Timing of Opalization at Lightning Ridge, Australia: New Evidence from Opalized Fossils

Round 1

Reviewer 1 Report

This manuscript provides evidence for the timing and processes involved in the precipitation of silica during opalization at Lightning Ridge, Australia. The study also has implications for understanding opalization at other sites in Australia and elsewhere, and provides a summary of the various types of organisms and styles of silicification involved in opalization of fossils in the mid-Cretaceous Griman Creek Formation.

The manuscript is generally well written and profusely illustrated. It summarizes the main theories presented in previous studies of opal formation and concludes that a mid-Cenozoic origin of the opals is most parsimonious.

Please be careful with the correct use of chronostratigraphic (rock) terms (Upper, Lower) vs geochronologic terms (Early, Late); also check the use of formal vs informal chronostratigraphic and geochronologic terms (Early vs early; Upper vs upper). 

Please carefully check that all references appear in the reference list and are correctly formatted – follow the style in a recent issue of the journal.

I think it is unlikely that the Finch claystone facies was deposited in active river channels or contains cross-bedding. Claystones typically accumulate in low-energy settings and the fine grain size is not conducive to the formation of cross-bedding.

The term ‘black opal’ is confusing to many people, since the opal is clearly not black and shows a great range of refractive colours. This needs to be explained better – or perhaps black opal is misleading and needs a better name (even though it is entrenched in the common parlance).

End of page 5: Some quite well preserved plant fossils ARE known from the mid-Cenozoic silcrete at Lightning Ridge – See Carpenter et al. 2012. Silcrete plant fossils from Lightning Ridge, New South Wales: new evidence for climate change and monsoon elements in the Australian Cenozoic. Australian Journal of Botany, 2011, 59, 399–425.

The authors may wish to clarify their interpretation of petrifaction vs permineralization with respect to silicified wood.

Where the term ‘mineralization’ is used in the text in relation to fossil plant/animal tissues, you need to make it clear whether you mean mineral replacement of organic tissues, or impregnation of tissues, or coating of tissues, or something else, or all of these together.

Figure 12 should have lettering for all images.

I regard the structures in Fig. 14H, I as not strictly ‘silicified cells’ but rather siliceous infillings of cell lumens’ It appears that all cell wall material has been lost leaving just the infillings of lumens and wall pitting (pores).

Figure 17 does not look particularly convincing for a fern rhizome. One would need to see some convincing C-shaped rachis vasculature, or some other anatomical features (e.g., surface scales or hairs; fibre bands, or one of the distinctive fern stele architectures) to convince readers that it is a fern rhizome.

Figure 18 is probably the least convincing element in the paper. These opalized structures appear to be septarian (or some similar) features. They lack the regular concentric/radial organization of angiosperm tissues, they have quite an odd range of “cell” shapes, and they lack any obvious pitting on "cell" walls that one might expect in angiosperms. I would delete this figure. I can see no convincing evidence of angiosperms among the fossils presented.

Page 25. The coprolites are interesting. They clearly contain plant matter and were presumably produced by herbivores. Macro-herbivore coprolites are rather rare in the fossil record owing to the lack of phosphates for cohesion, so one could elevate this to somewhat higher importance.

Fig. 27. These structures do not look like burrow fills. Certainly, the branched examples do not conform to the diagnosis of Skolithos. Their irregular, commonly knobbly, branched and twisted shapes suggest that these are more likely to represent casts of tree roots, and that would be more consistent with the non-marine depositional setting for at least some of the strata at Lightning Ridge.

Fig. 35. It always bugs me when I see figures with time running backwards (from right to left). Why not show time running in the direction that one normally reads?

I think most of these issues can be remedied quickly, after which this paper will make a useful contribution to debate on opalization processes.

I attach a copy-edited version of the manuscript with additional comments for the authors and editors.

