Reply to Shanmugam, G. “Review of research in internal-wave and internal-tide deposits of China:Discussion”

Zhen-Zhong Gao, You-Bin He, Xiang-Dong Li, Tai-Zhong Duan

1. School of Geosciences, Yangtze University, Jingzhou, 434023, China

2. School of Land Resource Engineering, Kunming University of Science and Technology, Kunming, 6500693, China 3. Petroleum Production and Research Institute of SINOPEC, Beijing, 100083, China

Abstract We are glad to know that our paper “Review of research in internal-wave and internal-tide deposits of China” (Gao et al., 2013) published in Journal of Palaeogeography has attracted close attention of international peers, and we noted that Shanmugam has provided a critical assessment of our paper. He claimed that “interpretations of ten ancient examples in China and one in the central Appalachians (USA) as deep-water deposits of internal waves and internal tides are unsustainable”, among many other comments and criticisms,and he concluded that “any interpretation of ancient strata as deposits of internal waves and internal tides is premature”. This article is aimed at responding to his major criticisms. We believe that constructive discussions will benefit the development of the study in internal-wave and internal-tide deposits. Unfortunately, Shanmugam seems not to follow this line of scientific criticism, but is trying to reject all research results in this subject based on his illogical and inconsistent reasoning, and distortion of others’ points of view. In this article, with facts and evidences, we will refuse the main wrong assertions of Shanmugam’s, for example, “the use of bidirectional cross-bedding as evidence for deposition by baroclinic currents in outcrop studies is sedimentologically erroneous ” and “any interpretation of ancient strata as deposits of internal waves and internal tides is premature”. We will also use typical characteristics of the ancient examples to demonstrate that they are certainly not turbidites, or contourites, or tsunami-related deposits, but internal-wave and internal-tide deposits as the most plausible to defend the rationality of our interpretation in our previous papers.

Key words internal-wave and internal-tide deposits, bidirectional cross-bedding, sedimentary succession, critical assessment

1 lntroduction＊

The study of deep-water clastic deposits, including gravity flow, contour currents, and internal-wave and internal-tide deposits, is currently a focus of interest in sedimentology. Compared to the study of gravity flows and contour current deposits, that of internal-wave and internal-tide deposits is a relatively new and under-explored field of research. In this relatively new research field, many issues still remain to be addressed further and in depth, and several important aspects need further improvements, as everyone would expect in a new research field. Even in almost matured oceanography do there ex-ist controversies and problems regarding the theory of internal-waves and internal-tides. Surely, the study of the ancient internal-wave and internal-tide deposits in the geological records, is expected to be filled with difficulties and controversies. We are thus pleased to see our paper, Review of research in internal-wave and internaltide deposits of China (Gaoet al., 2013), has been analyzed and discussed by Shanmugam (2014). We believe that constructive discussions will be beneficial to the development of research in internal-wave and internal-tide deposits.

Shanmugam discussed, queried and criticized the article (Gaoet al., 2013). Many of his basic points of criticism and comments regarding internal-wave and internal-tide deposits are exactly the same as in his other discussions (Shanmugam, 2013) on Pomaret al. (2012),or have nothing to do directly with our paper, such as his detailed explanation on basic concepts of internal-waves and internal-tides in his Section 3, and many comments on other work on turbidites, contourites, and other types of deposits. Pomaret al. (2013) have evidencedly demonstrated the illogical reasoning, distortion and inconsistency of Shanmugam’s critical points and comments,including his misunderstanding of basic physical processes associated with internal waves, his mechanical usage of uniformitarianism, and his baseless assertions.The exact same diseases have been repeated again in his discussion on our paper (Shanmugam, 2014). Therefore, we feel it is not worth our time and efforts further replying to those points. Here, we will discuss some of his questions and criticisms especially specific to our study on internal-wave and internal-tide deposits based on outcrops. We will focus our discussion corresponding to his sections 4-10, his abstract and his concluding section, while clarifying briefly our interpretation on internal-wave and internal-tide deposits in corresponding sections.

