Identification and correlation of tephras from the Plio-Pleistocene Shobudani Group, Kinokawa River, southwest Japan

Abstract

The abundances of 58 elements in volcanic glass shards from tephra samples of the Shobudani Group along the Kinokawa River, Japan, were analyzed using laser ablationinductively coupled plasma-mass spectrometry (LA-ICP-MS). The age of the Bodaitouge tephra near the Negoro Fault was estimated by fission-track (FT) dating, combined with U-Pb dating by LA-ICP-MS. The element patterns of the Gojo 4 and Shobudani 1 tephras are generally consistent with those of the Shishimuta-Pink and Ebisutouge-Fukuda tephras, respectively. Those of the Bodaitouge and Shin’ike tephras are similar to that of the Aira-Tn (AT) tephra. However, the age of the Bodaitouge tephra sample estimated by FT and U-Pb dating is 1.6-1.3 Ma, which is much older than the age of AT. The stratigraphic sequence of the Shin’ike tephra also suggests that it should be older than AT. However, in addition to the element abundances, incompletely hydrated volcanic glasses and the refractive index of coexisting ferrosilite suggest that the studied Shin’ike tephra sample is AT itself. The Shin’ike tephra may have been derived from a younger layer unconformably covering the Shobudani Group. The element pattern of the Nagayagawa tephra is similar to that of the Kikai-Akahoya rather than AT.