[The Journal of Geology, 1999, volume 107, p. 111–132] q 1999 by The University of Chicago. All rights reserved. 0022-1376/99/10701-0002$01.00

111

Debris Flow and Hyperconcentrated Flood-Flow Deposits in anAlluvial Fan, Northwestern Part of the Cretaceous Yongdong

Basin, Central Korea

Young Kwan Sohn, Chul Woo Rhee,1 and Bok Chul Kim2

Department of Earth and Environmental Sciences, Research Institute of Natural Sciences,Gyeongsang National University, Chinju 660-701, Korea

(e-mail: yksohn@nongae.gsnu.ac.kr)

A B S T R A C T

The alluvial-fan deposits in the Cretaceous Yongdong Basin, Korea, consist of conglomerates with a muddy or sandymatrix and sandstones with thick or thin laminations. The conglomerates and sandstones occur commonly in couplets,constituting apparently single sedimentation units. Facies transitions in the deposits can be summed up in a tripartitefacies sequence: (1) a clast-supported conglomerate with a muddy or sandy matrix and parallel clast alignment (faciesA and B), (2) a matrix-supported coarse-tail normally graded conglomerate with random clast orientation (facies C)and thickly stratified pebbly sandstone (facies D), and (3) laminated sandstone (facies E). The clast-supported con-glomerate is interpreted as deposits of debris flows dominated by frictional grain interactions. Development of per-vasive parallel clast alignment, lacking large floating clasts and inverse grading, suggests deposition via incrementalaggradation rather than en masse freezing. The matrix-supported conglomerate and thickly stratified sandstone areinterpreted as deposits of dense inertia layers or traction carpets developed beneath a high-concentration bipartiteflow. The laminated sandstone indicates traction sedimentation associated with dilute flows. The facies sequencetherefore suggests deposition from a composite sediment flow that comprises a preceding debris flow, a trailing wateryflow, and an intermediate flow between. The intermediate flow is regarded as a hyperconcentrated flow on the basisof its bipartite nature because a hyperconcentrated suspension has a meager yield strength and is prone to be densitystratified. The measured section comprises three depositional sequences, decameters thick and separated by thickmudstone beds, which could be interpreted in terms of fan evolution (progradation and retreat) under an influenceof changing sediment supply from a drainage basin. Close association of sediment type with constituent facies in thethree sequences suggests that composite sediment-flow deposits are favorably developed by sand-matrix debris flowsdrained from large and less rugged catchments.

Introduction

Debris flows are important depositional processeson alluvial fans, responsible for many coarse-grained deposits (Blackwelder 1928; Bull 1963;Hooke 1967; Gloppen and Steel 1981; Nemec andSteel 1984; Whipple and Dunne 1992; Blair and Mc-Pherson 1994). They are gravity-driven flows ofhighly concentrated mixtures of sediment and wa-ter, resulting in massive, poorly sorted, and gen-erally ungraded deposits with lobate fronts and lat-

Manuscript received August 12, 1997; accepted July 30, 1998.1 Department of Earth and Environmental Sciences, College

of Natural Sciences, Chungbuk National University, Cheongju361-763, Korea; e-mail: gloryees@cbucc.chungbuk.ac.kr.

2 Geological Research Division, Korea Institute of Geology,Mining and Materials, Taejon 305-350, Korea; e-mail: kbc@rock25t.kigam.re.kr.

eral levees (Sharp and Nobles 1953; Johnson 1970,1984; Fisher 1971; Costa 1984; Major 1997; Sohnet al. 1997). There is a wide spectrum of debrisflows, of which the mud-rich and clast-rich end-members can be modeled as viscoplastic fluid(Johnson 1970, 1984) or an inertial grain flow (Tak-ahashi 1978, 1981). Both models assume that thedeposition from a debris flow occurs abruptly or enmasse as the driving shear stress drops below theplastic yield strength (viscoplastic model) or asgrains lock up because of a decrease in the disper-sive pressure (inertial grain-flow model). Most de-bris flows are, however, neither a single-phase ho-mogeneous plastic substance nor an inertial grainflow that lacks the effects of interstitial fluid. In-stead, they are multiphase materials, in which a

112 Y . K . S O H N E T A L .

number of momentum transport processes operateinvolving both solid and fluid forces (Iverson andDenlinger 1987; Iverson 1997). It is also recognizedthat incremental aggradation by multiple waves orsurges is an important means of debris-flow em-placement (Wan and Wang 1994; Major 1997; Vall-ance and Scott 1997).

In contrast to debris flows, streamflows are dom-inated by fluid forces and deposit their sedimentloads on a grain-by-grain basis, resulting in well-sorted and stratified or cross-stratified deposits. Hy-perconcentrated flood flows having characteristicsintermediate between debris flows and streamflowshave also been recognized as an important agent ofsedimentation both in alluvial fans and in volcanicterrains (Bull 1963, 1977; Beverage and Culbertson1964; Harrison and Fritz 1982; Nemec and Mu-szynski 1982; Waitt et al. 1983; Ballance 1984; Ne-mec and Steel 1984; Pierson and Scott 1985; Smith1986, 1987a; Scott 1988; Todd 1989; Pierson et al.1990; Smith and Lowe 1991; Best 1992; Wan andWang 1994). These flows develop from either debrisflows or torrential streamflows by incorporatingwater or sediment along their flow paths. They pro-duce ungraded or coarse-tail normally graded graveldeposits and crudely or thickly stratified gravellysand deposits (Harrison and Fritz 1982; Pierson andScott 1985; Smith 1986; Smith and Lowe 1991; Best1992), similar to those of high-concentration tur-bidity currents (Lowe 1982; Postma et al. 1988;Chun and Chough 1992). In contrast to debrisflows, hyperconcentrated flood flows have meageryield strengths and are prone to be density stratifiedinto a high-concentration clast-rich lower part anda dilute finer-grained upper part (Smith 1986; Pier-son and Costa 1987; Wan and Wang 1994; Coussotand Meunier 1996). Recognition of such hybrid pro-cesses raises the possibility that couplets of con-glomerate and stratified sandstone may representsingle depositional events rather than multipleevents of different flows.

