New Insights into the Genetic Mechanism of the Miocene Mounded Stratigraphy in the Qiongdongnan Basin, Northern South China Sea

Round 1

Reviewer 1 Report

The manuscript is well written and presents a relevant topic in deep-water mounds. It is well structured, however, with figures difficult to read. Figure 1 is an example, in the image the contrast of strong colors and faded letters hinders the reading. I suggest increasing the font of the words in this figure. Still in this figure, says Figure 1. Tectonic units of the Qiongdongnan Basin (modified from [30-31]). In the caption there are only terms like uplift, lowuplift, magma... so, I think the name tectonic units is compromising because it makes the reader think about stratigraphic tectonic units of the basin.

Figure 3 is very good and the seismic interpretation too. In figure 4D, the H = 97m ? de Height is too close to the line, making it difficult to see. Same remark for Fig. 5B in Fig.5C, 5D and 5E.

In figure 6 some names are erased like Songnan low-uplift and Lingshui sub-basin, I suggest improving. Would that be bathymetry? Figure 7A is terrible to read Gullies. In general, I consider a good manuscript, with small details to fix and after that, I recommend acceptance.

Author Response

Question 1: It is well structured, however, with figures difficult to read. Figure 1 is an example, in the image the contrast of strong colors and faded letters hinders the reading. I suggest increasing the font of the words in this figure. Still in this figure, says Figure 1. Tectonic units of the Qiongdongnan Basin (modified from [30-31]). In the caption there are only terms like uplift, lowuplift, magma... so, I think the name tectonic units is compromising because it makes the reader think about stratigraphic tectonic units of the basin. 

Reply: Thank you very much for your constructive comment.

In the revised manuscript, we have modified and improved Figure 1 and made the text on the figure legible. Additionally, to allow readers to understand Figure 1, the relevant terms describing the tectonic units have been replaced by standard terminology described in previous studies. Tectonic units are divided into sags, highs, and uplifts. See the revised version for details.

 

Question 2: Figure 3 is very good and the seismic interpretation too. In figure 4D, the H = 97m ? de Height is too close to the line, making it difficult to see. Same remark for Fig. 5B in Fig.5C, 5D and 5E.

Reply: Thank you very much for your constructive comment. 

In the revised manuscript, we have modified and improved Figures 3-5 as well as figures 9,10 and 11 and made the text on the figure legible. 

 

Question 3: In figure 6 some names are erased like Songnan low-uplift and Lingshui sub-basin, I suggest improving. Would that be bathymetry? Figure 7A is terrible to read Gullies. In general, I consider a good manuscript, with small details to fix and after that, I recommend acceptance.

Reply: Thank you very much for your constructive comment. 

Due to the lack of seismic data in the western part of the basin, Figure 6 was mainly compiled based on the seismic data of the central and eastern parts of the basin and only reflects the part of the palaeogeomorphology. The Ledong Sag, Huangguang Sag, and part of the Lingshui Sag and Lingnan High are not shown in the diagram. Figure 6 reflects the palaeogeomorphology corresponding to the time equivalent to the interface T4, which was compiled according to the formation thickness between T4 and T3. Therefore, the values of the color bar in Figure 6 can not only reflect the burial depth of T4 after the deposition of the Huangliu Formation, but also can indicate the relative depth of the paleogeomorphology of T4 interface at the early stage of Huangliu Formation deposition. In the revised manuscript, Figure 6 and Figure 7A have been improved to make the text on the figure legible.

Author Response File: Author Response.docx

Reviewer 2 Report

 Thank you so much for giving me the chance to review the manuscript “New insights into the genetic mechanism of the Miocene mounded stratigraphy in the Qiongdongnan Basin, northern South China Sea”by Xu et al. This manuscript is very nicely written with sound data. In this manuscript, authors are reporting the mounds on the top of the Meishan Formation by using seismic data and they have also presented possible reasons for their formation. I have only three comments on this manuscript:

 

1.      On which basis authors identified such features as mounds. In other words, previous such studies should be included.

