地球環境研究,Vol.11(2009)

Distribution of Fluvial Terraces Formed in Approximately Same
Chronological Sequence in an Area of Scarce Tephra Occurrence−A Case Study of River Terrace Groupin the Eastern Hokuriku Region−
NAKAMURA Yosuke＊

Keywords: fluvial terrace, area of scarce tephra occurrence, eastern Hokuriku region
Nakata and Imaizumi, 2002). The basement rocks in

1. Introduction

the study area consist mainly of the Hida metamor-

Within the eastern Hokuriku region, an area of 100

phic (Jurassic) and granitic (Cretaceous) rocks. The

km NS and 150 km EW, bounded by Toyama Bay andEocene to Pliocene Hokuriku Group, composed of vol-Hida Mountains which is a well-known Quaternary

canic (early Miocene) and sedimentary (middle Mio-

mobile region, are developed cluster of reverse active

cene to middle Pleistocene) rocks, uncomformably

faults (Fig. 1: Research Group for Active Faults ofoverlies the pre-Tertiary rocks (Fig. 2: Kaseno et al,

Japan, 1980, 1991; Togo et al, 1998, 2003; Tsutsumi et1992).

al, 2002, 2003; Imaizumi et al, 2003; Ikeda et al., 2002;

These rocks are uncomformably overlain by theFig. 1
Topographic and active tectonic map of the eastern Hokuriku region [modified from Nakamura et al. (2008)]. Uo.F: Uozu
fault; Ku.F: Kurehayama fault; Ta.F: Takashozu fault; Ho.F: Horrinji fault; Mo.F: Morimoto-Togashi fault; Se.F: Sekidosan
fault; Us.F: Ushikubi fault; At.F: Atotsugawa fault; Ku.H: Kurikara Hill; Im.H: Imizu Hill; To.H: Togashi Hills. Inset: distribution of the active faults in central Japan. MTL: Median Tectonic Line; ISTL: H.F: Hanaore fault; N.F: Neodani fault; At.F:
Atotsugawa fault; Itoigawa-Shizuoka Tectonic Line; H.M: Hida Mountains.
＊

Faculty of Geo-environmental Science, Rissho University59Distribution of Fluvial Terraces Formed in Approximately Same Chronological Sequencein an Area of Scarce Tephra Occurrence (Nakamura)Fig. 2Geological map of the eastern Hokuriku region [modified from Kaseno et al. (1992)].

late Quaternary fluvial terrace deposits. Dating ofces between different catchment areas, stratigraphic

river terraces is essential to calculate the velocity ofpositions of regional tephra contained in terrace de-deformation for this area during late Quaternary.

posits and covering soil were heavily employed in thisHowever, tephra are rarely recognized in the terrace-

study. Terraces which were understood to haveforming or covering beds (so-called loam). Applica-

formed about the same time from the data oftion of tephrochronological criteria to date terraces isstratigraphic positions of contained regional tephra

therefore rather difficult. We have been engaged inand comparative positions of terrace surfaces wereobservations of regional tephra found in continuous

examined and compared with and the terraces werecore samples in covering beds to establish chronology

described in detail. The items examined include; dis-of the terraces and correlations.

tribution height, relative height from current river

For chronological study of terraces in an area

bed, gradient, degree of dissection, bed forming thewhere widespread volcanic ashes are rarely recog-

terrace, thickness of covering soil, kinds of gravel on

nized, degree of dissection, distribution, relative ele-

the terrace, average grain size, and degree of weather-vation from current river bed, elevation a.s.l., anding. In particular, details of the Terrace 6 (Nakamura

gradient obtained from air photos have been used in

and Okada, 2006), which is most widely distributed

the past. Studies of river terraces in east Hokuriku

and gives good reference surface in this region, is de-using air photos have been published in 'AMT',

scribed. Terrace 6 was formed just before the fall of'AQT', and 'DAF' before this study. In the 'AQT' andthe DKP (Daisen-Kurayoshi Pumice, 55 ka; Machida'DAF', the ages of river terrace formations have been

and Arai, 2003) and found in ten localities including

estimated from the methods mentioned above using

Kotatsuno surface (Kn3 terrace) in Kanazawa Plain

air photos and from these average deformation veloci-in the study area (Fig. 3).

