# Study on Sedimentary Evolution of the Hanjiang River Delta during the Late Quaternary
## 4 Discussion
### Sedimentary Evolution Process  
The results of OSL dating at the bottom of the ZK03 drilling hole show that the oldest Quaternary stratum in the Hanjiang Delta was in the MIS5 period. The grain size, elastic minerals and paleontological characteristics of sediments show that this section is a flood plain-floodplain deposit. During 90-70 ka BP, the global sea level was in a relatively high sea level period [13]. The strata in the study area during this period were alternately formed  
Figure 5: Grain size falling point map of Passeig C-M image of ZK03/ZK01 borehole sediments.  
by massive sand and mud, composed of mildly layered hard mud and silty sediments, and contained peat fragments, but no shells or marine indicators were found. It is speculated that this section is an estuarine-beach sedimentary environment. During this period, the study area experienced a change in the sedimentary environment from continental facies to marine facies. Due to the barrier of the third island mound, the influence range of this transgression was small, and no record of this transgression was found inside the delta plain.  
At approximately 70 ka BP, the Earth entered the last glacial period [13], the seawater withdrew from the delta plain, and a set of medium coarse sand-weathered clay layers was deposited. The global sea level was relatively low during the last glacial period, but there was still a period of relatively high sea level. The second transgression in the study area occurred after the late Pleistocene, and sedimentary strata with silty clay and muddy fine sand as the main sedimentary types were developed, representing the shallow marine sedimentary environment defined by the interactions between river and seawater tidal dynamics. At approximately 30 ka BP, this transgression reached its high sea level period [14; 15; 16]. Then, until the arrival of the last glacial maximum (26-17 ka BP), the sea level dropped to its lowest level [12; 13; 14], and a set of continental bottom gravel layers developed in the study area. Some boreholes also found porphyritic weathered clay and yellow-white clay sandy gravel layers.  
The Hanjiang River Delta began to accept postglacial deposition at approximately 10.4 ka BP, and the sea level began to rise rapidly, reaching the maximum transgression range at approximately 6.3 ka BP, during which time the sedimentary characteristics changed many times. The sedimentary evolution process of the study area since the Holocene can be divided into three stages from the old to new in chronological order. At approximately 10.4-8.5 ka BP, the end of the last glacial period, the global climate began to gradually become warm and humid, and at approximately 9.2 ka BP, after gradually transforming into marine deposits, the estuary environment developed. At approximately 8.5-6.3 ka BP, the composition of foraminiferal species represented a typical coastal shallow water combination, mainly warm water molecules, and contained a certain proportion of euryhaline transitional phase molecules, representing the coastal-shallow sea estuary sedimentary environment. At approximately 6.3-0 ka BP, large-scale regression led to the continuous diffusion of sediments carried by rivers to the open sea, and the strong wave action in the open sea led to the redistribution of sediments from the basin into the open sea. The sediments were distributed along both sides of the estuary and parallel to the coastline, forming a series of barrier sand dams. After the maximum transgression, the study area began to construct the Holocene delta and gradually formed the current land-sea pattern. In addition, dredging has had a certain impact on the deposition of coastal waters. Since we visited and investigated, no dredging has occurred in the coastal waters of the study area. Only the channel in the Rongjiang Estuary has been dredged on a small scale, which has little effect on the sedimentary evolution of the whole study area.