Sequence stratigraphy and facies analysis of the Gachsaran formation in Bostaneh Anticline, Northwest of Bandar- e Lengeh

Introduction
The Gachsaran Formation, known as the cap rock of the Asmari oil reservoirs, is the first formation of the Fars Group within the Zagros Basin and extends from the Dezful-Lurestan embayment to the Persian Gulf Basin. In the Fars province, the Gachsaran Formation shows significant variations in thickness, lithological characteristics, and fossil content particularly foraminifera compared to other parts of the Zagros. James and Wynd (1965) divided this formation into three members: Chehel, Champeh, and Mol. The Gachsaran Formation was deposited in two separate basins (the main and northern salt basin, and the southern Persian Gulf salt basin), which are not time-equivalent. Previous studies on this formation have primarily focused on geophysical properties, diagenesis, and sediment geochemistry, and relatively limited information is available regarding its sequence stratigraphy. Given the scarcity of such studies, detailed investigation of microfacies, depositional environment, and sequence stratigraphy can contribute to a better understanding of this formation and enhance exploration knowledge in the region. Accordingly, a subsurface stratigraphic section within the Bostaneh anticline, located in the southern salt basin of the Persian Gulf, was selected and studied. The Gachsaran Formation does not crop out in the study area; the subsurface section is situated at the core of the anticline, where the deposits of the Mishan Formation are exposed.
 
Materials and Methods
A total of 446 thin sections from 1288 meters of the Gachsaran Formation, obtained as drill cuttings, were examined in this study. The stratigraphic column of the studied section was constructed by integrating laboratory data with gamma-ray well logs and correlating these with the plotted composite log. Subsequently, microfacies analysis and sequence differentiation were carried out following the methodology of Flugel (2010). Carbonate rock nomenclature was based on the Dunham (1962) classification, while facies belt comparisons were made according to Flugel (2010).
Sequence-stratigraphic analysis was performed using the approaches of Hunt and Tucker (1992, 1995) and was correlated with the tectonostratigraphic megasequences proposed by Sharland et al. (2001). Gamma-ray logs were also employed to refine and accurately delineate certain lithological and sequence boundaries.
Results and Discussion
Stratigraphy: In the Bostaneh Anticline, the Gachsaran Formation, with a total thickness of 1,288 meters, is subdivided into the Chehel (1087 m), Champeh (170 m), and Mol (31 m) members. This formation consists of alternating layers of salt, anhydrite, limestone, argillaceous limestone, and marl, and it conformably overlies the Pabdeh Formation and underlies the Mishan Formation. Paleontological investigations resulted in the identification of 32 genera and 47 species of foraminifera. The presence of key index fossils such as Borelis melo curdica, Austrotrillina howchini, Peneroplis evolutus, Globorotalia praescitula, and Miogypsina sp. confirms an Early Miocene (Aquitanian–Burdigalian) age for this section.
Microfacies and Depositional Environment: Microscopic analyses led to the identification of five main microfacies, which belong to two major depositional environments: peritidal (Sabkha) and lagoonal settings.
Peritidal Microfacies

A) Alternating evaporite and mudstone layers:

This microfacies contains diverse anhydrite textures including laminated, lath-shaped, dispersed and isolated evaporitic crystals, needle-shaped, and swallowtail structures indicating highly evaporative conditions characteristic of peritidal Sabkha environments. The rhythmic alternation of layers reflects short-term fluctuations in relative water level. This microfacies corresponds to RMF25 of Flugel (2010).

B) Mudstone microfacies:

The dominant matrix is micrite with a mudstone texture, containing less than 1% carbonate and non-carbonate allochems. The absence of faunal diversity, together with the presence of iron oxide and detrital particles, suggests deposition in a very shallow, restricted environment with limited water circulation, likely a coastal Sabkha. This microfacies resembles RMF19 of Flugel (2010).
Lagoonal Microfacies

C) Miliolid wackestone:

The presence of porcelaneous foraminifera (e.g., miliolids) within a micritic matrix indicates deposition in a semi-enclosed, low-energy lagoon. This microfacies corresponds to RMF16 of Flugel (2010).

D) Bioclastic peloidal packstone-wackestone:

The presence of peloids and hyaline foraminifera within a micritic-sparry matrix suggests deposition in the middle parts of the lagoon. This microfacies is comparable to RMF20 of Flugel (2010).

E) Bioclastic wackestone-packstone:

An increased abundance of hyaline foraminifera suggests deposition in the outer lagoon, near sand shoals. This microfacies also corresponds to RMF20 of Flugel (2010).
Overall, the microfacies in the studied section transition gradually from one to another. This, together with the absence of reefal structures, as well as the lack of cortoids, oncoids, pisoids, and aggregate grains features typically associated with carbonate shelf settings and the absence of slump or gravity-induced deposits, indicates that sedimentation took place on a carbonate ramp comprising peritidal and lagoonal environments.
Sequence Stratigraphy: Based on relative sea-level variations and paleoenvironmental interpretations, two depositional sequences and three sequence boundaries (two SB1 and one SB2) were identified.
The first sequence, with a thickness of 1013 meters, includes LST, TST, and HST systems tracts and is bounded by SB1 and SB2. It begins with evaporitic and carbonate deposits of the Chehel Member. The upward increase in benthic foraminifera and miliolid abundance particularly within the TST and HST reflects variations in water depth and depositional energy. The transgressive surface (TS) and maximum flooding surface (MFS) occur at approximately 608 m and 538 m, respectively. These surfaces are identifiable on the gamma-ray log through distinct API shifts, with a marked decrease indicating the TS and an increase corresponding to the MFS.
The second sequence, with a thickness of 74 meters, contains TST and HST systems tracts and extends from the upper part of the Champeh Member into the deposits of the Mol Member.
The results of this study correlate well with the Ap11 tectonostratigraphic megasequence and the Ng10 and Ng20 maximum flooding surfaces proposed by Sharland et al. (2001). Although this research focuses on a single stratigraphic section, it provides a valuable foundation for future investigations and for developing a comprehensive analysis of the sedimentary basin.
 
Conclusion
The Gachsaran Formation in the Bostaneh Anticline section, with a total thickness of 1288 meters, comprises the Chehel, Champeh, and Mol members. In the studied subsurface interval, the formation consists of alternating layers of limestone, argillaceous limestone, marl, anhydrite, and a considerable thickness of salt, and it is stratigraphically positioned between the overlying Mishan Formation and the underlying Pabdeh Formation.
Microfacies analysis reveals five major microfacies, including evaporite-mudstone alternations, mudstone, 
miliolid wackestone, bioclastic-peloidal packstone-wackestone, and bioclastic wackestone-packstone, representing deposition in peritidal to lagoonal environments. The absence of cortoids, oncoids, pisoids, and aggregate grains features typical of carbonate shelf settings supports a carbonate ramp as the depositional model for this formation.
Sequence stratigraphic interpretations, integrated with paleontological evidence, demonstrate the presence of two depositional sequences: a lower complete sequence consisting of LST, TST, and HST, and an upper sequence comprising TST and HST. Additionally, three sequence boundaries were identified (two SB1 and one SB2).
The stratigraphic succession at this section correlates well with the Ap11 tectonostratigraphic megasequence and the maximum flooding surfaces Ng10 and Ng20 defined by Sharland et al. (2001). Although this investigation focuses on a single section, it provides a robust basis for future regional studies and contributes to a more comprehensive understanding of the sedimentary evolution of the basin.