the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Strato-structural evolution of the deep-water Orange Basin: Constraints from high-resolution 3D seismic data
- 1School of Geosciences, University of the Witwatersrand, Johannesburg, PBag 3, WITS, 2050, Republic of South Africa
- 2Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario, K1A 0E8, Canada
- 1School of Geosciences, University of the Witwatersrand, Johannesburg, PBag 3, WITS, 2050, Republic of South Africa
- 2Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario, K1A 0E8, Canada
Abstract. We use high-resolution 3D reflection seismic data to constrain the strato-structural evolution of the transitional and compressional domains of a Late Cretaceous deep-water fold-and-thrust belt (DWFTB) system and its influence on the overlying Cenozoic megasequence in the Orange Basin, South Africa. Multiple shale detachment surfaces have given rise to a complex structural framework, allowing for the redistribution of stress and strain, along progressive deformation and sedimentation. High-resolution 3D seismic data show that the compressional domain exhibits large-scale landward-dipping DWFTBs with faults initially detaching the Turonian shale detachment surface. Thrust sheets are segmented along strike by extensive oblique-slip faults which extend from the transitional domain into the down-dip compressional domain. the transitional domain is imaged as a complex region containing listric normal, thrust and oblique-slip faults. Many faults in the transitional domain have been reactivated to detach onto an older Albian shale detachment surface at depth, transferring stress and distributing strain during gravitational sliding. Smaller, localized fold-and-thrust belts are subsequently formed in the down-dip compressional domain directly below the kilometre-scale DWFTB system between the upper Turonian and lower Albian shale detachment surface. Mass erosional processes of the Cenozoic are confined above the transitional domain including a large, roughly slope-perpendicular Oligocene submarine canyon formed by turbidity currents, and a smaller series of slope-parallel, sinusoidal channel-like features in the Miocene formed by bottom currents. Normal and oblique-slip faults from the transitional domain have been reactivated to terminate at either one of the Oligocene or Miocene sequence boundaries. This, together with a present-day seafloor slump scar above a buried Late Cretaceous syncline, indicates how the stratigraphy and structural geometry of a buried DWFTB system controls fundamental sedimentary processes in an evolving continental margin.
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Nombuso Gladys Maduna et al.
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2022-180', Anonymous Referee #1, 02 Jun 2022
Using a set of 3D seismic reflection data this manuscript aims at constraining the strato-structural evolution of the transitional and compressional domains of a Late Cretaceous deep-water fold-and-thrust belt system and its influence of the overlying Cenozoic megasequence in the Orange Basin offshore SW Africa.
The writing, wording and phrasing very often is unclear. The authors often are not precise and leave room for misunderstandings. It, e.g., remain unclear what improvement in scientific knowledge will be achieved by the study. The authors cite a large amount of literature describing the tectonic setting and development (see, e.g., Seismic stratigraphy and Results and interpretation). The interpretation of sequences and stratigraphic markers is mainly a set of statements, there is no discussion. Most of the facts presented are cited from other studies – so what is new? This is really difficult to identify.
Furthermore, the authors are very focused on the tectonic influence on sedimentation and deposition. They large ignore the effect oceanic currents and water mass transport have on this. The authors only briefly venture in this direction but in an oversimplified way and they appear to lack an understanding of physical oceanography in general and of this region in particular.
The authors speak of high-resolution seismic reflection data. However, the data were collected with a sample rate of 2 ms (see Table 1, Nyquist frequency 250 Hz) and then resampled to 4 ms (see Table 2, Nyquist frequency 125 Hz). This certainly is not high-resolution! High-resolution seismic reflection data should be recoded at minimum 1 ms sample rate giving a Nyquist frequency of 500 Hz. And where is the use of lowering the vertical resolution of seismic data by resampling?
The paragraph on Seismic resolution is very simplistic and, in parts, wrong. A reputable reflection seismologist uses l/2 and not l/4 (or even l/8) to compute the vertical resolution as l/4 is a very theoretical value. I would like to see a spectrum of the seismic data since I seriously doubt that the dominant frequency observed is 50 Hz; please, provide evidence for this! Even so, the Fresnel Zone definitely is not half of the dominant wavelength but dependent on target depth! Please, see Yilmaz (2001), P. 1803. There the formula is given and states that . So, the Fresnel Zone is dependent on target depth z and the main frequency. So, even if we take the main frequency to be 48 m (serious doubts here) the Fresnel Zone in 1000 m water depth will be 155 m and in 2000 m water depth it will be 220 m. So, the assumptions of the authors of a constant Fresnel Zone of 24 m are wrong!
The data have undergone severe processing applying several deconvolution techniques, resampling, attenuations, and time variant scaling (see Table 2). This all may have enhanced the signal but it also destroyed the true reflection amplitude. Still, the authors interpret variations in reflection amplitude both laterally and vertically (e.g., line 176, 196, 212). This, however, is no longer possible.
There is more evidence that the authors are not on top of the seismic method. They have labelled the twoway travel time negative. That implies that the data have been recoded before the shots were trigger. This is nonsense (sorry for being so blunt). It is neither the habit to label depth in negative numbers. If you want to make clear that this is below seafloor, please use mbsf (metres below seafloor).
The authors use seismic attributes to aid their interpretation. They state that ‘seismic attributes are designed through mathematical manipulation’ but they are not designed but computed. And I hope the data have not been manipulated (even though the severe processing certainly manipulated the data).
Taking all this into account I have huge doubts about the interpretation of the seismic reflection data presented by the authors since they do not appear to have fully understood the concept and physics of the method or the possibilities and limits.
The interpretation of the data is presented before this is discussed. To me this appears awkward.
