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DEPOSITIONAL PROCESSES OF SUBMARINE DEBRIS FLOWS IN THE MIOCENE FAN DELTAS, POHANG BASIN, SE KOREA WITH SPECIAL REFERENCE TO FLOW TRANSFORMATION YOUNG K. SOHN Department of Earth and Environmental Sciences, Gyeongsang National University, Chinju 660-701, Korea e-mail: [email protected]

ABSTRACT: Subaqueous debris flows undergo various flow transformations, involving dilution and stripping of surface materials, penetration of ambient water into the flow interior, and detachment and disintegration of hydroplaning flow fronts. The surface transformation is a self-limiting process because the products of the process, such as an overriding suspended-sediment cloud or an armor of gravel at the flow front, inhibit effective working of the process. The degree of flow transformation therefore depends largely on whether a debris flow hydroplanes or not. For a subaqueous debris flow to hydroplane, its densiometric Froude number should be larger than 0.4, and the time scale of pore-pressure decay should be larger than the duration of a debris flow. In addition, a debris flow should be devoid of an extremely permeable girth of openwork gravel around the flow head because high pressures cannot be sustained underneath the gravelly material. Detailed sedimentological measurements and estimation of flow properties for three debris-flow beds in the Miocene fan deltas in SE Korea suggests that only a pebbly debris flow with a muddy (impermeable) matrix hydroplaned. On the other hand, bouldery debris flows are interpreted to have not hydroplaned irrespective of the nature of matrix. Nonhydroplaning debris flows were subject mainly to surface transformation and were outrun by surface-transformed suspendedsediment flows and debris-fall blocks after the flows entered a base-ofslope setting. Deposits of nonhydroplaning debris flows therefore overlie deposits of turbidity currents and debris falls. In the case of a hydroplaning debris flow, large chunks of debris could be detached from the flow front repetitively to form a series of small-volume flows that proceeded in front of the host debris flow. The preceding flows were promptly diluted to produce voluminous suspended-sediment clouds and were outrun by the faster-moving host debris flow. A deposit from a hydroplaning debris flow is therefore associated with thick and extensive turbiditic deposits that may either underlie or overlie the host debris-flow deposit. The turbiditic deposits associated with a hydroplaning debris flow are distinguished from those of a nonhydroplaning debris flow in that the former contain abundant gravel clasts and chunks of poorly sorted and clast-rich debris that cannot be suspended by the surface transformation process but were more likely derived from the detached fronts of a hydroplaning debris flow. These differences in sediment volume and grain size of turbiditic deposits and the stacking pattern of related debrites and turbidites provide a clue to the behavior of subaqueous debris flows.

INTRODUCTION

Debris flows are gravity-driven, high-concentration flows of sediment and water. There is a wide spectrum of debris flows, of which the mudrich and clast-rich end members can be modeled as a viscoplastic fluid (Johnson 1970, 1984) and a dilatant granular substance (Takahashi 1978, 1981), respectively. Most debris flows are, however, intimate mixtures of solid and fluid, involving both grain–grain and grain–fluid interactions (Hutter et al. 1996; Iverson 1997). Debris-flow processes therefore depend on the properties of grains (size, density, and volume fraction), interstitial fluid (density, viscosity, and volume fraction), and their mixtures (flow thickness, velocity or shear strain rate, and permeability) (Sohn 2000). Some of these properties change almost continuously as the slope gradient JOURNAL OF SEDIMENTARY RESEARCH, VOL. 70, NO. 3, MAY, 2000, P. 491–503 Copyright q 2000, SEPM (Society for Sedimentary Geology) 1073-130X/00/070-491/$03.00

and channel width change along the flow path and as sediment and water are incorporated into or removed from a debris flow (Fisher 1983; Smith and Lowe 1991). Of these factors, interaction with ambient water is the prime cause of flow transformation. For example, flow transformation of subaerial debris flows is usually caused by dilution at the leading edge of a debris flow as the flow encounters a perennial streamflow (Pierson and Scott 1985; Scott 1988; Smith and Lowe 1991; Best 1992). The flow is then transformed into a longitudinally segregated flow composed of a preceding hyperconcentrated flow and a trailing debris flow, resulting in a depositional sequence composed of stratified and finer-grained hyperconcentrated flow deposits and overlying massive and coarser-grained debrisflow deposits (Sohn et al. 1999). Compared with subaerial debris flows, conditions for and processes of flow transformation are far less understood in subaqueous debris flows. This is probably because natural debris flows in motion are rarely observable in subaqueous environments, and even experimental debris flows have been only scarcely studied in underwater conditions (e.g., Van der Knaap and Eijpe 1968; Hampton 1972; Mohrig et al. 1998; Mohrig et al. 1999). It can be assumed, however, that the flow transformation is much more likely in subaqueous environments because the entire surface of a debris flow is always in contact with ambient water and is subject to large flow resistance (Norem et al. 1990). Recent experiments by Mohrig et al. (1998) also show that the frontal part of a subaqueous debris flow can hydroplane over a wedge of ambient water sandwiched between the substrate and the overriding debris flow. The hydroplaning front has a high mobility and can be detached from the debris-flow body and disintegrate. Despite these favorable conditions for flow transformation, only a few examples have been reported from ancient subaqueous successions, from which processes of flow transformation can be inferred (Krause and Oldershaw 1979; Souquet et al. 1987; Sohn et al. 1997; Falk and Dorsey 1998). This is probably because flow transformation is recorded mostly as proximal–distal facies relationships (e.g., Chough et al. 1985; Piper et al. 1985) rather than as vertical facies sequences. In addition, unambiguous evidence for flow transformation is difficult to come by in vertical sequences because different lithofacies units produced by a single depositional event can commonly be misinterpreted as deposits of separate independent flows. Several mass-flow deposits in the base-of-slope setting of the Miocene fan deltas, SE Korea, provide an opportunity to investigate the processes of flow transformation in subaqueous debris flows. Each of these examples is composed of a meter-thick unit of conglomerate and overlying and/or underlying units of decimeter-thick sandy deposits. They occur as lenticular bodies that are encased in homogeneous hemipelagic mudstone, suggesting that the deposition of gravelly and sandy units was intimately associated in time and space, probably involving single events with multiple types of sediment gravity flows. Such composite flows are interpreted to be the result of flow transformation of subaqueous debris flows. This paper describes the characteristics of the composite mass-flow deposits, discusses the processes and conditions of flow transformation, and infers the development of depositional sequences in subaqueous debris flows. GEOLOGICAL SETTING

