Abstract
A thin sheet of clastic conglomerate has been located overlying the Pleistocene coral limestone in the intertidal zone, south of Yanbu on the west coast of Saudi Arabia. The conglomerate exhibits a polymodal grain-size distribution with gravel to cobble size clasts and sand-silt matrix. Grain and fabric characters of the conglomerate are consistent with a flash flood deposit rather than with a wave-processed nearshore sediment. Wadi Wasit draining the adjacent highlands and the coastal plain, covered with alluvial deposits derived from the Tertiary volcanic and metamorphic rocks, provided the coarse-grained sediments to the shelf. The carbonate cement in the conglomerate is composed of low Mg-calcite and appears to have been precipitated in non-marine conditions.