the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 04 Apr 2024
Landscape response to tectonic deformation and cyclic climate change since ca. 800 ka in the southern Central Andes
Abstract. Theory suggests that the response time of alluvial channel long-profiles to perturbations in climate is related to the magnitude of the forcing and the length of the system. Shorter systems may record a higher frequency of forcing compared to longer systems. Empirical field evidence that system length plays a role in the climate periodicity preserved within the sedimentary record is, however, sparse. The Toro Basin in the Eastern Cordillera of NW Argentina provides an opportunity to test these theoretical relationships as this single source-to-sink system contains a range of sediment deposits, located at varying distances from the source. A suite of eight alluvial fan deposits is preserved along the western flanks of the Sierra de Pascha. Farther downstream, a flight of cut-and-fill terraces have been linked to eccentricity-driven (100-kyr) climate cycles since ca. 500 ka. We applied cosmogenic radionuclide (10Be) exposure dating to the fan surfaces to explore (1) how channel responses to external perturbations may or may not propagate downstream, and (2) the differences in landscape response to forcing frequency as a function of channel length. We identified two generations of fan surfaces: the first (G1) records surface activity and abandonment between ca. 800 and 500 ka and the second (G2) within the last 100 kyr. G1 fans record a prolonged phase of net incision, which has been recognised throughout the Central Andes, and was likely triggered by enhanced 100-kyr global glacial cycles following the Mid-Pleistocene Transition. Relative fan surface stability followed, while 100-kyr cut-and-fill cycles occurred downstream, suggesting a disconnect in behaviour between the two channel reaches. G2 fans record higher frequency climate forcing, possibly the result of precessional forcing of climate (ca. 21/40-kyr timescales). The lack of a high-frequency signal farther downstream provides field support for theoretical predictions of a filtering of high-frequency climate forcing with increasing channel length. We show that multiple climate periodicities can be preserved within the sedimentary record of a single basin. Differences in the timing of alluvial fan and fluvial terrace development in the Toro Basin appears to be associated with how channel length affects fluvial response times to climate forcing as well as local controls on net incision, such as tectonic deformation.
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RC1: 'Comment on egusphere-2024-784', Burch Fisher, 03 May 2024
General Comments:
This manuscript by Orr et al. builds on a rich history of quality work done by this group tackling similar problems across the NW Argentinean Andes with the aim of using the rich geomorphic and tectonic settings of the region to constrain landscape responses and timing to climatic and tectonic perturbations. The paper is well written, reasoned, cited, and beautifully illustrated across all the figures. I really appreciate the care that this group of authors has taken with this work and think it should be accepted. The manuscript builds on previous work and datasets in a rigorous way to provide evidence for the preservation of varying geomorphic periodicities and processes within the same basin. I think the authors should be proud of their work.
The only weakness I find in the work is inherent to the methods and certainly no fault of the authors. Exposure dating of surfaces is just difficult, and even when you are as careful as these authors, there are just so many unknowns that can affect the CRN ages. This is especially apparent with the G1 fans. The authors rightfully acknowledge the messiness of these data and derive an incision rate from 800 ka to 500 ka that they argue is related to the MPT. The problem is that you have to explain away a lot of ages either through inheritance for the older ones or erosion for the younger ones. This in turn leaves you wondering which ones are reliable and how do you know? The authors state in line 516 that they “suggest that the G1 fan record can instead be used to capture an extended phase of net incision within the Sierra de Pascha tributaries.” Given the uncertainty in the ages in general it is hard to say that this incision occurred over 300 kyr or over 20 kyr. The point being that they invoke the MPT but without real good constraints you could invoke any number of drivers of incision over that period of time.
My only recommendation would be to temper the wording a bit (things like “likely”) with respect to large-scale climate drivers as there is a fair amount of ambiguity that results from this technique and the correlations between fan age climate proxies in Figure 7 are not always obvious. I would tend toward phrases like “we propose” as the authors have done in their key finding #2 in the conclusions and acknowledge that there is considerable ambiguity.
Nice effort and I look forward to seeing the paper in print!
Specific Comments:
-Any hypothesis why there is such a discrepancy between boulder dimensions between G1 and G2? Do the authors think this is systematic (source area difference?) or just a sampling bias? Is it evidence for widespread erosion of the G1 boulders?
-Figure 6 - I guess part A is informative for the reader but any comparison with the fan record in C is sort of laughable. Maybe consider what it adds to the story.
-Is it possible to put glacial extent in the Pascha headwaters on Figure 1B since you discuss local glaciations on line 592? Not sure but it looks like Martini et al., 2017 and Luna et al., 2018 could be put in there and give some spatial context for this part of the discussion.
-Figure 8B - You say “no significant geomorphic change recognized in the lower basin” but it appears in the diagram that there has been considerable mainstem Rio Toro aggradation from panel A to B. I think it is important to also detail what is going on along the mainstem in this figure as this is creating the local baselevel and presumably propagating any climatic signal between the two.
Line 693 - Doesn’t this fall in a drier period based on the panel A in Figure 7? It is a period of lower insolation which seems to correlate with warmer sea surface temp and less negative delta 18O. Have a look and see if you agree.
Conclusion point 3 - I don’t understand the data that is backing up the statement “the abandonment of the G2 fans is restricted to glacial periods”. In Figure 7 QF_5 and QF_6 both have pdfs that mostly predate any glacial ages. Is the idea that the onset of glaciation drove abandonment of these? Also what constrains the very long tail of the glacial PDF if there are no points out there? Is that curve taken from a more extensive regional study? You should state in the caption what constrains this PDF.
Conclusion point 5 - I think it needs to be pointed out in section 5.4 that the Rio Iruya dataset is different from the others in Figure 9 because it is not measuring the timing of formation or abandonment of a geomorphic feature but rather a nearly continuous chemical sediment signal of erosion from a catchment. I am still wrapping my head around what it all means in terms of comparing them but I think it is an important distinction to make.
Technical Corrections:
Line 520 - You say ~0.07 mm/yr of incision between 800 ka and 500 ka but then at line 557 it is 0.8 mm/yr. Just need to decide on the best value and be consistent.
Figure 9 - Fisher et al, 2016 should be Fisher et al., 2023 in the caption or the citation needs to be added for the 2016 AGU talk. The 2023 article is better to cite than the 2016 AGU talk though.
Fisher et al, 2023 reference is missing the last author in the reference. Should be… , and Lourens, L.J.
Citation: https://doi.org/10.5194/egusphere-2024-784-RC1 -
AC1: 'Reply on RC1', Elizabeth Orr, 07 Jun 2024
Response to Reviewer 1 Comments
General Comments
- This manuscript by Orr et al. builds on a rich history of quality work done by this group tackling similar problems across the NW Argentinean Andes with the aim of using the rich geomorphic and tectonic settings of the region to constrain landscape responses and timing to climatic and tectonic perturbations. The paper is well written, reasoned, cited, and beautifully illustrated across all the figures. I really appreciate the care that this group of authors has taken with this work and think it should be accepted. The manuscript builds on previous work and datasets in a rigorous way to provide evidence for the preservation of varying geomorphic periodicities and processes within the same basin. I think the authors should be proud of their work.
Thank you very much for your through review and constructive feedback on our manuscript. We have carefully addressed all of your comments, clarifying a number of aspects of the project. We believe that these adjustments have helped us to improve the manuscript – thank you!
- The only weakness I find in the work is inherent to the methods and certainly no fault of the authors. Exposure dating of surfaces is just difficult, and even when you are as careful as these authors, there are just so many unknowns that can affect the CRN ages. This is especially apparent with the G1 fans. The authors rightfully acknowledge the messiness of these data and derive an incision rate from 800 ka to 500 ka that they argue is related to the MPT. The problem is that you have to explain away a lot of ages either through inheritance for the older ones or erosion for the younger ones. This in turn leaves you wondering which ones are reliable and how do you know? The authors state in line 516 that they “suggest that the G1 fan record can instead be used to capture an extended phase of net incision within the Sierra de Pascha tributaries.” Given the uncertainty in the ages in general it is hard to say that this incision occurred over 300 kyr or over 20 kyr. The point being that they invoke the MPT but without real good constraints you could invoke any number of drivers of incision over that period of time.
