the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 14 May 2024
Overdeepening or tunnel valley of the Aare glacier on the northern margin of the European Alps: Basins, riegels, and slot canyons
Abstract. This work summarizes the results of an interdisciplinary project where we aimed to explore the origin of overdeepenings or tunnel valleys through a combination of a gravimetry survey, drillings, dating and a synthesis of previously published work. To this end, we focused on the Bern area, Switzerland, situated on the northern margin of the European Alps. In this region, multiple advances of piedmont glaciers during the Quaternary glaciations resulted in the carving of the main overdeepening of the Aare River valley (referred to as Aare main overdeepening). This bedrock depression is tens of km long and up to several hundreds of meters to a few kilometers wide. We found that in the Bern area, this main overdeepening is made up of two >200 m-deep troughs that are separated by a c. 5 km-long and up to 150 m-high transverse rocky ridge, interpreted as a riegel. The basins and the riegel are overlain by a >200 m- and 100 m-thick succession of Quaternary sediments, respectively. The bedrock itself is made up of a Late Oligocene to Early Miocene suite of consolidated clastic deposits, which are part of the Molasse foreland basin, whereas the Quaternary suite comprises a middle Pleistocene to Holocene succession of glacio-lacustrine gravel, sand and mud. A synthesis of published gravimetry data revealed that the upstream stoss side of the bedrock riegel is c. 50 % flatter than the downstream lee side. In addition, information from >100 deep drillings reaching depths >50 m suggests that the bedrock riegel is dissected by an anastomosing network of slot canyons. We propose that these canyons established the hydrological connection between the upstream and downstream basins during their formation. Based on published modelling results, we interpret that the riegels and canyons were formed through incision of subglacial meltwater during a glacier’s decay state, when large volumes of meltwater were released. Such a situation has repeatedly occurred since the Middle Pleistocene Transition approximately 800 ka ago, when large and erosive piedmont glaciers began to advance far into the foreland. This resulted in the deep carving of the inner-Alpine valleys, and additionally in the formation of overdeepenings on the plateau on the northern margin of the Alps.
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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RC1: 'Comment on egusphere-2024-683', Anonymous Referee #1, 05 Jun 2024
Dear authors, dear editorial team,
Thank you for the preparation and invitation for review, respectively, of this interesting manuscript. The issue of subglacial overdeepenings’ morphology, and notably their irregularity expressed in features such as sub-basins and riegels, is intriguing, relevant, and deserves researchers’ attention. The present submission is focused on a gravimetric study aimed at constraining the bedrock topography in the area of Bern, Switzerland, with regard to borehole and chronostratigraphic data. The authors argue that a bedrock riegel exists in the Bern city area that is dissected by a network of steeply incised gorges (‘slot canyons’).
While I agree with the majority of the explanation and argumentation that the authors provide, I am not convinced by the central premise of the manuscript, i.e. the existence of the bedrock riegel and slot canyons. The former is indicated by geophysical (gravimetric) data that show a relatively low negative anomaly interpreted as a relatively low depth to bedrock in the Bern city area. The latter are inferred from previous borehole logs that show a much greater drilled depth to bedrock. As far as I could gather from the manuscript, there is no drilling that would clearly support the existence of the riegel, as much as there is no geophysical evidence for the existence of the narrow gorges (due to limited resolution of the method). To me it appears not very likely that all deep boreholes in the area happened to strike narrow slot canyons in an otherwise massive bedrock riegel. I would argue that a more straightforward explanation for these findings would be i) a local underestimation of the depth of the Quaternary infill by the gravimetric study or ii) an misinterpretation of some commercial drillings by the respective contractor that led to overestimation of the drilled bedrock depth. I would insist that these possibilities have to be at least seriously discussed in the manuscript. For i), a more detailed characterisation of the sedimentary infill of the overdeepening (especially its heterogeneity) might be very helpful.
Concerning the proposed scenario of riegel and canyons in the Aare main overdeepening, I further have two rather general questions. Addressing these could in my opinion strengthen the line of reasoning considerably:
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As nicely shown in the manuscript, riegels and dissecting gorges are a relatively common feature of glacial valleys, and likely to be found in overdeepenings as well. To my knowledge, such gorges occur characteristically in high-strength rocks like crystallines or limestones, and the examples mentioned in the text refer to such rock types too. Are there known examples of (subglacial) slot canyons in rather low-strength, moderately consolidated, porous rocks such as the Molasse sandstones in the Alpine foreland?
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Are there known examples of anastomosing networks of slot canyons? I would imagine that the reactivation of a first, single incision through a riegel would always be ‘easier’ and thus more likely than the formation of a new incision, especially in such close proximity to the first that they might end up cross-cutting one another. Here, the relatively low erodibility of the Molasse sandstone could play a favourable role.
I hope that you find these remarks constructive, and that they will help make the manuscript more convincing. In addition, a list of minor comments will be included below.
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Title: ‘Overdeepening or tunnel valley of the Aare glacier…’ sounds a bit rough to me. Maybe ‘The Aare main overdeepening on the northern margin…’ would be a little bit more catchy. This is just a matter of taste, though.
