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

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Lithology of Sediments most have abundant microfossils and nannofossils. The clay content 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

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Figure 1. Physiographic map of South China Sea and adjacent areas, showing location of sampling stations.

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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 percentage) 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.

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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. ratio of the peak heights at 3.54-Á (004) reflection of chlorite and at 3.57-Á (002) reflection of kaolinite (Biscaye, 1965).1 For an es- CLAY MINERALOGY timation of the peak intensity of mixed-layer minerals, which appears in the range 11 to 18 A, an averaged value of the weighting Experimental Procedures factors of the component minerals was tentatively adopted; for example, an average value of 2.5 is arbitrarily assigned for Details of the experimental procedures for mineralogical analysis montmorillonite-illite mixed layer, and a value of 1.75 for illite- of fine-grained sediments can be found iri'Chen (1973). I present chlorite mixed layer. here a brief review of the procedure. About 5 g of sediment split Appendixes 1 and 22 give localities (geographic position and from the air-dried raw sample was dispersed in distilled water by depth below sea bottom) and mineral compositions of the analyzed ultrasonic agitation. Clay-sire particles were fractionated by the samples. In Appendix 2 the relative abundance of clay mineral sedimentation method according to Stoke's law. After the coarse components is represented as multiple height ratios. These may be particles had settled to the bottom, the clay suspension (<2-/xm translated into weight percentages. fraction) was decanted and thickened by evaporation. The thickened slurry (about 25% to 30% solid in suspension) was drawn off Mineral Composition and dropped onto glass slides or porcelain plates (the latter used for heat treatment above 600 °C) and rapidly dried with an infrared Illite, chlorite, smectite, kaolinite, and irregular mixed-layer clay lamp for about 15 min. A rapidly dried thick slurry helps reduce minerals are the essential components of the <2-/u.m clay fraction. errors of quantitative estimation due to segregation of different Serpentine and probably chamosite and pyrophyllite were also sizes of clay minerals into bands or layers. found in some samples. The nonclay minerals are quartz, plagio- X-ray diffractometer patterns of the glycolated oriented slides clase, K-feldspar, calcite, Mg-calcite, dolomite, aragonite, goethite, were used for measuring the peak intensities of the constituent 7-A amphibole, cristobalite, and a few other problematic unidentified (kaolinite and chlorite), 10-A (illite), and 17-A (smectite) minerals minerals. The occurrence of zeolite and gibbsite has so far not been for the purpose of making semiquantitative estimations of mineral confirmed in the studied area, although their occurrences in the abundance. The peak intensity represented by the measurement of tropical Pacific sediments have been reported by some workers multiple height (designated as 2H), modified from Schultz's (1964) (Rateev and others, 1969; Arrhenius, 1963; Pimm and Garrison, method, is a summation of the component peak heights, including 1971). the maximal height and subordinate heights on both sides of the maximum at intervals of every 0.5° 26 within the peak area above

the background (Fig. 3). This peak-height ratio can be converted 1 into relative abundance or weight percentage by multiplying each The peak-height ratio for kaolinite and chlorite in a 1:1 mixture is not part of the ratio by a reciprocal value of the "weighting factor," as always unity, because of the mineralogical variations mentioned above. For example, according to my empirical study (Chen, 1973), the ratio for proposed by Johns and others (1954) and modified by Biscaye kaolinite to Al-Mg chlorite (2.4% total Fe) is near unity, but the ratio of (1965). One must bear in mind, however, that no single set of kaolinite to Fe chlorite (39% total Fe) is nearly 1:2.5 weighting factors can be applied to all types of clays because of the differences in ionic substitutions, degrees of order in layer stacking, and particle sizes of different types of clays. The average clay- mineral compositions of each province, as shown in Table 1, were 2 Copies of these appendixes, GSA supplementary material 78-2, may be ordered from Documents Secretary, Geological Society of America, 3300 Penrose Place, Boulder, Colorado 80301.

TABLE 1. AVERAGE CLAY COMPOSITION AND RATIO VALUES OF PEAK HEIGHTS FOR CLAY-MINERAL PROVINCES IN SOUTH CHINA SEA

Clay Province Mineral A B C D E F

Kaolinite 6 12 15 21 18 20 Chlorite 25 21 20 19 20 23 Illite 65 55 42 35 30 47 Smectite and mixed layer 4 12 23 25 32 10

17 A/10 A <0.5 0.3-1.9 2-10 2-5 2-10 1.5 20 10 A/7 A >1 >1 <1 <1 <1 <1 Figure 3. Measurement of peak intensity by multiple peak-height method for quantitative x-ray analysis. Closely spaced chlorite (001) and Chlorite (004)/ glycolated smectite (001) reflections augment background reflections in this kaolinite (002) 6-5 5-3 4 4-2 3-1 3 range. Line ABC is practical baseline for peak-height measurement. Point B Note: Compositions in weight percent. is located at midpoint of line D-E.

