G. Shanmugam

50 years of the turbidite paradigm (1950s—1990s): deep-water processes and facies models—a critical perspective

Abstract Under the prevailing turbidite paradigm, the term turbidite (i.e., deposits of turbidity currents with Newtonian rheology and turbulent state) is used very loosely and is commonly applied to deposits of debris flows with plastic rheology and laminar state. For example, because high-density turbidity currents are defined on the basis of three different concepts (i.e., flow density, grain size, and driving force), there are no consistent criteria for recognition of their deposits. As a result, deep-water massive sands of debris-flow origin are routinely misinterpreted as high-density turbidites. The concept of waxing flow as a type of turbidity current is problematic because waxing flows are defined on the basis of velocity, not on fluid rheology and flow state. The waxing-flow concept allows inversely graded sands to be misinterpreted as turbidites. Perhaps, the most problematic issue is the use of alluvial channel traction bed forms observed in flume experiments as the analog for the five divisions of the Bouma Sequence (i.e., classic turbidites deposited from suspension). This is because flume experiments were conducted under equilibrium flow conditions, whereas natural turbidity currents deposit sediment under disequilibrium waning flow conditions. This and other problems of deep-water processes and facies models are addressed in this paper from the authors personal perspective. Classification of sediment-gravity flows into Newtonian flows (e.g., turbidity currents) and plastic flows (e.g., debris flows), based on fluid rheology and flow state, is a meaningful and practical approach. Although popular deep-water facies models are based on transport mechanisms, there are no standard criteria in the depositional record to reliably interpret transport mechanisms. According to existing turbidite-facies models, an ideal turbidite bed, which has normal grading, with gravel- to mud-size particles should contain a total of 16 divisions. However, no one has ever documented a complete turbidite bed with 16 divisions in modern or ancient deposits. Recognition of units deposited by deep-water bottom currents (also referred to as contour currents) is difficult. Traction structures are good indicators of bottom-current reworking, but distinguishing deposits of bottom currents from deposits of overbanking turbidity currents is difficult even though it has important implications for developing depositional models for hydrocarbon exploration and production. I consider sandy debris flows to be the dominant process responsible for transporting and depositing sands in the deep sea. Experiments on sandy debris flows suggest that low clay content (as little as 1%) is sufficient to provide the strength necessary for sandy debris flows. Deposits of experimental sandy debris flows are characterized by massive sand, sharp upper contacts, floating clasts, inverse grading, normal grading with clasts, and water-escape structures. As a counterpart to turbidite-dominated fan models suited for basinal settings, a slope model is proposed that is a debris-flow dominated setting with both non-channelized and channelized systems. Contrary to popular belief, deposits of sandy debris flows can be thick, areally extensive, clean (i.e., mud poor), and excellent reservoirs. High-frequency flows tend to develop amalgamated debris-flow deposits with lateral connectivity and sheet-like geometry. Submarine-fan models with turbidite channels and lobes have controlled our thinking for nearly 35 years, but I consider that these models are obsolete. The suprafan lobe concept was influential in both sedimentologic and sequence-stratigraphic circles because it provided a basis for constructing a general fan model and for linking mounded seismic facies with sheet-like turbidite sandstones. However, this concept recently was abandoned by its proponent, which has left the popular sequence-stratigraphic fan models with a shaky foundation. A paradigm shift is in order in the 21st century. This shift should involve the realization that thick deep-water massive sands are deposits of debris flows, not high-density turbidites. However, there are no standard vertical facies models that can be applied universally for either turbidites, contourites, or sandy debris flows. Science is a journey, whereas facies models terminate that journey and become the final destination.

