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The Bouma Sequence and the Turbidite Mind Set

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The Bouma Sequence and the Turbidite Mind Set 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.
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