2.      References dealing with identification of mounds using seismic data may cited at relevant places (see attached file).

3.      Some minor comments are given in the attached file.

I strongly recommend this manuscript for publication after minor revision.

 

Sincerely

Reviewer

 

Comments for author File: Comments.pdf

Author Response

Reviewer 2

Comments and Suggestions for Authors

Thank you so much for giving me the chance to review the manuscript “New insights into the genetic mechanism of the Miocene mounded stratigraphy in the Qiongdongnan Basin, northern South China Sea” by Xu et al. This manuscript is very nicely written with sound data. In this manuscript, authors are reporting the mounds on the top of the Meishan Formation by using seismic data and they have also presented possible reasons for their formation. I have only three comments on this manuscript:

 

Question 1: On which basis authors identified such features as mounds. In other words, previous such studies should be included.

Reply: Thank you very much for your constructive comment.

In the revised manuscript, we have added the seismic identification features of the mounds described in previous studies to the relevant places in section 4.2: The elevations tend to be present as the lenticular seismic reflectors that are the thickest in the middle part and thin toward both wings, showing the dome-like bedforms, which are common not only in the QDNB but also in the Pearl River Basin in the northern South China Sea [5,16,18]. Similar elevations were described as ‘mounds’ previously, and strata characterized by the presence of mounds are commonly said to have ‘mounded topography [3,18].

 

Question 2. References dealing with identification of mounds using seismic data may cited at relevant places (see attached file).

Reply: Thank you very much for your constructive comment.

In the revised manuscript, we have added the relevant references dealing with the identification of mounds using seismic data to the relevant places in sections 4.1-4.2. See the revised version for details.

 

Question 3. Some minor comments are given in the attached file.

Reply: Thank you very much for your reviews of the manuscript.

In the revised manuscript, the problems marked at relevant places have been addressed one by one. See the revised version for details.

Author Response File: Author Response.docx

Reviewer 3 Report

Szanowni Autorzy, przeczytałem niniejszy tekst z uwagą. Poniżej przedstawiam swoje spostrzeżenia:

Fig. 1. Opis do tego rysunku jest pomieszany są tam użyte formy geomorfologiczne i bliżej nieokreślone formy litologiczne. Co to znaczy „magma” może chodzi o intruzje magmowe, oznaczenie koloru wskazuje iż mogą to być intruzje kwaśne. Z kolei jeśli mają być obok basenów to może są to kopuły wulkaniczne? To trzeba uściślić.

Rozdział 2 metodyka jest napisany bardzo lakonicznie, w zasadzie po za ogólnymi informacjami brak jest informacji o metodyce, aparaturze i laboratorium gdzie uzyskano wyniki. To trzeba zmienić.

Analizując mikrofotografie 3-5 bardziej odnosi się wrażenie że mamy do czynienia z ruchami masowymi powodującymi osuwanie się skib materiału z powstałych zrębów na skutek prawdopodobnie tektoniki dysjunktywnej, która związana jest z ruchami ekstensyjnymi w odpowiedzi na prawdopodobny charakter skał podłoża. Ja tu nie widzę mouded a raczej osuwiska. Oczywiście mogę przyjąć, że się mylę, jednak autorzy powinni to lepiej udokumentować. Czy są dostępne rdzenie? Czy można by coś więcej napisać o stosunku warstw w omawianym dnie?

W kwestii subsydencji ważne jest by autorzy wskazali dokładniej na jakiej podstawie wykazano prędkość osiadania dna. Warto sprawdzić także jak wygląda miąższość poszczególnych warstw i w przypadku ruchów masowych (a także prawdopodobnie działania uskoków) wykazać różnice miąższości warstw najlepiej poprzez wykonanie modelowania. Autorzy w tym akapicie podali w zasadzie wynik do zaakceptowania przez czytelnika a priori.

Czy można precyzyjnie wyznaczyć okresy subsydencji i względnego spokoju? Na fig 3-5,7,9 wydaje się, że takie okresy musiały być.

