## 5 Interpretation and Discussion
### Paleoweathering Conditions  
Chemical weathering intensity in source rocks is mostly controlled by the composition of the source rock, period of weathering, climatic conditions and rates of tectonic uplift of the source region [35]. Calcium (Ca), sodium (Na), and potassium (K) are largely removed from source rocks during chemical weathering and the amount of these elements remaining in the sediments derived from the rocks aided as an indicator of the intensity of chemical weathering [36]. If siliciclastic sedimentary rocks are free from alkali related post-depositional modifications, their alkali contents (K\({}_{2}\)O + Na\({}_{2}\)O) and K\({}_{2}\)O/Na\({}_{2}\)O ratios should be considered as reliable indicators of the intensity of the source material weathering [37]. A few indices of weathering have been proposed based on the molecular proportions of mobile and immobile element oxides (Na\({}_{2}\)O, CaO, K\({}_{2}\)O, and Al\({}_{2}\)O\({}_{3}\)) to determine the degree of source rock weathering. Therefore, chemical compositions of weathering products in a sedimentary basin are anticipated to disclose the mobility of various elements during weathering [35]. The indices of weathering/alteration include chemical index alteration (CIA), chemical index weathering (CIW), and plagioclase index weathering (PIA). To determine the paleoweathering conditions for the studied samples, the formula for the weathering indices were used as follows:  
\[\text{CIA}=\ \left[\text{Al}_{2}\text{O}_{3}\ /\left(\text{Al}_{2}\text{O}_{3}+ \text{CaO}^{*}+\ \text{Na}_{2}\text{O}+\ \text{K}_{2}\text{O}\right)\right]\ \times 100\]  
\[\text{CIW}=\ \left[\text{Al}_{2}\text{O}_{3}\ /\ \left(\text{Al}_{2}\text{O}_{3}+ \text{CaO}^{*}+\ \text{Na}_{2}\text{O}\right)\right]\ \times 100\]  
\[\text{PIA}=\ \left[\left(\text{Al}_{2}\text{O}_{3}-\ \text{K}_{2}\text{O} \right)/\left(\text{Al}_{2}\text{O}_{3}+\text{CaO}^{*}+\ \text{Na}_{2}\text{O}-\text{K}_{2}\text{O}\right)\right]\ \times 100\]  
where CaO\({}^{*}\) is the content of CaO incorporated in silicate fraction. The calculated values for the chemical index alteration (CIA), chemical index weathering (CIW) and plagioclase index weathering (PIA) are presented in the supplementary data (Supplementary Materials Data; Table S5). The CIA is in a simplified way the ratio of primary minerals to secondary products (i.e., clay minerals). CIA values, usually ranging from about 50 for unweathered rocks up to 100 in highly weathered rocks. The CIA values for the studied samples range from 45.64% to 87.00%, suggesting weak to intense weathering conditions. Also, the binary plot of CIA against ICV shows that the samples are matured and have undergone weak to intense weathering conditions, mostly intense weathering (Figure 15). In addition, the weathering of feldspars to clay minerals can be monitored using the plagioclase index of alteration (PIA) [38; 39]. The PIA values of the studied samples range from 45.16% to 96.81% (averaging 88.58%). The PIA values indicate moderate to intense destruction of feldspars during transportation, sedimentation and diagenesis. The ternary plot of A-CN-K proposed by [3] is another way of evaluating the composition of the source rock as well as the mobility of elements during the process of chemical weathering of source materials and post-depositional chemical modification. The ternary plot of Al\({}_{2}\)O\({}_{3}\)-(CaO + Na\({}_{2}\)O)-K\({}_{2}\)O (exemplified as A-CN-K) shows that the studied samples plotted above the line joining plagioclase and K-feldspar (Figure 16). The weathering trendline of the samples tends towards the A-K boundary, suggesting the silicates (i.e., feldspar) have experienced moderate to high weathering, perhaps causing the leaching of calcium (Ca) and sodium (Na) out of plagioclase [38].