Little information is provided on the lithology. The authors just state that, e.g., a) the Albian surface represents a maximum flooding surface (how do they know this?) and b)is a shale detachment surface. This is just one example. What are the arguments for those interpretations? What knowledge is used and not presented?
Section 4.1.3 how does especially the younger Cenozoic tie in with other studies from this margin, e.g., (Hopkins and Cartwright, 2021; Weigelt and Uenzelmann-Neben, 2004, 2007a, b)? The discussion here is very centred on tectonically influenced sedimentation. It may be useful for the authors to broaden their reading.
Section 4.2.1 is very focused on complex attributes. In my opinion the authors got a bit carried away by the figures of the complex attributes and lost their feeling for resolution and other limits.
The whole discussion is a repetition of other people’s work.
Section 5 to me does not show any advance in scientific knowledge.
It would be helpful if the described features all were marked in the figures.
The References needs checking. For several references journal name, etc is missing, some references are listed twice.
Jungslager, 199 is missing from References
Figure3: Seismic depth limit is supposed to mean maximum penetration? Annotate the seismic markers discussed in the text please with the numbers used in the text
Figure 4 is obsolete
Figure 5: How were the type of the faults and the throw direction identified? Certainly not based on Fig 5a, which is rather chaotic. Travel time can never be negative! Show coordinates
Figs. 6-8 TWT never is negative! Same applies to depth; if needed use mbsf
Figs 9-12 are extremely confusing and not helpful at all. Please, omit
Figure 13: How were the ages assigned? Where does the anticline in 13d originate? Cold water usually has a higher density than warm water (annotation 13e)
Line 40 The authors state that this margin is largely underexplored since there is only one well per 4000 km2. This is meant regarding hydrocarbon exploration? Extensive sets of seismic reflection data have been collected along the margin and several scientific sites have been drilled. So, regarding the development of the passive continental margin a wealth of information is available.
Lines 44/45 what is the scientific importance of this?
Line 46 what is ‘early’ 2D seismic data
Lines 48/49 already said in line 46
Lines 58/59 ‘an in-depth examination of the transitional domain from a buried DWFTB system’ – from the DWFTB to what? Confusing
Line 93 this is a very confusing sentence. Rephrase
Line 102 this is only true until the onset of the Benguela Current in the Miocene. Please, study the right literature
Line 117/118 why was only a subset of the 3D survey used for this study? No argument given
Line 125 why were the data resampled to 4 ms? That reduced the vertical resolution by half!
Line 129 the velocity model used for depth conversion is rather crude. How was this derived? What is the reason for not using velocities derived during velocity analysis?
Line 151 ‘local deviation of the seismic signal’ – from what?
Line 170 ‘often always’ – which one?
Line 248ff how was constraining the ages carried out? And here it is stated that age assignment was difficult, but in the previous paragraphs ages were used very confidently…. It really would be nice to see a correlation of seismic data with ages and lithology at a well.
Line 268 how do you know the Oligocene unconformity was formed subaerially?
Line 270 or other mass transport?
Lines 278-280 this cannot be seen in the figure
Lines 280-282 see, e.g., (Weigelt and Uenzelmann-Neben, 2004)
Line 285 TWT can never be negative!
Lines 289-290 it is unclear how this detailed interpretation was derived
Line 292 ‘as explained previously’ – this was not explained but stated
Lines 294-296 and what is the importance of this?
Line 305 it is impossible to see this in the somewhat chaotic Figs 9 and 12
Line 310 to me the spoon-shaped feature looks over-interpreted
Line 314 in which sequence?
Line 321’ deviation from the normal trend’ – what is the normal trend?
Line 326 the authors cannot resolved metre-scaled displacements! The resolution of the seismic data does not allow this.
Line 329 mounded and chaotic geometry is not necessarily a sign for turbidites. There are other mass transport deposits
lines 335-337 this is no discussion!
Lines 341-343 repetition from lines 335/336
Lines 351-353 what does this mean? Confusing
Lines 372-394 this is all other people’s work but the manuscript rests on this. How were the ages identified?
Lines 417-420 there is no anticline in Fig. 7
Lines 439-442 all this is based on already published studies
Line 454 the canyon in Fig. 8 is definitely not fault controlled
Line 463 there are plenty of more recent studies based on higher quality data than Dingle et al., 1983
Line 465ff this whole discussion is focused on tectonic sealevel variations. How does the onset of Antarctic glaciation tie in, how the variability in Antarctic ice-sheet thickness and size, the variability in the location of the Southern Ocean frontal systems?
Line473 which existing planes of weakness? What caused them?
Line 484 it is interesting to see that the authors cite a paper from the Brazilian margin. There, the oceanographic system is quite different to the SW African. Why not cite papers which dealt with the SW African margin?
Line 480ff it appears to be assumed that deposition is mostly fault controlled. That is not the case for SW Africa, where upwelling, NADW, AABW and the Benguela Current significantly influence sediment transport and deposition.
Lines 490ff the ocean offshore SW Africa is strongly stratified, which results in baroclinic/geostrophic flow. Internal waves are not needed for this. Here, the authors think too complicated. I seriously doubt that tidal movements will affect deposition/erosion in 2000 m water depth. Tidal current further act not slope parallel.
Line 492 what is erosional undercutting?
Line 494 differences in temperature/salinity generally cause geostrophic flow, not only parallel to the slope
Lines 497-501 those erosional features may have been formed by AABW or NADW, not only by upwelling. Also see (Weigelt and Uenzelmann-Neben, 2004)
Line 501 and due to variability in the glaciation of Antarctica!
Line 502 in what water depth?
Lines 503-505 this is NADW!
Line509 a CDS show high sedimentation rates!
