## 5 Discussion  
### Sediment Maturity, Weathering and Sorting  
Source composition, chemical weathering and hydrological differentiation processes control the chemical composition of sedimentary suites together [12; 34]. With the increase in sediment maturity, there is an increase in quartz mineral instead of feldspar, mafic minerals, and lithic fragments in sediments, resulting in an increase in SiO\({}_{2}\) and decrease in other major elements, such as Al\({}_{2}\)O\({}_{3}\), Na\({}_{2}\)O, K\({}_{2}\)O, MgO, Fe\({}_{2}\)O\({}_{3}\), and CaO. The Na\({}_{2}\)O/K\({}_{2}\)O and Fe\({}_{2}\)O\({}_{3}\)/K\({}_{2}\)O ratios indicate the differentiation of orthoclase and Fe and Na-bearing minerals [29]. The sediments from the Songhua River have higher SiO\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\) (4.32 to 8.34 averaged at 6.75) and K\({}_{2}\)O/Na\({}_{2}\)O (0.85 to 2.88 averaged at 1.41) ratios than that of UCC (4.34 and 0.87), implying a higher compositional maturity. Both the Wenquanhe Formation and Guxiangtun Formation exhibit an increasing trend of compositional maturity upwards, reflected by changes in the SiO\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\) and K\({}_{2}\)O/Na\({}_{2}\)O ratios. These conclusions could also be proved by lower Fe\({}_{2}\)O\({}_{3}\)/K\({}_{2}\)O ratios, which range from 0.39 to 1.05.  
Moreover, the maturity and composition variations in sediments from the Songhua River have been influenced by the chemical weather process, as indicated by the chemical index of alteration (CIA). The CIA values of the sediments from the Songhua River range between 47 and 64 (Table 1), which is more than the CIA value of the UCC (about 46). In the A-CN-K diagram (Figure 6), the sediments follow the trend of the chemical weathering process, implying that the sediments have undergone a certain degree of chemical weathering. From the bottom to the top, the CIA values of the Wenquanhe and Guxiangtun Formations show an increased trend, indicating a gradual weathering of the source regions. The CIA variation in sediments from the Songhua River is mainly or partly controlled by lithology, because the sediments with a higher weathering degree have much more fine grains and clay materials. According to fossils of woolly rhinoceros and mammoth, the climate in the research area became cold from the early-middle Late Pleistocene to the late Late Pleistocene Epoch when the Guxiangtun Formation was deposited, and it became warm after the beginning of Holocene [25]. The climate during the last 2 Ma would influence the compositions of the sediments [35]. So, the climate factor that influenced the chemical weathering in the research area is a long-term action.  
As discussed in petrography, the detrital grains from the sediments are mainly sub-angular and subround, and the sediments are poorly sorted. To describe the sorting of  
Figure 6: The chemical index alteration (CIA) of the Guxiangyun and Wenquanhe Formations.  
sediments, the Th/Sc vs. Zr/Sc plot is adopted (Figure 7). The immobile elements, such as Th, Sc and Zr, have a short residence time in water, and are almost quantitatively transferred into the sediments [29]. For the magmatic rocks, the ratio of log (Th/Sc) vs. log (Zr/Sc) is about 1.93. In contrast, if zircon grains were added into the sediments, this ratio would be decreased steeply. Typically for UCC, the ratio of Th/Sc is \(\sim\)1 and for granitic rocks, the ratios are still higher. In Figure 7, the sediments are characterized by uniform Th/Sc (1.3-2.5) and slightly variable Zr/Sc (9.6-20.5) ratios. These data suggest that the sandstones were slightly enriched with zircon, indicting sorting during transport.