## 4 Discussion
Moreover, strong chemical weathering in the Yangtze River drainage area inevitably leads to the transformation of a large amount of biotite into weathered mica. Thus, the difference in weathering conditions is another important reason why the mass percentage of mica in the very fine sand fraction of the sediments in the Yangtze River Estuary is much higher than that in the very fine sand fraction of the sediments in the ancient Yellow River Estuary. This is also the main reason for the higher particle percentage of weathered mica in the very fine sand fraction of Yangtze River Estuary sediments and largely explains both the higher weathering degree of mica and the higher particle percentage of biotite. Third, in the Yangtze River Estuary, the flow velocity suddenly declines due to the low open flat trumpet-shaped terrain [59]. This terrain shape leads to the deposition of a large number of detritus [59]. Coupled with the controlling effect of seawater in the estuary and the disturbance of coastal currents, detrital mica easily affected by hydrodynamic forces also accumulates in this location. This is the third important reason for the higher mica content in the very fine sand fraction of the sediments in the Yangtze River Estuary. In contrast, mica originating from the Loess Plateau is resorted under the influence of stronger hydrodynamic forces in the estuary area of the ancient Yellow River. Moreover, due to the large differences in the particle size, density, shape, volume, and diameter-thickness ratio, the depositional velocity of mica is much lower than that of other minerals with the same particle size, such as quartz and feldspar. Thus, the particle size of mica settling in the very fine sand fraction is usually larger than that of other minerals, such as quartz and feldspar [28]. Research has demonstrated that the particle size difference between mica and other minerals in the same size fraction, such as quartz and feldspar, ranges from 1\(\upmu\) to 1.5\(\upmu\)b [27]. This is also an important reason for the lower mass percentage of mica in the very fine sand fraction of ancient Yellow River Estuary sediments. Specific Diagnostic Indices to Effectively Distinguish detritus between the Yangtze River Estuary and Ancient Yellow River Estuary  
Micas in detrital sediments are divided into muscovite and biotite. Muscovite exhibits the most stable chemical properties and a high erosion resistance [61]. Biotite with unstable chemical properties is easily weathered [61]. Therefore, the differences and changes in mica indicators, such as the content and/or ratio, provide a certain significance to indicate provenance conditions, such as the watershed climate type and hydrothermal conditions. Under the influence of the basic geology, climate and hydrodynamic forces in the source area, the detrital mica content and ratio in the very fine sand fraction of the sediments of the Yangtze River Estuary differ from those in the very fine sand fraction of the sediments in the ancient Yellow River Estuary. These differences make it possible to determine specific diagnostic indices for detritus to distinguish between these two large river estuaries. As shown in Figure 8, four mica indices, including the particle percentage of biotite, particle percentage of weathered mica, particle percentage ratio of weathered mica to the sum of biotite and weathered mica and particle percentage ratio of biotite to muscovite, significantly differ between the Yangtze River Estuary and ancient Yellow River Estuary. Specifically, in the very fine sand fraction of Yangtze River Estuary sediments, these mica indices ranged from 6.67% to 19.85%, from 42.14% to 65.71%, from 0.69 to 0.90, and from 0.13 to 0.56, respectively. However, in the very fine sand fraction of ancient Yellow River Estuary sediments, these indices ranged from 24.41% to 53.93%, from 9.74% to 39.81%, from 0.17 to 0.60, and from 0.64 to 2.09, respectively.