Review of research in internal-wave and internal-tide deposits of China: Discussion

G.Shanmugam

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

1 Introduction*

The topic of internal waves and internal tides is of considerable interest to both oceanographers and sedimentologists worldwide.In this context, the paper by Gaoet al.(2013)entitled “Review of research in internal-wave and internal-tide deposits of China” is of signif i cance not only for the Chinese readership but also for the international readership.However, their paper suffers from fundamental def i ciencies.In pointing out these problems and in advancing the primary mission of theJournal of Palaeogeography, which is to promote the communication and cooperation between Chinese and international scholars,I avail this opportunity by offering basic information and explanation.

The article by Gaoet al.(2013)is the first major review of research on ancient deposits.Therefore, a rigorous scrutiny of the review is imperative.Otherwise, the article will leave an indelible impression that the science of internal waves and internal tides is settled.From an oceanographic viewpoint, it is far from settled (Garrett and Kunze, 2007).From a sedimentological point of view, it is at a crisis stage(Shanmugam, 2008a, 2012a, 2012b, 2013a, 2013b, 2013c,2013d, 2013e, 2014)in the Thomas Kuhn’s (1996)f i ve stages of scientif i c revolutions: (1)random observations,(2)first paradigm, (3)crisis, (4)revolution, and (5)normal science.

1.1 Global data sets

Deep-water processes and facies models are full of confl icts (Shanmugam, 2012b).Eventually, all major con fl icts must be resolved.To this end, the subject of internal waves and internal tides has generated f i ve published debates, including this one, since 2008 (Table 1).Although various sedimentological issues raised in these debates are critical here, Gaoet al.(2013)have neglected to address them.In particular, a clear understanding of the origin of bottomcurrent reworked sands (BCRS), which include reworked sands by baroclinic currents (Shanmugam, 2013a), has direct implications for process sedimentology and petroleum geology.In this context, descriptions of deep-water strata from 35 case studies worldwide are considered (Figure 1,Table 1).These global data sets include 7832 meters of conventional cores from 123 wells, representing 32 petroleum fields.Finally, selected modern and ancient case studies of deep-water systems published by other researchers are discussed in illustrating the challenges in distinguishing baroclinic sands (see Section 9).Hopefully, this comprehensive discussion and related reply will motivate others to undertake future research.

1.2 Historical backgrounds

Gaoet al.(2013)state that “The study of internal waves has a long history in oceanography which can be traced back to the study of the interfacial wave theory by Stocks in1847 (Munk, 1981).” But Benjamin Franklin in 1762 was the first one who demonstrated that internal gravity waves on the interface between oil and water have a much longer period than do surface waves with the same wavelength(Phillips, 1974).Early observations of internal waves in nature have been attributed to Russell (1838)and even to earlier Viking times (Ekman, 1904).In the 20thcentury,the late Dr.John Ralph Apel is considered the “Father”of SEASAT (one of the earliest Earth-observing satellites by NASA)in the use of remote sensing for investigating the physics of internal waves and internal tides (Jackson,2004).

2 Fundamental concepts

2.1 Baroclinic oceans

Table 1 Part I: Case studies by other researchers that are used in this article (Locations: A, B, C, D and E, f i lled squares, see Figure 1).Part II: Case studies by the author based on conventional core and outcrop description worldwide (Locations: 1-13, f i lled circles,see Figure 1).Also note that traction structures of bottom-current origin, which may include baroclinic currents associated with internal waves and tides, are common in all 35 case studies listed.

Table 1, continued

Table 1, continued

The concept of ‘Barotropic vs.Baroclinic’ is of paramount importance in understanding currents associated with internal waves and internal tides (CIMAS, 2012).This is because these concepts are directly related to developing sedimentological criteria for recognizing ancient deposits.In an oceanographic context, barotropic currents are driven by the slope of the water surface, and these currents are typical of the well-mixed shallower (shelf)part of the ocean (Figure 2).In contrast, baroclinic currents are driven by the vertical variations in the density of the ocean water caused by changes in temperature and salinity.As a consequence, baroclinic currents are commonly associated with internal waves and internal tides that propagate along boundaries of density stratif i cations in the deeper part of the ocean (Figure 2).Baroclinic currents can occur in mid-ocean depths and along the ocean f l oor of continental slopes and submarine canyons.However, baroclinic currents do not occur along the deep abyssal f l oors (see Section 9.2).Despite its common usage in physical oceanography, the ‘baroclinic’ concept still remains an unfamiliar theme in process sedimentology.

