Reply to discussions by Zavala (2019) and by Van Loon, Hüeneke, and Mulder (2019) on Shanmugam, G. (2018, Journal of Palaeogeography, 7 (3): 197–238): ‘the hyperpycnite problem’
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In this reply, I respond to 18 issues associated with comments made by Zavala (e.g., inverse- to normally-graded sequence, origin of massive sands, experimental sandy debris flows, tidal rhythmites, facies models, etc.), and 10 issues associated with comments made by Van Loon et al. (e.g., 16 types of hyperpycnal flows, anthropogenic hyperpycnal flow, etc.).
KeywordsHyperpycnal flows Hyperpycnites Sandy debris flows Turbidity currents
American Association of Petroleum Geologists
High-density turbidity currents
Journal of Palaeogeography
Journal of Sedimentary Research
Massachusetts Institute of Technology
Marine and Petroleum Geology
National Aeronautics and Space Administration
St. Anthony Falls Laboratory
United States of America
United States Geological Survey
Shanmugam (2018a) has identified inherent problems associated with hyperpycnites in deep-water environments. Such controversial papers are bound to generate debates, which is normal. Unmitigated academic debates are an integral and a necessary part of advancing science. I have chosen a total of 28 issues for discussion. Hopefully, this discussion and related questions will help students of this domain in future research.
2 Result and discussion: reply to Zavala (2019)
2.1 Paper title
Zavala has chosen an awkward title for his discussion, which is “The new knowledge is written on sedimentary rocks – a comment on Shanmugam’s paper “The hyperpycnite problem”. According to the Cambridge Dictionary, the term “knowledge” is defined as follows: “Awareness, understanding, or information that has been obtained by experience or study, and that is either in a person’s mind or possessed by people generally“. Cambridge Dictionary, URL: https://dictionary.cambridge.org/us/dictionary/english/knowledge, accessed March 10, 2019.
In his book entitled “Use the Right Word”, Hayakawa (1968) states that, “Knowledge is more than a store of facts in the mind; it also includes the contribution of the mind in understanding data, perceiving relations, elaborating concepts, formulating principles, and making evaluations.”
Clearly, the word ‘knowledge’ refers to the acquired trait by the humankind, and one cannot write ‘knowledge’ on the rocks, as Zavala has falsely implied in his paper title.
2.2 What is a hyperpycnal flow?
Since the original description of hyperpycnal flows for deltaic settings (i.e., density of river water is greater than density of basin water) by Bates (1953), the basic definition of hyperpycnal flows is still elusive from a fluid dynamic point of view.
Zavala (2019) states that, “Clearly, we don’t equate turbidity currents (Newtonian turbulent flows) to hyperpycnal flows.” This statement only informs the readership what hyperpycnal flows are not. This statement does not define as to what is a hyperpycnal flow in terms of fluid mechanical properties. Zavala notes that he is addressing various problems discussed in my paper in a forthcoming paper of his. In that paper, perhaps Zavala could easily solve the basic problems by answering the following questions for the benefit of readership:
1) What is the fluid rheology of hyperpycnal flows and how does it differ from that of turbidity currents?
2) What is the flow state of hyperpycnal flows and how does it differ from that of turbidity currents?
3) What is the sediment concentration of hyperpycnal flows by volume and how does it differ from that of turbidity currents?
Given that the Yellow River is a delta, how tide-modulated hyperpycnal flows differ from the concept of conventional hyperpycnal flows that was originally advocated by Bates (1953) for deltaic environments?
What are the fluid dynamical properties of tide-modulated hyperpycnal flows?
What are the sedimentary characteristics of deposits of tide-modulated hyperpycnal flows?
2.3 What is a turbidity current?
Zavala has confused the basic concept of sediment-gravity flows and related processes, such as turbidity currents. For example:
Zavala (2019) has adopted the flawed classification of sediment-gravity flows by Mutti et al. (1999) in which all sediment-gravity flows are classified as “turbidity currents” (Fig. 2) without regard for fluid mechanics.