Ruchy o których autorzy piszą w dyskusji mogły mieć miejsce i też się z tym zgadzam, jednak ich impuls tworzenia się może nie koniecznie mieć charakter tylko związany z migracją prądów morskich ale także z impulsami tektonicznymi.  Nie wiem na ile zlityfikowane są omawiane osady ale może warto by sprawdzić także dane izotopowe które mogłyby dostarczyć informacji i działaniu uskoków?

Osiadanie terenu pod wpływem migracji magm w jego podłożu mogło wpłynąć na powstanie roztworów hydrotermalnych i propagacje izotopową tlenu, helu… Czy autorzy dysponują tymi danymi?
Fig. 10-12 faktycznie są przekonujące, że podany przez autorów scenariusz mógł mieć miejsce. Jednak wymaga on bardziej szczegółowych dowodów. Taki kształt jak obecnie mógł powstać także w wyniku innych działań.

Podsumowując chcę podkreślić iż niniejszy artykuł był dla mnie bardzo ciekawy. Doceniam prace i wkład autorów a moje uwagi maja w większości raczej charakter pytań zaciekawionego czytelnika niż samej oceny tekstu, nie mniej jednak prosiłbym się nad tymi moimi sugestiami pochylić.

 

Szanowna Redakcjo, niniejszy tekst ma moim zdaniem potencjał, choć zastanawiam się czy jego temat jest w zakresie czasopisma? To bardziej tekst o tektonice a nie zagadnieniach energetycznych. Chyba że rozumiemy tu energie środowiska i procesów dynamicznych. Generalnie po za kilkoma uwagami miałem wiele propozycji uściślenia pewnych danych. Uważam, że po ustosunkowaniu się autorów do moich poprawek tekst będzie się nadawał do druku. Chciałbym jednak móc go przeczytać po recenzji.

Dear Authors, I have read this text with attention. The following are my observations:

Fig. 1. The description to this figure is mixed up there are used geomorphological forms and unspecified lithological forms. What is meant by "magma" maybe it is about magma intrusions, the color designation indicates that they may be acid intrusions. On the other hand, if they are to be next to pools, perhaps they are volcanic domes? This needs to be made more specific.

Chapter 2 methodology is written very laconically, in fact, beyond general information there is no information about the methodology, apparatus and laboratory where the results were obtained. This needs to be changed.

Analyzing microphotographs 3-5, one gets more of the impression that we are dealing with mass movements causing the sliding of slices of material from the resulting logs due to probably disjunctive tectonics, which is associated with extensional movements in response to the probable nature of the bedrock. I don't see a mouded but rather a landslide here. Of course, I can assume that I am wrong, but the authors should document this better. Are there cores available? Could something more be written about the ratio of layers in the bottom in question?

In terms of subsidence, it would be important for the authors to indicate more precisely on what basis the subsidence velocity of the bottom was shown. It would also be useful to see what the thickness of the different layers looks like and, in the case of mass movements (and probably the action of faults), demonstrate the differences in the thickness of the layers preferably by performing modeling. The authors in this paragraph basically gave a result to be accepted by the reader a priori.

Can the periods of subsidence and relative quiescence be precisely determined? In figs. 3-5,7,9 it seems that such periods must have been.

The movements the authors write about in the discussion may have taken place, and I also agree, but their impulse of formation may not necessarily be only related to the migration of sea currents but also to tectonic impulses.  I don't know how lithified the sediments in question are, but perhaps it would be worth checking also isotopic data that could provide information and the action of the faults?

The subsidence of the area under the influence of magma migration in its bedrock could have influenced the formation of hydrothermal solutions and isotopic propagation of oxygen, helium... Do the authors have this data?

Figs. 10-12 are indeed convincing that the scenario given by the authors could have taken place. However, it requires more detailed evidence. The shape as it is now could also have been created by other activities.

In conclusion, I want to emphasize that I found this article very interesting. I appreciate the work and contribution of the authors, and my comments are mostly in the nature of questions from a curious reader rather than a mere evaluation of the text, but nevertheless I would ask you to consider my suggestions.

