150 Years (1872-2022) of research on deep-water processes, deposits, settings,triggers,and deformation:A difficult domain of progress, dichotomy, diversion, omission, and groupthink

G. Shanmugam

Department of Earth and Environmental Sciences,The University of Texas at Arlington,Arlington,TX 76019,USA

Abstract In capturing a snapshot of 150 years(1872-2022)of research on deep-water processes,deposits,settings, triggers, and deformation, the following 22 topics are selected: (1) H.M.S. Challenger expedition(1872-1876): The discovering of the “Challenger Deep” by the H.M.S. Challenger in the Mariana Trench has been the single most important achievement in deep-water research. (2) Five pioneers amid 50 notable contributors: R. A. Bagnold, J. E. Sanders, G. D. Klein, F. P. Shepard, and C. D. Hollister. (3) Mass transport:Mass-transport deposits(MTD)are the most important deep-water facies in terms of volume,geohazards,and petroleum reservoirs. (4) Gravity flows: There are six basic types,namely(a) hyperpycnal flows, (b)turbidity currents,(c)debris flows,(d)liquefied/fluidized flows,(e)grain flows,and(f)thermohaline contour currents.Sandy debrites are the most important petroleum reservoir facies.Despite their popularity,turbidites are not an important reservoir facies.(5)Kelvin-Helmholtz(KH)waves:Turbidites,related to KH waves,with internal hiatus are not qualified to function as predictive facies models;nor are they fit for stratigraphic correlations.(6) High-density turbidity currents (HDTC): Misclassification of density-stratified gravity flows with laminar debris flows and turbulent turbidity currents as HDTC is flawed. Experimental generation of density-stratified gravity flows in flume studies has debunked the concept of HDTC. (7) Classification of turbidites: Contrary to the popular groupthink, turbidites are exclusive deposits of turbidity currents. (8) Bottom currents: The four basic types of deep-marine bottom currents are: (a) thermohaline-induced geotropic contour currents, (b)wind-driven bottom currents,(c)tide-driven bottom currents(mostly in submarine canyons),and(d)internal wave/tide-driven baroclinic currents. (9) Classification of contourites: Contrary to the popular groupthink,contourites are the exclusive deposits of thermohaline-induced geotropic contour currents.(10)Tidal currents in submarine canyons: Their velocity measurements have been the single most important achievement in deep-water process sedimentology. (11) Modern and ancient systems: There is a dichotomy between rare observations of turbidity currents in modern settings and overwhelming cases of interpretations of ancient turbidites in outcrops and cores. The reason is that turbidity currents are truly rare in nature, but the omnipotent presence of turbidites in the ancient rock record is the manifestation of groupthink induced by the turbidite facies model (i.e., the Bouma Sequence). (12) Internal waves and tides: Despite their ubiquitous documentation in modern oceans, their ancient counterparts in outcrops are extremely rare. This is another dichotomy.(13) Hybrid flows: They are commonly developed by intersecting of down-slope gravity flows with along-slope contour currents.However,they are often misapplied to down-slope flow transformation of gravity flows.(14)Density(sediment)plumes:Deflected sediment plumes by wind forcing are common.Despite their importance in provenance studies, they are not adequately studied. (15) Hyperpycnal flows: They occur near the shoreline, next to the plunge point; but are of no relevance in deep-water environments. However, their importance in deep-marine settings is overhyped in recent literature. (16) Omission of erosional contact and internal hiatus: In order to promote genetic facies models that must not contain internal hiatuses, some researchers selectively omit internal hiatuses observed by the original authors. (17) Triggers of sediment failures: There are 22 types, but short-term triggers, such as earthquakes and meteorite impacts are more important than the conventional long-term trigger known as Eustasy. (18) Tsunami waves: Despite their sedimentologic importance,there are no reliable criteria for recognizing tsunami deposits in the ancient rock record. (19) Soft-Sediment Deformation Structures (SSDS): Although most SSDS are routinely interpreted as seismites,not all SSDS are caused by earthquakes.There are 10 other mechanisms,such as sediment loading,which can trigger liquefaction that can develop SSDS. (20) The Jackfork Group, Pennsylvanian, Ouachita Mountains, USA: Our reinterpretation of this classic North American flysch turbidites as MTD and bottomcurrent reworked sands has resulted in the longest academic debate with 42 printed pages in the AAPG Bulletin history since its founding in 1917.(21)Basin-floor fan model,Tertiary,North Sea:Our examination of nearly 12,000 ft (3658 m) of conventional core from Paleogene and Cretaceous deep-water sandstone reservoirs cored in 50 wells in 10 different areas or fields in the North Sea and Norwegian Sea reveals that these reservoirs are predominantly composed of MTDs,mainly sandy slumps and sandy debrites,and bottom-current reworked sands.Our core-seismic calibration debunked the conventional wisdom(groupthink)that basin-floor fans are composed of sandy turbidites in a sequence-stratigraphic framework. (22) Turbidite groupthink: A case study in illustrating how turbidite groupthink functions,without sound scientific methods,on the basis of published information on modern turbidity currents in Bute Inlet (fjord and estuary), British Columbia,Canada.

Keywords Mass transport, Gravity flows, Bottom currents, High-density turbidity currents, Contour currents, Tidal currents, Internal waves and tides, Kelvin-helmholtz waves, Hybrid flows, The Bouma Sequence,Tsunami waves, Deflected sediment plumes, Basin-floor fans, Tutbidite groupthink, Bute Inlet (BC, Canada),Soft-Sediment Deformation structures (SSDS)

1. Introduction

During the past 150 years (1872-2022), we have made not only enormous progress, but we have also met with (1) formidable difficulties in gathering data in the deep sea, (2) dichotomy between modern and ancient systems, (4) diversion from the original concepts, (4) omission of critical details, and (5) promotion of groupthink. In addressing these issues, the three basic objectives of this article are:

1) to provide a historical perspective,

2) to identify positive and negative aspects of past research, and

3) to offer a roadmap for future researchers.

1.1. A historical perspective

This article, in addition to incorpor ating contributions by countless other researchers, is a culmination of my previous contributions that dealt with history of research:

· A summary paper entitled“50 years of the turbidite paradigm (1950s-1990s): deep-water processes and facies models a critical perspective”published in the Marine and Petroleum Geology(Shanmugam,2000).

· A compilation of major contributions made on deep-water research during the period 1885-2005 in “Deep-Water Processes and Facies Models: Implications for Sandstone Petroleum Reservoirs”:Handbook of Petroleum Exploration and Production, Volume 5 (Shanmugam, (2006a, Chapter 2).

· A review paper on “Submarine fans: a critical retrospective (1950-2015)” published in the Journal of Palaeogeography (Shanmugam, 2016c).

· A comprehensive book on“Mass Transport,Gravity Flows, and Bottom Currents: Downslope and Alongslope Processes and Deposits” (Shanmugam,2021a).

1.2. Positive and negative aspects of past research

I have selected the following 22 topics to achieve this objective. By design, I have emphasized topics to which I have contributed directly during the past fifty years (1972-2022).

1) H.M.S. Challenger expedition (1872-1876)

2) Five pioneers amid 50 notable contributors

3) Mass transport

4) Gravity flows

5) Kelvin-Helmholtz waves

6) High-density turbidity currents (HDTC)

7) Classification of turbidites

8) Bottom currents

9) Classification of contourites

10) Tidal currents in submarine canyons

11) Modern and ancient systems

12) Internal waves and tides

13) Hybrid flows

14) Density(sediment)plumes

15) Hyperpycnal flows

16) Omission of erosional contact and internal hiatus

17) Triggers of sediment failures

18) Tsunami waves

19) Soft-Sediment Deformation Structures (SSDS)

20) The Jackfork Group, Pennsylvanian, Ouachita Mountains, USA

21) Basin-floor fan model, Tertiary, North Sea

22) Turbidite groupthink

1.3. A roadmap for future research

I have identified potential areas for future researchers throughout the article. In appreciating the diversity of this article, a sound knowledge of global history of science, soil mechanics, fluid mechanics,process sedimentology, laboratory flume experiments, outcrop and core description of rocks and sediments, physical oceanography, meteorology, satellite imagery, sediment plumes, seismicity, basin analysis, and petroleum geology would be beneficial to the reader.

1.4. Organization

Finally,this compendium is a hybrid in composition between an atlas (with 108 figures) and a review article (with 348 references). Although it has been classified as a “Review” to meet the Journal of Palaeogeography submission regulations,this article is not a review in the conventional sense. For example,this article does not explain the existing knowledge on deep-water systems based on all the published research. Nor does it emphasize the most recent advances in a particular domain (e.g., 3D seismic modeling). For an easy recognition, the year of publication of a figure is shown in a brown rectangle on the upper left.

2. H.M.S. Challenger expedition(1872-1876)

Since the birth of modern deep-sea exploration by the voyage of H.M.S. Challenger (December 21, 1872 to May 24, 1876) (Fig. 1), organized by the Royal Society of London and the Royal Navy (Murray and Renard, 1891), oceanographers have made considerable progress in understanding the world's oceans.

During the past 150 years, the single-most important progress on deep-sea exploration has been the discovery of the Challenger Deep, which is the deepest known point of the seabed in the Earth's hydrosphere with a depth of nearly 11 km (Fig. 1). The Challenger Deep is located in the Western Pacific Ocean near the island of Guam in the Mariana Trench.This discovery has paved the way for research on deep-water processes, deposits, settings, triggers,and deformation-the central theme of this contribution.

3. Five pioneers amid 50 notable contributors

I have identified five pioneering process sedimentologists/oceanographers in deep-water research,whose contributions have made indelible marks on the basic understanding of important basic processes.These pioneers are (Fig. 2):

Fig. 1 150 years (1872-2022) of deep-sea exploration. The discovery of the Challenger Deep was the forerunner to understanding deepmarine palaeogeography. From Shanmugam (2021a).

Fig. 2 Five pioneers of process sedimentology.

Table 1 50 Selected sedimentologists and oceanographers and their contributions on deep-water research during the past 150 years(1872-2022).This list does not represent total contribution by each author.Modified after Allen(1985);Apel(2002);Bagnold(1962);Bo uma(1962);Bouma et al.(1985);Briggs and Cline(1967);Curray and Moore(1974);Damuth et al.(1988);Dill et al.(1975);Dott(1963);Dzulynski et al.(1959);Strzebon′ski(2022);Embley(1980);Ewing et al.(1971);Forel(1885);Shanmugam(2013);Gill(1982);Gordon(2013);Hampton(1972);Haughton et al.(2009);Shanmugam(2012c);He et al.(2011);Heezen et al.(1966);de Castro et al.(2020);Hern′andez-Molina et al.(2013);Hollister(1967);Hsü(1989);Klein(1975);Kuenen(1957);Lonsdale et al.(1972);Lowe(1975);Lowe(1976a);Lowe(1976b);Lowe(1982);Lowe and Guy(2000);Marr et al.(2001);Middleton(1966);Middleton(1973);Middleton and Hampton(1973);Shanmugam and Moiola(1995);Mulder et al.(2003);Murray and Renard(1891);Mutti(1992);Natland(1967);Nelson et al.(1992);Nilsen et al.(1979);Normark et al.(1997);Pequegnat(1972);Pickering et al.(1989,1995);Pickering and Hiscott(2015);Piper(1975);Piper(1978);Piper and Brisco(1975);Piper and Aksu(1987);Piper et al.(1988);Piper et al.(1997);Piper et al.(2012);Shanmugam(1996);Postma et al.(1988);Viana and Rebesco(2007);Rebesco and Camerlenghi(2008);Sanders(1965);Shanmugam and Benedict(1978);Shanmugam and Walker (1978);Shanmugam and Walker(1980);Shanmugam and Lash(1982);Shanmugam and Benedict(1983);Shanmugam (1985);Shanmugam(1986);Shanmugam (1988);Shanmugam and Moiola(1985a);Shanmugam and Moiola(1985b);Shanmugam et al.(1985a);Shanmugam et al. (1985b);Shanmugam and McPherson(1987);Shanmugam et al.(1988a,b);Shanmugam et al.(1988c);Shanmugam(1996);Shanmugam (1997);Shanmugam(2002a);Shanmugam (2003);Shanmugam(2006a);Shanmugam (2006b);Shanmugam (2007);Shanmugam(2008a);Shanmugam (2008c);Shanmugam (2012a);Shanmugam (2012b);Shanmugam(2013);Shanmugam (2016a);Shanmugam(2016b);Shanmugam(2016c);Shanmugam(2017a);Shanmugam (2018a);Shanmugam(2018b);Shanmugam(2018c);Shanmugam(2019);Shanmugam(2020);Shanmugam(2021a);Shanmugam(2021b);Shanmugam(2021c);Shanmugam(2022a);Shanmugam(2022b);Shanmugam(2022d);Shepard et al.(1979);Southard and Stanley(1976);Stanley and Kelling,1978;Stanley(1980);Stanley(1981);Stanley (1993);Stanley and Moore(1983);Stanley et al.(1978);Shanmugam (2016a);Stow and Piper(1984);Stow and Fauge'res (1998);Stow et al. (2002);Stow and Fauge`res(2008);Walker(1992);Wüst (1933);Zenk(2008).

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1) R. A. Bagnold: Recognition of the importance of sediment concentration in typical turbidity currents (Fluid mechanics).

2) J. E. Sanders: Recognition of the importance of stratified gravity flows with a basal laminar and upper turbulent layers (Fluid mechanics).

3) G. D. Klein: Recognition of critical sedimentary features in identifying deposits of deep-marine contour currents and tidal currents in the ancient rock record (Outcrop and core).

4) F. P. Shepard: Velocity measurements of tidal currents in submarine canyons (Modern).

5) C. D. Hollister: Introduction of the contourite concept for deposits formed by the thermohalinedriven geotropic contour currents (Modern).

These pioneers and their works are amid 50 notable contributors in the world (Table 1).

4. Mass transport

Mass-transport deposits (MTD) have been documented not only on Earth but also on other planets,such as Mars and Jupiter (Fig. 3). The general term“mass transport” (Fig. 4) (i.e., slides, slumps, and debris flows)represents the failure,dislodgement,and downslope movement of either sediment or glacier under the influence of gravity(Fig.5).Mass transport is much more efficient in transporting large volumes of sediment of all sizes into the deep sea than turbidity currents(Fig. 6).

In soil mechanics (Duncan and Wright, 2005), a stable slope can be maintained only when the factor of safety for slope stability(F)is larger than or equal to 1(Fig. 7). The sliding motion of failed soil mass commences along the shear surface when the factor of safety (F) is less than 1(Fig. 7).

Where S=available shear strength,which depends on the soil weight, cohesion, friction angle, and porewater pressure; τ = equilibrium shear stress, which is the shear stress required to maintain a just-stable slope. It depends on the soil weight, pore-water pressure, and slope angle.

On the modern U. S. Atlantic Continental Slope,most slides occur on gentle slopes of less than 4°(Fig. 8) (Booth et al., 1993). On the modern seafloor(Fig. 9), fan-like distribution of MTD has been documented using Multibeam bathymetric images (Greene et al., 2006).

