宮城県中部および北部に分布する後期更新世広域テフラとその層位

Abstract

Widespread Tephra is a valuable time marker for tephrochronology and archaeology. Several fine ash fall deposit are distributed in central and northern part of Miyagi Pref. The authors have correlated them to widespread tephras by means of following methods. They are lithological description of tephras, measurements of refractive indices of glass shards and heavy minerals, and analyses of major elements chemical composition of glass shards using a microprobe analyzer. As a results, four late Pleistocene widespr ead tephras are discovered in this study area. They are AT, Aso-4, On-PmI and Toya. The authors described the stratigraphic positions of those widespread tephras in detail. And furthermore, they mentioned the significance that four late Pleistocene widespread tephras were discovered in this study area. The results are summarized as follows.1) In central part of Miyagi Pref., the stratigrafic sequence of AT ash, Kawasaki scoria layer, Aso-4 ash and Medeshima pumice layer occur in ascending order is confirmed. Kawasaki scoria and Medeshima Pumice are valuable marker tephras in that region.In northern part of Miyagi Pref., 10 tephras or tephra formations and their stratigraphic positions are recognized. They are, in ascending order, Hijiori pumice layer, Narugo·Katanuma-Uehara tephra, AT ash, Narugo-Yanagisawa tephra layer, Aso-4, Narugo-Nisaka tephra layer, Kitahara ash layer, On-PmI, Toya ash and Ichihasama pumice layer. Consequently, the late Pleistocene tephra stratigraphy in Miyagi Prefecture is linked with those in central and southwestern part of Japan.2) The stratigraphic relation between On-PmI and Toya ash is revealed for the first time to implicate the occurrence of marine terrace developed in ca 100 ka in a tectonically active region.