Sinkholes are more abundant, in the quaternary-covered basins with soluble bedrock. In the folded Zagros in Iran, the Most of plains are karstic. This research was carried out in an area of 155 km2 in location of old the Jaydar and the Gori Balmak lakes, in south of Poldokhtar city, SW Iran. The purpose of this study was to evaluate the factors affecting the formation of sinkhole. In this area are expanding lake sediments of the Seymareh landslide dams. The thickness of lake sediments have been estimated using geophysical studies include GPR with 10 MHz antenna and ERT, respectively an average 70 m and less than 10 m in the Jaydar and Gori Balmak plains. The bedrock of study area is the Gachsaran Formation (Middle Miocene) includes gypsum massive and marl. Uniaxial compressive strength (UCS) and tensile strength of gypsum section of bedrock is respectively 16.39 and 1.5 MPa. At least 4 joint sets are expanded in the bedrock. The source of the filling materials of the joints is eroded marl units of Gachsaran Formation and lake sediments. The dispersion of joints filling material is approved by Crumb, dual hydrometer and chemical tests. The dual consolidation test showed that the covering sediments of bed rock are collapsible. Pezometric data show that the average depth of groundwater in the past 10 years has loss about 3 m and lies at a depth of 20 m at ground level now. Field observations show less than 32 sinkholes in the study area
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