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[HCG28-P07] A consideration of the age of the higher terraces in the southeastern part of the Kashiwazaki plain, Niigata Prefecture, central Japan.
Keywords:middle Pleistocene, upper Pleistocene, tephra, Niigata Prefecture
1.Introduction
The Kashiwazaki plain (15 km in length and 4 – 7 km in width) locating in the middle part of Niigata Prefecture, central Japan is surrounded by Chuo Hills and Mt. Yoneyama.
The upper Pleistocene in this plain is described by Kashiwazaki plain Quaternary Formation Research Group (“KRG”, here in after) (1965) and Kishi et al. (1996), whereas the middle Pleistocene in this area has not been well studied, so far.
Our study implemented boring survey drilled to basement rock in order to grasp stratigraphy of the higher and middle terraces in the southeastern part of the Kashiwazaki plain.
2.Location of drilling
Kishi et al. (1996) classified terraces in the Kashiwazaki plain into the lower, middle and higher ones. The middle terrace is subdivided to MII surface, MI surface and MI+ surface from the lower, however several levels of higher terraces are lumped together as H surface. MI and MI+ surface and H surface are correlated to Yasuda surface and Oumigawa surface that are defined by KRG (1965), respectively.
The middle and higher terraces are present in study area. The higher terrace in this area is higher than MI surface by a level. Each three boring cores were drilled on MI and H surfaces.
3.Stratigraphy in the southeastern part of the Kashiwazaki plain
Boring survey results showed that sediments in study area can be divided into three layers based on fining-upward cycle. These three layers are MI terrace deposit, H terrace deposit and layer overlain by H terrace deposit, from the upper.
MI terrace deposit under MI surface is correlated to Yasuda Formation defined by KRG (1965), and this formation is subdivided into the lower and upper parts. Base of this formation shows coarse clastics consisting of gravels and coarse-grained sands, and silt or clay overlays them. Visible volcanic ash layer is absent in this formation.
H terrace deposit under H surface shows coarse clastics consisting of gravels at the base, and silt or clay overlying them. White-colored gravel layer consisting of well-rounded white-colored gravels is present in the lower part of this deposit. The combination of this white-colored gravel layer and overlying humic layer continues under Yasuda Formation. White fine vitric ash is present in the base of this layer.
Layer overlain by H terrace deposit is distributed only in the middle part of study area. Base of this layer shows coarse clastics consisting of gravels, and silt or clay overlays them. Visible volcanic ash layer is absent in this layer.
4.Analysis of tephra
White fine vitric ash in the base of H terrace deposit is correlated to Ata-Toihama tephra based on major chemical composition of glass shards. Presence or absence of volcanic ash in Yasuda Formation and loam overlying H terrace deposit is examined by particle composition analysis. This analysis revealed that cummingtonite is accumulated in the lower part of Yasuda Formation and in the base of loam. Cummingtonites are correlated to Iiduna Kamitaru c tephra based on their major chemical composition.
5.Summary
Our study results are summarized as follows.
・MI terrace deposit (Yasuda Formation) under MI surface is correlated to MIS5e based on Iiduna Kamitaru c tephra.
・H terrace deposit under H surface is correlated to MIS7 based on Ata-Toihama tephra. It continues under MI terrace deposit based on combination of characteristic facies.
・H terrace deposit under H terrace and layer overlain by H terrace deposits are present under MI deposits (Yasuda Formation) and both are called “paleo-Yasuda Formation”, herein.
References
Kashiwazaki plain Quaternary Formation Research Group. 1965. A study of the Quaternary formation at the Kashiwazaki Plain, Niigata Prefecture, Japan. Memoir of Niigata University. 10. 145-185.
Kishi, K., Miyawaki, R. and Miyawaki, A. 1996. The stratigraphy and paleoenvironmental reconstruction of the late Pleistocene deposits in Kashiwazaki plain, Niigata Prefecture. Quaternary Research. 35(1). 1-16.
The Kashiwazaki plain (15 km in length and 4 – 7 km in width) locating in the middle part of Niigata Prefecture, central Japan is surrounded by Chuo Hills and Mt. Yoneyama.
The upper Pleistocene in this plain is described by Kashiwazaki plain Quaternary Formation Research Group (“KRG”, here in after) (1965) and Kishi et al. (1996), whereas the middle Pleistocene in this area has not been well studied, so far.
Our study implemented boring survey drilled to basement rock in order to grasp stratigraphy of the higher and middle terraces in the southeastern part of the Kashiwazaki plain.
2.Location of drilling
Kishi et al. (1996) classified terraces in the Kashiwazaki plain into the lower, middle and higher ones. The middle terrace is subdivided to MII surface, MI surface and MI+ surface from the lower, however several levels of higher terraces are lumped together as H surface. MI and MI+ surface and H surface are correlated to Yasuda surface and Oumigawa surface that are defined by KRG (1965), respectively.
The middle and higher terraces are present in study area. The higher terrace in this area is higher than MI surface by a level. Each three boring cores were drilled on MI and H surfaces.
3.Stratigraphy in the southeastern part of the Kashiwazaki plain
Boring survey results showed that sediments in study area can be divided into three layers based on fining-upward cycle. These three layers are MI terrace deposit, H terrace deposit and layer overlain by H terrace deposit, from the upper.
MI terrace deposit under MI surface is correlated to Yasuda Formation defined by KRG (1965), and this formation is subdivided into the lower and upper parts. Base of this formation shows coarse clastics consisting of gravels and coarse-grained sands, and silt or clay overlays them. Visible volcanic ash layer is absent in this formation.
H terrace deposit under H surface shows coarse clastics consisting of gravels at the base, and silt or clay overlying them. White-colored gravel layer consisting of well-rounded white-colored gravels is present in the lower part of this deposit. The combination of this white-colored gravel layer and overlying humic layer continues under Yasuda Formation. White fine vitric ash is present in the base of this layer.
Layer overlain by H terrace deposit is distributed only in the middle part of study area. Base of this layer shows coarse clastics consisting of gravels, and silt or clay overlays them. Visible volcanic ash layer is absent in this layer.
4.Analysis of tephra
White fine vitric ash in the base of H terrace deposit is correlated to Ata-Toihama tephra based on major chemical composition of glass shards. Presence or absence of volcanic ash in Yasuda Formation and loam overlying H terrace deposit is examined by particle composition analysis. This analysis revealed that cummingtonite is accumulated in the lower part of Yasuda Formation and in the base of loam. Cummingtonites are correlated to Iiduna Kamitaru c tephra based on their major chemical composition.
5.Summary
Our study results are summarized as follows.
・MI terrace deposit (Yasuda Formation) under MI surface is correlated to MIS5e based on Iiduna Kamitaru c tephra.
・H terrace deposit under H surface is correlated to MIS7 based on Ata-Toihama tephra. It continues under MI terrace deposit based on combination of characteristic facies.
・H terrace deposit under H terrace and layer overlain by H terrace deposits are present under MI deposits (Yasuda Formation) and both are called “paleo-Yasuda Formation”, herein.
References
Kashiwazaki plain Quaternary Formation Research Group. 1965. A study of the Quaternary formation at the Kashiwazaki Plain, Niigata Prefecture, Japan. Memoir of Niigata University. 10. 145-185.
Kishi, K., Miyawaki, R. and Miyawaki, A. 1996. The stratigraphy and paleoenvironmental reconstruction of the late Pleistocene deposits in Kashiwazaki plain, Niigata Prefecture. Quaternary Research. 35(1). 1-16.