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Carbonate and Siliciclastic Clinothem Cycles and Their Relation With Evaporite Deposition, El Qaa Fault Block, Suez Rift, Egypt


The occurrence of mixed carbonate-siliciclastic-evaporite sedimentary systems is a common feature in many rift basins, yet their depositional controls remain poorly understood. The intercalation of evaporite bodies in carbonate and siliciclastic clinothem cycles are studied from Miocene exposures from the El Qaa Fault Block, Suez Rift, Egypt. Their facies distribution, rock body geometry and the depositional environment are analyzed in the context of an extensional basin. The observations made for the El Qaa case are expected to be relevant to other basins, especially as a potential analogue to marginal marine and hyper-saline rift basin successions (e.g. sub-salt hydrocarbon plays in the South Atlantic). The El Qaa Fault Block is characterized by the deposition of a deltaic complex derived from the eastern rift shoulder that prograded across the half-graben in a W to WNW direction. Clinoform height varied in time and space from 15-40 m in the oldest deposits closer to the fault border up to 120 m in the youngest westernmost lobes. Deltaic lobes are conglomerate- and sand- dominated. The occurrence of carbonate facies is typified by coral and algal bodies occupying the topsets and reworked bioclastic facies extending into the foresets of the lobes. Evaporites are found at the bottomsets of the youngest deltaic lobe units. They are exposed as massive anhydrite and gypsum beds up to 30 cm thick intercalated with mudstones that amalgamate together in distal direction, composing 5 to 40 m thick evaporite bodies which are several kilometers in lateral extent. The alternation of siliciclastic and carbonate facies defines progradational-aggradational cycles within the deltaic system that can be tracked into the evaporite beds. No evidences of topset subaerial exposure and incision are found, possibly indicating the subaqueous deposition of the evaporite units. The development of these cycles is associated to the deformation and subsidence of the hangingwall block caused by the extensional activity at the border fault system. The sedimentary and structural controls over this mixed depositional system responded not only to the local conditions of the El Qaa Fault Block realm, but were also related to the complex history of oceanic connections experienced by the Suez Rift through the Red Sea basin and Mediterranean Sea during the Miocene.