Resumen de Multiple dolomitization events of the Lower Carboniferous Um Bogma Formation, west central Sinai, Egypt
Mohamed N. Shaaban, Hanafy M. Holail, K. N. Sedeik, M. A. Rashed
The Lower Carboniferous Um Bogma Formation in west-central Sinai, Egypt displays extensive and pervasive dolomitization. The combined petrographic/isotopic/stratigraphic investigations of the dolomite succession in the studied area reveal the presence of two distinct dolomite types: dolomite type (I) is confined to the basal part of the succession being associated with economic ferro-manganese deposits, and dolomite type (II) is located near the upper part of the succession. Silt, clay and marl beds vertically separate the two dolomitic units.
The dominance of ooid and benthic fossil ghosts in the basal dolomite type (I) supports the suggestion that the CaCO3 precursor has been formed under shallow agitated subtidal setting. The terrigenous beds that overlie this basal unit indicate that that the area has been brought into a meteoric realm during a subsequent lowstand period even before the emplacement of the upper part of the formation. During this period considerable stabilization of the initial metastable CaCO3 phases into diagenetic low-Mg calcites has presumably took place.
The persistent stratigraphic distribution of dolomite type (II) throughout the entire area and the typical marine d13C values (+1.1 and 2.9%o PDB) are consistent with platform-wide pumping system within a subtidal setting. Early stabilization of the initial CaCO3 fabrics of the basal carbonate unit impeded their dolomitization during this stage. Subsequent shallow burial stabilization of dolomite type (II) is assessed from the well-developed stylolites, the non-planar crystal boundaries, the undulose extinction, the low porosity and the relatively depleted d18O (-5.1 to -7%o PDB) values. The initiation of dolomite type (I) was a post-burial process as indicated from the euhedral planar crystal boundaries, the high intercrystalline porosity and the absence of any compaction fabrics. The oxygen (3.3 to 4.7%o PDB) and carbon (+1.9 to 3.4%o PDB) isotopic values of this type of dolomite are possibly inherited from the CaCO3 precursor where dolomitization took place from more or less unmodified marine water during period of minimum sea level fluctuations.