- 2)According to Middleton and Hampton (1973), sediment-gravity flows are composed of four basic types, namely (a) turbidity currents, (b) debris flows, (c) grain flows, and (d) fluidized sediment flows (Fig. 2). Each flow type has a distinct set of fluid dynamical properties. Because Zavala has ignored the basic properties of sediment-gravity flows in his discussion, Zavala could classify all four types as turbidity currents. In other words, deposits of all four types would be called turbidites (Fig. 2), which is wrong. For example, deposits of debris flows should be classified as debrites, not turbidites.
I have adopted the Sanders’ (1965) classification in which only deposits of turbidity currents are called turbidites (Fig. 2). Sanders’ (1965) classification is consistent with principles adopted by eminent sedimentologists worldwide (e.g. Bagnold 1962; Dott Jr. 1963; Middleton and Hampton 1973; Allen 1985). Zavala (2019), however, is not among the list.
Zavala also classifies all turbidites as hyperpycnites (see Section 2.8).
2.4 Inversely- to normally-graded hyperpycnite sequence
In support of his false claim that inversely- to normally-graded sequence is the proof of hyperpycnite deposition, Zavala (2019) cites several published examples, all from the ancient sedimentary record. The issue is what kind of fluid mechanical properties that resulted in such a sequence? Zavala has never addressed this basic question. Previous explanations are flawed (Shanmugam 2018a). In fact, this was the issue that triggered the first debate on hyperpycnal flows with T. Mulder in 2002 (Shanmugam 2002b). I also debated this same issue in 2019 (Shanmugam 2019). There has been no solution to this problem during the past 17 years, although many papers have been published. For example contourite deposits also generate inversely- to normally-graded sequence (Shanmugam 2016), but Zavala has failed to address this basic issue.
2.5 Origin of massive sands and floating mudstone clasts
In defending the origin of massive sands by hyperpycnal flows, Zavala criticizes my paper (Shanmugam 1996) on sandy debris flows (also known as high-density turbidity currents) that was published 23 years ago. Renowned process sedimentologists did respond immediately to my publications on massive sands (e.g. Lowe 1997). Zavala had ample opportunities to debate these issues in 1996 or immediately thereafter, but he chose not to comment on these issues. In reality, these issues were raised by others and were responded to them in great detail by Shanmugam and Moiola (1997).
2.6 High-density turbidity currents (HDTC)
2.7 Experimental sandy debris flows
Group 2 would not (Kuenen 1951; Postma et al. 1988; Mutti et al. 1999; Zavala 2019). Postma et al. (1988) would combine both layers and classify them together as “High-density turbidity currents” (Fig. 3b).
2.8 What are coarse-grained hyperpycnites?
But the original authors (Zuffa et al. 2000) called these deposits as “turbidites”. In fact, the title of the paper by Zuffa et al. (2000) is “Turbidite megabeds in an oceanic rift valley recording Jokulhlaups of late Pleistocene glacial lakes of the western United States.”
The reason that Zavala used this example because Zuffa et al. (2000) called the process “hyperpycnally derived turbidity currents”. However, Zavala has failed to appreciate the basic tenet of process sedimentology, which is that deposits reflect physical conditions that existed at the final moment of deposition, which in this case are turbidity currents. This was the reason why that Zuffa et al. used the term turbidites, not hyperpycnites. A similar case on the Pacific Plate was described by Normark and Reid (2003).
On the one hand, Zavala (2019) states that, “Clearly, we don’t equate turbidity currents (Newtonian turbulent flows) to hyperpycnal flows.” But on the other hand, he calls turbidites as coarse-grained hyperpycnites. This practice is inconsistent.
In some circles, hyperpycnal flows are considered analogous to turbidity currents (Mulder et al. 2003; Steel et al. 2016). This is why a clear definition of hyperpycnal flows is imperative (Section 2.2).
