8 % for SiO2 and 15.2 % and 18.9 % for Al2O3) those reported
by Taylor and McLennan, (1985). Shales in the southern section are slightly more enrichment in SiO2, MgO and
Cl contents (averages 42.51, 3.48 and 0.64 %; respectively) as compared to those in the northern section
(averages 40.57, 2.78 and 0.27 %; respectively). The two sections have similar contents of Al2O3, Fe2O3 and
TiO2. Shales in the northern section are richer in the rest of major elements than those of the southern section.
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Fig. 3A
Fig. 3B
Fig. 3: (A) X-Ray Diffractograms of the oriented clay fractions of selected shale samples (Sm: smectite, K:
kaolinite, I: illite).
(B) (DTA) Differential Thermal Analysis, (DTG) Differential Thermogravimetry and (TG)
Thermogravimetry curves of the clay fractions of the selected shale samples.
SiO2/Al2O3 ratio for montmorillonite ranges from 2.80 to 3.31 (Felix, 1977). The reported ratios for the
studied shales are higher ranging from 2.31 to 5.05 (average 3.17), this confirms the results of which the
predominance XRD and SEM. The K2O/Al2O3 ratios reported for the studied shales range from 0.001 to 0.243
and average 0.131. These low values are attributed to the presence of illite in small concentrations (Cox et al.,
1995; Moosavirada et al., 2010 and Abou El-Anwar et al., 2014). The Strong positive correlations between
SiO2 and Al2O3, Fe2O3, TiO2 and MgO (r = 0.92, 0.92, 0.89 and 0.67; respectively) and moderate positive
correlation with K2O (r = 0.34) (Table 4) suggest the occurrence of these oxides in common phases, most likely
clay minerals. The moderate positive correlations between Al2O3 and both K2O and Na2O (r = 0.42 and 0.23;
respectively) are explained by the presence of illite in minor concentrations and suggest that the major part of
Na and K occurs in clay minerals (Liu et al., 2013 and El-Wekeil and Abou El-Anwar, 2013). Al2O3 % has a
distinct negative correlation with L.O.I. (r = - 0.96). This is attributed to the abundance of smectite.
CaO follows Al2O3 in abundance elements varying from 3.53 to 40.06 % (average 13.06 %). This average
is generally higher than those reported for (UCC) and (PAAS) (4.2 % and 1.3 %; respectively). Also, MgO
contents are occasionally high (average 3.13 %). This is attributed mostly to the abundance of Ca, Mgmontmorillonite and the occasional presence of dolomite, which confirmed with DTA results. CaO exhibits
very strong positive relationship with L.O.I. and P2O5 (r = 0.98 and 0.82; respectively, Table 4) indicating that a
major part of Ca exists in the form of carbonate and/or phosphate minerals.
The content of Fe2O3 (average 5.79 %) is generally higher than those reported for UCC while lower than
PAAS values (4.5 % and 6.5 %; respectively). its strong positive correlation with SiO2 (r = 0.92) and moderate
correlations with K2O and Na2O (0.31 and 0.20; respectively) suggest its presence mainly associated with clay
minerals. Also, moderately positive correlations of Fe2O3 with MgO, SO3, Th and Zr (r = 0.52, 0.34, 0.35 and
0.28; respectively) are attributed to its incorporation with heavy and trace elements. Also, the moderate positive
correlation between Fe2O3 and SO3 (r = 0.34) is explained by the occasional presence of pyrite which confirms
with SEM. TiO2 values (average 0.54 %) are closely similar to those reported for UCC (0.5 %) while lower than
those of PAAS (1 %). The markedly positive correlations between TiO2 and both Al2O3 and Fe2O3 (r = 0.89 and
0.94; respectively) suggests its enrichment during chemical weathering (cf. Eric 1991; Abou El-Anwar and
Samy, 2013 and El-Wekeil and Abou El-Anwar, 2013). These results from the effect of multiple cycles of
chemical and mechanical weathering of titanium-bearing silicates, commonly occurring in passive margin or
cratonic tectonic setting.
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Southern Section( Π )
Northern Section ( Ι )
Table 1: Chemical composition (%) of major elements of the studied shales.
Section
S. No.
SiO2
Al2O3
TiO2
Fe2O3
MnO
K2O
1
48.50
15.45
0.89
8.53
0.13
0.01
2
44.97
13.74
0.61
5.91
0.08
1.71
3
43.50
12.76
0.60
5.65
0.06
1.93
4
43.92
13.12
0.60
5.83
0.06
2.36
5
45.08
12.71
0.62
5.76
0.05
2.27
6
37.85
11.45
0.45
3.88
0.04
2.44
7
25.22
5.37
0.15
1.40
0.02
0.83
8
42.18
14.30
0.51
5.62
0.03
1.96
9
41.89
14.11
0.44
5.27
0.04
1.93
10
45.31
16.06
0.61
6.01
0.04
2.16
11
42.27
14.12
0.50
5.61
0.04
1.98
12
41.08
14.25
0.55
6.07
0.04
1.88
13
46.52
16.76
0.63
6.45
0.03
2.00
14
45.54
16.68
0.68
6.51
0.03
1.96
15
43.39
14.88
0.54
5.93
0.03
1.96
16
35.14
7.53
0.31
3.02
0.02
1.25
17
45.23
16.59
0.65
6.44
0.03
1.95
18
44.75
16.23
0.74
7.72
0.05
2.49
19
43.72
15.41
0.53
5.85
0.04
1.87
20
47.05
17.02
0.71
6.87
0.03
2.38
21
47.65
16.77
0.86
8.72
0.09
2.39
22
47.65
17.3
0.87
8.90
0.09
2.12
23
27.01
5.35
0.17
1.65
1.89
0.84
24
39.63
13.47
0.50
5.31
2.67
1.91
25
25.20
5.96
0.18
1.71
1.63
0.95
26
34.01
11.00
0.32
3.51
2.50
1.76
27
44.76
15.78
0.56
6.30
3.07
2.06
28
43.01
15.74
0.62
6.28
2.36
2.09
29
45.53
16.22
0.68
6.52
2.70
2.34
30
39.01
13.25
0.67
6.72
2.38
1.94
31
39.21
13.51
0.65
5.75
3.14
2.01
32
40.87
13.44
0.51
6.45
3.06
1.89
33
34.51
14.95
0.73
7.85
2.63
2.37
34
44.37
15.78
0.79
7.42
2.67
2.12
35
14.55
4.11
0.13
1.13
1.11
1.00
Min.
14.55
4.11
0.13
1.13
0.02
0.01
Max.
48.50
17.30
0.89
8.90
3.14
2.49
Aver.