Line 512 it is not true that erosional features of CDSs have been poorly studied
Lines 513-514 e.g., (Weigelt and Uenzelmann-Neben, 2004)
Lines 549-552 I do not agree. Most of the following was already known previoulsy
Hopkins, A., Cartwright, J., 2021. Large scale excavation of outer shelf sediments by bottom currents during the Late Miocene in the SE Atlantic. Geo-Marine Letters 41, 33.
Weigelt, E., Uenzelmann-Neben, G., 2004. Sediment deposits in the Cape Basin: Indications for shifting ocean currents? AAPG Bulletin 88, 765-780.
Weigelt, E., Uenzelmann-Neben, G., 2007a. Early Pliocene change of deposition style in the Cape Basin, southeastern Atlantic. Geological Society of America Bulletins 119, 1004-1013.
Weigelt, E., Uenzelmann-Neben, G., 2007b. Orbital forced cyclycity of reflector strength in the seismic records of the Cape Basin. Geophysical Research Letters 34.
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RC3: 'Reply on RC1', Chris Elders, 10 Jun 2022
In fairness to the authors, they are using commercially acquired and processed sesimic data. The processinag and resoltuion of the data is typical for such surveys, but different to that acquired for focussed studies of sallow sea floor sedimenst where higher reolutuioin data might be acquired
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RC4: 'Reply on RC3', Anonymous Referee #1, 10 Jun 2022
It is certainly correct that the authors use industry data, which show characteristic industry acquisition and processing parameters. However, those must not be ignored when interpreting the data. You can speak on high-resolution data, when the data has been resampled to 4 ms. You cannot interpret reflection amplitudes, which have been subject to several phases of deconvolution and time variant scaling. Furthermore, the lateral and vertical resolution are fixed terms. You cannot modify those Physics concepts according to your needs.
So, I still suggest to use much more caution in inter[reting the data and not get carried away by the possibilities of an interpretation software and complex attributes.
- AC6: 'Reply on RC4', Nombuso Maduna, 17 Sep 2022
- AC3: 'Reply on RC3', Nombuso Maduna, 17 Sep 2022
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RC4: 'Reply on RC3', Anonymous Referee #1, 10 Jun 2022
- AC2: 'Reply on RC1', Nombuso Maduna, 21 Jun 2022
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RC3: 'Reply on RC1', Chris Elders, 10 Jun 2022
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RC2: 'Comment on egusphere-2022-180', Chris Elders, 10 Jun 2022
The authors of this manuscript are to be congratulated for presenting some very elegant interpretation and visualisations of an interesting data set that images a the compressional and transitional (or translational?) domains of a large scale submarine slide complex.
I think that there are a few issues that the authors should consider in their interpretation and discussion of their observations. I feel that some of the inferences are somewhat circumstantial, and it would be useful if they could be better substantiated, or the alternatives considered. They are:
- Evidence that the small scale underlying thrusts in the Albain sequences are younger than overlying large scale thrust system.This seems counterintuitive, particularly given that thrusts normally cut up stratigraphy, particularly when a basal detachment becomes “locked”. Why could these older thrusts not simply be part of an older mass transport complex, which would be an equally valid (and more probable) interpretation of the data and the obsereved relationships?
- The use of the term transitional rather than translational to describe the domain between the extensional and compressional domains of a mass transport complex.Both are used in the literature, particularly for mass transport complexes in this region. “Transitional” may be valid where the zone is narrow, and extensional and compressional structures interact. “Translational” is more appropriate where the zone is wide and the sequences above the detachment are being displaced horizontally between the extensional and compressional domains. It think the situation described here is more akin to the latter, in which case the observations are particularly interesting. All the faults in this domain (even those interpreted as extensional) are highly oblique to the thrust faults in the compressional domain, and I suspect are also oblique to the extensional faults in the extensional domain, although this is not imaged in this data set. Are there any more extensive 2D surveys or existing maps that could be used to address this? If so, the inference that the translational domain consists mainly of oblique faults would be very interesting and innovative and would indicate the style of deformation that operates in this zone.
- The use of the term “spoon-shaped” to describe the plan view pattern of the oblique faults – I think this is confusing, and the inference of this geometry by extrapolation beyond the extend of the data set is geologically and mechanically unrealistic.I think it is better to confine the use of the term to the three dimensional geometry of individual fault planes that show curvature in three dimensions (up dip and along strike). This has previously been used to describe the geometry of linked extensional and oblique faults in the extensional domain.
- The inference of the sense of motion on oblique faults from the offset of thrusts.The implication in the manuscript is that the thrusts were originally contiguous structures that have subsequently been offset by the oblique faults. I think this is unlikely. I think it is more likely that they act like lateral ramps in thrust sheets and accommodate differences in displacement between originally offset thrusts. The actual displacement will depend on the nature of the offset, and will be variable. Transform faults between offset segments of a mid ocean ridge are also a good analogy in this respect.
- The use of the term “mass transport complex” to describe the turbidite and contourite deposits in the Oligocene and Miocene sequences – this is very confusing!The term mass transport complex should be restricted top large bodies of intact or semi-intact sediment transported down slope by gravitational processes, and be distinguished from sediment being transported by currents, that still may be gravitationally controlled (mass flow would be a better term for these if you prefer to avoid using terms such as turbidite and contourite). The Deep Water Fold and Thrust Belt is part of a mass transport complex, and the term should be restricted to that feature.
- Evidence for the control of the underlying structure on the younger canyons and contourite channels.The evidence seems to suggest that the Oligocene canyon has a different orientation to the underlying structure, so it is difficult to see the control. This therefore also reduces the likely control of the underlying structure on the Miocene margin-parallel channels. It would be better to use maps that superimpose the sedimentary features on underlying structure to establish these relationships, rather than inferring them from vertical sections where apparent relationships my just be an artefact of the location of a single section. The suggestion that the margin-parallel channels are influenced by strong tides also seems somewhat circumstantial. Are there any observations from the data that can be used to support this?
