الفهرس 
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Abstract
The studied Middle to Upper Miocene Abu Dabbab evaporites at the coastal plains of the Red Sea represent the middle belt that is bounded to the west by the Lower Miocene clastics of the Ranga Formation, and the carbonates of Um Mahara Formation, and to the east with a belt composed of Upper Miocene, Pliocene and Pleistocene sediments (Philobbos et al., 1985). These Miocene formations are unconformably overlying the Eocene, Cretaceous and Precambrian basement rocks to the west. The Abu Dabbab evaporites are characterized by a relatively simple and homogenous mineralogy primarily made up of secondary gypsum at quarries (Wadi Gasus, Wadi Teaban and Wadi Wizr), and anhydrite at rock exposure and at the top part of quarry facies (Aref et al., 2003). The primary deposited texture is represented by microbial laminites that are partially obscured by the massive texture of the secondary gypsum. The microbial laminites have a composition of calcite, dolomite, magnesite, celestite, or sulfides. The secondary gypsum is mainly represented by porphyrotopic, poikilotopic gypsum near the bottom of the quarries, and alabastrine, granular, selenite masses and satin spar gypsum veins near the top of the quarries (Aref et al., 2003). Due to the common absence of primary depositional features other than microbial laminites, the purity of the secondary gypsum beds and their thickness (~ 20 m), indicate that the gypsum was deposited dominantly in subaqueous rather than in a sabkha environment (Aref et al., 2003).
The present work deals with the geological features, sedimentological characteristics and origin of evaporite deposits in Wadi Gasus, Wadi Teaban, and Wadi Wizr areas, located along the Re Sea coastal plain between Safaga and Mersa Alam from north to south. The geomorphology, lithostratigraphy, tectonic setting and petrology were used to give a clear picture in the aim of throwing more light on the depositional environments of the different evaporite facies.
Geomorphologically, The studied areas are bounded on the east by the recent sediments of the shoreline of the Red Sea. Most of the sedimentary rocks in the three studied areas are of relatively moderate to low relief and its elevation decrease generally towards the shoreline. The evaporite hills and their caped mantle of anhydrite are the most conspicuous physical features of the areas. The studied areas are characterized by drainage patterns of dendritic type. The three main wadies Gasus, Teaban, and Wizr slope gently from west to east.
The three areas under study, wadi Gasus, wadi Teaban, and wadi Wizr contains economic gypsum deposits, with respect to the grade agent. The quantities of gypsum and anhydrite which used for industrial proposes (manufacture of gypsum for building proposes, in cement industry, for agricultures, …etc) composed of more than thousand millions ton of pure gypsum.
The exposed rock units at wadi Wizr are represented by the basement rocks (various igneous and metamorphic rocks; the pink granite is the common rock type), of Precambrian age. The Nubian Formation consists of two Members; (A) Taref Sandstone Member, Coniacian in age, outcrop at the base of the sedimentary sections, and (B) Quseir Clastic Member, The middle and the upper Cretaceous Formation in upper Egypt (Nakheil, Thebes, Esna, Tarawan, Dakhla, Duwi (phosphate) formations are not recorded and completely absent in wadi Wizr Area. The Nubian formation underlies unconformably the Middle Miocene rocks.
In wadi Teaban and wadi Gasus areas the Nubian formation, consists of the two Members Taref Sandstone Member and Quseir Clastic Member. The Cretaceous sediments (Nakheil, Thebes (the top part of all the high hills in the study areas), Esna, Tarawan, Duwi and Nubian formations) overlie unconformably the basement rocks and underlie unconformably the middle Miocene rocks. The phosphate beds in Duwi Formation are mined in both wadi Teaban and wadi Gasus areas by Red Sea Phosphate Company as open cast in some localities and as under ground mines on other localities for the production of Phosphate ore.
