Microfacies and geochemical evidence for original aragonite mineralogy of a foraminifera-dominated carbonate ramp system in the late Paleocene to Middle Eocene, Alborz basin, Iran

Abstract

The Ziyarat Formation is a Late Paleocene to Middle Eocene carbonate sequence, located in the north of Tochal Village (south-east of Tehran). The Ziyarat Formation with a total thickness of 212.5 m conformably overlies the Fajan conglomerate and is overlain by greenish tuffaceous siltstone of the Karj Formation. Petrographic studies have led to the recognition of 11 microfacies. Of these microfacies, five belong to inner ramp, four belong to middle ramp and two are located in the outer ramp. Due to the great diversity and abundance of large benthic foraminifera (e.g., Nummulites, Alveolina, Discocyclina), the Ziyarat Formation represents an example of “foraminifera-dominated carbonate ramp settings”. Paleobathymetry, light level and water energy were determined during deposition of Ziyarat carbonates based on the test shape and size variation of different types of benthic foraminifera and other components in different facies. Micritization, cementation, compaction, dissolution, dolomitization and fracturing are important diagenetic processes in the Ziyarat Formation, occurring in marine to meteoric and burial diagenetic environments. Geochemical studies indicate that Ziyarat carbonates were deposited in a shallow warm-water sub-tropical environment and aragonite was the original carbonate mineralogy with mean Sr values of 3,470 ppm. It should be noted that the maximum Sr content in abiotic calcite is 1,000 ppm. In Ziyarat limestone, dissolution as leaching is the most important diagenetic event in the evolution of porosity, particularly where Nummulite and other benthic foraminifera are abundant. This may indicate that the original carbonate mineralogy of Nummulites might be aragonite, rather than low-Mg calcite. In the Jahrum Formation of Iran and in the El Garia Formation in north-east of Libya, the widespread moldic porosity in Nummulite beds enhanced the reservoir potential of the carbonate sequence. Thus, it is proposed that the original carbonate mineralogy of nummulitid shells may be varied in different environments, and those with metastable mineralogy are excellent for forming hydrocarbon reservoirs. Bivariate plots of Mn versus Sr/Ca and δ18O values illustrated that Ziyarat limestones were affected by open diagenetic system with high water/rock interaction. Early burial diagenetic temperature calculation based on heaviest oxygen isotope values of micrite and δw of Eocene seawater of −0.85 SMOW showed that temperature was around 39 °C. Cathodluminescence studies of carbonate cements illustrated dull luminescence, because these carbonates were affected mainly by burial diagenesis. This statement is confirmed by δ18O and δ13C isotope evidences. Based on different facies and evidences such as absence of reefal facies, calciturbidite deposits, the occurrence of widespread tidal flat deposits and abundant micrites in all microfacies indicate that the Ziyarat Formation was deposited in a homoclinal carbonate ramp environment. In addition, many researchers suggested that due to extreme global warming during the Paleocene–Eocene time, platform margin coral reefs declined in low latitude and heterozoan calcitic foraminifers became abundant and, thus, widespread carbonate ramp systems developed.