Arab J Geosci (2015) 8:7973–7991 DOI 10.1007/s12517-015-1845-0
ORIGINAL PAPER
Sedimentological characteristics of continental sabkha, south Western Desert, Egypt Osama E. A. Attia & Hussien K. Hussien
Received: 29 September 2014 / Accepted: 9 February 2015 / Published online: 5 March 2015 # Saudi Society for Geosciences 2015
Abstract Continental sabkha is recorded in the extreme middle part of the south Western Desert of Egypt representing one of the most promising areas for sustainable development, especially for agriculture. The geomorphologic units of the area are formed under the influence of structural, lithological, and/ or climatic controls. These units include pediplain, depressions, mass-wasted blocks, residual hills, drainage lines, and aeolian landforms. The effective deflation process is reinforced by the lack of protective vegetation cover and the susceptibility of weakly consolidated rocks to wind removal. Different rock units covering the area range in age from Early Cretaceous to Quaternary. Quaternary deposits include playa/ sabkha deposits where the continental sabkha is divided into four zones A, B, C, and D. Underground waters coming from the north and east were progressively enriched in solute by interaction with the surrounding sedimentary and igneous rocks. Most of the evaporite minerals grow displacively as a result of evaporative pumping. Mineralogically, the continental sabkha salt complex includes gypsum, natroalunite, tamarugite, bloedite, eugsterite, nitratine, halite, and D’ansite. Some of the major and trace elements (SiO2, Al2O3, Fe2O3, MgO, CaO, Na2O, K2O, as well as Rb, Ba, Zr, Sr, Y, V, and Zn) were measured. The correlation and enrichment of these
O. E. A. Attia Department of Geology and Geophysics, College of Science, King Saud University, Riyadh, Saudi Arabia O. E. A. Attia (*) Department of Geology, Faculty of Science, Cairo University, Giza, Egypt e-mail:
[email protected] H. K. Hussien Egyptian Nuclear Materials Authority (ENMA), Cairo, Egypt
major and trace elements indicate the origin of the salt complex in the sabkha. Keywords Continental sabkha . Evaporites . Sedimentology . Geochemistry
Introduction The studied continental sabkha covers an area of about 2, 500 km2 between long. 29° 30′ and 29° 50′ E and lat. 22° 10′ and 22° 30′ N (Fig. 1). It locates at the extremely middle part of the south Western Desert of Egypt and to the north of the Egyptian-Sudanese Boundary, SE of Bir Kuryum, and east of Bir Safsaf and G. Nusab El Balgoum (Fig. 1). The studied area characterized by many desert beauty sight seen of palm and dome trees and many scattered oases where the supply of underground water (spring or Bir) close to the surface (0.5 to 2 m in depth). The studied area represents one of the most promising areas for sustainable development especially for agriculture, where water is found in few places at Bir Nakhlai, Bir El-Shab, Bir Kuryum, Bir Kiseiba, Bir Abu El-Hussein, and Bir Murr. With the exception of Bir Murr, which is very brackish, most of the Birs (wells) are fresh and of good taste. The area is accessible through three main asphaltic roads (Fig. 1). The first road is that previously known as the Camel track Darb El Arbain (Caravan Road) extending from Kharga Oasis at north and runs in an approximately south direction to Salema Oasis in Sudan. The second road starts from Aswan, branched westerly at a point 50 km of Abu Simbel City crossing to the north of Gebel El Nabta. This desert road continues further west passing through Bir Nakhlai and joins Darb El Arbain at Bir El-Shab. The third road is parallel to Darb El Arbain track starting from Dakhla Oasis through Bir Tarfawi to the east of El Uweinat farms. In addition, there is a
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28° 32º
30°
32°
34°
36°
29º 30´ 22º 30º
29º 40´
30º 00´ 22º 30´
29º 50´
250 265 250
30º
225 Bir Kuryim
250
28º
250
22º 20´
26º
225
Qena
250
Bir El Shab
225 To Aswan
22º 20´
Idfu
250 300 El Qara 285
225
225
Aswan
24º
Bir Dibis
22º To Oweinat
Road Study area
0
15 km
22º 10´
22º 10´
Legend: Fault Drainage line
Slope
Hill
Sand dunes Sub-plateau 204m
Playa Sub-plateau 208m
Plateau
Depression
Salt crust (sabkha)
Hamada desert
280
250
204
250
22º 00´ 29º 30´
To Salema
Well Asphaltic road
29º 40´
29º 50´
22º 00´ 30º 00´
Fig. 1 Location and geomorphic maps of Bir El-Shab area and its surroundings, south Western Desert, Egypt
subsidiary asphaltic road connecting Bir Tarfawi with the Camel road at Bir Abu El-Hussein. Desert tracks cross the south Western Desert joining the studied area with the famous Bir Nakhlai, Dungul, and Kurkur Oases. The studied area represents one of the most promising area for sustainable development, especially for agriculture, where water is found in few places at Bir Nakhlai, Bir El-Shab, Bir Kurayim, Bir Kiseiba, Bir Abu El-Hussein, and Bir Murr (Fig. 1). With the exception of Bir Murr, which is very brackish, most of the wells (Birs) are fresh and of good taste. The present work aims to study the geological, geomorphologic, and sedimentological characteristics of sabkha in Bir El-Shab area and their effects on the deposition of salt deposits. Also, it aims to study the mineralogical and geochemical characteristics of salt deposits in the studied area.