The shelf edge is the defining bathymetric boundary between the shallow mixed ocean and the deep stratif i ed(baroclinic)ocean (Figure 2).The shelf edge concept is not applicable to gently sloping carbonate ramp setting or to periods of sea-level lowstands.

According to the American Meteorological Society(Ocean Motion, 2012), a pycnocline is the interface between the mixed and the deep ocean layers where the density gradient is the greatest (Figure 2).The density gradient is caused either by differences in temperature (i.e., thermocline)or by salinity (i.e., halocline).The ocean’s uppermost 100 m or so is well mixed by wind-driven surface currents.In general, the deep-marine environment (i.e., >200 m in bathymetry)is vertically stratif i ed (Figure 2).

2.2 Internal waves and internal tides

Internal waves are gravity waves that oscillate along the interface between two water layers of different densities,known as pycnocline (Figure 2).Although pycnoclines are primary boundaries of density stratif i cation for the existence of internal waves, they are not essential in all cases.This is because any hydrostatically stable density stratif i -cation is suff i cient for sustaining internal waves (Garrett and Munk, 1979).In order to distinguish these additional boundaries from pycnoclines, the term ‘secondary density stratif i cation’ was introduced by Shanmugam (2013a)(Figure 2).Density stratif i cation in the water column of modern oceans is routinely recognized on high-resolution seismic profiles (Susantoet al., 2005).But there are no sedimentologic criteria to recognize paleo-pycnoclines in the ancient stratigraphic record.Internal waves are made visible at the sea surface through the effect of internal wave currents on surface roughness (Gargett and Hughes, 1972).Internal waves are common phenomena in coastal seas, open ocean, fjords, lakes, and the atmosphere.Internal tides are internal waves at a tidal frequency (Shepard, 1975).

Internal solitary waves or solitons, consisting of a single isolated wave, are ubiquitous in stratif i ed f l uids.Apel(2002)defined this class as follows: “Solitary waves are a class of nonsinusoidal,nonlinear,more or less isolated waves of complex shape,which occur commonly in nature.These waves maintain their coherence,and hence visibility,through nonlinear hydrodynamics and appear as long,quasilinear stripes in imagery.” Internal solitary waves travel in packets.The number of individual oscillations within the packet increases as its age increases,with one new oscillation added per Brunt-Väisälä period.The Brunt-Väisälä frequency or buoyancy frequency (e.g.,Apelet al., 2006; their equation 10)is expressed as follows:

Apel (2002)summarized the physical properties of internal solitary waves, and Shanmugam (2013a)has updated them.Internal solitary 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.The amplitudes are rank ordered, with the largest at the front of the packet and the smallest at its rear.The wavelengths and the crest lengths are also rank ordered, with the longest waves at the front of the group.Unlike surface waves, internal waves can stretch over tens of kilometers in length.Characteristically, a younger (smaller)wave packet follows an older (larger)packet forming a wave train in the Sulu Sea (Shanmugam, 2013a, his Figure 6).Unlike surface waves, internal waves can propagate not only horizontally, but also vertically and in any direction in between (Cacchione and Pratson, 2004).Although internal tides have large amplitudes in the deep ocean, their sea-surface height manifestations are only of a few centimeters (Ray and Mitchum, 1997).This is caused by the great increase in density difference between air and water at the sea-surface interface in comparison to the density difference between f l uids (i.e., water-water)at internal interfaces.For example, the density of water is 1000 times greater than that of air.