Clay Minerals

Illite. Illite, the most abundant clay mineral in the sediments of the South China Sea, has been verified substantially as a 2M polytype and dioctahedral. The diagnostic characteristics are very similar to those found in the Taiwan Strait (Chen, 1973). It is commonly thin and flaky with irregular or shredlike outlines (Fig. 7, A). Chlorite. The 14.15 A (001), 7.1 A (002), 4.7 A (003), and 3.5 A (004) are the most diagnostic chlorite peaks. Of samples dis- cussed here, however, the (001) is commonly superimposed by the broader smectite (001) peak, and the (002) and (004) peaks are superimposed by, respectively, kaolinite (001) and (002) reflections. Therefore, the accuracy of quantitative estimation may be questioned in some cases. In such situations, the weak (003) peak is useful as a reference for judging the presence and abundance of chlorite. Sedimentary chlorite from the South China Sea sediments can be dehydroxylated by heating for 1 hr at about 450 to 500 °C (Fig. 4, sample III-6). This procedure can greatly diminish the basal reflections so that the diagnostic peaks are similar to poorly crystalline chlorite and (or) Fe-rich chlorite (Brindley, 1961). The common occurrence of the South China Sea chlorite as minute fuzzy particles or aggregates of particles (Fig. 6) that can be completely dissolved in less than 1 hr in hot 6N HC1 (Fig. 7, D and E) also strengthens this inference. Some chlorite-rich samples (identified by x-ray), for example, from locality 7, contain abundant hexagonal pellets (Fig. 6, B) which are thought to be authigenic chlorite. Smectite. Most of the smectite from the South China Sea is characterized by broad (001) reflections appearing between 14.7 and 14.2 A (6.0° and 6.2° 20); this spacing is a little smaller than that of the normal montmorillonite. This basal reflection is expan- sible to 16.9 A by glycol solvation and is contracted to 9.9 A on heating to 300 °C. Its (060) spacing is about 1.50 A, indicating that its octahedral layer is of a dioctahedral type. This mineral is soluble in warm HC1 (pattern 14 in Fig. 5). These features suggest an Fe-rich variety of smectite, either an Fe-rich montmorillonite or a nontronite-like mineral similar to the smectite in the Philippine Sea (Huang and Chen, 1975). The occurrence of Fe-rich smectite in Holocene sediments is not uncommon in the Pacific Ocean (Arrhenius, 1963; Griffin and others, 1968; Aoki and others, 1974). When viewed under the electron microscope, both the segregated tiny flakes and aggregates of tiny laths or fibrous needles are commonly noted in samples of smectite-rich clays (Fig. 7, B and C). In addition to Fe-rich smectite, the occurrence of natural Na- montmorillonite was also noted in some samples (pattern 20 in Fig. 4). This mineral is diagnosed by its (001) peak at about 7.1° 26 (12.4 A), which is contractable by heating and expandible by glyco- lation as for other smectite minerals. The broad (001) peak of the 14.7- to 14.2-A smectite may be due partly to the mixed layering in its structure — that is, a Ca-smectite with interlayers of Na- smectite or illite.

Figure 4. Representative x-ray diffractometer patterns of clays, derived from continental sources, in province A (loc. 1 in East China Sea and loc. UI-6 in Taiwan Strait) and province B (Iocs. 19 and 20). Comparison of two patterns of locality 19 shows that vertical variation in clay composition is distinct. Heavier trace is of untreated sample; GLY = glycolated sample (heated to 350 or 600 °C). ALB = albite and other plagioclase, CAL = calcite, CHL = chlorite, ILL = illite, K-FEL = potassium feldspar, KAO = kaolinite, MONT = smectite, MNA = Na-montmorillonite, ML = mixed layer, QTZ = quartz.

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Kaolinite and Halloysite. The identification of kaolinite is sediments is thought to be poorly crystallized because its structure sometimes difficult when chlorite coexists in the same sample. Its can be collapsed upon heating at 500 °C for 1 hr, a temperature presence may be diagnosed, however, by the presence of a (002) lower than that required for typical well-crystallized kaolinite but peak at about 3.57 A (24.8° to 24.9° 20) in a well-resolved, slowly close to that of fireclay (MacKenzie, 1957; Smykatz-Kloss, 1974). scanned diffraction pattern (Biscaye, 1965). The (002) peak may Also, the characteristic hexagonal platy habit of kaolinite was seen become even clearer after the removal of the coexisting chlorite by only occasionally under an electron microscope (Fig. 7, F). Tubular dissolution with warm HC1 (Chen, 1973), but the (003) peak of particles, which are sparsely distributed in some samples from the kaolinite is generally too weak to detect. Kaolinite in the bottom Sunda shelf and the Gulf of Thailand, are identified as halloysite. Mixed-layer minerals. Mixed-layer clays are a minor component of most sea-bottom sediments, but they are widely distributed in the clay fraction of the South China Sea sediments. They give a broad and asymmetric peak at about 14 A or a plateau (or shoulder) between 10 and 14 A. This characteristic peak is due largely to random interstratification of montmorillonite with minor illite and, less commonly, with 14-A chlorite or vermiculite. The criteria for the identification of the mixed-layer clay minerals (Chen, 1973) in the studied samples are as follows. If the random mixed layer is composed of 10- and 17-A components (in glycolated sample), a weak shoulder may appear below 17 A (vIp ratio3 about zero; Fig. 5, pattern 42). This diffused zone can be contracted to 10 A by heating to 350 °C (pattern 19-1201cm in Fig. 4). If the random interstratification includes 17 and 14 A, or even 17, 14, and 10 A, an apronlike diffused zone shows up approximately between 14 and 17 A upon the background slope (vIp value is negative). In this case the diffused zone cannot be contracted completely to 10 A, but in- stead, a broad peak or a shoulder is commonly formed between 10 and 14 A (pattern 19-2cm in Fig. 4).

