EARTWSCIENCE

ELSEVIER Earth-Science Reviews 42 (1997) 201-229

The Bouma Sequence and the turbidite mind set

G. Shanmugam *

Mobil Technology Company, P. 0. Box 650232, Dallas, 7X 75265-0232, USA

Received 12 September 1996; accepted 28 April 1997

Abstract

Conventionally, the Bouma Sequence [Bouma, A.H., 1962. Sedimentology of some Flysch Deposits: A Graphic Approach to Facies Interpretation. Elsevier, Amsterdam, 168 pp.], composed of T,, T,, T,, Td, and T, divisions, is interpreted to be the product of a turbidity current. However, recent core and outcrop studies show that the complete and partial Bouma sequences can also be interpreted to be deposits formed by processes other than turbidity currents, such as sandy debris flows and bottom-current reworking. Many published examples of turbidites, most of them hydrocarbon-bearing sands, in the North Sea, the Norwegian Sea, offshore Nigeria, offshore Gabon, Gulf of Mexico, and the Ouachita Mountains, are being reinterpreted by the present author as dominantly deposits of sandy debris flows and bottom-current reworking with only a minor percentage of true turbidites (i.e., deposits of turbidity currents with fluidal or Newtonian rheology in which sediment is suspended by fluid turbulence). This reinterpretation is based on detailed description of 21,000 ft (6402 m) of conventional cores and 1200 ft (365 m> of outcrop sections. The predominance of interpreted turbidites in these areas by other workers can be attributed to the following: (1) loose applications of turbidity-current concepts without regard for fluid rheology, flow state, and sediment-support mechanism that result in a category of ‘turbidity currents’ that includes debris flows and bottom currents; (2) field description of deep-water sands using the Bouma Sequence (an interpretive model) that invariably leads to a model-driven turbidite interpretation; (3) the prevailing turbidite mind set that subcon- sciously forces one to routinely interpret most deep-water sands as some kind of turbidites; (4) the use of our inability to interpret transport mechanism from the depositional record as an excuse for assuming deep-water sands as deposits of turbidity currents; (5) the flawed concept of high-density turbidity currents that allows room for interpreting debris-flow deposits as turbidites; (6) the flawed comparison of subaerial river currents (fluid-gravity flows dominated by bed-load transport) with subaqueous turbidity currents (sediment-gravity flows dominated by suspended load transport) that results in misinterpreting ungraded or parallel-stratified deep-sea deposits as mrbidites; and (7) the attraction to use obsolete submarine-fan models with channels and lobes that require a turbidite interpretation. Although the turbidite paradigm is alive and well for now, the turhidites themselves are becoming an endangered facies! 0 1997 Elsevier Science B.V.

Keywords: turbidity currents; Bouma Sequence; submarine fans; debris flows

* Tel.: + I-214-951 3109; fax: + l-214-905 7058; e-mail: [email protected]

0012.8252/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SOOl2-8252(97)00010-X 1. Introduction sharp or gradational upper contact; and (3) whether the bed has mudstone clasts near the base or the top. The Bouma Sequence, which is interpreted to Even if one records all other information in addition represent the deposit of a turbidity current (Fig. 1). is to T;,. the notation, T, carries with it a powerful probably the single most widely used (or abused) message and a built-in interpretation that the bed was terminology for the field description of sands inter- deposited by a turbidity current. preted to be of deep-water origin; the interpretive On the other hand. if the same bed were to be term ‘deep-water sand’ will be used hereafter. The described without the T, notation, simply as concept of the Bouma Sequence is so deeply rooted ‘structureless, with a sharp upper contact, and con- in the psyche of geologists that the standard geologic taining floating mudstone clasts near the top,’ then practice of maintaining a distinction between de- the description stands alone without any attached scription and interpretation is often totally lost when interpretation to its origin. Thus the former descrip- it comes to describing deep-water sands. For exam- tion leaves one no choice but to interpret the bed as a ple, Miall (1995, p. 379) asks, ‘I . . who would now turbidite. whereas the latter description allows for object to the use of Bouma’s (1962) five divisions alternate interpretations, such as sandy debris flows. (A-E) as a framework for the field description of The Bouma Sequence represents an interpretive turbidites?” I, for one, would. depositional model for the deposit of a turbidity In an observational science like geology. we must current (Fig. 1). Therefore, describing a deep-water always maintain a clear distinction between descrip- sand unit as T, is like describing a cross-bedded sand tion and interpretation. This is particularly critical for unit as a ‘braided stream deposit.’ Because the deep-water sands whose depositional origins are Bouma divisions are now so routinely applied during much more complex than the published literature field descriptions, it is almost impossible to know saturated with turbidite terminology would indicate. how many of the published examples of ‘turbidites’ For example, if a bed is described in the field as T, actually represent deposits of true turbidity currents. division (i.e., Bouma division A), it is difficult to This skepticism stems from the fact that the com- know from that description alone: (1) whether the plete and partial ‘Bouma Sequence’ can be explained bed is structureless (i.e., sands that to the naked eye by processes other than true turbidity currents. I will appear to be devoid of primary structures) and un- return to this point below (see Section 4). graded or normally graded; (2) whether the bed has a About a decade ago, 1 questioned the validity of

- Bouma (1962) Middleton and Lowe (1962) This study Divisions Hampton (1973) I I I I Pelagic and Pelagic and emipelagic

Fig. I. Ideal Bouma Sequence showing T,. T,. T,, Td, and T, divisions. Conventional interpretation is that the entire sequence is a product of a turbidity current (Bouma, 1962; Walker, 1965; Middleton and Hampton, 1973). Lowe (1982) considers that the T, division is a product of a high-density turbidity current and the T,. T,. and T,, divisions are deposits of low-density turbidity currents. In this study, the T,, division is considered to be a product of a turbidity current only if it is normally graded, otherwise it is a product of a sandy debris flow; the T,. T,, and Td divisions are considered to he deposits of bottom-current reworking. See text for details. G. Shanmugam/Earth-Science Reviews 42 (1997) 201-229 203 using the turbidite facies scheme of Mutti and Ricci (Vail et al., 1991). From the standpoint of petroleum Lucchi (1972) for interpreting submarine-fan envi- industry, the principal attraction to submarine-fan ronments (Shanmugam et al., 1985). Since then, I models with channels and lobes is that the fan model have had the rare opportunity to describe deep-water can be used to predict the distribution of turbidite sands totaling nearly 22,000 ft (6.7 km) of rocks, sand (i.e., the occurrence of sheet-like lobe sands most of them hydrocarbon bearing, from a number of downdip from channels). However, incorrect inter- areas known for deep-water ‘turbidite’ deposition. pretation of deep-water sands as turbidites can lead They include Tertiary basin-floor fans in the North to erroneous distribution of sand, and can have nega- Sea (Shanmugam, 1995; Shanmugam et al., 1995a, tive economic consequences. Although Walker 19961, the Cretaceous in the Norwegian Sea (1992) himself abandoned his popular fan model, (Shanmugam et al., 1994, 19961, the Pliocene in many petroleum geologists still cling to this defunct offshore Nigeria (Shanmugam et al., 1995b), the fan model (e.g., Coleman et al., 1994; McGee et al., Pliocene in offshore Equatorial Guinea (Famakinwa 1994). I attribute this phenomenon to a prevailing et al., 1997; Shanmugam et al., 1997b), the Creta- mind set on turbid&es that forces one, for no appar- ceous in offshore Gabon, the Pliocene-Pleistocene ent geologic reason, to the turbidite-dominated fan in the Gulf of Mexico (Shanmugam et al., 1993; model. Shanmugam and Zimbrick, 19961, and the Pennsyl- Although some of the problems that I raise here vanian Jackfork Group in the Ouachita Mountains of were raised 30 years ago by Sanders (19651, they Arkansas and Oklahoma (Shanmugam and Moiola, were ignored by the research workers of that time as 1994, 1995). Most of these examples were previ- a matter of convenience. Consequently, the turbidite ously interpreted as turbidites by other workers. problem has compounded itself into a monstrous However, I have reinterpreted them to be deposits of level today. Any further postponing of this issue is sandy debris flows, slumps, and bottom currents; only going to worsen the problem. Hopefully, this turbidites are extremely rare. These new interpreta- critical review will re-open the much needed debate tions have led me to critically evaluate the funda- on the fundamentals of turbidite deposition toward mentals of turbidity currents and their deposits, in- establishing what we know and what we do not cluding the Bouma Sequence. know. By design, this is an opinion-oriented review arti- cle because examples that I use here are exclusively from my previous publications in which I advocated 2. Turbidity currents and debris flows sandy debris flow (see Shanmugam, 1996a) and 2.1. Definitions of turbidity currents bottom-current reworking processes for deep-water sands rather than conventional turbidity-current pro- The core of the problem is the meaning of the cesses. To my knowledge, there are no other publica- term ‘turbidity current’. What is a turbidity current? tions to cite on this subject matter of reinterpretation Surprisingly, the definitions of turbidity currents with of turbidites as deposits of sandy debris flows and emphasis on sediment-support mechanism and rheol- bottom currents. However, there are other workers ogy have remained remarkably consistent over the who had criticized the concepts of turbidity currents past four decades: (e.g., Ten Haaf, 1959; Sanders, 1965; Van der Lin- (1) 1960s: “Turbidity currents are defined as gen, 1969) and the Bouma Sequence (e.g., Hsu, density currents caused by sediment in turbulent 1989). suspension.” (Sanders, 1965, p. 193). The current trend in sedimentology and sequence (2) 1970s: “Turbidity currents, in which the sedi- stratigraphy is to ignore the fundamental problems of ment is supported mainly by the upward component turbidite concepts, but to cherish the misguided fan of fluid turbulence.” (Middleton and Hampton, 1973, models with turbidite channels and lobes. For exam- p. 2). ple, turbidites are considered to form the very foun- (3) 1980s: “Turbidity currents are sediment flows dation for submarine-fan models in both sedimentol- in which the grains are suspended by turbulence.” ogy (Walker, 1992, fig. 6) and sequence stratigraphy (Lowe, 1982, p. 282). (4) 1990s: “Turbidity currents are one type of is linear. Such materials with strength are considered sediment-gravity flow in which the sediment is held to be Bingham plastic. For Bingham plastics, the in suspension by fluid turbulence.” (Middleton, 1993, criterion for initiation of turbulence is based on both p. 89). “More importantly, if a flow is laminar or the Reynolds Number, R, and the Bingham Number, nonturbulent it can no longer be considered as a B (Fig. 2). Although debris flows can develop turbu- turbidity current” (Middleton, 1993, p. 93). lence (Enos, 1977), such flows are not diagnostic of It is clear that turbidity currents cannot exist most debris flows that are laminar (i.e., no fluid without turbulence. It is also evident that turbidity mixing across streamlines). Johnson (1970) favored a currents must transport sediment via suspended load. Bingham plastic rheologic model for debris flows. The rheology of a sediment-water mixture is 2.2. Rheology ogfzuids governed mainly by sediment concentration and to a lesser extent by grain size and the physical and Similar to fluid turbulence of turbidity currents, chemical properties of transported solids (Pierson fluidal (i.e., Newtonian) rheology of turbidity cur- and Costa, 1987, p. 4). Although, the rheology is a rents has also been suggested (Dott, 1963; Nardin et complex parameter and is difficult to measure accu- al., 1979; Lowe, 1982; Shanmugam and Moiola, rately (Phillips and Davies, 1991), it is useful in 1995). The rheology of fluids can be expressed as a distinguishing turbidity currents from debris flows. relationship between applied shear stress and rate of Therefore, there should not be any confusion as to shear strain (Fig. 2). Newtonian fluids (i.e., fluids what the term ‘turbidity current’ means in terms of with no inherent strength), like water, will begin to fluid rheology and sediment-support mechanism. A deform the moment shear stress is applied, and the turbidity current is a sediment-gravity flow with deformation is linear (Fig. 2). For Newtonian fluids. fluidal (i.e., Newtonian) rheology and turbulent state the criterion for initiation of turbulence is the in which sediment is held in suspension by jluid Reynolds Number, R (ratio between inertia and vis- rurbulence. cous forces), which is greater than 2000 (Fig. 2). In contrast to Newtonian fluids, some naturally 2.3. Problem areas occurring materials (i.e., fluids with strength) will not deform until yield stress has been exceeded (Fig. In simple terms, a turbidity current can be envis- 2); once the yield stress is exceeded the deformation aged as an evolving event over a space-time contin-