Chris Elders
Curtin Univerity
- AC1: 'Reply on RC2', Nombuso Maduna, 21 Jun 2022
- AC4: 'Reply on RC2', Nombuso Maduna, 17 Sep 2022
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EC1: 'Comment on egusphere-2022-180', CharLotte Krawczyk, 22 Jun 2022
Dear authors,
the review phase of your submission has been extended, since the current two reports for your manuscript deviate considerably. Therefore, I invited a third reviewer, and once we will have their report received, a decision can be made for the further processing of your manuscript.
All the best, Lotte Krawczyk.
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RC5: 'Comment on egusphere-2022-180', Anonymous Referee #3, 14 Aug 2022
Nombuso Maduna and co-authors have conducted a thorough study of a deep water fold-and-thrust belt in the Mesozoic Orange Basin, and the structural processes and features that are associated with this. The 3D seismic dataset interpreted here allows for more detailed analysis of this stratigraphy and interpretations that add to the current state of knowledge.
In this review, I recommend a careful check of how the results are presented, as sometimes they mix discussion points in this section, and that makes it difficult for a reader to discern what is new from what was previously published. I realise that this deep water fold-and-thrust belt has been published on before, but in the introduction I felt that a short description of what it is would be helpful for clarity. The reason for this is that although this is seemingly driven by gravitational activity, at first it appears strange in a context of Gondwana break-up to read about compression. In this regard, the composition of shale is important for the structural model of detachment. Have the shales been cored or sampled? I.e., how do we know that those reflectors and units are shale? A lot of your structural interpretation is based on the properties of this rock type, so please clarify this at the beginning. In the abstract too, I recommend up front adding a statement that this refers to gravity-driven compression on an extensional margin. Generally, as well, the angle of the slope required to generate these compressional faults seems to not be too steep. Perhaps look a little into this as well.
Specific comments:
Line 67: replace Lower with Early. And check the paper for consistency in this regard. The difference is that referring to time only, you’d say Early Cretaceous. But describing deposits, it would be Lower Cretaceous rocks (for example).
Offshore structural framework: there is a 2020 publication by Baby et al. too, that may be worth checking out: Baby, G., Guillocheau, F., Boulogne, C., Robin, C. and Dall'Asta, M., 2018. Uplift history of a transform continental margin revealed by the stratigraphic record: The case of the Agulhas transform margin along the Southern African Plateau. Tectonophysics 731, 104-130.
Lines 93 and 94: comprises (not comprises of)
Line 101: was eustatic sea-level change. Here I would also recommend leaving out the word ‘eustatic’, as even into drift there was appreciable uplift and subsidence in this area.
Line 115: between 2012 and 2013
Line 127: data is plural of datum
Methods: please clarify which were the methods you applied, and which were done by the petroleum company.
Results and interpretation: I think in this manuscript it will be clearer to separate the results from the interpretation.
Line 161: The study area lies offshore of northwest South Africa, along the continental slope
Line 204: this is an example of where I am unsure whether this is your interpretation, or one from elsewhere, and why I suggest you separate the results and lay out only new ideas in that section.
Line 214: what about synclines? Surely with anticlines there are also associated synclines. And are these anti- and synclines, or anti- and synforms?
Line 219: A and B2
The paragraph including line 230 is all discussion.
Line 238: how did you interpret that this is a MFS? I suggest in the results, explaining the reasoning behind assigning these surfaces and units. Was it based on geometry, or truncation of what is below, for example? You could also consider tabulating this sort of information, but either way I think it is important to say something about how you arrived at your assigned surfaces. Also for the methods, which sequence stratigraphic terminology and methods did you follow and why?
Line 279: Cenozoic unit
Line 278: rather than earliest, perhaps say basal sediments within the unit
In the structural framework you describe features at depths measured in both ms and metres. If you have done a time/depth conversion, please also include these depths on the figures of profiles.
Line 311: what is spoon shaped geometry? I am not sure this is a recognised term? And it is not obvious to me, without looking at the image to go with this, what that means anyway. Maybe refer to convex or concave instead.
Line 315: replace ‘compared to’ with ‘as’
Lines 319 and 320: another example of mixed interpretation in the results
Line 332: extremely is a bit too emotive
Line 371 onward: it is not clear to me which of these findings are new (from your work) and which were established previously. I suggest laying out what the accepted model was prior to your work, and then onward from that presenting the new model based on your data and interpretation thereof.
Line 393: this is the first time the Benguela Current is discussed and it needs to be introduced earlier in the regional setting. There is an appreciable amount of literature, in particular by Uenzelmann-Neben and colleagues, on the role of oceanographic circulation on seafloor sediments and I think your paper will benefit by including this in your interpretations and your context. This erosion is an important part of the story of deposition and preservation on this margin.
Line 403: differs against what? Are you referring to within the sequences, or between them, for example? Please be more clear here.
Line 427: we propose a third model that
Line 432: ‘much greater than 10 km’ does not say too much. Have you got a sense of at least how long this may be?
Ahead of section 5.3, and following the paragraph where you propose a third scenario, I feel that a section explaining how you can get extension and compression at the same time is necessary.
Line 466: shelf, rather than coastline?
Line 476: which river is this canyon associated with?
Lines 492-494: check this sentence. Erosional undercutting is singular, and the sentence reads a little awkwardly.
In the section about Miocene Benguela Upwelling, I suggest starting by saying that you are interpreting an analogous situation to the present, and then expand upon this rather than explaining it and then getting to what you are saying.
The paragraph of line 515 seems redundant to me, as this is not an analogue to what you are describing for the Orange Basin continental slope deposits.
Line 525: reference for the overpressured shales?