Nonclay Minerals

Quartz is relatively common in the clay-size fraction from the northern South China Sea, in the sediments off the mouth of the Mekong River (region F in Fig. 2), and in some places on the Sunda shelf. Its geographic distribution implies a terrigenous derivation. Under an electron microscope, a quartz particle was observed to be generally irregular in shape and thicker (darker) than clay minerals. Plagioclase, mainly albite, is relatively abundant in the region surrounding Luzon and a few other localities associated with volcanic sediments. Potassium feldspars were also found in some places, but they are not as abundant as plagioclase. Amphibole occurs only as an accessory component in samples associated with volcanic-source sediments. It is diagnosed by its (110) reflection at 8.38 A (10.5° 20) (Fig. 5, loc. 14 sample). Serpentine in moderate amounts is found only in the samples from locality 14 (Fig. 5). Calcite has a wide- spread distribution in the South China Sea. It was found even in the sediments below the carbonate compensation depth (about 4,000 m in the abyssal plain), such as at localities 5 and 19 (Fig. 4). Magnesium-calcite is identified from the chalky sediments in the deep basin and also from some of the clastic sediments in the shelf area. Its strongest x-ray peak (104) appears at 2.99 to 3.0 A. Ac- cordingly, it: is estimated to contain about 15 mol % MgC03 (Goldsmith and others, 1955). Many lath-shaped crystals in the clay-size fraction of a chalky mud at locality 24 is thought to be Mg-calcite. Aragonite is occasionally found associated with Mg- calcite in the chalky sediments, and dolomite is found only in a few shelf samples.

3 The vip ratio is the ratio of the height of the peak above background (v) to the depth of the valley (p) on the low-angle side (Biscaye, 1965).

Figure 5. Representative x-ray diffractometer patterns of smectite-rich clays from province C (loc. 14), D (loc. 50), and E (loc. 29) and continental clay from province F (loc. 42). AMP = amphiboles, SERP = serpentine, 18 15 DEHY = dehydrated, HCL = warm HC1 treated. For other notations, see 2e Cuka Figure 4.

SIZE AND SHAPE OF CLAY-SIZED PARTICLES province is surrounded by the smectite-rich sediments on the Sunda shelf. As observed under an electron microscope, clay particles with As shown in Figure 2, the increase in smectite content from north sizes in the range of 0.5 to 1.5 /xm are most dominant, but sizes less to south is the most striking feature of the mineral variation. If we than 0.1 /xm are still fairly abundant. The clay particles are gener- use the value of the peak-height ratio of 17 A/10 A peaks as a prov- ally irregular in shape and have fuzzy margins (Figs. 6, 7). Particles ince index, this ratio is less than 0.5 in province A, between 0.3 and with euhedral shapes or bounded by cleavage traces were only oc- 1.9 in province B, and mostly from 2 to 10 in provinces C, D, and casionally seen. Among the latter types, hexagonal kaolinite plates E. On the other hand, the content of chlorite and illite decreases (Fig. 7, F), aggregates of slender laths or needles of Fe-rich smectite (Fig. 7, B), and tiny (<0.1 /xm) hexagons, probably chlorite, (Fig. 6, B) were noted. The minute crystallites of the latter two minerals appear to be formed in the marine environment. The fuzzy to blurry fringes on the clay particles mentioned above may also be incipient crystallites grown on a detrital particle at the depositional site by diagenetic processes.

REGIONAL DISTRIBUTION OF CLAY MINERALS

The clay-mineral composition of bottom sediments, consisting of about 65% illite, 25% chlorite, and 10% other clay minerals, including kaolinite, smectite, and mixed-layer minerals, is very uni- form over the extensive continental shelf from the East China Sea southwestward to the southern end of the Taiwan Strait. In contrast, however, the smectite content increases seaward from the shelf break (Chen, 1973; Aoki and Oinuma, 1974). In the South China Sea the clay-mineral assemblages become complex and highly diversified in different areas, making it possible to divide the area into six clay-mineral provinces or lithotopes. These provinces (Fig. 2) are (A) the South China continental shelf, (B) the South China Sea Basin (or China Basin), embracing most of the central abyssal basin, (C) the sea surrounding the Philippines, (D) the Sunda shelf, (E) the Gulf of Thailand, and (F) the subaqueous part of the delta off the mouth of the Mekong River.

