Biostratigraphy and palaeoecological interpretation of the Miocene–Pleistocene sequence at El-Dabaa, northwestern Egypt

Abstract. Thirteen shallow boreholes were drilled by the Qattara Project Authority (QPA) on the top of the second limestone ridge in the El-Dabaa area, along the Mediterranean coast, northwest Egypt. Five foraminiferal biozones could be recognized in ascending stratigraphic order: the Heterostegina costata, Praeorbulina sicana, Borelis melo melo, Globigerinoides obliquus extremus zones and an un-named zone, embracing the Middle Miocene Marmarica Formation and the Pliocene–Pleistocene Alexandria Formation.

The deposition of the fossiliferous carbonate rocks of the Marmarica Formation probably took place in a warm water, inner shelf environment (0–20 m palaeodepth) subjected to some current activity, and with salinity ranging from normal to slightly hypersaline (35–50‰). The sediments of the oncolitic/shelly rocks were deposited in slightly deeper water than the overlying fossiliferous rocks and in less agitated conditions, as indicated from the high percentages of rotaliids and rarity of miliolids.

The foraminiferal associations in the Alexandria Formation indicate a clear, relatively agitated, near-shore marine depositional environment in a vegetated inner neritic zone (10–25 m palaeodepth) with 25°C surface water isotherms. On the other hand, the accumulation of the clayey sediments and some geosoils (basal part of the Alexandria Formation) occurred in less agitated, intertidal water conditions, as deduced from the occurrence of planktic foraminiferal species as Globigerinoides trilobus trilobus, G. trilobus immaturus, G. obliquus extremus, G. obliquus obliquus, Globorotalia inflata, and Orbulina universa.