Abstract
Few of the Japanese Pliocene to early Pleistocene tephra beds have been correlated to their source volcanoes, mainly because of the rapid degradation of proximal ejecta after eruptions. The present paper discusses the source areas of non-marine, distal tephra beds intercalated in the Plio-Pleistocene Second Setouchi Supergroup, central Japan, by focusing on the facies and thickness variations of both pyroclastic 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 material by fluvial activity—a significant process during tephra bed formation. 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 reworked 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 derived 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 alluvial 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 important clues (although indirect) to the source area, eruptive style, history, magnitude, and paleogeographical background of ancient tephras and eruptions.
