珠江三角洲全新世海-陆沉积模式转换及其年代

Holocene sedimentary environment transform and onset time of Pearl River Delta progradation

全新世是现代珠江三角洲平原形成的重要阶段,西江、北江和东江的河流物源为全新世海-陆交互相沉积提供了最主要的物质来源。然而,珠江三角洲早全新世以来的海进-海退沉积序列反映的沉积速率变化,以及退积-进积过程和起始年代因三角洲地区复杂的地形地貌而存在时空差异。此外,许多地点钻孔揭示出海-陆沉积模式转换过程中存在大量的沉积间断。本文对珠江三角洲的两个典型钻孔进行了详细的岩性、粒度等环境指标的分析,结合区域内20余个钻孔的横向对比,对全新世的海-陆沉积环境与模式转换进行了深入探讨。结果表明,全新世海相沉积底部年代为穿时性分布,在伶仃洋约为11 kaBP,海侵沿着深切河谷深入到番禺-三水一带的年代延迟至9~8 kaBP;高海面期不同地点存在三角洲前缘相的河口湾、潮坪等多种沉积环境,同时,早—中全新世三角洲前缘沉积环境受到古地形影响,在一些地点存在许多极高沉积速率的堆积体(可达~4 cm/a)。全新世沉积由海进的退积模式转变为海退的进积模式在大量钻孔中都有明显体现,通常表现为细粒组分的粉砂含量增多,磁化率快速升高,海洋有孔虫含量降低等;其沉积环境通常由滨海相转变为网状水系下的分流间湾、决口扇或河流冲积相。由潮汐作用为主的三角洲前缘沉积模式转换至平原相的发生时间主要集中在4~3 kaBP,并且许多地点的沉积动力转换的接触界面表现为明显的沉积间断,即晚全新世强烈的河流冲积和三角洲平原快速推进导致对原先较老沉积物的侵蚀作用。

The Pearl River Delta is mainly formed in Holocene by the sediments from the Xijiang,Beijiang and Dongjiang rivers as the results of sea-land interaction.Owing to the complexity of topography in the deltaic area,there remain some key questions to be solved,such as the spatio-temporal variation in depositional processes,onset time of delta progradation,variation in sedimentation rate and transgression-regression patterns.Boreholes at many sites have revealed that there are significant sedimentary discontinuities during the transformation from marine to fluvial environment and vice versa.In this paper,detailed investigation is devoted to the lithology,grain size distribution,magnetic susceptibility,microfossils and chronology of two typical cores,upon the correlation with over 20 cores from the region.Sea-land interaction models are established for the deltaic sequences.Results show that the age of the bottom of the marine sediments is diachronous from place to place.It is about 11 kaBP in the Lingding estuary,but 9~8 kaBP in the Panyu-Sanshui area due to the time delay during sea water transgression following the incised-valley;Various sedimentary environments such as estuaries and tidal flats as the components of the delta front facies appeared in different localities during high sea-level period.Affected by the land topography,sedimentation rate in some places may be as high as^4 cm/a.In many drilling holes,the transformation from marine progradation to retrogradation is often observed,while the fine sand components increase in contents,magnetic susceptibility rapidly increases,and the amount of ocean foraminifera decreases.At the same time,facies changes occur from coastal facies to tributary bays,crevasse splay,or river alluvial facies under a reticulated water system.The delta front facies,mostly tide-dominated,transformed to the delta plain facies around 4~3 kaBP,with distinct discontinuities,indicating the intensification of late Holocene underwater erosion by the Pearl River.