Initialising ...
Initialising ...
Initialising ...
Initialising ...
Initialising ...
Initialising ...
Initialising ...
Saegusa, Yoshie*; Sugai, Toshihiko*; Ogami, Takashi*; Kashima, Kaoru*; Sasao, Eiji
Quaternary International, 230(1-2), p.67 - 77, 2011/01
Times Cited Count：8 Percentile：23.96(Geography, Physical)We reconstructed Holocene environmental changes by analyzing diatom assemblages of four drilling cores from the Nobi Plain, central Japan. We identified five diatom assemblage zones: (1) freshwater species dominant; (2) increase of marine and brackish-marine species, indicating transgression; (3) marine species dominant; (4) increase of freshwater species, indicating marine regression as a result of delta progradation; and (5) freshwater species dominant, in ascending order. It became clear that the shoreline migrated landward (transgression) faster than it migrated seaward (regression) by the comparison the diatom assemblages and lithological units. The transgressive diatom assemblage changes (decrease in marine-brackish water species) took up to 1000 years, whereas regressive changes required only a few hundred years. Diatom analysis is useful for reconstructing not only Holocene sea-level changes and sedimentary environments but also local geographic effects.
Niwa, Yuichi*; Sugai, Toshihiko*; Saegusa, Yoshie*; Ogami, Takashi*; Sasao, Eiji
Quaternary International, 230(1-2), p.78 - 86, 2011/01
Times Cited Count：10 Percentile：28.92(Geography, Physical)We assessed the usefulness of electrical conductivity (EC) of stirred Holocene deltaic sediments from three cores from the Nobi Plain, Japan, as a proxy for marine transgression and regression. Marine deposits showed high EC, terrestrial deposits low EC, and brackish deposits intermediate. Because it was positively correlated with the percentage of marine diatom species, EC in inner bay deposits primarily reflected salinity. In the YM core (the youngest of the three cores), EC of the inner bay deposits was weakly positively correlated with clay content. EC of inner bay clayey deposits was generally higher in YM than in KZN, suggesting that the lower EC in older clayey sediments results from compaction. These findings suggest that the EC values of fine sediments initially record salinity but may decrease gradually over time under the influence of compaction. Thus, to reconstruct the original salinity, the effects of compaction and of grain size distribution should be evaluated.
C datingsOgami, Takashi*; Sugai, Toshihiko*; Fujiwara, Osamu*; Yamaguchi, Masaaki*; Sasao, Eiji
Chigaku Zasshi, 118(4), p.665 - 685, 2009/00
The depositional process of the latest Pleistocene to Holocene shallow marine and fluvial sequences is investigated by using five drill cores from the Kiso river delta, central Japan. Based on facies analysis, the sediments were classified into 5 units; A: basal gravel, B: fluvial to intertidal sand and silt, C: transgressive lag deposit, prodelta mud or sandy silt, D: delta-front-slope sandy silt or sand, delta-front-platform sand, E: delta-plain to fluvial sand and silt. Detailed age-depth curves of each cores are reconstructed based on 107 AMS C ages. The curves are divided into four sections; I: mainly consists of unit B with rapid accumulation (7.3-21.4 mm/yr), II: mainly consists of unit C with moderate accumulation (2.4-2.8 mm/yr), III: mainly consists of unit D with rapid accumulation (6.7-17.8 mm/yr), IV: consists of unit D and E with moderate accumulation (1.3-3.9 mm/yr). The section boundaries of I/II and II/III reflect inundation into the bay and achievement of progradational delta-front slope, respectively. The expanding rate of the bay is 10 m/yr during 10,200-7,900 cal yrs BP. The progradation rates of the delta are 3-4 m/yr (6.500-4,100 cal yrs BP), 5 m/yr (4,100-1,300 cal yrs BP), 10 m/yr (1,300 cal yrs BP to the present). These indicate the transition from transgression to regression occurred between 7,800 to 7,300 cal yrs BP. The geological cross section along the Kiso river coupled with isochrones indicates following history. (1) 10,000-7,280 cal yrs BP (K-Ah horizon): During the expansion of the bay, onlap of unit C on unit B is visible, and unit C overlapped all the core sites. (2) 7,280 cal yrs BP to present: Isochrones younger than 6,500 cal yrs BP are similar each other and cross unit boundaries of C/D and D/E. This demonstrates units C, D and E are contemporaneous heterotopic facies formed by progradational delta.
Niwa, Yuichi*; Tajikara, Masayoshi; Yasue, Kenichi; Ogami, Takashi*; Sugai, Toshihiko*
no journal, ,
no abstracts in English
Ogami, Takashi*; Tajikara, Masayoshi; Yasue, Kenichi; Niwa, Yuichi*; Sugai, Toshihiko*
no journal, ,
no abstracts in English
Niwa, Yuichi*; Sugai, Toshihiko; Ogami, Takashi*; Tajikara, Masayoshi; Yasue, Kenichi; Fujiwara, Osamu*
no journal, ,
no abstracts in English
Niwa, Yuichi*; Sugai, Toshihiko; Ogami, Takashi*; Tajikara, Masayoshi; Yasue, Kenichi; Fujiwara, Osamu*
no journal, ,
no abstracts in English
Niwa, Yuichi*; Sugai, Toshihiko; Ogami, Takashi*; Tajikara, Masayoshi; Yasue, Kenichi
no journal, ,
no abstracts in English
Niwa, Yuichi*; Sugai, Toshihiko*; Ogami, Takashi*; Tajikara, Masayoshi*; Yasue, Kenichi; Fujiwara, Osamu*
no journal, ,
Analyses of drilling cores and C-14 ages from a lowland along the Yoro fault system fringing the Nobi plain suggest that five subsidence events occurred in the middle to late Holocene. The cores are represented by a prograding delta sequence that has formed on the footwall side of the Yoro fault system. Vertical changes of sedimentary facies, grain size distribution and Electronic Conductivity (EC) value of sediment samples suggest that temporal relative sea-level rise occurred. Trend of relative-sea level lowering during middle to late Holocene indicates that cause of these sea-level rise events can be coseismic subsidence.
