Flow transformation and depositional organization of debris flow ...

17

堆

積

J. Sed.

学

研

Soc.

究,59号,17-26,

Japan,

No.

2004

59,

17-26,

Flow

2004

transformation

and

depositional

flow-hyperconcentrated in volcanic and

fan-delta

Middle

The

volcanic, of the

lacustrine

Yachiho

centrated-flow deposits flow

and

streamflow

deposits,

cending

is

segmentation

Key

words:

flow are A depositional

commonly

of the flow

debris

by

flow,

setting,

deposits,

flows.

Yachiho

that

transformation,

Niigata

Ikarashi of Natural

Higashisumiyoshi-ku, †Present logy,

address: Kyoto

606-8502,

Osaka Department

University, Japan

2-cho

May 13, 2004 Snowy Areas

8050, Niigata

950-

, Nagai Park 546-0034, Japan

1-23,

History of

deposits, the

flow preceded

dilute

initial

flow,

in as-

longitudinal

debris

volcanic

hyperconflow.

fan-delta

Group

21, 2004, Accepted: for Hazards in

University,

2181, Japan **Osaka Museum

with

depositional

debris flow in the cohesionless debris-

streamflow

the

debris-

coexist

and

indicates

from

Forma-

hypercon-

with

usually

generation

normal

debris

segregated

and

hyperconcentrated-flow

to cohesionless comprising

hyperconcentrated

Volcaniclastic hydrological remobilization and resedimentation processes, comprising debris flows, hyperconcentrated flows, and flood flows (sometimes called lahar, Smith and Fritz, 1989), are capable of transporting voluminous sediment loads immediately in the aftermath of volcanic eruptions (Smith, 1986; Scott, 1988; Smith and Lowe, 1991; Nakayama and Yoshikawa, 1997; Kataoka and Nakajo, 2002; Man-

January Institute

the

Middle

organization

organization

Cohesionless

Introduction

Received: *Research

and

This

a streamflow

flow

The

deposits that

closely related organization

observed.

composite

followed

indicates

and

debris-flow

depositional

Hyperconcentrated-flow This

Japan

Lower

include facies.

and

Lower

central

Nakajo**

commonly

fan-delta-plain


order,

centrated

upper

Pleistocene

in the Pleistocene

Japan

deposits.

of hyperconcentrated fan-delta setting.

of debris spectrum

Group,

Takeshi

its characteristics

deposits.

debris-flow

processes volcanic flow

by

and

deposits

central

in the

recognized

cohesionless

fan-delta

The

Yachiho

Kataoka*†

Group,

deposits

are

setting:

Formations, Kyoko

tions

organization

flow-streamflow

Geology

Kitashirakawa-Oiwake,

and

MineraKyoto

,

ville, 2002). Since the processes can give potential hazards even far distant from the sediment source areas, understanding the behavior, dynamics, and depositional processes of such flows has been attempted by observations of modern volcanogenic debris flows and hyperconcen trated flows (Pierson and Scott, 1985; Scott, 1988; Scott et al., 1996) and by analysis of ancient volcaniclastic sedimentary successions in volcaniclastic fan, volcaniclastic apron, and even more distal alluvial-plain settings (Smith, 1986, 1988; Kataoka and Nakajo, 2000; Lirer et al., 2001). The volcanogenic hyperconcentrated flow deposits often characterize the nonmarine successions in Japan and arc-adjacent areas. Nonetheless, there exist scanty number of studies on the hyperconcentrated flow deposits and its related deposits in Japan due to unawareness to identify and less curiosity in hydrological sedimentary processes of volcaniclastic sediments.

18

Kataoka

Apart

from

these,

intermediate flow

hyperconcentrated

between

and

dilute

bertson,1964; centrated searchers

flow

high-concentration

streamflow Smith

and Nakajo

is

(Beverage

and

and Lowe,1991).

concentration

a flow (Beverage

Cul-

Hypercon-

logy (Pierson 3) transport

of solid particles

and Culbertson,

and Costa,

within

hyperconcentrated

including

flow in an ancient

we

suc-

concentration or rheology of flow from deposits. Therefore, identification of of observation

measure

relies

upon

of the deposits

ship between

debris

been

considered

Lowe,

still remains

depositional previously

1991; Sohn

relation-

processes (e.g.,

Smith

and complexity

the present

depositional

bris

flow

based

and

on the

example Yachiho setting,

processes

of inter-

analysis

Group

with

of volcanogenic

de-

flow

of ancient

of volcanic

The

the Pleistocene

lacustrine

We propose

that

fan-delta recogni-

tion of hyperconcentrated flow deposits can be supported by finding evidence such as 1) high sediment sis

concentration

within

of characteristics

a flow on the ba-

of depositional

and 2) intermediate

condition

bris flow and dilute

streamflow

style of debris-flow,

hyperconcentrated-flow,

associated the

deposits.

formation

paper

of depositional

the flow transformation hyperconcentrated-flow, uum.