The debris-flow deposits described in this paper are from the submarine fan deltas in the Miocene Pohang Basin, SE Korea (Choe and Chough 1988; Hwang et al. 1995; Sohn et al. 1997; Sohn 2000, Sohn et al. un-

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FIG. 1.—Location and geologic maps of the Pohang Basin. A) The Pohang Basin is located along the eastern continental margin of Korea. The basin formed by pullapart along two strike-slip faults during the back-arc opening of the East Sea (Sea of Japan) (Yoon and Chough 1995). The basement escarpment was the principal displacement zone, along which the 2000-m-deep Ulleung back-arc basin was opened. B) Enlargement of study area showing outlines of fan-delta systems. The nonmarine and shallow-marine sediments accumulated prior to 17 Ma, when the eastern continental margin of Korea experienced strike-slip deformation, whereas the fan-delta conglomerates and the overlying sequences formed after the downfaulting of the Pohang Basin associated with the cessation of the strike-slip deformation (Kim 1999; Lee et al. 1999; Sohn et al. unpublished work). The nonmarine strata were severely block-faulted during the tectonic event whereas the overlying sequences are undeformed, preserving the original depositional slope angles of strata. Circles indicate the locations of measured deposits (Beds A, B, and C). Arrows superimposed above the circles represent the flow direction of debris flows determined by bed attitudes and trends of chute axes.

published work). The Pohang Basin is the largest Tertiary basin in South Korea, produced by pull-apart during the back-arc opening of the East Sea (Sea of Japan) (Yoon and Chough 1995) (Fig. 1A). Nonmarine and shallow marine (,50 m deep) sediments accumulated in the basin during the early Miocene until a bathyal (.500 m deep) condition was produced by a downfaulting of the basin floor ca. 17 Myr ago (Kim 1999; Lee et al. 1999; Sohn et al. unpublished work). Several fan deltas began to develop along the western margin of the basin where a high-gradient slope was produced and the rate of sediment supply increased drastically from the uplifted footwall. Sedimentation in the basin continued until ca. 10 Ma (Lee et al. 1988), when the basin was regionally uplifted because of the onset of contractile deformation along the southern part of the East Sea (Jolivet et al. 1994). During the ca. 7 Myr period of deep-marine sedimentation, the fan deltas were subject to several cycles of sea-level fall and rise, resulting in development of lowstand depositional systems (submarine fans) at the foot of some fan deltas (Sohn et al., unpublished work). The debris-flow deposits described in this paper (designated as Beds A to C) originated from the Maesan and Gohyeon fan deltas (Fig. 1B). The

Maesan fan delta consists of ramp-type delta-slope deposits, dipping at ca. 108, whereas the Gohyeon fan delta is of Gilbert type, comprising flat-lying topsets and steeply inclined (20–308) foresets. Beds A and B, derived from the Maesan fan delta, are composed mainly of granitic clasts and rich in cobble- to boulder-size clasts, whereas Bed C, derived from the Gohyeon fan delta, is rich in pebble-size clasts composed of sedimentary and volcanic rocks. The measured deposits occur near the outer margin of these fan deltas, mostly beyond the high-gradient realms. They have a lenticular (chute-filling or channel-filling) geometry and are encased in homogeneous hemipelagic mudstone. COMPOSITE MASS-FLOW DEPOSITS

Bed A: Pebble–Boulder Conglomerate with Muddy Sand Matrix Description.—Bed A consists of two layers: a sandstone layer, 30–65 cm thick, at the base and a conglomerate layer above it, which is up to 4 m thick (Fig. 2A). The bed is overlain and underlain by homogeneous hemipelagic mudstone and has a wedge-shaped geometry, thickest near the

DEPOSITIONAL PROCESSES OF SUBMARINE DEBRIS FLOWS right part and pinching out toward the left. The measured outcrop is transverse to flow direction and extends ca. 80 m. The conglomerate consists of clast-supported pebble-to-boulder gravel, of which the maximum clast is 1.6 m in the long axis. The matrix is composed of poorly sorted granule and sand that contains 5–8 wt. % mud. In the right part of the conglomerate (Figs. 2B, 3), large gravel clasts are concentrated in indistinct bands, probably formed by superposition of inversely graded subunits that are pebbly at the base and bouldery at the top (vertical arrows; Fig. 2B). The gravel clasts are either flat-lying or imbricated. The imbrication is accentuated toward the right margin. In the central part (Fig. 2C), the conglomerate is inversely graded from clast-supported fine pebble at the base to clast-supported cobble and boulder at the top with large protruding clasts. Internal layering and clast imbrication are lacking in this part. The conglomerate gradually thins and terminates toward the left. Figure 2D shows an isolated pod of gravel at the left margin of the conglomerate. The gravel pod has a mound-like geometry with a relatively flat base and a convex-upward top. It is composed of clast-supported cobble and boulder gravel with scarce pebble gravel. The interstices are occupied by mud that is indistinguishable from the underlying and overlying mudstone, suggesting that the interstices were originally empty but later infiltrated by the background mud. The sandstone bed beneath the conglomerate is laterally discontinuous, occurring only in the middle to right part and abruptly terminating before reaching the right margin of Bed A (Fig. 2A). The lower contact is sharp and irregular because of erosion and penecontemporaneous deformation, whereas the upper contact is relatively planar (Fig. 3). The sandstone is coarse-tail normally graded from pebbly sandstone at the base to sandstone at the top in the central part of the exposure (Fig. 2C) and inversely-tonormally graded in the right part (Fig. 2B). The sandstone contains several cobble–boulder clasts (max. 70 cm in the long axis) that are identical in lithology (granite) to those in the conglomerate (Figs. 2B, 3). Interpretation.—Poor sorting, inverse grading, large protruding clasts, and abrupt lateral termination in the conglomerate are characteristic features of debris-flow deposits (Johnson 1970, 1984). The imbrication in the right margin of the bed is probably due to compressional shear straining of debris during emplacement (Nemec 1990). The wedge-shaped geometry of the conglomerate without a prominent scour suggests passive filling of a preexisting depression by a sluggish debris flow. Internal layering in the conglomerate (Fig. 2B) was probably produced by stacking of several debrisflow surges, as observed in natural and experimental debris flows (Li and Yuan 1983; Davies 1986, 1990; Wan and Wang 1994; Major 1997). Restricted occurrence of the layering suggests that a different number of surges arrived at different parts of the exposure, probably controlled by preexisting topography. The pod of gravel clasts (Fig. 2D) is interpreted to comprise coarse clasts that were collected to the flow front and then pushed aside by the following part of debris flow to form a lateral levee. Levees generally consist of the coarsest gravel in a debris flow and commonly show openwork textures (Whipple and Dunne 1992; Iverson 1997; Major 1997). The grain size and texture of the gravel pod are thus similar to those of levee materials. The sandstone layer beneath the conglomerate is interpreted as a deposit of a dilute, fine-grained flow that proceeded in front of the gravelly debris flow at the site of deposition. The irregularly scoured lower contact and normal grading are suggestive of the turbulent and erosive nature of the flow. The inverse-to-normal grading in the right part of the layer (Fig. 2B) was probably produced by transport lag of coarse particles (Hand 1997) rather than by inertial grain interactions in a dense dispersion. Several outsize clasts are too large and exotic to be transported solely by a dilute turbulent flow. They were more likely transported by rolling, sliding, and saltation under the pull of gravity and partly aided by the coexistent turbulent flow. These clast movements can be called ‘‘debris fall’’ (Holmes 1965; Nemec 1990; Kim et al. 1995; Sohn et al. 1997). The coarse clasts most likely were supplied from the top and front of a debris flow because

