Isotopic and Sedimentologic Proxies of Palaeoexposure Events and Oceanic Anoxic Event 2 (OAE2) in the Cenomanian–Turonian Neritic Carbonates (the Sarvak Formation) in the Abadan Plain, Zagros Basin, Iran

Abstract

The shallow marine carbonate deposits of the Sarvak Formation have been deposited in an active tectonic setting under warm and humid climatic conditions during the Cenomanian–Turonian (C–T). The focus of this study is on the sedimentological and geochemical analysis of this formation in the Abadan Plain, Zagros Basin. Petrographic studies led to the identification of five microfacies deposited in the restricted lagoon, tidal flat, reef, shoal and open marine environments. They indicate deposition of the Sarvak Formation on a homoclinal ramp–like carbonate platform. Sequence stratigraphy studies have led to the identification of two third-order sequences named Cenomanian sequence (DSS-1) and Turonian sequence (DSS-2), along with six fourth-order sequences. The upper boundary of DSS-1, known as the C–T disconformity (CT-ES), is subjected to meteoric diagenetic processes, as evidenced by meteoric dissolution (karstification), collapsed brecciation and the development of palaeosols. The upper SB of DSS-2 is marked by the mid-Turonian disconformity (mT-ES), characterised by silicification, brecciation, meteoric dissolution and iron oxide staining of core samples. The significant decreases in δ¹⁸O_carb and δ¹³C_carb values indicate a strong effect of meteoric diagenesis beneath the palaeoexposure surfaces. The calculation of sea surface temperatures (SSTs) based on the δ¹⁸O_carb values measured from the unaltered carbonates indicated that the average SST for the C–T is 31.8°C and 30.9°C in K-01 and K-02 wells, respectively. A decrease of about 1.5°C is measured during the C–T transition, which indicate a cooling trend of the Late Cretaceous in the Tethyan realm. The carbon isotopic pattern observed in the analysed sections shows correlation with previous studies conducted in the Tethyan region. Additionally, the isotopic composition indicative of Oceanic Anoxic Event 2 (OAE2) is identified around the C–T boundary. Nevertheless, the signature of OAE2 has been somewhat obscured by diagenetic processes associated with C–T palaeoexposure.