Review Reports - The End of Paleogene White River Group Deposition in Wyoming and Nebraska, USA: A Distal Record of the Collapse and Emplacement of the Markagunt Gravity Slide at 23 Ma

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Reviewer Comment

The manuscript entitled “The End of Paleogene White River Group Deposition in Wyoming and Nebraska, USA: A Distal Record of the Collapse of the Emplacement of the Markagunt Gravity Slide at 23 Ma” presents new DZ U-Pb data (4 samples; ~200 grains each) from the uppermost White River Group (Brule Member) at the White River Group/Arikaree contact in eastern Wyoming and northwest Nebraska. The paper mainly focused on the maximum depositional age of ~23.07 ± 0.46 Ma, overlapping the independently constrained emplacement age of the Markagunt Gravity Slide (23.05 +0.22/−0.20 Ma). The authors relate this temporal match and detrital zircon age spectra as an evidence for forced drainage reorganization and sediment routing due to catastrophic collapse of the Marysvale Volcanic Field and marking this as an end of White River group deposition and provenance shift into the Arikaree Group. The dataset is valuable, however I have few comments.

Page 4. Line 122-125. Samples are described as “directly beneath” the Arikaree but no measured sections, distances below contact, or site-specific stratigraphic metadata are provided. Sample locations on a basic stratigraphic logs from the study site must be added. Or describe the approximate distance below the contact from where samples were collected.
Page 4. Line 127-130. Methods refer key details to a website. However, authors need to describe the discordance ratio, standards and rejection criteria to exclude some DZ ages from the main analyses within the Method section.
Page 5. Line 154-162. The author used several MDA age estimators and then averages the age. This is not the standard practice. The author needs to select and justify the preferred estimator and describe the limitations in others.
Page 7. Line 197-199. The manuscript states White River group deposition “ceased” at ~23.07 Ma. This is a very strong statement considering the MDA provides only a maximum age, which means depositional age should be younger than MDA. This should be rephrased as: “uppermost sampled White River Group yields a MDA of ~23.1 Ma,” not cessation.
The DZ provenance claims mainly rely on visual comparisons (Fig. 5). The manuscript should include at least one quantitative comparison. Similarity matrix, KS and Kuiper test, MDS to reflect statistical soundness of the data.
Detrital zircon U-Pb data set should be made available in the supplementary file.

The manuscript is overall satisfactory and publishable. However, it requires minor revision to improve the stratigraphic control and quantitative comparison. I suggest minor revisions.

Author Response

The manuscript entitled “The End of Paleogene White River Group Deposition in Wyoming and Nebraska, USA: A Distal Record of the Collapse of the Emplacement of the Markagunt Gravity Slide at 23 Ma” presents new DZ U-Pb data (4 samples; ~200 grains each) from the uppermost White River Group (Brule Member) at the White River Group/Arikaree contact in eastern Wyoming and northwest Nebraska. The paper mainly focused on the maximum depositional age of ~23.07 ± 0.46 Ma, overlapping the independently constrained emplacement age of the Markagunt Gravity Slide (23.05 +0.22/−0.20 Ma). The authors relate this temporal match and detrital zircon age spectra as an evidence for forced drainage reorganization and sediment routing due to catastrophic collapse of the Marysvale Volcanic Field and marking this as an end of White River group deposition and provenance shift into the Arikaree Group. The dataset is valuable, however I have few comments.

Page 4. Line 122-125. Samples are described as “directly beneath” the Arikaree but no measured sections, distances below contact, or site-specific stratigraphic metadata are provided. Sample locations on a basic stratigraphic logs from the study site must be added. Or describe the approximate distance below the contact from where samples were collected.

This has been clarified

Page 4. Line 127-130. Methods refer key details to a website. However, authors need to describe the discordance ratio, standards and rejection criteria to exclude some DZ ages from the main analyses within the Method section.

The full methodology is added.

Page 5. Line 154-162. The author used several MDA age estimators and then averages the age. This is not the standard practice. The author needs to select and justify the preferred estimator and describe the limitations in others.

This is becoming the standard practice. There was an entire session at GSA this year that focused on MDAs. The consensus is that we report all estimates based on the most commonly used methods. I have removed the language that may suggest the mean MDA as the representative MDA and emphasized the youngest grain cluster age, explaining the rationale behind choosing this as the reported MDA.

