دانشگاه اصفهان پژوهش های چینه نگاری و رسوب شناسی 2008-7888 42 2 2026 06 22 Depositional environment evolution and biogeochemical changes across the Permian–Triassic transition in the central Persian Gulf تحول محیط رسوبی و تغییرات زیستی- ژئوشیمیایی در گذر پرمین- تریاس خلیج‎‍فارس مرکزی 17 50 29992 10.22108/jssr.2025.146573.1320 FA نوش آفرین حقیقت دکتری زمین‌شناسی، گروه زمین‌شناسی، دانشکده علوم زمین، دانشگاه خوارزمی، تهران، ایران حسین هاشمی دانشیار، گروه زمین‎‍شناسی، دانشکده علوم زمین، دانشگاه خوارزمی‎‍، تهران، ایران وحید توکلی استاد، دانشکدۀ زمین‎‍شناسی، دانشکدگان علوم، دانشگاه تهران، تهران، ایران Journal Article 2025 09 05 Abstract In this study, the stratigraphic distribution of foraminifera and the environmental evolution of the Permian–Triassic boundary (PTB) successions in the central Persian Gulf (upper Dalan and the basal part of the Kangan formations) were investigated through the integration of micropaleontological, microfacies, and isotopic analyses (δ¹³C, δ¹⁸O, and ⁸⁷Sr/⁸⁶Sr). The results reveal three local composite biozones within the upper Dalan and lower Kangan formations, along with the abundance of the index taxon Paradagmarita, which shows biogeographic affinity with approximately coeval strata in Turkey, the Caucasus, Saudi Arabia, and Oman. Microfacies analysis identified three facies belts including lagoonal, subtidal shoal, and tidal flat, indicating a general shallowing trend from lagoonal toward peritidal settings. A simultaneous decrease in δ¹³C and δ¹⁸O values was observed near the boundary, coinciding with the biotic crisis and the development of oxygen-depleted conditions. The increase in ⁸⁷Sr/⁸⁶Sr ratios across the boundary suggests enhanced influx of continental materials and intensified chemical weathering. The results also indicate a moderate increase in ooid size and significant changes in foraminiferal assemblages, reflecting a relatively shallow, warm, stressed, and oxygen-deficient environment. The novelty of this study lies in the combined application of three independent datasets, including sedimentological, geochemical, and paleontological, from multiple subsurface sections, enabling a more precise reconstruction of the environmental evolution and biotic events at the onset of the Triassic. Keywords: Foraminifera, Microfacies, Ooids, Carbon and oxygen isotopes, ⁸⁷Sr/⁸⁶Sr, Permian–Triassic boundary, Persian Gulf Introduction The Permian–Triassic transition marks the most profound biotic crisis in Earth’s history, with over 90% of marine species becoming extinct. This event significantly transformed carbonate platforms and their sedimentary environments. The Dalan and Kangan formations, representing the Late Permian to Early Triassic succession in the Zagros and Persian Gulf regions, record these changes in detail. Previous works have discussed the sedimentological and geochemical evolution of this interval (Abdolmaleki and Tavakoli 2016; Rafiei et al. 2016; Tavakoli et al. 2018; Haghighat et al. 2020). However, few studies have combined micropaleontological, isotopic, and microfacies evidence from multiple subsurface sections to elucidate the precise environmental evolution during this critical interval. The present study provides a comprehensive reconstruction of the Permian–Triassic boundary (PTB) based on integrated data from four wells in the central Persian Gulf. Materials & Methods The study is based on petrographic and geochemical analyses of 2,500 thin sections from the Dalan and Kangan formations in four wells (A, B, E, and F). Samples were taken at 30 cm intervals to ensure high stratigraphic resolution. Foraminiferal assemblages were identified under transmitted and polarized light microscopes. Microfacies were described and classified following Folk (1959), Dunham (1962), and Embry & Klovan (1971). Stable isotopes (δ¹³C and δ¹⁸O) were analyzed on micritic matrix samples to minimize diagenetic alteration. Samples with micritic fabrics (wackestones and fine packstones) were preferred due to their low permeability and better preservation of primary isotopic composition. Strontium isotope ratios (⁸⁷Sr/⁸⁶Sr) were determined from wells B, E, and F. Petrographic and geochemical screening confirmed the absence of secondary