Dolomitization of Middle Miocene buildups, Um Gheig area, Red Sea coast, Egypt

Middle Miocene reefal carbonates of the Um Mahara Formation at Um Gheig area near the Red Sea coast of Egypt had a complex history of dolomitization. Various dolomite petrotypes are recorded including; 1) dolomite replacing allochems with complete fabric preservation, 2) mimic dolomite replacement for originally Mg-calcite radial fibrous cements, 3) fibrous, span, and micrite dolomite cements that fill some secondary pores, and 4) dolomite matrix which possesses non-planar crystal boundaries and polymodal size distribution. The investigated dolomites exhibit a narrow delta(18)O range (-3.6 to -6.3 parts per thousand PDB) relative to their delta(13)C values(1.2 to -11.2 parts per thousand PDB). The petrographic and isotopic characteristics are in accord with multiple phases of dolomitization, related to sealevel variations. The first dolomitization phase was an early and pervasive event that took place during a stillstand period within the zone of circulating marine pen-fluids ahead of a mixing zone. This resulted in dolomitization of the bioclasts and early Mg-calcite marine cements. Excellent fabric preservation of these components is attributed to pre-dolomitization diagenesis. Silica interruption for this early dolomitization phase is indicated by the sharp contacts observed between the silica minerals and dolomite and the presence of dolomite inclusions within quartz. As sealevel continued to drop, the mixing-zone migrated downward through the dolomitized reefal body resulting in recrystallization of early dolomites and precipitation of some dolomite cements of limited extent. This late dolomitization phase was followed by meteoric diagenesis as the ground water continued to migrate basinwards. During this period of diagenesis meteoric calcite cements, that show relatively depleted delta(18)O and delta(13)C values (-6.6 to -9.3 and -5.3 to -7.9 parts per thousand PDB respectively), were precipitated within some secondary vugs.