Composition and Characteristic of the Surficial Sediments in the Southern Corniche of Jeddah, Red Sea Coast

This work discusses the composition and characteristic of the surficial sediments in the southern corniche of Jeddah, Saudi Red Sea coast, in an attempt to infer the surficial distribution pattern of minerals and provenance of sediments. Twenty-six superficial sediments samples were collected from backreef and forereef areas and were analyzed for grain size, CaCO3 content, and mineralogy. The textural of grain size range from gravel to mud fraction. The mud-dominated substrates (<63 µm) occur generally in the back-reef area near the shoreline (sheltered area) and in the lagoon. Gravel rich-sediments are mostly found in forereef regions. The highest content of aragonite and Mg-calcite occur in the forereef area, probably because to suitability the forereef region for chemical and biochemical precipitation of these minerals. High Mg-calcite and Dolomite are low in both the regions. The pyrite occurs in lagoon; this indicates the reductive conditions in this part. However, on the contrary the percentage of carbonate minerals were low in the backreef-flat area, which could be attributed to the supply of non-carbonate terrigenous materials. The terrigenous material contains quartz, k-feldspar, plagioclase and amphibole minerals and are dominant in backreef-flat area with averages of 12.7%, 7.13%, 2.93% and 0.65%, respectively. Their abundance could be attributed to the supply of terrigenous materials by Aeolian deposits and intermittent Wadis.


Int r oduct i on
The southern Corniche of Jeddah (SCJ) is located south of Jeddah City on the w estern side of the Saudi Arabian Red Sea coast (Fig. 1). It is situated in arid and hot a clim ate w ith scarce rainfall and no perennial river runoff.
The shallow w ater environm ent of the SCJ are areas of extensive carbonate production and accumulation as indicated by the occurrence of m any reefal and calcareous deposits. The shallow w ater carbonate deposits com prise m ainly of biogenic sand enriched in calcareous algae, coral debris, skeletons rem ain of m olluscs, and foraminiferal tests (Bahafzallah and El-Askary, 1981;Behairy, 1984, 1986;Basaham , 2009;Al-Dubai, 2011;Bantan and Abu-Zied, 2014). Chem ical and biochem ical carbonate deposits also take place in t he coastal hypersaline shallow areas (W inter et al., 1983;Ellis and M illim an, 1985;Basaham , 2009). W right and Burchette (1996) reported that carbonate deposits are very frequent and consist of biogenic carbonate (detrital bioclastics and non-detrital calcareous organism s), peloids, coated grains (e.g., ooids), aggregates, lithified clastics and m atrix (m ud-grade carbonate).
Other constituents of the bottom sedim ents of the SCJ consist of terrigenous sedim ent. They are lim ited and conveyed by aeolian processes from an adjacent area that are covered by old alluvium and fan deposits derived from the nearby m etam orphosed sedim ents and basic volcanics of the Jeddah Group (El-Sabrouti, 1983). Durgaprasada Rao and Behairy (1982) mentioned that aeolian transport is a m ajor supplier w ith a m axim um of 0.06 g m-2 day-1 of the m odern non-biogenic nearshore sedim ents of the Saudi Red Sea coast. M oreover, terrigenous sediments are supplied by an interm ittent w adis (ephem eral rivers) in the southern part of the SCJ, especially during the rainy seasons (M orcos, 1970). How ever, influx from the w adis has little im portance since m ost of the sedim ents transported by w adis occasional not reach directly to the shoreline and m ay rem ain trapped in the neighboring coastal plains. Terrigenous particles are also introduced by tidal m ovem ents (Phleger, 1969) or by w ave erosion of old reefal lim estone t erraces that occupying the coastal plain (Durgaprasada and Behairy, 1982).
Therefore, the present study is intended to investigate of characteristics and distribution of m inerals pattern in the bott om sedim ents of the forereef and backreef regions of the SCJ. The The southern Corniche of Jeddah (SCJ) is located ~10 km south of Jeddah City on the eastern side of the Saudi Arabian Red Sea coast (Fig. 1). It extends to a length of 20 km south of the Jeddah City. The present study deals only w ith the m arine coastal (backreef) area of the SCJ that occurs betw een Jazirt Ghurab and Al-Sarum Guard Station, w hich is located betw een latitudes 21 o 10` to 21 o 20` N and longitudes 39 o 10` to 39 o 06` E ( Fig. 1). It occupies a total area of approxim ately 43 km 2 w ith length of 19.5 km and extends into the sea of ~1.5 km w ide. It is divided by the breaker zone (coral reef crest) into back-reef region (a narrow coastal strip) including tidal flats and Ghurab lagoon. The w idth of back-reef area varies considerably but w ith an average of 500 m . The bottom topography (bathymetry) of the study area is variable due to the dominance of coral reef comm unities (Fig. 1). W ater depth of the back-reef area varies from 0.0 (shoreline) to 2 m , w hereas w ater depth of the forereef area reaches 60 m . After the reef crest (breaker zone), the sea bottom descends rapidly reaching m axim um depths (60m ) in the study area (Fig. 1). The central part of SCJ, especially at Al-Budhai area is covered by m angrove trees (Fig. 1). The eastern side is bordered by old reefal lim estone terraces (0.5-1m in height), covering a vast area from the shore to about 10 km inland (Basaham , 2004). These terraces w ere probably form ed during the latest Pleistocene high sea level stand (Skipw ith, 1973). This old reefal lim estone overlies a bed of conglom erate w ith argillaceous cem ent, w hich is in turn underlain by basalt s of the trap series (El-Sabrouti, 1983).

