Eman W. Al-Berzanje Petrography, Microfacies, Depositional Environment and Diagenetic Evolution of the Upper Triassic Kurra Chine and Baluti Formations, Northern Iraq

Abstract

The Upper Triassic carbonate succession of northern Iraq represents an important record of the northeastern Arabian Plate passive margin and provides valuable information about carbonate platform evolution during the development of the Neo-Tethys Ocean. This study investigates the petrography, microfacies characteristics, depositional environments, and diagenetic evolution of the Baluti and Kurra Chine formations based on detailed microscopic examination of carbonate thin sections from the Ranya and Sirwan sections, northern Iraq. Petrographic analysis reveals that the studied succession is dominated by limestone, dolomitic limestone, and dolostone affected by extensive diagenetic modification. Six major microfacies were recognized within both formations based on carbonate texture, allochem composition, Standard Microfacies (SMF), and Wilson facies zones. The identified microfacies indicate deposition within a shallow homoclinal carbonate ramp extending from restricted lagoonal environments to open lagoon and shoal-margin settings. The Kurra Chine Formation records relatively open inner-ramp conditions, characterized by transitions from restricted lagoon (FZ8) through open lagoon (FZ7) to shoal-margin environments (FZ6). In contrast, the Baluti Formation represents a more restricted carbonate system dominated by lagoonal conditions with intermittent siliciclastic input and localized open marine influence. Petrographic observations indicate that both formations experienced complex diagenetic histories involving dolomitization, recrystallization, calcite cementation, dissolution, compaction, and fracturing. Dolomitization represents the dominant diagenetic process and played a significant role in modifying primary carbonate fabrics and developing secondary intercrystalline porosity. Dissolution locally enhanced pore development, whereas late calcite cementation and compaction reduced pore connectivity. The results demonstrate that the Baluti and Kurra Chine formations record cyclic changes in depositional conditions related to relative sea-level fluctuations during the Late Triassic. The integration of petrographic and microfacies analysis provides a comprehensive understanding of carbonate platform evolution, diagenetic modification, and reservoir-quality controls within Upper Triassic carbonate successions of northern Iraq.