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gravel clasts concentrated at the flow front and protruding above the flow top are very susceptible to remobilization (Sohn et al. 1997). Bed B: Cobble-Boulder Conglomerate with Sand Matrix Description.—Bed B rests on homogeneous hemipelagic mudstone with an overall concave-up lenticular geometry (Figs. 4, 5). The bed is about 3 m thick in the left half and gradually pinches out toward the right with a lateral extension of about 35 meters. Figure 4 shows an almost complete cross section of the bed cut roughly perpendicular to the flow direction. The lower contact of the bed is erosive and forms a 3-m-deep, overhanging channel wall at the left margin (Fig. 5A). At the right margin, where the bed is less than 1 m thick, it also rests on a deeply scoured surface (Fig. 4). The bed consists mostly of cobble- and boulder-size clasts and very rare pebble clasts. The matrix consists of moderately sorted, medium- to coarse-grained sand. The bed has an internal discontinuity in the middle part, which is dipping toward the left and marked by a contrast in gravel content (arrows; Fig. 4). The conglomerate above the discontinuity is clast-supported and rich in meter-size boulders. The conglomerate is inversely graded from tightly packed cobble gravel to loosely packed boulder gravel and is characterized by protrusion of large clasts (Fig. 5). Although the conglomerate is cobbly in the lower part, a few large boulder clasts are present at the base, lying directly on the mudstone substrate or contained in the basal sandstone layer (Fig. 5B). A decimeter-thick sandstone layer is present beneath the conglomerate (Figs. 4, 5B). It is composed of moderately sorted medium to coarse sand that is identical to the matrix of the conglomerate. The sandstone is massive, ungraded, and laterally discontinuous. The conglomerate below the discontinuity is finer-grained and matrix-rich, and is overlain by a thin sand layer and a mud drape. The sandstone layer is slightly finergrained than the matrix of the conglomerate. Interpretation.—The lenticular geometry of the conglomerate with an overhanging margin suggests either deep scouring or modification of a preexisting chute by the debris flow. This process may have been augmented by rapid fluctuations in shear and normal stresses associated with grain collisions at the base of the bouldery debris flow. The asymmetry of the channel cross section is reminiscent of a river channel with a cutbank on one side. It seems possible that the debris flow moved along a curved chute, shifting its center of flow toward the outer (left) margin of the bend. The inclined discontinuity (arrows; Fig. 4) is probably indicative of lateral migration of the center of flow toward the cutbank as well as aggradation from a series of surges. The sand deposit, resting on a scoured surface, suggests that a turbulent and highly erosive flow proceeded in advance of the gravelly debris flow. A few boulder clasts lying directly on the mudstone substrate (Fig. 5B) may be deposits of a debris fall, which occurred simultaneously with the turbulent flow. Bed C: Pebble Conglomerate with Muddy Sand Matrix Description.—Bed C is encased in homogeneous mudstone with an overall concave-up lenticular geometry. The measured outcrop (Fig. 6) is nearly flow-transverse and extends laterally about 120 m along a valley wall. The bed is composed of pebbly sandstone, less than 1 m thick in the left part (first and second rows in Figure 6), which passes laterally into a tripartite bed, 2–3 m thick, that is composed of a sandy or fine gravelly division at the base, a pebble conglomerate division in the middle, and a pebbly sandstone division at the top (Fig. 7A). The pebbly sandstone in the left is normally graded and has an irregular lower contact because of scour and loading (Fig. 6). The fine gravel clasts are randomly oriented, and stratification is not developed. The lower part of the sandstone contains ca. 50 wt. % gravel, consisting mainly of granule and fine-pebble gravel, whereas the upper part contains ca. 15 wt. % gravel (mostly granule). The sand is normally graded from very coarse sand at

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FIG. 2.—Drawings of Bed A. A) Overview of Bed A, composed of a basal sandstone layer and an overlying conglomerate unit. The conglomerate has a wedge-shaped geometry, abruptly pinching out toward the left (northwest) and thickening toward the right (southeast). Flow direction is into the page. Details of abruptly terminating left part, inversely graded central part, and internally layered right part are shown in enlarged sketches below. B) Rightmost part of Bed A, characterized by internal layering in the conglomerate, which disappears toward the right. The internal subunits (boundaries indicated by solid lines) are inversely graded (vertical arrows). There is a discontinuous sandstone layer containing several outsize clasts beneath the conglomerate with erosional scours at base. C) Inversely graded middle part of Bed A conglomerate with large protruding boulders. The conglomerate is underlain by a normally graded sandstone layer. D) A mound-like gravel pod at the left margin of Bed A. The interstices of the gravel pod are filled by hemipelagic mud that is identical to the surrounding mudstone.

