|Dear SE science editor,|
I have reviewed the Mignard et al. manuscript about the Ogooue Fan offshore Gabon. I found the data of great interest and some of the conclusions regarding 1) topographic control on depositional architecture, and 2) the control of sea level on canyon-fan activity very interesting. I have numerous minor to moderate recommendations for revision. In general:
--The authors throw a lot of data, descriptions, and interpretations into this paper. It might be a good idea to break this into multiple papers, and potentially focus this paper on a single interpretation and insight. For example, the topographic control on depositional architecture and geomorphology is interesting and could be the focus. Alternatively, the idea that canyon-fan systems are active at both high and low sea level is interesting and might be the focus of the paper.
--The manuscript is vague and ambiguous in places. What is “subtle?” How slow is “very slow?” I pose numerous questions for the authors to address in my line-by-line review below.
--I think that the authors could better justify the novelty and importance of their work. For example, see lines 73-75. Why exactly does the impact of subtle changes in slope on fans still need to be understood? What’s the outstanding problem the authors are addressing? How is this study different/special compared to any of the other studies describing fans?
--The authors seemed to mix observations and interpretations in the results section. I recommend that they clearly provide observations and wait until the interpretation and discussion section to interpret processes from bathymetry, echofacies, and core facies.
--The authors need to elaborate some of their interpretations. Some were vague to me and some interpretations seemed contradictory. See specific comments below starting around the interpretation and discussion section starting at line 411.
--Some grammatical errors I tried to point out. I recommend a more thorough revision of these and other errors I might have missed.
Detailed line-by-line review
Line 9. What does “important” mean? What constitutes an “important” change in slope gradient?
Line 11. How has velocity been constrained/documented in ancient environments? I am not aware of these studies.
Line 11. Is half a degree really “subtle?” I recall that gradients around half a degree correspond with the development of swift gravity flows and supercritical bedforms, like antidunes and cyclic steps.
Postma, George, and Kick Kleverlaan. "Supercritical flows and their control on the architecture and facies of small-radius sand-rich fan lobes." Sedimentary Geology 364 (2018): 53-70.
Line 18. Do the authors mean “Detailed mapping of a near-seafloor seismic dataset...?” Also, are these data seismic-reflection data or what? Probably indicate -reflection.
Line 21. Why is apostrophe before “sedimentary load” on this line?
Line 24. “... more complex fan architecture...” compared to what? More “complex” than what?
Line 27. “most energetic turbidity currents...” How do the authors know the relative “energy” of the turbidity currents?
Line 30. Should be a semicolon at the end of this line (replacing a comma).
Line 31. Replace with “... turbidite sands are preferentially deposited on the floor of the channel...”
Lines 32-33. How do the authors know that distal depocenters received only the upper, fine-grained parts of flows? How do they know the flows themselves that tend to reach the distal depocenters were not simply relatively fine grained overall?
Line 34. No need to hyphenate sea level. It is not a compound modifier in this context.
Line 36. Just because canyon heads are separated from the coast does not rule out submarine mass movements at highstand.
Line 38. Remove “the” in “the longshore drift.”
Line 48. Replace “first” with “early.”
Lines 51-54. Rephrase: “However, studies of modern and ancient fans showed...” Also, what’s the different between “modern” and “ancient?”
Mutti, Emiliano, and William R. Normark. "Comparing examples of modern and ancient turbidite systems: problems and concepts." In Marine clastic sedimentology, pp. 1-38. Springer, Dordrecht, 1987.
Line 56. Mini-basin should be plural.
Line 61. What does “successive depocenters” mean?
Line 62. What are “stepped slopes” (also, no need to hyphenate; revise this erroneous hyphenation throughout)?
Line 68. What exactly is “very slow?”
Line 71. What’s “subtle?”
Line 72. “... deep-sea fan organization...” or “... the organization of deep-sea fans...”
Line 73. No evidence for a “growing number of studies describing these systems.” How do you know the number is growing?
Lines 73-75. Why does the impact of subtle changes in gradient need to be better understood? As an aside, I’d replace “apprehended” with “understood.”
