Paleoenvironment and paleobiogeography of the Lower Cretaceous carbonate successions of northern Tethyan margin: Examples from Northeastern and Central Iran

Lower Cretaceous carbonate successions outcrop extensively along the northern Tethyan margin in the Kopet-Dagh (NE Iran) and Yazd Block (Central Iran). These carbonate units are not well constraint in terms of stratigraphic, sedimentological, geochemical, and paleobiogeographic view point. To improve our knowledge, a detailed multidisciplinary study has been presented on two roughly coeval stratigraphic units in the two different areas, and correlated with other tethyan carbonates. The two stratigraphic units (Taft and Tirgan formations) have been deposited along a carbonate shelf with microfacies associations ranging from the outer-shelf to the inner-shelf settings. Diachrony in ages at the base and top of the stratigraphic units suggests the long-term subsidence effects during the Early Cretaceous, followed by a Cimmerian phase of epi-orogenesis, and also the remnant paleoreliefs. New biostratigraphic data coupled with stable isotope δ13C records show that the onset of carbonate production started much earlier than previously thought (Valanginian rather than Barremian). Our results also permit us to correlate the lower Aptian stratigraphic units with the OAE1a. The worldwide distribution of index microfossils including benthic foraminifera and colomiellids reported on paleobiogeographic reconstructions. They have been used to define paleobiogeographic provinces for the Valanginian, Hauterivian and Aptian intervals and show predominant basin connections. Significantly, the allochem content of the lithostratigraphic units and their phosphorus accumulation show phases of important heterozoan to photozoan carbonate production, which are recorded in other localities of the Tethyan realm as the consequence of global paleoceanographic changes.