Intertidal conglomerate south of Yanbu — An episodic clastic deposit in the eastern Red Sea

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. The void filling low Mg-calcite exhibits a coarsening of crystal size away from the grains towards pore interiors. The 14C dating of the carbonate material gave an age of 3580±80 yr B.P.. All evidences suggest that the conglomerate was deposited on the emergent reef flat during an episodic flood stage of the Wadi during the Middle Holocene pluvial period on the west coast of Saudi Arabia.