In Antarctica,Macdonald et al.(1003)documented a spectacular outcrop example showing sheet-like geometry of an ancient (Jurassic) sandy submarine slide (1000 m long and 50 m thick) encased in deepwater mudstone facies (Fig. 10). Another outcropbased Jurassic slide (Fig. 11A) has been interpreted to travel several km from shallow-water to deep-water environments in Italy(Fig.11B).Such large sand bodies have good potential to serve as deep-water petroleum reservoirs.

Fig. 3 Diagram showing four planets (i.e., Venus, Earth, Mars and Jupiter's moon) with observed mass-transport deposits (MTD). Source:NASA. Heading and arrows by Shanmugam. From Shanmugam (2021a).

Fig. 4 Generalized distributions of processes. From Shanmugam (2021a). Diagram credit: Principales medios sedimentarios.svg.

Fig. 5 Gravity-driven downslope processes in deep-marine (＞200 m) environments. From Shanmugam et al. (1994).

Fig. 6 Comparison of human transport on land (A) with gravity driven sediment transport under water (B). From Shanmugam (2015).

Fig.7 A)Plot showing that the shear strength of the soil(s)is composed of frictional(φ)and cohesive(c)components;B)Conceptual diagram showing that a stable slope can be maintained only when the factor of safety for slope stability (F) is larger than or equal to 1 (Duncan and Wright, 2005). This sliding motion of failed soil mass commerces along the shear surface when the factor of safety (F) is less than 1. From Shanmugam (2014).

Fig.8 Histogram showing frequency distribution of submarine slides with increasing slope angle,U.S.Atlantic Continental Slope.Note most slides occur on gentle slopes of less than 4°. From Booth et al. (1993).

Large tongue-shaped debrite sediment bodies have been mapped on the Norwegian (Fig. 12) and on the U.S. Atlantic Margin (Embley, 1980). Such modern analogues are of great value in interpreting ancient sediment geometries of MTD.

Fig. 9 EM300 Multibeam bathymetric image showing fan-shaped MTD. From Greene et al. (2006).

Fig.10 Outcrop photograph showing sheet-like geometry of an ancient sandy submarine slide(1000 m long and 50 m thick)encased in deep water mudstone facies.Note the large sandstone sheet with rotated/slumped edge(left).Ablation Point Formation,Kimmeridgian(Jurassic),Alexander, Antarctica. Photo courtesy of D. J. M. Macdonald. From Macdonald et al. (1993). Gamma ray motif and other labels by G.Shanmugam.

Fig. 11 A) Jurassic slide block in southern Italy. Arrow shows a person (T. Teale). Photo by G. Shanmugam; B) A depositional model for the slide block (Teale and Young, 1987). Elsevier. Copyright Clearance Center's RightsLink. Licensee: G. Shanmugam. License number:5374391146753, License date: August 22, 2022.

In this MTD domain, phenomenal improvements in our understanding of sedimentological processes have been made through theoretical (e.g., Terzaghi, 1950;Varnes, 1958; Bagnold, 1962; Dott, 1963; Sanders,1965; Nardin et al., 1979; Lowe, 1982; Shanmugam,1996; Iverson, 1997; Hungr et al., 2001; Dasgupta,2003), experimental (e.g., Bagnold, 1962; Kuenen,1966; Middleton, 1970; Hampton, 1972: Lanteaume et al., 1967; Melosh, 1979; Postma et al., 1988;Major and Iverson, 1999; Shanmugam, 2000; Marr et al., 2001), and modern and ancient field observational (Heim, 1882, 1932; Sharpe, 1938; Shepard and Dill, 1966; Helwig, 1970; Fisher, 1971; Lewis, 1971;Jacobi, 1976; Warme et al., 1978; Cook, 1979;Woodcock, 1979; Damuth and Embley, 1981;Underwood and Bachman, 1982; Prior and Coleman,1984; McPherson et al., 1987; Surlyk, 1987; Moore et al., 1989; Friedman et al., 1992; Macdonald et al., 1993; Schwab et al., 1993; Maltman, 1994;Cruden and Varnes, 1996; Hampton et al., 1996;Elverh?i et al.,1997;Elverhoi et al.,2002;Piper et al.,1997; Gee et al., 1999; Sultan et al., 2004; Hurst et al., 2005; Locat and Lee, 2005; Masson et al.,2006; Solheim, 2006; Welbon et al., 2007;Moscardelli and Wood, 2008; Twichell et al., 2009;Gamboa et al., 2010; Mosher et al., 2010; Cossey,2011; Meckel, 2011; Zou et al., 2012; Alsop and Marco, 2013; Hughes-Clarke et al., 2014; Madrussani et al., 2018; Palladino et al., 2019; Moore et al.,2019; Ogata et al., 2019; Purkis et al., 2022, among others)studies.Mass-transport deposits(MTD)are the most important deep-water facies in terms of volume,geohazards, and petroleum reservoirs (Hampton et al., 1996; Elverh?i et al., 1997; Piper et al., 1997;Meckel, 2011; Shanmugam, 2015, 2021a).

Fig. 12 Map showing modern debris tongues on the Norwegian-Barents Sea Continental Margin. From Elverhoi et al. (1997). Modified by Shanmugam (2006a).

Fig. 13 Types of gravity flows. From Shanmugam (2020).

Fig. 14 Origin of Antarctic Bottom Water (AABW) as downslope gravity flows. Modified after Gordon (2013) and Purkey et al. (2018).

Fig.15 Underwater photographs showing a pocket of rounded cobbles up to 15 cm in diameter in massive sandy matrix at a depth of 130 m(427 ft) in Los Frailes Canyon, Baja California. Photo by R. F. Dill. From Shepard and Dill (1966). Published in Shanmugam (2012a).

Fig.16 Underwater photograph showing a cascading sand fall at a depth of 40 m(130 ft)in gully leading down into San Lucas Canyon,Baja California.Such events are analogous to pure grain flows.Photo by R.F.Dill.From Shepard and Dill(1966).Published in Shanmugam(2012a).

Fig. 17 Rheology (stress-strain relationships) of Newtonian fluids and Bingham plastics. Graph shows that the fundamental rheological difference between debris flows (Bingham plastics) and turbidity currents (Newtonian fluids) is that debris flows exhibit strength, whereas turbidity currents do not. Reynolds number is used for determining whether a flow is turbulent(turbidity current) or laminar(debris flow)in state. From Shanmugam (1997). Elsevier. Copyright Clearance Center's RightsLink. Licensee: G. Shanmugam. License number:5378780879658, License Date: August 30, 2022.

Fig. 18 Depth-velocity diagram showing laminar and turbulent fields of fluids (partly after Allen, 1984; Enos, 1977). From Shanmugam(2012a).

Fig.19 Turbidity currents are truly turbulent in state in which grains are in suspension(upper part).However,the basal flowing-grain layers are laminar in state and they are not turbidity currents (Sanders, 1965). Sanders'definition is adopted in this article.

Fig.20 Hydraulics of experimental turbidity currents.A)Turbidity currents surge;B)Steady uniform flow;C)Flow in and around the head;D)Schematic subdivisions of turbidity current;E)Photo showing head,neck,and body of an experimental turbidity current.Credit:A),B),C)and D) from Middleton and Hampton (1973). E) From experiments by M. L. Natland. Photo courtesy of G. C. Brown.

Fig.21 Turbidity currents.Modified after Allen(1985).Elsevier,Copyright Clearance RightsLink,Licensee:G.Shanmugam.License number:5374471080278, License date: August 22, 2022.

Fig.22 A)Front view of experimental turbidity current showing turbulent state;B)Map view showing fan geometry;Arrow=channel mouth;C)Core photo of silty turbidite layers showing normal grading(arrow).Experiments in A)and B)by M.L.Natland.Photos of turbidity currents courtesy of G. C. Brown. Core photo by G. Shanmugam.

5. Gravity flows

Gravity flows are the most consequential sedimentary phenomena in the geologic record. From a sedimentological perspective, gravity flows are ubiquitous in both subaerial and subaqueous environments.Importantly, gravity flows dominate in shelf, slope,and basin environments. They are caused not only by sediment density, but also by changes in temperature and salinity. Furthermore, density-driven flows travel not only downslope, but also alongslope (Figs. 13 and 14). There are six basic types of gravity (density)flows (Shanmugam,2020).The density value cited for each example below is to provide a relative sense,and they should not be considered typical of the given example:

Fig. 23 Thin-bedded turbidites. Photo by G. Shanmugam.

Fig. 24 A) Walther's Law of Facies: Vertical succession of facies reflects lateral changes in environment (No hiatus) (Middleton, 1973). If a sequence contains hiatus,it cannot be used in stratigraphic correlation;B)Sequence with hiatus is disqualified from being used as a predictive facies model (Walker, 1992).

Fig.25 A)The Bouma Sequence with five divisions(Bouma,1962);B)Submarine fan setting showing distribution of five divisions.It can be used as a predictive model using the Walther's Law only if the sequence is continuous without an internal hiatus (Middleton, 1973). From Shanmugam (2016c).

1) hyperpycnal flows (ρ): 0.025 g cm-3(Wright and Nittrouer, 1995),

2) turbidity currents (ρ): 1.1 g cm-3(Kuenen, 1966),

3) debris flows (ρ): 2.0 g cm-3(Hampton, 1972),

4) liquefied/fluidized flows (ρ): 1.8 g cm-3(Breien et al.,2010),

5) grain flows (ρ): 2.1-2.3 g cm-3(Parsons et al.,2001), and

6) Antarctic bottom water (thermohaline contour currents, THCC) (ρ): 0.03 g cm-3(Purkey et al.,2018) (Fig. 14).

All six types of gravity flows and their deposits,definitions, origins, identification markers, and problems are discussed by Shanmugam (2020). In modern submarine canyons, Shepard and Dill (1966) documented active sandy debris flows(Fig.15)and sand fall(Fig. 16) by underwater photographs. Unlike MTDs(slides, debris flows, grain flows, etc.), turbidity currents have never been documented in modern deepsea environments using underwater photographs convincingly. However, Grotzinger et al. (2007) used the photograph on sand fall (Fig. 16) as evidence for initiating sandy turbidity currents that could develop submarine fans, without any empirical data on turbidity currents. Important contributions on gravity flows are listed in Table 1.

5.1. Turbidity currents

Because of the skewed importance given to turbidity currents and their deposits (i.e., turbidites)by other researchers during the past 70 years, some basic fluid dynamical properties of turbidity currents are considered here briefly for clarity. Turbidity current is a sediment-gravity flow with Newtonian rheology(Fig.17)and turbulent state(Fig.18)in which sediment is supported by turbulence(Fig.19)and from which deposition occurs through suspension settling(Dott, 1963; Sanders, 1965; Middleton and Hampton,1973; Shanmugam, 1996).

Fig.26 A)Unit 2 with measured details of Unit2;B)Outcrop photo showing contorted layers at the base.Peira Cava is the type locality for the Bouma Sequence in the Maritime Alps in SE France. From Shanmugam (2002a). Elsevier, Copyright Clearance Center's RightsLink, Licensee: G. Shanmugam. License number:5373451372529, License date: August 21, 2022.

In laboratory experiments (Middleton and Hampton, 1973), surge-type turbidity currents show well-developed head, neck, and body components(Fig. 20). Turbidity currents exhibit unsteady and nonuniform flow behavior (Fig. 21). Muddy turbidity currents tend to spread out and develop fan geometry in unconfined environments (Fig. 22B). Fine-grained turbidites exhibit sheet-like geometry in basin-plain environments (Fig. 23), which are strikingly different from tongue geometry of debrites (Fig. 12). As they flow downslope,turbidity currents(Fig.21)invariably entrain ambient fluid (seawater) in their frontal head portion due to turbulent mixing (Allen, 1985). With increasing fluid content,plastic debris flows may tend to become Newtonian turbidity currents.However,not all turbidity currents evolve from debris flows. Some turbidity currents may evolve directly from sediment failures. Although turbidity currents may constitute a distal end-member in basinal areas, they can occur in any part of the system (i.e., shelf edge, slope, and basin). Turbidites are commonly recognized by their normal grading (Fig. 22C). Although these properties are reasonable for muddy or fine-grained turbidites,they are not applicable to coarse-grained sediments.The reason is that the concept of “High-density turbidity currents (HDTC)” is flawed (Shanmugam,1996), as explained with flume experiments in Section 7 below.

5.2. Walther's law

Walther's Law of Facies (named after Johannes Walther [1860-1937]), states that the vertical succession of facies reflects their lateral changes in environment (Fig. 24). This law is applicable only to those sequences that represent continuous deposition without internal hiatus (Middleton, 1973). If a sequence contains hiatus, it cannot be used in stratigraphic correlations. Also, a sequence with hiatus is disqualified from being used as a predictive facies model (Walker, 1992). In other words, the popular Bouma Sequence (Fig. 25) is rendered useless if it contains internal hiatus.For example,the middle cutout Bouma Sequence(Walker,1965)is disqualified as a facies model.

Fig. 27 A) Sedimentological log of amalgamated sandstone Unit 7 showing basal inverse grading overlain by an interval of complex normal grading with floating granules and mudstone clasts, parallel laminae, and lenticular layers. Note sudden increase in grain size at 5 m. Note conventional description using Bouma notations (Ta, Tb, and Tc); B) Outcrop photograph of Unit 7 showing sheet-like geometry; C) Outcrop photograph of Unit 7 showing basal inversely graded interval in coarse-to granule-grade sandstone; D) Outcrop photograph of a pocket of clasts and matrix in the middle of the unit. Arrow shows stratigraphic position of photo; E) Outcrop photograph of Unit 7 showing a floating mudstone clast in the middle of the unit. Annot Sandstone (Eocene-Oligocene), Peira Cava area, French Maritime Alps. SE France. Figures from Shanmugam (2002a). With permission from Elsevier. From Shanmugam (2021c). Elsevier, Copyright Clearance Center's RightsLink, Licensee: G. Shanmugam. License number: 5373451372529, License date: August 21, 2022.

Fig. 28 A) Measured field details of Unit8; B) Outcrop photo showing a pocket of gravel that is interpreted as MTD (sandy debrite). From Shanmugam (2002a), Elsevier, Copyright Clearance Center's RightsLink, Licensee: G. Shanmugam. License number: 5373451372529, License date: August 21, 2022.

Fig.29 A)Unit 2 with measured details for Unit2;B)Outcrop photo showing double mud layers(DML).Peira Cava is the type locality for the Bouma Sequence in the Maritime Alps in SE France. From Shanmugam (2002a and 2021c). DML indicates tidal deposition (Visser, 1980).Elsevier,Copyright Clearance Center's RightsLink,Licensee:G.Shanmugam.License number:5373451372529,License date:August 21,2022.

Fig. 30 A) Sedimentological log of an amalgamated sandstone unit 10; B) Outcrop photograph showing sigmoidal cross-bedding with mud(mica) drapes. Annot Sandstone (Eocene Oligocene), Peira Cava area, French Maritime Alps. From Shanmugam (2002a). Elsevier, Copyright Clearance Center's RightsLink, Licensee: G. Shanmugam. License number: 5373451372529, License date: August 21, 2022.

