Geochemistry, microfacies and diagenetic evidences for original aragonite mineralogy and open diagenetic system of Lower Cretaceous carbonates Fahliyan Formation (Kuh-e Siah area, Zagros Basin, South Iran)

Abstract

The Fahliyan Formation is an important reservoir unit of Cretaceous strata in the south of Iran (Zagros Mountains). This formation with the age of Lower Cretaceous overlies unconformably the Hith Formation and is conformably covered by the Gadvan Formation in the studied area. To evaluate the original carbonate mineralogy, paleotemperature, and diagenetic environment in Kuh-e Siah anticline in Boushehr Province, the Fahliyan Formation was investigated by petrographic and geochemical analyses. Petrographic studies led to the recognition of 25 microfacies that were deposited in four facies belts: tidal flat, lagoon, and shoal in inner ramp and shallow open marine in mid-ramp environment. Of these microfacies, twenty-one belong to inner ramp and four are located in the mid ramp. Calcareous algae and benthic foraminifera are abundant in the shallow-marine carbonates of the Fahliyan Formation. Evidences such as absence of calciturbidite deposits and reefal facies, the occurrence of widespread tidal flat deposits, gradual facies changes, and abundant micrites indicate that the Fahliyan Formation was deposited in a homoclinal carbonate ramp environment. Abundant aragonite skeletal and non-skeletal components and presence of dolomites in Fahliyan carbonates indicate original aragonite mineralogy. Radial ooids generated in low-energy environments similar to modern aragonitic forms. Deformed and spalled ooids as well as shattered micritic envelopes indicate aragonite dissolution during meteoric diagenesis. Isopachous and fibrous sparry calcite cements resemble modern aragonite morphologies. Micritization, geopetal fabric, bioturbation, neomorphism, compaction, cementation, dissolution, dolomitization, dedolomitization, and fracturing are diagenetic processes in the Fahliyan Formation, occurring in marine to meteoric and burial diagenetic environments. Cementation, dissolution, and dolomitization are the main diagenetic processes that affected the original texture. Values of major (Ca and Mg) and minor (Sr, Na, Fe and Mn) elements (such as high Sr value) and δ18O and δ13C designate that the Fahliyan carbonates were deposited in a shallow warm-water sub-tropical environment and aragonite was the original carbonate mineralogy. Variations of Sr/Ca and δ18O values versus Mn suggest that diagenetic alteration must have occurred in an open diagenetic system, with high water–rock interaction. Dolomitization, dissolution and cementation are the main diagenetic features observed in the Fahliyan Formation of Kuh-e Siah and confirm high water–rock interaction in open diagenetic system. Dissolution as leaching is the most important diagenetic event in the evolution of porosity, particularly where green algae and other benthic foraminifera are abundant. This may indicate that the original carbonate mineralogy of the mentioned bioclasts might be aragonite, rather than low-Mg calcite. Temperature calculation based on the oxygen isotope value of the least-altered sample, using Anderson and Arthur (Stable isotopes in sedimentary geology, vol 10. Society of Economic Paleontologists and Mineralogists. Short Course, pp 1–151, 1983) equation, indicates paleotemperature to be around 29.1 °C.