the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 26 Apr 2024
Popigai and Chicxulub craters: multiple impacts and their associated grabens
Abstract. More advanced data (gravity field model EIGEN 6C4 with GOCE gradiometry data instead of EGM2008) and more sophisticated method (using a set of the gravity aspects instead of the gravity anomalies and the radial second derivative of the disturbing potential only) enable a deeper study of various geological features, here the impact craters Chicxulub and Popigai. We confirm our results from 2010, extend them, and offer more complicated models, namely by means of the gravity strike angles. Both craters are double or multiple craters. The probable impactor direction is from NE for Chicxulub and SE-NW for Popigai. The both crater formations seem to be associated with impact induced tectonics that triggered development of impact grabens.
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RC1: 'Comment on egusphere-2024-866', Anonymous Referee #1, 28 May 2024
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I was excited to read a paper discussing both Popagai and Chicxulub given these are two of the largest preserved impact basins on Earth. The authors are experts in gravity methods and do a very nice job laying out the efforts to provide update gravity anomaly maps for each of the two impact structures. However, the inferences the authors go on to make about both structures include a fatal flaw of not constraining their results with other available data that would allow for additional constraints and in this case refute their ideas of multiple impacts in the region of either of these craters. Additionally the discussions of both craters is a strange combination of specific details that are not really important to their regional story and missing other papers and data that could test their proposed ideas. For instance at Popagai the whole discussion about diamonds as a shock indicator is unneeded and instead a discussion of the structural geology, erosion depth, drilling results, etc should have been included to allow for aspects like interrogating the new gravity map for what it represents in terms of Popagai's structure. You should a very interesting low gravity ring that could represent the annualar trough for instance or could represent a highly shocked peak ring (an exciting possibility!)...yet instead of diving into such intriguing results, the authors focus on other features in the region that can be explained tectonically and do not have any evidence of an impact origin. For Chicxulub, the authors similarly focus on a detail that is not resolvable in their gravity map (the Holocene aged karst feature the ring of cenotes, which exist due to caves formed within the inner ring of the crater), but then do not detail the ways in which their new gravity map differs from previous ones that might be illustrative of the crater heterogeneities. For instance its a cool opportunity to consider the relative contributions from impact trajectory versus target heterogeneity. Instead they suggest the pre-existing sedimentary basin to the NE of the crater might be a second impact despite seismic images across this feature which show no structure evidence for any kind of impact (no rim faults, terrace zone, central uplift, etc- see the papers already referenced for plenty of evidence its not an impact). Just because it is a gravity low does not mean it's an impact...it usually just means it's a sedimentary basin. Moreover the authors then try to attribute the pre-existing rift that lies south of the crater and likely continued through the area the impact formed as somehow caused by the impact. This structure has been drilled and imaged on seismic data is a known tectonic graben of Jurassic to Cretaceous age, not made by the Cretaceous-Paleogene impact. In effect the features being highlighted adjacent to Chicxulub are well studied in other papers and do not correlated in age and have data specifically demonstrating these are not of impact origin. Thus in both the case of Popagai and Chicxulub the authors have not used their new data to examine exciting details of impact structure and processes where we know there are impacts to study and instead have gone down the proverbial rabbit hole to chase circular gravity lows arguing for impacts where the geologic data in both locations do not show any evidence. I suggest to reject this paper as it stands and recommend to the authors to rebuild their paper using these gravity anomaly maps to improve our knowledge of Chicxulub and Popagai instead.
Citation: https://doi.org/10.5194/egusphere-2024-866-RC1 -
AC1: 'Reply on RC1', Jaroslav Klokocnik, 06 Jun 2024
reply
Reply of the authors of
Popigai and Chicxulub craters: multiple impacts and their associated grabens
RC1: 'Comment on egusphere-2024-866', Anonymous Referee #1, 28 May 2024.
Thank you for the detailed review. We are ready to account your critical comments and if possible further comments based on other suggestions. We can make deep changes in the manuscript. We are not experts on the impact craters; these results are only one example of our applications of the gravity aspects to test various features on the Earth, the Moon, and Mars (see references in our manuscript). It is well possible that we missed some important references from really a great amount of them, so we would be happy if you are more specific in your review and would recommend what is missing and should be newly mentioned.
