The Sedimentary Record CLASSIFICATION OF GRAVITY FLOWS: BACKGROUND From Turbid to Lucid: AND REVIEW The initial definitions of currents A Straightforward Approach to Sediment and debris flows were purely descriptive, with- out being specific about the physical proper- Gravity Flows and Their Deposits ties of the current. Therefore, from the begin- M. Royhan Gani ning, questions revolved around what should Department of Geosciences be the main basis of classification of sediment University of Texas at Dallas gravity flows. Different authors emphasized P.O. Box, 830688, Richardson, TX, 75083-0688, USA different parameters in their classification [email protected] schemes (sediment concentration: Bagnold, 1962; rheology: Dott, 1963; fluid : Sanders, 1965; sediment-support mechanisms: ABSTRACT Middleton and Hampton, 1973; combination Deepwater sediment gravity flows are categorized on the basis of a combination of four of rheology and sediment-support mechanism: parameters – sediment concentration, sediment-support mechanism, flow state (laminar Lowe, 1982; combination of physical flow or turbulent), and rheology. Because there is no agreement among sedimentologists properties and sediment-support mechanism: about which of these parameters should be the decisive one, one school’s Mulder and Alexander, 2001). Among the become another school’s debrites, and vice-versa. Except for rheology, all of these four most important parameters (sediment parameters change gradationally from one end member to another.Therefore, rheologi- concentration, sediment-support mechanism, cal classification of sediment gravity flows should be the most straightforward and the flow state, and rheology) of sediment gravity least controversial.These flows can be either Newtonian (i.e., turbidity currents), or flows, sediment concentration (by volume) non-Newtonian (i.e., debris flows). However, identification of flow rheology by examining directly affects other three parameters. the deposits may not be easy.Although we may confidently identify some rocks as tur- Therefore, sediment concentration appears to bidites and others as debrites, there are some transitional deposits, here called densites, be the most pragmatic parameter for defining that share both the characteristics of turbidites and debrites. Densites are the deposits the various types. Unfortunately, we can not of dense flows, which are rheologically stratified flows having a composite rheology of establish specific threshold values for various Newtonian fluids and non-Newtonian fluids. Moreover, the absence of a general term for types of sediment gravity flows (Shanmugam, all types of deposits has resulted in overuse and misuse of the 1996) because grain size and concentration of term .The term ‘gravite’ is proposed here for deposits of any kind of sediment minerals offset these threshold values. gravity flow, irrespective of their depositional environment. Sediment-support mechanisms include matrix strength, dispersive grain pressure, escaping pore fluid, and fluid turbulence. INTRODUCTION fluid is moved by gravity dragging the sedi- These mechanisms may change gradationally The term ‘’ was introduced ment along, whereas in former gravity moves with increasing fluid content, and more than by Johnson (1938) and applied to a current the sediment, which drags the fluid along. A one support mechanism may operate simulta- generated due to turbid or muddy water. turbidity current is only one type of sediment neously for a specific type of sediment gravity Later, Kuenen (1957) introduced the term gravity flow. The center point of the turbidite flow. Similarly, the flow state may change gra- ‘turbidite’ for the deposit of a turbidity cur- controversy lies in the classification scheme of dationally and back-and-forth between a lami- rent, and Bouma (1962) introduced a classic sediment gravity flows, which is so far poorly nar state and a turbulent state with the change five-fold vertical facies model for turbidites. constrained and a bit ambiguous. This contro- of sediment concentrations or basin slopes. Soon, the terms ‘turbidites,’ ‘Bouma versy can lead to erroneous numerical model- On the other hand, the rheology of sediment sequences,’ and ‘deepwater deposits’ became ing of sediment gravity flows frequently used gravity flows is expressible in a straightforward almost synonymous in many published in submarine construction and hydrocarbon and simplified mathematical way in a 2-D accounts. Although the overuse and misuse of exploration because specific mathematical for- graph (Fig. 1). Most importantly, the rheologi- the terms ‘turbidity current’ and ‘turbidite’ mulae govern specific types of sediment gravi- cal types do not vary gradationally among was first indicated by Sanders (1965), the tur- ty flows. Therefore, it is felt that this classifica- each other. Therefore, rheology may be the bidite controversy has recently caught wide tion scheme needs to be reviewed to clarify the one parameter that can be used least ambigu- attention (e.g., Shanmugam, 2000; Lowe and controversy. In this paper, I take a simple and ously to define various types of sediment grav- Guy, 2000; Kneller and Buckee, 2000; Mulder straightforward approach to classifying sedi- ity flows. and Alexander, 2001). ment gravity flows. I also suggest some key According to Figure 1, there are only two Sediment gravity flows play a major role in depositional features on which flow types can basic types of rheology in sediment gravity transporting and depositing in be interpreted least equivocally. Although the flows – Newtonian and non-Newtonian. If a deepwater environments, and can be defined suggestions made here are applicable irrespec- sediment gravity flow deforms instantly with as a complex mixture of sediment and fluid tive of depositional environments, this paper applied stress and develops a linear relation- that flows down slope due to the action of mainly deals with deepwater sediment gravity ship between shear stress and strain rate, it is gravity. Sediment gravity flows are different flows and their deposits. called a Newtonian fluid. Any deviation from from fluid gravity flows, because in the latter, this characteristic results in non-Newtonian

