Depositional Paleoenvironment and Model of Early Cretaceous Fine-Grained Sediments in the Central Sulu Orogenic Belt: Insights from Sedimentology, Organic Petrology, and Geochemistry

The third member of the Laiyang Formation (K1l3) within the Sulu orogenic belt has been continuously depositing fine-grained sediments. However, the depositional characteristics and formation mechanisms of this Late Mesozoic fine-grained rock series remain poorly understood. In this study, the integration of sedimentology with petrology and geochemistry of fine-grained rocks belonging to K1l3 in the southern Riqingwei Basin allows us to analyze the macro- and microsedimentary structures, paleoenvironmental evolution, and dynamic sedimentary processes. The evidence indicates that gravity flow was the key mechanism for the long-distance transport of fine-grained materials in K1l3, along the gentle slope in deep-water settings. The fine-grained mud-rich sedimentary system in this area consists of laminated and massive siliciclastic fine-grained rocks. The TOC content is generally low (0.46–2.20%, avg. 0.97%), and most samples are “organic-bearing” fine-grained sedimentary rocks. Amorphous organic matter is the dominant maceral component, followed by opaque phytoclasts and collinite. By utilizing CIA and C-value to assess paleoclimate, Ti content, Al content, K/Al, and Rb/Al to characterize detrital flux, Sr/Ba to interpret paleosalinity, V/(V + Ni), VEF, and UEF to analyze redox conditions, and CuEF, ZnEF, and NiEF to evaluate paleoproductivity, the depositional environment of the K1l3 fine-grained rocks was reconstructed. The climate during this period was relatively arid with limited terrestrial input. Moreover, the seawater was anoxic, and the productivity was at a low to moderate level. The vertical variations in the geochemical profile of Well LK-1 indicate that the primary productivity and redox conditions significantly influenced the organic matter enrichment in the fine-grained sediments. Turbidity currents are important processes for the transport of clasts and terrestrial organic matter to the distal region of the deep-water setting. Based on the aforementioned interpretations, the depositional model of K1l3 in the southern Riqingwei Basin was established, providing new insights into the origin of deep-water fine-grained rocks.