Fig. 31 A) Sedimentological log of an amalgamated sandstone unit showing sigmoidal cross-bedding with tangential toeset. Note inverse grading below and lenticular above; B) Outcrop photograph showing mica-draped (dark colored) stratification. Note inversely graded gravel layer below (bottom arrow). Arrows show stratigraphic position of photo. Annot Sandstone, Peira Cava area, French Maritime Alps. From Shanmugam (2002a) Elsevier. Copyright Clearance Center's RightsLink. Licensee: G. Shanmugam. License number: 5373451372529, License date: August 21, 2022.

5.3. The Bouma Sequence

In proposing the first turbidite facies model (i.e.,the Bouma Sequence)(Fig.25),Bouma(1962)used the Annot Sandstone [Gr`es d’ Annot Formation (Eocene--Oligocene), exposed in the Peira-Cava area and vicinity of the French Maritime Alps.Later,he extended his study to Switzerland and other areas in Europe.Bouma (1962) documented his field observations in eight photographic plates, seven of which contain outcrop photographs from the Peira Cava type locality(Plates A, B, C, D, E, F, and H) and the eighth one contains outcrop photographs from Switzerland(Plate G).

Although the Annot Sandstone appears to show normal grading,detailed description offers a different story (Shanmugam, 2002a). Simply put, the Bouma Sequence is an extreme example of omission. This model excludes all the important details from a process sedimentological point. The key features that contradict the conventional turbidite interpretation of the Annot Sandstone are (see Shanmugam, 2002a,2021a,b, c):

· Basal contorted layers (Fig. 26) (Shanmugam,2002a): The contorted intervals beneath the sandstone are interpreted to be a product of shearing and slumping produced by stress exerted by overriding mass flows.Large-scale shear structures have been reported in the Annot Sandstone in other areas as well (Clark and Stanbrook, 2001).

· Basal inverse grading (Fig. 27C): A combined mechanism of dispersive pressure,matrix strength,hindered settling, and buoyant lift would explain the development of inverse grading. The inverse grading is attributed to a plastic debris-flow origin.

· Basal normal grading: This is a rare feature.Because these sandstone intervals are not only amalgamated but composed of complex internal features, no simple origin is meaningful.

· Lenticular layers (Shanmugam, 2002a): Lenticular layers with quartzose granules (Fig. 28A) in sandstone may be interpreted as deposits of non-Newtonian flows with strength. The presence of planar fabric supports the laminar state of flow(Fisher, 1971). By simply describing these sedimentary features without using the “Bouma” divisions,lenticular layers would be interpreted to be deposits of plastic laminar flows.

· Pockets of gravel(Fig.28B):Unit 8 with pockets of gravel cannot be explained by a single waning turbidity current. The depositing flow must have had enough flow strength to support granules near its upper part. The pockets of gravels near the top of the bed reflect freezing of a plastic flow.

· Floating armored mudstone balls (Fig. 29A)(Shanmugam, 2002a): Stanley et al. (1978) interpreted armored mudstone balls in the Annot Sandstone to be associated with the filling of canyons by mass flows (MTD).

Fig. 32 Model for tidal bundles in shallow-marine environments. This model has been adopted to explain sigmoidal cross-bedding in deepwater Annot Sandstone in SE France by Shanmugam (2002a, 2003). Original diagram from Terwindt (1981). Modified by Banerjee (1989) and further simplified by Shanmugam et al. (2000).

Fig. 33 Kelvin-Helmholtz clouds look like ocean waves. Photo taken on M5, south of Brimingham/Black country driving towards Woecester(UK)on March 28,2022 around sunset by Erms Hammersley.Photo credit:EarthSky and Matty Hammersley.With email permission for use from the owner obtained.

· Floating mudstone clasts (Fig. 27E): In Unit 7, intervals of floating mudstone clasts are interpreted as deposits of plastic debris flows.A combination of dispersive pressure, matrix strength, hindered settling,and buoyant lift is proposed as the cause of floating clasts.

· Floating quartzose granules (Shanmugam, 2002a):Even a single floating quartzose granule in a quartzrich sandy matrix is of rheologic and hydrodynamic significance. In the Annot Sandstone, quartzose granules floating in a sandy matrix are evidence that the flow had strength and that settling of the grains is hindered.Because Unit 7 contains floating quartz granules and amalgamation surfaces, it has been interpreted to be deposits of multiple episodes of sandy debris flows and bottom currents.

· Double mud layers (DMLs, Fig. 29B): There are no analogous divisions for DMLs in the Bouma Sequence.In the Annot Sandstone,DMLs have been interpreted to be deposits of deep-marine tidal currents (Shanmugam, 2003). Hydrodynamically,turbidity currents are unsuitable to explain the DML.

· Sigmoidal cross-bedding (Figs. 30 and 31): There are no analogous divisions for sigmoidal crossbedding in the Bouma Sequence. These bedforms,typical of tidal currents in estuarine environments(Fig. 32) (Terwindt,1981; Banerjee, 1989).

Fig. 34 Image of Kelvin-Helmholz waves and bottom sediment layer. From Ge et al. (2022).

Fig.35 Problems with the concept of high-density turbidity currents(HDTC)See Racki(2003).A)Over lapping sediment concentration.From Shanmugam(1996);B)Stratified flows with laminar layer at the base.From Postma et al.(1988);Middleton(1993);Pierson and Costa(1987).Elsevier,Copyright Clearance Center's RightsLink,Licensee:G.Shanmugam.License number:5373461175993,License date:August 21,2022.

Although the turbidite facies model advocates a simple origin by turbidity currents,details of the Annot Sandstone clearly reveal a complex origin by processes involving slumping, sandy debris flows, and tidal bottom currents.Deposits of true turbidity currents,with normal grading, are extremely rare. Our observations are nothing new. Stanley (1963, 1975) was one of the early researchers to recognize the importance of slumps,debris flows,grain flows,and liquefied flows in the origin of the Annot Sandstone, SE France. In a recent comprehensive study,Etienne et al.(2012,p.3)have acknowledged the complex origin of the Annot Sandstone.The Annot Sandstone units are up to 1200 m thick(Inglis et al.,1981)and composed of siliciclastic deposits resulting from various types of gravity flows such as slumps, debris flows (Hampton, 1972;Middleton and Hampton,1973),slurry flows(Lowe and Guy, 2000), high-density turbidity currents (Kuenen,1966; Middleton, 1967; Lowe, 1982; Postma, 1986)also defined as sandy debris flows (Shanmugam, 1996,2000) and low-density turbidites (Bouma, 1962;Middleton and Hampton, 1973) (i.e. classical turbidites).

There are no theoretical (Sanders, 1965; Van der Lingen, 1969; Shanmugam, 1997; Hsü, 2004), experimental (Leclair and Arnott, 2005), or observational(Shanmugam, 2002a, 2006a) basis for validating the complete Bouma Sequence. Leclair and Arnott (2005,p.4) state that “…the debate on the upward change from massive (Ta)to parallel laminated (Tb)sand in a Bouma-type turbidity remains unresolved.” Therefore, the problem remains as to how one can explain deep-water units that show a partial Bouma Sequence composed of a basal massive division and an upper parallel-laminated division. In areas in which both downslope sandy debris flows and alongslope-bottom currents operate concurrently, the reworking of the tops of sandy debris flows by bottom currents may be expected. Such a scenario could generate a basal massive sand division and an upper reworked division,mimicking a partial Bouma Sequence (Shanmugam,2006a, 2012a). The reworking of deep-water sands by bottom currents has been suggested by other researchers as well (e.g., Stanley, 1993; Ito, 2002;Martin-Chivelet et al., 2008).

Fig.36 Flume used in the experiments of sandy debris.See Shanmugam (2000)and Marr et al.(2001)for details on experiments. Photo by Shanmugam.

Fig. 37 Experimental density-stratified gravity flows showing three types of sandy debris flows (weak, moderate and strong) and their properties. From Shanmugam (2000, 2002a). Elsevier. Copyright Clearance Center's RightsLink.Licensee: G. Shanmugam. License number:5374380555141. License date: August 22, 2022.

Fig.38 Downslope transformation of sandy debris flow into stratified flows with upper turbulent turbidity current and lower laminar sandy debris flow. See Shanmugam (2000) and Marr et al. (2001).

In summary, the most influential turbidite facies model (i.e., the Bouma Sequence) was acceptable in 1962 when our understanding of deep-water processes was limited.However,given the cumulative knowledge that we have acquired during the past 60 years,such as double mud layers(Fig.29)and sigmoidal cross-bedding(Figs. 30 and 31) in the Annot Sandstone, the Bouma Sequence is obsolete in 2022. Under this umbrella of new knowledge, the Bouma Sequence can no longer function as a genetic facies model for turbidity currents and their deposits.Although,it was reasonable in 1962,at a time of limited knowledge on deep-water processes,to introduce a simplistic turbidite facies model,it is unreasonable to apply this model to the rock record in 2022.

Fig.39 Diversion caused by confusing nomenclature used for turbidites by Mutti et al.(1999).From Shanmugam(2002).Elsevier.Copyright Clearance Center's RightsLink. Licensee: G. Shanmugam. License number: 5374381438606. License date: August 22, 2022.

Fig. 40 Three major types of turbidite facies models based on grain size. From Shanmugam (2000). Elsevier. Copyright Clearance Center's RightsLink. Licensee: G. Shanmugam. License number: 5373670166643. License date: August 21, 2022.

6. Kelvin-Helmholtz waves

The Kelvin-Helmholtz instability defines a fluid instability in nature. It occurs when there is velocity shear in a single continuous fluid or in a velocity difference across the interface between two fluids.Kelvin-Helmholtz instabilities are visible as billow clouds in the atmospheres of planets,such as in cloud formations on Earth. They also develop waves in the oceans. Hwang et al. (2012) reported the first in situ observation of Kelvin-Helmholtz waves (i.e., KH waves) at high-latitude magnetopause.

In fluid dynamics, the onset of instability and transition to turbulent flow within fluids of different densities moving at different speeds(Drazin,2003).If the density and velocity vary continuously in space(with the lighter layers uppermost,so that the fluid is RT-stable), the dynamics of the Kelvin-Helmholtz instability can be described by the Taylor-Goldstein equation as below:

Fig. 41 Process continuum in turbidity currents and related division. Modified after Lowe (1982). From Shanmugam (2000). Elsevier.Copyright Clearance Center's RightsLink. Licensee: G. Shanmugam. License number: 5373670166643. License date: August 21, 2022.

Fig. 42 Global overturning circulation of water masses. Note Gulf of Cadiz, which is the type locality for the contourite facies model(Faug`eres et al., 1984). Modified after Talley (2013).

Fig. 43 Conceptual model of the Southern Ocean showing three vertical segments, composed of the upper surface currents, the middle deep-water masses and the lower bottom currents,forming a vertical continuum (left).Modified after Hannes Grobe,April 7,2000.Http://en.wikipedia.org/wiki/File.Antarctic_bottom_water_hg.png (Accessed 18.05.11). From Shanmugam (2012a).

Fig. 44 A) Western Boundary Undercurrent (red arrows) along the U.S. Atlantic margin. From Flood et al. (1974); B) Traction structures in the Atlantic contourites. Pleistocene, Continental rise off Georges Bank, U.S. Atlantic Margin. Vema 18-374,710 cm, water depth 4756 m.From Bouma and Hollister (1973). Elsevier. Copyright Clearance Center's RightsLink. Licensee: G. Shanmugam. License number:5374490138205. License date: August 22, 2022.

the Brunt-V¨ais¨al¨a frequency, U is the horizontal parallel velocity, k is the wave number, c is the eigenvalue parameter of the problem, φ is complex amplitude of the stream function.

In a recent occurrence in the UK,Kelvin-Helmholtz clouds look like ocean waves in the sky (Fig. 33)(EarthSky and Matty Hammersley). Retrieved May 28,2022.

In a recent study (Fig. 34), Ge et al. (2022) stated that “Here, we demonstrate, on the basis of a highresolution advanced numerical CFD (computational fluid dynamics) simulation and rock-record examples,that the depositional event in reality involves many brief episodes of nondeposition.The reason is inherent hydraulic fluctuations of turbidity current energy driven by interfacial Kelvin-Helmholtz waves.”What is the practical significance of these “turbidites with hiatuses” associated with Kelvin-Helmholtz waves?Conventionally, a genetic facies model is designed for a single depositional event,without internal hiatuses.A classic example is the turbidite facies model or the“Bouma Sequence”(Bouma,1962)(Fig.25).According to Middleton (1973), Walther's Law is not meaningful for sequences with internal hiatuses. In other words,Walther's Law is not meaningful for these “turbidites with hiatuses” discussed by Ge et al. (2022). Importantly, these turbidites are problematic in stratigraphic correlations.

Fig. 45 A) Sea surface temperature (SST) image showing the Loop Current in the Gulf of Mexico and the axis of the Gulf Stream in the Atlantic Ocean along the U.S. Continental margin on March12, 2011. From Shanmugam (2012a); B) Location map of the Ewing Bank and adjacent areas in the Northern Gulf of Mexico.Solid Yellow dots show locations of cores used by Shanmugam et al.(1993).Elsevier.Copyright Clearance Center's RightsLink. Licensee: G. Shanmugam. License number: 5374280624784. License date: August 22, 2022.

7. High-density turbidity currents (HDTC)

Fig.46 Image of sea surface height(cm)showing that the warm waters of the Loop Current(red)were 5-75 cm higher than the surrounding water when Hurricane Katrina passed through the Loop Current during August 26, 27,28 and 29, 2005. Note that the wind speeds (mi/hr) of Hurricane Katrina increased dramatically as it passed over the warm waters of the Loop Current toward the Gulf Coast. Hurricane Katrina's wind speed is highlighted by SaffirSimpson Scale categories 2-5. The image was produced by a University of Colorado at Boulder team and processed at CU-Boulder's Colorado Center for Astrodynamics Research(CCAR).Source:http://www.colorado.edu/news/releases/2005/358.html (accessed 31.05.11). After Shanmugam (2012a).

Turbidity currents are characterized by low sediment concentration, commonly below 9% sediment concentration by volume (Fig. 35A) (Bagnold, 1962).Experimental concentrations that exceed this limit cannot be considered normal turbidity currents. They are commonly mass flows or sandy debris flows(Shanmugam,1996).Therefore,experiments on“highdensity turbidity currents”(Fig.35B)by Postma et al.(1988)is a diversion because their concept represents sandy debris flows(Shanmugam,1996).Sanders(1965)recognized the importance of density-stratified gravity flows with basal laminar and upper turbulent layers(Fig. 19). Our flume experiments (Fig. 36) on sandy debris flows confirmed Sanders’concept by developing density-stratified flows(Figs.37 and 38)(Shanmugam,2000; Marr et al., 2001). Such flows are mislabeled as“high-density turbidity currents”by other researchers(Fig. 35B). Our flume experiments on sandy debris flows(Shanmugam,2000;Marr et al.,2001)have been a major achievement in process sedimentology.This is because finally it provided clarity to the long-standing,confused concept of“high-density turbidity currents”.

