- Email: [email protected]
Rapid formation of river terraces in non-welded ignimbrite along the Hishida River, Kyushu, Japan
Geomorphology 30 Ž1999. 291–304
Rapid formation of river terraces in non-welded ignimbrite along the Hishida River, Kyushu, Japan Shozo Yokoyama
)
Department of Geography, Faculty of Education, Kumamoto UniÕersity, Kumamoto 860-8555, Japan Received 10 April 1998; received in revised form 23 March 1999; accepted 1 April 1999
Abstract River terraces are well preserved in the middle to lower reaches of the Hishida River, that flows through the extensive ignimbrite field produced by the large-scale Ito eruption about 24,500 years ago from Aira caldera in southern Kyushu, Japan. The terraces are erosional and formed during dissection of the initial topography of the Ito ignimbrite. These occur within the upper non-welded zone of the Ito ignimbrite, about 20–60 m below the depositional surface and usually more than 20 m above the welded zone. Streams associated with terrace formation were ephemeral, because perennial streams are considered highly unlikely within the porous non-welded ignimbrite. All tephra layers on the original ignimbrite depositional surface also cover the terraces. This indicates that the formation of terraces preceded the accumulation of the oldest tephra, the age of which is about 23,000 years old. High erodibility of the non-welded Ito ignimbrite and absence of vegetation on the initial surface would have permitted a rapid dissection of the initial topography and the formation of the river terraces. Thus, it is interpreted that the river terraces were formed very rapidly in a period of far less than 1500 years immediately after the deposition of the Ito ignimbrite and before cover of new vegetation on the depositional surface. q 1999 Elsevier Science B.V. All rights reserved. Keywords: river terrace; non-welded ignimbrite; high erodibility; Hishida River
1. Introduction The formation of erosional terraces consists of two processes, Ž1. lateral cutting by which a step Žtread. is formed and, Ž2. downcutting by which a scarp is produced. While tectonic or climatic stability for a considerable period Žstream equilibrium. is
)
Fax: q81-096-342-2529. E-m ail address: yokosho@ educ.kum am oto-u.ac.jp ŽS. Yokoyama.
generally assumed for the formation of the former, changes in these factors Ždisequilibrium. are postulated to cause the latter Že.g., Howard et al., 1968, p. 1117.. However, where banks are easily eroded, time and stability are not essential factors in the formation of river terraces ŽRitter, 1986, p. 272., and tectonic and climatic changes are not always necessary for downcutting. Highly erodible materials such as mudflow deposits and alluvium, for example, may permit the formation of erosional terraces in a very short period irrespective of tectonism or climatic change. River
0169-555Xr99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 5 5 5 X Ž 9 9 . 0 0 0 3 7 - 9
S. Yokoyamar Geomorphology 30 (1999) 291–304
292
Fig. 1. Map of southern Kyushu showing distribution of the Ito ignimbrite Žstippled. Žafter Yokoyama, 1972. and location of the study area Žhorizontally striped.. Dashed line indicates the outline of Aira caldera as proposed by Matumoto Ž1943..
terraces along the Abe, Joganji, and Ura Rivers, Japan ŽMachida, 1966., those near Pyramid Lake, Nevada ŽBorn and Ritter, 1970., and those along the Yuba River, California ŽChorley et al., 1984, p. 359. are some documented examples. Depositional terraces can also be formed very rapidly due to rapid aggradation and the succeeding downcutting into alluvial fills. Such examples have been reported from Waiho River, New Zealand ŽGage, 1970., and Douglas Creek, Colorado ŽWomack and Schumm, 1977.. In all these examples, terraces were formed in less than 100 years, and are in soft sediments. It is clear that these terraces owe their rapid formation to the soft sediments. This indicates that the erodibility of materials should be considered in interpretation of river terrace formation.
Fig. 2. Generalized geologic map of the study area and surrounding region. Location is given in Fig. 1. Map is based on 1:500,000 Geological map Sheet 15 Kagoshima ŽImai et al., 1980..
Non-welded ignimbrites formed by explosive volcanic eruptions are generally composed of loose accumulations of volcanic ash and pumice, and are highly susceptible to water erosion. Because of the high erodibility and absence of vegetation after an eruption, the initial depositional topography of nonwelded large ignimbrite, which is generally flat, is considered to experience very rapid dissection into ignimbrite plateaus immediately after its formation ŽYokoyama, 1985.. This is well exemplified by a very rapid erosion of ignimbrite from the 1991 erup-
Table 1 X X Temperature and precipitation data at Miyakonojo Ž31844 N, 131805 E. a Monthly means: 1961–1990
Temperature Ž8C. Precipitation Žmm. a
Jan.
Feb.
Mar.
Apr.
May
June
July
Aug.
Sept.
Oct.
Nov.
Dec.
Annual
5.0 60.4
6.5 91.1
9.8 150.8
15.1 223.3
18.8 253.6
22.2 424.9
26.0 343.1
26.1 340.6
23.2 262.6
17.7 115.3
12.3 78.5
6.9 51.5
15.8 2395.8
ŽAfter Climatic Table of Japan, Volume 1, Japan Meteorological Agency, 1991..
S. Yokoyamar Geomorphology 30 (1999) 291–304
tions of Mount Pinatubo, Philippines Že.g., Umbal and Rodolfo, 1996; Rodolfo et al., 1996.. Many river terraces and reworked ignimbrites developed extensively in non-welded ignimbrite fields are interpreted to be the products during such rapid
293
dissection independent of tectonism or climatic change. However, few studies on non-welded ignimbrites from this standpoint have been carried out. Extensive areas in southern Kyushu ŽFig. 1. are covered by the large-scale Ito ignimbrite erupted
Fig. 3. Distribution of river terraces in the middle to lower reaches of the Hishida River. Horizontally striped areas are terraces surfaces. Numerals on terrace surfaces indicate the altitude Žin meters. of the terrace surface. Dotted lines are contours Žin meters. for the reconstructed depositional surface of the Ito ignimbrite.
294
S. Yokoyamar Geomorphology 30 (1999) 291–304
S. Yokoyamar Geomorphology 30 (1999) 291–304
from Aira caldera ŽMatumoto, 1943. about 24,500 yr B.P. Ž14 C date; Ikeda et al., 1995.. River terraces are developed along other rivers in the ignimbrite field. These terraces were obviously formed by the rivers during dissection of the depositional topography of the ignimbrite, and are therefore, younger than 24,500 yr B.P. River terraces developed in the middle to lower reaches of the Hishida River in Osumi Peninsula are the most typical, and the good preservation of the terraces makes this area ideal for studying terraces in non-welded ignimbrites. In this paper, the rate and processes of terrace development along the Hishida River are discussed.