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l. 48f: ‘Erosional work of [predominantly?] glaciers with support by subglacial meltwater’ somewhat contradicts the definition of a tunnel valley as being a product of subglacial meltwater erosion (e.g. Cofaigh, 1996, Prog. Phys. Geogr.; van der Vegt et al., 2012, Geol. Soc. Spec. Pub.).
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l. 54f: I would generally suggest to avoid terms such as valley (if not speaking of tunnel valleys) or channel (e.g. l. 115) that have a strong fluvial connection, and prefer, for example, trough or basin.
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l. 66ff: Worth clarifying that ‘modern landscapes’ refers specifically to glaciated or previously glaciated landscapes, and to overdeepened as well as non-overdeepened settings?
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l. 76f: ‘Meltwater as an important erosional mechanism’ – does that refer to glacial erosion in general, or to overdeepening erosion specifically?
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Fig. 1: - I would recommend to also label the riegel at the cirque outlet to stress the ubiquity of riegels as a glacial landform. - ‘Situation immediately following a full glacial period´ could be ambiguous. Maybe ‘Situation at peak glacial conditions’ or ‘Situation at the onset of ice decay’ would be clearer. Similarly, I would use ‘… started to decay’ instead of ‘… started to melt’. - ‘Will be filled by lacustrine sediments and will eventually host a lake’ – other way around.
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l. 88f: ‘Bedrock structure’ could be mistakable. Better use ‘bedrock topography’.
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l. 101: ‘Bedrock swells cross the thalweg of valleys’, or similar, might be clearer than ‘bedrock swells occur at the base of valleys’.
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l. 108: ‘6° versus 20°’ instead of ‘20° versus 6°’.
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Fig. 2: Small insets with longitudinal sections over the respective riegels showcasing their topography would make this figure better accessible.
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Fig. 3: Using a single harmonised coordinate system would be helpful to the international readership. So would highlighting the extent of 3c on 3b (as done for 3b on 3a).
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l. 129f (and l. 389): What is meant with a connection between both sub-basins, and why was it ruled out? Could the swell between Bern and Meikirch not be another glacially formed riegel?
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l. 137f: ‘The contact between the UMM and the LFM gently dips towards the south’ – does this also apply to the bedding of both? Does this dip angle fit the shallow stoss-side of the riegel, as a potential lithological control?
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l. 204: How does the W-E Gürbe-Aare transect show a change in the gravity signal from upstream to downstream (i.e. largely S-N)? Also, as indicated before, a comparison of the gravity signal with the sedimentary infill composition and architecture might be very helpful at this point.
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l. 207 (and similarly in the caption of Fig. 4 and in l. 221): ‘The signals of the overdeepening signal decreases’ is ambiguous, as they become less negative – better to use ‘the absolute value of the signal´, or similar?
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l. 219: ‘Bümpliz side channel’ – using ‘tributary’ (as previously for the Gürbe) instead of ‘side’ would be more specific. See also third comment.
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l. 228f: ‘Indicated by the thickness of the Quaternary’ – is there any direct measurement of the thickness (i.e. a borehole), or is this based on the gravity anomaly? If so, I would rephrase this.
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l. 237f: Could different sediment fill properties, e.g. deltaic gravels vs. glaciolacustrine sands, play a role in the change of the gravity signal?
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l. 251f: ‘Can only be resolved by invoking the occurrence of a plateau at shallow elevations that is dissected by one or multiple slot canyons’. As stated above, I disagree here. This needs to be argued better.
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l. 278: ‘Response of the erosion by glaciers’ – whose response? Better to use ‘product’?
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Fig. 6: What is the horizontal scale and vertical exaggeration?
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l. 329f: ‘… such conditions most likely prevailed at the front of piedmont glaciers’ – but also in inner-Alpine valleys where riegels and slot canyons occur?
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l. 364: ‘Elevation’ instead of ‘depth’?
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l. 389ff (and l. 466f): Are incised slot canyons really necessary for two sub-basins to be considered linked or connected? As you describe well later on, there are mechanisms that allow subglacial water to travel up adverse slopes, such as that on a riegel, that do not require the existence of a gorge.
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l. 407: It might be worthwhile to add that this uppermost limit was most probably not reached in the case of the Aare main overdeepening, as some foreland overdeepenings are significantly deeper (e.g. underneath Lake Constance; Fabbri et al., 2021, Front. Earth Sci.)
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l. 477: What specifically is meant by ‘such structures’?
Citation: https://doi.org/10.5194/egusphere-2024-683-RC1 -
AC1: 'Reply on RC1', Fritz Schlunegger, 26 Jun 2024
Dear Reviewer Many thanks for the very constructive and detailed comments on our manuscript. Please find below a reply of how we will handle the various concerns and questions.
Reviewer’s comment: While I agree with the majority of the explanation and argumentation that the authors provide, I am not convinced by the central premise of the manuscript, i.e. the existence of the bedrock riegel and slot canyons. The former is indicated by geophysical (gravimetric) data that show a relatively low negative anomaly interpreted as a relatively low depth to bedrock in the Bern city area. The latter are inferred from previous borehole logs that show a much greater drilled depth to bedrock. As far as I could gather from the manuscript, there is no drilling that would clearly support the existence of the riegel, as much as there is no geophysical evidence for the existence of the narrow gorges (due to limited resolution of the method). To me it appears not very likely that all deep boreholes in the area happened to strike narrow slot canyons in an otherwise massive bedrock riegel.
