Calcareous nannofossil changes in response to the spreading of the South China Sea basin during Eocene-Oligocene

Abstract

Numerous models have been proposed to constrain the process of the South China Sea formation, yet the Paleogene paleoenvironmental researches are hindered by the paucity of long, continuous, well-dated geological records. In this work, Eocene to Oligocene nannofossil biostratigraphy and assemblage changes in a sediment section from the northern South China Sea were studied to shed light on these issues. We established an age model to confine the studied section between ~40 and 22 Ma based on 17 calcareous nannofossil datum levels. The age model enabled to date the most prominent seismic reflector T60 centered at ~26.8 Ma, a regional tectonic event interpreted as corresponding to the “Ridge Jump Event” in the early phase of South China Sea spreading. Changes in nannofossil abundance and assemblage compositions imply continuous transgression and deepening of water depth from the Late Eocene to Early Miocene. During the Late Eocene, the studied area was a neritic sea with relatively eutrophic conditions supporting abundant R. lockeri group and C. pelagicus. From ~34 Ma to ~27 Ma, during the Oligocene, the earlier neritic environment was replaced by relative warm and oligotrophic conditions in a shallow sea, as indicated by higher concentration of C. floridanus, Sphenolithus spp. and Helicosphaera spp. Since ~26 Ma, pelagic conditions prevailed, small Reticulofenestra (<5 μm) became dominant, and Discoaster spp. also increased in response to the deepened water depth.