dolomitization or recrystallization features in the selected samples, following the procedures described in Tavakoli (2015) and Abdolmaleki and Tavakoli (2016). Discussion of Results & Conclusions Foraminiferal Assemblages: Micropaleontological analysis revealed 51 species (41 genera) of benthic foraminifera dominated by Globivalvulina, Dagmarita, and Paradagmarita. Two major biozones were defined: the Glomomidiellopsis–Paradagmarita Assemblage Zone (Changhsingian) and the Microconchus–Ammodiscus kalhori Assemblage Zone (Griesbachian). These assemblages correlate well with equivalent zones in Saudi Arabia, Oman, Turkey, and the Caucasus (Haghighat et al. 2020). The disappearance of Late Permian taxa and the dominance of opportunistic forms (Ammodiscus kalhori, Microconchus phlyctaena) above the boundary indicate severe environmental stress and low-oxygen conditions. Microfacies Evolution: Nine microfacies types were recognized, ranging from ooid and bioclastic grainstones to thrombolitic boundstones and dolomitic mudstones. The upward transition from subtidal grainstones to peritidal thrombolitic facies indicates progressive shallowing. Ooid grainstones in the uppermost Dalan suggest deposition in high-energy shoal settings. In contrast, microbial boundstones in the lowermost Kangan represent early recovery of carbonate production under oxygen-depleted, restricted conditions. This pattern matches earlier models for the PTB carbonates in the Persian Gulf (Tavakoli et al. 2018; Davoodi et al. 2024). Isotopic Trends: The isotopic curves show a pronounced negative excursion in δ¹³C (from +4‰ to −1‰) and δ¹⁸O, synchronous with a rise in ⁸⁷Sr/⁸⁶Sr ratios across the PTB. The δ¹³C drop corresponds to global disturbances in the carbon cycle, potentially related to methane release, enhanced continental weathering, and volcanogenic CO₂ emissions (Tavakoli and Rahimpour-Bonab 2012). The gradual depletion of δ¹⁸O toward the boundary likely reflects rising seawater temperature and meteoric diagenesis under greenhouse conditions (Abdolmaleki and Tavakoli 2016; Naderi-Khujin et al. 2016). Elevated ⁸⁷Sr/⁸⁶Sr ratios across the boundary further support intensified continental weathering and influx of radiogenic strontium during the end-Permian climatic crisis. Integrated Interpretation: Integration of faunal, sedimentological, and isotopic data indicates a shift from a well-oxygenated, open-marine platform during the Late Permian to a restricted, shallow, and stressed lagoonal system in the Early Triassic. The decline in biodiversity, increase in microbial facies, and negative isotopic excursions reflect the combined effects of eustatic fall, climatic warming, and oceanic anoxia. These changes represent the regional expression of global end-Permian perturbations. The integrated multiproxy approach employed here refines previous reconstructions (Haghighat et al. 2020; Nazemi et al. 2021; Davoodi et al. 2024; Rezvannia et al. 2025; Shahkaram et al. 2025) and provides a more complete picture of the environmental transition in the central Persian Gulf. در این پژوهش، پراکندگی چینه‌شناسی روزنبران و تحول محیطی توالی‌های مرز پرمین–تریاس در بخش مرکزی خلیج‎‍فارس (عضو بالایی دالان و قاعدۀ سازند کنگان) با تلفیق مطالعات فسیل‌شناسی، ریزرخساره‌نگاری و آنالیزهای ایزوتوپی δ¹³C)، δ¹⁸O و (87Sr/86Sr بررسی شد. نتایج، سه زیست‌زون تجمعی محلی در دالان بالایی و قاعدۀ کنگان و فراوانی شاخصِ Paradagmarita را نشان می‌دهد که هم‌پوشانی زیست‌جغرافیایی با پهنه‌های تقریباً هم‌عهد در ترکیه، قفقاز، عربستان و عمان دارد. تحلیل ریزرخساره‌ها، سه کمربند رخساره‌ای کولاب، پشته‌های زیرآبی و پهنه کشندی را مشخص کرد که دلالت بر روندِ کاهش عمق از رخسارۀ کولاب به‎‍‎‍سوی پیراکشندی دارد. کاهش هم‌زمان δ¹³C و δ¹⁸O در نزدیکی مرز مشاهده شد که با بحران زیستی و شرایط کم‌اکسیژن منطبق است. افزایش نسبت87Sr/86Sr در مرز نشان‌دهندۀ افزایش نفوذ مواد قاره‌ای و شدت‎‍یافتن فرسایش شیمیایی است. نتایج نشان‌دهندۀ رشد متوسط اندازۀ اائیدها و تغییرات جمعیتی روزن‌بران و محیطی نسبتاً کم‌عمق، گرم، پرتنش و کم‌اکسیژن است. نوآوری این مطالعه در کاربرد هم‌زمان سه حوزۀ داده‌ای (رسوب‌شناسی، ژئوشیمی، فسیل‌شناسی) در چند برش زیرسطحی است که بازسازی دقیق‌تر تحول محیطی و رخدادهای زیستی آغاز تریاس را ممکن می‌کند.

https://jssr.ui.ac.ir/article_29992_4650b140c77c71beede580144da021fa.pdf