The modern coral colonization of the Bajuni Barrier Island (Southern Somalia): A facies model for carbonate-quartzose sedimentation

The Somali coast south of Kisimayo has been long investigated and a modem depositional model is proposed. Several evolutive phases have been identified, corresponding to eustatic sea-level variations starting from isotope stage 5e, which led to the present coastal setting, characterized by mixed carbonate-terrigenous sedimentation and conditioned by climatic factors. The sea level fall below the continental shelf edge during the last glacial period caused subaerial exposure, erosion and weathering of the sedimentary substratum. As a consequence a braided fluvial net developed on the coastal belt and coastal dune ridges migrated toward the shelf edge, influencing the distribution pattern of the modern sedimentary and ecological environments. The Holocene inundation of the shelf caused the drowning of the channels, the overflowing of the interdune areas, the formation of the Bajuni barrier island, of wide coastal lagoons and channelized tidal flats. The relation among inherited morphology, supply of terrigenous material and development of coral communities is shown in a series of ecological profiles and sketch maps of facies distribution. The depositional systems of outer shelf. marine passes, coastal sound and braided channels have been investigated in detail in some key areas. The types of coral colonization as well as their interaction with seagrass meadows have been examined. In outer shelf corals grow in form of fringing reefs and coral carpets around the abraded flat of the islands and along some morphological steps corresponding to old sea-level stillstands. In the marine passes crossing the barrier island, the hard substrate is colonized by fringing and pinnacle reefs, coral carpets and knobs and their distribution is controlled by the energy dispersion of the tidal currents. In the coastal sound corals grow to form at places small fringing reefs and carpets along the protected edges of the islands and near the channel mouths, whereas patch and table reefs are found on isolated erosion remnants inside wide Thalassodendron meadows. The braided channelized area of the coastal belt is characterized by the development of mangal and salt flat depositional systems where the facies distribution is controlled by the extension of the tidal prism.