Comments for author File: Comments.pdf

There are a few general recommendations concerning the grammar. For example, the authors should avoid using temporal terms (often, sometimes, frequently, occasionally, etc) when describing fossils or strata. If you write that the rocks are often marly, then it implies that they might not be if you come back tomorrow. Better to use terms of generality (commonly, normally, generally, in many cases sporadically, etc). Similarly, replace temporal terms, such as ‘while’ with alternatives, such as ‘although’, ‘whereas’ or ‘and’.

Please carefully check the text for consistency with regard to American vs UK English spellings.

Please also check the use of hyphens vs en dashes, and Ma vs Myrs. Use ‘sedimentary rock’ for consolidated (lithified) material. ‘Sediments’ refer to unconsolidated material.

A few lines in the text are written in the style of a figure caption, e.g., “Fig. X shows ….”. These lines should be rephrased with reference to the figure at the end.

Author Response

We greatly appreciate your evaluation of our manuscript, particular the level of detail in the PDF attachment. We have paid close attention to you comments, and adopted virtually all of your recommendations. In several instances, we have added new wording or discussions to address issues that you raised. The manuscript is better as a result of your wisdom and insights. I will limit the point-by point response to the major recommendations. Please see the attached file.

Author Response File: Author Response.pdf

Reviewer 2 Report

    This paper reported new discovery from opalized fossils to reveal the Timing of Opalization at Lightning Ridge, Australia. They ascertained the age of opalization Lightning Ridge at as the Neogene by extensive opal deposits in the Great Artesian Basin in Australia. New paleontological discoveries include a surprising level of cellular detail in plant fossils, preservation of individual tracheids as opal casts, first evidence of angiosperm fossils at Lightning Ridge, and the first report of Early Cretaceous coprolites from New South Wales, Australia. This study is detailed and interesting, and has valuable implications.

    In my opinion, there are many minor problems or errors in the manuscript. It is recommended to check the revised format, word spelling, as well as spacing and punctuation before submitting the manuscript. Some revisions need to be addressed before accepted for publication.

Comment 1: What does "these" mean in line 72? What does "resulted in" mean in line 401? What does "the" mean in line 602? I guess that the author did not cancel the editing mode of WORD. Please check this issue in the full manuscript.

Comment 2: For the sentence as "deposited over 40-50 Ma during the Early-mid Cretaceous" in lines 92-93, I suggest authors provide an exact deposition age here. Also, “the Early-mid Cretaceous” should be revised to "the Early-Mid Cretaceous". Please check similar issues throughout the manuscript.

Comment 3: In the section of "3. Site History", more references should be added in suitable position.

Comment 4: It is suggested that the style of scale in figures should be uniform such as in Figure 12. Especially, the scale is lacked in Figure 7. I recommend to use scale bar without characters in the Figures and add "scale bar =" to their annotation.

Comment 5: In line 561, "Inother cases" misses a space, please check this issue in the full text of the manuscript.

Comment 6: In line 635, why is "Figure 21" presented as red?

Comment 7: In line 676, What does the mean of "SEM image[?] "?

Comment 8: In line 775, change "-Many" to "Many"?

Author Response

Thanks for the very helpful reviews. We have accommodated almost all of your recommendations. One exception is the format for scale bars. My personal preference is to show labelled scale bars so that readers do not have to refer to the figure caption to see the value of the bar. This is especially useful when a plate contains multiple images taken over a range of magnifications. For that reason, the scale bars represent  a range of sizes, so that it doesn't work to just say "scale bars = X size".

We have cleaned up many minor problems regarding spelling, punctuation, etc. The multitude of word spacing errors is created by the full-justification format create by MDPI editors. For some reason, there is a mismatch in the justification between my computer and the MDPI computers. I have found that this is a recurring problem through the editing process, and that spacing errors can only be resolved at the final galley proof stage. Until then, every revision falls victim to a new set of word spacing errors.

Thanks for your wise advice for improving the manuscript. Your work is much appreciated.

Best wishes, George

Reviewer 3 Report

I enjoyed reading your manuscript entitled "Timing of Opalization at Lightning Ridge, Australia: New Evidence from Opalized Fossils". This is a well-written, well-organized and well-illustrated manuscript about a range of exceptionally well-preserved opalized plants, invertebrates, vertebrates, and trace fossils.