40.57
13.46
0.56
5.67
0.94
1.86
36
39.15
13.36
0.52
5.96
0.04
1.86
37
42.55
15.25
0.57
6.70
0.04
2.04
38
46.02
14.42
0.61
6.39
0.06
0.60
39
43.03
12.58
0.54
5.17
0.06
1.50
40
38.06
10.81
0.51
5.06
0.07
1.56
41
40.16
12.37
0.49
5.02
0.07
1.48
42
48.67
14.08
0.84
7.57
0.06
1.99
43
41.13
11.18
0.48
3.95
0.05
1.43
44
41.19
12.23
0.57
4.89
0.06
1.50
45
47.74
15.28
0.65
6.33
0.05
2.08
46
38.45
14.36
0.57
5.56
0.04
1.92
47
40.55
15.43
0.58
6.84
0.04
1.99
48
46.42
15.82
0.81
6.42
0.06
0.73
49
37.86
10.92
0.53
5.08
0.07
1.43
50
41.16
12.97
0.46
5.04
0.07
1.28
51
47.67
15.08
0.64
6.97
0.06
1.99
52
40.83
12.18
0.68
4.95
0.05
1.23
53
40.29
12.83
0.57
6.89
0.06
1.40
54
46.84
14.28
0.55
7.33
0.05
2.09
Min.
37.86
15.82
0.46
3.95
0.04
0.60
Max.
48.67
15.82
0.84
7.57
0.07
2.09
Aver.
42.51
13.44
0.59
5.90
0.06
1.58
Total Min.
14.55
4.11
0.13
1.13
0.02
0.01
Total Max.
48.67
17.30
0.89
8.90
3.14
2.49
Total Aver.
41.54
13.45
0.54
5.79
0.50
1.72
Note: N.D. = Not Detected
Na2O
0.81
0.43
0.29
0.26
0.33
0.36
0.60
1.10
0.50
0.27
1.25
1.34
0.61
1.09
0.90
0.20
0.89
1.28
0.40
0.62
1.04
1.70
0.88
1.18
0.46
0.63
0.54
0.75
0.85
2.49
3.52
3.72
1.52
1.17
0.82
0.20
3.72
0.99
0.84
0.98
0.60
0.46
0.21
0.33
0.39
0.36
0.53
0.43
0.74
0.78
0.85
0.20
0.43
0.38
0.35
0.43
0.44
0.20
0.98
0.51
0.20
3.72
0.75
MgO
2.95
3.17
4.04
3.53
4.63
3.49
1.75
3.06
2.87
2.57
2.38
3.08
2.77
2.47
2.56
2.19
2.64
2.93
3.26
2.77
3.13
3.20
1.89
2.67
1.63
2.50
3.07
2.36
2.70
2.38
3.14
3.06
2.63
2.67
1.11
1.11
4.63
2.78
2.45
2.92
3.51
3.41
2.87
4.50
3.22
3.25
3.01
4.01
2.85
3.82
3.72
2.95
4.82
3.20
3.65
3.81
4.11
2.45
4.82
3.48
1.11
4.82
3.13
CaO
4.51
11.83
11.71
12.25
10.67
17.04
32.83
11.00
13.03
9.69
13.62
10.73
8.60
8.21
11.77
25.92
8.87
6.84
11.06
7.28
4.38
3.53
30.78
15.16
32.29
21.10
8.86
10.82
8.52
12.90
9.70
11.08
8.62
8.76
40.06
3.53
40.06
13.54
14.16
9.63
9.26
13.23
18.26
12.94
7.62
16.27
14.55
7.01
14.84
9.85
9.45
19.36
12.74
8.62
17.27
15.55
8.21
7.01
19.36
12.57
3.53
40.06
13.06
P2O5
0.13
0.15
0.14
0.14
0.15
0.14
0.53
0.20
0.46
0.18
0.21
0.17
0.29
0.25
0.23
0.64
0.16
0.18
0.27
0.14
0.12
0.12
0.42
0.28
0.45
0.33
0.22
0.7
0.21
0.19
0.19
0.27
0.19
0.19
0.50
0.12
0.70
0.26
0.25
0.17
0.13
0.15
0.15
0.13
0.16
0.15
0.14
0.12
0.23
0.16
0.14
0.15
0.13
0.15
0.14
0.12
0.13
0.12
0.25
0.15
0.12
0.70
0.21
CL
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
0.30
1.60
0.02
0.26
1.50
1.55
0.31
0.32
0.21
0.22
0.21
0.23
0.32
0.23
0.95
0.53
0.32
0.31
0.22
N.D.
1.60
0.27
1.31
1.50
0.45
0.35
0.08
0.02
0.13
0.09
0.50
0.40
1.25
1.31
0.49
0.08
0.02
0.12
0.09
3.50
0.41
0.02
3.50
0.64
N.D
3.50
0.46
SO3
0.32
0.57
1.02
0.25
0.22
0.15
0.15
0.75
0.60
0.95
0.52
1.85
0.63
1.75
1.04
0.18
0.77
0.60
0.47
0.54
1.03
1.25
0.26
0.93
0.26
0.54
1.56
1.65
0.94
1.09
1.98
0.80
2.23
1.03
0.30
0.15
2.23
0.83
0.59
0.33
0.62
0.58
0.49
0.78
0.31
0.22
0.23
0.25
0.61
0.41
0.72
0.47
0.98
0.32
0.22
0.21
0.26
0.21
0.98
0.45
0.15
2.23
0.64
L.O.I.
15.29
16.22
17.62
17.12
17.00
22.19
30.65
17.11
17.94
15.55
16.97
16.34
14.10
14.13
16.17
23.13
15.04
13.85
16.69
13.98
11.76
11.11
30.04
18.31
30.51
23.79
15.66
15.87
14.76
18.73
19.24
17.50
14.66
14.93
35.69
11.11
35.69
18.28
18.42
16.53
15.90
18.33
21.46
21.17
14.40
20.99
20.02
15.25
18.58
18.24
14.37
20.90
19.90
14.80
18.36
14.34
15.30
14.34
21.46
17.75
11.11
35.69
18.28
5354
J. Appl. Sci. Res., 9(8): 5344-5369, 2013
The studied shales contain high concentrations of MgO and P2O3 (averages 3.13 and 0.21 %; respectively)
as compared with the corresponding values reported for the UCC and PAAS (averages 2.2 and 0.16 %;
respectively). Also, their contents of Na2O and K2O (averages 0.75 and1.72 %; respectively, Table 1) are lower
than those given for the UCC (3.9 and 3.4 %; respectively) and PAAS (1.2 and 3.7 %; respectively). The
moderate positive correlations between Na and both SO3 and Cl (0.62 and 0.40; respectively) strongly suggest
that part of Na is present as halite which is confirmable with the results of SEM (Plate I: F). The relative
depletion in these elements is related to their high mobility during weathering processes (Cullers, 1988). The
high concentrations of Mn and P (averages 0.50 and 0.21 %; respectively) as compared with the corresponding
values reported for the UCC (0.08 and 0.16 %; respectively) and PAAS (0.11 and 0.16 %; respectively) indicate
that these elements are progressively concentrated during chemical weathering.
Southern Section ( Π )
Northern Section ( Ι )
Table 2: Elemental ratios of the major elements, CIA and IVC in the studied shales.
