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

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Sedimentological Significance and Brine Chemistry of Recent Coastal Sabkha, Northwest Libya JKAU: Mar. Sci., Vol. 22, No. 2, pp: 135-158 (2011 A.D. / 1432 A.H.) DOI : 10.4197/Mar. 22-2.8 Sedimentological Significance and Brine Chemistry of Recent Coastal Sabkha, Northwest Libya Mohamed Abdel Galil and Esmail El-Fergany Faculty of Science at El-Khums, Misratah University, Libya [email protected] Abstract. Sabkha deposits occupy the relatively low topographic areas and are separated from the sea by coastal sand dunes. The sabkha sediments are relatively finer compared to those of the coastal sand dunes and beach. Grain size grading with improvement in sorting occurs in the direction of sediment drift landward. The brines of the saline pans are of recent marine water origin. During spring and summer months, due to evaporation, the water level in the saline basin is lowered to a level below or nearly equal to that of the Mediterranean Sea from which the waters seep into the Salina. The brackish waters are partially or completely evaporated which lead to deposition of evaporite minerals in the saline basins and the surrounding sabkha plains. In autumn and winter months, the Salina is filled with water and the surrounding sabkha plain is moistened with seawater seepage and sporadic rainfall. These waters led to partial dissolution of the former summer deposited halite and/or gypsum. The halite crusts in the coastal saline pans are subjected to dissolution during seawater and/or meteoric water flood stage, and to cementation during the desiccation stage. The resulting dissolution and re-precipitation features are diagnostics of the ephemeral saline pan halite. The salinities increase from the sea landward (46.9 g/l, up to 180.4 g/l and up to 323.8 g/l for the seawater, coastal sand dunes pans, and the sabkha brines respectively). Accompanying the increase in salinities is the very high concentration of Na+ and Cl- ions. The brines are highly saturated with NaCl, which favors a dominant halite precipitation (65.02- 78.12%), while bicarbonate salts are traces (0.13- 0.79 %). Introduction Sabkha is an Arabic word for salt flat area. The study coastal sabkha is situated in an extensive sabkha plain about 6 Km to the east of Zuwarah 135 136 Mohamed Abdel Galil and Esmail El-Fergany city and about 50 Km to the east of Tunisia border (Fig. 1). Deposition and dissolution of the evaporite minerals in the recent deposits are interpreted using the saline pan cycle (Lowenstein and Hardie, 1985), which consists of a flood stage (brackish lake), an evaporative concentration stage (saline lake), a desiccation stage (dry saline pan) and return to a flood stage (brackish lake). As evaporation and halite crystallization continue, the saline lake shrinks, ultimately drying out (Fig. 2). According to Meteorological Authority data of Zuwarah station, the study area has a Mediterranean climate where arid to semi-arid 0 conditions are predominating. The average temperature rises to 50 C 0 during summer months, while it drops to 20 C during the winter. December and January are the wettest months and rain is often concentrated in a few heavy showers. Wind speed increases in November until April causing dust storms. The aim of the present work is to study the field relationships, textural characteristics of Zuwarah sabkha sediments and to delineate the water origin from the brine chemistry. Materials and Methods Twelve water samples were collected from sabkha brines, from sand dune pans and from the seawater. The collected samples were + + ++ ++ - - - - analyzed for the major ions (Na , K , Ca , Mg , Cl , SO4 , HCO3 and - - CO3 ). All concentrations are expressed as equivalent per million (epm= ppm/ equivalent weight), whereas the salinity is expressed as gram per liter (g/l). Results of the chemical analyses were recalculated to e % of major cations and major anions (epm of specific cation or anion/sum of epm of cations or anions) to interpret the origin of brines. Moreover, fifteen sediment samples were collected from beach, coastal sand dunes and sabkha plain. The collected sediment samples were washed by distilled water several times, dried and mechanically analyzed using a Ro-Tap shaker at half phi interval according to Folk and Ward (1957). Five salt samples were examined by using X-ray diffraction analysis. Also, Scanning Electron Microscope (SEM) photographs were carried out for selected seven salt samples. Sedimentological Significance and Brine Chemistry of Recent Coastal Sabkha, … 137 Fig. 1. Location map of the study area and schematic cross-section showing the general distribution of Quaternary deposits (Anketell and Ghellali, 1991). Lithostratigraphy Ephemeral saline pans occupy the lowest topographic depressions in the sabkha plain. According to Anketell and Ghellali (1991) Jeffara, Gargaresh and the upper member of Qasr Al-Haj formations cap the Plio- Pleistocene deposits (Fig. 1). These deposits are capped by a veneer of Holocene superficial deposits comprising recent sand dunes, wadi deposits and sabkha (El-Hinnawy and Cheshitev, 1975). Sabkha deposits occupy the relatively low topographic areas and are separated from the sea by coastal sand dunes (Fig. 2). The dry sabkha is frequent in the interdune areas and characterized by halophytes. Landward, the sabkha deposits are underlain by silt of Jeffara Formation (Fig. 3). 