the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 07 Jul 2022
Tectonostratigraphic evolution of the Slyne Basin
Conor M. O'Sullivan
Conrad J. Childs
Muhammad Mudasar Saqab
John J. Walsh
Patrick M. Shannon
Abstract. The Slyne Basin, located offshore NW Ireland, is a narrow and elongate basin composed of a series of interconnected grabens and half-grabens, separated by transfer zones coincident with deep Caledonian-aged crustal structures. The basin is the product of a complex, polyphase structural evolution stretching from the Permian to the Miocene. Relatively low-strain episodic rifting occurred in the Late Permian and the latest Triassic to Middle Jurassic, with the main phase of rifting occurring in the Late Jurassic. These extensional events were punctuated by periods of tectonic quiescence during the Early Triassic, and regional uplift and erosion during the late Middle Jurassic. Late Jurassic strain was primarily accommodated by several kilometres of slip on the basin-bounding faults, which formed through the breaching of relay ramps between left-stepping fault segments developed during earlier Permian and Early-Mid Jurassic rift phases. Following the cessation of rifting at the end of the Jurassic, the area experienced kilometre-scale uplift and erosion during the Early Cretaceous and second, less-severe phase of denudation during the Palaeocene. These post-rift events formed a distinct regional post-rift unconformity and resulted in a reduced post-rift sedimentary section. The structural evolution of the Slyne Basin is influenced by pre-existing Caledonian structures at a high angle to the basinal trend. The basin illustrates a rarely documented style of fault reactivation in which basin-bounding faults are oblique to the earlier structural trend, but the initial fault segments are parallel to this trend. The result is a reversal of the sense of stepping of the initial fault segments generally associated with basement control on basin-bounding faults.
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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Conor M. O'Sullivan et al.
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2022-581', Tiago Alves, 15 Jul 2022
Dear authors,
I reviewed your paper soon after it was sent to me as your study area is of great interest. I found your paper well organised and clear, though needing to be somewhat 'retouched' in a few sections. I found a few typos and minor aspects, which I will comment in detail below. All in all, a more complete undertanding of the Slyne Basin is paramount to chracaterising the history of North Atlantic rifting, so your work is comendable and welcome to the geological community.
Detailed comments (see attached .pdf for reference):
1. The abstract refers to two tectonic events, but only one post-rift unconformity. Please, check this part.
2. There are published examples in the literature of Paleozoic fault zones controlling the formation of rift basins in deep offshore Portugal. The most documented of such examples is the Messejana Fault Zone in SW Portugal - see Pereira and Alves (2013). Crustal deformation and submarine canyon incision in a Meso-Cenozoic first-order transfer zone (SW Iberia, North Atlantic Ocean). Tectonophysics 601, 148-162.
3. In specific points in the text, I felt that a map of the Caledonian sutire zones and basement structures may be of use to your reasoning. Please, check the literature for one such maps - an example being Figure 4 in Alves et al. (2022). Analysis of a basement fault zone with geothermal potential in the Southern North Sea. Geothermics 102, 102398. Could you find one such maps for the region around Ireland/West GB?
4. Figure 2 looks slightly tentative as the tectonic events in the right-hand column seem not to correlate well with the unconformities and units to the left. There are some incorrect detail regarding the ages of rifting and ocean spreading in West Iberia and the Central Atlantic. I would suggest you to review the ages of these tectonic events.
5. An example of item 4. above is clear in Lines 168-170, in which you are recording an Albian-age succession that is associated with the break-up process in the Bay of Biscay. This continetal-breakup process was terminated in the Cenomanian-Turonian and is part of a Breakup Sequence in NW Iberia (see Alves and Cunha, 2018, EPSL; Alves et al., 2020, JMPG). Thus, your stratigraphic recorded in Ireland is being influenced by events that happened in SW Europe.
6. The mid part of the paper is informative and very nice to read. Some quantification is missing, though. The role of salt in basin inversion needs to the be quantified and compared with similar basins in Portugal (Lusitanian Basin) and Southern North Sea (Broad Fourteens Basin and the Dutch offshore). Again, as an example Alves et al. (2003) Post-Jurassic tectono-sedimentary evolution of the Northern Lusitanian Basin (Western Iberian margin). Basin Research 15, 227-249 developed such a comparison to find out that a value of horizontal shortening around 10% was able to preserve multipel petroleum systems in the Dutch offshore basins, while disrupting trap potential in the Lusitanian Basin - in great part due to the generalised exhumation of the basin during the Cenozoic, accompanying basin inversion.
Could you add information on the impact of salt as a facilitator of such inversion? Are you dealing with a thin-skinned tectonic style (Alves et al., 2002. Marine and Petroleum Geology) in your study area, or this is apparent only in parts of your study area? Is the tectonic shortening in your region concentrated close to basin-bounding structures, or more widespread.
As a final remark, some parts of the paper are excellent and honest accounts of the geological evolution of the Slyne Basin. A better link between basement Caledonian structures (via a map) and the major fault trends in the study area will significantly improve this paper. Similarly, the role of salt in the subsidence and basin inversion histories of the study area should be developed in more detail. This is a very interesting article just needing a minor to moderate re-focus of some aspects.
