HR: 0800h
AN: H51C-1137    [Abstracts]
TI: The pattern of coral reef development at two sites in the Seychelles
AU: * Collier, J S
EM: jenny.collier@ic.ac.uk
AF: Dept. Earth Sciences & Engineering, Imperial College, University of London, RSM Building, Prince Consort Road, London, SW7 2BP United Kingdom
AU: Humber, S R
EM: s.r.humber@ic.ac.uk
AF: Dept. Earth Sciences & Engineering, Imperial College, University of London, RSM Building, Prince Consort Road, London, SW7 2BP United Kingdom
AB: Extensive drilling and dating of modern reef complexes worldwide have shown them to be just thin veneers of Holocene material deposited on older Pleistocene limestone. Where the Pleistocene substrate has been imaged seismically it is often characterised by rough karst topography that is thought to be the result of exposure to subaerial weathering during sea-level low stands. The importance of this antecedent karst topography relative to organic growth processes in controlling present day reef morphology has been much debated. We present high-resolution side-scan backscatter and 2D bathymetric survey results from two sites in the Seychelles (western Indian Ocean). These sites are relatively protected and reef morphology is not modified by tropical storms. The sonogram mosaics reveal areas of uniform backscatter intensity, together with regions of coarse image texture and isolated objects. Based on image texture and gross morphology, the mosaics have been classified into two categories of sediment (fine and coarse sand), seagrass beds, and four categories of reef (low relief reefs, patches, pinnacles, high relief continuous reef). The interpretation was ground truthed by independent diver observations and sediment grain size analysis. The pattern of reef development in these two leeward reefs is complex, with no shore-parallel zoning. The total spatial coverage of the reef classes is only 15% of the total surveyed area. Within these mapped reef areas, actual hard coral cover is estimated to be $<$ 50%, so the actual hard-ground coverage is relatively small. Within the various reef types, no simple relationship between morphological parameters such as spatial area, perimeter water depth and height was found. We speculate that the dominant control on reef development is antecedent granite or karst limestone topography.
DE: 3000 MARINE GEOLOGY AND GEOPHYSICS
DE: 3022 Marine sediments--processes and transport
DE: 3045 Seafloor morphology and bottom photography
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting