Diagenetic evolution of Upper Cretaceous carbonate sequences (Sarvak Formation) in the Abadan Plain, Iran: evidence from petrographic, elemental, stable, and strontium isotopic data

This study investigates the diagenetic evolution and geochemical variations within the Sarvak Formation by integrating the core, thin-section, scanning electron microscopy, X-ray diffraction, trace elements, carbon-oxygen, and strontium isotopic data from three wells in the Abadan Plain. The formation displays diverse diagenetic processes, influenced by multiple exposure surface condition including the Cenomanian-Turonian boundary (CT-ES) and middle Turonian (mT-ES). Meteoric diagenesis, shaped by tectonic activities and eustatic sea level changes, played a pivotal role, leading to dissolution (karstification), brecciation, cementation, and palaeosol formation. Geochemical analysis revealed significant elemental variations across the studied wells. Magnesium (Mg) exhibited peaks at sequence boundaries, responding to facies changes, while iron (Fe) and manganese (Mn) concentrations increased below disconformities, reflecting changes in redox conditions and karstification. Sodium (Na) concentrations varied with facies changes, showing peaks at sequence boundaries, indicative of palaeoexposure events. Strontium (Sr) isotopes aided in delineating stratigraphic positions of palaeoexposure surfaces and absolute dating of sequences. Increases in⁸⁷Sr/⁸⁶Sr ratios below disconformities suggested subaerial exposure events, with shorter durations at CT-ES and longer durations at mT-ES, contributing to dissolution features development. The δ¹³C_carb and δ¹⁸O_carb values indicated severe meteoric diagenesis below sequence boundaries, characterized by dissolution and reprecipitation of calcium carbonate as low-magnesium calcite (LMC) cements. Cross plots of δ¹³C_carb vs. δ¹⁸O_carb displayed inverted-J patterns, indicative of meteoric diagenesis effects. Comparison with previous studies highlighted similar trends in geochemical variations, with notable differences in element concentrations between wells. This study enhances our understanding of diagenetic history of the Sarvak Formation in the Abadan Plain and, in a larger scale, provides insights into diagenetic evolution of palaeoexposure-related carbonate sequences.