پديد آورندگان :
عبدي، ليلا نويسنده كارشناس ارشد رسوب‌شناسي و سنگ‌شناسي رسوبي، دانشگاه تهران , , رحيم‌پور بناب ، حسين نويسنده استاد، دانشكده زمين‌شناسي، دانشگاه تهران , , يوسفيراد، مصطفي نويسنده استاديار، گروه زمين‌شناسي، دانشگاه پيام نور، اراك , , ميرمحمد مكي ، محسن نويسنده استاديار، دانشكده جغرافيا، جغرافياي خاك، دانشگاه همبولت، برلين، آلمان ,
چكيده لاتين :
Introduction
Playas are shallow, ephemeral lakes that form in arid and semi-arid regions. These inland basins generally occur in the lowest part of an internally drained desert following seasonal or episodic storm events (Reeves 1978; Shaw and Thomas 1989). Playa/lake deposits are of particular interest for palaeoclimate research, since even small changes in the evaporation or precipitation rates may lead to major changes in lake-level and salinity (Battarbee, 1999). In addition, changes in the hydrologic budget of a closed drainage basin maybe indicated by its sediments (Last, 1989). One of these famous inland basins in Iran is the Meyghan Playa located 15 km NE of Arak city, in bordering between central Iran and Sanandaj-Sirjan zones. Because of this exceptional tectonic situation Meyghan playa is formed as a depression surrounded by great variety of sedimentary and metamorphic lithology and igneous rocks of Central Iran zone (Emami, 1991). Meyghan Playa is about 100-110 km2 and it’s surrounded by mountains about 2000-3000 meters high. This playa formed in a tectonic depression that drainages extensive catchment area, Similar too many other cases (Eugster and Hardie 1978; Valeo-Garces et al. 2000). Meighan Playa is surrounded by mountains, alluvial fans, dry and saline mudflats. Whether goes down to about -30°C in winter and it gets up to about +40°C in summer (IRIMO 2008). Presently it is very shallow basin (Maximum depth in wet season ?1.5 m) and receives average annual precipitation of about 320 mm, while the average annual evaporation is about 2070 mm (Abdi 2010). Water influx into the basin is mainly via ground water discharges, ephemeral rivers and direct precipitation. Due to domination of the semi-arid climate with low seasonal precipitations, groundwater system is one of the major suppliers for Meyghan water (Abdi 2010). For few months, in rainy season, it turns into a playa lake but in dry season changes in to a salt flat, so it is also called Kavir-e-Meyghan.
Material and Methods
To study, 30 sediment samples were collected from the 7 boreholes with maximum depth 520 cm in the Meyghan playa’s bed. Sediment samples were studied for mineralogical, geochemical and sedimentological characteristic include granolometry, calcimetery, organic matter content, pH, EC and physical characteristics of sediments.
Discussion of Results and Conclusions
Meyghan playa’s sediments are mostly of chemicals (evaporites) and detrital type. Amount of evaporative minerals in surface sediments is greater than the clastic minerals. This study shows chemical sediments are formed mainly during periods of low water level and condition that lake rarely is fed from the land. While detrital sediments of the basin were deposited during periods of high water level and were transported by rivers and floods as bed load and suspended load to playa then deposited. Results showed that the playa’s bed sediments are very fine-grained (mostly silt and clay) and contains detrital minerals such as quartz, muscovite and clays. Illite and Chlorite of detrital origin are the most important clay minerals in the playa. Evaporate minerals are Calcite, Gypsum, Halite, Glauberite, Thernadite, Polyhalite and Natron.
Although various playa deposits such as beach ridges and basin fills have been used in palaeoclimate studies (Shoffner, 2000), playa evaporites are one of the most common deposits used for these studies. Calcite which forms as a primary precipitate in relatively humid climate is indicative of low salinities and freshwater influxes to the lake. On the whole, the carbonate-rich (mainly calcite) facies characterize low salinity environment accompanied with a higher lake stand but the sulphate-rich (mainly gypsum) facies indicates higher salinity and so a lower lake level (Sinha et al., 2006). The clay minerals distribution in young sediments is mainly an indication of the climate and weathering pattern of the source area. Illite and chlorite are considered common by products of weathering reactions with low hydrolysis typical of cool to temperate and/or dry climates (Li et al., 2000; Khormali et al., 2005). Deconinck et al. (2005) pointed out that the increasing Illite content is due to cold weather. Considering sedimentology and mineralogy of profiles with depth, despite visible fluctuations due to climate changes (dry and wet seasons), indicates a relatively regular trend of these parameters along the boreholes. So that toward depth, abundance of detrital minerals, calcium carbonate, and mean of grain size is increased while organic matter and mean of grain’s sorting decrease. These changes along with the reduction of evaporative minerals especially minerals such as gypsum and Halite toward depth, indicate that there was a high water level and cold-humid climate in the past (late Pleistocene) in the Meyghan playa, and/or maybe they are representation of presence a lake with fresh water or Low salinity in the past which its salinity has increased over time.
Keywords: Meyghan Playa, Depositional trend, climate changes, Quaternary.