Fan-deltas and braid deltas: Varieties of coarse-grained deltas

Two types of coarse-grained deltas are recognized: fan-deltas and braid deltas. Fan-deltas are gravel-rich deltas formed where an alluvial fan is deposited directly into a standing body of water from an adjacent highland. They occupy a space between the highland (usually a fault-bounded margin) and the standing body of water. In contrast, braid deltas (here introduced) are gravel-rich deltas that form where a braided fluvial system progrades into a standing body of water. Braid deltas have no necessary relationship with alluvial fans, as exemplified by fluvioglacial braid deltas. Braid deltas have previously been classified as fan-deltas even though the geomorphic and sedimentologic settings of the two systems can be vastly different. Braid deltas are a common present-day geomorphic feature and are abundant in the geological record. Fan-deltas and braid deltas can be distinguished in the rock record by distinctive subaerial components of these depositional systems; the shoreline and subaqueous components of both are similar. Fan-delta sequences have a subaerial component that is an alluvial-fan facies comprising interbedded sheetflood, debris-flow, and braided-channel deposits. Fan-deltas produce small (a few tens of square kilometres), wedge-shaped bodies of sediment, commonly displaying high variability in paleocurrent patterns and abrupt changes in facies. The deposits are generally very coarse grained (with large out-sized clasts), very poorly sorted, matrix-rich, polymictic, heterolithic, partially cemented by penecontemporaneous carbonate, and have low porosity and permeability. Braid-deltas, in contrast, have a subaerial component consisting entirely of braided-river or braidplain facies. Their deposits display better sorting, roundness, and clast orientation than do fan-delta sediments; they lack a muddy matrix; they display size grading and bar migration; they commonly have a sheet geometry with high lateral continuity (tens to hundreds of square kilometres); and they exhibit moderate to high porosity and permeability. Valuable paleogeographic and tectonic information concerning the proximity of highlands and major fault zones may be misinterpreted or lost if these two coarse-grained deltaic systems are not differentiated.

Sequence of structures in fine-grained turbidites: Comparison of recent deep-sea and ancient flysch sediments

Abstract A comparative study of the sequence of sedimentary structures in ancient and modern fine-grained turbidites is made in three contrasting areas. They are (1) Holocene and Pleistocene deep-sea muds of the Nova Scotian Slope and Rise, (2) Middle Ordovician Sevier Shale of the Valley and Ridge Province of the Southern Appalachians, and (3) Cambro-Ordovician Halifax Slate of the Meguma Group in Nova Scotia. A standard sequence of structures is proposed for fine-grained turbidites. The complete sequence has nine sub-divisions that are here termed T0 to T8. “The lower subdivision (T0) comprises a silt lamina which has a sharp, scoured and load-cast base, internal parallel-lamination and cross-lamination, and a sharp current-lineated or wavy surface with ‘fading-ripples’ (= Type C etc. …).” (= Type C ripple-drift cross-lamination, Jopling and Walker, 1968). The overlying sequence shows textural and compositional grading through alternating silt and mud laminae. A convolute-laminated sub-division (T1) is overlain by low-amplitude climbing ripples (T2), thin regular laminae (T3), thin indistinct laminae (T4), and thin wipsy or convolute laminae (T5). The topmost three divisions, graded mud (T6), ungraded mud (T7) and bioturbated mud (T8), do not have silt laminae but rare patchy silt lenses and silt pseudonodules and a thin zone of micro-burrowing near the upper surface. The proposed sequence is analogous to the Bouma (1962) structural scheme for sandy turbidites and is approximately equivalent to Boumas (C)DE divisions. The repetition of partial sequences characterizes different parts of the slope/base-of-slope/basin plain environment, and represents deposition from different stages of evolution of a large, muddy, turbidity flow. Microstructural detail and sequence are well preserved in ancient and even slightly metamorphosed sediments. Their recognition is important for determining depositional processes and for palaeoenvironmental interpretation.

Significance of Coniferous Rain Forests and Related Organic Matter in Generating Commercial Quantities of Oil, Gippsland Basin, Australia

Contrary to the conventional belief that humic coal generates primarily gas, 3 billion bbl of recoverable oil has been discovered in the humic coaly succession of the fluviodeltaic Latrobe Group (Upper Cretaceous-Tertiary) that serves as both the reservoir and the source for hydrocarbons in the offshore Gippsland basin of southeastern Australia. Evidence for generation of liquid hydrocarbons from the coaly succession includes: (1) similarity of n-alkane distribution in the oil and in the coal extracts; (2) high wax content of oil (up to 27% by weight); (3) high ratio of pristane/phytane in oil (5-6); and (4) dominance of C29 steranes in the oil. In the Gippsland basin, coniferous rain forests dominated by kauri vegetation flourished in a raised bog setting. Present temperate climate and kauri vegetation of New Zealand are considered to be the modern analog to the Gippsland basin. The coniferous vegetation provided large quantities of hydrogen-rich exinite macerals, such as cutinite and resinite, with potential to generate oil. High rainfall, raised ground-water level, low oxygen, high acidity, and low-nutrient conditions of a raised bog setting were suitable for preserving organic matter. A comparison of gas chromatograms of oils in the Gippsland basin with gas chromatograms of oils generated by hydrous pyrolysis in the laboratory from the immature source rocks suggests that the paraffinic fraction of the oil was derived from coal, and the naphthenic fraction was derived chiefly from resin.

