Depositional history and climate change in the Quaternary sequence of Meyghan playa

To study، 30 sediment samples were collected from the 7 boreholes with maximum depth 520 cm in the Meyghan playa bed which is located in 15 km northeastern part of Arak. Sediment samples were studied for sedimentological characteristic include mineralogy، granolometry، calcimetery، organic matter content، pH، EC and physical characteristics of sediments. Meyghan playa sediments are mostly of chemicals and detrital. Percentage of evaporative minerals in surface sediments is greater than the clastic minerals. Sediments of this playa are mainly chemical (evaporites) and clastic components. Amount of evaporative minerals in surface sediments is greater than the clastic minerals. This study shows، Chemical sediments are formed Mainlyو during periods of low water levels and sometimes، the lake is fed from the land. While castic sediments of the basin were deposited during periods of high water levels and transported by rivers and floods as bed load and suspended load to playa and deposited. Considering sedimentology and mineralogy profiles with depth despite visible fluctuations due to climate change (dry and wet seasons)، indicative almost regular process of parameters along the borehole. So that with depth abundance of detrital minerals، calcium carbonate، and mean of grain size is increased while organic matter and mean of grain’s sorting is decreased. These factors along with the reduction of evaporative minerals with depth، especially minerals with high solubility such as gypsum and Halite indicate there was a high water level and cold - humid climate for the past (late Pleistocene) in the Meighan playa and/or maybe there is a lake with fresh water or Low salinity in the past that its salinity has increased over time.