The manuscript could be significantly changed if we get a chance to publish it.
- We guarantee our computations of the gravity aspects, but we cannot be sure about our new interpretations. You are expert, we cannot compete.
- You wrote: …a fatal flaw of not constraining their results with other available data that would allow for additional constraints and in this case refute their ideas of multiple impacts in the region of either of these craters. Based on the seismic profiles, you conclude (a majority opinion) that the gravity low NE of Chicxulub or the southern tail are pre-impact structures. It looks like we did not explain well our results, namely those following from the strike angles. In a big contrast to the gravity anomalies, showing simply a gravity low for NE of Chicxulub or the tail, the strike angles react with a halo as for any other impact crater, not an arbitrary circular-like gravity low.
We know majority opinion that the gravity low NE of the main Chicxulub crater and the southern anomaly (the tail) are pre-impact structures. It would be naive to base our hypothesis that there is more than one crater only on gravity anomalies. But also the strike angles behave as expected for the impact crater (see above and Figs. 2b–e).
- We do not work only with the gravity anomalies but with the gravity aspects, a set of functions of the disturbing gravitational field potential. May we recommend the reader to read the theory of the gravity aspects in Klokočník al (2017) and (2020) or in Supplement S1 (which was submitted to SE EGU Discussion together with the main text)? The difference between the gravity anomalies and the gravity aspects is like the difference between a bicycle and a Cadillac.
- Although we now drive a Cadillac and not a bicycle, we are aware that the gravity data alone cannot solve the inverse task in a unique way. We always need additional data. It is emphasized in our text.
We agree with the following critics and would change our text accordingly:
…Additionally, the discussions of both craters is a strange combination of specific details that are not really important to their regional story and missing other papers and data that could test their proposed ideas…
We need, however, a more specific review. We would be happy for recommendations of missing papers, not only for a general, critical remarks like … in both the case of Popagai and Chicxulub the authors have not used their new data to examine exciting details of impact structure and processes where we know there are impacts to study and instead have gone down the proverbial rabbit hole to chase circular gravity lows arguing for impacts where the geologic data in both locations do not show any evidence.
Would it be possible to take our hypothesis about Chicxulub and Popigai as a working hypothesis, a minority opinion, and test it with a new look? If you assume that a crater is a double/multiple crater, you probably will conduct your research in another way/style than if you stick to the traditional, majority view. We know that against an impact origin of Chicxulub II speak the seismic profiles. But we found only 1-2 such profiles NE of the main Chicxulub crater roughly in the direction where we predict the second crater (the profiles from e.g. Gullick et al 2008, 2016). We do not know about any seismic profile crossing the southern tail. In a big contrast to this, the network of seismic profiles crossing the main Chicxulub crater is dense. For the case of Popigai, we did not find any seismic profile at all. The magnetic field over the Popigai region is based (probably till now) on data from a 2.5 km grid compilation of the former Soviet Union.
Kind regards: the authors.
Status: this preprint is open for discussion.
Popigai and Chicxulub craters: multiple impacts and their associated grabens
Jaroslav Klokocnik, Vaclav Cilek, Jan Kostelecky, and Ales Bezdek
Status: open (until 21 Jun 2024) RC1: 'Comment on egusphere-2024-866', Anonymous Referee #1, 28 May 2024.