Submarine fans: Characteristics, models, classification, and reservoir potential

Abstract Submarine-fan sequences are important hydrocarbon reservoirs throughout the world. Submarine-fan sequences may be interpreted from bed-thickness trends, turbidite facies associations, log motifs, and seismic-reflection profiles. Turbidites occurring predominantly in channels and lobes (or sheet sands) constitute the major portion of submarine-fan sequences. Thinning- and thickening-upward trends are suggestive of channel and lobe deposition, respectively. Mounded seismic reflections are commonly indicative of lower-fan depositional lobes. Fan models are discussed in terms of modern and ancient fans, attached and detached lobes, highly efficient and poorly efficient systems, and transverse and longitudinal fans. In general, depositional lobes are considered to be attached to feeder channels. Submarine fans can be classified into four types based on their tectonic settings: (1) immature passive-margin fans (North Sea type); (2) mature passive-margin fans (Atlantic type); (3) active-margin fans (Pacific type); and (4) mixed-setting fans. Immature passive-margin fans (e.g., Balder, North Sea), and active-margin fans (e.g., Navy, Pacific Ocean) are usually small, sand-rich, and possess well developed lobes. Mature passive-margin fans (e.g., Amazon, Atlantic Ocean) are large, mud-rich, and do not develop typical lobes. However, sheet sands are common in the lower-fan regions of mature passive-margin fans. Mixed-setting fans display characteristics of either Atlantic type (e.g., Bengal, Bay of Bengal), or Pacific type (Orinoco, Caribbean), or both. Conventional channel-lobe models may not be applicable to fans associated with mature passive margins. Submarine fans develop primarily during periods of low sea level on both active- and passive-margin settings. Consequently, hydrocarbon-bearing fan sequences are associated generally with global lowstands of sea level. Channel-fill sandstones in most tectonic settings are potential reservoirs. Lobes exhibit the most favorable reservoir quality in terms of sand content, lateral continuity, and porosity development. Lower-fan sheet sands may also make good reservoirs. Quartz-rich sandstones of mature passive-margin fans are most likely to preserve depositional porosity, whereas lithic sandstones of active-margin fans may not.

The Bouma Sequence and the turbidite mind set

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 Ta, Tb, Tc, Td, and Te 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 turbidites; 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 turbidites themselves are becoming an endangered facies!

Experiments on subaqueous sandy gravity flows: The role of clay and water content in flow dynamics and depositional structures

Deep-water deposits consisting mainly of massive sand are commonly identified as deposits of turbidity currents (i.e., turbidites). Speculation has risen in recent years as to whether some of these massive sandy deposits could have instead been deposited by debris flows. This possibility is explored here by examining the flow mechanics of sand-rich subaqueous gravity flows by means of laboratory experiments. In these experiments, sandy gravity flows were generated when well-mixed slurries of sand, clay, and water were released into a tank filled with tap water and allowed to flow under gravity over a slope that declined from 4.6° to 0.0°. The observed flow mechanics and resulting depositional features were strongly tied to the “coherence” of the debris flows (i.e., the ability of the slurry to resist being eroded and broken apart by the shear and pressure undergone by the flow). Low water content and high clay content resulted in strongly coherent debris flows, whereas high water content and low clay content resulted in weakly coherent flows. As little as 0.7 to 5 wt% of bentonite clay or 7 to 25 wt% of kaolinite clay at water contents ranging from 25 to 40 wt% was required to generate coherent gravity flows. Weakly coherent and moderately coherent flows produced significant, low-concentration subsidiary turbidity currents, and their deposits developed coarse- tail grading, water-escape structures, and minor increases in thickness at the base of the slope. Strongly coherent debris flows commonly hydroplaned and generated only minor subsidiary turbidity currents. Their deposits were structureless and ungraded, commonly showing tension cracks, compression ridges, water-escape structures, detached slide blocks, and a significant increase in thickness at the base of the slope. Application of distorted geometric scaling suggests that many aspects of these experiments appropriately scale up to the field scale of natural submarine debris flows.

