Geochronological and geochemical characterization of paleo-rivers deposits during rifting of the South China Sea

Abstract

We examined an International Ocean Discovery Program (IODP) drilling core from Site U1501, located on the distal margin of the northern South China Sea (SCS) basin to unravel the sediment provenance evolution in the Paleogene and the evolution of river catchments during basin opening. We attempt to understand the major factors driving river development in a rift basin by utilizing provenance tools to constrain sediment transport pathways and compare these with the regional tectonics during the Paleogene in order to resolve competing models for drainage evolution and test their relationships with the evolving topography of SW China and the SE Tibetan Plateau. For this purpose, ten samples were collected from a 200-m-thick, syn-rift Eocene/pre-Eocene interval. Detrital zircon U-Pb data were collected by LA-ICP-MS to identify the sediment provenance and differentiate fluvial sources. Bulk rock geochemistry data was utilized to shed light on chemical weathering conditions and compositional maturity to further decipher sediment transportation patterns. We compare our data with adjacent IODP Site U1435 and several industrial boreholes located in the Pearl River Mouth Basin (PRMB). We applied multiple statistical tests, including K-S, Monte Carlo mixing and multidimensional scaling testing, to evaluate U-Pb age spectra similarities and to estimate endmember contributions from a variety of source areas. Our results from Site U1501 show that sediments deposited as fluvial sands during the rifting stage, were predominantly derived from the East Cathaysia block, probably from local sources. A progressive increase in older detrital zircon U-Pb ages peaks (>200 Ma) was observed at Site U1435 and in PRMB strata, signaling a spatial shift in sediment provenance from east to west occurring between the late Eocene and the early Oligocene. This trend reflects a transition in sediment delivery from local small-catchment streams to a more regional drainage eroding the east and north of the South China Block. Westward drainage expansion is likely impacted by the uplift of the Tibetan Plateau.