Özet:

A very significant quantity of dolomites and dolomitic limestones of variable origins occurs in the syn-rift Neogene carbonate platforms of the NW Red Sea coast and the Gulf of Suez. These are more common in the Um Diheisi Member of the Ranga Formation (early Miocene), the Um Mahara Formation (middle Miocene), the Um Gheig Formation (late Miocene), and the Dashet El Dabaa Member of the Shagra Formation (Pliocene). Based on their stratigraphic distribution, petrography, and cathodoluminescent characteristics, as supported by geochemical data, syn-rift irregular morphotectonic relief, and the eustasy, four distinct types of dolomites have been identified and were interpreted as follows: 1) Syndepositional stratiform dolomicrite of the Um Diheisi Member occurred in structurally half graben restricted basins favoring structural sites of cyclic peritidal dolomitization that developed in response to sea transgression and then short-term sea level fall of each cycle period. 2) Regional replacive dolomite of the Um Mahara Formation involved mixing meteoric, marine, and hypersaline waters associated with the highstand sea level and then the repeated emergence of the platform and the temporal drowning with restricted marine waters. 3) Mixed syndepositional, replacive, and void-filling cement dolomites of the Um Gheig Formation, which suggest three dolomitization events:- i- an early, penecontemporaneous dolomite associated with highstand phases; ii- diagenetic dolomites formed by mixing meteoric and hypersaline reflux waters during lowstands and falling sea level, and iii- the latest dolomitization phase that occurred as dolomite cement involving hydrothermal fluids during the exposure periods. 4) Replacive dolomite of the Dashet El Dabaa Member, which involved mixtures of hypersaline and marine waters. Paleoclimate played an important role in the dolomitization of the Neogene sediments. An arid climate prevailed during the dolomite of the Um Diheisi Member and the early dolomitization phases of the Um Gheig Formation. A humid climate predominated in the replacive dolomite of the Um Mahara Formation, in which the meteoric groundwater allowed the dissolution of precursor limestones and subsequent formation of extensive bodies of dolomites. Arid conditions also prevailed during the dolomitization of the Dashet El Dabaa Member, as evidenced by the lack of dissolution and the abundance of well-preserved unstable feldspars.

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