Ten Turbidite Myths

Abstract During the past 50 years, the turbidite paradigm has promoted many myths related to deep-water turbidite deposition. John E. Sanders (1926–1999), a pioneering process sedimentologist, first uncovered many of these turbidite myths. This paper provides a reality check by undoing 10 of these turbidite myths. Myth No. 1: turbidity currents are non-turbulent flows with multiple sediment-support mechanisms. Reality: turbidity currents are turbulent flows in which turbulence is the principal sediment-support mechanism. Myth No. 2: turbidites are deposits of debris flows, grain flows, fluidized flows, and turbidity currents. Reality: turbidites are the exclusive deposits of turbidity currents. Myth No. 3: turbidity currents are high-velocity flows and therefore they elude documentation. Reality: turbidity currents operate under a wide range of velocity conditions. Myth No. 4: high-density turbidity currents are true turbidity currents. Reality: Ph. H. Kuenen (1950) introduced the concept of “turbidity currents of high density” based on experimental debris flows, not turbidity currents. High-density turbidity currents are sandy debris flows. Myth No. 5: slurry flows are high-density turbidity currents. Reality: slurry-flows are debris flows. Myth No. 6: flute structures are indicative of turbidite deposition. Reality: flute structures are indicative only of flow erosion, not deposition. Myth No. 7: normal grading is a product of multiple depositional events. Reality: normal grading is the product of a single depositional event. Myth No. 8: cross-bedding is a product of turbidity currents. Reality: cross-bedding is a product of traction deposition from bottom currents. Myth No. 9: turbidite facies models are useful tools for interpreting deposits of turbidity currents. Reality: a reexamination of the Annot Sandstone in SE France, which served as the basis for developing the first turbidite facies model, suggests a complex depositional origin by plastic flows and bottom currents. Myth No. 10: turbidite facies can be interpreted using seismic facies and geometries. Reality: individual turbidity-current depositional events, commonly centimeters to decimeters in thickness, cannot be resolved in seismic data. All turbidite myths promote falsehood and should be abandoned.

Eustatic control of turbidites and winnowed turbidites

Global changes in sea level, primarily the results of tectonism and glaciation, control deep-sea sedimentation. During periods of low sea level the frequency of turbidity currents is greatly increased. Episodes of low sea level also cause vigorous contour currents, which winnow away the fines of turbidites. In the rock record, the occurrence of most turbidites and winnowed turbidites closely corresponds to global lowstands of paleo-sea level. This observation may be useful in predicting the occurrence of deep-sea reservoir facies in the geologic record.

Deep-marine tidal bottom currents and their reworked sands in modern and ancient submarine canyons

Abstract Submarine canyons provide a unique setting for tidal processes to operate from shallow-marine to deep-marine environments. In modern canyons, current-meter measurements at varying water depths (46–4200 m) show a close correlation between the timing of up- and down-canyon currents and the timing of semi-diurnal tides. These tidal bottom currents in submarine canyons commonly attain maximum velocities of 25–50 cm/s. Based on core and outcrop studies of modern and ancient deep-marine deposits, it is proposed here that sand-mud rhythmites, double mud layers, climbing ripples, mud-draped ripples, alternation of parallel and cross-laminae, sigmoidal cross-bedding with mud drapes, internal erosional surfaces, lenticular bedding, and flaser bedding can be used to interpret deposits of deep-marine tidal currents. This approach is an alternative to the conventional approach in which most deep-water traction structures (e.g. climbing ripples and cross-bedding) would be attributed to deposition from turbidity currents. Underwater photographs show active mass flows (i.e. slides, slumps, grain flows, and debris flows) in modern canyons. Box cores taken from modern submarine canyons (e.g. La Jolla, California) and conventional cores and outcrops of ancient canyon-fill facies (Oua Iboe, Pliocene, Nigeria and the Annot Sandstone, Eocene–Oligocene, SE France) contain deposits of both tidal processes and mass flows. This facies association in the rock record can be used as a criterion for recognizing submarine canyon settings. In a channel-mouth environment, deep-marine tidal deposits are likely to develop elongate bars that are aligned parallel to the channel axis within the channel, whereas turbidites are more likely to develop depositional lobes that are aligned perpendicular to channel axis. Turbidite depositional lobes are much larger than the channel width, whereas tidal sand bars are much smaller than the channel width. Therefore, the wrong use of a turbidite lobe model with sheet geometry in lieu of a tidal bar model with bar geometry will result in an unrealistic overestimation of sandstone reservoirs in deep-water exploration.