Signification hydrologique et climatique des carbonates continentaux quaternaires de la bordure du désert oriental égyptien (côte de la mer Rouge)

The western coast of the Red Sea, S of Quseir, displays important, continental Quaternary deposits (alluvial cones, fluviatile filling of of ancient valleys) associated with Quaternary, uplifted, fringing reefs and with various continental carbonates. The mutual relationships of these formations were influenced by complex tectonic movements (Red Sea rifting), glacio-eustatic oscillations of sea level and climatic changes. - Several sections across fluviatile formations and alluvial cones (Igla, Umm Gheig, Nahari, Khalilat el Bahari) and some additional observations (Ambagi, Greifat, Khalilat el Ghibli, Ghadir, Gemal etc.) were used as a basis for a tentative reconstruction of the palaeogeographical and palaeoclimatological conditions along this coast. The continental carbonate facies are the following: - limestone - cemented sandstones, displaying erosional features and plane and oblique stratification surfaces indicative of long-lived, active, fluviatile channels; - micritic, travertinous limestones with fossil, reed tuffs (reed - limestone) and algal concretions (stromatolites) showing decimetric, karstic cavities filled with speleothems. Silty layers containing Melanoides shells are interbedded with these limestones; - vertical, cylindrical calcareous root - coatings (subcutanic figures, «rhizoconcretions») occur within distinctive alluvial layers (debris flow and braided stream deposits) and are frequently associated with centimetric, limestone-cemented sandstone beds which are parallel to stratification. Moreover, calcrete belonging to the «laminar horizon» type occurs on the surface of emergent reefs of the Last Interglacial. It is interpreted as a surficial desert stromatolite formation rather than a pedological accumulation. In the sedimentary bodies studied, carbonates are an effect of more or less humid climatic conditions, the necessary migration of ions necessitating a minimum of hydrocirculation. Under the present, hyperarid conditions, carbonate cementations are extremely limited (poorly calcareous, centimetric rhizoconcretions around minor roots). Less arid conditions must have prevailed during the precipitation of the formations observed: (1) predominantly arid climate for the laminar horizon (desert stromatolite) developed on the Pleistocene reefs and for the root sleeves in alluvial cone deposits; (2) more humid climate for the cementations of fluviatile sands and for the travertinous, lacustrine limestones (fresh-water stromatolites). The cementation of sediments of the alluvial cones by phreatic waters may occur under any of these climates.