We will include a new figure showing the contour lines of equal Bouguer anomaly values, which have calculated using the density of bedrock (2500 kg/m3, Bandou et al., 2022) as reference density and not the 2670 kg/m3 standard density value that is used per default for such corrections. The pattern of the contour lines will clearly document that the target overdeepening is generally broad and deep upstream of Bern, shallow beneath the city of Bern, and then narrow and deep downstream of it, similar to what we display in Figure 4 where we illustrate the pattern of the residual gravity anomalies related to the Quaternary sediments. This will be used as a further, partly independent piece of evidence that such a bedrock riegel exists beneath Bern. We will also present, in a new figure, the log of one drilled core and the gamma-log of another core, both of which document that Quaternary sequences have been encountered in deep drillings beneath the city of Bern. In addition, we will include the gravimetry profile of the Bern-2 section (see Figure 4 in the submitted manuscript for location), originally published in Bandou et al. (2023), where a second residual gravity anomaly with a short wavelength has been identified by c. 10 gravity stations. We will document that this secondary residual anomaly occurs beneath a large-wavelength residual gravity anomaly. Bandou et al. (2023) considered the anomaly with a large wavelength as the gravity signal of the bedrock riegel, whereas the short-wavelength anomaly beneath it illustrates the possible occurrence of a slot canyon, filled by Quaternary sediments. In summary, we are confronted with the situation that there is a bedrock riegel confirmed by several partly independent pieces of evidence, but that thick Quaternary deposits have been encountered in some deep drillings as well (and have also been detected in the Bern-2 gravity profile). We thus prefer an interpretation where a bedrock riegel is cut by narrow slot canyons filled with Quaternary sediments (see other options below), as such a scenario adequately combines the findings from our gravity survey and drilling information. The bedrock riegel beneath Bern has indeed not been confirmed by a drilling. However, the shallow bedrock is situated beneath public park where deep drillings have not been conducted.
Reviewer’s comment: I would argue that a more straightforward explanation for these findings would be i) a local underestimation of the depth of the Quaternary infill by the gravimetric study
We discard this hypothesis, because the regional Bouguer anomaly map where Bouguer corrections are accomplished with the bedrock’s density clearly document that the bedrock is shallower in Bern than south and north of it – please see our reply above. We can rule out that the occurrence of Quaternary sediments with a much higher density (at least twice the density of the other Quaternary deposits) is responsible for this low gravity signal beneath Bern as this would invoke the occurrence of Quaternary sequences made up of a stack of e.g., highly compacted glacial till deposits. Such a scenario would offer an unrealistic situation that cannot not be combined with currently available information about the geologic architecture in the region. Therefore, we consider this option as not really realistic.
Reviewer’s comment: or ii) an misinterpretation of some commercial drillings by the respective contractor that led to overestimation of the drilled bedrock depth.
We will present information from a core and a gamma-log (both from deep drillings), which clearly document that at some sites, the bedrock-Quaternary contact is very deep. It is possible that for some deep drillings, the contact between the Quaternary sediments and the Molasse bedrock has been misinterpreted by the respective contractors. However, we consider it unrealistic that this has been the case for nearly all drillings. Therefore we consider the option offered by the reviewer as not really realistic.
Reviewer’s comment: I would insist that these possibilities have to be at least seriously discussed in the manuscript. For i), a more detailed characterisation of the sedimentary infill of the overdeepening (especially its heterogeneity) might be very helpful.
We will certainly do this. We will take the opportunity to include new figures (contour lines of Bouguer anomaly values where we used the bedrock density of 2500 kg/m3 for Bouguer corrections, logs of two drillings, and the residual gravity anomaly pattern of the Bern-2 profile of Bandou et al., 2023) to sustain our original interpretation.
Reviewer’s comment: Concerning the proposed scenario of riegel and canyons in the Aare main overdeepening, I further have two rather general questions. Addressing these could in my opinion strengthen the line of reasoning considerably: As nicely shown in the manuscript, riegels and dissecting gorges are a relatively common feature of glacial valleys, and likely to be found in overdeepenings as well. To my knowledge, such gorges occur characteristically in high-strength rocks like crystallines or limestones, and the examples mentioned in the text refer to such rock types too. Are there known examples of (subglacial) slot canyons in rather low-strength, moderately consolidated, porous rocks such as the Molasse sandstones in the Alpine foreland?
We think that the downstream end of Lake Lucerne is such an example where the Upper Marine Molasse bedrock (the same unit as in the Bern area) forms a riegel that is cut by the Reuss River. We will include this example in our paper.
Reviewer's comment: Are there known examples of anastomosing networks of slot canyons? I would imagine that the reactivation of a first, single incision through a riegel would always be ‘easier’ and thus more likely than the formation of a new incision, especially in such close proximity to the first that they might end up cross-cutting one another. Here, the relatively low erodibility of the Molasse sandstone could play a favourable role.
We indeed consider the riegel/slot canyon ensemble near Meiringen to show such features (Figure 2a). We will outline this more clearly in the revised version of our manuscript. All other minor comments and suggestions will be considered upon drafting the revision of the paper.