ties were calculated.
In previous study, relative ages were deduced basedon conventional method as well for terraces distrib-

2. Method and description of the river terraces

uted in a single catchment area. However, for estab-

As a general role, the fluvial terraces are easy tolishing chronological order and correlation of terra-

form in the glacial period in the study area, and this60

地球環境研究,Vol.11(2009)

Fig. 3
Columnar sections of Terrace 6 deposits in the eastern Hokuriku region. Tephras not visible in the field are marked by mineral and glass concentration zones within the Loamy soil sequence. AT: Aira-Tanzawa Tephra, DKP: Daisen-Kurayoshi Pumice. Location of the profiles are shown in Fig.4-Fig.7.

theory confirmed Japan and New Zealand by using

In this study, as with those preceding studies, thethe tephrochronology (Hirakawa and Ono, 1974;

relative ages of river terraces distributed within the

Yanagida, 1981; Sugai, 1992; Eden and Hammond,same catchment area were estimated with the same

2003; Litchfield and Berryman, 2005, 2006). But, inmethod. However, chronology of terraces and correla-studying river terraces for classification, chronology,tion with those in different catchment areas have been

and correlation in an area with poorly developed

carried out with an emphasis on stratigraphy of re-tephra, air photos are most commonly used to readgional tephra correlating with widespread volcanic

off the data on degrees of dissection, distribution,ashes.

relative heights from current river bed, heights, andgradients.

Comparisons were made among terraces estimatedto be formed at about the same time from strati-

The river terraces distributed within the study areagraphic positions of regional tephra and from the re-have been studied for classification and correlation inlationships of different terrace surfaces for items

the 'Atlas of quaternary marine terraces in the Japa-

describe above. At the same time classification of ter-nese islands' (AMT: Koike and Machida, 2001), 'Atlasraces relying on air photos was examined on accuracyof Quaternary Thrust Faults in Japan' (AQT: Ikeda

and dependence on judgments of individuals. We stud-

et al, 2002), and 'Digital active fault map of Japan'

ied most widely distributed Terrace 6 (Table 1). Close(DAF: Nakata and Imaizumi, 2002). In particular, av-

inspection of Table 1 and Fig. 3 reveals that there is aerage uplift and subsidence deformation velocities ofbig difference in distribution pattern of terraces

active faults have been estimated from the ages of ter-which were formed at the same time.

races based on the estimated formation ages of terraces.

61

Distribution of Fluvial Terraces Formed in Approximately Same Chronological Sequencein an Area of Scarce Tephra Occurrence (Nakamura)

Table 1
Formation age and correlation of fluvial terraces in the eastern part of Hokuriku region. The following widespread ash deposits are distributed in the fluvial terrace deposits and overlying loamy soil in the chronological order: the AT (AiraTanzawa Tephra, 26-29 ka;), the DKP (Daisen-Kurayoshi Pumice, 55 ka), the K-Tz (Kikai-Tozurahara Tephra, 75-95 ka), theSK (Sanbe-Kisuki Tephra, 80-100 ka), and the Dpm (Tateyama-D Tephra, 130 ka).
Distal ash

TonamiToyama(west)

Toyama(east)

Terrace 12

To9

Tw9

Te9

Terrace 11

To8

Tw8

Te8

Terrace

Kn7

To7

Tw7

Te7

Terrace 9

Kn6

To6

Tw6

Te6

DKP (55ka)

Terrace 8

Kn5

To5

Tw5

K−Tz (95ka)

Terrace 7

Kn4

To4

Terrace 6

Kn3

To3

Tw4

Te4

Terrace 5

Kn2

Terrace 4

Kn1

To2

Tw3

Te3

AT (26−29 ka)(SK) (100ka)(Tateyama Dpm)
(120−130ka)

Terrace 10

Kanazawa

Te5

Terrace 3

Te2

Terrace 2
Terrace 1

Kn3 Surface (Kanazawa Plain: Fig. 4)

