, Volume 5, Issue 3, pp 127-150

The modern reef complex, Jeddah area, Red Sea: a facies model for carbonate sedimentation on embryonic passive margins

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The modern reef complex north of Jeddah comprises an offshore knoll platform and a fringing reef, subdivised into several depositional zones: tops and upper flanks of offshore reefs; lower flanks of offshore reefs and nearby inter-reef areas; fringing forereef, reef flat and backreef zones, and beach. Sixty-seven sediment samples were collected from the different zones and have been analysed in order to define relationships between the distribution of sedimentary facies and the depositional environments, and to furnish a reliable facies model by using multivariate analysis. Six types and subtypes have been objectively differentiated on the basis of total biogenic component and foraminiferal associations. Grain size data allowed us to discriminate three textural types, whereas five chemotypes have been recognized according to trace element concentration. Regarding the offshore reef platform, poorly sorted, medium sands of molluscan-coralline algal-Amphistegina and Cd types are restricted to the lower flanks of buildups and to the adjacent inter-reef deposits, whereas the tops and upper flanks of theses buildups are characterized by moderately sorted, coarse sands of coralline algal-Tubipora-Amphistegina-encrusting foraminiferal-bryozoan types, with a Mn chemotype. Concerning the fringing reef system, backreef areas exhibit poorly sorted, fine sands of molluscan-Ammonia-Peneroplis and Fe-Cu types. Moderately sorted, coarse sands of coralgal-Calcarina-Spiroloculina and Fe-Zn types are found on the reef flat. The forereef zone is characterized by poorly sorted, fine sand of Triloculina-encrusting foraminiferal-bryozoan and Zn-Mn types. The lateral limits of the various biotypes roughly coincide with the distribution of the relevant living organic communities. Trace elements appear to be either bound to the reef-associated silicate fractions or incorporated into the carbonate skeletons. On the basis of prevailing water conditions, physiography, biological and sedimentological attributes, the fringing reef can be regarded as an asymmetrical structure, with bidimensional (lateral and vertical) facies zonation; in contrast, the offshore platform is a symmetrical structure, with one dimensional (depth-dependent) facies zonation. This system is believed to represent a modern example of a laterally undifferentiated, offshore reef tract in a relatively enclosed basin, at an embryonic passive continental margin.