Thank you for your feedback. Working with an age dataset with such broad age distributions is a challenge. By resampling the Qf_1 depth profile and integrating it with the new boulder ages, we were able to constrain the age of the oldest G1 fan surface. This then served as a benchmark for the rest of the dataset, enabling us to interpret the remainder of the fan record with increased confidence. Based on the fan stratigraphy and our observations of the surfaces and boulders (see Supplement 2 and 3), we identified the boulders which were likely affected by erosion (TB19_06, TB19_08, TB19_14) or inheritance (TB19_02, TB19_04). This enabled us to constrain a phase of net incision (ca. 800- 500 ka), during which each of the G1 fans were active at some point, before being abandoned. As stated in the manuscript, applying 21Ne to some of these surfaces may help to reveal the complexities in burial history.
We acknowledge that there remain some uncertainties about the ca. 800-500 ka net incisional phase. Importantly, we are not suggesting a period of continuous incision during this time. Given our confidence in the Qf_1 dating (and that the other surfaces broadly fit into the a. 800-500 ka window), we argue that the G1 fans nicely capture a phase of net incision, before a period of lower geomorphic activity. The onset of G1 fan activity and abandonment appears to coincide with the MPT. We believe that the depth profile results are robust and support the idea of the onset of rapid incision around 800 ka. How fast it proceeded is less clear, but some preliminary 21Ne data from a new ongoing project (https://doi.org/10.5194/egusphere-egu24-14778) suggests that the incision was faster than 100-kyr cycles would imply. We feel as though this argument is compelling because similar phases of net incision linked to the MPT are recorded throughout the Andes. Throughout the manuscript, we acknowledge that there are other drivers during this time that may have contributed to the net incision. However, we have revised the wording in this section to clarify our arguments.
L534: ‘Given these complexities in the fan chronostratigraphy, rather than identifying discrete phases of aggradation and incision for each fan surface, we suggest that the G1 fan record can instead be used to capture a phase of net incision within the Sierra de Pascha tributaries. Crucially, this is unlikely continuous incision, but rather phase of net incision, which was punctuated by the formation of individual surfaces, possibly controlled by higher frequency climate cyclicity (e.g. 100-kyr). If so, this would imply periods of faster incision through the fill’.
- My only recommendation would be to temper the wording a bit (things like “likely”) with respect to large-scale climate drivers as there is a fair amount of ambiguity that results from this technique and the correlations between fan age climate proxies in Figure 7 are not always obvious. I would tend toward phrases like “we propose” as the authors have done in their key finding #2 in the conclusions and acknowledge that there is considerable ambiguity.
Thank you for the recommendation. As advised, we have adjusted the wording slightly to acknowledge remaining uncertainties in the results. We feel that Section 5.3 approaches this argument cautiously and draws on evidence from other studies to suggest a regional event. Rather than saying that the MPT has driven the incision entirely, we argue that there is compelling evidence of a potential link between them. We also acknowledge that other drivers (e.g. tectonic activity) may also contribute here. We have adjusted a couple of paragraphs to emphasise that we are being cautious with our interpretations.
L754: ‘Enhanced incision likely linked to the MPT has also been recognised at other locations in the Central Andes (Fig. 1A), including the Casa Grande Basin (23°S) in the Eastern Cordillera, the Salinas Grandes Basin (23.5°S) of the Puna Plateau (Pingel et al., 2019b), and the Iglesia (30.5°S) and Calingasta (32°S) basins in the Western Precordillera (Terrizzano et al., 2017; Peri et al., 2022).’
L770: ‘While it is not possible to discount a tectonic influence on landscape change in the upper Toro basin entirely due to some chronological ambiguity in the datasets and inherent challenges of deconvolving different forcing mechanisms, the links between MPT climate and incision, and its expression elsewhere in the Andes and beyond, is compelling.’
Specific Comments:
- Any hypothesis why there is such a discrepancy between boulder dimensions between G1 and G2? Do the authors think this is systematic (source area difference?) or just a sampling bias? Is it evidence for widespread erosion of the G1 boulders?
The G1 fan surfaces appeared more weathered and eroded (with some boulder spallation) than G2. This is expected, given that the surfaces could be over ca. 400 kyr older and likely explains why the G1 boulders were noticeably smaller than those on G2. The boulder size is unlikely to reflect a difference in source area, but might relate to the hillslope processes (or even the size of debris flow) that transported them. We made a concerted effort to sample large, tabular, stable boulders that stood tall of the fan surface, with minimal evidence of weathering and surface spallation. While erosion may have affected some of the boulders (see response to Q2), our careful sampling strategy means that we can be more confident that the boulder ages reflect periods of surface activity and/or stability.
Nevertheless, we are finding that the 21Ne data, in combination with 10Be data for individual boulders, can help to constrain boulder erosion rates (which, so far, are extremely low). We are looking forward to seeing whether those data give more insight into the differences in boulder sizes, but those data and accompanying analysis go beyond the scope of this manuscript.
- Figure 6 - I guess part A is informative for the reader but any comparison with the fan record in C is sort of laughable. Maybe consider what it adds to the story.
Thank you for the feedback. We believe that it is important to include this data in the figure as it reinforces our argument that assigning G1 surfaces individually to particular forcing or events is not appropriate. Fig. 6 also includes some important geomorphic and isotope data which is used to explore some of the alternative drivers of incision. On the whole we think this figure is appropriate, and helps to capture some of the climate, tectonic and geomorphic history of the basin on these very long timescales.
- Is it possible to put glacial extent in the Pascha headwaters on Figure 1B since you discuss local glaciations on line 592? Not sure but it looks like Martini et al., 2017 and Luna et al., 2018 could be put in there and give some spatial context for this part of the discussion.
The past glacier extents in the tributaries have not been mapped or dated, although this presents an exciting avenue for future research. While the studies by Martini et al. (2017) and Luna et al. (2018) offer excellent glacier chronologies for local basins, we are hesitant to extrapolate their data and apply their observations to the Toro Basin. Instead, we suggest that in line with regional glacial records (D’Arcy et al., 2019a), the Pascha tributaries were likely glaciated for periods in the last ca. 100 kyr.
- Figure 8B - You say “no significant geomorphic change recognized in the lower basin” but it appears in the diagram that there has been considerable mainstem Rio Toro aggradation from panel A to B. I think it is important to also detail what is going on along the mainstem in this figure as this is creating the local baselevel and presumably propagating any climatic signal between the two.
Thank you for catching this error! The terraces record significant aggradation in the lower basin at this time (as reflected in the figure and later discussion). The local base level rose in line with this aggradation. The phrase "No significant geomorphic change in the lower basin" refers to the absence of an incision record. This has been clarified in the figure caption.
L605: ‘Aggradation was recorded in the lower basin (Tofelde et al., 2017).’
- Line 693 - Doesn’t this fall in a drier period based on the panel A in Figure 7? It is a period of lower insolation which seems to correlate with warmer sea surface temp and less negative delta 18O. Have a look and see if you agree.
Thank you for catching this omission. We have adjusted this statement to reflect your comments.
L723: ‘This points to a modest phase of net incision in several Sierra de Pascha catchments during a dry interglacial period (Fritz et al., 2007).’
- Conclusion point 3 - I don’t understand the data that is backing up the statement “the abandonment of the G2 fans is restricted to glacial periods”. In Figure 7 QF_5 and QF_6 both have pdfs that mostly predate any glacial ages. Is the idea that the onset of glaciation drove abandonment of these?
Thank you for this comment. D’Arcy et al. (2019a) argue that glaciation is this region broadly in phase with insolation cycles, periods of SASM strengthening and/or northern hemispheric events. While there are no glacial records local to Toro that recognise a glacial event between 70 and 60 ka, the climatic conditions were sufficient to sustain glaciers. Glaciers were also recorded in the Central Andes at this time (D’Arcy et al., 2019a).