Line 82. I wouldn’t say “poorly studied.” It just hasn’t been studied. That’s different than if there were studies, but they were poor (i.e., not useful/erroneous).
Lines 88-93. I understand the objectives of the paper, but why is this of broad scientific importance or interest? Lines 73-75 state that subtle (how subtle is subtle?) changes in gradient need to be better understood, but why? The authors need to better highlight the significance of this work. If they think gradient needs to be better understood, they need to explain why. Convince the reader this study is worth reading. Furthermore, how exactly can this study be used to develop predictive models for stepped slopes? Finally, one degree is not a small slope gradient. I recall that continental rises, across which many submarine fans form, are less than a degree. Two degrees is about as steep as passive margins get. Perhaps see the following references of slopes of continental slopes and rises for context:
O'Grady, Damian B., James PM Syvitski, Lincoln F. Pratson, and J. F. Sarg. "Categorizing the morphologic variability of siliciclastic passive continental margins." Geology 28, no. 3 (2000): 207-210.
Pratson, Lincoln F., and William F. Haxby. "What is the slope of the US continental slope?." Geology 24, no. 1 (1996): 3-6.
Figure 1. Where is the path of the longitudinal profile in part b? Why are the color scales for the bathymetry different (one very blue, another brown to yellow to green). I’m also not a fan of the brown-yellow-green color map for bathymetry. The authors probably want a single hue sequential color map. They should consult this amazing website for improvement: http://colorbrewer2.org/. What is the scale of the topography in part a? The figure overall seems a bit grainy and low resolution. It would be nice to sharpen up especially the bathymetry of the Ogooue turbidite system.
Line 101. “... within the Gondwana craton...” or simply “... within Gondwana...”
Line 102. Syn-rfit deposits are probably overlain by sedimentary rocks, not sediments.
Line 107. Canyon, singular, incision.
Line 109. Sediment, singular, depocenters.
Line 110. Replace or with and? Is it possible all these rivers delivered sediment to canyons? Not one or the other?
Line 113. Developed, past tense (not develops).
Line 114. Deep? How deep is deep? Is this referring to present water depth or paleowater depth?
Line 119. Replace “realized” with another verb.
Line 119. Replace “evidenced” with another verb. Awkward sentence.
Line 127. Provide reference for the concave up slope profile of other passive margins. Might I suggest this or a reference therein (see also aforementioned O’Grady and/or Pratson and Haxby references):
Covault, Jacob A., Eitan Shelef, Miles Traer, Stephen M. Hubbard, Brian W. Romans, and Andrea Fildani. "Deep-water channel run-out length: Insights from seafloor geomorphology." Journal of Sedimentary Research 82, no. 1 (2012): 21-36.
Line 127. Seven degrees is very steep. Where exactly are these measurements coming from? Within the canyon-channel system of the fan or the regional margin slope?
Line 136. Do the authors have a figure to illustrate this sinuous train of pock marks?
Line 138. Rephrase: “... is the third largest...”
Line 142. “... low slope gradient...” How low?
Line 145. How do lakes contribute to muddy composition?
Line 147. See this for the river’s sediment load:
Milliman, John D., and Katherine L. Farnsworth. River discharge to the coastal ocean: a global synthesis. Cambridge University Press, 2013.
Lines 147-150. I do not understand how the Bateke Sand controls the muddy sediment load.
Lines 152-153. Explain how waves transport sediment northward. Are these longshore currents? How are they causing sediments to be transported northward?
Line 155. Remove “of” in “of 50 km long.”
Figure 2. Why not make the bathymetry in part a the same color as in figure 1? Also, the authors should avoid a rainbow color bar like this. Please see articles like this explaining bias associated with a rainbow color bar: https://www.climate-lab-book.ac.uk/2014/end-of-the-rainbow/. These are too low resolution. The authors need to make these more clear/higher resolution. I recommend that the authors make bathymetry color bars the same in all figures.
Figure 3. This is nice, but it’s yet another bathymetry color bar. However, this is the best color bar yet. Perhaps use this same color bar for all bathymetric images?