2. Physical setting The drainage basin of the Hishida River, about 400 km2 , is located southeast of Aira caldera ŽFig. 1.. The Hishida River is about 50 km in length and rises on the eastern rim of Aira caldera. It flows southeastward in its upper reaches, and then flows southward into Shibushi Bay ŽFig. 2.. Meteorological data during 30 years Ž1961–1990. at Miyakonojo, located about 30 km north of Shibusi, show that the mean annual temperature is 15.88C, and the annual precipitation is about 2400 mm ŽTable 1.. Annual discharge of the Hashida River is estimated to be about 7 = 10 8 m3 ŽSaito and Maruyama, 1976.. The dominant landforms in the drainage basin of the Hishida River are extensive Ito ignimbrite plateaus ŽYokoyama, 1974.. The altitudes of the plateaus exceed 400 m around the eastern rim of Aira caldera. In general, the plateaus dip gently Ž- 28. to the east, and fall below 200 m in altitude about 20 km east of the caldera rim. Along the main stream of the Hishida River, the plateaus lower gradually southward to less than 50 m near the river mouth on the coast of Shibushi Bay ŽFig. 3.. This decrease in plateau altitude represents largely the
295
original gently-dipping depositional surface of the Ito ignimbrite sheet. The Ito ignimbrite in the study area is typically composed of an upper non-welded zone and a lower welded zone. The upper non-welded zone generally exceeds 50 m in thickness and has a maximum thickness of about 100 m. The welded zone exceeds 20 m in thickness in places, but the maximum thickness of this unit is unknown, because beds of most modern streams are in the welded zone. The upper non-welded zone is a poorly consolidated deposit composed mainly of white volcanic ash and pumice, and is popularly called ‘Shirasu’ , a Japanese term which means white sand. The term ‘Shirasu’ is adopted in this paper to describe the non-welded portion of the Ito ignimbrite. The basement rocks underlying the Ito ignimbrite consist mainly of sandstone and shale ŽMesozoic to Tertiary., andesite lava ŽPliocene to Early Pleistocene., and ignimbrites ŽPliocene to Late Pleistocene. ŽFig. 2.. Sandstone and shale are the most extensive basement rocks forming mountains in the upper reaches of the Hishida River, and some isolated island-like hills and mountains protruding through the ignimbrite plateaus. Tephra layers several meters in thickness, derived mainly from Sakurajima volcano, cover both the ignimbrite plateaus and basement mountains over most of the study area.
3. Distribution of terraces Terraces are mainly distributed in the middle to lower reaches of the Hishida River, which are here defined as the drainage basins above and below the major junction, about 11 km from the river mouth, respectively ŽFig. 3.. Few terraces are found in the upper reaches; the upper drainage basin located outside of Fig. 3.
Fig. 4. Geologic and topographic cross sections of river terraces along the Hishida River. Dotted line indicates the altitude Žin meters. of the depositional surface of the Ito ignimbrite. Thickness of terrace deposits Žheavy line. is exaggerated. Tephra cover forming the land surface is X omitted. TB on the H–H cross section is the range of ‘terrace belt’ defined in the text. Inset gives positions of profiles.
296
S. Yokoyamar Geomorphology 30 (1999) 291–304
Several unpaired terraces are developed in the middle to lower 15 km reaches ŽFig. 4.. Detailed subdivision and correlation of the terraces is difficult, because the number of terraces is variable. In contrast, a single seemingly paired terraces are found in the lowermost 6 km reaches ŽFig. 4I–IX .. The terrace belt, here defined as the zone between the outer boundaries of terraces developed on both sides of the valley, widens in the downstream direction ŽFigs. 3 and 4.. The terrace belt is considerably wider than the present river valley. About 5 km above the river mouth, for example, the former which is approximately 1.5 km, while the latter is 0.5 km.
4. Longitudinal profile of terraces Fig. 5 represents the longitudinal profile of terrace surfaces, together with profiles of the present river bed and the depositional surface of Ito ignimbrite along the Hishida River. The position of the welded zone underneath the Shirasu is also shown. A remarkable characteristic of the terraces is that they occur within Shirasu ŽFig. 5.. In other words,
the paleo-Hishida River flowed within the level of Shirasu, 20–60 m below the depositional surface and generally, more than 20 m above the welded zone ŽFig. 6.. This presents a striking contrast to the present channel, which flows mostly through the welded zone. The longitudinal profile of the terraces is rather simple compared with the present river bed, which shows a considerable gradient change. This is because the paleo-Hishida River flowed entirely within the highly erodible Shirasu, whereas the present river flows through the welded zone and is constrained in its downcutting by the resistant welded zone, in which the degree of welding Ž‘hardness’. is spatially variable. The overall gradient of the paleo-Hishida River bed is about 5–10 = 10y3 . This gradient is much steeper compared with 7 = 10y4 for the present river bed of the lowermost reaches within the alluvium.
5. Terrace deposits Terrace deposits are composed of gravel beds, sand beds, and mud beds of various thickness.
Fig. 5. Longitudinal profiles of depositional surface of the Ito ignimbrite, river terrace surfaces, and present river bed in the middle to lower reaches of the Hishida River. Profiles are along the main stream of the Hishida River ŽThe eastern branch above the major junction ŽFig. 3. is the main stream... Because of the difficulty of detailed subdivision and correlation of terraces, they are grouped together, and longitudinal profile of the terrace surfaces is shown as a belt Žobliquely striped.. Vertically-striped zone indicates the welded zone of the Ito ignimbrite.
S. Yokoyamar Geomorphology 30 (1999) 291–304
297
Fig. 6. River terraces and the Ito ignimbrite plateau about 1.7 km downstream from the major junction. The terraces on both sides of the valley are bridged. Difference in elevation between the surface of the ignimbrite plateau Žcovered with Japanese cedars. and the terrace surface is about 60 m. The bridge piers are set within the welded zone of the Ito ignimbrite.
Fig. 7. Horizontal sandy beds. Sorting of the deposits of individual beds is variable. The handle of the hammer is 25 cm long.
298
S. Yokoyamar Geomorphology 30 (1999) 291–304
Mostly, the beds are horizontal. The gravel beds consist of pumice, andesites, sandstone and shale. The materials coincide with the components of Shirasu, i.e., pumice clasts Žusually less than 20 cm in size., lithic clasts Žless than 5 cm; mainly andesites, shale, and sandstone., and smaller particles derived from them. This indicates that the deposits are reworked Shirasu, which is self-evident, since the drainage area of the Hishida River is mostly covered with Shirasu. The terrace deposits are poorly consolidated. Grain-size characteristics and the proportion of lithics to pumice varies between beds. Some beds are composed of well sorted sands, while others are poorly sorted ŽFig. 7. resembling the laharic deposits of Mount Pinatubo ŽRodolfo et al., 1996; Fig. 13.. In contrast to lithic gravels which are usually subangular to angular indicating little abrasion during fluvial trasnport, pumice gravels are mostly rounded due to abrasion. Cross-bedding is often observed within the gravel beds and sand beds. Outcrop observations show that the boundary between the terrace deposits and the underlying Shirasu is usually horizontal ŽFig. 8., indicating nearly uni-
form thicknesses of the deposits in a limited area. The thickness of the deposits is less than 3 m in most places ŽFig. 9.. The uniformly-thin deposits indicate one of the properties of erosional terraces ŽRitter, 1986, p. 269..