- Also what constrains the very long tail of the glacial PDF if there are no points out there? Is that curve taken from a more extensive regional study? You should state in the caption what constrains this PDF.
For clarity, we have removed the long tail on the glacier PDF.
- Conclusion point 5 - I think it needs to be pointed out in section 5.4 that the Rio Iruya dataset is different from the others in Figure 9 because it is not measuring the timing of formation or abandonment of a geomorphic feature but rather a nearly continuous chemical sediment signal of erosion from a catchment. I am still wrapping my head around what it all means in terms of comparing them but I think it is an important distinction to make.
Thanks for highlighting this and we agree that this is an important distinction to make. We have added this clarification in the Fig. 9 caption.
L777: ‘Unlike the other records of aggradation and incision, the Iruya record is derived from the basin's sedimentary record and is a paleo-erosion dataset.’
Technical Corrections:
- Line 520 - You say ~0.07 mm/yr of incision between 800 ka and 500 ka but then at line 557 it is 0.8 mm/yr. Just need to decide on the best value and be consistent.
Thank you for catching this error. This incision rate is now quoted throughout the manuscript as 0.7 mm/yr.
- Figure 9 - Fisher et al, 2016 should be Fisher et al., 2023 in the caption or the citation needs to be added for the 2016 AGU talk. The 2023 article is better to cite than the 2016 AGU talk though.
The Fisher et al. (2023) publication is now cited throughout the manuscript.
- Fisher et al, 2023 reference is missing the last author in the reference. Should be… , and Lourens, L.J.
The reference has now been revised to include the last author.
Citation: https://doi.org/10.5194/egusphere-2024-784-AC1 -
RC2: 'Reply on AC1', Burch Fisher, 11 Jun 2024
I appreciate the thoughtful comments and it sounds good to me. It will be interesting to see what the Ne data reveal when they are done. Nice work!
Citation: https://doi.org/10.5194/egusphere-2024-784-RC2
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AC1: 'Reply on RC1', Elizabeth Orr, 07 Jun 2024
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CC1: 'Comment on egusphere-2024-784', Heiko Pingel, 21 May 2024
Hi everybody,
I thought I had posted this comment earlier, but I can't see it in the comments list, so I'm posting it again. I apologize for the inconvenience.
Congrats on this interesting study. Here are some minor comments meant to improve this work in the paragraphs related with the geological setting.
Line 123: I wonder what DeCelles et al., 2011 doing here, as they have not studied nor mentioned the course and nature of the Rio Toro
Lines 129-132: possibly site primary literature, or indicate by „reviewed in“
Lines 132-134 „The Solá fault has been active since at least the Pliocene, and tectonic deformation from the Miocene to mid-Pleistocene has been recorded along the San Bernardo and Gólgota faults (Marrett and Strecker 2000).“ Comment: ca. 10-9 Ma growth strata in the Agujas Cgl. in the Qda Cerro Bayo, along the Sola Fault indicate fault activity since at least Late Miocene times. Similarly, pre-13 Ma growth strata within the Barres Sst along the Golgota Fault set the minimum timing for deformation in the area (see DeCelles et al., 2011; Pearson et al., 2013; Pingel et al., 2020).
Lines 157-159: (1) There is no major N-trending anticline separating the Río Toro from the fan deposits, as indicated here in the text and in the geological map in Fig. 2. The topographic expression causing this separation is formed by west-tilted basin strata (Barres; Agujas; volcanics of the Las Burras-Almagro-El Toro magmatic complex; and Alfarcito Conglomerates), likely deformed via basin-internal shortening in response to contraction along the Golgota fault. The map in Fig. 2 itself shows no repetition of the strata, only westward younging, as would be expected for W-tilted strata.
(2) Something is wrong with the Mio-Pliocene basin stratigraphy: The Agujas Conglomerates are missing entirely from the record. The middle Miocene Barres sst is overlain by the Late Miocene Agujas Cgl, followed by the Alfarcitos cgl. ; all units are intercalated with volcanics of various kind.
(3) The literature presented to underpin the commented statement is partly inadequate: a) DeCelles et al.’s part on the Toro Basin is from the Qda. Cerro Bayo along the Sola Fault near Est. Maury not discussing the upper Toro Basin; b) Robledo et al. is a paleontological work dealing with flora and insect trace fossils of a Mio-Pliocene section along the Sola Fault near Gobernador Solá; c) Mazzuoli et al., is the only reference that fits, as they have dated the volcanics in the center of the ridge. None of the sources presented mention an anticline.
Line 171: „South American low-level jet (SALLJ)“ in Fig. 2 it is called LLAJ–Low-Level Andean Jet
Fig. 1: Label for Almagro Range is positioned wrong.
Fig. 2: The legend states that the unit below the Miocene volcanics is the Barres Fm. This is not correct. It is mainly the Agujas Conglomerates plus the underlying Barres sandstone.
Best regards, Heiko Pingel
Citation: https://doi.org/10.5194/egusphere-2024-784-CC1 -
AC2: 'Reply on CC1', Elizabeth Orr, 09 Jun 2024
Response to Reviewer 2 Comments
- Congrats on this interesting study. Here are some minor comments meant to improve this work in the paragraphs related with the geological setting.
Thank you very much for reviewing our manuscript and providing some helpful feedback on the sections relating to Toro’s geological setting. We have addressed each of your comments and made the necessary changes to the revised manuscript.
- Line 123: I wonder what DeCelles et al., 2011 doing here, as they have not studied nor mentioned the course and nature of the Rio Toro .
Thanks for catching this error. We have removed the citation.
- Lines 129-132: possibly site primary literature, or indicate by „reviewed in“
Thank you for the suggestion. We have added the Alonso (1992) citation, which helped to inform the adapted geological map in Fig. 2A.
Alonso, R.N. Estratigrafía del Cenozoico de la cuenca de Pastos Grandes (Puna Salteña) con énfasis en la Formación. Revista de la Asociación Geológica Argentina, 47(2), 189-199.1992.
- Lines 132-134 „The Solá fault has been active since at least the Pliocene, and tectonic deformation from the Miocene to mid-Pleistocene has been recorded along the San Bernardo and Gólgota faults (Marrett and Strecker 2000).“ Comment: ca. 10-9 Ma growth strata in the Agujas Cgl. in the Qda Cerro Bayo, along the Sola Fault indicate fault activity since at least Late Miocene times. Similarly, pre-13 Ma growth strata within the Barres Sst along the Golgota Fault set the minimum timing for deformation in the area (see DeCelles et al., 2011; Pearson et al., 2013; Pingel et al., 2020).
Thank you for highlighting these omissions. We have adjusted the paragraph as recommended. We have also added the additional citations.
L135: The Solá fault has been active since at least the Late Miocene, and tectonic deformation from the Miocene to mid-Pleistocene has been recorded along the San Bernardo and Gólgota faults (Marrett and Strecker 2000; DeCelles et al., 2011; Pearson et al., 2013; Pingel et al., 2020). The Gólgota fault reactivated after ca. 0.98 Ma (Hilley and Strecker 2005).
- Lines 157-159: (1) There is no major N-trending anticline separating the Río Toro from the fan deposits, as indicated here in the text and in the geological map in Fig. 2. The topographic expression causing this separation is formed by west-tilted basin strata (Barres; Agujas; volcanics of the Las Burras-Almagro-El Toro magmatic complex; and Alfarcito Conglomerates), likely deformed via basin-internal shortening in response to contraction along the Golgota fault. The map in Fig. 2 itself shows no repetition of the strata, only westward younging, as would be expected for W-tilted strata.
Thank you very much for your comment. We have carefully reviewed the geologic maps of the basin and agree with your argument. We have therefore removed the mapped anticline from Fig. 2A and adjusted the manuscript text.
L163: ‘The Middle Miocene Barres Sandstone and Agujas Conglomerates, interbedded with lava flows, and the Pliocene-Pleistocene Alfarcito Conglomerates make up the west-tilted strata, which lay between the fan deposits and the Río Toro (Fig. 2A; Hilley and Strecker, 2005; Mazzuoli et al., 2008).’
