AAPG Middle East Region, Second EAGE/AAPG Hydrocarbon Seals of the Middle East Workshop

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The Development of Seals and Overpressured Traps in Evaporite Mélanges and Tectonic Rafts Associated with the Opening of the Red Sea

Abstract

Within the Red Sea, the Late Miocene age Maqna Group was deposited as a series of shallow marine sediments consisting of intercalated carbonates, neritic shales, anhydrites and turbidite sandstones. In many areas a prominent unconformity marks the transition between the syn-rift Burqan Formation and the late rift Maqna Group sediments so that the Maqna Group forms a “cap” to the rifted fault block architecture of the Red Sea. After the deposition of the Maqna Group a significant down to the west rotation took place within the Red Sea which resulted in a regionally extensive westward dip to the base of the Maqna Group. During this Middle Miocene period the Red Sea also became separated from the Tethys which resulted in a restricted marine environment and the deposition of the Mansiyah Formation layered evaporites throughout the Red Sea. The existence of a westward dipping unconformity surface at the base of the Maqna Group combined with laterally extensive impermeable units (e.g. anhydrites within the Maqna Group) and the deposition of impermeable layered evaporites on top of the Maqna Group created ideal conditions for raft tectonics and evaporite mélanges. As the Mansiyah evaporites were deposited, de-watering of the underlying Maqna Group strata was probably inhibited by the Mansiyah halite acting as an impermeable top seal. The entire Maqna Group had the potential to become overpressured which would have promoted the development of a basal detachment or decollement on the basal Maqna Group unconformity. Additionally, laterally extensive anhydrite and halite layers within the Maqna Group could have formed internal detachment surfaces. Recent 3D seismic data from the Arabian side of the Red Sea shows that the Maqna Group responded to the development of overpressure and detachments by forming evaporite mélanges (blocks of sedimentary rock entrained within halite) and tectonic rafts.