Formation mechanisms and geological significance of the Lower Triassic fine-grained mixed sedimentary rocks in the Central Uplift of the South Yellow Sea Basin, eastern China.

• During the Early Triassic, the study area is dominated by fine-grained mixed rocks that formed in a storm-controlled neritic shelf environment. • Mixed sediments formed via facies mixing and punctuated mixing. • From southeast to northwest, the South Yellow Sea Basin contains four paleogeographic units: coast, carbonate platform, shallow shelf, and deep-water basin. • The Qinglong Formation strata exhibit weak hydrocarbon generation and strong sealing. Drilling results from borehole CSDP-2 show that typical fine-grained mixed rocks occur in the Lower Triassic succession in the Central Uplift of the South Yellow Sea Basin, which reveals the storm event and possible double-provenance supply, and advances the previous understanding. We document the rock types, sedimentary structures and textures, and formation mechanisms of fine-grained mixed rocks on the basis of core and thin-section observations, XRD whole-rock geochemical, major and trace element, and laser particle-size analyses. During the Early Triassic, a storm-controlled neritic shelf and restricted platform were the main sedimentary environments of the Central Uplift, and the deposition was dominated by fine-grained mixed rocks, with some argillaceous siltstone and silt-bearing limestone. Formation of the fine-grained mixed rocks is attributed to facies mixing and punctuated mixing under the comprehensive influence of the paleogeographic background, paleoclimate variations, eustatic sea-level change, sediment supply, and storm events. Facies mixing is apparent in the succession as alternations of restricted platform, evaporated platform, brackish platform, and lagoon deposits. Mixed rocks formed by punctuated mixing exhibit an orderly succession of turbidity current, debris flow, storm current, and weak tractive current sediments. The geological significances of this research are twofold. In the South Yellow Sea Basin, the neritic shelf that formed in the Central Uplift gradually changed to a shallow-water carbonate platform in the Southern Depression and Wunansha Uplift. A deep-water basin formed in the Northern Depression. This palaeogeographic pattern is consistent with that of the Lower Yangtze continental region, revealing that two possible ancient provenances, the Cathaysia Wuyi Terrane and the North China Craton, may exist simultaneously. The Lower Triassic strata are characterized by weak hydrocarbon generation and strong sealing, and could act as caprocks for the underlying Permian hydrocarbon system. [ABSTRACT FROM AUTHOR]