6. Tephra cover Tephra and volcanic soils several meters in thickness uniformly cover both the depositional and terrace surfaces. Stratigraphic sections of the tephra and other deposits at some representative localities on both the depositional and terrace surfaces are shown in Fig. 10. Tephra layers are mostly volcanic ash and pumice derived from nearby Sakurajima volcano, located about 30 km west of the study area ŽFig. 1.. Sakurajima is a post-Ito ignimbrite volcano, which has actively continued tephra eruptions. A rhyolitic tephra called ‘Akahoya’, a nickname for the distinctive orange pumice and ash layers which usually lie between black soils in the upper horizon of every section, has been established as product of an eruption from Kikai caldera located in the sea about 100
Fig. 8. Terrace deposits overlying the Ito ignimbrite and overlain by tephra. Thickness of the terrace deposits Žsand and gravel. is approximately 80 cm. A hammer is in the horizon of the terrace deposits in the center.
S. Yokoyamar Geomorphology 30 (1999) 291–304
299
Fig. 9. Map showing the thickness Žin meters. of terrace deposits.
km south of Sakurajima. The Akahoya is a widespread tephra about 6300 years old with an extensive distribution throughout southwestern Japan ŽMachida,1983; Machida and Arai, 1978..
No significant difference can be found in the number or thickness of tephra layers between the Ito depositional surface and the terrace surfaces ŽFig. 10.. Field investigation reveals that the tephra layers
300
S. Yokoyamar Geomorphology 30 (1999) 291–304
S. Yokoyamar Geomorphology 30 (1999) 291–304
in the study area cover most of the land other than alluvial plains uniformly in accord with the relief of the present topography. In summary, the tephra cover indicates that the formation of river terraces and general present topography preceded the accumulation of all tephra. Therefore, the tephra cover in this area cannot be used to differentiate between the depositional surface and terrace surfaces, and to subdivide or correlate terraces.
7. Discussion and conclusions 7.1. Characteristics of the paleo-Hishida RiÕer It is a noteworthy characteristic that the paleoHishida River flowed entirely within Shirasu, in contrast to the present river which flows through the welded zone in most places. This places a significant constraint on nature of the paleo-Hishida River. Shirasu is very porous Žtypically 1.37 void ratio. and cannot be an impermeable bed in itself. All the springs in the Shirasu area are associated with the existence of underlying impermeable beds such as the Ito welded zone or basement rocks. Since river terraces stand within the Shirasu, it is apparent that the paleo-Hishida River flowed above the water table. This means that the paleo-Hishida River was not a perennial stream like the present Hishida River, but an ephemeral stream which flowed only when rainfall exceeded the infiltration capacity of the Shirasu. In this sense, the paleo-Hishida River may have been very similar to ephemeral streams common in arid and semiarid regions Že.g., Hadley, 1968.. 7.2. Age, duration, and process of riÕer terrace formation Okuno et al. Ž1997. summarized the stratigraphy of Sakurajima tephra group found in the northern part of Osumi Peninsula ŽFig. 1. including the upper
301
reaches of the Hishida River, and presented the 14 C dates of the individual tephra layers. The date of the oldest tephra or the age of the beginning of the volcanic activity at Sakurajima is 23,000 yr B.P., shortly after the eruption of Ito pyroclastic flow Ž24,500 yr B.P... Tephra-stratigraphical research reveals that the oldest tephra is found near the bottom of the lowest ash layer in the study area. The formation of river terraces preceded the deposition of the lowest tephra, because the terraces are covered by it. It follows, therefore, that the river terraces were formed during a very short period between 24,500 and 23,000 years ago. In order to specify the time of terrace formation in more detail within the above tephrochronological constraints, some idea about the presumed time of river terrace formation are outlined below on the basis of the characteristics of the original Ito ignimbrite depositional surface. Non-welded ignimbrite usually suffers very intense erosion by water flow, if the surface is bare and unprotected. The ignimbrite, erupted in 1991 from Mount Pinatubo is a good example for this. Rapid erosion of the ignimbrite and the associated lahars, which occurred immediately after the eruption, have been described in many papers ŽNewhall and Punongbayan, 1996.. An artificial cut in nonwelded ignimbrite for civil engineering works presents another example. On a bare and flat land of Shirasu, for example, where vegetation and tephra covers have been removed for construction, gully networks or badlands form after several rain storm or even after a single event. Palynological studies reveal that the study area belonged to the cool temperate forest Ždeciduous broad-leaved forest. zone during the period of 25,000–20,000 yr B.P., while it now belongs to the warm temperate evergreen broadleaved forest zone ŽHatanaka, 1985.. However, no data were available concerning the Late Pleistocene precipitation and flood characteristics in the study area.
Fig. 10. Columnar sections for tephra and other deposits on both terrace and depositional surfaces. Number over each column is locality shown in the index map. Localities 1, 4, 5, 12, 14, and 19 are on the depositional surface of the Ito ignimbrite, while others are on the terrace surfaces. Reworked Shirasu at localities 1, 4, and 5 are thinly laminated ash and sand beds which are interpreted as sheetwash. Ah: ‘Akahoya’ tephra ŽSee text for explanation..
302
S. Yokoyamar Geomorphology 30 (1999) 291–304
Fig. 11. Conceptual diagrams showing the process of river terrace formation in the upper non-welded zone Ž‘Shirasu’. of the Ito ignimbrite. Initial topography before dissection. Formation of shallow valley within Shirasu by an ephemeral stream. Headward advance of an incised river with perennial flow Žhorizontally striped.. Formation of river terraces with the headward advance of the incised river.
It is not known when and what kind of plants grew on the newly-formed depositional surface of the Ito ignimbrite. In any case, absence of vegetation would have continued for some years on the depositional surface. Thus, the newly-formed depositional topography of Ito ignimbrite must have suffered very rapid dissection by water flows associated with heavy rain. A model of valley and river terrace formation in Ito ignimbrite is illustrated in Fig. 11. Incipient streams flowed either on the depositional surface or through shallow valleys within Shirasu ŽFig. 11- .. The streams were ephemeral associated with heavy rains, because perennial supply of groundwater was
not available at this shallow level in the Shirasu. Many of the streamflows at this stage were probably similar to lahars which occurred around Mount Pinatubo immediately after the eruption. Considering the extreme erodibility of Shirasu, it is probable that stream downcutting into the distal Shirasu proceeded very rapidly, and the valley floor immediately reached either base level or the underlying welded zone or basement rocks, which resisted downcutting. Streams in such incised valleys were perennial due to continual supply of ground water at this depth ŽFig.11- .. Deep dissection of nearly flat or very gently inclined depositional topography in upper drainage areas could only be attained by headward
Table 2 Documented rates of headward extension of guilles in the non-welded zone of Ito ignimbrite Descriptions Gully erosion 150 m long, 30 m wide, and 7 m deep occurred in an hour. Gully 200 m long, 50 m in average width, and 20 m in average depth was formed due to piping which continued for 4 months. Gully may extend headwards more than 100 m in a single heavy storm. Gully may extend headwards more than several tens of meters in a single heavy storm.
References Tamachi Ž1953. Yamanouchi and Kimura Ž1969. Koide Ž1973.
