Detection of the widespread tephra beds since Miocene in Japanese Islands and the investigation of the modes of their distribution and deposition

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1. Detection of the wideaspread tephra beds since Miocene in Japanese Islands was achieved through the Japan Sea side, the Pacific Ocean Side and cenrtal moun taineous regions, such as Niigata region, Akita region, Boso Peninsula, Kakegawa region, Nagano region and so on. In results, more than 10 widearea tephras were correlated among these regions. Especially, the Znp-Ohta Tephra Bed(4.0Ma) and the Omine-SK110 Tephra Bed(1.6Ma) were detected widely, making useful datum plan es in geological and geoecological reconstruction.2, As for the investigation of the modes of dispersion and sedimentation were achieved especially on the recognition of subaqueous sediment gravity flow depos its and their mechanism of transportation and sedimentation. In results, several types of gravity flows were clarified: the Joetsu Ash and the SK050 Tephra show typical turbidite character, while the SK110 Tephra was supposed to flow down as nearly plug flow condition. In addition, the Omine-SK110 Tephra was possibly suspcted that it erupted on-land in the western Nagano region, and entered in suba queous environment in the Niigata region, and further flowed down to the Akita region along the submarine trough 450km far from the on-land volcanic source The results of these investigations was reported at the 17th International Sedi mentological Congress on August, 2006.