4 | September 2004 The Sedimentary Record the flow rheology of a deposit by examining its depositional features may be challenging. There are sediment gravity flow deposits that share both the characters of turbidites and debrites (deposits of debris flows). If we follow the turbidite controversy for the last ten years, it becomes obvious that these hybrid deposits (Non-Newtonian Bingham Plastic) are the main issue of debate. A plethora of ter- Cohesive minology (e.g., high density turbidity cur- rents, sandy debris flows, slurry flows, concen- trated density flows) has been applied to these

Shear stress rocks. Most of these deposits originate from (Newtonian fluid) rheologically stratified (or bipartite) sediment gravity flows (e.g., Sanders, 1965; Tinterri et al., 2003) with commonly a lower zone of Turbidity current non-Newtonian dilatant fluid (non-cohesive debris flow) overlain by a Newtonian fluid (turbidity current). Because these types of Yield strength Non-cohesive debris(Non-Newtonian flow dilatant fluid) flows can frequently change the intra-flow rhe- ological boundaries, and can generate a single Strain rate (du/dy) event bed, a separate name is needed for these flows and their deposits. In this study, these Figure 1. Basic types of rheology in sediment gravity flows. According to this diagram, turbidity currents are Newtonian fluids, whereas debris flows are not. Debris flows can be either non-Newtonian Bingham flows are called ‘dense flows’ (after Allen, plastics (cohesive debris flows; e.g., mud flows) with a certain yield strength, or non-Newtonian dilatant 1997), as they show an intermediate density fluids (non-cohesive debris flows; e.g., grain flow) without any yield strength. (due to intermediate sediment concentration) between turbidity currents and debris flows rheology. Sediment gravity flows can show two issue of turbidite controversy. (Fig. 2A), and their deposits are named ‘den- types of non-Newtonian rheology (Fig. 1). In It may be easy to determine the rheology of sites.’ However, it is emphasized that accord- a non-Newtonian Bingham plastic, a critical flows in the laboratory. However, interpreting ing to rheology there are only two basic types value of shear stress (called yield stress) has to be crossed before there is any deformation, Non-Newtonian dilatant fluid after which the deformation is linear (i.e., a A ? Newtonian fluid Bingham plastic Bingham plastic is a combination of an ideal ? ? Figure 2. plastic and a Newtonian fluid). In a non- 0203010 40 50 60 70 80 90 Distribution of dif- ferent types of sedi- Newtonian dilatant fluid there is no yield Gravel ment gravity flows strength, but the deformation is nonlinear in 2-D space of sedi- with applied stress in such a way that it Coarse sand ment concentration becomes progressively harder to deform the vs. grain size (A), fluid (Fig. 1). Applying the above concepts, I Fine sand Dense flow Debris flow and Reynolds num- recommend that sediment gravity flows with (Hybrid of ber vs. Froude num- Silt turbidity current Grain size ber (B). Note that Newtonian rheology should be called ‘turbidi- and non-cohesive debris flow) dense flows (rheolog- ty currents,’ and those with non-Newtonian Clay ically stratified sedi- rheology should be called ‘debris flows’ (Fig. Turbidity current ment gravity flows) 1). Debris flows can be divided further into 0 10 20 30 40 50 60 70 80 90 100 occupy an interme- Sediment concentration (% by volume) ‘cohesive debris flows’ (non-Newtonian diate position between turbidity Bingham plastics), and ‘non-cohesive debris B 104 currents and debris flows’ (non-Newtonian dilatant fluids) (Fig. flows. For conven- 1). This ‘cohesiveness’ of debris flows generally Turbidity current ience, rheological types are shown for