Section
S. No.
SiO2/Al2O3
MgO/Al2O3
K2O/Al2O3
Al2O3/TiO2
1
3.14
0.19
0.001
17.36
2
3.27
0.23
0.124
22.52
3
3.41
0.32
0.151
21.27
4
3.35
0.27
0.180
21.87
5
3.55
0.36
0.179
20.50
6
3.31
0.30
0.213
25.44
7
4.70
0.33
0.155
35.80
8
2.95
0.21
0.137
28.04
9
2.97
0.20
0.137
32.07
10
2.82
0.16
0.134
26.33
11
2.99
0.17
0.140
28.24
12
2.88
0.22
0.132
25.91
13
2.78
0.17
0.119
26.60
14
2.73
0.15
0.118
24.53
15
2.92
0.17
0.132
27.56
16
4.67
0.29
0.166
24.29
17
2.73
0.16
0.118
25.52
18
2.76
0.18
0.153
21.93
19
2.84
0.21
0.121
29.08
20
2.76
0.16
0.140
23.97
21
2.84
0.19
0.143
19.50
22
2.75
0.18
0.123
19.89
23
5.05
0.35
0.157
31.47
24
2.94
0.20
0.142
26.94
25
4.23
0.27
0.159
33.11
26
3.09
0.23
0.160
34.38
27
2.84
0.19
0.131
28.18
28
2.73
0.15
0.133
25.39
29
2.81
0.17
0.144
23.85
30
2.94
0.18
0.146
19.78
31
2.90
0.23
0.149
20.78
32
3.04
0.23
0.141
26.35
33
2.31
0.18
0.159
20.48
34
2.81
0.17
0.134
19.97
35
3.54
0.27
0.243
31.62
Min.
2.31
0.15
0.001
17.36
Max.
5.05
0.36
0.243
35.80
Aver.
3.15
0.22
0.143
25.44
36
2.93
0.18
0.139
25.69
37
2.79
0.19
0.134
26.75
38
3.19
0.24
0.042
23.64
39
3.42
0.27
0.119
23.30
40
3.52
0.27
0.144
21.20
41
3.25
0.36
0.120
25.24
42
3.46
0.23
0.141
16.76
43
3.68
0.29
0.128
23.29
44
3.37
0.25
0.123
21.46
45
3.12
0.26
0.136
23.51
46
2.68
0.20
0.134
25.19
47
2.63
0.25
0.129
26.60
48
2.93
0.24
0.046
19.53
49
3.47
0.27
0.131
20.60
50
3.17
0.37
0.099
28.20
51
3.16
0.21
0.132
23.56
52
3.35
0.30
0.101
17.91
53
3.14
0.30
0.109
22.51
54
3.28
0.29
0.146
25.96
Min.
2.63
0.18
0.042
16.76
Max.
3.68
0.37
0.146
28.20
Aver.
3.19
0.26
0.119
23.21
Total Min.
2.31
0.15
0.001
16.76
Total Max.
5.05
0.37
0.243
35.80
Total Aver.
3.17
0.24
0.131
24.33
K2O/Na2O
0.012
3.977
6.655
9.077
6.879
6.778
1.383
1.782
3.860
8.000
1.584
1.403
3.279
1.798
2.178
6.250
2.191
1.945
4.675
3.839
2.298
1.247
0.955
1.619
2.065
2.794
3.815
2.787
2.753
0.779
0.571
0.508
1.559
1.812
1.220
0.012
9.077
2.980
2.214
2.082
1.000
3.261
7.429
4.485
5.103
3.972
2.830
4.837
2.595
2.551
0.859
7.150
2.977
5.237
3.514
3.256
4.750
0.859
7.429
3.690
0.012
9.077
3.340
MgO/Al2O3
0.19
0.23
0.32
0.27
0.36
0.30
0.33
0.21
0.20
0.16
0.17
0.22
0.17
0.15
0.17
0.29
0.16
0.18
0.21
0.16
0.19
0.18
0.35
0.20
0.27
0.23
0.19
0.15
0.17
0.18
0.23
0.23
0.18
0.17
0.27
0.15
0.36
0.22
0.18
0.19
0.24
0.27
0.27
0.36
0.23
0.29
0.25
0.26
0.20
0.25
0.24
0.27
0.37
0.21
0.30
0.30
0.29
0.18
0.37
0.26
0.15
0.37
0.20
K2O/Al2O3
0.001
0.120
0.150
0.180
0.180
0.210
0.150
0.140
0.140
0.130
0.140
0.130
0.120
0.120
0.130
0.170
0.120
0.150
0.120
0.140
0.140
0.120
0.160
0.140
0.160
0.160
0.130
0.130
0.140
0.150
0.150
0.140
0.160
0.130
0.240
0.001
0.240
0.140
0.140
0.130
0.040
0.120
0.140
0.120
0.140
0.130
0.120
0.140
0.130
0.130
0.050
0.130
0.100
0.130
0.100
0.110
0.150
0.040
0.150
0.120
0.001
0.240
0.060
Ti/Al
0.07
0.05
0.05
0.05
0.06
0.04
0.03
0.04
0.04
0.04
0.04
0.04
0.04
0.05
0.04
0.05
0.04
0.05
0.04
0.05
0.06
0.06
0.04
0.04
0.03
0.03
0.04
0.04
0.05
0.06
0.05
0.04
0.06
0.06
0.04
0.03
0.07
0.05
0.04
0.04
0.05
0.05
0.05
0.04
0.07
0.05
0.05
0.05
0.04
0.04
0.06
0.05
0.04
0.05
0.06
0.05
0.04
0.04
0.07
0.05
0.03
0.07
0.04
IVC
1.10
1.68
1.86
1.85
1.87
2.38
6.97
1.59
1.68
1.29
1.76
1.62
1.22
1.22
1.56
4.33
1.25
1.31
1.46
1.17
1.18
1.13
7.09
2.15
6.49
2.91
1.51
1.57
1.46
2.17
2.02
2.18
1.71
1.57
11.00
1.10
11.00
2.44
1.89
1.46
1.42
1.89
2.59
1.97
1.48
2.26
2.01
1.30
1.81
1.51
1.34
2.66
1.88
1.41
2.26
2.19
1.56
1.30
2.66
1.84
1.10
11.00
2.14
CIA
93.47
84.92
83.62
81.90
81.42
78.80
68.32
80.52
80.81
85.20
79.46
80.01
84.01
82.45
81.76
73.39
84.00
79.72
84.58
83.84
82.05
80.99
67.64
78.68
70.07
78.29
83.85
78.78
81.80
73.37
69.57
68.61
77.78
81.13
61.81
61.81
93.47
79.05
80.68
81.95
90.81
84.77
83.93
85.66
83.91
84.25
84.11
84.75
82.15
83.32
89.48
84.98
86.81
84.96
86.75
86.11
83.66
80.68
90.81
84.90
61.81
93.47
81.94
(ii) Trace elements:
The shales of the southern section are significantly more enriched in Zr and Th, to a much lesser extent
(averages 483 and 15 ppm; respectively) as compared with those in the northern section (averages 54 and 7
5355
J. Appl. Sci. Res., 9(8): 5344-5369, 2013
ppm; respectively). This may be attributed to the high content of zircon in the southern section. This indicates
that intensive chemical weathering from those shales was inherited rocks rich in felsic minerals through.