S. Yokoyamar Geomorphology 30 (1999) 291–304
extension of such deep incised valleys. In an early stage of dissection, therefore, little or very shallow dissection must have occurred above the deeply incised valleys, and ephemeral streams would have flowed repeatedly in shallow valleys following heavy rains. The ephemeral streams would have widened the valley rapidly by lateral cutting caused by channel migration at a shallow level within the Shirasu. The shallow valley floors changed into river terraces with the headward advance of the incised valleys ŽFig.11- .. Thus, the time for terrace-formation depended on the rate at which the incised valleys advanced upstream. It is difficult to know precisely how long it took for the incised valleys to extend more than 20 km upstream. Under present conditions, the rate of extension of gullies within a 1st–2nd order drainage basin exceeds 100 m in a single heavy storm ŽTable 2.. This suggests that the rate of headward advance of the incised valleys with a much wider drainage area could be extremely high. It is considered, therefore, that the extension of the incised valleys into the upper reaches, and the formation of river terraces, was accomplished rapidly in a very short period of far less than 1500 years, immediately after the deposition of Ito ignimbrite and before the significant cover of new vegetation developed on the depositional surface. It is not exactly clear why unpaired and paired terraces are found in the middle to lower and lowermost reaches, respectively. The following are two probable cases. One case is that the stream in the upper course was unstable because of steeper gradients, and continued downcutting to form unpaired terraces, while stream level in the lower course remained essentially constant. Another case is that the higher terraces which had been formed in the lowermost reaches were eroded away due to lateral cutting by a lower level stream which became the paired terraces with a wide terrace belt.
Acknowledgements This work was supported by the Grant-in-Aid for Scientific Research of the Ministry of Education ŽC.
303
ŽNo. 60580202.. I thank T. Sunamura for valuable comments, and S. Self for providing thoughtful and constructive reviews. Tom Gardner and Jamie Woodward are thanked for their comments, which helped to improve the manuscript.
References Born, S.M., Ritter, D.F., 1970. Modern terrace development near Pyramid Lake, Nevada, and its geological implications. Geol. Soc. Am. Bull. 81, 1233–1242. Chorley, R.J., Schumm, S.A., Sugden, D.E., 1984. Geomorphology. Methuen, London and New York, 605 pp. Gage, M., 1970. The tempo of geomorphic change. J. Geol. 78, 619–625. Hadley, R.F., 1968. Ephemeral streams. In: Fairbridge, R.W. ŽEd.., The Encyclopedia of Geomorphology. Reinhold Book, New York, pp. 312–314. Hatanaka, K., 1985. Palynological studies on the vegetational succession since the Wurm ¨ glacial age in Kyushu and adjacent areas. Journal of the Faculty of Literature Kitakyushu University ŽSeries B. 18, 29–71. Howard, A.D., Fairbridge, R.W., Quinn, J.H., 1968. Terraces, fluvial introduction. In: Fairbridge, R.W. ŽEd.., The Encyclopedia of Geomorphology. Reinhold Book, New York, pp. 1117–1124. Ikeda, A., Okuno, M., Nakamura, T., Tsutsui, M., Kobayashi, T., 1995. Accelerator mass spectrometric 14 C dating of charred wood in the Osumi pumice fall and the Ito ignimbrite from Aira caldera, southern Kyushu, Japan. The Quaternary Research 34, 377–379 Žin Japanese.. Imai, I., Teraoka, Y., Ono, K., Matsui, K., Okumura, K., 1980. 1:500,000 Geological map Sheet 15 Kagoshima. 2nd edn. Geological Survey of Japan. Koide, H., 1973. The Land of Japan—Its Nature and Development, 1st volume. University of Tokyo Press, 287 pp. Žin Japanese.. Machida, H., 1966. Rapid erosional development of mountain slopes and valleys caused by large landslides in Japan. Geographical Reports of Tokyo Metropolitan University 1, 55–78. Machida, H., 1983. Tephra studies, short review. Journal of Geography 92, 441–447 Žin Japanese.. Machida, H., Arai, H., 1978. Akahoya ash—a Holocene widespread tephra erupted from the Kikai caldera, south Kyushu, Japan. The Quaternary Research 17, 143–163 Žin Japanese.. Matumoto, T., 1943. The four gigantic caldera volcanoes of Kyushu. Japanese Journal of Geology and Geography 19, Special number, 57 pp. Newhall, C.G., Punongbayan, R.S., ŽEds.., 1996. Fire and Mud: Eruptions and Lahars of Mount Pinatubo, Philippines, Philippine Institute of Volcanology and Seismology, Quezon City, and University of Washington Press, Seattle and London, 1126 pp. Okuno, M., Nakamura, T., Moriwaki, H., Kobayashi, T., 1997.
304
S. Yokoyamar Geomorphology 30 (1999) 291–304
AMS radiocarbon dating of the Sakurajima tephra group, southern Kyushu, Japan. Nuclear Instruments and Methods in Physics Research B 123, 470–474. Ritter, D.F., 1986. Process Geomorphology Ž2nd edn.., Dubuque, Wm. C. Brown Publishers, 579 pp. Rodolfo, K.S., Umbal, J.V., Alonso, R.A., Remotigue, C.T., Paladio-Melosantos, M.L., Salvador, J.H.G., Evangelista, D., Miller, Y., 1996. Two years of lahars on the western flank of Mount Pinatubo: Initiation, flow processes, deposits, and attendant geomorphic and hydraulic changes, in Fire and Mud: Eruptions and Lahars of Mount Pinatubo, Philippines. In: Newhall, C.G., Punongbayan, R.S. ŽEds.., Philippine Institute of Volcanology and Seismology, Quezon City, and University of Washington Press, Seattle and London, pp. 989–1014. Saito, K., Maruyama, Y., 1976. Hydrogeologic study in the Kimotsuki River basin and its vicinity. Engineering Geology 17 Ž2., 1–10 Žin Japanese.. Tamachi, S., 1953. Countermeasures against disasters in Shirasu area Ž2nd interim report.: Report on the disaster research in Shirasu area, and the related data, 3rd series. The planning Department of Kagoshima Prefecture, 37–53 Žin Japanese.. Umbal, J.V., Rodolfo, K.S., 1996. The 1991 lahars of southwest-
ern Mount Pinatubo and evolution of the lahar-dammed Mapanuepe Lake, in Fire and Mud: Eruptions and Lahars of Mount Pinatubo, Philippines. In: Newhall, C.G., Punongbayan, R.S. ŽEds.., Philippine Institute of Volcanology and Seismology, Quezon City, and University of Washington Press, Seattle and London, pp. 951–970. Womack, W.R., Schumm, S.A., 1977. Terraces of Douglas Creek, Northwestern Colorado: an example of episodic erosion. Geology 5, 72–76. Yamanouchi, T., Kimura, D., 1969. Disaster problems of the ‘Shirasu’ loam. Journal of the Japan Society of Civil Engineers 54 Ž11., 9–20 Žin Japanese.. Yokoyama, S., 1972. Flow and emplacement mechanism of Ito pyroclastic flow in southern Kyushu, Japan. Tokyo Geography Papers 16, 127–167 Žin Japanese.. Yokoyama, S., 1974. Mode of movement and emplacement of Ito pyroclasatic flow from Aira caldera, Japan. The Science Reports of the Tokyo Kyoiku Daigaku, Section C ŽGeography, Geology and Mineralogy. 12 Ž114., 17–62. Yokoyama, S., 1985. Geomorphological aspects of large-scale pyroclastic flow deposits: a review. Trans. Jpn. Geomorph. Union 6, 131–152 Žin Japanese..
