| چکیده انگلیسی مقاله |
Geological time has been evaporited sediments form by solar-driven concentration of a surface or nearsurface brine. Large, thick and extensive deposits were dominated by rock-salt (mega-halite) or anhydrite (mega-sulfate) deposits tend to be marine evaporites and can be associated with extensive deposits of potash salts (mega-potash). Examinations of sedimentologic and mineralogic characteristics of the closed-basin cores due to the very high susceptibility to precipitation and evaporation balance are beneficial for determination and evolution of late Quaternary sedimentary specification. Playa defined as an intra-continental arid zone basin with a negative water balance for over half of each year, dry for over 75% of the time, with a capillary fringe close enough to the surface such that evaporation will cause water to discharge, usually resulting in evaporites. The negative water balance includes all sources of precipitation, surface water flow and groundwater flow minus evaporation and evapotranspiration. Hoz-e-Soltan Lake, with 195 km2 catchment area and 25-50 cm depth, locates at 85 km of southwest of Tehran-Qom highway, in Central Part of Iran. It is sited between 34º56´ and 35º31´ north and 50º53´ and 51º20´ east at western-north of Hoz-e-Masileh. The maximum superficial relief is about 1940 m to the north and 1150 to the south. It is surrounded by Ali-Abad and Kushk-e-Nosrat mountains in north and northeast, Manzarieh and Chahar mountains in west and Mohammad-Abad and Badamcheh mountains in south and southeast. Orogenic movements and volcanic eruptions at the end of Cretaceous and early Eocene led to formation of Eocene volcanics in the north and some central parts of Iran, and particularly around the Hoz-e-Soltan basin. As a result of faulting takng place following the volcanism at this area, a graben was formed, which was later filled by the salt Lake. This graben formed a closed basin, which has since become partially filled with interbedded terrigenous-clastic and evaporite sediments. The evaporate facies present conform approximately to a ″tear-drop pattern″. This research was conducted with the aim to determine sedimentary facies and identify evaporate minerals as well as the change in the type of mineral composition within the sediments that were formed during the Holocene period reflecting the balance in water input and output in the studied basin. Materials and Methods In this study, 9 core samples (max length 700 cm) were selected from the intact bottom of the Lake using an auger core sampler. The core samples were then dissected in halves longitudinally, underneath the sediments, Playa environment was detected. In total, 113 sub-samples were prepared from various core samples of sedimentary facies, which were then analyzed using grain and mineral analytical methods such as granulometery and mineralogy (XRD). High-resolution digital photographs of cores were performed with 12 megapixel Canon camera imaging. Samples were dried in an oven at 60 ̊C for 48 hours in laboratory. Analysis of particles larger than 63 micron and smaller than 63 micron (silt and clay) was performed by using a wet Vibratory Sieve Shaker ANALYSETTE 19 and Laser Particle Sizer ANALYSETTE 22, respectively. Also the analyses of all clastic, carbonate and evaporite minerals were determined semi-quantitative. Sedimentary components were studied by binocular Nikon microscope with a maximum magnification of 94.5 (6.3 × 1.5 × 10) and scanning electron microscope. All of analyses were done in the laboratory of Geological Survey of Iran. Results and Discussion After obtaining the results of different laboratory studies such as granulometery, mineralogy, chemical analysis, scanning electron microscope, morphometry and morphoscopy from 3 cores (HS1, HS2 and HS3) and processing the data, final investigations were done. Sediments in the central part of the Lake is almost entirely chemical (evaporate and carbonate) and in the margin of the Lake is detrital-chemical. Quartz is the most abundant mineral with different amounts in the studied cores. Feldspar is less abundant. The highest amounts of detrital sediments were seen in the margin of the Lake, particularly in western-north, west and western-south. There is alternation of detrital sediments with chemical and biochemical sediments resulting from the wet and drought periods. Between the carbonate minerals, calcite is the most abundant mineral as cement in the alluvial fans and sand dunes. The amount of calcite is increased from the center to the east and eastern-south of the Lake. Quartz and calcite dominated in mud flat and saline mud flat sub-environments. Halite with an area of 75 km2 is the most abundant lithofacies in the Lake. There are two main types of halite: massive halite and hoper halite. Gypsum is in the lens form, semi-translucent prismatic crystals in white, grey and yellowish brown colors. Halite and gypsum dominated in salt pan and ephemeral Lake sub-environment. Generally, the colour of sediment in mud flat sub-environment is brownish red, saline mud flat is red and yellow with large crystals of diagenetic gypsum, salt pan is white, green and grey and ephemeral Lake is light to dark grey. Conclusion On the basis of granulometery results, 5 sedimentary types were recognized in the sub-surface sediments: slight gravel-bearing sandy mud, slight gravel-bearing muddy sand, mud, sandy mud, and muddy sand. The results of these analysis showed that the Lake sediments are composed of 3 types of sediments: detrital, carbonate and evaporate sediments. Quartz, feldspar and clay minerals dominated of detrital sediment, but other minerals such as mica, magnetite, hematite, amphibole, zeolite, anatase and pyroxene were found. Calcite dominated of carbonate sediment and in some samples aragonite were found. Calcite, gypsum and halite dominated of evaporitic minerals, but other minerals such as anhydrite, carnalite, polyhalite, tenardite, basanite and hexa-hydrate were found. |