Methods of study Aerial photographs (scale of 1:100,000 and field works) were used to construct the geomorphic and drainage maps for the studied area. The collected sediments were examined under binocular microscope for primary categorization of the
sediments as well as the preliminary determination of their mineralogical components. Then, thin sections were prepared for detailed petrographic. Evaporite thin sections for petrographic study were made under dry and cool conditions using epoxy cement. All samples are studied by transmitted polarized light microscope and supplemented by scanning electron microscope (SEM) and Xray diffraction (XRD) studies for some evaporite salts to identify their mineralogical composition, especially those of complicated composition (Bir El-Shabsabkha samples). SEM (Philips® XL30) was used to examine some individual mineral grains with resolution of 3.5 nm at 30 kV. In addition, semi-quantitative microchemical analysis was performed on the examined mineral grains using the energydispersive X-ray (EDAX) system of the SEM. More precise results were obtained by the basic EDAX automatic peak identification and the true standard less quantification using ZAF matrix correction routines. XRD analyses were carried out for bulk powder samples. An XRD unit (PW3710/31), with generator (PW 1830), scintillation counter (PW 3020), and nickel-filtered Cu target tube (PW 2233/20) at 40 kV and 30 mA was used. The scans were limited to the range from 5° to 64° scanning rate of 2° 2θ/
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menthe resulting d-spacing values of the analyzed samples were compared with the data of Powder Diffraction File (PDF)®, 1982, for mineral identification (γ-Ray Scintillometer (model GR 101 A)). The radioactivity is expressed in counts per second (CPS). Geochemically, about 32 representative samples from different zones of the sabkha were subjected to complete chemical analysis to determine their major oxides content and some trace elements. The major oxides (SiO2, Al2O3, Fe2O3, FeO, MnO, MgO, CaO, Na2O, K2O, P2O5, and SO3) and loss of ignition (LOI) were determined according to the procedures of Shapiro and Brannock (1962). The precision of determination is within ±5 %. The determination of trace elements (Sr, Cu, Pb, and Zn) were carried by the X-ray fluorescence technique (XRF) model Philips® (PW1410) X-ray spectrometer in the Nuclear Materials Authority lab, Egypt, adopting the techniques of Norrish and Chappell (1966).
Geomorphology Western Desert is a huge platform with a mean elevation of 500 m above sea level, consisting of thick layers of sedimentary rocks (Said 1962). The present morphology of the Western Desert is inherited from ancient, contrasted morphogenetic systems, and it responses to climatic changes (Kröpelin 1993). Aridity has been the dominating factor in the Quaternary climate in the Western Sahara around the tropic of Cancer where this hyper-arid area receives less than 2-mm average annual rainfall (Kröpelin 1993). The southern part of the Western Desert of Egypt occupies the center of the largest hyper-arid region on the earth. It has the features of lithology-controlled scarp landforms with ridges separated from each other by several kilometers. Sandstone with a slight northward regional slope and dip makes up the largest part of the exposed and subsurface strata. The incident solar energy is capable of evaporating more than 200 times the amount of received precipitation (Henning and Flohn 1977; Kehl and Bornkamm 1993). Exogenetic drainage lines are absent, and internal drainages are restricted to the depressions. Devoid of strong relief, barren rocky surface with remnant hills are the main geomorphologic units (Fig. 1) and most of the area can be considered as a hamada desert (Fig. 2). The studied area can be differentiated geomorphologically into the following: (1) southern Nakhlai-Shab pediplain, (2) depressions, (3) residual hills, (4) aeolian landforms, (5) salt crust, and 6) playa deposits which mainly created by structural, lithological, and/or climatic controls.
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Fig. 2 Hamada desert with remnant hills in the studied area
and occasionally covered with loose sands or ripple marks (Fig. 3) with the occurrence of some exceptions (few igneous and volcanic outcrops and also relatively small patches of Quaternary deposits). This pediplain extends further south and west toward the great African Sahara. In most of its parts, the pediplain surface is nearly flat, sloping gently to the north and dissected by faults (Fig. 1). The relief of the pediplain is relatively high toward south, near the Sudan borders, where it decreases gradually to the north. At lat. 22° 00′, elevations of 340 m were recorded, whereas south of lat. 22° 30′, the elevation of the ground surface varies from 200 to 280 m to the far west with an average 250 m. The area is characterized by its internal drainage patterns. The streams are none flowing owing to the extreme dryness of this part of the Western Desert. Denderitic drainage pattern is the main pattern on this pediplain surface. Depressions The depressions are mainly represented by Kiseiba depression to the north and small scattered depression to the south (Fig. 1) (Hussein 2002). Kiseiba depression has a triangular shape with its apex pointing to the NE direction. This depression extends to the north direction for about 15 km with an average width of about 5 km. Southern depression includes the most mapped area (Fig. 1). It has a triangular shape with its apex pointing NE covering an area of about 460 km2. The floor of the depression is undulated and generally slopped to the north and northwest, while other local inward slopes are also recorded. The surface of the depression is generally made up of
Southern Nakhlai-Shab pediplain This pediplain covers the southern part of the studied area. The pediplain surface is covered with Nubia sandstone beds
Fig. 3 Part from Nakhlai-Shab pediplain covered with sand sheet
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Cretaceous sandstone which latter covered with Quaternary sand sheet with the development of sand dunes, sabkha, and playa (Fig. 4). The origin of these depressions is still controversial (Walther 1912; Abu Al-Izz 1971). The proposed hypothesis of wind, as a powerful agent of excavation of large closed depressions of the Western Desert of Egypt, has been widely accepted (Glennie 1970; Haynes 1980, 1982). Also, the fluvial action on semi-enclosed Dakhla and Kharga depressions by a Pliocene river system formerly draining in a southwesterly direction was proposed by Said (1983), or by the Miocene Fluvial action in the Qattara depression, which draining from the south, near the Gilf El-Kebir (Albritton et al. 1990). Afifi (2001) in his study of Nusab El-Balgoum and its surroundings suggested that a great NNW paleo-river (channel) located at the boundaries of the Nusab El-Balgoum and Bir Abu ElHussein areas. The delta of this river was located at Atmout El-Kebiesh area. He also concluded that the wind action is limited to remove materials from the depression. In the present work, the effective deflation process in the studied depressions is reinforced by the lack of a protective vegetation cover and the susceptibility of weakly consolidated rocks to wind removal (Fig. 5). This is similar to that described by Haynes (1982). The following evidences indicate the role of deflation in the origin of the studied depressions: 1. They have an elongated kidney-like shape, transverse to the prevailing winds. 2. The presence of a relatively steep cliff on the upwind side and a gentler slope on the downwind side. 3. The occurrence of crescentic sand dunes (lunettes) and other sand dunes on the downwind side. 4. Grain composition of the dunes is closely similar to that of the nearby bedrocks.