8. Classification of turbidites

In the original classification of sediment-gravity flows by Middleton and Hampton (1973), there are four distinct types of gravity flows,including turbidity currents (Fig. 39). Conventionally, turbidites are considered to be deposits of turbidity currents(Kuenen, 1957; Sanders, 1965). However, Mutti et al.(1999) applied the term “turbidites” to deposits of all four types of sediment-gravity flows without regard for fluid mechanics (Fig. 39). In other words, debrites are turbidites! This is certainly a diversion. After 65 years of research on turbidites since the term was first introduced in 1957 by Kuenen, we still don't have consensus as to what the term “turbidite” means!

8.1. Types of turbidites

In a paper entitled“Ten turbidite myths”published in the Earth-Science Reviews (Shanmugam, 2002a), I pointed out that there are 34 types of stratified flows in HDTC. There are fluxoturbidites in the Carpathians(see reinterpretation by Strzebo′nski, 2022), seismoturbidites (Mutti et al., 1984), and “problematica”(Stanley et al.,1978)turbidites.Dzulynski et al.(1959)introduced the term “fluxoturbidites”and it has been variously interpreted to represent sand avalanches(Hsü, 2004) to MTD (Strzebo′nski, 2022). By counting the most recent turbidite type with hiatus (Ge et al.,2022), there are at least 36 types of turbidites (i.e.,one low-density turbidite, one turbidie with hiatus,and 34 high-density turbidites). This overzealous affinity towards turbidites is rather unique.

In practice, turbidites are identified under three broad categories based on grain size (Fig. 40). They are:

(a) Coarse-grained turbidites (Lowe, 1982).

(b) Medium-grained turbidites (Bouma, 1962).

Fig.47 Undersea photograph showing possible mud-draped(arrow)current at 3091 m water depth in the Gulf of Mexico.AlaminosCruise69-A-13,St.35.From Pequegnat(1972).From Shanmugam(2012a).Elsevier.Copyright Clearance Center's RightsLink.Licensee:G.Shanmugam.License number:5374280624784. License date: August 22, 2022.

(c) Fine-grained turbidites(Stow and Shanmugam,1980). Previously, Piper (1972) identified finegrained turbidites. According to Lowe (1982),turbidites are a consequence of a process continuum in terms of grain size (Fig. 41).

Despite this enormity of turbidite types, some researchers tend to ignore these problems and imply that all turbidites are one and the same (e.g., Rodrigues et al., 2022; Shanmugam, 2022b).

Fig.48 Photograph showing flaser bedding.Note the presence of mud in ripple troughs(arrow).Upper Pliocene,Ewing Bank Block826,Gulf of Mexico. From Shanmugam et al. (1993).

9. Bottom currents

9.1. The thermohaline circulation

Stommel(1958)first developed the concept of the global circulation of thermohaline water masses and the vertical transformation of light surface waters into heavy deep-water masses in the oceans. Broecker(1991) presented a unifying concept of the global oceanic “conveyer belt” by linking the wind-driven surface circulation with the thermohaline-driven deep circulation regimes. The large-scale horizontal transport of water masses, which also sink and rise at select locations, is known as the “thermohaline circulation”or THC. Aspects of thermohaline circulation are discussed by Zenk (2008). The global overturning circulation has been presented by Talley (2013)(Fig. 42).

9.2. Vertical continuum

A sound knowledge of global ocean surface currents is critical for understanding ocean bottom currents (Gill, 1982; Apel, 1987). This is because surface currents and bottom currents are interrelated entities in the world's oceans.For example,the three segments of the Southern Ocean, composed of (1) the upper surface currents, (2) the middle deep-water masses,and (3) the lower bottom currents, form a vertical continuum (Fig. 43).

Fig.49 Summary of traction of features interpreted as indicative of deep-water bottom current reworking by all types of bottom currents.Each feature occurs randomly and should not be considered as part of a vertical facies model. From Shanmugam et al. (1993).

9.3. Deep-marine bottom currents

The four basic types of deep-marine bottom currents are:(1)thermohaline-induced geotropic contour currents, (2) wind-driven bottom currents, (3) tidedriven bottom currents (mostly in submarine canyons), and (4) internal wave/tide-driven baroclinic currents (Southard and Stanley, 1976; Shanmugam,2008a). Traction structures are common in deposits of all four types of bottom currents, including the Atlantic contourites (Fig. 44). In the Gulf of Mexico(Fig. 45) with wind-driven Loop Current (Figs. 45 and 46), there are traction deposits both on the modern seafloor (Fig. 47) and in the subsurface (Figs. 48 and 49). However, there are no diagnostic sedimentological or seismic criteria for distinguishing ancient contourites from the other three types.Double mud layers are a reliable criterion for recognizing deep-marine tidalites in cores and outcrops (Visser, 1980).Shanmugam et al. (1993) have documented the importance of bottom-current reworking and related traction structures in the Ewing Bank area, Gulf of Mexico.

9.4. The contourite problem

Contourites are deposits of thermohaline-driven geotropic contour currents (Hollister, 1967). Contourites can be muddy or sandy in texture,siliciclastic,or calciclastic in composition. The Gulf of C′adiz(Fig. 50) is the type locality for the contourite facies model based on muddy lithofacies (Faug`eres et al.,1984; Stow and Faug`eres, 2008). Despite being the type locality for contour currents, there are no genuine contour currents in the Gulf of C′adiz (Zenk,2008). Other problems associated with the contourite facies model, such as internal hiatus (Fig. 51), were addressed by Shanmugam (2016a). The presence of hiatus at the bottom of C3 division (Fig. 51), which corresponds to maximum velocity(Stow and Faug`eres,2008, their Fig. 13.9), is a problem in a facies model.The reason is that a true depositional unit cannot have hiatus in the middle, representing certain period of missing time.Walther's Law cannot be applied to such units(Middleton,1973).Therefore,it cannot function as a predictive facies model (Walker, 1992). Finally,the true representation of fluid mechanics of depositional processes is the primary sedimentary structures(Sanders,1963).Disappointingly,the contourite facies model is not based on primary sedimentary structures(Fig. 51), but on changing grain size and extensive bioturbation. Post-depositional bioturbation is common in deep-sea sediments. There is nothing unique about bioturbation that reflects deposition (reworking) associated with contour-following geostrophic bottom currents.

Fig.50 Schematic diagram showing the location of Gulf of Cadiz and complex transport nature of the Mediterranean Outflow Water(MOW),involving three stages of evolution: (1) channel-current stage (2) mixing and spreading (i.e., transition) stage, and (3) genuine contourcurrent stage (see Zenk, 2008, his Fig. 4.10). From Shanmugam (2016a). Elsevier. Copyright Clearance Center's RightsLink Licensee: G.Shanmugam. License number: 5374250708256, License Date: August 22, 2022.

Fig.51 Contourite facies model showing five divisions(C1-C5)and the position of internal hiatus at the bottom of C3 division(red arrow).From Stow and Faug`eres(2008).Elsevier.Copyright Clearance Center's RightsLink.Licensee:G.Shanmugam.License number:5374260798266.License date: August 22, 2022.

Fig. 52 Diversion caused by confusing nomenclature used for contourites by Lovell and Stow (1981). Modified after Shanmugam (2016a);Klein (1971). Elsevier. Copyright Clearance Center's RightsLink. Licensee: G. Shanmugam. License number: 5374250708256, License Date:August 22, 2022.

Fig.53 Type 1 shelf-incising,river-associated Congo(Zaire)Canyon.Compiled from Harris and Whiteway(2011).From Shanmugam(2021a).Elsevier.Copyright Clearance Center's RightsLink.Licensee:G.Shanmugam.License number:5374241466123.License date:August 22,2022.

10. Classification of contourites

There are two schools of thought in defining contourites.

Fig.54 Types 2 and 3 canyons on the slope of the Gulf of Lion,northern Mediterranean Sea.They are spaced less than 10 km apart from each other. Source: Compiled from Harris and Whiteway (2011). From Shanmugam (2021a). Elsevier. Copyright Clearance Center's RightsLink.Licensee: G. Shanmugam. License number:5374241466123. License date: August 22, 2022.

Fig. 55 A) Conceptual diagram showing cross-section of a submarine canyon with ebb and flood tidal currents (opposing arrows). Shepard et al. (1979) measured current velocities in 25 submarine canyons at water depths ranging from 46 to 4200 m by suspending current meters commonly 3 m above the sea bottom. Measured maximum velocities commonly range from 25 to 50 cm/s. From Shanmugam (2003); B)Time-velocity plot from data obtained at 448 m in the Hueneme Canyon,California,showing excellent correlation between the timing of upand down canyon on currents and the timing of tides obtained from tide tables(solid curve).3mAB=Velocity measurements were made 3m above sea bottom.From Shepard et al.(1979).Elsevier.Copyright Clearance Center's RightsLink.Licensee:G.Shanmugam.License number:5373460448321. License date: August 21, 2022.

1) The original school by Hollister (1967) who introduced the term “contourite”. By his definition,contourites represent the deposits of thermohalineinduced geotropic contour currents. I follow this original definition.

2) The later school by Lovell and Stow (1981, p. 349)who conclude that “Contourite: a bed deposited significantly reworked by a current that is persistent in time and space and flows along slope in relatively deep water(certainly below wave base).The water may be fresh or salt; the cause of the current is not necessarily critical to the application of the term.”I have used Italics for the last phrase to emphasize their point that contourites can be produced by any kind of bottom current (Fig. 52),irrespective of their origin (i.e., thermohaline,wind,tide,or baroclinic).Stow et al.(2008,p.144)explicitly state that“Bottom(contour)currents are those currents that operate as part of either the normal thermohaline circulation or wind-driven circulation systems.” This is a diversion. The confusion surrounding the term “contourite” has been addressed by Shanmugam (2016a).

Fig. 56 Edop Field (A) with submarine canyon (B) filled with slump facies (C). From Shanmugam (2017a).

Fig. 57 Edop Field with submarine canyon filled with tidalite facies composed of double mud layers (DML). From Shanmugam (2003).Elsevier.Copyright Clearance Center's RightsLink.Licensee:G.Shanmugam.License number:5373460448321.License date:August 21,2022.

Fig.58 A)Index map showing locations of the Krishna-Godavari(KG)Basin and the KG-D6 block on the eastern continental margin of India;B) Map showing location of our study area in the Block KG-D6; C) Root mean-square (RMS) seismic amplitude map of our study area showing locations of cored wells 1, 2, and 3. RMS map represents the entire reservoir (400 ms time window). Amplitude color code: bright red, high amplitude(gas-charged sandy lithologies);yellow,intermediate amplitude(mixed lithologies);blue-to-dull green,low amplitude(non sandy or muddy lithologies). Sinuous and lobate planform geometries are present. Note position of well 2 in a sinuous form. The seismic profile,which passes through well 2,represents an oblique strike section across a sinuous form(submarine canyon).From Shanmugam et al.(2009).

11. Tidal currents in submarine canyons

A comprehensive study of submarine canyons worldwide (Figs. 53 and 54) was carried out by Harris and Whiteway (2011). According to Harris and Whiteway (2011), canyons exhibit an impressive array of statistics from their length and spacing to their slope, depth range, dendricity, and sinuosity.Active continental margins contain 44.2% of all canyons(2586)and passive margins contain 38.4%(2244).Canyons are steeper, shorter, more dendritic, and more closely spaced on active than on passive continental margins. River-associated, shelf-incising canyons are more numerous on active continental margins (n5119) than on passive margins (n534). They are most common on the western margins of South and North America where they comprise 11.7%and 8.6%of canyons, respectively. A variety of deposits, such as slumps, debrites, tidalites, and hemipelagites, can accumulate within submarine canyons.

Fig. 59 Bathymetric image of our study area showing locations of three cored wells (red dots), widespread distribution of mass-transport deposits (i.e., slides, slumps, and debrites), and incipient submarine canyons on the modern upper-slope setting just seaward of the shelf edge.Linked occurrences of headwall scarps(slide scars)near the shelf edge,chutes immediately downslope of slide scars,and slide blocks immediately downslope of chutes are evident. Mass-transport deposits show slope-confined lobate forms in inter-canyon areas. Background scale (0, 500, 1000, 1500, 2000, and 2500 m) represents present-day water depths. From Shanmugam et al. (2009).

Fig.60 A)Sedimentological log of core 8 for the interval 2072-2077.5 m in well 2 showing alternation of sand(lithofacies 3)and mudstone(lithofacies 4)intervals with continuous presence of double mud layers(DML).Note floating sandstone rock fragments and mudstone clasts in a basal mudstone interval(lithofacies 2).The cored interval represents core 8 of canyon-fill deposits in seismic profile(Fig.12).See Fig.4 for explanation of symbols; B) Lithofacies 3 core photograph showing rhythmic bedding (rhythmites) and double mud layers (DML, arrows) in sand. Neap (thin) bundle; Spring (thick) bundle. From Shanmugam et al. (2009).

Fig.61 Seismic profile showing boundaries of a major erosional feature of Pliocene age,which we have interpreted as a submarine canyon on the upper-slope environment. Cored intervals are shown by yellow boxes on the wireline log of well 2.The southeast canyon wall,which corresponds to the contact between cores 10 and 11(rectangle box,see Fig.62 for details),is characterized by slump folds,sand injections,and other sediment deformation in core. Both walls of the canyon are aligned in trend with underlying normal faults. Immediately beneath the canyon,a seismic unit(with cores 12,13,and 14)exhibits continuous and parallel reflections.This seismic unit,which is 1750 m long or wide,is composed primarily of sandy debrites in core 14 in the inter-canyon environments.This NW-SE seismic profile represents an oblique strike section across a sinuous canyon with well 2. See Fig. 58 for location and Fig. 62 for model. From Shanmugam et al. (2009).

One of the most important progresses made in deep-water research in the 20th century was the documentation of tidal currents in submarine canyons.Shepard et al.(1979)measured current velocities in 25 submarine canyons worldwide at water depths ranging from 46 to 4200 m by suspending current meters,usually 3 m above the sea bottom (Fig. 55A). Shepard et al. (1979) also documented systematically that upand down-canyon currents closely correlated with timing of tides (Fig. 55B). These canyons include the Hydrographer,Hudson,Wilmington,and Congo(Zaire)in the Atlantic Ocean; and the Monterey, Hueneme(Fig. 55B), Redondo, La Jolla/Scripps, and Hawaii canyons in the Pacific Ocean. Maximum velocities of up-and down-canyon currents commonly ranged from 25 to 50 cm s-1(Fig. 55A). Keller and Shepard (1978)reported velocities as high as 7075 cm s-1, velocities sufficient to transport even coarse-grained sand,from the Hydrographer Canyon. In the Niger Delta area of West Africa, five modern submarine canyons (Avon,Mahin, Niger, Qua Iboe, and Calabar) have been recognized. In the Calabar River, the tidal range is 2.8 m and tidal flows with reversible currents are common (Allen, 1965). In the Calabar Estuary,maximum ebb-current velocities range from 60 to 280 cm s-1,and flood current velocities range from 30 to 150 cm s-1. These velocities are strong enough to transport particles of sand and gravel size.The Calabar Estuary has a deep-water counterpart that cuts through sediments of the outer shelf and slope,forming the modern Calabar Submarine canyon.Thus,as they do in the Congo Canyon to the south, tidal currents are likely to operate in the Calabar and Qua Iboe Canyons.