Many sediment flows cannot, however, be cat-egorized into one of these end-member types be-cause individual flows may comprise more thanone flow type at an instant in time and are subjectto a series of flow transformation during transport(Beaty 1963; Pierson 1980, 1986; Pierson and Scott1985; Smith 1986, 1987b; Scott 1988; Pierson et al.1990; Smith and Lowe 1991; Best 1992; Wan andWang 1994). Depositional events on alluvial fansare therefore a complex mixture of different pro-cesses, such as debris flows, hyperconcentratedflood flows, and streamflows. Understanding thenature of these composite events, such as the ev-olution of flow properties with time and space and

the interaction between different parts of a flow andits environment, is crucial for proper interpretationof alluvial-fan facies and processes.

The alluvial-fan succession on the northwesternmargin of the Cretaceous Yongdong Basin, Korea(fig. 1), consists of conglomerates with muddy orsandy matrix and sandstones with thick or thinlaminations that are interpreted to be deposits ofdebris flows, highly concentrated bipartite flows,and dilute streamflows. The conglomerate andsandstone units commonly occur in couplets, theformer gradationally overlain by the latter, appar-ently constituting single sedimentation units.These facies transitions suggest sequential depo-sition from a composite sediment flow that com-prises a preceding debris flow, a trailing dilute flow,and probably with a hyperconcentrated flow be-tween. This study documents the sedimentarycharacteristics of the deposits and infers the char-acters and depositional processes of composite sed-iment flows on alluvial-fan surfaces.

Geologic Setting

In the southwestern Korean Peninsula, several Cre-taceous basins were formed along a series of NE-SW trending strike-slip faults (fig. 1). The YongdongBasin (ca. 10 # 40 km2 in area) is an elongate half-graben basin filled with siliciclastic, nonmarine de-posits (ca. 6–8 km thick) (Lee and Paik 1989, 1990;Chun et al. 1993). To the southeast, the depositsare in fault contact with Precambrian gneiss andschist (Shimamura 1924; Yun and Park 1968; Wonand Kim 1969; Kim and Hwang 1986). On thenorthwestern margin, they unconformably overliephyllites and limestones of unknown age, Permianmetasandstones, and Permian to Jurassic graniticrocks (Shimamura 1924; Won and Kim 1969; Kimet al. 1978; Hong et al. 1980; Kim and Hwang 1986;Kim and Lee 1986; Jin et al. 1993; Shin and Jin1995). They are intruded by hypabyssal and vol-canic rocks, such as quartz porphyry, rhyolite, andtrachytic andesite (fig. 1b). The occurrence of fossilflora (coniferous gymnosperms and angiosperms)and fauna (gastropods, estherids, ostracods, andcharophytes) suggests that the deposits wereformed in Neocomian to Aptian-Albian times un-der temperate climatic conditions (Chun et al.1993; Choi et al. 1995).

In the southern part of the basin, the basin-fillsuccessions can be divided into two allostrati-graphic units (alloformations 1 and 2) by a well-developed paleosol horizon, which is tens of metersthick and extensively exposed across the basin (fig.1b; Kim 1996). Each alloformation comprises var-

Journal of Geology D E B R I S A N D F L O O D - F L O W D E P O S I T S 113

Figure 1. a, Distribution of Cretaceous nonmarine basins and fault pattern in the Korean Peninsula (modified afterKorea Institute of Geology, Mining, and Materials 1995; Choi 1996). Numbers indicate Cretaceous basins. 1, Pungam;2, Eumsung; 3, Kongju; 4, Puyeo; 5, Yongdong; 6, Jinan; 7, Neungju; 8, Kyongsang. b, Distribution of dominantlithologic units in the southern part of the Yongdong Basin. I, Conglomerate; IIa–i, conglomerate/gravelly sandstone;IIIa, b, gravelly sandstone; IVa–d, gravelly sandstone/reddish mudstone; V, sandstone; and VI, mudstone. Paleoflowpatterns measured from clast imbrications and attitude of channel walls or trough axes are indicated by arrows. Themeasured section (marked by asterisk) is present in the unit IIg adjacent to the northern basin boundary.

114 Y . K . S O H N E T A L .

ious lithologic units of conglomerate, (gravelly)sandstone, and mudstone, distinguished from oneanother on the basis of dominant sedimentary fa-cies and/or clast composition. Apparent paleoflowdirections (fig. 1b) and the overall distributionalpattern of lithologic units in each alloformation(fig. 1b) indicate that discrete coarse-grained de-posits (conglomerate and gravelly sandstone) alongthe basin margin grade laterally into fine-graineddeposits (mudstone) in the basin center (Kim 1996).Detailed facies analysis in the southern part of theYongdong Basin reveals that each alloformationrepresents a depositional system comprising allu-vial fans/fan deltas along the basin margin, down-stream braided rivers, and lacustrine depositionalenvironments in the basin center (Kim 1996).

In this article, we describe in detail a road-cutsection in the lithologic unit IIg (fig. 1b), whichconsists of conglomerates, gravelly sandstones, andsubordinate amounts of sandstones. The unit is in-terpreted to represent an alluvial-fan depositionalenvironment on the basis of its occurrence adjacentto the Jurassic basement of granodiorite to the west,the presence of debris-flow deposits, and the overallsediment dispersal pattern in the study area (fig.1b). Although the geometry and dimension of theconglomerate body are unclear because of postde-positional deformation, the conglomerate body fitsinto the widely used definition of an alluvial fanas “a body of detrital sediments built up at a moun-tain base by a mountain stream” (Blissenbach 1954;Nilsen 1982).

Facies Description and Interpretation

Facies A: Clast-Supported Conglomerate with MuddyMatrix. The conglomerates of this facies are me-dium to thick bedded (0.3–1 m thick) and are gen-erally massive and ungraded (fig. 2). Unit bounda-ries are mostly parallel sided, but some units haveslightly scoured bases and undulatory upper con-tacts with positive primary relief. Rarely, units ofthis facies pinch out abruptly (fig. 3; arrows). Clastsare mostly pebble to cobble sized; boulder-sizeclasts are less abundant; large mudstone chips areabundant in some units; large protruding clasts arenot observed. Clasts are subangular to subroundedand generally elongate. They are mostly in tightcontact and are aligned parallel to bedding planes(fig. 4). This facies occurs mainly in the middlesequence as thick amalgamated packets (fig. 2).Thin discontinuous units of sandy deposits (faciesD or E) commonly overlie this facies. Sparingly, thematrix of facies A conglomerates becomes sandier

upward, gradually passing into facies Bconglomerates.