the lower part to medium to coarse sand at the upper part. A small portion of the deposits is, however, ungraded either in the gravel fraction or in the sand fraction (left part of first row in Figure 6). The ungraded portion contains more abundant gravel clasts and a large mudstone chip in a more poorly sorted (muddier) matrix, distinguished from the rest of the deposits. It is also noteworthy that gravel clasts are inversely-to-normally graded on both sides of the ungraded deposit. The pebbly sandstone gradually thickens and coarsens toward the right, transforming into a thick-bedded tripartite bed (third row in Figure 6). The conglomerate division of the tripartite bed is composed of clastsupported pebble-to-cobble gravel with rare boulders (Fig. 7A). The gravel clasts show variable grading patterns, either inverse-to-normal grading, basal or overall inverse grading, or normal grading (Fig. 6). The clasts generally lack preferred orientation, but some of large clasts are aligned parallel to the bedding plane. The lower contact is irregular because of scour and loading. The matrix of the conglomerate generally consists of poorly sorted muddy sand but shows marked variations in grain-size characteristics both vertically and laterally. In the major part of the conglomerate, the matrix is muddier in the lower half, containing ca. 10 wt. % mud, and becomes less muddy upward, containing ca. 5–8 wt. % mud. In other parts of the conglomerate, the matrix is sandy throughout the unit. The finer-grained division beneath the conglomerate is decimeters thick and can be divided into two subunits (S1 and S2 in Figure 7B). They are laterally discontinuous, occurring only in two places and extending less than 5 meters (third row in Figure 6). The lower subunit (S1) consists of granules and coarse sand lacking large gravel clasts, whereas the upper subunit (S2) contains abundant granules and pebble clasts in a poorly sorted

(but not muddy) coarse sand matrix. The upper subunit is distinguished from the overlying pebble conglomerate by a contrast in matrix grain size. Both subunits are massive and ungraded and bounded by prominent erosional contacts. The pebbly sandstone division above the conglomerate (S3 in Figure 7A) is a few decimeters to 1 m thick and consists of poorly sorted medium to coarse sand and sparse pebble to boulder clasts. The gravel clasts are floating in the middle of or protruding above the layer, resulting in local inverse or inverse-to-normal grading (Fig. 6). The layer is, however, mostly massive. The contact between the layer and the underlying conglomerate is generally gradational. The contact is locally deformed by large flame structures (third row in Figure 6). Interpretation.—The conglomerate is characterized by lateral variations of grading patterns. These variations were probably caused by lateral variations in the material properties of the debris flow, as manifested by the lateral heterogeneity of the matrix grain size. The lateral heterogeneity of matrix may be due to either incomplete mixing of a heterogeneous parent material during transport or nonuniform removal of fines from the debrisflow matrix associated with localized incorporation of ambient water and elutriation of fines by escaping interstitial fluid. On the other hand, the vertical change of matrix grain size from muddy to sandy in some part of the conglomerate is interpreted to be due to vertical aggradation from a longitudinally segregated flow that comprises a muddier frontal part and a sandier rear part (Vallance and Scott 1997; Sohn et al. 1999). Development of flame structures indicates that the debris-flow deposit was not immediately consolidated after deposition, having a high excess pore pressure and

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FIG. 2.—Continued.

FIG. 3.—Photograph of the right part of Bed A. The contacts between the conglomerate (G), the basal sandstone layer (S), and the underlying mudstone (M) are indicated by broken and solid lines. Outsize boulder clasts in the sandstone layer are indicated by arrows. The conglomerate is 4 m thick. See Figure 2A for location of the photograph.

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FIG. 4.—Drawing of Bed B, characterized by abundant boulder clasts and deep scour into the underlying mudstone. The conglomerate has an overhanging channel wall at the left margin and an inclined discontinuity at the central part. The conglomerate is inversely graded in the left part from tightly packed cobble gravel in the lower part to loosely packed boulder gravel in the upper part. There is a thin sandstone layer beneath the conglomerate. Flow direction is into the page.

being liquefied by the loading of the overlying sand deposit (Major and Iverson 1997). The sandy and fine pebbly deposits beneath the conglomerate are interpreted to have resulted from finer-grained and more dilute flows that proceeded in front of the gravelly debris flow. The lower subunit (S1; Fig. 7B), which is composed mostly of sand, was probably deposited by a dilute and turbulent flow whereas the upper subunit (S2; Fig. 7B), which is poorly sorted and contains abundant fine pebble clasts, was deposited by a higherconcentration flow. Both of these flows are interpreted to have been erosive, forming scoured lower contacts. These flows deposited their sediment very rapidly, resulting in massive and ungraded deposits. The pebbly sandstone layer above the conglomerate (S3; Fig. 7A) is interpreted to be deposits of a turbidity current generated by various flowtransformation processes of the gravelly debris flow. Sparsely scattered cobble- to boulder-size clasts, occasionally protruding above the top of the layer, are interpreted as deposits of debris-fall processes, which operated simultaneously with the turbidity current. The outsize clasts occur mostly above the inversely graded portions of the conglomerate, suggesting that the clasts were derived from the protruding clasts lying on the inversely graded portions of the debris-flow deposit. The clasts were probably transported along the flow path, moving only a short distance across the flow path. The normally graded pebbly sandstone to the left of the conglomerate (first and second rows in Figure 6) is also interpreted to be deposits of a turbidity current generated by various flow-transformation processes. The ungraded deposit is, however, interpreted as an incompletely disintegrated block of debris that was detached from the host debris flow. The inverseto-normal grading on both sides of the ungraded deposit can be explained by localized supply of gravel clasts from the detached block during deposition of the turbidity current.

