Planktonic foraminiferal biostratigraphy of the upper Turonian-lower Santonian succession in the Lorestan Zone of the Zagros sedimentary Basin, SW Iran

Abstract

The Surgah Formation was studied in four sections (Tang-e-Shirin Ab, Kuh-e-Surgah Holostem Valley, and Sarkan) from the NW-SE of the Zagros sedimentary Basin. Forty-one planktonic foraminiferal species belonging to eighteen genera were recognized. These species were used to identify three zones across the Surgah Formation from the bottom to the top: Dicarinella primitiva – Marginotruncana sigali Zone, Dicarinella concavata Zone, and Dicarinella asymetrica Zone. Based on these data, the Surgah Formation was assigned to the Late Turonian-Santonian age. The base of the Coniacian Stage projects within the lower part of the Dicarinella concavata Zone and is traced at the first occurrence (FO) of Pseudotextularia nuttalli. Owing to the absence of Pseudotextularia nuttalli at the Sarkan well, we utilized the extinction of Whiteinella aprica to estimate the base of the Coniacian Stage. The Coniacian/Santonian (C/S) boundary is placed within the basal part of the Dicarinella asymetrica Zone based on the FO of Globotruncana linneiana. The base of the Santonian Stage denotes marked radiation in the globotruncanids among the planktonic foraminiferal assemblage. Great lithofacies and thickness variations have been observed within the D. concavata Zone. These variations likely indicate that the shoreline was located northwest of the Lorestan province during the Coniacian. In contrast, the southwest extreme of the Lorestan province was characterized by deep-water settings.