The

between

records dense

;

de-

by superposition and

also addresses organization

by

in between debris-flow, and streamflow contin-

fluvial

volcaniclastic

volcano

were

affected

volcanism between ∼1.25Ma

comprising consists

sediment,

and

by basin

subsidence

of the ancestral and

1.17Ma

Tatesh(Col-

laborative Research Group for Yatsugatake, 1988 ; Kakihara and Collaborative Research Group for Yatsugatake, 2001). The lacustrine-fan-delta system prograded northeastward, based on the paleocurrent data (Susanto, 1994). Debris-flow and hyperconcentrated-flow de-

Facies description

deposits

deposits.

here is from

Japan.

organideals

hyperconcentrated

presented central

contribution

Yatsugatake

central

posits are included in upper fan-delta-plain facies in the lacustrine fan-delta succession (Susanto, 1994). These deposits commonly form couplets, and are well suited for considering depositional processes and flow transformation between debris flow and hyperconcentrated flow.

and there

pretation and discussion on depositional zation with flow transformation. the

have

et al., 1999), although,

vagueness

Therefore,

in-

flow and hyperconcentrated

flow as well as their

(Fig. 1) and mainly

ina

(Smith,

1986; Sohn et al., 1999). Spatiotemporal

stratovolcanoes

Mount

with episodic

sediment ancient

flow deposits

Group,

the north-northeast

base of the complex

1994), which

because

tegration

directly

Yachiho around

(lower Pleistocene) of this group consist of volcanic, lacustrine fan-delta successions (Susanto,

cession,

hyperconcentrated

cannot

is distributed

lava (Collaborative Research Group for Yatsugatake, 1988). The Lower and Middle Formations

grain support mechanism (Smith, 1986; Smith and Lowe,1991). It is difficult, however, to recognize

background

Plio-Pleistocene

of lacustrine,

1988); and

processes

The Japan,

1964); 2) rheo-

1987; Costa,

and depositional

Geologic

debris

flow has been defined by many rebased on various criteria such as 1)

sediment

2004

and interpretation

In this paper, debris-flow deposits, hyperconcentrated-flow deposits and associated deposits in the upper fan-delta-plain facies are divided into five major sedimentary facies. Measured sections are shown in Fig. 2. Facies A: Matrix-supported gravel with muddy matrix Description: Facies A is 0.2m thick and is represented by poorly sorted and matrix-supported gravel with muddy matrix. Facies A overlies facies C and underlies facies B. The bed of this facies lacks internal stratifications. Fine-pebble size clasts in the lower part grade upward into granule-size grains, showing coarse-tail normal grading. In the upper part of the bed, clasts locally show a clast-supported framework. Contacts with the underlying deposits are planar

J. Sed. Soc. Japan,

No. 59

Fig. 1 Geologic 1988). Alphabets

Depositional organization

of debris-flow, hyperconcentrated

map of the study area (modified indicate the sites of representative

and sharp.

from Collaborative Research Group columnar sections in Fig. 2.

supported

framework

cate that the original Interpretation:

The general

this facies indicate and Johnson, lance

and

internal

flow deposition

1976; Nemec

Scott

and

of debris.

are

of

(Rodin

Steel, 1984; Val-

1997). Poor sorting

stratification

sedimentation ding may bris flow

debris

characteristics

suggestive

Coarse-tail

and

lack of of rapid

normal

gra-

suggest incremental deposition of de(Vallance and Scott, 1997). Matrix-

flow, and streamflow

and

deposits

19

for Yatsugatake

muddy

matrix

flow was relatively

indi-

viscous

(Johnson, 1970; Lowe, 1982; Capra and Macias, 2000) so that clasts were probably supported by matrix served

strength

within

clast concentrations

ity of solid particle

the

flow. suggest

concentration

Locally

ob-

heterogenein the flow.