CONDITIONS FOR FLOW TRANSFORMATION

Types of Flow Transformation Flow transformation refers to changes in the physics and material properties of a sediment flow across the flow depth, width, and length caused by variations in substrate geometry, such as slope angle and channel width, and by incorporation, removal, or internal segregation of materials (Fisher 1983; Smith and Lowe 1991). Interaction of a flow with its ambient fluid seems especially important as a cause of flow transformation, leading not only to changes in solid and fluid fractions but also to changes in the physics and rheology of the flow, eventually resulting in turbulent–laminar transitions and spatial segregation of materials. Possible types of interaction with ambient fluid in subaqueous debris flows include (1) dilution and stripping of surface materials (Hampton 1972), (2) penetration of water into the interior of a flow through clefts developed along the flow front (Allen 1971; Britter and Simpson 1981; Simpson and Britter 1982), and (3) detachment and disintegration of a hydroplaning flow front (Mohrig et al. 1998). The surface transformation (process 1) reduces the total volume of a debris flow to the benefit of suspended-sediment clouds or currents generated above it, but does not appear to change significantly the material properties of a debris flow. Hampton (1972) demonstrated that the surface transformation is accomplished mainly by removal of debris from the flow front and ejection of it into the overlying water. He also showed that the degree of debris removal is insignificant behind the flow front in small experimental flows. Removal of debris may be inhibited even from the flow front if large gravel clasts are concentrated there and form a resistant armor enveloping the snout of a debris flow. The frontal concentration of gravel clasts can be produced not only by several clast-focusing mechanisms (Bagnold 1968; Suwa 1988) but also by the surface transformation

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FIG. 5.—A) Left part of Bed B conglomerate (G), showing a deep scour into the underlying mudstone (M), which forms an overhanging margin (arrowed). The conglomerate is inversely graded from tightly packed cobble gravel in the lower part to loosely packed boulder gravel in the upper part. B) Closer view of inversely graded Bed B conglomerate (G) with large boulders protruding into the overlying pebble conglomerate (PG). There is a thin discontinuous sandstone layer (S) beneath the Bed B conglomerate. A few boulder clasts (arrowed) lie directly above the mudstone substrate or are contained in the sandstone layer. Hammer (circled) for scale. See Figure 4 for location of the photographs.

itself because this process selectively removes fine grains and leaves behind coarse grains. Thus, many factors suppress the effective working of surface transformation, making it a self-limiting process. Process 2 may be an effective mechanism of flow dilution that can substantially alter the material properties of a debris flow by increasing the volume but decreasing the strength or viscosity of the debris. A flow may be vertically segregated or gravity-transformed by this process to form a bipartite flow and eventually diluted into a turbidity current. However, the development of lobe-and-cleft structures, which facilitate penetration of ambient water, has not yet been demonstrated experimentally in debris flows. Process 3 seems to be a very efficient and experimentally verified means of flow dilution that can tear apart the front of a subaqueous debris flow repetitively as long as hydroplaning occurs, thereby enhancing the transformation of a debris flow into a turbidity current (Mohrig et al. 1998). The water layer that penetrates underneath a debris flow acts not only as a lubricating layer for the hydroplaning debris flow but also as a conduit for incorporation of ambient water. The high pressures developed in the penetrating water layer probably help the water permeate into the interior of a debris flow. Process 3 can therefore play the same role as that of process 2 in altering the material properties of a debris flow. Part of the

ambient water incorporated in this way may not be uniformly soaked but escape through the upper surface of a debris flow along with fine-grained interstitial materials, further changing the rheological properties of the debris flow. Conditions for Hydroplaning Hydroplaning appears to affect significantly the flow transformation processes in subaqueous debris flows. To assess the possible role of hydroplaning in flow transformation, a densiometric Froude number and the time scale of pore-pressure decay in the Pohang debris flows were estimated (Table 1). Mohrig et al. (1998) suggest that a densiometric Froude number (Frd) can be used as a criterion for hydroplaning of a debris flow, which is defined as Frd 5

v

!1

2

rd 2 1 gh cos u rf

(1)

where n is the velocity of debris flow, rd is the density of debris flow (ca.

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FIG. 6.—Drawing of Bed C, showing a tripartite (third row) or bipartite (fourth and fifth rows) layering and an extensive wing of pebbly sandstone (first and second rows). The tripartite deposit consists of basal fine-grained layers (S1 and S2), a pebble–cobble conglomerate in the middle, and an overlying pebbly sandstone layer (S3). S1 and S2 layers are laterally discontinuous and have scoured lower contacts. The pebble–cobble conglomerate is variably graded. Pebbly sandstone of S3 contains protruding outsize clasts. The pebbly sandstone wing (first and second rows) is normally graded in both the gravel and the sand fractions, but part of the layer is ungraded, clast-rich, and poorly sorted (muddy).

2000 kg/m3), rf is the density of ambient fluid (1035 kg/m3), g is the acceleration due to gravity (9.8 m/s2), h is the debris flow thickness, and u is the slope angle. The densiometric Froude number can be regarded as the ratio of the hydrodynamic pressure at the leading edge of a debris flow, rfv2/2, to the average submerged load of the debris flow, (rd 2 rf)gh cos u. Hydroplaning occurs as the hydrodynamic pressure approaches that of the submerged debris load. Mohrig et al. (1998) demonstrated that hydroplaning can occur when the Froude number exceeds 0.4. The Pohang debris flows are estimated to have had a densiometric Froude number between 0.23 and 2.3 for Bed A and C debris flows and between 0.46 and 4.6 for Bed B debris flow (Table 1) when they are assumed to have experienced the typical range of shear strain rates for debris flows, which is between 1 s21 and 10 s21 (Johnson 1970; O’Brien and Julien 1988; Phillips and Davies 1991). This estimation suggests that the debris flows had a sufficiently large densiometric Froude number when they were fast-moving along the high-gradient fan-delta slopes. A large densiometric Froude number is a necessary but not a sufficient condition for hydroplaning. It is also necessary to estimate the time scale of pore-pressure decay (Iverson and LaHusen 1989; Mohrig et al. 1998) to determine whether high pressures can be sustained in the thin layer of water penetrating underneath the advancing front of a debris flow. The time scale R is defined as R5

d 2m kE

(2)