Areal Variation of Clay Composition

Province A, an extension of the continental shelf from the East China Sea, has essentially the same clay-mineral composition as that of the Taiwan Strait and the East China Sea mentioned above. These provinces have the highest content of illite (>60%) and the lowest content of smectite (<5%) of all provinces examined. Province B includes the central part of the South China Sea and extends northeastward, passing through the Bashi Channel, to in- clude the sea between Taiwan and Ryukyu and also the adjacent northwestern Philippine Sea (Province I in Huang and Chen, 1975). Illite and chlorite are reduced by 5% to 10% in this province, as compared with province A, but smectite and kaolinite increase relatively; the smectite content is largely between 5% and 15% in this province. In provinces C, D, and E, smectite becomes very promi- nent, ranging from 20% to 50%. Illite, on the contrary, is reduced to only 30% to 35% on the average. The lowest illite content, about 14%, was found at localities 11 and 14 in province C, an area where volcanic-derived minerals and glassy ashes are abundant in the sediments. In provinces D, E, and F, the kaolinite content averages about 20%, but it was as high as about 50% at locality 60 east of the Strait of Malacca (Fig. 2). Province F, the subaqueous part of the Mekong Delta, has a clay-mineral assemblage similar to the northern continental suite in provinces A and B, except that it contains more kaolinite. This

Figure 6. Electron micrographs showing general morphology of clay particles from China Basin. A — locality 7, 530 cm; scale (arrows) = 2 /xm; B — locality 7, 910 cm; scale = 1 /xm; C — locality 9, 910 cm; scale = 2 /xm. In B, note numerous small hexagonal pellets, which may be authigenic chlorite.

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southward. The peak-height ratio of 10 A (illite)/7 A (chlorite + was plotted in a triangular diagram. The extent of variation in kaolinite) is generally greater than unity in province A and in part composition among the clays from the surface sediments of each of province B, but the ratio becomes smaller than unity in provinces province is shown by a line and the letter of the province. The clays C, D, and E. The average peak-height ratio of chlorite to kaolinite from provinces A, B, and F are in common with a smaller 17 A/(10 is about 5:1 in the region north of lat 20°N, and about 2.5:1 to 1:1 A + 7 A) ratio and are inclined to spread toward the left side of the in the region between lat 20°N and 10°N. Farther south, in some diagram. The clays from provinces C, D, and E, however, generally parts of the Sunda shelf, the content of kaolinite may reach five with greater values of 17 A/(10 A + 7 A), spread mainly in the right times that of chlorite (Fig. 8; Table 1). side of the diagram. The clustering of points representing samples The average clay-mineral composition of the sediments for each from different provinces manifests the regional control on the province, calculated in weight percentage, is listed in Table 1 and clay-mineral assemblages and shows the contrast between the shown in histograms in Figure 2. Asian continent-derived clays (provinces A, B, and F) and the Variation in the clay-mineral composition for samples within a archipelago-derived clays (provinces C, D, and E). The spreading province as well as between provinces from the South China Sea as extent of the different compositions of these two groups of clays, related to provenance factor is depicted by a ternary diagram in including samples from various levels below the sea bottom, are Figure 9. The peak-height (1H) ratio of 7 A (kaolinite + chlorite), encircled by the two heavy lines 1 (solid) and 2 (broken) in Figure 10 A (illite), and 17 A (smectite and minor mixed-layer minerals), 9. The points that fall in the overlapping area enclosed by line 1 and

ft. * c

Figure 7. Electron micrographs of clay from South China Sea. A — Thin illite flakes with irregular outlines, from locality 25, 160 cm. B — Note existence of aggregates of tiny slender laths or needles (indicated by arrows) in clay fraction from locality 25, 600 cm, which is believed to be authigenic Fe-rich smectite. C — Smectite-rich clay from Gulf of Thailand, locality 31. Scale bars in A, B, and C = 1 /J-m. D and E — Clay sample from locality 36; D is original clay before HC1 treatment, and E is clay after warm HC1 dissolution. Note that tiny particles of both chlorite and smectite are substantially removed by this treatment. Residual particles are chiefly quartz. F — Kaolinite-rich clay from locality 60, 168 cm, with occasional hexagonal platelets of kaolinite. Scale bars in D, E, and F = 2 p.m.

line 2 are mostly the clays from provinces B and C and the lower vestigated samples does not appear to correlate with the lithologic levels of province F. variation of sediments, the variation in water depth within a province, the environmental energy conditions, or diagenetic changes. Vertical Variation of Clay Composition Judging from the geographic as well as physiographic distribution pattern of the clay-mineral assemblages in the South China Sea, the In addition to the areal variation of clay minerals in the South essential control on the variation of mineral assemblages in the China Sea, a distinctive vertical variation exists in some cores. The sea-bottom sediments is provenance factors. Two main source best examples are observed in the cores from localities 19 and 25 in areas are contributing sediments to the South China Sea, and they the central South China Sea, the lengths of which are 12 m and 6 m, respectively. Figure 10 shows the tendency of the mineral variation o from the top to the bottom. The upper part generally has a higher smectite content than the lower part. In contrast, the lower part contains more mixed-layer clay minerals and (or) better crystallized chlorite and illite (verified by the sharpness of the basal reflections; see Fig. 4, comparing samples 19-2cm and 19-1201cm). The vertical modifications in mineral composition and degree of crystallinity are postulated to be the result of diagenesis.

