Stable isotopes and rare-earth elements of carbonate rocks, Paleocene-Eocene Kolosh Formation, High Folded Zone, NE Iraq: implication for depositional environment and Diagenesis

Petrographic and geochemical studies were carried out on the Paleocene-Eocene carbonates of the Kolosh Formation in the High Folded Zone to infer the environment of deposition, diagenetic overprints, and their origin. The petrographic study revealed five main microfacies; packstone, wackestone, mudstone, grainstone, and crystalline carbonate. There are different depositional environments from shoal to shallow marine environments (mid-ramp conditions) that are adequate for mixing continental material with seawater. Several evidences of diagenetic processes were observed, such as micritization, neomorphism, and dolomitization. Petrographic and geochemical analyses show the dominance of calcite and dolomite minerals. The contents of major oxides and the concentrations of trace elements were normalized to Post-Archean Australian Shale (PAAS) values. The Paleocene-Eocene carbonates are enriched in CaO, MgO, Cr, and Ni, whereas are highly depleted in Na2O, K2O, TiO2, Al2O3, Rb, Ba, Th, and Zr. The low contents of Al2O3 and SiO2 refer to a low siliciclastic involvement during the deposition of the Paleocene-Eocene carbonate. The REEs average content is lower (15.60 ppm) than that of marine carbonates (28 ppm). The REE patterns (normalized to PAAS) display a seawater-like type, while a slightly lower average Y/Ho value (33.66) relative to that of typical seawater (similar to 44 to 74) refers to the alteration of the seawater by the freshwater. The REEs of this study are comparable with the limestone of the Middle Eocene Fulra Formation and Arabian Sea carbonates. Paleocene-Eocene carbonates have delta C-13(PDB) ranging between - 13.31 and 1.25 parts per thousand(PDB), and delta O-18 from - 6.30 to 0.12 parts per thousand(PDB). The negative values and no correlation between delta C-13 and delta O-18 indicate that modification of the primordial composition of the isotopes is not a result of diagenesis only, but other factors, such as temperature, and organic matter have an important role. The negative Ce/Ce*, Th/U ratios, and authigenic U suggest their accompanying the deposition of the carbonates in oxidizing conditions. The paleotemperature of the studied carbonates (similar to 12 to 42 degrees C) is slightly higher than other global records in the Paleocene-Eocene, which may propose the variations in the composition of oxygen isotope and modification in the geochemical characteristics during the post-carbonate deposition time.

Automated summary

Petrographic and geochemical studies were conducted on the Paleocene-Eocene carbonates of the Kolosh Formation in the High Folded Zone. The study revealed the depositional environment, diagenetic processes, and mineral composition of the carbonates. The carbonates showed different microfacies and were enriched in CaO, MgO, Cr, and Ni, while depleted in Na2O, K2O, TiO2, Al2O3, Rb, Ba, Th, and Zr. The REE patterns were similar to seawater, but with a slightly lower Y/Ho value, indicating alteration by freshwater. The δC-13 and δO-18 values suggested modification of the isotopic composition by factors such as temperature and organic matter. The study also found that the carbonates were deposited in oxidizing conditions and had a higher paleotemperature compared to global records.