2.3 Process sedimentology

Process sedimentology is the founding principle behind all process interpretations of sedimentary rocks (see details in Shanmugam, 2006a, Chapter 1).Basic requirements of this discipline are (1)a knowledge of physics, in particular, soil mechanics and f l uid mechanics (Sanders, 1963;Brush, 1965), (2)the routine application of uniformitarianism, (3)objective description of the rock, (4)documentation of excruciating details in sedimentological logs, (5)pragmatic interpretation of processes using sedimentary structures, (6)the absolute exclusion of facies models, and(7)the use of common sense.

The term “tidalite” was originally introduced for alternating units of traction and suspension deposition from shallow-water tidal currents (Klein, 1971).The genetic term “internal tidalites” is appropriate for deposits of internal tidal currents.Deposits of baroclinic currents, associated with both internal waves and internal tides, could be termed “baroclinites” (Shanmugam, 2013a).

3 Evidence for oceanic pycnoclines

The supreme evidence for interpreting internal-wave and internal-tide deposits in the rock record is the physical evidence for oceanic pycnoclines (Shanmugam, 2012a).Without that evidence for density stratif i cation, no difference between a surface tidalite formed by surface (barotropic)tides on a shallow-marine shelf and an internal tidalite formed by internal (baroclinic)tides in a deep-marine slope or canyon environment exists.The interpretations of ancient strata as deposits of internal waves and internal tides by Gao and his colleagues (Gao and Eriksson, 1991;Gaoet al., 2013; Heet al., 2011)were not based on the ultimate evidence for pycnoclines.Shanmugam (2012a)debated this problem with reference to interpretation of Ordovician deposits in China by Heet al.(2011).In their reply, Heet al.(2012)conceded that “Conclusive evidence for the existence of a pycnocline in our stratigraphic record is currently lacking.Because the absence of proof is not proof of the contrary,it is unreasonable to use this as a basis to negate the possibility that these deposits may have been generated by internal waves and internal tides.”

4 Distinguishing between internalwave and internal-tide deposits

Gaoet al.(2013)treat both internal-wave and internaltide deposits as one and the same.Internal waves can be distinguished from internal tides in modern oceans by monitoring tidal frequency.However, the distinction between deposits associated with internal waves and those associated with internal tides in the ancient stratigraphic record has never been resolved using core studies of modern analogs.For this reason, Heet al.(2008, 2011)have combined characteristic structures of both internalwave and internal-tide deposits together.These sedimentary structures are (1)bidirectional cross-lamination, (2)cross-lamination dipping upslope, (3)multidirectional cross-lamination, (4)f l aser bedding, (5)wavy bedding, (6)lenticular bedding, (7)double mud layers, and (8)reactivation surfaces.The problem is that these sedimentary structures are associated with deposits of tidal currents not only in shallow-marine environments (Reineck and Wunderlich, 1968; Klein, 1970; Visser, 1980; Terwindt, 1981;Allen, 1982; Nio and Yang, 1991; Dalrymple, 1992; Alexanderet al., 1998; Archer, 1998; Shanmugamet al., 2000;Davis and Dalrymple, 2012)but also in deep-marine environments (Klein, 1975; Cowanet al., 1998; Shanmugam,2003; Shanmugamet al.2009; Mutti and Carminatti,2012).This is an important area of future sedimentological research.

5 Bidirectional cross-bedding

The review by Gaoet al.(2013)is symptomatic of research with chronic problems dealing with deposits of internal waves and internal tides, which include the first facies model of an Ordovician internal-tide deposit from the central Appalachians (Figure 1, location A).For the first time, bidirectional cross-bedding was related to internal-tide origin in submarine channels and canyons in the Appalachian study (Gao and Eriksson, 1991).However,bidirectional current directions of baroclinic currents associated with modern internal waves and internal tides are still murky.

Gaoet al.(2013)claim that “The most typical sedimentary structures of internal-wave and internal-tide deposits are bidirectional cross beds(Figures2, 3)and unidirectional cross-beds with laminae dipping up the submarine canyon or regional slope…” This claim is based strictly from their study of ancient stratigraphic record, without any validation from modern analogs.Although Shepardet al.(1979)documented along-canyon bidirectional tidal currents in submarine canyons, it is unclear as to whether these currents are barotropic or baroclinic in origin (Shanmugam, 2013a, 2013b).

