At the boundary between Jurassic and Cretaceous, because of Late Kimmerian movements, the sea ragressed from many areas of North and Cantral Iran, and many continental areas rose,
which included a number of differentiated basins. The boundary between Jurassic and Cretaceous in these parts of Iran is usually marked by a big diastem or by red, clastic and evaporitic sediments. The whole or part of which may be of Neocomian age. Usually. there is also a sharp uncomformity between the Orbitolina limestone and the older formations.
Continuous sedimentation from Jurassic te Cretaceous and marine Sediments of Neocomian age were, until recently, known only in south - southwest ( Zagros) and Northeast (Kopet
Dagh) Iran. New discovries have been made in the western Alborz (Talesh) and southern Sabzewar areas.
Early Kimmerian movements from the late Middle Triassic onwards separated the sedimentary area of south and southwest Iran from that of Central and North Iran- Both of these regions then took different lines of development tectonically and lithologically. The sedimentary basin of Kopet- Dagh has much in common with that of the Zagros.In Barremian and Aptian times an extensive transgression of the sea took place. This may have been one of the biggest in the Iranian Cretaceous. At that time a shallow sea existed over almost the whole of Iranian territory, and uniform Orbitolina limestones were everywhere deposited. The Orbitolina limestones are usually followed by a shaly and ammonitiferous sequence which belongs mostly to the Albian and which has a wide distribution in Northeast , Southwest and Central Iran- Sediments of this age have not been proved with certainty in the Alborz , and possibly they have been eroded as a result of the tectonic movements which took place at the end of the Albian.
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