2 About “Lack of evidence for oceanic pycnoclines”

Although density interface or density gradient is necessary for the formation of internal waves (Duet al., 2011;Dykstra, 2012), the criteria have not been established yet to recognize pycnoclines in the stratigraphic record in the present study.

Shanmugam claims that “the supreme evidence for interpreting internal-wave and internal-tide deposits in the rock record is the physical evidence for oceanic pycnoclines”, and “The interpretations of ancient strata as deposits of internal waves and internal tides by Gao and his colleagues were not based on the ultimate evidence for pycnoclines”, therefore Gao and his colleagues’ interpretations “are unsustainable”. This reasoning is obviously improper, and illogical. Shanmugam made the exact same claims and comments on other author’s work (Shanmugam, 2013), and got a well-evidenced response from Pomaret al. (2013) that clearly shows Shanmugam’s assertion illogical and incorrect.

Shanmugam ignored that interpretation of grading beds as turbidites had lacked “physical evidence for turbidity current”, or hummocky stratification beds as tempestites lacked “physical evidence for a storm”. As Pomaret al.(2013) pointed out “…interpreting the breaking of internal waves through sedimentary structures within a rock succession implies recognizing the occurrence of pycnocline”,the same geological method in interpreting ancient strata has been used and proved useful in modern geology as in recognizing oceanic anoxia events, salinity crisis events,and so on. In the future, it may be possible to recognize paleo-pycnocline, if internal-wave and -tide deposits can be studied more and more first, without “the supreme evidence for oceanic pycnoclines” as Shanmugam claimed as necessary to study the deposits.

3 About “Distinguishing between internal-wave and internal-tide deposits”

Shanmugam claimed“Gaoet al. (2013) treat both internal-wave and internal-tide deposits as one and the same”.This claim is not true and is a distortion. Firstly, the authors are clear about the concepts of internal tides and internal waves, and never treat them as one and the same.Secondly, when interpreting the examples, we have tried to separate the internal-tide from internal-wave deposits if the evidence was found, such as the one in the central Appalachians. If the evidence cannot be found to separate internal-tide from internal-wave deposits, we have to treat them all as “internal-tide and internal-wave deposits”.“Seek the truth from facts”, this is absolutely scientific attitude. Because the period and frequency of ancient internal waves could not be obtained yet, we have to infer something just according to depositional features. Nevertheless, we agree with Shanmugam that it is an important area of future sedimentological research to distinguish between internal-wave and internal-tide deposits in the strata record.

4 Sedimentary structures (vs. Sections 6 and 7 in Shanmugam’s paper, 2014)

Regarding bidirectional cross-bedding, Shanmugam claimed “the presumed genetic link between bidirectional cross-bedding and internal tidal currents has not yet been established using modern analogs”, “the development of bidirectional cross-bedding in modern submarine canyons or channels by internal waves or internal tides has never been documented using sediment core”, and therefore, “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”. These assertive statements of Shanmugam are also totally illogical and distortional. We agree that the directions of propagation of internal waves are highly variable with respect to the shoreline, the shelf edge, and the channel axis, this does not exclude that we can recognize the deposits formed by internal-waves and -tides that do propagate updip continental slope or along submarine canyon and channels, and interact with the seafloor. If Shanmugam paid a little more attention on Gaoet al.(2013)’s review and the papers referred, he could have found that all 11 examples were from submarine canyon/channel or slope settings, the interpretation were integrative, and we never claimed that bidirectional crossbedding is the only criterion to identify internal-wave and -tide deposits.

1) About “bidirectional current directions of baroclinic currents associated with modern internal-waves and internal-tides are still murky”, this is obviously inconsistent with the facts. The research team of F. P. Shepard carried out a large-scale submarine survey and measurements in 1970s. They observed bidirectional tidal currents in many submarine canyons (Figure 1). The cycle is semidiurnal tide or diurnal tide and same as the local surface tidal cycle. Importantly, the phase is different. The depth of these bidirectional tidal currents can be up to several kilometers. These periodic bidirectional currents occur at certain depth, not every depth. Therefore these are internal-tides, not surface tides. Clearly it shows that“published empirical data” indicate “bidirectional currents were associated with modern internal-waves and internal-tides”. These bidirectional currents, with their velocity measured at a maximum of 20 cm/s, certainly would be able to generate ripples or small dunes. Though not observed, it is not hard to postulate from process sedimentology that bidirectional cross-bedding can be formed in this type of environment.

