--> ABSTRACT: Diagenesis of the Middle Eocene Nummulite Bank of Giza Pyramids Plateau, Egypt--Petrologic and <SUP>18</SUP>O/<SUP>16</SUP>O Evidence, by Hanafy Mahmoud Holail; #91003 (1990).

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ABSTRACT: Diagenesis of the Middle Eocene Nummulite Bank of Giza Pyramids Plateau, Egypt--Petrologic and 18O/16O Evidence

Hanafy Mahmoud Holail

The middle Eocene Nummulite Bank of Giza Pyramids plateau has been extensively diagenetically altered. The result of a multidisciplinary study which included petrolgraphy, cathodoluminescence, and 16O/16O and 13C/12C ratio measurement combined with trace element data, provides information about the origin and nature of the diagenetic fluids as well as the magnitude and timing of the diagenetic phases.

A sequence of petrologic features coupled with the geochemical characteristics suggests a progression of diagenetic fluids from marine to meteoric. The micrite rock-matrix isotopic compositions deviate from an initial marine composition (-2.90 ^pmil ^dgr18O; +5.0^pmil ^dgr13C PDB) along covarient trends that reflect alteration by diagenetic pore waters, whereas the Nummulite grains show isotopic composition similar to an initial marine composition (-1,80 ^pmil ^dgr18O; +2.10 ^pmil^dgr13C PDB).

This early diagenetic alteration of the rock matrix was followed by replacement dolomitization of the nummulite grains and rock matrix. Limpid, euhedral, cathodoluminescence zoning dolomite precipitated in intraparticle and interparticle porosity subsequent to replacement dolomitization. The oxygen isotope ratios of the dolomite range from +1.10 to -0.60 ^pmil PDB, whereas the carbon isotope ratios range from +1.80 to +0.90 ^pmil PDB. These isotopic ratios with the trace element values suggest that dolomitization occurred in marine-dominated mixed waters at low temperatures. The final diagenetic phase precipitated was a

blocky calcite spar cement. The oxygen and carbon isotope ratios of this calcite cement represent diagenesis in a freshwater zone.

These diagenetic alterations and the biofabric features affected the porosity and permeability of the Nummelite Bank rock.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990