L616: ‘To elaborate on the first possibility, the Sierra de Pascha catchments are positioned behind and perpendicular to west-tilted and deformed basin strata (Barres Sandstone, Agujas and Alfracito Conglomerates, lava flows) (Fig. 2A). In concert with the work by Hilley and Strecker (2005), we suggest that channel incision through the resistant sedimentary units accelerated sometime between 0.98 and 0.8 Ma. Once this incision propagated upstream, the removal of weakly consolidated sedimentary units in the upper basin was likely efficient (Hilley and Strecker, 2005). This evolving topography could therefore help to explain the net incision needed in the upper Toro Basin to preserve the alluvial fan surfaces between ca. 800 to 500 ka, and why terrace levels in the lower basin are not recognised during this time interval.’
- Something is wrong with the Mio-Pliocene basin stratigraphy: The Agujas Conglomerates are missing entirely from the record. The middle Miocene Barres sst is overlain by the Late Miocene Agujas Cgl, followed by the Alfarcitos cgl. ; all units are intercalated with volcanics of various kind.
Thank you for catching this oversight. We have adjusted the legend of Fig 2A and included the Agujas Conglomerate in the main text.
L163: ‘The Middle Miocene Barres Sandstone and Agujas Conglomerates, interbedded with lava flows, and the Pliocene-Pleistocene Alfarcito Conglomerates make up the west-tilted strata, which lay between the fan deposits and the Río Toro (Fig. 2A; Hilley and Strecker, 2005; Mazzuoli et al., 2008).’Resistant Barres, Agujas and Alfarcito units characterise several erosional surfaces that stand ~700 m above the modern river channel.
- The literature presented to underpin the commented statement is partly inadequate: a) DeCelles et al.’s part on the Toro Basin is from the Qda. Cerro Bayo along the Sola Fault near Est. Maury not discussing the upper Toro Basin; b) Robledo et al. is a paleontological work dealing with flora and insect trace fossils of a Mio-Pliocene section along the Sola Fault near Gobernador Solá; c) Mazzuoli et al., is the only reference that fits, as they have dated the volcanics in the center of the ridge. None of the sources presented mention an anticline.
Thank you for your comments. We have removed the relevant citations.
- Line 171: „South American low-level jet (SALLJ)“ in Fig. 2 it is called LLAJ–Low-Level Andean Jet.
This has been changed in the revised manuscript.
- 1: Label for Almagro Range is positioned wrong.
The position of the label has been adjusted.
- 2: The legend states that the unit below the Miocene volcanics is the Barres Fm. This is not correct. It is mainly the Agujas Conglomerates plus the underlying Barres sandstone.
Thank you for catching this. We have adjusted the legend to be consistent with the geological map in Pingel et al., (2020) (Fig. 3).
Citation: https://doi.org/10.5194/egusphere-2024-784-AC2
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AC2: 'Reply on CC1', Elizabeth Orr, 09 Jun 2024
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RC3: 'Comment on egusphere-2024-784', Gregoire Messager, 30 Jun 2024
- Scientific significance (Good): the paper discusses the importance of alluvial records in capturing climate variability relative to the length of rivers within an orogenic system, specifically in broken foreland setting. This topic is intriguing because the organization and spatial patterns of alluvial terraces and fans may be indicative of various driving factors, including tectonic activity, climatic changes, and autogenic processes. The results presented in the manuscript add valuable insights into the interpretation of alluvial sequences during the late Pleistocene.
- Scientific quality (Good):
- Methods:
- The authors have mapped alluvial fans using satellite imagery along with DEMs and have dated these features using Cosmogenic Radionuclide 10Be derived from surface samples and a depth profile. One can regret that the analysis of the surface samples did not also incorporate 26Al, as this dual measurement could have provided a more comprehensive discussion of the laboratory errors or potential sample inheritance issues that might affect the 10Be age estimates.
- The authors should introduce here how the depth profile provides more reliable results and how to use the results versus the surface exposure age.
- Are there any erosional rates available for this area? If so, this could impact the CRN ages.
- Furthermore, the authors distinguish between the ages of the alluvial fans’ aggradation and their ages of abandonment. This aspect is quite interesting, but the method by which the abandonment ages were determined remains unclear to me. Although the authors cite earlier studies, it would be beneficial to elaborate on the methodology and clarify the uncertainties associated with these results, as they play a crucial role in the discussions presented in the paper.
- Eventually, in Figures 6C and 7C, the authors display areas of probable age for the terraces using the Probability Density Functions. Could you remind here the concept?
- Results: I find the section 4 dedicated to the results, not well adjusted. The section starts with a conclusion of the results comparing two generations of fans (G1 and G2). This helps to split the description of the results in two subsections dedicated to G1 fans and then G2 fans but it is confusing in the reading as all information and discussion are given at the same time. You have first to show that you are working on alluvial fans, then you date them. I would rewrite the section 4 as follow:
- Section 4.1: a description of the alluvial fans morphology, arrangement (cut and fill, fill-cut, strath), sediment types/size, slope illustrated with pictures (Qf_1 and Qf_5 are not shown on Figure 3) from the field or extracted from satellite images. In this section, it might be good to have a longitudinal incision profile of the alluvial fans versus their nearest stream (height of alluvial fans with regards to the stream bed rock versus distance to the river outlet with significant vertical exaggeration) since you give those values in the text. This could fit in Figure 3.
- Section 4.2: CRN ages of the surface activity and abandonment of the individual alluvial fans described in 4.1 and discuss potential inheritances/erosional factors.
- The identification of two generations of fans is either a conclusion to this section 4 (4.3) or part of the discussion.
- Discussion: The discussion presented in the paper is engaging but occasionally difficult to follow, largely due to the absence of visual aids to clarify the various hypotheses being examined. In my view, the paper's findings predominantly indicate a renewed phase of incision after 750 ka, which the authors attribute to the shift in the duration of glacial cycles during the Middle Pleistocene Transition (1.2-0.8 Ma). Additionally, the research points to a depositional hiatus between 500 and 100 ka in the tributaries of the lower Toro Basin, whereas during the same period, alluvial terraces were developing along the Toro River in the upper basin. To explain this spatio-temporal variabilities, the authors work on three hypotheses: (i) tectonic uplift in the upper reaches, (ii) upstream/downstream incision/aggradation feedbacks, (iii) response time to base level variabilities due to climate variabilities.
- The authors favor hypothesis (iii) but the impact of the base level variability in the Lerma Valley at the Toro Basin outlet has been poorly investigated. There is a possibility that autogenic processes or the reorganization of the drainage network may control a sudden drop in base level. Additionally, the contribution of tectonic uplift at the basin's outlet versus the river erosion power, as detailed by Hilley and Strecker (2005), needs consideration. If these two processes sustain a sufficiently elevated base level, the system's response time could be significantly prolonged, which would give predominance to hypothesis (ii).
- The authors dismiss hypothesis (i), yet this requires further clarification since tectonic uplift rates comprise 50 to 80% of the incision rates. Consequently, a question arises as to why tectonic uplift couldn't be responsible for multiple generations of alluvial fans. It would be beneficial for the authors to explain this reasoning in more detail.
- To aid in comprehending the various hypotheses, I recommend including additional figures:
- A table that succinctly compares the periods of activity and abandonment of the alluvial fans with the ages of the downstream terraces as identified in Tofelde et al. (2017).
- A composite figure that visually represents the three hypotheses could be particularly helpful. This could take the form of schematic longitudinal profiles extending from the lower Toro Basin to the Lerma Valley, including the downstream tectonic barriers. The figure should illustrate how the longitudinal profile has evolved over time and how these changes correspond to the alluvial records. We should not rule out that all three hypotheses may contribute simultaneously.