2) Although Shanmugam’s statement “the presumed genetic link between bidirectional cross-bedding and internal tidal currents has not yet been established using modern analogs” may be still true at the present day, this certainly cannot be the reason to reject bidirectional cross-bedding as discriminating characteristics of internal-tide and internal-wave deposits.

Currently our knowledge on the deep ocean is still limited, including the modern deep-water deposits. Different from the study of the shallow water deposits, only a few sedimentologists have observed and studied modern deep water deposits. Overall, we know very little about it. However, we not only believe in “the present is the key to the past”, but also “the past is the key to the present” in sedimentology or geology in general. Both philosophies or methodologies have long been proven successful in geosciences (extended uniformitarianism).

The understanding and interpretation of the sedimentary phenomena do not always start from the modern sediments, then to ancient rocks. For instance, in the study of hummocky cross-stratification, it is intensive study in ancient strata that helped postulate their origin related to storms, without modern sediment core-based analog study first (Walker, 1985). There are many similar examples to mention in geology. Therefore, it is not proper for Shanmugam to deny all research results just because the research is only based on the stratigraphic record without through a modern sediment core study to validate it first.

Shanmugam said “the use of bidirectional cross-bedding as evidence for deposition by baroclinic currents in outcrop studies is sedimentologically erroneous.”Any interpretation of the stratigraphic record is an integrative task. So in deep-water environments, especially in canyons or channels, it is the most plausible interpretation that these bidirectional sedimentary structures represent deposits from internal-tides and internal-waves. Turbidity currents, contour currents, wind-driven flows or the storm flows can be excluded obviously. Are there any other types of currents that can create bidirectional cross-bedding in deep-water environment? We do not know of any yet. Obviously, in terms of the knowledge we have in the deep ocean environment, internal-tides and internal-waves are most suitable processes producing these bidirectional sedimentary structures. The simple denial by Shanmugam is not convincing.

Figure 1 Time-velocity curves of alternating up- and down-canyons (bars), showing a clear relationship between the current and the tide (solid curve). A-Station 28 in Hueneme Canyon, depth 448 m, 3 m above the bottom; B-Station 123 in San Clemente rift valley,depth 1646 m, 3 m above the bottom (modified from Shepard et al., 1979).

To further clarify our opinions, the typical characteristics of internal-wave and internal-tide deposits are briefly summarized as below. Internal-wave and internal-tide deposits are well developed in diverse environments of deep-water ocean. The grain-size is rather fine, such as very fine sand to medium sand with minor coarse sand in internal-tide deposits in submarine channels. They contain specific directed structures: bidirectional cross-lamination or cross-bedding. And unidirectional cross-bedding, flaser-, wavy- and lenticular bedding can usually be observed. In different environments and hydrodynamic conditions, their sedimentary characteristics and formation mechanism are different, forming different vertical successions. Interpretation is integrative, either bidirectional cross-laminations, or other traction structures are only part of the evidence.

5 About “Vertical facies models”

Shanmugam seriously criticized vertical facies models that the authors summarized and proposed 11 points to question them. Due to their irrelevance and our limited space, we just discuss one of them.

Shanmugam thought “What is so unique about the upward-coarsening trend with bidirectional cross-bedding?For example, upward-coarsening trends with bidirectional cross-bedding 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)”.

