139736eo.pdf (20MB) - Japan Oceanographic Data Center

On the African side, Precambrian shield occupies about two-thirds the width of the EgyptianDesert. The thin coastal strip between the shield and the Red Sea is occupied by Miocene andyounger sediments (Fig. 1). On the Arabian side, the shield is affected by an enormous shear zone.The shield on the east is covered by extensive Tertiary and Quaternary basaltic lava flows. Towardsthe Red Sea an irregular fault-controlled strip is occupied by Miocene sediments and Quaternaryalluvial sediments and gravels. According to SAID (1969), the Pleistocene deposits of the Red Seacoastal plain include emergent thick sections of alternating massive coral reefs and gravel beds.AKKAD and DARDIER (1966) described four gravel beds along the Egyptian coastal plain. Asimilar succession was recorded along the coastal plain between Jeddah and Yanbu to the north(BEHAIRY, 1983). In addition to the old interbedded coral reefs, there are a series of LatePleistocene reefs on wave-cut terraces of different heights above the modern sea.The Red Sea is unique amongst the seas of the world in that no permanent streams flow intoit; only winds, mostly northwesterly, and torrential rains transport temgenous sediments (SHUKRIand HIGAZY, 1944a). A considerable amount of sediment in the Red Sea may be aeolian, but theactual amount is not known. During summer, northerly winds prevail through the whole Red Sea,while duflng the winter the prevailing wind is only northerly in the northern half of the sea(MORCOS, 1970).RED SEA SEDIMENTSSTOFFERS and ROSS (1974) listed the major sedimentary constituents of the Red Seasediments (LEG 23B Drilling) as follows:Biogenic constituents: Nannofossils, foraminifera, pteropods, siliceous fossils, otherfossils.Volcanogenic constituents: Tuffites, volcanic ash, montmorillonite, cristobalite, zeolites.Terrigenous constituents: Quartz, feldspar, rock fragments, mica, heavy minerals, clayminerals.Authigenic minerals: Sulfide minerals, aragonite, Mg-calcite, protodolomite, dolomite,quartz, chalcedony.Evaporitic minerals: Magnesite, gypsum, anhydrite, halite, polyhalite.Brine precipitates: Fe-montmorillonite, goethite, hematite, siderite, rhodochrosite, pyrite,sphalerite, anhydrite.The great range in composition of Red Sea sediments reflects depositional environment aswell as the source of deriviation. Therefore the summarized list presented by STOFFERS and ROSS(1974) does not necessarily apply to the entire Red Sea. This list also does not include reefalsediments, which are of prime interest as their production is enormously high in the Red Sea.The surficial deposits of the northern Red Sea can be placed into the followingcatagories :1. Normal sediments of the northern Red Sea; 2. Reefal sediments; and 3. Hydrothermal deposits.NORMAL, SEDIMENTS OF THE NORTHERN RED SEAThe recent sediments of the northern Red Sea were described by SAID (1951) as whitish"coral muds" of high calcium carbonate content; these sediments do not possess significant quantitiesof chemically precipitated CaC03. The coarse fractions of these sediments (>20p.) consist largely offragmentary organic remains. On the other hand, MILLIMAN et al. (1969) concluded thatinorganically precipitated carbonates may account for more than half the deep-sea carbonates in thecentral and southern Red Sea. However, the paucity of lithic fragments, and the relatively lowamount of magnesian calcite in cores north of 24"N (e.g. core C61-69G in MILLIMAN et al., 1969)suggest a lower rate of carbonate precipitation in the northern Red Sea. This supports SAID'Sconclusions, but one cannot ignore the minor occurrence of inorganic carbonate sediments in thisregion. Organic carbonate sediments also are found in the Gulf of Suez (MOHAMED, 1980).The texture of northern Red Sea surface sediments (based on fifty samples, Fig. 3), varieswidely SHUKRI and HIGAZY, 1944a). Sediments of the most northern section of the Red Sea are342