Residual hills The most prominent topographic features within the studied depression are the large number of residual hills. These hills are usually isolated, conical, and/or have flat-topped surfaces
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(Fig. 6). Several hills of low relief composed of sandstone, siltstone, and claystone are laid down on the western side of the depression. Aeolian landforms Sand dunes and sand accumulation cover most of the investigated area. Sand dunes are concentrated around Bir Kuryim and Bir El-Shab (Fig. 1). These sand dunes are fixed with dense networks of plant rootlets (Fig. 7a, b). Sand dunes are generally of the longitudinal to barchans types (Fig. 8). The individual longitudinal sand dune ranges from 250- to 500-m length and up to 10-m height. Generally, the sand dunes strike in a NNW-SSE direction and sometimes deviated to the N-S direction. Salt crust Salt crust is recorded on the ground surface around Bir Dibis, Bir El-Shab, and Bir Kuryim (Fig. 9). The salt crust is formed by capillary action under the influence of evaporative pumping in area where the groundwater is recorded at shallow depths (about 1 m). Playa/sabkha deposits Playa deposits are made up of horizontal alternating laminations of soft friable sand, clay, and silt with frequent plant remains. These deposits are mainly recorded in Bir El-Shab area where they represent the main cultivated lands. Sometimes, gypsum crust observed at the top these deposits (Fig. 10). Litho-stratigraphically, the studied area is a cuesta type landscape ranging in age from EarlyCretaceous to Quaternary. Consequently, more light should be carried out on the stratigraphy in the study area to help deducing the evolution of the sabkha under investigation. The area under study is covered with the following rock units from top to bottom (El Deftar 1988): Quaternary deposits
Tertiary Upper Cretaceous
Playa Sabkha Salt deposits Aeolian sand accumulation Basalt Nubia Formation El-Borg Formation Abu Ballas Formation
Basement rocks
Fig. 4 Photo looking N shows Bir El-Shab depression with scattered palm and doom trees with recent vegetation
The basement rocks in the south Western Desert form relatively small linear hills. Most of them are of low lands rounded hillocks of older and younger granites (Fig. 11a, b). Their contacts with the overlying sedimentary rocks are mostly marked by some faults that follow the general E-W and NE-
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Fig. 5 Weakly consolidated rock removal by wind, it is used in vegetation in Bir El-Shab
SW trends (Issawi 1971). The basement rocks outcrop to the NE of Bir El-Shabin the form of small elongated patches of medium to very coarse-grained weathered biotite granite and syenites with pink color. The area intersected with several dikes including quartz porphyry dikes, aptites, and pegmatite dikes (Fig. 11c, d). Some basaltic dykes trending NE-SW cut the granitic rocks. These basaltic dykes are mainly of Tertiary age. Due to the undulation character of the basement rocks surface in the studied area, many rock types of different depositional environments are observed ranging from fluvial, land/shore and even restricted very shallow sedimentary sections. Abu Ballas Formation (Barthel and Boettcher 1978) is well developed in Bir Kurayim area consisting of sandy clay, clay, and silt with fine-grained sandstone intercalation. In Dibis area, this formation overlained unconformable by medium to coarse grained, moderately compact thick bedded sandstone of Nubia Formation with the presence of thin band of paleosol (Fig. 12a) with the occurrence of plant remains at the basal part. El-Borg Formation is well developed in the studied area, especially, to the south of Bir El-Shab area at G. El Qara (Fig. 1). Its sequence from base to top is as follows: 1. White kaolinitic sandstone 2. Thick dark yellow, fine-grained, moderately hard, laminated sandstone with rootlets fossils 3. Dark gray gypseous claystone with small dark yellow rootlets (Fig. 4b) 4. Moderately hard, thinly laminated, variegated sandstone
p
5. Very fine-grained, slightly hard, thinly laminated, gypseouskaolinitic sandstone 6. Very fine-grained, soft grading upward to thinly laminated clayey sandstone, grayish white sandstone with plant roots 7. Yellowish to brownish gray, moderately hard, fine- to medium-grained, cracked, jointed, contains leaf prints sandstone (Fig. 4c) 8. Dark gray, large blocks of conglomerates (Fig. 12d) In the western part of the studied area, El-Borg Formation is well developed, especially in Bir Dibis area, overlying unconformable Abu Ballas Formation. It is composed mainly of brick red, medium to coarse-grained, moderately hard sandstone, capped by a huge boulder of dark grey, massive, very hard, ill-sorted conglomerates, and cemented by ferruginous sandstones. Early Cretaceous age is assigned to this formation as deduced from comparison with other literatures (Klitzsch and Lejal-Nicol 1984), the stratigraphic position of this formation and the presence of plant remains. Nubia Formation in the studied area is well developed to the east and southeast of Bir El-Shab. It consists of three units (47.73-m thick) from base to top as follows: 1. Clayey sandstone and silty clay which grading upward to gypseous claystone lower unit 2. Sandy clay middle unit with plant trace 3. Laminated sandstone capped by a huge boulder of conglomerates upper unit (Fig. 13) The Quaternary period is distinguished by its severe climatic changes (Klitzsch 1984; Haynes 1982). These changes are alternatives from wet to dry episodes that were expressed on the surface of the studied area by different kinds of features and deposits such as aeolian deposits (sand dunes, sand sheets, sand shadows, etc), playa deposits, and salt crust (sabkha sediments). The present study will focus on the Quaternary salt crust.
