General comments - overall quality of the preprint

This paper describes a high resolution microstructural study of the fine-scale processes underlying shear failure of claystones, specifically the Opalinus Clay (Shaly facies).  The microstructural (BIB-SEM) work is beautifully executed and presented in terms of micrographs.  The study forms a valuable contribution in that very little has been done to date on the micro-mechanisms that control failure of such materials. Moreover,  what has been done previously mainly addresses samples deformed to large strains beyond failure, rather than the failure process itself. The present work focuses specifically on the microstructures and micro-mechanisms associated with initiation of shear failure and shear localization at low strains up to the peak stress, thus providing fundamental insight into and a physical basis for the formulation and testing of quantitative micromechanical  models in future. Given the importance of understanding clay and claystone failure and associated permeability changes in the context of geological storage systems, the present work adds a useful observational dimension to purely continuum approaches to shear failure and localization in these materials.  It is nice work, it is well presented, well illustrated and mostly well written. It certainly deserves to be published. At the same time, however, there are a substantial number of scientific and technical issues that I feel the authors need to address before the pre-print would be acceptable for full publication. These are mostly minor but quite numerous. For this reason, I recommend major revisions.

SCIENTIFIC COMMENTS

General scientific points that require attention are as follows:

A) The authors often describe their work as addressing the microphysics or micromechanics of shear failure of the OPA material. However, there is no physics or mechanics in the paper at all – it is purely qualitative and conceptual. Rather, the authors address the MICRO-MECHANISMS involved in deformation and failure of the material studied and should really use this term to describe their work (it is a micro-mechanistic study).

B) The experimental sample investigated was tested unconsolidated and undrained at 4 MPa confining pressure. There is no discussion as to how these test conditions (hence observed processes) might relate to conditions of interest in the subsurface, such as caprock or repository conditions. Perhaps the effective stress would be comparable (low) due to consolidation effects, but some discussion of this would be welcome – as would the likely pore pressure evolution in the experiment (perhaps based on volumetric strain data). 

C) The description of the experiment does not seem quite right in terms of boundary conditions applied to the sample. The authors seem to be saying that the test was carried out at constant confining pressure and at controlled circumferential strain rate, which is equivalent to controlled radial strain rate. Do the authors mean that axial loading system was servocontrolled to provide a constant radial expansion (or contraction rate)?  This seems unlikely to me. Or do they mean that a constant axial strain rate (or stress rate) was imposed and the radial expansion/contraction was simply measured (not controlled)?

D) The terminology used to describe the various cracks, fractures and shear bands at different scales is somewhat confusing to the reader (at least this one!) and not very systematic. I would strongly recommend defining the terminology to be used for these features at an early stage in the manuscript. It would be useful if the authors could define cracks versus microcracks, as well as defining crack character – i.e.  whether individual cracks are Mode I (opening) or Mode II (shear) cracks or mixed mode.  The term “shear band” should also be defined and appropriately distinguished from a shear crack – at least in a morphological sense. Voids that open between grains due to local dilatation should also be defined as something like dilatational or dilated pores.  Of course, the observed features may ALL have been influenced by unloading (removing axial stress and/or confining pressure), so that needs to be mentioned.

E) While it is clear that many of the observed cracks and shear bands are directly associated with the evolution of the macroscopic shear failure process, some features that the authors link to shear failure could potentially be present in the starting material or could have been caused by the application of confining pressure (i.e. to the hydrostatic component of consolidation). Crushed fossils, disrupted pyrite framboids, buckling of phyllosilicates etc all come to mind here. I do not doubt the authors’ interpretation necessarily, but it would be desirable to add a few micrographs AT LEAST of an undeformed control sample to prove that the above features are not present in the starting material. Ideally one would like to see a few shots of a hydrostatically loaded microstructure too, if easily available.

F) Last but not least, some brief discussion is needed as to how fluid flow from consolidating regions of the sample to dilating regions of the sample may have facilitated shear localization (as opposed to dilatancy hardening), as this could be a key component in controlling the failure behavior and strength. Such aspects would be essential for any quantitative microphysical modelling in future, and notably for understanding strain rate effects.

Detailed scientific comments/questions

1) Title: This seems unnecessarily long and not especially informative to me.  Would something like “High resolution BIB-SEM study of micromechanisms leading to shear failure of Opalinus Clay in a triaxial test” not be more accurate and sufficient??

2) Abstract: Best clarify that failure occurred by shear failure and mention orientation of the shear farcture/band network (line 21).

