Isotope geochemistry of the Miocene and Quaternary carbonate rocks in Rabigh area, Red Sea coast, Saudi Arabia

Abstract

The Rabigh area, a coastal region north of Jeddah city, Saudi Arabia contains raised Quaternary coral reefal terraces and reworked coral fragments mixed with sand and gravel. This area has a thin exposure Lower Miocene shallow marine carbonate rocks that laterally pass into evaporites. The Miocene carbonate and evaporite rocks conformably overly the Lower Miocene siliciclastic sequence, are in turn capped by the Harrat basaltic boulders. The Miocene carbonates are made up of dolomitic packstone, wackestone and mudstone, whereas the overlying Quaternary reefal terraces are composed of coral boundstone and grainstones.The Quaternary reefal terraces of Rabigh area have been dated using the uranium-series dating method to obtain precise dates for these corals. The calculated ages (128, 212 and 235ka) indicate that deposition took place during high sea level stands associated with interglacial times during Oxygen Isotope Stages (OIS) 5 and 7. The youngest age (128ka) clearly corresponds to stage 5e of the last interglacial period. The obtained ages correlate well with those of the emerged reefs on the Sudanese and Egyptian coasts at the western side of the Red Sea. The broad distribution of wet climate, pluvial deposits on the continents and high sea level stands indicate a wide geographical range of the interglacial events of the Oxygen Isotope Stages (OIS) 5 and 7.The oxygen and carbon isotopic composition of the Miocene and Quaternary carbonate rocks in Rabigh area show a broad range of δ13C and δ18O. The Quaternary carbonate rocks have significantly higher δ13C than the Miocene ones, but low δ13C values of the Miocene samples likely indicate a high contribution of carbon from organic sources at the time of deposition. Linear trends are evident in both groups of samples supporting the likelihood of secondary alteration.