depends on the clay concentration of the Turbulent possible operational flows. Although some workers (e.g., 3 10 ranges of sediment Hampton, 1975; Baas and Best, 2002) concentrations; how- Dense flow showed that as little as 2-4% clay (by volume) ever, the percentage can generate yield strength in the flows, fur- of clay within the ther research is needed to clarify the matter. So 102 bulk sediments (not far, we know the least about the numerical and Reynolds number shown in Fig. 2A) is Debris flow an important factor experimental modeling of non-Newtonian Laminar controlling the flow dilatant sediment gravity flows (e.g., grain rheology (after Allen, flows) and their deposits. I suspect that it is a 101 1997). 0.1 Supercritical critical loophole in understanding the evolu- Subcritical 1 10 tion of sediment gravity flows; hence it is an

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Dominant sediment- Rheology Flow Type Deposits support mechanism

Subcatagories: - Low-concentration (<1%) & muddy (e.g., fluid mud ?) Turbidity - Low-concentration (0.2-3%) & medium- Newtonian current grained (Hyperpycnal flow: Mulder et al., Turbidite Fluid turbulence fluid (mostly 2003) turbulent) - Low-concentration & fine-grained (Stow & Shanmugam, 1980) - Medium-grained classic (Bouma, 1962)

Variously named as: Variable - High-density turbidity currents (Lowe, 1982) Dispersive grain (Partly non- - Sandy debris flows (Shanmugam, 1996) pressure, fluid Dense flow - Slurry flows (Lowe & Guy, 2000) Newtonian fluid, Densite Gravite turbulence, (partly laminar, - Concentrated density flows (Mulder & partly Newtonian escaping pore fluid, partly turbulent) Alexander, 2001) (This study) (Gani, 2003) fluid) - Liquified flows /fluidized flows matrix strength Non-Newtonian Non-cohesive debris flow Dispersive (e.g., grain flow) dilatant fluid Debris flow grain pressure Debrite (mostly Increasing fluid content Bingham laminar) Cohesive debris flow Matrix strength plastic Slide and Bingham plastic Slide and slump slump depostis Matrix strength

Figure 3. Classification of sediment gravity flows with a simplified nomenclature for the flow types as well as their deposits. Flow rheology is the basis of this clas- sification (Fig. 1). Direction of increasing fluid content is roughly analogous to the down-slope evolution of sediment gravity flows. Flow states and sediment-sup- port mechanisms are incorporated to give a comprehensive picture of the nature of these flows. For the range of sediment concentrations of these flows, see Fig. 2A.

of sediment gravity flows – turbidity currents about which depositional features are the key Therefore, successful interpretation of sedi- and debris flows. in determining the types of sediment gravity ment gravity flow deposits depends on how As mentioned earlier, there are no threshold flows. While dealing with rocks, sedimentolo- accurately we can establish a link between values of sediment concentration (by volume) gists deduce the processes of depositions based physics (of the process) and sedimentology (of in constraining the types of sediment gravity on observable criteria of the deposits. the product). Rheology and sediment-support flows. A range of sediment concentration val- ues, which can vary according to grain size, is Figure 4. Links between physics (of processes) and sedimentology (of products) of sediment gravity flows. suggested for turbidity currents, dense flows, Different rheological properties and sediment support mechanisms can generate depositional features and debris flows (Fig. 2A). In general, with diagnostic to specific types of sediment gravity flows. increasing sediment concentration, a turbidity Link current can transform into a dense flow, and Physics (of flows) Sedimentology (of products) then into a debris flow. Similarly, depending Well sorting with no 'floating' on the Froude numbers, these three flows can No yield strength; no clasts; top part always Newtonian fluid be both turbulent and laminar (Fig. 2B). freezing shows normal distribution However, turbidity currents are mostly turbu- grading lent, whereas debris flows are mostly laminar. Frictional freezing No normal distribution Non-Newtonian but no yield strength; Based on flow rheology and incorporating grading; layer by layer Rheology dilatant fluid freezes from the the concept of dense flows, a simplified tabular accretion classification of sediment gravity flows is gen- bottom up erated (Fig. 3). Because most of the sediment En masse freezing due Poor sorting; preserved flow gravity flows originate from slides and slumps, to yield strength; plug morphology; sharp upper these are included at the bottom of this classifi- Bingham plastic flow; freezes from the boundary; boulder cation. The classification also shows the domi- top down projecting through the top nant sediment-support mechanism and flow Differential grain Normal distribution grading; state for each of the types to give a comprehen- settling from Fluid turbulence well sorting sive picture about the nature of these flows. suspension Sediment- Escaping pore Leaves escape marks Dish and pillar structures; DIAGNOSTIC FEATURES OF support fluid convolution DIFFERENT SEDIMENT mechanism Dispersive Larger the grain, Inverse grading GRAVITY FLOW DEPOSITS grain pressure greater the liftoff One reason for the turbidite controversy is the Supports Matrix-supported 'floating' Matrix strength lack of consensus among sedimentologists large/outsized clasts clasts; poor sorting 6 | September 2004 The Sedimentary Record

