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Tectono-Stratigraphic Evolution of the Southern Red Sea, Jizan Basin, Saudi Arabia

Abstract

The Red Sea evolved from a Tertiary rift basin that evolved into a passive margin and resulted in the separation of the Arabian plate from Africa. This study utilizes seismic and borehole data to characterize the Jizan area in the southern Red Sea and to shed light on its tectono-sedimentary history The rifting developed half-graben basins that are bounded by NW-SE faults and are filled with Oligo-Miocene deposits. Jizan is a half-graben marginal basin located in the southeastern side of the Red Sea. This basin is bounded by a NE-dipping fault system and is filled with the Jizan Group Oligo-Miocene volcanic and siliciclastic deposits, then blanketed by interbedded siliciclastics and evaporites of the Maqna Group. Continental rifting started during the Oligocene and continued into the Early Miocene Epoch. Seismic data reveals a wedge-shaped geometry of the early basin fill, thickening towards the half-graben-bounding fault, reflecting fault block rotation and synrift sedimentation. Based on field and core studies, the continental rift deposits consist of volcanic basalts and volcanoclastic sediments interbedded with organic-rich shales, coal, siltstone and fine grained sandstones (Jizan Group). Fresh-water algae are common in the shales, whereas sporadic Ostracods and amorphous organic matter appear toward the top of the Jizan Group. These facies record the development of anoxic lakes that were gradually filled and likely evolved into transitional marine environments, while being periodically subjected to volcanic activity. Transverse rivers, similar to modern ones, are inferred to have transferred sediments into the lakes. An analogy can be drawn between these early rift sediments and the present day East African Rift lacustrine and lava flow deposits. Post-rift deposits of the Maqna Group unconformably overlay the Jizan Group. Seismic data reveals stratigraphic architecture indicative of sedimentation during a tectonically quiescent period in a sagging basin. Biostratigraphic data from boreholes penetrating the basin margin suggest a hiatus along the boundary between the two Groups. Cored intervals from the Maqna Group consist of slumps, debrites, turbidites and bioturbated mudstones, reflecting sedimentation in a deep-marine environment. Seismic stratigraphic analysis hints at an axial channel-levee system and shows transverse incised valleys. The latter reach up to 3 kilometers in width and 300 meters in depth, and occur in the shallow parts of the hanging-wall block. The retrogradational stacking of Maqna facies and the gradual upward transition into evaporites reflect backfilling of the valleys, gradual retreat of the Maqna clastic systems and a progressive shift toward an arid climate and hydrological restriction in the basin. The base Maqna unconformity and the intra-Maqna incisions are inferred to have bypassed potential reservoir sands into the deeper parts of the Jizan basin toward the west. Continuous sedimentation is believed to have occurred therein.