Quaternary salt crust Fig. 6 Isolated hills with conical and flat-topped surface (arrows). Playa sediment (p) in the low land is partially covered with sand sheet near Bir El-Shab, South Western Desert
The term Shab means in Arabic Balum-rich deposit.^ Therefore, the Bedouin assign the term BBir El-Shab^ to indicate
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a
b
Fig. 7 Dune mound fixed by palm and dome trees near a Bir Kuryim and b Bir El-Shab south Western Desert
alum-rich deposits around the Bir location. The sabkha of Bir El-Shab occupies more or less circular shallow basin extending in a nearly N-S direction for about 5 km, with average width of about 5 km covers an area of about 25 km2 (Fig. 14). According to field investigations and drilling 13 trenches ranging in depth from 1 to 2 m (Figs. 14 and 15), the sabkha of Bir El-Shabcan was divided into four zones A, B, C, and D. The surface of zone BA^ is covered by wind-blown sand which seems to be drifted from different distances. Its color is whitish to yellowish and yellow. Darker cables of hard compact ferruginous sandstone coming from the hills are intermixed with this sand. Laterally, this sand cover encrusted with salt Bkorchef^ resulting from the infiltration of the salt carrying solutions to the surface followed by evaporation (Fig. 16a, b). The presence of this korchef indicates the existence of salts deposits with depth in larger amounts than the areas covered by sand. This encrustation (2–5-cm thickness) changes with depth from a hard compact upper surface to well crystalline salt crystals cementing sand grains (Fig. 15). The korchef crust is more or less continuous in some places, while in others, it is intermittent forming irregular patterns. In further other places, it is in the form of circular up-arched structures or BMelons^ of varying diameters (20–60 cm) containing pure salt (snow white crystals) in their interior (Fig. 16c). Below the surface sediments, the salt bed forms a continuous or discontinuous layer of 5- to 30-cm thickness changing in color from snow white to whitish yellow or gray (Fig. 15). It is rather
continuous in the western, southwestern, and southern parts of the area while it is intermittent in other parts forming large buried structures (Melons) of 20–70-cm diameter (Fig. 16c). A section in one of these Melons showed that it is formed of an outer dark colored crust, an intermediate pare of pearlcolored polycrystalline aggregate of alum of mixed with minor amount of sand, and an interior of polycrystalline salt precipitates (Fig. 16d). Zone A of the sabkha represented by trenches T1, T2, and T3 (Figs. 14 and 15). These trenches show that the whole sequence composed of pale gray silt and siltstone with the occurrence of highly imbedded plant remains with depth. This is overlain, sometimes, by laminated, yellowish to reddish brown sandstone with white alum bed. This, in turn, is overlain by yellowish white alum crust (Fig. 16e). Alum domes recorded with depth in the trench to the south boundary of this zone (T3, Fig. 15). There is general increase in the content of sand and sandstone from north to south. Zone BB^ of the sabkha locates to the south of zone A (Figs. 14 and 15). It is represented by trenches T4, T5, and T6 (Fig. 15). The trenches show that this zone is composed of wet, dark red to violet, medium- to fine-grained silt and sand at the base to pale red sand at the top encrusted with white fine-grained alum which capped, sometimes, with gypsum crust (Fig. 17a). Scattered and patches of gypsum is recorded
Fig. 8 Longitudinal dune (arrows) fixed by palm, domes trees, and vegetation at Bir El-Shab, south Western Desert
Fig. 9 White efflorescent salt crust (arrows) on the top of the ground at Bir El-Shab, south Western Desert
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Fig. 10 Trench shows horizontal lamination yellow to brown silt and sand with gypsum crust at top (arrows). Playa sediments filling a deflation hollows at Bir El-Shab, south Western Desert
with depth. In the middle part of this zone, the top sandstone is interlaminated with black organic material which overlain
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alayer of yellowish sandstone with the occurrence of scattered white alum crystals (Fig. 17a). Zone BC^ of the sabkha is to the east of zone B (Figs. 14 and 15). It is represented by trenches T7, T8, and T9 (Figs. 15 and 17b). This zone consists of gray to yellowish brown mudstone intercalated with iron oxides (Fig. 17b). This is capped with yellowish to yellow silt and sand with intercalated with fine-grained alum and gypsum to the top (Fig. 17c). Black crenulated microbial layer is recorded at the top of the sediments to the west of this zone (T7, Fig. 15) which overlain acicular gypsum crystals in sandy layer. Zone BD^ of the sabkha is to the east of zone A and to the north of zone C (Fig. 14). The gypsum crust is white, from 5 up to 20 cm thick, and it is usually display polygonal fracture and/ or ridges (tepee structure). These tepee structures result from volume expansion and contraction. This zone is represented by trenches T10, T11, T12, and T13 (Figs. 14 and 15). It is composed of gray to yellow silt and sand in the lower part (Fig. 10) overlain by black lamination with evaporites in the form of nodules, acicular crystals or forming crust at the top (Fig. 17d). Consequently, from the above description, Bir El-Shab basin can be subdivided into two main parts, alum-rich part to the west represented by zones A and B (Fig. 14) and gypsumrich part to the east represented by zones C and D (Fig. 14). Sabkha zones A and B of Bir El-Shab separated by sand dunes as deduces from the top view and cross section of Bir El-Shab basin (Fig. 18).
a
b
c
d
Fig. 11 Field photos taken NE of Bir El-Shab area, south Western Desert show a large exposure of low elevated weathered granite hillocks, b close-up view of a with the surrounding sand sheet, c dike (arrows) of quartz porphyry trending NE-SW, and d weathered apatite and pegmatite dykes
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a
b
c
d
Fig. 12 a Basal part of Abu Balls Formation shows paleosol beds, north Bir Dibis, south Western Desert. b Surface weathering of ferruginous silicified rootlets (arrow) in the clay stone of El-Borg Formation, at Bir
Kurayim locality. c Stem leaf print (arrow) in the clay stone of El-Borg Formation, at Bir Kuryum locality. d Dark gray conglomerates at the top part of El-Borg Formation, south Western Desert
Mineralogical characteristics of Bir El-Shab sabkha salt complex
Gypsum {CaSO4·2H2O} occurs as tabular crystals of monoclinic crystal and characterized by yellowish white color with relief higher than the canda balsam and weak birefringence. Gypsum crystals occur as scattered rosette crystals in the beds and as cements (Fig. 19a) that grow between detrital quartz grains. Also, gypsum occurs as continuous crust covering the recent sediments (Fig. 17d), composed of acicular crystals arranged in aggregates (Fig. 19b). It is also characterized by vitreous luster. It could be identified by XRD giving its characteristic peaks (Fig. 20a). Natroalunite {(Na,K)Al3(SO4)·2(OH)6} occurs as rhomblike, trigonal, and hexagonal crystals of yellowish white and grayish white colors (Fig. 19c, d). It is characterized by conchoidal fractures and high brittleness which is responsible for its occurrence as small conchoidal fragments. It exhibits a characteristic pearly luster which distinguishes nautroalunite from the other minerals. It is found associating gypsum (Fig. 20a). Tamarugite {Na2Al(SO4)2·OH·3H2O} is a hydrous sulfate of sodium and aluminum occurring as tabular or short prismatic colorless of monoclinic crystal system (Fig. 19e, f). XRD analysis confirmed the existence of this mineral composition (Fig. 20b).
The identification of the constituting minerals of El-Shab sabkha sediments complex by using the techniques mentioned above revealed that the sabkha salt complex includes gypsum, natroalunite, tamarugite, bloedite, eugsterite, nitratine, halite, and D’ansite (Table 1).