In the study areas two Middle Miocene formations, were recorded, 1-Gebel El Rusas Formation (consists of the lower clastics and upper carbonate units0, and 2 – Abu Dabbab Formation that represents the main sedimentary outcrops of the studied areas. It consists of gypsum and anhydrite with some carbonate intercalations in the upper part. The Miocene – Pliocene (Samh formation), in the studied areas lie along the Red Sea coastal plain parallel to the Red Sea. The Pliocene (Gabir Formation) overlies conformably the coral reefs of Samh Formation Pliocene.
The evaporite rocks are differentiated into the following facies; 1 - Massive gypsum,
2 - Regular laminated gypsum, 3 - Microbial laminated gypsum, 4 – Stromatolitic gypsum,
5 - Nodular mosaic anhydrite, and 6 - Enterolithic anhydrite nodules
Diagenetically, the evaporite rocks have been affected during the three main stages; syndepositional stage, burial stage and uplift stage.
Petrographic investigation show that the Middle to Upper Miocene Abu Dabbab evaporites of the northern Red Sea coastal plain of Egypt are composed mainly of secondary gypsum rocks that are mantled with 2-3 m thick anhydrite deposit. Several fabric types of secondary gypsum are recorded; these are porphyrotopic, poikilotopic, alabastrine, and granular, selenitic and satin spar gypsum veins. All these types occur in the gypsum sequence, with predominance of porphyrotopic and poikilotopic gypsum near the base, and alabastrine, selenitic granular and satin spar gypsum veins near the top. Fractures inherited in the evaporite rocks due to unloading accompanying uplifting and eastward tilting, and that accompanied rifting, favored pathways for percolation of meteoric water during pluvial periods. The coarse porphyrotopic and poikilotopic gypsum were formed during early exhumation of the rock at near equilibrium condition by slow crystal growth.
The fine alabastrine gypsum is formed in an active phreatic water condition and postdates the coarse porphyrotopic and poikilotopic gypsum by their dissolution and rapid crystal growth under disequilibrium condition. The gypsum veins represent excess volume of sulfate that resulted from conversion of anhydrite into gypsum. The selenite pockets represent the latest secondary gypsum rock, formed by the dissolution of all the above-mentioned types and their later recrystallization under phreatic conditions, with very slow crystal growth under equilibrium conditions.
With regard to the geochemical characteristics of the studied gypsum, relatively higher concentration of FeO, MnO, k2O, MgO, TiO2, Sr, and Zr are observed. This indicates that the parent brine, from which the Red Sea gypsum was deposited, had relatively higher concentration of these elements. The only possibility to maintain such high concentration of these elements is their derivation from the nearby basement rocks and the pre-Middle Miocene rocks of the Red Sea region via meteoric water. Therefore, meteoric water over the Red Sea basement rocks and the pre-Middle Miocene rocks led to the leaching of the minor and trace elements, which was flushed to the evaporite basins. Deposition of these evaporites from mixing of meteoric water enriched in minor and trace elements, with marine water lead to precipitation of gypsum highly enriched in minor and trace elements. Further on, during burial diagenesis of the Red Sea gypsum to anhydrite, followed by exhumation to secondary gypsum (Aref et al., 2003), the necessary waters that hydrate the anhydrite are also enriched in trace and minor elements coming from the Red Sea basement rocks and the pre-Middle Miocene rocks during the Pliocene and Pleistocene rainy periods.
Environmentally, the studied evaporite facies in the Red Sea region is either deposited in supratidal sabkha environment (nodular mosaic anhydrite and nodular enterolithic anhydrite) or shallow subaqueous (subtidal to intertidal) environment (regular laminated gypsum, microbial laminated gypsum, stromatolitic gypsum). In the supratidal sabkha environment, deposition of primary gypsum nodules is by displacive or replacive growth in siliciclastic host sediment and primary dolomite. Continued growth of the gypsum nodules led to the formation of enterolithic gypsum nodules. In the subaqueous environment, deposition of gypsum took place from the brine-air interface or from the sediment-water interface as fibrous or prismatic crystals. These crystals are either draped over microbial laminites or deposited over mud or silt laminae that led to the formation of couplets of gypsum-microbial laminites.