I was excited to read a paper discussing both Popagai and Chicxulub given these are two of the largest preserved impact basins on Earth. The authors are experts in gravity methods and do a very nice job laying out the efforts to provide update gravity anomaly maps for each of the two impact structures. However, the inferences the authors go on to make about both structures include a fatal flaw of not constraining their results with other available data that would allow for additional constraints and in this case refute their ideas of multiple impacts in the region of either of these craters. Additionally the discussions of both craters is a strange combination of specific details that are not really important to their regional story and missing other papers and data that could test their proposed ideas. For instance at Popagai the whole discussion about diamonds as a shock indicator is unneeded and instead a discussion of the structural geology, erosion depth, drilling results, etc should have been included to allow for aspects like interrogating the new gravity map for what it represents in terms of Popagai's structure. You should a very interesting low gravity ring that could represent the annualar trough for instance or could represent a highly shocked peak ring (an exciting possibility!)...yet instead of diving into such intriguing results, the authors focus on other features in the region that can be explained tectonically and do not have any evidence of an impact origin. For Chicxulub, the authors similarly focus on a detail that is not resolvable in their gravity map (the Holocene aged karst feature the ring of cenotes, which exist due to caves formed within the inner ring of the crater), but then do not detail the ways in which their new gravity map differs from previous ones that might be illustrative of the crater heterogeneities. For instance its a cool opportunity to consider the relative contributions from impact trajectory versus target heterogeneity. Instead they suggest the pre-existing sedimentary basin to the NE of the crater might be a second impact despite seismic images across this feature which show no structure evidence for any kind of impact (no rim faults, terrace zone, central uplift, etc- see the papers already referenced for plenty of evidence its not an impact). Just because it is a gravity low does not mean it's an impact...it usually just means it's a sedimentary basin. Moreover the authors then try to attribute the pre-existing rift that lies south of the crater and likely continued through the area the impact formed as somehow caused by the impact. This structure has been drilled and imaged on seismic data is a known tectonic graben of Jurassic to Cretaceous age, not made by the Cretaceous-Paleogene impact. In effect the features being highlighted adjacent to Chicxulub are well studied in other papers and do not correlated in age and have data specifically demonstrating these are not of impact origin. Thus in both the case of Popagai and Chicxulub the authors have not used their new data to examine exciting details of impact structure and processes where we know there are impacts to study and instead have gone down the proverbial rabbit hole to chase circular gravity lows arguing for impacts where the geologic data in both locations do not show any evidence. I suggest to reject this paper as it stands and recommend to the authors to rebuild their paper using these gravity anomaly maps to improve our knowledge of Chicxulub and Popagai instead.
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AC1: 'Reply on RC1', Jaroslav Klokocnik, 06 Jun 2024
reply
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CC1: 'Comment on egusphere-2024-866', Lev Eppelbaum, 07 Jul 2024
reply
The satellite gravimetry epoch opened a new page in regional gravity data analysis. Previous gravity data were characterized by numerous white spots (with data absence), often - low accuracy, and technical errors at the boundaries of zones of various gravity surveys (carried out at different times with different accuracy). The modern satellite-derived gravity data are observed with a regular grid with the same accuracy. It allows them to apply their analysis of different procedures and transformations developed by Klokochnik et al. and successfully tested in numerous models and field examples.
Many hundreds (and maybe – thousands) of publications suggested the examination of Popigai (Russia) and Chicxulub (Gulf of Mexico) craters. Numerous investigators proposed various physical-geological models of these phenomenal structures. I must note that the physics of the impact structures has not been fully investigated till the present. These investigations are continuing, and it is unlikely that an end will be drawn here in the near future. Of course, the tectonic interpretation of the impact areas may be different. In any case, the paper by Klokochnik et al. is helpful because it provides data for the following different variants of interpretation based on specific observed data.
I support this paper's publication.
Citation: https://doi.org/10.5194/egusphere-2024-866-CC1 -
AC2: 'Reply on CC1', Jaroslav Klokocnik, 08 Jul 2024
reply
Thank you for your support. There are two important factors in the gravity field investigation: better data and a better method. Data: the EIGEN 6C4 gravity model with the GOCE gradiometry data represents improvement over its predecessor EGM2008 by about one order in the ground resolution and precision, namely for the polar areas. Method: the gravity aspects describe the density anomalies (causative bodies) more completely than only the gravity anomalies can do. Combined together, with the improved data and the method, we can come with new results (namely with the combed gravity strike angles). They require a revision of existing interpretations. This is exactly what we do.
Citation: https://doi.org/10.5194/egusphere-2024-866-AC2
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AC2: 'Reply on CC1', Jaroslav Klokocnik, 08 Jul 2024
reply
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