On the other hand, shales of the northern section are have higher contents of Sr, V, Cr, Zn, Ni, Cu, Co and
Rb (averages 401, 158, 112, 100, 80, 59, 26 and 26 ppm; respectively) than those of the southern section
(averages 393, 24, 12, 35, 40, 23, 11 and 15 ppm; respectively). Generally, the concentrations of Sr, V, Cr, Zn,
Ni, Cu and Co trace elements in the northern section are higher than those reported for UCC (336, 60, 35, 71,
20, 25 and 10 ppm; respectively) and PAAS (200, 150, 110, 85, 20, 50, and 23 ppm; respectively). Strong
positive correlation is display between V and other trace elements Cr, Cu, Co, Ni, Zn and Rb (r = 0.93, 0.77,
0.72, 0.68, 0.61 and 0.55; respectively). The enrichment in these elements and their positive correlations may be
related to inheritance from source rocks rich in mafic minerals.
Southern Section( Π )
Northern Section( Ι )
Table 3: Concentrations (ppm) of the trace elements and their elemental ratios for some selected shale samples.
Section
S. No.
Co
Cr
Cu
Ni
V
Zn
Sr
Th
Rb
Zr
V/(V+Ni)
V/Cr
1
30 100
50
70 140 102 320
8
30
40
0.67
1.4
3
19 145
56
55 157
90 420
12
45
23
0.74
1.08
5
32 160
40
85 210
85 430
7
25
35
0.71
1.31
8
19 133
65
75 254
54 390
5
22
60
0.77
1.91
9
22 101
52
80 120
80 620
4
17
20
0.6
1.19
11
18
99
55
88 145
99 510
9
19 100
0.62
1.46
13
29 145
44
88 124 105 400
3
18
43
0.58
0.86
15
33 135
74
95 180 110 360
4
20
50
0.65
1.33
18
25
85
46
98 150
95 390
7
40
65
0.6
1.76
20
38
50
48 100
99 188 400
9
29 120
0.5
1.98
22
22 137
85
87 220 160 210
6
17
40
0.72
1.61
28
17
84
77
65 110
99 710
4
12
80
0.63
1.31
30
37 150
58
53 234
97 310
10
38
45
0.82
1.56
32
23
97
63
84 128
45 290
8
24
55
0.6
1.32
34
25
56
74
76
95
98 250
7
27
33
0.56
1.7
Aver.
26 112
59
80 158 100 401
7
26
54
0.65
1.45
36
11
12
30
55
90
25 300
15
14 340
0.62
7.5
38
15
15
15
53
28
24 340
17
13 620
0.35
1.87
39
10
12
45
48
18
30 690
13
14 410
0.27
1.5
41
14
9
33
62
17
54 310
12
15 510
0.22
1.89
43
9
14
26
25
25
65 370
9
19 680
0.5
1.79
45
7
10
29
30
19
48 345
7
22 390
0.39
1.9
47
9
12
11
22
10
34 400
19
15 650
0.31
0.83
48
12
10
14
45
14
39 510
20
12 400
0.24
1.4
50
14
13
12
61
15
25 480
13
9 210
0.2
1.15
52
16
14
22
35
20
33 240
16
17 540
0.36
1.43
53
8
13
16
33
15
22 420
18
14 710
0.31
1.15
54
10
15
18
15
12
24 310
20
15 340
0.44
0.8
Aver.
11
12
23
40
24
35 393
15
15 483
0.35
1.93
Total Min.
7
9
11
15
10
22 210
3
9
20
0.2
0.8
Total Max.
38 160
85 100 254 188 710
20
45 710
0.82
7.5
Total Aver.
19
61
41
60
91
68 397
11
20 269
0.5
1.71
SiO2
TiO2
Al2O3
Fe2O3
MnO
K2O
Na2O
MgO
CaO
P2O5
CL
SO3
L.O.I
Co
Cr
Cu
Ni
V
Zn
Sr
Th
Rb
Zr
Table 4: Correlation coefficients of the major and trace elements in the studied shales.
SiO2 TiO2 Al2O3 Fe2O3
MnO
K2 O
Na2O
MgO
CaO
P2O5
1.00
0.89 1.00
0.92 0.89
1.00
0.92 0.94
0.93
1.00
-0.25 -0.17
-0.13
-0.19
1.00
0.34 0.23
0.42
0.31
0.09
1.00
-0.02 0.16
0.23
0.20
0.37
0.00
1.00
0.67 0.50
0.53
0.52
-0.38
0.23
-0.29
1.00
-0.97 -0.90
-0.97
-0.96
0.20
-0.38
-0.14
-0.64
1.00
-0.77 -0.76
-0.78
-0.78
0.25
-0.31
0.02
-0.74
0.82
1.00
0.13 0.22
0.30
0.31
-0.01
0.23
0.40
0.01
-0.23
-0.16
0.27 0.30
0.48
0.34
0.10
0.24
0.62
-0.01
-0.38
-0.22
-0.98 -0.91
-0.96
-0.95
0.22
-0.40
-0.12
-0.60
0.98
0.76
-0.02 0.06
0.03
0.07
0.29
0.12
0.46
-0.28
-0.04
0.23
-0.15 -0.19
-0.14
-0.22
0.36
0.14
0.26
-0.25
0.13
0.26
-0.46 -0.38
-0.34
-0.40
0.57
0.10
0.35
-0.67
0.40
0.49
-0.27 -0.21
-0.24
-0.22
0.17
0.05
0.31
-0.44
0.23
0.44
-0.17 -0.20
-0.15
-0.23
0.38
0.20
0.25
-0.31
0.15
0.30
-0.20 -0.13
-0.15
-0.13
0.26
0.05
0.40
-0.45
0.16
0.36
-0.13 -0.19
-0.14
-0.24
-0.13
0.00
-0.41
0.05
0.17
0.02
0.37 0.33
0.26
0.35
-0.28
-0.14
-0.37
0.45
-0.29
-0.48
0.03 0.09
0.01
0.05
0.16
0.10
0.08
-0.26
-0.01
0.26
0.35 0.34
0.27
0.28
-0.37
-0.10
-0.31
0.44
-0.28
-0.44
Ni/Co
2.33
2.89
2.66
3.95
3.64
4.89
3.03
2.88
3.92
2.63
3.95
3.82
1.43
3.65
3.04
3.25
5
3.53
4.8
4.43
2.78
4.29
2.44
3.75
4.36
2.19
4.13
1.5
3.6
1.43
5
3.42
CL
SO3
L.O.I
Co
1.00
0.02
-0.24
-0.10
-0.38
-0.18
-0.17
-0.22
0.07
-0.21
0.15
-0.04
0.21
1.00
-0.40
0.40
0.22
0.15
0.33
0.12
0.27
-0.02
-0.25
0.00
-0.26
1.00
-0.03
0.15
0.43
0.23
0.17
0.16
0.15
-0.33
-0.06
-0.32
1.00
0.72
0.58
0.75
0.72
0.76
-0.26
-0.58
0.57
-0.74
Cr/Th
12.5
12.08
22.86
26.6
25.25
11
48.33
33.75
12.14
5.56
22.83
21
15
12.13
8
19.27
0.8
0.88
0.92
0.75
1.56
1.43
0.63
0.5
1
0.88
0.72
0.75
0.9
0.5
48.33
9.77
Cr
Th/Co
0.267
0.632
0.219
0.263
0.182
0.5
0.103
0.121
0.28
0.237
0.273
0.235
0.27
0.348
0.28
0.281
1.364
1.133
1.3
0.857
1
1
2.111
1.667
0.929
1
2.25
2
1.384
0.103
2.25
0.833
Cu
Ni
1.00
0.71 1.00
0.64 0.68 1.00
0.93 0.77 0.68
0.61 0.69 0.70
-0.10 -0.08 -0.07
-0.68 -0.78 -0.72
0.57 0.39 0.32
-0.82 -0.78 -0.77
Cu/Zn
0.49
0.62
0.47
1.20
0.65
0.56
0.42
0.67
0.48
0.26
0.53
0.78
0.6
1.4
0.76
0.66
1.2
0.63
1.5
0.61
0.4
0.6
0.32
0.36
0.48
0.67
0.73
0.75
0.69
0.26
1.5
0.67
V
Rb/Sr
0.09
0.11
0.06
0.06
0.03
0.04
0.05
0.06
0.10
0.07
0.08
0.02
0.12
0.08
0.11
0.07
0.05
0.04
0.02
0.05
0.05
0.06
0.04
0.02
0.02
0.07
0.03
0.05
0.04
0.02
0.12
0.05
Zn
TiO2/Zr
222.5
152.5
269.57
128.57
25
305
55
158.14
62
132.31
14.17
80
85
175.56
23.64
119.03
105.69
17.94
8.23
20.49
12.75
8.38
20.77
8.15
11.5
30.48
12.59
7.75
22.06
7.75
305
70.54
Sr
Th
Rb
Zr
1.00
0.61 1.00
-0.22 -0.15 1.00
-0.67 -0.65 -0.05 1.00
0.55 0.47 -0.29 -0.32 1.00
-0.80 -0.65 0.05 0.72 -0.51
1.00
(iii) Normalized pattern for Post-Archaean Australian Shale (PAAS):
The concentrations of the major and trace elements in the studied samples have been normalized to the