Tiago M. Alves
3D Seismic Lab - Cardiff University
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AC1: 'Reply on RC1', Conor O'Sullivan, 16 Sep 2022
Re: Manuscript Submission
Dear Editor,
Please find enclosed our manuscript titled ‘Tectonostratigraphic evolution of the Slyne Basin’ by Conor O’Sullivan (corresponding author), Conrad Childs, Muhammad Saqab, John Walsh and Patrick Shannon, which we have submitted for peer-review and publication in the Solid Earth Journal. There has been significant study into the development of rift basins along the European Atlantic margin from offshore Portugal to offshore northern Norway. The Irish Atlantic margin has by comparison seen conspicuously little investigation in this area, owing to the historic lack of seismic data and the relatively limited number of well penetrations. Since then, significantly more seismic data has been acquired and additional exploration wells have been drilled, providing the materials and impetus for a re-evaluation of these basins.
Our manuscript is a comprehensive investigation of the structural evolution of the Slyne Basin, located offshore north-western Ireland. We incorporate the most recent seismic reflection data alongside recent hydrocarbon exploration wells to investigate the location, shape and age of major structures which define this basin. We also investigate the upper-crustal character of this Mesozoic rift basin relative to deep, crustal-scale structures believed to be related to the Caledonian Orogeny. We demonstrate a rarely documented style of fault reactivation where basin-bounding faults are oblique to the earlier structural trend, but the initial fault segments are parallel to this trend. This manuscript will be of interest to several geoscientists working on the Irish Atlantic margin and adjacent jurisdictions, as well as the global community investigating fault analysis and general structural geology.
We think this paper will be a valid contribution to your journal and hope that you will consider it positively. Alongside this submission we have included a list of potential reviewers and would be grateful of any additional reviewers of your choice. We look forward to hearing from you.
Yours sincerely,
Dr. Conor O’Sullivan (on behalf of the co-authors)
Structural Geologist
Petroleum Experts Ltd.,
Petex House,
10 Logie Mill,
Edinburgh,
EH7 4NJ,
United Kingdom.
Telephone: +44 (0) 7479343956 / +353 (0) 851441181
Email: cmnosullivan@gmail.com
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AC2: 'Reply on RC1', Conor O'Sullivan, 16 Sep 2022
Re: Manuscript Revision
Dear Editor,
Apologies for the late resubmission of our manuscript to your journal, my, co-authors and I hope it can still be considered as a revision of our initial submission. My co-authors and I thank you for securing two excellent reviewers for our manuscript. We are very appreciative of the feedback our manuscript has received from both reviewers and an associate editor of Basin Research. We have found this feedback both positive and constructive and feel it has greatly refined our original submission. Please see below a summary of the changes we have made to the manuscript, alongside an attached table outlining the specific responses to reviewer comments in detail.
In general, we have:
- Highlighted uncertainty and data resolution more clearly.
- Clarified our use of specific salt tectonics terminology to improve accuracy and avoid misconception.
- Revised the text to remove typos and make arguments more focused.
- Improved detail, clarity and information on all figures.
With these revisions in mind, we hope this manuscript has suitably addressed the shortcomings of our previous submission and satisfies the reviewers’ comments. The co-authors and I are still confident this manuscript will be an important contribution to the understanding of salt tectonics and salt distribution on the European Atlantic margin. We look forward to hearing from you and please do not hesitate to contact me if you require any additional material or clarification.
Yours sincerely,
Conor O’Sullivan (on behalf of the co-authors)
Irish Centre for Research in Applied Geosciences (iCRAG)
O’Brien Centre for Science (East)
University College Dublin
Belfield, Dublin 4, Ireland.
Telephone: +353 (0) 85 144 1181
Email: conor.osullivan@icrag-centre.org
Citation: https://doi.org/10.5194/egusphere-2022-581-AC2 -
AC4: 'Reply on RC1', Conor O'Sullivan, 16 Sep 2022
Please disregard the previous two comments, the wrong text was uploaded!
Re: Manuscript Revision
Dear Editor,
My co-authors and I thank you for securing two excellent reviewers for our manuscript. We are very appreciative of the feedback our manuscript has received from both. We have found this feedback both positive and constructive and feel it has greatly refined our original submission. Please see below a summary of the changes we have made to the manuscript, alongside an attached table outlining the specific responses to reviewer comments in detail.
In general, we have:
- Improved detail, clarity and information on all figures.
- Revised the text to remove typos and make arguments more focused.
- Refined the terminology when referring to particular tectonic events to avoid confusion.
- Added more quantification regarding post-rift salt-related fault movement.
- Added more detail, both in-text and in figures, to describe the basement structure in the study area.
With these revisions in mind, we hope this manuscript has suitably addressed the shortcomings of our previous submission and satisfies the reviewers’ comments. The co-authors and I are still confident this manuscript will be an important contribution to the understanding of salt tectonics and salt distribution on the European Atlantic margin. We look forward to hearing from you and please do not hesitate to contact me if you require any additional material or clarification.