The first thing we should know is that this question is a serious distortion, because there is no “upwardcoarsening trend with bidirectional cross-bedding” in internal-tide and internal-wave deposits. In our article,there are just “a coarsening-up and then fining-up succession maybe with bidirectional cross-bedding”(Gaoet al., 2013). Therefore, we cannot answer such question that never exits. There are two types of coarsening-up and then fining-up succession (Figure 2). One is a coarsening-up and then fining-up succession (bidirectional graded succession) (Figure 2a, 2b). The basic feature of coarsening-up and then fining-up succession is that the dominant grain size is fine sand, and the coarsest part is located in the middle, where the grain size decreases both upwards and downwards (Figure 2a, 2b).These features suggest a weak-＞strong-＞ weak hydrodynamic condition (Gaoet al., 2000), which is closely related to the period of internal-wave and internal-tide.Another is a coarsening-up and then fining-up succession with couplets of sandstone and mudstone (bidirectional graded couplet succession) (Figure 2c), which is usually developed in very gentle and open areas. In this environment, the velocities of bidirectional currents caused by internal-waves and internal-tides are less than in channelized environments, but the slack-water periods between current reversals are longer. Mud layers deposited from suspension during the “tidal stillstand”and sand layer deposited during the “flood tide” or “ebb tide” are interbedded and preserved. And because of the control exerted by the longer period, these frequently alternating beds also display symmetrical graded couplet successions (Figure 2c) (Gaoet al., 1998). This is a vertical succession that is controlled by double periods of internal-waves and internal-tides.

Maybe Shanmugam raises the question unintentionally, but should not he read the article carefully before he criticizes it? Should not he make self-examination?Nevertheless, our vertical facies successions are simply summarized from our 11 real working examples only,not a big dataset. This means they may not be necessa-rily applicable to other regions and settings, but they are real existence, and can be used as a basis for comparative study.

Figure 2 Coarsening-up and then fining-up successions of internal-tide and internal-wave deposits (simplified from Gao et al., 2013).a-Coarsening-up and then fining-up succession consisting of cross-laminated sandstone; b-Coarsening-up and then fining-up succession consisting of medium-scale and small-scale cross-laminations; c-Coarsening-up and then fining-up succession consisting of sandstone and mudstone couplets.

6 About “The interpretation of ancient strata by Gao and his colleagues were made without validation from modern analogs”

Shanmugam stated “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 report yet on the study of modern internal-tide and internal-wave deposits is one of weaknesses in this study subject, but we should not stop explaining the deposition in the stratigraphic record just for this reason. We cannot deny its existence in the stratal record just because we have not found its analog in modern environments. For example, sedimentological studies suggest that the epeiric sea is recognized as being developed well in ancient records, purely postulated from the carbonate rock study without typical modern analog since the modern carbonates are developed almost all in marginal seas. The typical modern epeiric sea has not been found yet (Feng, 1982; Heckel, 1972).Does it mean carbonate deposition in ancient epeiric sea does not exist at all? Why should the phenomenon in the stratigraphic record be interpreted only after modern analogs are found? Depositional characteristics of sedimentary rocks (mainly the texture of rocks and sedimentary structures) can be explained by using the principles of sedimentology. For instance, flute casts often appear in turbidites and they can indicate flow directions; the sedimentary structures in contourites can point to the flow directions along the slope direction.Deep-water deposits with bidirectional cross-lamination (up- and down-slope dipping) obviously have been influenced by currents flowing in up- and down-slope directions. It would be the most plausible if these deposits are explained as internal-tide deposits according to sedimentary theory. For example, Cacchioneet al.(1988) reported internal-tides and sediment movement on Horizon Guyot, Mid-Pacific Mountains. Bádenaset al. (2012)also reported a facies model for internalites(internal-wave deposits) on a gently sloping carbonate ramp (Upper Hurassic, Ricla, NE Spain). These obviously follow a similar rational line doing scientific research in geosciences.

7 Distinguishing internal-wave and internal-tide deposits from others in deep water settings

Shanmugam claimed that in these examples from our article, “Real potential exists for misinterpreting deep-marine baroclinic sands as turbidites, contourites, tsunamirelated deposits,etc.” and said “any interpretation of ancient strata as deposits of internal-waves and internal-tides is premature”.