3) Last statement about LIMITED similarity with natural fault zone microstructure and inferred role of pressure solution in nature is not very well supported in the main text.  Perhaps add some additional evidence in the main text?

4) Intro:  I advise using the term “radioactive waste” rather than “nuclear waste”.   Nuclear suggests high level waste risk which is often not the case.  Atomic nuclei are also hardly waste.

5) Page 2, line 48.  Microstructure is perhaps the wrong term here.  Mineral composition would be accurate. Line 52 – what measure of visible pore size shows a power law distribution??? Lines 61-62 – compressive loading of samples in what orientations with respect to bedding?  And what is meant by plastic flow – please define as it has many meanings.

6) Page 3, Line 83: Is “slickensides” the correct term here for these very fine striations in the mirror slip surface?  Are they optically visible or only in SEM and sub-optical? Line 88 - do you mean smectite interlayer water here?  Please clarify.

7) Page 4, Line 124: Mixed layer silicates?? Do you mean mixed layer smectites and smectite-illite??? Last line: what is the pore fluid?  A brine with what composition?

8) When first describing fracture/crack/bedding/loading orientation, please define what you mean by orientation or inclination in terms of an angle (say theta) defined in a small diagram.  Present usage is confusing, perhaps due to language. It is important to be consistent in the use of this terminology throughout the paper. Confusing at present.

9) Section 2.2.  Please clarify boundary conditions imposed in the experiment – see point B above.

10) Section 2.3, first line.  Mechanically stabilized with epoxy ?

11) Section 3.1, last 3 lines.  Specify branching fractures as the green ones if that is what is meant (all look branching to me). Fragments are only CRUDELY lens-shaped – best say this perhaps.  Please clarify what is meant by “relay fractures”.  

12) Section 3.2, first paragraph: How long were the samples stored before study ?? Please indicate in the material description. What reacted to produce gypsum?  How do you know this occurred during sample storage?? Could storage have had any other effects that could corrupt the microstructure?

13)Page 6, Line 183: Microcracks were often oblique to bedding…. Which set is meant here?

14) At several points in the description of the microstructure (Results – Figs 4 and 5), reference is made to increased porosity and grain-matrix separation at specific sites around clasts and clast-like/sigmoidal micro-lithons.  These sites seem to correspond to interfaces under local extension – i.e. at roughly 45 degrees to the overall (macroscopic) shear failure plane orientation.  This should be described as such, if indeed a widespread feature, making reference to previous work that has observed and constructed models for this type of feature (e.g. Den Hartog & Spiers 2014; Haines et al., 2013). These features have key mechanical significance in defining the onset of mechanical weakening due to a decrease in dilation angle (see papers by Den Hartog et al).

15) Page 8, line 235: “Plastic reorientation of clay aggregates”. What exactly is meant here? Why is this plastic not frictional intergranular sliding? “Ductile” deformation is inferred from reorientation to form an SPO,  I guess? The term “plastic” is used elsewhere in the ms to refer to intracrystalline plasticity of the phyllosilicate grains>  perhaps make more consistent? Line 236 – “grain boundary sliding”. This term is generally used for a high T process involving grain boundary dislocations.  Better use “intergranular sliding” here as it is likely a frictional or fluid-lubricated sliding process.  Lines 239-242 – I am surprised that no reference is made here to the top quality work on deformation of claystones such as COX by the Paris Est (Ecole des Pont) Team (recent papers by Philippe Braun, Pierre Delage and co-workers). Relevant work should be added.

16) Page 9, Line 260: It is assumed that unloading elasticity is equal throughout the sample.  Perhaps make this “roughly equal” as the elastic stiffness of the porous zones will certainly be much lower than the undeformed matrix. Line 268-269 – reference is made here to the amounts of elastic versus inelastic deformation in the sample.  This could be evaluated from the elastic part of the unloading curve, perhaps.  Not done? Could this be added to add a littlle more quantitative information?

17) Section 4.3 - Micro-mechanical model.  This is a misnomer, I feel, as the model presented is a conceptual model containing  no real micromechanics – a term that implies a quantitative approach.  This header is perhaps best modified to “4.3 Micro-mechanistic model” - which is probably what is meant by the authors. Perhaps it would be wise to add to this section the caveat that the conceptual model developed is based on a low P experiment with no pre-consolidation, and to clarify the imposed boundary conditions (constant P, controlled axial or radial? strain rate etc etc).  It has to be recognized that different microstructures may develop under different conditions, or it should be argued why this would not be the case.

