Stable isotopic composition of carbonate-cemented recent beachrock along the Mediterranean and the Red Sea coasts of Egypt

Abstract

Recent beachrocks on the Mediterranean and the Red Sea coasts of Egypt have been studies. In the Mediterranean beachrock skeletal particles (mainly molluscan fragments) are the most abundant grain type comprising approximately 60% of the framework grains; intraclasts make up the remaining 40%. In contrast, the Red Sea beachrocks are dominantly composed of igneous and metamorphic crystalline basement rock fragments that range in dimension from large boulders in proximal parts of the fan to gravels, sands and silts in the distal parts. High energy conditions as well as the presence of relatively coarse, high porous and permeable sediments ensure that adequate volumes of supersaturated water are able to move through the pore system to accomplish the cementation process. Textural study (light and SEM microscope) on samples from both localities reveals that high-Mg calcite cements occur as thick to thin bladed isopachous crusts, which sometimes occlude all pore spaces. Aragonite cements are also present as thin circumgranular acicular crusts. However, some samples along the Red Sea coast contain high-Mg calcite displaying scalenohedral crystal forms. The isotopic composition of these beachrock cements generally show high values of δ 18O and δ 13C. In the Mediterranean beachrock cements the δ 18O and δ 13C values range from –0.4 to + 1.2 and from + 1.0 to + 2.1% PDB, respectively, whereas the δ 18O and δ 13C values of Red Sea beachrock cements range from −0.1 to + 1.2 and from + 2.1 to | 4.5% PDB, respectively. The oxygen and carbon isotopes, together with the texture and mineralogical composition of these cements, are good indicators of the conditions of primary cementation and reflect the gross of stable isotopic and chemical composition of marine water.