Many thanks for the constructive review.
References: Bandou, D., Schlunegger, F., Kissling, E., Marti, U., Schwenk, M., Schläfli, P., Douillet, G., and Mair, Three-dimensional gravity modelling of a Quaternary overdeepening fill in the Bern area of Switzerland discloses two stages of glacial carving. Scientific Rep., 12, 1441, https://doi.org/10.1038/s41598-022-04830-x, 2022.
Bandou, D., Schlunegger, F., Kissling, E., Marti, U., Reber, R., and Pfander J.: Overdeepenings in the Swiss plateau: U-shaped geometries underlain by inner gorges. Swiss. J. Geosci., 116, 19, https://doi.org/10.1186/s00015-023-00447-y, 2023.
Citation: https://doi.org/10.5194/egusphere-2024-683-AC1
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RC2: 'Comment on egusphere-2024-683', Anonymous Referee #2, 26 Jul 2024
Please find my comments in the attached manuscript. I look forward to seeing a revised version of the manuscript!
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AC2: 'Reply on RC2', Fritz Schlunegger, 31 Jul 2024
Dear Reviewer
Many thanks for the very constructive and detailed comments on our manuscript. Please find below a reply of how we will handle the various concerns and questions.
Reviewer’s comment: Reviewer 1 has already raised the issue of whether the interpreted riegel could be the result of either an underestimation of the Quaternary infill by gravimetry and the misinterpretation of drillings and I note that you have already responded to these comments. So I don’t want to dwell on this topic too much. But I welcome the introduction of new data that would support your interpretation and while I don’t necessarily disagree with your interpretation, I would appreciate a brief discussion of the possible alternative interpretations to highlight some of the uncertainties and be more transparent to the reader. So I look forward to reading this addition.
Our reply: This will be done, and additional material will be presented to support our interpretation. Please also see our response to the previous review.
Reviewer’s comment: On Figure 7, I would appreciate the location of deep drillings being labelled (in a similar style to Figure 6). This would allow the reader to easily see the ground-truthed evidence supporting the relatively low Quaternary infill (if any boreholes do show this) and location of slot canyons. While this information can be found in Figure 5, it can be quite difficult to interpret this data from a map figure and I would appreciate it plotted onto the cross-profiles as well.
Our reply: This will be done.
Reviewer’s comment: For example, on the Kehrsatz cross-profile you reconstruct a single slot canyon but on Figure 5 there don’t appear to be any drillings which would support this reconstruction? If there is a drilling that supports it then could you label it on Figure 7c, even if the drilling is one of those located a few hundred metres up or downstream of the drawn cross-profile.
Our reply: We will project one deep drilling situated between the Kehrsatz and Airport profiles onto these profiles. However, this has to be considered with caution because the projection distance is quite large (nearly 1 km).
Reviewer’s comment: Section 5.5 Chronological framework. A synthesis of dating evidence is mentioned in both the abstract and introduction of the paper, so I expected the discussion of the chronological constraints to be more important to the paper. But, this section feels a little bit like a brief review of the existing constraints and doesn’t really significantly add anything new or contribute to your aim of exploring the origin of these overdeepenings. While it is important to understand the timing of glaciations that formed these overdeepenings, I believe that this section could be simplified and cut to about half it’s current length. In its current length I felt like it distracted from the main message of the manuscript. In your conclusions section, you barely mention the chronology and I think this illustrates how this section isn’t a key element of your manuscript and it’s location in the discussion section is taking away from the message. Another alternative to shortening this section could be moving it earlier in the manuscript, perhaps after Section 2 on ‘Riegels and slot canyons in the Alpine valleys…’
Our reply: We indeed agree. We will shorten this section as suggested and include it into section 2 as suggested.
Reviewer’s comment: The evolution of the slot canyon network across the bedrock riegel. Across Figure 7, there appears to be an evolution of the subglacial drainage network that incised these slots canyons as it passes over the riegel. At the Aare cross-section, upstream of the riegel there appears to be no evidence of a channelised subglacial drainage network incising into the bedrock (distributed subglacial drainage network or just no drilling data which identified a channel?). As you pass over the riegel there is the anastomosing network of slot canyons that you reconstruct. Then downstream of the riegel on the Bremgarten cross-profile, there is a single slot canyon (possibly?). I am aware that these reconstructed networks are very dependent on drilling data which struck one of these canyons and so this reconstruction has quite large uncertainties, but, I would be interested to see a description of this evolution of the channel network. If you believe that your data is reliable enough to reconstruct it, if not feel free to ignore this.
What are the processes that could drive these changes in network and how it relates to the changing bed slope? Additionally, how could these variations in the subglacial drainage network affected ice flow and erosion? A lot of attention of the manuscript is focused on the overdeepenings themselves, but these slot canyons are also highlighted as a key feature so I would be interested to hear a bit more about the drainage system, if you feel confident enough to reconstruct it!
Our reply: Our database is too sparse to enter into such a discussion. We would need further detailed information about the ages and the sedimentary architecture of the Quaternary fill. We have no data to constrain these boundary conditions and therefore decided not enter in such a discussion (which would be highly speculative), but we add a related note at the end of the paper.