2.9 Tidal rhythmites
In defending his field example of lofting rhymites in Orinoco Fan, Zavala argues that tidal currents cannot operate in deep-water environments. Clearly, Zavala is unfamiliar with the classic work of Shepard and Dill (1966) and Shepard et al. (1979) on tidal currents in submarine canyons. In understanding tide-induced bottom currents, Shepard et al. (1979) measured current velocities in 25 submarine canyons worldwide at water depths ranging from 46 to 4200 m by suspended current meters, usually 3 m above the sea bottom. Maximum velocities of up- and down-canyon currents commonly ranged from 25 to 50 cm . s− 1 (Shepard et al. 1979).
Klein (1975), based on studies of DSDP (Leg 30, Sites 288 and 289) cores, suggested that current ripples, micro-cross laminae, mud drapes, flaser bedding, lenticular bedding, and parallel laminae reflect alternate traction and suspension deposition from tidal bottom currents in basinal deep-marine environments. Deep-water tidal rhythmites have been documented from both modern and ancient examples worldwide (Shanmugam 2003), which include the Pliocene reservoir sands in upper-slope canyon environments, offshore Krishna-Godavari Basin, Bay of Bengal (Shanmugam et al. 2009).
2.10 Modern examples
Perhaps the best modern example of a sediment plume that has been dissipated by ocean currents is in the Rio de Plata Estuary that is located between Argentina and Uruguay (Shanmugam 2018a, 2018b). I have documented this spectacular example in my paper. This modern plume is of significance in this debate for two reasons:
2) Zavala has never acknowledged this obvious contradiction in any of his papers. Why?
2.11 Facies models of intrabasinal and extrabasinal turbidites
Zavala’s skewed emphasis of hyperpycnites in the ancient sedimentary record tends to promote circular reasoning (see Section 3.8 below). This was the same approach that had led to the disaster of turbidite research that wasted away decades of valuable efforts (see review by Shanmugam 2006, 2012). We have learned a hard lesson from studying turbidites in the ancient rock record and of focusing on meritless genetic facies models (Bouma 1962; Walker 1978; Mutti 1992).
Specifically, in proposing his facies models of intrabasinal and extrabasinal turbidites, Zavala has failed to take into account the importance of submarine canyons in advocating submarine fan models (Shanmugam 2018a). Instead of providing empirical data for verification, he cites his own earlier publication as the proof that the facies models must be correct with the following statement (Zavala 2019): “Intrabasinal and extrabasinal turbidites display diagnostic characteristics in their deposits that allow a clear differentiation between them (Zavala and Arcuri 2016).” The issue here is the role of submarine canyons.
2.12 Representation of tsunami map as core photo of floating clasts
In disagreeing with my interpretation of a core photo with floating clasts as sandy debris flows, Zavala (2019) states that “Shanmugam (2015) in his Fig. 15 (here reproduced in Fig. 1), provides an example of a core photograph of massive fine-grained sandstone showing a large floating mudstone clast with a planar clast fabric (Fig. 1), a typical bi-modal deposit.”
In his References, the cited reference is listed as follows:
“Shanmugam 2015. The landslide problem. Journal of Palaeogeography 4 (2): 109–166.”
The Fig. 15 caption in Shanmugam (2015) reads as “Map showing the site of Chicxulub meteorite impact at the K-T boundary in Yucatan, Mexico,...” In other words, the tsunami map has nothing to do with floating clasts. In an academic debate, such a careless citation of references should be avoided.
2.13 Representation of mass-transport deposits (MTD) as hyperpycnites
In supporting his false notion that there are numerous cases of published examples of coarse-grained hyperpycnites, Zavala (2019) states that “Shanmugam ignores one of the best known documentation of coarse grained hyperpycnal flows of different fan deltas in British Columbia, Canada (Prior and Bornhold 1990;...”