- Methods:
One aspect that requires further clarification is the rationale behind the G2 fans reflecting climate periodicity of 20 to 40 kyr, as outlined in section 5.4. While initially introduced as a tentative hypothesis, this conclusion seems to be presented later as a more definitive outcome. Additionally, this premise is based on the constrained age distribution of the G2 fans, which is specified as between 21 and 40 kyr in section 5.1.2. However, there is a discrepancy, as the ages from CRN dating and the derived abandonment ages indicate a narrower span, with differences between terraces typically ranging from 5 to 20 kyr.
To clarify, section 5.1.2 should delve deeper into addressing the uncertainties associated with the ages, which could include potential inheritance effects or erosion impacts. It should also expound upon the aggradation activity duration, the precise timing of terrace abandonment, and how these periods correlate with global climate benchmarks such as Marine Isotope Stages (MIS), which have been utilized for the G1 fans' chronology.
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AC3: 'Reply on RC3', Elizabeth Orr, 16 Jul 2024
Thank you very much for your thorough review and constructive feedback on our manuscript. We have carefully addressed all of your comments (see attached file), clarifying a number of aspects of the project. We believe that your feedback has helped us to strengthen the manuscript – thank you!
Status: closed
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RC1: 'Comment on egusphere-2024-784', Burch Fisher, 03 May 2024
General Comments:
This manuscript by Orr et al. builds on a rich history of quality work done by this group tackling similar problems across the NW Argentinean Andes with the aim of using the rich geomorphic and tectonic settings of the region to constrain landscape responses and timing to climatic and tectonic perturbations. The paper is well written, reasoned, cited, and beautifully illustrated across all the figures. I really appreciate the care that this group of authors has taken with this work and think it should be accepted. The manuscript builds on previous work and datasets in a rigorous way to provide evidence for the preservation of varying geomorphic periodicities and processes within the same basin. I think the authors should be proud of their work.
The only weakness I find in the work is inherent to the methods and certainly no fault of the authors. Exposure dating of surfaces is just difficult, and even when you are as careful as these authors, there are just so many unknowns that can affect the CRN ages. This is especially apparent with the G1 fans. The authors rightfully acknowledge the messiness of these data and derive an incision rate from 800 ka to 500 ka that they argue is related to the MPT. The problem is that you have to explain away a lot of ages either through inheritance for the older ones or erosion for the younger ones. This in turn leaves you wondering which ones are reliable and how do you know? The authors state in line 516 that they “suggest that the G1 fan record can instead be used to capture an extended phase of net incision within the Sierra de Pascha tributaries.” Given the uncertainty in the ages in general it is hard to say that this incision occurred over 300 kyr or over 20 kyr. The point being that they invoke the MPT but without real good constraints you could invoke any number of drivers of incision over that period of time.
My only recommendation would be to temper the wording a bit (things like “likely”) with respect to large-scale climate drivers as there is a fair amount of ambiguity that results from this technique and the correlations between fan age climate proxies in Figure 7 are not always obvious. I would tend toward phrases like “we propose” as the authors have done in their key finding #2 in the conclusions and acknowledge that there is considerable ambiguity.
Nice effort and I look forward to seeing the paper in print!
Specific Comments:
-Any hypothesis why there is such a discrepancy between boulder dimensions between G1 and G2? Do the authors think this is systematic (source area difference?) or just a sampling bias? Is it evidence for widespread erosion of the G1 boulders?
-Figure 6 - I guess part A is informative for the reader but any comparison with the fan record in C is sort of laughable. Maybe consider what it adds to the story.
-Is it possible to put glacial extent in the Pascha headwaters on Figure 1B since you discuss local glaciations on line 592? Not sure but it looks like Martini et al., 2017 and Luna et al., 2018 could be put in there and give some spatial context for this part of the discussion.
-Figure 8B - You say “no significant geomorphic change recognized in the lower basin” but it appears in the diagram that there has been considerable mainstem Rio Toro aggradation from panel A to B. I think it is important to also detail what is going on along the mainstem in this figure as this is creating the local baselevel and presumably propagating any climatic signal between the two.
Line 693 - Doesn’t this fall in a drier period based on the panel A in Figure 7? It is a period of lower insolation which seems to correlate with warmer sea surface temp and less negative delta 18O. Have a look and see if you agree.
Conclusion point 3 - I don’t understand the data that is backing up the statement “the abandonment of the G2 fans is restricted to glacial periods”. In Figure 7 QF_5 and QF_6 both have pdfs that mostly predate any glacial ages. Is the idea that the onset of glaciation drove abandonment of these? Also what constrains the very long tail of the glacial PDF if there are no points out there? Is that curve taken from a more extensive regional study? You should state in the caption what constrains this PDF.
Conclusion point 5 - I think it needs to be pointed out in section 5.4 that the Rio Iruya dataset is different from the others in Figure 9 because it is not measuring the timing of formation or abandonment of a geomorphic feature but rather a nearly continuous chemical sediment signal of erosion from a catchment. I am still wrapping my head around what it all means in terms of comparing them but I think it is an important distinction to make.
Technical Corrections:
Line 520 - You say ~0.07 mm/yr of incision between 800 ka and 500 ka but then at line 557 it is 0.8 mm/yr. Just need to decide on the best value and be consistent.
Figure 9 - Fisher et al, 2016 should be Fisher et al., 2023 in the caption or the citation needs to be added for the 2016 AGU talk. The 2023 article is better to cite than the 2016 AGU talk though.
Fisher et al, 2023 reference is missing the last author in the reference. Should be… , and Lourens, L.J.
Citation: https://doi.org/10.5194/egusphere-2024-784-RC1 -
AC1: 'Reply on RC1', Elizabeth Orr, 07 Jun 2024
Response to Reviewer 1 Comments
General Comments
- This manuscript by Orr et al. builds on a rich history of quality work done by this group tackling similar problems across the NW Argentinean Andes with the aim of using the rich geomorphic and tectonic settings of the region to constrain landscape responses and timing to climatic and tectonic perturbations. The paper is well written, reasoned, cited, and beautifully illustrated across all the figures. I really appreciate the care that this group of authors has taken with this work and think it should be accepted. The manuscript builds on previous work and datasets in a rigorous way to provide evidence for the preservation of varying geomorphic periodicities and processes within the same basin. I think the authors should be proud of their work.
Thank you very much for your through review and constructive feedback on our manuscript. We have carefully addressed all of your comments, clarifying a number of aspects of the project. We believe that these adjustments have helped us to improve the manuscript – thank you!
- The only weakness I find in the work is inherent to the methods and certainly no fault of the authors. Exposure dating of surfaces is just difficult, and even when you are as careful as these authors, there are just so many unknowns that can affect the CRN ages. This is especially apparent with the G1 fans. The authors rightfully acknowledge the messiness of these data and derive an incision rate from 800 ka to 500 ka that they argue is related to the MPT. The problem is that you have to explain away a lot of ages either through inheritance for the older ones or erosion for the younger ones. This in turn leaves you wondering which ones are reliable and how do you know? The authors state in line 516 that they “suggest that the G1 fan record can instead be used to capture an extended phase of net incision within the Sierra de Pascha tributaries.” Given the uncertainty in the ages in general it is hard to say that this incision occurred over 300 kyr or over 20 kyr. The point being that they invoke the MPT but without real good constraints you could invoke any number of drivers of incision over that period of time.
Thank you for your feedback. Working with an age dataset with such broad age distributions is a challenge. By resampling the Qf_1 depth profile and integrating it with the new boulder ages, we were able to constrain the age of the oldest G1 fan surface. This then served as a benchmark for the rest of the dataset, enabling us to interpret the remainder of the fan record with increased confidence. Based on the fan stratigraphy and our observations of the surfaces and boulders (see Supplement 2 and 3), we identified the boulders which were likely affected by erosion (TB19_06, TB19_08, TB19_14) or inheritance (TB19_02, TB19_04). This enabled us to constrain a phase of net incision (ca. 800- 500 ka), during which each of the G1 fans were active at some point, before being abandoned. As stated in the manuscript, applying 21Ne to some of these surfaces may help to reveal the complexities in burial history.
