GEOLOGICAL AND PHYSICAL CONSTRAINTS ON BED-LOAD COMPOSITION IN THE RED RIVER DRAINAGE SYSTEM, SOUTH CHINA AND VIETNAM

We collected and analyzed 20 bed-load samples from the Lo, Da and Red rivers and their tributaries in order to understand various factors affecting bed-load composition. These three rivers collectively represent the Red River drainage system in South China and Vietnam. Cluster analyses based on major element chemistry reveal two different sample groups: one SiO₂ enriched, and another Al₂O₃ enriched group. The SiO₂ enriched (>73 wt %) group of samples drains areas that are dominated by sedimentary rocks of Mesozoic and Paleozoic ages; while the Al₂O₃-rich (>10 wt %) group of samples drains areas with primarily metamorphic and felsic igneous rocks. Petrographic analyses of the sand-sized fraction of the bed-load reveals that the ratios of Q: F: Rf (quartz: feldspar: rock-fragments) plot in the lithic-arenite zone, and are relatively immature compared to sands from rivers draining similar climatic regions. The arithmetic average for Q: F: Rf for the SiO₂-rich group averages 46:4:50 and for the Al₂O₃-rich group is 38:5:57. The ratios of mature/immature [Q/ (F + Rf)] framework grains shows a positive correlation with SiO₂ (R²=0.57), within the SiO₂­-rich group; however, no such correlation is seen in the Al₂O₃-rich group. These results highlight the roles of recycled/sedimentary rocks on the sources of quartz, and therefore SiO₂ to the bed-load. The Al₂O₃-rich group shows a correlation (R²=0.76) between average precipitation and Chemical Index of Alteration (CIA). The CIA indicates the degree of chemical alteration the sediment has undergone; therefore, the correlation suggests that precipitation might be a significant factor affecting the bed-load chemistry in basins dominated by primary (metamorphic/igneous) rocks. Based on our results, we suggest that in the Red River drainage system, lithology is the most important parameter influencing bed-load composition. Precipitation emerges as an important control on bed-load in river systems that have significant amounts metamorphic and felsic igneous rocks in their drainages, but it is less effective in areas that are composed of recycled and sedimentary rocks. Other factors such as runoff, slope and basin area do not show a relationship with bed-load, suggesting that these factors are less important in affecting sediment composition.