Fig. 13 Upper unite of Nubia Formation, showing white moderately hard laminated sandstone, west Bir El-Shab, south Western Desert
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29° 45´ 30˝
29° 46´ 00˝
29° 46´ 30˝
29° 47´ 00˝
22° 20´ 00˝
T13
To Kharga
´
22° 19 30˝
T12
T1
Zone D
To east Oweinat
22° 18´ 30˝
Zone A
T2
22° 19´ 00˝
T11
T10
T3
T9 T4
T8 T5 T6
Zone C
T7
Zone B
LEGEND: Sand sheet
Sand dunes
Hills
Farms
Road 1
Zone A
Zone B
Zone C
Zone D
T1 Trench # 1
Fig. 14 Detailed map of Bir El-Shab playa/sabkha zonation and trench locations
Bloedite {Na2 Mg (SO4) 4H2O} occurs as monoclinic crystals displaying several colors such as white, gray, or colorless with white streak. It is characterized by conchoidal fractures and vitreous luster (Fig. 19g). Its composition is confirmed by XRD analysis (Fig. 20c). Eugsterite {Na4Ca (SO4)3·2H2O} occurs as acicular crystals of rhombic forms and could be fairly distinguished by XRD analysis (Fig. 20d). Nitratine {NaNO3} identified only in XRD analysis by its characteristic peaks (Fig. 20e). Halite {NaCl} forms coarse crystals of isometric system. It displays several colors such as white, colorless, and pink with glassy luster and white streak. It is characterized by perfect cleavage in the three directions of the cubic form. XRD analysis shows its characteristic peaks (Fig. 20f). D’ansite {Na21Mg(SO4)10Cl3} is very difficult to be identified microscopically, but the XRD analysis could distinguish it and showed its characteristic peak (Fig. 20g).
Geochemistry of Sabkha salt complex Some of the major and trace elements (SiO2, Al2O3, Fe2O3, MgO, CaO, Na2O, K2O, as well as Rb, Ba, Zr, Sr, Y, V, and Zn) were measured in 33 representative samples to throw more light on the geochemistry of the studied sabkha salt complex. These samples represent the different facies types (surface and subsurface) of the studied sabkha salt complex of Bir El-Shab. Zone A is represented by ten samples (Table 2), zone B by nine (Table 3), zone C by six samples (Table 4), and zone D by seven samples (Table 5) from about 13 trenches drilled in the sabkha. The analytical results of are shown in Tables 2, 3, 4, and 5. Major element geochemistry: The total SiO2 content have ranges 40.68–62, 55.04–70.15, 58.8–73.6, and 50.57–85.58 % of zones A, B, C, and D of Bir El-Shab sabkha sediments, respectively (Tables 2, 3, 4, and 5,
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Fig. 15 Lithologic logs represent the dug trenches in the different zones of Bir El-Shab sabkha
respectively). This indicates that zones C and D are characterized by much more siliciclastic component mainly in the form of detrital quartz grains (Tables 4 and 5). The total Al2O3 content in the four zones have ranges 12.41–16.30, 11.40– 17.04, 10.8–15.60, and 2.81–16.06 %, respectively (Tables 2, 3, 4, and 5, respectively) which consistent with that of SiO2. There is a negative correlation between SiO2 and
Al2O3 (Fig. 21 (1)) indicating that these elements are related to the siliciclastic materials. The total iron content in the form of Fe2O3 ranges 2.4– 9.71, 3.1–11.89, 2.4–8.3, and 1.2–4.24 % in zones A, B, C, and D, respectively (Tables 2, 3, 4, and 5, respectively). It shows negative correlation with SiO2 for all zones (Fig. 21 (2)), indicating that the depositional environment poorly in
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a
c
b
e
d
Fig. 16 Incrustation of wind-blown sands with salts in zone A of Bir ElShab sabkha, south Western Desert, Egypt. a Polygonal tepee structure. b Hard compacted surface of sabkha due to salt encrustation. c Outer darkcolored part of circular up-arched structures or “Melons” (arrows) of salts. d Section in one of circular up-arched structures or Melons
a
showing the inner part of pure salt (snow white crystals of polycrystalline aggregate of alum) with minor contamination of sand. e Trench in zone A showing a horizontal sequence from wet yellowish sand layers with dark thin organic matter laminations capped by 15 cm from alum deposits in Bir El-Shab, south Western Desert, Egypt
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b Gyp
m
S
d
Fig. 17 a Trench in zone “B” showing horizontal layering of wet yellowish, dark red silt, and sand at the base to pale red sand at the top (S) intercalated with white fine-grained alum (Alu) and capped with gypsum crust (Gyp). Notice the appearance of water at the base representing water table in the area. b Trench in zone “C” shows intercalation of iron oxides with mudstone (m) at the base and
horizontal laminations of yellowish silt and sand intercalated with white fine-grained alum and capped with gypsum crust. c Close-up view of b showing gypsum and fine-grained alum crust at the top (arrow). d Trench in zone “D” showing fibrous gypsum crystals forming crust at the top (arrows)
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Arab J Geosci (2015) 8:7973–7991
Fig. 18 Schematic sketch of Bir El-Shab basin
To Owineat
To Kharga
a
To Tushkia
b NW SE
Legend: Shab
Gypsum
siliciclastic materials. Also, its negative correlation with Al2O3 (Fig. 21 (3)) shows mainly in zone B, indicating that
Table 1 Microprobe analyses for the most common minerals in ElShab sabkha salt complex Mineral
Gypsum Natroalunite Tamarugite Bloedite Eugsterite
SiO2 % Al2O3 % Fe2O3 % MgO % CaO % K2O % NaO % Y (ppm) Zr (ppm) Sr (ppm) Zn (ppm)
67.9 1.45 1.51 ND 9.95 0.213 0.558 ND 277 220 ND
82.7 2.51 2.33 0.129 3.49 0.294 0.459 ND 291 153 ND
