Early–mid Cretaceous calcareous nannofossils from the central Tethys: biostratigraphy and ecology

Abstract

Abstract The Early–mid Cretaceous Garau and Kazhdumi Formations of the Zagros Basin (western and southwestern Iran) have been studied for calcareous nannofossils and their assemblage composition. The 722 m thick Garau Formation was investigated in the Kabir-Kuh section (northwestern Zagros), the 197 m thick Kazhdumi Formation was studied in the Maghar section (southwestern Zagros). Based on the calcareous nannofossil assemblages, the Garau Formation can be attributed to the Valanginian to Lower Aptian. The sequence yields moderately diverse nannofossil assemblages which are closely related to those of the western Tethys. The assemblages encountered in the current study differ by the absence or rarity of several taxa, which are otherwise common in the western Tethys. On the other hand, some taxa (e.g., Rhagodiscus asper, Lithraphidites carniolensis and small Zeugrhabdotus spp.) are more common than elsewhere in Tethys. Two phases of decline of nannoconids, observed in other parts of the Tethys as well, are reported for the late Barremian and early Aptian. Warm water species like R. asper are quite common, while taxa preferring cool to temperate water like Repagulum parvidentatum are entirely absent. The Kazhdumi Formation spans the interval from the upper Lower Aptian to the Lower Cenomanian. In the Kazhdumi Formation the index taxa Prediscosphaera columnata and Corollithion kennedyi, which are quite rare, are used to define the Aptian–Albian and the Albian–Cenomanian boundary. The Nannoconus truitti acme, known from various sites around the globe (South Atlantic, western Tethys, Boreal Realm), is recorded from the late Aptian.