Rapid formation of river terraces in non-welded ignimbrite along the Hishida River, Kyushu, Japan Shozo Yokoyama
)
Department of Geography, Faculty of Education, Kumamoto UniÕersity, Kumamoto 860-8555, Japan Received 10 April 1998; received in revised form 23 March 1999; accepted 1 April 1999
Abstract River terraces are well preserved in the middle to lower reaches of the Hishida River, that flows through the extensive ignimbrite field produced by the large-scale Ito eruption about 24,500 years ago from Aira caldera in southern Kyushu, Japan. The terraces are erosional and formed during dissection of the initial topography of the Ito ignimbrite. These occur within the upper non-welded zone of the Ito ignimbrite, about 20–60 m below the depositional surface and usually more than 20 m above the welded zone. Streams associated with terrace formation were ephemeral, because perennial streams are considered highly unlikely within the porous non-welded ignimbrite. All tephra layers on the original ignimbrite depositional surface also cover the terraces. This indicates that the formation of terraces preceded the accumulation of the oldest tephra, the age of which is about 23,000 years old. High erodibility of the non-welded Ito ignimbrite and absence of vegetation on the initial surface would have permitted a rapid dissection of the initial topography and the formation of the river terraces. Thus, it is interpreted that the river terraces were formed very rapidly in a period of far less than 1500 years immediately after the deposition of the Ito ignimbrite and before cover of new vegetation on the depositional surface. q 1999 Elsevier Science B.V. All rights reserved. Keywords: river terrace; non-welded ignimbrite; high erodibility; Hishida River
1. Introduction The formation of erosional terraces consists of two processes, Ž1. lateral cutting by which a step Žtread. is formed and, Ž2. downcutting by which a scarp is produced. While tectonic or climatic stability for a considerable period Žstream equilibrium. is
)
Fax: q81-096-342-2529. E-m ail address: yokosho@ educ.kum am oto-u.ac.jp ŽS. Yokoyama.
generally assumed for the formation of the former, changes in these factors Ždisequilibrium. are postulated to cause the latter Že.g., Howard et al., 1968, p. 1117.. However, where banks are easily eroded, time and stability are not essential factors in the formation of river terraces ŽRitter, 1986, p. 272., and tectonic and climatic changes are not always necessary for downcutting. Highly erodible materials such as mudflow deposits and alluvium, for example, may permit the formation of erosional terraces in a very short period irrespective of tectonism or climatic change. River
0169-555Xr99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 5 5 5 X Ž 9 9 . 0 0 0 3 7 - 9
S. Yokoyamar Geomorphology 30 (1999) 291–304
292
Fig. 1. Map of southern Kyushu showing distribution of the Ito ignimbrite Žstippled. Žafter Yokoyama, 1972. and location of the study area Žhorizontally striped.. Dashed line indicates the outline of Aira caldera as proposed by Matumoto Ž1943..
terraces along the Abe, Joganji, and Ura Rivers, Japan ŽMachida, 1966., those near Pyramid Lake, Nevada ŽBorn and Ritter, 1970., and those along the Yuba River, California ŽChorley et al., 1984, p. 359. are some documented examples. Depositional terraces can also be formed very rapidly due to rapid aggradation and the succeeding downcutting into alluvial fills. Such examples have been reported from Waiho River, New Zealand ŽGage, 1970., and Douglas Creek, Colorado ŽWomack and Schumm, 1977.. In all these examples, terraces were formed in less than 100 years, and are in soft sediments. It is clear that these terraces owe their rapid formation to the soft sediments. This indicates that the erodibility of materials should be considered in interpretation of river terrace formation.
Fig. 2. Generalized geologic map of the study area and surrounding region. Location is given in Fig. 1. Map is based on 1:500,000 Geological map Sheet 15 Kagoshima ŽImai et al., 1980..
Non-welded ignimbrites formed by explosive volcanic eruptions are generally composed of loose accumulations of volcanic ash and pumice, and are highly susceptible to water erosion. Because of the high erodibility and absence of vegetation after an eruption, the initial depositional topography of nonwelded large ignimbrite, which is generally flat, is considered to experience very rapid dissection into ignimbrite plateaus immediately after its formation ŽYokoyama, 1985.. This is well exemplified by a very rapid erosion of ignimbrite from the 1991 erup-
Table 1 X X Temperature and precipitation data at Miyakonojo Ž31844 N, 131805 E. a Monthly means: 1961–1990
Temperature Ž8C. Precipitation Žmm. a
Jan.
Feb.
Mar.
Apr.
May
June
July
Aug.
Sept.
Oct.
Nov.
Dec.
Annual
5.0 60.4
6.5 91.1
9.8 150.8
15.1 223.3
18.8 253.6
22.2 424.9
26.0 343.1
26.1 340.6
23.2 262.6
17.7 115.3
12.3 78.5
6.9 51.5
15.8 2395.8
ŽAfter Climatic Table of Japan, Volume 1, Japan Meteorological Agency, 1991..
S. Yokoyamar Geomorphology 30 (1999) 291–304
tions of Mount Pinatubo, Philippines Že.g., Umbal and Rodolfo, 1996; Rodolfo et al., 1996.. Many river terraces and reworked ignimbrites developed extensively in non-welded ignimbrite fields are interpreted to be the products during such rapid
293
dissection independent of tectonism or climatic change. However, few studies on non-welded ignimbrites from this standpoint have been carried out. Extensive areas in southern Kyushu ŽFig. 1. are covered by the large-scale Ito ignimbrite erupted
Fig. 3. Distribution of river terraces in the middle to lower reaches of the Hishida River. Horizontally striped areas are terraces surfaces. Numerals on terrace surfaces indicate the altitude Žin meters. of the terrace surface. Dotted lines are contours Žin meters. for the reconstructed depositional surface of the Ito ignimbrite.
294
S. Yokoyamar Geomorphology 30 (1999) 291–304
S. Yokoyamar Geomorphology 30 (1999) 291–304
from Aira caldera ŽMatumoto, 1943. about 24,500 yr B.P. Ž14 C date; Ikeda et al., 1995.. River terraces are developed along other rivers in the ignimbrite field. These terraces were obviously formed by the rivers during dissection of the depositional topography of the ignimbrite, and are therefore, younger than 24,500 yr B.P. River terraces developed in the middle to lower reaches of the Hishida River in Osumi Peninsula are the most typical, and the good preservation of the terraces makes this area ideal for studying terraces in non-welded ignimbrites. In this paper, the rate and processes of terrace development along the Hishida River are discussed.