0.3 18.1 0.13 0.14 0.3 ND 17.4 131 ND ND ND
2.73 15.1 0.36 3.08 1.13 0.019 14.4 235 206 ND 747
21.0 9.25 1.67 2.02 3.84 0.09 12.3 197 186 133 322
Sand dunes
Vegetation
Bedrock
the clay minerals did not play the major role in controlling the Fe2O3 distribution. However, it shows positive correlation with Al2O3 mainly in zones A, C, and D. This suggests that iron is related to clay minerals. The CaO content have ranges 1.98–11.79, 1.91–6.29, 0.5– 6.59, and 1.1–15.7 % in zones A, B, C, and D, respectively (Tables 2, 3, 4, and 5, respectively). On the other hand, the MgO content have ranges 1.01–2.93, 0.6–4.02, 0.2–1.8, and 0.1–3 %, respectively (Table 2). Both CaO and MgO show negative correlation with SiO2 (Fig. 21 (4 and 5)), while the correlations of CaO and MgO with both Al2O3 and Fe2O3 content are difficult to detect (Fig. 21 (6, 7, 8, and 9)). Also, CaO shows positive correlation with MgO (Fig. 21 (10)). All these relations could be explained as a result of mainly the authigenic origin of Ca and Mg either as carbonates and/or sulfates. The Na2O content in the surface samples is relatively higher than the subsurface samples in the studied zones A, B, C, and D ranges 0.23–4.67, 0.49–4.67, 0.6–3.2, and 0.5– 4 %, respectively (Tables 2, 3, 4, and 5, respectively). It is
Arab J Geosci (2015) 8:7973–7991
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b
a Qz Qz
Qz
Gy
Q Qz 30 µm m
200 µm
c
d
Nat
Nat
5 µm
400 µm
f
e
Taam
Tam
200 µm
2 µm m
g
3 µm Fig. 19 a Microphotograph showing gypsum crystals (Gy) cementing quartz grains (Qz), nicols crossed. b SEM image of tabular gypsum crystals. c Microphotograph showing rhomb-like natroalunite crystals (Nat), plane polarized light. d SEM image of trigonal natroalunite
crystal (Nat). e Tabular or short prismatic tamarugite crystal (Tam), plane polarized light. f SEM image of prismatic tamarugite crystals. g SEM image of bloedite crystals (arrows)
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Arab J Geosci (2015) 8:7973–7991
a
d
b
e
c
f
g
Fig. 20 Representative XRD chart of Bir El-Shab sabkha salt complex
found that there is a positive correlation between Na2O + K2O and Al2O3 (Fig. 21 (11)), but there is also a poor or negative correlation between Na2O and K2O (Fig. 21 (12)). This suggesting that these elements are not only sequestered in different types of clay minerals but also precipitated from the sabkha brines. Trace element geochemistry Trace element contents are variable in the different zones (Tables 2, 3, 4, and 5). This is illustrated by the variation
diagrams plotted between the different trace elements versus the major oxides and versus each other (Fig. 21). The Rb content of the surface samples in different studied zones ranges 7–36, 10–33, 11–35, and 11–22 ppm (Tables 2, 3, 4, and 5, respectively). It is found that Rb has positive correlation with Al2O3 and K2O in the sediments of zone D (Fig. 21 (13 and 14)) and positive correlation with Ba and Sr in the sediments of zones A and D (Fig. 21 (15 and 16)). Rb shows negative correlation Ba and K2O especially in zones A, B, and C, also between Ba and SiO2, but the negative correlation between Rb and Sr show in zones B and C. The
Arab J Geosci (2015) 8:7973–7991 Table 2 Major oxides (wt%) and trace elements (ppm) of recent salt complex in zone BA^ of Bir ElShab sabkha
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Zone BA^ Sample no.
T1.1
T1.2
T1.3
T2.1
T3.3
T3.4
T3.5
T3.6
T3.7
T3.8
40.68 15.16 4.21
52.17 14.8 2.7
56.74 16.24 4.4
56.3 16.3 4.75
62 14.87 8.13
60.07 14.92 9.63
60.28 14.52 9.71
48.56 13.22 3.41
53.3 13.94 2.4
55.05 12.41 2.86
2.93 11.79 4.67 2.93
2.4 7.3 3.4 4.7
2.58 6.81 4.14 2.42
2.88 6.84 4.11 2.28
0.95 2.02 0.51 3.21
1.05 1.98 0.56 3.03
1.01 2 0.61 3.02
1.95 8.96 0.63 2.86
0.72 4.6 2.3 4.3
2.2 7.42 0.48 3.17
LOI 2.68 Trace elements (ppm)
2.9
3.61
3.33
5.94
6.25
6.21
2.86
3.1
3.05
1929 28
2589 34
1688 9
1084 11
78.00 7
604 18
83 36
77 14
78 7
Major oxides (wt%) SiO2 Al2O3 Fe2O3 MgO CaO Na2O K2O
Ba Rb
330 12
Sr
19
64
80
47
105
69
40
28
94
69
Y Zr
77 71
59 255
11 199
2 175
9 192
2 96
11 112
53 23
2 188
2 96
Zn V
7 34
52 156
22 227
11 147
4 35
21 5
3 38
298 5
32 6
21 5
difference in correlation between these elements related to sabkha brines which formed sabkha salt complex. The Sr content in different studied zones ranges 19–105, 2– 78,14–47, and 29–140 ppm, respectively (Tables 2, 3, 4, and 5, respectively). Sr and CaO show negative correlation in zones A and D (Fig. 21 (17)) suggested that the CaO
disappeared in both zones. However, in zones B and C, this relation shows positive correlation (Fig. 21 (17)), indicating the authigenic nature and direct coprecipitation of Sr with CaO from sabkha brines. The Ba content show more or less similar distribution to that of SiO2, Fe2O3, and Rb, where it ranges 77–2,589, 78–1,
Table 3 Major oxides (wt%) and trace elements (ppm) of recent salt complex in zone BB^ of Bir ElShab sabkha
T5.2
T6.3
T6.4
T6.5
T6.6
T6.7
T6.8
T6.9
58.3
59.76
62.51
60.81
58.46
60.49
60.05
70.15
12.34 11.89 1.04 2.63 0.7 3.24 6.28
13.84 10.73 1.11 2.12 0.49 3.09 5.81
14.99 6.92 1.02 2.24 0.62 3.11 5.89
14.27 9.2 1.13 1.91 0.49 3.13 6.11
13.23 8.5 3.61 2.35 3.21 2.1 5.12
14.91 7.94 1.04 2.84 0.51 3.2 5.75
15.45 7.86 1.17 2.55 0.46 3.17 6.08
11.4 3.1 0.6 0.84 0.8 4.4 2.5
118 10 78 2 245 24 9
1256 16 73 2 172 4 64
918 18 23 2 411 3 56
84.00 12 2 2 191 15 4
83.00 33 17 2 243 20 5
114.00 29 18 2 337 20 5
87 20 18 2 333 16 6
78 13 11 2 96 24 3
Zone BB^ Sample no.