Reynolds Number:

R=elt” P Shear R > 2000 = Turbulent R < 500 = Laminar Stress z Bingham Number:

B+

R = 1OOOB = Turbulent R/B = I%*/ K=l 000 = Turbulent

K= Strength p = Viscosity p = Density u = Velocity dt_&y = Rate of Change of Velocity D = Flow Thickness

Rate of Shear Strain (du/dy)

Fig. 2. Rheology (stress-strain relationships) of Newtonian fluids (turbidity currents) and Bingham plastics (debris flows). Compiled from several sources (Dott, 1963; Enos, 1977: Pierson and Costa. 1987; Phillips and Davies, 1991; Middleton and Wilcock, 1994). A fundamental rheological difference between debris flows (Bingham plastics) and turbidity currents (Newtonian fluids) is that debris flows exhibit strength, whereas turbidity currents do not. In general, turbidity currents are turbulent, and debris flows are laminar in state. G. Shanmugam/Earth-Science Reviews 42 (1997) 201-229 205 uum: it starts commonly in deep-water environments, Considering that debris flows and turbidity currents gathers momentum, perhaps erodes at first, then have distinctly different rheological and dynamical deposits, and finally dies. Because currents may properties, these two processes and their deposits increase or decrease in their energy conditions in should not be treated as one and the same. Turbidity time and space when they encounter an obstacle or a currents are fluidal (i.e., Newtonian) in rheology change in sea-floor gradient (e.g., Shanmugam and (Dott, 1963; Lowe, 1979; Nardin et al., 1979; Shan- Moiola, 1985, fig. 3; see also Shanmugam and mugam and Moiola, 1995, 19971, whereas debris Moiola, 19881, the same current may undergo flow flows are plastic in rheology (Do& 1963; Johnson, transformation and may not always maintain a tur- 1970) or they represent non-Newtonian fluids (Cous- bidity-current status by being fully turbulent or being sot and Meunier, 1996). Turbidity currents are con- fluidal over its entire life cycle (see Section 7.1). sidered as two-phase flow (water and solid), whereas Therefore, the concept of turbidity current is applica- debris flows are one-phase flow in which the whole ble only to those currents that exhibit the rheological mass undergoes large and continuous deformation and dynamical properties of turbidity currents in (Coussot and Meunier, 1996). Turbidity currents are space and time. Waning flows, for example, are fully turbulent in state (Middleton, 1993), whereas qualified to be true turbidity currents; however, de- debris flows are laminar (i.e., no fluid mixing across pletive waxing flows of Kneller (1995) should not be streamlines) in state (Johnson, 1970; Carter, 1975; considered true turbidity currents. This is because Middleton and Wilcock, 1994). Sediment in turbidity Kneller (1995, p. 37) equates waxing flows with currents is held in suspension by fluid turbulence traction carpets in explaining inverse grading; how- (Middleton and Hampton, 1973), whereas sediment ever, traction carpets are neither fluidal nor turbulent in debris flows is supported by matrix strength, (see Shanmugam, 1996a). From a depositional point dispersive pressure, buoyancy (Middleton, 1993). of view, waxing (accelerating) flows are not impor- Turbidity currents transport mainly fine-grained sedi- tant because turbidity currents commonly begin to ment because turbulence is their only sediment-sup- deposit sediment as they start to loose energy slowly port mechanism, whereas debris flows are capable of (i.e., waning currents). In other words, deposition of transporting sediment of all sizes because of their turbidites is an indication that the current is no multiple sediment-support mechanisms (matrix longer waxing or accelerating (Hsu, 1989). In some strength, dispersive pressure, buoyancy) and their cases, the most erosive part of the flow occurs during fluid strength. Turbidity currents in which grain-to- waxing flow (Valiance and Scott, 1997). grain contact is rare, whereas in debris flows grain- Another problem with the classification of turbid- to-grain contact is frequent. Turbidity currents in ity currents by Kneller (1995) is the category of which sediment concentration is low (l--23% by ‘uniform’ flows. If one accepts the definition of volume, Middleton, 1967, 1993), whereas in debris Middleton (1993) that a turbidity current must be flows sediment concentration is high (50~90%, in turbulent, then there cannot be a ‘uniform’ turbidity general, Coussot and Meunier, 1996). Turbidity cur- current. As opposed to laminar flows, turbulent flows rents in which sediment is settled from suspension are always nonuniform because of entrainment of grain by grain (Middleton and Hampton, 1973), overlying water (van Kessel and Kranenburg, 1996). whereas ,sediment in debris flows is deposited via This practice of lumping accelerating (waxing) flows freezing en masse (Johnson, 1970). and uniform flows under the term ‘turbidity current’ is a major source of confusion in the literature. 3. Deposits of turbidity currents 2.4. Turbidity currents vs. debris flows Deposition from turbidity currents commonly oc- Perhaps, the single most source of confusion is curs through sediment fallout from suspension the use of the term ‘turbidite’ for deposits of debris (Kuenen and Migliorini, 1950; Dott, 1963). In a truly flows (e.g., Labaume et al., 1987; Mutti, 19921, and turbulent fluidal flow, coarse- and fine-grained parti- traction carpets (Lowe, 1982; Postma et al., 1988). cles tend to settle separately during deposition de- 206 G. Shanmugam / Earth-Sciencr Rel~iews 42 C1997) 203-229