2. Physical setting The drainage basin of the Hishida River, about 400 km2 , is located southeast of Aira caldera ŽFig. 1.. The Hishida River is about 50 km in length and rises on the eastern rim of Aira caldera. It flows southeastward in its upper reaches, and then flows southward into Shibushi Bay ŽFig. 2.. Meteorological data during 30 years Ž1961–1990. at Miyakonojo, located about 30 km north of Shibusi, show that the mean annual temperature is 15.88C, and the annual precipitation is about 2400 mm ŽTable 1.. Annual discharge of the Hashida River is estimated to be about 7 = 10 8 m3 ŽSaito and Maruyama, 1976.. The dominant landforms in the drainage basin of the Hishida River are extensive Ito ignimbrite plateaus ŽYokoyama, 1974.. The altitudes of the plateaus exceed 400 m around the eastern rim of Aira caldera. In general, the plateaus dip gently Ž- 28. to the east, and fall below 200 m in altitude about 20 km east of the caldera rim. Along the main stream of the Hishida River, the plateaus lower gradually southward to less than 50 m near the river mouth on the coast of Shibushi Bay ŽFig. 3.. This decrease in plateau altitude represents largely the
295
original gently-dipping depositional surface of the Ito ignimbrite sheet. The Ito ignimbrite in the study area is typically composed of an upper non-welded zone and a lower welded zone. The upper non-welded zone generally exceeds 50 m in thickness and has a maximum thickness of about 100 m. The welded zone exceeds 20 m in thickness in places, but the maximum thickness of this unit is unknown, because beds of most modern streams are in the welded zone. The upper non-welded zone is a poorly consolidated deposit composed mainly of white volcanic ash and pumice, and is popularly called ‘Shirasu’ , a Japanese term which means white sand. The term ‘Shirasu’ is adopted in this paper to describe the non-welded portion of the Ito ignimbrite. The basement rocks underlying the Ito ignimbrite consist mainly of sandstone and shale ŽMesozoic to Tertiary., andesite lava ŽPliocene to Early Pleistocene., and ignimbrites ŽPliocene to Late Pleistocene. ŽFig. 2.. Sandstone and shale are the most extensive basement rocks forming mountains in the upper reaches of the Hishida River, and some isolated island-like hills and mountains protruding through the ignimbrite plateaus. Tephra layers several meters in thickness, derived mainly from Sakurajima volcano, cover both the ignimbrite plateaus and basement mountains over most of the study area.
3. Distribution of terraces Terraces are mainly distributed in the middle to lower reaches of the Hishida River, which are here defined as the drainage basins above and below the major junction, about 11 km from the river mouth, respectively ŽFig. 3.. Few terraces are found in the upper reaches; the upper drainage basin located outside of Fig. 3.
Fig. 4. Geologic and topographic cross sections of river terraces along the Hishida River. Dotted line indicates the altitude Žin meters. of the depositional surface of the Ito ignimbrite. Thickness of terrace deposits Žheavy line. is exaggerated. Tephra cover forming the land surface is X omitted. TB on the H–H cross section is the range of ‘terrace belt’ defined in the text. Inset gives positions of profiles.
296
S. Yokoyamar Geomorphology 30 (1999) 291–304
Several unpaired terraces are developed in the middle to lower 15 km reaches ŽFig. 4.. Detailed subdivision and correlation of the terraces is difficult, because the number of terraces is variable. In contrast, a single seemingly paired terraces are found in the lowermost 6 km reaches ŽFig. 4I–IX .. The terrace belt, here defined as the zone between the outer boundaries of terraces developed on both sides of the valley, widens in the downstream direction ŽFigs. 3 and 4.. The terrace belt is considerably wider than the present river valley. About 5 km above the river mouth, for example, the former which is approximately 1.5 km, while the latter is 0.5 km.
4. Longitudinal profile of terraces Fig. 5 represents the longitudinal profile of terrace surfaces, together with profiles of the present river bed and the depositional surface of Ito ignimbrite along the Hishida River. The position of the welded zone underneath the Shirasu is also shown. A remarkable characteristic of the terraces is that they occur within Shirasu ŽFig. 5.. In other words,
the paleo-Hishida River flowed within the level of Shirasu, 20–60 m below the depositional surface and generally, more than 20 m above the welded zone ŽFig. 6.. This presents a striking contrast to the present channel, which flows mostly through the welded zone. The longitudinal profile of the terraces is rather simple compared with the present river bed, which shows a considerable gradient change. This is because the paleo-Hishida River flowed entirely within the highly erodible Shirasu, whereas the present river flows through the welded zone and is constrained in its downcutting by the resistant welded zone, in which the degree of welding Ž‘hardness’. is spatially variable. The overall gradient of the paleo-Hishida River bed is about 5–10 = 10y3 . This gradient is much steeper compared with 7 = 10y4 for the present river bed of the lowermost reaches within the alluvium.
5. Terrace deposits Terrace deposits are composed of gravel beds, sand beds, and mud beds of various thickness.
Fig. 5. Longitudinal profiles of depositional surface of the Ito ignimbrite, river terrace surfaces, and present river bed in the middle to lower reaches of the Hishida River. Profiles are along the main stream of the Hishida River ŽThe eastern branch above the major junction ŽFig. 3. is the main stream... Because of the difficulty of detailed subdivision and correlation of terraces, they are grouped together, and longitudinal profile of the terrace surfaces is shown as a belt Žobliquely striped.. Vertically-striped zone indicates the welded zone of the Ito ignimbrite.
S. Yokoyamar Geomorphology 30 (1999) 291–304
297
Fig. 6. River terraces and the Ito ignimbrite plateau about 1.7 km downstream from the major junction. The terraces on both sides of the valley are bridged. Difference in elevation between the surface of the ignimbrite plateau Žcovered with Japanese cedars. and the terrace surface is about 60 m. The bridge piers are set within the welded zone of the Ito ignimbrite.
Fig. 7. Horizontal sandy beds. Sorting of the deposits of individual beds is variable. The handle of the hammer is 25 cm long.
298
S. Yokoyamar Geomorphology 30 (1999) 291–304
Mostly, the beds are horizontal. The gravel beds consist of pumice, andesites, sandstone and shale. The materials coincide with the components of Shirasu, i.e., pumice clasts Žusually less than 20 cm in size., lithic clasts Žless than 5 cm; mainly andesites, shale, and sandstone., and smaller particles derived from them. This indicates that the deposits are reworked Shirasu, which is self-evident, since the drainage area of the Hishida River is mostly covered with Shirasu. The terrace deposits are poorly consolidated. Grain-size characteristics and the proportion of lithics to pumice varies between beds. Some beds are composed of well sorted sands, while others are poorly sorted ŽFig. 7. resembling the laharic deposits of Mount Pinatubo ŽRodolfo et al., 1996; Fig. 13.. In contrast to lithic gravels which are usually subangular to angular indicating little abrasion during fluvial trasnport, pumice gravels are mostly rounded due to abrasion. Cross-bedding is often observed within the gravel beds and sand beds. Outcrop observations show that the boundary between the terrace deposits and the underlying Shirasu is usually horizontal ŽFig. 8., indicating nearly uni-
form thicknesses of the deposits in a limited area. The thickness of the deposits is less than 3 m in most places ŽFig. 9.. The uniformly-thin deposits indicate one of the properties of erosional terraces ŽRitter, 1986, p. 269..
