VARIATIONS IN BIOGENIC COMPONENTS OF LATE MIOCENE-HOLOCENE SEDIMENTS FROM SHENHU AREA IN THE NORTHERN SOUTH CHINA SEA AND THEIR GEOLOGICAL IMPLICATION

Late Miocene to Holocene microfossils from sediments recovered in 4 holes (BY1, BY2, BY3 and BY4) from the Shenhu area of the northern South China Sea were analyzed. Four microfossil groups (calcareous fossils:foraminifera and calcareous nannofossils;siliceous fossils:diatoms and radiolarians) represent major biogenic components of the sediments studied. Remarkable spatio-temporal variations in abundance of calcareous and siliceous components were recognized; siliceous fossils are nearly absent in late Miocene-Pliocene sediments, whereas they are abundant in sediments of last 0.40 Ma; and low abundance of large sized (>0.15 mm) foraminifera was also observed in late Miocene sediments and it increased in the Pliocene-Pleistocene sediments. High abundance of siliceous fossils in the late Pleistocene sediments indicated high paleo-productivity in water surface of this period. Low abundance of calcareous fossils is probably caused by large input of terrigenous materials, however, abundance of calcareous fossils varied in the Pliocene with sites, for example, more abundant in holes BY1 and By2 and less in BY3 and BY4, indicating complicated input sources during this period. Furthermore, abundant calcareous fossils were found in the hydrate-bearing sediment layers whose 20%~40% of pore volume was filled with gas hydrate.It is inferred that abundant calcareous fossils, with grain sizes larger than fine silt or clay, might increase pore spaces of sediments for containing abundant gas hydrates.