Kuenen ( 1953) This Study bedding. Normal grading (Kuenen and Migliorini, 1950) is the most reliable criterion to interpret fluidal rheology and suspension deposition of turbidity cur- rents (Dott, 1963). Although normal grading has been reported from deposits of debris flows (Val- lance and Scott, 19971, these deposits also contain floating clasts that are absent in turbidites. Following Kuenen (1953), it is a common prac- tice to interpret an entire bed as a turbidite (Fig. - 3C,E) even if grading is restricted only to the upper- most portion of the bed (Fig. 3D,F). For example, if Graded a 2 m thick sand bed has a 2 cm thick normally graded top, 1 would interpret only the 2 cm graded t top as a turbidite. The origin of the underlying sand requires independent evaluation using its own fea- Ungraded tures. In other words, the 2 cm graded top does not reveal anything about the depositional origin of the underlying sand. I have seen examples in which thick ‘massive’ sands with rafted ungraded clasts ‘1 (i.e.. debris-flow origin) have thin graded tops (i.e.. turbidity-current origin). In order to interpret a nor- Graded mally graded bed as a turbidite, one must describe the graded bed with precision; no exceptions should t be allowed in terms of floating quartz granules or rafted mudstone clasts within the ‘graded’ unit. Ungraded Deposits of turbidity currents are called turbidites (Bouma, 1962). In spite of this simple and straight- forward definition, the term ‘turbidite’ means differ- ent things to different people. To some, turbidite means any deep-water sand, and to others, turbidite Fig. 3. Three varieties of normally graded bedding (A, C, E) means a deep-water channel or lobe sand, but in this selected from Kuenen (1953). Vertical arrows show intervals of ‘normal grading’. Using grain sizes shown in (A). CC). and (E), a study turbidite means a deposit of a turbidity current. more realistic nature of grading (vertical arrows) is suggested in this study (B, D, and F). Note that the entire bed is normally graded only in case (B). In (D) and (F) cases, only the top 4. The Bouma Sequence: alternate interpretations portions are normally graded (small vertical arrows); the bulk is ungraded, or complexly graded. In this study, only normally graded portions (shaded grey) are considered deposits of turbidity Bouma (1962) established a standard sequence of currents. Fining-upward sequences, composed of multiple deposi- sedimentary structures for deposits of turbidity cur- tional events (D), should not be considered a single eortnally rents based on his study of 106 1 beds in the Mar- graded bed (C). Grain sizes in (B), (I)). and (F) are added in this itime Alps of southern France. Conventionally, the study. Bouma Sequence and its five divisions (Fig. I). namely T,. T,, T,, Td, and T,, are considered to be pending on their fall velocities. This causes deposits the product of a single turbidity-current event of turbidity currents to be characterized by normal (Bouma, 1962; Walker, 196.5; Mutti and Ricci Luc- size grading (i.e., upward decline in grain size) and chi, 1972; Middleton and Hampton, 1973, 1976). In gradational upper contacts (Fig. 3A,B). Because tur- the Bouma Sequence, only the T, division, if graded, bulent turbidity flows behave as Newtonian fluids, represents deposition from suspension; the other three one could infer Newtonian rheology from graded divisions, composed of horizontal and ripple lamina- G. Shanmugam/ Earth-Science Reviews 42 (1997) 201-229 201 tions (i.e., T,, T,, T,), are products of mostly trac- tion resulted from rapid ‘freezing’. The sediment- tion, or combined traction and suspension. In the support mechanism at the time of deposition of type area, however, less than 10% of the 1061 beds massive beds from high-concentration flows was dis- show the complete Bouma Sequence (e.g., T,, T,, persive pressure (Middleton, 1967, p. 495). This is T,, Td, T,). Most are top-absent (e.g., T,, T,, T,), expected because concentrations of 44% by volume middle-absent (e.g., T,, T,), or base-absent (e.g., T,, were used in the experiment for high-concentration Tdr T,) sequences (Walker, 1965). flows, which is within the range where dispersive pressures are important (Middleton, 1967, p. 480). 4.1. Debris-jlow origin of T, division At high concentrations (i.e., above 30% by volume), The basal Bouma division (T,) is defined as a the dispersions do not behave as a Newtonian fluid massive or a graded bed (Fig. 1). The origin of the (Middleton, 1967, p. 479). Middleton’s (1967) exper- Bouma T, division is controversial. It has been vari- imental flows meet all the criteria for mass flows ously ascribed to: (1) turbidity currents (Bouma, (i.e., debris flows), as defined by Dott (1963): (1) 1962); (2) antidune phase of the upper flow regime they are non-Newtonian flows that exhibit pseudo- (Harms and Fahnestock, 1965; Walker, 1967); (3) plastic behavior; (2) they are high-concentration grain flows (Stauffer, 1967); (4) pseudo-plastic quick flows in which the sediment is supported by disper- bed (Middleton, 1967); (5) density- modified grain sive pressure; and (3) they are deposited by ‘freez- flows (Lowe, 1976); (6) high-density turbidity cur- ing’. For these reasons, I ascribed the massive T, rents (Lowe, 1982); (7) upper-plane-bed conditions division to a sandy debris-flow origin (Shanmugam, under high rates of sediment feed (Amott and Hand, 1996b). Therefore, the routine interpretation of un- 1989); and (8) sandy debris flow (Shanmugam and graded massive deep-water sands as turbidites, and Moiola, 1995; Shanmugam, 1996b). more specifically, as T, divisions, is misleading. For Of these, Bouma’s (1962) turbidity-current inter- example, one could interpret the Paleocene (west of pretation is valid only if the bed is normally graded. Shetlands) sandstone bed and its thin mud cap as T, The antidune hypothesis can be eliminated based on and T, divisions, respectively (Fig. 4). However, empirical evidence (Allen, 1991). Stauffer (1967) floating quartz granules and rafted mudstone clasts suggested that the T, division is a product of grain indicate flow strength, and therefore, the sandstone flows. According to Lowe (19821, grain flows are should be interpreted as a product of sandy debris considered to exhibit plastic rheology. If so, it is flow (Shanmugam et al., 1995a). difficult to consider the T, division as a deposit of a In explaining the origin of ‘massive’ sands by turbidity current that is thought to exhibit fluidal processes other than turbidity currents, Sanders rheology. (1965, p. 193) argued that there are no physical Middleton and Hampton (1976, p. 215) indicated mechanisms known by which sand being transported that the T, division, if massive, may owe its origin to by turbulent suspension can be deposited without a transient flow composed of grain flows, fluidized passing through the tractional ranges in the process flows, and debris flows. Based on experiments, Mid- of deposition. dleton (1967) proposed that massiue portions of Mutti and Nilsen (1981) explained the floating turbidite beds could result from high-concentration mudstone clasts in T, sandstone by “deposition en ‘turbidity flows’. Following Middleton’s (1967) sug- masse of the denser portion of turbidity currents gestion, the ‘massive’ sands of deep-sea sequences freezes the rip up clasts”. Because en masse deposi- are routinely interpreted as deposits of ‘high-density tion by freezing is characteristic of plastic flows turbidity currents’ (Mutti and Ricci Lucchi, 1972; rather than fluidal flows, I would attribute these Lowe, 1982; Pickering et al., 19891, or as the basal rafted clasts to a sandy debris-flow origin. Also, a Bouma division (i.e., T,). planar clast fabric, attributed to the T, division by Middleton (1967) reasoned that massive beds were Mutti and Nilsen (198 l), is more indicative of lami- deposited because of the formation of an expanded nar flow conditions in plastic debris flows than ‘quick’ bed that behaves as a pseudo-plastic unit. turbulent flow conditions in turbidity currents (John- Middleton (1967, p. 495) also suggested that deposi- son, 1970; Fisher, 1971; Shanmugam and Benedict, 1978). In the past, the turbidite paradigm was so rent’ is a misnomer, and it should be replaced by the influential that even the strong field criteria for term ‘sandy debris flows’ to avoid confusion regard- plastic rheology (e.g., floating mudstone clasts) and ing the rheology of fluids and the state of flow laminar flow conditions (e.g., planar clast fabric). (Shanmugam, 1996a). which are characteristic of debris flows, were as- In short, what some may interpret as the basal cribed to turbidity currents (e.g., Mutti and Nilsen, division of the Bouma Sequence (i.e., Ta) may in 1981). reality represent deposits not only of turbidity cur- According to Lowe (1982), the T, division is not rents but also debris flows. Herein lies the challenge part of the Bouma Sequence. Lowe (1982) consid- of distinguishing one from the other. Field evidence ered the T, division as a deposit of a high-density for rheology of the flow (plastic vs. fluidal), state of turbidity current, and the overlying T,, T,, and T,, the flow (laminar vs. turbulent), sediment-support divisions as deposits of a low-density turbidity cur- mechanism (matrix strength/dispersive pressure vs. rent (Fig. 1). In terms of fluid rheology, high-density fluid turbulence), and depositional mechanism (freez- turbidity currents are interpreted to represent plastic ing vs. settling) should be looked at critically in flows (Shanmugam and Moiola, 1995, fig. 7). 1 also distinguishing whether the T, division was deposited suggested that the term ‘high-density turbidity cur- by a debris flow or by a turbidity current.

Normal grading --- (Turbid&) Te? 1-1111-1--1

Ta? Rafted mudstone clasts of different sizes and floating quartz granules (Sandy debris flow)

Fig. 4. A massive sandstone unit with a thin mudstone cap would conventionally be interpreted as T, and T, Bouma divisions. Concentration of mudstone clasts near the top of the sandstone unit would be interpreted as T, division using Mutti and Nilsen (1981) criteria. However, these floating mudstone clasts of various sizes and probably of similar density, and floating quartz granules (arrows) in a fine-grained sand matrix suggest flow strength. Note that the upper sandstone unit shows normal grading (vertical arrow), indicating deposition from turbidity current (i.e., turbidite). Paleocene, west of the Shetlands. From Shanmugam et al. (1995a). G. Shanmugam / Earth-Science Reviews 42 (1997) 201-229 209

4.2. Bottom-current origin of T,,, T, and Td divisions Therefore, not all bed-load deposits can be inter- preted routinely as turbidite beds; associated normal A common problem in interpreting deep-water grading is the key in establishing a turbidity-current sands is the occurrence of current ripple laminae, origin. However, climbing ripples may be used as a usually interpreted as representing the T, division, criterion for turbidity-current deposition (Sanders, and parallel laminae, usually interpreted as represent- 1963, 1965). This is because when sands fall out of ing the T, and T,, divisions. Walker (1992, p. 242) suspension while the turbidity current is moving routinely classifies these rippled beds as ‘thin-be- within the ripple bedform range, climbing ripples are dded turbidites’ without regard for their true origin produced. Such ripples could be distinguished from (i.e., turbidity current vs. bottom current). The term the tractionally formed ripples by bottom currents. bottom current refers to currents unrelated to turbid- One popular interpretation is that the rippled sands ity currents in the deep sea, and bottom-current represent overbank turbidity-current deposits of a deposits are characterized by traction features channel-levee complex in the Gulf of Mexico (e.g., (Hollister, 1967; Shanmugam et al., 1993). Natland Shew et al., 1994). However, rippled sands have also (1967) distinguished these deposits with traction been interpreted as products of bottom-current re- structures as ‘tractionites’ from ‘turbidites’. working in the Gulf of Mexico (e.g., Shanmugam et Shepard et al. (1969) interpreted laminated sand al., 1993). Hsu (1989) also suggested that the rippled with concentration of heavy minerals as deposits of sands of the T, division can be deposited by marine bottom-current reworking in the La Jolla canyon. bottom currents unrelated to turbidity currents. The Bottom-current measurements of velocities in the La routine interpretation of discrete rippled sands as Jolla canyon show 34 cm per second (Shepard and levee deposits needs reevaluation; it assumes deposi- Marshall, 1969); such velocities are sufficient to tion from turbidity currents without evidence. I do erode and transport fine-grained sand. not claim that ripples cannot occur on levees, but I In deep-water settings, if a sharp-based bed in do suggest that ripples can also occur as discrete which a planar-laminated interval (T,) passes up into units formed by bottom currents (Fig. 51, unrelated a rippled interval CT,), it is a common practice to to levees or turbidity currents. In other words, the interpret such traction features as a product of decel- occurrence of ripples in a deep-water sequence is not erating turbulent flow dropping sediment from sus- ironclad proof for levees, as is widely believed under pension onto a well defined sediment bed and then the current turbidite paradigm. transporting sediment as bed load under the overly- In addition to traction structures, such as cross- ing flowing suspension. Under this scenario, the bedding, horizontal lamination, and isolated current settled sediment from a turbidity current will pre- ripples, bottom-current reworked sands also exhibit serve its original depositional features (e.g., normal internal erosional surfaces indicating pulses of in- grading) only if the sediment is halted from any creased current energy, mud offshoots suggesting further movement; however, if the settled sediment is oscillating energy conditions, variable current direc- subjected to further transport as bed load, it will not tions, and sharp upper contacts (Shanmugam et al., only lose its original depositional features of turbid- 1993). Many of these features are difficult to explain ity currents (e.g., normal grading) but also will de- by downslope-flowing decelerating turbidity cur- velop new (i.e., reworked) depositional features that rents. An important attribute of these traction struc- will be analogous to rippled bedding of bottom tures is that they commonly occur in discrete units currents. This is because traction structures caused unassociated with normally graded beds. In the ab- by bed-load transport are diagnostic of bottom-cur- sence of associated graded beds, it is difficult to rent deposits as well (Hollister, 1967; Shanmugam et envision deposition by turbidity currents. Although al., 1993). In such cases, it is difficult to recognize both turbidity currents and bottom currents exist in from the depositional record whether sands in a the deep sea, it is easier to explain features like mud parallel-laminated or ripple-laminated interval were offshoots and variable current directions by bottom originally transported as suspended load of a turbid- currents than by decelerating turbidity currents. ity current or as bed load of a bottom current. Lenticular laminae caused by starved ripples that 210 G. Shanmugum / Earth-Science Rer,itws 42 (I9971 201-229

