Shanmugam 1997 ESR the Bouma Sequence and the Turbidite Mind

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

EARTWSCIENCE

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 subconsciously 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

The Bouma Sequence, which is interpreted to represent the deposit of a turbidity current (Fig. 1). is probably the single most widely used (or abused) terminology for the field description of sands interpreted to be of deep-water origin; the interpretive term ‘deep-water sand’ will be used hereafter. The concept of the Bouma Sequence is so deeply rooted in the psyche of geologists that the standard geologic practice of maintaining a distinction between description and interpretation is often totally lost when it comes to describing deep-water sands. For example, Miall (1995, p. 379) asks, ‘I . . who would now object to the use of Bouma’s (1962) five divisions (A-E) as a framework for the field description of turbidites?” I, for one, would.

In an observational science like geology. we must always maintain a clear distinction between description and interpretation. This is particularly critical for deep-water sands whose depositional origins are much more complex than the published literature saturated with turbidite terminology would indicate. For example, if a bed is described in the field as T, division (i.e., Bouma division A), it is difficult to know from that description alone: (1) whether the bed is structureless (i.e., sands that to the naked eye appear to be devoid of primary structures) and ungraded or normally graded; (2) whether the bed has a

sharp or gradational upper contact; and (3) whether the bed has mudstone clasts near the base or the top. Even if one records all other information in addition to T;,. the notation, T, carries with it a powerful message and a built-in interpretation that the bed was deposited by a turbidity current.

On the other hand. if the same bed were to be described without the T, notation, simply as ‘structureless, with a sharp upper contact, and con- taining floating mudstone clasts near the top,’ then the description stands alone without any attached interpretation to its origin. Thus the former description leaves one no choice but to interpret the bed as a turbidite. whereas the latter description allows for alternate interpretations, such as sandy debris flows.

The Bouma Sequence represents an interpretive depositional model for the deposit of a turbidity current (Fig. 1). Therefore, describing a deep-water sand unit as T, is like describing a cross-bedded sand unit as a ‘braided stream deposit.’ Because the Bouma divisions are now so routinely applied during field descriptions, it is almost impossible to know how many of the published examples of ‘turbidites’ actually represent deposits of true turbidity currents. This skepticism stems from the fact that the complete and partial ‘Bouma Sequence’ can be explained by processes other than true turbidity currents. I will return to this point below (see Section 4).

About a decade ago, 1 questioned the validity of

- Bouma (1962)

Divisions Middleton and

Hampton (1973) Lowe (1962) This study

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 Lucchi (1972) for interpreting submarine-fan environments (Shanmugam et al., 1985). Since then, I have had the rare opportunity to describe deep-water sands totaling nearly 22,000 ft (6.7 km) of rocks, most of them hydrocarbon bearing, from a number of areas known for deep-water ‘turbidite’ deposition. They include Tertiary basin-floor fans in the North Sea (Shanmugam, 1995; Shanmugam et al., 1995a, 19961, the Cretaceous in the Norwegian Sea (Shanmugam et al., 1994, 19961, the Pliocene in offshore Nigeria (Shanmugam et al., 1995b), the Pliocene in offshore Equatorial Guinea (Famakinwa et al., 1997; Shanmugam et al., 1997b), the Creta- ceous in offshore Gabon, the Pliocene-Pleistocene in the Gulf of Mexico (Shanmugam et al., 1993; Shanmugam and Zimbrick, 19961, and the Pennsyl- vanian Jackfork Group in the Ouachita Mountains of Arkansas and Oklahoma (Shanmugam and Moiola, 1994, 1995). Most of these examples were previously interpreted as turbidites by other workers. However, I have reinterpreted them to be deposits of sandy debris flows, slumps, and bottom currents; turbidites are extremely rare. These new interpretations have led me to critically evaluate the fundamentals of turbidity currents and their deposits, including the Bouma Sequence.

By design, this is an opinion-oriented review article because examples that I use here are exclusively from my previous publications in which I advocated sandy debris flow (see Shanmugam, 1996a) and bottom-current reworking processes for deep-water sands rather than conventional turbidity-current processes. To my knowledge, there are no other publications to cite on this subject matter of reinterpretation of turbidites as deposits of sandy debris flows and bottom currents. However, there are other workers who had criticized the concepts of turbidity currents (e.g., Ten Haaf, 1959; Sanders, 1965; Van der Lin- gen, 1969) and the Bouma Sequence (e.g., Hsu, 1989).

The current trend in sedimentology and sequence stratigraphy is to ignore the fundamental problems of turbidite concepts, but to cherish the misguided fan models with turbidite channels and lobes. For example, turbidites are considered to form the very foundation for submarine-fan models in both sedimentology (Walker, 1992, fig. 6) and sequence stratigraphy

(Vail et al., 1991). From the standpoint of petroleum industry, the principal attraction to submarine-fan models with channels and lobes is that the fan model can be used to predict the distribution of turbidite sand (i.e., the occurrence of sheet-like lobe sands downdip from channels). However, incorrect interpretation of deep-water sands as turbidites can lead to erroneous distribution of sand, and can have nega- tive economic consequences. Although Walker (1992) himself abandoned his popular fan model, many petroleum geologists still cling to this defunct fan model (e.g., Coleman et al., 1994; McGee et al., 1994). I attribute this phenomenon to a prevailing mind set on turbid&es that forces one, for no appar- ent geologic reason, to the turbidite-dominated fan model.

Although some of the problems that I raise here were raised 30 years ago by Sanders (19651, they were ignored by the research workers of that time as a matter of convenience. Consequently, the turbidite problem has compounded itself into a monstrous level today. Any further postponing of this issue is only going to worsen the problem. Hopefully, this critical review will re-open the much needed debate on the fundamentals of turbidite deposition toward establishing what we know and what we do not know.

2. Turbidity currents and debris flows

2.1. Definitions of turbidity currents

The core of the problem is the meaning of the term ‘turbidity current’. What is a turbidity current? Surprisingly, the definitions of turbidity currents with emphasis on sediment-support mechanism and rheology have remained remarkably consistent over the past four decades:

(1) 1960s: “Turbidity currents are defined as density currents caused by sediment in turbulent suspension.” (Sanders, 1965, p. 193).

(2) 1970s: “Turbidity currents, in which the sediment is supported mainly by the upward component of fluid turbulence.” (Middleton and Hampton, 1973, p. 2).

(3) 1980s: “Turbidity currents are sediment flows in which the grains are suspended by turbulence.” (Lowe, 1982, p. 282).

(4) 1990s: “Turbidity currents are one type of sediment-gravity flow in which the sediment is held in suspension by fluid turbulence.” (Middleton, 1993, p. 89). “More importantly, if a flow is laminar or nonturbulent it can no longer be considered as a turbidity current” (Middleton, 1993, p. 93).

It is clear that turbidity currents cannot exist without turbulence. It is also evident that turbidity currents must transport sediment via suspended load.

2.2. Rheology ogfzuids

Similar to fluid turbulence of turbidity currents, fluidal (i.e., Newtonian) rheology of turbidity currents has also been suggested (Dott, 1963; Nardin et al., 1979; Lowe, 1982; Shanmugam and Moiola, 1995). The rheology of fluids can be expressed as a relationship between applied shear stress and rate of shear strain (Fig. 2). Newtonian fluids (i.e., fluids with no inherent strength), like water, will begin to deform the moment shear stress is applied, and the deformation is linear (Fig. 2). For Newtonian fluids. the criterion for initiation of turbulence is the Reynolds Number, R (ratio between inertia and viscous forces), which is greater than 2000 (Fig. 2).

In contrast to Newtonian fluids, some naturally occurring materials (i.e., fluids with strength) will not deform until yield stress has been exceeded (Fig. 2); once the yield stress is exceeded the deformation

P

is linear. Such materials with strength are considered to be Bingham plastic. For Bingham plastics, the criterion for initiation of turbulence is based on both the Reynolds Number, R, and the Bingham Number, B (Fig. 2). Although debris flows can develop turbulence (Enos, 1977), such flows are not diagnostic of most debris flows that are laminar (i.e., no fluid mixing across streamlines). Johnson (1970) favored a Bingham plastic rheologic model for debris flows.

