Foraminiferal Biostratigraphy of the Gachsaran Formation in Bostaneh Anticline, northwest of‎‎ Bandar- e Lengeh

The first study on the Gachsaran Formation was first done by Pilgrim (1908), and others continued investigations from the 1920s (e.g., Richardson, 1924; Furon, 1941; Watson, 1960; James & Wynd, 1965; Setudehnia, 1972) Stratigraphic and sedimentologic studies of the Gachsaran Formation were carried out by Gill & Ala (1972), Kashfi (1980) and Motiei (1993). The studies were continued by Bahroudi & Koyi (2004), Moussavi Harami et al. (2010), Bahadori et al. (2011), Torabi et al. (2012), Gorjian et al. (2012), Aghdam et al. (2013) and Soleimani & Bahadori (2015). Most of these studies considered for geophysical properties, digenesis, and the geochemistry of deposits. There are a few biostratigraphic studies. Hence, the study of biostratigraphy and age determination of the Gachsaran Formation in this area improves understanding of this formation and provides useful and valuable information for exploration.The study area is located in the Zagros zone and the Coastal Fars as a sub-basin. Herein, we investigate the Gachsaran Formation for the identification of foraminiferal taxa, faunal assemblages, and biozones at a subsurface section in Bostaneh anticline located in the northwest of Bandar- e Lengeh.

In this research, 446 thin sections of well cutting were examined. Taxonomy of foraminifera was carried out in the micropaleontology laboratory of the Kharazmi University, using Loeblich and Tappan (1988), Henson (1950), Kennett & Sirinivasan (1983), Huang (1964), Adams & Bourgeois (1967), Papp & Schmid (1984), Bolli & Saunders (1985), Postuma (1971), Daneshian & Ramezani Dana (2007). Based on Gamma-ray logs and laboratory results, the lithostratigraphic boundaries were determined. The resulted biostratigraphy of foraminifers was followed and compared to those presented by Wynd (1965) and Adams & Bourgeois (1967).

The thickness of the Gachsaran Formation in Bostaneh anticline is 1288 m and consists of limestone, argillaceous limestone, anhydrite, salt deposits, marl, and thin layered shale. The studied sequence thickness is divided to Chehel, Champeh, and Mol members. In the investigated section, Gachsaran Formation underlies the Mishan Formation and overlies the Pabdeh Formation. Totally, 30 genera and 42 species of benthic foraminifera, and two genera and five species of planktic foraminifera were identified, from which the following zones have been identified.