Shanmugam not only lacks relevant geological field trips, but also ignores the description of these typical examples of ancient deposits. Here we do not see any evidence on which Shanmugam was based on to have made his assertion. We have no way to prevent Shanmugam from “misinterpreting deep-marine baroclinic sands as turbidites, contourites, tsunami-related deposits,etc.”.However, we think we have demonstrated our 11 examples of internal-wave and internal-tide deposits distinguished from other deep-water deposits such as turbidites and contourites.

The following descriptions in our paper are some of the basic features of 11 internal-tide and internal-wave deposits that had been found. Everyone can judge to see if they are the features of turbidites, contourites or storm deposits,or they are matching the flow features of internal-tides and internal-waves.

To these deposits formed in deep water sedimentary environments, should the bidirectional cross-beddings be the feature of turbidites, contour current or storm deposits?

On the vertical sequence in terms of these deposits, can we interpret “the coarsening-up and then fining-up succession consisting of cross-laminated sandstone” as the result of turbidites, contourites, or storm deposits (Gaoet al.,2013, Figure 4A)?

Can coarsening-up and then fining-up succession consisting of sandstone and mudstone couplets occurred in deep-water deposits, be interpreted as any other deep-water deposits, but as the internal-tide deposits (Gaoet al.,2013, Figure 4B)?

Flaser bedding, wavy bedding and lenticular bedding occurring in deep-water deposits are the special sedimentary structures of turbidites, contourites, or storm deposits,or more probably related to deep water tide action?

As far as we understand, to our existing knowledge,it is most plausible that these deposits be interpreted as originating from internal-tides and internal-waves. It is too assertive to say “any interpretation of ancient strata as deposits of internal-waves and internal-tides is premature”. Is Shanmugam trying to impede advance of science in internal-wave and internal-tide deposits by rejecting any research in this subject?

8 Conclusions

1) Shanmugam not only ignored the detailed description of characteristics of 11 ancient examples from our study of internal-wave and internal-tide deposits, but also lacked relevant field geological investigations. Just based on 332 published references he claimed to have read, he denies all the research findings of internal-tide and internal-wave deposits that dozens of professionals have worked on over the last 20 years. And he even said “any interpretation of ancient strata as deposits of internal-waves and internaltides is premature”. All the conclusions he made are lack of rationale.

2) All typical features of the 11 ancient examples (Gao and Ekisson,1991; Gaoet al.,1996, 1997; Heet al., 1998,2003, 2005, 2007, 2011; Jinet al., 2002; Guoet al., 2004;Liet al., 2009), including bidirectional cross-bedding,flaser-, wavy-, and lenticular bedding, coarsening-up and then fining-up succession consisting of sandstone and mudstone couplets, coarsening-up and then fining-up succession consisting of cross-laminated sandstone, cannot be explained by the features of turbidites, contourites, or storm deposits. These features are instead compatible with hydraulic characteristics of internal-tides and internalwaves. So it is proper or most plausible to interpret them as internal-tide and internal-wave deposits in the deep-water settings.

3) The study of internal-tide and internal-wave deposits is a relatively new research subject of sedimentology. So far it only has over 20 years of history. Compared with gravity flow deposits and contourites in deep-water settings, the research in internal-tide and internal-wave deposits is just beginning and the examples are quite limited.Thus, this is an area that needs to be strengthened. Currently, we should enhance the discovery and research of recent sediments, but survey in deep ocean is very difficult and expensive. This should be one of the important reasons that we do not know much about the modern deposits of internal-tides and internal-waves. It was mentioned in our original paper that only when we combine the study of internal-tide and internal-wave deposits in the stratigraphic record with the research of the modern deposits and flume experiments, can we make the research more complete and fruitful. Nevertheless at the present stage, we should not try to deny all the research findings of internal-tide and internal-wave deposits from the stratigraphic record, just because it lacks modern deposition examples. What we should do is to improve it gradually.

Acknowledgements

This research was supported by the China National Natural Science Foundation (Nos. 40672071 and 41072086)and the Research Fund for the Doctoral Program of Higher Education in China (No. 20104220110002). We express our sincerest gratitude to Prof. Zeng-Zhao Feng and an anonymous reviewer for their constructive comments.