T4.2
Major oxides (wt%) 55.04 SiO2 Al2O3 17.04 3.9 Fe2O3 MgO 4.02 CaO 6.29 4.67 Na2O 2.93 K2O LOI 3.7 Trace elements (ppm) Ba 735 Rb 12 Sr 54 Y 5 Zr 73 Zn 6 V 28
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Arab J Geosci (2015) 8:7973–7991
Table 4 Major oxides (wt%) and trace elements (ppm) of recent salt complex in zone BC^ of Bir El-Shab sabkha Zone BC^ Sample no.
T7.1
T8.1
T8.2
T8.3
T9.1
T9.3
Major oxides (wt%) SiO2 Al2O3 Fe2O3
64.0 15.6 2.4
66.3 11.5 4.7
72.8 10.8 3.3
73.6 11.1 3
60.0 14.76 3
58.8 15 8.3
MgO CaO Na2O K2O
1.8 2.2 3.1 6.4
1.2 4.2 2.2 5.4
0.5 0.5 0.8 4.2
0.2 1.9 0.6 4.6
1.7 6.59 1.6 2.53
0.5 1.3 3.2 3.3
2.7
3
2.9
5.52
3.8
LOI 3.2 Trace elements (ppm) Ba Rb
853 11
65 35
80 11
76 17
1006 19
2109 21
Sr Y Zr
47 29 76
32 109 407
14 2 348
17 2 199
39 2 355
33 2 392
V
28 54
260 6
13 8
12 7
4 119
5 223
256, 65–2,109, and 38–1,830 ppm in zones A, B, C, and D, respectively (Tables 2, 3, 4, and 5, respectively). The positive correlation between the Ba and Rb contents (Fig. 21 (15)) indicates the multi-genetic nature of Ba in these deposits. In addition, its higher enrichments in zones A and D deposits can
be related to higher biogenic activity similar to iron enrichments (Tables 2 and 5). Ba and Sr show positive correlation (Fig. 21 (18)) whereas there is negative correlation between Ba and K2O (Fig. 21 (19)). However, there is positive correlation in some zones and negative correlation in other zones between Ba and SiO2 contents (Fig. 21 (20)) as well as the enrichment of Ba in zones A and D than the other zones. This could be related to the presence of detrital barite derived from bedrocks. The Zr content ranges 23–255, 73–411, 76–407, and 83– 729 ppm in zones A, B, C, and D, respectively (Tables 2, 3, 4, and 5, respectively). Zr and SiO2 show positive correlation (Fig. 21 (21)). The Y content ranges 2–77, 2–5, 2–109, and 2–62 ppm in zones A, B, C, and D, respectively (Tables 2, 3, 4, and 5, respectively). Y content shows negative correlation with both SiO2 and Zr contents (Fig. 21 (22 and 23)). The distribution of these elements is related to the siliciclastic deposits in these zones. The V content ranges 5–227, 3–64, 6–223, and 2–206 ppm in zones A, B, C, and D, respectively (Tables 2, 3, 4, and 5, respectively). V content shows positive correlations with both SiO2 and Fe2O3 contents (Fig. 21 (24 and 25)). This suggests their detrital nature. However, the significant enrichment of V in most zone deposits can be related to different source rocks. The Zn contents ranges 3–298, 3–24, 4–260, and 3– 342 ppm in zones A, B, C, and D, respectively (Tables 2, 3, 4, and 5, respectively). Zn shows negative correlation with both Fe2O3 and Ba (Fig. 21 (26 and 27)). These elements are carried to the depositional sites adsorbed on clay minerals.
Table 5 Major oxides (wt%) and trace elements (ppm) of recent salt complex in zone BD^ of Bir El-Shab sabkha
Summary and conclusions
Zone BD^ T10.1
T10.5
T11.2
T12.1
T12.2
T13.1
T13.4
55.08 16.06 4.24 3 7.71 4 2.2
58.75 14.61 3.05 1.63 7.02 3.23 2.46
85.58 2.81 1.2 1.05 2.7 0.56 1.86
60.5 14.11 3.02 1.64 6.05 3.01 2.1
73.2 10.2 2.9 0.1 1.1 0.5 4.4
50.57 12.81 3.2 1.05 15.7 0.55 1.81
55.5 12.4 2.4 1.4 5.1 2.9 3.9
3.72 1830 13 98 2 452 3 206
5.01 488 11 29 2 83 4 27
1.66 464 13 29 11 144 3 32
5.92 1181 15 56 2 148 5 45
3.50 218 20 140 2 576 37 15
1.61 38 22 50 62 187 342 3
3.4 228 22 91 6 729 129 17
Bir El Shab area occupies the depression between the elevated lands in the southern Nakhlai-Shab pediplain, souh Western Desert, Egypt. The area is characterized by a large number of residual hills which are usually isolated, conical, and/or flattopped surfaces (Fig. 6). Several hills of low relief composed of sandstone, siltstone, and claystone are laid down on the western side of the depression. Sand dunes and sand accumulations cover most of the studied area (Fig. 1). The area is covered with different rock units ranging in age from Early Cretaceous to Quaternary. These rock units are classified stratigraphically into the following rock units from top to bottom: Quaternary deposits (playa, sabkha, salt deposits, and aeolian sand accumulations), Tertiary (basalt), Upper Cretaceous (Nubia Formation), Lower Cretaceous (El-Borg Formation and Abu Ballas Formation), and the basement rocks (granites). The area is intersected by several dikes includes quartz porphyry dikes (Fig. 11). Apatites and pegmatite dikes have been observed.
