Calcareous nannofossil and planktonic foraminifera biostratigraphy through the Middle to Late Eocene transition of Fayum area, Western Desert, Egypt

As a result of the occurrence of the nannofossil marker species; Isthmolithus recurvus only in side views below and above the first appearance of Chiasmolithus oamaruensis at both Naalun and Guta section-I, this species is not reliable to define the NP18/NP19 zonal boundary. At Guta section-II, the Middle/Upper Eocene boundary can be delineated by the first appearance of Globigerinatheka semiinvoluta above the first occurrence of Isthmolithus recurvus in both plane and side views.