Writers : Khadem.Nasrollah
Refference : Geosciences Scientific Quarterly Journal,No.:6,P.:6

Abstract :
An attempt is made here to introduc, a new classification for the alluvial deposits of the Tehran region.At the outset of he Pliocen. sedimentation, the Alborz Range did not possess the high altitude of the present day.lt mostly comprised of gentle topography, undulating valleys with either fine- grained or semi- course grained alluviums, lacking any flood- deposlt.lt seems that, wram, humid Pontlan climate were dominant in the area.Moreover, it appears that it was mostly covered by altered rocks.The latter consists of mainly conglomerate, arenaceous clay and argillaceous silts.
The climate changes at a particular stage in the formation of conglomerate and silt, e.changes occurred in the warm, humid climatic condition, similar to what exists to day. Moreover, black, calcareous shale crops out in the eastern part of the area, while volcanic rocks, such as tuff and agglomerate crops lut in the middle part. This new climatic environment brought about gentle slope and piedamont zone, eliminating clayey material from the rock.However, the presence of faults, joints and fractures coupled with erosive effects on these rocks brought about fine to medium- grained rocks.Following this, scre.s and alluvium in water, constitute more or less angular grains with sandy cement.The latter alluviums display well- bedding, covering the piedmont zone.The contact with the previously mentioned alluvial deposit. i.e.conglomerate and silt which essentionally is the result of faulting, folding and uplifting, as for example occurning In Abbas Abad, Shahrak-e-Gharb, Ghaytarieh and others, is an unconformity.Moreover, the tectonic event during Pliocene, caused folding and faulting in the latter alluviums, which followed by orogenic movements, erosion, and emplacement of Manzarieh igneous body.
Since, no components of the latter intrusion is observed in the old alluvial deposit, It seems that the mentioned rocks were emplaced after the formation of the alluviums.
Due to orogenic movements, resulting In changing topography and alluvial deposit, coarse, heterogeneous sediments of the middle portion of the area under investigation, covers the eroded surface of ancient alldviums.ln other areas, such as Sohanak and Quchak Pass, these alluviums become white, tine grained and unsorted.
Apart from Bagh-e-Faise Valley, all the alluvial deposits so far formed, are devoid of any rocks older than the Eocene. The origin of thier derivation is entirely from Karaj Formation.Thus, it may be concluded that, up to the present stage, the entire area of Alborz under consideration, were more or less covered by the Karaj Formation.Due to thrustings, no pre- Eocene rocks were exposed then.
Due to repeated faultings, in the long run, the above- mentioned alluvial deposits, caused considerable changes in th. topography and along the rivers. The vertical movements of the Alborz,resulted in terrace formation on either sides of the rivers.
Subject List : Alluvial deposites