Petrographic Properties of Tephras, Leg 56, Deep Sea Drilling Project

Abstract

Examining volcanic materials in deep sea sediments may be one of the most important tasks of the Deep Sea Drilling Project. The investigation of volcanic ash near young source volcanoes is particularly helpful in enabling us to infer the history of volcanism in and around the island arcs. In the area of the Japanese islands volcanic deposits are usually distributed east of the source by prevailing westerly winds. It is also possible that some deep sea tephra has its source in a large, already known land volcanism. During Leg 56, volcanic ash layers from the Miocene to Recent were recovered at Sites 434, 435, and 436 in the northwestern Pacific. In addition, piston core (KH77-1-6) containing two tephra layers was raised by the R/V Hakuho-Maru from the Ocean Research Institute, University of Tokyo. The core site (39°41.4'N, 145 °22.2 'E) 30 km southwest of DSDP Site 436 is on the outer swell beyond the Japan Trench. Figure 1 shows the location of all these sites. The present work comprises the following results with respect to these tephras: (1) petrographic study of deep sea tephra layers; (2) examination of characteristic trends in tephra deposition from Miocene to Recent; (3) correlation of tephras among cores. Tephrochronological correlation and identification have been made by many investigators. For instance, Ninkovich (1968) successfully correlated deep sea tephra layers to a Pleistocene eruption from New Zealand. One of the present authors (Furuta, 1976) demonstrated that petrographic composition of constituent minerals and magnetic properties of Ti-Fe oxides can be used to identify specific tephra events, which permits stratigraphic correlation from one section to another. Meanwhile, identification and correlation among land tephras have long been a main subject in tephrochronology. Westgate (1977), for instance, reported that the Holocene tephra layers in the western territory of Canada could be identified according to significant compositional differences of volcanic glass. Aramaki and Ui (1976) suggested that discrimination of tephra layers in apparently similar petrographic features could be done by using the Ca-Mg-Fe ratio of pyroxene in the pyroclastic deposits of southern Kyushu. Machida and Arai (1976) reported that the refractive indices of volcanic glass, orthopyroxene, and hornblende phenocrysts might be an effective means for correlating widespread tephra layers. By this method they succeeded in tracing a tephra land layer extending more than 1000 km. 40N