Abstract: Abu Shaar is a pervasively dolomitized Miocene carbonate platform that veneers tilted Precambrian basement blocks on the western margin of the Gulf of Suez. This reef-rimmed complex developed in an active margin setting with facies composition and distribution controlled by eustasy and local tectonics. The first stage of platform development is recorded by the Kharasa Member which resulted from initial marine onlap as aprons of mixed coarse-grained siliciclastics and carbonate. These sediments pass gradually upward into open-platform carbonates and a reef-dominated platform margin. A second phase of platform development is recorded by the complex stratigraphy of the Esh el Mellaha Member, which also included prolific reef growth in platform margin and back reef areas. Following a brief period of subaerial exposure, a third phase of platform development, the Bali'h Member, saw only minor reef development and soon became dominated by restricted carbonate and evaporite facies. The fourth and final stage of development is recorded on the platform margin by an enigmatic Chaotic Breccia Member, a unit composed of the insoluble material of a formerly extensive evaporite sequence. Microfabrics of allochems that were originally Mg-calcite or calcite in composition were generally well preserved whereas originally aragonitic allochems were replaced by dolomite with significant loss of microfabric or dissolved resulting in moldic pores. These rocks were probably dolomitized by marine to hypersaline fluids. These fluids could have refluxed into the complex from overlying shallow subtidal or sabkha environments. Alternatively, fluids could have originated as hypersaline brines while the platform lay exposed during an extensive evaporative phase accompanying Middle to Late Miocene sea-level lowstand. These fluids were subsequently introduced into the carbonate complex during transgression, causing dissolution of aragonite and dolomitization. In order to explain the wide ranging and negative δ 18 O data, these sediments are thought to have later recrystallized, involving meteoric or more likely hydrothermal waters. Sedimentary facies and diagenesis of the carbonates at Abu Shaar are strikingly similar to other Miocene reefs described from elsewhere in the Red Sea and western Mediterranean areas. The reasons for this, although unclear, likely include regional tectonic effects operating against a backdrop of fluctuating sea levels.