Coarse-silt-fraction mineralogy at DSDP Legs 56 and 57 Holes

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).

Geochemistry of the Japan Trench sediments at DSDP Legs 56-57 Holes

The main objective of this investigation was to study distribution of main chemical constituents and several minor elements in sediment sections drilled during DSDP Legs 56 and 57 in the Japan Trench, in order to infer geochemical features of different lithologic types of sediments, and to find out how the geochemistry is associated with major lithologic constituents, such as terrigenous detrital matter, clay, volcanic ash, and biogenic particles. The geochemical data may help to indicate the nature of the sediments and to interpret sedimentation processes. The analyzed samples seem to be representative of most lithologic units, sub-units, and sediment types drilled at all sites on both legs, except for some shallow-water deposits at Sites 438 and 439. We analyzed bulk-sediment composition by X-ray fluorescence (Kuzmina and Turanskaya) and routine wet-chemical methods (Mikhailov); amorphous SiO2, extracted in a boiling sodium carbonate solution (Analythical Laboratory, P. P. Shirshov Institute of Oceanology); Cr, Zn, Cu, Ni, Co, and Al by atomic absorption (Gordeev); and Sn, Pb, Zn, Cu, Ni, Co, Cr, V, B, and Ag by quantitative spectrographic analyses in both bulk samples and granulometric fractions (Mikhailov). In addition, Fe, Ti, Mn, and CaCO3 have been determined in selected samples by routine wet-chemical methods (Analytical Laboratory, P. P. Shirshov Institute of Oceanology). Murdmaa was responsible for interpretation of the results.

(Table 1) Geochemistry of ash layers at DSDP Leg 57 Holes

The study of the main characteristics of ash layers in Leg 57 cores shows that they are suitable for an analysis of the effect on eruptive activity of their distribution. We found (1) sediment recovery good and ash layers numerous; (2) sedimentary environment generally free from terrigenous clastic material; (3) reworking limited; (4) volcanic glass very acidic, ranging from rhyolitic to rhyodacitic composition; and (5) alteration and diagenesis negligible above the lower Miocene. The curves of explosive volcanic activity in Holes 438, 439, and 440 display two stages of high activity: an early one around 16 m.y. and a late one starting 5 m.y. B.P., both stages being separated by an upper Miocene quiescence. Detail in these results is limited by the chemical composition of the glass and accounts only for trends in explosive acid volcanism. Nevertheless, results are roughly in agreement with other data from the Northwest Pacific, although some discrepancies in the correlation of intensity of the episodes occur. The data from Leg 57 support the hypothesis of synchronous pulses in explosive volcanism.