Yours sincerely,
Dr. Conor O’Sullivan (on behalf of the co-authors)
Structural Geologist
Petroleum Experts Ltd.,
Petex House,
10 Logie Mill,
Edinburgh,
EH7 4NJ,
United Kingdom.
Telephone: +44 (0) 7479343956 / +353 (0) 851441181
Email: cmnosullivan@gmail.com
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AC1: 'Reply on RC1', Conor O'Sullivan, 16 Sep 2022
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RC2: 'Comment on egusphere-2022-581', Amir Joffe, 01 Aug 2022
Dear Conor (and all the other authors),
It was a pleasure to be invited to review your work, and thoroughly enjoyed reading your manuscript. As I am not an expert in this geographic area, but do have interest in basin evolution, the interaction of normal and strike-slip faults, I have read and reviewed this manuscript as such. I believe that some of the comments might be trivial to you, but please consider that with such a general title and excellent work you have performed, this manuscript has a potential to draw students and other geologist as a ‘first, must read’ introduction paper to this basin. Therefore, some of my comments focus on adding a little more clarity to a non-expert of the development of western Europe reader.
Overall, I think the manuscript has a potential to significantly contribute to the understanding NW-Europe, and the relationship between multiple extension phases with inherited structural relief. The authors had excess to an extensive set of datasets, which they used were well, even if I think that more use could have been used with showcasing more thickness maps from the 3D volumes (see detailed comments). The text is structures very well, with excellent figures that support the authors claims, and help the reader to follow the text.
Some clarificatios are needed on the impact of the Caledonian structures on the development of the basin bounding faults. Are these older structures act as weakness zones? Do you think there is any affect to transtension/transoression on the transfer zones location? And finally, why would you think initial segmentation is preserved in the Northern Slyne basin but not in the southern?
As stated above, this work is novel and should become a key reference for future studies of western Europe and rift basins. I hope the authors find my comments are useful. I have included my details below, so please so do not hesitate to get in touch if any of my comments are unclear or you would just like to discuss about them.
Best of luck with the resubmission process,
Amir Joffe
Amir.joffe18@imperial.ac.uk
Detailed comments:
- Line 14: I’d add what age is “Caledonian”. It would really help phrase it in the context of later events.
- Line 15: I’d potentially add “Initial” at the beginning of this sentence. It helps a non-expert understand they should expect a sequential sequence of rifting.
- Lines 35-36: You are referring here to Variscian orogenic and the opening of the Atlantic Ocean, but Figure 1A legend says Palaeozoic & Pre-Cambrian basement. I think that it would make it easier to add an age to Variscian orogeny in the text, even if this is still early in the introduction. This comment will also help understand the relationship between the Caledonian and Variscian orogenies in the next sentence.
- Line 39: The transition here is not easy. Are you referring to the Caledonian or Variscian structures? I’d maybe write something like “Later rift event had either reactivated Caledonian/Variscian age structure if oriented optimally (REFs) or were segmented, hindering fault growth, in cases rift structures were oblique (REFs)”.
- Line 55: I would suggest adding: “The Slyne basin (XX km long and YY km wide)…”
- Line 58: You make it sounds like transfer zones are specific description of these structures located in the Slye basin. I think that you could make it a little more coherent by: “In the case of the Slyne basin, these transfer zones have been…”
- Line 60-62: I’m sure I miss understand something here, but it reads like you’re suggesting that transfer zones are areas of reactivation of pre-existing zones of weakness. To my understanding, transfer zone area areas where normal faults transfer strain (Morley et al. 1990; Childs et al. 2019). Could you make it clearer what is your definition of transfer zones?
- Line 92-93: Can you highlight the location of the Caledonian terrane boundaries are in Figure 1.
- Line 94-102: Very clear description to a great figure.
- Line 112: I’d add a reference to Figure 1B at the end of this sentence. I might also think if there is a way to somehow highlight in that great figure what are the Caledonian structures? It would really help differentiating between them and the younger faults.
- Line 149: I’d change to: “The Carboniferous mudstones are overlain…” just to make it clear you are not referring to the Silurian metasediments you ended the sentence with.
- Line 200: The horizons are not in Figure 2. It would be great to have them in.
- Line 260: please add reference to Figure 1B at the end of this. This is a very cool observation, but complicated to understand without the figures in hand, so reference to figures is helpful.
- Lines 273 – 274: Not sure that I completely agree. You said that the exact location of the GGFZ is not that clear in that area, I don’t think you can conclude that it acted as a barrier to the propagation of the Slyne Embayment basin bounding fault.
- Lines 274 – 275: I’d just add why you think they are linked. I guess you think that because of their geographical location? If so, you could add something like: “As the GGFZ transect the CSTZ and is located between the fault bounding the Slyne Embayment and the southernmost segment of fault system bounding the Northern Slyne Sub-basin, there is probable structural link between these fault systems.”