turbidites (Pickering et al., 1989). The absence of cross-bedding is ascribed to various causes, such as flows being too rapid (Walker, 1965), or flows being too thin (Walker, 1965), or flows being too fine- grained (Walton, 1967). Hsu (1989) explained the absence of cross-bedding in turbidites by a critical Froude Number; for example, in turbidity currents that flow fast enough to transport sand in suspension (i.e., Froude Number > 0.35-0.40), dune bedforms cannot develop (see also Section 6.3). Large-scale features (1 O-80 m in height), such as ‘migrating mud waves’ or ‘abyssal bedforms’ in the deep sea, have been reported (e.g., Klaus and Led- better, 1988; Flood, 1988). However, they should not be equated with dune bedforms in rivers that create cross-bedding due to bed-load transport of granular material, which must be at least 125 Frn in grain size (i.e., fine sand). Deep-sea migrating waves are composed primarily of silt and clay, and therefore they do not have the necessary grain size to generate cross-bedding. Mud waves are ascribed to sculpting of muddy sea floor by deep bottom currents, such as the Antarctic Bottom Water (AABW) in the Argen- tine Basin (Klaus and Ledbetter, 1988). Piper et al. (1988) suggested that deep-sea gravel waves in the Grand Banks area are products of bed-load transport by turbidity currents, analogous to dune bedforms in subaerial rivers, involving traction Fig. 5. Core photograph showing discrete sand layers comprised processes. The implication is that these gravel waves of current ripples with variable dip directions suggesting multiple current directions. Preserved (lower arrow) and eroded (upper are composed of cross-bedding; however, no core arrow) tops of ripples indicate variable energy conditions of the information is available to prove the presence of current. Note the absence of normally graded units. All these sand cross-bedding in these gravel waves. Hsu (1989) layers could be interpreted as the ‘T,’ division; however, these proposed an alternate, debris avalanche, origin for traction structures are considered evidence for bottom-current the gravel waves in the Grand Banks area. reworking. Middle Pleistocene, Ewing Bank Block 826, Gulf of Mexico. From Shanmugam et al. (1993). Giant sediment waves (5 m in height), composed of sand and boulders, on the continental margin off Nice (southern France) were ascribed to deposition could be interpreted as representing the T, division by ‘sediment flows’ (Malinvemo et al., 1988). Sedi- are common in the Pliocene and Pleistocene of the ment flows are a combination of both debris flow Gulf of Mexico. However, lenticular laminae, inter- and turbidity current (Malinvemo et al., 1988). How- preted to be of bottom-current origin, have been ever, it is not clear how these sediment waves were reported from DSDP leg 28, Site 268, in Antarctica deposited by two rheologically different flows (i.e., (Piper and Brisco, 1975). Starved ripples are more plastic debris flows and fluidal turbidity currents). likely to be formed by relatively sediment-free So, what does the complete Bouma Sequence ‘clear-water’ bottom currents than by sediment-laden really mean in terms of its turbidity-current origin? turbidity currents (Shanmugam et al., 1993). The Ta division, if massive, I would prefer to inter- It is well known that large-scale cross-bedding pret it as a product of sandy debris flow when it (dune bedforms) is generally absent in Bouma-type shows evidence for plastic rheology and laminar G. Shanmugam/Earth-Science Reaiews 42 (19971201-229 211 flow conditions. However, the T, division, if nor- debris flows and other deep-sea processes. mally graded, implies deposition from a turbidity It is not surprising, therefore, that in many cases current. Because the T, division can be both massive the term ‘turbidite’ actually refers to deposits of (i.e., debris flow) and graded (i.e., turbidite) under debris flow (i.e., plastic rheology and laminar state) the current Bouma Sequence (Fig. 11, it is awkward and traction processes that are unrelated to true to use the same T, division for deposits of both turbidity currents as defined earlier. The classic cases debris flows and turbidity currents. are the ‘fluxoturbidite’ (Dzulynski et al., 19591, and If the T,, T,, and Td divisions occur as part of a ‘atypical turbidite’ (Stanley et al., 1978) that refer to complete Bouma Sequence with a basal graded divi- deposits of complex mass flow processes, such as sion CT,), which is extremely rare in the rock record, slumps, debris flows, and sand flows. I have selected the sequence may be interpreted to represent deposits seven other published examples to demonstrate my of turbidity currents. However, if these traction struc- point (Fig. 6). They include: (1) ‘megaturbidites’ tures occur as discrete units unassociated with basal referring to deposits of large-scale debris flows with graded beds, or if they occur in association with plastic rheology (Labaume et al., 1987); (2) ‘high- basal massive beds (i.e., sandy debris flows), I would density turbidity currents’ referring to inertia flows prefer to interpret them as traction deposits formed (i.e., laminar state) (Postma et al., 1988); (31 ‘high- by reworking of bottom currents. In other words, density turbidity current’ referring to ‘traction car- even the complete Bouma Sequence can be inter- pet’ (i.e., laminar state) that is not part of turbulent preted as deposits of processes other than turbidity suspension (Lowe, 1982); (4) Bouma T, referring to currents (Shanmugam, 1996b). pseudo-plastic quick bed and freezing deposition (Middleton, 1967); (5) Bouma T, referring to lami- nar flows and freezing deposition (Mutti and Nilsen, 5. The turbidite mind set 1981); (6) Bouma T, referring to traction processes (Shew et al., 1994); and (7) ‘turbidity currents’ The long-standing belief (i.e., the mind set) that referring to non-turbulent (i.e., laminar state) flows most deep-water sands are products of turbidity cur- (McCave and Jones, 1988). Turbidity currents sim- rents in a submarine-fan setting appears to be over- ply cannot exist without turbulence (see Middleton, stated. There are historical reasons for this mind set. 19931, and therefore, non-turbulent turbidity currents In an important discussion on a paper by J.E. Sanders, of McCave and Jones (1988) are the ultimate exam- for example, PH.H. Kuenen states, “Deposits from ple of an oxymoron. all kinds and combinations of currents falling under It should be clear from these examples that any the definition of turbidity currents are turbidites deep-water deposit can be interpreted as a product of [italics mine], whether there was bottom traction, a turbidity current, no matter what the rheology or laminar flow, non-turbulent flow etc. involved or sediment-support mechanism of the flow is. On the not.” (see Sanders, 1965, p. 218). This non-rigorous, other hand, if we wish to broaden the definition of ‘anything goes’, approach of lumping turbulent and ‘turbidity current’ to include all kinds of deep-sea laminar flows under the umbrella term ‘turbidity processes, then there is no need for a classification of current’ allows for inclusion of debris flows under sediment-gravity flows based on rheology and sedi- ‘turbidity currents’. This approach is quite prevalent ment-support mechanism. Until we resolve this fun- today. Mutti (1992, p. 401, for example, states, damental issue, most deep-water deposits will con- “Cohesive debris flows and turbidity currents should tinue to be interpreted as turbidites whether these therefore be considered the two main mechanisms sediments were deposited by turbidity currents or responsible for having transported and deposited the not. This explains why many examples that I have bulk of turbidite [italics mine] sediments.” Our fail- reinterpreted as debris flows were previously inter- ure to distinguish turbidity currents from debris flows preted as turbidites by other workers. The primary based on fluid rheology, state of the flow, and reason for interpreting any deep-water deposit as sediment-support mechanism, has resulted in a spe- some kind of turbidite is that it allows us to place cial category of ‘turbidity currents’ that includes these deposits in a predictable submarine-fan setting. G Shmmugun~ / Eurth-Science KW~CM..Y 42 ( I9971 201-229

Lithofacies Process Interpretation

Normally Turbidity Current I graded sand (Fluidal rheology) Megaturbidite 2Y&to m Debris flow (Labaume et al., 1987) Breccia (Plastic rheology)

I

Traction 3

Normally Suspension High-density turbidite graded sand (Postma et al., 1988) Inertia flow Gravelly sand (Laminar state) i

Inversely Traction carpet High-density turbidite graded (Laminar state) (Lowe, 1982) sand

Pseudoplastic Bouma Ta Massive quick bed (Middleton, 1967) sand (Plastic rheology)

Sand with Freezing Bouma Ta rafted clasts (Laminar state) (Mutti & Nilsen, 1981)

Rippled Traction Bouma Tc sand (Bed load) (Shew et al., 1994)

Non turbulent Muddy turbidite F”Fded (Plastic rheology) (!&Cave & Jones, 1988)

Fig. 6. Compilation of published examples showing that any deep-waler deposit, no matter what its primary sedimentary features are, can be interpreted as some kind of ‘turbidite’. Left-hand column shows published examples with various lithofacies and associated features. Middle column shows depositional processes suggested by the original authors. Right-hand column shows turbidite interpretation by the original authors. I have constructed diagrams of massive sand, rippled sand, and ungraded mud, and added information on fluid rheology, flow state, and nature of sediment load for this study (given in parentheses in the middle column). Note that sand with normal grading as wells as sand with inverse grading have been interpreted to be a turbidite.

