Abstract
Middle to Late Pleistocene coral reefs stretch in three discontinuously elevated units above the present sea level between Duba and Sharma along the Red Sea coast, northwest Saudi Arabia. They correspond to MIS 5, MIS 7, and MIS 9 of the deep sea cores. Each unit exhibits prominent terraces as a result of the onlap during different sea levels, erosion during transgression, and tectonic uplift. Framestones, bafflestones, bindstones, packstones, grainstones and floatstones of coral, algal, and other bioclasts are the main microfacies types recorded from these reefal units and their internal sediment. The lower and the upper units exhibited a vertical pattern of a transgressive sequence, starting at the base with a coral assemblage of coral rock zone and graded into the upper reef slope community at the top, while the middle one exhibited a regressive sequence. Corals progressively changed from aragonite in the lower unit to calcite in the upper one by meteoric leaching of the trabeculae, and the cements changed from marine aragonitic needles and peloidal Mg-calcite in the lower unit to blocky and dog teeth cements and silica in the middle and upper units. The gravel lenses within coral colonies of the upper unit with plagioclase, epidote, and fractured quartz grains are attributed to a granitic province from the hinterland mountains, driven to the reef during rainy periods.