Abstract
Subsea carbonate cements vary in their types and distribution in a modern coral reef formation north Jeddah, Saudi Arabia. The petrographic characteristics of cemented reefs were studied by scanning electron microscope and X-ray diffraction methods. The cements consist basically of acicular aragonite and magnesium calcite containing 17 mole % MgCO. The predominant cement in the Red Sea reefs is aragonite which is generally restricted to intraskeletal cavities of coral, whereas the Mg-calcite develops as a cryptocrystalline cement filling and coating the reef components.