Our affinity to fan models embedded in our psyche describing deep-water sands using the Bouma Se- subconsciously drives us to interpret any deep-water quence. This description is model-driven. Once a bed deposit as a turbidite (Fig. 6). is described using the Bouma Sequence, the in- A possible reason for the overwhelming number evitable interpretation would always be a turbidite- of published examples of turbidites is the practice of dominated submarine-fan model. Field studies car- G. Shanmugam/ Earth-Science Reviews 42 (1997) 201-229 213 ried out during the past 25 years by many on the also have discovered that beds described as individ- Pennsylvanian Jackfork Group in the Ouachita ual Bouma T, divisions earlier are generally com- Mountains of Arkansas and Oklahoma serve to illus- prised of amalgamated events of complex origin, and trate my point. This deep-water sequence was con- that ‘complete Bouma Sequences’ are actually com- sidered a classic ‘flysch’ in North America (Cline, posed of multiple depositional events; this contrasts 1970; Morris, 1977) and was thought to be deposited sharply with the conventional view that the complete by turbidity currents in a submarine-fan setting by Bouma Sequence represents a single depositional many workers including me (Morris, 1977; Moiola event. Our reinterpretation of the Jackfork Group has and Shanmugam, 1984; Shanmugam et al., 1988a; created substantial controversy (see Shanmugam and Jordan et al., 1991; DeVries and Bouma, 1992; Moiola, 1997). Mutti, 1992). In an earlier study (Moiola and Shan- In addition to the outcrop study of the Jackfork, mugam, 19841, we described (1200 ft or 365 m) beds examination of conventional cores from a number of of the Jackfork Group using the Bouma divisions. reservoirs previously interpreted as classic examples We described structureless sandstone beds as the T,, of submarine fans in the North Sea and claimed to be and beds with ripple laminations as the T, divisions. composed of turbidites (e.g., Frigg Field, Walker, Not surprisingly, like many others, we ended up 1992, fig. 6) revealed that 80-90% of the sediments publishing a turbidite-dominated submarine-fan can be interpreted as deposits of sandy debris flows model for the Jackfork (Moiola and Shanmugam, and slumps; turbidites comprise less than 1% 1984). However, the extreme rarity of normally (Shanmugam et al., 1995a). The predominance of graded beds in the Jackfork has been an unresolved interpreted turbidites in these areas by other workers issue until recently (Shanmugam and Moiola, 1995). can be attributed to the following: (1) loose applica- To resolve the issue of the missing normally tions of turbidity-current concepts without regard for graded beds in the Jackfork, we began to slab the fluid rheology, flow state, and sediment-support ‘massive’ sandstone beds and examined them in mechanism that result in a category of ‘turbidity polished samples and thin sections. To our surprise, currents’ that includes debris flows and bottom cur- normally graded beds are truly rare in these sand- rents; (2) field description of deep-water sands using stones. These samples, however, revealed many new, the Bouma Sequence that invariably results in diagnostic features, including: (1) concentration of model-driven turbidite interpretations: (3) the pre- rafted mudstone clasts near the tops of sandstone vailing mind set that most deep-water sands were beds (flow strength, rigid plug); (2) inverse grading deposited by some kind of turbidity currents; and (4) of clasts (flow strength and buoyant lift); (3) planar the attraction to obsolete submarine-fan models with clast fabric (laminar flow); (4) contorted bedding turbidite channels and lobes. Sure, there are tur- (plastic deformation); and (5) moderate to high detri- bidites in these areas, but they are very rare and tal matrix (plastic flow). These features cannot be comprise less than 1% of the nearly 2 1,000 ft (6402 interpreted using the Bouma divisions. We then reex- m) of cored intervals that I described in detail at a amined the entire measured sections systematically scale of 1 : 25 or 1 : 40. A few rare beds of turbidites in the field in light of the new information obtained do not make a submarine fan! from the slabs and thin sections. This second time Another reason for the prevailing perception that around, we avoided use of the Bouma Sequence in turbidites are the most common deep-water facies is our field description. As a result, the massive sands the misuse of the terminology ‘depositional lobe’ for (i.e., T, division before) were reinterpreted as sandy modem fans, such as the Mississippi Fan (Nelson et debris flows, and the rippled/parallel laminated al., 1992). The concept of depositional lobe was sands (i.e., T, and Td divisions before) were reinter- derived mainly from the ‘classic’ areas for turbidite preted as bottom-current reworked sands. We have deposition, such as the Miocene Mamoso-arenacea now reinterpreted the classic submarine-fan setting Formation in Italy (Ricci Lucchi, 198 1) and the attributed to the Jackfork as a slope setting domi- Eocene Hecho Group in Spain (Mutti, 1977). By nated by sandy debris flows and slumps (Shanmu- conventional definition, depositional lobes are domi- gam and Moiola, 1995). From the Jackfork study, we nated by classic turbidites (i.e., beds exhibiting nor- ma1 grading with all five divisions of the Bouma depositional lobes (Shanmugam and Moiola, 199 1). Sequence) with thickening-up trends (Mutti and Ricci The outer fan areas of the Mississippi Fan, for Lucchi, 1972; Mutti, 1977; Shanmugam and Moiola. example, are commonly used as the modern analog 1991). Ironically, even in the classic areas for tur- for turbidite fans with sheet-like geometries bidite lobe deposition, such as the Miocene (Shanmugam et al.. 1988b). Such a notion was based Mamoso-arenacea Formation in the northern Apen- strictly on parallel and continuous reflection patterns nines (Ricci Lucchi, 1981), I find that normally observed on seismic profiles (Shanmugam et al., graded bedding is very rare. I988b). However. new SeaMARC 1A sidescan-sonar Piston and gravity cores (Fig. 7) taken from the data (Twichell et al., 19921, and piston and gravity ‘depositional lobe’ of the modern Mississippi Fan cores (Nelson et al., 1992; Schwab et al., 1996) (Schwab et al., 1996) shows a dominance of debris taken from channels in the outer Mississippi Fan flows (Fig. 8). Therefore, the use of the term ‘de- reveal the following: (1) the terminus of the Missis- positional lobe’ for areas dominated by debris flows sippi Fan is not sheet-like as previously thought; (2) perpetuates the notion that turbidites are more com- contrary to popular belief, the terminus of the Mis- mon in modem fans than they actually are. I am not sissippi Fan is channelized; (3) channels in the termi- aware of a single modem fan in which depositional nus of the Mississippi Fan are filled with debris lobes are composed of classic turbidites with thick- tlows for the most part (Fig. 8); and (4) debris flows ening-up trends that have been documented using can travel hundreds of kilometers on gentle slopes. long continuous cores. I am even unsure whether The channel-fill debris flows also support the view there are such things called ‘typical depositional that the processes that cut the channels were not lobes’ or even ‘typical submarine fans’ in modern necessarily the same processes that filled the chan- oceans! The term ‘lobe’, similar to the term ‘turbi- nels because debris flows are generally non-erosive. dite’, is used loosely without any precision. The lobe The non-erosive nature of subaqueous debris flows problem was discussed elsewhere (Shanmugam. can be attributed to hydroplaning (Mohrig et al., 1990; Shanmugam and Moiola, 1991). 1997). Conventionally, sheet-like geometries are associ- Channel forms (e.g., sinuous) observed on ampli- ated with turbidites deposited at the terminus of a tude extraction maps of subsurface data may not submarine fan. These sheet sands are also known as necessarily reveal anything about the nature of chan-

PC39 GC50 PC28

TC43 GC51 GC44 PC29 PC38 PC37

PC52 PC53 PC54

0 PC: Piston Cores . GC and TC: Gravity Cores

Fig. 7. Map showing location of piston and gravity cores taken from channels in outer Mississippi Fan, Gulf of Mexico. Compiled from Twichell et al. (1992) and Schwab et al. (1996). G. Shanmugam / Earth-Science Reviews 42 (1997) 201-229 215

Cores of S !O f Lobe 8 PC29 1 GC44 1 TC43 1 GC51 I- I =R=

60 ‘2; a- 70 D>T .-I T=O% 2 60 3 s 50 ‘Z 3 :: 40 $ 30

(Percentage of facies was calculated usmg aata from Schwab et al., 1996)