The rheology of a sediment-water mixture is governed mainly by sediment concentration and to a lesser extent by grain size and the physical and chemical properties of transported solids (Pierson and Costa, 1987, p. 4). Although, the rheology is a complex parameter and is difficult to measure accu- rately (Phillips and Davies, 1991), it is useful in distinguishing turbidity currents from debris flows. Therefore, there should not be any confusion as to what the term ‘turbidity current’ means in terms of fluid rheology and sediment-support mechanism. A turbidity current is a sediment-gravity flow with fluidal (i.e., Newtonian) rheology and turbulent state in which sediment is held in suspension by jluid rurbulence.

2.3. Problem areas

In simple terms, a turbidity current can be envis- aged as an evolving event over a space-time contin-

Reynolds Number:

R=elt”

Shear Stress z

Rate of Shear Strain (du/dy)

R > 2000 = Turbulent R < 500 = Laminar

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

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.


uum: it starts commonly in deep-water environments, gathers momentum, perhaps erodes at first, then deposits, and finally dies. Because currents may increase or decrease in their energy conditions in time and space when they encounter an obstacle or a change in sea-floor gradient (e.g., Shanmugam and Moiola, 1985, fig. 3; see also Shanmugam and Moiola, 19881, the same current may undergo flow transformation and may not always maintain a turbidity-current status by being fully turbulent or being fluidal over its entire life cycle (see Section 7.1). Therefore, the concept of turbidity current is applicable only to those currents that exhibit the rheological and dynamical properties of turbidity currents in space and time. Waning flows, for example, are qualified to be true turbidity currents; however, de- pletive waxing flows of Kneller (1995) should not be considered true turbidity currents. This is because Kneller (1995, p. 37) equates waxing flows with traction carpets in explaining inverse grading; however, traction carpets are neither fluidal nor turbulent (see Shanmugam, 1996a). From a depositional point of view, waxing (accelerating) flows are not important because turbidity currents commonly begin to deposit sediment as they start to loose energy slowly (i.e., waning currents). In other words, deposition of turbidites is an indication that the current is no longer waxing or accelerating (Hsu, 1989). In some cases, the most erosive part of the flow occurs during waxing flow (Valiance and Scott, 1997).

Another problem with the classification of turbidity currents by Kneller (1995) is the category of ‘uniform’ flows. If one accepts the definition of Middleton (1993) that a turbidity current must be turbulent, then there cannot be a ‘uniform’ turbidity current. As opposed to laminar flows, turbulent flows are always nonuniform because of entrainment of overlying water (van Kessel and Kranenburg, 1996). This practice of lumping accelerating (waxing) flows and uniform flows under the term ‘turbidity current’ is a major source of confusion in the literature.

2.4. Turbidity currents vs. debris flows

Perhaps, the single most source of confusion is the use of the term ‘turbidite’ for deposits of debris flows (e.g., Labaume et al., 1987; Mutti, 19921, and traction carpets (Lowe, 1982; Postma et al., 1988).

Considering that debris flows and turbidity currents have distinctly different rheological and dynamical properties, these two processes and their deposits should not be treated as one and the same. Turbidity currents are fluidal (i.e., Newtonian) in rheology (Dott, 1963; Lowe, 1979; Nardin et al., 1979; Shan- mugam and Moiola, 1995, 19971, whereas debris flows are plastic in rheology (Do& 1963; Johnson, 1970) or they represent non-Newtonian fluids (Cous- sot and Meunier, 1996). Turbidity currents are considered as two-phase flow (water and solid), whereas debris flows are one-phase flow in which the whole mass undergoes large and continuous deformation (Coussot and Meunier, 1996). Turbidity currents are fully turbulent in state (Middleton, 1993), whereas debris flows are laminar (i.e., no fluid mixing across streamlines) in state (Johnson, 1970; Carter, 1975; Middleton and Wilcock, 1994). Sediment in turbidity currents is held in suspension by fluid turbulence (Middleton and Hampton, 1973), whereas sediment in debris flows is supported by matrix strength, dispersive pressure, buoyancy (Middleton, 1993). Turbidity currents transport mainly fine-grained sediment because turbulence is their only sediment-support mechanism, whereas debris flows are capable of transporting sediment of all sizes because of their multiple sediment-support mechanisms (matrix strength, dispersive pressure, buoyancy) and their fluid strength. Turbidity currents in which grain-to- grain contact is rare, whereas in debris flows grain- to-grain contact is frequent. Turbidity currents in which sediment concentration is low (l--23% by volume, Middleton, 1967, 1993), whereas in debris flows sediment concentration is high (50~90%, in general, Coussot and Meunier, 1996). Turbidity currents in which sediment is settled from suspension grain by grain (Middleton and Hampton, 1973), whereas ,sediment in debris flows is deposited via freezing en masse (Johnson, 1970).

3. Deposits of turbidity currents

Deposition from turbidity currents commonly occurs through sediment fallout from suspension (Kuenen and Migliorini, 1950; Dott, 1963). In a truly turbulent fluidal flow, coarse- and fine-grained parti- cles tend to settle separately during deposition de-

206 G. Shanmugam / Earth-Sciencr Rel~iews 42 C 1997) 203-229

Kuenen ( 1953) This Study

-

‘1 t Graded

Ungraded

t Graded

Ungraded

Fig. 3. Three varieties of normally graded bedding (A, C, E) selected from Kuenen (1953). Vertical arrows show intervals of ‘normal grading’. Using grain sizes shown in (A). CC). and (E), a 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 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 currents. Fining-upward sequences, composed of multiple depositional events (D), should not be considered a single eortnally graded bed (C). Grain sizes in (B), (I)). and (F) are added in this study.

pending on their fall velocities. This causes deposits of turbidity currents to be characterized by normal size grading (i.e., upward decline in grain size) and gradational upper contacts (Fig. 3A,B). Because turbulent turbidity flows behave as Newtonian fluids, one could infer Newtonian rheology from graded

bedding. Normal grading (Kuenen and Migliorini, 1950) is the most reliable criterion to interpret fluidal rheology and suspension deposition of turbidity currents (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 practice 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 a 2 m thick sand bed has a 2 cm thick normally graded top, 1 would interpret only the 2 cm graded top as a turbidite. The origin of the underlying sand requires independent evaluation using its own features. 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 (i.e.. debris-flow origin) have thin graded tops (i.e.. turbidity-current origin). In order to interpret a normally graded bed as a turbidite, one must describe the graded bed with precision; no exceptions should be allowed in terms of floating quartz granules or rafted mudstone clasts within the ‘graded’ unit.

Deposits of turbidity currents are called turbidites (Bouma, 1962). In spite of this simple and straight- forward definition, the term ‘turbidite’ means different things to different people. To some, turbidite means any deep-water sand, and to others, turbidite means a deep-water channel or lobe sand, but in this study turbidite means a deposit of a turbidity current.

4. The Bouma Sequence: alternate interpretations

Bouma (1962) established a standard sequence of sedimentary structures for deposits of turbidity currents based on his study of 106 1 beds in the Mar- itime Alps of southern France. Conventionally, the Bouma Sequence and its five divisions (Fig. I). namely T,. T,, T,, Td, and T,, are considered to be the product of a single turbidity-current event (Bouma, 1962; Walker, 196.5; Mutti and Ricci Luc- chi, 1972; Middleton and Hampton, 1973, 1976). In the Bouma Sequence, only the T, division, if graded, represents deposition from suspension; the other three 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 traction, or combined traction and suspension. In the type area, however, less than 10% of the 1061 beds show the complete Bouma Sequence (e.g., T,, T,, T,, Td, T,). Most are top-absent (e.g., T,, T,, T,), middle-absent (e.g., T,, T,), or base-absent (e.g., T,, Tdr T,) sequences (Walker, 1965).