Tw2
To1

Tw1

Te1

The terrace at Morishin is about 200 to 260 m and 50

This is a terrace surface distributed as narrow

to 120 m a.s.l at Higashi-Tonami Hills with relative

strips of about 100 to 800 m in NW −SE direction

heights from current river bed being 20 to 40 m. The

along Sai and Asano Rivers flowing throughterrace deposits consisting of gravels are covered with

Kanazawa City (Fig. 4: Nakamura et al, 2003a). The

about 1 to 1.5 m thick loamy soil. In the top to middle

heights are from 50 m (Kenrokuen) to 110 m (Tsu-part of the soil is found the AT and near the bottomchishimizu) with relative heights from the current

the DKP (Fig. 3).

river bed being 40 to 60 m (Sangawa, 1986). It is covered by terrace deposits about 5 m thick of volcanic

Tw4 terrace (Western margin of Toyama Plain: Fig. 6)

products of rhyolitic and andesitic nature. DifferenceThis surface is distributed in an area from Ko-

from Noda terrace deposits lies in containing richer

nagasawa to Hiraoka and in the fan near Sakainoshin

andesitic gravels than rhyolitic ones. Small amount of(Fig. 6: Nakamura et al, 2003b). The height of thistuff gravels are also found in it. On the terrace de-surface is from about 45 to 60 m with relative height

posit, loamy soil about 0.5 to 1.0 m thick is distrib-from current river bed being 20 to 30 m. This surface

uted. Within the upper part of the loamy soil the AT

straddles between Ida River and Yamada River(Aira-Tanzawa Tephra, 26-29 ka; Machida and Arai,catchments. Terrace deposit consists of mainly gran-1976, 2003), and near the base DKP tephra are inter-ite gravels (average size about 10 to 15 cm) covered bycalated (Fig. 3).

loamy soil of 1 to 1.5 m thick. Near the top to the middle of the loamy soil occurs the AT and near the bot-To3 terrace (Tonami Plain: Fig. 5)

tom the DKT (Fig. 3).

This is a surface continuous over 12 km near
Morishin along Yamada River in the Tonami Plain

Te4 terrace (Eastern margin of Toyama Plain: Fig. 7)(Fig. 5: Nakamura et al, 2002). This is a fluvial ter-This surface is distributed continuously for about

race formed on a fan made by a tributary of Yamada

10 km from Hirono, Osakino, and on the left bank ofRiver flowing out from Takashozu Mountains andKatakai and Kurobe Rivers (Fig. 7: nakamura, 2005).

main course of Sho River. The surface shows less dis-Compared to Taikaijino surface and older ones, this issection as compared To2 terrace, and well preserved.

less dissected and well preserved. The height and62

地球環境研究,Vol.11(2009)

Fig. 4
Geomorphological map and profiles of deformed terraces (Inset) of the Kanazawa City area. The average vertical slip rate
for the Morimoto-Togashi fault zone is estimated to be 0.5-0.8 mm/yr. Kn1: Kn1 terrace (Terrace 4); Kn2: Kn2 terrace (Terrace 5); Kn3: Kn3 terrace (Terrace 6); Kn4: Kn4 terrace (terrace 7); Kn5: Kn5 terrace (Terrace 8); Kn6: Kn6 terrace (Terrace9); Kn7: Kn7 terrace (Terrace 10-11).

relative height from current river bed (shown in the(Fig. 3).

brackets) are 20 to 150 m (15 to 50 m) in Hirano, 50 to
240 m (20 to 80 m) in Osakino, 40 to 160 m (10 to 50m) in Ishigaki, 30 to 100 m (5 to 30 m) in Tenjinnoshin, and 15 to 100 m (5 to 40 m) in Maezawa, respectively.