20

Kataoka

and Nakajo

Fig. 2 Columnar logs of the measured section, Loc. A to E. sections corresponding with facies code (facies A to E).

2004

Alphabets

beside

the columnar

J. Sed. Soc. Japan,

Depositional organization of debris-flow, hypereoncentrated

No. 59


deposits

21

Interpretation: The characteristics of this facies are suggestive of deposition from cohesionless debris flow (Lowe, 1979; Nemec and Steel, 1984; Sohn et al., 1999). Reverse grading of the bed indicates that clasts were mainly supported by dispersive pressure arising from clast collision due to high sediment concentration (Bagnold, 1954; Lowe, 1982). Normal grading may have resulted from more watery turbulent condition in part of the flow (Nemec and Steel, 1984).

Fig. 3 Inverse grading in cohesionless debris flow deposits (Facies B). Out-size boulder clasts are included in the top part of the bed.

Facies matrix

B: Clast-supported

Description: than sandy

Facies

2m thick, matrix.

are

with more than inversely mally

including

1m in diameter

is absent. graded

graded.

0.2 to more facies

upward sorted, outsized

with

A, C, D,

into facies pebble-

to

boulders

(Fig. 3). Internal

The beds are commonly

or

locally

Basal

contacts

ing beds are sharp, with the overlying

sandy

gravel

overlies

passes of poorly

clasts

stratification

B deposits,

This facies

B consists

cobble-sized

with

clast-supported

and E, and commonly C. Facies

gravel

planar facies

ungraded

or nor-

Facies C: Crudely stratified pebbly sand Description: This facies, 0.2 to 2m thick, refers to diffusely, horizontally interbedded, fine to coarse sand layers and granule to fine pebble gravel layers (Fig. 4). Sorting of the deposits is better than that of facies A and B, but poorer than facies D. This facies commonly rests on facies B and passes upward into facies D. Individual sand and gravel layers are commonly 0.5 to 3cm thick, and lack internal structures and erosional surfaces. Outsized cobble clasts (up to 20cm) are locally found. No scours are present beneath these outsized clasts. Interpretation: The characteristics of this facies are indicative of hyperconcentrated flow deposition of sediment (Smith, 1986; Smith and Lowe, 1991; Sohn et al., 1999). The crude stratification resulted from alternation of finergrained and coarser-grained layers, and absence of major trough or planar cross-stratification and major internal scours and stratal truncations in spite of the presence of outsized cobble clasts imply that sedimentation was neither under debris flow nor under normal streamflow. This stratification of the deposits implies a plane bedform accumulation as a consequence of rapid sediment fallout from suspension that did not develop a high relief bedform. These imply that the facies C deposits attributed to rapid accumulation of solid particles under high-sedimentladen flow.

with the underly-

or erosional. Contacts C are gradational.

Facies D: Cross-stratified gravel and sand Description: This facies includes well

to

Kataoka

22

and Nakajo

Fig. 4 Crude horizontal stratification flow deposits (Facies C). Ruler is 60cm

crudely

cross-stratified

granule deposits

to fine pebble gravel beds. The facies are 0.1 to 0.4m thick and well sorted.

Cross-stratification low-angle type. beds have monly

medium

includes Low-angle

sheet-like

overlies

sand

and

high-angle type to cross-bedded sand

geometry.

facies

beds

This facies

B or C and underlies

comfacies

E. Interpretation: sorting

Cross-stratification

is suggestive

and bedform

of traction

migration

under

tion (Allen, 1969; Collinson Cross-stratification

and

sedimentation

streamflow

condi-

and Thompson,

formed

by

1982).

bedform

migra-

tion under a normal streamflow condition. low-angle cross-stratification was probably ed by a shallow Facies

Description: cludes

and fast sheet-like

E: Massive

or thinly

Facies

massive

clay

laminated

and

mud thick,

horizontally

thinly laminated silt beds. This facies overlies facies C or D deposits. Interpretation: nated fine-grained from suspension delta

plain

The

massive

beds most sedimentation

or during

inter-flood

The form-

flow.

E, 0.1 to 0.7m beds

good

or thinly

inand

usually lami-

probably resulted in inactive fanintervals.

2004

of hyperconcentrated long.

