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

normally sorted, with median diameters ranging between 0.015 and 0.78 mm. However, samplesnear the entrance of the Gulf of Aqaba are generally fine-grained. There is also a remarkabledifference in texture between the sediments of the Gulf of Suez and the Gulf of Aqaba.SHUKRI and HIGAZY showed that the texture controls the distribution of some chemicalconstituents; organic matter (0.27%- 1.7%; MOHAMED, 1949) is concentrated in the finer fractions,whereas carbonates (50%-95%; MOHAMED, 1949) are concentrated in the coarse fractions. Waterdepth also controls the sediment distribution in the northern Red Sea, with increasing of depthassociated with a decrease in median diameter; sediments tend to be better sorted in the shallow areas(SHUKRI and HIGAZY, 1944a).The mineralogy of the northern Red Sea sediments was studied by SHUKRI and HIGAZY(1944b), SAID (1951) and MILLIMAN et al. (1969). SHUKRI and HIGAZY (1944b) found 38types of heavy and light minerals in the sand fractions. The light minerals are composed essentiallyof angular quartz indicating a crystalline rock source. Next in abundance are muscovite and thefeldspars. Heavy fraction minerals are dominated by iron minerals (ilmenite, magnetite, hematite andlimonite). Pyrite averages about 43% of the heavy residues of the Gulf of Suez but is less dominantin the northern Red Sea. The grains are rounded and range between 13 and 61% of the total heavyfractions. Biotite ranges between 2 and 58% and occurs in dark brown and green varieties.Amphiboles are represented by hornblende and actinolite, and range between 5 and 48%. Bothmonoclinic (augite, diopside and diallage) and rhombic pyroxenes (hyperthene, enstatite) are found;their total relative frequencies vary from 2 to 30%. Epidote is represented by pistachite, zoisite andclinozoisite; its total frequency varies from 1 to 10%. Minor occurrences of chlorite, tourmaline,sphene, apatite, zircon, rutile, garnet, staurolite, kyanite, fluorite, glauconite, spinel, olivine, anataseand rock fragments are recorded. The distribution of minerals in the northern Red Sea sedimentsalong the different sections is shown in Figure 4.The minerals of the Red Sea surface sediments are mainly derived from crystalline rocks.According to SHUKRI and HIGAZY (1944b), the mineral assemblage of a sediment depends onthree main factors: a) Nature of distributive rocks; b) Type of weathering and mode of transportation;and c) Post-depositional processes, leading to simplification of detrital grains and increase ofauthigenic minerals.The detrital minerals are transported to the Red Sea mainly by wind, though heavy rains andwaves undoubtedly contribute smaller amounts. The mineral constituents of the sediments areuniform in all parts of the sea and they all display the same general character, whether collected fromthe southern, middle or northern portions of the Red Sea, Local conditions, however, give rise tominor variations in the frequencies or types of minerals.The carbonate mineralogy of the sediments in the northern Red Sea shows that aragonite isthe most common mineral and is definitely of organic origin; calcite is also present as well as minoramounts of dolomite-ankerite mineral group (SHUKRI and HIGAZY, 1944b). On the other hand,the finer calcareous fraction is mainly calcitic, with very little aragonite. SAID (1951) mentioned thatthe coarser fractions, from which some of the fine fractions is derived, consist mainly of aragonite.In neither of these studies was magnesian calcite distinguished from calcite.The carbonate mineralogy of the northern Red Sea sediments might indicate two differentsources of organic fractions, one is the nannofossils and benthic foraminifera (mostly calcite) and theother is the coarse coral remains (mostly aragonite). According to MILLIMAN et al. (1969) thedeep-sea sediments of the Red Sea have the unique presence of lithified carbonate layers, which aremostly aragonitic. GEVIRTZ and FRIEDMAN (1966) estimated that 11% of the Red Sea sedimentsare lithified and aragonite is the dominant mineral. Aragonite is dominant in the coarse fraction, whilemagnesian calcite becomes dominant in the fine fractions (MILLIMAN et al., 1969). This variationof carbonate minerals relative to the size agrees with earlier observations of SAID (195 1).REEFAL SEDIMENTS IN THE NORTHERN RED SEAOne of the major contributors to nearshore and shelf sediments in the Red Sea are coral reefs.SAID (1951) reported that recent Red Sea sediments are generally coral muds and that the coarserfractions of these sediments are composed largely of coral fragments. MERGNER (1984) defined theRed Sea as a "coral sea" regarding its topography, climate and hydrography, and throughout its343