珠江三角洲ZK201-2孔晚更新世以来微体生物群与古环境

对珠江三角洲中山地区ZK201-2钻孔微体生物群的分布特征及壳体保存状况进行了研究,共发现有孔虫12属23种,介形类16属19种。根据优势属种的分布及部分特殊种的生态环境指示意义,识别出7个有孔虫组合和3个介形虫组合,分别指示了不同的沉积环境。应用上述分析结果,并结合岩性变化特征,恢复了晚更新世晚期以来研究区沉积环境演变过程。晚更新世晚期,在ZK201-2钻孔河床相砂砾沉积之上的细粒沉积物中发现海相腹足类及双壳类生物碎片,判断是受到海水影响的河口湾沉积;末次盛冰期海平面下降,原有海相沉积暴露地表风化形成杂色黏土沉积层;冰后期首先出现河漫滩至滨海河湾相沉积,未发现钙质微体生物化石;约9 cal.kaBP后开始进入潮汐河口湾至前三角洲沉积阶段,沉积速率较快,有孔虫和介形类以壳体保存较好的半咸水-浅海种为主,其丰度、分异度及属种组合变化受海平面变化和河流输砂影响频繁;至全新世中期7 cal.kaBP前后珠江三角洲中部出现最大海侵,有孔虫和介形类丰度、分异度值出现相对高值;全新世晚期海平面逐渐下降,研究区进入低速沉积阶段,依次发育河口沙坝、潮下带至滨海湿地、三角洲平原相沉积,期间发现多期风暴潮沉积,见大量异地分子。

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.