18) Fig 9 vs. Fig 1. This depicts the micromechanistic model proposed.  However, the stress strain-curves used to illustrate the various stages of evolution looks rather different from the original experimental data depicted in Fig.1.  Is the stress axis perhaps the total axial stress as opposed to the differential stress plotted in Fig. 1??  Please clarify as the stress levels mentioned seem not to match up as is. It would be useful to clarify the orientation of Sigma-1 in the microstructural sketches also.  In Fig. 1,  how was the stress level labelled “crack initiation” established?  This should be mentioned somewhere?

19) Section 5 - Implications and conclusions. As indicated above and in my general scientific comments, this section would benefit from some brief consideration of the extent to which the observed processes and proposed mechanistic model can be expected to apply under shallow upper crustal (geo-storage system) conditions. The authors do state that more work is needed on consolidated samples and on naturally sheared/faulted samples to test the broader applicability of the model.  However, in future experiments, would not higher P-T conditions and deformation/loading rate not be important too, bearing in mind issues like smectite hydration state changes and internal fluid pressure equilibration, for example? 

20) Still Section 5: In Line 353 of page 12, how does the present study support implementing damage into constitutive modelling in the references mentioned? Do the authors mean “support” here or “provide input to constrain” such models, perhaps?

21) Figures 2-7: It would be useful if the orientation of the principal compression direction is indicated in all key micrographs, either directly in the micrographs or appropriately via the captions. Angles defining fracture and shear band orientations should be marked on key micrographs also, so that the intended meaning of terms like inclination (to what? Horizontal? Vertical? Bedding? ) is clear. What is OM in Fig. 5a?

TECHNICAL ISSUES (language, typographics etc)

Overall the paper is well written and in good English.  Nonetheless a few small improvements can be made as follows:

i) Fault gouge spelling: Gouge NOT Gauge

ii) “Associated with” is the correct usage, not “associated to”

iii) “Indicated “should not be used to mean “correspond to”. Use the latter.

iv) A typo: Saddle-reef pores. Better define the term also as it is from structural geology and is not familiar in rock mechanics.

v) “Acting on”… not “acting to..”.

vi) “growed” >>>> grew

vii) Page 11 – reformulate first sentence.  English awkward/unclear.

Finally, I would recommend a thorough spelling/grammar check at the end of the road.

I wish the authors good luck in  revising the paper and hope that my comments are helpful and not too burdensome.  Please note that I have elected not to do a second review, not out of disinterest but from "review overload".    

REVIEWERS’ COMMENTS

The authors present a sketch of a grain-scale deformation model for Opalinus Clay based on one sample that has been deformed in a triaxial test and whose postexperimental microstructure was analysed using broad-ion-beam scanning electron microscopy. Figures show the strain-stress evolution of the sample failure (shear / mixed mode), detailed micrographs and a generalized scheme that comprises the author’s findings.

The main finding is that the laboratory deformed sample developed dilational bands, while such a feature has not been reported for naturally faulted samples of the same material. It suggests that at least in the early evolution of shear of OPA there is an increase in porosity and permeability.

The second finding is that there are microstructural indicators for ductile behaviour. Foremost bending of elongated clay grains.

General appreciation

The manuscript is short and yet comprehensive. It is well structured, the English is good. The origin of all presented data is mostly clear and exceptionally well visualized.

Hence, I suggest to accept this manuscript with minor changes.

A detailed, line-by-line review with additional comments can be found in the attached word file.

Main critical points:

