10:45 〜 12:15
[SEM15-P04] 粘性残留磁化に基づく東京低地下における後期更新世河川成礫層の年代
キーワード：粘性残留磁化、河川成礫層、東京低地
Fluvial gravel deposits filling late Pleistocene incised-valleys beneath coast planes of Japan are significant key marker beds to correlate strata laterally, reconstruct basin evolutions, and evaluate active tectonics. However, the age constraint of fluvial gravel beds is very difficult due to the lack of organic material, microfossils, and tephra beds, and thus largely depends on the sequence stratigraphic approach. Therefore, many of these depositional ages and spatial-temporal variations of the sediment distribution are unclear. In this study, we attempted to directly decide the depositional age of lowstand fluvial gravel beds, by a viscous remanent magnetization (VRM) dating approach, which has been applied to estimate the reworking age of massive boulders by geological hazards.
Igneous pebbles were collected from all-core boring samples GS-KNJ-1 and GS-AMG-1, drilled in the Tokyo Lowland. The pebbles were cut into specimens with approximately 1 cm3. To infer magnetic minerals in specimens and those magnetic domains, we conducted magnetic measurements including hysteresis curves, first order reversal curves (FORCs), thermomagnetic (Js-T) curves, stepwise isothermal remanent magnetization (IRM) acquisition experiments, and viscous decay experiments of IRM at temperatures. Natural remanent magnetizations (NRMs) were measured during progressive thermal demagnetization experiments up to 600 ℃ in 5–20 ℃-increments.
The Js-T curve (Curie temperature), the IRM acquisition curve (magnetic coercivity spectra), and FORC diagram (magnetic domain distribution) exhibit that examined specimens mainly contain titanomagnetite and/or poor-Ti titanomagnetite with vortex state and multi-domain (MD) sizes. In ThD results, a low-temperature component in an NRM, which is probably a VRM, was clearly distinguished from a high-temperature primary component at ca 230 ℃. Based on the result of the viscous decay experiment of IRM, we determined a parameter to calibrate the effect of vortex state and MD size titanomagnetites to the age estimation. Using the demagnetization temperature of the VRM, calibration parameter, and an observed temperature at 50 m deep in the Tokyo Lowland, the depositional age of the pebble was calculated as late MIS 2, which is generally consistent with inferred age based on C-14 dating obtained from mud beds above and below the fluvial gravel bed. The accuracy of age constraints will be improved by increasing the number of specimens.
Igneous pebbles were collected from all-core boring samples GS-KNJ-1 and GS-AMG-1, drilled in the Tokyo Lowland. The pebbles were cut into specimens with approximately 1 cm3. To infer magnetic minerals in specimens and those magnetic domains, we conducted magnetic measurements including hysteresis curves, first order reversal curves (FORCs), thermomagnetic (Js-T) curves, stepwise isothermal remanent magnetization (IRM) acquisition experiments, and viscous decay experiments of IRM at temperatures. Natural remanent magnetizations (NRMs) were measured during progressive thermal demagnetization experiments up to 600 ℃ in 5–20 ℃-increments.
The Js-T curve (Curie temperature), the IRM acquisition curve (magnetic coercivity spectra), and FORC diagram (magnetic domain distribution) exhibit that examined specimens mainly contain titanomagnetite and/or poor-Ti titanomagnetite with vortex state and multi-domain (MD) sizes. In ThD results, a low-temperature component in an NRM, which is probably a VRM, was clearly distinguished from a high-temperature primary component at ca 230 ℃. Based on the result of the viscous decay experiment of IRM, we determined a parameter to calibrate the effect of vortex state and MD size titanomagnetites to the age estimation. Using the demagnetization temperature of the VRM, calibration parameter, and an observed temperature at 50 m deep in the Tokyo Lowland, the depositional age of the pebble was calculated as late MIS 2, which is generally consistent with inferred age based on C-14 dating obtained from mud beds above and below the fluvial gravel bed. The accuracy of age constraints will be improved by increasing the number of specimens.