In the Eocene part of the Siwa Oasis, the larger foraminifera are represented by the genera NummulitesArxina, Operculina, Sphaerogypsina, Asterocyclina, Grzybowskia, Silvestriella, Gaziryina and Discocyclina inorder of abundance. Operculina continues up to the early Oligocene as modern representatives in tropicalregions, while the other genera became extinct. Nevertheless, the most common larger foraminiferalgenus Lepidocyclina (Nephrolepidina) appears only in the lowermost Oligocene
In spite of the Eocene–Oligocene (E/O) transition is thought to have been attended by major continentalcooling at northern middle and high latitudes, we discover that at the Siwa Oasis, there is a clear warmingtrend from the late Eocene (extinction level of Nummulites, Sphaerogypsina, Asterocyclina, Grzybowskia Silvestriella and Discocyclina) to the early Oligocene is observed due to the high abundance ofOperculina and occurrence of kaolinite and gypsiferous shale deposits in both Qatrani and El Qara formations (Oligocene) at this transition. The El Qara Formation is a new rock unit proposed herein for theOligocene (Rupelian age) in the first time.
Several episodes of volcanic activity occurred in Egypt during the Cenozoic. Mid Tertiary volcanicity was widespread and a number of successive volcanic pulses are starting in the late Eocene. The release of mantle CO2 from this very active volcanic episode may have in fact directly caused the warm Eocene–Oligocene greenhouse climate effect