1. Although dilation bands in OPA are shown for the first time, the finding of high permeability in the early strain evolution is not a novel finding. Veins along fractures in the so-called Main Fault of the Mont Terri Rock Laboratory have long been reported as indicators for paleo-fluid flow by numerous authors (see attached word file). This should be addressed.
2. The use of the term “ductile”: The stress-strain curve shows bulk brittle behaviour, yet there are two indicators for an uncritical strain evolution in parts of the sample: (1) bend and delaminated clay grains as well as (2) finely distributed brittle shear within deformation bands / cataclastic flow. These three scale-depended phenomenon (bulk, grain-scale and deformation band-scale) could be better distinguished and elaborated more. For each occasion of the term “ductile”: To what scale does this refer to? Optional it could also be addressed: At what condition (P, T, strain rate) might a rather stiff mica grain bend? At what condition does a calcite grain break? At what condition does sub-critical fracture grow happen instead of seismical fracturing?
3. Terminology: There is a little inconsistency in the use of deformation band, deformed zone, strain zone, shear zone, fracture and crack. I gave some suggestions on how to improve that in the world file. From du Bernard (2002), there is the term “dilation band”, too.
4. A statement on subsample positioning is missing. Might there be a spatial relation on strain localization and strain rate to the subsample position? The crack likely nucleated at the sample center, where there is a high abundancy of fractures, that distribute the bulk strain while at the sample edge there is only one fracture. Has brittle deformation started once that first single fracture reached the samples’ edge to create one sample through-going, cohesionless discontinuity? Might a shear zone at the samples’ edge represents a more brittle deformation, while a shear zone from the samples’ center might have preserved more of the prior-to-failure, uncritical fracture growth microstructure? A subsample off-center in the middle, such as in Fig. 6 providing insight in the most less and slowly strained shear zones?
5. The title sould be shortend, see attached word file

I do not wish to remain anonymus and wish the authors good luck for the publication of this nice work.

General Comments

The authors present a thorough and high resolution study of the deformation mechanisms associated with the Opalinus Clay using a combination of triaxial tests and post deformation image analysis. The study builds on previous work, but presents a new set of data which will be highly valuable to the community, and for that reason, in my opinion, it deserves to be published. However, there are some scientific questions/comments which I would like to see addressed before the MS is accepted for publication, which I present in the specific and technical comments below. In addition to these I have three main comments:

• The authors should consider the role that mechanical anisotropy has on their results. It is mentioned briefly in the introduction, but then very little attention is given to this thereafter. I feel that some of the observations, for example the orientation of tensile cracks being oblique to the loading direction, could be explained by the mechanical anisotropy present within the material. I have suggested some studies which the authors may find useful on the subject.
• There is an inconsistency when describing the orientation of structures or processes, where the authors use the sample orientation, bedding orientation and loading orientation throughout. I found this confusing when reading the MS and I would recommend that the authors use the bedding orientation only. There is one point (which I highlight below) where it is also appropriate to refer to structures/processes relative to the loading direction as well, but apart from that I would just stick to bedding orientation.
• At the end of the MS the authors run the risk of undermining their own great work by suggesting that their results do not compare well with naturally deformed samples of OPA. This may be the case, but then you need to stress why your contribution is still valuable, and give more details on why you think there are differences, and what can be done in any further work. The authors give a vague answer to this, but I would suggest that they end on a stronger note.

Apart from the inconsistency mentioned above, the MS is in the main well written, with a good structure and relevant (and useful) figures. There are some typos and minor technical amendments which I suggest in the detailed comments below.  



I wish the authors the best of luck and I look forward to seeing this work published soon!

Specific and Technical Comments

Line 18 – Why test at this orientation? This is also a general question, not just specific to this line.

Line 28 – Could you briefly state what the major effect on permeability evolution is in the abstract?

Line 51 – Are the bedding parallel cracks likely to exist at depth? Or are they only present on exhumed material that has therefore been unloaded? This may be pedantic but the way this reads at the moment suggests that these exist at depth, like the pore space in the fossils, pyrite or clay matrix (which all presumably exist at depth).

Lines 61-64 – There has been a renewed interest in looking at failure in shales at different orientations see the following papers:

Nejati, A. Aminzadeh, F. Amann, M.O. Saar, T. Driesner (2020) Mode I fracture growth in anisotropic rocks: Theory and experiment. Int J Solids Struct, 195 pp. 74-90, 10.1016/j.ijsolstr.2020.03.004

Gehne S, Forbes Inskip ND, Benson PM, Meredith PG, Koor N (2020) Fluid-driven tensile fracture and fracture toughness in Nash point shale at elevated pressure. J Geophys Res: Solid Earth 125:1–11. https ://doi.org/10.1029/2019J B018971

Forbes Inskip ND, Meredith PG, Chandler MR, Gudmundsson A (2018) Fracture properties of Nash Point shale as a function of orientation to bedding. J Geophys Res: Solid Earth. https ://doi. org/10.1029/2018J B0159 43

Chandler MR, Meredith PG, Brantut N, Crawford BR (2016) Fracture toughness anisotropy in shale. J Geophys Res: Solid Earth 121:1–24. https ://doi.org/10.1002/2015J B0127 56

Although these studies mainly use unconfined tests, they demonstrate how the fabric, and alignment of grains in shales has a significant effect on fracture propagation and mode of failure. Although I do not think an in depth discussion of these papers is required, they should at least be mentioned here.  