There are four major types of submarine canyons based on the position of canyon heads, the role of surface (barotropic)tide, the role of internal (baroclinic)tide,etc.(Figure 3), which should be taken into account when interpret-ing ancient rock record.

Unlike barotropic tidal currents that f l ow along the axis of the canyon, baroclinic currents f l ow across the canyon and in a direction parallel to the shelf break (Allen and Durrieu de Madron, 2009).Selected examples of crosscanyon currents are: (1)Hydrographer Canyon, U.S.Atlantic (Wunsch and Webb, 1979); (2)Monterey Canyon,U.S.Pacif i c (Kunzeet al., 2002); and (3)Gaoping Canyon, Taiwan (Leeet al., 2009).In such canyons, baroclinic currents cannot generate bidirectional cross-bedding.

Importantly, the development of bidirectional crossbedding in modern submarine canyons or channels by internal waves or internal tides has never been documented using sediment core.In the modern Gulf of Cadiz (Figure 1, location B), where internal waves and internal tides are active today (Alvarado-Bustos, 2011; Sanchez-Garridoet al., 2011; Quaresma and Pichon, 2013), Stowet al.(2013)reported current reversal associated with internal tidal currents in the modern Cadiz Channel, but they did not report bidirectional cross-bedding in the core.Therefore, the presumed genetic link between bidirectional cross-bedding and internal tidal currents has not yet been established using modern analogs.

Satellite images of modern internal waves reveal that the directions of propagation of internal waves are highly variable with respect to the shoreline, the shelf edge,and the channel axis (Figure 4).Selected examples are:(1)internal waves that propagate towards the shoreline,(2)internal waves that propagate away from the shoreline or the shelf edge, (3)internal waves that propagate nearly parallel to the shoreline, (4)internal waves that propagate in the direction parallel to the strait axis or channel axis,controlled by a sill, (5)internal waves that propagate in the same direction on both sides of a strait, controlled by a sill, and (6)two wave trains of internal waves propagating in opposite directions from the point of origin, a sill, in a strait (Figure 4).But there is no systematic linking of wavepropagation directions seen as the sea-surface manifestations on satellite images with their respective inf l uence on internal sedimentary structures (i.e., dip directions)in the depositional bedforms on the modern seaf l oor.This lack of a link between the direction of wave propagation along pycnoclines and the direction of current movement on the seaf l oor is further compounded by the presence of local sills on the seaf l oor that invariably control the direction of wave propagation (Figure 4D, 4E, and 4F).Furthermore,Dykstra (2012, his Figure 14.3b caption)state that “If more than one wave is present in the ocean at different depths,which can occur in well-stratif i ed water with signif i cant seaf l oor topography(e.g.Robertson,2005),current directions along the seaf l oor can become quite complicated.”Under this umbrella of knowledge vacuum on current directions, the use of bidirectional cross-bedding as evidence for deposition by baroclinic currents in outcrop studies is sedimentologically erroneous.

6 Traction structures

Gaoet al.(2013, their Table 1)and Gaoet al.(1998)claim that various traction structures (e.g., unidirectional cross-bedding, cross-laminated lenses,etc.)are evidence of internal waves.This claim is false because traction structures have been documented to form by other deep-water processes (Shanmugam, 2012b).There are four types of deep-water bottom currents, namely (1)thermohaline-induced geostrophic bottom currents (i.e., contour currents),(2)wind-driven bottom currents, (3)deep-marine tidal bottom currents, and (4)baroclinic currents, are considered(Shanmugam, 2008a).All four types of bottom currents can develop traction structures (Figure 5).For example:

Traction structures are considered to be an integral part of contourites (i.e., deposits of contour currents)(Hubert,1964; Hollister, 1967; Hsü, 1989; Mutti, 1992; Mutti and Carminatti, 2012; Ito, 2002; Martın-Chivelet,et al..2008;Shanmugam, 2000, 2008a).