138 Mohamed Abdel Galil and Esmail El-Fergany Coastal sand dune Salt crust Fig. 2. Coastal sand dune separated a relatively low area from the sea. Note, as evaporation continues, the saline lake shrinks. Jeffara Formation Salt crust Fig. 3. A trench in the coastal sabkha showing salt crust contains crystals of evaporite minerals overlain Jeffara Formation. Sedimentological Significance and Brine Chemistry of Recent Coastal Sabkha, … 139 Lowenstein and Hardie (1985) grouped the layered evaporites in three depositional settings: (1) deep perennial (density stratified) basins, (2) shallow perennial lakes or lagoons, and ephemeral saline pans. Deposition of the evaporite minerals in Zuwarah sabkha took place in shallow and flat basins that are normally dry in summer and flooded with water in winter. These basins represent the ephemeral saline lake sub- environment of Hardie et al. (1978), and the ephemeral saline pans of Lowenstein and Hardie (1985). The saline pans range in size from a few square meters to hundreds of square meters depending on the amount of ground water seepage, the slope, and the surface area of the evaporite basin. The saline pans usually occupy the center of the evaporite basin, but may be shifted towards the margin depending on the topographic location of small depressions within the basin. The saline pans are filled with water in winter and floored with layered salt in summer (Fig. 4). As a result of continued evaporation, the saline pans are encrusted with halite crusts, hence they can be termed halite pans, similar to that described by Lowenstein and Hardie (1985) and Smoot and Lowenstein (1991). The saline pan zones are surrounded with brine saturated mudflats that are covered with scattered halophytes surround the saline pan (Fig. 5). Near the coastal saline basin, the water table is close to the surface of the sabkha sediments. The main supply to the coastal saline basin and the surrounding sabkha sediments is either through storm flooding of sea water, seawater seepage or high tide seawater spray, in addition to minor input from groundwater inflow through the highly permeable fluvial and dune sands after torrential rains. The inflow of both marine and nonmarine waters into the saline pans causes the level of groundwater to rise (Basyoni and Mousa, 2009). During spring and summer months, due to evaporation, the water level in the saline basin is lowered to a level below or nearly equal to that of the Mediterranean Sea from which the waters seep into the saline pans. The brackish waters are partially or completely evaporated which lead to deposition of evaporite minerals in the saline basins and the surrounding sabkha plains (Fig. 4 and 5). X-ray examinations show a dominant halite while bicarbonate salts are traces. In autumn and winter months, the saline pans are partially filled with water and the surrounding sabkha plain is moistened with seawater seepage and sporadic rainfall. These 140 Mohamed Abdel Galil and Esmail El-Fergany waters led to partial dissolution of the former summer deposited halite and/or gypsum (Fig. 6). Fig. 4. Saline pan floored with layered salt in summer. This pan will be partially filled with water in winter. Saline pan Halophytes Fig. 5. Scattered halophytes surround the saline pan. Sedimentological Significance and Brine Chemistry of Recent Coastal Sabkha, … 141 Fig. 6. The halite crusts are subjected to dissolution during meteoric water flood stage. Note the isolated remnant salt. Halite continues to precipitate from the groundwater brine as clear, void filling cement and displacive crystals within mud (Lowenstein and Hardie, 1985; Casas and Lowenstein, 1989; Smoot and Lowenstein, 1991). During the flood stage, dilute floodwater is pounded in the saline pan and dissolves the underlying halite crusts (Fig. 7 to 10). The textural features produced during the flood stage include: (1) Horizontal truncation surface, (2) cavities formed by dissolution, and (3) mud partings between and within evaporite crystals (Casas and Lowenstein, 1989). Fig. 7. SEM photomicrograph cavities Fig. 8. SEM photomicrograph of partial formed by dissolution (arrows). dissolution of cubic halite crystals. 142 Mohamed Abdel Galil and Esmail El-Fergany Fig. 9. SEM photomicrograph of the cleavage Fig. 10. SEM photomicrograph of cavities planes in halite showing dissolution. and partial surface dissolution. During the evaporative concentration stage, the ephemeral pans reach saturation with respect to halite and turns into saline pans. Crystallization starts at the brine surface as small plates and hopper crystals, which sink to the bottom (Fig. 11), and as bottom growth of chevrons and cornets (Arthurton, 1973). When the brine reaches saturation with respect to halite, halite crystallizes at the brine-air interface as millimeter-sized rectangular and square-shaped plates and pyramidal hoppers (Arthurton, 1973).
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