General characteristics of this facies, such as themassive and ungraded nature, poor sorting, muddymatrix, and rare positive primary relief and taperedmargins, are suggestive of emplacement by debrisflows (Johnson 1970, 1984). The tight clast-sup-ported fabric suggests that the debris flows had highmechanical strengths imparted from grain friction.Clast collision is interpreted to have been ham-pered by highly viscous muddy matrix, resulting inthe lack of inverse grading and clast imbrication.The muddy matrix probably enhanced the mobilityof debris flows by lubricating frictional clast inter-actions and maintaining high pore fluid pressures,rather than increasing the mechanical strengths ofdebris. The pervasive parallel-to-bedding clastalignment throughout the conglomerate units,lacking large floating or protruding clasts and sub-vertical clast alignment, suggests that the debrisflows experienced full laminar shear before depo-sition (Fisher 1971; Enos 1977), without developinga nondeforming plug at the upper part.

Facies B: Clast-Supported Conglomerate with SandyMatrix. Facies B conglomerates share many char-acteristics in common with facies A conglomer-ates, having similar clast sizes and shape, fabric,bed thickness, and bed geometry, except that thematrix is composed of poorly sorted granular coarsesand. Scoured bases are also more common. Thisfacies commonly passes upward into matrix-sup-ported conglomerates or stratified sandstones (fa-cies C or D; fig. 5). This facies is most common inthe upper sequence (fig. 2).

This facies is suggestive of cohesionless debrisflows dominated by frictional grain interactions(Kim et al. 1995; Sohn et al. 1997). The lack ofinverse grading and imbrication of clasts is proba-bly due to suppression of clast collision. The well-developed parallel clast alignment suggests fric-tional interaction among clasts along deposit mar-gins (snout/lobe) (Major 1998).

Facies C: Sand-Matrix-Supported Conglomerate.This facies refers to medium- to thick-bedded(0.3–1 m thick), pebble-to-cobble conglomerateswhose clasts are either supported by sand matrixor in loose contacts. Gravel clasts are poorly alignedrelative to those of facies A and B conglomerates.This facies occurs either as basal portions of up-ward-fining units (figs. 6, 7) or as solitary units. Inthe former, facies C conglomerates have scouredbases and are overlain by stratified units of faciesD and E. They are coarse-tail normally graded andlaterally discontinuous. The upper contacts showundulatory primary relief. In the latter, conglom-

Journal of Geology D E B R I S A N D F L O O D - F L O W D E P O S I T S 115

Figure 2. Columnar log of the measured section (for location, see fig. 1). The section comprises three sequences(lower, middle, and upper sequences) bounded by thick homogeneous mudstone units. Capital letters (A–F) representfacies codes. Numbers represent stratigraphic levels above the base of the measured section in meters. M 5

; ; .mudstone S 5 sandstone G 5 conglomerate

erates have lenticular geometry with positive pri-mary relief and abrupt lateral terminations. Grad-ing is mostly absent. This facies is common in thelower sequence (fig. 2).

The coarse-tail normally graded conglomerates

overlain by stratified sandy deposits are interpretedas deposits of high-concentration dispersion of sandand gravel that were developed beneath a turbulentheavily sediment-laden flow (Lowe 1982; Todd1989). Since gravel clasts are difficult to fully sup-

116 Y . K . S O H N E T A L .

Figure 2 (Continued)

port by turbulence, they are concentrated towardthe base of a turbulent flow to form a dense inertialayer. Gravel clasts in the layer can be effectivelysegregated according to their sizes under a hinderedsettling condition (Druitt 1995). The coarse-tailnormal grading in this facies probably resulted fromabrupt immobilization of the size-graded inertialayer rather than from prolonged suspension sedi-

mentation. The overall random clast orientationand undulatory upper contacts support this inter-pretation. However, the solitary units with pro-nounced lenticular geometry resulted more likelyfrom cohesionless debris flows that are not nec-essarily associated with a turbulent flow.

Facies D: Stratified Pebbly Sandstone. This faciesincludes well- to crudely stratified pebbly sand-

Journal of Geology D E B R I S A N D F L O O D - F L O W D E P O S I T S 117

Figure 2 (Continued)

stones and sandstones. They occur mostly as thin,discontinuous interbeds (0.1–0.3 m thick) betweenother facies units, gradually or abruptly overlyingunits of facies A–C (fig. 3). Stratification is plane-parallel or low-angle inclined relative to bedding

planes. Internal truncation surfaces are rare. Meter-thick units occur in the lower sequence, where thestratification is defined by repetition of centime-ters-thick, generally ungraded, coarse- to fine-grained sand layers. Pebble clasts are either sparsely

118 Y . K . S O H N E T A L .

Figure 3. Sketch of part of the middle sequence. Stacked conglomerate beds (facies A and B) are either continuous(lower left part) or abruptly terminating (open arrows) and are draped by (pebbly) sandstone layers (facies D and E).Conglomerate beds overlying a topographic relief (facies B, bracketed) compensate for the relief and are laterallyoffset, as shown by inclined layers of mudstone chips. A bouldery conglomerate bed in the upper part is rapidlypinching out and suggests selective migration of large clasts to the lateral margins of a debris flow. For stratigraphicposition, see figure 2.

present or form discontinuous trains. Their longaxes are aligned parallel to bedding planes (fig. 7).

The centimeters-thick stratification, lackingangle-of-repose cross-bedding and internal trunca-tion surfaces, has been observed in many catas-tropic flood-flow deposits and attributed to rapidaccumulation under a swift, heavily sediment-laden turbulent flow (Harrison and Fritz 1982; Pier-son and Scott 1985; Smith 1986, 1987a; Blair 1987;Smith and Lowe 1991; Best 1992). Best (1992) andSmith (1986, 1987a) interpreted the stratificationto be due to migration of low-relief bedformsformed during the transition from dunes to upper-stage plane beds (Bridge and Best 1988) on the basis

of lenticularity and the interdigitating nature of thestratification. The centimeters-thick stratificationin this facies, which lacks these features, may how-ever be a result of repetitive deposition from thin,short-lived traction carpets developed beneath aturbulent flow that carried abundant sediment loadand fluctuated in the suspended-load fallout rate(Hiscott 1994; Sohn 1997).

