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Minerals in Bottom Sediments of the South China Sea

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Minerals in Bottom Sediments of the South China Sea Minerals in bottom sediments of the South China Sea PEI-YUAN CHEN* Department of Geology, National Taiwan University, Taipei, Taiwan, Republic of China ABSTRACT scattered throughout. Physiographically it can be divided into four main regions: (1) the South China Sea Basin (China Basin) in the The clay fraction separated from bottom sediments of the South center, (2) the South China shelf on the northwest, (3) the Sunda China Sea consists of illite, chlorite, kaolinite, smectite, and mixed- shelf on the southwest, and (4) the Gulf of Thailand, a northwest- layer clay minerals. The relative abundance of these clay compo- ern extension of the Sunda shelf (Fig. 1). nents varies in different regions and also with depth below the bot- The shelf areas are flat; water depths are generally less than 80 m tom. Six different clay-mineral provinces can be recognized. Illite on the South China shelf, less than 50 m in the Gulf of Thailand, and chlorite derived from the Asiatic continent are predominant on and 40 to 50 m on the Sunda shelf (see Fig. 1, and Emery and the continental shelf beyond the China coast, but they diminish Niino, 1963). The bathymetric contours descend abruptly beyond southward to the Sunda shelf. Fe-rich smectite and subordinate the border of the shelf, ranging from 150 to 200 m at the shelf edge, kaolinite, derived mainly from the igneous material of the tropical to about 1,000 m at the foot of the side slope, to about 4,500 m in archipelagoes, are the principal minerals in the equatorial region. the center of the basin (Chase and Menard, 1969). The eastern They prevail on the Sunda shelf and in the Gulf of Thailand (Gulf border of the basin is deeper. The steep-sided Manila Trench of Siam) but not in the deltaic area beyond the mouth of the (5,000 m at the deepest point) and the broader and shallower West Mekong River. Here, the archipelagic smectite-kaolinite suite is Luzon Trough (about 2,500 m deep), are oriented roughly parallel overlapped by a continental illite-rich suite discharged by the to the western coast of Luzon (Ludwig and others, 1967). On the Mekong River. High-smectite clays also form a halo surrounding floor of the basin there are broad, hilly areas. Most of the basement the Philippine volcanic arc. The clay-mineral assemblage in the cen- irregularities are due to the presence of fault blocks, volcanoes, and tral China Basin is transitional, but it is more akin to the northern calcareous reefs (Parke and others, 1971; Emery and Ben- (continental) suite than to the southern (archipelagic) suite, on the Avraham, 1972). basis of its clay-mineral composition. Therefore, it is clear that According to Emery (1968) and Emery and Ben-Avraham provenance controls the pattern of clay-mineral distribution in the (1972), the sediments in the shelf regions are chiefly relicts from the South China Sea. glacially lowered sea level. The sediments in the Gulf of Thailand, and also in the inner side and depressions of the shelf areas, how- INTRODUCTION ever, are chiefly recent deposits (Emery and Niino, 1963). The cored sediments studied are mostly Holocene, but Pleistocene or The distribution of clay minerals in the Pacific Ocean has been possibly even Pliocene sediments may be present at some sites (Em- studied by several workers (Gorbunova, 1963; Griffin and ery and Ben-Avraham, 1972). The foraminifera separated from Goldberg, 1963; Oinuma and Kobayashi, 1966; Griffin and others, these samples have been studied by C. Y. Huang. Numerous 1968; Heath, 1969; Rateev and others, 1969; Fan, 1972; Aoki and Holocene to Pleistocene species, including Globorotalia trun- others, 1974), but none of these studies covers the area in the mar- catulinoides, have been identified. ginal seas along the eastern Asian continent from the Yellow Sea in the north to the South China Sea in the south. This study, which SAMPLE COLLECTION, LOCATION, incorporates the results of two previous studies (Chen, 1973; AND LITHOLOGY Huang and Chen, 1975), presents data on this area. Collection and Geographic Distribution of Samples PHYSIOGRAPHIC AND GEOLOGIC SETTING OF SOUTH CHINA SEA A total of 191 samples from 62 stations were analyzed for clay-mineral composition (Fig. 1). Of these, 148 samples from 29 Samples were collected throughout the South China Sea (Fig. 1), stations, mostly from the abyssal area of the South China Sea but from the southern part of the Taiwan Strait (Formosa Strait) to the also including one station on the shelf of the East China Sea, were northern part of the Java Sea (Sunda Sea), and from the western cored by the R/V Conrad and by the R/V Vema of Lamont-Doherty coast of the Philippines to the Indochina coast