Author Response

Reviewer 3

Dear Authors, I have read this text with attention. The following are my observations:

 

Question 1: Fig. 1. The description to this figure is mixed up there are used geomorphological forms and unspecified lithological forms. What is meant by "magma" maybe it is about magma intrusions, the color designation indicates that they may be acid intrusions. On the other hand, if they are to be next to pools, perhaps they are volcanic domes? This needs to be made more specific.

Reply: Thank you very much for your constructive comment.

In the revised manuscript, we have modified and improved Figure 1. To allow readers to understand Figure 1, the relevant terms describing the tectonic units have been replaced by standard terminology described in previous studies. Tectonic units are divided into sags, highs, and uplifts. See the revised version for details. Additionally, thanks to the expert for pointing out the error in color designation associated with the magmas. After our examination, we found that the magmas shown in the previous figure are mainly seamounts composed of basaltic volcanic rocks, which have been verified in previous studies, and their legend has been modified in the revised manuscript.

 

Question 2: Chapter 2 methodology is written very laconically, in fact, beyond general information there is no information about the methodology, apparatus and laboratory where the results were obtained. This needs to be changed.

Reply: Thank you very much for your constructive comment.

In the revised manuscript, we have rewritten the Chapter data and methods, including seismic data acquisition, processing, interpretation software, subsidence analysis technology, etc. It is believed that the methodology for this study has been improved. See the revised version for details.

 

Question 3: Analyzing microphotographs 3-5, one gets more of the impression that we are dealing with mass movements causing the sliding of slices of material from the resulting logs due to probably disjunctive tectonics, which is associated with extensional movements in response to the probable nature of the bedrock. I don't see a mouded but rather a landslide here. Of course, I can assume that I am wrong, but the authors should document this better. Are there cores available? Could something more be written about the ratio of layers in the bottom in question?

Reply: Thank you very much for your constructive comment.

In many previous studies, the listric growth faults commonly developed in response to the extensional movements, which likely control the displacement and rotation of the slider. On the other hand, sediments can also slide under their own gravity. In this study, the formation of the mounds was considered to be associated with the latter (landslide, as the reviewer mentioned here). Based on the available data, this study believes that the sliding of the uppermost unconsolidated layer was related to the steepening of slopes rather than extensional movements (there is no conclusive evidence of regional stretching movements at that time). Combined with the characteristics of the mound shown by seismic data and the tectonic activity analysis, it is concluded that the differential subsidence caused the steepening of the slope, which drove the gravity slip of the sediments, and in this process, the uppermost deposits glided along a bedding-parallel detachment surface, creating a number of listric detachment faults that ceased downward to this surface. The uppermost layer was cut into a range of tilted fault blocks with tops constituting a seemingly mounded topography. The detachment surface is probably the boundary between the shallow unconsolidated deposits and the deep lithified strata. These statements were mentioned in the chapter “Discussion” of the article. Unfortunately, we don't have log and core data on the Meishan Formation at the moment. The reviewer's suggestions were excellent. If we have the core data, some soft-sediment deformation structures are likely observed in the mounds, which can be used to indicate the sliding of sediment related to the gravity instability during the synsedimentary stage and can further support our findings. Even though, as we mentioned in the article, seismic data show that soft-sediment deformation associated with gravity gliding might have occurred in the mounds with their internal reflections showing convex-up fold structures and disturbed bedding (Figure 3–5, Figure 11A–C). Such phenomenon can be related to the rollover anticlines unique to the listric growth faults or associated with the local compressive stresses pervasive in slide blocks which cause internal beds to buckle. These features are consistent with the gravity slide tectonics shown in some important examples documented previously.

 

Question 4: In terms of subsidence, it would be important for the authors to indicate more precisely on what basis the subsidence velocity of the bottom was shown. It would also be useful to see what the thickness of the different layers looks like and, in the case of mass movements (and probably the action of faults), demonstrate the differences in the thickness of the layers preferably by performing modeling. The authors in this paragraph basically gave a result to be accepted by the reader a priori.

Reply: Thank you very much for your constructive comment.

