Dolomitization of Middle Eocene carbonate rocks, Abu Roash area, Egypt

• Autores: Ahmed S. Mansour, Hanafy M. Holail
• Localización: Carbonates and Evaporites, ISSN 0891-2556, Vol. 19, Nº. 2, 2004, págs. 151-161
• Idioma: inglés
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• Resumen

  • Dolomitization is the most striking diagenetic process affecting the late Middle Eocene shelly limestone beds at Abu Roash and its neighboring areas. Petrographic investigations reveal that these rocks represent shelly-wackstone to packstone microfacies, that they were deposited probably within a shallow subtidal setting. The rock matrix has been subjected to both selective and complete dolomitization and replaced by equigranular mosaics of fine-to medium-grained dolomite (crystalline planar-s dolomite texture). Meanwhile, the embedded shell fragments show high resistance to dolomitization and their original calcite composition is mostly preserved. The petrographic features of the dolomite texture suggest that these dolomites were formed under near surface and low temperature conditions.

    Oxygen and carbon isotopic values of the calcite framework grains (-3.8 to -2.0%o and +1.9 to +2.5%o PDB, respectively) reflect marine values with some degree of alteration by the effect of slightly18O-depleted diagenetic water. Meanwhile, oxygen and carbon isotopic signatures of the studied dolomites (+0.1 to +1.5%o and +1.7 to +2.3%o PDB, respectively) are heavier relative to those of marine limestones. The isotopic signature of dolomites combined with petrographic data is in accord with their formation from modified-marine fluids during early diagenesis.