Sequence stratigraphy of the Gurpi Formation in the Lendeh area (NW of Dehdasht) and its comparison with Aghar (SW of Firozabad) and Kuh-Siah (N of Dehdasht) Anticlines

The Gurpi Formation, the hydrocarbon source rock in the southwest Iran, is extended in Folded Zagros in Provinces Khuzestan, Lurestan, and Fars (James & Wynd, 1965). James & and Wynd (1965) measured the Gurpi Formation at the type section (49°13΄47˝ E, 32°26΄50˝N). The formation at the type section includes grey to blue marl and shale beds and occasionally thin beds of argillaceous limestone (Aghanabati, 2006). The Formation overlies the Ilam Formation and is disconformably overlain by the Pabdeh Formation at the type section. The marl and carbonate rocks of the Gurpi Formation were formed in a deep marine environment. Microfauna of the Gurpi Formation was studied by Kalantary (1976), Vaziri Moghaddam (2002), Darvishzadeh et al. (2007) and Fereydoonpour et al. (2104). The main objective of this paper is to identify depositional sequences by the study of planktonic foraminifera, planktonic to benthic ratio (P/B), percentage of morphotype groups of planktonic foraminifera and the carbonate content.

The Gurpi Formation in the study area is located at 14 km North of Lendeh city (50°28′ 94˝ E, 31° 01 ′11˝ N). The thickness of the Gurpi Formation at the Lendeh area (NW of Dehdasht) is 221m. In the study area, the formation overlies the Ilam Formation and is disconformably overlain by the Pabdeh Formation. Two hundred and eighteen of soft and hard samples were collected for this study. One hundred and eighty-eight of thin sections were prepared to analyze the planktonic foraminifera in hard samples. About 100 g of each soft sample was processed. The samples were dried, soaked in hydrogen peroxide (10%). The disaggregated samples were washed through a 63 μm, 125 μm, and 250 μm sieves. About 300 individuals were picked up for each sample in two size fractions (125–250 μm). The species and genera of planktonic foraminifera were identified based on Sliter (1989), Robozynki & Caron (1995), and Premoli Silva & and Verga (2004). The parameters such as the planktonic morphogroups, CaCO3 content, and P/B ratio were used for palaeodepth and sea-level changes of the Gurpi Formation (Mabrouk et al., 2007; Hart & and Baily, 1979; Mancini & and Tew, 1996; Mancini et al., 1996).

The study of planktonic foraminifera of the Gurpi Formation in North of Lendeh area led to the recognition of nine planktonic foraminifera biozones, that included: Dicarinella concavata Zone, Dicarinella asymetrica Zone, Globotruncanita elevata Zone, Globotruncana ventricosa Zone, Radotruncana calcarata Zone, Globotruncanella havanensis Zone, Globotruncana aegyptiaca Zone, Gansserina gansseri Zone, and Contusotruncana contusa Racemiguembelina fructicosa Zone. These biozones indicate that the Gurpi Formation deposited during the Santonian to Middle Maastrichtian. Based on the distribution of morphotype groups of planktonic foraminifera, planktonic to benthic ratio (P/B), and content of carbonate, nine third-order depositional sequences have been recognized. The sequences are comparable to those recognized in other parts of the Zagros Basin.