Facies E: Thinly Stratified Sandstone. This faciesincludes thinly stratified or horizontally laminated,coarse- to fine-grained sandstones, centimeters todecimeters thick (figs. 7, 8). They mostly overlieunits of facies B and D gradationally and only rarelyoccur as solitary beds. They commonly show fin-

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Figure 4. Photograph showing amalgamated units of facies A conglomerates with clasts aligned parallel to beddingplanes. Note that large clasts are concentrated in the lower part (near scale). For stratigraphic position, see figure 2.Hammer is 30 cm long.

ing-up trends and are devoid of gravel clasts. Thinunits are generally discontinuous, laterally extend-ing less than a few meters, whereas thick units arelaterally persistent. Thick units are abundant in thelower sequence.

Deposits similar to this facies are common inalluvial fans and have been attributed to waning-stage sheetfloods or water flows, dewatering of de-bris flows, or reworking of surficial deposits bysheetwash during storms (Gloppen and Steel 1981;Ballance 1984; Wells 1984). Common gradationalcontacts with the underlying units of facies B or Dsuggest that this facies resulted mostly from waterflows attendant to preceding sediment flows. Thehorizontal lamination was most likely formed un-der upper-plane-bed conditions. The lack of angle-of-repose cross-bedding and channel structures in-dicates sporadic shallow sheet flows (Ballance1984).

Facies F: Fine-Grained Homogeneous Deposits.This facies includes all fine-grained deposits inter-calated between gravelly and sandy beds. Except forunits along scoured contacts, they are bounded byflat bases and tops. They are generally homoge-neous and purple but appear to be stratified whendiffuse bands of sand are intercalated. They rangein grain size from clay to muddy sand and, in somecases, contain scattered granule to fine pebbleclasts and thin sheetlike layers of massive or strat-

ified gravelly sandstones. This facies occurs eitheras decimeter-thick discontinuous interbeds or asmeter-thick, laterally persistent beds along the se-quence boundaries (22–24-m and 46–48-m intervalsin the measured section; fig. 2).

This facies most probably formed on inactive seg-ments of alluvial-fan surfaces that were inundatedonly by occasional floods. Thick units forming thesequence boundaries indicate a long period of fanabandonment and slow suspension sedimentation.Thin discontinuous gravelly sandstone sheetsbounded by flat base and top are indicative of sheet-flood deposits on distal fan surfaces.

Depositional Processes of CompositeSediment Flows

Facies Relationships. Individual sedimentationunits in the measured section consist of one to sev-eral facies units. The most commonly observedtype is graded and stratified conglomerate/sand-stone units, in which conglomerates (facies A–C)are either sharply overlain by sandstones (facies Dand E) or gradually decrease in clast size and con-tent toward the sandstone divisions (figs. 5–8). Theconglomerate divisions are generally thicker thanthe sandstone divisions. Three representative bedtypes (fig. 9) were obtained by summing up faciestransitions (fig. 10) and by comparison with real

120 Y . K . S O H N E T A L .

Figure 5. Photograph showing gradual facies transition from clast-supported conglomerate (facies B) to matrix-supported conglomerate (facies C) and crudely stratified pebbly sandstone (facies D). The facies C unit is wedgeshaped. For stratigraphic position, see figure 2. Hammer is 30 cm long.

occurrences in the outcrops. The relationship be-tween these bed types and individual facies couldbe determined by vertical facies transitions in sev-eral beds that are composed of more than four faciesunits (figs. 7, 8) as well as by the summation of allfacies transitions in the measured section (fig. 10).The facies sequence is tripartite, consisting of mas-sive and clast-supported conglomerates withmuddy or sandy matrix in the lower part (facies Aor B), sand-matrix-rich and normally graded con-glomerates and stratified gravelly sandstones in themiddle part (facies C and D), and thinly stratifiedor laminated sandstones in the upper part (facies E)(fig. 11). The facies sequence can be regarded as anidealized depositional sequence produced by a com-posite sediment flow that comprises a debris flow,a streamflow, and an intermediate flow conditionbetween. The facies sequence depicted in figure 10is only a vertical expression of the facies relation-ships that may exist more commonly as proximal-distal relationships in the field rather than as ver-tical successions in a single locality.

Organization of Flow Types. The facies relation-ships suggest that the responsible sediment flowscomprised different flow types, either across the

flow depth (vertically) or along the flow length (lon-gitudinally). We surmise that the flows were es-sentially longitudinally segregated, comprising apreceding debris flow and a trailing dilute flow (fig.12c). Such a flow segregation was documented bya number of workers (Beaty 1963; Okuda et al. 1980;Pierson 1980, 1986; Suwa and Okuda 1983; Suwa1988; Best 1992; Wan and Wang 1994). They notethat the flow head or the preceding debris flow hasa steep front and contains the densest slurry andthe coarsest particles. The flow head consumesmuch kinetic energy and hence moves slower thanthe main body, occasionally slowing the flow andallowing the slurry behind to pile up to great depths(Pierson 1980, 1986; Wan and Wang 1994). The flowbody is more fluidal than the head and changes intoa progressively more dilute tail that accounts forthe recessional limb of a slurry-flood wave. Thebody, having intermediate characteristics betweenthe preceding debris flow and the trailing waterflow, may be vertically segregated into a dense in-ertia layer and a dilute overlying flow as a resultof the meager yield strength of interstitial fluid (dis-cussed later). Such a vertical segregation seems un-

Journal of Geology D E B R I S A N D F L O O D - F L O W D E P O S I T S 121

Figure 6. Photograph of matrix-supported conglomer-ates (facies C) gradationally overlain by stratified sand-stones (facies D or E). Note the erosional lower contactof the facies C conglomerate. For stratigraphic position,see figure 2. Scale is 10 cm long.

stable and transient, being maintained as long asthe overlying flow is contained inside the high-relief front and lateral levees of the preceding debrisflow. Because there is a marked difference in mo-bility between the basal layer with high mechanicalstrengths and the overlying fluid layer, the over-lying fluid layer may spill over the debris-flow frontand levees and proceed in front of the underlyingbasal layer, once the relief of the debris-flow frontand lateral levees is subdued via lateral spreadingand deposition. The facies sequence in figure 11can thus be produced by a sediment flow with afull gamut of flow types segregated longitudinallyalong the flow length and partly across the flowdepth (fig. 12c).