Page 7. Line 197-199. The manuscript states White River group deposition “ceased” at ~23.07 Ma. This is a very strong statement considering the MDA provides only a maximum age, which means depositional age should be younger than MDA. This should be rephrased as: “uppermost sampled White River Group yields a MDA of ~23.1 Ma,” not cessation.

We replaced at with after.

The DZ provenance claims mainly rely on visual comparisons (Fig. 5). The manuscript should include at least one quantitative comparison. Similarity matrix, KS and Kuiper test, MDS to reflect statistical soundness of the data.

We have added an MDS plot of the 50 Ma and younger zircons from the relevant samples to show the procession of events.

Detrital zircon U-Pb data set should be made available in the supplementary file.

This will be the case.

The manuscript is overall satisfactory and publishable. However, it requires minor revision to improve the stratigraphic control and quantitative comparison. I suggest minor revisions.

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have submitted a manuscript regarding the provenance and evolution of the White River Group Deposition in Wyoming and Nebraska, USA. They have employed detrital zircon geochronology to identify the age of deposition relative to the emplacement of the Markagunt Gravity Slide in Utah’s Marysvale Volcanic Field. They provide interesting data on the evolution of the area and the river systems that may have contributed to sedimentation in different time periods.

Their work is very interesting, yet there are certain issues that need to be addressed in order for the manuscript to be accepted for publication:

There is no supplementary data regarding zircon geochronological data, although such material is clearly stated in the manuscript (lines 131-132). I hacve tried to downloaded through the manuscript, yet no such file exists.
The authors need to provide details on how the depositional age of zircons is calculated. Their work is based on U-Pb geochronology on zircons. Yet, no data is presented on how the depositional age of zircons is calculated, which is far different than the zircon formation age that is most commonly calculated and used.
There are no microscopic images of the zircon grains analyzed. At the same time there are no data regarding, shape, size and texture of zircon grains in order to discriminate between possible magmatic sources.
The authors state that they have collected only four (4) samples. They make no note on the number of zircon grains collected and analyzed for U-Pb dating. At the same time, this number of samples is very small in order to cover such a large area.

Please refer to the revised version for details.

Comments for author File: Comments.pdf

Author Response

Their work is very interesting, yet there are certain issues that need to be addressed in order for the manuscript to be accepted for publication:

There is no supplementary data regarding zircon geochronological data, although such material is clearly stated in the manuscript (lines 131-132). I hacve tried to downloaded through the manuscript, yet no such file exists.

Sorry about this. We will make sure that the data is available.

The authors need to provide details on how the depositional age of zircons is calculated. Their work is based on U-Pb geochronology on zircons. Yet, no data is presented on how the depositional age of zircons is calculated, which is far different than the zircon formation age that is most commonly calculated and used.

The method and rational for these calculations are in the text. To report maximum depositional age of the four samples, we use the Matlab-based program DZmda to present 8 different MDA calculations (Fig. 4): (1) youngest single zircon grain (YSG), based on the youngest individual age and uncertainty; (2) youngest graphical peak (YPP), based on the youngest Probability Density Plot (PDP) peak age; (3) youngest gaussian fit (YGF), based on a gaussian curve fit to the youngest PDP mode; (4) the youngest cluster of zircon grains with overlapping error estimates at 2-sigma (YGC2s); (5) the youngest cluster of three zircon grains with overlapping 2-sigma error estimates (Y3Zo); (6) youngest three zircons (Y3Za), based on the weighted mean of the youngest three zircons regardless of uncertainty; (7) Tau (TAU), a method based on the weighted mean and uncertainty of all ages that fall within the probability minima of the youngest PDP age peak; and (8) youngest statistical population (YSP), based on the weighted mean of the youngest group of ages with a mean square deviation closest to one. These statistical methods are further described in [45-46].

There are no microscopic images of the zircon grains analyzed. At the same time there are no data regarding, shape, size and texture of zircon grains in order to discriminate between possible magmatic sources.

We find that the U-Pb ages are the best way to discriminate among sources.

The authors state that they have collected only four (4) samples. They make no note on the number of zircon grains collected and analyzed for U-Pb dating. At the same time, this number of samples is very small in order to cover such a large area.