Figure 4. The green letters against the pink to purple boxes of backscatter facies are hard to see. Choose a color like white or black or move the letters out of the pink/purple boxes.
Line 191. Is echofacies plural? I think so, because the authors define multiple. If so, rephrase: “... the 3.5 kHz echofacies have been...”
Line 203. The authors need to rephrase and correct erroneous punctuation: “... Mignard et al. (2017), and new AMS...”
Line 208. I believe that the authors mean to say “medial and distal parts of the system.”
Line 209. Datings should be dates.
Line 209. Need a different verb than “realized.” Awkward English in this context.
Lines 210-212. I am unsure, but I think it might not be necessary to apply a reservoir correction if you’re calibrating the C14 dates. The authors need to explain why it is necessary to add a reservoir correction if the dates have already been calibrated.
Line 228. No commas after authors. This is a common error throughout the manuscript and the authors need to revise it.
Lines 224-257. Recommend simply put all this information into a table. A lot of the explanation of the facies is redundant with the name of the facies.
Figure 6. Too low resolution. Difficult to see features in the gradient map. Make this bigger and/or increase the resolution.
Line 287. Why is westward capitalized?
Line 288. Is it common to measure sinuosity over 2 km long segments like this? If so, provide a supporting reference.
Line 301. No need for comma after “depth” and before “presents.”
Line 314. I do not understand the sentence “... with no scar of down-flank mass deposits.”
Lines 331-333. This long sentence is difficult for me to follow. I recommend that the authors rewrite for clarity, potentially breaking the sentence into two.
Figure 10. What is the vertical exaggeration of this perspective image?
Figure 11. The authors describe reflectors, but I believe that they mean reflections: see https://www.glossary.oilfield.slb.com/en/Terms/r/reflector.aspx versus https://www.glossary.oilfield.slb.com/en/Terms/r/reflection.aspx.
Lines 346-410. This section on “echofacies” seems to be a results section, but there is a lot of interpretation mixed in. The authors should make more of an effort to clearly separate observations from interpretation. They have an “interpretation” section starting on line 411. They should try to put their interpretations in that section.
Line 358. I’m confused how an echofacies can change along an isobath (line of equal depth) and slope can increase. Isn’t slope consistent along the isobath? Perhaps the sentence could be rewritten for clarity? Also, what’s the difference between echofacies and acoustic facies sensu https://onlinelibrary.wiley.com/doi/abs/10.1046/j.1365-3091.1999.00203.x? Is there a difference?
Lines 364-366. I do not understand the evidence to support the interpretation that waves in echofacies I’ are a result of post-depositional deformation. First, what is the evidence? Second, exactly what type of post-depositional deformation? Third, how can the authors rule out some kind of current reworking?
Line 368. I believe here and throughout the authors mean “reflection” not “reflector.”
Line 372. The authors need to explain why echofacies III is coarse grained. How do they know? I thought they lacked groundtruth samples (see their statement on lines 367-368 about the lack of sampling).
Lines 373-374. Why are hyperbolic “facies” certainly due to irregular/steep seafloor? Explain.
Line 377. Remove “respectively.”
Line 380. Why is echofacies hyphenated here and not throughout? The authors need to be consistent.
Lines 380-381. Exactly what mass flow processes resulted in structureless deposits? The authors need to explain.
Line 384. What do the authors mean by “numerous detrital debris?”
Line 393. What exactly is “rough echo character?” Moreover, I do not understand how a rough echo character can have a “specific organization” comprising multiple aggradational stacked transparent units. This sentence is unclear to me.
Line 395. Lobes should be singular, lobe.
Line 397. Why do the authors say several decimenters, plural, but several meter, singular? They should be consistent here and throughout.
Line 399. “Bedded-rough echofacies” seems awkward to me. Rephrase.
Lines 406-407. The authors need to explain the origin of hyperbolic facies from steep slopes/topography.
Line 410. Sliding should be interpreted on the basis of the internal deformation of the deposit, not the limited area of coverage.
Line 411-end. The interpretations in the “sedimentary processes along the fan” have a lot of basic observations. For example, lines 534-539 are predominantly observations... Have the authors considered putting the observations in this section in the preceding results section?
