Abstract

Miocene reef and platform carbonates exposed along the Red Sea coast at Gebel Abu Shaar and West Gemsa are entirely dolostone while exhibiting all the textures and fabrics of a metastable suite of carbonate sediments that has undergone meteoric diagenesis. The microfabrics of former Mg calcite fibrous marine cements and bioclasts and calcite particles are well preserved and commonly mimetically replaced. Originally aragonitic components typically illustrate little to no microfabric retention or are leached, yielding moldic to vuggy porosity. Dolomite cement and replacement crystals have inclusion-rich or dissolved cores and relatively inclusion-free rims. Most crystals are anhedral, and many have an irregular microtopography generated by leaching of crystal surfaces. Carbon and oxygen isotope values vary widely. Many samples have the low positive values characteristic of marine precipitates, but others are depleted in 13 C and 18 0, reaching minimum delta 13 C and delta 18 O values of -8.9 and -8.0%0 (PDB), respectively. The origin of these dolostones can be explained by two hypotheses. The first is dolomitization of a partially stabilized or unaltered Mg calcite and aragonite sediment in a zone of mixed freshwater and seawater. Following dolomite formation, the progressive lowering of sea level and resultant freshening of the mixing zone caused leaching of dolomite crystal cores and corrosion of crystal faces. The second hypothesis invokes two dolomitization events--an earlier, pervasive dolomitization by marine or hypersaline waters, followed by recrystallization in meteoric or hydrothermal fluids, and then selective dissolution. Both hypotheses are consistent with the geologic setting of these sediments, but the pervasive nature of the dolostone favors the recrystallization hypothesis. Vadose calcite cement, ferric oxide/hydroxide crusts, and secondary evaporite cement are the latest precipitates, although they occlude only a minor amount of primary and secondary porosity.

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