Diagenetic trends of the Pleistocene calcareous ridges, Mersa Matruh area, Egypt

Abstract

Ooids are a major constituent of the Pleistocene carbonates along the northwestern coastal zone of Egypt. The Mersa Matruh area was selected for a detailed diagenetic study of these rocks. δ 18O, δ 13C and Sr values were determined on whole-rock samples on manually separated ooids and calcite cements. Plerographic examination shows that these rocks are partly consolidated ooid grainstones with low-Mg calcite cement in granular voids. Oxygen isotopic and Sr values for the whole rock (+ 0.5% vs. PDB and 9300 ppm, respectively) are consistent with data on limestones of the same age. In contrast, oxygen isotopic values for ooids (ave. +0.5% vs. PDB) are heavier relative to values for low-Mg calcite cements (ave. −1.4% vs. PDB), and Sr contents are higher for ooids (10,000 ppm). In addition, δ 13C-values are identical for the whole rock and ooids (ave. +4.5% vs. PDB), and heavier relative to δ 13C-values of the calcite cements (ranging from +1.4 to +3.8% vs. PDB). These data may suggest that calcite cements are formed in non-diluted pore water under relatively high rock/water ratios.