There are several examples of longitudinally seg-regated flows, of which the organization of flowtypes is opposite to that implied by the facies se-quence. These flows were mostly initiated by tur-

bulent water floods and then transformed into de-bris flows (lahars) by incorporating sediments. Thedebris flows transformed farther downcurrent intohyperconcentrated flood flows (lahar-runout) bymixing of the leading edges of the debris flows withperennial streamflows (Pierson and Scott 1985;Smith 1986; Scott 1988; Pierson et al. 1990; Best1992; Scott et al. 1995). These flows therefore com-prised a peak flow of hyperconcentrated flood flowfollowed by a recessional debris flow (fig. 12a). Inthe resultant deposits, hyperconcentrated flood-flow deposits thicken downcurrent, whereas theoverlying debris flow deposits thin and disappeardowncurrent, producing an inversely graded tran-sitional facies between (figs. 12b and 13). The op-posite organization of flow types seems to have re-sulted from the difference in depositional setting.The debris flows documented in the above studieswere mainly initiated by catastrophic events andhad large volumes. They mostly flowed and depos-ited their sediments along a river valley where theywere subject to continuous mixing with ambientwater. The facies sequence in this study is, how-ever, applicable to noncatastrophic flows that hadalready passed through river valleys in catchmentareas. The flows spread out laterally and moved onunconfined alluvial-fan surfaces, where abundantwater for flow dilution is not available. Therefore,these flows had only a small chance to be dilutedat the leading edges, thus resulting in differentorganization of flow types and depositionalsequences.

Deposition from Debris Flows. Interpretation ofdebris-flow deposits has relied mainly on the vis-coplastic flow model (Johnson 1970, 1984) and theinertial grain-flow model (Takahashi 1978, 1981).Both models assume that the emplacement of adebris flow occurs via en masse freezing as the driv-ing shear stress drops below the yield strength ofthe viscoplastic substance or as the dispersive pres-sure drops and grains lock up frictionally. Althoughthe applicability of these models has been ques-tioned recently (Iverson 1997), many features of de-bris-flow deposits can be explained by these mod-els, including lobate geometry, basal or overallinverse grading, large floating or protruding clasts,high-angle or subvertical clast imbrication, verticalvariation in clast alignment, semicircular channelstructures, and the presence of large friable sedi-ment blocks and clast clumps (Johnson 1970, 1984;Naylor 1980; Nemec et al. 1980; Postma et al. 1983;Nemec and Steel 1984; Hiscott and James 1985; Casand Landis 1987; Nemec 1990; Sohn and Chough1992; Kim et al. 1995).

Facies A and B conglomerates, however, show

122 Y . K . S O H N E T A L .

Figure 7. Photograph showing a fining-upward unit consisting of facies A (not shown), C, D, and E. Note dispersedelongate clasts in the facies D unit, which are aligned parallel to the gradational facies boundaries. For stratigraphicposition, see figure 2. The pen is 13 cm long.

few of these features but have generally thin sheet-like geometry and parallel-aligned gravel clastsfrom base to top of units. These features can beexplained if deposition has occurred incrementallyfrom a debris flow. Recent experiments and fieldobservations show that a debris flow usually con-sists of a number of surges or roll waves, and itsdeposition occurs not by abrupt freezing of the en-tire flow but by incremental aggradation of indi-vidual surges (Li and Yuan 1983; Davies 1986, 1990;Wan and Wang 1994; Major 1997). Each surgeshoulders aside deposits of earlier surges, spreadsout, and leaves a deposit that is thinner and longerthan the surge itself. However, the cumulative de-posit thickness produced by a series of surges maygreatly exceed the average flow thickness. Depositsof earlier surges do not consolidate before the ar-rival of later surges, resulting in amalgamation ofmany surge layers into a single massive layer (Ma-jor 1997). A similar process of incremental depo-sition has also been observed in muddy debris flows(Li and Yuan 1983; Wan and Wang 1994), suggestingthat such a process is common in both cohesiveand cohesionless debris flows.

We interpret the facies A and B conglomerates tobe mainly incrementally aggraded materials, rather

than frozen masses of debris flows, and thus haveresulted in several peculiar features. The well-developed parallel alignment of clasts is probablyrelated to high rates of laminar shear straining ofindividual surges (Fisher 1971; Enos 1977) associ-ated with spreading or extensional deformationduring emplacement and subsequent smearing bylater surges before the deposits of earlier surges con-solidate. Combined with the incremental aggra-dation, successive shouldering aside of earlier de-posits by later surges prevents the protrusion oflarge clasts. Large clasts are therefore expected tobe found mostly along the front and lateral marginsof a deposit rather than on the top. An abruptlypinching-out bed composed of large boulders (fig.3) may be an example of clast concentration alongthe flow margin. The lack of basal or overall inversegrading may also be related to incremental depo-sition because vertical distribution of clast sizesthroughout a bed is determined by the grain sizedistribution in individual surges. Vallance andScott (1997) showed, for example, that normal grad-ing can be produced by incremental deposition ofa longitudinally segregated cohesive debris flow.We suggest that the development of pervasive par-allel-to-bedding clast alignment and the lack of

Journal of Geology D E B R I S A N D F L O O D - F L O W D E P O S I T S 123

Figure 8. Photograph of a fining-upward bed composedof clast-supported conglomerate with muddy matrix (fa-cies A; base not shown) and sand matrix (facies B) over-lain by thickly stratified pebbly sandstone with gravelbands (facies D) and laminated sandstone (facies E). Thisoutcrop is located 10 m apart from the measured sectionand correlated to the 28–30-m interval of the columnarsection of figure 2. Pencil is 13 cm long.

large floating clasts and inverse grading, in spite oftight clast support, are possible evidence for incre-mental deposition.

