552 grain size samples and 33 mineral quality defined samples have been collected in the Co To island, Tonkin Bay of Vietnam, to study the distribution and the sediment provenance. The field investigation method, the grain analysis, and the detrital composition examination method have been used to study the characteristics of the surface sediments in Co To island. The obtained results show that the surface sediments in the area consist of ten different fields, namely: sandy gravel, gravelly sand, sand, gravelly muddy sand, sand mixed gravel, muddy sand, gravelly mud mixed sand, silty sand, muddy sandy gravel, and sandy mud. The mineral component method shows that quartz is the main contain of the surface sediment (72,0÷90,6%), less rock detritus, felspat, mica, and very few of the shells. It is to confirm that the continent is the main primary material source of the study area. The distribution of the sediment is complicated. As the East-Southeast side of the research area, the sediment distribution following gravity way, coarse grain near the shore (gravel, sandy gravel,…), finer grain far off the shore (gravelly muddy sand, silty sand,…), choice level is very low. Which This is saying that the movement of sediment in the area is dominant, ; the flow energy is governed by the topography near the island and seafloor. In tMhe meanwhile, the west-northwest side, the topography of seafloor is the complicated, coarse sediment grains (gravel, sand), ); the roundness is bad, which is saying that the hydrodynamic flow there plays an important role in the sediment distribution.
The marine area surrounding Ly Son island, of Quang Ngai province demonstrates steep and complex sea - floor morphology due to the presence of submarine coral reefs developed above the Ly Son paleo - volcano. The data of grain - size and mineral compositon analysis show that surface sediments of the Ly Son island are subdivided into 09 different sediment groups: muddy sandy gravel, sand, gravelly sand, gravel mixed sand and gravelly muddy sand, gravelly mud mixed sand, muddy sand, silty sand, gravelly mud, whose major composition is dominated by quartz (15.0÷71.0%), shelf fragments (27.0÷81.0%), minor lithic fragments and feldspar. This indicates that surface sediments can come from multi - origins in this studied area. The distribution of those sedimentary groups is relatively complex. The northern region mainly contains coarser grained sediments (i.e., sanddy gravel, sand, etc.), while the southern region hightly apears finer grained sediments (i.e., mud and silt). The coarser grained sediments such as muddy sandy gravel and gravelly sand, which are mainly coral fragment and often concentrated in the steep terrain around the emerged islands, while finer grained sediments more like sand, silty sand, etc. mostly deposited inside the submarine incised channels and slopes. The data from heavy mineral analysis show that the main valuable placer are ilmenite, zircon, etc. but their grades are very low and hence little economic prospect (ilmenite accounts for 23.57÷23.83 g/m3; zircon accounts for 0.33÷4.53 g/m3); Minerals used as construction materials are concentrated mainly in coarse - grained sedimentary fields (sand, gravelly sand and gravel mixed sand) with economic potential to serve construction needs on Ly Son island.
Morphological characteristics of the southwestern deep-depression East Sea have been defined based on the subdivision into co-origin surfaces. The results show that, the study area has 16 morphological units, including: Horizontal surface, slightly inclined surface, continental shelf accumulation, 200÷300 m depth; The surface is slightly inclined and wavy accumulates the outer shelf, 300÷700 m deep; Horizontal surface, abrasive, 500÷700 m (Guyot); Abrasive horizontal surface (Guyot), depth 1,300÷1,600 m; Abrasive horizontal surface (Guyot), depth -2,000 m; The surface of the ancient volcanic crest is distributed at different depths; Young volcanic surface, 1,200÷3,000 m deep; Wavy, accumulative plain, continental rise, 1,100÷1,800 m deep; Plain transport - accumulation plain, depth 1,100÷2,300 m; The smooth plain transports accumulates, depth 2,300÷3,000 m; The plain is divided by underground hills and mountains in the north, 2,000÷2,600 m deep; The plain is strongly dissected of underground mountains, 1,700÷2,600 m deep; Deep depression surface splits; Tectonic slope surface, continental slope 800÷1,400 m depth; Slope surface of the Northwestern underground mountain range 1,800÷2,600 m; Slope surface of the Southeastern underground mountain range 2000÷2900 m. Based on the morphological characteristics of the study area, field investigation, and analytical results allowed us to capture the potential areas of the Fe - Mn nodule and crust, namely: morphological units such as Guyot, young volcanic surface are supposed to be the prospect of the Fe - Mn crust while the deepwater surfaces demonstrate favorable place for Fe - Mn nodule can produce accumulation.
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