Line 86 – What is the range of confining stresses tested? You quote the strain range (>20%) so you should also quote the stress range.

Line 99 – This should be “gouge” not “gauge”

Line 101 – I don’t think “modelled experimentally” is the best phrase here, perhaps “has been analysed experimentally by use of direct shear tests on samples sheared both parallel......”

Line 128 – Why did you choose to core the material in this orientation? Are your tests on samples in this orientation the most representative of what you would expect on structures found at Mont Terri and elsewhere in the OPA? As you mentioned earlier, and in the references I provide above, loading orientation is known to be a significant contributor to failure (fracture mode, strength etc) in transversely isotropic rocks. It would be good to show the reasoning for your sample orientation choice here.

Line 134 – Similar to my comment above, why did you choose a circumferential displacement rate of 0.08 mm/min. Many failure processes are heavily rate dependent, are these rates relevant to the application of your study? Or is this rate defined in an ISRM suggested method?

Line 139 – This section relates only to sample preparation for the image analysis, and so should be titled as such. Something along the lines of “Sample preparation for image analysis”. Furthermore, I would also change section 2.1 to something like “Material description and core sample preparation” or “Material description and sample preparation for mechanical testing” if you want to keep the two types of sample preparation separate. Either that or you could remove lines 126 – 130 from section 2.1 and incorporate it with section 2.3 for a more general sample preparation section, but this should then come before the section on Triaxial testing.

Line 153 – Do you really need to use the acronym ROI for regions of interest? I do not think it saves much in the way of words, and in my opinion the use of another acronym is confusing for the reader. You also only use it once in the whole MS.

Line 163 – I would use either sub-horizontal or parallel to bedding, and not both. My preference would be to always use an orientation related to the bedding, so parallel in this case. The reason being that the bedding relates to the fabric of the rock and will remain (relatively) constant, whereas horizontal or vertical can be different depending on how you orientate your sample. Also be consistent then with how you define both fracture sets, i.e. oblique to bedding (fracture set 1) and parallel to bedding (fracture set 2).

Line 205 – This should be “saddle reef pores” rather than “raddle reef pores” I think. I have to admit, this is not a term I am familiar with. Do you need to define this, or at least annotate examples in Figure 7?

Lines 211 – 214 – It is not clear to me where these porosities have been measured. Could the sub-areas used in this analysis be marked on Figure 7?

Lines 219 and 224 – I think you have mixed up fracture sets 1 and 2 here. In section 3.1 you define fracture set 1 as the set oblique to bedding while fracture set 2 is parallel to bedding. Please amend to be consistent.

Line 226 – Here you go back to defining features from the horizontal rather than bedding. Switching between the two is confusing, so please stick to one or the other. As mentioned previously I think orientating with regards to the bedding is better.

Line 230 – Further to my previous comment: You now use the bedding direction rather than horizontal. You use both within the same paragraph, please be consistent.

Lines 233 – 234 – “In general, the deformation of both the clay-rich matrix and larger quartz, calcite and mica grains is brittle, ductile or a combination”, brittle, ductile or a combination covers everything and so this sentence in its current form is rather redundant in my opinion. I understand what you mean, in that you do not just have one type of deformation and that both occur, but maybe then consider re-writing the sentence to something like “We observe both brittle and ductile deformation in the clay-rich matrix and larger quartz, calcite and mica grains, and so deformation is not solely brittle or ductile.”

You also list clay-rich matrix, quartz, calcite and mica here, what else is there of significant quantities? Do you not see deformation in the Iron rich minerals and/or feldspars? If not is this because they account for <10% of the rock (from section 2.1.), and therefore could this simply be a sampling bias? I just wonder if you need to specifically list clay-rich matrix, quartz, calcite and mica, or if you could just say that you observe both brittle and ductile deformation.

Finally, I personally do not think you need to say a combination of both, as I believe it is covered when saying that you observe brittle and ductile deformation. However, if you feel strongly about keeping that in that is fine.

Lines 265 – 268 – Similar to my comments above, the way that the first sentence here is written is a bit redundant as elastic and inelastic deformation covers everything. Again, I understand what you mean in that both occur, but then consider re-writing to something like “We observed that both elastic and inelastic deformation occurred during testing, and that both occurred simultaneously”. You mention in line 268 that both occur simultaneously but have you got evidence for this?