11.1. Edop Field, Offshore Nigeria

In an ancient submarine canyon recognized in the Edop Field, Nigeria (Fig. 56A), cores show slump fold suggesting MTD (Fig. 56C) and well-developed double mud layers(DML)(Fig.57),indicating tidal deposition.DMLs are unique to shallow-water tidal environments and have been ascribed to alternating ebb and flood tidal currents with extreme time-velocity asymmetry in subtidal settings (Visser, 1980). Double mud layers are sedimentary structures of tidalite origin and they have been recognized in both shallow-water environments(Visser,1980)and in deep-water facies(Fig.57).

11.2. Krishna-Godavari (KG) Basin, Bay of Bengal, India

In the Krishna-Godavari (KG) Basin, Bay of Bengal,India (Fig. 58), modern settings have well-developed canyons and MTDs (Fig. 59). Double mud layers suggesting tidalites are present in cores (Fig. 60) in a submarine canyon environment (Figs. 61 and 62)(Shanmugam et al.,2009).The KG Basin contains both sandy debrites and tidalites in the petroleumproducing reservoir. Canyon-fill facies are characterized by the close association of sandy debrites and tidalites in the KG Basin (Fig. 60). Reservoir sands,composed mostly of amalgamated units of sandy debrites,are thick(up to 32 m),and low in mud matrix(less than 1% by volume). The best reservoir facies is composed of sandy debrites. This facies exhibits high values of measured porosities (35%-40%) and permeabilities(850-18,700 mD).Sandy tidalites and related bottom-current reworked facies exhibit moderate porosity(31%-40%) and permeability (525-6930 mD).Muddy tidalites are poor reservoirs(Shanmugam et al.,2009).

Fig.62 Integration of seismic data(Fig.61)and core data for the interval 2107-2113 m in well 2 showing the position of southeast canyon wall, which intersects the core interval near the contact between core 10 and 11 at 2111 m. Severe sediment deformation is evident both below and above the canyon wall.The lack of core recovery at the canyon wall may be due to extreme sediment deformation.The canyon-fill facies is composed of sandy debrites(lithofacies 1),sandy tidalites(lithofacies 3),and muddy slumps(lithofacies 2).The inter-canyon facies is composed of muddy slumps and debrites with sand injective (lithofacies 2) in the upper part of core 11. From Shanmugam et al. (2009).

Fig. 63 A dichotomy. Comparison of number of direct observations on deep-water tidal and turbidity currents in modern settings with number of interpreted deep-water tidalites and turbidites in the ancient sedimentary record.Importantly,Shepard et al.(1979)documented 25,000 h of velocity measurements of mostly tidal currents from 25 submarine canyons around the world. But no such robust dataset on modern turbidity currents exists (see Section 23 below on Turbidite groupthink). From Shanmugam (2021a). Dykstra (2012); Heezen et al.(1964); Mutti and Ricci Lucchi (1972).

Fig. 64 A) Index map; B) Satellite in image of internal waves, Andaman Sea. Image credit: NASA.

Fig.65 A)Barotropic waves;B)Baroclinic waves;C)Explanation.A theoretical progress was made by Gill(1982)who proposed that density stratifications in the world's oceans can be used to explain baroclinic waves along pycnoclines. From Shanmugam (2013).

Fig. 66 Baroclinic currents (A) and ripple bedforms (B) associated with internal waves and tides. From Shanmugam (2013).

12. Modern and ancient systems

Shepard et al. (1979) documented 25,000 h of velocity measurements of mostly tidal currents from over 150 stations located in 25 submarine canyons around the world.This seminal study by Shepard et al.(1979)has never been matched by any other studies on velocity measurements of modern turbidity currents in submarine canyons,including by the most recent study on the Monterey Canyon (Maier et al., 2019). The previous 12 case studies of modern turbidity currents that were published during a period of 56 years(1952-2008) (e.g., Heezen and Ewing, 1952; Inman et al., 1976; Hay et al., 1982; Normark, 1989;Parsons et al., 2003; Xu et al., 2004, among others)were also unconvincing (see explanations by Shanmugam, 2012a). This extreme rarity of velocity measurements of modern turbidity currents is truly mystifying and a dichotomy given the fact that thousands of ancient strata were interpreted routinely as turbidites in the sedimentary record (Fig. 63).

Empirical data on turbidity currents and on tidal currents gathered from modern deep-water settings,when compared to their ancient interpreted counterparts(Fig.63),defy the doctrine of uniformitarianism.Clearly, the driving force behind most turbidite interpretations has been the Bouma Sequence and related groupthink, not empirical data and pragmatism.

Fig. 67 A) Modified classification of canyons after Harris and Whiteway (2011); B) and C) from Shanmugam (2012a). Elsevier. Copyright Clearance Center's RightsLink. Licensee: G. Shanmugam. License number: 5374280624784. License date: August 22, 2022.

Fig. 68 Maps showing the variable directions of propagation of internal waves. From Shanmugam (2013).

Fig. 69 Internal waves breaking over the Mid-Ocean Ridge, Brazil Basin. Note absence of internal waves over the smooth abyssal plains.Modified after St. Laurent et al. (2012).

Fig. 70 Common occurrence of traction structures in deep-marine facies. From Shanmugam (2021a, b).

Fig. 71 Hybrid flows originally proposed by Shanmugam et al. (1993).

13. Internal waves and tides

13.1. Occurrence

Fig.72 Misapplication of hybrid-flow concept to downslope flow transformation.Elsevier.Copyright Clearance Center's RightsLink.Licesee:G. Shanmugam. License number:5373470097097. License date: August 21, 2022.

Fig. 73 Hybrid flows do not represent flow transformation. Modified after Shanmugam (2021a); Fallgatter et al. (2017); Fonnesu et al.(2016); Stow and Smillie (2020); Talling (2013).

Fig. 74 A diversion caused by mixed system with intersecting down-slope turbidity currents and along-slope bottom currents proposed by Rodrigues et al. (2022, their Fig. 18). They assumed bottom current is a single process. However, bottom currents are composed of four processes that include tidal currents which do not flow along-slope (Shanmugam, 2008a). See Shanmugam (2022b).

Fig. 75 Summary diagram showing 14 general types of plumes that include 12 marine examples and two lacustrine examples. From Shanmugam (2018b).

Apel(2002),Apel et al.(2006),and Jackson(2004)have published comprehensive accounts of internal waves and tides worldwide. A sedimentlogic and oceanographic review was provided by Shanmugam(2013). Internal waves are gravity waves that oscillate along oceanic pycnoclines. Internal tides are internal waves with a tidal frequency. Internal solitary waves (i.e., solitons), the most common type, are commonly generated near the shelf edge (100-200 m in bathymetry) and in the deep ocean over areas of seafloor irregularities, such as mid-ocean ridges, seamounts, and guyots. Empirical data from 51 locations(Shanmugam, 2013) in the Atlantic, Pacific, Indian,Arctic, and Antarctic oceans reveal that internal solitary waves travel in packets, such as the one in the Andaman Sea (Fig. 64). Internal waves commonly exhibit (1) higher wave amplitudes (5-50 m) than surface waves (<2 m), (2) longer wavelengths(0.5-15 km) than surface waves (100 m), (3) longer wave periods(5-50 min)than surface waves(9-10 s),and(4)higher wave speeds(0.5-2 m s-1)than surface waves (25 cm s-1). Maximum speeds of 48 cm s-1for baroclinic currents were measured on guyots.

13.2. Stratified oceans

Gill(1982)discussed the basic differences between barotropic (surface) waves that develop at the air-water interface and baroclinic (internal) waves that develop at the water-water interface (Fig. 65).Fluid parcels in the entire water column move together in the same direction and with same velocity in a surface wave,whereas fluid parcels in shallow and deep layers of the water column move in opposite directions and with different velocities in an internal wave (Fig. 65). The surface displacement and interface displacement are the same for a surface wave,while the interface displacements are large for internal wave. Although the free surface movement associated with the baroclinic mode is only 1/400 of the interface movement, this is still sufficient for baroclinic motions to be detectable by sea-surface changes(Wunsch and Gill, 1976).

13.3. Depositional framework

Baroclinites represent deposits of baroclinic currents induced by internal waves and tides(Shanmugam, 2013). A preliminary depositional framework is proposed for continental slopes, submarine canyons, and guyots (Fig. 66A). To date, the only convincing photographic documentation of sedimentary bedforms, attributed to internal waves and internal tides,in modern deep-marine environments has been from seamounts and guyots in the Pacific Ocean(Menard, 1952; Lonsdale et al., 1972). These photographs are the only empirical foundation for gaining insights into depositional processes (e.g., traction vs.suspension) associated with internal waves and internal tides. Photographic data show the occurrence of discrete ripple chains(Fig.66B).However,core-based sedimentological studies of modern sediments emplaced by baroclinic currents on continental slopes,in submarine canyons, and on submarine guyots are lacking.There are no cogent sedimentologic or seismic criteria for distinguishing ancient baroclinites. Basic challenges are that internal waves tend to be more frequent in Type 3 submarine canyons (Fig. 67) of Harris and Whiteway(2011)and that their propagation directions are highly variable (Fig. 68). These are certainly immediate areas of research.

Fig. 76 A) Deflecting Elwha River plume; B) Elwha River without plume. From Shanmugam (2019). Aerial photo: T. Roorda.

13.4. Ocean-floor topography

In terms of ocean-floor topography, breaking of internal waves over the Mid-Atlantic Ridge has been documented by St. Laurent et al. (2012) in the South Atlantic(Fig.69).But these waves are absent over the smooth abyssal plains (Fig. 69). These differences are of significance in interpreting paleogeography.

13.5. Recognition

Holbrook and Fer (2005) have used seismic images of the Norwegian Sea water column to show reflections that capture snapshots of fine structure displacements due to internal waves. Horizontal wave number spectra derived from digitized reflection horizons in the open ocean compare favorably to the Garrett-Munk tow spectrum of oceanic internal wave displacements.Spectra within 10 km laterally and 200 m vertically of the continental slope show enhanced energy likely associated with internal wave-sloping boundary interactions. In modern oceans (Cacchione et al., 2002), internal waves and tides do develop ripple bedforms (Fig. 66B) (Lonsdale et al., 1972).However, recognition of ancient deep-water deposits of internal waves and tides in the sedimentary record is rare(Gao et al.,1998;He et al.,2011;Pomar et al.,2012). The other problem is that traction structures are common in deposits of all four types of bottom currents as well as in turbidites (Fig. 70). This is another area of future research.

Fig. 77 Deflected sediment transport vs. conventional source to sink downslope transport. From Shanmugam (2019).

Fig. 78 A) Distinction between fan deltas and braid deltas, After McPherson et al., 1987; B) and C) Photographs are courtesy of John. G.Mcpherson.

Fig.79 A)Continental margin;B)Close-up view showing plunge point(red dot)and hyperpycnal flows near the shoreline.From Shanmugam(2021a). Elsevier. Copyright Clearance Center's RightsLink. Licensee: G. Shanmugam. License number:5374470231989. License date: August 22, 2022.

14. Hybrid flows

An important progress was made by recognizing hybrid flows(Fig.71)that are commonly developed by intersecting of downslope gravity flows with alongslope contour currents (Shanmugam et al., 1993).Hybridite is an amalgamated offspring deposit of two hydrodynamically different flow types, such as downslope sandy debris flows or turbidity currents and along-slope contour currents (i.e., hybrid flows).However, some authors (e.g., Haughton et al., 2009)equate hybrid flows with flow transformation in gravity flows(Fig.72).This controversy necessitates the basic description of the term “hybrid”etymologically.

According to the Cambridge Dictionary, the term“hybrid”represents the hybrid offspring byproducts of two different plants, animals, or other entities(https://dictionary.cambridge.org/dictionary/learner-english/hybrid, accessed June 2, 2020). In animals,for example,a mule is the hybrid offspring of a male donkey and a female horse. By contrast, a debris flow often transforms downslope into a stratified flow with a lower debris-flow layer and an upper turbidity-current layer(see Fig.38),as documented by Norem et al. (1990) and by our flume experiments(Shanmugam, 2000). In other words, the concept of“hybrid” begins with two different parent species yielding a single hybrid offspring,whereas the concept of “flow transformation” begins with a single parent flow that transforms downslope into two sedimentgravity flows (Fig. 73).

Bottom-current reworking is the key function of hybrid flows. Traction structures are common in hybridizes (Shanmugam et al., 1993; Fuhrman et al.,2020; Strzebo′nski, 2022), and they are petroleum reservoirs in the Gulf of Mexico (Shanmugam et al.,1993).

Fig. 80 Hyperpycnite facies model with internal erosional contact shown by a red arrow. Modified after Mulder et al. (2003). Elsevier.Copyright Clearance Center's RightsLink. Licensee: G. Shanmugam. License number:5374241011219. License date: August 22, 2022.

Fig.81 Omission of internal erosional contact from the hyperpycnite facies model and omission of internal hiatus from the contourite facies model. Compare with Fig. 80 for the original hyperpycnite facies model. From Rodríguez-Tovar (2022).

images/BZ_62_390_371_433_403.pngimages/BZ_62_386_2132_428_2164.png

Fig. 82 Map showing the site of Chicxulub meteorite impact at the K-Pg boundary in Yucatan, Mexico. From Shanmugam (2021a). Elsevier.Copyright Clearance Center's RightsLink. Licensee: G. Shanmugam. License number:4375370557807. License date: August 22, 2022.

Fig. 83 Conceptual diagrams showing the frequency of occurrences of shelf edges with increasing bathymetry for incoming internal waves and tides. From Shanmugam (2013).

Fig. 84 The predictive model, developed at NASA's Goddard Space Flight Center in Greenbelt, Maryland, estimates potential landslide activity triggered by rainfall. Rainfall is the most widespread trigger of MTD aound the world. If conditions beneath the Earth's surface are already unstable, heavy rains act as the last straw that causes mud, rocks or debris-or all combined-to move rapidly down mountains and hillsides as MTD. Source: NASA. From Shanmugam (2021a).

Fig.85 Illustration of the 1979 sediment failure that occurred at the Nice International Airport in southern France.The Nice sediment failure has been attributed to a combination of both external and internal factors(Dan et al.,2007).A)Internal(in situ)lithologic factor composed of clay and sand layers;B)Human factor involving the building of airport embankment;C)External meteorological and internal geotechnical factors. From Shanmugam (2015).

Fig.86 Tsunami depositional model.A)Stages 1,2 and 3;B)Stage 4 showing coastal(boulder)and deep-water deposition.See Fig.87 for an exotic boulder. Modified after Shanmugam (2006b).