The process of incremental aggradation seems es-pecially accentuated in composite sediment flowsbecause different parts of a flow with different rhe-ologies are not likely to be deposited all at once intime and space but to form a composite bed throughseveral phases of deposition under different con-ditions. The generation of thin sheetlike geometryin debris-flow deposits seems also improved by thecomposite nature of a sediment flow. Researchershave observed that the more fluid and mobile flowbody pushes the head of less mobile debris flowforward (Pierson 1980, 1986; Wan and Wang 1994).This behavior may help the debris flow spread itsmaterials over broader areas compared with a sol-

itary or unified debris flow. Otherwise, the follow-ing flow may modify the deposits of the precedingdebris flow to produce thinner and broader deposits.It is suggestive that debris flow deposits associatedwith composite sediment-flow events show thinsheetlike geometry (e.g., Ballance 1984; Wells1984).

Deposition from Hyperconcentrated Flood Flows.Recognition of hyperconcentrated flow depositsfrom ancient sequences is hampered by the diffi-culties in properly estimating the flow properties,such as sediment concentration and rheology, onthe basis of deposit characteristics. It is thereforenecessary to infer the hyperconcentrated flow pro-cesses on an approximate and indirect basis. Therehave been many attempts to define hyperconcen-trated flood flows, using various criteria such assediment concentration (20–60 vol %; Beverage andCulbertson 1964), rheology (Pierson and Costa1987), transport and depositional processes (Smith1986), and a combination of energy slope and sed-iment concentration (Wan and Wang 1994). In gen-eral, these studies show that (1) a hyperconcen-trated flood flow is a two-phase or multiphase flow,in which solid particles (sand and gravel) and in-terstitial fluid (water, silt, and clay) behave inde-pendently (Wan and Wang 1994; Coussot and Meu-nier 1996); (2) interstitial fluid possesses only ameager yield strength and acts as a transportingmedium (Pierson and Costa 1987); and (3) the roleof turbulence can be important as a grain-support-ing mechanism (Smith 1986). Therefore, grains ofdifferent sizes can behave differently in a hyper-concentrated flood flow, leading to density strati-fication or bipartite division of the flow into a denseand coarse-grained lower part and a dilute and finer-grained upper part. Druitt (1995) demonstrated ex-perimentally that a bidispersion, having a solid con-centration between 20 vol % and 44 vol %,segregates into clusters of different grains, whichthen separate by gravitational convection.

Recognition of hyperconcentrated flow depositsfrom ancient sequences can thus be achieved byfinding evidence for bipartite division of a flow (e.g.,Todd 1989). This approach is indirect but may bemore practical in identifying supposed hypercon-centrated flood-flow deposits because criteria forhyperconcentrations, such as sediment concentra-tions and rheologies (viscosity and yield strength),can hardly be inferred from deposit characteristics.Furthermore, a single rheology or sediment con-centration is not sufficient to describe or charac-terize a hyperconcentrated flow that is prone to bebipartite. One has to remember, however, that abipartite flow may not necessarily be in a hyper-

124 Y . K . S O H N E T A L .

Figure 9. Three representative bed types common in the middle sequence (bed type I), the upper sequence (bedtype II), and the lower sequence (bed type III). They are composed of relatively thick conglomerate divisions overlainby thin sandstone divisions.

concentrated condition sensu stricto (Beverage andCulbertson 1964) because a bipartite flow can begenerated by an excess of sediment supply towardthe flow base relative to the deposition rate, irre-spective of the sediment concentration or rheologyof the flow (Sohn 1997).

The dense basal layer has a similar rheology andsimilar momentum transport processes to those ofa debris flow (Lowe 1982; Smith 1986; Smith andLowe 1991), leading some researchers (Shanmugam1996, 1997) to identify the basal layer with a debrisflow. There are some differences, however, such asin the source of shear stress, direction of granulartemperature dissipation, shape of velocity profile,vertical distribution of shear stress, and sedimentconcentration profile (Sohn 1997). It thereforeseems rash to identify the dense basal part of abipartite flow with a unified debris flow simply onthe basis that a single flow should not be consideredto represent multiple rheologies.

Some of facies C and D deposits in the Yongdongalluvial fan could be interpreted with some cer-tainty to be deposits of intermediate or hypercon-centrated flows because they are sandwiched be-tween debris-flow deposits (facies A and B) andstreamflow deposits (facies E) within apparentlysingle sedimentation units (e.g., figs. 7, 8). If thesedeposits were to occur solitarily, it would not beeasy to interpret them as hyperconcentrated flood-flow deposits because similar deposits can also beproduced by high-magnitude streamflows. Facies Cand D deposits are also interpreted to have resulted

from the dense basal part or traction carpet of abipartite flow. It is not possible to infer the overallsediment concentration or rheology of the flow, butthe flow is estimated to have had a sediment con-centration much larger than that of a normalstreamflow.

Depositional Setting of the CompositeSediment Flows

Description of Sequences. The measured sectioncan be divided into three sequences by meter-thickreddish mudstone beds (facies F): lower (0–22-m in-terval of the measured section), middle (24–46 m),and upper (48–55 m) sequences (fig. 2). The threesequences are comparable on scale to the “se-quence” of Heward (1978). Each sequence is dis-tinct in facies composition, clast size, and beddingpattern, as described below.

The lower sequence is characterized by thesmallest content and size of gravel clasts comparedwith the overlying sequences. It consists of alter-nations of disorganized conglomerates, decimetersto a meter thick, and stratified (gravelly) sand-stones, many of them forming graded and stratifiedunits. The conglomerates are generally sand ma-trix–supported, rest mostly on erosional surfaces,and show marked lateral variations in unit thick-ness. Gravel clasts are mostly granitic and thecoarse sand- to granule-sized fraction of the matrixappears to have been derived from the same graniticrocks. This sequence contains two coarsening-to-

Journal of Geology D E B R I S A N D F L O O D - F L O W D E P O S I T S 125

Figure 10. Number of facies transitions in the measured section

fining upward cycles (8–13 m and 18–22 m) de-marcated by decimeter-thick purple mudstones.Such cycles are suggestive of sporadic emplace-ment of coarse-grained sediments, probably byshifting debris-flow lobes across fan surface.

The middle sequence contains the most abun-dant and coarsest gravel clasts and is dominated byamalgamated disorganized conglomerates, 2–3 mthick, with a muddy or sandy matrix (fig. 2). Con-glomerate beds are bounded by slightly erosionalbases and relatively flat tops. Each depositionalunit can be discerned by the contrast in texture orby the presence of mudstone chips and lenticularsandstone interbeds. Pebble- to boulder-sized clastsare tightly packed, with elongate ones aligned par-allel to bedding planes. Clasts consist of granite(48%), slate (35%), and metasandstone (17%) frag-ments. Interbedded mudstones are rare and thinand tend to drape underlying conglomerate bedswith positive relief (near 43 m; fig. 3). Mudstonechips are relatively abundant and range in size fromgranule to boulder. They are in some cases cross-bedded (fig. 3).

