Gypsum salina-coral reef relationships during the Last Interglacial (Marine Isotopic Stage 5e) on the Egyptian Red Sea coast: a Quaternary analogue for Neogene marginal evaporites?

In the tectonically stable area of the Egyptian coast of the Red Sea, the uppermost marine deposits of the Last Interglacial (Eemian, 5e = 5.5 Marine Isotopic Substage) consist of two contrasting sedimentary units: a subcontinuous fringing coral reef and, on its backward side, shelly and subaqueous gypsum deposited in erosive depressions elongated parallel to the shore. The entrenched geometry and the marine to brackish evolution of the pre-evaporite faunas indicate that during this very short period of time, a brief sea-level drop responsible for the limited erosion separated the succession of reef to marine gypsum and that the coastal area underwent several climatic variations. The marine ingression into drowned valleys (early MIS 5.51) developed khor environments (non-brackish lagoon on the Arabic arid shores) which were quickly replaced by highly restricted conditions leading to gypsum precipitation. Permanence of the salina waters and seasonal lamination interrupting crystal growth suggest an anchialine salina system (littoral haline lakes) fed by permanent seeping of sea-water, balanced by refluxing dense brines. A semi-arid climate is assumed, with seasonal rains allowing limited continental sulphate contribution (from Miocene gypsum outcrops). Both climate and sea-level changes were responsible for the complex evolution of this area. Compiled data enable generalization concerning the climatic oscillations during the Last Interglacial sedimentation. A model derived from these Red Sea Quaternary deposits, as an analogue for the marginal Miocene evaporite deposits in the Mediterranean and Red Sea, is discussed. (C) 2001 Elsevier Science B.V. All rights reserved.