QUATERNARY DUNE CARBONATES FROM THE MEDITERRANEAN COAST OF EGYPT - PETROGRAPHY AND DIAGENESIS

The Quaternary carbonates of the Mediterranean coast of Egypt between Alexandria and Salum appear as parallel limestone ridges rising up to 100 m above sea level. These ridges are dominated by dunal carbonates which differ not only in their primary composition but also by distinc t grades of meteoric water diagenesis. Oolitic facies dominates the yo unger aeolianites of the first and second ridges. Bioclastic facies wi th abundant coralline algae, benthonic foraminifers, molluscs, echinod erms and intraclasts represents the major rock type in the older aeoli anites. Features of meteoric water diagenesis include precipitation of increasing amounts of void-filling low Mg-calcite spat, dissolution o f aragonite and stabilization of aragonite and high Mg-calcite to low Mg-calcite. Aeolianites below paleosol horizons contain abundant calcr ete cements, micritized fossils and detrital grains which are commonly corroded and replaced by calcite. Three stages of progressive meteori c diagenesis are recognised in the Egyptian Quaternary aeolianites. In stage 1 minor precipitation of low Mg-calcite occurs at the grain bou ndaries. Stage 2 is marked by partial dissolution of aragonite, partia l loss of high Mg-calcite and precipitation of low Mg-calcite in some pore spaces. In stage 3, most of the remaining pores are occluded by c ementation. At the end of stage 3, Mg is removed from high Mg-calcite but some aragonite still persists. The early vadose cements are repres ented by miniscus, bridge and pendant cements. The phreatic cements we re precipitated as bladed spar in the isopachous rims and equant spar in the intergranular and mouldic porosity. The late vadose cements are represented by micritic cements that were related to calcrete formati on. Elemental behaviour during meteoric water diagenesis indicates tha t it leads to a gradual decrease in bulk Sr along with Na in progressi vely altered aeolianites. Mn distribution is controlled by the carbona te mineralogy (aragonite versus calcite) as well as the proximity of t he aeolianites to the overlying paleosol horizons.