In the Ewing Bank 826 area of the Gulf of Mexico,tractions structures in the Plio-Pleistocene intervals have been interpreted to be by the wind-driven Loop Current(Shanmugamet al., 1993a, 1993b)(Table 1).These deposits are characterized by cross-bedding, ripple lamination,and horizontal lamination (Figure 5).

In the Krishna-Godavari Basin in the Bay of Bengal,traction structures in the Pliocene sandy intervals have been related to deep-marine tidal currents (Shanmugamet al., 2009).

On the Horizon Guyot in the Pacif i c Ocean, traction bedforms have been attributed to reworking by internal tidal currents (Lonsdaleet al., 1972).These are baroclinic sands.

The presence of traction structures in cores and outcrops have long been recognized as evidence for bottomcurrent reworked sands in deep-water strata (Hsü, 1964,2008; Hubert, 1964; Klein, 1966; Hollister, 1967; Natland,1967; Piper and Brisco, 1975; Shanmugamet al., 1993a,1993b; Shanmugam, 2008a; Martın-Chiveletet al., 2008;Mutti and Carminatti, 2011).Hsü (1964)argued that traction structures in deep-marine sands were more meaningful as deposits of bottom currents than of turbidity currents.Traction structures are common in deep-water petroleum reservoirs worldwide (Table 1).The challenge is how to distinguish parallel laminae formed by contour currents from those formed by wind-driven bottom currents in the ancient stratigraphic record.

Gaoet al.(2013)state, “The grain-size of sandstone(grainstone)of internal-tide and internal-wave deposit origin is similar to that of fine-grained turbidites and sandy contourites.Distinguishing correctly internal-tide and internal-wave deposits,turbidites and contourites is also the key to recognizing internal-tide and internal-wave deposits.There are distinctions between them in terms of sedimentary structures,relationships between the direction of directional sedimentary structures and palaeogeographical patterns,vertical successions, bioturbation,and so on.” However, distinguishing contourites from other deep-water deposits is impractical (Shanmugam, 2012b).

Stowet al.(2013)have interpreted sands and gravels as“sandy contourites” in the Gulf of Cadiz (Figure 1, location B).Although this site served as the birthplace for the first contourite facies model (Faugèreset al., 1984), the Gulf of Cadiz is an extremely complex deep-water environment with multiple interactive processes (e.g., Mediterranean Outf l ow Water (MOW), internal waves, internal tides,etc.)and with intricate submarine channels, ridges, and sills.For these reasons, there are no objective criteria to distinguish traction structures formed by contour currents from those formed by internal waves or internal tides.Bioturbation is not unique to contourites (Shanmugam, 2012b, 2013a).Therefore, there are no objective criteria to distinguish contourites from baroclinic sands.

Furthermore, there are process-sedimentological challenges in distinguishing tsunami-related deep-water deposits with traction structures from other deposits (Shanmugam, 2006a, 2006b, 2012c).By ignoring this wealth of published information on traction structures associated with various bottom currents, Gaoet al.(2013)promote a falsehood on the link between traction structures and internal waves, without a critical analysis.

7 Vertical facies models

Gaoet al.(2013)state that “Four basic sedimentary successions of internal-wave and internal-tide deposits arerecognized,which include: (1)a coarsening-up and then fining-up succession(bidirectional graded succession),(2)a fining-up succession(unidirectional graded succession), (3)a coarsening-up and then fining-up succession with couplets of sandstone and mudstone(bidirectional graded couplet succession),and(4)a mudstone-oolitic limestone-mudstone succession(Figure4.)” These trends are strictly from study of outcrops in China and the central Appalachians (USA).I have provided detailed critiques of these models elsewhere (Shanmugam, 2012a, 2013a,2013b).There are fundamental questions that still remain to be addressed.

1)Why the four vertical trends are considered indicative of deposits of internal waves and internal tides?

2)Are there modern analogs to support these vertical trends?

3)Are there theoretical solutions that can explain these vertical trends?

4)Are there laboratory experimental works that can replicate these vertical trends?

5)Are there differences in vertical trends between internal-wave and internal-tide deposits?

6)What is so unique about the lithofacies “a mudstone-oolitic limestone-mudstone succession” (their Figure 4f)that implies deposition from internal waves or internal tides?

