珠江三角洲中部大鳌平原晚第四纪古生物记录及环境演化

LATE QUATERNARY PALEONTOLOGY AND ENVIRONMENTAL CHANGES IN DA'AO PLAIN,MIDDLE PEARL RIVER DELTA,SOUTHERN CHINA

珠江三角洲是中国最主要的三角洲之一,人口密集,在未来极易受到海平面上升的影响,因此该地区的海平面变化研究尤为重要。有孔虫、介形类等微体动物是半咸水中最重要的生物类群,对海平面变化非常敏感,虽然已有许多涉及研究区微体化石的研究,但该地区微体化石定量分析还未开展,详细的古环境演化过程亦有待阐明。本文利用取自珠江三角洲中部大鳌子平原的5个钻孔的岩芯进行分析,对其中PRD05和PRD04钻孔进行详细的微体动物群定量分析和宏体动物群分析,结合各钻孔沉积学及磁化率特征及其他3个钻孔测年样品的微体、宏体动物群分析,讨论了珠江三角洲中部的环境演化过程。微体动物根据已有水深分布资料,划为两种类型分别代表近岸和远岸两种类型,借以讨论海平面变化。结果显示,珠江三角洲中部晚第四纪的古环境由钻孔中从下到上7个层位的划分体现为3个沉积阶段:1)低地沉积阶段(14340cal.aB.P.前),包括基岩及其风化层(层位A),末次冰期前和期间的河流沉积环境(层位B);2)全新世海侵阶段(14340～2860cal.aB.P.),包括障壁后沉积(层位C,14340～6480cal.aB.P.)和河口湾沉积(层位D,6480～2860cal.aB.P.);3)高地沉积阶段(2860cal.aB.P.至今),包括砂坝环境(层位E,2860～230cal.aB.P.)和三角洲平原沉积(层位F,230cal.aB.P.至今)。其中全新世海侵阶段的河口湾沉积,主要由微体动物群特征反映了更高分辨率的环境演化,包括全新世伊始的快速海侵(D1亚层);海侵速率降低(D2亚层);海平面显著上升(D3亚层);海平面范围最大(D4亚层),海平面最高(约5700cal.aB.P.);海平面快速降低(D5亚层);小规模海平面波动(D6亚层)。

Pearl River Delta is one of the main deltas in China,which hosts a very dense population and is one of the the most important ecnomic areas of the country.Being only several metres above sea-level,the area is substantially easy to be influenced by sea-level change in the future.So the study of sea-level change of the Pearl River Delta is significant.Ostracods and foraminifers are the only representatives of the aquatic meiofauna which fossilise readily and they have been described as the most important microfossils in brackish water environments.Microfaunal study about Pearl River Delta begun in the late 1980＇s.Quantitative analysis of Pearl River estuary has been carried out,yet of microfauna to examine sea-level changes lack in the study area.Da＇ao Plain,located closer to the open sea therefore has more marine influence than the other two sub-plains（Panyu Plain and Shunde Plain）of middle Pearl River Delta.In this area,sea-level changes and corresponding quantitative micropaleontological characteristics have not been discussed yet and detailed paleoenvironmental change is unknown. Therefore,this research is attempt to fill the gaps.Five continuously-cored boreholes were drilled from subsurface of present middle Pearl River Delta,and are located at Da＇ao Sha（cores PRD05 and PRD04）and the northwest of Wugui Mount（cores PRD03,PRD02,and PRD01,eastward）.Integrated micropaleontological,sedimentological,and magnetic susceptibility investigations of these cores and detailed micropaleontological quantitative analysis of two of them（PRD05 and PRD04）are discussed in order to examine the paleoenvironmental characteristics in the study area,and totally 297 samples were investigated for both micropaleontological and mollusk analysis respectively.The results of the paper enable for the first time the detailed reconstruction of sequence-stratigraphic architecture as well as sea-level change and paleoenvironmental evolution in the Middle Pearl River Delta during the Late Quaternary. Totally 34 species from 17 genus of foraminifers and 59 species from 34 genus ostracods were identified in the cores. According to distribution of water depth,microfaunas are divided into two groups（Group Ⅰ,〈20m｜Group Ⅱ, 〉20m） to discuss environmental changes.Microfaunal characteristics especially foraminifers allow refining paleoenvironmental interpretation.Detailed correlation of cores PRD01～05 allow to depict the Late Quaternary paleogeographic evolution of the study area.The vertical stacking of seven units depicts the paleoenvironmental evolution of the study area from the Last Glacial Maximum to present.Unit A corresponds to base rock and its weathered residue.Unit B shows fluvial environment deposited during last glacial period.Unit C（14340～6480cal.aB.P.）generally marks a back-barrier environment.Unit D（6480～2860cal.aB.P.）primarily represents an estuary environment over the study area,corresponding to the Holocene transgression,while Unit E（2860～230cal.aB.P.）represents sand barrier deposition in the west part of the study area,suggesting a continuous regression.Unit F（230cal.aB.P.～0）equals to present overbank delta plain deposit in all cores.These units are considered to be 3 periods： Lowstand deposits（Unit A and B）,Transgression deposits（Unit C and D）and Highstand deposits（Unit E and F）.Therefore Unit C～F corresponds to the post-Last Glacial Maximum deposition.Division of microfauna into two groups allow prodelta（esturine）environment differentiating into six subunits which represent different sea-level fluctuations.Subunit D1（6480～6340cal.aB.P.）： sea-level rose rapidly｜Subunit D2（6340～6120cal.aB.P.）： sea-level rise slowed down｜Subunit D3（6120～5670cal.aB.P.）： sea-level rose rapidly again｜Subunit D4（5670～4950cal.aB.P.）： largest extension of transgression was reach and then sea-level fell gradually｜Subunit D5（4950～4140cal.aB.P.）： sea-level fell greatly｜Subunit D6（4140～2860cal.aB.P.）： a small scale sea-level fluctuation occurred.Maximum marine ingression is recorded at about 5700cal.aB.P.Five subunits of Unit E,primarily represented sand barrier environment in Da＇ao Sha,are identified by micropaleontological and lithology as well.Subunit E1（2860～1720cal.aB.P.）mark the onset of Pearl River Delta and Subunit E2（1720～1030cal.aB.P.）correspond to a further sea-level fall.Subunit E3（1030～900cal.aB.P.）marks a transitional littoral environment at location of Core PRD04,probably implying a small-scale sea-level rise.Subunit E4（900～460cal.aB.P.）and Subunit E5（460～230cal.aB.P.）equals to sea-level fall.But Subunit E5,with some marine influence implies a weak sea-level rise.