西南日本の中・古生界層状チャートの研究（総説）

とくに層状チャートに記録された海洋無酸素事変

抄録

Recent progress in biostratigraphic, sedimetologic and geochemical investigations on bedded cherts of the Tanba-Mino Terrane of Southwest Japan are reviewed, and Oceanic Anoxic Event (OAE) recorded in the chert sequences is also presented. This may have two significant meanings i. e. to elucidate tectonics of Jurassic terranes and to evaluate an environmental change of system and condition of oceans during the Phanerozoic time. Radiolarians and spongy spicules are important composition of bedded cherts, of which Late Permian and Triassic ones indicate apparent alignment of tests deposited under the influence of bottom currents. The presumed current direction is from west to east, and this direction is approximately equal among the examined Tanba rocks, although the bedded cherts are embedded in the Jurassic melange as clasts or blocks. This points that bedded cherts could be remained their original attitude and may help to understand the mechanism of accretion of oceanic plate stratigraphy. The OAE was appeared in the P/T boundary and in lower Toarcian (Lower Jurassic), both of which are clearly marked by black organic cherts or mudstones, and in underlying sequence, bedded cherts gradually change their color from red to grey/black via greenish grey, and vice versa in overlying sequence. Late Permian shifting of sedimentary environment from oxic to oxygen depleted (or anoxic), can be linked to decreased rate of plate genesis, and the OAE at terminal Permian recorded in chert sequence shows strong coincidence to extinction of radiolarians. The Toarcian OAE is well known in shallow marine sediments in the world, and the Toyora Group, Yamaoku Formation and Kuruma Group are important examples in Southwest and Central Japan. Black organic mudstones in both bedded chert sequence and these shallow marine sediments characteristically produced acritarchs known as a cause of a "red tide". This plankton can act as an important member of primary production, and at the same time, its intence blooming might be a trigger of the OAE.