In a recent study, Rodrigues et al. (2022) have proposed a new classification for mixed turbiditecontourite depositional systems based on their notion that there is only one type of tubidite and that there is only one type of contourite in deep-marine systems(Fig. 74). However, there are at least 36 turbidite types and 4 contourite types in the published literature. Furthermore, they have used the term“bottom current”to represent a single unique current,which flows along-slope. Nevertheless, there are 4 major types of bottom currents(thermohaline contour currents,tidal currents,baroclinic currents,and winddriven currents). With the exception of contour currents, the other three currents do not and cannot flow along-slope.The problem here is that the authors have failed to provide a clear and precise definition of the terms “turbidite”, “contourite”, and “bottom current”. Consequently, their classification has added a new layer of confusion to an already muddled domain of turbidite-contourite research(Shanmugam,2022b).

15. Density (sediment) plumes

Fig. 87 Tsunami-emplaced exotic boulder. From Frohlich et al. (2009).

Fig. 88 Tsunami deposits. From Shanmugam (2021b). Elsevier. Copyright Clearance Center's RightsLink. Licensee: G. Shanmugam. License number:5374480811371. License date: August22, 2022

Fig. 89 Soft-sediment deformation structures (SSDS). From Logan (1863).

Fig.90 Core photograph showing interbedded occurrence of deformed(convolute bedding)sandstone and siltstone(light gray) layers with undeformed claystone (dark gray) layers. Paleocence, U. K. North Sea. Figure from Shanmugam (2012a). Elsevier. Copyright Clearance Center's RightsLink. Licesee: G. Shanmugam. License number:5374370901708. License date: August 22, 2022.

Fig. 91 Seismites. A) and B) Outcrop photographs; C) Sketch. From Alsop and Marco (2013). Elsevier. Copyright Clearance Center's RightsLink. Licesee: G. Shanmugam. License number: 5374240118824. License date: August 22, 2022.

Fig.92 A)Study locations of the Jackfork Group in Oklahoma and Arkansas,USA.From Shanmugam and Moiola(1995);B)Stratigraphy of the Jackfork Group.

Satellite images have revealed that density (sediment)plumes are complex(Fig.75)and are commonly deflected away from the normal downslope direction in 18 out of 29 cases at river mouths (Shanmugam,2018a). These examples are Brisbane, Congo, Connecticut, Dart, Ebro, Eel, Elwha, Finesse, Guadalquivir, Krishna-Godavari, Mississippi, Morns, Rio de la Plata (Estuary), Pearl, Rhone, Tiber, Yellow, and Yangtze rivers (Fig. 75). As a consequence, current directions change drastically and sediment distribution occurs on only one side of river mouths, such as the Elwha River plume(Fig.76).Sediment transport is diverted by a plethora of 22 oceanographic, meteorological, and anthropogenic external factors. Empirical data show that wind forcing is the most dominant factor (Shanmugam, 2019; Foreman et al., 2008).Other factors are tidal currents, ocean currents, and coastal upwelling. Deflection of sediment plumes defies the conventional use of paleocurrent directions in determining sediment transport and provenance in the ancient sedimentary record (Fig. 77). Failure to recognize deflected sediment plumes in the rock record could result in construction of erroneous depositional models with economic implications for reservoir prediction in petroleum exploration (Shanmugam,2019, 2021a).

An important related topic is the distinction between fan deltas and braid deltas (Fig. 78) and their implications for petroleum exploration (McPherson et al., 1987). Braid deltas develop important petroleum reservoirs, such as the Ivishak Formation in the Prudhoe Bay Field, Alaska (Shanmugam and Higgins,1988). Braid deltas also have a great potential to develop as deep-water reservoirs near the shelf edge during lowstands of sea level. The reasons are that lowstands represented a time when sea level was falling quite rapidly thereby exposing vast areas of the former shelf to wholesale erosion. The rivers would have incised which would have kept them relatively confined but that meant that their velocity would have been high and their erosive ability would have also been high. All this was at a time when the uplands were probably undergoing relatively high erosion because of the inferred climate change …. colder,with the tree-line lowering and increased glacial action and weathering, both mechanical and chemical.This led to an increase in sediment supply to the rivers causing the rivers to braid because of the overload of sediment(John G.McPherson,e-mail communication;July 12,2022).Saller et al.(2004)discussed the linked lowstand delta to basin-floor fan deposition, offshore Indonesia. Lowstand braid deltas are a potential area of future research.

Fig.93 A)Flute castsas sole marks in the Jackfork have been used as evidence for turbidity currents.However,bottom currents could also generate such sole transport direction.Photo by Shanmugam;B)Outcrop photography showing floating quartzite pebble(arrow)in sandstone,which is indicative of flow strength in debris flows.Pennsylvanian Jackfork Group.Ouachita Mountains.From Shanmugam and Moiola(1995).

16. Hyperpycnal flows

In advocating a rational theory for delta formation,Bates (1953) suggested three types (1) hypopycnal plume for floating river water that has lower density than basin water, (2) homopycnal plume for mixing river water that has equal density as basin water, and(3)hyperpycnal plume for sinking river water that has higher density than basin water. Although Middleton and Hampton (1973) did not consider hyperpycnal flows in their original classification of sediment-gravity flows (Fig. 39), hyperpycnal flows are indeed sediment-gravity flows (Fig. 79).

The term“hyperpycnite”was introduced by Mulder et al. (2002) in an academic debate with me(Shanmugam, 2002b) on the origin of inverse grading by hyperpycnal flows. Mulder et al. (2002) attempted to differentiate “hyperpycnites” deposited by hyperpycnal turbidity currents from “classic turbidites”deposited from failure-related turbidity currents.The problem is that triggering mechanisms of turbidity currents (or any other processes) cannot be determined from the depositional record (Shanmugam,2015, 2016a,b).

Fig. 94 A) Duplex-like structures in the Jackfork caused by synsedimentary slumping. From Shanmugam et al. (1988); B) Outcrop photography showing sandstone clast (arrow) in mudstone which is indicative of flow strength in debris flows. Pennsylvanian Jackfork Group.Ouachita Mountains. Red scale = 15 cm. From Shanmugam and Moiola (1995).

Invariably, hyperpycnal flows (i.e., sedimentgravity flows) and their deposits (hyperpycnites)occur immediately seaward of the plunge point near the shoreline(Fig.79).The hyperpycnite facies model(Fig. 80) was introduced by Mulder et al. (2003).However, the model is problematic for the following four reasons. (1) Hyperpycnal flows have never been documented to reach the deep sea in modern settings(Shanmugam, 2018b, c). (2) The hyperpycnite facies model contains internal erosional contact (Fig. 80),and therefore cannot function as a predictive facies model (Walker, 1992). (3) Laboratory experiments were unable to validate the model(Lamb et al.,2010).(4) There are at least 16 types of hyperpycnal flows(e.g., density flow, underflow, high-density hyperpycnal plume, high-turbid mass flow, tide-modulated hyperpycnal flow, cyclone-induced hyperpycnal turbidity current, multi-layer hyperpycnal flows,etc.), without an underpinning principle of fluid dynamics(Shanmugam,2018b).The hyperpycnite facies model is obsolete.

Fig. 95 Types of sigmoidal deformation structures (duplex)in the Jackfork Group. From Shanmugam (2021a).

17. Omission of erosional contact and internal hiatus

By his impressive paper title“Ichnological analysis:A tool to characterize deep-marine processes and sediments”, Rodríguez-Tovar (2022) has implied that his paper is about a comprehensive review of all deepmarine facies.But in reality,he has omitted important tidalites, baroclinites, and hybridites. Also, he has ignored all the mass-transport deposits(MTD),such as slides, slumps, and debris flows. More importantly, he has relied on vertical facies models (e.g., the Bouma Sequence) for distinguishing one facies (e.g., turbidites)from the others.What is troubling is that he has omitted the key features,such as internal hiatus from the original contourite model and erosional contact from the original hyperpycnite facies model(Fig. 81).It is worth noting that units with internal hiatuses or erosional contacts are disqualified from application of the Walther's Law (Middleton, 1973). In deep-water research, one cannot practice omission of previous observations in order to promote one's skewed narrative. This is because deep-water research tends to advance in incremental stages by building on knowledge accumulated by previous scholars.

18. Triggers of sediment failures

A triggering mechanism or a trigger is defined in this article as “the primary process that causes the necessary changes in the physical, chemical, and geotechnical properties of the soil,which results in the loss of shear strength that initiates the sediment failure and movement.” Commonly, triggering processes are considered “external” with respect to the site of failure. In continental margins, several triggering mechanisms may operate concurrently or in tandem(e.g., earthquake-triggered tsunamis). Sowers (1979)articulated the challenge of identifying the single mechanism that is responsible for the failure as follows: “In most cases, several ‘causes’ exist simultaneously; therefore, attempting to decide which one finally produced failure is not only difficult but also technically incorrect. Often the final factor is nothing more than a trigger that sets a body of earth in motion that was already on the verge of failure. Calling the final factor the cause is like calling the match that lit the fuse that detonated the dynamite that destroyed the building the cause of the disaster.”Although more than one triggering mechanism can cause a single process(e.g.,debris flows)at a given site,there are no objective criteria to distinguish either the triggering mechanism or the transport process from the depositional record yet (Shanmugam, 2006a, 2012a;Mulder et al., 2011).

Fig. 96 An unconventional model. From Shanmugam and Moiola (1995).

Fig. 97 Sequence-stratigraphic models for deep-water systems vs. empirical data. From Shanmugam et al. (1995).

Fig.98 Seismic profile showing mounded geometry.Note position of Well 214/28-01 used in the core study.From Shanmugam et al.(1995).

On continental margins, there are 22 possible triggers of sediment failures that can generate downslope gravity flows and related sediment deformation(Table 2). Short-term triggers are composed of earthquakes, meteorite impacts (Fig. 82), internal waves (Fig. 83), rainfall (Fig. 84), human activity(Fig. 85), volcanic activities, tsunami waves (see Section 19), tropical cyclones, etc. Intermediate-term triggers are composed of tectonics, glacial maxima,etc.Long-term events are sea-level changes.However,the prevailing notion that deep-water deposits develop during periods of sea-level lowstands is a myth.The geologic reality is that frequent short-term events that last for only a few minutes to several hours or days (e.g., earthquakes, meteorite impacts, tsunamis, tropical cyclones, etc.) are more important in controlling deposition of deep-water sands than sporadic long-term events that last for thousands to millions of years (e.g., lowstand systems tract)(Shanmugam, 2015, 2021a).

Fig. 99 A) and B) Core photographs showing slump-folded heterolithic (sand and mud) facies and associated sand injection, Paleocene,Faeroe Basin, U. K. Continental Margin. From Shanmugam et al. (1995).

19. Tsunami waves

Tsunamis are oceanographic phenomena that represent a water wave or series of waves, with long wavelengths and long periods,caused by an impulsive vertical displacement of the body of water by earthquakes, landslides, volcanic explosions, or extraterrestrial (meteorite) impacts. Earthquakes commonly generate tsunamis through the transfer of large-scale elastic deformation associated with rupture to potential energy within the water column (Geist, 2005).The two prominent tsunamis of the 21st century were triggered by M59 earthquakes in (1) off West coast of Sumatra on December 26, 2004 and (2) offshore Honshu, Japan on March 11, 2011. Wave heights of the 2004 Indian Ocean Tsunami reached up to 15m.

A tsunami wave can trigger a number of transportational processes, such as overwash surge, backwash flow, debris flow, turbidity current, and bottom current. These processes, in turn, will emplace sediment from a variety of depositional mechanisms,namely sudden freezing,settling from suspension,and bedload or traction (Shanmugam, 2012b). The transport of tsunami-induced sediment into the deep sea(Kastens and Cita, 1981), which includes mass transport, was discussed by Shanmugam (2006b). Tsunamirelated deposition in deep-water environments may be explained in four progressive steps (Fig. 86):

1) triggering stage,

2) tsunami stage,

3) transformation stage, and

4) depositional stage

During the triggering stage, earthquakes, volcanic explosions, undersea landslides, and meteorite impacts can trigger displacement of a large quantity of water either up or down, causing tsunami waves.During the tsunami stage,tsunami waves carry energy traveling through the water, but these waves do not move the water, nor do they transport sediment.

Fig. 100 Sedimentological log showing a lower MTD unit and an upper bottom-current reworked unit. Paleocene, Faeroe Basin, U.K.Continental Margin.

Fig.101 Core photo(A)and sedimentological log(B)of a basal contact of a Tertiary sand showing evidence for shearing(i.e.,slide).North Sea. Photo by Shanmugam.

During the transformation stage, the tsunami waves erode and incorporate sediment into the incoming wave. The enormous tsunami waves are important triggering mechanisms of sediment failures.The advancing wave front from a tsunami is capable of generating large hydrodynamic pressures on the seafloor that would produce soil movement sand slope instabilities (Wright and Rathje, 2003). The transformation stage is evident in sediment-rich backwash flows during the 2004 Indian Ocean tsunami at Kalutara Beach, southwestern Sri Lanka (Shanmugam, 2006b,his Fig.2).The incoming ocean waters are clearly blue in color (implying sediment-free), but these waters transform into brown in color near the coast because of their incorporation of sediment.The transformation to brown color is the result of the wave breaking,and the wave will break in different water depths according to its wavelength and seafloor irregularities.Frohlich et al. (2009) documented huge exotic boulders from the Tongatapu Island, southwest Pacific where the largest boulder has dimensions of 15 m × 39 m x 311 m (Fig. 87). Frohlich et al. (2009)estimated masses of boulders to be in the range of 70-1600 metric tons. Such boulder emplacement could be attributed to the transformation stage and related sediment emplacement. These tsunamiinduced backwash flows should not be confused with hyperpycnal flows introduced by river waters (Bates,1953). Although many sedimentary features are considered to be reliable criteria for recognizing potential paleo-tsunami deposits, similar features are also common in cyclone-induced deposits.At present,paleo-tsunami deposits cannot be distinguished from paleo-cyclone deposits using sedimentological features alone (Fig. 88), without historical information(Shanmugam,2008a,2012b;see also Bourgeois et al.,1988). This is an important area for future research.

20. Soft-sediment deformation structures(SSDS)

Since the detailed treatment of the topic of softsediment deformation structures (SSDS; also known as penecontemporaneous or synsedimentary deformation structures) in terms of physics and sedimentology by Allen (1984), SSDS have received considerable attention worldwide (e.g., Maltman,1994; Collinson, 1994; Alfaro et al., 2016;Shanmugam, 2016b,2017a).

Soft-sediment deformation structures (SSDS)(Fig. 89), commonly associated with deep-water deposits, have been the focus of attention for over 150 years. Existing unconstrained definitions allow one to classify a wide range of features under the umbrella phrase “SSDS.” As a consequence, a plethora of at least 120 different types of SSDS (e.g., convolute bedding, slump folds (Fig. 90), load casts, dish-andpillar structures, pockmarks, raindrop imprints,explosive sand-gravel craters, clastic injections,crushed and deformed stromatolites, brecciated clasts,etc.)have been recognized in strata ranging in age from Paleoproterozoic to the present time(Shanmugam, 2017a).