4.1. Debris-jlow origin of T, division

The basal Bouma division (T,) is defined as a massive or a graded bed (Fig. 1). The origin of the Bouma T, division is controversial. It has been variously ascribed to: (1) turbidity currents (Bouma, 1962); (2) antidune phase of the upper flow regime (Harms and Fahnestock, 1965; Walker, 1967); (3) grain flows (Stauffer, 1967); (4) pseudo-plastic quick bed (Middleton, 1967); (5) density- modified grain flows (Lowe, 1976); (6) high-density turbidity currents (Lowe, 1982); (7) upper-plane-bed conditions under high rates of sediment feed (Amott and Hand, 1989); and (8) sandy debris flow (Shanmugam and Moiola, 1995; Shanmugam, 1996b).

Of these, Bouma’s (1962) turbidity-current interpretation is valid only if the bed is normally graded. The antidune hypothesis can be eliminated based on empirical evidence (Allen, 1991). Stauffer (1967) suggested that the T, division is a product of grain flows. According to Lowe (19821, grain flows are considered to exhibit plastic rheology. If so, it is difficult to consider the T, division as a deposit of a turbidity current that is thought to exhibit fluidal rheology.

Middleton and Hampton (1976, p. 215) indicated that the T, division, if massive, may owe its origin to a transient flow composed of grain flows, fluidized flows, and debris flows. Based on experiments, Mid- dleton (1967) proposed that massiue portions of turbidite beds could result from high-concentration ‘turbidity flows’. Following Middleton’s (1967) sug- gestion, the ‘massive’ sands of deep-sea sequences are routinely interpreted as deposits of ‘high-density turbidity currents’ (Mutti and Ricci Lucchi, 1972; Lowe, 1982; Pickering et al., 19891, or as the basal Bouma division (i.e., T,).

Middleton (1967) reasoned that massive beds were deposited because of the formation of an expanded ‘quick’ bed that behaves as a pseudo-plastic unit. Middleton (1967, p. 495) also suggested that deposi-

tion resulted from rapid ‘freezing’. The sediment- support mechanism at the time of deposition of massive beds from high-concentration flows was dispersive pressure (Middleton, 1967, p. 495). This is expected because concentrations of 44% by volume were used in the experiment for high-concentration flows, which is within the range where dispersive pressures are important (Middleton, 1967, p. 480). At high concentrations (i.e., above 30% by volume), the dispersions do not behave as a Newtonian fluid (Middleton, 1967, p. 479). Middleton’s (1967) experimental flows meet all the criteria for mass flows (i.e., debris flows), as defined by Dott (1963): (1) they are non-Newtonian flows that exhibit pseudo- plastic behavior; (2) they are high-concentration flows in which the sediment is supported by dispersive pressure; and (3) they are deposited by ‘freezing’. For these reasons, I ascribed the massive T, division to a sandy debris-flow origin (Shanmugam, 1996b). Therefore, the routine interpretation of ungraded massive deep-water sands as turbidites, and more specifically, as T, divisions, is misleading. For example, one could interpret the Paleocene (west of Shetlands) sandstone bed and its thin mud cap as T, and T, divisions, respectively (Fig. 4). However, floating quartz granules and rafted mudstone clasts indicate flow strength, and therefore, the sandstone should be interpreted as a product of sandy debris flow (Shanmugam et al., 1995a).

In explaining the origin of ‘massive’ sands by processes other than turbidity currents, Sanders (1965, p. 193) argued that there are no physical mechanisms known by which sand being transported by turbulent suspension can be deposited without passing through the tractional ranges in the process of deposition.

Mutti and Nilsen (1981) explained the floating mudstone clasts in T, sandstone by “deposition en masse of the denser portion of turbidity currents freezes the rip up clasts”. Because en masse deposition by freezing is characteristic of plastic flows rather than fluidal flows, I would attribute these rafted clasts to a sandy debris-flow origin. Also, a planar clast fabric, attributed to the T, division by Mutti and Nilsen (198 l), is more indicative of laminar flow conditions in plastic debris flows than turbulent flow conditions in turbidity currents (John- son, 1970; Fisher, 1971; Shanmugam and Benedict,

1978). In the past, the turbidite paradigm was so influential that even the strong field criteria for plastic rheology (e.g., floating mudstone clasts) and laminar flow conditions (e.g., planar clast fabric). which are characteristic of debris flows, were ascribed to turbidity currents (e.g., Mutti and Nilsen, 1981).

According to Lowe (1982), the T, division is not part of the Bouma Sequence. Lowe (1982) considered the T, division as a deposit of a high-density turbidity current, and the overlying T,, T,, and T,, divisions as deposits of a low-density turbidity current (Fig. 1). In terms of fluid rheology, high-density turbidity currents are interpreted to represent plastic flows (Shanmugam and Moiola, 1995, fig. 7). 1 also suggested that the term ‘high-density turbidity cur-

rent’ is a misnomer, and it should be replaced by the term ‘sandy debris flows’ to avoid confusion regard- ing the rheology of fluids and the state of flow (Shanmugam, 1996a).

In short, what some may interpret as the basal division of the Bouma Sequence (i.e., Ta) may in reality represent deposits not only of turbidity currents but also debris flows. Herein lies the challenge of distinguishing one from the other. Field evidence for rheology of the flow (plastic vs. fluidal), state of the flow (laminar vs. turbulent), sediment-support mechanism (matrix strength/dispersive pressure vs. fluid turbulence), and depositional mechanism (freezing vs. settling) should be looked at critically in distinguishing whether the T, division was deposited by a debris flow or by a turbidity current.

--- Te?

Ta?

Normal grading (Turbid&)

1-1111-1--1

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

A common problem in interpreting deep-water sands is the occurrence of current ripple laminae, usually interpreted as representing the T, division, and parallel laminae, usually interpreted as representing the T, and T,, divisions. Walker (1992, p. 242) routinely classifies these rippled beds as ‘thin-bedded turbidites’ without regard for their true origin (i.e., turbidity current vs. bottom current). The term bottom current refers to currents unrelated to turbidity currents in the deep sea, and bottom-current deposits are characterized by traction features (Hollister, 1967; Shanmugam et al., 1993). Natland (1967) distinguished these deposits with traction structures as ‘tractionites’ from ‘turbidites’.

Shepard et al. (1969) interpreted laminated sand with concentration of heavy minerals as deposits of bottom-current reworking in the La Jolla canyon. Bottom-current measurements of velocities in the La Jolla canyon show 34 cm per second (Shepard and Marshall, 1969); such velocities are sufficient to erode and transport fine-grained sand.

In deep-water settings, if a sharp-based bed in which a planar-laminated interval (T,) passes up into a rippled interval CT,), it is a common practice to interpret such traction features as a product of decelerating turbulent flow dropping sediment from suspension onto a well defined sediment bed and then transporting sediment as bed load under the overlying flowing suspension. Under this scenario, the settled sediment from a turbidity current will preserve its original depositional features (e.g., normal grading) only if the sediment is halted from any further movement; however, if the settled sediment is subjected to further transport as bed load, it will not only lose its original depositional features of turbidity currents (e.g., normal grading) but also will develop new (i.e., reworked) depositional features that will be analogous to rippled bedding of bottom currents. This is because traction structures caused by bed-load transport are diagnostic of bottom-current deposits as well (Hollister, 1967; Shanmugam et al., 1993). In such cases, it is difficult to recognize from the depositional record whether sands in a parallel-laminated or ripple-laminated interval were originally transported as suspended load of a turbidity current or as bed load of a bottom current.

Therefore, not all bed-load deposits can be interpreted routinely as turbidite beds; associated normal grading is the key in establishing a turbidity-current origin. However, climbing ripples may be used as a criterion for turbidity-current deposition (Sanders, 1963, 1965). This is because when sands fall out of suspension while the turbidity current is moving within the ripple bedform range, climbing ripples are produced. Such ripples could be distinguished from the tractionally formed ripples by bottom currents.

