11:30 AM - 11:45 AM
[HGM04-09] Formation of the aggradational terraces composed of the reworked Ito pyroclastic flow deposits
★Invited Papers
Keywords:Ignimbrite plateau, Re-sedimentation, Lahar deposit, Miyakonojo Basin, Reworked Shirasu
1. Introduction
Volcaniclastics are eroded rapidly following eruptions and redeposited on downstream areas. In the process, strath terraces with relatively thin sand and gravel beds are commonly formed in ignimbrite plateaus (Yokoyama, 1999).
On the other hand, closed landforms such as basins, depressions, and water areas once appeared after pyroclastic flow deposition, and accumulation terraces were formed to fill these areas. This presentation will discuss the process of terrace formation, the re-sedimentation of pyroclastic flow deposits, and the filling of depression.
2. Landforms in the Miyakonojo basin
The Miyakonojo Basin in southern Kyushu is located upstream of the Oyodo River, about 20 km east of the Aira Caldera, which erupted the Ito pyroclastic flow deposit (A-Ito). The A-Ito depositional surface remains on the plateau around the basin, whereas the plateau in the central part of the basin consists of three to four fluvial terrace surfaces.
The upper terraces are composed of the A-Ito and its re-sedimented deposits. In the western part of the basin, thick sand and gravel beds (>10 m) are distributed. The basin's northern and southern parts contain clay layers (Kino and Ota, 1977). These layers have been collectively called ""reworked shirasu"" or ""stratified shirasu,"" but the depositional process and environment have not been clarified.
3. Methods
Geologic columns of the Miyakonojo Basin were collected to determine the elevation and thickness of the strata (A-Ito, fluvial deposits, and tephric loess). The author used publicly available data (KuniJiban, Geo-Station, National Geo-Information Center) and publications (""Kyushu Geotechnical Information Sharing Database,"" ""National Groundwater (Deep Well) Data Book,"" ""Hydrogeological map and explanatory text of the Miyakonojo basin""), and geological columns provided by the Miyazaki Prefecture Construction Technology Center.
4. Results
The top of the extant A-Ito was around 120 m asl in the central part of the basin. The A-Ito depositional surface was preserved only above 150 m asl.
Fluvial deposits, which are reworked A-Ito, reached a thickness of 30 m on the western basin plateau. Conversely, the thickness was relatively thin (<5 m) at the basin margin and in the southern part. They were composed of sand or gravel with a large amount of pumice. The outcrops showed clear cross-bedding, and the particles were rounded.
On the left bank of the Oyodo River from Otobo to Kamizuru, clay or silt layers with a thickness of about 3 m were distributed at an elevation of about 130 m.
5. Formation process of accumulation terraces
The processes of sedimentary terrace formation by the rework of A-Ito deposits are currently considered as follows.
1) The depositional surface of A-Ito in the central part of the Miyakonojo basin was about 30 m lower than that of the marginal part, and a depression was formed at the time of A-Ito deposition.
2) The depression was rapidly filled by the inflow of sediment derived from the A-Ito, which was eroded by the rivers.
3) After the depression filled, runoff from the basin began, and the non-welded part of A-Ito at the basin outlet was quickly eroded. The base level dropped to near the top of the welded part.
4) The rivers incised, and terraces were formed by 26 ka BP.
The formation and fill of depressions can successfully explain the terrace formation and the distribution of thick fluvial deposits.
References
Kino, Y. and Ota, R. (1977) Geological Map of Japan 1:50000, Miyakonojo. Geological Survey of Japan.
Yokoyama, S. (1999) Rapid formation of river terraces in non-welded ignimbrite along the Hishida River, Kyushu, Japan. Geomorphology 30: 291-304.
Acknowledgement
I would like to thank Mr. Nonaka and Ms. Watanabe for their assistance. This work was supported by JSPS KAKENHI Grant Number 24K16208.
Volcaniclastics are eroded rapidly following eruptions and redeposited on downstream areas. In the process, strath terraces with relatively thin sand and gravel beds are commonly formed in ignimbrite plateaus (Yokoyama, 1999).
On the other hand, closed landforms such as basins, depressions, and water areas once appeared after pyroclastic flow deposition, and accumulation terraces were formed to fill these areas. This presentation will discuss the process of terrace formation, the re-sedimentation of pyroclastic flow deposits, and the filling of depression.
2. Landforms in the Miyakonojo basin
The Miyakonojo Basin in southern Kyushu is located upstream of the Oyodo River, about 20 km east of the Aira Caldera, which erupted the Ito pyroclastic flow deposit (A-Ito). The A-Ito depositional surface remains on the plateau around the basin, whereas the plateau in the central part of the basin consists of three to four fluvial terrace surfaces.
The upper terraces are composed of the A-Ito and its re-sedimented deposits. In the western part of the basin, thick sand and gravel beds (>10 m) are distributed. The basin's northern and southern parts contain clay layers (Kino and Ota, 1977). These layers have been collectively called ""reworked shirasu"" or ""stratified shirasu,"" but the depositional process and environment have not been clarified.
3. Methods
Geologic columns of the Miyakonojo Basin were collected to determine the elevation and thickness of the strata (A-Ito, fluvial deposits, and tephric loess). The author used publicly available data (KuniJiban, Geo-Station, National Geo-Information Center) and publications (""Kyushu Geotechnical Information Sharing Database,"" ""National Groundwater (Deep Well) Data Book,"" ""Hydrogeological map and explanatory text of the Miyakonojo basin""), and geological columns provided by the Miyazaki Prefecture Construction Technology Center.
4. Results
The top of the extant A-Ito was around 120 m asl in the central part of the basin. The A-Ito depositional surface was preserved only above 150 m asl.
Fluvial deposits, which are reworked A-Ito, reached a thickness of 30 m on the western basin plateau. Conversely, the thickness was relatively thin (<5 m) at the basin margin and in the southern part. They were composed of sand or gravel with a large amount of pumice. The outcrops showed clear cross-bedding, and the particles were rounded.
On the left bank of the Oyodo River from Otobo to Kamizuru, clay or silt layers with a thickness of about 3 m were distributed at an elevation of about 130 m.
5. Formation process of accumulation terraces
The processes of sedimentary terrace formation by the rework of A-Ito deposits are currently considered as follows.
1) The depositional surface of A-Ito in the central part of the Miyakonojo basin was about 30 m lower than that of the marginal part, and a depression was formed at the time of A-Ito deposition.
2) The depression was rapidly filled by the inflow of sediment derived from the A-Ito, which was eroded by the rivers.
3) After the depression filled, runoff from the basin began, and the non-welded part of A-Ito at the basin outlet was quickly eroded. The base level dropped to near the top of the welded part.
4) The rivers incised, and terraces were formed by 26 ka BP.
The formation and fill of depressions can successfully explain the terrace formation and the distribution of thick fluvial deposits.
References
Kino, Y. and Ota, R. (1977) Geological Map of Japan 1:50000, Miyakonojo. Geological Survey of Japan.
Yokoyama, S. (1999) Rapid formation of river terraces in non-welded ignimbrite along the Hishida River, Kyushu, Japan. Geomorphology 30: 291-304.
Acknowledgement
I would like to thank Mr. Nonaka and Ms. Watanabe for their assistance. This work was supported by JSPS KAKENHI Grant Number 24K16208.