where d is a characteristic length that can be identified with the average

flow depth, m is the viscosity of the interstitial fluid, which includes silt and clay, k is the permeability of debris, and E is the uniaxial compression modulus or the stiffness of debris. The viscosity m for the interstitial fluid of Beds A to C debris flows was obtained by empirical formulas proposed by Thomas (1965), Barnea and Mizrahi (1973), and Metzner (1985). The interstitial fluid in the Beds A and C debris flows, which comprise ca. 5–10 wt. % mud in the muddy sand matrix, is estimated to have had a relative viscosity of 2 according to the formulas. This corresponds to a dynamic viscosity of 0.002 Pa·s. The interstitial fluid in the Bed B debris flow, which comprises well-sorted sand matrix, is estimated to have had a dynamic viscosity of water (0.001 Pa·s). The permeability of the debris-flow mixtures was estimated using an empirical method recommended by Masch and Denny (1966), which defines the permeability as a function of median grain diameter and particle sorting. The permeabilities of the matrices of Bed A, B, and C debris flows are estimated to be 10211 m2, 5 3 10210 m2, and 10211 m2, respectively. As for the stiffness of debris mixtures, a value of 105 Pa was used to yield a minimum estimate for the time scale, following Mohrig et al. (1998). The time scale of pore-pressure decay is estimated to be 2000, 80, and 2000 seconds for Bed A, B, and C debris flows, respectively (Table 1). The time scale is larger than the duration of debris flows for Bed A and C debris flows when they are assumed to have traveled ca. 1 or 2 km from the subaerial–subaqueous transition region to the depositional site with a mean shear rate between 1 and 10 s21. Bed A and C debris flows are therefore interpreted to have been sufficiently impermeable to build up and sustain high pressures in the thin water layer underneath the debris flows. On the other hand, the time scale of pore-pressure decay is smaller than

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FIG. 7.—A) Photograph of Bed C showing a tripartite layering. S2 is a laterally discontinuous pebbly sandstone layer; G is a clast-supported pebble–cobble conglomerate with a muddy sand matrix and random clast fabric; S3 is a pebbly sandstone layer gradationally overlying the conglomerate. Note the presence of large protruding boulders at the top of the conglomerate. B) Close-up of the basal part of Bed C, consisting of two fine-grained layers (S1 and S2) beneath the conglomerate (G). Note the erosional contacts between the fine-grained layers, the conglomerate, and the underlying mudstone (M). Hammer for scale. See Figure 6 for location of the photographs.

the supposed flow duration for Bed B debris flow, suggesting that the permeability of the debris-flow matrix was too high to facilitate hydroplaning. Effects of Material Segregation The style of flow transformation may change depending on the physical properties of debris-flow mixtures, such as clast sizes, viscosity of interstitial fluid, and permeability. Distribution of these properties in a debris flow is generally nonuniform because of vertical and longitudinal segregation of constituent materials. It is therefore necessary to consider how flow transformation is affected by the nonuniform distribution of material properties. Observations of subaerial debris flows show that the head of a debris flow has a steep front and contains the coarsest particles; the body is more fluidal than the head and transforms into a progressively more dilute tail (Okuda et al. 1980; Pierson 1980, 1986; Best 1992; Wan and Wang 1994; Vallance and Scott 1997). In addition, focusing of large clasts to the flow front (Bagnold 1968; Suwa 1988) and shouldering aside of these clasts along the flow margins (Major 1997) produce an arcuate frontal dam and lateral levees of coarse clasts. They are commonly devoid of matrix and almost totally lack excess pore pressures (Iverson 1997; Major and Iverson

1997). The front and lateral margins of a debris flow therefore consume kinetic energy very rapidly through frictional and collisional interactions of clasts. As a result, the flow front moves much slower than the following part, retarding the overall flow and allowing the slurry behind to pile up to great depths (Pierson 1980, 1986; Wan and Wang 1994). This behavior is in marked contrast to that of hydroplaning subaqueous debris flows, in which the frontal parts have a higher mobility, move faster, and can be detached from the main body of the flows (Mohrig et al. 1998). If similar material segregation occurs in subaqueous debris flows, the flow front will be much more enriched in coarse clasts and devoid of matrix because interstitial materials between coarse clasts can be more efficiently winnowed into the ambient water because of high shear stresses on the snout of subaqueous debris flows. The resultant dam and levees of openwork clasts will not hydroplane because of their extremely high permeability. In addition, they may shove off ambient water from the substrate, leaving only a small chance of hydroplaning for the rest of the flow even if the greater part of a debris flow has material properties that are suitable for hydroplaning. The surface transformation will also be hampered because the mixing process, mainly accomplished by removal of debris from the flow front, cannot but be resisted by the clasts that armor the snout of a debris flow.

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FIG. 8.—Spatial organization of different flow types produced by flow transformation of subaqueous debris flows. A) A debris flow rich in coarse gravel clasts may become longitudinally segregated, developing a frontal concentration of large clasts. Such a debris flow is not likely to hydroplane and hence is subject to mainly surface transformation. When the debris flow enters a base-of-slope setting, suspended-sediment clouds (1) and coarse gravel clasts derived from the front of the debris flow (2) may outrun the debris flow (3), resulting in a depositional sequence that comprises a turbidity-current and debris-fall deposit beneath a debris-flow deposit (columns). B) A debris flow composed of small gravel clasts and an impermeable (muddy) matrix may hydroplane, involving various flow transformation processes. Various types of fine-grained or dilute flows (1, 2, 4) may develop around the debris flow (3), producing voluminous flow-transformed deposits beneath and above a debris-flow deposit (column). Relative velocities of various parts of the composite flow are indicated by arrows.