One popular interpretation is that the rippled sands represent overbank turbidity-current deposits of a channel-levee complex in the Gulf of Mexico (e.g., Shew et al., 1994). However, rippled sands have also been interpreted as products of bottom-current reworking in the Gulf of Mexico (e.g., Shanmugam et al., 1993). Hsu (1989) also suggested that the rippled sands of the T, division can be deposited by marine bottom currents unrelated to turbidity currents. The routine interpretation of discrete rippled sands as levee deposits needs reevaluation; it assumes deposition from turbidity currents without evidence. I do not claim that ripples cannot occur on levees, but I do suggest that ripples can also occur as discrete units formed by bottom currents (Fig. 51, unrelated to levees or turbidity currents. In other words, the occurrence of ripples in a deep-water sequence is not ironclad proof for levees, as is widely believed under the current turbidite paradigm.

In addition to traction structures, such as cross- bedding, horizontal lamination, and isolated current ripples, bottom-current reworked sands also exhibit internal erosional surfaces indicating pulses of in- creased current energy, mud offshoots suggesting oscillating energy conditions, variable current directions, and sharp upper contacts (Shanmugam et al., 1993). Many of these features are difficult to explain by downslope-flowing decelerating turbidity currents. An important attribute of these traction structures is that they commonly occur in discrete units unassociated with normally graded beds. In the absence of associated graded beds, it is difficult to envision deposition by turbidity currents. Although both turbidity currents and bottom currents exist in the deep sea, it is easier to explain features like mud offshoots and variable current directions by bottom currents than by decelerating turbidity currents.

Lenticular laminae caused by starved ripples that

210 G. Shanmugum / Earth-Science Rer,itws 42 (I9971 201-229

Fig. 5. Core photograph showing discrete sand layers comprised of current ripples with variable dip directions suggesting multiple current directions. Preserved (lower arrow) and eroded (upper arrow) tops of ripples indicate variable energy conditions of the current. Note the absence of normally graded units. All these sand layers could be interpreted as the ‘T,’ division; however, these traction structures are considered evidence for bottom-current reworking. Middle Pleistocene, Ewing Bank Block 826, Gulf of Mexico. From Shanmugam et al. (1993).

could be interpreted as representing the T, division are common in the Pliocene and Pleistocene of the Gulf of Mexico. However, lenticular laminae, interpreted to be of bottom-current origin, have been reported from DSDP leg 28, Site 268, in Antarctica (Piper and Brisco, 1975). Starved ripples are more likely to be formed by relatively sediment-free ‘clear-water’ bottom currents than by sediment-laden turbidity currents (Shanmugam et al., 1993).

It is well known that large-scale cross-bedding (dune bedforms) is generally absent in Bouma-type

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 processes. The implication is that these gravel waves are composed of cross-bedding; however, no core information is available to prove the presence of cross-bedding in these gravel waves. Hsu (1989) proposed an alternate, debris avalanche, origin for the gravel waves in the Grand Banks area.

Giant sediment waves (5 m in height), composed of sand and boulders, on the continental margin off Nice (southern France) were ascribed to deposition by ‘sediment flows’ (Malinvemo et al., 1988). Sedi- ment flows are a combination of both debris flow and turbidity current (Malinvemo et al., 1988). How- ever, it is not clear how these sediment waves were deposited by two rheologically different flows (i.e., plastic debris flows and fluidal turbidity currents).

So, what does the complete Bouma Sequence really mean in terms of its turbidity-current origin? The Ta division, if massive, I would prefer to interpret it as a product of sandy debris flow when it shows evidence for plastic rheology and laminar

G. Shanmugam/Earth-Science Reaiews 42 (19971201-229 211

flow conditions. However, the T, division, if normally graded, implies deposition from a turbidity current. Because the T, division can be both massive (i.e., debris flow) and graded (i.e., turbidite) under the current Bouma Sequence (Fig. 11, it is awkward to use the same T, division for deposits of both debris flows and turbidity currents.

If the T,, T,, and Td divisions occur as part of a complete Bouma Sequence with a basal graded division CT,), which is extremely rare in the rock record, the sequence may be interpreted to represent deposits of turbidity currents. However, if these traction structures occur as discrete units unassociated with basal graded beds, or if they occur in association with basal massive beds (i.e., sandy debris flows), I would prefer to interpret them as traction deposits formed by reworking of bottom currents. In other words, even the complete Bouma Sequence can be interpreted as deposits of processes other than turbidity currents (Shanmugam, 1996b).

5. The turbidite mind set

The long-standing belief (i.e., the mind set) that most deep-water sands are products of turbidity currents in a submarine-fan setting appears to be over- stated. There are historical reasons for this mind set. In an important discussion on a paper by J.E. Sanders, for example, PH.H. Kuenen states, “Deposits from all kinds and combinations of currents falling under the definition of turbidity currents are turbidites [italics mine], whether there was bottom traction, laminar flow, non-turbulent flow etc. involved or not.” (see Sanders, 1965, p. 218). This non-rigorous, ‘anything goes’, approach of lumping turbulent and laminar flows under the umbrella term ‘turbidity current’ allows for inclusion of debris flows under ‘turbidity currents’. This approach is quite prevalent today. Mutti (1992, p. 401, for example, states, “Cohesive debris flows and turbidity currents should therefore be considered the two main mechanisms responsible for having transported and deposited the bulk of turbidite [italics mine] sediments.” Our failure to distinguish turbidity currents from debris flows based on fluid rheology, state of the flow, and sediment-support mechanism, has resulted in a special category of ‘turbidity currents’ that includes

debris flows and other deep-sea processes. It is not surprising, therefore, that in many cases

the term ‘turbidite’ actually refers to deposits of debris flow (i.e., plastic rheology and laminar state) and traction processes that are unrelated to true turbidity currents as defined earlier. The classic cases are the ‘fluxoturbidite’ (Dzulynski et al., 19591, and ‘atypical turbidite’ (Stanley et al., 1978) that refer to deposits of complex mass flow processes, such as slumps, debris flows, and sand flows. I have selected seven other published examples to demonstrate my point (Fig. 6). They include: (1) ‘megaturbidites’ referring to deposits of large-scale debris flows with plastic rheology (Labaume et al., 1987); (2) ‘high- density turbidity currents’ referring to inertia flows (i.e., laminar state) (Postma et al., 1988); (31 ‘high- density turbidity current’ referring to ‘traction carpet’ (i.e., laminar state) that is not part of turbulent suspension (Lowe, 1982); (4) Bouma T, referring to pseudo-plastic quick bed and freezing deposition (Middleton, 1967); (5) Bouma T, referring to laminar flows and freezing deposition (Mutti and Nilsen, 1981); (6) Bouma T, referring to traction processes (Shew et al., 1994); and (7) ‘turbidity currents’ referring to non-turbulent (i.e., laminar state) flows (McCave and Jones, 1988). Turbidity currents simply cannot exist without turbulence (see Middleton, 19931, and therefore, non-turbulent turbidity currents of McCave and Jones (1988) are the ultimate example of an oxymoron.

It should be clear from these examples that any deep-water deposit can be interpreted as a product of a turbidity current, no matter what the rheology or sediment-support mechanism of the flow is. On the other hand, if we wish to broaden the definition of ‘turbidity current’ to include all kinds of deep-sea processes, then there is no need for a classification of sediment-gravity flows based on rheology and sediment-support mechanism. Until we resolve this fundamental issue, most deep-water deposits will continue to be interpreted as turbidites whether these sediments were deposited by turbidity currents or not. This explains why many examples that I have reinterpreted as debris flows were previously interpreted as turbidites by other workers. The primary reason for interpreting any deep-water deposit as some kind of turbidite is that it allows us to place these deposits in a predictable submarine-fan setting.

