Bathymetric Survey and Ground-Truthing of the Al-Kharrar Lagoon, Red Sea Coast of Saudi Arabia

SGS-TR-2008-8 Najeeb M.A. Rasul, Mansour I. Abushosha, Omar A. Al-Hazmi, Salem M. Al-Nomani, Fares Z. Bahareth, Nawaf A. Widinly, Rami R. Akbar, Abdulnasser S. Qutub, and Radwan Al-Farawati
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The Al-Kharrar Lagoon is mostly covered with fine sediments (mud to sandy mud) except at the entrance, where coarser sediments (sand to gravelly sand) dominate.  Mangrove (Avicennia marine) stands are scattered around the islands that trap finer sediments, whereas various species of corals are common in the lagoon.  The northerly and northwesterly winds influence the movement of water and fine sediments in the lagoon to the adjacent sabkhas in the south, particularly during spring tides. Strong tidal currents as strong as 1m/sec prevail at the entrance of the lagoon, which control the bathymetry particularly at the mouth, and the winnowing of the fine sediments and their transportation to the margins where they settle down forming levee like structures.  Detrital sediments are transported into the lagoon by the wadis during sporadic rainfall, whereas frequent dust storms contribute sufficient quartz.  Two sedimentary environments have been recognized based on the dispersal pattern: i) the finer terrigenous environment, and ii) the coarser biogenic environment.  The mineralogical composition of the bottom sediments is relatively uniform, whereas the minor variation in the mineralogical make-up is caused by the mixing from the local igneous-metamorphic source rocks and the environment of deposition.  The sediment veneer has two prominent sources: biogenic and aeolian that owes their distribution to the transit mechanism and the reworking process.  However, a third but a minor source is also envisioned - the evaporite deposits from the adjoining sabkhas. The calcium carbonate is abundant in the form of aragonite and high - Mg calcite.  The clay minerals are derived from the wadis during flash floods and are possibly transported by the wind.  Iron incorporated in fine sediments is dispersed into the lagoon by runoff from the wadis that have igneous-metamorphic rock exposures. Generally, with an increase in calcium carbonate the total organic carbon decreases.  The sediments are also stained gray-black because of a reducing environment, especially close to the mangrove stands. The total organic carbon has a positive correlation with the sediment texture and sediment color, i.e. in finer and dark colored sediment total organic carbon is higher because finer sediment preserves organic matter well.  The sources of total organic carbon are the organic debris.  There is an abundant input of trace metals in the Al-Kharrar Lagoon, possibly from the wadis.  The fine sediments tend to show higher values because of the tendency to absorb the metals more efficiently.



Rasul, N.M.A.,  Abushosha, M.I., Al-Hazmi, O.A., Al-Nomani, S.M., Bahareth, F.Z., Widinly, N.A., Qutub, A.S. and Farawati, R.K., 2010,  Bathymetric survey and ground-truthing of the Al-Kharrar lagoon, Red Sea coast of Saudi Arabia: Saudi Geological Survey Technical Report SGS-TR-2008-8, 77 p., 51 figs., 14 tables, 11 apps.