Hanafy M. Holail - Volume 6, January, 1998

Radiaxial fibrous calcite cement of Middle Eocene limestone, Giza Pyramids
Plateau, Egypt: Isotopic composition and implication to its origin
Hanafy M. Holail

Geology Dept., Fac. Sciences, Alexandria Univ., Egypt

Radiaxial fibrous calcite cement is common in the Middle Eocene nummulitic limestones of Egypt, and occurs mostly in intergranular voids, fossil molds and large cavities. The cement exhibits distinctive petrographic characteristics, and trace element and isotope geochemistry. The lack of pseudomorphic aragonite fabrics, the abundance of microdolomite inclusions, the presence of the length segments of high-Mg calcite relics and the low Sr contents (660-780 ppm) suggest that the studied cement types contain imprints of former high-Mg calcite. Isotopic compositions of the radiaxial fibrous calcite cement Nummulite grains and micrite matrix allow comparison of environments of precipitation with Middle Eocene marine environments. Isotopic data of unaltered RFC cement ( ¹⁸O range -1.4 to -2.9 % PDB;¹³C range +1.5 to +3.9 % PDB ) and well-preserved Nummulites ( ¹⁸O range - l.6 to -3.1% PDB; ¹³C range +1.1 to + 2.8 % 0 PDB) provide an estimate of the isotopic composition of marine carbonate during the Middle Eocene time.

The oxygen isotopic composition (~ 2.4 % PDB) of the RFC cements reflects equilibrium precipitation at realistic temperatures (19 -28^oC) from water of normal marine isotopic composition. Meanwhile, evidence for a marine origin of the studicd RFC cements consists of a relative 3.7 mol % MgCO₃ memory, a relative enriched ¹⁸O composition and early timing of precipitation. Finally, stabilization of these cements has not significantly changed the initial marine isotopic composition and probably occurrcd during shallow burial with marine water.