7)What are the criteria to distinguish deposits of barotropic tidal currents in shallow-marine environments from those of baroclinic tidal currents in deep-marine environments?

8)What is so unique about the upward-coarsening trend with bidirectional cross-bedding (their Figure 4a)? For example, upward-coarsening trends with bidirectional crossbedding have been documented in estuarine tidal sand bars(Shanmugamet al., 2000, their Figure 9).Upward-coarsening trends are also considered typical of storm deposits(Bádenas and Aurell, 2001; Pomaret al., 2012).

9)The uncertainty of outcrop-based vertical facies models has long been recognized for storm (Dott and Bougeois, 1982), f l uvial (Miall, 1985)and turbidite (Shanmugamet al., 1985)deposits.What is the reason for ignoring this tumultuous sedimentological history behind vertical facies models?

Heet al.(2011, their Figure 11)proposed an idealized vertical facies model that closely mimics the Ta, Tc, and Te divisions of the turbidite facies model, known as the ‘Bouma Sequence’ (see Shanmugam, 2013a, his Figure 15D).Even the classic ‘Bouma Sequence’ (Bouma, 1962)is considered obsolete due to lack of theoretical, experimental,and empirical basis (Hsü, 1964, 1989, 2004, 2008; Sanders, 1965; Van der Lingen, 1969; Leclair and Arnott, 2005;Shanmugam, 1997a, 2000, 2002, 2006a, 2012b, 2013e).In spite of these controversies, what is the reason for adopting the ‘Bouma Sequence’ in the study by Heet al.(2011)?

Given the fact that the very existence of sandy and gravelly turbidity currents has never been documented in modern oceans, the outcrop-based turbidite facies models(Bouma, 1962; Lowe, 1982)and their more recent versions (Tallinget al., 2012, their Figure 3)and explanations(Postmaet al., 2014)are irrelevant for interpreting ancient rock record objectively worldwide (Shanmugam, 2014).The turbidite facies models, which are nothing more than a groupthink, have suppressed scientif i c curiosity during the past 50 years by averting novel observations and by preventing innovative interpretations.Analogous to turbidite facies models, facies models proposed for deposits of internal waves and internal tides will impose similar limitations due to a lack of scientif i c foundation.

8 Sea-level changes

Gaoet al.(2013)claim that “With a rise in sea level, the distance from sediment source areas to depositional areas gradually increases, coarse-grained clasts are stranded closer to source areas, and internal waves and internal tides become dominant in reworking fine-grained gravity-f l ow deposits.” In order to evaluate the validity of this claim, one needs to evaluate the origin of internal waves and internal tides.

Internal waves are triggered by natural forces like (a)wind (meteorological force), (b)tide (astronomical force),(c)tropical cyclones (Namet al., 2007), (d)tsunamis(Santek and Winguth, 2007), (e)river plumes (Nash and Moum, 2005), and by man-made activities like sailing ships (Apel and Gjessing, 1989).Tropical cyclones also inf l uenced the generation of internal tides (Davidson and Holloway, 2003).The problem is that these triggering mechanisms are not unique to a period of rise in sea level.

Empirical data on tropical cyclones (meteorological phenomena)and tsunamis (oceanographic phenomena)from the Indian, Atlantic, and Pacif i c Oceans reveal that they are common events today (Shanmugam, 2008b).Because tsunamis are most commonly triggered by earthquakes (e.g., 2004 Indian Ocean Tsunami triggered by the Sumatra-Andaman Earthquake), no relationship can exist between sea level changes and the timing of tsunamis (Shanmugam, 2007, 2008b).In other words, tsunamitriggered internal waves can occur irrespective of sea-level changes.

Short-term events, such as earthquake-triggered tsunamis, last only for several hours or days.On the other hand, long-term events, such as sea-level changes, last for thousands to millions of years (Shanmugam, 2012b,2012c, 2014).Therefore, numerous short-term tsunamis can occur during a single long-term rise in sea level.But there are no criteria to distinguish internal-wave deposits associated with earthquake-induced tsunamis in the stratigraphic record.