Also could pore compression (line 266) not be inelastic rather than just elastic? Or do you consider inelastic pore compression as pore collapse?

Line 274 – Here you now use angle with respect to the maximum principle (presumably – but need to state this, as in engineering σ₁ is commonly the maximum principle tensile stress) compressive stress σ₁ in addition to angle from bedding and horizontal as before. I can understand why you may want to use σ₁/loading direction here as well, but then you should also indicate which fracture set these structures relate to (Fracture set 1?).

Line 284 – It is interesting that your microstructural analysis suggests that tensile cracks form as obliquely rather than vertically (parallel to the loading direction) orientated cracks. You do not mention this here but could this not be down to the transversely isotropic nature of the material, and that you load the sample oblique to bedding? Literature on the fracture toughness of shales at different angles to bedding suggest that during crack growth there is a competition for a tensile fracture to form parallel to load and parallel to the plane of weakness i.e. the short-transverse orientation. As a result, the actual fracture orientation lies somewhere between the two. Again, see the following:

Nejati, A. Aminzadeh, F. Amann, M.O. Saar, T. Driesner (2020) Mode I fracture growth in anisotropic rocks: Theory and experiment. Int J Solids Struct, 195 pp. 74-90, 10.1016/j.ijsolstr.2020.03.004

Forbes Inskip ND, Meredith PG, Chandler MR, Gudmundsson A (2018) Fracture properties of Nash Point shale as a function of orientation to bedding. J Geophys Res: Solid Earth. https ://doi. org/10.1029/2018J B0159 43

Chandler MR, Meredith PG, Brantut N, Crawford BR (2016) Fracture toughness anisotropy in shale. J Geophys Res: Solid Earth 121:1–24. https ://doi.org/10.1002/2015J B0127 56

Again, these studies are unconfined but I think that the formation of (apparent) tensile fractures in these studies may explain some of the observations that you have here with regards to the orientation of tensile cracks.

Line 292 – Should be “changed” not “changeed”

Line 320 – You use the British spelling of characterised here (with an s rather than a z) but use American spelling elsewhere (line 14, 84). Generally you use American spelling throughout, but be consistent.

347 – You use the term obliquely orientated here, but obliquely orientated to what, bedding, horizontal or σ₁? You use all three in the MS, and it may be that the cracks are oblique to all three, but you should state what the cracks are oblique to here.

Lines 351 – 352 – Here you state that there are few similarities with naturally deformed OPA. The manuscript and study are really interesting, however as you point out in your abstract important questions remain, particularly in relating data gathered in the lab to that observed in the subsurface. Your statement here suggests that you are not able to apply the results of your work to subsurface processes on a larger scale, which in my opinion undermines the great work that you have done. Although it may be true that your results do not corroborate well with naturally deformed OPA, you should expand on why you think this is, and how then your results are relevant. In the final lines of the MS you briefly suggest what could be done next, but you could expand on this to suggest how this study can better inform what types of experiments should be carried out next to tackle these open questions.

Line 521 - 522 – change “was conducted circumferential-displacement-controlled...” to “was conducted using a circumferential-displacement-control rate of 0.08 mm/min...”.  

Figure 3 right – The axis is labelled “angle” but what angle is this referring to? It is not made clear in the figure caption and given that the horizontal and the bedding direction are within 10° of each other it isn’t immediately obvious which angle you are referring to. Please re label the axis “fracture angle to bedding/horizontal (as appropriate)”

Lines 536 – 537 – Here you just say “the oblique orientated fracture set” If you compare both fracture sets to the horizontal (which you do in line 537), they are both oblique i.e. not parallel or perpendicular to the horizontal. This also goes back to my previous comments of use one frame of reference only, bedding or horizontal, throughout. Bedding would be my preference.

Figure 4 – Is Figure 4 (b) an inset from Figure 4 (a)? If so indicate where it is from.

Also put a scale in Figures (c) and (d)

Figure 6 – Should the text in Figure 6 (a) be “Bent mica” rather than “Bend mica”, this is also true for the figure caption and line 195 in the main text.

 Line 565 – Here you reference (a, a’, a”) but only a and a’ are shown in the figure. Also this may seem pedantic, but in the figure you use the prime symbol whereas you use the apostrophe in the figure caption, this should be consistent.

Figure 9 – I really like this figure, it explains nicely the features that you observe in your samples and describes the processes that have taken place. You also show an example of a saddle reef pore, but is there a ‘real life’ example that you could show in figure 7?