average contents in the Post-Archaean Australian Shale (PAAS) reported by Taylor and McLennan (1985) (Fig.
4). The PAAS normalized pattern for the studied Esna shales could be classified into four classes. The first class
comprises elements which are more enriched in the northern section relative to the PAAS (>1). These include
the major elements Mn and P; and trace elements Co, Cr, Cu, Ni, V, Zn and Rb. The enrichment in these trace
elements may be related to inheritances from mafic source rocks; most probably andesitic-basaltic volcanic
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J. Appl. Sci. Res., 9(8): 5344-5369, 2013
rocks. This agrees with the findings of El-Barkooky (1986), El- Kelani and El-Bakry (2000), Liu et al., (2009)
and El-Wekeil and Abou El-Anwar (2013). The second class comprises Ca and Sr the concentrations of which
are mainly similar in the northern and southern sections and higher when compared with those reported for the
PAAS. This enrichment may be related to partial derivation from nearby carbonate rocks. The third class
comprises the major elements Ti, Fe, K and Na the concentrations of which in the two sections are relatively
lower than those of the PAAS. The leaching of these mobile elements Ti, Fe, K and Na indicated that the high
mobility during weathering processes (Cullers, 1988). The fourth class includes Zr which is relatively enriched
in shales of the southern section. This may be explained by the presence of more felsic constitutes, (cf. Abou ElAnwar and Samy 2013).
Fig. 4: Concentrations of the major and trace elements in the studied shales normalized to the averages given by
Taylor and McLennan (1985) for PAAS.
Discussion o f geochemistry:
(A) Sedimentary History:
The differences in the clay mineral distributions can be related to weathering, sedimentation, burial,
diagenesis, hydrothermal alterations, and tectonic processes (Velde and Meunier, 2008 and Song et al., 2012).
(1) Provenance:
Chemical compositions of the sediments are a powerful indicator for determinate the provenance and
tectonic setting of sedimentary basins. Immobile elements and REEs, are favored in the provenance analyses
because they are resistant to geochemical fractionation during weathering, erosion and post-depositional
processes, and may faithfully reflect the feature of sources.
(i) Source Rocks:
The major elements concentrations show no marked variations between the two sections. On the contrary,
trace element contents vary remarkably reflecting difference in the nature of source rocks. The concentration of
zirconium is used for characterizing the nature and composition of source rocks (Hayashi et al., 1997). Felsic
rocks have higher Zr concentrations as compared to those of the mafic rocks. The zirconium content varies from
210 to 710 ppm (average 483 ppm) in the southern section shales, while it ranges from 20 to 120 ppm (average
54 ppm) in the northern section shales. This suggests a contribution from felsic source rock for the shales of the
southern section and from more mafic rocks for those of the northern section. The TiO2/Zr ratio generally
decreases with increasing SiO2 content. The rocks are > 200 for mafic igneous rocks, 195-55 for inter-mediate
igneous rocks and to < 55 for felsic rocks (Hayashi et al., 1997).
Table (3) shows that the TiO2/Zr ratio is 22 for the southern section shales, suggesting a felsic source. The
ratios for the northern section are up to 269.57 (average 119) matches with inter-mediate to mafic igneous
rocks. Also, the TiO2 versus Zr plot of the marine shales (Fig. 5) represents predominantly felsic source rocks
for the southern section shales and predominantly more intermediate to mafic rocks for those of the northern
section (cf. Hayashi et al., 1997).
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J. Appl. Sci. Res., 9(8): 5344-5369, 2013
The markedly higher contents of Zr (average of 483 ppm) in the southern section shales as compared to
those in the northern section shales (average of 54 ppm) can be attributed to the occurrence of markedly higher
concentrations of zircon which indicates high detrital input in the depositional basin close to the source area
and/or more intensive chemical weathering during the formation of these shales. The enrichment in Zr (average
483 ppm) and Th, to a lesser extent (average 15 ppm) is attributed most probably to derivation from Gebel
Oweinat rocks. The positive correlations between Zr and Th, Ti, Fe and Mg (r= 0.72, 0.34, 0.28 and 0.44;
respectively) may be the result of substitutions of these elements for Zr in Zr-bearing phases (mainly zircon).
Fig. 5: Plot of TiO2 versus Zr for the studied Esna shales.
(ii) Climatic conditions and sediment maturity:
The major element composition of the studied shale samples are controlled mainly by their contents of clay
minerals rather than the non-clay silicate phases. The original characters and maturity of the sediments together
with the prevailed climatic conditions can be recognized by calculating ICV (Index of Compositional Variation)
proposed by Cox et al., (1995):
ICV = (Fe2O3 +K2O+Na2O+ CaO +MgO +MnO)/Al2O3.
The more mature shales exhibit low ICV values (<1.0). For the studied Esna shales, the calculated ICV
values range from1.1 to 11 (average 2.14), Table (2). Consequently, these shales are considered to be relatively
immature.