In short, a rise in sea level is irrelevant to understanding internal-wave sedimentation in deep-water environments.

9 International literatures

The purpose of this section is to demonstrate that Gaoetal.(2013)tend to evade relevant international literatures in their review.I also use this section to illustrate sedimentological challenges that still exist in distinguishing types of bottom-current reworked sands in the ancient sedimentary record.

9.1 Stratif i ed oceans

In a comprehensive review of deep-water bottom currents and their deposits, Shanmugam (2008a)observed that “Gao et al.(1998)interpreted ancient strata with bidirectional cross-bedding,fl aser bedding,wavy bedding,and lenticular bedding as deposits of internal tides based on associated deep-water turbidite and slump facies.The key to interpreting deposits of‘internal tides’or baroclinic currents in the rock record is the vidence for tidal currents in a stratif i ed deep ocean.Without that evidence for density stratif i cation,there is no difference between a tidal deposit formed by surface(barotropic)tide in a shallow-marine shelf and a tidal deposit formed by internal(baroclinic)tide in a deep-marine slope or canyon environment.” Howver, Gaoet al.(2013)have failed to acknowledge this basic weakness in their interpretation.If they disagree with the above assessment, then they need to refute the appraisal with counter reasoning and field data.

9.2 Seaf l oor topography

Gao Zhenzhong, as a co-author of Heet al.(2011), proposed an abyssal-basin palaeogeography, characterized by internal-tide and internal-wave deposits, for the the Middle Ordovician Xujiajuan Formation of the Xiangshan Group, Ningxia, China (Figure 1, location D).Such interpretations totally ignore relevant publications on the role of seaf l oor topography in generating internal waves.For example, Polzinet al.(1997)have documented that the turbulent mixing of internal waves is concentrated over the rough seaf l oor topography of the Mid-Atlantic Ridge in the Brazil Basin, South Atlantic Ocean.Using the crossisopycnal data from the Brazil Basin (see Polzinet al.,1997, their Figure 2), Jayneet al.(2004)have illustrated the concept of turbulent mixing in the Brazil Basin.Clearly, ubiquitous internal waves are generated over the midocean ridge by the tides f l owing over rough topography,whereas internal waves are conspicuously absent over the smooth abyssal f l oor (St.Laurentet al.(2012, their Figure 1; Turnewitschet al., 2013)(see also Figure 6).Empirical data also show that internal waves and internal tides are common over submarine guyots in the Pacif i c Ocean(Lonsdaleet al., 1972), but absent or insignif i cant over the flat abyssal f l oor (Figure 6).Heet al.(2011)did not provide any evidence of submarine guyots or seamounts surrounded by abyssal-basin palaeogeography during the Middle Ordovician in explaining internal-wave deposits.Shanmugam (2012a)pointed out this shortcoming in their paper (Heet al., 2011), which has become a source of lively debate inGeo-Marine Letters(Shanmugam, 2012a; Heet al., 2012).Surprisingly, Gaoet al.(2013)did not cite the discussion and reply in their review.Nor did they explain the origin of internal waves and internal tides over the flat abyssal f l oors in the Middle Ordovician of the Xiangshan Group, Ningxia, China.

9.3 High-velocity currents

There are other relevant articles on physical oceanography and process sedimentology (e.g., Dykstra, 2012;Mulderet al., 2012; Pomaret al., 2012), which Gaoet al.(2013)did not cite.For example, Mulderet al.(2012)believed that high-velocity currents in submarine canyons in the modern Bay of Biscay were related to internal tides,but did not provide empirical data in distinguishing baroclinic currents from barotropic currents.