Facies relationships and flow transformation of composite flows Vertical facies variation in the measured section from the Lower and Middle Formations in the Yachiho Group (Fig. 2) generally shows that the facies unit changes upward from cohesion less debris-flow deposits (facies B), through hyperconcentrated-flow deposits (facies C), into normal-streamflow deposits (facies D), displaying fining-upward sequences. Also, hyperconcentrated flow deposits (facies C) are apparently coincident with cohesionless debris flow deposits (facies B). This indicates that the generation and depositional processes of hyperconcentrated flows were closely related to cohesionless debris flows. Contacts between facies B (cohesionless debris flow), facies C (hyperconcentrated flow), and facies D (streamflow) are commonly gradual and lack thick streamflow deposits or palaeosols. These characteristics indicate that there was no major hiatus during deposition of the three successive facies. The vertical facies relationship suggests that different types of flows coexisted temporally and spatially, and probably they constituted a longitudinally segregated flow (Fig. 5). In cohesionless flow, low viscosity probably en-

J. Sed. Soc. Japan,

Fig.

5

Schematic

streamflow

in

cohesionless

hanced

Nemec,

depositional

setting

of

processes the

Yachiho

flow-hyperconcentrated

flow, during

longitudinally segregated resulting in concentration the front

for

fandelta

segregation

debris

Depositional organization of debris-flow, hyperconcentrated

model the

debris

grain

sionless

No. 59

within

flow-normal

the flow.

traveling,

tends

into several of boulder

of the flow (Pierson,

Cohe-

1986; Suwa,

1990). Also, hyperconcentrated

to be

portions, clasts at 1988;

flow de-

posits are commonly overlain by normal streamflow deposits. This is suggestive of more reduction in contents a resultant

predominance

and complete The debris

of solid particles

leading

grain-by-grain phase

flow, which

density.

Hyperconcentrated

of traction

streamflow

because

of its higher

flow part

forms

the

of cohesionless-debris-flow

the lower

part, hyperconcentrated-flow streamflow

deposits

in

deposits deposits

in

segregation

23

deposits

flowconsists

of

continuum.

the upper part (Fig. 5). Thus, hyperconcentrated flow deposits in volcanic lacustrine fandelta setting of the Lower and Middle Formations are envisaged to have been related with cohesionless debris flow processes, and vertical superposition of deposits suggest lateral (longitudinal) flow segregation and transformation. Conclusions Debris-flow

sedimentation.

its consist

and normal

flow-hyperconcentrated

Longitudinal

processes

middle phase, and more diluted streamflow characterizes the trailing phase. The resultant depos-

in the middle,

debris

in the flow and

of the flow is cohesionless fastest

of Group.


and


de-

posits in the Pleistocene Lower and Middle Formations of the Yachiho Group, central Japan commonly occur in upper fan-delta-plain facies. Recognition of hyperconcentrated flow deposits can be judged from characteristics of the deposit itself and depositional organization with debris flow and streamflow deposits. Hyperconcen trated flow deposits show crude horizontal strati-

Kataoka

24

fications consisting of alternation of centimeter thick fine-grained layers and granule-pebble layers. The deposits lack internal truncation surfaces in spite of the inclusion of outsized cobble clasts. The crude horizontal stratifications with isolated clast are evidence for rapid deposition of solid particle from high sediment concentrated flow. The hyperconcentrated flow deposits in upper fan-delta-plain facces are usually coincident with cohesionless debris flow deposits. This shows the close relationship between cohesionless debris flow and hyperconcentrated flow in their depositional processes. The depositional organization usually comprises cohesionless debris flow deposits, hyperconcentrated flow deposits, and normal streamflow deposits in ascending order. This organization indicates the longitudinal segmentation of the composite flows during flow traveling. Acknowledgments We thank to Professor Wataru Maejima for his comments and discussion on the early version of the manuscript. Two anonymous reviewers and the editor are acknowledged for suggestion and careful editorial reading that improved the paper. Financial support was partly provided by Osaka City University (a Special Grant for Postgraduate