The intensity of chemical weathering is affected by the tectonic activity and climatic conditions in the
source area. The degree of chemical weathering was increased by decreasing the tectonic activity and/or change
in climate towards warm and humid conditions (Jacobson, et al., 2003). The relationship between SiO2 and
(Al2O3 +K2O+Na2O) was used to recognize the climatic conditions (Suttner and Dutta, 1986). Figure (6) reveals
that the studied shale samples are relatively immature and plot in the semiarid climate field. This conforms well
the ICV values.
Clay minerals in sediments can provide important constraints on continental weathering under different
climatic conditions (Singer, 1980 & 1984; Chamley, 1989; Thiry, 2000 and Wang and Yang, 2013). Smectite
may be formed under different chemical and climatic conditions. Illite is interpreted to form in relatively very
cold or hot-dry climate, while a hot and humid climate leads to a stronger chemical weathering favoring
formation of kaolinite. Moreover, the variety of clay minerals produced by hydrothermal alteration and tectonic
processes (faulting) can be significant, but is confined to very local areas; whereas, weathering could cause a
widespread and high-degree variation in clay minerals (Chamley, 1989 and Song et al., 2012). Consequently, it
can be concluded that the abundances of smectite and, to a lesser extent kaolinite in shales of the two studied
sections are related to warmer with different chemical environments.
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J. Appl. Sci. Res., 9(8): 5344-5369, 2013
Fig. 6: Plots of the studied shales on the diagram proposed by Suttner and Dutta (1986)
(iii) Tectonic setting:
Provenance of the mud rocks can be evaluated using the major element discrimination scheme of
Bhatia (1983) and Roser and Korsch (1988). Plotting the studied shale samples on the discrimination diagram
of tectonic setting proposed by Bhatia (1983) revealed that most of them fall into the passive margin and
oceanic island arc fields (Fig. 7). The oceanic island arc can be adequately discerned from the passive margin
setting by using the range of major and trace element composition normalized to the upper continental crust
values (Floyd et al., 1989). The chemical compositions of some shale samples show enrichment in Ca, K and Sr
and, consequently, lie in the field of oceanic island arc. Plotting of the studied samples on the binary diagram
proposed by Roser and Korsch (1986) shows that all of them fall into the passive margin provenance (Fig. 8).
This makes it most probable that the studied shales were deposited chiefly in a passive margin setting. The
passive margin comprises rifted continental margins and is developed along the edges of the continents and
remnant ocean basins adjacent to collision orogens (Bhatia, 1983). Sedimentary basins adjacent to the oceanic
island arc or island arcs partly formed on thin continental crust called arcs (Dickinson and Seely, 1979). In this
tectonic setting, a volcanic arc separates the fore-arc from an oceanic back-arc basin.
Fig. 7: Plots of F1 vs. F2 discriminate on the diagram proposed by Bhatia (1983)
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J. Appl. Sci. Res., 9(8): 5344-5369, 2013
Fig. 8: Plots of F1 vs. F2 discriminate on the diagram proposed by Roser and Korsch (1986)
In the provenance discrimination diagram of Roser and Korsch (1986) the formulated discriminant
functions (bivariates F1 and F2) are based on the concentrations of both immobile and variably mobile major
elements. Applying this diagram (Fig. 9) revealed that all the northern section samples lie in the intermediate
igneous field, only two samples fall into the mafic igneous and one sample in the quartzose sedimentary fields.
Most of the southern section samples lie in the felsic igneous field while three samples fall into the quartzose
sedimentary field. The Th/Co verses Zr/Co plots can be used to discriminate between the felsic and mafic
source rocks (McLennan et al., 1980; Bhatia and Crook, 1986 and Borges et al., 2008). Figure (10) shows the
predominantly mafic characters of the source rocks for the northern section shales being most related to the
PAAS and the more felsic nature of the source rocks of the southern section shales being mainly related to the
UCC.
The K2O/Al2O3 ratios of sediments can be used as a marker of the original composition of earlier sediments
(Cox et al., 1995). In shales, this ratio indicates the relative predominance of alkali feldspars versus plagioclases
and clays. The K2O/Al2O3 ratios for clay minerals and feldspars are different (0.0 - 0.3 and 0.3 -0.9;
respectively). In the alkali feldspars, the ratios range from 0.3 to 0.9, illite approximately 0.3 and other clay
minerals nearly zero. K2O/Al2O3 ratio > 0.5, suggests the relative dominance of alkali feldspars in the original
shales. In contrast, ratios < 0.3 suggest minimal alkali feldspar contents. The calculated K 2O/Al2O3 ratios for
the studied shales in the northern and southern sections average 0.14 and 0.12; respectively (Table 2), which
suggests the rarity of K-feldspars and predominance of clay minerals in the source rocks.
Fig. 9: Plots of F1 vs. F2 discriminate on the diagram proposed by Roser and Korsch (1986)
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J. Appl. Sci. Res., 9(8): 5344-5369, 2013
Fig. 10: Plots of Zr/Co vs. Th/Co on the diagram proposed by McLennan et al., (1993)
Ti and Al elements are generally considered to be highly immobile during weathering and diagenesis
(Young and Nesbitt, 1988) and hence, these elements can be used as an indicator of provenance. Ti/ Al ratios
are higher in basic rocks than in acidic rocks. The Ti/Al ratios in shales of the southern and northern sections
average 0.05 and 0.04; respectively (Table 2), which is matches the PAAS (average 0.052). On the other hand,
applying the SiO2/ Al2O3 versus K2O/Na2O bivariate diagram proposed by Wronkiewicz and Condie (1987)
revealed that the shale samples plot in the field of Phanerozoic-Proterozoic shales (Fig. 11). Also, the plot
indicates that the studied shales are more related to the PAAS, which conforms with the finding using Ti/ Al
ratio.
Fig. 11: Plots of SiO2/Al2O3 versus K2O/Na2O on the diagram proposed by Wronkiewicz and Condie (1987)
The values of Al/Ti ratio of igneous rocks gradually increase with increasing SiO2 contents (Holland,
1984). These values increase from 3 - 8 in mafic igneous rocks, 8 - 21 in intermediate igneous rocks and 21 - 70
in felsic igneous rocks.
The Al/Ti ratios for the studied shales range from 16.67 to 35.8 (Table 2). These values indicate that the
studied samples are related to intermediate igneous and felsic rocks which is conformable with the results
obtained from using by the discriminants and diagram of Roser and Korsch (1986), (Fig. 8).
The concentrations of Cr and Ni in shales indicates the absorption of these ions by the clay particles during
weathering of ultramafic rocks containing Cr and Ni minerals (Garver et al., 1996). Consequently, the high Cr
and Ni concentrations and their positive correlation can be used as a marker of mafic sources for the sediments.
In the studied northern section, the positive correlation between Cr and Ni (0.64, Table 4) and their relatively
high concentrations enrichment (averages 112 and 80 ppm; respectively) suggest derivation from mafic sources.