- Lines 280 – 283: A bit long and convoluted. Could you split into two sentences, stopping after the reference to the figure. Does the HBFC has a sinistral component? If so, I think it might be helpful to add that in the text and add strike-slip arrows to figure 1B (and also to the other basement faults).
- Line 295 – 296: Figure 2 shows two small rectangles indicating Triassic salt in the southern basin, so maybe worthwhile removing them from Figure 2. A reference to Figure 9A could also be useful here.
- Line 333 – 335: could you please add reference to Figure/other paper like you did in the next sentence? As I would guess that most readers of this manuscript are not salt-tectonic experts, it would really help demonstrate your point.
- Chapter 6: The transition between the results (Chapter 5) and Chapter 6 makes the reading a little difficult as it is not clear what is the difference between Chapter 6 (Structural Evolution of the Slyne Basin) and Chapter 3 (Geological Settings). This is because Chapter 6 is followed by a Discussion chapter (Chapter 7). I think could easily be fixed with a sentence or two describing the role of Chapter 6 (Maybe worth thinking about making it a sub-chapter within the Discussion).
- Chapter 6: I think that adding thickness maps for each unit will be very useful to illustrate the relationship between the different structural elements. These cross-sections are helping, and are doing a good job at making the point you are trying to make, but as you have such an abundance of seismic data it would be really cool to see that. I understand this would require a significant amount of work, so I leave that to the authors discretion.
- Chapter 6: Great Glen is strike slip. Does the HBFC and the SUAG are also strike slip? If so, do they have the same direction? If no what is their original offset and how would you think that influence the location of the later faults? Does transtension/transoression have influenced the location of the basin-bounding faults or the geometry of the Slyne basin?
- Chapter 6: Adding a block diagram/simplified map similar to Figure 14 which demonstrates the evolution which you can use refer back throughout Chapter 6 would be useful. As there almost no use of the 3D volumes, I think that as the paper is titled tecono-stratigraphy of the basin, this sort of figure would be very helpful to a non-expert reader.
- Lines 361 – 364: I think that either a thickness map or illustration on the cross-section is needed here. It’s very hard to judge based on the cross-sections provided if the thickness was salt-tectonic related or related to other extension related structures occurring at the time.
- Lines 394 – 396: This is an interesting and non-trivial observation! At first, I was certain the Late Jurassic had growth strata, but I totally agree. Does that mean this thick unit is pre-kinematic to the main extension phase? These thickness changes are not trivial, and I would suggest adding some indication for this in the cross-sections to help readers understand that. I’d also think that a little more details on why you think that growth strata are present in the Central and Northern Slyne basins, but not in the Northern basin. Or how these faults in the Northern basin had accumulated such thick strata with not apparent growth at all. Sounds like a key feature not only to the understanding of the basin development, but also to the strain interaction between the two sub-basins!
- Line 402: “this fault”, does that mean the basin-bounding fault or the intra-basinal listric fault?
- Line 435 – 437: Not clear what syn-rift episode you are referring to. A reference to a figure is missing here.
- Lines 513 – 519: A simplified figure is missing here to help explain this text. I think you can add more text/details to Figure 13 B&C to show how the angle between extension and pre-existing structure will affect the resulted structures. Also, as this is a critical text to your assumptions, if you could add a reference here it would help (maybe to the analogue modelling you wrote).
- Lines 525 – 531: I couldn’t follow the differences between Figure 13B&C, or fully understand the text. An example for the confusion is: “… characteristic of extension oblique to the basement fabric; the key feature is that the overall orientation of the structure is parallel to the basement structure”. It’s not very clear who are the structures, what is the basement fabric and what are the basement structures, mainly because it’s not very clear from Figure 13B what you are referring to in the text. Amending the text or adding arrows/text in the figures could help.
- Line 542: an earlier explanation on what is the Caledonian trend (potentially in the Geological settings) would have made it much easier to understand. See my previous comments.
- Lines 552 – 555: Could you suggest why the initial segmentation is preserved in the Northern Slyne basin but not in the southern? Is it the activation of the Great Glen?
- General: There are multiple syn-rift episodes, it would be easier if you could number/give age throughout the text when you refer to them. It’s not easy to follow ‘main syn-rift’ or ‘syn-rift’, especially when the rifting episodes are not coeval in all sub-basins.
Citation: https://doi.org/10.5194/egusphere-2022-581-RC2 -
AC3: 'Reply on RC2', Conor O'Sullivan, 16 Sep 2022
Re: Manuscript Revision
Dear Editor,
My co-authors and I thank you for securing two excellent reviewers for our manuscript. We are very appreciative of the feedback our manuscript has received from both. We have found this feedback both positive and constructive and feel it has greatly refined our original submission. Please see below a summary of the changes we have made to the manuscript, alongside an attached table outlining the specific responses to reviewer comments in detail.
In general, we have:
- Improved detail, clarity and information on all figures.
- Revised the text to remove typos and make arguments more focused.
- Refined the terminology when referring to particular tectonic events to avoid confusion.