Arab J Geosci (2015) 8:7973–7991 1
20
7989
2
18
18
3
20
4
4
5
8
MgO %
12
12
CaO %
12
Fe2O3%
Fe2O3 (%)
Al2O3%
16
10
2
6
6 4
80
60
0
120
MgO %
CaO %
CaO %
4
10
0 10
20
0
5
8
11
10
0
SiO2%
5
10
15
12
18
50
13
8
90
10
2
0 5
10
15
0
4
8
12
16
35
45
CaO %
14
15
3000
6
10
70
SiO2 %
Fe 2O3 %
14
4
0 30
90
2
0 25 0
20
70
4
Al2O3 %
Al2O3 %
K2O %
10
50
9
4
2
15
6
14
4
2000
1000
2
0
2 0
5
10
15
20
25
2
0
2
4
6
0 5
15
25
Na2O %
Al2O3 %
16
200
2
6
17
20
35
45
200
150
18
8
20
10
30
40
5
15
25
Rb (ppm)
19
20
100
6
K2O %
Sr (ppm)
CaO %
10
Rb (ppm)
150
100
0
0
Rb( ppm)
100
SiO2 %
6
Sr (ppm)
8
Fe 2O3 %
Al2O3 % 18
20
0
0
0
10
Al2O3 %
7
20
10
Na2O+K2O %
90
SiO2%
6
20
0 30
0
0 30
90
MgO %
70
SiO2%
Ba (ppm)
60
MgO %
50
K2O %
0 40
4
80
60
50
2
50
25
35
45
0
100
21
1000
22
100
80
800
SiO2 %
SiO 2%
80
60
2000
40 0
3000
1000
2000
3000
0
1000
B a (ppm)
B a( ppm)
Zr (ppm)
100
0
200
Sr (ppm)
Rb( ppm)
23
100
2000
3000
B a (ppm)
24
20
25
400
80
SiO2 %
15
0
0
0
5
0
Fe2O3 %
0
60
60
10
-4 0 0
-8 0 0
40
40
-800
-400
0
400
-200
800
-100
26
100
200
-200
-100
10
0
100
40 200 -300
Y (ppm)
-150
0
150
300
0 -300
-100
100
300
V (ppm)
V (ppm)
29
28
27
3000
Ba (ppm)
Fe2O3 %
20
0
Y( ppm)
Zr (ppm)
30
2000
1000
0
0 0
300
600
900
Zn (ppm)
0
200
400
600
800
Zn (ppm)
Legend: = area "A"
= area "B"
= area "C"
= area "D"
Fig. 21 Variation diagrams for major and trace elements of the surface samples in different zones of Bir El-Shab sabkha deposits
El-Borg Formation is well developed to the south of Bir ElShab area at G. El Qara composing of kaolinitic sandstone, white in color at the base and very fine sandy size. Nubia Formation is well developed to the east and southeast of Bir
El-Shab. In the present work, Quaternary deposits including playa/sabkha deposits are distributed in many parts of the studied areas where Bir El-Shab sabkha is divided into four zones: A, B, C, and D (Fig. 14).
7990
Zone A of the sabkha characterized by the prevailing of sandstone, silt and siltstone with the occurrence of plant remains with depth (Figs. 14 and 15). This zone characterized by the occurrence of alum crust (Fig. 16e) which underlain by alum beds. Alum domes recorded with depth in the trench to the southern boundary of this zone (T3, Fig. 15). In this zone, there is generally an increase in the content of sands and sandstone from north to south. Zone B of the sabkha located to the south of zone A (Figs. 14 and 15). This zone includes wet silt and sands at the base to encrusted sand at the top with alum and the latter sometimes capped with gypsum crust (Fig. 17a). In this zone, scattered and patches of gypsum is recorded with depth. In the middle part of this zone, the top sandstone is interlaminated with black organic material which overlain a layer of yellowish sandstone with the occurrence of scattered white alum crystals (Fig. 17a). Zone C of the sabkha is to the east of zone B (Figs. 14 and 15) including mudstone intercalated with iron oxides (Fig. 17a). This is capped with alum and gypsum to the top (Fig. 17a). Black crenulated microbial layer is recorded at the top of the sediments to the west of this zone which overlain acicular gypsum crystals in sandy layer. Zone D of the sabkha is to the east of zone A and to the north of zone C (Fig. 14). This zone includes gypsum crust which usually displays polygonal fracture and/or ridges (tepee structure). These tepee structures result from volume expansion and contraction. Mineralogically, Bir El-Shab salt complex includes qypsum, natroalunite, tamarugite, bloedite, eugsterite, nitratine, halite, and D’ansite. The existence of eugsterite in the surface horizon could be aided by high Na/Ca ratios (more than 4) and increased Cl levels (Vergouwen 1981). Also, eugsterite is a very common salt mineral which forms during evaporation of nonalkaline water at 60 °C (Vergouwen 1981; Fitzpatrick et al. 2010). The presence of tamarugite indicates sufficiently strong acid conditions (Topper et al. 2014). The geochemical characteristics of Quaternary salt complex in Bir El-Shab are studied. The geochemistry of major and trace elements of sabkha deposits indicates that SiO2 and Al2O3 elements are related to the siliciclastic component. The clay minerals did not play the major role in controlling the Fe2O3 distribution in zone B of the sabkha. However, at zones A, C, and D, the iron is related to clay minerals. The Ca and Mg elements are mainly of authigenic origin either as carbonates and/or sulfates. The Na2O and K2O elements are not only incorporated in different types of clay minerals but also precipitated from the sabkha brines. The difference in correlation between Rb and Sr elements is related to sabkha brines which formed sabkha salt complex. The Sr element in zone A and zone D indicates that the Ca content disappears in both areas. However, in zones B and C, it indicates its authigenic origin and direct coprecipitation with Ca from sabkha brines. The
Arab J Geosci (2015) 8:7973–7991
correlation between Ba and Rb contents is indicative of the multigenetic origin of Ba in these deposits. Enrichment of Ba in both zones A and D than the other areas could be related to presence of detrital barite derived from bedrocks. The Zr and Y elements are related to the siliciclastic component in these deposits. The V content can be related to derivation from different source rocks. The Zn is carried to the depositional sites and then adsorbed on clay minerals. Acknowledgments The authors would like to express their grateful to all the stuff members and employee of Nuclear Materials Authority (NMA) of Egypt for kind assistance and great help during the progress of this work.
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7991 Topper RPM, Lugli S, Manzi V, Roveri M, Meijer PT (2014) Precessional control of Sr ratios in marginal basins during the Messinian Salinity Crisis? Geochem Geophys Geosys. doi:10. 1002/2013GC005192 Vergouwen L (1981) Eugsterite, a new salt mineral. Amer Mineralog 66: 632–636 Walther J (1912) Das gesetz der wüstenbildung in Gegenwart und Vorzeit. Quele and Meyer, Leipzig, 342 p