3. Discussion
The height of this surface is widely variable from 15
to 260 m (Table 2). However, the height is largely

At Tenjinnoshin, cross section of terrace deposit

controlled by the position of localities within the riverconsisting of gravels of granite, gabbro, and gneiss iscourse. Therefore the variety does not mean much.

exposed. Average sizes of gravels at this locality are

The relative heights from current river bed areabout 10 to 20 cm (maximum 70 cm) and the thick-mostly 20 to 40 m. But, at Osakino at the eastern edge

ness of the deposit is about 20 m. Irrespective ofof Toyama Plain the relative height reaches 80 m, andlithologic differences, the gravels are little weathered

at Maezawa, which also lies on the eastern edge ofand a little more angular than those in the current

Toyama Plain, it is only 5 m (Table 2). The difference

river bed. At Maezawa a cross section is also exposed.

may be interpreted by uplift of hanging wall by active

The lithologies of gravels include granite, gabbro,

fault and flexure cliff buried under the terrace withgneiss, and andesite, but the proportion of granite to

the result of decreasing relative height. There areother lithologies is smaller than that in Tenjinnoshin.

cases where other terrace surfaces formed within 20

Average size of gravels at this locality is about 20 cm

to 30 thousand years after the formation of Terrace 6(maximum 60 cm) and the thickness of the terrace de-show similar relative heights.

posit is at least 15 m. At this locality, up to 1 m thick

The gradients of terrace surfaces are widely differ-loamy soil covers the terrace deposit, and near the top

ent from 9 to 54 ‰ (Table 2). They are dependent onof the soil occurs the AT, and near the base the DKP

erosional ability of the river, position of the terrace

63Distribution of Fluvial Terraces Formed in Approximately Same Chronological Sequencein an Area of Scarce Tephra Occurrence (Nakamura)

Table 2
Summary of comparisons of the Terrace 6 in the eastern Hokuriku region and 'ftn' in AMT (n refers to figures 2 to 6) means
MISn, while 'L' in AQT and DAF is approximately MIS 2 time.
No

Area

①

Kanazawa②

TonamiRiver

Relative
Gradientheights (m)(‰)

Degrees ofdissection

Thick of terrace
deposits (m)Terrace

Height (m)SaiKotatsuno

50−11020−40

12weakly dissected

5

Sho

Tokumanshin50−12020−40

9slightly dissected＞5③

Yamada (Ton)

Morishin200−260

20−40

35intensely dissected＞5④ Toyama (W)Yamaga (Toy)

Konagasawa

30−65

20−30

n.d

slightly dissected

＜10

⑤

Ida

Hiraoka

60−90

30−40

17

intensely dissected＞5⑥ Toyama (E)Kamiichi

Hirono20−15015−50

30slightly dissected

10−20

⑦

Hayatsuki

Osakino50−24020−8054partialy dissected＞15⑧

KatakaiIshigaki30−17520−40

32slightly dissected10−30

Tenjinnoshin

20−12015−50

44partialy dissected20

Maezawa

15−90

5−40

28

partialy dissected

＞15

⑨
⑩

KurobeTable 2 (continue)
No

Thick ofloamy soil (m)RiverGravelAverage size
(Gravel) (cm)Degrees ofweathering

VerticalAMT
displacement (m)

AQT

DAF

①

0.5−1

Sai

R, T, A, etc.

10−20

intensely weathered

n.d

ft2

n.d

n.d

②

0.5−0.7

Sho

A, G, etc.

10−15

relatively fresh

3−5

ft3

L

0.1million

③

＜0.5

Yamada (Ton)

n.d

④

1.0

Yamaga (Toy) A, G, M, etc.

⑤

0.5−1.0

Ida

A, G, T, etc.

⑥1.0−1.5

KamiichiG, A, R, etc.

⑦0.5−0.7

Hayatsuki

G, GA, A, etc.

⑧0.5−1.0

Katakai

G, GA, A, etc.

⑨

0.5−0.7⑩

0.5−1.0

Kurobe

n.d

n.d

9

n.d

n.d

0.02million

10−15

relatively fresh

15−20

ft2

n.d

n.d

10−15

n.d

＞5

n.d

n.d

0.1million

10−20

intensely weathered

10

ft4−5

L

n.d

15−30

relatively weathered

22

tf6

n.d

L

10−25

sedimentary rocks only

20

ft4−5

n.d

n.d

G, GA, A, etc.

