LITHOLOGIC FEATURES OF SEDIMENTS CHARACTERIZED BY HIGH SEDIMENTATION RATES SINCE THE LAST GLACIAL MAXIMUM FROM DONGSHA AREA OF THE SOUTH CHINA SEA

A total of 14 piston cores, with lengths ranging from 658 to 937cm and with water depths from 1 616 to 3 426 m, were obtained by several exploration cruises for gas hydrate survey from 2001 to 2004 in the Dongsha area of the South China Sea. Ages of these sediment cores were determined by means of AMS ¹⁴C dating and stable oxygen isotope stratigraphy of plankton foraminifera and CaCO₃ stratigraphy. Results from visual core description, smear slide and grain size analyses indicate that sedimentary features of Holocene sediments, including sedimentary components and sedimentation rates are varing with time and location. Three sedimentary sub-areas with different sedimentary features were recognized. In AreaⅠ, Holocene sediments are characterized by clayey silt, with rare biogenic components and low CaCO₃ (less than 10%), but with common authigenic pyrite and high sedimentation rates (41.7 cm/ka). In Area Ⅱ, Holocene sediment sequences are dominated by clayey silt, interbedded with turbidite layers of sandy silt, silty sand and sand. Abundant turbidite layers were observed in cores HD77, HD133 and GC10. In addition, contents of feldspar and quartz and biogenic calcareous component (mainly foraminifers) in the sediments increase obviously. In this area, a sedimentation rate of 31.6 cm/ka was estimated for the Holocene sediments. In Area Ⅲ, Holocene sediments contain abundant biogenic components and rare feldspar and quartz minerals, showing features that are commonly observed in deep-sea sediments. CaCO₃ content varies from 10% to 30% and sedimentation rate of Holocene is 20 cm/ka.