Students)

to the first author. References

Allen,J.R.L., 1969:Somerecent advances in the physics of sedimentation.Proceedingsof the Geologists'Association, 80, 1-42. Bagnold, R.A., 1954:Experiments on a gravity-free dispersion of large solid spheres in a Newtonian fluid under shear. Proceedingsof the RoyalSocietyof London,Series A, 225,49-63. Beverage,J.P. and Culbertson,J.K., 1964:Hyperconcentrations of suspended sediment. Journal of the Hydraulics Division,Proceedingsof the AmericanSocietyof CivilEngineers,90, HY6,117-128. Capra, L. and Macias,J.L., 2000:Pleistocenecohesivedebris flows at Nevado de Toluca Volcano, central Mexico. Journal of Volcanology and GeothermalResearch,102,149 -168. CollaborativeResearch Group for Yatsugatake,1988:Plioce-

and Nakajo

2004

ne-Lower Pleistocene Around Mts. Yatsugatake -Especially on the Yachiho Group-. Monograph of the Association for the Geological Collaboration in Japan, no. 34,15-52. (In Japanese with English abstract.) Collinson, J.D. and Thompson, D.B., 1982: Sedimentary Structures. George Allen & Unwin, London, 194p. Costa, J.E., 1988: Rheologic, geomorphic and sedimentologic differentiation of water floods, hyperconcentrated flows, and debris flows. In Baker, V.R., Kochel, R.C. and Patton, P.C., eds., Flood Geomorphology, J. Wiley and Sons, New York, 113-122. Johnson, A.M., 1970: Physical processes in geology. Freeman, Cooper and Company, San Francisco, California, 577p. Kakihara, H. and Collaborative Research Group for Yatsugatake, 2001: The volcanism of Mts. Yatsugatake. Daishiki, 33, 9-12. (In Japanese.) Kataoka, K. and Nakajo, T., 2000: Depositional processes of the debris-flow and hyperconcentrated flow deposits, the Ebisutoge-Fukuda tephra (Karegawa volcanic ash) in the Tokai Group, Plio-Pleistocene, central Japan. Journal of Geological Society of Japan, 106, 897-900. (In Japanese with English abstract.) Kataoka, K. and Nakajo, T., 2002: Volcaniclastic resedimentation in distal fluvial basins induced by large-volume explosive volcanism: the Ebisutoge-Fukuda tephra, Plio-Pleistocene boundary, central Japan. Sedimentology, 49, 319-334. Lirer, L., Vinci, A., Alberico, I., Gifuni, T., Bellucci, F., Petrosino, P. and Tinterri, R., 2001: Occurrence of intereruption debris flow and hyperconcentrated flood-flow deposits on Vesuvio volcano, Italy. Sedimentary Geology, 139, 151-167. Lowe, D.R., 1979: Sediment gravity flows: their classification and some problems of application to natural flows and deposits. In Doyle, L.J. and Pilkey, O.H., Jr., eds., Geology of Continental Slopes, SEPM Special Publication, no. 27, 75-82. Lowe, D.R., 1982: Sediment gravity flows: II. Depositional models with special reference to the deposits of highdensity turbidity currents. Journal of Sedimentary Petrology, 52, 279-297. Manville, V., 2002: Sedimentary and geomorphic responses to ignimbrite emplacement: readjustment of the Waikato River after the A.D. 181 Taupo eruption, New Zealand. Journal of Geology, 110, 519-541. Nakayama, K. and Yoshikawa, S., 1997: Depositional processes of primary to reworked volcaniclastics on an alluvial plain: an example from the Lower Pliocene Ohta tephra bed of the Tokai Group, central Japan. Sedimentary Geology, 107, 211-229. Nemec, W., 1990: Aspects of sediment movement on steep delta slopes. In Collela, A. and Prior, D.B., eds., Coarse-

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Depositional organization

of debris-flow, hyperconcentrated

grained deltas, Special Publication of the International Association of Sedimentologists, no. 10, 29-73. Nemec, W. and Steel, R.J., 1984: Alluvial and coastal conglomerates: their significant features and some comments on gravelly mass-flow deposits. In Koster, E.H. and Steel, R.J., eds., Sedimentology of Gravels and Conglomerates, Canadian Society of Petroleum Geologists Memoir, no. 10, 1-31. Pierson, T.C., 1986: Flow behavior of channelized debris flows, Mount St. Helens, Washington. In Abrahams, A.D., Hillslope Processes, Allen & Unwin, Boston, 269-296. Pierson, T.C. and Costa, J.E., 1987: A rheologic classification of subaerial sediment-water flows. Geological Society of America Reviews in Engineering Geology, 7, 1-12. Pierson, T.C. and Scott, K.M., 1985: Downstream dilution of a lahar: transition from debris flow to hyperconcentrated streamflow. Water Resources Research, 21, 15111524. Rodin, J.D. and Johnson, A.M., 1976: The ability of debris, heavily freighted with coarse clastic material, to flow on gentle slopes. Sedimentology, 23, 213-234. Scott, K.M., 1988: Origins, behavior, and sedimentology of lahar and lahar-runout flows in the Toutle-Cowlitz River system. US. Geological Survey Professional Paper, 1447-A, 74p. Scott, K.M., Janda, R.J., de la Cruz, E.G., Gabinete, E., Eto, I., Isada, M., Sexon, M. and Hadley, K.C., 1996: Channel and sedimentation responses to large volumes of 1991 volcanic deposits on the east flank of Mount Pinatubo. In Newhall, C.G. and Punongbayan, R.S., eds., Fire and Mud -Eruptions and Lahars of Mount Pinatubo , Philippines, Philippine

