Writers : Aghanabati,A.
Refference : Geosciences Scientific Quarterly Journal,No.:19,P.:2
Publishing Year : 1374

Abstract :
The Jurassic deposits of fran can be divided Into two major groups. One displays a complete sedimentary cycle, the deposit of which took place during Late Triassic to Middle Jurassic times, and It is called the Shemshak Group. While the second group or next sedimentary cycle, mostly composed of limestone- marl facies and being called the Megu Group.
Various Field-evidences dearly demonstrates that during Bathonian, accompanied contemporaneously with the Middle Cimmerian tectonic event, the upper part of paralic deposit of Shemshak Group came to an end. Subsequently, a cessation in sedimentation, as well as an erosional cycle took place and once again the Jurassic Sea transgressed a large parts of central Iran with medium depth.
The lithological characteristics, as well as the superposition of the two groups mentioned above, heralds a quiscent condition during their deposition, so that in some localities, the boundery between the Shemshak Group and Megu Group is apparently transitional and continuous. But nevertheless, in other areas, such as central and eastern Alborz uplands, as well as large areas of central Iran, the onset of Bathonian transgression is marked with conglomeratic and or coarse- grained sandstone, and even locally as rather weak unconformity.
The outcrops of this period is widespread in the Alborz, consisting mostly of more or less shaly marl, ammonitiferous many limestone and limestone and are called Dalichai and Lar Formations respectively.
In central Iran, the deposit of the second sedimentary cycle, mamely the Jurassic Megu Group consists of ammonitiferous limestone which are characterized as key- beds, so much so that it may be imtroduced as a new lithostratigraphic unit.
Subject List : Stratigraphy