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Growth Anatomy of a Modern Rift Basin Carbonate Platform (Al Wahj, Red Sea): Interplay of Rift Faulting, Salt Tectonics, Eustacy, and Climate


Carbonates deposited along rift settings contain important hydrocarbon reservoirs in places such as offshore Brazil, Angola, Norway, as well as the Red Sea where rifting is still ongoing presently. Carbonate platforms along the Red Sea shore may hence serve a modern analogue to elucidate the impact of a complex interplay between rift faulting, salt tectonics, eustacy and climate on platform architecture and reservoir distribution. The Al Wahj (AW) carbonate platform is with a length and width of about 60 km by 30 km, the largest attached carbonate platform found at the eastern margin of the Red Sea rift. It is located right in front of the largest drainage system of the Arabian shield covering approx. 42% of all the drainage area. Evaluation of satellite images in combination with a two week field mapping during July 2018, and dating of carbonate reefs and coastal clastic establishes; a) AW platform is delineated by a raised rim consisting of reefs, small internal lagoons and carbonate islands, b) two coral terraces 3-4 m apart at AW coastline, and c) gravel terraces with varying grain size (boulder to sand size) adjacent to AW coastline. While the location of the outer rim is determined by faults and half grabens which initiated during Late Oligocene Rifting, its existence as a carbonate growth feature is likely related to levees associated with a siliciclastic fan that was drawn out to sea during a period of sea-level lowstand and wet climate. Evidence from other localities along Red Sea coastline demonstrates that carbonates have established directly on top of siliciclastics fan deltas in the present as well as recent past (Pleistocene). The gravel terraces exposed onshore containing bolder-sized sediments were derived from the raised basement of pre-Cambrian to Mesozoic sedimentary cover. Several chains of reefs and islands within the lagoon and associated siliciclastic terraces and wadi incisions on land suggests that this theme was repeated several times with siliciclastics being drawn into the lagoon during lowstands and carbonates developing on the levees of the fans during transgression and subsequent highstand. However, movement of siliciclastics into the lagoon occurs not only during sea-level lowstands but may also be triggered by wet climate periods during highstands. This is evidenced by a stranded belt of recent reefs found at the current beach line and essentially at sea level. The reefs appear to have been smothered by the influx of siliciclastics during a wet climate period which affected the hinterland recently with sea-level already near present day height. A final puzzle peace of the assemblage are several isolated carbonate islands found in the AW lagoon which are not part of the levee chains. They are interpreted to have been initiated by the movement of Miocene Rift basin salt which underlies the platform sequences.