CONTROLLING FACTORS IN DISTRIBUTION OF CLAY MINERALS

According to the mineralogical characteristics of the South China Sea sediments, terrigenous detritus, including subordinate volcani- clasts, contributes a substantial part of the bottom sediments. However, calcareous sediments, mainly biogenic skeletons with calcite cement, are dominant in some places. Siliceous remains (diatoms) and authigenic noncarbonate minerals or mineraloids are rarely seen in the samples. On close examination, the clay-mineral composition of the in-

Figure 9. Ternary diagram showing extent of variation in clay-mineral composition of days from each day-mineral province in South China Sea. Composition is expressed by peak-hdght ratio (percentage) in x-ray pattern among 7 Â, 10 Â, and 17 Â minerals. D, = samples from eastern Sunda shelf, D2 = samples from western Sunda shelf. See text for further explana- tion.

7Â

i Figure 8. Selected x-ray diffractometer patterns scanned at 0.25° 26 per min. between 24.5° and 25.5° 26 angle, showing increase of kaolinite and decrease of chlorite from north to south in South China Sea (see Fig. 1 for sample locations).

Figure 10. Ternary diagram trend of vertical variation of day-mineral composition in cores V-19-132 (loc. 19) and V-19-134 (loc. 25). Note dus- tering of points into two groups. Group representing upper stratigraphie levels has larger smectite (17 Â) content.

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have markedly different geologic characteristics. The northern China and Indochina to the Asian continental shelf and the China source is the Asian continent, and the southern source is the ar- Basin. Farther southward, however, in the southwest part of the chipalegoes or volcanic arcs that bound the South China Sea on the South China Sea, the dispersion of the continental sediments is in- eastern and southern sides. terrupted and diluted by the archipelago-derived clay-mineral suite. Thus, the striking contrasts among lithotopes of province F and Continental Source provinces D and E (as described above) can be explained as the superposition of the northern inland-derived illite-and chlorite-rich Characteristic Mineral Assemblage. The mineral composition sediments discharged by the high-sediment yield Mekong River of the sediments derived from the Asian continent is characterized onto the smectite and kaolinite-dominant sediments of the Sunda by the abundance of illite and chlorite and rather scarce kaolinite shelf (see below). This is a good example of the provenance control and smectite in the clay fraction. Quartz is very abundant, but it is on the distribution of clay minerals in the South China Sea. substantially restricted to the >2-/xm fractions. Feldspars are present in all size fractions in amounts ranging from moderate to Archipelago Source scarce. The distribution of the continental-source sediments is con- tinuous from the East China Sea southward along the coast to the Characteristic Mineral Assemblage. The clay-mineral as- South China Sea, reaching as far as the Mekong Delta. The mineral semblages in the sea surrounding the Philippine Islands and in the composition of the fine sediments is remarkably homogeneous over southwestern South China Sea (provinces C, D, and E) are charac- a vast area, but the composition of coarser sandy sediments may terized by a high content of smectite and (or) kaolinite. The two vary somewhat in different regions (Wang, 1960; Emery and major constituents in the continental assemblage, illite and chlorite, Niino, 1963; Chou, 1971, 1972; Chen and Chen, 1971). The pos- are much reduced in these regions. There is a halo of smectite-rich tulated relationship of the clay-mineral assemblage in the northern sediments extending across latitudes and enveloping the Philippine South China Sea to land sources on the eastern Asian continent is Islands. This zone grades into the lower-smectite and higher-illite supported by data showing illite and chlorite as the two major provinces westward in the South China Sea and northeastward in constitutents of the clays in soils and argillaceous rocks from main- the Philippine Sea. land China and Taiwan (Chen, 1972a, 1972b, 1973). The pre- Origin of Smectite. The predominance of smectite in the sedi- dominance of illite (hydromica) in the soils of the Asian continent ments in the equatorial Pacific has been described by Gourbunova was also mentioned by Gradusov (1974). According to his estima- (1963), Griffin and others (1968), Rateev and others (1969), and tion, in most of China illite constitutes more than 70% of total clay Huang and Chen (1975). The origin of