Diagenesis of the middle eocene "nummulite bank" of The giza pyramids plateau, Egypt: petrologic and 18o/16o evidence

Abstract

The Middle Eocene "nummulite bank" of the Giza Pyramids Plateau has been extensively altered diagenetically. The result of a study involving petrography, cathodoluminescence, and 18O/160 and 13C/12C data, provide information about origin and nature of the diagenetic fluids, as well as magnitude and timing of the diagenetic stages. A sequence of petrologic features coupled with the.geochemical characters suggests a progression of diagenetic fluids from marine to meteoric. The micrite rock matrix isotopic compositions depart from an initial marine composition (-2.9%o 8I8O and +0.5%o 813C PDB) along covariant trends that reflect slight alteration by diagenetic pore waters. The nummulite grains show isotopic composition close to the isotopic composition of marine carbonates (-1.8%o 818O and + 1.9%o 813C PDB). The early diagenetic alteration of the rock matrix was followed by partial to complete dolomitization of the limestone matrix and nummulite grains. The oxygen isotopic ratios of the matrix-replacive dolomites range from -0.6 to +1.2%o PDB, whereas the carbon isotopic ratios range from +0.8 to +3.0%o PDB. These isotopic ratios suggest that dolomitization occurred in marine-dominated mixed waters at low temperatures. The final diagenetic phase involve the precipitation of a blocky calcite spar cements (ave. -9.6%o 818O and -4.2%o 813C PDB).