- Added more quantification regarding post-rift salt-related fault movement.
- Added more detail, both in-text and in figures, to describe the basement structure in the study area.
With these revisions in mind, we hope this manuscript has suitably addressed the shortcomings of our previous submission and satisfies the reviewers’ comments. The co-authors and I are still confident this manuscript will be an important contribution to the understanding of salt tectonics and salt distribution on the European Atlantic margin. We look forward to hearing from you and please do not hesitate to contact me if you require any additional material or clarification.
Yours sincerely,
Dr. Conor O’Sullivan (on behalf of the co-authors)
Structural Geologist
Petroleum Experts Ltd.,
Petex House,
10 Logie Mill,
Edinburgh,
EH7 4NJ,
United Kingdom.
Telephone: +44 (0) 7479343956 / +353 (0) 851441181
Email: cmnosullivan@gmail.com
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2022-581', Tiago Alves, 15 Jul 2022
Dear authors,
I reviewed your paper soon after it was sent to me as your study area is of great interest. I found your paper well organised and clear, though needing to be somewhat 'retouched' in a few sections. I found a few typos and minor aspects, which I will comment in detail below. All in all, a more complete undertanding of the Slyne Basin is paramount to chracaterising the history of North Atlantic rifting, so your work is comendable and welcome to the geological community.
Detailed comments (see attached .pdf for reference):
1. The abstract refers to two tectonic events, but only one post-rift unconformity. Please, check this part.
2. There are published examples in the literature of Paleozoic fault zones controlling the formation of rift basins in deep offshore Portugal. The most documented of such examples is the Messejana Fault Zone in SW Portugal - see Pereira and Alves (2013). Crustal deformation and submarine canyon incision in a Meso-Cenozoic first-order transfer zone (SW Iberia, North Atlantic Ocean). Tectonophysics 601, 148-162.
3. In specific points in the text, I felt that a map of the Caledonian sutire zones and basement structures may be of use to your reasoning. Please, check the literature for one such maps - an example being Figure 4 in Alves et al. (2022). Analysis of a basement fault zone with geothermal potential in the Southern North Sea. Geothermics 102, 102398. Could you find one such maps for the region around Ireland/West GB?
4. Figure 2 looks slightly tentative as the tectonic events in the right-hand column seem not to correlate well with the unconformities and units to the left. There are some incorrect detail regarding the ages of rifting and ocean spreading in West Iberia and the Central Atlantic. I would suggest you to review the ages of these tectonic events.
5. An example of item 4. above is clear in Lines 168-170, in which you are recording an Albian-age succession that is associated with the break-up process in the Bay of Biscay. This continetal-breakup process was terminated in the Cenomanian-Turonian and is part of a Breakup Sequence in NW Iberia (see Alves and Cunha, 2018, EPSL; Alves et al., 2020, JMPG). Thus, your stratigraphic recorded in Ireland is being influenced by events that happened in SW Europe.
6. The mid part of the paper is informative and very nice to read. Some quantification is missing, though. The role of salt in basin inversion needs to the be quantified and compared with similar basins in Portugal (Lusitanian Basin) and Southern North Sea (Broad Fourteens Basin and the Dutch offshore). Again, as an example Alves et al. (2003) Post-Jurassic tectono-sedimentary evolution of the Northern Lusitanian Basin (Western Iberian margin). Basin Research 15, 227-249 developed such a comparison to find out that a value of horizontal shortening around 10% was able to preserve multipel petroleum systems in the Dutch offshore basins, while disrupting trap potential in the Lusitanian Basin - in great part due to the generalised exhumation of the basin during the Cenozoic, accompanying basin inversion.
Could you add information on the impact of salt as a facilitator of such inversion? Are you dealing with a thin-skinned tectonic style (Alves et al., 2002. Marine and Petroleum Geology) in your study area, or this is apparent only in parts of your study area? Is the tectonic shortening in your region concentrated close to basin-bounding structures, or more widespread.
As a final remark, some parts of the paper are excellent and honest accounts of the geological evolution of the Slyne Basin. A better link between basement Caledonian structures (via a map) and the major fault trends in the study area will significantly improve this paper. Similarly, the role of salt in the subsidence and basin inversion histories of the study area should be developed in more detail. This is a very interesting article just needing a minor to moderate re-focus of some aspects.
Tiago M. Alves
3D Seismic Lab - Cardiff University
-
AC1: 'Reply on RC1', Conor O'Sullivan, 16 Sep 2022
Re: Manuscript Submission
Dear Editor,
Please find enclosed our manuscript titled ‘Tectonostratigraphic evolution of the Slyne Basin’ by Conor O’Sullivan (corresponding author), Conrad Childs, Muhammad Saqab, John Walsh and Patrick Shannon, which we have submitted for peer-review and publication in the Solid Earth Journal. There has been significant study into the development of rift basins along the European Atlantic margin from offshore Portugal to offshore northern Norway. The Irish Atlantic margin has by comparison seen conspicuously little investigation in this area, owing to the historic lack of seismic data and the relatively limited number of well penetrations. Since then, significantly more seismic data has been acquired and additional exploration wells have been drilled, providing the materials and impetus for a re-evaluation of these basins.
