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Though quantitative statistical analysis of skeletal carbonate sediments is under a cloud of uncertainty, the results thereof, in conjunction with copious field observations, elucidate the causes of sediment distribution in and adjacent to the Mombasa, Kenya, reef/platform complex and aid definition of hydrodynamic and ecologic environments. The recent surface sediments have been analyzed to determine their textural and genetic composition. They are made up of two components: biogenic carbonate material (molluscan debris, coralgal, Halimeda and Foraminifera) and fluvio-terrigenous quartz (from the pre-Quaternary of the immediate hinterland).
The fringing reef is divisible into a northern and southern sector by the Ras Iwa Tine promontory. Four sediment populations are present which are distinct in skeletal origin, textural composition, and position on the reef. Sediment samples close to the berm abound in Halimeda fragments, whereas the carbonate fraction on the outer platform (1 km away) and the channel are dominated by molluscan fragments.
Size distribution and statistical analysis of the sediments indicate contrasting physical environments on either side of the Ras, even though geomorphologically they are similar. Sediments in the southern sector are generally better sorted (moderate to good), nearly symmetrically to slightly coarsely skewed, and unimodal, whereas the sediments in the northern sector are badly sorted, coarsely skewed, and bimodal in the shallower lagoonal area.
The distribution patterns reflect the physical oceanographic parameters within the reef, the areal coverage by vegetation, and the sediment source. Pure populations having the best sorting values are on the beach and within the southern lagoon. Sediments finer than 4^phgr are scarce within the reef.
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