چكيده لاتين :
Introduction
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.
Materials and Methods
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).
Results and Discussion
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. At 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.
Austrotrillina howchini - Peneroplis evolutus assemblage Zone (biozone no. 60, Wynd, 1965)
This biozone from 1258 m. to 522 m. depth, consists of the upper part of Chehel Member and lower part of Champeh Member. The biozone is distinguished by the appearance, accompaniment and the extinction of Austrotrillina howchini and Peneroplis evolutus with Aquitanian age in the stratigraphic subsurface section. This biozone is equivalent to biozone no. 2 (Miogypsinoides – Archaias - valvulinid Assemblage Zone) of Adams & Bourgeois (1967) which defined for Asmari Formation.
This biozone includes two Assemblage Subzones that are as follows:
2a. Elphidium sp.14 - Miogypsina Assemblage Subzone This subzone with 74 m. thickness (from 596 m. to 522 m. depth) belongs to middle part of Champeh Member. In this part, Elphidium sp. 14 and Miogypsina sp. are abundant. Therefore, this is equivalent to subzone no. 2a.
2b. Archaias asmaricus - Archaias hensoni Assemblage Subzone
In the study well, from 1288 m. to 596 m. depth, is comparable to subzone no. 2b because of the presence of Archaias sp.
Borelis melo curdica Zone (biozone no. 62, Wynd, 1965)
This biozone (from 522 m. to 450 m. depth) includes the upper part of the Champeh to the bottom of the Mol members. According to the first appearance of Borelis melo curdica this part of Gachsaran Formation belongs to Burdigalian.
Faunas of the Fars Group (faunal assemblages no. 63, Wynd, 1965)
Wynd (1965) did not define the biozones for the Fars Group but described the main faunal elements which may be found in the formations of this group. In other words, based on microscopic and macroscopic faunas which found in the Gachsaran Formation, an assemblage was listed no. 63. This fossil assemblage includes Rotalia - Nonion and locally Chlorophyte assemblage from 450 to 406 m. depth, in the upper part of studying subsurface section with 44 m. thickness due to the abundance of Rotalia viennotti, Rotalia sp. and presence of Nonion sp. So an Early Miocene age (Burdigalian) is suggested for this part. In Bostaneh well, biozone no. 62 and faunal assemblages no. 63 of Wynd (1965) is comparable to biozone no. 1 of Adams & Bourgeois (Borelis melo group - Meandropsina iranica Assemblage Zone).
The assemblage zone from 522 m. to 406 m. depth assigned to Burdigalian, including a part of the Champeh and deposits of Mol members. The presence of Borelis melo curdica with Dendritina rangi, Peneroplis farsensis and Valvulinid sp.2 defines biozone no. 1 Adams & Bourgeois (1967). Therefore, the Gachsaran Formation belongs to the Lower Miocene (Aquitanian to Burdigalian) because of the mentioned index foraminiferal taxa.
