Lower and middle Eocene ironstone sequences of the Naqb and Qazzun formations from the north-east Bahariya Depression, Western Desert, Egypt, represent a proxy for early Palaeogene climate and sea-level changes. These sequences represent the only Palaeogene economic ooidal ironstone record of the Southern Tethys. These ironstone sequences rest unconformably on three structurally controlled Cenomanian palaeohighs (for example, the Gedida, Harra and Ghorabi mines) and formed on the inner ramp of a carbonate platform. These palaeohighs were exposed and subjected to subaerial lateritic weathering from the Cenomanian to early Eocene. The lower and middle Eocene ironstone sequences consist of quiet water ironstone facies overlain by higher energy ironstone facies. The distribution of low-energy ironstone facies is controlled by depositional relief. These deposits consist of lagoonal, burrow-mottled mud-ironstone and laterally equivalent tidal flat, stromatolitic ironstones. The agitated water ironstone facies consist of shallow subtidal-intertidal nummulitic-ooidal-oncoidal and back-barrier storm-generated fossiliferous ironstones. The formation of these marginal marine sequences was associated with major marine transgressive-regressive megacycles that separated by subaerial exposure and lateritic weathering. The formation of lateritic palaeosols with their characteristic dissolution and reprecipitation features, such as colloform texture and alveolar voids, implies periods of humid and warm climate followed major marine regressions. The formation of the lower to middle Eocene ironstone succession and the associated lateritic palaeosols can be linked to the early Palaeogene global warming and eustatic sea-level changes. The reworking of the middle Eocene palaeosol and the deposition of the upper Eocene phosphate-rich glauconitic sandstones of the overlying Hamra Formation may record the initial stages of the palaeoclimatic transition from greenhouse to icehouse conditions.
ZusammenfassungKarstph~inomene und Kalkkrusten (Caliche) haben sich auf den Oberfl~chen der Gesteine des Mittelmioz~ins im Gebiet zwischen Quseir und Mersa Alam entlang der K~iste des Roten Meeres in )~gypten entwickelt. Eingehende Untersuchungen yon Kalkkrusten Profilen iassen unterschiedliche Strukturen und Gef~ige erkennen, die sowohi zyklische Bildung belegen aIs auch diagenetische Kristallisations-Differentiation und Deformation. Das Anftreten dieser Caliche-Bildungen legt nahe, dab die mittelmioziinen Gesteine im Untersuchnngsgebiet abwechselnd regenreichen Perioden und solchen mit starker Evaporation ausgesetzt waren. AbstractKarstified features and surficial calcareous crust (caliche) develop on the upper most surfaces of the Middle Miocene rocks in the area between Quseir and Mersa Alam along the Red Sea coast, Egypt. Detalied studies of the caliche profile revealed various textures and fabrics denoting a cyclicity in deposition as well as diagenetic differentiation crystallization and deformation. The occurrence of these caIiche deposits suggest that the Middle Miocene rocks in the study area have been subjected to alternating periods of rainfall and intense evaporation. R~sum~Des structures karstiques et des crofites calcaires superficielles (caliches), se sont form&s ~t la surface des roches du Mio&ne moyen dans la r6gion comprise entre Ouseir et Mersa Alam, le long de la c6te de la met Rouge, en Egypte. L'&ude d&aill& de profils de la caliche r&~le des structures varifies qui indiquent un dSp6t cyclique ainsi qu'une cristallisation diff6rentielle et une dSformation diag6n&ique. La prfsence de tels dSp6ts de caliche suggSre que les roches du Miocene moyen de cette rfgion ont &6 soumises alternativement ~t des p6riodes de pluie et d'&aporation intense. KpaTKOe coxep~aHHe
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