Institute

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the University of Washington Press, 971-988. Smith, G.A., 1986: Coarse-grained nonmarine volcaniclastic sediment: terminology and depositional process. Geological Society of America, Bulletin, 97, 1-10. Smith, G.A., 1988: Sedimentology of proximal to distal volcaniclastics dispersed across an active foldbelt: Ellensburg Formation (late Miocene), central Washington. Sedimentology, 35, 953-977. Smith, G.A, and Fritz, W.J., 1989: Volcanic influences on terrestrial sedimentation. Geology, 17, 375-376. Smith, G.A. and Lowe, D.R., 1991: Lahars: volcano-hydrologic events and deposition in the debris flow-hyperconcentrated flow continuum. In Fisher, R.V. and Smith, G.A., eds., Sedimentation in Volcanic Settings, SEPM Special Publication, no. 45, 59-70. Sohn, Y.K., Rhee, C.W. and Kim, B.C., 1999: Debris flow and hyperconcentrated flood-flow deposits in an alluvial fan, northwestern part of the Cretaceous Yongdong basin, central Korea. Journal of Geology, 107, 111-132. Susanto, E.E., 1994: Volcanogenic sedimentation of a fan delta complex recorded in the stratigraphic succession of Lower and Middle Formations of the Yachiho Group, central Japan. Journal of Geosciences Osaka City University, 37, 145-164. Suwa, H., 1988: Focusing mechanism of large boulders to a debris-flow fronts. Transactions, Japanese Geomorphological Union, 9, 151-178. Vallance, J.W. and Scott, K.M., 1997: The Osceola Mudflow from Mount Rainier: sedimentology and hazard implications of a huge-clay-rich debris flow. Geological Society of America, Bulletin, 109, 143-163.

Kataoka

26

and Nakajo

2004

土石流・ハイパーコンセントレイティッド流・河川流の分化と堆積累重様式 ― 前期更新世,八

千穂層群下部・中部累層の例 ―

片岡香子・中条武司,2004,堆積学研究,No. 59, 17-26 Kataoka, K. and Nakajo, T., 2004: Flow transformation and depositional organization of debris flow-hyperconcentrated flow-streamflow spectrum in volcanic fan-delta setting: The Pleistocene Lower and Middle Formations, Yachiho Group, central Japan. Jour. Sed. Soc. Japan, No. 59, 17-26 前期更新世,八千穂層群下部・中部累層の火山性湖成ファンデルタ堆積物は上部ファンデルタプレーン相に土石流堆積物・ハイパーコンセントレイティッド流堆積物を含む.ハ

イパーコ

ンセントレイティッド流堆積物はその特徴とそれに関係した他の堆積物との累重関係によって認定できた.ハ

イパーコンセントレイティッド流堆積物は,不

明瞭な成層構造をもち,数cm

程度の細粒層と細礫 ∼ 中礫層の互層状を示す.こ

の堆積物は,ア

礫を含むが,内部には侵食面などをもたない.こ

のことは,高濃集した流れから粒子が急速に

沈積したことを示す.こ

ウトサイズクラストとして大

こでのハイパーコンセントレイティッド流堆積物は多くの場合,非

着性土石流堆積物に付随して見られる.このことは,ハ

粘

イパーコンセントレイティッド流の発

生と堆積過程が非粘着性土石流と密接に関係していることを示す.こ

こでは下位から,非粘着

性土石流堆積物・ハイパーコンセントレイティッド流堆積物・河川流堆積物と累重するのが認められる.この累重様式は複合した流れの流下方向への分化を表しており,先行して流れた非粘着性土石流の背後に,そ

こから分化したハイパーコンセントレイティッド流があり,そのさ

らに後を河川流が流下したと考えられる.