2007 Volume 58 Issue 5-6 Pages 133-188
The middle to upper Miocene tephrostratigraphy was established for the tephra beds intercalated in the Nomura Formation exposed along the Shinazawagawa River in the Tsugawa area, Niigata Prefecture, Japan. Field occurrence, petrographical characteristics (grain-size distribution, mineral composition, heavy mineral composition and shape of volcanic glass shards) and chemical composition of volcanic glass shards were systematically described for the 80 tephra beds. Furthermore, precise numerical age was calculated for each tephra bed on the basis of detailed diatom biochronology of the Nomura Formation. From 12 Ma to 7.5 Ma four stages was discriminated based on the frequency of tephra beds. A tendency of upward decrease of K2O content was recognized in the glass shards in the Nomura Formation.
