Holocene delta evolution and sequence stratigraphy of the Pearl River Delta in South China

We analyzed the sedimentary characteristics and chronostratigraphy of the Pearl River Delta in South China, and discussed the Holocene stratigraphic characteristics and the filling process since the Holocene, based on 90 boreholes. Our results showed that the maximum transgressive surface of the Pearl River Delta was located in the soft marine silt horizons, whereas it was shown as an erosion surface in areas with tidal scour. From bottom to top, Holocene sedimentary sequences were fluvial facies, floodplain-estuary facies, and estuary-delta facies in the paleo-valley, while the paleo-interfluve sequences were described as littoral facies, estuary facies, and delta facies. Vertical accumulation sequences of Holocene sediments were shown primarily as positive tapering-shaped sequences with no typical foreset, topset, or other sedimentary sequences of the Gilbert Delta. The paleo-Pearl River estuary had different evolution models in the highstand systems tract (HST) compared with other large deltas worldwide, in that it is dominated by a unique and complex geomorphologic boundary, and that it is a multi-sourced delta compounded by the independent and parallel development of fluvial sedimentary bodies with different scales. A huge estuary was formed in the present Pearl River Delta area when the post-glacial transgression reached maximum at about 6 ka BP. The estuary was divided into two parts by obstruction of a series of islands, an upper part composed of the semi-enclosed inner paleo-bay connected to rivers and a lower part consisting of the outer paleo-bay connected to the ocean. Both of these areas were interlinked by only a few narrows, in which the fluvial sediments are accumulated primarily in the inner paleo-bay. During 6-2 ka BP, the sedimentation in various regions of the inner paleo-bay occurred simultaneously with independent development under the remodeling and transformation effect of complex boundaries on river and ocean dynamics. After 2 ka BP, the evolution of the delta is no longer a simple natural process, but rather a common remodeling process accomplished by nature and humans.

Sustained growth of the largest uninhabited alluvial island in the Changjiang Estuary under the drastic reduction of river discharged sediment

The evolution of the Changjiang Delta is obviously affected by current rapidly rising sea level and drastically declining river discharge,and it is also vital for the sustainable development of Shanghai and the Changjiang River Economic Belt,which represent China’s economic development leader and major national strategic area,respectively.In this paper,the growth pattern of Jiuduansha Island,the largest uninhabited alluvial island in the Changjiang Estuary,is studied in terms of the change in elevation,position and area over the past 50 years through using satellite-derived instantaneous shoreline positions and high/low tide exposure areas based on 497 satellite images from 1974 to 2020;and the influencing factors and future development patterns are further discussed by comparison with other alluvial islands or sandbars in the estuary.The results show that Jiuduansha Island has maintained a rapid or even accelerated area growth rate,although the sediment discharge of the Changjiang River has sharply decreased in recent decades.This sustained growth is mainly attributed to the existence of the estuarine turbidity maximum zone,coarsening fluvial sediment,onshore sediment replenishment by tide,cone-like geomorphology of Jiuduansha Island,and siltation promotion effect of vegetation.The growth rate of the low tide exposure area of Jiuduansha Island increased from 1.9 km^(2)a^(−1) in 1974–1990 to 3.0 km^(2)a^(−1) in 1990–2020,and the growth rate of the high tide exposure area reached as high as 3.7 km^(2)a^(−1) in 1994–2020.The implementation of the Deep-Water Channel Project has significantly affected the evolution of Jiuduansha Island,including shifting the heads of Shangsha and Zhongxiasha from severe retreat to rapid accretion,and promoting tidal flat progradation seaward of the Jiangyanansha and Zhongxiasha,but restricting the growth of the low tide exposure area of Jiuduansha Island.