Deep-water resedimentation of anhydrite and gypsum deposits in the Middle Miocene (Belayim Formation) of the Red Sea, Egypt

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ABSTRACT

The Middle Miocene evaporites in the Red Sea rift were deposited within a complex system of fault-bounded basins that were episodically active during sedimentation. Such a tectonic framework is known to be highly favourable to resedimentation processes. An offshore petroleum well in the north-western Red Sea has cored, below a massive salt unit, an anhydrite-bearing succession which provides an excellent opportunity to study the processes of gravity induced redeposition of Ca-sulphates in a deep basin. Anhydrite deposits, interbedded with siliciclastic layers and thin halite layers, are composed of resedimented facies ranging from fine-grained laminated sediments to coarse-grained breccias. The components derive from the reworking of shelf sediments deposited initially in shallow water to supratidal settings on the surface and edges of structural highs bordering depressions: proximal siliciclastic deposits with interstitial anhydrite (cement patches, nodules) or gypsum and dolostones with early diagenetic anhydrite facies (nodular, chicken-wire) formed in sabkha conditions, interstitially grown gypsum crystals and subaqueous gypsum crusts precipitated in hypersaline ponds, and diatom-rich oozes formed in marine, shallow-water conditions. The homogeneity of the stable isotope composition and petrography of sulphates argue for the initial crystallization of Ca-sulphates within brines of the same origin and in closely interconnected sedimentary settings. The unconsolidated sediments redeposited as slope-foot accumulations were carried both as anhydrite (nodules, soft masses, various fragments, individual grains or crystals released by disintegration of large masses) and gypsum (crystalline aggregates or single crystals) later converted to anhydrite during burial. Layers of chaotic breccia are interpreted as the result of seismic events, whereas the fine-grained deposits could be related to redistribution by nepheloid layers of suspensions of finer grains released by disintegration of the soft anhydrite masses during downslope transport, or of in situ deposits removed by the turbiditic flows.

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