We acknowledge that there remain some uncertainties about the ca. 800-500 ka net incisional phase. Importantly, we are not suggesting a period of continuous incision during this time. Given our confidence in the Qf_1 dating (and that the other surfaces broadly fit into the a. 800-500 ka window), we argue that the G1 fans nicely capture a phase of net incision, before a period of lower geomorphic activity. The onset of G1 fan activity and abandonment appears to coincide with the MPT. We believe that the depth profile results are robust and support the idea of the onset of rapid incision around 800 ka. How fast it proceeded is less clear, but some preliminary 21Ne data from a new ongoing project (https://doi.org/10.5194/egusphere-egu24-14778) suggests that the incision was faster than 100-kyr cycles would imply. We feel as though this argument is compelling because similar phases of net incision linked to the MPT are recorded throughout the Andes. Throughout the manuscript, we acknowledge that there are other drivers during this time that may have contributed to the net incision. However, we have revised the wording in this section to clarify our arguments.
L534: ‘Given these complexities in the fan chronostratigraphy, rather than identifying discrete phases of aggradation and incision for each fan surface, we suggest that the G1 fan record can instead be used to capture a phase of net incision within the Sierra de Pascha tributaries. Crucially, this is unlikely continuous incision, but rather phase of net incision, which was punctuated by the formation of individual surfaces, possibly controlled by higher frequency climate cyclicity (e.g. 100-kyr). If so, this would imply periods of faster incision through the fill’.
- My only recommendation would be to temper the wording a bit (things like “likely”) with respect to large-scale climate drivers as there is a fair amount of ambiguity that results from this technique and the correlations between fan age climate proxies in Figure 7 are not always obvious. I would tend toward phrases like “we propose” as the authors have done in their key finding #2 in the conclusions and acknowledge that there is considerable ambiguity.
Thank you for the recommendation. As advised, we have adjusted the wording slightly to acknowledge remaining uncertainties in the results. We feel that Section 5.3 approaches this argument cautiously and draws on evidence from other studies to suggest a regional event. Rather than saying that the MPT has driven the incision entirely, we argue that there is compelling evidence of a potential link between them. We also acknowledge that other drivers (e.g. tectonic activity) may also contribute here. We have adjusted a couple of paragraphs to emphasise that we are being cautious with our interpretations.
L754: ‘Enhanced incision likely linked to the MPT has also been recognised at other locations in the Central Andes (Fig. 1A), including the Casa Grande Basin (23°S) in the Eastern Cordillera, the Salinas Grandes Basin (23.5°S) of the Puna Plateau (Pingel et al., 2019b), and the Iglesia (30.5°S) and Calingasta (32°S) basins in the Western Precordillera (Terrizzano et al., 2017; Peri et al., 2022).’
L770: ‘While it is not possible to discount a tectonic influence on landscape change in the upper Toro basin entirely due to some chronological ambiguity in the datasets and inherent challenges of deconvolving different forcing mechanisms, the links between MPT climate and incision, and its expression elsewhere in the Andes and beyond, is compelling.’
Specific Comments:
- Any hypothesis why there is such a discrepancy between boulder dimensions between G1 and G2? Do the authors think this is systematic (source area difference?) or just a sampling bias? Is it evidence for widespread erosion of the G1 boulders?
The G1 fan surfaces appeared more weathered and eroded (with some boulder spallation) than G2. This is expected, given that the surfaces could be over ca. 400 kyr older and likely explains why the G1 boulders were noticeably smaller than those on G2. The boulder size is unlikely to reflect a difference in source area, but might relate to the hillslope processes (or even the size of debris flow) that transported them. We made a concerted effort to sample large, tabular, stable boulders that stood tall of the fan surface, with minimal evidence of weathering and surface spallation. While erosion may have affected some of the boulders (see response to Q2), our careful sampling strategy means that we can be more confident that the boulder ages reflect periods of surface activity and/or stability.
Nevertheless, we are finding that the 21Ne data, in combination with 10Be data for individual boulders, can help to constrain boulder erosion rates (which, so far, are extremely low). We are looking forward to seeing whether those data give more insight into the differences in boulder sizes, but those data and accompanying analysis go beyond the scope of this manuscript.
- Figure 6 - I guess part A is informative for the reader but any comparison with the fan record in C is sort of laughable. Maybe consider what it adds to the story.
Thank you for the feedback. We believe that it is important to include this data in the figure as it reinforces our argument that assigning G1 surfaces individually to particular forcing or events is not appropriate. Fig. 6 also includes some important geomorphic and isotope data which is used to explore some of the alternative drivers of incision. On the whole we think this figure is appropriate, and helps to capture some of the climate, tectonic and geomorphic history of the basin on these very long timescales.
- Is it possible to put glacial extent in the Pascha headwaters on Figure 1B since you discuss local glaciations on line 592? Not sure but it looks like Martini et al., 2017 and Luna et al., 2018 could be put in there and give some spatial context for this part of the discussion.
The past glacier extents in the tributaries have not been mapped or dated, although this presents an exciting avenue for future research. While the studies by Martini et al. (2017) and Luna et al. (2018) offer excellent glacier chronologies for local basins, we are hesitant to extrapolate their data and apply their observations to the Toro Basin. Instead, we suggest that in line with regional glacial records (D’Arcy et al., 2019a), the Pascha tributaries were likely glaciated for periods in the last ca. 100 kyr.
- Figure 8B - You say “no significant geomorphic change recognized in the lower basin” but it appears in the diagram that there has been considerable mainstem Rio Toro aggradation from panel A to B. I think it is important to also detail what is going on along the mainstem in this figure as this is creating the local baselevel and presumably propagating any climatic signal between the two.
Thank you for catching this error! The terraces record significant aggradation in the lower basin at this time (as reflected in the figure and later discussion). The local base level rose in line with this aggradation. The phrase "No significant geomorphic change in the lower basin" refers to the absence of an incision record. This has been clarified in the figure caption.
L605: ‘Aggradation was recorded in the lower basin (Tofelde et al., 2017).’
- Line 693 - Doesn’t this fall in a drier period based on the panel A in Figure 7? It is a period of lower insolation which seems to correlate with warmer sea surface temp and less negative delta 18O. Have a look and see if you agree.
Thank you for catching this omission. We have adjusted this statement to reflect your comments.
L723: ‘This points to a modest phase of net incision in several Sierra de Pascha catchments during a dry interglacial period (Fritz et al., 2007).’
- Conclusion point 3 - I don’t understand the data that is backing up the statement “the abandonment of the G2 fans is restricted to glacial periods”. In Figure 7 QF_5 and QF_6 both have pdfs that mostly predate any glacial ages. Is the idea that the onset of glaciation drove abandonment of these?
Thank you for this comment. D’Arcy et al. (2019a) argue that glaciation is this region broadly in phase with insolation cycles, periods of SASM strengthening and/or northern hemispheric events. While there are no glacial records local to Toro that recognise a glacial event between 70 and 60 ka, the climatic conditions were sufficient to sustain glaciers. Glaciers were also recorded in the Central Andes at this time (D’Arcy et al., 2019a).
- Also what constrains the very long tail of the glacial PDF if there are no points out there? Is that curve taken from a more extensive regional study? You should state in the caption what constrains this PDF.
For clarity, we have removed the long tail on the glacier PDF.
- Conclusion point 5 - I think it needs to be pointed out in section 5.4 that the Rio Iruya dataset is different from the others in Figure 9 because it is not measuring the timing of formation or abandonment of a geomorphic feature but rather a nearly continuous chemical sediment signal of erosion from a catchment. I am still wrapping my head around what it all means in terms of comparing them but I think it is an important distinction to make.
Thanks for highlighting this and we agree that this is an important distinction to make. We have added this clarification in the Fig. 9 caption.
L777: ‘Unlike the other records of aggradation and incision, the Iruya record is derived from the basin's sedimentary record and is a paleo-erosion dataset.’
Technical Corrections:
- Line 520 - You say ~0.07 mm/yr of incision between 800 ka and 500 ka but then at line 557 it is 0.8 mm/yr. Just need to decide on the best value and be consistent.