The upper sequence consists of stacked, sheet-like conglomerates separated by (pebbly) sandstonewedges. It is characterized by fining-up units, 50–70cm thick, composed of clast-supported conglom-erates with sandy matrix and overlying stratifiedconglomerates or pebbly sandstones (fig. 2). Thestratified deposits compensate for local relief pro-duced by underlying conglomerates and are com-monly scoured by overlying conglomerates. Theconglomerates occur mostly as discrete beds, com-pared with the amalgamated beds of the middle

sequence. Clasts are better sorted and finer grained(cobble sized) than those of the middle sequence.

Development of Sequences. Contrasting deposi-tional sequences demarcated by thick interveningmudstone beds suggest several phases of aggrada-tion of coarse-grained deposits during alluvial-fanprogradation interrupted by migration (switching)and abandonment of an alluvial-fan segment (He-ward 1978). The vertical variations in sequencecharacteristics, such as overall clast size, bed thick-ness, and stacked nature, can be principally attrib-uted to the downfan and down-flow variations ofalluvial-fan processes (Heward 1978). It is generallyreported that hyperconcentrated and dilute flowsbecome more common toward the distal part of analluvial fan (e.g., Meyer and Wells 1995). Such atrend has been generally attributed to the differ-ences in mobility between solitary debris flows andwater flows but can also be produced by differentialdeposition from a composite sediment flow. It issupposed that the frontal debris-flow part of a com-posite sediment flow is deposited near the proximalpart of an alluvial fan, via either forming laterallevees or paving its materials along the flow path,whereas the more fluid rear part can continue tomove farther downfan.

Facies occurrences in the three sequences sug-gest that the coarsest sequence (the middle se-quence) dominated by mud-matrix debris-flowdeposits (facies A) represents the proximal allu-vial-fan facies, whereas the fine-grained lower se-quence dominated by hyperconcentrated flood-flow deposits (facies C and D) represents a moredistal facies. The coarsening- and thickening-up-

126 Y . K . S O H N E T A L .

Figure 11. Facies sequence obtained by the summation of facies transitions within individual sedimentation units(fig. 10) and comparison with real occurrences (e.g., figs. 7, 8), with brief descriptions and interpretations on the rightcolumns. The facies sequence represents an idealized sequence of facies that can be produced by a composite debrisflow–hyperconcentrated flood flow–streamflow event.

ward trend from the lower to the middle se-quences is therefore interpreted to represent pro-gradation of a proximal fan segment over distalfan deposits. The abrupt changes in clast com-position from granitic (lower sequence) to polym-ictic (middle sequence) are, however, interpretedto have been caused by the changes in drainagebasin characteristics—especially the source-rocklithology. The transition from the middle to theupper sequences is characterized by fining ofclast sizes and thinning of bed thicknesses butmost notably by the changes in the matrix typefrom muddy to sandy. There are only a few con-glomerate units with sandy matrices in the mid-dle sequence, whereas the upper sequence totallylacks conglomerates with a muddy matrix. Thefining- and thinning-upward trend from the mid-dle to the upper sequences is probably attribut-able to the retreat of an active fan segment, butthe abrupt changes in the matrix type are morelikely caused by the changes in drainage basincharacteristics.

Traditionally, alluvial fans have been divided

into fluvial dominated (or wet) and debris flowdominated (or dry), depending on prevalent depo-sitional processes (Bull 1972, 1977; Schumm 1977;Gloppen and Steel 1981; Kostaschuk et al. 1986).The alluvial-fan sequences in the Yongdong Basin,comprising deposits of both debris flows and diluteflows, have characteristics of both wet and dry al-luvial fans. This does not, however, mean that theYongdong alluvial fan experienced drastic changesin climatic or tectonic regimes. It is recognized thatboth types of alluvial fan can occur simultaneouslywithin a single geographical area because deposi-tional processes on an alluvial fan are primarilydetermined by local sediment types and flood hy-drographs of associated drainage basins (Kostas-chuk et al. 1986; Wells and Harvey 1987). An al-luvial-fan succession may also show considerabletemporal variations in constituent facies as its con-tributing drainage basin evolves. In this respect, sig-nificant stratigraphic variations in dominant faciesas well as in clast composition and matrix type canbe caused by changes in the drainage basin char-acteristics—especially the source rock lithology, re-

Journal of Geology D E B R I S A N D F L O O D - F L O W D E P O S I T S 127

Figure 12. Comparison of two contrasting styles of debris flow–hyperconcentrated flood flow–streamflow events.a, A hyperconcentrated flood flow is generated by the dilution at the leading edge of a debris flow that entered ariver valley. The resultant sediment flow therefore comprises a preceding hyperconcentrated flood flow, followed bya recessional debris flow. As the dilution front propagates behind a debris flow, the entire flow may transform intoa hyperconcentrated flow (Pierson and Scott 1985; Smith 1986, 1987b; Scott 1988). b, The resultant deposits fromthis sediment flow consist of downstream-thickening (and eventually thinning) hyperconcentrated flood-flow depositsoverlain by upstream-thickening debris-flow deposits. Four columns, showing representative facies in proximal todistal parts, are given. c, This study suggests a different organization of flow types with a debris flow preceding inadvance of a hyperconcentrated flood flow and streamflow. d, The resultant deposits are envisaged to comprise debris-flow deposits in the proximal part, which are successively overlain and replaced by hyperconcentrated flood flow andstreamflow deposits toward the downstream direction. Four representative columns are given. The transition facies(col. 2) is markedly different from that of b. The two models may differ greatly in time scale and dimension andshould not be compared on a footing of equality.

lief, and drainage density. Changes in these factorsresult in different flood hydrographs (e.g., flashy vs.sluggish) and different transport modes of (coarse-grained) sediments from the catchment (Ritter etal. 1995). The changes in clast composition and ma-trix type in the three sequences of the measuredsection were most likely produced by successiveexposure of different source rocks, caused by retreatof an alluvial fan and lowering of a drainage basin

following repeated tectonic uplift, which then ledto changes in the nature of sediment supply withtime. Sediments of the lower and upper sequences,mostly lacking a mud fraction, were probablydrained from large and less rugged catchments,whereas those of the middle sequence dominatedby mud-rich debris-flow deposits were from smalland steep (rugged) catchments (Kostaschuk et al.1986; Allen and Hovius 1998).