Fig. 8. Histograms showing dominance of debris-flow facies in cores taken from channels in the outer Mississippi Fan (see Fig. 7 for location of cores). Percentages of facies were calculated by the author using published data from Schwab et al. (1996). Note that all nine cores contain debris flows, whereas only three cores comprise turbidites. In seven out of nine cores, the amount of debris-flow facies far exceeds the amount of turbidite facies. In core CC 44. debris flows comprise 100%. This facies distribution has important implications for submarine-fan models. See text for details. nel fill (i.e., sand vs. mud, or turbidite vs. debris the complete Bouma Sequence, which is very rare in flow, etc.). Also, what appears to be sheet-like on nature, can be explained by a non-turbidity-current seismic scale (e.g., Mississippi Fan, Shanmugam et origin (e.g., basal debris-flow deposit with overlying al., 1988b) may actually be composed of channelized bottom-current deposit, see Shanmugam, 1996b). To bodies on smaller scales (e.g., Mississippi Fan, my knowledge, no one has ever reproduced the Twichell et al., 1992) that are beyond the resolution complete Bouma Sequence deposited from suspen- of conventional seismic data. For these reasons, de- sion by a single turbidity current in the laboratory veloping depositional models using seismic geome- (e.g., Kuenen, 1966; Middleton, 1967; Luthi, 1981). tries alone without the benefit of core should proceed with caution. The prevailing turbidite mind set can be at- 6. A test of turbid& interpretation tributed, at least in part, to the constant promotion of the Bouma Sequence in synthesis articles on facies 6.1. Normal grading models. For example, Walker (1984, Walker, 1992)) continually suggests that the Bouma Sequence is the Normal size grading is the only reliable criterion one example that fulfills all four functions of a facies to interpret a deep-water sand as a turbid&. The model so well (i.e., norm, framework, predictor, and following test can be administered to measure the basis for environmental interpretation). Does it? The validity of an interpretation of a deep-water sand as a Bouma Sequence can serve all four functions of a turbidite. If an interpretation is based on observa- facies model only if we assume that there are no tions, such as the presence of sharp basal contact, processes other than turbidity currents that can gen- normal size grading, and gradational upper contact, erate various divisions of the Bouma Sequence. As I then the sand may be reasonably interpreted as a have tried to show, such an assumption is false. Even turbidite (Fig. 3A,B). But if a turbidite interpretation 216 G. Shanmugan~ / Earth-Sciencv h’eriewx 42 (I 997) 201-229 is based on excuses, such as the normal grading is tures in the past are probably not the processes that absent because of uniform grain size, or the normal will fill them in the future. Furthermore, scour sur- grading is absent because of short distance of trans- faces can also be created by processes other than port, or the normal grading is absent because of turbidity currents, such as geostrophic currents sediment deformation, or the normal grading is ab- (Myrow and Southard. 1996), and bottom currents sent because of bioturbation, or the gradational upper (Klein, 1966). Regardless, interpretations of origin of contact is absent because of possible erosion and so deep-water sands should be based on their internal on, then it is not possible to demonstrate that it is a depositional features, not on their erosional basal turbidite. contacts or sole marks. My experience is that even in sandstones with nearly uniform grain size, there are enough features 6.3. Turbidity currents L‘S ricer currents indicating plastic rheology and laminar flow condi- tions to interpret them as deposits of sandy debris Another common practice is to compare deposits flows. The practice of using the absence of normal of subaqueous turbidity currents with those of sub- grading as the basis for interpreting turbidites defies aerial river currents on the ground that both currents the very foundation of geologic interpretation based are turbulent, and therefore their deposits must be on observation. The problem with this twisted logic quite similar. This is not true. River currents and is that it allows one to interpret a variety of deep- turbidity currents are not one and the same, although water deposits as turbidites, irrespective of whether both are turbulent. River currents are low in sus- the bed shows normal grading or not (Fig. 6). pended sediment (l-5%), whereas turbidity currents are relatively high in suspended sediment (l-23%, 6.2. Erosional ~1s.depositional ,features see Shanmugam, 1996a), although both currents are considered to be Newtonian in rheology. River cur- It is a common practice to interpret deep-water rents are ,fluid-gravity flows, whereas turbidity cur- sands that contain flutes and scour surfaces as tur- rents are .yediment-gravity flows (Middleton, 1993). bidites (e.g., Hiscott and Middleton, 1979; Shan- In river currents, sand and gravel fractions are trans- mugam and Moiola, 1995). ported primarily by bed load (traction) mechanism, According to this conventional wisdom. scour and therefore river deposits are characterized by surfaces at the bottom of a sand can be used to infer dune bedforms (cross-bedding). In contrast, sands in deposition of the sand by turbulent flows (Hiscott turbidity currents are transported by suspended load, and Middleton, 1979; Shanmugam and Moiola. and thus sandy turbidites show a general lack of 1995). However, I now question this wisdom for two cross-bedding. reasons (see Shanmugam and Moiola, 1997). First, Lowe (1982) claimed that high-density turbidity large-scale erosional surfaces can also be created by currents can generate cross-bedding, but the concept mass movements (e.g., slump scars), not just by of high-density turbidity currents is highly controver- turbulent flows; distinguishing the origin of erosional sial (see Section 7.2, see also Shanmugam, 1996a). surfaces by mass movements vs. large-scale turbu- High sediment concentration (C > 20-25 vol.%) in lent flows is almost an impossible task in outcrop. high-density turbidity currents not only results in Second, although small-scale scour surfaces and non-Newtonian behavior (e.g., Rutgers, 1962), but flutes may suggest turbulent state of the flow in also tends to damp the turbulence in the lower part some cases, this does not mean that the sand that of the flow (Postma et al., 1988). High sediment rests on a scour surface was deposited by the same concentration increases the flow strength and de- turbulent flow that created the scour surface (Sanders, creases its boundary resistance, such that the flow’s 1965, p. 209); for example, scour surfaces can be behavior can be effectively supercritical even though created by turbulent flows and filled later by debris its Froude Number may appear to be less than unity flows or other processes. Modem unfilled submarine or subcritical (Nemec, 1990). Furthermore, flow den- channels and canyons are a testimony to the fact that sity and velocity are highly variable within high-den- the processes that had created these erosional fea- sity turbidity currents. For these reasons, the applica- G. Shanmugam / Earth-Science Reviews 42 (1997) 201-229 217 tion of ‘Newtonian’ Froude Number to high-density it may subsequently develop grain-dispersive charac- flows may be questionable (Nemec, 1990). This is teristics. Then, as shear rates are reduced, say, by a important here because the presence or absence of reduction in bed slope or by jamming of coarse cross-bedding in turbidites has been explained by a grains in the channel, the flow may once again critical Froude Number (see Section 4.2). exhibit plastic-viscoplastic behavior.” In other It is a common practice to use a gravel in an words, a debris flow may transform into a grain alluvial conglomerate to estimate the velocity of flow, and then back to a debris flow again. Similarly, river currents on the basis of the Shield diagram; transformation of grain flow to turbidity current and however, the same approach to estimate the speed of returning to grain flow during the last stages of sediment-gravity flow on the basis of the largest deposition has been suggested by Middleton (1970). clast in a deep-water sequence is not meaningful Such transformations are common in mass flows. (Hsu, 1989). Similarly, velocity-size diagrams (e.g., In experiments on ‘high-density turbidity cur- Harms et al., 1975) are meant for bed-load domi- rents’, for example, the basal laminar flow that de- nated river currents, and may not be applicable to posited the traction carpet was initially fully turbu- turbidity currents in which suspended load is the lent, but during the depositional stage the turbulent dominant mode of transport. flow was transformed into quasi-plastic laminar flow Normal grading is rare in river deposits (Hein, (Postma et al., 1988). Similarly, experimental studies 1984) because of dominant bed-load transport, show that massive sands were transported by turbid- whereas normal grading is common in. turbidites ity currents, but were deposited by freezing (Vrolijk because of dominant suspended load transport. Float- and Southard, 1982). In other words, these massive ing pebbles and clasts are common in river deposits sands could be considered analogous to the Bouma because of bed-load transport in which grain size T, division during transport by turbidity currents, but does not play a major role during deposition. In their freezing mode of deposition could be inter- contrast, floating pebbles are absent in turbidites preted as deposition from a sandy debris flow. This because pebbles tend to settle first in comparison to flow transformation from turbulent to laminar state is sand during deposition from suspension. shown in Fig. 9B. Experimental studies also show that plastic debris flows can be diluted to develop fluidal turbidity 7. Topics of future research currents during transport (Hampton, 1972). Flow transformations can occur in both density-stratified 7. I. Recognition of transport mechanism (Postma et al., 1988) and density-unified flows in subaqueous environments. Clearly, some sediment- Contrary to popular belief, there are no estab- gravity flows undergo flow transformation (i.e., lam- lished criteria for recognizing transport mechanism inar to turbulent and vice versa) prior to deposition from the depositional record (Middleton and Hamp- (Fig. 9B,C). In a dd’1t’ ion, there are sediment-gravity ton, 1973; Lowe, 1982; Postma, 1986; Middleton, flows that do not undergo any flow transformation 1993; Shanmugam, 1996a). In fact, any discussion of (Fig. 9A,D). Th e c h a 11enge in the depositional record long-distance sediment transport in this review is is how to distinguish flows that underwent flow meaningless because all that we can infer from the transformation from flows that did not. Whether a depositional record is flow rheology and state that flow underwent transformation or not, its deposit existed during the final moments of deposition. In will reflect flow conditions that existed only during theory, we tend to assume that depositional processes the final moments of deposition (Fig. 9). Conse- must be the same as transportational processes. In quently, evidence for flow transformation is not pre- reality, however, such assumptions may not be true served in the final deposit (Fig. 9B,C), and there are in all cases because of flow transformation during no field criteria to infer transport processes from the transport. depositional record (Shanmugam, 1996a). Phillips and Davies (1991, p. 109) noted, “ . . . al- Many of us use this universal constraint as a though a flow may start as a viscous plastic material license to assume that all deep-water sands must 218 G. Shanmu,gam /Em-th-Scienc~r Rrl~iettx 42 (I9971 201-229

Transportation Deposition sands for understanding their depositional origin; we A Laminar can simply assume that all deep-water sands are turbidites. Disappointingly, many in the geologic *\p community do precisely that (Hiscott et al., 1997). As a result, the geologic literature is saturated with Debris Flow examples of ‘turbidites’, irrespective of whether these sediments were transported and deposited by turbid- Turbulent ity currents or by some other processes. This is perhaps the single most important area of future ‘Bp research on deep-water facies. Because flow transformations occur commonly in Debris Flow nature, there is a need for two sets of nomenclature

Laminar for sediment-gravity flows, one for transport pro- cesses and the other for depositional processes. Until we develop much needed criteria and nomenclature for distinguishing transport processes from deposi-

Turbidite tional processes, we are forced to use transport terms like debris flows and turbidity currents for deposi-

Turbulent 4 tional processes. However, one must clearly specify whether a process term, such as debris flow, is being used to represent either mechanics of transportation or mechanics of deposition, or both. In this paper. Turbidite process terms are used to represent only mechanics

Fig. 9. Hypothetical scenarios showing two cases Itirh flow of deposition. transformation (B and C) and two cases without flow transforma- tion (A and D) in subaqueous debris Rows and turbidity currents. 7.2. High-density turbidity currents Differences and similarities in flow conditions between transporta- tion and deposition are shown by laminar (i.e.. debris flow) and The concept of high-density turbidity currents is a turbulent (i.e., turbidity current) states of flow. Arrows indicate flow direction. Generalized deposits of turbidity currents with highly confusing one because the density may vary normal grading and debris flows with inverse grading are shown dramatically through the flow, and it was the focus on the right. Note that deposits reflect only flow conditions that of a recent critical perspective article (Shanmugam, existed during the final stages of deposition, The case of flow 1996a). The problem here is that no one is able to transformation from turbulent to laminar state (B) is partly based define high-density turbidity current in terms of its on experiments by Vrolijk and Southard (1982) and Postma et al. (1988). The case of flow transformation from laminar to turbulent density, rheology, or sediment-support mechanism state (C) is partly based on experiments by Hampton (1972). Flow (see Shanmugam, 1996a). Although many (e.g., transformations can occur in both density-stratified (Postma et al., Postma et al., 1988) recognize that a ‘high-density 1988) and density-unified flows, See Fig. I I for evolution 01 turbidity current’ is a density-stratified flow com- turbidity currents from debris flows in density-stratified flows. At posed of a lower layer (i.e., high-concentration, plas- present, there are no field criteria to infer transport processes from the depositional record because evidence for flow transformation tic, laminar) and an upper layer (i.e., low-concentra- is not preserved in the final deposit, From Shanmugam and tion. fluidal, turbulent), they still fail to appreciate Moiola ( 1997). that the basal high-concentration layer (i.e., traction carpet) cannot be a turbidity current because of its plastic rheology and laminar flow state (Fig. 10). have been transported by turbidity currents but un- Because sediment-gravity flows are classified on the derwent late-stage plastic deformation to resemble basis of rheology and sediment-support mechanism debris-flow deposits. If we continue to follow such (Dott, 1963; Lowe, 1979, 1982), a single flow (i.e.. an assumption-based (i.e., model-driven) interpreta- high-density turbidity current) should not be consid- tion, then there is no need to examine deep-water ered to represent both Newtonian and non-Newto- G. Shanmugam / Earth-Science Reviews 42 (I 9971201-229 219

Non- NeWloll

Fig. 10. (A) Differing interpretation of experimental ‘high-density turbidity currents’ of Postma et al. (1988). Note floating mudstone clasts near the tops of sandy debris flows. Reasons for differing interpretation are discussed by Shanmugam (1996a). (B) According to Postma et al. (1988). lower and upper layers represent non-Newtonian and Newtonian rheology, respectively. (C) According to Postma et al. (19881, lower and upper layers represent laminar and turbulent states, respectively. The basal laminar layer (i.e., sandy debris flow in this study) is va.riously termed as inertia-flow layer, traction carpet, flowing-grain layer, etc., by various authors (see Shanmugam, 1996a). (D) Interpretation of Postma et al. (1988). Because sediment-gravity flows are classified on the basis of rheology and sediment-support mechanism (Lowe, 1982), a single flow (i.e., high-density turbidity current) cannot be both Newtonian and non-Newtonian in rheology, and laminar and turbulent in state at a given point in time and space.

nian rheology, and both laminar and turbulent state turbidity current’) in the rock record when there are at a given point in time and space. two types of debris flows. The common type is a The importance of density-stratified flows is that density-unified debris flow with plastic rheology and turbidity currents and the underlying debris flows laminar state that freezes during deposition causing may travel at different speeds at different points in sharp upper contacts, rafted clasts, floating quartz space depending on the slope. For example, the granules, planar and random clast fabric, inverse lower debris flow will travel ahead of the overriding grading, basal shear zone, etc. The other type is a turbidity current in the steeper slopes, but behind the density-stratified ‘high-density turbidity current’ in turbidity currents along gentler slopes (Norem et al., which the style of deposition can lead to a highly 1990). As a consequence, turbidity currents evolved concentrated, sheared sediment-water mixture near from debris flows may outrun debris flows (Fig. 11). the bottom (i.e., traction carpet) that also possesses Therefore, even if there are deposits that show a rheological and dynamical properties of a debris basal debris-flow unit overlain by a turbidite unit flow, which also produces sharp upper contacts, (see Postma et al., 19881, it is difficult to prove that planar fabric, rafted clasts, etc. these two units were deposited by genetically related ‘High-density turbidity currents’ and their traction density-stratified flows. At present, there are no carpets constitute an important area of future re- means to decipher whether a turbidite bed was de- search. Basic issues remain: (1) what are high-den- posited by a current that was generated far away at sity turbidity currents in terms of flow density, fluid the source or by a turbidity current that was gener- rheology, and sediment-support mechanism? (2) what ated locally from a debris flow. This has important are traction carpets in terms of fluid rheology and implications for interpreting provenance of sediment sediment-support mechanism? (3) are traction car- deposited by turbidity currents. pets part of a turbidity current or a separate entity? The other issue is how to recognize debris-flow (4) are traction carpets analogous to bed-load trans- component of a stratified flow (i.e., ‘high-density port? and (5) how can we differentiate deposits of 220

