As so ci ate Ed i tor-Wojciech Granoszewski A di verse Late Oligocene to Early Mio cene cal car e ous nannofossil as sem blage was ex am ined from the Qom For ma tion in the Cen tral Iran Ba sin, and the Oligocene-Mio cene bound ary was iden ti fied based on the quan ti ta tive anal y sis of the as semblages in 303 smear slides. Eleven well-es tab lished cal car e ous nannofossil bio-events are de lin eated in the Up per Oligocene through Lower Mio cene. The re sults clearly show that the High est Oc cur rence (HO) of Sphenolithus delphix is the clos est bio-event to the bound ary as tra di tion ally de lin eated on the lithostratigraphic cri te ria, and pro vides a dis tinct biohorizon be low it. The Low est Oc cur rence (LO) of the spe cies Discoaster druggii is the old est Mio cene bio-event that is observed shortly af ter the HO of S. delphix, show ing that cal car e ous nannofossils are well suited for ap prox i mat ing the Oligocene-Mio cene bound ary in the Qom For ma tion. The Oligocene-Mio cene bound ary is placed in the up per part of Sub-mem ber "c1" in all three sec tions stud ied here and it is trace able through out the Cen tral Iran Ba sin, which makes a poten tially re li able marker ho ri zon for se quence strati graphic and hy dro car bon stud ies in the area.

Arkhangelskiella confusa Arkhangelskiella maastrichtianArkhangelskiella cymbiformis Ahmullerella sp. Ahmullerella alboradiata Ahmullerella regularis Ceratolithoides indiensis Cretarhabdus conicusChiastozygus synquadriperforatus Chiastozygus sp. Cribrocorona echinus Cribrosphaerella ehrenbergii Cylindralithus nudus Eiffelittus turriseiffelii Eiffelittus

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