Our manuscript is a comprehensive investigation of the structural evolution of the Slyne Basin, located offshore north-western Ireland. We incorporate the most recent seismic reflection data alongside recent hydrocarbon exploration wells to investigate the location, shape and age of major structures which define this basin. We also investigate the upper-crustal character of this Mesozoic rift basin relative to deep, crustal-scale structures believed to be related to the Caledonian Orogeny. We demonstrate a rarely documented style of fault reactivation where basin-bounding faults are oblique to the earlier structural trend, but the initial fault segments are parallel to this trend. This manuscript will be of interest to several geoscientists working on the Irish Atlantic margin and adjacent jurisdictions, as well as the global community investigating fault analysis and general structural geology.
We think this paper will be a valid contribution to your journal and hope that you will consider it positively. Alongside this submission we have included a list of potential reviewers and would be grateful of any additional reviewers of your choice. We look forward to hearing from you.
Yours sincerely,
Dr. Conor O’Sullivan (on behalf of the co-authors)
Structural Geologist
Petroleum Experts Ltd.,
Petex House,
10 Logie Mill,
Edinburgh,
EH7 4NJ,
United Kingdom.
Telephone: +44 (0) 7479343956 / +353 (0) 851441181
Email: cmnosullivan@gmail.com
-
AC2: 'Reply on RC1', Conor O'Sullivan, 16 Sep 2022
Re: Manuscript Revision
Dear Editor,
Apologies for the late resubmission of our manuscript to your journal, my, co-authors and I hope it can still be considered as a revision of our initial submission. My co-authors and I thank you for securing two excellent reviewers for our manuscript. We are very appreciative of the feedback our manuscript has received from both reviewers and an associate editor of Basin Research. We have found this feedback both positive and constructive and feel it has greatly refined our original submission. Please see below a summary of the changes we have made to the manuscript, alongside an attached table outlining the specific responses to reviewer comments in detail.
In general, we have:
- Highlighted uncertainty and data resolution more clearly.
- Clarified our use of specific salt tectonics terminology to improve accuracy and avoid misconception.
- Revised the text to remove typos and make arguments more focused.
- Improved detail, clarity and information on all figures.
With these revisions in mind, we hope this manuscript has suitably addressed the shortcomings of our previous submission and satisfies the reviewers’ comments. The co-authors and I are still confident this manuscript will be an important contribution to the understanding of salt tectonics and salt distribution on the European Atlantic margin. We look forward to hearing from you and please do not hesitate to contact me if you require any additional material or clarification.
Yours sincerely,
Conor O’Sullivan (on behalf of the co-authors)
Irish Centre for Research in Applied Geosciences (iCRAG)
O’Brien Centre for Science (East)
University College Dublin
Belfield, Dublin 4, Ireland.
Telephone: +353 (0) 85 144 1181
Email: conor.osullivan@icrag-centre.org
Citation: https://doi.org/10.5194/egusphere-2022-581-AC2 -
AC4: 'Reply on RC1', Conor O'Sullivan, 16 Sep 2022
Please disregard the previous two comments, the wrong text was uploaded!
Re: Manuscript Revision
Dear Editor,
My co-authors and I thank you for securing two excellent reviewers for our manuscript. We are very appreciative of the feedback our manuscript has received from both. We have found this feedback both positive and constructive and feel it has greatly refined our original submission. Please see below a summary of the changes we have made to the manuscript, alongside an attached table outlining the specific responses to reviewer comments in detail.
In general, we have:
- Improved detail, clarity and information on all figures.
- Revised the text to remove typos and make arguments more focused.
- Refined the terminology when referring to particular tectonic events to avoid confusion.
- Added more quantification regarding post-rift salt-related fault movement.
- Added more detail, both in-text and in figures, to describe the basement structure in the study area.
With these revisions in mind, we hope this manuscript has suitably addressed the shortcomings of our previous submission and satisfies the reviewers’ comments. The co-authors and I are still confident this manuscript will be an important contribution to the understanding of salt tectonics and salt distribution on the European Atlantic margin. We look forward to hearing from you and please do not hesitate to contact me if you require any additional material or clarification.
Yours sincerely,
Dr. Conor O’Sullivan (on behalf of the co-authors)
Structural Geologist
Petroleum Experts Ltd.,
Petex House,
10 Logie Mill,
Edinburgh,
EH7 4NJ,
United Kingdom.
Telephone: +44 (0) 7479343956 / +353 (0) 851441181
Email: cmnosullivan@gmail.com
-
AC1: 'Reply on RC1', Conor O'Sullivan, 16 Sep 2022
-
RC2: 'Comment on egusphere-2022-581', Amir Joffe, 01 Aug 2022
Dear Conor (and all the other authors),
It was a pleasure to be invited to review your work, and thoroughly enjoyed reading your manuscript. As I am not an expert in this geographic area, but do have interest in basin evolution, the interaction of normal and strike-slip faults, I have read and reviewed this manuscript as such. I believe that some of the comments might be trivial to you, but please consider that with such a general title and excellent work you have performed, this manuscript has a potential to draw students and other geologist as a ‘first, must read’ introduction paper to this basin. Therefore, some of my comments focus on adding a little more clarity to a non-expert of the development of western Europe reader.
