## 5 Conclusions  
Detrital mica contents were interestingly displayed and studied to identify two large rivers, namely the ancient Yellow River and the Yangtze River. Micas indices were selected and applied as provenance indicators to determine sediment contribution from the two rivers to the Subei coastal plain.  
The contents and ratios of mica minerals in the very fine sand fraction of the sediments in the Yangtze River Estuary differed from those in the very fine sand fraction of the sediments in the ancient Yellow River Estuary. This difference could be attributed to a combination of different factors, such as basic geology, climate and estuarine hydrodynamics, in these two drainage area.  
Four diagnostic indices were selected and used to effectively distinguish detritus in the Yangtze River Estuary and ancient Yellow River Estuary; these indices include 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. These indices ranged from 6.67% to 19.85%, 42.14% to 65.71%, 0.69 to 0.90, and 0.13 to 0.56, respectively, in the very fine fraction of Yangtze River Estuary sediments and 24.41% to 53.93%, 9.74% to 39.81%, 0.17 to 0.60, and 0.64 to 2.09, respectively, in ancient Yellow River Estuary sediments.  
In the Subei littoral plain area, northward from the Yangtze River Estuary to SBY11 in Yangkou town, Rudong County, detrital micas were mainly affected by the Yangtze River. Moreover, in the Subei littoral plain area, southward from the ancient Yellow River Estuary to SBY12 in Bengcha town, Rudong County, detrital micas were mainly affected by the ancient Yellow River. In addition, the main mixing area of the Yangtze River and the ancient Yellow River occurred between these two sampling sites. The demarcation line between these areas could also be located in this region.  
The above specific diagnostic indices of detrital mica could be employed for quantitative analysis of the provenances of detritus and their contributions.  
Detrital mica in certain grain size fractions of sediments could be used for detritus identification, mixed area determination and provenance analysis.  
Figure 11: Scatter diagram for calculated values versus measured values of specific diagnostic indexes of detrital mica in the very fine sand fraction from sediments of the Subei littoral plain area (95% confidence).  
**Author Contributions:** Conceptualization, Z.D. and L.Z.; Writing--Original manuscript Preparation, Z.D. and L.Z.; Writing--Review and Editing, L.Z. and B.J.; Ideas and experimental funds, L.Z. and B.J.; Data processing and Figures, Z.D. and L.Z.; Sampling and Experiments, L.Z. and B.J. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This study was supported by National Natural Science Foundation of China (41576057), Guangdong Basic and Applied Basic Research Foundation (2020A1515011077), Scientific Research Projects of Jiaying University (2020 KJY05, 2019 KY05), Yantai Key Research and Development Plan (2019 XDHZ103).  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** It and all the data presented in this study are available on request from the corresponding author.  
**Acknowledgments:** The authors would like to thank editors and all anonymous reviewers for their constructive suggestions and comments that helped to improve the quality of this paper.  
**Conflicts of Interest:** The authors declare no conflict of interest.