Reviewer’s comment: As a final note, while it is clear the authors have worked hard to integrate a lot of data for this paper, at times the manuscript does not fully reflect their work and can be a little sloppy. In the line-by-line comments, I highlight a few sentences where there are typos or the meaning of the sentence might be unclear. Additionally, I spent only a couple of minutes checking the citations and reference list and I noticed two or three mistakes already (that are listed in the minor comments section). I am sure there are other mistakes in the citations/reference list that I did not spot. Please can you thoroughly proof-read and double-check this for the next version of the manuscript.
Our reply: Thanks for noting. We carefully check the revised manuscript of typos, errors and correct citations. All additional minor comments will be addressed upon improving our manuscript. Thank you very much for the comments and suggestions! They are very helpful.
Citation: https://doi.org/10.5194/egusphere-2024-683-AC2
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AC2: 'Reply on RC2', Fritz Schlunegger, 31 Jul 2024
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2024-683', Anonymous Referee #1, 05 Jun 2024
Dear authors, dear editorial team,
Thank you for the preparation and invitation for review, respectively, of this interesting manuscript. The issue of subglacial overdeepenings’ morphology, and notably their irregularity expressed in features such as sub-basins and riegels, is intriguing, relevant, and deserves researchers’ attention. The present submission is focused on a gravimetric study aimed at constraining the bedrock topography in the area of Bern, Switzerland, with regard to borehole and chronostratigraphic data. The authors argue that a bedrock riegel exists in the Bern city area that is dissected by a network of steeply incised gorges (‘slot canyons’).
While I agree with the majority of the explanation and argumentation that the authors provide, I am not convinced by the central premise of the manuscript, i.e. the existence of the bedrock riegel and slot canyons. The former is indicated by geophysical (gravimetric) data that show a relatively low negative anomaly interpreted as a relatively low depth to bedrock in the Bern city area. The latter are inferred from previous borehole logs that show a much greater drilled depth to bedrock. As far as I could gather from the manuscript, there is no drilling that would clearly support the existence of the riegel, as much as there is no geophysical evidence for the existence of the narrow gorges (due to limited resolution of the method). To me it appears not very likely that all deep boreholes in the area happened to strike narrow slot canyons in an otherwise massive bedrock riegel. I would argue that a more straightforward explanation for these findings would be i) a local underestimation of the depth of the Quaternary infill by the gravimetric study or ii) an misinterpretation of some commercial drillings by the respective contractor that led to overestimation of the drilled bedrock depth. I would insist that these possibilities have to be at least seriously discussed in the manuscript. For i), a more detailed characterisation of the sedimentary infill of the overdeepening (especially its heterogeneity) might be very helpful.
Concerning the proposed scenario of riegel and canyons in the Aare main overdeepening, I further have two rather general questions. Addressing these could in my opinion strengthen the line of reasoning considerably:
-
As nicely shown in the manuscript, riegels and dissecting gorges are a relatively common feature of glacial valleys, and likely to be found in overdeepenings as well. To my knowledge, such gorges occur characteristically in high-strength rocks like crystallines or limestones, and the examples mentioned in the text refer to such rock types too. Are there known examples of (subglacial) slot canyons in rather low-strength, moderately consolidated, porous rocks such as the Molasse sandstones in the Alpine foreland?
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Are there known examples of anastomosing networks of slot canyons? I would imagine that the reactivation of a first, single incision through a riegel would always be ‘easier’ and thus more likely than the formation of a new incision, especially in such close proximity to the first that they might end up cross-cutting one another. Here, the relatively low erodibility of the Molasse sandstone could play a favourable role.
I hope that you find these remarks constructive, and that they will help make the manuscript more convincing. In addition, a list of minor comments will be included below.
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Title: ‘Overdeepening or tunnel valley of the Aare glacier…’ sounds a bit rough to me. Maybe ‘The Aare main overdeepening on the northern margin…’ would be a little bit more catchy. This is just a matter of taste, though.
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l. 48f: ‘Erosional work of [predominantly?] glaciers with support by subglacial meltwater’ somewhat contradicts the definition of a tunnel valley as being a product of subglacial meltwater erosion (e.g. Cofaigh, 1996, Prog. Phys. Geogr.; van der Vegt et al., 2012, Geol. Soc. Spec. Pub.).
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l. 54f: I would generally suggest to avoid terms such as valley (if not speaking of tunnel valleys) or channel (e.g. l. 115) that have a strong fluvial connection, and prefer, for example, trough or basin.
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l. 66ff: Worth clarifying that ‘modern landscapes’ refers specifically to glaciated or previously glaciated landscapes, and to overdeepened as well as non-overdeepened settings?
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l. 76f: ‘Meltwater as an important erosional mechanism’ – does that refer to glacial erosion in general, or to overdeepening erosion specifically?
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Fig. 1: - I would recommend to also label the riegel at the cirque outlet to stress the ubiquity of riegels as a glacial landform. - ‘Situation immediately following a full glacial period´ could be ambiguous. Maybe ‘Situation at peak glacial conditions’ or ‘Situation at the onset of ice decay’ would be clearer. Similarly, I would use ‘… started to decay’ instead of ‘… started to melt’. - ‘Will be filled by lacustrine sediments and will eventually host a lake’ – other way around.