In section 5.4, it is not very clear whether you are suggesting that this deformation is ongoing, or that it took place during deposition and now may get reactivation along its planes of weakness. I think the latter, but please make this clearer?
Table 3: Perhaps add which of these units have been sampled – e.g., you have space in the rows below the ages. Alternatively, if there is well / borehole data, add a column for that or state in the caption that all units have been sampled geologically.
Figure 2: please add an inset box of where figure 2 is onto figure 1.
Figure 3: I think this should come ahead of table 3 in the text.
Figure 4: as in the methods text, I recommend indicating which of these techniques were done by you, and which were already applied to the data.
Figure 5: Make the text and black lines in panel B bolder and I suggest that the same positions of profiles should also be shown on A. Please also add a small inset of where this is?
Figure 11: Can you link this canyon to a specific river? If so, please name it.
Figure 13 is excellent. Please just add a modern coastline position for orientation?
I enjoyed the opportunity to review this manuscript and certainly hope to see the paper published, following revision.
Kind regards.
- AC5: 'Reply on RC5', Nombuso Maduna, 17 Sep 2022
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2022-180', Anonymous Referee #1, 02 Jun 2022
Using a set of 3D seismic reflection data this manuscript aims at constraining the strato-structural evolution of the transitional and compressional domains of a Late Cretaceous deep-water fold-and-thrust belt system and its influence of the overlying Cenozoic megasequence in the Orange Basin offshore SW Africa.
The writing, wording and phrasing very often is unclear. The authors often are not precise and leave room for misunderstandings. It, e.g., remain unclear what improvement in scientific knowledge will be achieved by the study. The authors cite a large amount of literature describing the tectonic setting and development (see, e.g., Seismic stratigraphy and Results and interpretation). The interpretation of sequences and stratigraphic markers is mainly a set of statements, there is no discussion. Most of the facts presented are cited from other studies – so what is new? This is really difficult to identify.
Furthermore, the authors are very focused on the tectonic influence on sedimentation and deposition. They large ignore the effect oceanic currents and water mass transport have on this. The authors only briefly venture in this direction but in an oversimplified way and they appear to lack an understanding of physical oceanography in general and of this region in particular.
The authors speak of high-resolution seismic reflection data. However, the data were collected with a sample rate of 2 ms (see Table 1, Nyquist frequency 250 Hz) and then resampled to 4 ms (see Table 2, Nyquist frequency 125 Hz). This certainly is not high-resolution! High-resolution seismic reflection data should be recoded at minimum 1 ms sample rate giving a Nyquist frequency of 500 Hz. And where is the use of lowering the vertical resolution of seismic data by resampling?
The paragraph on Seismic resolution is very simplistic and, in parts, wrong. A reputable reflection seismologist uses l/2 and not l/4 (or even l/8) to compute the vertical resolution as l/4 is a very theoretical value. I would like to see a spectrum of the seismic data since I seriously doubt that the dominant frequency observed is 50 Hz; please, provide evidence for this! Even so, the Fresnel Zone definitely is not half of the dominant wavelength but dependent on target depth! Please, see Yilmaz (2001), P. 1803. There the formula is given and states that . So, the Fresnel Zone is dependent on target depth z and the main frequency. So, even if we take the main frequency to be 48 m (serious doubts here) the Fresnel Zone in 1000 m water depth will be 155 m and in 2000 m water depth it will be 220 m. So, the assumptions of the authors of a constant Fresnel Zone of 24 m are wrong!
The data have undergone severe processing applying several deconvolution techniques, resampling, attenuations, and time variant scaling (see Table 2). This all may have enhanced the signal but it also destroyed the true reflection amplitude. Still, the authors interpret variations in reflection amplitude both laterally and vertically (e.g., line 176, 196, 212). This, however, is no longer possible.
There is more evidence that the authors are not on top of the seismic method. They have labelled the twoway travel time negative. That implies that the data have been recoded before the shots were trigger. This is nonsense (sorry for being so blunt). It is neither the habit to label depth in negative numbers. If you want to make clear that this is below seafloor, please use mbsf (metres below seafloor).
The authors use seismic attributes to aid their interpretation. They state that ‘seismic attributes are designed through mathematical manipulation’ but they are not designed but computed. And I hope the data have not been manipulated (even though the severe processing certainly manipulated the data).
Taking all this into account I have huge doubts about the interpretation of the seismic reflection data presented by the authors since they do not appear to have fully understood the concept and physics of the method or the possibilities and limits.
The interpretation of the data is presented before this is discussed. To me this appears awkward.
Little information is provided on the lithology. The authors just state that, e.g., a) the Albian surface represents a maximum flooding surface (how do they know this?) and b)is a shale detachment surface. This is just one example. What are the arguments for those interpretations? What knowledge is used and not presented?
Section 4.1.3 how does especially the younger Cenozoic tie in with other studies from this margin, e.g., (Hopkins and Cartwright, 2021; Weigelt and Uenzelmann-Neben, 2004, 2007a, b)? The discussion here is very centred on tectonically influenced sedimentation. It may be useful for the authors to broaden their reading.
Section 4.2.1 is very focused on complex attributes. In my opinion the authors got a bit carried away by the figures of the complex attributes and lost their feeling for resolution and other limits.
The whole discussion is a repetition of other people’s work.
Section 5 to me does not show any advance in scientific knowledge.
It would be helpful if the described features all were marked in the figures.
The References needs checking. For several references journal name, etc is missing, some references are listed twice.
