Depositional Characteristics And Environmental Evolution Of Ganjiang Delta In Poyang Lake

The depositional environment evolution of deltas is a hot topic in today’s geoscience research.Ganjiang Delta is the most well-developed river delta around Poyang Lake,the lar gest freshwater lake in China.It is also the main collection area of Ganjiang source sediments.As a link between Poyang Lake and Ganjiang River,it contains abundant geological and environmental information and is an ideal place to study environmental evolution of inland areas.Taking the plain area of Ganjiang River Delta as the research object,this paper uses environmental proxy indicators such as grain size,organic matter content,uranium-thorium content and magnetic susceptibility,combined with previous research results,and under the constraints of different depth formation ages of the profile,to explore the interaction between the sedimentation in the estuary of the Ganjiang River Delta and the changes of the lake surface,and to reveal its environmental significance.The following understandings are obtained:1)According to the lithology,grain size,uranium thorium content and magnetic susceptibility of the sediments,the Ganjiang Delta sediments can be successively divided into shore-shallow lacustrine facies→semi-deep lacustrine facies→semi-deep lacustrine facies→shore-shallow lacustrine facies→delta-plain facies.The grain size of the sediments shows an overall pattern of change,from fine at the top to coarse at the bottom.As the sampling location is located at the boundary of Ganjiang Delta plain and Poyang Lake,the delta plain facies sediments are relatively fine,mainly clay and silty sand.Under the double influence of Poyang Lake and Ganjiang River,lacustrine sediments are relatively coarse,mainly silty sand and fine sand.2)The uranium-thorium content,ratio,grain size and organic matter content in the sediments of Ganjiang Delta have a good corresponding relationship,which is capable of distinguishing detrital sources,reflecting the oxidation-reduction environment,and distinguishing deltaic facies from lacustrine facies.The uranium thorium content in the sediments of Ganjiang Delta is significantly higher than that in the soil background,which may be related to the origin of Ganjiang River system and its flow through the South China block where uranium-rich rock bodies are widely developed.3)There is a good correlation between magnetic susceptibility and grain size in the sediments of Ganjiang River Delta,and the variation of magnetic susceptibility is mainly affected by climate change and lake level in Poyang Lake region.The magnetic susceptibility fluctuates greatly in the delta plain facies deposition,and the ferromagnetic minerals in the surface sediments are oxidized and transformed into ferrimanganous nodules with a low magnetic susceptibility.Low magnetic susceptibility values in lacustrine sediments indicate a higher water surface and a warm,moist climate.