The relatively high concentrations of V, Cr, Ni, Co, Cr and Zn in these shales can be attributed to their
abundance in the mafic source rocks and which were subjected to intensive chemical weathering (cf. Liu et al.,
2013; Abou El- Anwar and Samy, 2013; El-Wekeil and Abou El-Anwar, 2013 and Abou El-Anwar et al.,
2014). In shales of the southern section, the high concentrations of the trace elements Zr and Th (averages 483
and 15 ppm; respectively) suggest derivation from felsic sources. The Precambrian crystalline basement at
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J. Appl. Sci. Res., 9(8): 5344-5369, 2013
Gebel Oweinat is composed mainly of an association of metasediments, metavolcanics, metagabbros and
serpentinites which are intruded by some younger volcanic activities of granites, granodiorites and syenites
(Tawadros and Ezzat, 2001). So, it is likely that the studied shales were derived from rocks of Gebel Oweinat.
(2) Chemical mobility and weathering trends:
The chemical composition of bed rocks are likely to alter during the weathering processes in basins, mainly
due to loss of mobile elements (Das and Krishnaswami, 2007) and size sorting (Bouchez et al., 2011 and
Tripathy et al., 2013). An assessment of the extent of weathering loss undergone by detrital components of
sediments can be made by comparing the CIA (Chemical Index of Alteration) values of sediment sections with
those of source materials. CIA provides information on the intensity of chemical weathering the sediments have
undergone (Nesbitt and Young, 1982) and is calculated as:
CIA = [Al2O3/( Al2O3 +‫‏‬CaO* + ‫‏‬Na2O ‫‏‬+ K2O)] x 100
where: CaO* represents CaO associated with the silicate fraction of the sample.
The calculated CIA values for the studied shales indicate that those of the southern section (average 93)
were affected by a more intensive chemical weathering than the northern one (average 79) Table (2). The higher
zircon content of these sediments is also compatible with their higher values of CIA index. The marked
abundance of smectite in the studied shales is related mainly to the effect of intensive chemical weathering
especially for shales in the southern section.
Geochemical classification and maturity of the studied Esna shales and determining their textural and
geochemical maturity greatly help identifying their origin and the influence of weathering. The
combination of the Index of Compositional Variability (ICV) of Cox et al., (1995) and (CIA) the
Chemical Index of Alteration (Nesbitt and Young, 1982, 1984) can also be used to evaluate the sediment
maturity and weathering intensity. ICV values of >0.84 are typical of major rock-forming minerals such
as feldspars, amphiboles and pyroxenes, whereas values <0.84 are typical of alteration products such as
kaolinite, illite and muscovite. Table (2) shows that shales of the northern section have ICV values ranging
from 1.1 to 11 (average 2.44) and CIA values varying from 62 to 93 (average 79 %). On the other hand,
those of the studied southern section have ICV values ranging from 1.30 to 2.66 (average 1.84) and CIA values
varying from 81 to 91 % (average 85 %). The ICV values for the both sections are thus greater than those
given by (Taylor and McLennan, 1985) for PAAS (Post-Archaean Australian Shale), (ICV= 0.85), (Fig. 12).
Generally, This implies that the studied shales are relatively geochemically immature, and were derived
from high intensive weathered source and more related to PAAS in source bed rocks as shown in Figure (12).
Consequently, the above discussion suggests a common immature source for shales in both sections and those
of the southern section were subjected to more intensive weathering.
The Ti/Al ratios can be used as a useful marker of provenance of clay sediments as both elements are
highly immobile during weathering and diagenesis (Young and Nesbitt, 1988). The Ti/ Al ratio is higher in
basic rocks than in acidic rocks. Table (2) shows that the average values of Ti/Al ratio for shales in the northern
and southern sections (0.05 and 0.04; respectively), are similar to that of PAAS (0.052) while lower than those
given for the UCC, NASC and basalt, which is conformable with the results shown in figure (12).
Fig. 12: CIA verses IVC for the studied Esna shales
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The elemental concentrations in sediments result from the competing influences of provenance, weathering,
sorting and diagenesis (Quinby-Hunt et al., 1991). The studied shales are generally enriched in immobile
elements (Si and Al). These elements can continue to exist throughout intensive chemical weathering and
diagenesis (Cullers, 2000). Their concentrations in sediments are used as a measure for the decrease of detrital
input. SiO2, Al2O3 and TiO2 tend to form together the main constituents of the studied shales and are normally
related to clays of terrigenous origin. Amajor (1987) used the TiO2 versus Al2O3 binary plot to distinguish
between granitic and basaltic source rocks. Applying shows this plot for the studied shales (Fig. 13) reveals that
they were inherited mainly from predominately basaltic granitic to granitic basaltic rocks. This disagrees with
the findings of El-Wekeil and Abou El-Anwar (2013). Also, the composition of the studied Esna shales is more
interrelated to the PAAS for those in the northern section and to the UCC for the southern section shales.
Fig. 13: Plot of TiO2 versus Al2O3 on the diagram adopted by Amajor (1987)
(3) Depositional environments:
The geochemical ratios; Ni/Co, V/Cr, and V/(V + Ni) can be used as a redox index to indicate variable
paleoredox conditions (Jones and Manning, 1994). Low values (<5) of Ni/Co suggest oxic environments,
whereas higher values are indicative of suboxic and anoxic environments (Nath et al., 1987). Values of V/Cr >
2 correspond to anoxic depositional conditions, while lower values are indicative of more oxidizing conditions
(Jones and Manning, 1994). Lewan (1984) demonstrated that V/ (V + Ni) for organic matter that accumulated
under euxinic conditions should be greater than 0.5. Hatch and Leventhal (1992) compared V/(V + Ni) ratios
with other geochemical redox indicators (including degree of pyritization) and suggested ratios > 0.84 for
euxinic conditions, 0.54-0.82 for anoxic waters, and 0.46-0.60 for dyoxic conditions. Table (3) shows that the
V/ (V + Ni) values recorded (0.82) indicate that they were deposited under euxinic conditions which favored
partial pyritization which is conformed with SEM results.
Shales of the northern section have ratios of trace metals V/Cr, Ni/Co and V/ (V + Ni) averaging 1.45, 3.25
and 0.65; respectively. This reflects the prevalence of oxic to anoxic depositional conditions (Table3). On the
other hand, the ratios recorded for the southern sections shales average 1.93, 3.60 and 0.35; respectively
indicating the predominance of oxic to dyoxic depositional conditions.
The concentrations of the high redox sensitive metals (V, Zn, Cr, Co, Ni and Sr) in the studied Esna shales
are similar to those reported for the modern black mud directly associated with marine hydrothermal vents,
resulting from their mineralization by hydrothermal solutions. This is in agreement with the findings of Wei et
al., (1995);Fleurance et al., (2013) and Abou El-Anwar et al., (2014).
The relation between K2O/Al2O3 and MgO/Al2O3 was used by Roaldset (1978) to differentiate between
marine and non-marine clay. Applying this relation (Fig. 14) revealed that the studied shale samples lie in the
marine water field. This conforms with the results obtained by using the redox sensitive metals V, Zn, Cr, Co,
Ni and Sr concentrations.
In conclusion, the high concentration of V, Zn, Cr, Co, Ni and Sr and the ratios of trace metals Ni/Co,
V/Cr, V/(V + Ni) and K2O/Al2O3 and MgO/Al2O3 plotting reflect deposition under oxic to anoxic or dyoxic
marine conditions coupled with the effect of hydrothermal solutions .