Overall, I think the manuscript has a potential to significantly contribute to the understanding NW-Europe, and the relationship between multiple extension phases with inherited structural relief. The authors had excess to an extensive set of datasets, which they used were well, even if I think that more use could have been used with showcasing more thickness maps from the 3D volumes (see detailed comments). The text is structures very well, with excellent figures that support the authors claims, and help the reader to follow the text.
Some clarificatios are needed on the impact of the Caledonian structures on the development of the basin bounding faults. Are these older structures act as weakness zones? Do you think there is any affect to transtension/transoression on the transfer zones location? And finally, why would you think initial segmentation is preserved in the Northern Slyne basin but not in the southern?
As stated above, this work is novel and should become a key reference for future studies of western Europe and rift basins. I hope the authors find my comments are useful. I have included my details below, so please so do not hesitate to get in touch if any of my comments are unclear or you would just like to discuss about them.
Best of luck with the resubmission process,
Amir Joffe
Amir.joffe18@imperial.ac.uk
Detailed comments:
- Line 14: I’d add what age is “Caledonian”. It would really help phrase it in the context of later events.
- Line 15: I’d potentially add “Initial” at the beginning of this sentence. It helps a non-expert understand they should expect a sequential sequence of rifting.
- Lines 35-36: You are referring here to Variscian orogenic and the opening of the Atlantic Ocean, but Figure 1A legend says Palaeozoic & Pre-Cambrian basement. I think that it would make it easier to add an age to Variscian orogeny in the text, even if this is still early in the introduction. This comment will also help understand the relationship between the Caledonian and Variscian orogenies in the next sentence.
- Line 39: The transition here is not easy. Are you referring to the Caledonian or Variscian structures? I’d maybe write something like “Later rift event had either reactivated Caledonian/Variscian age structure if oriented optimally (REFs) or were segmented, hindering fault growth, in cases rift structures were oblique (REFs)”.
- Line 55: I would suggest adding: “The Slyne basin (XX km long and YY km wide)…”
- Line 58: You make it sounds like transfer zones are specific description of these structures located in the Slye basin. I think that you could make it a little more coherent by: “In the case of the Slyne basin, these transfer zones have been…”
- Line 60-62: I’m sure I miss understand something here, but it reads like you’re suggesting that transfer zones are areas of reactivation of pre-existing zones of weakness. To my understanding, transfer zone area areas where normal faults transfer strain (Morley et al. 1990; Childs et al. 2019). Could you make it clearer what is your definition of transfer zones?
- Line 92-93: Can you highlight the location of the Caledonian terrane boundaries are in Figure 1.
- Line 94-102: Very clear description to a great figure.
- Line 112: I’d add a reference to Figure 1B at the end of this sentence. I might also think if there is a way to somehow highlight in that great figure what are the Caledonian structures? It would really help differentiating between them and the younger faults.
- Line 149: I’d change to: “The Carboniferous mudstones are overlain…” just to make it clear you are not referring to the Silurian metasediments you ended the sentence with.
- Line 200: The horizons are not in Figure 2. It would be great to have them in.
- Line 260: please add reference to Figure 1B at the end of this. This is a very cool observation, but complicated to understand without the figures in hand, so reference to figures is helpful.
- Lines 273 – 274: Not sure that I completely agree. You said that the exact location of the GGFZ is not that clear in that area, I don’t think you can conclude that it acted as a barrier to the propagation of the Slyne Embayment basin bounding fault.
- Lines 274 – 275: I’d just add why you think they are linked. I guess you think that because of their geographical location? If so, you could add something like: “As the GGFZ transect the CSTZ and is located between the fault bounding the Slyne Embayment and the southernmost segment of fault system bounding the Northern Slyne Sub-basin, there is probable structural link between these fault systems.”
- Lines 280 – 283: A bit long and convoluted. Could you split into two sentences, stopping after the reference to the figure. Does the HBFC has a sinistral component? If so, I think it might be helpful to add that in the text and add strike-slip arrows to figure 1B (and also to the other basement faults).
- Line 295 – 296: Figure 2 shows two small rectangles indicating Triassic salt in the southern basin, so maybe worthwhile removing them from Figure 2. A reference to Figure 9A could also be useful here.
- Line 333 – 335: could you please add reference to Figure/other paper like you did in the next sentence? As I would guess that most readers of this manuscript are not salt-tectonic experts, it would really help demonstrate your point.
- Chapter 6: The transition between the results (Chapter 5) and Chapter 6 makes the reading a little difficult as it is not clear what is the difference between Chapter 6 (Structural Evolution of the Slyne Basin) and Chapter 3 (Geological Settings). This is because Chapter 6 is followed by a Discussion chapter (Chapter 7). I think could easily be fixed with a sentence or two describing the role of Chapter 6 (Maybe worth thinking about making it a sub-chapter within the Discussion).