6. Tephra cover Tephra and volcanic soils several meters in thickness uniformly cover both the depositional and terrace surfaces. Stratigraphic sections of the tephra and other deposits at some representative localities on both the depositional and terrace surfaces are shown in Fig. 10. Tephra layers are mostly volcanic ash and pumice derived from nearby Sakurajima volcano, located about 30 km west of the study area ŽFig. 1.. Sakurajima is a post-Ito ignimbrite volcano, which has actively continued tephra eruptions. A rhyolitic tephra called ‘Akahoya’, a nickname for the distinctive orange pumice and ash layers which usually lie between black soils in the upper horizon of every section, has been established as product of an eruption from Kikai caldera located in the sea about 100
Fig. 8. Terrace deposits overlying the Ito ignimbrite and overlain by tephra. Thickness of the terrace deposits Žsand and gravel. is approximately 80 cm. A hammer is in the horizon of the terrace deposits in the center.
S. Yokoyamar Geomorphology 30 (1999) 291–304
299
Fig. 9. Map showing the thickness Žin meters. of terrace deposits.
km south of Sakurajima. The Akahoya is a widespread tephra about 6300 years old with an extensive distribution throughout southwestern Japan ŽMachida,1983; Machida and Arai, 1978..
No significant difference can be found in the number or thickness of tephra layers between the Ito depositional surface and the terrace surfaces ŽFig. 10.. Field investigation reveals that the tephra layers
300
S. Yokoyamar Geomorphology 30 (1999) 291–304
S. Yokoyamar Geomorphology 30 (1999) 291–304
in the study area cover most of the land other than alluvial plains uniformly in accord with the relief of the present topography. In summary, the tephra cover indicates that the formation of river terraces and general present topography preceded the accumulation of all tephra. Therefore, the tephra cover in this area cannot be used to differentiate between the depositional surface and terrace surfaces, and to subdivide or correlate terraces.
7. Discussion and conclusions 7.1. Characteristics of the paleo-Hishida RiÕer It is a noteworthy characteristic that the paleoHishida River flowed entirely within Shirasu, in contrast to the present river which flows through the welded zone in most places. This places a significant constraint on nature of the paleo-Hishida River. Shirasu is very porous Žtypically 1.37 void ratio. and cannot be an impermeable bed in itself. All the springs in the Shirasu area are associated with the existence of underlying impermeable beds such as the Ito welded zone or basement rocks. Since river terraces stand within the Shirasu, it is apparent that the paleo-Hishida River flowed above the water table. This means that the paleo-Hishida River was not a perennial stream like the present Hishida River, but an ephemeral stream which flowed only when rainfall exceeded the infiltration capacity of the Shirasu. In this sense, the paleo-Hishida River may have been very similar to ephemeral streams common in arid and semiarid regions Že.g., Hadley, 1968.. 7.2. Age, duration, and process of riÕer terrace formation Okuno et al. Ž1997. summarized the stratigraphy of Sakurajima tephra group found in the northern part of Osumi Peninsula ŽFig. 1. including the upper
301
reaches of the Hishida River, and presented the 14 C dates of the individual tephra layers. The date of the oldest tephra or the age of the beginning of the volcanic activity at Sakurajima is 23,000 yr B.P., shortly after the eruption of Ito pyroclastic flow Ž24,500 yr B.P... Tephra-stratigraphical research reveals that the oldest tephra is found near the bottom of the lowest ash layer in the study area. The formation of river terraces preceded the deposition of the lowest tephra, because the terraces are covered by it. It follows, therefore, that the river terraces were formed during a very short period between 24,500 and 23,000 years ago. In order to specify the time of terrace formation in more detail within the above tephrochronological constraints, some idea about the presumed time of river terrace formation are outlined below on the basis of the characteristics of the original Ito ignimbrite depositional surface. Non-welded ignimbrite usually suffers very intense erosion by water flow, if the surface is bare and unprotected. The ignimbrite, erupted in 1991 from Mount Pinatubo is a good example for this. Rapid erosion of the ignimbrite and the associated lahars, which occurred immediately after the eruption, have been described in many papers ŽNewhall and Punongbayan, 1996.. An artificial cut in nonwelded ignimbrite for civil engineering works presents another example. On a bare and flat land of Shirasu, for example, where vegetation and tephra covers have been removed for construction, gully networks or badlands form after several rain storm or even after a single event. Palynological studies reveal that the study area belonged to the cool temperate forest Ždeciduous broad-leaved forest. zone during the period of 25,000–20,000 yr B.P., while it now belongs to the warm temperate evergreen broadleaved forest zone ŽHatanaka, 1985.. However, no data were available concerning the Late Pleistocene precipitation and flood characteristics in the study area.
Fig. 10. Columnar sections for tephra and other deposits on both terrace and depositional surfaces. Number over each column is locality shown in the index map. Localities 1, 4, 5, 12, 14, and 19 are on the depositional surface of the Ito ignimbrite, while others are on the terrace surfaces. Reworked Shirasu at localities 1, 4, and 5 are thinly laminated ash and sand beds which are interpreted as sheetwash. Ah: ‘Akahoya’ tephra ŽSee text for explanation..
302
S. Yokoyamar Geomorphology 30 (1999) 291–304
Fig. 11. Conceptual diagrams showing the process of river terrace formation in the upper non-welded zone Ž‘Shirasu’. of the Ito ignimbrite. Initial topography before dissection. Formation of shallow valley within Shirasu by an ephemeral stream. Headward advance of an incised river with perennial flow Žhorizontally striped.. Formation of river terraces with the headward advance of the incised river.
It is not known when and what kind of plants grew on the newly-formed depositional surface of the Ito ignimbrite. In any case, absence of vegetation would have continued for some years on the depositional surface. Thus, the newly-formed depositional topography of Ito ignimbrite must have suffered very rapid dissection by water flows associated with heavy rain. A model of valley and river terrace formation in Ito ignimbrite is illustrated in Fig. 11. Incipient streams flowed either on the depositional surface or through shallow valleys within Shirasu ŽFig. 11- .. The streams were ephemeral associated with heavy rains, because perennial supply of groundwater was
not available at this shallow level in the Shirasu. Many of the streamflows at this stage were probably similar to lahars which occurred around Mount Pinatubo immediately after the eruption. Considering the extreme erodibility of Shirasu, it is probable that stream downcutting into the distal Shirasu proceeded very rapidly, and the valley floor immediately reached either base level or the underlying welded zone or basement rocks, which resisted downcutting. Streams in such incised valleys were perennial due to continual supply of ground water at this depth ŽFig.11- .. Deep dissection of nearly flat or very gently inclined depositional topography in upper drainage areas could only be attained by headward
Table 2 Documented rates of headward extension of guilles in the non-welded zone of Ito ignimbrite Descriptions Gully erosion 150 m long, 30 m wide, and 7 m deep occurred in an hour. Gully 200 m long, 50 m in average width, and 20 m in average depth was formed due to piping which continued for 4 months. Gully may extend headwards more than 100 m in a single heavy storm. Gully may extend headwards more than several tens of meters in a single heavy storm.
References Tamachi Ž1953. Yamanouchi and Kimura Ž1969. Koide Ž1973.
