Biostratigraphy of Campanian—Maastrichtian sequences and facies analysis in Anaran and Samand Anticlines, Zagros, Iran

Abstract

Gurpi Formation is an important sedimentary sequence in the Zagros area of southwest Iran. It provides effective cap rock at the top of the Bangestan reservoirs and acts as a possible source for younger hydrocarbon reservoirs. Integrated facies analysis and paleontological study of this formation are presented in two surface sections located on the Samand and Anaran anticlines of Lorestan Province. The formation has mainly composed of 203–298 m of argillaceous limestones with two clean limestone members of “Seymareh” and “Imam Hassan.” In the biostratigraphic part of this study, 51 species related to 16 genera of planktonic foraminifers in the Dareh Vorazan section and 44 species belong to 16 genera of planktonic foraminifera in Pahleh section were distinguished. Accordingly, four biozones have been determined in the Dareh Vorazan and the Pahleh sections which are Globotruncana aegyptiaca, Gansserina gansseri, Contusotruncana contusa, and Abathomphalus mayaroensis. Consequently, the Gurpi Formation has an age of Late Campanian–Late Maastrichtian. The location of the Cretaceous–Paleogene (K-Pg) boundary is detected at the top of this formation. Macroscopic field observations integrated with microscopic studies have resulted in the recognition of four microfacies types that belong to open lagoon, shoal, proximal, and distal open marine environments of shallow inner to deep basinal settings. Depositional facies and environments of studied sections are interpreted in a regional depositional model.