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l. 88f: ‘Bedrock structure’ could be mistakable. Better use ‘bedrock topography’.
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l. 101: ‘Bedrock swells cross the thalweg of valleys’, or similar, might be clearer than ‘bedrock swells occur at the base of valleys’.
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l. 108: ‘6° versus 20°’ instead of ‘20° versus 6°’.
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Fig. 2: Small insets with longitudinal sections over the respective riegels showcasing their topography would make this figure better accessible.
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Fig. 3: Using a single harmonised coordinate system would be helpful to the international readership. So would highlighting the extent of 3c on 3b (as done for 3b on 3a).
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l. 129f (and l. 389): What is meant with a connection between both sub-basins, and why was it ruled out? Could the swell between Bern and Meikirch not be another glacially formed riegel?
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l. 137f: ‘The contact between the UMM and the LFM gently dips towards the south’ – does this also apply to the bedding of both? Does this dip angle fit the shallow stoss-side of the riegel, as a potential lithological control?
-
l. 204: How does the W-E Gürbe-Aare transect show a change in the gravity signal from upstream to downstream (i.e. largely S-N)? Also, as indicated before, a comparison of the gravity signal with the sedimentary infill composition and architecture might be very helpful at this point.
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l. 207 (and similarly in the caption of Fig. 4 and in l. 221): ‘The signals of the overdeepening signal decreases’ is ambiguous, as they become less negative – better to use ‘the absolute value of the signal´, or similar?
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l. 219: ‘Bümpliz side channel’ – using ‘tributary’ (as previously for the Gürbe) instead of ‘side’ would be more specific. See also third comment.
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l. 228f: ‘Indicated by the thickness of the Quaternary’ – is there any direct measurement of the thickness (i.e. a borehole), or is this based on the gravity anomaly? If so, I would rephrase this.
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l. 237f: Could different sediment fill properties, e.g. deltaic gravels vs. glaciolacustrine sands, play a role in the change of the gravity signal?
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l. 251f: ‘Can only be resolved by invoking the occurrence of a plateau at shallow elevations that is dissected by one or multiple slot canyons’. As stated above, I disagree here. This needs to be argued better.
-
l. 278: ‘Response of the erosion by glaciers’ – whose response? Better to use ‘product’?
-
Fig. 6: What is the horizontal scale and vertical exaggeration?
-
l. 329f: ‘… such conditions most likely prevailed at the front of piedmont glaciers’ – but also in inner-Alpine valleys where riegels and slot canyons occur?
-
l. 364: ‘Elevation’ instead of ‘depth’?
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l. 389ff (and l. 466f): Are incised slot canyons really necessary for two sub-basins to be considered linked or connected? As you describe well later on, there are mechanisms that allow subglacial water to travel up adverse slopes, such as that on a riegel, that do not require the existence of a gorge.
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l. 407: It might be worthwhile to add that this uppermost limit was most probably not reached in the case of the Aare main overdeepening, as some foreland overdeepenings are significantly deeper (e.g. underneath Lake Constance; Fabbri et al., 2021, Front. Earth Sci.)
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l. 477: What specifically is meant by ‘such structures’?
Citation: https://doi.org/10.5194/egusphere-2024-683-RC1 -
AC1: 'Reply on RC1', Fritz Schlunegger, 26 Jun 2024
Dear Reviewer Many thanks for the very constructive and detailed comments on our manuscript. Please find below a reply of how we will handle the various concerns and questions.
Reviewer’s comment: While I agree with the majority of the explanation and argumentation that the authors provide, I am not convinced by the central premise of the manuscript, i.e. the existence of the bedrock riegel and slot canyons. The former is indicated by geophysical (gravimetric) data that show a relatively low negative anomaly interpreted as a relatively low depth to bedrock in the Bern city area. The latter are inferred from previous borehole logs that show a much greater drilled depth to bedrock. As far as I could gather from the manuscript, there is no drilling that would clearly support the existence of the riegel, as much as there is no geophysical evidence for the existence of the narrow gorges (due to limited resolution of the method). To me it appears not very likely that all deep boreholes in the area happened to strike narrow slot canyons in an otherwise massive bedrock riegel.
We will include a new figure showing the contour lines of equal Bouguer anomaly values, which have calculated using the density of bedrock (2500 kg/m3, Bandou et al., 2022) as reference density and not the 2670 kg/m3 standard density value that is used per default for such corrections. The pattern of the contour lines will clearly document that the target overdeepening is generally broad and deep upstream of Bern, shallow beneath the city of Bern, and then narrow and deep downstream of it, similar to what we display in Figure 4 where we illustrate the pattern of the residual gravity anomalies related to the Quaternary sediments. This will be used as a further, partly independent piece of evidence that such a bedrock riegel exists beneath Bern. We will also present, in a new figure, the log of one drilled core and the gamma-log of another core, both of which document that Quaternary sequences have been encountered in deep drillings beneath the city of Bern. In addition, we will include the gravimetry profile of the Bern-2 section (see Figure 4 in the submitted manuscript for location), originally published in Bandou et al. (2023), where a second residual gravity anomaly with a short wavelength has been identified by c. 10 gravity stations. We will document that this secondary residual anomaly occurs beneath a large-wavelength residual gravity anomaly. Bandou et al. (2023) considered the anomaly with a large wavelength as the gravity signal of the bedrock riegel, whereas the short-wavelength anomaly beneath it illustrates the possible occurrence of a slot canyon, filled by Quaternary sediments. In summary, we are confronted with the situation that there is a bedrock riegel confirmed by several partly independent pieces of evidence, but that thick Quaternary deposits have been encountered in some deep drillings as well (and have also been detected in the Bern-2 gravity profile). We thus prefer an interpretation where a bedrock riegel is cut by narrow slot canyons filled with Quaternary sediments (see other options below), as such a scenario adequately combines the findings from our gravity survey and drilling information. The bedrock riegel beneath Bern has indeed not been confirmed by a drilling. However, the shallow bedrock is situated beneath public park where deep drillings have not been conducted.
