Mineral assemblage of the Tama-I Loam Formation in Kanto Plain, central Japan and correlation of some tephra layers intercalated in the Formation

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  • 多摩Iローム層の鉱物組成とテフラの対比
  • 多摩1ローム層の鉱物組成とテフラの対比
  • タマ 1 ロームソウ ノ コウブツ ソセイ ト テフラ ノ タイヒ

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Abstract

Tephra layers in the Middle Pleistcene Tama-I Loam Formation, central Japan have been characterized petrographically to aid correlation. The loam deposits themselves were also examined petrographically to detect dispersed tephra layers not readily apparent in outcrop. This work identified 28 pumice and vitric marker tephras, here termed the Tama 101 to Tama 128 tephras. Based on their petrographic properties, some can be correlated with widespread tephras occurring elsewhere in Japan. The Tama 101, Tama 116, Tama 119/122 tephras are biotite-bearing and crystal-rich, and are correlated with the C_1, B_0, and Apms tephras, respectively, all of which originated from the North Japan Alps. Tama 128 tephra is an olivine- and pyroxene-rich basaltic tephra, which is consequently correlated with the Ol.Py.C.A. tephra from the Yatsugatake volcanic center. Vitric Tama 124 tephra is correlative with the Kkt tephra, which originated from Kakuto Caldera in south Kyusyu. Five tephras in the lower half of the Tama-I Loam Formation are equivalent to tephras in basin sediments in Boso Peninsula. The proposed correlations are Tama 101 to Ks22, Tama 114 to HG, Tama 115 to GK, Tama 116 to J4 and Tama 117 to J4up. Published radiometric age of the correlative tephras and oxygen isotope stratigraphy of Boso sediments suggest that the Tama-I Loam Formation ranges in age 0.7 to 0.35Ma.

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