Thank you for catching this error. This incision rate is now quoted throughout the manuscript as 0.7 mm/yr.
- Figure 9 - Fisher et al, 2016 should be Fisher et al., 2023 in the caption or the citation needs to be added for the 2016 AGU talk. The 2023 article is better to cite than the 2016 AGU talk though.
The Fisher et al. (2023) publication is now cited throughout the manuscript.
- Fisher et al, 2023 reference is missing the last author in the reference. Should be… , and Lourens, L.J.
The reference has now been revised to include the last author.
Citation: https://doi.org/10.5194/egusphere-2024-784-AC1 -
RC2: 'Reply on AC1', Burch Fisher, 11 Jun 2024
I appreciate the thoughtful comments and it sounds good to me. It will be interesting to see what the Ne data reveal when they are done. Nice work!
Citation: https://doi.org/10.5194/egusphere-2024-784-RC2
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AC1: 'Reply on RC1', Elizabeth Orr, 07 Jun 2024
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CC1: 'Comment on egusphere-2024-784', Heiko Pingel, 21 May 2024
Hi everybody,
I thought I had posted this comment earlier, but I can't see it in the comments list, so I'm posting it again. I apologize for the inconvenience.
Congrats on this interesting study. Here are some minor comments meant to improve this work in the paragraphs related with the geological setting.
Line 123: I wonder what DeCelles et al., 2011 doing here, as they have not studied nor mentioned the course and nature of the Rio Toro
Lines 129-132: possibly site primary literature, or indicate by „reviewed in“
Lines 132-134 „The Solá fault has been active since at least the Pliocene, and tectonic deformation from the Miocene to mid-Pleistocene has been recorded along the San Bernardo and Gólgota faults (Marrett and Strecker 2000).“ Comment: ca. 10-9 Ma growth strata in the Agujas Cgl. in the Qda Cerro Bayo, along the Sola Fault indicate fault activity since at least Late Miocene times. Similarly, pre-13 Ma growth strata within the Barres Sst along the Golgota Fault set the minimum timing for deformation in the area (see DeCelles et al., 2011; Pearson et al., 2013; Pingel et al., 2020).
Lines 157-159: (1) There is no major N-trending anticline separating the Río Toro from the fan deposits, as indicated here in the text and in the geological map in Fig. 2. The topographic expression causing this separation is formed by west-tilted basin strata (Barres; Agujas; volcanics of the Las Burras-Almagro-El Toro magmatic complex; and Alfarcito Conglomerates), likely deformed via basin-internal shortening in response to contraction along the Golgota fault. The map in Fig. 2 itself shows no repetition of the strata, only westward younging, as would be expected for W-tilted strata.
(2) Something is wrong with the Mio-Pliocene basin stratigraphy: The Agujas Conglomerates are missing entirely from the record. The middle Miocene Barres sst is overlain by the Late Miocene Agujas Cgl, followed by the Alfarcitos cgl. ; all units are intercalated with volcanics of various kind.
(3) The literature presented to underpin the commented statement is partly inadequate: a) DeCelles et al.’s part on the Toro Basin is from the Qda. Cerro Bayo along the Sola Fault near Est. Maury not discussing the upper Toro Basin; b) Robledo et al. is a paleontological work dealing with flora and insect trace fossils of a Mio-Pliocene section along the Sola Fault near Gobernador Solá; c) Mazzuoli et al., is the only reference that fits, as they have dated the volcanics in the center of the ridge. None of the sources presented mention an anticline.
Line 171: „South American low-level jet (SALLJ)“ in Fig. 2 it is called LLAJ–Low-Level Andean Jet
Fig. 1: Label for Almagro Range is positioned wrong.
Fig. 2: The legend states that the unit below the Miocene volcanics is the Barres Fm. This is not correct. It is mainly the Agujas Conglomerates plus the underlying Barres sandstone.
Best regards, Heiko Pingel
Citation: https://doi.org/10.5194/egusphere-2024-784-CC1 -
AC2: 'Reply on CC1', Elizabeth Orr, 09 Jun 2024
Response to Reviewer 2 Comments
- Congrats on this interesting study. Here are some minor comments meant to improve this work in the paragraphs related with the geological setting.
Thank you very much for reviewing our manuscript and providing some helpful feedback on the sections relating to Toro’s geological setting. We have addressed each of your comments and made the necessary changes to the revised manuscript.
- Line 123: I wonder what DeCelles et al., 2011 doing here, as they have not studied nor mentioned the course and nature of the Rio Toro .
Thanks for catching this error. We have removed the citation.
- Lines 129-132: possibly site primary literature, or indicate by „reviewed in“
Thank you for the suggestion. We have added the Alonso (1992) citation, which helped to inform the adapted geological map in Fig. 2A.
Alonso, R.N. Estratigrafía del Cenozoico de la cuenca de Pastos Grandes (Puna Salteña) con énfasis en la Formación. Revista de la Asociación Geológica Argentina, 47(2), 189-199.1992.
- Lines 132-134 „The Solá fault has been active since at least the Pliocene, and tectonic deformation from the Miocene to mid-Pleistocene has been recorded along the San Bernardo and Gólgota faults (Marrett and Strecker 2000).“ Comment: ca. 10-9 Ma growth strata in the Agujas Cgl. in the Qda Cerro Bayo, along the Sola Fault indicate fault activity since at least Late Miocene times. Similarly, pre-13 Ma growth strata within the Barres Sst along the Golgota Fault set the minimum timing for deformation in the area (see DeCelles et al., 2011; Pearson et al., 2013; Pingel et al., 2020).
Thank you for highlighting these omissions. We have adjusted the paragraph as recommended. We have also added the additional citations.
L135: The Solá fault has been active since at least the Late Miocene, and tectonic deformation from the Miocene to mid-Pleistocene has been recorded along the San Bernardo and Gólgota faults (Marrett and Strecker 2000; DeCelles et al., 2011; Pearson et al., 2013; Pingel et al., 2020). The Gólgota fault reactivated after ca. 0.98 Ma (Hilley and Strecker 2005).
- Lines 157-159: (1) There is no major N-trending anticline separating the Río Toro from the fan deposits, as indicated here in the text and in the geological map in Fig. 2. The topographic expression causing this separation is formed by west-tilted basin strata (Barres; Agujas; volcanics of the Las Burras-Almagro-El Toro magmatic complex; and Alfarcito Conglomerates), likely deformed via basin-internal shortening in response to contraction along the Golgota fault. The map in Fig. 2 itself shows no repetition of the strata, only westward younging, as would be expected for W-tilted strata.
Thank you very much for your comment. We have carefully reviewed the geologic maps of the basin and agree with your argument. We have therefore removed the mapped anticline from Fig. 2A and adjusted the manuscript text.
L163: ‘The Middle Miocene Barres Sandstone and Agujas Conglomerates, interbedded with lava flows, and the Pliocene-Pleistocene Alfarcito Conglomerates make up the west-tilted strata, which lay between the fan deposits and the Río Toro (Fig. 2A; Hilley and Strecker, 2005; Mazzuoli et al., 2008).’
L616: ‘To elaborate on the first possibility, the Sierra de Pascha catchments are positioned behind and perpendicular to west-tilted and deformed basin strata (Barres Sandstone, Agujas and Alfracito Conglomerates, lava flows) (Fig. 2A). In concert with the work by Hilley and Strecker (2005), we suggest that channel incision through the resistant sedimentary units accelerated sometime between 0.98 and 0.8 Ma. Once this incision propagated upstream, the removal of weakly consolidated sedimentary units in the upper basin was likely efficient (Hilley and Strecker, 2005). This evolving topography could therefore help to explain the net incision needed in the upper Toro Basin to preserve the alluvial fan surfaces between ca. 800 to 500 ka, and why terrace levels in the lower basin are not recognised during this time interval.’