10−20

relatively fresh

15−20

ft5

n.d

n.d

G, GA, A, etc.

20

relatively fresh

15

ft4−5

L

n.d

within the catchment, and velocity of crustal defor-

degrees of weathering of gravels in the terrace depos-

mation. Therefore, correlation using the gradients is

its vary considerably from location to location. It

not reliable. The degrees of dissection of the terraces

seems that the weathering are controlled not only by

are similar except for a few small areas, but as withthe ages of formation but also lithologies and/or

relative heights, terraces formed in other ages could

quality of permeating water.

show similar degrees of dissection. Thicknesses of ter-

In conclusion, there are wide varieties in distribut-

race deposits and covering soil beds could be as muching heights, gradients, thickness in terrace deposits,

as 2 to 3 times different. Furthermore, accurate esti-

thickness of covering soil, variety of gravels in ter-mations of those aren't easy unless in areas with goodrace deposits, and degrees of weathering in gravels

exposures are available. Even in this study, many offor the terraces formed at about the same time. On

the thickness data of covering beds were obtained

the other hand, relative heights from current riverthrough drilling.

bed, degrees of dissection of terrace surfaces, and av-

The lithologies of gravels are dependent of geology

erage sizes of terrace gravels are less variable. How-of provenance areas. The descriptions were just for

ever, some of those may show similar values inreference and cannot be used for correlations. The av-

terraces formed within 20 to 30 thousand years oferage sizes of gravels are 10 to 20 cm irrespective of

Terrace 6 formation. Therefore, extra care must be

lithologies for most areas. On the other hand, thetaken if correlation is tried solely based on these.

64地球環境研究,Vol.11(2009)

Fig. 5
Geomorphological map and profiles of deformed terraces (Inset) of the southern part of Tonami Plain. The Profiles of deformed terraces across the Takashozu fault and Hohrinji fault are shown after Nakamura (2001). The average vertical slip
rate for the Takashozu and Hohrinji fault zone are estimated to be 0.1-0.3 mm/yr and 0.1-0.4 mm/yr, respectively. To1: To1
terrace (Terrace 1); To2: To2 terrace (Terrace 4); To3: To3 terrace (Terrace 6); To4: To4 terrace (terrace 7); To5: To5 terrace(Terrace 8); To6: To6 terrace (Terrace 9); To7: To7 terrace (Terrace 10); To8: To8 terrace (terrace 11); To9: To9 terrace (Terrace 12).

Next, we examined classification and correlation ofwhile in the eastern edge of Toyama Plain slightly

terraces solely based air photos as well as individual

older than the formation of Terrace 6. In AQT, only

differences. In Table 2 is shown comparisons of esti-a few estimations were given, but all of them were

mates of formation of the Terrace 6 of this paper withgiven younger ages than those for Terrace 6. In DAF,

those in the AMT, AQT, and DAF. By the way 'ft n'

only a few ages of terrace formation were given, but

in AMT (n refers to figures 2 to 6) means MIS n

those spread from 20 to 100 thousand years BP.

(Chappell, J., and Shackleton, N. J., 1986) while 'L' in
AQT and DAF is approximately MIS 2 time.

Lastly, difference by individuals on reading the air
photos are examined. As hitherto mentioned, all threeFirst examined is the accuracy of classification anddocuments gave widely scattered results on the age ofcorrelation. The age of formation of Terrace 6 was de-formation for terraces in a restricted area. For exam-

termined using tephrochronology in this paper to beple, in Hirano and Osakino, AMT gave MIS 4 to 5 (ftMIS 3 to MIS 4. In AMT the terrace of the similar age

4 to 5), while AQT MIS 2 (L surface) for those in theare shown in only four areas from Tokuma Shin,former. For the latter, AMT gave an age of MIS6 (ftHirono, Ishigaki, and Maesawa. Other estimated ages6), while DAF about MIS 2 (L surface).

varies widely from 20 thousand years to MIS 6.
Next examined are individual studies. In AMT,
those were estimated to have formed about the same