Fig. 102 Features associated with mass-transport deposits (MTD) in the North Sea cores. From Shanmugam et al. (1995).

Two factors that control the origin of SSDS are prelithification deformation and liquidization. A sedimentological compendium of 140 case studies of SSDS worldwide, which include 30 case studies of scientific drilling at sea (Deep Sea Drilling Project/ODP/IODP),published during a period between 1863 and 2017,has yielded at least 31 different origins.Earthquakes have remained the single most dominant cause of SSDS because of the prevailing “seismite” mindset(Shanmugam, 2016b). Although genuine seismites are present in the Dead Sea Basin(Fig.91),not all SSDS are products of seismicity. For example (Shanmugam,2017a), sand injection has been reported from the subducting Philippine Sea Plate. Sand injection has also been reported at the base of sandy slide due to sediment loading.The remaining unresolved issue is in distinguishing SSDS formed by earthquakes (i.e., true seismites) from those formed by mass transport and from those that formed by tectonics(Alsop and Marco,2013; Shanmugam, 2017a).

21. The Jackfork Group, Pennsylvanian,Ouachita Mountains, USA

The Pennsylvanian Jackfork Group in the Ouachita Mountains of Arkansas and Oklahoma (Fig. 92) has conventionally been interpreted by many workers(e.g., Briggs and Cline, 1967), including us(Shanmugam and Moiola, 1988), as a classic flysch sequence dominated by turbidites in a submarine fan setting; however, normal size grading and Bouma sequences, indicative of turbidite deposition, are essentially absent in these sandstone beds.Commonly,well-developed flute casts as sole marks in the Jackfork(Fig.93A)have been used as evidence for turbidity currents. However, bottom currents could also generate such sole marks (Klein, 1966; Shanmugam,2002a). Our field study of the Jackfork Group revealed abundant evidence for MTD in the form of:

Fig. 103 Plot showing the abundance of slump and debris flow facies in the North Sea and North Atlantic cores. Note influence of bottom currents in the Faeroe cores. From Shanmugam et al. (1995).

· floating quartzite pebbles (Fig. 93B),

· sigmoidal deformation(duplex)structures(Figs.94 and 95) (Shanmugam et al., 1988c),

· floating exotic sandstone clasts (Fig. 94B),

· sharp and irregular upper bedding contacts,

· inverse size grading,

· floating mudstone clasts,

· a planar clast fabric,

· lateral pinch-out geometries,

· moderate-to-high detrital matrix (up to 25%), and

· contorted layers.

All these features indicate sand emplacement by debris flows (mass flows) and slumps. Mud matrix in these sandstones was sufficient to provide cohesive strength to the flow. Discrete units of current ripples and horizontal laminae have been interpreted to represent traction processes associated with bottomcurrent reworking. As a consequence, we admitted our earlier misinterpretations as a turbidite fan and proposed a totally different debrite model(Shanmugam and Moiola,1995).This was an epiphany.

The dominance of sandy debris-flow and slump deposits (nearly 70% at DeGray Spillway section) and bottom-current reworked deposits (40% at Kiamichi Mountain section) and the lack of turbidites in the Jackfork Group have led us to propose a slope setting.Our rejection of a submarine fan setting has important implications for predicting sand-body geometry and continuity because deposits of fluidal turbidity currents in fans are laterally more continuous than those of plastic debris flows and slumps on slopes. A turbidite-dominated fan model would predict an outer fan environment with laterally continuous, sheet-like sandstones for the Jackfork Group in southern Oklahoma and western Arkansas,whereas a debrite/slump model (Fig. 96) would predict predominantly a slope environment with disconnected sandstone bodies for the same area.

Our (Shanmugam and Moiola, 1995) controversial reinterpretation had resulted in five lively discussions by some of the leading authorities in the field, which included the following researchers:

· Bouma et al. (1997)

· Coleman(1997).

· D'Agostino and Jordan (1997)

· Lowe (1997).

· Slatt et al. (1997).

Fig. 104 A) Index map of North America showing Vancouver Island in British Columbia, Canada; B) Map showing Bute Inlet with Homathko and Southgate Rivers in the mainland Canada.Note Seymour Narrows(Spring tidal range:5.1 m)and Campbell River(Spring tidal range:4.6 m)near the mouth of Bute Inlet.Tofino:Spring tidal range:4.1 m.Port Hardy:Spring tidal range:5.6 m.Entire map area represents marcrotidal environment. Tidal range data from Thomson (1981) (see Fig. 105). Map credit: Wikipedia. Color labels by G. Shanmugam; C) Map showing Bute Inlet study area by Pope et al. (2022). Note source and sink are outside of the study area. Map from Pope et al. (2022). Color labels by G. Shanmugam.

We promptly responded to all criticisms(Shanmugam and Moiola, 1997). These academic discussions had resulted in 42 printed pages in the AAPG Bulletin.It is worth noting that no other paper in the AAPG Bulletin history (1917-present) has generated this much controversy.

Deep-water genetic facies models that were proposed for ideal end-member processes, such as the turbidite facies model(Bouma,1962),the contourite facies model (Faug`eres et al., 1984), and the hyperpycnite facies model (Mulder et al., 2003), are obsolete (Shanmugam, 1997, 2002a, 2021a, b, c). The popularity of genetic facies models is a manifestation of groupthink. Groupthink is the policy of outright rejection of unconventional ideas by a group of people in order to maintain conformity of ideas within the group. This practice is very prevalent in sedimentology in protecting the orthodoxy of turbidite facies models. An example is our reinterpretation of the classic Ouachita flysch in the USA that was originally rejected by the GSA Bulletin,but was later published by the AAPG Bulletin (Fig. 96) (Shanmugam and Moiola, 1995). Groupthink invariably influences peer-review process in academic journals, including medicine, chemistry, and geology (Shanmugam,2022d).

Fig.105 Spring tidal range of Johnstone Strait region,Canada.1.Bull Harbour;2.Malcoolm Island;3.PortMacnaill;4.Wevnton Passage;5.Hardwicke and YorkcIslands; 6. Hclmken Island; 7. Sunderland Channel;8. Nodales Channel; 9. Duncan Bay and Campbell River area; 10.ButeInlet Study area(rectangle)covered by Pope et al.(2022,their Fig.1B);11.Seymour Narrows;12.Strait of Georgia;13.ChathamPt.,14.Klsey Bay; 15. Johnstone Strait;16. Alert Bay; 17. Port Hardy;18. Cape Scott; 19. Discovery Passage. Tofino is located on the west Coast of Vancouver Island (Fig. 104B). Tidal range table. L. 18 = Location 18; Broken line in Queen Charlotte Strait gives sounding line for bottom profiles. Map and tidal range data are from Thomson (1981). Additional labels by Shanmugam.

22. Basin-floor fan model, Tertiary, North Sea

Sequence-stratigraphic models, based on seismic and log data (Vail et al., 1991), are a diversion from understanding the true depositional origin of reservoirs based on the ground truth (i.e., rocks)(Shanmugam et al., 1995). The concept of basin-floor fan (Vail et al., 1991) in a sequence-stratigraphic template (Fig. 97) has been popular both in the academia (Catuneanu, 2006) and in the petroleum industry (Saller et al., 2004). The conventional basinfloor fans are believed to be composed of sandy turbidites (Vail et al., 1991). Because of uncertainties surrounding these models,Mobil Oil Company initiated a major field study in the early 1990s to understand the depositional origin of deep-water strata in the North Sea and adjacent regions that include the Faeroe Basin,West of the Shetland Islands(Shanmugam et al.,1995).

Many of the cored reservoirs either have been previously interpreted as basin-floor fans or exhibit seismic (e.g., mounded forms) (Fig. 98) and wirelinelog signatures (e.g., blocky motif) and stratal relationships (e.g., downlap onto sequence boundary)indicating basin-floor fans within a sequencestratigraphic framework. This model predicts that basin-floor fans are predominantly composed of sandrich turbidites with laterally extensive, sheet-like geometries. However, calibration of sedimentary facies in our long (400-700 ft) cores with seismic and wireline-log signatures through several of these basinfloor fans (including the Gryphon-Forth, Frigg, and Faeroe areas) shows that these features are actually composed almost exclusively of MTDs consisting mainly of slumps and debris flows.

Fig.106 Groupthink model for Bute Inlet showing the pre-conceived conclusion of turbidity currents,irrespective of alternative processes.Mass transport = Slide, Slump and Debris flow (Shanmugam et al., 1994).

Our examination of nearly 12,000 ft (3658 m) of conventional core from Paleogene and Cretaceous deep-water sandstone reservoirs cored in 50 wells in 10 different areas or fields reveals that these reservoirs are predominantly composed of MTDs, mainly sandy slumps and sandy debris flows (Shanmugam et al., 1995). Classic turbidites are extremely rare and comprise less than 1% of all cores. Sedimentary features indicating slump and debris-flow origin include the following:

· sand units with sharp upper contacts;

· slump folds(Fig. 99);

· pockets of mud clasts and quartzite pebbles(Fig. 100);

· discordant, steeply dipping layers (up to 60°)(Fig. 101);

· glide planes (Fig. 101B);

· shear zones;

· brecciated clasts(Fig.102);

· clastic injections;

· floating mudstone clasts;

· planar clast fabric;

· inverse grading of clasts;and

· moderate-to-high matrix content (5%_30%).

Our core studies thus,underscore the complexities of deep-water depositional systems with an abundance of MTD (Fig. 103) and indicate that model-driven interpretation of remotely sensed data (i.e., seismic and wireline logs) to predict specific sedimentary facies and depositional features should proceed with caution. Process sedimentological interpretation,using long sediment cores, is commonly critical for determining the true origin and distribution of reservoir sands.

Our (Shanmugam et al., 1995) reinterpretation of massive sands as MTDs in basin-floor fans of the North Sea had resulted in a major discussion by eminent sedimentologits composed of Hiscott et al.(1997)and in a reply by Shanmugam et al.(1997).This debate was mostly about HDTCs. Our study also attracted global attention in the petroleum industry, including the UK Department of Trade and Industry(DTI).In response to an invitation from the UK Department of Trade and Industry (DTI), I organized a deep-water sandstone workshop in Edinburgh, Scotland, for petroleum geoscientists from various countries in Europe in 1995(October).This DTI-sponsored workshop utilized cores from the U.K. Atlantic Margin (Faeroe Basin) that contain deposits of sandy MTDs and bottom-current reworked sands (Shanmugam et al., 1995). Participants from both the industry and academia attended,which included H. E. Clifton (1988) from the USA(formerly with the USGS). The workshop provided the participants a rare opportunity to appreciate the value of core-seismic calibration in interpreting deep-water processes without the distraction of sequencestratigraphic models. Finally, advances in 3D seismic modeling of deep-water sandstones (e.g., Zhang et al., 2020) have their limitations as well. In the real world,the true quality of the reservoir can only be validated by the ground truth via core-seismic calibration.

Fig. 107 Uniformitarianism. From Shanmugam (2021a).

Fig. 108 Acknowledgements.

23. Turbidite groupthink

Groupthink refers to forced harmony of opinion,by suppressing diversity, within a group of people. This psychological phenomenon is prevalent among religious, political, business, medical, and geological communities. This section, which focuses on deepwater research with emphasis on turbidite groupthink, deals with a recent paper published in the journal Science Advances. The article entitled “First source-to-sink monitoring shows dense head controls sediment flux and runout in turbidity currents”,in the view of this author, illustrates how a group of 22 authors (Pope et al., 2022) were influenced by the“groupthink”phenomenon. Under this mindset, these researchers were willing to forego the scientific method by ignoring previous geologic knowledge in achieving their pre-determined conclusion. Although there are nine potential depositional processes in operation in Bute Inlet,the authors have opted for only one, namely turbidity currents. They have totally ignored a wealth of published data,accumulated over 125 years, on the influence of strong tidal currents near the mouth of Bute Inlet.The region represents a macro-tidal environment with a spring tidal range in excess of 4 m. This topic is divided into the following themes:

23.1. Bute Inlet as both a fjord and an estuary

The Bute Inlet is both a fjord and an estuary. It is located on the mainland British Columbia, Canada,north of Vancouver Island(Fig.104),. The head of the inlet is fed by freshwater and sediment delivered by the Homathko and Southgate Rivers.It is 80 km(50 mi)long from the estuaries of the two rivers at the head of the inlet,to the mouth.Bute Inlet is merely a merged seaward conduit of the Homathko and Southgate estuaries(Fig.104B).The word“estuary”is derived from the Latin word aestuarium meaning tidal inlet of the sea.The key attribute of an estuary is that it serves as a setting where saltwater from the sea mixes with freshwater from the river(Fairbridge,1980).Salinity is the standard measure of saltwater. In this context,Tabata and Pickard (1957) reported their findings in their article entitled “The Physical Oceanography of Bute Inlet,British Columbia”as follows:“Distributions of salinity, temperature, and oxygen of Bute Inlet based on twelve oceanographic surveys between the period August 1950 to July 1953 have been examined.The salinity structures of the shallow water during the various seasons can be classified under two main groups, one occurring at periods of small river runoff and the other at periods of large river runoff. In general,the surface salinity increases to seaward and with depth during all seasons. The surface water along the western shore is almost always observed to be less saline than along the eastern shore.The salinity of the deep water is 30.6%-30.8‰ during both periods. The seasonal fluctuation of salinity at the surface is well marked but below a depth of 60 feet no obvious cycle exists.” Therefore, Bute Inlet is an estuary with a stratified water column.Saltwater incursions from the sea can readily occur because the mouth of the inlet has open access to the Pacific Ocean. For example,Lafond and Pickford (2011) reported that there were deep-water exchanges (＞100 m) between Bute Inlet and the Strait of Georgia. Bute Inlet is connected to the Strait of Georgia through Sutil Channel.The Strait of Georgia is connected to the Pacific Ocean through the Strait of Juan de Fuca(Fig.104B).Details on tidal currents in the Strait of Georgia and in the Strait of Juan de Fuca were discussed by various researchers(Thomson,1994;Thomson et al.,2007;Warrick et al.,2011; Ryan et al., 2019). Schafer, Cole, and Syvitski(1989) described the estuarine-mouth processes in Bute Inlet. According to Schafer, Cole, and Syvitski(1989), “Sediment on sills near the mouth of Knight and Bute Inlets are reworked morainal deposits facies that are presently being modified by tidal currents that also serve to concentrate indigenous calcareous foraminifera populations into foraminiferal lag deposits.”.The mouth of Bute Inlet and surrounding bays,straits,passages, and rivers are characteristically representing a macro-tidal environment with a spring tidal range in excess of 4 m(Thomson,1981)(Fig.105,see spring tidal range table).