. Flow Density

~ m Turbidity Current 1 Fig. Il. Bottom profile: a schematic diagram showing evolution of turbidity currents from debris flows in three stages (initiation. stratification, and separation). Top profile: a schematic plot of flow thickness vs. flow density for three stages. Density stratification is well developed (top middle) when low-density turbidity current occurs above high-density debris flows (bottom middle). When a turbidity current separates (bottom right) from debris flows (bottom middle), the separated turbidity current with uniform density within the flow is considered a density-unified flow (top right). However, this density-unified flow could again become density-stratified by accumulation of sediment, and thus creating a high-density layer at its base. Such high-density layers are variously termed traction carpet, inertia-flow layer, etc. (see Shanmugam, 1996a for a critique). Modified after Norem et al. (1990).

traction carpets from those of sandy debris flows and and it is composed of both grain flow and cohesive grain flows? debris tlow. In this type, flows are considered purely for their rheological behavior irrespective of their 7.3. Theoretical model of sandy debris ,flou sediment-support mechanisms. The second type refers to flows based exclusively on sediment-sup- High-density turbidity currents are considered to port mechanism. For example, a grain flow is one be sandy debris flows from a rheological point of end-member type rheological debris flow in which view (Shanmugam, 1996a). Therefore, a theoretical dispersive pressure (frictional strength) is the pri- model of sandy debris flow is discussed here. Two mary sediment-support mechanism (Fig. 12). whereas different theoretical models have been developed to a cohesive debris flow is the other end-member type explain debris flows (Fig. 12): (1) the model by rheological debris flow in which mud matrix (cohe- Johnson (1970) explains cohesive debris flows that sive strength) is the primary sediment-support mech- behave as a Bingham plastic; and (2) the model by anism (Fig. 12). Bagnold (1956) explains grain flows (i.e., cohesion- In nature, however, rheological debris flows tend less debris flows) (see also Bagnold, 1966; Friedman to be something in between cohesive debris flows and Sanders, 1978; Friedman et al., 1992). Although and grain flows. In general, intermediate types of these two models do not explain all aspects of flows have not been studied (Fig. 12). A limitation complex debris flows in nature, they do serve a of this end-member type scheme is that it does not useful purpose of understanding end-member types. allow provision to interpret a thick, deep-water, Lowe (1979, fig. 3) proposed a classification of ‘massive’ sand with low clay content either as a sediment-gravity flows based on rheology and sedi- product of grain flow or as a product of cohesive ment-support mechanism. Lowe used the term ‘de- debris flow. This is because grain flows cannot bris flow’ for two different types. The first type develop thick massive beds, and these flows require refers to rheological debris flow (i.e., plastic flows), steep slopes (Fig. 12); cohesive debris flows require G. Shanmugam/Earth-Science Reviews 42 (1997) 201-229 221

Grain Flow Debris Flow (Bagnold, 1956) (Johnson, 1970) I Theoretical Flow Type

Plastic (Cohesive Strength:

I This I 5--- Natural Debris Flow - Study I sandy Debris Flow 1 MuddyDebris Flow Fig. 12. Theoretical vs. natural debris flows. Theoretically, grain flows and debris flows (i.e., cohesive debris flows) can be considered to be two end-members of rheological ‘debris flows’ (Lowe, 1979). Following Lowe (19791, the rheologic term ‘plastic’ is used for both grain flows (frictional strength) and debris flows (cohesive strength). Sandy debris flows are considered to represent an intermediate position between end-member types, and therefore, multiple sediment support mechanisms are proposed for sandy debris flows. An advantage of this concept is that it requires neither the steep slopes required for grain flows nor the high matrix content necessary for cohesive debris flows. Note that intermediate types of flows have not been studied prior to this study.

high clay content. Consequently, ‘massive’ sands flow (Fig. 121, with multiple sediment-support mech- with low clay content are interpreted as high-density anisms, such as cohesive strength, frictional strength, turbidites even if they do not exhibit any evidence and buoyancy. Therefore, cohesion is not always the for deposition from turbidity currents (Shanmugam, principal sediment-support mechanism in sandy de- 1996a). bris flow. This means that high clay content is not a Hampton (1975, p. 843) stated, “Most real debris prerequisite for a sandy debris flow (see Shanmugam flows are probably combination debris flow-grain and Moiola, 1997). The concept of sandy debris flow flows in the sense of Middleton and Hampton’s with a low matrix content was first suggested by (1973 and in press) idealized terminology and there- Hampton (19751, who also presented supporting ex- fore involve at least two mechanisms of grain sup- perimental data, mechanical arguments, and theoreti- port, implying greater competences . . . ” To accom- cal considerations. modate the intermediate type of rheological debris The advantage of the sandy debris-flow concept is flows, I have defined sandy debris flow (Shanmu- that it can be used to explain a wide range of gam, 1996a). A debris flow with a minimum of problematical submarine ‘massive’ sands with fea- 25-30% sand can be considered to be a sandy debris tures indicative of plastic rheology, and with only a flow. The term sandy debris flow is being used in a small percentage of mud matrix. This concept also rheological sense (i.e., a flow with strength). The alleviates the problem of interpreting ‘grain flow’ sandy debris flow (i.e., a rheological term) is not type deposits without requiring steep slopes of over equal to the cohesive debris flow by Lowe (1979) 20”. However, there is a need to conduct experiments (i.e., a sediment-support term>. Sandy debris flow is in establishing various parameters that control sandy not an end-member type; it is a transitional type debris flow. For example: (1) what are the boundary between cohesive debris flow and cohesionless grain conditions required for sandy debris flows? (2) what is the minimum amount of clay needed for sandy still preserve the original graded bedding because debris flows? (3) can we measure the strength of they move as coherent mass. sandy debris flows? (2) Concentration of rafted mudstone clasts near the tops of massive sandstone beds has been used to 7.4. Recognition of deposits of sandy debris ,flow.s infer flow strength in both ancient (Fig. 14) and modem (Fig. 15) environments at the time of deposi- Deposits of sandy debris flows and related slumps tion. Occurrences of rafted clasts in the deposit can and slides have been recognized using the following be explained by freezing of mass flows that had criteria (Shanmugam et al., 1995a, 1997a; Shan- rafted clasts at different levels within the tlow. This mugam and Moiola, 1995, 1997): is because that flows with measurable strength are (1) Massive (ungraded) sand with a basal zone of capable of supporting clasts of different sizes and shearing (Fig. 13) has been used to infer mass move- weights at any levels within the flow (Lawson, 198 1). ments as slide/slump on a glide plane. Such a (3) Inverse grading of mudstone clasts (Fig. 15) feature is unlikely to develop in turbidites because has been used to infer flow strength and buoyant lift settling of grains from turbulent suspension does not in sandy debris flows. One might argue that these cause basal deformation during deposition. After de- mudstone clasts might have been derived from col- position, however, turbidites can undergo remobiliza- lapse of adjacent channel walls, implying that clasts tion as slide/slump; such remobilized units would had nothing to do with depositional processes. How-

Massive szmd

Shearing

Mudstone

Fig. 13. c :ore photogr ‘alJh showing basal shearing zone of a massive (ungraded) sand. This has been interpreted to I‘ WE;est n lass move ‘ments as sllide/: slump along a decollement surface (primary glide plane). Note sand dike at the bottom of the sand. Mas! jive sar Id CO1 rafted mud clasts and !;tf :ep layers near the top (not shown). Eocene, North Sea. G. Shanmugam / Earth-Science Reviews42 (I 997) 201-229 223

(4) Floating quartz pebbles and granules in fine- grained sandstone (Fig. 4) have been used to infer flow strength. Sandy debris flows are capable of supporting and transporting grains of any sizes and Rafted mudstone weights at any levels within the flow because of their clasts of different sizes combined frictional and cohesive strength. In other in fii-grained massive sandstone words, we can infer plastic rheology of fluids from (Sandy debris flow) floating pebbles and granules in deep-water sands (i.e., pebbly sand) and muds (i.e., pebbly mud). Even if a sand unit contains only a few floating quartz pebbles, it provides an important piece of informa- tion on the nature of flow and mechanics of deposi- tion. Commonly, we tend to ignore the significance of a few isolated floating quartz pebbles in deep-water sands on the ground that similar isolated pebbles have been observed in fluvial deposits. Such a rea- soning is flawed because fluvial currents and turbid- ity currents are not one and the same; they are fundamentally different processes (see Section 6

Fig. 14. Core photograph showing rafted mudstone clasts of Rafted mudstone different sizes near the top of a massive sandstone unit. This has &as& of different sizes been interpreted to indicate flow strength and deposition by .&Ifine-grained sand freezing in a sandy debris flow. Note that long axes of clasts are (Sandy debris flow) aligned parallel to bedding (i.e., planar clast fabric), which indi- cate laminar flow. Paleocene, North Sea.

ever, there is no reason to believe that random wall collapse would produce inverse grading of clasts. There is also no reason to believe that deposition of clasts from turbidity currents would produce inverse grading. In the case of La Jolla submarine-fan valley in offshore California (Shepard et al., 19691, mud- stone clasts were broken off the steep walls and got transported downslope toward the valley axis by sliding and slumping. Because density of mudstone clasts is different from density of quartz sands, the grading of clasts should be treated independently from the grading of quartz sand in interpreting depositional processes. Fig. 15. Core photograph showing rafted mudstone clasts of different sizes near the top of a sand unit. This has been inter- There are cases where quartz sands show normal preted to indicate flow strength and deposition by freezing in a grading, but mudstone clasts within the normally sandy debris flow. Note inverse grading of clasts. Modem intras- graded sand unit show inverse grading. lope area, Gulf of Mexico. 224 G. Shunmugam/ Eurth-Scrence Hwirws 42 (1097) 201-229

above). In cross-bedded sandstone units of lluvial preted as high-density turbidites, owe their origin to origin, the occurrence of isolated pebbles and clasts sandy debris flow. This is because as little as 2% can easily be explained by bed-load (traction) trans- matrix can provide the necessary strength to the flow port. However, such an explanation is not valid for (Hampton, 1975). My views on sandy debris flow turbidite deposits because turbidity currents are dom- are based mainly on observation of features in deep- inated by suspended load, not bed-load transport. water sands that indicate plastic rheology, laminar (5) Planar clast fabric (Fig. 14) has been used to state, and ‘freezing’ during deposition. However, infer laminar flow (Fisher, 197 1). experiments of sandy debris flows are necessary to (6) Preservation of fragile shale clasts suggests firmly establish the behavior of flows and their laminar flow (Enos, 19771. deposits. (7) Irregular upper contacts and lateral pinch-out geometries indicate freezing of primary relief that is 7.5. Depositional models ofdebris flows common in debris flows. (8) Detrital matrix is indicative of high-concentra- Unlike submarine fans with organized turbidite tion flow and plastic rheology. packages in channels and lobes (Mutti and Ricci Evidence for laminar flow and flow strength with Lucchi, 19721, debris flows are disorganized (Fig. rafted clasts and planar fabric makes a strong case 16). Debris-flow dominated systems can be broadly for a debris flow. classified into (1) non-channelized and (2) channel- Many deep-marine fine-grained massive sands ized types (Fig. 16). Most deep-water reservoirs in with the above features, which are routinely inter- the North Sea (Shanmugam et al., 1995a), Norwe-

