Writers : Seyed- Emami, K.
Refference : Geosciences Scientific Quarterly Journal,No.:21-22,P.:50
Publishing Year : 1375

Abstract :
Middle Cretaceous Sedimentary rocks are widely distributed in Central Iran. They constitute a more or less continuos sequence, being bounded by two events, one at the base (i.e. late Cimmerian) and the other at the top (i.e. Austrian). The sequence attaining thicknesses of several hundred meters up to bOOm arid more. It can be roughly divided into three Iithostratgaphic units: a basal unit of pink sandstones and conglomerates (? Hauterivianf Barremian), a middle unit of predominantly cliff forming Rudist- Orbitolina - bearing limestones (Barremian- Aptian) and an upper unit of dark glauconitic claystones and shales (upper Aptian- AIbian).
The basal pink and van- coloured sandstones and conglomerates (few meters to 50Gm) transgress everywhere upon an erodeded paleorelief. The latter is established by the late- Cimmerian and or probably, even mid- Cimmerian (Lutian) orogenic movements. Upwards the detrital sequence gradually passes into well- bedded to massive and cliff- forming limestones (Rudist- Orbitolina bearing Iimestones) in Urgonian fades (30Gm to l000rn thick).
The peak of the Urgonian facies in Central Iran is reached during Barremian and early Aptian, where extensive shallow- water carbonate platforms were established on the middle and late Cimmerian Paleorelief.
Many sea- level fluctuations characterize the middle and upper Aptian rocks, which are indicated by several siliciclastic and many intercalations. The Urgonian facies ceases rather abruptly, around the Aptian- Albian boundary. It is followed by dark claystones and calcareous shales (150. 20Gm).
Cenomanian Stage is once again of transgressive nature everywhere. It consists of glauconitic sandstones, limestones and mans, transgressing on a partly eroded Albian or older strata (Austrian event).
Variations in thickness and lithology, with occasional metamorphism and volcanic activity may reflect an active, synsedimentary tectonic (niftings) environment.
The paleobiogeographic relationships of the ammonite faunas allow determination of the paleogeographic position in Central Iran for the Middle Cretaceous.
As a result of prevailing Urgonian fades, in Barremian and Aptian times ammonites are very scarce. The fauna consists of more of less cosmopolitan generas belonging to the families Deshayesitidae, Cheloniceratidae and Parahoplitidae.
In the early Albian the boreal ammonites i.e. Leymeriellidae and Hoplitidae show close relations
to the East Gaspian Area as well as North Europe.
In the middle AIbian, besides cosmopolitan families, such as Brancoceratidae and Lyeiiiceratidae the boreal Hoplitidae are present as wefl. During late Albian besides scarce Hoplitidae, the dominance of the Desmoceratidae and Hamitidae is remarkable. Thus these show clear relationship to the Meditteranean area (Hamites- Beudanticeras Province) of Wiedmann (1988).
Subject List : Paleogeography