Sam ples and t echni ques of st udy
This w ork is based on data from 26 superficial sedim ents sam ples. They w ere collected from the backreef and forereef of the SCJ, using a stainless steel Van Veen grab sam pler (HYDRO BIOS KIEL) on board of fishing boat and by hand in the shallow er areas. The coordinates of the sedim ent sam ples (  The m ineralogy w as determ ined in the bulk pow dered sub-sam ples that less t han 0.063 m m by X-ray diffraction (XRD) using a Shim adzu 6000 Xray generator. The sub-sam ples w ere placed on glass holders and scanned from 2 to 60o 2θ at a speed of 2o/m inute using a Cu-K α _ radiation tube, Ni filter and a voltage of 30 kV and 20 mA. The relative percentages of the various minerals in pow dered sub-sam ples w ere determ ined by m easuring the heights of the m ain reflections according of M illiman (1974) and Tucker (1988). Graphic grain size param eters w ere det erm ined according to Folk`s classification (Folk, 1980) after m echanical sieving of backreef and forereef sedim ents.
Calcium carbonate (CaCO3) concentrations w ere determ ined by treating a know n w eight of each sub-sam ple w ith an appropriate volume of cold dilute hydrochloric acid (1M HCl) using a Calcim eter (W IKA) w ith an accuracy of ± 0.25%.

Sedi m ent gr ai n si ze
The raw w eights for each grain-size class w ere converted to w eight percent for each sub -sam ple and classification to four sizes m ud (silt and clay), coarse sand, fine sand and gravel. The m edian, standard error, standard deviation and m ean of the grain size for each sub-sam ple w ere calculated using SPSS v.11.5/2002 for (Window s-based softw are). Contour m aps w ere plotted for the sedim ents and m inerals using Rem ote Sensing (IM AGINE 8.7), ArcGIS 9 and SURFER v.8 (Golden Softw are). Triangle diagram of distribution of gravel, sand and m ud w ere plotted using Golden Softw are Grapher 5 Softw are.