Figure 4 lists the number of zircons per sample. Figure 5 lists the number of Tertiary zircons used in the analysis.

Please refer to the revised version for details.

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have provided a revised version of the manuscript, yet with very little added details relative to the first draft.

For instance:

- No supplementary material is provided, although it is clearly mentioned in both the text and in their answers on the 1st round of review.

- There are still comments of the first round review that have not been addressed (e.g., figure 4)

- There is no table in the text with the acquired data. The authors only state the collection of four samples. Still, a table with the number of zircons analyzed and the corresponding data is required.

- Most importantly, they have not provided a detailed explanation of their methodology employed for their study. This is needed in the manuscript. They provide a relevant explanation in lines 134-145, yet they could provide more details on how such data may be used in a sedimentological study.

Please refer to the attached revised version for details.

Comments for author File: Comments.pdf

Author Response

No supplementary material is provided, although it is clearly mentioned in both the text and in their answers on the 1st round of review.

Apologies, I believe it did get submitted at some point, but it seems to not have been included with our latest submission. It has been included.

There are still comments of the first round review that have not been addressed (e.g., figure 4)

If you are referring to in-text comments from your first review, I have addressed those by line number below. Apologies for the oversight. As for figure 4, I'm not sure what you are referring to. There are no comments from either of the round one reviews referring to figure 4.

There is no table in the text with the acquired data. The authors only state the collection of four samples. Still, a table with the number of zircons analyzed and the corresponding data is required.

The number of zircons analyzed is displayed on Fig. 4B (n=). I have added additional wording to the text and figure caption to make this clearer.

Most importantly, they have not provided a detailed explanation of their methodology employed for their study. This is needed in the manuscript. They provide a relevant explanation in lines 134-145, yet they could provide more details on how such data may be used in a sedimentological study.

Detrital zircon studies utilizing maximum depositional ages (in which the formation age of a sedimentary rock cannot be older than youngest grain/population of grains) and provenance analysis (matching of detrital grains to potential sources) are widespread and common-practice, and we believe an expanded methodology explaining well-established practices in the field would detract from the paper, especially considering this has been submitted to a special issue on detrital geochronology and given the shorter form factor of this manuscript. We have revised the methodology and results section for clarity and have expanded the supplementary files, but in the text we encourage readers to refer to the literature cited therein that explains these methods in greater detail.

In-text commentary replies (round 1):

44: This is described in the methods and results. As this is in the introduction, it would not be appropriate to go in too much detail here.

Figure 1: It is standard practice to use "explanation" over "legend" on geologic maps. The study area(s) are marked by the yellow dots for this study, spanning both Wyoming and Nebraska, and the broader regional context is needed. An inset map would be unnecessary.

82: This is part of the background to our earlier studies on the region as in the preceding sentence.

Figure 3: These are presented for textural context for the sampled sandstones in this study like in A-C. I do not have access to these thin sections, so I would not be able to accurately identify minerals beyond using the photos presented here.

123: We combine these four samples with over a dozen others from across the region.

127-128: We explain our methods for calculating the maximum depositional age in the methods. These are standard-practice applications for assessing the depositional age of sedimentary rocks.

131: addressed above

134: This is explained in the following text, and we encourage readers to refer to the papers explaining these methods in detail in line 145 for additional information on these methods.

148-152: As explained in the introduction and background, the main source areas are Eocene to Miocene in age. Added a few words for clarification.

155-156: Addressed above

169: I am not able to add images of the zircons to the manuscript at this time. However, this stratigraphic unit and the sources of these zircons have been well-described in the cited literature. these are largely tuffaceous zircons deposited by eolian processes.

186: fixed

In-text commentary replies (round 2):

Figure 3: Addressed above

127: addressed above

Methods comments: The methods have been revised and we have included a supplementary file describing the geochronology methods in greater detail.

216: addressed above

238: addressed above

239-250: We largely do not include interpretations with the results. Please refer to the discussion. We do not state here that TAU is considered reliable, only that it has the smallest 2-sigma error in our analysis. I have revised some of the language to expand on why the youngest grain cluster MDA is reliable and encourage readers to refer to the cited literature which explains these methods in greater detail.

253: addressed