The manuscript needs only minor corrections:

Line 12: Fossil plant remains

Line 72: these discrepancies in

Line 85: Early Cretaceous

Line 118: 100.2–96.6 Ma

Fig. 1B: Please add some geographic coordinates to the map.

Fig. 2: I recommend using chronostratigraphic terms for the stratigraphic ages of the units (e.g., “Lower Cretaceous” not “Early Cretaceous”).

Line 363: delicate topologies

Line 484: primarily opal

Line 565: (B)

Line 672: Lower Cretaceous

Line 755: Lower Cretaceous

Line 762: Lower Cretaceous

Author Response

Thanks for your careful review of our manuscript. We have  made the corrections you suggest (e.g., Lines 12, 72,85,118, 363, 484, 565. Geographic coordinates have been added to the Figure 1 map. We are taking a close look at the issue of "Lower Cretaceous" versus "Early Cretaceous", because there is some diversity of opinion among the four peer reviewers regarding this topic. I suspect that resolution of this issue will depend on the results of the second round of reviews.

Thanks again, on behalf of my coauthor Elizabeth Smith, we greatly appreciate your constructive approach to reviewing and your attention to detail.

Best wishes, George

Reviewer 4 Report

This is a thorough review page and of interest to anyone interested in the topic of  opal formation, but it would benefit from some stated objectives for the study and more analysis on how the materials can provide greater insight of those  topics.

The study would benefit by shortening the discussion of Virgin Valley, Nevada opal. Although the amount of documentation is impressive, the opal types are sufficiently different and the depositional setting is different, making an extended discussion of Virgin Valley opal of limited value to the focus on Australian opal. 

The reference to tectonics as a generator of fractures and deformation of the sedimentary deposits should be softened. The term is generally used for deformation from  regional forces acting on crustal rocks, which may not be important. Deformation of sedimentary strata by forces related to compaction and dewatering can generate the fractures and fluid intrusion features mentioned in the report and I'd like to see more consideration of that source of sediment deformation. Especially so, since there is much mention of diagenesis that involves changes in volume of sediment and migration of fluids. The forces involved can be of local origin. Local sources of force to deform sediment is expected and should be evaluated.

Author Response

Thanks for your insightful comments. We have made a variety of edits and re-wordings based on the comments of three other reviewers. In regard to the main points that you raised:

The study would benefit by shortening the discussion of Virgin Valley, Nevada opal. Although the amount of documentation is impressive, the opal types are sufficiently different and the depositional setting is different, making an extended discussion of Virgin Valley opal of limited value to the focus on Australian opal. 

Reply: Our main point here is that although Australian opal is commonly described as being geologically unique, opal deposits occur throughout the world, and the thermodynamics and petrogenesis of opal at all of these sites are governed by fundamental parameters. Virgin Valley  opal deposits are important to consider, because the mines have been producing precious opal for as many years as Australian opal.  Both locations are in arid regions that were once occupied by large inland lakes at a time of milder, wetter climate The coexistence of opal-A and opal-CT in the late Miocene deposits, and to opal-CT structure of the precious opal makes Virgin Valley an important comparative site. And , the fact that the online-only format for Minerals means that text length is not the limitation that it would be for a printed journal.

The reference to tectonics as a generator of fractures and deformation of the sedimentary deposits should be softened. The term is generally used for deformation from  regional forces acting on crustal rocks, which may not be important. Deformation of sedimentary strata by forces related to compaction and dewatering can generate the fractures and fluid intrusion features mentioned in the report and I'd like to see more consideration of that source of sediment deformation. Especially so, since there is much mention of diagenesis that involves changes in volume of sediment and migration of fluids. The forces involved can be of local origin. Local sources of force to deform sediment is expected and should be evaluated.

Reply: This is an important point. We have rewritten this part of the manuscript in accordance with your recommendations.

Again, thanks for your suggestions for improving the manuscript. We appreciate your help.

Best wishes, George