smectite is generally related minerals in the clay fraction from the C horizon and about 30% to to volcanic activity or the alteration of volcanic materials by hydro- more than 70% in the A horizon of the soil profiles. The clay com- thermal, weathering, or halmyrotic processes. The abundance of position of the alluvial clays and soils in the Mekong Delta provides smectite surrounding the Philippines is paralleled by the common additional verification of the predominance of illite and chlorite in occurrence of volcaniclastic sediments offshore from Luzon Island sediments derived from the continent (Post and Sloane, 1971; (Iocs. 6, 12, 16, and 17). This suggests that volcanoes of the Uehara and others, 1974). Philippine arc are the principal sources for smectite in the sur- Transportation, Dispersion, and Mixing of Sediments. The dis- rounding seas. The abundance of smectite on the Sunda shelf may integrated rock materials on the Asian continent are carried into be related to volcanism in Indonesia. The abundance of smectite in the seas chiefly by rivers, but subordinately by wind. The great the Gulf of Thailand is uncertain. majority of the sediments derived from eastern Asia (mainly China) It is postulated that in the South China Sea and adjacent seas, the are carried by eastward-flowing rivers that empty into the Yellow smectite in the sea-bottom sediments results mainly from the land- Sea and the East China Sea. A much smaller proportion of the con- derived weathering products of andesitic and basaltic lavas and tinental sediments, which are derived from southern China and pyroclastics rather than from the transformation of glass shards in part of Indochina, are transported by southwestward-flowing rivers sea water. This is corroborated by the fact that the ash layers en- that empty into the South China Sea. Among the latter rivers, the countered in the cores from the South China Sea do not contain Mekong has the largest sediment yield, but it is only one-tenth that more smectite than the non-ashy layers. If any smectite is being of the Huang-Ho (Holeman, 1968). formed in the sea it must be derived either from the clay-sized vol- The South China Sea obtains its sediments not only from the riv- canic dust fallout, as I postulated previously (Chen, 1970), or from ers that empty directly into it, but also from rivers that discharge authigenic processes such as that suggested by Aoki and others into the East China Sea and even the Yellow Sea. The suspended (1974). sediments in these seas are transported by the southward-flowing Sources of Kaolinite. The main source area of kaolinite is the nearshore current along the China coast (Niino and Emery, 1961), Indonesian and Malaysian islands. The abundance of kaolinite in passing through the Taiwan Strait into the northern part of the the shallow sediments of the Strait of Malacca along the Sumatra South China Sea. coast and the decrease of kaolinite content outward from the shore The southwestward dispersion of sediments along the west side with increasing water depth was reported by Keller and Richards of the Taiwan Strait is shown in ERTS-1 photographs of this region (1967). The kaolinite deposits in the large granitic area of Belitung (nos. 534015625, 534015655, 535020255, and others, EROS Island were investigated by Murray (1974). The proximity of Data Center, U.S. Geological Survey), taken at 920 km above the localities 60 and 61 to the eastern end of the Strait of Malacca and Earth's surface. These pictures show clearly that sediments emptied Belitung Island may explain the high kaolinite content (about 25% into the sea by mainland rivers from Chekiang and Fukien Prov- to 50%) in the 4-m-long cores. inces are dispersed over a wider area on the southern side of the It can be concluded that the abundance of smectite and kaolinite river mouths than on their northern sides. Also, the submarine bars in the Sunda shelf sediments is closely related to the volcanism and and spits lying on the shallow sea bottom in a zone from the coast the tropical humid climate of the south sea archipelago. The deeply outward 20 to 50 km, become narrower or pointed to the south. weathered extensive Quaternary rhyolitic to andesitic volcanic This shows the effect of the southward current on the transporta- rocks and older granitic intrusive rocks in Sumatra, Java, and adja- tion of bottom sediments in this area. Because of currents, the con- cent islands, and also the extensive lateritic soils and kaolin clays tinental clay-mineral suite disperses southward from the shores of derived from altered granitic rocks in the Malay Peninsula, are

