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Abstract :

The depositional environment and the diagenetic history of the Middle Jurassic carbonate (Bir Maghara Formation), north Sinai, Egypt, have been evaluated through comprehensive petrographic and geochemical studies of oncoid grains. Petrographically theoncoids are composed of micrite and vary from subspherical to spherical bodies. The abundance of the oncoids and their close association with ooids, pellets, and shelf fauna indicate that the Middle Jurassic carbonate sequence was deposited in a shal low marine environment within the photic zone.

The oncoids are characterized by relatively low Fe and Mn concentrations, normal Sr concentrations, light δ18O values (from –3.9 to –5.5%o PDB), and heavy δ13C values (from+2.2 to+3.1 %o PDB). Enhanced magnesium concentrations (3.6 mol % MgCO3) in these oncoids, relative to carbonate matrix (0.6 mol% MgCO3), suggest their original Mg–calcite mineralogy. The δ18O and δ13C values show a narrow range, and the oncoid grains from the same stratigraphic level do not yield any large difference in their isotopic composition. Isotopic comparison of these oncoids (mean δ18O = –5.4%o; mean δ13C =+2.1 %o PDB) with the sparry calcite cements (mean δ18O = –9.8%o; mean δ13C =+0.6%o PDB) suggests that meteoric modification was coincident with the development of subaerial exposure and meteoric influx during emergence of the carbonate platform. Such diagenetic modification is probably responsible for 18O depletion of the oncoids and the carbonate matrix.

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