Tephrochronological Study on “Middle Terrace” and “Ancient Dune” in the Takada Plain, Niigata Prefecture, Central Japan

Abstract

The late Pleistocene Series in the Takada plain, Niigata prefecture, central Japan is represented by constituents of the “Middle Terrace” called the Hirayama Terrace and the “Ancient Dune (Old Dune)”. The Hirayama Terrace, consisting of the Hirayama Formation, is distributed around the Takada plain, and the Ancient Dune, consisting of the Katamachi Sand bed, is distributed along the coast of the Japan sea from Joetsu to Kakizaki (Fig. 1).
Many works have been published about the Hirayama Formation and the Katamachi Sand bed, but there are some problems on their formation ages and sedimental environments remaining unsolved. Generally, the Middle Terrace and the Ancient Dune are important for the consideration of the late Pleistocene geohistory in Japan. In this study, the writers try to determine the formation ages of the Middle Terrace and the Ancient Dune in this district. Sedimentology of their constituents will be discussed in detail in the next paper.
Tephrochronological approach is usually effective for regional geologic chronology. Five dated marker tephra layers were found out in this district. They are Aira Tn Ash (AT) (21, 000-22, 000 y. B. P.), Daisen Kurayoshi Pumice (DKP) (45, 000-47, 000 y. B. P.), Katamachi Pumice (KtP) (65, 000-75, 000 y. B. P.), Hanabusa Pumice (HB) (approximately 100, 000 y. B. P.), and Kamitaru Pumice (KT) (130, 000-150, 000 y. B. P.). The Hanabusa Pumice and the Kamitaru Pumice are eruptives respectively from Myoko and Iizuna volcanoes, and the others have their source in the volcanoes far away from the Myoko volcano group. From the stratigraphic relationships to these marker tephra layers (Fig. 3), the chronology of the Hirayama Terrace, the Ainokaze Terrace heigher than the Hirayama Terrace, and the Ancient Dune is resulted as follows..
1). The Kamitaru Pumice layer (KT) lies just above the Ainokaze Formation at Haizuka west of Joetsu city. From this fact, The Ainokaze Terrace plain, a part at least, is estimated to have been formed about 130, 000-150, 000 y. B. P.,
2). The Hirayama Formation interbeds the Hanabusa Pumice layer (HB) within the uppermost part at the former type locality, Hirayama west of Joetsu city, and therefore the Hirayama Terrace plain may be formed about 100, 000 y. B. P..
3). The upper part of the the Katamachi Sand bed includes the Katamachi Pumice layer (KtP) (Fig. 4), and is covered with the Daisen Kurayoshi Pumice (DKP) and the Aira Tn Ash (AT). The lower part is considered to have almost same age as the Hirayama Formation judging from their stratigraphic relation and sedimental environment. Accordingly, the Ancient Dune in the Takada plain is estimated to have been formed during the period from approximately 100, 000 y. B. P. to 50, 000 y. B. P.. This period corresponds with time of the marine regression which followed the Shimosueyoshi transgression.