Oligocene Calcareous Nannofossils from IODP Site U1435: Implications for the Initial Opening of the South China Sea

Abstract:

International Ocean Discovery Program Expedition 349 cored Site U1435 on a structural high along the northern continent/ocean boundary in the East Subbasin of the South China Sea. At this site, thin Neogene and Quaternary sediment cover (~35 m thick) characterized by clays and manganese nodules is underlain by ~40 m of Oligocene deep marine nannofossil-rich clay and claystone. Calcareous nannofossils are abundant and moderately to well preserved throughout much of the Oligocene sequence, although their overall abundance decreases near the base of the unit. The diverse assemblage is characterized by abundant reticulofenestrids, common Cyclicargolithus floridanus, Cyclicargolithus abisectus, and Zygrhablithus bijugatus, with fewer numbers of species belonging to the genera Coccolithus, Discoaster, and Helicosphaera. The presence of well-preserved sphenoliths allows for good biostratigraphic control through the section, which we correlate to nannofossil Zones NP21 to NP25. The base of the sequence in Section 349-U1435A-9R-2 is dated to between 32.92 and 33.43 Ma based on the presence of Coccolithus formosus and absence of abundant Clausicoccus distichus. The Oligocene section at Site U1435 unconformably overlies a unit comprised dominantly of sandstone of unknown age due to a paucity of calcareous and siliceous planktonic marine microfossils. The presence of brackish water benthic foraminifers in some samples suggests a nearshore environment of deposition, substantially different from the deepwater (>2500 m) environment inferred for the Oligocene rocks based on the trace fossil assemblage. We infer that the abrupt transition from shallow to deepwater facies at this site represents the breakup unconformity for the initial opening of the South China Sea, indicating that seafloor spreading began at ~33 Ma.