4. Conclusion

time to slightly younger age in Kanazawa Plain,

In conclusion, classification and correlation of topo-

Tonami Plain, and western edge of Toyama Plain,graphic surfaces solely based on air photos are

65Distribution of Fluvial Terraces Formed in Approximately Same Chronological Sequencein an Area of Scarce Tephra Occurrence (Nakamura)Fig. 6
Geomorphological map and profiles of deformed terraces (Inset) of the Kurehayama fault. The Profiles of deformed terraces across the Kurehayama fault are shown after Nakamura et al. (2003b). The average vertical slip rate for the
Kurehayama fault zone is estimated to be 0.1-0.4 mm/yr. Tw1: Tw1 terrace (Terrace 1); Tw2: Tw2 terrace (Terrace 2); Tw3:
Tw3 terrace (Terrace 3); Tw4: Tw4 terrace (terrace 6); Tw5: Tw5 terrace (Terrace 8); Tw6: Tw6 terrace (Terrace 9); Tw7: Tw7
terrace (Terrace 10); Tw8: Tw8 terrace (terrace 11); Tw9: Tw9 terrace (Terrace 12).

extremely difficult if not impossible and individual
difference is large. Therefore, judgement solely on air
photo data has a limitation without absolute age
data, and on site survey is essential to get reliable re-Quaternary Takigami Granodiorite in the Japan Alps,
central Japan. Geology, 20, 657-660.
Harayama, S., Ohyabu, K., Miyama, Y., Adachi, H.,
Shukuwa, R., 2003. Eastward tilting and uplifting after
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Hida Mountain range. Quat. Res. 42, 127-140 (in Japanesewith English abstract).

Acknowledgement

Hirakawa, K., and Ono, Y., 1974. The landform evolution of

I greatly thank Dr. A. Okada, Dr. K. Takemura, and Dr.
S. Fijii for their discussions and helpful suggestions. Y.
Tanaka, N. Sugito, S. Doshida, A. Matsuoka, T. Imamura,
Y. Hattori, and T. Inoue assisted us in the field.

the Tokachi Plain. Geogr. Rev. Jpn. 47, 607-632 (in Japanese with English abstract).
Ikeda, Y., Imaizumi, T., Togo, M., Hirakawa, K., Miyauchi,T., Sato, H., editors, 2002. Atlas of Quaternary Thrust
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1:25,000 Active fault map in urban area, TOYAMA. Technical Rep. Geogr. Surv. Inst. of Japan, D1-No.396.
Tsutsumi, H., Togo, M, Imaizumi, T., Kaneda, H.,
Nakamura, Y., Hirouchi, D., 2003. 1:25,000 Active fault
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Yanagida, M., 1981. On the Formation Processes of River

Togo, M., Ikeda, Y.,Yasutaka, Imaizumi, T., Sawa, H.,

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Japan. Geogr. Rev. Jpn. 54, 423-436 (in Japanese withof active faults along the Morimoto-Togashi fault zone,

English abstract).

68地球環境研究,Vol.11(2009)

Distribution of Fluvial Terraces Formed in Approximately Same
Chronological Sequence in an Area of Scarce Tephra Occurrence−A Case Study of River Terrace Group in the Eastern Hokuriku Region−
NAKAMURA Yosuke＊＊

Faculty of Geo-environmental Science, Rissho University

Abstract:
Fluvial terraces are widely distributed in the eastern part of Hokuriku region, composed of the
Toyama, Tonami, and Kanazawa Plain, northern part of Central Japan. In this area, the age of terraces has not been reported, as volcanic ashes are rarely visible within terrace deposits. In the recent study, we carried out a drilling survey on these terraces to obtain samples of the overlying
loamy soil and upper part of terrace deposits and we extracted some well-known widespread volcanic ash, from which we were able to estimate the approximate age of the terraces. Relative ages
were estimated by previous study based on conventional method as well for terraces distributed ina single catchment area. However, for establishing chronological order and correlation of terraces
between different catchment areas, stratigraphic positions of regional tephra contained in terrace
deposits and covering soil were heavily employed in this study. In particular, details of the Terrace
6, which is most widely distributed and gives good reference surface in this region, is described.
Terrace 6 was formed just before the fall of the DKP tephra and found in ten localities in the study
area.