Line 413. The authors need to consult Piper and Normark (2001: AAPG Bulletin), who show the importance of sandy/muddy architectural elements of submarine fans, rather than simply characterizing entire fans as muddy or sandy. The bottom line is that many (all?) fans have sandy and muddy architectural elements, whether they are classic “muddy” fans like the Amazon, or classic “sandy” fans like the La Jolla or Hueneme(sp?). I recommend that the authors avoid characterizing entire fans as sandy or muddy.
Line 416. Replace sediment processes with sedimentary processes.
Line 434. Canyons should be singular, canyon. Otherwise, the authors need an apostrophe, canyons’.
Lines 433-440. I do not understand the difference between mass wasting in type 1 canyons and failures/headward erosion processes in type 2 canyons. Aren’t failures related to mass wasting?
Line 438. The authors say that type 2 canyon morphology is related to depositional processes (hemipleagic settling and dilute turbidity currents). That said, they stress that these canyon systems are predominantly erosional and reflect erosional processes (lines 425-426). The processes of type 2 canyon formation seem to contradict the earlier assessment of these canyon systems on the upper slope (lines 425-426).
Lines 443-444. I do not understand how fine-grained sedimentation created sinuous pockmark trains. Explain.
Lines 447-450. I do not understand how the development of the Mandji Island favored the construction of the Cape Lopez type 1 canyon.
Line 452. Why use “extrernal levee” terminology? A levee is a levee. Do these channels have any other type of levee (i.e., internal levees)?
Lines 452-454. Why did decreasing slope promote the development of levees? What is the process control on levee formation?
Lines 455-456. What is the evidence that the channels are erosive? How do the authors know the channel confinement is not simply a result of levee growth?
Line 459. Rephrase: “... turbidity currents frequently overspill.” The authors need to explain why high reflectivity indicates frequently overspilling turbidity currents.
Line 466. Rephrase: “... and inhibits channel avulsion.”
Line 468: Rephrase: “... to allow transport of sediment...” Also, how exactly does entrenched morphology of a channel keep energy high?
Line 475. Should be channel, singular, at the end of this line: channel system.
Line 478. Reduces the erosional power of what? Unclear. Presumably: “... reduces the erosional power of the turbidity current?”
Lines 488. I do not understand what is meant by “bedded rough to rough echo-facies.” These are types of facies, right? But what does “bedded rough” mean exactly? Also, again, echofacies should not be hyphenated to be consistent with the majority of the manuscript.
Lines 498-501. I do not understand how the authors know that the upper valley migrated upstream by retrogressive erosion. Perhaps clarify?
Lines 503-505. I do not understand this sentence.
Lines 511-512. What are lobe elements? Perhaps refer to work by Prelat to explain.
Line 525. What do the authors mean by “same scheme?” Do they mean “pattern?”
Lines 527-529. I do not understand the evidence for repeated spillover of the fine-grained tops of turbidity currents. The authors need to explain.
Line 536. What do the authors mean by “scheme?”
Lines 537-538. Rephrase: “... caused by Mount Loiret...”
Line 556. No need to hyphenate “stepped slope.”
Lines 560-562. Recommend citing Deptuck et al. (2012: SEPM) also.
Line 568 and throughout. I believe that the authors mean to cite Deptuck et al. (2012), not Deptuck (2012).
Line 569. They (not he).
Line 571. Sediment, singular.
Line 572. Sediment, singular.
Lines 579-582. This is a bit of an awkward sentence. Rephrase, particularly these bits: “conversely... constitutes additional stresses.”
Lines 592. How is salt tectonic activity weak?
Lines 598-664. This section seems redundant with the preceding interpretations and results. Perhaps it could be shortened or removed and aspects could be included in the initial interpretation section “Sedimentary processes along the fan.”
Lines 665-704. This is interesting. Consider focusing the entire paper (or another paper) on this aspect.
Line 668. No hyphen in “relative low sea level.” Sea level is not a compound modifier. Revise here and throughout.
Lines 670-672. Comma after e.g. and no parentheses within parentheses.