A linkage between source volcanoes and alluvial basins, based on the facies and thickness variations of distal tephra beds of the Plio-Pleistocene Second Setouchi Supergroup, central Japan

Abstract

片岡香子・長橋良隆，2012，鮮新・更新統第二瀬戸内累層群中の遠方テフラ層の層相・層 厚変化からみた火山砕屑物供給源と沖積堆積場との関係．地質雑， 118， 139–156． （Kataoka, K. S. and Nagahashi, Y., 2012, A linkage between source volcanoes and alluvial basins, based on the facies and thickness variations of distal tephra beds of the Plio-Pleistocene Second Setouchi Supergroup, central Japan. Jour. Geol. Soc. Japan, 118, 139–156.） 本研究は，鮮新・更新統第二瀬戸内累層群 （大阪層群・古琵琶湖層群・東海層群） の非海 成層中の22テフラ層準における層相の再検討を行い，降下テフラ部と再堆積部の側方へ の層厚変化から，大阪・古琵琶湖・東海の3つの堆積盆への火山砕屑物流入経路を考慮し， 供給源となる火山地域の推定を行った．多くのテフラ層は，厚層の再堆積部を持っている ことから，テフラの空中飛来だけでなく，河川系を通じた運搬・堆積がテフラ層の形成に 重要であることが明らかとなった．検討したテフラ層では，堆積盆近傍の火砕流堆積物の 再堆積に由来するテフラ層15層準，九州南部や東北などの遠方の給源からの降下テフラ 層5層準，を判断した．テフラ層形成の背後にある噴出源の位置や噴火様式，古地理 （古水 系） をより理解するためにも，降下テフラだけでなく，水流作用により堆積した再堆積部に 着目することが重要である Few of the Japanese Pliocene to early Pleistocene tephra beds have been correlated to their source volcanoes, mainly because of the rap- id degradation of proximal ejecta after eruptions. The present paper discusses the source areas of non-marine, distal tephra beds interca- lated in the Plio-Pleistocene Second Setouchi Supergroup, central Japan, by focusing on the facies and thickness variations of both py- roclastic fall deposits and fluvially reworked deposits. Many of the examined tephra beds have thick (1–10 m) reworked/resedimented portions, indicating the transportation and deposition of tephra ma- terial by fluvial activity—a significant process during tephra bed for- mation. The tephra beds are divided into the following types based on the ratio of the thickness of the primary fall part to that of the re- worked part: A-type, tephra consisting mainly of reworked part; B-type, reworked part dominant; C-type, fall part dominant; and D-type, tephra consisting mainly of fall part. The presence of pumice pebbles and cobbles in an A-type bed suggests the tephra was de- rived mainly from the resedimentation of ignimbrite emplaced by the nearest source volcano. The C- and D-type beds indicate eruptive activity at distant volcanic areas, with the consequence that the allu- vial area received ashfall only. The present results indicate that 15 of the examined tephra beds were derived from large ignimbrites, and 5 from southern Kyushu or Tohoku (mainly distal ashfall). Thus, the sedimentology of distal resedimented tephra beds provides impor- tant clues (although indirect) to the source area, eruptive style, histo- ry, magnitude, and paleogeographical background of ancient tephras and eruptions. Keywords: