5:15 PM - 6:45 PM
[HGM03-P01] Formation age of the fluvial terraces in the Kasanohara Ignimbrite Plateau, Kyushu, Japan
Keywords:Pyroclastic plateau, Fluvial terrace, Shirasu, Ito pyroclastic flow, Deepening, Kushira River
1. Landforms in the Kasanohara Ignimbrite Plateau
The Kasanohara Ignimbrite Plateau is a representative pyroclastic flow plateau in southern Kyushu, which was dissected by the Kimotsuki River and Kushira River and their tributaries after the Ito (30 ka) and the Ata (110 ka) pyroclastic flows deposit into the Kimotsuki Plain. In contrast to the Ata welded ignimbrite, this area's Ito pyroclastic flow deposit (A-Ito) is non-welded and easily eroded. The plateau is mainly composed of higher terraces (Kasanohara and Shinbori terraces), including the depositional surface of A-Ito, and the remainder is the small-scale lower terraces along river valleys (Yokoyama 2000). These terraces and their terrace deposits indicate the incision process along the Kushira River after the deposition of A-Ito.
2. Research Objectives
Previous studies have considered that the Kasanohara Plateau has formed quickly due to the rapid incision of rivers after the deposition of A-Ito. However, there needs to be more investigation of the terrace deposits that indicate the rapid formation of the plateau. This study aims to clarify the formation period of the lower terraces based on the ages of tephra and paleosols in the lower part of the loam layers and to examine the length of time for river incision in the ignimbrite.
3. Methods
Outcrops were observed at four sites (Fig. 1) on the lower terraces (Uraishihara, Tsurumine, Hirase, and Iwahiro) and two sites on the Shinbori terrace (Nakano and Nakago). At Uraishihara, Tsurumine, and Hirase, paleosols were collected at the boundary with the sand and gravel layer and just below the tephra. AMS 14C dating was performed by Paleo Labs, Inc. Tephras were collected at five sites except Nakago, and oxides of 9 major elements (Na, Mg, Al, Si, K, Ca, Ti, Mn, Fe) in volcanic glass were determined using SEM-EDS (JEM-6390LA, JEOL Ltd.), weight ratios were measured at 20-30 grass fragments per sample. The same measurements were made for volcanic glasses from the Kikai Akahoya tephra (K-Ah), Sakurajima Satsuma tephra (Sz-P14), and Sakurajima Takatoge 6 tephra (Sz-P17), collected at an outcrop in Horikiri, Tarumizu City.
4. Results
At two sites on the Shinbori terrace and at Uraishihara and Tsurumine on the lower terrace face, a 10-50 cm thick pumice layer consisting of yellowish-white pumice up to 3 cm thick was observed, and the chemical composition of them was resembling that of Sz-P14. An orange glassy volcanic ash layer was observed at Iwahiro, with results similar to K-Ah's. A volcanic ash layer with similar characteristics was also observed at Hirase. However, the chemical composition varied widely from one glass to another, with some overlap with that of K-Ah.
The calibrated ages of paleosol for the area between the gravel layer and the fall tephra layer were 13300-9800 yBP at Uraishihara, 15500-10500 yBP at Tsurumine, and 10300-9200 yBP at Hirase.
5. Formation periods of the lower terraces
Based on the results, the pumice layers found at Uraishihara and Tsurumine were identified with Sz-P14, and the volcanic ash layer at Iwahiro was identified with K-Ah. The volcanic ash layer of Hirase contains K-Ah and other tephra and is a reworked deposit. The formation periods of lower terraces are estimated to be 16000 to 13000 yBP on the upstream side and 10000 to 7300 yBP on the downstream side. It is unlikely that this was immediately after the deposition of A-Ito. A possible reason for the delayed river incision is the resistance due to buried Ata welded ignimbrite.
Reference
Yokoyama, S. 2000. Rapid Formation of Kasanohara Ignimbrite Plateau, South Kyushu, Japan. Transactions, Japanese Geomorphological Union 21: 277–290.