23.2. The geologic concept of source-to-sink

Pope et al. (2022) claimed that they have documented the importance of turbidity currents from source-to-sink in the Bute Inlet study area.They have misused the phrase “source-to-sink”in their study for the following reasons:

a) The Bute Inlet study area is located in between the northern source and the southern sink regions(Fig. 104C). The true source region represents the provenance of two rivers.The proximal mooring M6 is located near the mouths of two rivers (see their Fig. 1). The two river mouths are not the true source or provenance(See Fig.104C).River mouths are the source of hyperpycnal flows (Bates, 1953).

b) The term“sink”refers to a depositional sink,which is the final destination or the endpoint of a sediment package in a depositional system. A classic example of a sink is a submarine fan in a basin-plain environment.In this case,the“source”is in the terrestrial setting, ultimately in mountainous uplands. The sediment is transported downslope via the fluvial system and delivered to the coast where it may be reworked on the shelf and delivered to slope canyons, thereby feeding basin-floor or slope submarine fans (the “sink”)(Fig. 4). The sediment feed from the rivers to the canyons may be more direct during lowstand conditions when the shelf is either very narrow or nonexistent.

c) In short, the Bute Inlet study area, with six moorings M1-M6, represents sediment-transfer zone.Pope et al. (2022) did not gather data outside the Bute Inlet(channel),where the true source and sink areas are located(See Fig. 104C).

23.3. The sedimentology of turbidity currents

Starting with the paper title “First source-to-sink monitoring shows dense head controls sediment flux and runout in turbidity currents”, Pope et al. (2022)have kept using the term “turbidity currents” for flow events without defining what they are at the outset in terms of fluid rheology (Dott, 1963), flow state (Sanders, 1965), sediment concentration(Bagnold,1962),density stratification(Sanders,1965;Postma et al., 1988), and flow mixing at head (Allen,1985). Characteristics of turbidity currents and their deposits were described earlier (Figs. 17-23). Once the basic properties of a turbidity current are defined,one can easily verify whether the measured events at Bute Inlet are indeed turbidity currents or something else (e.g., sandy debris flows or baroclinic tidal currents).Their lack of definition of turbidity currents has allowed Pope et al. (2022) to describe any event observed in their ADCP monitoring as a “turbidity current”.

23.4. Empirical photographic documentation from Bute Inlet

Photography has been widely utilized to document the flow characteristics and deposits of sediment -gravity flows. Shepard and Dill (1966) documented active sandy debris flows (Fig. 15) and sand fall(Fig. 16) in submarine canyons. Although Pope et al.(2022) measured 95 events with their ADCP but failed to include a single photograph of these events. Photographs are a relatively cheap and easy means by which to distinguish debris flows from turbidity currents in density-stratified flows.

23.5. Direct measurements of sediment concentration in the inlet

A fundamental property of turbidity currents is their sediment concentration because turbidity currents are sediment-gravity flows (Middleton and Hampton, 1973). Typical turbidity currents have a sediment concentration of less than 9% by volume(Bagnold,1962).However,Pope et al.(2022)state that

“… key characteristics, such as sediment concentration, that drive the flow have not been measured or derived … It has therefore not been possible to develop a model that robustly captures the full range of turbidity current characteristics and how or why flows may evolve from one set of characteristics to another.” Pope et al. (2022) directly measured flow velocity, flow height, and duration using six ADCPs moored at locations from the Homathko Delta down to a terminal channel lobe. These measurements were then used to derive other key characteristics that control turbidity-current behavior using a modified Ch′ezy formula as proxy.

23.6. Use of the Che′zy formula

The Ch′ezy formula, developed by French physicist and engineer Antoine de Ch′ezy(1718-1798), is based on experimental data collected from the canal of Courpalet and from the river Seine.It describes mean flow velocity in turbulent open-channel flow(Chanson,2004). It is used broadly in fields related to fluid mechanics.Open channels refer to any open conduit,such as rivers.An open-channel flow is a type of liquid flow within a conduit with a free surface (Chow et al.,2008). A free surface is the surface of a fluid that is subject to zero parallel shear stress, such as the interface between two homogeneous fluids. An example would be a river, namely a body of water(liquid)and the air in the atmosphere.

The Ch′ezy formula is written as:

Where V is average velocity;C is the Ch′ezy coefficient;Rhis the hydraulic radius; and Sois the hydraulic gradient.

There are fundamental problems with applying the Ch′ezy formula to turbidity currents.Firstly,the Ch′ezy formula (modified or not) is specific to open-channel flows. It does not apply to deep-water turbidity currents. Secondly, many turbidity currents are not confined in channels.As a rule,submarine fans develop at the point where turbidity currents exit their confining slope channels(Fig.22B).Thirdly,the Ch′ezy formula cannot differentiate the precise sedimentconcentration difference between the basal laminar layer(i.e.,sandy debris flows)and the upper turbulent layer (i.e., turbidity currents) in density-stratified gravity flows (Fig. 19). Finally, the application of the Ch′ezy formula to Bute estuary with stratified water column, induced by salinity and temperature, is complicated. In other words, the events measured by ADCP in Bute Inlet may not be true turbidity flows but rather sandy debris flows or baroclinic tidal currents(see following section).In summary,the Ch′ezy formula is invalid in the context to which it was applied by Pope et al. (2022).

23.7. The regional influence of strong tidal currents

The Canadian fjords are influenced by exceptionally strong tidal currents(Thomson and Huggett,1980;Huggett and Woodward, 1981; Thomson, 1981;Foreman et al., 1992, 2004, 2012; Stacey, 2005;Sutherland et al., 2007). As mentioned earlier, strong currents are no surprise given that the entire region is a macro-tidal environment with a tidal range of over 4 m (Fig. 105). The first tidal current measurements were made from the HMS Nymphe in Seymour Narrows,located just south of the mouth of Bute Inlet, during the summer of 1895 (Thomson, 1981). Thomson and Huggett (1980) reported M2semidiurnal motions in a stratified Johnstone Strait (Fig. 104B). They recorded westward mean flows up to 30 cm s-1in the upper 100 m of the channel and eastward mean flows up to 20 cm s-1in the lower 200 m.

In the Johnstone Strait (Fig. 105), according to Huggett and Woodward (1981), “The larger flood currents (85-110 cm/s) have a duration of 8～ hours, but the smaller flood currents maintain their maximum speed for 2～hours to 3 hours.The larger ebb currents(50-70 cm/s)have an average duration of 5 h with the diurnal inequality only half(20 cm/s)that of the flood currents.” Despite the availability of rich datasets on tidal currents in the area of Bute Inlet, Pope et al.(2022) have chosen to totally ignore tidal processes.

Although tide gauges have been maintained at numerous locations for varying lengths of time (e.g.,Foreman et al., 2012, their Fig. 2 and their Table 1),moored current-meter measurements and salinity and temperature profiles have been sparse in the central portion of the region, largely because of the strong tidal currents and the potential danger their deployment and recovery poses to small vessels. It is worth noting that the strong tidal influence is a regional phenomenon and not a local one (Figs. 104 and 105).

23.8. Equipment damage as evidence for turbidity currents

In a previous study, Prior et al. (1987) used this criterion as evidence for turbidity currents in Bute Inlet. The idea was first proposed by Inman et al.(1976) based on the concept that turbidity currents were so powerful that they invariably destroyed instruments in submarine environments. It is now thought that much of the destruction is actually caused by other sediment-gravity events including slides,slumps,and debris flows,not turbidity currents.In fact, Bute Inlet is an ideal location for mass movements, as evidenced by a long runout proglacial landslide in 2020 (Petley, 2020).

23.9. Sediment core from the inlet

The underpinning purpose of studying modern depositional systems is that the sedimentary processes and their deposits provide the vital clues with which to better interpret the ancient record. For example,much is known about fluvial rocks because we can directly observe the processes and deposits of modern rivers. Pope et al. (2022) failed to recover the vital sediment cores associated with specific flow events observed in the ADCP monitoring. Instead, they examined cores taken in an earlier study by Prior et al.(1987), published some 30 years before the present ADCP measurements.

In another study,Hage et al.(2020)recovered Box cores from the floor of Bute Inlet in 2016.They called sandy deposits in the Box cores as “turbidites”. Hage et al. (2020) also relied on ASCP data, and also failed to provide empirical photographic evidence for turbidity currents that presumably deposited the sands.In both cases,one could interpret the sediments in the Box cores as “turbidites”; with or without the ADCP data! The crucial link between each flow event measured by ADCP and respective deposit is missing in both studies.

23.10. Application of the Bouma Sequence

Pope et al.(2022)have applied what are now highly questionable interpretations of the Bouma Sequence(Shanmugam,1997,2002a)to the 1987 Bute Inlet core.Pope et al. (2022) reported Ta and Tb divisions of the Bouma Sequence in cores previously obtained. But,based on experimental basis,Leclair and Arnott(2005,p. 4) cautioned that “… the debate on the upward change from massive (Ta) to parallel laminated (Tb)sand in a Bouma-type turbidite remains unresolved.”No one has ever generated the complete Bouma Sequence in flume experiments.In cases like this,Pope et al. (2022) should, at least, acknowledge the fundamental defects of the model. Sixty years of research on turbidites following Bouma's landmark studies (1962) have greatly modified the original interpretations of the Bouma cycle. Sedimentary structures in the Annot Sandstone which hosts the Bouma Sequence, including double mud layers (Fig. 29) and sigmoidal cross-bedding (Fig. 31) provide direct evidence for alternate interpretations, such as tidal processes (Shanmugam, 2002a).

As discussed earlier, given the vast variety of turbidite types it would be naive to interpret the Bute Inlet deposits as of the Bouma type. The Bouma Sequence should no longer function as a genetic facies model for all turbidity currents and their deposits.

23.11. The issues of high-density turbidity currents

Pope et al.(2022)have advocated traction carpets in turbidity currents.Traction carpets are analogous to the flowing-grain layer of Sanders (1965) in densitystratified flows (Fig. 19). Traction carpets and modified grain flows are different names for the nowdefunct concept of “high-density turbidity currents”(Shanmugam, 1996). A Mobil-funded flume experimental study, at the St. Anthony Falls Laboratory at the University of Minnesota during 1996-1998(Fig. 36), on sandy debris flows has clarified the issue of density-stratified gravity flows (Shanmugam, 2000;Marr et al.,2001).There is a clear distinction between basal sandy debris flows with high sediment concentration and upper turbulent low-concentration turbidity currents (Fig. 38). The basal debris flows vary in thickness and there are three types (Fig. 37)(Shanmugam, 2000). These basal debrite layers are analogous to the “dense” layers described by Pope et al. (2022).

23.12. Groupthink model

The claim of modern turbidity currents in Bute Inlet by Pope et al.(2022) remains unproven. They have provided no scientific data to establish the true nature of submarine flows in the Inlet and their work consists of a lot of speculation and conjecture. It is suggested that the reasoning behind the conclusions reached by Pope et al. (2022) is that of a turbidite groupthink(Fig.106)in which all alternative interpretations have been filtered out of the consideration. In summary,their paper contributes little to our understanding of deep-water sedimentation.

24. A roadmap for future researchers

During the past 150 years, we have made impressive advances in deep-water research. However, we have also blunted our own progress by diversion,omission and groupthink.A way forward is to eliminate our overzealous approach based on outcrop-based facies models, and allow ourselves a free flow of ideas for interpreting geologically significant deep-water processes that have been documented empirically in modern systems, such as MTD, tidal currents, baroclinic currents, and thermohaline currents. In short, our goal is to use the known (modern)to interpret the unknown(ancient)(Fig.107),not vice versa.

During a period of 36 years (1983-2019), I have raised many questions in 38 academic discussions(Shanmugam, 2021a). In 2022, two comments dealt with deep-water systems (Shanmugam, 2022a, b).They are all potential research topics for future researchers. Despite inherent challenges, studies of deep-water processes involving tidal currents, internal waves, tsunami waves, cyclonic waves, rivermouth density plumes, and sediment deformation are still a new frontier of research for understanding sedimentary basins and their resource potential.

A video on experiments on sandy debris flows is available online:

You Tube URL site https://youtu.be/uMO7jffZwK0.

Details of the experiments are given in a book(Shanmugam,2021a).Additional details are published

in Shanmugam (2000)and Marr et al. (2001).

Length of video: 19 min. There is no audio to this video.

Location of experiments: St. Anthony Falls Laboratory (SAFL), University of Minnesota. Minneapolis,Minnesota.

Period: 1996-1998.

Funding Institutions: Mobil Oil Company, Dallas,Texas, and Office of Naval Research.

Mobil Scientist: G. Shanmugam.

Project Director: Prof. Gary Parker.

Student researchers:Jeff Marr and Peter Harff.

Video production:Jeff Marr(see video in Appendix A).

Conflicts of interest

The author declares that he has no competing interests.

Author's contributions

The author read and approved the final manuscript.

Funding

This project did not receive any funding.

Availability of data and materials

Data sharing is not applicable to this article as no datasets were generated or analyzed during the current study.on my deep-water research since the 1980s(Shanmugam, 1918d). I thank the Organizing Committee of 5th ICP, Prof. You-Bin He, Yangtze University,Wuhan(China)and Editor Dr.Yuan Wang for inviting me to deliver the lecture at the 5th ICP(Fig.108).Dr.Yuan Wang also did an excellent editing of this complex and long paper. I also thank Xiu-Fang Hu, Editor,Journal of Palaeogeography, for processing the manuscript.I am thankful to Elsevier Journal Manager,Lydia Xu (許祎昕), Beijing. China for her excellent handling of this paper during production. I also thank Elsevier Data Administrator, Edward Roobert and the Elsevier Production Team in Chennai,India for their meticulous corrections and production of this long paper. I am grateful to two journal reviewers, Prof. Santanu Banerjee, IIT Bombay (India), and Prof. You-Bin He,Yangtze University, Wuhan (China), for their critical and helpful comments. John G. McPherson (Melbourne,Australia)is thanked for valuable discussion on lowstand braid deltas and for providing photographs of braid deltas. John G. McPherson and D. W. Kirkland reviewed the “Turbidite Groupthink” section of this article and provided helpful comments.As always,I am thankful to my wife Jean Shanmugam for her general comments. G. Shanmugam is an Associate-Editor-in-Chief of the Journal of Palaeogeography. Over the years, he has published several book reviews (Shanmugam, 1980, 2001, 2002b, 2009, 2011, 2022c;Shanmugam and Moiola, 1979).

In honoring John. E. Sanders and other geologic pioneers, Prof. Gerald M. Friedman organized a“Conference on the History of Geologic Pioneers” at the Rensselaer Center of Applied Geology, Troy, New York in 2000. At that conference, by invitation from Prof. G. M. Friedman and George Devries Klein, I presented a lecture entitled “John E. Sanders and the turbidite controversy”.

Acknowledgements

This article, which is based on a keynote lecture delivered at the 5th ICP on May 14, Saturday,9:50-10:20 AM (Beijing Time), 2022, Wuhan, China,reflects my research interest on soil mechanics and mass-transport deposits(MTD)that began in 1965 at IIT Bombay in India and continued in the USA since 1970.My sincere thanks to hundreds of colleagues worldwide for their help in the field, and in the laboratory in studying deep-water deposits during the past 50 years.The late George Devries Klein had a profound influence