For a debris flow to develop a girth of openwork clasts, it should contain abundant coarse clasts of variable grain size, and the coarse clasts should be effectively transferred to the flow front. There are two hypotheses for the focusing of large clasts to the flow front. Bagnold (1968) suggested that large clasts tend to drift toward the region of least shear rate, that is, toward the upper part of a debris flow. Because the upper part of a debris flow moves faster, the large clasts should drift toward the flow front. For this mechanism to be operative, the debris flow should be in the graininertia regime, being dominated by collisional clast interactions. Suwa (1988) presented a different view on the focusing mechanism of large clasts. He suggested that large clasts attain velocities higher than that of the flow front, thereby focusing themselves to the flow front. His analysis also suggests that the focusing mechanism operates more effectively in variably sized materials because the large size ratio of overlying clasts to underlying clasts creates an easier pivoting condition for the overlying clasts. It is therefore inferred that a debris flow that is in the grain-inertia regime and composed of an abundance of variously sized clasts can form TABLE 1.—Estimation of flow parameters based on physical properties of debris flows. Parameters Bulk density of debris1 Debris flow thickness 2 Debris flow velocity3 Slope angle Dynamic viscosity of interstitial fluid Hydraulic permeability Stiffness of debris mixture Travel distance of debris flow Duration of debris flows Densiometric Froude number Time scale of pore-pressure decay

Symbol & Unit

Bed A

Bed B

Bed C

rd (kg/m3) h (m) n (m/s) u (deg) m (Pa s) k (m 2) E (Pa) (m) (s) Frd R (s)

2000 1 1–10 10 0.002 10211 105 2000 200–2000 0.23–2.3 2000

2000 2 2–20 10 0.001 5 3 10210 105 2000 100–1000 0.46–4.6 80

2000 1 1–10 20 0.002 10211 105 1000 100–1000 0.23–2.3 2000

1 A debris flow is assumed to be a poorly sorted bidisperse suspension, having a sediment volume fraction of 0.6. 2 The thickness of internal subunits (ca. 1 m) was used for debris-flow thickness for Bed A and C. 3 The debris-flow velocity was obtained by multiplying supposed debris-flow thickness and typical shear strain rates of debris flows (1–10 s21).

a girth of openwork clasts that inhibits hydroplaning and surface transformation. DEPOSITIONAL SEQUENCES FROM SUBAQUEOUS DEBRIS FLOWS

Bed B debris flow satisfies many conditions for nonhydroplaning, such as small time scale of pore-pressure decay and an abundance of coarse clasts. On the other hand, Bed C debris flow satisfies the conditions for hydroplaning, such as large time scale of pore-pressure decay and small clast sizes. Bed A debris flow shows characteristics intermediate between the two, having a large time scale of pore-pressure decay but an abundance of coarse clasts of variable sizes. It is interpreted that Bed A debris flow did not hydroplane, even with a large densiometric Froude number and a large time scale of pore-pressure decay, because of the formation of a highly permeable girth of clasts around the flow front and margin. A pod of openwork gravel at the left margin of Bed A (Fig. 2D) is suggestive of effective segregation of coarse clasts in the debris flow. The vertically stacked layers of composite mass-flow deposits probably reflect temporal changes in flow type and conditions at the depositional sites and can be used to infer the pattern of longitudinal segregation in the flows. There are significant differences in the vertical sequences between the hydroplaning and nonhydroplaning debris-flow deposits, suggesting that organization of flow types (debris flow, turbidity current, and debris fall) can be very different between the hydroplaning and nonhydroplaning debris flows. Nonhydroplaning Debris Flows Bed A consists of a sandstone layer with some outsize clasts overlain by a conglomerate with local internal layering (Fig. 2). The bed is indicative of the arrival and deposition of a sandy turbidity current, together with rolling or saltating gravel clasts (debris fall), prior to the emplacement of a gravelly debris flow. Bed B also comprises a thin sandstone layer at its base (Fig. 5B), which is overlain by and amalgamated with a conglomerate. The sandstone contains a few boulder clasts that lie directly on the

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501

FIG. 8.—Continued.

mud substrate. These features suggest that a sandy flow and debris-fall blocks arrived the depositional site earlier than the bouldery debris flow (Fig. 8A). These relationships between the sandy and gravelly deposits can be explained by the low mobility of the debris flows relative to the dilute flows and debris falls they generated. The low mobility is probably related with nonhydroplaning of Bed A and B debris flows. Flow transformation therefore could involve only surface transformation. The bouldery upper surfaces of the debris flows may have induced large shear stresses on the upper surfaces, but the formation of clast-packed flow fronts may have hindered further progress of this process. The suspended materials generated by surface transformation moved downslope as a turbidity current. Although the turbidity current was generated behind the front of the debris flows, it could outrun the slow-moving debris flows probably after the debris flows entered a low-gradient depositional surface (Fig. 8A). Virtual absence of sand deposits above the Bed A and B conglomerates suggests that almost all of the suspended material could proceed in front of the decelerating debris flows. Some of the large gravel clasts that were concentrated along the flow front could be separated from the debris flows and move in front of the debris flows. They were emplaced in topographic lows as debris-fall deposits. Hydroplaning Debris Flows Bed C comprises thin, fine-grained layers beneath a conglomerate, suggesting that some fine-grained and dilute flows moved in front of a gravelly debris flow. The basal layers are, however, different from those of Bed A and B in that they are poorly sorted, contain much fine gravel, but do not contain outsize clasts. Hence, the materials of the basal layers are interpreted to have been derived directly from the frontal part of Bed C debris flow, probably associated with detachment and disintegration of the hydroplaning flow front (Fig. 8B) rather than from the suspension above the body and tail of the flow. Superposition of a poorly sorted, fine pebbly subunit (S2; Fig. 7B) above a moderately sorted sandy subunit (S1; Fig. 7B) suggests that the foremost flow had incorporated more ambient water, facilitating the size segregation of clasts, whereas the following flow was less evolved (diluted) and more akin to the original debris flow. These flows are interpreted to have dissipated very rapidly because of their small volumes and have been outrun and cannibalized by the following debris flow and its suspension.