- Chapter 6: I think that adding thickness maps for each unit will be very useful to illustrate the relationship between the different structural elements. These cross-sections are helping, and are doing a good job at making the point you are trying to make, but as you have such an abundance of seismic data it would be really cool to see that. I understand this would require a significant amount of work, so I leave that to the authors discretion.
- Chapter 6: Great Glen is strike slip. Does the HBFC and the SUAG are also strike slip? If so, do they have the same direction? If no what is their original offset and how would you think that influence the location of the later faults? Does transtension/transoression have influenced the location of the basin-bounding faults or the geometry of the Slyne basin?
- Chapter 6: Adding a block diagram/simplified map similar to Figure 14 which demonstrates the evolution which you can use refer back throughout Chapter 6 would be useful. As there almost no use of the 3D volumes, I think that as the paper is titled tecono-stratigraphy of the basin, this sort of figure would be very helpful to a non-expert reader.
- Lines 361 – 364: I think that either a thickness map or illustration on the cross-section is needed here. It’s very hard to judge based on the cross-sections provided if the thickness was salt-tectonic related or related to other extension related structures occurring at the time.
- Lines 394 – 396: This is an interesting and non-trivial observation! At first, I was certain the Late Jurassic had growth strata, but I totally agree. Does that mean this thick unit is pre-kinematic to the main extension phase? These thickness changes are not trivial, and I would suggest adding some indication for this in the cross-sections to help readers understand that. I’d also think that a little more details on why you think that growth strata are present in the Central and Northern Slyne basins, but not in the Northern basin. Or how these faults in the Northern basin had accumulated such thick strata with not apparent growth at all. Sounds like a key feature not only to the understanding of the basin development, but also to the strain interaction between the two sub-basins!
- Line 402: “this fault”, does that mean the basin-bounding fault or the intra-basinal listric fault?
- Line 435 – 437: Not clear what syn-rift episode you are referring to. A reference to a figure is missing here.
- Lines 513 – 519: A simplified figure is missing here to help explain this text. I think you can add more text/details to Figure 13 B&C to show how the angle between extension and pre-existing structure will affect the resulted structures. Also, as this is a critical text to your assumptions, if you could add a reference here it would help (maybe to the analogue modelling you wrote).
- Lines 525 – 531: I couldn’t follow the differences between Figure 13B&C, or fully understand the text. An example for the confusion is: “… characteristic of extension oblique to the basement fabric; the key feature is that the overall orientation of the structure is parallel to the basement structure”. It’s not very clear who are the structures, what is the basement fabric and what are the basement structures, mainly because it’s not very clear from Figure 13B what you are referring to in the text. Amending the text or adding arrows/text in the figures could help.
- Line 542: an earlier explanation on what is the Caledonian trend (potentially in the Geological settings) would have made it much easier to understand. See my previous comments.
- Lines 552 – 555: Could you suggest why the initial segmentation is preserved in the Northern Slyne basin but not in the southern? Is it the activation of the Great Glen?
- General: There are multiple syn-rift episodes, it would be easier if you could number/give age throughout the text when you refer to them. It’s not easy to follow ‘main syn-rift’ or ‘syn-rift’, especially when the rifting episodes are not coeval in all sub-basins.
Citation: https://doi.org/10.5194/egusphere-2022-581-RC2 -
AC3: 'Reply on RC2', Conor O'Sullivan, 16 Sep 2022
Re: Manuscript Revision
Dear Editor,
My co-authors and I thank you for securing two excellent reviewers for our manuscript. We are very appreciative of the feedback our manuscript has received from both. We have found this feedback both positive and constructive and feel it has greatly refined our original submission. Please see below a summary of the changes we have made to the manuscript, alongside an attached table outlining the specific responses to reviewer comments in detail.
In general, we have:
- Improved detail, clarity and information on all figures.
- Revised the text to remove typos and make arguments more focused.
- Refined the terminology when referring to particular tectonic events to avoid confusion.
- Added more quantification regarding post-rift salt-related fault movement.
- Added more detail, both in-text and in figures, to describe the basement structure in the study area.
With these revisions in mind, we hope this manuscript has suitably addressed the shortcomings of our previous submission and satisfies the reviewers’ comments. The co-authors and I are still confident this manuscript will be an important contribution to the understanding of salt tectonics and salt distribution on the European Atlantic margin. We look forward to hearing from you and please do not hesitate to contact me if you require any additional material or clarification.
Yours sincerely,
Dr. Conor O’Sullivan (on behalf of the co-authors)
Structural Geologist
Petroleum Experts Ltd.,
Petex House,
10 Logie Mill,
Edinburgh,
EH7 4NJ,
United Kingdom.
Telephone: +44 (0) 7479343956 / +353 (0) 851441181
Email: cmnosullivan@gmail.com
Peer review completion
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Conor M. O'Sullivan et al.
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