Characteristics and paleoenvironmental conditions of the well-preserved Early Mesoproterozoic Fengjiawan Formation stromatolites in the southern North China Craton

• Published in: Precambrian research 2024-06, Vol.405, p.107370, Article 107370
• Main Authors: Khaled, Ahmed, Li, Rongxi, Zhao, Bangsheng, Liu, Futian, Wu, Xiaoli, Zhang, Yanni, Qin, Xiaoli, Yu, Qiang
• Format: Article
• Language: English
• Subjects: Early Mesoproterozoic; Geochemistry; Growth mechanisms; Paleoenvironments; southern North China Craton; Stromatolites
• ISSN: 0301-9268; 1872-7433
• DOI: 10.1016/j.precamres.2024.107370

•The lower part of the Fengjiawan Formation contains abundant stromatolitic dolostones that possess well-preserved primary structures.•The depositional environment was an open shallow marine associated with a passive continental margin setting.•The stromatolitic dolostones are characterized by suboxic environment with minimal detritus contamination and weak post-depositional alteration.•Multiple depositional factors contributed to the growth and excellent preservation of the stromatolite-dominated microbialites. Stromatolites in the lower part of the Early Mesoproterozoic Fengjiawan Formation in the southern North China Craton (NCC) contain abundant microbial dolostones with well-preserved primary structures. This study comprehensively investigates the macromorphological, petrographical, mineralogical, and geochemical characteristics of these pure stromatolite-dominated microbialites for the first time. The results reveal diverse macromorphological structures distributed from the bottom to the top as conical (lower intertidal to upper subtidal zones), hemispherical domal (lower supratidal zone), spherical domal (upper to middle intertidal zone), and more abundant columnar (upper to middle subtidal zone) stromatolites. The mineral composition of these stromatolites indicates that they are pure dolostones, predominantly consisting of dolomite (avg. 94.24 wt%). These stromatolite-dominated microbialites were subjected to minimal detrital contamination and weak post-depositional alteration as documented by different geochemical multiproxy evaluations. This indicates that they have effectively retained the composition of the seawater in which they were initially formed. Geochemical indicators, such as Y/Ho, La/La*, Gd/Gd* and Ce/Ce* strongly suggest deposition of the stromatolites in an open shallow marine environment characterized by a passive continental margin setting. Different elemental ratios, such as Ce/Ce*, MoEF, UEF, MoEF/UEF, imply the stromatolites formation under a suboxic redox environment, where the photosynthesis of cyanobacteria likely played a significant role in the slight oxygenation of the seawater during their formation. These oxygenation levels decreased as water depth increased, with more nutrient supply promoting from deeper waters. The formation and growth of the stromatolite-dominated microbialites were significantly influenced by various depositional conditions, including sea level fluctuations, biological processes, suboxic redox env