9.4 Hummocky cross-stratif i cation

In explaining the origin of hummocky cross-stratif i cation (HCS), Heet al.(2011 with Gao as a co-author)state,“It probably represents the product of combined fl ows generated by the interaction of short-period internal waves with turbidity currents.” Although the origin of HCS has been controversial (Shanmugam, 2013a, 2013b), Gaoet al.(2013)totally ignored the controversies.For example,Harmset al.(1975)first proposed that HCS was a product of storm deposition.However, Morsilli and Pomar (2012)attributed the origin of HCS to internal waves.Following this trend, Pomaret al.(2012)reinterpreted the Upper Jurassic “storm” strata, exposed near Ricla in NE Spain(Figure 1, location C)with hummocky cross-stratif i cation(HCS), as ‘internal-wave deposits’.Their reinterpretation implies that associated HCS in these strata was also formed by internal waves.Pomaret al.(2013)did not justify the origin of HCS by internal waves with supporting data and convincing explanation on mechanics of deposition.

9.5 Modern analogs

In developing facies models for ancient internal-wave and internal-tide deposits, Gaoet al.(2013)need to evaluate published empirical data on modern internal waves and internal tides.For example, Salleret al.(2006)interpreted petroleum-producing Miocene sands with parallel and cross laminae as turbidites using the turbidite facies model of Bouma (1962)in the Kutei Basin (Figure 1, location E).The problem here is that Salleret al.(2006)have overlooked the existence of empirical data on bottom currents associated with various modern oceanographic phenomena in the Makassar Strait (Figure 1, location E).They are (1)documented Indonesian throughf l ow (Gordon,2005), (2)observed internal waves (Hatayama, 2004), (3)observed internal tides (Rayet al., 2005), and (4)measured velocities of deep tidal currents (Nummedal and Teas,2001; Wajsowiczet al., 2003).These data are relevant in interpreting Miocene sands alternatively as deep-marine tidalites or baroclinic sands (Shanmugam, 2008c, 2014).

9.6 Seismic wave geometry

In discussing large-scale seismic geometry, Gaoet al.(2013)state that “…interpreting some deep-sea large-scale sediment waves as having an internal-wave origin…”Sediment waves associated with internal waves and internal tides are poorly understood from a process sedimentology viewpoint (Shanmugam, 2012b).At present, no objective criteria exist for distinguishing wave geometry created by internal tidal currents from wave geometry created by contour currents (Nielsonet al., 2008)or by turbidity currents (Normarket al., 1980)using seismic profiles alone(Shanmugam, 2013a).This field remains an important area of future research.

10 Concluding remarks

Empirical data on the physical characteristics of modern internal waves and internal tides from 51 regions of the oceans of the world, descriptions of core and outcrop worldwide carried out by the author, and selected case studies published by other researchers have resulted in the following key conclusions:

Core-based sedimentologic studies of modern sediments emplaced by baroclinic currents on continental slopes, in submarine canyons, and on submarine guyots are totally lacking.

No cogent sedimentologic or seismic criteria exist for interpreting ancient strata as of internal-wave and internaltide deposits in outcrops or cores.

Outcrop-based vertical facies models proposed for ancient deposits are untenable due to an absolute lack of theoretical, experimental, and empirical foundation.At this embryonic stage of our understanding of internal waves and internal tides in terms of their depositional characteristics, the promotion of vertical facies models with lingering questions (see Section 7)is like putting the cart before the horse!

Real potential exists for misinterpreting deep-marine baroclinic sands as turbidites, contourites, tsunami-related deposits,etc.

The interpretation of ancient strata by Gao and his colleagues were made without validation from modern analogs (i.e., without uniformitarianism).

In light of these conclusions, it would be helpful, if Gaoet al.could respond to the issues raised in this discussion and could explain the basis for their interpretation.

In future research, it is imperative to select appropriate modern deep-marine settings for understanding the link between baroclinic currents and their deposits.At such settings, field research must be carried out by obtaining physical measurements of currents and by documenting disposition of sedimentary structures in long sediment cores.It is also necessary to conduct laboratory experiments for understanding depositional mechanics of sedimentary structures formed by baroclinic currents.Such a coordinated approach is likely to yield the much-needed clarity.

Acknowledgements

I thank Prof.Zeng-Zhao Feng (Editor-in-Chief)and Yuan Wang of the journal for their encouragement and help.I also thank two anonymous journal reviewers for their critical and helpful comments.As always, I am grateful to my wife Jean Shanmugam for her general comments.

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