Viscous remanent magnetization dating of the subsurface fluvial gravel beds

2024 Fiscal Year Final Research Report

• Project/Area Number: 22K14107
• Research Category: Grant-in-Aid for Early-Career Scientists
• Allocation Type: Multi-year Fund
• Review Section: Basic Section 17030:Human geosciences-related
• Research Institution: National Institute of Advanced Industrial Science and Technology
• Principal Investigator: Haneda Yuki 国立研究開発法人産業技術総合研究所, 地質調査総合センター, 研究員 (50884029)
• Project Period (FY): 2022-04-01 – 2025-03-31
• Keywords: 河川成礫層 / 残留磁化 / 沖積層 / 年代測定

Outline of Final Research Achievements

Fluvial gravel deposits filling late Pleistocene incised valleys beneath coast planes are significant key marker beds to correlate strata laterally, reconstruct basin evolutions, and evaluate active tectonics in Japan. However, dating these fluvial gravel deposits is very difficult because of the lack of organic material, microfossils and tephra beds. In this study, we attempted to constrain the depositional age of lowstand fluvial gravel beds beneath the Tokyo Lowland, by a viscous remanent magnetization (VRM) dating approach. We measured remanent magnetization of gravels, which is a signal of the ancient geomagnetic field, and evaluated these rock-magnetic properties. The resultant depositional age of an igneous pebble is between 20,000 and 40,000 years ago, consistent with the age inferred from radiocarbon ages and the sedimentary model of the alluvium basal gravel bed. In the future, it is necessary to evaluate the limits of the VRM dating approach.

Free Research Field

古地磁気学

Academic Significance and Societal Importance of the Research Achievements

本研究の大きな意義の一つは、本手法の最も適した試料が地下に埋没した河川成礫層であることを示した点である。これにより、本手法のパラメータが持つ不確実性を軽減することができる。そのため、地盤の活構造評価に用いられてきた平野地下の礫層を具体的な年代値に基づいて対比し、地下地質構造を詳細に復元されることが期待される。さらに、本手法はより細かい礫や砂サイズ粒子に含まれる磁性鉱物への適用も可能である。すなわち、礫層だけでなく、地下や海底の砂層の堆積年代制約用いることで、河川流路の発達、古気候変動、古地震編年などへの寄与が期待される。