Stratigraphic Subdivision of the Pleistocene Miyata Formation Based on Lithology and Unconformity, Miura Peninsula, with Special Reference to the Radiometric Age of the Intercalated Funakubo Tuff

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  • 三浦半島第四系宮田層中の不整合と挟在する“船久保タフ(Fn）”のFt, U-Pb, K-Ar 年代
  • ミウラ ハントウ ダイヨンケイ ミヤタソウ チュウ ノ フセイゴウ ト キョウザイスル"フネ クボ タフ(Fn)"ノ Ft, U-Pb, K-Ar ネンダイ

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Abstract

The Miyata Formation distributed in Miura Peninsula is subdivided into 4 units based on lithology and unconformity. They consist of light yellowish brown tuffaceous sandstone (A), dark gray brownish tuffaceous sandy gravel (B), alternation of dark brown sandy gravel and light gray brownish mudstone (D), and pale reddish brown tuffaceous fi ne-grained sandstone (C), in ascending order. Their stratigraphic relationships are angular unconformities, respectively. Their valley-shaped erosion surfaces are considered to be due to global sea-level change related to glacial and interglacial periods. They are similar to ones known in the Sagami Group in the southern part of the Tama hills. Those precise geologic ages are still unknown, and then radiometric age dating was attempted using zircon minerals of the Funakubo Tuff (Fn), intercalated in the uppermost C unit of the Miyata Formation. As a result, their FT and U-Pb ages by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) are 0.41 Ma.

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