Reviewer’s comment: I would argue that a more straightforward explanation for these findings would be i) a local underestimation of the depth of the Quaternary infill by the gravimetric study
We discard this hypothesis, because the regional Bouguer anomaly map where Bouguer corrections are accomplished with the bedrock’s density clearly document that the bedrock is shallower in Bern than south and north of it – please see our reply above. We can rule out that the occurrence of Quaternary sediments with a much higher density (at least twice the density of the other Quaternary deposits) is responsible for this low gravity signal beneath Bern as this would invoke the occurrence of Quaternary sequences made up of a stack of e.g., highly compacted glacial till deposits. Such a scenario would offer an unrealistic situation that cannot not be combined with currently available information about the geologic architecture in the region. Therefore, we consider this option as not really realistic.
Reviewer’s comment: or ii) an misinterpretation of some commercial drillings by the respective contractor that led to overestimation of the drilled bedrock depth.
We will present information from a core and a gamma-log (both from deep drillings), which clearly document that at some sites, the bedrock-Quaternary contact is very deep. It is possible that for some deep drillings, the contact between the Quaternary sediments and the Molasse bedrock has been misinterpreted by the respective contractors. However, we consider it unrealistic that this has been the case for nearly all drillings. Therefore we consider the option offered by the reviewer as not really realistic.
Reviewer’s comment: I would insist that these possibilities have to be at least seriously discussed in the manuscript. For i), a more detailed characterisation of the sedimentary infill of the overdeepening (especially its heterogeneity) might be very helpful.
We will certainly do this. We will take the opportunity to include new figures (contour lines of Bouguer anomaly values where we used the bedrock density of 2500 kg/m3 for Bouguer corrections, logs of two drillings, and the residual gravity anomaly pattern of the Bern-2 profile of Bandou et al., 2023) to sustain our original interpretation.
Reviewer’s comment: Concerning the proposed scenario of riegel and canyons in the Aare main overdeepening, I further have two rather general questions. Addressing these could in my opinion strengthen the line of reasoning considerably: As nicely shown in the manuscript, riegels and dissecting gorges are a relatively common feature of glacial valleys, and likely to be found in overdeepenings as well. To my knowledge, such gorges occur characteristically in high-strength rocks like crystallines or limestones, and the examples mentioned in the text refer to such rock types too. Are there known examples of (subglacial) slot canyons in rather low-strength, moderately consolidated, porous rocks such as the Molasse sandstones in the Alpine foreland?
We think that the downstream end of Lake Lucerne is such an example where the Upper Marine Molasse bedrock (the same unit as in the Bern area) forms a riegel that is cut by the Reuss River. We will include this example in our paper.
Reviewer's comment: Are there known examples of anastomosing networks of slot canyons? I would imagine that the reactivation of a first, single incision through a riegel would always be ‘easier’ and thus more likely than the formation of a new incision, especially in such close proximity to the first that they might end up cross-cutting one another. Here, the relatively low erodibility of the Molasse sandstone could play a favourable role.
We indeed consider the riegel/slot canyon ensemble near Meiringen to show such features (Figure 2a). We will outline this more clearly in the revised version of our manuscript. All other minor comments and suggestions will be considered upon drafting the revision of the paper.
Many thanks for the constructive review.
References: Bandou, D., Schlunegger, F., Kissling, E., Marti, U., Schwenk, M., Schläfli, P., Douillet, G., and Mair, Three-dimensional gravity modelling of a Quaternary overdeepening fill in the Bern area of Switzerland discloses two stages of glacial carving. Scientific Rep., 12, 1441, https://doi.org/10.1038/s41598-022-04830-x, 2022.
Bandou, D., Schlunegger, F., Kissling, E., Marti, U., Reber, R., and Pfander J.: Overdeepenings in the Swiss plateau: U-shaped geometries underlain by inner gorges. Swiss. J. Geosci., 116, 19, https://doi.org/10.1186/s00015-023-00447-y, 2023.
Citation: https://doi.org/10.5194/egusphere-2024-683-AC1
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RC2: 'Comment on egusphere-2024-683', Anonymous Referee #2, 26 Jul 2024
Please find my comments in the attached manuscript. I look forward to seeing a revised version of the manuscript!