S. Yokoyamar Geomorphology 30 (1999) 291–304
extension of such deep incised valleys. In an early stage of dissection, therefore, little or very shallow dissection must have occurred above the deeply incised valleys, and ephemeral streams would have flowed repeatedly in shallow valleys following heavy rains. The ephemeral streams would have widened the valley rapidly by lateral cutting caused by channel migration at a shallow level within the Shirasu. The shallow valley floors changed into river terraces with the headward advance of the incised valleys ŽFig.11- .. Thus, the time for terrace-formation depended on the rate at which the incised valleys advanced upstream. It is difficult to know precisely how long it took for the incised valleys to extend more than 20 km upstream. Under present conditions, the rate of extension of gullies within a 1st–2nd order drainage basin exceeds 100 m in a single heavy storm ŽTable 2.. This suggests that the rate of headward advance of the incised valleys with a much wider drainage area could be extremely high. It is considered, therefore, that the extension of the incised valleys into the upper reaches, and the formation of river terraces, was accomplished rapidly in a very short period of far less than 1500 years, immediately after the deposition of Ito ignimbrite and before the significant cover of new vegetation developed on the depositional surface. It is not exactly clear why unpaired and paired terraces are found in the middle to lower and lowermost reaches, respectively. The following are two probable cases. One case is that the stream in the upper course was unstable because of steeper gradients, and continued downcutting to form unpaired terraces, while stream level in the lower course remained essentially constant. Another case is that the higher terraces which had been formed in the lowermost reaches were eroded away due to lateral cutting by a lower level stream which became the paired terraces with a wide terrace belt.
Acknowledgements This work was supported by the Grant-in-Aid for Scientific Research of the Ministry of Education ŽC.
303
ŽNo. 60580202.. I thank T. Sunamura for valuable comments, and S. Self for providing thoughtful and constructive reviews. Tom Gardner and Jamie Woodward are thanked for their comments, which helped to improve the manuscript.
References Born, S.M., Ritter, D.F., 1970. Modern terrace development near Pyramid Lake, Nevada, and its geological implications. Geol. Soc. Am. Bull. 81, 1233–1242. Chorley, R.J., Schumm, S.A., Sugden, D.E., 1984. Geomorphology. Methuen, London and New York, 605 pp. Gage, M., 1970. The tempo of geomorphic change. J. Geol. 78, 619–625. Hadley, R.F., 1968. Ephemeral streams. In: Fairbridge, R.W. ŽEd.., The Encyclopedia of Geomorphology. Reinhold Book, New York, pp. 312–314. Hatanaka, K., 1985. Palynological studies on the vegetational succession since the Wurm ¨ glacial age in Kyushu and adjacent areas. Journal of the Faculty of Literature Kitakyushu University ŽSeries B. 18, 29–71. Howard, A.D., Fairbridge, R.W., Quinn, J.H., 1968. Terraces, fluvial introduction. In: Fairbridge, R.W. ŽEd.., The Encyclopedia of Geomorphology. Reinhold Book, New York, pp. 1117–1124. Ikeda, A., Okuno, M., Nakamura, T., Tsutsui, M., Kobayashi, T., 1995. Accelerator mass spectrometric 14 C dating of charred wood in the Osumi pumice fall and the Ito ignimbrite from Aira caldera, southern Kyushu, Japan. The Quaternary Research 34, 377–379 Žin Japanese.. Imai, I., Teraoka, Y., Ono, K., Matsui, K., Okumura, K., 1980. 1:500,000 Geological map Sheet 15 Kagoshima. 2nd edn. Geological Survey of Japan. Koide, H., 1973. The Land of Japan—Its Nature and Development, 1st volume. University of Tokyo Press, 287 pp. Žin Japanese.. Machida, H., 1966. Rapid erosional development of mountain slopes and valleys caused by large landslides in Japan. Geographical Reports of Tokyo Metropolitan University 1, 55–78. Machida, H., 1983. Tephra studies, short review. Journal of Geography 92, 441–447 Žin Japanese.. Machida, H., Arai, H., 1978. Akahoya ash—a Holocene widespread tephra erupted from the Kikai caldera, south Kyushu, Japan. The Quaternary Research 17, 143–163 Žin Japanese.. Matumoto, T., 1943. The four gigantic caldera volcanoes of Kyushu. Japanese Journal of Geology and Geography 19, Special number, 57 pp. Newhall, C.G., Punongbayan, R.S., ŽEds.., 1996. Fire and Mud: Eruptions and Lahars of Mount Pinatubo, Philippines, Philippine Institute of Volcanology and Seismology, Quezon City, and University of Washington Press, Seattle and London, 1126 pp. Okuno, M., Nakamura, T., Moriwaki, H., Kobayashi, T., 1997.
304
S. Yokoyamar Geomorphology 30 (1999) 291–304
AMS radiocarbon dating of the Sakurajima tephra group, southern Kyushu, Japan. Nuclear Instruments and Methods in Physics Research B 123, 470–474. Ritter, D.F., 1986. Process Geomorphology Ž2nd edn.., Dubuque, Wm. C. Brown Publishers, 579 pp. Rodolfo, K.S., Umbal, J.V., Alonso, R.A., Remotigue, C.T., Paladio-Melosantos, M.L., Salvador, J.H.G., Evangelista, D., Miller, Y., 1996. Two years of lahars on the western flank of Mount Pinatubo: Initiation, flow processes, deposits, and attendant geomorphic and hydraulic changes, in Fire and Mud: Eruptions and Lahars of Mount Pinatubo, Philippines. In: Newhall, C.G., Punongbayan, R.S. ŽEds.., Philippine Institute of Volcanology and Seismology, Quezon City, and University of Washington Press, Seattle and London, pp. 989–1014. Saito, K., Maruyama, Y., 1976. Hydrogeologic study in the Kimotsuki River basin and its vicinity. Engineering Geology 17 Ž2., 1–10 Žin Japanese.. Tamachi, S., 1953. Countermeasures against disasters in Shirasu area Ž2nd interim report.: Report on the disaster research in Shirasu area, and the related data, 3rd series. The planning Department of Kagoshima Prefecture, 37–53 Žin Japanese.. Umbal, J.V., Rodolfo, K.S., 1996. The 1991 lahars of southwest-
ern Mount Pinatubo and evolution of the lahar-dammed Mapanuepe Lake, in Fire and Mud: Eruptions and Lahars of Mount Pinatubo, Philippines. In: Newhall, C.G., Punongbayan, R.S. ŽEds.., Philippine Institute of Volcanology and Seismology, Quezon City, and University of Washington Press, Seattle and London, pp. 951–970. Womack, W.R., Schumm, S.A., 1977. Terraces of Douglas Creek, Northwestern Colorado: an example of episodic erosion. Geology 5, 72–76. Yamanouchi, T., Kimura, D., 1969. Disaster problems of the ‘Shirasu’ loam. Journal of the Japan Society of Civil Engineers 54 Ž11., 9–20 Žin Japanese.. Yokoyama, S., 1972. Flow and emplacement mechanism of Ito pyroclastic flow in southern Kyushu, Japan. Tokyo Geography Papers 16, 127–167 Žin Japanese.. Yokoyama, S., 1974. Mode of movement and emplacement of Ito pyroclasatic flow from Aira caldera, Japan. The Science Reports of the Tokyo Kyoiku Daigaku, Section C ŽGeography, Geology and Mineralogy. 12 Ž114., 17–62. Yokoyama, S., 1985. Geomorphological aspects of large-scale pyroclastic flow deposits: a review. Trans. Jpn. Geomorph. Union 6, 131–152 Žin Japanese..