possible sources for this clay-mineral suite. Rateev and others scope. The existence of authigenic or diagenetic clay minerals in the (1969), referring to the common association of smectite and kao- South China Sea sediments is likely, but the quantity of this mate- linite, called this assemblage an "equatorial-type" suite. rial is insignificant. The occurrence of smectite-rich clays is related to volcanism, but the smectite minerals originate chiefly from Transitional Source alteration of volcanic rocks on land rather than from the halmyrotic alteration of glass shards in the sea. The China Basin in the central abyssal part of the South China Some olive-gray mud and calcareous ooze have been found on Sea may be considered a transitional area in the distribution of clay the abyssal plain beneath the calcium carbonate compensation composition because it contains a mixture of both continental and depth. They appear not to be originally abyssal sediments but archipelagic sediments, but its composition is more akin to the rather to have been deposited in a shallow environment. The cause northern assemblage. This inference is based on the evidence that for their occurrence at the present depth may be due to either sub- continent-derived illite and chlorite are more dominant in the sedi- marine sliding or basin subsidence. ments of the China Basin than are smectite and kaolinite derived from the archipelagoes. The clayey and silty sediments in the ACKNOWLEDGMENTS abyssal plain may be brought in mainly by subaqueous processes from the shelf area on the mainland side. Windblown dust from the This research was financially supported by the National Science mainland during the winter monsoon and volcanic fallout from the Council, Republic of China. The Institute of Oceanography, Na- volcanoes on the neighboring archipelagoes may be subordinate tional Taiwan University, and the Core Laboratory of the contributors to the deep-sea sediments, but volcanic fallout is not a Lamont-Doherty Geological Observatory provided the sediment significant part of the clay-size sediments in the China Basin. Sedi- samples for this investigation. Coring in the South China Sea by ments in the surrounding shallow sea might be conveyed and dis- Lamont-Doherty was supported by U.S. Office of Naval Research persed into deep water by tides, waves, and currents, but probably contract CN00014-67-A-0108-0004 and National Science Foun- the most important source is submarine slumping (Emery and dation Grant GA-35454. I thank Sam Boggs and P. E. Biscaye for Ben-Avraham, 1972). critically reading the manuscript and also T. W. Huang and T. T. In the central South China Sea, calcareous mud or chalky ooze, Lin for assistance in the laboratory. The election micrographs were mainly biogenic but also of chemical origin, are present in addition prepared in the Electron Microscope Laboratory of the National to terrigenous sediments. These carbonate sediments are chiefly Taiwan University. Pleistocene in age (based on unpublished information from the Core Laboratory of the Lamont-Doherty Observatory and on REFERENCES CITED foraminifera identification in my laboratory at National Taiwan University). They contain very little clay minerals. Illite is the dom- Aoki, S., and Oinuma, K., 1974, Clay mineral composition in Recent inant or only mineral in the insoluble residues. marine sediments around Nansei-Syoto Islands, south of Kyusyu, Ja- pan: Geol. Soc. Japan Jour., v. 80, p. 57-63. Aoki, S., Kohyama, N., and Sudo, T., 1974, An iron-rich montmorillonite SUMMARY AND CONCLUSIONS in a sediment core from the northeastern Pacific: Deep-Sea Research, v. 21, p. 865-875. The bottom sediments in the South China Sea are principally Arrhenius, G., 1963, Pelagic sediments, in Hill, M. N., ed., The sea, Vol. 3: Pleistocene to Holocene clastic sediments, with subordinate chalky New York, Interscience, p. 655-727. carbonate sediments in the central part. Illite, chlorite, smectite, Biscaye, P. E., 1965, Mineralogy and sedimentation of recent deep-sea clay kaolinite, and irregular mixed-layer clay minerals are present in in the Atlantic Ocean and adjacent seas and oceans: Geol. Soc. almost all samples but vary in relative abundance from area to area. America Bull., v. 76, p. 803-832. Six clay-mineral lithotopes, or provinces, can be established. Brindley, G. W., 1961, Chlorite minerals, in Brown, G., ed., The x-ray identification and crystal structures of clay minerals: London, Miner- Illite and chlorite are the two major components derived from alogical Society, p. 242-296. the continent. They are dominant in the northern part of the South Chase, T. E., and Menard, H. W., 1969, Bathymetric atlas of the north- China Sea and are most abundant on the continental shelf fringing western Pacific Ocean: Washington, D.C., U.S. Naval Oceanographic the coast of China. Smectite and kaolinite represent the tropical or Office, 50 sheets. equatorial-type clay-mineral suite derived mainly from the ar- Chen, J. C., and Chen, C., 1971, Mineralogy, geochemistry and paleontol- chipelagoes to the east and south of the South China Sea. The dis- ogy of shelf sediments of the South China Sea and Taiwan Strait: Acta tribution pattern of clay-mineral assemblages in the South China Oceanog. Taiwanica, no. 1, p. 33-53. Sea is controlled mainly by provenance. The distribution pattern Chen, P. Y., 1970, Petrography of the upper Eocene ash beds from Gon- displays a tendency toward latitudal zoning, a feature reported by zales and Fayette Counties, Texas: Geol. Soc. China Proc., no. 13, earlier workers in the other parts of the Pacific equatorial region. p. 23-33. 1972a, Transformation of clay minerals in shallow sea surrounding This distributional pattern of clay minerals has resulted from Quemoy Island, Fukien, China: Internat. Clay Conf., Madrid, 1972, clay-mineral compositions of the two main sources, a northern Abs., p. 62. continental Asian source and a southern tropical archipelagic 1972b, Clay minerals from the alterations of mafic and intermediate source. The compositional difference, in turn, is the consequence of igneous rocks in Taiwan and neighborning islands: Geol. Soc. China the climate control and the influence of the various rock associa- Proc., no. 15, p. 45-64. tions in the source areas. In the South China Sea, however, the zo- 1973, Clay minerals distribution in the sea-bottom sediments nation has a traverse orientation surrounding the Philippine ar- neighboring Taiwan Island and northern South China Sea: Acta chipelago, reflecting the control of areal tectonic frameworks on Oceanog. Taiwanica, no. 3, p. 25—64. the distributional pattern in addition to the provenance factor. Chou, J. T., 1971, Recent sediments in the shallow part of the South China Sea: Geol. Soc. China, Proc., no. 14, p. 99-117. The clay-sized sediments are believed to be mainly land-derived, 1972, Sediments of the Taiwan Strait and the south part of the Taiwan as verified by the existence of abundant 2M illite and also the sub- Basin: U.N. ECAFE, CCOP Tech. Bull., no. 6, p. 75-97. ordinate irregular-shaped quartz. The clay particles are generally Emery, K. O., 1968, Relict sediments on continental shelves of the world: fragmental shreds with irregular outlines; very rarely do particles Am. Assoc. Petroleum Geologists Bull., v. 52, p. 445-464. show crystal-surface-bounded outlines under the electron micro- Emery, K. O., and Ben Avraham, Z., 1972, Structure and stratigraphy of