G Shmmugun~ / Eurth-Science KW~CM..Y 42 ( I9971 201-229

Lithofacies

I Y&to 2 m

I

Normally graded sand

Breccia

Normally graded sand

Gravelly sand

Inversely graded sand

Massive sand

Sand with rafted clasts

Rippled sand

F”Fded

Process Interpretation

Turbidity Current (Fluidal rheology)

Debris flow (Plastic rheology)

Megaturbidite (Labaume et al., 1987)

Traction 3

Suspension

i

High-density turbidite (Postma et al., 1988)

Inertia flow (Laminar state)

Traction carpet (Laminar state)

High-density turbidite (Lowe, 1982)

Pseudoplastic quick bed (Plastic rheology)

Bouma Ta (Middleton, 1967)

Freezing (Laminar state)

Bouma Ta (Mutti & Nilsen, 1981)

Traction (Bed load)

Bouma Tc (Shew et al., 1994)

Non turbulent (Plastic rheology)

Muddy turbidite (!&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-


ried out during the past 25 years by many on the Pennsylvanian Jackfork Group in the Ouachita Mountains of Arkansas and Oklahoma serve to illus- trate my point. This deep-water sequence was considered a classic ‘flysch’ in North America (Cline, 1970; Morris, 1977) and was thought to be deposited by turbidity currents in a submarine-fan setting by many workers including me (Morris, 1977; Moiola and Shanmugam, 1984; Shanmugam et al., 1988a; Jordan et al., 1991; DeVries and Bouma, 1992; Mutti, 1992). In an earlier study (Moiola and Shan- mugam, 19841, we described (1200 ft or 365 m) beds of the Jackfork Group using the Bouma divisions. We described structureless sandstone beds as the T,, and beds with ripple laminations as the T, divisions. Not surprisingly, like many others, we ended up publishing a turbidite-dominated submarine-fan model for the Jackfork (Moiola and Shanmugam, 1984). However, the extreme rarity of normally graded beds in the Jackfork has been an unresolved issue until recently (Shanmugam and Moiola, 1995).

To resolve the issue of the missing normally graded beds in the Jackfork, we began to slab the ‘massive’ sandstone beds and examined them in polished samples and thin sections. To our surprise, normally graded beds are truly rare in these sandstones. These samples, however, revealed many new, diagnostic features, including: (1) concentration of rafted mudstone clasts near the tops of sandstone beds (flow strength, rigid plug); (2) inverse grading of clasts (flow strength and buoyant lift); (3) planar clast fabric (laminar flow); (4) contorted bedding (plastic deformation); and (5) moderate to high detrital matrix (plastic flow). These features cannot be interpreted using the Bouma divisions. We then reex- amined the entire measured sections systematically in the field in light of the new information obtained from the slabs and thin sections. This second time around, we avoided use of the Bouma Sequence in our field description. As a result, the massive sands (i.e., T, division before) were reinterpreted as sandy debris flows, and the rippled/parallel laminated sands (i.e., T, and Td divisions before) were reinterpreted as bottom-current reworked sands. We have now reinterpreted the classic submarine-fan setting attributed to the Jackfork as a slope setting dominated by sandy debris flows and slumps (Shanmu- gam and Moiola, 1995). From the Jackfork study, we

also have discovered that beds described as individ- ual Bouma T, divisions earlier are generally comprised of amalgamated events of complex origin, and that ‘complete Bouma Sequences’ are actually composed of multiple depositional events; this contrasts sharply with the conventional view that the complete Bouma Sequence represents a single depositional event. Our reinterpretation of the Jackfork Group has created substantial controversy (see Shanmugam and Moiola, 1997).

In addition to the outcrop study of the Jackfork, examination of conventional cores from a number of reservoirs previously interpreted as classic examples of submarine fans in the North Sea and claimed to be composed of turbidites (e.g., Frigg Field, Walker, 1992, fig. 6) revealed that 80-90% of the sediments can be interpreted as deposits of sandy debris flows and slumps; turbidites comprise less than 1% (Shanmugam et al., 1995a). 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 that invariably results in model-driven turbidite interpretations: (3) the prevailing mind set that most deep-water sands were deposited by some kind of turbidity currents; and (4) the attraction to obsolete submarine-fan models with turbidite channels and lobes. Sure, there are turbidites in these areas, but they are very rare and comprise less than 1% of the nearly 2 1,000 ft (6402 m) of cored intervals that I described in detail at a scale of 1 : 25 or 1 : 40. A few rare beds of turbidites do not make a submarine fan!

Another reason for the prevailing perception that turbidites are the most common deep-water facies is the misuse of the terminology ‘depositional lobe’ for modem fans, such as the Mississippi Fan (Nelson et al., 1992). The concept of depositional lobe was derived mainly from the ‘classic’ areas for turbidite deposition, such as the Miocene Mamoso-arenacea Formation in Italy (Ricci Lucchi, 198 1) and the Eocene Hecho Group in Spain (Mutti, 1977). By conventional definition, depositional lobes are dominated by classic turbidites (i.e., beds exhibiting nor-

ma1 grading with all five divisions of the Bouma Sequence) with thickening-up trends (Mutti and Ricci Lucchi, 1972; Mutti, 1977; Shanmugam and Moiola. 1991). Ironically, even in the classic areas for turbidite lobe deposition, such as the Miocene Mamoso-arenacea Formation in the northern Apen- nines (Ricci Lucchi, 1981), I find that normally graded bedding is very rare.

Piston and gravity cores (Fig. 7) taken from the ‘depositional lobe’ of the modern Mississippi Fan (Schwab et al., 1996) shows a dominance of debris flows (Fig. 8). Therefore, the use of the term ‘depositional lobe’ for areas dominated by debris flows perpetuates the notion that turbidites are more common in modem fans than they actually are. I am not aware of a single modem fan in which depositional lobes are composed of classic turbidites with thickening-up trends that have been documented using long continuous cores. I am even unsure whether there are such things called ‘typical depositional lobes’ or even ‘typical submarine fans’ in modern oceans! The term ‘lobe’, similar to the term ‘turbidite’, is used loosely without any precision. The lobe problem was discussed elsewhere (Shanmugam. 1990; Shanmugam and Moiola, 1991).

Conventionally, sheet-like geometries are associated with turbidites deposited at the terminus of a submarine fan. These sheet sands are also known as

depositional lobes (Shanmugam and Moiola, 199 1). The outer fan areas of the Mississippi Fan, for example, are commonly used as the modern analog for turbidite fans with sheet-like geometries (Shanmugam et al.. 1988b). Such a notion was based strictly on parallel and continuous reflection patterns observed on seismic profiles (Shanmugam et al., I988b). However. new SeaMARC 1 A sidescan-sonar data (Twichell et al., 19921, and piston and gravity cores (Nelson et al., 1992; Schwab et al., 1996) taken from channels in the outer Mississippi Fan reveal the following: (1) the terminus of the Missis- sippi Fan is not sheet-like as previously thought; (2) contrary to popular belief, the terminus of the Mis- sissippi Fan is channelized; (3) channels in the terminus of the Mississippi Fan are filled with debris tlows for the most part (Fig. 8); and (4) debris flows can travel hundreds of kilometers on gentle slopes. The channel-fill debris flows also support the view that the processes that cut the channels were not necessarily the same processes that filled the channels because debris flows are generally non-erosive. The non-erosive nature of subaqueous debris flows can be attributed to hydroplaning (Mohrig et al., 1997).

Channel forms (e.g., sinuous) observed on ampli- tude extraction maps of subsurface data may not 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).


60 ‘2; a-

I 70

.- 2 60

3 s 50 ‘Z 3 :: 40

$ 30

Cores of S

=R=

D>T T=O%

!O f Lobe 8 PC29 1 GC44 1 TC43 1 GC51 I- I

(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 flow, etc.). Also, what appears to be sheet-like on seismic scale (e.g., Mississippi Fan, Shanmugam et al., 1988b) may actually be composed of channelized bodies on smaller scales (e.g., Mississippi Fan, Twichell et al., 1992) that are beyond the resolution of conventional seismic data. For these reasons, de- veloping depositional models using seismic geometries alone without the benefit of core should proceed with caution.

