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ABSTRACT: Evolution of the Japan Sea: Results of Ocean Drilling Program Legs 127 and 128

James C. Ingle, Kenneth Pisciotto, Kiyoshi Suyehiro, Kensaku Tamaki

Unifying objectives of Ocean Drilling Program Legs 127 and 128 in the Japan Sea were to (1) assess the style and dynamics of marginal sea formation in a continental arc setting and (2) decipher the parallel depositional and paleoceanographic history of the sea. Six sites were drilled in the eastern and central areas of the sea including the Japan and Yamato basins, Okishiri and Oki ridges, and Kita-Yamato Trough within the larger Yamato Rise. In addition, downhole geophysical experiments were conducted to establish the character of deep crust and upper mantle structure. Drilling resulted in penetration of the complete Neogene sedimentary sequence in the sea as well as underlying volcanic and interbedded sedimentary rocks representing acoustic basement in many areas. Lower Miocene back-arc basalts recovered at Site 794 are interpreted as true igneous basement beneath the Yamato Basin. Preliminary analysis of data from Legs 127 and 128, together with seismic reflection profiles and information from adjacent onshore sequences indicates that (1) the Japan Sea formed by rifting in the late Oligocene-early Miocene, accompanied by widespread volcanism marked by the Green Tuff deposits of Japan, (2) rapid subsidence to lower bathyal water depths occurred in the early to early middle Miocene, (3) unusually uniform calcareous and siliceous hemipelagic sediments were deposited throughout the sea during the early Miocene through the early Pliocene with organic-rich facies signaling suboxic phases in bottom-water evolution, and (4) compressional tectonics were initiat d in the eastern sea in the latest Pliocene-early Pleistocene with coincident and widespread appearance of cyclic oxic/anoxic deposits likely reflecting tectonic, eustatic, and climatic control of circulation, productivity, and water-mass character in the sea.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990