ABSTRACT: The Nukhul Formation crops out in west-central Sinai (Egypt), but its age is inadequately resolved due to the scarcity of diagnostic planktonic foraminifera and calcareous nannofossil taxa, and the common occurrence of reworked specimens. The detailed analysis of the calcareous nannofossil content of one section measured in Wadi Baba, located in west-central Sinai, allows identification of 41 calcareous nannofossil species belonging to 18 different genera. Our results indicate that this section spans from Zone NP25 (Sphenolithus ciperoensis) to Zone NN2 (Discoaster druggii), suggesting a Chattian and Aquitanian ages for the Nukhul Formation in the studied section. Eleven bioevents are recognized throughout the studied section which have improved the biostratigraphic resolution attainable using standard zonation. In particular, the following biohorizons are found in the lower part of the succession, within Zone NP25 (Sphenolithus ciperoensis): LCO of Sphenolithus ciperoensis, LO of Chiasmolithus altus, LCO of Cyclicargolithus abisectus, LO of Reticulofenestra hesslandii, LO of Reticulofenestra bisecta, LO of Zygrhablithus bijugatus, FO of Triquetrorhabdulus carinatus, FO of Helicosphaera truempyi, FO of Helicosphaera granulata, LO of Pontosphaera enormis and LO of Calcidiscus gallagheri). Two additional bioevents were detected in the upper part of the Nukhul Formation within Zone NN2: FO of Discoaster druggii and FO of Helicosphaera ampliaperta). These data are consistent with the hypothesis that the Suez Rift System was started in the late Oligocene, as the deposition of the Nukhul Formation was started due to the opening of the Gulf basin during the Chattian. A prominent unconformity surface is traceable near the top of the Nukhul Formation at Wadi Baba section that indicates a hiatus of ca. 2.7 Myr based on the biostratigraphic data.
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