Biostratigraphy and sequence stratigraphy of the Lower Carboniferous deposits in the Alborz zone based on benthic foraminifera and isotopic geochemistry

Understanding the cyclical nature of changes in sea-level regime leads to awareness of historical land events. In the functional basin, it leads to predictions about the presence of source rocks and hydrocarbon reservoirs in sedimentary sequences. Cycles are formed during a process of falling to rising sea level. The study of sedimentary cycles and understanding the causes of their formation is done in the form of cyclic stratigraphy. Relative changes in sea level were the first active mechanism for the formation of marine sedimentary cycles and sequences. In general, cyclic sedimentary patterns are the product of tectonic and climatic processes, followed by global and local changes in sea level.

 The study of cycles and their formation controlling factors in this article has been done using qualitative studies such as field and laboratory evidence of carbonate deposits of Mobarak Formation. In order to achieve the types of cycles and processes affecting their formation, facies and facies association, depositional environment, isotopic geochemistry and relative sensors were determined using benthic foraminifera as well as sequence stratigraphy.
Discussions and

Studies on field evidence such as sediment erosion pattern, lateral continuity of layers, sedimentary and biological structures, as well as petrographic studies of facies and facies association obtained in this formation, four third-order sedimentary sequences of S1 to S4 were identified. The sequences did not have a low- stand systems tract (LST) and were identified by a highstand systems tract (HST) and transgressive systems tract (TST) that could be controlled by the interaction between eustatic sea-level changes and climate conditions. Depositional sequences show retro-gradational (transgressive systems tract) and prograditional (high-stand systems tract) stacking patterns. Together, these sequences cover a large regressive cycle from the basins / open marine facies to the tidal zone. The distribution and geometry of the Tournaisian-Visean Mobarak Formation imply deposition on a low-angle, relatively low-energy homoclinal carbonate ramp. This interpretation is based on the characteristics of the constituting facies and their gradual lateral and vertical changes. Based on the facies analysis and their associations, four facies’ belts (associations) can be recognized, i.e., basinal environments, outer ramp (deep subtidal associations), mid ramp (shallow subtidal associations), and inner ramp. The study of fossils in sedimentary deposits is one of the oldest and most common methods for determining the relative age of sediments.  By examining the distribution of fossils in stratigraphic units and organizing stratigraphy in units based on their fossil content, the age of the units can be determined. According to this, the bio-stratigraphic investigation shows 9 biozones (MFZ1- MFZ9) belonging to the Hastarian, Ivorian, and Moliniacian sub-stages based on the classification of Mississippian Foraminiferal Zones (MFZ). A good agreement between TSTs and HSTs, and δ13C chemo-stratigraphy implies that sea level fluctuations are the main mechanism that control the δ13C fluctuations in the Mobarak Formation.  Integration of these data indicates that the sequences has been driven by the eustatic sea-level changes and suggests a basic for the regional sequence stratigraphic correlations. The relative ages of the studied calcareous strata were obtained in the Kalariz sections. They are related to the three sub-stage of Hastarian, Ivorian, and Moliniacian. In addition, the study of oxygen and carbon isotopes, which was performed in the Kalariz section, was used to determine the sequence levels and the long-term temperature of the Lower Carboniferous deposits. Therefore, by studying the isotopic values of oxygen and carbon, it is found that the analyzed samples of this formation experienced the least amount of alteration and diagenesis and were close to the waters of the lower Carboniferous Sea.

In this study, 12 main facies were identified and classified into 5 facies association. These facies association are spread on a homoclinal ramp carbonate platform. Examining the field and laboratory evidence, four third-order sedimentary sequences S1, S2, S3 and S4 were identified by the high-stand systems tract (HST) and transgressive systems tract (TST) categories. According to the subdivision of Mississippian foraminiferal zones (MFZ), 9 biozones (MFZ1- MFZ9) were identified in this study, which belong to the subgroups of Hastarian, Ivorian and Moliniacian. In addition, by studying the isotopic values ​​of oxygen and carbon, it was found that the analyzed samples of this formation experienced the least amount of alteration and diagenesis and were close to the waters of the lower Carboniferous Sea. Finally, the studies showed that the main mechanism controlling the fluctuations of isotopic values, cycles and sequences of the third order are the Eustatic fluctuations of the water surface.