In reality, Prior and Bornhold (1990, p. 75) state that “The subaqueous morphology and geometry of developing Holocene fan deltas in fjords in British Columbia are used to interpret underwater sediment-dispersal processes. The fans are constructed by combinations of processes occurring with various frequencies and magnitudes, including subaqueous debris avalanching, inertia flows, turbidity flows, slope failure and settling of suspensions from buoyant plumes.” Clearly, the above study by Prior and Bornhold is about mass-transport deposits (MTD), not hyperpycnal flows.
2.14 Representation of turbidity currents as hyperpycnal flows
In documenting modern hyperpycnal flows in deep water, Zavala (2019) states that “Khripounoff et al. (2003) documented on March 8, 2001, a sediment laden turbidity current in the Congo canyon travelling at 121 cm/sec at 4000 m depth, 150 m above the channel floor, transporting quartz rich well sorted fine grained sand (150–200 μm) and large plant debris (wood, leaves, roots). This single flow was sustained for10 days.” In direct contradiction, Zavala (2019) also states that, “Clearly, we don’t equate turbidity currents (Newtonian turbulent flows) to hyperpycnal flows.” On the one hand, Zavala considers turbidity currents as hyperpycnal flows, and on the other hand, he does not. His inconsistent philosophy can be confusing to the reader.
2.15 Representation of triggering mechanism as depositional process
The primary deep-water depositional process in this case was turbidity currents, not hyperpycnal flows. The reason is the presence of turbidites in U.S.G.S. cores (Normark et al. 2009).
Covault et al. (2010) attributed the initiation of sediment-gravity flows to three possible triggering mechanisms, namely (a) earthquakes associated with tectonic activities, (b) hyperpycnal fluvial effluents, and (c) deltaic buildup and related sediment failure.
Zavala selectively emphasized the hyperpycnal origin and ignored the other two. More importantly, Zavala confused the triggering mechanism with the depositional process.
According to Covault et al. 2010, p. 248), “The contribution and timing of fluvial effluents relative to mass-wasting processes in the canyon and steep outer shelf to the initiation of sediment gravity flows in the Newport system are unknown.” Clearly, Zavala has overlooked the critical factor that the importance of hypepycnal flows is unknown in this case study.
Turbidity currents, irrespective of their differences in triggering mechanisms, will invariably possess the same fluid mechanical properties. This is the reason why Zavala needs to define hyperpycnal flows in terms of fluid rheology, flow state, and sediment concentration.
In conclusion, even if a sediment-gravity flow is triggered solely by hyperpycnal discharge of a river, the resulting flow still has to be classified based on fluid mechanics at the point of deposition, not on triggering mechanisms at the point of initiation.
2.16 Original authors’ conclusion
In documenting hyperpycnites, Zavala (2019) cited an example of deposits associated with the failure of the Malpasset Dam in the Mediterranean (Mulder et al. 2009). But the primary conclusion of Mulder et al. (2009) was stated as “The deposits associated with the Malpasset Dam failure strongly differ from classical hyperpycnites...”. Mulder et al. (2009) also concluded that “the process is closer to a slide-triggered, surge like flow forming a hyperconcentrated flow,” In other words, this case is not a hyperpycnite, as Zavala alleges.
2.17 Earthquake-triggered flows
In conflict with the original concept of hyperpycnal flows designed strictly for river-discharged sediment flows (Bates 1953), Zavala (2019) has confused the issue with an example from Taiwan in which normal river discharges are dominated by earthquake-triggered higher sediment concentrations (Dadson et al. 2005).
2.18 Recognition of hyperpycnal flows on seismic profiles
Zavala (2019) cites Sacchi et al. (2009) as an example of hyperpycnal flows in Italy. This study was based on interpretation of hyperpycnal flows on high-resolution seismic profiles. What are the seismic criteria that distinguish hyperpycnal flows from turbidity currents on seismic profiles in terms of fluid mechanics?