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the China Basin: U.N. ECAFE, CCOP Tech. Bull., v. 6, p. 117-140. Niino, H., and Emergy, K. O., 1961, Sediments of shallow portions of East Emery, K. O., and Niino, H., 1963, Sediments of the Gulf of Thailand and China and South China Seas: Geol. Soc. America Bull., v. 72, adjacent continental shelf: Geol. Soc. America Bull., v. 74, p. 541 — p. 731-762. 554. Oinuma, K., and Kobayashi, K., 1966, Quantitative study of clay minerals Fan, P. F., 1972, Mineralogy of Cretaceous to Holocene deep-sea clays and in some Recent marine sediments and sedimentary rocks from Japan: clay minerals from the Atlantic and Pacific Oceans: Clay Minerals Clays and Clay Minerals Conf., 14th, Berkeley, Calif., Proc., p. 209- Conf. 21st, Woods Hole, Mass., Abs., p. 13. 219. Goldsmith, J. R., Graf, D. L., and Joensuu, O. I., 1955, The occurrence of Parke, M. L., Jr., Emergy, K. O., Szymankiewicz, R., and Reynolds, L. M., magnesian calcite in nature: Geochim et Cosmochim. Acta, v. 7, 1971, Structural framework of continental margin in South China Sea: p. 212-230. Am. Assoc. Petroleum Geologists Bull., v. 55, p. 723-751. Gorbunova, I. N., 1963, Clay minerals in the sediments of the Pacific Pimm, A. C., and Garrison, R. E., 1971, Sedimentology synthesis: Lithol- Ocean: Litologjya i Poleznyye Iskopayemyye, no. 1, p. 28-42. ogy, chemistry and physical properties of sediments in the northwest- Gradusov, B. P., 1974, A tentative study of clay mineral distribution in soils ern Pacific Ocean, in Fischer, A. G., and others, Initial reports of the of the world: Geoderma, v. 12, p. 49—55. Deep Sea Drilling Project, Vol. 6: Washington, D.C., U.S. Gov. Print- Griffin, J., and Goldberg, E. D., 1963, Clay-mineral distribution in the ing Office, p. 1131-1305. Pacific Ocean, in Hill, M. N., The sea, Vol. 3: New York, Interscience, Post, J. L., and Sloane, R. L., 1971, The nature of clay soils from Mekong p. 728-741. Delta, An Giang Province, South Vietnam: Clays and Clay Minerals, Griffiin, J., Windom, H., and Goldberg, E. D., 1968, The distribution of v. 19, p. 21-29. clay minerals in the world ocean: Deep-Sea Research, v. 15, p. 433— Rateev, M. A., Gorbunova, Z. N., Lisitzyn, A. P., and Nosov, G. L., 1969, 459. The distribution of clay minerals in the oceans: Sedimentology, v. 13, Heath, G. R., 1969, Mineralogy of Cenozoic deep-sea sediments from the p. 21-43. equatorial Pacific Ocean: Geol. Soc. America Bull., v. 80, p. 1997- Schultz, L. G., 1964, Quantitative interpretation of mineralogical composi- 2017. tion form x-ray and chemical data for the Pierre Shale: U.S. Geol. Sur- Holeman, J. N., 1968, The sediment yield of major rivers of the world: vey Prof. Paper 391-C, 31 p. Water Resources Research, v. 4, p. 737-747. Smykatz-Kloss, W., 1974, Differential thermal analysis, application and re- Huang, T. W., and Chen, P. Y., 1975, The abyssal clay minerals in the west sults in mineralogy: New York, Springer-Verlag, 185 p. Philippine Sea: Acta Oceanog. Taiwanica, no. 5, p. 37-63. Uehara, G., Nishina, H., Melvin, S., Tsuji, G., and Gordon, Y., 1974, La Johns, W. D., Grim, R. E., and Bradley, W. F., 1954, Quantitative estima- composition du limon du Mekong et son role possible comme une tions of clay minerals by diffraction methods: Jour. Sed. Petrology, source de substances nutritives dans les sols du delta (Report submit- v. 24, p. 242-251. ted to Committee of Research and Coordination of Lower Mekong Keller, G. H., and Richards, A. F., 1967, Sediments of Malacca Strait, Basin, Bangkok, Thailand, from Department of Agronomy and Pedol- southeast Asia: Jour. Sed. Petrology, v. 37, p. 102-127. ogy, University of Hawaii, Honolulu): 100 p. Ludwig, W. J., Hayes, D. E., and Ewing, J. I., 1967, The Manila Trench and Wang, C. S., 1960, Sand fraction study of the shelf sediments of the China West Luzon Trough — I, Bathmetry and sediment distribution: Coast: Geol. Soc. China Proc., no. 4, p. 33-49. Deep-Sea Research, v. 14, p. 533-544. Mackenzie, R. C., 1957, The differential thermal investigation of clays: London, Mineralogical Society, 456 p. MANUSCRIPT RECEIVED BY THE SOCIETY JULY 21, 1976 Murray, H. H., 1974, Residual kaolin formation on the island of Belitung, REVISED MANUSCRIPT RECEIVED JANUARY 31, 1977 Indonesia: Clay Minerals Conf., 23rd, Cleveland 1974, Abs., p. 49. MANUSCRIPT ACCEPTED MARCH 11, 1977

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