The prevailing turbidite mind set can be attributed, at least in part, to the constant promotion of the Bouma Sequence in synthesis articles on facies models. For example, Walker (1984, Walker, 1992)) continually suggests that the Bouma Sequence is the one example that fulfills all four functions of a facies model so well (i.e., norm, framework, predictor, and basis for environmental interpretation). Does it? The Bouma Sequence can serve all four functions of a facies model only if we assume that there are no processes other than turbidity currents that can generate various divisions of the Bouma Sequence. As I have tried to show, such an assumption is false. Even

the complete Bouma Sequence, which is very rare in nature, can be explained by a non-turbidity-current origin (e.g., basal debris-flow deposit with overlying bottom-current deposit, see Shanmugam, 1996b). To my knowledge, no one has ever reproduced the complete Bouma Sequence deposited from suspension by a single turbidity current in the laboratory (e.g., Kuenen, 1966; Middleton, 1967; Luthi, 1981).

6. A test of turbid& interpretation

6.1. Normal grading

Normal size grading is the only reliable criterion to interpret a deep-water sand as a turbid&. The following test can be administered to measure the validity of an interpretation of a deep-water sand as a turbidite. If an interpretation is based on observations, such as the presence of sharp basal contact, normal size grading, and gradational upper contact, then the sand may be reasonably interpreted as a 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 indicating plastic rheology and laminar flow conditions to interpret them as deposits of sandy debris flows. The practice of using the absence of normal grading as the basis for interpreting turbidites defies the very foundation of geologic interpretation based on observation. The problem with this twisted logic is that it allows one to interpret a variety of deep- water deposits as turbidites, irrespective of whether the bed shows normal grading or not (Fig. 6).

6.3. Turbidity currents L ‘S ricer currents

6.2. Erosional ~1s. depositional ,features

It is a common practice to interpret deep-water sands that contain flutes and scour surfaces as turbidites (e.g., Hiscott and Middleton, 1979; Shan- mugam and Moiola, 1995).

According to this conventional wisdom. scour surfaces at the bottom of a sand can be used to infer deposition of the sand by turbulent flows (Hiscott and Middleton, 1979; Shanmugam and Moiola. 1995). However, I now question this wisdom for two reasons (see Shanmugam and Moiola, 1997). First, large-scale erosional surfaces can also be created by mass movements (e.g., slump scars), not just by turbulent flows; distinguishing the origin of erosional surfaces by mass movements vs. large-scale turbulent flows is almost an impossible task in outcrop. Second, although small-scale scour surfaces and flutes may suggest turbulent state of the flow in some cases, this does not mean that the sand that rests on a scour surface was deposited by the same turbulent flow that created the scour surface (Sanders, 1965, p. 209); for example, scour surfaces can be created by turbulent flows and filled later by debris flows or other processes. Modem unfilled submarine channels and canyons are a testimony to the fact that the processes that had created these erosional fea-

Another common practice is to compare deposits of subaqueous turbidity currents with those of subaerial river currents on the ground that both currents are turbulent, and therefore their deposits must be quite similar. This is not true. River currents and turbidity currents are not one and the same, although both are turbulent. River currents are low in suspended sediment (l-5%), whereas turbidity currents are relatively high in suspended sediment (l-23%, see Shanmugam, 1996a), although both currents are considered to be Newtonian in rheology. River currents are ,fluid-gravity flows, whereas turbidity currents are .yediment-gravity flows (Middleton, 1993). In river currents, sand and gravel fractions are transported primarily by bed load (traction) mechanism, and therefore river deposits are characterized by dune bedforms (cross-bedding). In contrast, sands in turbidity currents are transported by suspended load, and thus sandy turbidites show a general lack of cross-bedding.

Lowe (1982) claimed that high-density turbidity currents can generate cross-bedding, but the concept of high-density turbidity currents is highly controversial (see Section 7.2, see also Shanmugam, 1996a). High sediment concentration (C > 20-25 vol.%) in high-density turbidity currents not only results in non-Newtonian behavior (e.g., Rutgers, 1962), but also tends to damp the turbulence in the lower part of the flow (Postma et al., 1988). High sediment concentration increases the flow strength and de- creases its boundary resistance, such that the flow’s behavior can be effectively supercritical even though its Froude Number may appear to be less than unity or subcritical (Nemec, 1990). Furthermore, flow density and velocity are highly variable within high-density turbidity currents. For these reasons, the applica-


tion of ‘Newtonian’ Froude Number to high-density flows may be questionable (Nemec, 1990). This is important here because the presence or absence of cross-bedding in turbidites has been explained by a critical Froude Number (see Section 4.2).

It is a common practice to use a gravel in an alluvial conglomerate to estimate the velocity of river currents on the basis of the Shield diagram; however, the same approach to estimate the speed of sediment-gravity flow on the basis of the largest clast in a deep-water sequence is not meaningful (Hsu, 1989). Similarly, velocity-size diagrams (e.g., Harms et al., 1975) are meant for bed-load dominated river currents, and may not be applicable to turbidity currents in which suspended load is the dominant mode of transport.

it may subsequently develop grain-dispersive characteristics. Then, as shear rates are reduced, say, by a reduction in bed slope or by jamming of coarse grains in the channel, the flow may once again exhibit plastic-viscoplastic behavior.” In other words, a debris flow may transform into a grain flow, and then back to a debris flow again. Similarly, transformation of grain flow to turbidity current and returning to grain flow during the last stages of deposition has been suggested by Middleton (1970). Such transformations are common in mass flows.

Normal grading is rare in river deposits (Hein, 1984) because of dominant bed-load transport, whereas normal grading is common in. turbidites because of dominant suspended load transport. Float- ing pebbles and clasts are common in river deposits because of bed-load transport in which grain size does not play a major role during deposition. In contrast, floating pebbles are absent in turbidites because pebbles tend to settle first in comparison to sand during deposition from suspension.

In experiments on ‘high-density turbidity currents’, for example, the basal laminar flow that deposited the traction carpet was initially fully turbulent, but during the depositional stage the turbulent flow was transformed into quasi-plastic laminar flow (Postma et al., 1988). Similarly, experimental studies show that massive sands were transported by turbidity currents, but were deposited by freezing (Vrolijk and Southard, 1982). In other words, these massive sands could be considered analogous to the Bouma T, division during transport by turbidity currents, but their freezing mode of deposition could be interpreted as deposition from a sandy debris flow. This flow transformation from turbulent to laminar state is shown in Fig. 9B.

7. Topics of future research

7. I. Recognition of transport mechanism

Contrary to popular belief, there are no established criteria for recognizing transport mechanism from the depositional record (Middleton and Hamp- ton, 1973; Lowe, 1982; Postma, 1986; Middleton, 1993; Shanmugam, 1996a). In fact, any discussion of long-distance sediment transport in this review is meaningless because all that we can infer from the depositional record is flow rheology and state that existed during the final moments of deposition. In theory, we tend to assume that depositional processes must be the same as transportational processes. In reality, however, such assumptions may not be true in all cases because of flow transformation during transport.