The conglomerate of Bed C is overlain by a meter-thick sand layer that contains outsize gravel clasts (S3; Fig. 7A). The sand layer and the gravel clasts are interpreted to be deposits of a turbidity current and a debris fall, respectively. The presence of these deposits above the conglomerate is in contrast to the depositional sequences in Bed A and B deposits, suggesting that the debris flow had a higher mobility and could move faster than the turbidity current and debris fall. There are several reasons for the high mobility of Bed C debris flow. First, this flow had material properties suitable for hydroplaning and could glide over a lubricating water layer; second, this flow experienced relatively small shear resistance at the upper surface because of small clast size; third, it consumed its kinetic energy much less rapidly than Bed A and B debris flows because of its smaller clast sizes; it could attain a higher velocity, passing through the highgradient foreset slope of a Gilbert-type fan delta; and finally, a flow-retarding dam of openwork clasts may not have formed along the flow front. The Bed C debris flow appears to have been subject to various flow transformation processes, including detachment and disintegration of hydroplaning flow front, dilution of the flow interior due to incorporation of water, elutriation of fines by escaping pore fluid, and surface transformation. The volume of debris that was diluted and suspended into the overlying water was much larger than that of Bed A or B debris flows. These materials could produce extensive deposits alongside and above the conglomerate (Fig. 6). It is noteworthy that these deposits contain considerable amounts of pebbles and granules, which are difficult to be supplied by surface transformation or elutriation processes. The coarse grains are therefore interpreted to have been derived from the detached blocks that were shed from the hydroplaning debris flow. An ungraded part of conglomerate in the turbidite wing is interpreted as a remnant of such blocks. DISCUSSION AND CONCLUSIONS

Subaqueous debris flows undergo various flow transformations, including surface transformation, incorporation of ambient water into the flow interior, detachment and disintegration of hydroplaning flow front, and elutriation of fines by escaping pore fluid. The type and degree of flow transformation are strongly controlled by whether a debris flow hydroplanes. When a debris flow is not hydroplaning, surface transformation is the major cause of flow transformation. The efficiency of surface transformation is, however, reduced by the sheath of a concurrently moving suspended-sediment cloud or by an armor of gravel that surrounds the flow head, both

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of which are generated by the surface transformation process itself. Surface transformation is therefore a self-limiting process and is not likely to transform a whole body of debris flow into a turbidity current or produce a thick turbidite layer associated with a debris-flow deposit. Previously, thick turbidite caps above debris-flow deposits have been interpreted mostly in terms of surface transformation (e.g., Krause and Oldershaw 1979; Souquet et al. 1987). These interpretations need to be questioned and reevaluated. On the other hand, effective flow dilution and alteration of material properties can occur in hydroplaning debris flows by detachment and disintegration of flow front, incorporation of ambient water through the water layer that penetrates underneath the flow front, and elutriation of fines by escaping pore fluid as well as by surface transformation. It has been suggested that hydroplaning of a debris flow should require a large densiometric Froude number (.0.4) and sufficiently impermeable debris for which the time scale of pore-pressure decay should be greater than the flow duration (Mohrig et al. 1998). Nonuniform distribution of material properties in a debris flow seems to be another variable that determines the behavior of a debris flow. For example, a collision-dominated debris flow that contains abundant gravel clasts of various sizes can be easily fringed by an extremely permeable dam of large clasts and may never hydroplane even if the debris flow has material properties that are suitable for hydroplaning. The dilution processes associated with hydroplaning debris flows are selfescalating because the front of a debris flow generated after the detachment of a former front may have more suitable material properties for hydroplaning, such as finer grain size, more abundant fines, and more fluidal behavior. Bed A and B debris flows are composed mainly of bouldery gravel clasts and are interpreted to have been nonhydroplaning. Therefore, these debris flows were subject mainly to surface transformation. The surface-transformed materials formed a turbidity current that could move in front of the debris flow, forming a thin sandy layer beneath the debris-flow deposit. Outrunning by the turbidity current was possible because the debris flows were nonhydroplaning, had high apparent viscosities or mechanical strengths because of an abundance of large gravel clasts, and consumed kinetic energy very rapidly. The sandy layers contain outsize boulder clasts that are interpreted to have been derived from the fronts and tops of the debris flows and deposited as debris-fall deposits. The presence of outsize debris-fall blocks beneath debris-flow deposits implies that clasts were efficiently segregated and transferred to the flow top and front. A vertical sequence composed of thin sandy deposits with outsize clasts and overlying conglomerates (columns in Figure 8A) may represent an ideal depositional sequence produced by a nonhydroplaning debris flow. The Akrech submarine debris-flow deposit reported by Padgett et al. (1977) is an example of such a sequence, consisting of a base of granulestone with floating cobbles and pebbles and an overlying zone of clast-rich conglomerate. Bed C debris flow is composed mostly of pebble gravel in an impermeable matrix and is interpreted to have hydroplaned. Therefore, this debris flow experienced various transformation processes. Abundant coarse and fine debris was shed from the flow front and flow top to form dilute flows. These flows formed extensive turbidity-current deposits above and alongside the debris-flow deposit. Some of these flows could move in front of the debris flow temporarily, forming finer-grained deposits beneath gravelly debris-flow deposits. The presence of turbidity-current deposits mostly above the debris-flow deposits is due to high mobility of the hydroplaning debris flow. A vertical sequence composed of a conglomerate and thick overlying sandy deposits (column in Figure 8B) most likely results from hydroplaning debris flows. Deposits of hydroplaning debris flows may show thin fine-grained basal layers beneath the conglomerate and contain detached blocks of debris in the turbidite. There may be many more factors that control the processes of flow transformation in subaqueous debris flows. An important one seems to be the development of hydraulic jumps. It is generally accepted that sediment gravity flows experience extreme changes in the physics and material prop-

erties when they pass through a hydraulic jump (Komar 1971; Weirich 1988, 1989). The debris flows studied in this paper flowed on high-gradient fan-delta slopes in a supercritical condition and probably passed through a hydraulic jump before they came to rest. It is supposed that the debris flows decelerated drastically and were outrun by the accompanying turbidity currents after the break in surface slope. Further research is necessary to assess the role of hydraulic jump in producing depositional features and vertical sequences in flow-transformed, composite debris flows. The role of surging or development of roll waves (Davies 1986, 1990) should also be considered to properly assess the role of various flow transformation processes. It is an especially intriguing question to ask whether the fronts of individual surges hydroplane or not. If they do, flow dilution would be greatly amplified in surging debris flows. ACKNOWLEDGMENTS

Field work for this research was supported by the Korea Science and Engineering Foundation. This research was completed while the author was a visiting scientist at the US Geological Survey Cascades Volcano Observatory, and was made possible by a postdoctoral research fellowship provided by the same foundation. Mr. S.B. Kim spent many hours in the field, assisting measurements of the deposits and providing helpful discussions. Dr. I.G. Hwang introduced the debris-flow deposits to the author. This manuscript was improved by thoughtful and critical reviews by Drs. D. Mohrig, M. Hampton, and R. Dorsey. All the help is gratefully acknowledged. REFERENCES

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