3 Result and discussion: reply to Van Loon, Hüeneke and Mulder (2019)
3.1 Definition of 16 types of hyperpycnal flows
There is a fundamental philosophical difference between myself and Van Loon et al. on hyperpycnal flows. For example, I am of the view that in order to effectively debate this issue, there must be a common agreement on the definition of hyperpycnal flows. But there is no such common ground. The problem is that there are 16 types of hyperpycnal flows (Shanmugam 2018a). Unlike me, Van Loon et al. consider that 16 types of hyperpycnal flows are not a problem. In this context, they state that “it remains unclear why the existence of different type would pose a problem regarding the fact that they are all sediment-gravity flows and that they can form deposits with their own sedimentary characteristics (as far as the flow types and/or conditions differ!).”
What is difference in sedimentary characteristics between deposits of single-layer and double-layer flows?
What is difference in sedimentary characteristics between deposits of single-layer and multi-layer flows?
What is difference in sedimentary characteristics between deposits of double-layer and multi-layer flows?
What is difference in sedimentary characteristics between deposits of double-layer and tide-modulated hyperpycnal flows?
What are the sedimentary characteristics of deposits of internal waves associated with tide-modulated hyperpycnal flows (Wang et al. 2010)?, to mention a few issues.
Baroclinic currents associated with internal waves and tides (Wang et al. 2010) are one of the most controversial topics that resulted in lively scientific debates (Shanmugam 2014a, 2014b). Van Loon et al. have avoided discussing these natural complexities.
Historically, the less the knowledge about a particular process that we possess, the more the number of flow types that we propose. Analogous to 16 types of hyperpycnal flows, there are 78 types of mass-transport deposits (MTD), which include over 34 types of high-density turbidity currents (see Section 2.6 above), in the published literature (Shanmugam 2012, his Table 2.2). Unless Van Loon et al. can define the fluid mechanics of each flow type and explain corresponding deposits in terms of sedimentary structures, this debate is futile.
3.2 A review article
Van Loon et al. (2019) assert that my paper is not a review article. They are correct in that my paper is not strictly a 100% review in the conventional sense. However, I published it as a review article in the Journal of Palaeogeography (JOPG) with the approval of journal editors and reviewers. The reason is that JOPG has only four categories of manuscript submission, namely 1) original article, 2) review, 3) brief report, and 4) academic discussion. My paper is a hybrid type, comprised of original contribution, review, and discussion. However, my paper can be better classified as a review category than as the other types.
3.3 Anthropogenic “hyperpycnal flows”
The main focus of the discussion by Van loon et al. (2019, their Fig. 1) is on a modern example of anthropogenic hyperpycnal flows. This example is totally out-of-place in this debate because I did not consider anthropogenic flows in my original paper (Shanmugam 2018a), where I only discussed natural flows. However, I am compelled to discuss anthropogenic flows in this reply.
Yang et al. (2018) claimed that hyperpycnal flows were associated with a dam built by human activity (i.e., anthropogenic). However, the concept of hyperpycnal is meant for natural river-mouth processes (Bates 1953), not for artificial flows associated with an anthropogenic dam.
In their Fig. 1 caption, Van Loon et al. claim that their Fig. 1 is from an article by Yang et al. (2018) published in Marine and Petroleum Geology (MPG) and from a website “www.quanjing.com“. But this figure is absent in the MPG article by Yang et al. (2018). Quanjing is a Chinese language website that also does not display their Fig. 1. So, what is the source of their Fig. 1?
The color difference between upper and lower layers could be due to any number of factors (e.g., differences in composition, grain size, sediment concentration, etc.). Hyperpycnal flows are defined on the basis of flow density (Bates 1953), not color.
