Variations in Plio-Pleistocene Deposition in the Red Sea

The thickness of deep-water Plio-Pleistocene (PP) sediments in the Red Sea varies somewhat, as expected from increased biogenic pelagic production rates in the south and with input of aeolian and fluvial sediments through the Tokar Gap in the Sudanese hills. Otherwise, however, the sediment distribution does not obviously reflect the likely pattern of sediment input (from the positions of wind gaps through the Red Sea hills and fluvial drainage basin outlets). We use localized seismic surveys to investigate sediment distribution of two areas in more detail. The first, located near the coast of Egypt, utilized 3D seismic data collected for oil and gas exploration. The data reveal a pattern of sediment deposition that is unrelated to drainage basins of the adjacent hills. Instead, deposition here has been strongly affected by halokinetics, with sediment filling evaporite depressions that are elongated sub-parallel with the coast. For the second, Chirp sediment profiler data allow study of finer scale Pleistocene sedimentation around Thetis Deep in the central Red Sea. The data contain a common sequence of reflections, which suggests that hemipelagic sedimentation has been almost uniform about the deep. The seismic time interval between the seabed and one reflection at ~20-30 ms sub-bottom was mapped out and varies little either side of the deep, but does reveal a systematic thickening of the interval with increasing water depth. The data also reveal structures indicating localized slope failure and sediment flow deposits, as well as tectonic disruptions. From correlations of reflections with sea level curves and sediment core data, we suggest that the slope failures occurred in the Late Pleistocene after Marine Isotope Stage (MIS) 12 and probably before MIS 6. We suggest that these slopes likely failed because of seismic ground accelerations. Applying a pseudo-static slope stability model and assuming shear strengths of comparable carbonate-rich sediments, we estimate the potential acceleration and earthquake magnitude. The results suggest that the very low incidence of historical earthquakes in the central Red Sea is not entirely representative of the Late Pleistocene.