128 Y . K . S O H N E T A L .

Figure 13. Photograph of 1980 Mount St. Helens lahar deposit exposed along the North Fork Turtle River, Wash-ington. The lower part of the deposit is finer-grained and inversely graded and erosionally overlies pre-1980 muddeposits (below the dotted line). This part is interpreted as being composed of deposits of a hyperconcentrated floodflow (lahar runout) that proceeded a debris flow. The gravelly debris-flow deposit gradually overlies the hypercon-centrated flood-flow deposits, forming an overall inversely graded “transition” facies. The pencil is 17 cm long.

Implications of the Composite Sediment Flow Depos-its. There is a close association between the ma-trix type of conglomerates and the depositional pro-cesses in the measured section. The conglomeratesin the lower and upper sequences, mostly havingsandy matrices, are commonly intercalated withhyperconcentrated flood-flow deposits (facies C andD), whereas the conglomerates in the middle se-quence, which comprise muddy matrices, generallylack interbeds of dilute-flow deposits. This asso-ciation suggests that sand-matrix debris flows aremore prone to transform into hyperconcentratedflows. In general, noncohesive flows tend to trans-form easily into dilute flows via dilution, whereascohesive flows remain debris flows to their termini(Pierson 1986; Scott 1988; Vallance and Scott 1997).The availability of silt-clay fractions in the drainagearea therefore seems important in determiningtypes and spacial distribution of facies.

Flow behavior of debris flows from a drainagebasin to its associated fan is largely governed bylongitudinal profiles of tributaries because thechannel slope regulates transportation forces (trac-

tive force and the slope-direction component of sed-iment weight) that affect runout distances and ge-omorphic processes (erosion and deposition) of thedebris flows (Ohmori and Shimazu 1994; Cenderelliand Kite 1998). In channels of an exponential func-tion type, large curvature may significantly de-crease transportation forces downstream, and thusupstream debris flows cannot runout to the gentledownstream segment (alluvial-fan surface) unlessthey are transformed into fluid, turbidity flows(Ohmori and Shimazu 1994). According to Ohmoriand Shimazu (1994), tributaries of an exponentialfunction type develop in a less rugged drainage ba-sin. We infer that the facies variations among thethree sequences reflect the changes in relief devel-opment and thus the changes in the longitudinalprofiles of tributaries in the catchment area withtime.

According to recent experiments on cohesionlessdebris flows (Major 1997), temporally distinct de-bris flows as well as contemporaneous ones pro-duce mostly amalgamated beds without distinct in-

Journal of Geology D E B R I S A N D F L O O D - F L O W D E P O S I T S 129

ternal boundaries that demarcate flow waves/events even though their deposits are emplaced byincremental sedimentation rather than en massefreezing. Similar amalgamated conglomerates (fa-cies A and B) are contained in the middle sequenceof the measured section. They, however, commonlyshow textural variations in vertical section (figs. 2,3). Relatively distinct internal contacts and prev-alent clast alignments of the debris-flow depositsmight have resulted from prolonged incrementalsedimentation of a number of longitudinally well-sorted debris flows (Major 1997). That is, they mayresult from discrete waves of debris flows ratherthan from closely spaced surges of a single flowevent. Such a distinction of debris flows is circum-stantially supported by the facies successions (figs.9, 11) obtained from the measured section, repre-senting a composite debris flow–hyperconcentratedflood flow–streamflow event. In this respect, pres-ence or absence of the facies succession can be anindirect measure in determining whether flowunits of debris-flow deposits can be correctlyrecognized.

Conclusions

Alluvial-fan deposits in the Cretaceous YongdongBasin, Korea, consist of conglomerates and sand-stones deposited by debris flows, intermediateflows, and dilute flows. Some of the conglomeratesand sandstones constitute single sedimentationunits, suggestive of composite sediment-flow pro-cesses. The composite sediment flows are inferredto have been longitudinally segregated into a pre-ceding debris flow and a trailing dilute flow, withan intermediate flow between. The deposits fromthe preceding debris-flow part of the compositeflow are distinguished from the deposits of solitarydebris flows in that they have pervasive parallelalignment of clasts and thin sheetlike geometry butlack basal inverse grading, large floating clasts, andother features of classical debris-flow deposits.These differences were most likely produced by in-

cremental aggradation of debris flows and modifi-cation of debris-flow deposits by later surges ofcomposite sediment flows. The deposits of inter-mediate flows show evidence for deposition frombipartite flows that comprise dense inertia layersor traction carpets beneath dilute and turbulentflows. We suggest that the bipartite flow can beapproximately equated with a hyperconcentratedflow based on the fact that a hyperconcentratedsuspension has a meager yield strength and is proneto be density stratified. The relationship betweendensity stratification and rheology (viscosity, yieldstrength, and sediment concentration) of a flowneeds to be established.

As far as can be judged from exposures of theYongdong alluvial-fan succession, deposition fromthe composite sediment flows produces spatiotem-poral variations in facies composition both as a re-sult of an alluvial-fan evolution and as a result offlow transformation over travel distances: thelower sequence dominated by sandy-to-gravelly hy-perconcentrated flow deposits mostly with graniticclasts, the middle sequence dominated by coarse-grained debris-flow deposits with mud matrix andpolymictic gravel clasts, and the upper sequencedominated by sand-matrix debris-flow deposits.The coarsening to fining of the sequences reflectsprogradation and retreat of an alluvial fan, but thechanges in clast composition and matrix type morelikely resulted from the changes in the charactersof supplied sediments as different source rockswere exposed in the drainage area.

A C K N O W L E D G M E N T S

This research was supported through grants to Y.K. Sohn (project BSRI-96-5402) and C. W. Rhee(BSRI-96-5417) by the Basic Science Research In-stitute Program, Ministry of Education, Korea. Thisarticle benefited from helpful review and com-ments by an anonymous reviewer, as well as com-ments by and discussions with S. B. Kim and J.Major.

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