Mineralogical variations in the unconsolidated sediments of El Qasr reef, north of Jeddah, west coast of Saudi Arabia

Carbonate mineralogy of El Qasr reef sediments, north of Jeddah on the west coast of Saudi Arabia, was studied by X-ray analysis. Although the mineralogical composition varies only little, there are significant differences in the proportions of aragonite and high Mg-calcite between the environments. Lagoon sediments, which contain relatively more fine-grained material, are characterized by higher aragonite concentrations than the reef flat sediments. Decreasing grain size and increasing aragonite contents towards the centre of the lagoon suggest a transport of aragonitic mud from the shallow reef flat to the lagoon. The amount of fine fraction and occasionally the nature of the coarse fraction rather than the sand-sized skeletal material controls the aragonite concentrations in the reef sediments. Low Mg-calcite, which occurs in minor quantities and is erratically distributed in the reef sediments, is derived from the Pleistocene coral limestone in the coastal plain. Aragonite abundances in the sediments may be considered to delineate lagoon and reef flat environments in a coral reef complex.