珠江三角洲中部晚更新世以来的有孔虫记录与古环境演化

摘要

Late Quaternary sedimental records,especially Late Pleistocene transgressive records, are well preserved in Boreholes PRD20 and PRD1 7 drilled in central Pearl River Delta.Based on the com-prehensive analysis of foraminifera and lithological characteristics of Boreholes PRD20 and PRD1 7,com-bining with previous researches,palaeoenvironmental changes since the Late Pleistocene have been recon-structed,with the Late Pleistocene transgression highlighted.Before 44 500 cal a BP,a fluvial environ-ment developed,indicated by coarse-grained sediments.Between 44500 and 21 300 cal a BP,a drowned valley estuary setting developed with the sea water intrusion into the study area,which was corresponding to the transgression of the Wurm Sub-Interglacial Stage.Two short-term transgression-regression circles can be recognized based on foraminiferal data.It can be deduced that Boreholes PRD20 and PRD1 7 were loca-ted in the fluvial-tidal interaction area of the drowned valley estuary during this period.A mottled clay layer in Borehole PRD1 7 and a conglomeratic sand layer in Borehole PRD20 ware sandwiched between Upper Pleistocene and Holocene sediments,which were most likely formed by subaerial oxidation of the underly-ing sediments during the last glacial maximum.The study area was influenced by postglacial sea level rise after around 8000 cal a BP,which was recorded by a peat layer in Borehole PRD20.The foraminferal and sedimental records of Borehole PRD20 suggest a bay head environment during 8000-5500 cal a BP.Since then,a deltaic plain facies developed in the study area.As a result of the tectonic uplift,the core site of Borehole PRD1 7 showed a higher topography and did not accept sediments until 4700 cal a BP,which is much later than in other parts of the Pearl River Delta.During this time,the study area could be occasion-ally influenced by tidal water with sporadic foraminifera tests.%珠江三角洲中部钻孔 PRD20和 PRD17晚第四纪以来的沉积记录较为完整，尤其是晚更新世海侵记录良好。以其为研究对象，对两孔的有孔虫微体动物群记录和沉积特征进行了分析和对比，并结合前人研究成果，重建珠江三角洲晚更新世以来的古环境演变，并着重分析了晚更新世海侵事件。约44500 cal a BP以前，研究区为河流环境，发育河道砂砾沉积。约44500—21300 cal a BP，发生晚更新世海侵，形成溺谷型河口湾，两孔沉积物中有孔虫的垂直分布记录了水体条件发生的多次变化，可辨识出2次海侵海退次级旋回；其中 PRD20孔中有孔虫以滨岸广盐型占绝对优势，PRD17孔中以滨岸广盐型为主、海相—半咸水型有孔虫占有一定比例，表明研究区位于溺谷湾中径流与潮流共同作用的区域，且 PRD20孔受径流影响更大。约21300 cal a BP后，受末次冰盛期海退影响，研究区暴露于地表遭受风化剥蚀，风化产物在 PRD20孔表现为1层褐红色砂砾，在PRD17孔表现为1层花斑黏土。约8000 cal a BP，海平面回升导致河水滞留形成沼泽环境，PRD20孔发育1层泥炭层。约8000—5500 cal a BP，PRD20孔发育受径流影响较大的河口湾湾头沉积，仅在全新世海侵最盛时含有少量有孔虫记录。约5500 cal a BP后，研究区发育三角洲平原沉积。受构造抬升影响，PRD17孔地势较高，于约4700 cal a BP才开始重新接受沉积，零星出现潮水搬运而来的有孔虫。