Gravite

Debrite Densite Turbidite T urbidite Debrite

A B C D E F G H Mudstone Clasts

Increasing fluid content / decreasing sediment concentration

Figure 5. Simplified lithologic models for gravites (deposits of sediment gravity flows). A: cohesive debrite; B-C: non-cohesive debrites; D-F: densites (deposits of bipartite sediment gravity flows); G: turbidite (); H: turbidite (deposit of hyperpycnal flow). Note that the arrowed direction is analogous to the down-slope evolution of sediment gravity flows. See text for discussion. mechanisms of flows suggest a number of Lowe, 1982), there are other views that sup- divisions. However, these beds show distribu- links, which, in turn, point to a set of key port non-Newtonian fluid rheology (e.g., tion grading only at the top parts, and the rest depositional features for each type of sediment Allen, 1997). In this study, for the sake of sim- of these beds are either massive or inversely gravity flow deposits (Fig. 4). Based on these plicity, I consider debris flows as sediment graded (Fig. 5D-F). As part of the turbidite diagnostic features, the concepts described in gravity flows whose rheology is not controversy, various flow names have been the previous section, and on numerous pub- Newtonian (Fig. 1). Therefore, debrites can proposed in explaining the depositional mech- lished works, I suggest the following terminol- include both cohesive debrites (Bingham plas- anism of these deposits (Fig. 3). The features ogy be applied for the deposits of different tic rheology) and non-cohesive debrites (non- of these beds are diagnostic of rheologically sediment gravity flows. Newtonian dilatant fluid rheology). In gener- stratified/bipartite sediment gravity flows (i.e., al, a gravite bed that does not show any distri- dense flows) and are here called densites. Gravite bution grading even in the uppermost part is a Gravite is defined as a sediment or rock debrite (Fig. 5). Cohesive debrites are relative- Turbidite deposited from a sediment gravity flow (Gani, ly easy to identify. Most importantly, because When the term ‘turbidite’ was introduced 2003). It is an umbrella term that incorporates of the yield strength of the flow, they contain (Kuenen, 1957) for deposits of turbidity cur- all sediment gravity flow deposits (including ‘floating,’ outsized clasts in a muddy matrix rents, it became popular in geological litera- slide and slump deposits) irrespective of their (Figs. 5A, 6A). These deposits show poor sort- ture. I propose using turbidity currents only depositional environment (Fig. 5). The ing with rare, if any, coarse-tail grading. On for sediment gravity flows with Newtonian absence of such a concise, general term result- the other hand, non-cohesive debrites are rela- rheology. Therefore, I recommend restricting ed in overuse and misuse of the term ‘tur- tively mud-free (e.g., grain flow the term turbidites to only those gravites that bidite’ in geological literature. For example, deposits) that show inverse grading because of suggest a Newtonian rheology of the deposit- although submarine fans consist of different the dispersive grain pressure (Fig. 5C). ing currents. Turbidity currents have long types of sediment gravity flow deposits, the Generally, non-cohesive debrites aggrade layer been regarded as surge-type waning currents. term ‘turbidite systems’ has been used inter- by layer (~

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Debrite mal grading analogous to a waxing-then-wan- (journal editors) for reviewing the manuscript. A ing hydrograph (Mulder et al., 2003; A work of this type is heavily footed on the Fig. 5H). Hyperpycnal flows are regarded as shoulders of earlier workers on sediment gravi-

