Study of sedimentology of Neogene deposites of Siahkooh and Gachab

Marl formations are widespread in Iran. High potential to erodibility of marl formations in watershed basins of Iran cause many problems including loss of soil quality in agricultural and pasture lands and forests، increasing the risk of landslides and mass movements، reducing the quality of ground and surface water، sediment load increases entered into the reservoirs، desertification areas due to intensive erosion and watershed management plan failure due to inadequate knowledge of the susceptibility to erosion the formations. The study area، Gatchab and Siahkooh Mountains situated in the northwest of central playa of Iran (Kavir plain). It is located in southwest of the Semnan city with 74 km distance. The widespread of the geological unit in the area is upper red formation with three members M1، M2 and M3. With respect to sedimentology finding on textures of Neogene sediments، they are sandy silt، silt، sandy mud and mud particles. The examination of distribution of various particle size of the sediments have shown that these sediments deposited in shallow water and lagoonal environments with changing of the depth، that changed to high dynamic environments condition of channel rivers. The results of analyses have shown that the percent abundance of lime in these sediments change from 24. 28% to 34. 98%. Because of the low contents of lime compound and clay particles، it is better to called them salty and gypsiferous siltstone and mudstone instead the marl term for these sediments.