Experimental studies also show that plastic debris flows can be diluted to develop fluidal turbidity currents during transport (Hampton, 1972). Flow transformations can occur in both density-stratified (Postma et al., 1988) and density-unified flows in subaqueous environments. Clearly, some sediment- gravity flows undergo flow transformation (i.e., laminar to turbulent and vice versa) prior to deposition (Fig. 9B,C). In dd’t’ a 1 ion, there are sediment-gravity flows that do not undergo any flow transformation (Fig. 9A,D). Th e c a h 11 enge in the depositional record is how to distinguish flows that underwent flow transformation from flows that did not. Whether a flow underwent transformation or not, its deposit will reflect flow conditions that existed only during the final moments of deposition (Fig. 9). Conse- quently, evidence for flow transformation is not preserved in the final deposit (Fig. 9B,C), and there are no field criteria to infer transport processes from the 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 A Laminar

Deposition

*\p

Debris Flow

Turbulent

‘Bp

Debris Flow

Laminar

Turbidite

Turbulent 4

Turbidite

Fig. 9. Hypothetical scenarios showing two cases Itirh flow

transformation (B and C) and two cases without flow transformation (A and D) in subaqueous debris Rows and turbidity currents. Differences and similarities in flow conditions between transportation and deposition are shown by laminar (i.e.. debris flow) and turbulent (i.e., turbidity current) states of flow. Arrows indicate flow direction. Generalized deposits of turbidity currents with normal grading and debris flows with inverse grading are shown on the right. Note that deposits reflect only flow conditions that existed during the final stages of deposition, The case of flow transformation from turbulent to laminar state (B) is partly based on experiments by Vrolijk and Southard (1982) and Postma et al. (1988). The case of flow transformation from laminar to turbulent state (C) is partly based on experiments by Hampton (1972). Flow transformations can occur in both density-stratified (Postma et al., 1988) and density-unified flows, See Fig. I I for evolution 01 turbidity currents from debris flows in density-stratified flows. At present, there are no field criteria to infer transport processes from the depositional record because evidence for flow transformation is not preserved in the final deposit, From Shanmugam and Moiola ( 1997).

have been transported by turbidity currents but underwent late-stage plastic deformation to resemble debris-flow deposits. If we continue to follow such an assumption-based (i.e., model-driven) interpretation, then there is no need to examine deep-water

sands for understanding their depositional origin; we can simply assume that all deep-water sands are turbidites. Disappointingly, many in the geologic community do precisely that (Hiscott et al., 1997). As a result, the geologic literature is saturated with examples of ‘turbidites’, irrespective of whether these sediments were transported and deposited by turbidity currents or by some other processes. This is perhaps the single most important area of future research on deep-water facies.

Because flow transformations occur commonly in nature, there is a need for two sets of nomenclature for sediment-gravity flows, one for transport processes and the other for depositional processes. Until we develop much needed criteria and nomenclature for distinguishing transport processes from depositional processes, we are forced to use transport terms like debris flows and turbidity currents for depositional 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. process terms are used to represent only mechanics of deposition.

7.2. High-density turbidity currents

The concept of high-density turbidity currents is a highly confusing one because the density may vary dramatically through the flow, and it was the focus of a recent critical perspective article (Shanmugam, 1996a). The problem here is that no one is able to define high-density turbidity current in terms of its density, rheology, or sediment-support mechanism (see Shanmugam, 1996a). Although many (e.g., Postma et al., 1988) recognize that a ‘high-density turbidity current’ is a density-stratified flow composed of a lower layer (i.e., high-concentration, plastic, laminar) and an upper layer (i.e., low-concentration. fluidal, turbulent), they still fail to appreciate 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). Because sediment-gravity flows are classified on the basis of rheology and sediment-support mechanism (Dott, 1963; Lowe, 1979, 1982), a single flow (i.e.. high-density turbidity current) should not be considered to represent both Newtonian and non-Newto-

G. Shanmugam / Earth-Science Reviews 42 (I 9971201-229

Non- NeWloll

219

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 at a given point in time and space.

The importance of density-stratified flows is that turbidity currents and the underlying debris flows may travel at different speeds at different points in space depending on the slope. For example, the lower debris flow will travel ahead of the overriding turbidity current in the steeper slopes, but behind the turbidity currents along gentler slopes (Norem et al., 1990). As a consequence, turbidity currents evolved from debris flows may outrun debris flows (Fig. 11). Therefore, even if there are deposits that show a basal debris-flow unit overlain by a turbidite unit (see Postma et al., 19881, it is difficult to prove that these two units were deposited by genetically related density-stratified flows. At present, there are no means to decipher whether a turbidite bed was deposited by a current that was generated far away at the source or by a turbidity current that was generated locally from a debris flow. This has important implications for interpreting provenance of sediment deposited by turbidity currents.

The other issue is how to recognize debris-flow component of a stratified flow (i.e., ‘high-density

turbidity current’) in the rock record when there are two types of debris flows. The common type is a density-unified debris flow with plastic rheology and laminar state that freezes during deposition causing sharp upper contacts, rafted clasts, floating quartz granules, planar and random clast fabric, inverse grading, basal shear zone, etc. The other type is a density-stratified ‘high-density turbidity current’ in which the style of deposition can lead to a highly concentrated, sheared sediment-water mixture near the bottom (i.e., traction carpet) that also possesses rheological and dynamical properties of a debris flow, which also produces sharp upper contacts, planar fabric, rafted clasts, etc.

‘High-density turbidity currents’ and their traction carpets constitute an important area of future research. Basic issues remain: (1) what are high-density turbidity currents in terms of flow density, fluid rheology, and sediment-support mechanism? (2) what are traction carpets in terms of fluid rheology and sediment-support mechanism? (3) are traction carpets part of a turbidity current or a separate entity? (4) are traction carpets analogous to bed-load transport? 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 grain flows?

7.3. Theoretical model of sandy debris ,flou

High-density turbidity currents are considered to be sandy debris flows from a rheological point of view (Shanmugam, 1996a). Therefore, a theoretical model of sandy debris flow is discussed here. Two different theoretical models have been developed to explain debris flows (Fig. 12): (1) the model by Johnson (1970) explains cohesive debris flows that behave as a Bingham plastic; and (2) the model by Bagnold (1956) explains grain flows (i.e., cohesionless debris flows) (see also Bagnold, 1966; Friedman and Sanders, 1978; Friedman et al., 1992). Although these two models do not explain all aspects of complex debris flows in nature, they do serve a useful purpose of understanding end-member types.

Lowe (1979, fig. 3) proposed a classification of sediment-gravity flows based on rheology and sediment-support mechanism. Lowe used the term ‘debris flow’ for two different types. The first type refers to rheological debris flow (i.e., plastic flows),

and it is composed of both grain flow and cohesive debris tlow. In this type, flows are considered purely for their rheological behavior irrespective of their sediment-support mechanisms. The second type refers to flows based exclusively on sediment-support mechanism. For example, a grain flow is one end-member type rheological debris flow in which dispersive pressure (frictional strength) is the primary sediment-support mechanism (Fig. 12). whereas a cohesive debris flow is the other end-member type rheological debris flow in which mud matrix (cohesive strength) is the primary sediment-support mechanism (Fig. 12).

In nature, however, rheological debris flows tend to be something in between cohesive debris flows and grain flows. In general, intermediate types of flows have not been studied (Fig. 12). A limitation of this end-member type scheme is that it does not allow provision to interpret a thick, deep-water, ‘massive’ sand with low clay content either as a product of grain flow or as a product of cohesive debris flow. This is because grain flows cannot develop thick massive beds, and these flows require steep slopes (Fig. 12); cohesive debris flows require


Grain Flow (Bagnold, 1956) I

Debris Flow (Johnson, 1970)

Theoretical Flow Type

Plastic (Cohesive Strength:

I This I 5--- Natural Debris Flow -

I Study sandy Debris Flow 1 Muddy Debris 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 with low clay content are interpreted as high-density turbidites even if they do not exhibit any evidence for deposition from turbidity currents (Shanmugam, 1996a).

Hampton (1975, p. 843) stated, “Most real debris flows are probably combination debris flow-grain flows in the sense of Middleton and Hampton’s (1973 and in press) idealized terminology and therefore involve at least two mechanisms of grain support, implying greater competences . . . ” To accom- modate the intermediate type of rheological debris flows, I have defined sandy debris flow (Shanmu- gam, 1996a). A debris flow with a minimum of 25-30% sand can be considered to be a sandy debris flow. The term sandy debris flow is being used in a rheological sense (i.e., a flow with strength). The sandy debris flow (i.e., a rheological term) is not equal to the cohesive debris flow by Lowe (1979) (i.e., a sediment-support term>. Sandy debris flow is not an end-member type; it is a transitional type between cohesive debris flow and cohesionless grain

flow (Fig. 121, with multiple sediment-support mechanisms, such as cohesive strength, frictional strength, and buoyancy. Therefore, cohesion is not always the principal sediment-support mechanism in sandy debris flow. This means that high clay content is not a prerequisite for a sandy debris flow (see Shanmugam and Moiola, 1997). The concept of sandy debris flow with a low matrix content was first suggested by Hampton (19751, who also presented supporting experimental data, mechanical arguments, and theoretical considerations.