3 cm low-concentration (0.2-3% by volume) and ty flows. While writing this paper, I was a medium-grained turbidity currents (Fig. 3). research assistant at The University of Texas at Therefore, their deposits (Fig. 5H) should not Dallas. This is contribution number 1026 of be confused with deposits of dense flows (Figs. the Geosciences Department, University of 5D-F). However, more study is needed for Texas at Dallas. I wish to express my gratitude successful identification of ancient hyperpyc- to Nahid Sultana, my wife, for editorial assis- nites. Future research is also necessary in order tance. Densite to identify deposits of fluid muds commonly B developed on modern continental shelves REFERENCES ALLEN, P.A., 1997, Earth surface processes: Blackwell, London, 404 p. (Traykovski et al., 2000) from ancient records. BAGNOLD, R.A., 1962, Auto-suspension of transported sediment: tur- Because sediment concentration of fluid muds bidity currents: Royal Society of London Proceedings, Series A265, p. T 315-319. urbidite is very low (<<1% by volume), it is debatable BAAS, J.H., 2004, Conditions for formation of massive turbiditic sand- stones by primary depositional processes: Sedimentary Geology, v. whether these should be regarded as fluid 166, p. 293-310. BAAS, J.H., and BEST, J.L., 2002, Turbulence modulation in clay-rich gravity flows or sediment gravity flows. sediment-laden flows and some implications for sediment : Debrite Nonetheless, if these mudrock beds show nor- Journal of Sedimentary Research, v. 72, p. 336-340. BOUMA, A.H., 1962, Sedimentology of some flysch deposits: a graphic mal grading they are best identified as tur- approach to facies interpretation: Elsevier, Amsterdam, 168 p. 14 cm BOUMA, A.H., Stone, C.G., (Eds.) 2000, Fine-grained turbidite sys- bidites. Alternatively, if these beds are ungrad- tems: American Association of Geologists, Memoir 72, Turbidite 342 p. ed they are debrite, in which case the sediment DOTT, R.H., Jr., 1963, Dynamics of subaqueous gravity depositional C Tc concentration of fluid muds should exceed 2% processes: American Association of Petroleum Geologists, Bulletin, v. 47, p. 104-128. (cf. Baas and Best, 2002). GANI, M.R., 2003, Crisis for a general term referring to all types of sedi- ment gravity flow deposits: gravite: Geological Society of America, Tb Abstracts with Programs, v. 34, No. 7, p. 171. CONCLUSIONS GANI, M.R., and ALAM, M.M., 1999, Trench-slope controlled deep-sea clastics in the exposed lower Surma Group in the southeastern fold The overuse and misuse of the term ‘turbidite’ belt of the Bengal Basin, Bangladesh: Sedimentary Geology, v. 127, p. 221-236.

