Impact of the Paleocene–Eocene Thermal Maximum (PETM) on organic petrographic characteristics of the basal Pabdeh Formation at Kuh-e-Gurpi Anticline, SW Iran

AbstractThis study investigates the impact of the Paleocene–Eocene Thermal Maximum (PETM) event on the organic petrographic characteristics of the base Pabdeh Formation in the Tang-e-Hati section, located at the Kuh-e-Gurpi Anticline, SW Iran. To delineate the Paleocene–Eocene boundary, nanofossil analyses were employed. Additionally, the collected samples from the studied section were investigated using organic petrographic methods under reflected white light. The nanofossil results indicate that the Paleocene–Eocene boundary is situated approximately 26.5 meters from the base of the Pabdeh Formation, specifically between subzones NP9a and NP9b, marked by the presence of the key species: Discoaster araneus, Rhomboaster cuspis, and R. spineus. Furthermore, the organic petrographic results suggest that the studied marls were deposited under oxidizing conditions. Results from this study are consistent with the presence of a grey marl with relatively darker color, lower fossil concentration, and higher organic matter concentrations compared to the lower and upper parts. Results from this study conclusively suggest that deposition of the grey marl was associated with a short-term relative sea-level fall, which in turn led to an increased sedimentation rate in the basin and a greater influx of terrestrial organic matter.Keywords: Purple shale, Organic petrography, Paleocene–Eocene boundary, PETM incident, Tang-e-Hati IntroductionOrganic geochemistry, focusing on the study of organic compounds in sediments and their interactions with geological processes, plays a vital role in hydrocarbon exploration (Peters et al. 2005). One of the key analytical approaches in this field is organic petrography, which enables the identification of macerals, kerogen types, thermal maturity, and palaeo-depositional environments of potential source rocks (Hackley & Cardott 2016). The Paleocene–Eocene Thermal Maximum (PETM) was a short-lived but intense global warming event, associated with a 5–8 °C rise in global temperatures, ocean acidification, and major disruptions in the carbon cycle (Zachos et al. 2008).In southwestern Iran, the Pabdeh Formation, with its continuous marl and purple shale strata, provides an excellent opportunity to assess variations in organic matter preservation across the PETM (Motiei 1993). Although numerous studies have investigated the geochemical properties of the Pabdeh Formation (Alizadeh et al. 2012; Safaei-Farouji et al. 2022; Hosseiny et al. 2024), little is known about its organic petrographic characteristics, particularly across the Paleocene–Eocene boundary. This study aims to fill that gap by examining the organic matter variations and palaeo-depositional conditions during the PETM by means of organic petrography techniques. Material & Methods In this study, 24 samples were systematically collected from the base of the Pabdeh Formation at the Tang-e-Hati section (southern flank of the Kuh-e-Gurpi Anticline). Sampling intervals were generally less than 5 meters; however, in the vicinity of the Paleocene–Eocene boundary, the interval was reduced to approximately 1 meter or less to allow for a more detailed investigation of PETM-related changes. The collected samples, consisting of both consolidated and unconsolidated materials, were transferred to the laboratory for calcareous nannofossil and organic petrographic analyses. Calcareous nannofossils were prepared using the standard smear slide technique (Bown & Young, 1998), and examined under a polarized Olympus BX60 microscope at 1250× magnification. Species identification was carried out based on established references (Perch-Nielsen 1985; Agnini et al. 2014) and biozonation and boundary placement followed the schemes of Martini (1971), Romein (1971), and Aubry (1998). For organic petrographic studies, polished pellets were prepared from small fragments (approximately 1.5 × 1.5 cm) of the collected samples. These fragments were embedded in a 2:1 mixture of epoxy resin and hardener using standard protocols. After 24 hours, the samples were removed from the molds and polished according to the standard procedures (Bustin et al. 1985; Taylor et al. 1998). Petrographic observations were performed using a Zeiss Axioplan II microscope at 100× magnification under oil immersion. Discussion of Results & ConclusionsIn this study, the Paleocene–Eocene boundary was identified at approximately 26.5 meters above the base of the Pabdeh Formation, based on the recognition of nannofossil subzones NP9a and NP9b. Organic petrography revealed three distinct sections differing in organic matter content and color: i) a lower purple shale with low organic matter contents which was precipitated under oxidizing conditions, ii) a middle grey marl with higher organic content linked to the PETM event and increased water acidity, and iii) an upper purple shale with decreased organic content and a return to oxidizing conditions. Changes in color and organic matter content correspond to fluctuations in the relative sea level, sedimentation rate, and pH of the water. During deposition of the lower purple shale, higher sea levels and lower sedimentation rates favored good fossil preservation. During the PETM interval, sea level dropped, sedimentation rate increased, and water acidity increased, resulting in reduced fossil preservation and increased terrestrial organic matter input. After the PETM, sea level rose again, terrestrial organic matter input decreased, and fossil preservation improved. These results highlight the interplay of sea-level changes, sedimentation rates, and water pH in controlling the organic petrographic characteristics of the base Pabdeh Formation during the PETM, providing valuable insights for reconstructing palaeo-depositional environments in this part of the Zagros Basin.