Acknowledgment
I would like to thank Professor Toshihiko Sugai and his laboratory members at the Graduate School of Frontier Sciences, the University of Tokyo, for allowing the use of SEM-EDS for chemical analysis.
This study was supported by JSPS KAKENHI Grant Number 19J20125.
The Kasanohara Ignimbrite Plateau is a representative pyroclastic flow plateau in southern Kyushu, which was dissected by the Kimotsuki River and Kushira River and their tributaries after the Ito (30 ka) and the Ata (110 ka) pyroclastic flows deposit into the Kimotsuki Plain. In contrast to the Ata welded ignimbrite, this area's Ito pyroclastic flow deposit (A-Ito) is non-welded and easily eroded. The plateau is mainly composed of higher terraces (Kasanohara and Shinbori terraces), including the depositional surface of A-Ito, and the remainder is the small-scale lower terraces along river valleys (Yokoyama 2000). These terraces and their terrace deposits indicate the incision process along the Kushira River after the deposition of A-Ito.
2. Research Objectives
Previous studies have considered that the Kasanohara Plateau has formed quickly due to the rapid incision of rivers after the deposition of A-Ito. However, there needs to be more investigation of the terrace deposits that indicate the rapid formation of the plateau. This study aims to clarify the formation period of the lower terraces based on the ages of tephra and paleosols in the lower part of the loam layers and to examine the length of time for river incision in the ignimbrite.
3. Methods
Outcrops were observed at four sites (Fig. 1) on the lower terraces (Uraishihara, Tsurumine, Hirase, and Iwahiro) and two sites on the Shinbori terrace (Nakano and Nakago). At Uraishihara, Tsurumine, and Hirase, paleosols were collected at the boundary with the sand and gravel layer and just below the tephra. AMS 14C dating was performed by Paleo Labs, Inc. Tephras were collected at five sites except Nakago, and oxides of 9 major elements (Na, Mg, Al, Si, K, Ca, Ti, Mn, Fe) in volcanic glass were determined using SEM-EDS (JEM-6390LA, JEOL Ltd.), weight ratios were measured at 20-30 grass fragments per sample. The same measurements were made for volcanic glasses from the Kikai Akahoya tephra (K-Ah), Sakurajima Satsuma tephra (Sz-P14), and Sakurajima Takatoge 6 tephra (Sz-P17), collected at an outcrop in Horikiri, Tarumizu City.
4. Results
At two sites on the Shinbori terrace and at Uraishihara and Tsurumine on the lower terrace face, a 10-50 cm thick pumice layer consisting of yellowish-white pumice up to 3 cm thick was observed, and the chemical composition of them was resembling that of Sz-P14. An orange glassy volcanic ash layer was observed at Iwahiro, with results similar to K-Ah's. A volcanic ash layer with similar characteristics was also observed at Hirase. However, the chemical composition varied widely from one glass to another, with some overlap with that of K-Ah.
The calibrated ages of paleosol for the area between the gravel layer and the fall tephra layer were 13300-9800 yBP at Uraishihara, 15500-10500 yBP at Tsurumine, and 10300-9200 yBP at Hirase.
5. Formation periods of the lower terraces
Based on the results, the pumice layers found at Uraishihara and Tsurumine were identified with Sz-P14, and the volcanic ash layer at Iwahiro was identified with K-Ah. The volcanic ash layer of Hirase contains K-Ah and other tephra and is a reworked deposit. The formation periods of lower terraces are estimated to be 16000 to 13000 yBP on the upstream side and 10000 to 7300 yBP on the downstream side. It is unlikely that this was immediately after the deposition of A-Ito. A possible reason for the delayed river incision is the resistance due to buried Ata welded ignimbrite.
Reference
Yokoyama, S. 2000. Rapid Formation of Kasanohara Ignimbrite Plateau, South Kyushu, Japan. Transactions, Japanese Geomorphological Union 21: 277–290.
Acknowledgment
I would like to thank Professor Toshihiko Sugai and his laboratory members at the Graduate School of Frontier Sciences, the University of Tokyo, for allowing the use of SEM-EDS for chemical analysis.
This study was supported by JSPS KAKENHI Grant Number 19J20125.