5 cm for many types of sediment gravity flow HAMPTON, M.A., 1975, Competence of fine-grained debris flows: Ta Journal of Sedimentary Petrology, v. 45, p. 834-844. deposits has resulted in what I refer to here as JOHNSON, D., 1938, The origin of submarine . Journal of the turbidite controversy. This controversy is Geomorphology, v. 1, p. 111-340. KNELLER, B.C., and BRANNEY, M.J., 1995, Sustained high-density rooted in the classification scheme of sediment turbidity currents and the deposition of thick massive sands: Sedimentology, v. 42, p. 607-616. Figure 6. (A) Wackestone cohesive debrite in the gravity flows, which is poorly constrained and KNELLER, B., and BUCKEE, C., 2000, The structure and fluid base-of-slope deposits of Permian Lamar somewhat contradictory. I suggest a simple mechanics of turbidity currents: a review of some recent studies and Member, Guadalupe Mountains, their geological implications: Sedimentology, v. 47, p. 62-94. but well-constrained classification of sediment KUENEN, P.H., 1957, Sole markings of graded graywacke beds. Journal west Texas. Poorly sorted skeletal grains floating of Geology, v. 65, p. 231-258. within matrix mud along with ungraded and gravity flows based on flow rheology. Turbidity LOWE, D.R., 1982, Sediment gravity flows, II. Depositional models with special reference to the deposits of high-density turbidity cur- sharp upper boundary indicate a Bingham plas- currents are Newtonian fluids, whereas debris rents: Journal of Sedimentary Petrology, v. 52, p. 279–297. tic rheology of the depositing flow. Compare with flows are not. Debris flows can be of two LOWE, D.R., and GUY, M., 2000, Slurry-flows deposits in the Britannia Formation (Lower Cretaceous), North Sea: a new perspective on the Fig. 5A. (B) Delta front densite in the Upper types: a cohesive debris flow (a non- turbidity current and debris flow problem: Sedimentology, v. 47, p. Cretaceous Wall Creek Formation of central Newtonian Bingham plastic) or a non-cohe- 31-70. Wyoming. The lower layer of this bed is ungrad- MIDDLETON, G.V., and HAMPTON, M.A., 1973, Sediment gravity sive debris flow (non-Newtonian dilatant flows: mechanics of flow and deposition, in Middleton, G.V., and ed with floating mud clasts, hence indicates non- Bouma A.H., eds., Turbidites and deep-water : Newtonian flow rheology. The upper layer shows fluid). Some sediment gravity flows may be Proceedings of Pacific Section Society of Economic Paleontologists and Mineralogists, Los Angeles, p. 1-38. distribution grading with flat stratification indi- rheologically stratified (or bipartite) in nature, MULDER, T., and ALEXANDER, J., 2001, The physical character of cating Newtonian flow rheology. Note that as so that commonly a lower layer of non- subaqueous sedimentary density flows and their deposits: bedding plane has not developed between these Sedimentology, v. 2001, v. 48, p. 269-299. Newtonian rheology is overlain by a layer of MULDER, T., SYVITSKI, J.P.M., MIGEON, S., FAUGERES, J.C., and two layers, the entire bed is called a densite SAVOYE, B., 2003, Marine hyperpycnal flows: initiation, behavior (hybrid of debrite and turbidite; compare with Newtonian rheology. These types of flows are and related deposits. A review: Marine and petroleum Geology, v. 20, p. 861-882. Figs. 5E-F). (C) Turbidite in Miocene base-of- called dense flows and their deposits are SANDERS, J.E., 1965, Primary formed by tur- slope deposits of the Bengal Basin, Bangladesh named densites. Identification of flow rheolo- bidity currents and related resedimentation mechanisms, in Middleton, G.V., ed., Primary sedimentary structures and their (modified from Gani and Alam, 1999). The gy from ancient deposits may not be an easy hydrodynamic interpretation: Society of Economic Paleontologists entire bed shows distribution grading with the and Mineralogists Special Publication, v. 12, p. 192-219. task. Nonetheless, there are some diagnostic SHANMUGAM, G., 1996, High-density turbidity currents: are they development of Bouma Tabc divisions and with- depositional features of debrites, turbidites, sandy debris flows?: Journal of Sedimentary Research, v. 66, p. 2-10. out any floating clasts in Ta division. These indi- SHANMUGAM, G., 2000, 50 years of turbidite paradigm (1950s – cate differential grain settling from a Newtonian and densites that relate to the physics of corre- 1990s): deep-water processes and facies models – a critical perspec- tive: Marine & Petroleum Geology, v. 17, p. 285-342. fluid. Compare with Fig. 5G. sponding flows. In order to avoid confusion, if STOW, D.A.V., and SHANMUGAM, G., 1980, Sequence of structures in fine-grained turbidites: comparison of recent deep-sea and ancient it is not possible to determine the category of flysch sediments: Sedimentary Geology. v. 25, p. 23–42. rents’ are densites (Figs. 5D-F). However, the a sediment gravity flow deposit, we should TINTERRI, R., DRAGO, M., CONSONNI, A., DAVOLI, G., and MUTTI, E., 2003, Modeling subaqueous bipartite sediment gravity top part of these deposits should show normal simply call the deposit a gravite. flows on the basis of outcrop constraints: first results: Marine and Petroleum Geology, v. 20, p. 911-933. grading (e.g., Baas, 2004), otherwise they are TRAYKOVSKI, P., GEYER, W.R., IRISH, J.D., and LYNCH, J.F., 2000, The role of wave-induced density-driven fluid mud flows for cross- debrites. The only real-world example of ACKNOWLEDGMENTS shelf on the Eel : Continental Shelf quasi-steady turbidity currents are the hyper- I would like to thank J. Bhattacharya for help- Research, v. 20, p. 2113-2140. pycnal flows produced during river . ful discussion on the topic. I am grateful to C. These hyperpycnites show reverse-then-nor- Stone, and S. A. Leslie and L.E. Babcock

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