Miocene coral reefs and reef corals of the south-western Gulf of Suez and north-western Red Sea: distribution, diversity and regional environmental controls
Résumé
Coral reefs developed within the Gulf of Suez-northern Red Sea region during a relatively brief time interval corresponding to the maximum of the Middle Miocene worldwide marine transgression which was also associated with phase of a warm global climate. The reefs occur in mixed carbonate-siliciclastic sequences which belong to the marine Miocene syn-rift unit (Group B or Upper Rudeis-Kareem Formations) and possibly extend from Langhian to early Serravallian age. Studied reef locations extend along the western coast of the Gulf of Suez and north-western Red Sea from north of Hurghada to the Abu Ghusun-Ras Honkorab area. Reef distribution and development within the region appear to be highly controlled by the tectono-sedimentary regional setting of the early rift system. In particular, the structural framework strongly controlled the regional palaeotopography which in turn constrained the transport of siliciclastic sediments and the circulation of regional and local open-marine waters. Coral reefs typically occur both on structural and sedimentary palaeohighs (horsts and fronts of fan deltas, respectively), and preferentially developed on the north-east side of fault blocks facing the open-marine waters of the axial rift zone. Reef growth was generally favoured at locations sheltered from the main siliciclastic supply from the south-west rift shoulder, while transport of terrigenous material was concentrated within palaeo-depressions (half-graben and graben). Coral assemblages appear to be restricted to small-sized fringing reefs, thin biostromes, or shallow marine facies with scattered coral colonies. In most areas, the steep north-east facing footwall slopes seem to have limited reef progradation. The coral fauna shows an entirely Miocene Mediterranean affinity with no apparent faunal overlap with the Miocene reef coral fauna from the neighbouring Indo-Pacific. Material collected from the studied areas includes 16 genera and 27 species, a similar richness to Middle Miocene reef coral faunas from the western Mediterranean. Coral reefs and their coral fauna became definitively extinct within the Gulf of Suez - northern Red Sea region during the Serravallian, as a result of an increasing restriction of marine conditions which eventually led to evaporite sedimentation. This was directly related to the gradual closure of the Suez Isthmus, probably due to both eustatic and tectonic movements, which finally isolated the Gulf of Suez - Red Sea from the Mediterranean basin. Thus coral faunas became extinct in this region much earlier than in the Mediterranean generally, where reef corals occurred until the end of the Miocene.