Fig. 16. Proposed depositional model for debris-flow dominated systems (non-channelized and channelized). In non-channelized systems, sandy debris flows are expected to occur downdip from sand-rich shelf (modified after Shanmugam, 1996b). In channelized systems, sandy debris flows are expected to occur mainly within channels and at their terminus. Although debris flows may generate lobate sand bodies, they are not analogous to typical depositional lobes formed by classical turbidity currents in submarine fans (e.g., Mutti and Ricci Lucchi, 1972). Different oil-water contacts CO\ W) may be encountered in debris-flow reservoirs because of their lateral discontinuity. However, there are cases where debris-flow reservoirs are sheet-like because of good vertical and lateral connectivity caused by amalgamation of sand units. G. Shanmugam/ Earth-Science Reviews 42 (1997) 201-229 225 gian Sea (Shanmugam et al., 1994), Gulf of Mexico as discussed by Walker (1973) more than two decades (Famakinwa et al., 1997; Shanmugam and Zimbrick, ago, is still alive and well in the minds of many 19961, and offshore Equatorial Guinea (Shanmugam sedimentologists and sequence stratigraphers (see et al., 1997b) are considered to be non-channelized Shanmugam et al., 1997c), the turbid&es themselves type. Channelized type includes certain intervals in are becoming an endangered facies! Perhaps, it is the Edop Field in offshore Nigeria (Shanmugam et time to quit the practice of model-driven interpreta- al., 1995b), and the modem outer Mississippi Fan tion and begin the practice of observation-driven (Fig. 7). In this slump and debris-flow dominated interpretation. slope model, nature of shelf (sand rich vs. mud rich), sea-floor topography (smooth vs. irregular), and de- positional process (settling vs. freezing), tend to Acknowledgements control sand distribution and geometry. Contrary to popular belief, sandy debris flows can be thick, I thank J.E. Sanders, three other reviewers, and areally extensive, and excellent reservoirs (Shanmu- editor G.M. Friedman for their constructive com- gam and Zimbrick, 1996). High-frequency flows tend ments; R.J. Moiola, J.E. Damuth, and G. Zimbrick to develop amalgamated debris-flow deposits with for reviewing earlier versions of the manuscript; lateral connectivity and sheet-like geometry. M.K. Lindsey for drafting; G.K. Baker for manage- According to the model (Fig. 161, amalgamated rial support, and Mobil for granting permission to sandy debris flows may be predicted to occur publish this paper. I wish to thank my wife, Jean, for downdip from a sand-rich shelf. Experimental stud- editorial assistance. This critical review would not ies of subaqueous debris flows have shown that have been possible without the help of numerous hydroplaning can dramatically reduce the bed drag, colleagues who assisted in describing cores and out- and thus increase head velocity (Mohrig et al., 1997). crops worldwide. This would explain why subaqueous debris flows can travel faster and farther on gentle slopes than subaerial debris flows. Future research should also References focus on establishing not only dimensions and ge- ometries of debris-flow deposits, but also their seis- Allen, J.R.L., 1991. The Bouma division A and the possible mic and log attributes in order to predict them in duration of turbidity currents. J. Sediment. Petrol. 61, 291-295. frontier areas of exploration. Arnott, R.W.C., Hand, B.M., 1989. Bedforms, primary structures and grain fabric in the presence of suspended sediment rain. J. Sediment. Petrol. 59, 1062-1069. Bagnold, R.A., 1956. The flow of cohesionless grains in fluids. 8. A critical perspective Philos. Trans. R. Sot. London, Ser. A 249, 235-297. Bagnold, R.A., 1966. An approach to the sediment transport The long-standing practice of describing deep- problem from general physics. U.S. Geol. Surv., Prof. Pap. 411-L 11-137. water sequences using letters (Bouma T,, T,, T,, Td, Bouma, A.H., 1962. Sedimentology of some Flysch Deposits: A and T, divisions) and numbers (S,, S,, S, of Lowe, Graphic Approach to Facies Interpretation. Elsevier, Amster- 1982) creates an unrealistic deep-sea environment dam, 168 pp. flooded with ‘turbidites’. In areas that are believed to Carter, R.M., 1975. A discussion and classification of subaqueous include some of the classic examples of ‘turbidites’ mass-transport with particular application to gram flow, slurry flow, and fluxoturbidites. Earth-Sci. Rev. 11, 145-177. and ‘submarine fans’ in the rock record (e.g., the Cline, L.M., 1970. Sedimentary features of Late Paleozoic flysch, Miocene Mamoso-arenacea Formation in the north- Ouachita Mountains, Oklahoma. In: Lajoie, J. (Ed.), Flysch em Apennines, the Paleogene Frigg Fan in the North Sedimentology in North America. Geol. Assoc. Can. Spec. Sea, the Pliocene-Pleistocene sequences in the Gulf Pap. 7, 85-101. of Mexico, the Pennsylvanian Jackfork Group in Coleman, J.L. Jr., Swearingen, G.V., Breckon, C.E., 1994. The Jackfork Formation of Arkansas-a test for the Walker- Arkansas and Oklahoma), the greater the number of Mum-Vail models for deep-sea fan deposition. Field Guide deep-water sands I examine the fewer the number of Book, Geol. Sot. Am. South-Central Sect. Mtg., Little Rock, turbidites I interpret. Although the turbidite paradigm, AK, 56 pp. 226 (;. Shmmugum / Earth-Science Ruiews 42 (19971 201-229

Coussot, P., Meunier, M., 1996. Recognition, classification and Hsu. K.J.. 1989. Physical Principles of Sedimentology. Springer- mechanical description of debris flows. Earth-X Rev. 40. Verlag, New York, NY, 233 pp. 209-227. Johnson, A.M., 1970. Physical Processes in Geology. Freeman. DeVries, M.B., Bouma, A.H., 1992. Lateral correlation trends in Cooper, San Francisco, CA, 577 pp. bedded and massive turbidites, with an example from DeGray Jordan, D.W.. Lowe. D.R.. Slatt, R.M.. Stone, C.G.. D’Agostino. Lake, Arkansas. Gulf Coast Assoc. Geol. Sot. Trans. 42, A., Scheihing, M.H.. Gillepsie, R.H., 1991. Scales of geologi- 789-79 1, cal heterogeneity of Pennsylvanian Jackfork Group, Ouachita Dott, R.H. Jr., 1963. Dynamics of subaqueous gravity depositional Mountains, Arkansas: applications to field development and processes. Am. Assoc. Pet. Geol. Bull. 47. 104-128. exploration for deep-water sandstones. Dallas Geol. Sot. Field Dzulynski, S., Ksiaakiewicz, M., Kuenen, Ph.H.. 1959. 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H.J., 1992. Characteristics of a sandy depositional lobe on the and facies models. In: Walker, R.G. (Ed.), Facies Models, 2nd outer Mississippi Fan from Sea MARC IA sidescan sonar ed. Geosci. Can., Repr. Ser. 1, l-9. images. Geology 20, 689-692. Walker, R.G., 1992. Turbidites and submarine fans. In: Walker, Vail, P.R., Audemard, F., Bowman, S.A., Eisner, P.N., Perez-Cruz, R.G., James, N.P. (Eds.), Facies Models: Response to Sea C., 1991. The stratigraphic signatures of tectonics, eustacy and Level Change. Geol. Assoc. Can., pp. 239-263. sedimentology-an overview. In: Einsele, G., Ricken, W., Walton, E.K., 1967. The sequence of internal structures in tur- Seilacher, A. (Eds.), Cycles and Events in Stratigraphy. bidites. Scott. J. Geol. 3, 306-317. Springer-Verlag, Berlin, pp. 618-659. G. (Shari) Shanmugam is a Geological Valiance, J.W., Scott, K.M., 1997. The Oseola mudflow from Scientist with Mobil Oil Technology Mount Rainier: sedimentology and hazard implications of a Company in Dallas where he joined Mo- huge clay-rich debris flow. Geol. Sot. Am. Bull. 109, 143-163. bil in 1978. He holds degrees from An- van Kessel, T., Kranenburg, C., 1996. Gravity current of fluid namalai University in south India (B.Sc, mud on sloping bed. J. Hydraul. Eng., ASCE 123, 710-717. Geology and Chemistry), Indian Insti- Van der Lingen, G.J., 1969. The turbidite problem. N. Z. J. Geol. tute of Technology in Bombay (M.Sc., Geophys. 12, 7-50. Applied Geology), Ohio University in Vrolijk, P.J., Southard, J.B., 1982. Experiments on sand deposi- Athens (M.Sc., Geology), and the Uni- tion by high-velocity surges. 1 lth Int. Congr. Sedimentology, versity of Tennessee in Knoxville Abstr., Hamilton, Ont., Session 4. (Ph.D., Geology). His publications (75 Walker, R.G., 1965. The origin and significance of the internal papers and 63 abstracts) cover a wide sedimentary structures of turbidites. Proc. Yorkshire Geol. rsnge of topics on petroleum exploration and production. His Sot. 35, l-32. puiblications primarily focus on the origin and distribution of Walker, R.G., 1967. Turbidite sedimentary structures and their deep-water sands; however, he has also published articles on relationships to proximal and distal environments. J. Sediment. porosity development from chert dissolution, erosional uncon- Petrol. 37, 25-43. formities, foredeep tectonics, Mn distribution, tide-dominated es- Walker, R.G.. 1973. Mopping up the turbidite mess. In: Ginsburg, tuarine facies, coniferous rain forests and oil generation from R.N. (Ed.), Evolving Concepts in Sedimentology. The Johns coaly source rocks. Hopkins University Press, Baltimore, MD, pp. l-37. Walker, R.G., 1984. General introduction: facies, facies sequences