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

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.