keywords: fluvial terrace, area of scarce tephra occurrence, eastern Hokuriku region

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Distribution of Fluvial Terraces Formed in Approximately Same Chronological Sequencein an Area of Scarce Tephra Occurrence (Nakamura)

ほぼ同一時代に形成された河成段丘面の分布形態の比較に関する研究〜北陸地方東部の河成段丘群を事例として〜
中村
＊

要

洋介＊

立正大学地球環境科学部

旨：
一般に､ 広域火山灰等の年代試料が乏しい地域において河成段丘面の分類・編年・対比を行う場合には､ 一般的には空中写真判読によって段丘面の開析形態､ 分布､ 現河床からの比高､ 比高､ ならびに勾配等を手掛かりにして記載を行う｡ 北陸地方東部に分布する河成段丘面は､ 筆者らの一連の研究の遂行以前に､ 主として空中写真判読に基づき､ ｢日本の海成段丘アトラス｣ (小池､ 町田編､ 2001)､｢第四紀逆断層アトラス｣ (池田ほか､ 2002)､ ｢活断層詳細デジタルマップ｣ (中田､ 今泉編､ 2002) 等によって､ 分類・対比がなされている｡ また､ 第四紀逆断層アトラスや活断層デジタルマップでは､ 上述のような手法で推定された河成段丘の形成時期をもとに､ 活断層の平均変位速度が算出されている｡
本研究においても､ 同一河川の流域に分布する河成段丘面の相対的な区分は､ 上述の先行研究と同様に､ 段丘面の開析形態､ 分布､ 現河床からの比高､ 比高､ ならびに勾配等を手掛かりにして行っている｡
しかしながら､ 段丘面の編年や他の河川の流域の段丘面との対比には､ 段丘面構成層中や被覆土壌層中
に挟在する広域火山灰の層位を重視した｡ 本研究では､ 広域火山灰の層位や上下の段丘面との関係より､ほぼ同時期に形成されたと考えられる段丘面を比較し､ 分布高度､ 比高､ 勾配､ 開析程度､ 段丘構成層
ならびに被覆土壌層の層厚､ ならびに段丘礫の種類､ 平均粒径､ ならびにその風化程度等にどれほどの相違が認められるかを整理した｡
今回は､ 本研究地域で最も広範囲に分布し､ 各平野間の段丘面同士の良好な対比基準となっている､
６面 (中村・岡田､ 2006) の比較を行った｡ ６面は､ DKP 降下直前に形成された段丘面で､ 本研究地
域には金沢平野の小立野面 (Kn３) を始め､ 合計10ヶ所に分布する (図３､ 表２)｡ 表２よりほぼ同時期に形成された段丘面であっても､ 分布高度､ 勾配､ 段丘面構成層の層厚､ 被覆土壌層の層厚､ 段丘礫の種類､ ならびにその風化程度には､ 地域によってばらつきが認められる｡ 一方､ 現河床からの比高､段丘面の開析程度､ 段丘礫の平均粒径には､ 地域毎のばらつきが少ない｡ ただし､ 他の時代 (面形成
前後２〜３万年間) に形成された段丘面でも､ これらと同等の値を示す場合があり､ それらとの区別を行なう際には細心の注意が必要である｡
続いて､ 空中写真判読を中心とした地形面の分類・対比の難しさについて検討する｡ 表２に､ 各先行研究による､ 本研究における面相当の段丘面の形成時期に関する推定結果を記す｡ 本研究で火山灰層序から求めた面の形成時期 (MIS３〜４) に近いものは､ 日本の海成段丘アトラスによって推定された徳万新､ 広野､ 石垣､ ならびに前沢の４地域のみである｡ その他に推定された値は､ ２万年前〜MIS
６までと非常に大きなばらつきが認められる｡ また､ 各既存研究間における同一の段丘面の年代推定も有意に差が大きい｡

キーワード：河成段丘､ 火山灰火山灰稀産地域､ 北陸地方東部

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