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The Link Between Carbonates, Evaporites and Siliciclastics in Arid Rift Basins; El Qaa Fault Block, Suez Rift, Egypt


Mixed carbonate-evaporite-siliciclastic depositional systems represent a challenge for most of the existing depositional models for rift basins as these models are heavily built on the assumption that deposition occurs under relatively humid climatic conditions. Understanding the fine balance between precipitation and evaporation rates in arid to semi-arid basins becomes crucial in order to correctly evaluate the role of water chemistry in the deposition of carbonates and evaporites or the seasonally controlled ephemeral processes that source terrigenous clasts to the basin. This study focuses on the identification of the main drivers controlling the evolution and interaction between coarse-grained fan deltas and associated coral-algal bodies, a footwall crest carbonate platform and evaporites from Miocene exposures of the El Qaa half-graben, Suez Rift, Sinai Peninsula, Egypt. The outcome of this case study is a depositional model of wide applicability in arid rift basins with hyper-saline and marginal marine successions (e.g. sub-salt hydrocarbon plays in the South Atlantic). A coarse-grained deltaic succession with coral-algal bodies occupying the topset of the 30 to 120 m thick lobes typifies the eastern margin of the El Qaa hal-graben. The deltaic lobes prograde westwards towards the hangingwall dipslope, overlying prodelta deposits characterized by mudstone and evaporite dominated units. Structural derived variations in bathymetry along strike the hangingwall dipslope determine the occurrence of a footwall crest reefal carbonate platform towards the south and subaqueous mudstone-evaporite successions to the north. The evaporite bodies are composed of amalgamated massive anhydrite and gypsum beds. They range in thickness from 5 to 40 m and can extend laterally for several kilometers. Water chemistry was determined by both regional and local controls. On the one hand, the intermittent oceanic connection of the Suez Rift through the Red Sea basin and Mediterranean Sea during the Miocene regulated the composition of sea-water in the basin. On the other hand, hangingwall deformation and syncline growth in El Qaa created local restrictions on the circulation of sea-water within the half-graben, promoting the deposition of the evaporites. Fault growth and hangingwall tilt controlled the availability of accommodation space for the deposition of the deltaic lobes and the footwall crest carbonate platform.