MICROFAUNAS IN HOLE ZK201-2 AT ZHUJIANG RIVER DELTA SINCE LATE PLEISTOCENE AND THEIR IMPLICATIONS FOR PALEOENVIRONMENTS

The distribution and preservation of microfaunas in the borehole of ZK201-2 from the Zhongshan area of the Zhujiang River Delta are quantitatively analyzed in this paper. A total of 12 foraminiferal genera including 23 species, and 16 ostracod genera including 19 species have been identified. According to the distribution of dominant species and some special species with ecological significance, 7 foraminiferal assemblages and 3 ostracods assemblages are recognized respectively. Different assemblages reflect different types of sedimentary environments. Based on above analyzed results and the sedimentary properties, the paleoenvironmental changes in the studied area since late Pleistocene are finally reconstructed and summarized. Some fragments of marine gastropods and bivalves are also found in fine sediments above the fluvial sand and gravel deposits during late Pleistocene, indicating an estuarine environment. With the sea level dropping rapidly during the Last Glacial Maximum(LGM)period, the late Pleistocene marine sediments were weathered into mottled-clay sediments. After the LGM, alluvial flat or sluggish river-bend environment appeared at the early postglacial period and foraminifera and ostracods species were absent in this period. Probably from 9 cal.kaBP, the palaeoenvironments changed from tidal estuary to front delta conditions with relatively high sedimentation rate. Dominated by abundant and well-preserved marine or brackish species during this period, the abundance, diversity and assemblages of foraminifera and ostracods were influenced frequently by the sea level changes and sediment yield. According to the relative high abundance of foraminifera and ostracod, the marine transgression maximum occurred at approximately 7 cal.kaBP in the middle Holocene. With the sea level dropped since late Holocene, the study area gradually experienced the palaeoenvironments of river-mouth bar, tidal flat, coastal-wetland and delta plain in succesion, and the sedimentation rate was relative low. Allochthonous species were also identified during this period, indicating the storm surges in the studied area.