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AC2: 'Reply on RC2', Fritz Schlunegger, 31 Jul 2024
Dear Reviewer
Many thanks for the very constructive and detailed comments on our manuscript. Please find below a reply of how we will handle the various concerns and questions.
Reviewer’s comment: Reviewer 1 has already raised the issue of whether the interpreted riegel could be the result of either an underestimation of the Quaternary infill by gravimetry and the misinterpretation of drillings and I note that you have already responded to these comments. So I don’t want to dwell on this topic too much. But I welcome the introduction of new data that would support your interpretation and while I don’t necessarily disagree with your interpretation, I would appreciate a brief discussion of the possible alternative interpretations to highlight some of the uncertainties and be more transparent to the reader. So I look forward to reading this addition.
Our reply: This will be done, and additional material will be presented to support our interpretation. Please also see our response to the previous review.
Reviewer’s comment: On Figure 7, I would appreciate the location of deep drillings being labelled (in a similar style to Figure 6). This would allow the reader to easily see the ground-truthed evidence supporting the relatively low Quaternary infill (if any boreholes do show this) and location of slot canyons. While this information can be found in Figure 5, it can be quite difficult to interpret this data from a map figure and I would appreciate it plotted onto the cross-profiles as well.
Our reply: This will be done.
Reviewer’s comment: For example, on the Kehrsatz cross-profile you reconstruct a single slot canyon but on Figure 5 there don’t appear to be any drillings which would support this reconstruction? If there is a drilling that supports it then could you label it on Figure 7c, even if the drilling is one of those located a few hundred metres up or downstream of the drawn cross-profile.
Our reply: We will project one deep drilling situated between the Kehrsatz and Airport profiles onto these profiles. However, this has to be considered with caution because the projection distance is quite large (nearly 1 km).
Reviewer’s comment: Section 5.5 Chronological framework. A synthesis of dating evidence is mentioned in both the abstract and introduction of the paper, so I expected the discussion of the chronological constraints to be more important to the paper. But, this section feels a little bit like a brief review of the existing constraints and doesn’t really significantly add anything new or contribute to your aim of exploring the origin of these overdeepenings. While it is important to understand the timing of glaciations that formed these overdeepenings, I believe that this section could be simplified and cut to about half it’s current length. In its current length I felt like it distracted from the main message of the manuscript. In your conclusions section, you barely mention the chronology and I think this illustrates how this section isn’t a key element of your manuscript and it’s location in the discussion section is taking away from the message. Another alternative to shortening this section could be moving it earlier in the manuscript, perhaps after Section 2 on ‘Riegels and slot canyons in the Alpine valleys…’
Our reply: We indeed agree. We will shorten this section as suggested and include it into section 2 as suggested.
Reviewer’s comment: The evolution of the slot canyon network across the bedrock riegel. Across Figure 7, there appears to be an evolution of the subglacial drainage network that incised these slots canyons as it passes over the riegel. At the Aare cross-section, upstream of the riegel there appears to be no evidence of a channelised subglacial drainage network incising into the bedrock (distributed subglacial drainage network or just no drilling data which identified a channel?). As you pass over the riegel there is the anastomosing network of slot canyons that you reconstruct. Then downstream of the riegel on the Bremgarten cross-profile, there is a single slot canyon (possibly?). I am aware that these reconstructed networks are very dependent on drilling data which struck one of these canyons and so this reconstruction has quite large uncertainties, but, I would be interested to see a description of this evolution of the channel network. If you believe that your data is reliable enough to reconstruct it, if not feel free to ignore this.
What are the processes that could drive these changes in network and how it relates to the changing bed slope? Additionally, how could these variations in the subglacial drainage network affected ice flow and erosion? A lot of attention of the manuscript is focused on the overdeepenings themselves, but these slot canyons are also highlighted as a key feature so I would be interested to hear a bit more about the drainage system, if you feel confident enough to reconstruct it!
Our reply: Our database is too sparse to enter into such a discussion. We would need further detailed information about the ages and the sedimentary architecture of the Quaternary fill. We have no data to constrain these boundary conditions and therefore decided not enter in such a discussion (which would be highly speculative), but we add a related note at the end of the paper.
Reviewer’s comment: As a final note, while it is clear the authors have worked hard to integrate a lot of data for this paper, at times the manuscript does not fully reflect their work and can be a little sloppy. In the line-by-line comments, I highlight a few sentences where there are typos or the meaning of the sentence might be unclear. Additionally, I spent only a couple of minutes checking the citations and reference list and I noticed two or three mistakes already (that are listed in the minor comments section). I am sure there are other mistakes in the citations/reference list that I did not spot. Please can you thoroughly proof-read and double-check this for the next version of the manuscript.
Our reply: Thanks for noting. We carefully check the revised manuscript of typos, errors and correct citations. All additional minor comments will be addressed upon improving our manuscript. Thank you very much for the comments and suggestions! They are very helpful.
Citation: https://doi.org/10.5194/egusphere-2024-683-AC2
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AC2: 'Reply on RC2', Fritz Schlunegger, 31 Jul 2024
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Fritz Schlunegger
Edi Kissling
Dimitri Tibo Bandou
Guilhem Amin Douillet
David Mair
Urs Marti
Regina Reber
Patrick Fabian Schläfli
Michael Alfred Schwenk
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