The clay fraction separated from bottom sediments of the South China Sea consists of illite, chlorite, kaolinite, smectite, and mixed-layer clay minerals. The relative abundance of these clay components varies in different regions and also with depth below the bottom. Six different clay-mineral provinces can be recognized. Illite and chlorite derived from the Asiatic continent are predominant on the continental shelf beyond the China coast, but they diminish southward to the Sunda shelf. Fe-rich smectite and subordinate kaolinite, derived mainly from the igneous material of the tropical archipelagoes, are the principal minerals in the equatorial region. They prevail on the Sunda shelf and in the Gulf of Thailand (Gulf of Siam) but not in the deltaic area beyond the mouth of the Mekong River. Here, the archipelagic smectite-kaolinite suite is overlapped by a continental illite-rich suite discharged by the Mekong River. High-smectite clays also form a halo surrounding the Philippine volcanic arc. The clay-mineral assemblage in the central China Basin is transitional, but it is more akin to the northern (continental) suite than to the southern (archipelagic) suite, on the basis of its clay-mineral composition. Therefore, it is clear that provenance controls the pattern of clay-mineral distribution in the South China Sea.

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