Result s Sediment characteristics and substrate types
The coarse sand fraction (150 µm to 2 mm ) predominates in the northern part of the studied area, representing about 53% of the total sedim ent fraction (Fig. 2). The fine sand fraction (63-150 µm) predominates in the lagoon and the southern part of the studied areas, representing about 25% of the total sedim ent fraction (Fig. 2). The m ud-dominated substrates (<63 µm) occur generally in t he back-reef area near the shoreline (sheltered area) and in the lagoon, representing about 5% of the total sedim ent fraction (Fig.2). How ever, gravel rich-sedim ents are m ostly found in forereef regions such as that of Al-Budhai area w it h an average of 17% (Fig. 2). In general, the sand and gravel sedim ents dominate studied part of the SCJ. M icroscopic studies indicate that the sand sedim ent (>63 µm) is com posed m ainly of bioclastic m aterials, such as foraminiferal tests, ostracod carapaces, pelecypod and gastropod shells, sponges, tubes of polychaetes, corals debris and coralline algae. Lithic particles of sand are relatively few . The bottom sedim ents of the lagoon and those occurring near the sew age outlet are characterized by having grey to blackish colors. According to Folk`s classification m ethod (Folk, 1980), seven textural classes are identified (Fig. 3). M ost of the sam ples are generally vary betw een sand to gravelly sand, w hile few sam ples w ere m uddy sand. Overall, the backreef zone of the SCJ is generally flat and covered by thin layer of soft sedim ents overlying hard substrates.

Calci um car bonat e
The bottom sedim ents of the SCJ consist m ainly of calcium carbonate (CaCO 3 ). Their contents vary from 59 to 99%, w ith a m ean value of 84.5%. The high content of CaCO 3 occurs in forereef area, the low content (59%) of CaCO 3 occurs in the lagoon (Fig. 4). The calcium carbonate in the bottom sedim ents of the Al-Budhai area ranges from 73 to 94%. The highest value of CaCO 3 in the bottom surface sedim ents of the SCJ is usually linked w ith the coarse-grained sedim ents (Fig. 4).

M i ner al com posi t ion
The m ineralogy of the studied sedim ent sam ples from the forereef and backreef-flat region of the SCJ is characterized by the existence of carbonates m inerals (e.g., aragonite, Mg-calcite, calcite and dolomite), plagioclase, am phibole, K-feldspar, quartz, pyrite and halite (Table 1). Overall, carbonate m inerals represent m ore than 70% of the bottom surface sedim ents.
Sam ples from the forereef area show higher aragonite and higher M g-calcite content than t he sam ples from the backreef-flat. The average percentage of aragonite ranges from 37.47 to 79.85%, and the highest percentage of aragonite occur in the forereef part and decreases tow ards the shoreline of the studied area. Mg-calcite content varies from 9.69% to 32.96 w ith an average of 18.31%. Dolom ite occurs in considerable am ounts; especially in the surface sedim ent sam ples w hich collected from backreef-flat. How ever, dolom ite and calcite occur in minor content, especially in the sam ples collected from backreef-flat and shoreline, w ith averages of 0.87 and 1.08%, respectively ( Table  1).
The non-carbonate m ineralogy of the bulk surface bottom sedim ents is dominated by terrigenous quartz K-feldspar, plagioclase and am phibole m inerals. They occur in considerable am ount, especially in the surface sedim ent sam ples w hich collected from backreef-flat and shoreline (Table 1). It is apparent that quartz and K-feldspar are m ore in abundance in the backreef-flat part and shoreline, especially in Al-Budhai area (Fig. 5) w ith average am ounts of 12.71% and 7.13% respectively; w hile plagioclase and amphibole m inerals are abundant along the northern part of the studied area.
Halite occurs in considerable am ounts in m ost of the studied sam ples, it constitutes about 4.42% of the w hole sedim ents of the SCJ. In som e sam ples, pyrite is present in lesser am ounts, especially in the northern part in Ghurab lagoon. It form s 0.34% of the bulk m ineralogy of the sam ples. Therefore, it can be noted that the SCJ sedim ents are composed m ainly of aragonite, M g-calcite, quartz and Kfeldspar.

