FORMATIVE AGE AND RATE OF DEFORMATION OF FLUVIAL TERRACES ALONG THE LOWER STREAM OF THE SHINANO RIVER, NORTHEAST JAPAN

Abstract

Fluvial terraces are widely distributed along the lower stream of the Shinano River from Ojiya city to Nagaoka city, Niigata prefecture, northeast Japan. This area has experienced active crustal movement since the Tertiary period, and the deformations of terrace plains by active folds and faults are distinctly recognized. Therefore, this area is well known as one of the best fields in Japan for studies on active crustal movement, especially on active folds. Many works have been published about fluvial terraces and active crustal movement in this area since pioneer works by Ikebe (1942) and Otsuka (1942), but few detailed studies about formative age of fluvial terraces have been published except Ota (1969). Tephrochronological approach is usually effective for regional geologic chronology. The purposes of this paper are to determine the formative age of fluvial terraces by means of tephrochronological approach, and to calculate the deformation rate of terrace plains. Main results are summarized as follows. 1. Three dated marker tephra layers were found out in this area. They are Aira-Tn ash (AT, ca. 21, 000 22_??_000 y. B. P.), Daisen Kurayoshi pumice (DKP, ca. 45, 000_??_47, 000 y. B. P.), and Nakago pumice layer (NG, ca. 130, 000_??_150, 000 y. B. P.). 2. Based on the stratigraphic relationships between terraces and these marker tephra layers, formative ages of fluvial terraces were determined as follows; A Terrace (Koshijippara terrace): ca. 130, 000_??_150, 000 y. B. P., B Terrace (Kowadappara Terrace): ca. 50, 000_??_60, 000 y. B. P., C Terrace (Funaokayama Terrace): ca. 21, 000_??_25, 000 y. B. P., D_??_F Terraces: younger than 20, 000 y. B. P., perhaps Holocene. 3. The average rate of the northward tilting of the fluvial terraces in the region is recalculated as about 0.15_??_0.17mm/km•year during the past 130, 000_??_150, 000 years, and about 0.13_??_0.16mm/km•year during the past 50, 000_??_60, 000 years. Both values correspond well with the rate of the tilting deduced from the precise relevelings (0.19 or 0.15mm/km•year, Mizoue et at., 1980). 4. The average rates of the tilting at the west wing of the Ojiya syncline (Ikebe, 1940) during the past 130, 000_??_150, 000 years and 50, 000_??_60, 000 years are 0.4‰•1, 000 years and 0.4_??_0.5‰•1, 000 years, respectively. 5. The above-mentioned facts in 3 and 4 imply that the active folding in this area has proceeded at a nearly constant rate during at least the past 150, 000 years. 6. The Katakai faults (Ota, 1969) cutting A Terrace have displaced at the order of 0.01_??_0.1m/1, 000 years.