In the revised manuscript, we used the backstripping to create the balanced cross-sections before the tectonic subsidence reconstruction. And the balanced cross-sections corresponding to the subsidence curves have been added to the revised manuscript, shown in Fig.8, to show the differences in the thickness of the layers.

 

Question 5: Can the periods of subsidence and relative quiescence be precisely determined? In figs. 3-5,7,9 it seems that such periods must have been.

Reply: Thank you very much for your constructive comment.

The rate of tectonic subsidence is mainly calculated based on the thickness of the original strata and deposition time. In our research, the tectonic subsidence of the basin is relatively fast in the syn-rift stage, and the positions with the maximum subsidence rate is controlled by the boundary fault. In the Early and Middle Miocene (Sanya and Meishan Formations), the tectonic subsidence was relatively static, but gradually accelerated after the late Miocene. And there was a larger subsidence rate in the center of the basin depression, which resulted in the gravity instability of the sediments in the slope zone. The calculated values of subsidence may contain errors, but they can reflect the subsidence differences in different periods and regions of the basin. In the revised manuscript, we have modified the chapter “4.3 Tectonic subsidence” which involves the periods of subsidence and relative quiescence of the basin.

 

Question 6: The movements the authors write about in the discussion may have taken place, and I also agree, but their impulse of formation may not necessarily be only related to the migration of sea currents but also to tectonic impulses.  I don't know how lithified the sediments in question are, but perhaps it would be worth checking also isotopic data that could provide information and the action of the faults?

Reply: Thank you very much for your constructive comment.

In many studies, the deep water mounds were considered to be interpreted as sediment waves generated by bottom currents, but lacked the overwhelming proof supporting the existence of the bottom currents at that time. In our study, we tend to think that the formation of the mound is related to tectonic activity. The reviewer's suggestion is very good, but we do not have isotope data at this stage to date the faulting. However, the main time of fault activity can be roughly judged by the cutting relationship between the fault and sequence boundary (the time corresponding to T4).

 

Question 7: The subsidence of the area under the influence of magma migration in its bedrock could have influenced the formation of hydrothermal solutions and isotopic propagation of oxygen, helium... Do the authors have this data?

Reply: Thank you very much for your constructive comment.

The reviewer's suggestion is very good. Magmatic rock samples are also what we have always wanted to obtain, but unfortunately, so far, there is no drilling sampling for the intrusions to determine their activity period. Even though, the samples from adjacent areas indicated that the magmatism typically occurred in the Miocene and more recently. Additionally, according to the cutting relationship between the volcanic ejection structures and the sequence interface, it can be estimated that the main activity period of the volcanos was concentrated at 15.5 Ma, 10.5 Ma and 5.5 Ma.

 

Question 8: Figs. 10-12 are indeed convincing that the scenario given by the authors could have taken place. However, it requires more detailed evidence. The shape as it is now could also have been created by other activities.

Reply: Thank you very much for your constructive comment.

At present, numerous models were proposed over the years to interpret the origin of mounds in the study area, such as carbonate reefs, channel incision, sea currents, and so on, but there is no unified conclusion. The main reason is the lack of cores associated with the mounded stratigraphy and magmatic rocks. Like previous studies, our research also lacks these data at present. We realized that the model proposed previously may not be suitable for this area, so we developed a new model. This model is mainly based on the recognition of seismic reflection characteristics and may lack more detailed evidence as expected by the reviewer, such as cores and isotopic data. Our original intention is to provide a new idea about the formation of mounds for researchers to discuss. The data we can collect so far have been shown in the paper. We will continue to follow this research until new data is collected and hope more detailed evidence will be given.

 

In conclusion, I want to emphasize that I found this article very interesting. I appreciate the work and contribution of the authors, and my comments are mostly in the nature of questions from a curious reader rather than a mere evaluation of the text, but nevertheless I would ask you to consider my suggestions.

Reply: Thank the reviewer for the attention and suggestions on this study. On the basis of the existing research materials, we have made great efforts to improve this paper according to your suggestions, and hope that our answers to your questions can help you.

Author Response File: Author Response.docx