- Something is wrong with the Mio-Pliocene basin stratigraphy: The Agujas Conglomerates are missing entirely from the record. The middle Miocene Barres sst is overlain by the Late Miocene Agujas Cgl, followed by the Alfarcitos cgl. ; all units are intercalated with volcanics of various kind.
Thank you for catching this oversight. We have adjusted the legend of Fig 2A and included the Agujas Conglomerate in the main text.
L163: ‘The Middle Miocene Barres Sandstone and Agujas Conglomerates, interbedded with lava flows, and the Pliocene-Pleistocene Alfarcito Conglomerates make up the west-tilted strata, which lay between the fan deposits and the Río Toro (Fig. 2A; Hilley and Strecker, 2005; Mazzuoli et al., 2008).’Resistant Barres, Agujas and Alfarcito units characterise several erosional surfaces that stand ~700 m above the modern river channel.
- The literature presented to underpin the commented statement is partly inadequate: a) DeCelles et al.’s part on the Toro Basin is from the Qda. Cerro Bayo along the Sola Fault near Est. Maury not discussing the upper Toro Basin; b) Robledo et al. is a paleontological work dealing with flora and insect trace fossils of a Mio-Pliocene section along the Sola Fault near Gobernador Solá; c) Mazzuoli et al., is the only reference that fits, as they have dated the volcanics in the center of the ridge. None of the sources presented mention an anticline.
Thank you for your comments. We have removed the relevant citations.
- Line 171: „South American low-level jet (SALLJ)“ in Fig. 2 it is called LLAJ–Low-Level Andean Jet.
This has been changed in the revised manuscript.
- 1: Label for Almagro Range is positioned wrong.
The position of the label has been adjusted.
- 2: The legend states that the unit below the Miocene volcanics is the Barres Fm. This is not correct. It is mainly the Agujas Conglomerates plus the underlying Barres sandstone.
Thank you for catching this. We have adjusted the legend to be consistent with the geological map in Pingel et al., (2020) (Fig. 3).
Citation: https://doi.org/10.5194/egusphere-2024-784-AC2
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AC2: 'Reply on CC1', Elizabeth Orr, 09 Jun 2024
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RC3: 'Comment on egusphere-2024-784', Gregoire Messager, 30 Jun 2024
- Scientific significance (Good): the paper discusses the importance of alluvial records in capturing climate variability relative to the length of rivers within an orogenic system, specifically in broken foreland setting. This topic is intriguing because the organization and spatial patterns of alluvial terraces and fans may be indicative of various driving factors, including tectonic activity, climatic changes, and autogenic processes. The results presented in the manuscript add valuable insights into the interpretation of alluvial sequences during the late Pleistocene.
- Scientific quality (Good):
- Methods:
- The authors have mapped alluvial fans using satellite imagery along with DEMs and have dated these features using Cosmogenic Radionuclide 10Be derived from surface samples and a depth profile. One can regret that the analysis of the surface samples did not also incorporate 26Al, as this dual measurement could have provided a more comprehensive discussion of the laboratory errors or potential sample inheritance issues that might affect the 10Be age estimates.
- The authors should introduce here how the depth profile provides more reliable results and how to use the results versus the surface exposure age.
- Are there any erosional rates available for this area? If so, this could impact the CRN ages.
- Furthermore, the authors distinguish between the ages of the alluvial fans’ aggradation and their ages of abandonment. This aspect is quite interesting, but the method by which the abandonment ages were determined remains unclear to me. Although the authors cite earlier studies, it would be beneficial to elaborate on the methodology and clarify the uncertainties associated with these results, as they play a crucial role in the discussions presented in the paper.
- Eventually, in Figures 6C and 7C, the authors display areas of probable age for the terraces using the Probability Density Functions. Could you remind here the concept?
- Results: I find the section 4 dedicated to the results, not well adjusted. The section starts with a conclusion of the results comparing two generations of fans (G1 and G2). This helps to split the description of the results in two subsections dedicated to G1 fans and then G2 fans but it is confusing in the reading as all information and discussion are given at the same time. You have first to show that you are working on alluvial fans, then you date them. I would rewrite the section 4 as follow:
- Section 4.1: a description of the alluvial fans morphology, arrangement (cut and fill, fill-cut, strath), sediment types/size, slope illustrated with pictures (Qf_1 and Qf_5 are not shown on Figure 3) from the field or extracted from satellite images. In this section, it might be good to have a longitudinal incision profile of the alluvial fans versus their nearest stream (height of alluvial fans with regards to the stream bed rock versus distance to the river outlet with significant vertical exaggeration) since you give those values in the text. This could fit in Figure 3.
- Section 4.2: CRN ages of the surface activity and abandonment of the individual alluvial fans described in 4.1 and discuss potential inheritances/erosional factors.
- The identification of two generations of fans is either a conclusion to this section 4 (4.3) or part of the discussion.
- Discussion: The discussion presented in the paper is engaging but occasionally difficult to follow, largely due to the absence of visual aids to clarify the various hypotheses being examined. In my view, the paper's findings predominantly indicate a renewed phase of incision after 750 ka, which the authors attribute to the shift in the duration of glacial cycles during the Middle Pleistocene Transition (1.2-0.8 Ma). Additionally, the research points to a depositional hiatus between 500 and 100 ka in the tributaries of the lower Toro Basin, whereas during the same period, alluvial terraces were developing along the Toro River in the upper basin. To explain this spatio-temporal variabilities, the authors work on three hypotheses: (i) tectonic uplift in the upper reaches, (ii) upstream/downstream incision/aggradation feedbacks, (iii) response time to base level variabilities due to climate variabilities.
- The authors favor hypothesis (iii) but the impact of the base level variability in the Lerma Valley at the Toro Basin outlet has been poorly investigated. There is a possibility that autogenic processes or the reorganization of the drainage network may control a sudden drop in base level. Additionally, the contribution of tectonic uplift at the basin's outlet versus the river erosion power, as detailed by Hilley and Strecker (2005), needs consideration. If these two processes sustain a sufficiently elevated base level, the system's response time could be significantly prolonged, which would give predominance to hypothesis (ii).
- The authors dismiss hypothesis (i), yet this requires further clarification since tectonic uplift rates comprise 50 to 80% of the incision rates. Consequently, a question arises as to why tectonic uplift couldn't be responsible for multiple generations of alluvial fans. It would be beneficial for the authors to explain this reasoning in more detail.
- To aid in comprehending the various hypotheses, I recommend including additional figures:
- A table that succinctly compares the periods of activity and abandonment of the alluvial fans with the ages of the downstream terraces as identified in Tofelde et al. (2017).
- A composite figure that visually represents the three hypotheses could be particularly helpful. This could take the form of schematic longitudinal profiles extending from the lower Toro Basin to the Lerma Valley, including the downstream tectonic barriers. The figure should illustrate how the longitudinal profile has evolved over time and how these changes correspond to the alluvial records. We should not rule out that all three hypotheses may contribute simultaneously.
- Methods:
One aspect that requires further clarification is the rationale behind the G2 fans reflecting climate periodicity of 20 to 40 kyr, as outlined in section 5.4. While initially introduced as a tentative hypothesis, this conclusion seems to be presented later as a more definitive outcome. Additionally, this premise is based on the constrained age distribution of the G2 fans, which is specified as between 21 and 40 kyr in section 5.1.2. However, there is a discrepancy, as the ages from CRN dating and the derived abandonment ages indicate a narrower span, with differences between terraces typically ranging from 5 to 20 kyr.
To clarify, section 5.1.2 should delve deeper into addressing the uncertainties associated with the ages, which could include potential inheritance effects or erosion impacts. It should also expound upon the aggradation activity duration, the precise timing of terrace abandonment, and how these periods correlate with global climate benchmarks such as Marine Isotope Stages (MIS), which have been utilized for the G1 fans' chronology.
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AC3: 'Reply on RC3', Elizabeth Orr, 16 Jul 2024
Thank you very much for your thorough review and constructive feedback on our manuscript. We have carefully addressed all of your comments (see attached file), clarifying a number of aspects of the project. We believe that your feedback has helped us to strengthen the manuscript – thank you!
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