Jungslager, 199 is missing from References
Figure3: Seismic depth limit is supposed to mean maximum penetration? Annotate the seismic markers discussed in the text please with the numbers used in the text
Figure 4 is obsolete
Figure 5: How were the type of the faults and the throw direction identified? Certainly not based on Fig 5a, which is rather chaotic. Travel time can never be negative! Show coordinates
Figs. 6-8 TWT never is negative! Same applies to depth; if needed use mbsf
Figs 9-12 are extremely confusing and not helpful at all. Please, omit
Figure 13: How were the ages assigned? Where does the anticline in 13d originate? Cold water usually has a higher density than warm water (annotation 13e)
Line 40 The authors state that this margin is largely underexplored since there is only one well per 4000 km2. This is meant regarding hydrocarbon exploration? Extensive sets of seismic reflection data have been collected along the margin and several scientific sites have been drilled. So, regarding the development of the passive continental margin a wealth of information is available.
Lines 44/45 what is the scientific importance of this?
Line 46 what is ‘early’ 2D seismic data
Lines 48/49 already said in line 46
Lines 58/59 ‘an in-depth examination of the transitional domain from a buried DWFTB system’ – from the DWFTB to what? Confusing
Line 93 this is a very confusing sentence. Rephrase
Line 102 this is only true until the onset of the Benguela Current in the Miocene. Please, study the right literature
Line 117/118 why was only a subset of the 3D survey used for this study? No argument given
Line 125 why were the data resampled to 4 ms? That reduced the vertical resolution by half!
Line 129 the velocity model used for depth conversion is rather crude. How was this derived? What is the reason for not using velocities derived during velocity analysis?
Line 151 ‘local deviation of the seismic signal’ – from what?
Line 170 ‘often always’ – which one?
Line 248ff how was constraining the ages carried out? And here it is stated that age assignment was difficult, but in the previous paragraphs ages were used very confidently…. It really would be nice to see a correlation of seismic data with ages and lithology at a well.
Line 268 how do you know the Oligocene unconformity was formed subaerially?
Line 270 or other mass transport?
Lines 278-280 this cannot be seen in the figure
Lines 280-282 see, e.g., (Weigelt and Uenzelmann-Neben, 2004)
Line 285 TWT can never be negative!
Lines 289-290 it is unclear how this detailed interpretation was derived
Line 292 ‘as explained previously’ – this was not explained but stated
Lines 294-296 and what is the importance of this?
Line 305 it is impossible to see this in the somewhat chaotic Figs 9 and 12
Line 310 to me the spoon-shaped feature looks over-interpreted
Line 314 in which sequence?
Line 321’ deviation from the normal trend’ – what is the normal trend?
Line 326 the authors cannot resolved metre-scaled displacements! The resolution of the seismic data does not allow this.
Line 329 mounded and chaotic geometry is not necessarily a sign for turbidites. There are other mass transport deposits
lines 335-337 this is no discussion!
Lines 341-343 repetition from lines 335/336
Lines 351-353 what does this mean? Confusing
Lines 372-394 this is all other people’s work but the manuscript rests on this. How were the ages identified?
Lines 417-420 there is no anticline in Fig. 7
Lines 439-442 all this is based on already published studies
Line 454 the canyon in Fig. 8 is definitely not fault controlled
Line 463 there are plenty of more recent studies based on higher quality data than Dingle et al., 1983
Line 465ff this whole discussion is focused on tectonic sealevel variations. How does the onset of Antarctic glaciation tie in, how the variability in Antarctic ice-sheet thickness and size, the variability in the location of the Southern Ocean frontal systems?
Line473 which existing planes of weakness? What caused them?
Line 484 it is interesting to see that the authors cite a paper from the Brazilian margin. There, the oceanographic system is quite different to the SW African. Why not cite papers which dealt with the SW African margin?
Line 480ff it appears to be assumed that deposition is mostly fault controlled. That is not the case for SW Africa, where upwelling, NADW, AABW and the Benguela Current significantly influence sediment transport and deposition.
Lines 490ff the ocean offshore SW Africa is strongly stratified, which results in baroclinic/geostrophic flow. Internal waves are not needed for this. Here, the authors think too complicated. I seriously doubt that tidal movements will affect deposition/erosion in 2000 m water depth. Tidal current further act not slope parallel.
Line 492 what is erosional undercutting?
Line 494 differences in temperature/salinity generally cause geostrophic flow, not only parallel to the slope
Lines 497-501 those erosional features may have been formed by AABW or NADW, not only by upwelling. Also see (Weigelt and Uenzelmann-Neben, 2004)
Line 501 and due to variability in the glaciation of Antarctica!
Line 502 in what water depth?
Lines 503-505 this is NADW!
Line509 a CDS show high sedimentation rates!
Line 512 it is not true that erosional features of CDSs have been poorly studied
Lines 513-514 e.g., (Weigelt and Uenzelmann-Neben, 2004)
Lines 549-552 I do not agree. Most of the following was already known previoulsy
Hopkins, A., Cartwright, J., 2021. Large scale excavation of outer shelf sediments by bottom currents during the Late Miocene in the SE Atlantic. Geo-Marine Letters 41, 33.
Weigelt, E., Uenzelmann-Neben, G., 2004. Sediment deposits in the Cape Basin: Indications for shifting ocean currents? AAPG Bulletin 88, 765-780.
Weigelt, E., Uenzelmann-Neben, G., 2007a. Early Pliocene change of deposition style in the Cape Basin, southeastern Atlantic. Geological Society of America Bulletins 119, 1004-1013.
Weigelt, E., Uenzelmann-Neben, G., 2007b. Orbital forced cyclycity of reflector strength in the seismic records of the Cape Basin. Geophysical Research Letters 34.