Di scussi on and Conclusi ons
The studied area of the SCJ can be divided into tw o m ajor zones, nam ely: (1) The backreef-flat zone, w hich is considered as sheltered-low energy zone and (2) the forereef zone, w hich is considered as an open sea. The superficial sedim ents of the backreef-flat of SCJ are m uddy sand and consist m ainly of carbonate and terrigenous m inerals. Carbonate m inerals are represent ed by M g-calcite, calcite, aragonite, and dolomite, w hile terrigenous m ineral is represent ed by quartz, K-feldspar, plagioclase and am phibole. M oreover, the presence of halite and pyrite m inerals in sedim ents of the backreef -flat zone is also noticed.
The m ineralogy of the backreef-flat zone show s significant variations in their distribution and com positions, in the northern part of the studied area especially in lagoon, it contains som e pyrite m inerals that could be attributed to the occurrence of reducing conditions in this lagoon due to dominance of organic m atters.
Terrigenous m aterials in backreef-flat are dominated by quartz and K-feldspar and low abundance of plagioclase and am phibole; these m inerals could be attributed to the nature of the source rocks, such as igneous and volcanic basalts rocks, w hich form ed the bordering mountains of the Red Sea, and apparently transported to backreef-flat area by interm ittent w adi stream and w ind. M any studies indicated that these m inerals are intim ately associated w ith terrigenous deposition and introduced to sea-m arginal flat environm ents in the Red Sea by w inds (Sneh andFriedm an, 1985 andPhleger, 1969). W adi Fatim a is dry interm ittent stream and flow s during the rainy season in the m ain w adi and the southern part of the SCJ. The abundance and supplies of the terrigenous m aterials in the backreef-flat area could be responsible for the low abundance of aragonite and M g-calcite. This low average carbonate m inerals in backreef-flat are probably due to dilution by terrigenous input w hich transported by w ind and seasonal freshw ater discharge to the coastal area (Khalaf and Ala, 1980). How ever, this dilution by the terrigenous material gradually decreases tow ards the forereef region. M oreover, inorganic precipitation of carbonate deposits (carbonate m inerals) in seaw ater is inhibited by the presence of dissolved organic and inorganic chem icals (Suess 1973;Rushdi et al. 1992). The relative abundance of the halite w as close to the shoreline; especially in shallow w at er sheltered area could be attributed to high rates of evaporation, and very low precipitation and runoff. The concentration of pyrite m ineral in backreefflat area, especially in Ghurab lagoon could be attributed to the early diagentic processes that occur as response to the reducing environm ental conditions. Basaham (1998) reported that pyrite in the surface sedim ents of Al-Arbaeen lagoon w as a product of early diagenetic reactions in highly reducing environm ent. The recent superficial sedim ents in forereef area of the SCJ are gravely sand and consist m ainly of carbonate m aterials, w hile the terrigenous m atters (e.g., quartz, K-feldspar, plagioclase and am phibole) are rare in this region. The high value of aragonite and high magnesium calcite in forereef zone sedim ents could be attributed to high degree of saturation of seaw ater w ith respect to calcium carbonate (Rushdi et al., 1992). It is noted that aragonite and M g-calcite form ed the m ajority am ong other carbonate m inerals in forereef region, w here it has been show n that aragonite is kinetically favored to precipitate in a solution w ith m agnesium -to-calcium concentration ratio m ore than 4 such as in seaw at er (Kitano 1964;Rushdi 1993;Rushdi et al. 1992).
Based on the previous discussion, it is suggested that the m ineralogical com positions of surface sedim ents of the SCJ are derived from the follow ing sources: Fluvial deposits w hich discharged in the southern part of the SCJ by W adi Fatim a, chem ical and biochem ical precipitation from the sea w ater, and eroded the reefal terraces on the shoreline and the reef crest (breaker zone). The low value of aragonite and M g-calcite in backreef-flat sedim ents could be attributed to supply of the terrigenous m aterials. On other hand, the abundance of aragonite and M g-calcite in forereef area, probably due to the high levels of seaw ater saturation w ith respect calcium carbonate and the decline of the terrigenous inputs.