Geological structure and late quaternary geomorphological evolution of the Farasan Islands Continental Shelf, South Red Sea, SW Saudi Arabia

A marine geological-geophysical survey of selected areas on the Saudi Arabian continental shelf of the Farasan Archipelago in the southeastern Red Sea has been conducted in the framework of the ERC-funded DISPERSE project. The aim was to explore systematically the geological, tectonic and sedimentary structure of the shelf and provide insights into the submerged prehistoric landscapes. The survey targeted features believed to be of significance in relation to the archaeological potential of the submerged landscape, including geological structure, palaeoenvironment, and sea-level change. Swath bathymetry, seismic and subbottom profiling data, side scan sonar imaging and sediment coring indicate that the flat areas belonging to the 70-90 m deep shelf may have developed as part of an erosive marine terrace during the latter part of MIS 3, between 30 and 45 ka BP. A second terrace lying at 115-120 m water-depth is associated with MIS 2, whereas a third one, mapped at about 40 m water-depth, may correspond to the MIS 5.1 period, 80-85 ka BP. Extensional tectonics, possibly driven by basin-ward flow of underlying Miocene evaporites below the shelf, is responsible for the rupturing of the latter along NW-SE trending faults. The resulting fault-bounded blocks, which are composed of Plio-Quaternary rocks, were dragged and drifted away from the shelf edge to create isolated flat-topped ridges surrounded by steep slopes and troughs. The largest part of the shelf along with the 90 m deep, flat tops of the ridges were exposed when the sea-level was at 115-120 m bpsl (below present sea level) during the Last Glacial Maximum (LGM). This geomorphological configuration may also be valid for the previous low sea-level period, 140 ka BP (MIS 6). Shallow and deep depressions and valleys on the main terrace of the shelf, which formed by solution of evaporite diapirs or domes, were permanent or ephemeral lakes when the shelf was exposed during MIS 2. Similar lakes possibly formed in the many deep sinkholes which occur on the available hydrographic charts along the 120 km wide and several-hundred-km long Farasan shelf. Finally, the presence of valleys and canyons on the seafloor of the survey areas indicates erosion of the shelf under subaerial conditions due to surface water-flow.