Their frequent use of terms “hyperpycnites” and “hyperpycnal flows” in the comment by Van Loon et al. (2019) is problematic because the term “hyperpycnal flows” has never been defined (see Section 2.2 above). The reason is that 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 (Wang et al. 2010), cyclone-induced hyperpycnal turbidity current, multi-layer hyperpycnal flows, etc., see Shanmugam 2018a, 2018b), without an underpinning principle of fluid mechanics. Which one of the 16 types best represents the flow types shown in their Fig. 1? Why?
Their Fig. 1 with two layers of different colors could be due to differences in density. In fact, Gao et al. (2015) proposed density-stratified hyperpycnal flows. Did Van Loon et al. measure the density difference between upper and lower layers? Density-stratified flows are a source of major controversy in debating the origin of high-density turbidity currents (Shanmugam and Moiola 1997; Shanmugam et al. 1997).
What is the difference in flow state (turbulent vs. laminar) between two layers that is critical in distinguishing turbidity currents from debris flows? (Middleton and Hampton 1973; Middleton 1993; Middleton and Wilcock 1994)?
What is the difference in sediment concentration by volume between two layers that is critical in defining turbidity currents?
In summary, all designations of flow types must be based on principles of physics and empirical data.
3.4 Anthropogenic Elwha sediment plume, Strait of Juan de Fuca
Distinguishing hyperpycnites from turbidites or contourites is still a major challenge. Serious scientific discussions on hyperpycnal flows must be based on measured physical properties of natural flows, not on color difference between two layers in artificial conditions.
Despite the uncertain nature of flow types, an important lesson learned from the Elwha sediment plume is that external factors are critical in redirecting sediment transport. The deflection of Elwha plume to the east (Fig. 6c) could be attributed to tidal currents in this estuarine environment (Cannon 1978; Thomson et al. 2007; Warrick et al. 2011). Also, summer upwelling winds move easterly into the Strait of Juan de Fuca. Such summer winds could also explain deflecting sediment plume to the east of the Elwha River mouth. The reason is that winds reach a maximum speed of 8 m s− 1 off Vancouver Island with increasing magnitudes eastward in the Strait of Juan de Fuca (Foreman et al. 2008).
3.5 Tidal shear fronts and internal waves
Van Loon et al. (2019) defend the occurrence of hyperpycnal flows amid the presence of tidal shear fronts in the Yellow River in China. The reality is that tidal shear fronts do indeed serve as natural barriers for sediment transport by hyperpycnal flows. According to Wang et al. (2010), tide-modulated hyperpycnal flows are associated with internal waves (Wang et al. 2010). As mentioned earlier, baroclinic currents associated with internal waves and tides are one of the most controversial topics that resulted in spirited scientific debates (Shanmugam 2014a, 2014b). Van Loon et al. have avoided discussing these natural complexities.
3.6 Turbidite facies models
Van Loon et al. strongly believe in the origin of complete Bouma Sequence by turbidity currents, which is their prerogative. However, no one has ever generated the ubiquitous Bouma Sequence with five divisions by turbidity currents in a laboratory flume during the past 82 years, since the first experiment on density flows by Kuenen (1937). Nor has any one documented the complete Bouma Sequence from modern deep-sea sediments. And yet, according to Van Loon et al., there are thousands of examples of ancient turbidites in the published literature. Why? The answer is simple. The application of the turbidite facies model (i.e., the Bouma Sequence) to ancient sedimentary record guarantees a model-driven turbidite interpretation with a 100% success. For example, if one describes a deep-water sand as Ta, there is only one possible interpretation, which is that the Ta interval represents the basal part of a turbidite bed. In other words, one begins rock description with a turbidite facies model and ends up with interpreting the bed as a turbidite. This kind of logic is called ‘circular reasoning’.
3.7 Scientific documentation
They (Yang et al. 2017; their Fig. 8) used “dark shale” as a grain-size term. However, neither dark color nor shale is a grain-size term.