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RC3: 'Reply on RC1', Chris Elders, 10 Jun 2022
In fairness to the authors, they are using commercially acquired and processed sesimic data. The processinag and resoltuion of the data is typical for such surveys, but different to that acquired for focussed studies of sallow sea floor sedimenst where higher reolutuioin data might be acquired
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RC4: 'Reply on RC3', Anonymous Referee #1, 10 Jun 2022
It is certainly correct that the authors use industry data, which show characteristic industry acquisition and processing parameters. However, those must not be ignored when interpreting the data. You can speak on high-resolution data, when the data has been resampled to 4 ms. You cannot interpret reflection amplitudes, which have been subject to several phases of deconvolution and time variant scaling. Furthermore, the lateral and vertical resolution are fixed terms. You cannot modify those Physics concepts according to your needs.
So, I still suggest to use much more caution in inter[reting the data and not get carried away by the possibilities of an interpretation software and complex attributes.
- AC6: 'Reply on RC4', Nombuso Maduna, 17 Sep 2022
- AC3: 'Reply on RC3', Nombuso Maduna, 17 Sep 2022
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RC4: 'Reply on RC3', Anonymous Referee #1, 10 Jun 2022
- AC2: 'Reply on RC1', Nombuso Maduna, 21 Jun 2022
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RC3: 'Reply on RC1', Chris Elders, 10 Jun 2022
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RC2: 'Comment on egusphere-2022-180', Chris Elders, 10 Jun 2022
The authors of this manuscript are to be congratulated for presenting some very elegant interpretation and visualisations of an interesting data set that images a the compressional and transitional (or translational?) domains of a large scale submarine slide complex.
I think that there are a few issues that the authors should consider in their interpretation and discussion of their observations. I feel that some of the inferences are somewhat circumstantial, and it would be useful if they could be better substantiated, or the alternatives considered. They are:
- Evidence that the small scale underlying thrusts in the Albain sequences are younger than overlying large scale thrust system.This seems counterintuitive, particularly given that thrusts normally cut up stratigraphy, particularly when a basal detachment becomes “locked”. Why could these older thrusts not simply be part of an older mass transport complex, which would be an equally valid (and more probable) interpretation of the data and the obsereved relationships?
- The use of the term transitional rather than translational to describe the domain between the extensional and compressional domains of a mass transport complex.Both are used in the literature, particularly for mass transport complexes in this region. “Transitional” may be valid where the zone is narrow, and extensional and compressional structures interact. “Translational” is more appropriate where the zone is wide and the sequences above the detachment are being displaced horizontally between the extensional and compressional domains. It think the situation described here is more akin to the latter, in which case the observations are particularly interesting. All the faults in this domain (even those interpreted as extensional) are highly oblique to the thrust faults in the compressional domain, and I suspect are also oblique to the extensional faults in the extensional domain, although this is not imaged in this data set. Are there any more extensive 2D surveys or existing maps that could be used to address this? If so, the inference that the translational domain consists mainly of oblique faults would be very interesting and innovative and would indicate the style of deformation that operates in this zone.
- The use of the term “spoon-shaped” to describe the plan view pattern of the oblique faults – I think this is confusing, and the inference of this geometry by extrapolation beyond the extend of the data set is geologically and mechanically unrealistic.I think it is better to confine the use of the term to the three dimensional geometry of individual fault planes that show curvature in three dimensions (up dip and along strike). This has previously been used to describe the geometry of linked extensional and oblique faults in the extensional domain.
- The inference of the sense of motion on oblique faults from the offset of thrusts.The implication in the manuscript is that the thrusts were originally contiguous structures that have subsequently been offset by the oblique faults. I think this is unlikely. I think it is more likely that they act like lateral ramps in thrust sheets and accommodate differences in displacement between originally offset thrusts. The actual displacement will depend on the nature of the offset, and will be variable. Transform faults between offset segments of a mid ocean ridge are also a good analogy in this respect.
- The use of the term “mass transport complex” to describe the turbidite and contourite deposits in the Oligocene and Miocene sequences – this is very confusing!The term mass transport complex should be restricted top large bodies of intact or semi-intact sediment transported down slope by gravitational processes, and be distinguished from sediment being transported by currents, that still may be gravitationally controlled (mass flow would be a better term for these if you prefer to avoid using terms such as turbidite and contourite). The Deep Water Fold and Thrust Belt is part of a mass transport complex, and the term should be restricted to that feature.
- Evidence for the control of the underlying structure on the younger canyons and contourite channels.The evidence seems to suggest that the Oligocene canyon has a different orientation to the underlying structure, so it is difficult to see the control. This therefore also reduces the likely control of the underlying structure on the Miocene margin-parallel channels. It would be better to use maps that superimpose the sedimentary features on underlying structure to establish these relationships, rather than inferring them from vertical sections where apparent relationships my just be an artefact of the location of a single section. The suggestion that the margin-parallel channels are influenced by strong tides also seems somewhat circumstantial. Are there any observations from the data that can be used to support this?
Chris Elders
Curtin Univerity
- AC1: 'Reply on RC2', Nombuso Maduna, 21 Jun 2022
- AC4: 'Reply on RC2', Nombuso Maduna, 17 Sep 2022
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EC1: 'Comment on egusphere-2022-180', CharLotte Krawczyk, 22 Jun 2022
Dear authors,
the review phase of your submission has been extended, since the current two reports for your manuscript deviate considerably. Therefore, I invited a third reviewer, and once we will have their report received, a decision can be made for the further processing of your manuscript.
All the best, Lotte Krawczyk.
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RC5: 'Comment on egusphere-2022-180', Anonymous Referee #3, 14 Aug 2022
Nombuso Maduna and co-authors have conducted a thorough study of a deep water fold-and-thrust belt in the Mesozoic Orange Basin, and the structural processes and features that are associated with this. The 3D seismic dataset interpreted here allows for more detailed analysis of this stratigraphy and interpretations that add to the current state of knowledge.
