5:15 PM - 6:30 PM
[HGM03-P06] The dividing mountain of the Seto Inland Sea region in the last glacial period
Keywords:Seto Inland Sea, dividing mountain, freshwater fish, Parabotia curtus
Due to the low sea levels in the glacial period, Shikoku and Honsyu were connected, and the rivers facing Seto Inland Sea merged into east flowing or west flowing main river and flowed into the Pacific Ocean. Iseki (1956) and Kuwashiro (1959) indicated that the dividing mountain in Setouchi region during the last glacial period existed near Shiwaku Islands with rivers west of Takahashi River flowing west and rivers east of Asahi River flowing east. On the other hand, Hirayama et al. (2003) indicated that the western end of the fish phase on the east side represented by Parabotia curtus is said to be Ashida River located on the west side of the Shiwaku Islands, contradicting the paleo topography considered by Iseki (1956). Therefore, this paper examined the location of the dividing mountain in the Seto Inland Sea region in the last glacial period, based on boring logs in Okayama Plain area and along the Seto Ohashi Bridge and the Ichthyofauna of the river in Kasaoka City, Okayama Prefecture.
Boring logs in Okayama Plain are showed that Takahashi River run possibility the east of Sosha, fujito's narrow pass, or the current Takahashi River. However, clear valley topography, such as in Kanto Plain, cannot be confirmed, and the clarify palaeocurrent system of the main stream more detailed data accumulation is necessary.
Then, the sea floor along the Seto Ohashi Bridge in Bisan Seto does not almost find the covered deposits, for the erosion in the tidal power. Thus, it means to be difficult to estimate the paleo topography based on the current sea floor.
As a result of fish sampling surveys at 11 locations on the four rivers in Kasaoka City, 12 species of pure freshwater fish were caught. These Ichthyofaunas show the feature of the western side of Seto Inland Sea, and the dividing mountain in the last glacial period is located on the east of Kasaoka City, and Ashida River flowed west.
The reason why fishes living on the east side of Seto Inland Sea, such as Parabotia curtus, live in Ashida River is that Takahashi River merged Asahikawa River during the last glacial period, or that Takahashi River jointed to Sasagase River in the aggradational stage in Holocene. The invasion from Takahashi River to Ashida River was thought to have occurred with the transition of flow path on alluvial fan in Ibara City in Holocene.
Boring logs in Okayama Plain are showed that Takahashi River run possibility the east of Sosha, fujito's narrow pass, or the current Takahashi River. However, clear valley topography, such as in Kanto Plain, cannot be confirmed, and the clarify palaeocurrent system of the main stream more detailed data accumulation is necessary.
Then, the sea floor along the Seto Ohashi Bridge in Bisan Seto does not almost find the covered deposits, for the erosion in the tidal power. Thus, it means to be difficult to estimate the paleo topography based on the current sea floor.
As a result of fish sampling surveys at 11 locations on the four rivers in Kasaoka City, 12 species of pure freshwater fish were caught. These Ichthyofaunas show the feature of the western side of Seto Inland Sea, and the dividing mountain in the last glacial period is located on the east of Kasaoka City, and Ashida River flowed west.
The reason why fishes living on the east side of Seto Inland Sea, such as Parabotia curtus, live in Ashida River is that Takahashi River merged Asahikawa River during the last glacial period, or that Takahashi River jointed to Sasagase River in the aggradational stage in Holocene. The invasion from Takahashi River to Ashida River was thought to have occurred with the transition of flow path on alluvial fan in Ibara City in Holocene.