Sedimentology, geochronology, and provenance of the late Cenozoic “Yangtze Gravel”: Implications for Lower Yangtze River reorganization and tectonic evolution in southeast China

The evolution of the Yangtze River, the longest river in Asia, provides a spectacular example for understanding the Cenozoic interaction between tectonics, climate, and surficial processes. The oldest Lower Yangtze deposits, characterized by ∼100-m-thick sequences of unconsolidated conglomerate, sandstone, and siltstone, referred to as “Yangtze Gravel,” have been recently dated >23 Ma, indicating a pre-Miocene establishment of a through-going river. However, the link between river integration and tectonic evolution has never been established due to the limited study of these sediments. Here, we report sedimentology, geochronology, and provenance of the Yangtze Gravel based on 17 stratigraphic sections exposed along the Lower Yangtze River. Our new chronostratigraphic results, including 40Ar/39Ar ages from the overlying basalt and fossil-based stratigraphic correlation, suggest an early-middle Miocene age for these sediments. Detailed analysis of lithofacies reveals several sequences of coarse-grained channel-belt deposits (channel fills and bars), indicating braided alluvial deposition across the Jianghan Basin, North Jiangsu-South Yellow Sea Basin, and East China Sea Shelf Basin. This ancient Lower Yangtze River is further characterized by petrography and detrital zircon U-Pb geochronology results which show similar provenance and erosion pattern as the present-day Yangtze River. However, the ancient river in early-middle Miocene is an alluvial, bedload-dominated braided river with higher stream power and a more prolonged course flowing into the East China Sea Shelf Basin. These differences between ancient and modern Lower Yangtze River reflect varied climate and paleogeography in southeast China during the late Cenozoic. Compared with the Paleogene red-colored, halite-bearing, Ephedripite pollen-dominated, lacustrine deposits in Jianghan Basin and North Jiangsu-South Yellow Sea Basin, the deposition of yellow to green-colored, coarse-grained, arboreal pollen, and wood-dominated Yangtze Gravel indicates a drainage reorganization from hydrologically closed lakes to a through-going river system during late Oligocene to early Miocene. During Paleogene, rift basins were filled by alluvial and fluvial-lacustrine deposition with restricted flow distance and local sources. From late Oligocene to early-middle Miocene, the post-rift subsidence opens a path for the ancient Lower Yangtze River connecting the Jianghan Basin, North Jiangsu-South Yellow Sea Basin, and East China Sea Shelf Basin. We attribute the drainage reorganization of the Lower Yangtze River to be a surficial response to Cenozoic tectonics, particularly the western Pacific subduction, in southeast China. The deposition of the widespread, coarse-grained Yangtze Gravel is probably due to the combined effects of catchment expansion and strong monsoonal climate in East Asia.