In this review, I recommend a careful check of how the results are presented, as sometimes they mix discussion points in this section, and that makes it difficult for a reader to discern what is new from what was previously published. I realise that this deep water fold-and-thrust belt has been published on before, but in the introduction I felt that a short description of what it is would be helpful for clarity. The reason for this is that although this is seemingly driven by gravitational activity, at first it appears strange in a context of Gondwana break-up to read about compression. In this regard, the composition of shale is important for the structural model of detachment. Have the shales been cored or sampled? I.e., how do we know that those reflectors and units are shale? A lot of your structural interpretation is based on the properties of this rock type, so please clarify this at the beginning. In the abstract too, I recommend up front adding a statement that this refers to gravity-driven compression on an extensional margin. Generally, as well, the angle of the slope required to generate these compressional faults seems to not be too steep. Perhaps look a little into this as well.
Specific comments:
Line 67: replace Lower with Early. And check the paper for consistency in this regard. The difference is that referring to time only, you’d say Early Cretaceous. But describing deposits, it would be Lower Cretaceous rocks (for example).
Offshore structural framework: there is a 2020 publication by Baby et al. too, that may be worth checking out: Baby, G., Guillocheau, F., Boulogne, C., Robin, C. and Dall'Asta, M., 2018. Uplift history of a transform continental margin revealed by the stratigraphic record: The case of the Agulhas transform margin along the Southern African Plateau. Tectonophysics 731, 104-130.
Lines 93 and 94: comprises (not comprises of)
Line 101: was eustatic sea-level change. Here I would also recommend leaving out the word ‘eustatic’, as even into drift there was appreciable uplift and subsidence in this area.
Line 115: between 2012 and 2013
Line 127: data is plural of datum
Methods: please clarify which were the methods you applied, and which were done by the petroleum company.
Results and interpretation: I think in this manuscript it will be clearer to separate the results from the interpretation.
Line 161: The study area lies offshore of northwest South Africa, along the continental slope
Line 204: this is an example of where I am unsure whether this is your interpretation, or one from elsewhere, and why I suggest you separate the results and lay out only new ideas in that section.
Line 214: what about synclines? Surely with anticlines there are also associated synclines. And are these anti- and synclines, or anti- and synforms?
Line 219: A and B2
The paragraph including line 230 is all discussion.
Line 238: how did you interpret that this is a MFS? I suggest in the results, explaining the reasoning behind assigning these surfaces and units. Was it based on geometry, or truncation of what is below, for example? You could also consider tabulating this sort of information, but either way I think it is important to say something about how you arrived at your assigned surfaces. Also for the methods, which sequence stratigraphic terminology and methods did you follow and why?
Line 279: Cenozoic unit
Line 278: rather than earliest, perhaps say basal sediments within the unit
In the structural framework you describe features at depths measured in both ms and metres. If you have done a time/depth conversion, please also include these depths on the figures of profiles.
Line 311: what is spoon shaped geometry? I am not sure this is a recognised term? And it is not obvious to me, without looking at the image to go with this, what that means anyway. Maybe refer to convex or concave instead.
Line 315: replace ‘compared to’ with ‘as’
Lines 319 and 320: another example of mixed interpretation in the results
Line 332: extremely is a bit too emotive
Line 371 onward: it is not clear to me which of these findings are new (from your work) and which were established previously. I suggest laying out what the accepted model was prior to your work, and then onward from that presenting the new model based on your data and interpretation thereof.
Line 393: this is the first time the Benguela Current is discussed and it needs to be introduced earlier in the regional setting. There is an appreciable amount of literature, in particular by Uenzelmann-Neben and colleagues, on the role of oceanographic circulation on seafloor sediments and I think your paper will benefit by including this in your interpretations and your context. This erosion is an important part of the story of deposition and preservation on this margin.
Line 403: differs against what? Are you referring to within the sequences, or between them, for example? Please be more clear here.
Line 427: we propose a third model that
Line 432: ‘much greater than 10 km’ does not say too much. Have you got a sense of at least how long this may be?
Ahead of section 5.3, and following the paragraph where you propose a third scenario, I feel that a section explaining how you can get extension and compression at the same time is necessary.
Line 466: shelf, rather than coastline?
Line 476: which river is this canyon associated with?
Lines 492-494: check this sentence. Erosional undercutting is singular, and the sentence reads a little awkwardly.
In the section about Miocene Benguela Upwelling, I suggest starting by saying that you are interpreting an analogous situation to the present, and then expand upon this rather than explaining it and then getting to what you are saying.
The paragraph of line 515 seems redundant to me, as this is not an analogue to what you are describing for the Orange Basin continental slope deposits.
Line 525: reference for the overpressured shales?
In section 5.4, it is not very clear whether you are suggesting that this deformation is ongoing, or that it took place during deposition and now may get reactivation along its planes of weakness. I think the latter, but please make this clearer?
Table 3: Perhaps add which of these units have been sampled – e.g., you have space in the rows below the ages. Alternatively, if there is well / borehole data, add a column for that or state in the caption that all units have been sampled geologically.
Figure 2: please add an inset box of where figure 2 is onto figure 1.
Figure 3: I think this should come ahead of table 3 in the text.
Figure 4: as in the methods text, I recommend indicating which of these techniques were done by you, and which were already applied to the data.
Figure 5: Make the text and black lines in panel B bolder and I suggest that the same positions of profiles should also be shown on A. Please also add a small inset of where this is?
Figure 11: Can you link this canyon to a specific river? If so, please name it.
Figure 13 is excellent. Please just add a modern coastline position for orientation?
I enjoyed the opportunity to review this manuscript and certainly hope to see the paper published, following revision.
Kind regards.
- AC5: 'Reply on RC5', Nombuso Maduna, 17 Sep 2022
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