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Soviet sedimentologists use the term "coarse silt" to denote the size fraction 0.1 to 0.05 mm (50-100 μm). Petelin (1961) has shown that this fraction is most diagnostic for terrigeneous and volcanogenic mineral assemblages and provinces in Recent deep-sea sediments, because of its greatest variability of both heavy and light non-opaque minerals, which may be easily identified by the common immersion method. We believe that the fraction is suitable for mineralogical study of unconsolidated and friable sediments from DSDP cores as well, if the objective is to investigate their source area and transporation tracks. In the case of fine-grained oceanic sediments, mineral composition of the coarse silt does not differ markedly from that of the "coarse fraction" (>62 μm). We studied the coarse-silt fraction from Japan Trench sediments drilled during DSDP Legs 56 and 57, to determine the origin of clastic minerals, which are supposed to be both volcanic and terrigeneous. Some authigenic minerals were also found, and we discuss them briefly in this chapter. We separated the 0.1 to 0.05 mm fraction by wet sieving, after dispersion of sediment samples in water with tripolyphosphate solution. Heavy and light fractions were separated using bromoform with specific gravity 2.9. We then studied both heavy and light fractions by the immersion method, counting 300 to 500 grains in each, if available. The results are represented as grainfrequency percentages in Table 1. We also recalculated the data on the basis of non-opaque heavy minerals and characteristic ratios (Table 2). The studied set of samples comprises most of the Neogene to Quaternary lithologic units, distinguished on both legs, as well as a minor amount of Miocene turbidites and upper Oligocene shallow-water deposits from Site 439. The coarse-silt fraction is too scarce to be counted in the pelagic clay drilled at Site 436, and the Cretaceous shales (Site 439) and cherts (Site 436) are not friable enough to be dispersed.

Coarse-Silt-Fraction Mineralogy of Japan Trench Sediments, Deep Sea Drilling Project Legs 56 and 57