The advantage of the sandy debris-flow concept is that it can be used to explain a wide range of problematical submarine ‘massive’ sands with features indicative of plastic rheology, and with only a small percentage of mud matrix. This concept also alleviates the problem of interpreting ‘grain flow’ type deposits without requiring steep slopes of over 20”. However, there is a need to conduct experiments in establishing various parameters that control sandy debris flow. For example: (1) what are the boundary conditions required for sandy debris flows? (2) what

is the minimum amount of clay needed for sandy debris flows? (3) can we measure the strength of sandy debris flows?

7.4. Recognition of deposits of sandy debris ,flow.s

Deposits of sandy debris flows and related slumps and slides have been recognized using the following criteria (Shanmugam et al., 1995a, 1997a; Shan- mugam and Moiola, 1995, 1997):

(1) Massive (ungraded) sand with a basal zone of shearing (Fig. 13) has been used to infer mass movements as slide/slump on a glide plane. Such a feature is unlikely to develop in turbidites because settling of grains from turbulent suspension does not cause basal deformation during deposition. After deposition, however, turbidites can undergo remobiliza- tion as slide/slump; such remobilized units would

still preserve the original graded bedding because they move as coherent mass.

(2) Concentration of rafted mudstone clasts near the tops of massive sandstone beds has been used to infer flow strength in both ancient (Fig. 14) and modem (Fig. 15) environments at the time of deposition. Occurrences of rafted clasts in the deposit can be explained by freezing of mass flows that had rafted clasts at different levels within the tlow. This is because that flows with measurable strength are capable of supporting clasts of different sizes and weights at any levels within the flow (Lawson, 198 1).

(3) Inverse grading of mudstone clasts (Fig. 15) has been used to infer flow strength and buoyant lift in sandy debris flows. One might argue that these mudstone clasts might have been derived from collapse of adjacent channel walls, implying that clasts had nothing to do with depositional processes. How-

Fig. as sl mud

13. c lide/:

Massive sz md

Shearing

Mudstone

move :ore photogr ‘al Jh showing basal shearing zone of a massive (ungraded) sand. This has been interpreted to I slump along a decollement surface (primary glide plane). Note sand dike at the bottom of the sand. Mas! clasts and ! ;tf :ep layers near the top (not shown). Eocene, North Sea.

‘WE ;est n lass jive sar Id CO1

‘ments rafted

G. Shanmugam / Earth-Science Reviews 42 (I 997) 201-229 223

Rafted mudstone clasts of different sizes in fii-grained massive sandstone (Sandy debris flow)

(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 weights at any levels within the flow because of their combined frictional and cohesive strength. In other words, we can infer plastic rheology of fluids from 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 information on the nature of flow and mechanics of deposition. 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 turbidity currents are not one and the same; they are fundamentally different processes (see Section 6

Fig. 14. Core photograph showing rafted mudstone clasts of different sizes near the top of a massive sandstone unit. This has been interpreted to indicate flow strength and deposition by freezing in a sandy debris flow. Note that long axes of clasts are aligned parallel to bedding (i.e., planar clast fabric), which indicate 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, mudstone 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. There are cases where quartz sands show normal grading, but mudstone clasts within the normally graded sand unit show inverse grading.

Rafted mudstone &as& of different sizes .&I fine-grained sand (Sandy debris flow)

Fig. 15. Core photograph showing rafted mudstone clasts of different sizes near the top of a sand unit. This has been interpreted to indicate flow strength and deposition by freezing in a sandy debris flow. Note inverse grading of clasts. Modem intras- 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 common in debris flows.

7.5. Depositional models of debris flows

(8) Detrital matrix is indicative of high-concentration flow and plastic rheology.

Evidence for laminar flow and flow strength with rafted clasts and planar fabric makes a strong case for a debris flow.

Many deep-marine fine-grained massive sands with the above features, which are routinely inter-

Unlike submarine fans with organized turbidite packages in channels and lobes (Mutti and Ricci Lucchi, 19721, debris flows are disorganized (Fig. 16). Debris-flow dominated systems can be broadly classified into (1) non-channelized and (2) channelized types (Fig. 16). Most deep-water reservoirs in 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.


gian Sea (Shanmugam et al., 1994), Gulf of Mexico (Famakinwa et al., 1997; Shanmugam and Zimbrick, 19961, and offshore Equatorial Guinea (Shanmugam et al., 1997b) are considered to be non-channelized type. Channelized type includes certain intervals in the Edop Field in offshore Nigeria (Shanmugam et al., 1995b), and the modem outer Mississippi Fan (Fig. 7). In this slump and debris-flow dominated slope model, nature of shelf (sand rich vs. mud rich), sea-floor topography (smooth vs. irregular), and depositional process (settling vs. freezing), tend to control sand distribution and geometry. Contrary to popular belief, sandy debris flows can be thick, areally extensive, and excellent reservoirs (Shanmu- gam and Zimbrick, 1996). High-frequency flows tend to develop amalgamated debris-flow deposits with lateral connectivity and sheet-like geometry.

According to the model (Fig. 161, amalgamated sandy debris flows may be predicted to occur downdip from a sand-rich shelf. Experimental studies of subaqueous debris flows have shown that hydroplaning can dramatically reduce the bed drag, and thus increase head velocity (Mohrig et al., 1997). This would explain why subaqueous debris flows can travel faster and farther on gentle slopes than subaerial debris flows. Future research should also focus on establishing not only dimensions and geometries of debris-flow deposits, but also their seismic and log attributes in order to predict them in frontier areas of exploration.

8. A critical perspective

The long-standing practice of describing deep- water sequences using letters (Bouma T,, T,, T,, Td, and T, divisions) and numbers (S,, S,, S, of Lowe, 1982) creates an unrealistic deep-sea environment flooded with ‘turbidites’. In areas that are believed to include some of the classic examples of ‘turbidites’ and ‘submarine fans’ in the rock record (e.g., the Miocene Mamoso-arenacea Formation in the north- em Apennines, the Paleogene Frigg Fan in the North Sea, the Pliocene-Pleistocene sequences in the Gulf of Mexico, the Pennsylvanian Jackfork Group in Arkansas and Oklahoma), the greater the number of deep-water sands I examine the fewer the number of turbidites I interpret. Although the turbidite paradigm,

as discussed by Walker (1973) more than two decades ago, is still alive and well in the minds of many sedimentologists and sequence stratigraphers (see Shanmugam et al., 1997c), the turbid&es themselves are becoming an endangered facies! Perhaps, it is time to quit the practice of model-driven interpretation and begin the practice of observation-driven interpretation.

Acknowledgements

I thank J.E. Sanders, three other reviewers, and editor G.M. Friedman for their constructive com- ments; R.J. Moiola, J.E. Damuth, and G. Zimbrick for reviewing earlier versions of the manuscript; M.K. Lindsey for drafting; G.K. Baker for manage- rial support, and Mobil for granting permission to publish this paper. I wish to thank my wife, Jean, for editorial assistance. This critical review would not have been possible without the help of numerous colleagues who assisted in describing cores and out- crops worldwide.

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G. (Shari) Shanmugam is a Geological Scientist with Mobil Oil Technology Company in Dallas where he joined Mo- bil in 1978. He holds degrees from An- namalai University in south India (B.Sc, Geology and Chemistry), Indian Insti- tute of Technology in Bombay (M.Sc., Applied Geology), Ohio University in Athens (M.Sc., Geology), and the Uni- versity of Tennessee in Knoxville (Ph.D., Geology). His publications (75 papers and 63 abstracts) cover a wide

rsnge of topics on petroleum exploration and production. His puiblications primarily focus on the origin and distribution of deep-water sands; however, he has also published articles on porosity development from chert dissolution, erosional uncon- formities, foredeep tectonics, Mn distribution, tide-dominated es- tuarine facies, coniferous rain forests and oil generation from coaly source rocks.

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Shanmugam 1997 ESR the Bouma Sequence and the Turbidite Mind