Conventionally, the term shale has been used for indurated lithofacies with fissility (Folk 1968, p. 31). Fissility is not a grain-size term.
Importantly, the term shale can represent different grain sizes, such as “clay-shale,” “silt-shale,” and “sandy-silt-shale” (see Table 2 in Folk 1968).
Yang et al. (2017) also used “tuff” as a grain-size term. However, tuff is a genetic term reserved for volcanic ash, not for grain size.
3.8 Copyright clearance
Van Loon et al. note that I used a figure from an article by Yang et al. (2017) without obtaining permission from the authors. The figure in question was published in the AAPG Bulletin. According to AAPG Bulletin Permission guidelines to authors, “If you want to use a single figure, a brief paragraph, or a single table from an AAPG publication in a paper in another publication, AAPG considers this to be fair usage, and you need no formal permission.” ‘Fair usage’ policy has been adopted by many international journals. For all other figures, I did obtain formal permissions from Copyright Clearance Center’s RightsLink (see figure captions for license numbers and dates in my article, Shanmugam 2018a).
3.9 Importance of satellite images
Van Loon et al. (2019) also criticize my article for using satellite images, which was a major objective of my paper (Shanmugam 2018a). I used satellite images as a proxy to fill the knowledge gap on sediment transport in marine and lacutrine environments (Shanmugam 2018a, 2018b). Ironically, Mulder et al. 2003, their Fig. 1) used an aerial photograph of hyperpycnal discharge in Lake Tanganyika (Tanzania) as the very first figure in their article on hyperpycnal flows. Such comments are hypocritical.
3.10 Flawed recognition criteria
Finally, in their conclusions, Van Loon et al. are disappointed that I used only one photograph of hyperpycnites in my 42-page long paper (Shanmugam 2018a). I admit that I could have used more examples. The reason that I used only one figure (Shanmugam 2018a, his Fig. 14b) was to illustrate the absurdity of interpreting a sediment as hyperpycnites without a reliable criterion. In that example, the authors (Yang et al. 2017) used an “erosional surface” within a depositional unit. The presence of an erosional surface within a single hyperpycnite depositional unit is antithetical to the basic principles of stratigraphy and sedimentation (Krumbein and Sloss 1963). It is sedimentologically meaningless to relate layers above and below an erosional surface, with a break in deposition in the middle, to the same process. My point was that there are no reliable criteria for recognizing hyperpycnites. Without criteria, one cannot recognize hyperpycnites. Without hyperpycnites, there cannot be photographs of hyperpycnites. Therefore, whether I used one or a thousand photographs is irrelevant!
Despite a deluge of incoming literature on hyperpycnal flows, our understanding of these flows is still muddled.
5 Methods/experimental study
A Mobil-funded experimental flume study was carried out at St. Anthony Falls Laboratory (SAFL), University of Minnesota (1996–1998) under the direction of Professor G. Parker to evaluate the fluid dynamical properties of sandy debris flows. Results were published in two major articles (Shanmugam 2000; Marr et al. 2001).
I thank Prof. Zeng-Zhao Feng (Editor-in-Chief of the Journal of Palaeogeography; China University of Petroleum, Beijing) and the Editor Yuan Wang for their detailed critical comments that greatly improved the quality of this paper. I wish to thank Editor Yuan Wang for her excellent editing. I also thank three journal reviewers for their comments. I thank Tom Roorda for giving permission to use his aerial photograph of spectacular sediment plume that was triggered by the demolition of Elwha Dam in 2012 (Fig. 6c) and the Elwha River mouth without sediment plume in 2019 (Fig. 6d). Tom Roorda also provided Meta Data for Elwha plume photographs. I thank D. W. Kirkland and J. E. Damuth for their comments on the usage of the word ‘Knowledge’. As always, I am thankful to my wife Jean Shanmugam for her general comments.
The author read and approved the final manuscript.
The author declares that he has no competing interests.
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