The impact of eustasy, tectonics, and paleoclimate on the dolomitization of the syn-rift Neogene carbonate platforms in the Red Sea coastal area, Egypt

Abstract

A very significant quantity of dolomites and dolomitic limestones of variable origins occurs in the syn-rift Neogene carbonateplatforms of the NW Red Sea coast and the Gulf of Suez .These are more common in the Um Diheisi Member of the Ranga Formation (earlyMiocene), the Um Mahara Formation (middle Miocene), the Um Gheig Formation (late Miocene), and the Dashet El Dabaa Memberof 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 beenidentified and were interpreted as follows: 1) Syndepositional stratiform dolomicrite of the Um Diheisi Member occurred in structurallyhalf graben restricted basins favoring structural sites of cyclic peritidal dolomitization that developed in response to sea transgressionand then short-term sea level fall of each cycle period. 2) Regional replacive dolomite of the Um Mahara Formation involved mixingmeteoric, marine, and hypersaline waters associated with the highstand sea level and then the repeated emergence of the platform andthe temporal drowning with restricted marine waters. 3) Mixed syndepositional, replacive, and void-filling cement dolomites of the UmGheig Formation, which suggest three dolomitization events:- i- an early, penecontemporaneous dolomite associated with highstandphases; ii- diagenetic dolomites formed by mixing meteoric and hypersaline reflux waters during lowstands and falling sea level, andiii- 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 playedan important role in the dolomitization of the Neogene sediments. An arid climate prevailed during the dolomite of the Um DiheisiMember and the early dolomitization phases of the Um Gheig Formation. A humid climate predominated in the replacive dolomiteof the Um Mahara Formation, in which the meteoric groundwater allowed the dissolution of precursor limestones and subsequentformation 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.