Petrography, Geochemistry, and Stratigraphy of the Paleocene-Eocene Carbonate and Evaporite Deposits in the South of Iraq



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The Paleocene and Eocene carbonate and evaporite rocks in southern Iraq were deposited on a shallow carbonate platform that occupied the southwest margin of the Mesopotamian foreland basin. Tens of meters thick shallowing-upward cycles were identified based on the evaporite-rich carbonates representing cycle tops. The original depositional textures for the carbonates have been destroyed by pervasive dolomitization. The identified microfacies, after diagenetic overprint, are foraminiferal biodolomicrite, sparse peloidal and foraminiferal biodolomicrite, peldolomicrite, and peloidal and foraminiferal biodolosparite. The Paleocene-Eocene dolomites have a wide spectrum of crystal sizes that range from micritic, aphanocrystalline, to 100 μm size crystals. The dolomite crystals are mainly planar-s to non-planar and they show syntaxial overgrowth cement in some horizons. Other diagenetic processes accompanying dolomitization throughout the section are allochem dissolution, evaporite precipitation and replacement, authigenic quartz precipitation, and pore-filling and -lining calcite and dolomite cementation. This study also focused on documenting and evaluating the geochemical characteristics of the dolomites. X-ray analysis showed that dolomite is the dominant mineral in all of the representative microfacies. ICP-AES results indicate that the dolomites are nearly stoichiometric with the highest mole % CaCO3 modal class between 50% and 51%. The dolomites are associated with evaporites and yet have probably stabilized in contact with seawater or mixed meteoric and seawater rather than hypersaline seawater. This conclusion is based on their Sr+2 and Na+2 content (avgs. 102 and 600 ppm, respectively). The Fe-enrichment in the dolomites (avg. 135 ppm) also indicates that dolomitizing fluids had a component of non-marine water. The wide range of the trace element values suggest that dolomitization had taken place in contact with fluids that had different chemistries and at different rock-water interaction ratios. The dolomitization settings were enriched in organic carbon based on the highest modal class of δ13C values between -6 and -7 ‰ PDB. Dolomitization in contact with meteoric water can be responsible for the dominantly negative δ13C values. However, δ18O values with their highest modal class between 0 and 2 ‰ PDB disagrees with this interpretation. Therefore, the δ13C and δ18O values are probably the result of dolomitization within sulfate-reducing bacterial zone.



Stratigraphy, Iraq, Petrography, Geology