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*

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。

Foraminiferal records and palaeoenvironmental changes since the Late Pleistocene in central Pearl River Delta

Late Quaternary sedimental records,especially Late Pleistocene transgressive records,are well preserved in Boreholes PRD20 and PRD17 drilled in central Pearl River Delta. Based on the comprehensive analysis of foraminifera and lithological characteristics of Boreholes PRD20 and PRD17,combining with previous researches,palaeoenvironmental changes since the Late Pleistocene have been reconstructed,with the Late Pleistocene transgression highlighted. Before 44500 cal a BP, a fluvial environment developed,indicated by coarse-grained sediments. Between 44,500 and 21300 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 PRD17 were located in the fluvial-tidal interaction area of the drowned valley estuary during this period. A mottled clay layer in Borehole PRD17 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 underlying 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 8000-5500cal 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 PRD17 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 occasionally influenced by tidal water with sporadic foraminifera tests.