to the west, includ- Geological Observatory. Other samples were dredged or cored ing the Gulf of Thailand (Gulf of Siam). The area is about from the shelf areas, including 33 stations on the South China shelf, 3,400,000 km2. Gulf of Thailand, and Sunda Sea, by the R/V Chiu Liang of the In- The bottom topography of the South China Sea is irregular and stitute of Oceanography, National Taiwan University. complicated, consisting of abyssal basins in the centrl part, a flat Three samples of each core (from the top, middle, and bottom of continental shelf in the northwestern and southern margins, the core) were selected for mineralogical analysis. Besides these troughs and trenches in the eastern part, and seamounts and ridges samples, five other long cores from localities 6, 7, 9, 19, and 25 (Fig. 1) were selected and sampled at intervals ranging from 10 cm * Present address: Indiana Geological Survey, Bloomington, Indiana to 100 cm, in order to study the vertical variation in mineral com- 47401. position. Geological Society of America Bulletin, v. 89, p. 211-222, 10 figs., 1 table, February 1978, Doc. no. 80206. 211 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/89/2/211/3444191/i0016-7606-89-2-211.pdf by guest on 30 September 2021 212 PEI-YUAN CHEN Lithology of Sediments most have abundant microfossils and nannofossils. The clay con- tent of the shelf sediments is generally very low (5% to 15%) in The samples range in category from noncalcareous clastic sedi- provinces A, E, and F (Fig. 2), but it is higher in province D (30% to ments to highly calcareous marl and chalky ooze. The grain size of 60%). Sediments from the deep basin (province B, water depth the shelf sediments is generally coarser than that of the basin sedi- >1,000 m) have variable clay content, ranging from 10% to 85; ments, ranging from slightly muddy sand to granular and sandy the sediments with low clay fraction (<20%) mostly contain abun- mud. The shelf sediments are commonly very shelly. Sediments dant microfossils and clastic volcanic minerals, such as plagioclase, from the basin floor generally range in particle size from sandy mud pyroxene, hornblende, magnetite, and glass shards. to silty clay. They contain few fragments of molluscan shells, but Samples collected from reef areas, at depths ranging from a few • LOCATION AND NUMBER OF CORING STATION O LOCATION AND NUMBER OF DREDGING STATION VOLCANO 200" SHELF BREAK AND REEF BANK ,Z000' 0EPTH ,N METERS BELOW SEA LEVEL lôcf Figure 1. Physiographic map of South China Sea and adjacent areas, showing location of sampling stations. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/89/2/211/3444191/i0016-7606-89-2-211.pdf by guest on 30 September 2021 MINERALS IN BOTTOM SEDIMENTS OF THE SOUTH CHINA SEA 213 hundred metres to more than 2,000 m below sea level, are generally CaC03, and consist of abundant coccoliths and a few other nan- whitish, highly calcareous chalky mud and foraminiferal ooze. noplanktons, subordinate micritic CaC03, and accessory clay min- They were encountered at localities 13, 21, 23, 24, and 27 (Figs. 1, erals. Volcanic glass shards are common accessory constituents in 2). These fine-grained, calcareous sediments contain about 10% to many samples. Several layers of sediments composed essentially of 45% clay-size particles, which are composed of 63% to 68% silt-size, glassy ash are encountered in the area surrounding Luzon Figure 2. Map showing division of day-mineral provinces in South China Sea. Histograms represent average clay-mineral composition (weight percent- age) in each province. Relative abundance of illite, smectite, chlorite, and kaolinite in each locality is indicated by symbols of peak-height rations 17 A/10 A, 7 A/10 A, and chlorite (004)/kaolinite (002). Sample localities not indicated in Figure 1 are from Chen (1973) and Huang and Chen (1975). Solid triangles indicate localities of clay samples studied by Aola and Oinuma (1974) and Oinuma and Kobayashi (1966), whose clay compositions fall in composition range of province A. Samples from province A commonly show abundance of chlorite; they are mostly categorized as S-5 and S-6 and are not labeled in figure; all samples of other categories are appropriately labeled. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/89/2/211/3444191/i0016-7606-89-2-211.pdf by guest on 30 September 2021 214 PEI-YUAN CHEN Island (Iocs. 6, 9, 12, and 16), and in the southern Sunda shelf (loc. calculated in weight percentage using the following factors: 2.5 for 60). Some samples (Iocs. 5, 14, 15, 17, and 60) also contain abun- 7 Á, 1 for 10 Á, and 4 for 17Á. The 7-Á peak height is arbitrarily dant volcanic minerals, such as plagioclase, hornblende, hyper- divided between kaolinite and chlorite by factors obtained from the sthene, augite, and magnetite, and a few rock fragments.
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