Gulfs of the northern red sea: Depositional settings of abrupt siliciclastic-carbonate transitions

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The two narrow gulfs of the northern Red Sea, Gulf of Suez and Gulf of Aqaba, have had different tectonic histories, but both display active interfingering of siliciclastic and carbonate facies. In an early stage of rifting, these embryonic seas are flanked by rugged mountains (to 2,000 m) and narrow coastal plains (generally <10 km wide) built of alluvial fans composed of poorly sorted siliciclastic debris, mostly from crystalline basement rocks. An arid setting promotes aperiodic transport of siliciclastic sediments as well as deposition of evaporites (coastal sabkhas) and carbonates (reefs and associated sediments). Gulf margins prograde by a combination of rapid fan deposition during flash floods and subsequent carbonate stabilization of terrigenous fans and cones during intervening periods. High-resolution seismic and side-scan sonar data suggest that narrow pathways for sediment transport through the reefs are continually active. They accommodate most of the sediment transport to deep water during small discharge events. Large flash floods may completely overwash carbonates at the distal ends of fans, requiring renewed reef development. Rapid siliciclastic deposition, coupled with biological and chemical binding of carbonates as well as their tendency toward vertical buildups, results in steep slopes along the gulf margins. The Gulf of Suez is shallow (<100 m), and a relatively broad (>12 km) and geometrically complicated strait separates it from the northern Red Sea. In contrast, the Gulf of Aqaba is deep (< 1,800 m) and has a very narrow strait. Although both basins result from rifting associated with opening of the Red Sea, the Gulf of Suez is dominated by normal faults and tilted blocks, whereas the Gulf of Aqaba formed primarily by strike-slip displacements, with minor movements perpendicular to its extension. Seismic and borehole data confirm that the Gulf of Suez is a grabenlike structure that has filled with nearly 6 km of dominantly siliciclastic sediment since Miocene times. An evaporite unit over 1 km thick and numerous thin carbonate horizons, as well as local reef buildups, interfinger with the noncarbonates. New data suggest that local basins within the Gulf of Aqaba have as much fill as the Gulf of Suez. Turbidites and foraminifer muds presently are filling the deepest basins. The Gulf of Suez contains numerous carbonate platforms, which are probably seated on subtle gulf-parallel structures. Some of these features suggest that they are the initial stages of much larger carbonate platforms that will develop as rifting continues. Modern physical processes–strong axial winds (<30 m/sec), an energetic gulf-parallel wave field, and vigorous tidal currents (>50 cm/sec)–tend to streamline reefs and sediment bodies, creating spindle shaped carbonate platforms. The Gulf of Aqaba has no mid-gulf platforms, but a complex of reef-dominated carbonates exists on gulf-normal structural blocks at the Strait of Tiran. A cross-section reduction of this already narrow strait by lowering of sea level, reef growth, and/or sedimentation could drastically change the basin-filling process by increasing salinity to the point of eventual evaporite deposition. © 1988, Elsevier Science & Technology. All rights reserved.

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Developments in Sedimentology

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