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Fig. 14: Plots of the studied shales on the diagram proposed by Roaldset (1978)
Genesis of clays:
The data obtained from the petrographic, mineralogic and geochemical studies indicate that the clay
minerals which constitute the Esna Shale at the El-Quss Abu Said depression are almost entirely of allogenic
(detrital) origin. Smectite was derived most probably from basic and ultrabasic volcanic rocks in source areas
by chemical weathering of mica, feldspars and Fe, Mg- minerals under hot, predominantly arid to semiarid
conditions and restricted drainage (Weaver, 1989 and Abu- Zeid and Dabous, 2005). Also, smectites may form
in the lower parts of the weathering profiles developed on basic rocks under humid condition (Weaver, 1989).
Kaolinite originated in the source area through intense chemical weathering of feldspars and micas in acidic
igneous and metamorphic rocks or from their detrital weathering products under tropical to subtropical humid
climatic conditions with abundant rainfall and relatively high degree of leaching (Hendriks, 1985; Marzouk,
1985; Chamley, 1989; El-Wekeil, 1993 and Ghandour et al., 2004). Illite is typically formed in areas of high
relief where physical erosion is predominant in cold and / or dry climate (Weaver, 1989). Also, it can be formed
by chemical weathering in soils rich in K- feldspars and mica under the same climatic conditions (Pettijohn,
1975 and Weaver, 1989). Illite is the highest stable clay mineral being resistant to weathering in soils under
severe conditions (Lee, 2002; Akarish, 2011 and El-Wekeil and Abou El-Anwar, 2013).
Applying the diagram proposed by Suttner and Dutta(1986) revealed that, the studied shales plot in the
semiarid field (Fig.6). This climatic zone is considered to be not suitable for the formation of kaolinite. ElWekeil and Abou El-Anwar (2013) explained the formation of kaolinite under semiarid conditions based on the
hypothesis of Klitzsch( 1990); Heckel ( 2008) and Sahney et al.,( 2010). They emphasized that during the Late
Carboniferous and in connection with the Hercynian structural event, large parts of central and southern Egypt
were uplifted and the sea retreated which led to changes in the environments of a large part of Egypt. This was
accompanied by a change in climate from hot to humid during the early period of Late Carboniferous to cool
and arid in the later period. Also, the studied shales of the southern section plot in the felsic field (Figs.5 and 9).
Wilson and Pittman ( 1977); Eswaran ( 1979); Glasmann (1982); Colman (1982); Eggleton et al., (1987) and
Ruff and Christensen ( 2007) stated that chemical weathering of basaltic andesite or some form of altered basalt
(plagioclase- and clinopyroxene- rich basalt) can be rich in dioctahedral smectite and / or especially similar
amorphous silica phases. Such processes are controlled by fluid movement through pores which must be
extremely slow (Borchardt, 1977), as well as microenvironmental conditions and mineral transformations
during near –surface rock weathering appear to be largely determined by soil microenvironmental factors with
large – scale climatic factors acting as modifiers (Eswaran and DeConinck, 1971).
In conclusion, it is believed that the origin of the detrital clay minerals in the studied sediments may be
attributed to the extensive weathering of the crystalline igneous and metamorphic rocks of Gebel Oweinat and
to a much lesser extent from the low- relief sandy plain and the scattered hill rocks exposed to the east and north
of the study area. The resulted materials were carried by fluvial action which finally interfered and admixed
with marine environment (open marine facies). The shales were later on affected by diagenesis especially those
in southern area, during the deposition and deep burial stages. The differences in abundances of these clays may
be related to changes in the nature of source rocks, climatic conditions and the transgression and regression of
the sea during the Early Eocene.
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(B) Environmental Impact (contamination and pollution):
The index of geoaccumulation (Igeo) designed by Müller (1969) has been useful to evaluate the
contamination of an environment sediments by Loska et al. (2004).
Igeo = log2 (Cn /1:5Bn)
Where: Cn is the measured concentration of a given metal in sediment and Bn represents its geochemical
background concentration. In this study, the concentrations of elements in PAAS (Taylor and McLennan, 1995)
were used as background values. According to Müller (1981), the geoaccumulation index in relation to
pollution extent is classified into seven classes. These are: <0 unpolluted, 0-1 unpolluted to moderatelypolluted, 1-2 moderately-polluted, 2-3 moderately- to strongly-polluted, 3-4 strongly-polluted, 4-5 strongly-to
very strongly-polluted and >5 very strongly polluted (Müller, 1969).
Applying the geoaccumulation indices for the studied Esna shales (Fig. 15), revealed that, the majority of
the elements are classified as unpolluted to moderately -polluted. However, Mn in the shales of the northern
section (1.67) is moderately –to strongly -polluted. In addition, Zr in those of the southern section (4.63) is
classified as strongly -to very strongly-polluted. Generally, the geoaccumulation indices reveal that shales of the
northern section unpolluted to strongly polluted whereas those in the southern section are unpolluted to very
strongly polluted.
Fig. 15: The geoaccumulation indices (Igeo) for the studied Esna shales
Conclusions:
The Esna Shale in El-Quss Abu Said depression, Farafra Oasis is overlying unconformably by the Tarawan
Formation and conformably with gradationally contact underlain by the Farafra Formation. It consists
predominantly of vari -colored shales with intercalations of argillaceous limestone.
The Esna Shales are texturally classified as mudstones. Mineralogically, these shales consist mainly of
detrital smectite with subordinate kaolinite and traces of illite. Generally, they are moderately- crystallized
suggesting that they are mainly detrital in origin.
The most important factors controlling the mineralogical and geochemical composition of the shale
in Esna shales are the composition of the bedrock and the possible occurrence of an old weathering
crust. SiO2 Al2O3, Na2O and K2O are lower content whereas CaO and P2O5 are higher than the values reported
for the Upper Continental Crust (UCC) and Post Archaean Australian Shale (PAAS). Average values of V, Cr,
Zn, Ni, Cu. Co and Rb are strongly enriched in respect to UCC in northern section. This can be attributed to the
abundance of mafic components in the source area and the intensive chemical weathering of source rocks.
Average values of Zr and, to a much lesser extent Th are enriched in respect to UCC in south section. This
indicated that it subjected to intensive chemical weathering than the northern shales section and related to
inheritance from source rocks rich in felsic minerals. The studied shales are generally relatively an immature,
unpolluted to very strong polluted. They were deposited in Phanerozoic-Proterozoic under oxic to anoxic or
dyoxic marine conditions. They are product of intensive chemical weathering of metamorphic and igneous
(granitic and basaltic) rocks coupled with hydrothermal solution. Also, the composition of the Esna shales is
more related to the PAAS for northern section shales and to the UCC for southern section. The provenance
constituted a part of passive margin and was characterized in semiarid climate. For the above reason, the studied
area was subjected to chemical weathering and pollution.
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Recommendations:
The Esna shales in the study areas contain high percentages of Ca- smectite and low percentage of Nasmectite. Therefore, the authors recommend conducting further studies on these huge mudstone exposures
especially measuring their petrophysical properties to assess the probability of their use for many industrial
purposes especially the production of cements and ceramics.
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