Depositional and Diagenetic Models of Carbonate Platforms
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.
Figures & Tables
Middle East Models of Jurassic/Cretaceous Carbonate Systems
This volume will interest tectonic modelers, stratigraphers, sedimentologists, and explorationists. It is the product of the international conference of “Jurassic/Cretaceous Carbonate Platform-Basin Systems, Middle East Models” that was convened in December 1997 jointly by SEPM (Society for Sedimentary Geology) and the United Arab Emirates University in Al Ain, United Arab Emirates. The twenty-three papers present new data and interpretations arranged in three sections: 1) sequence stratigraphy, cyclostratigraphy, chronostratigraphy, and tectonic influences, 2) depositional and diagenetic models of carbonate platforms, and 3) hydrocarbon habitat and exploration/development case studies. New tectonic models of the Arabian Basin, new stratigraphic and sequence stratigraphic reference sections, new geochemical and source rock data, and new reservoir data are presented. New geologic models make this set of papers relevant to geoscientists working outside of Arabia also.