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Extensive precipitation of aragonite and high-Mg calcite (12-14% MgC0 3 ) cements in the intertidal sediments ofthe Gulf of Aqaba, Egypt and the Arabian Gulf, Qatar results in the formation of dominant beachrock exposures. The 20-60 ern thick beachrocks in both areas are parallel to the shoreline and slope gently seaward. The 14 C dating values show that the cement of the Gulf ofAqaba beachrock (2470±60y) are rather older than those ofthe Arabian Gulf (1360±45y). Framework grains in the Gulf of Aqaba beachrock are moderate to unsorted coarse terrigenous rock fragments, which differ than the unsorted carbonate particles ofthe Arabian Gulfbeachrock. Carbonate cements in both the Aqaba and the Arabian Gulfs display the same architecture, which comprises: 1) thin isopachous crust made up of high-Mg calcite mosaics and/or aragonite needles that surround grains and 2) intergranular cryptocrystalline high-Mg calcites, which fill the rest of the pores. Minor dolomite mosaics may associate with the intergranular cement. The co-existence of the aragonite needles, of the isopachous crust, with the micritized grains and micritic envelopes is evidence that marine phreatic processes are dominant in the intertidal zone and that lithification has started in this zone. The bi-mineralic composition of the isopachous crust in the Aqaba beachrock is attributed mairily to kinetic factors (i.e. the rate of supply of carbonate ions) and to the composition of the substrate and/or organic control in the beachrock of the Arabian Gulf. Some physico-chemical, kinetic, hydrologic and biologic factors are believed to be effective in controlling the precipitation rates ofthe isopachous cement. The oxygen and carbon isotopic composition of the intergranular high-Mg calcite cement of the Aqaba Gulf (+2.0 to -1.6 and +2.9 to +4.4 %0 PDB respectively) is in accord with their precipitation in equilibrium with marine water. However, the relatively depleted 0 180 (-0.5 to -3 %0 PDB) and 013C (+0.3 to +2.2 %0 PDB) values of the intergranular high-Mg calcite cement of the beachrock of the Arabian Gulf is attributed to extraneous source of bicarbonate ions. The minor dolomite rhombs are formed directly from seawater within microenvironments created in response to the release of Mg 2 + ions to the pore water following the partial dissolution of some high-Mg calcite carbonate particles.

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Middle Miocene reefal carbonates of the Urn Mahara Formation at Urn Gheig area near the Red Sea coast of Egypt had a complex history ofdolomitization. Various dolomite petrotypes are recorded including; I) dolomite replacing allochems with complete fabric preservation, 2) mimic dolomite replacement for originally Mg-calcite radial fibrous cements, 3) fibrous, spar, and micrite dolomite cements that fill some secondary pores, and 4) dolomite matrix which possesses non-planar crystal boundaries and poly modal size distribution. The investigated dolomites exhibit a narrow 0 18 0 range (-3.6 to -6.3 %0 PDB) relative to their OUC values (1.2 to -11.2%0PDB). The petrographic and isotopic characteristics are in accord with multiple phases ofdolomitization, related to sealevel variations. The first dolomitization phase was an early and pervasive event that took place during a stillstand period within the zone of circulating marine pore-fluids ahead of a mixing zone. This resulted in dolomitization ofthe bioclasts and early Mg-calcite marine cements. Excellent fabric preservation ofthese components is attributed to pre-dolomitization diagenesis. Silica interruption for this early dolomitization phase is indicated by the sharp contacts observed between the silica minerals and dolomite and the presence of dolomite inclusions within quartz.As sealevel continued to drop, the mixing-zone migrated downward through the dolomitized reefal body resulting in recrystallization ofearly dolomites and precipitation of some dolomite cements oflimited extent. This late dolomitization phase was followed by meteoric diagenesis as the ground water continued to migrate basinwards. During this period ofdiagenesis meteoric calcite cements, that show relatively depleted 0 18 0 and oUC values (-6.6 to -9.3 and-5.3 to -7.9%0PDB respectively), were precipitated within some secondary vugs.