WELL-DOCUMENTED TRAVEL HISTORY OF MESOZOIC PELAGIC CHERT IN JAPAN - FROM REMOTE OCEAN TO SUBDUCTION ZONE

The Mino-Tanba belt in southwest Japan, a segment of the Cordilleran-type orogenic chain of Jurassic east Asia, is composed mainly of a Middle-Upper Jurassic subduction-accretion complex in which Triassic and Lower Jurassic bedded radiolarian cherts occur as large allochthonous units structurally interlayered with Middle-Upper Jurassic clastic rocks. High-resolution microfossil (conodont and radiolaria) research has identified very low average sedimentation rates of about 0.5 g/cm2/1000 yr in the chert units, similar to those of modern pelagic sediments accumulated in open ocean environments. Judging from the low average sedimentation rate, high purity of biogenic silica, long duration of continuous deposition (> 50 m.y.), and wide along-strike extent (> 1000 km), the bedded radiolarian cherts in the Mino-Tanba belt are best understood as ancient pelagic sediments that accumulated in open ocean environments; accordingly, the alleged origin in smaller marginal basins is untenable. Upward lithologic change from bedded chert to overlying siliceous mudstone in the uppermost portion of chert sequences suggests the gradual landward approach of the oceanic plate toward a trench. The tectonic interlayering of these cherts and coarse-grained terrigenous clastics is a secondary feature that was added through duplexing-underplating in the subduction zone. On the basis of the primary stratigraphy and field occurrence of Triassic bedded chert in the Mino-Tanba belt, newly proposed are an idealized oceanic plate stratigraphy and a generalized travel history of a Cordilleran-type bedded chert from its birth at a mid-oceanic ridge to its demise at a subduction zone.