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SSP3.21

Bedrock depressions are common features of past and modern glacial landscapes. They are often referred to as overdeepenings and act as important terrestrial archives. Which processes control the formation and geometry of glacial overdeepenings? How did they evolve over time? Which chronological and environmental information can be derived from the sedimentary record? These are the questions that will be addressed in this session.

The timing, extent and driving mechanisms for the last major glacial cycle are increasingly better understood but remain poorly constrained for previous cycles. The early conceptual models, initially adopted to understand older glaciations, neglected much of the spatial and temporal complexity of glaciations. Furthermore, they suffered from a lack of constraining data, which is mainly due to the surficial incompleteness of the terrestrial records.
Some of these limitations may be overcome by studying the sedimentary infill of subglacially formed basins. It is generally accepted that glacial processes, supported by subglacial water, have carved these overdeepenings. However, considerable uncertainties remain concerning the erosional mechanisms and physical constraints.
The sedimentary record in overdeepenings is diverse, including glacial, glacio-lacustrine and fluvial sediments. Investigated records suggest that many overdeepened basins contain a multi-cycle infilling and erosion history. Overdeepenings may therefore act as sediment storages on the timescale of several glacial-interglacial cycles, and provide a valuable record of a landscape’s glacial history. The combination of sedimentological, geophysical, and chronological methods together with the application of landscape evolution models provides new insights into the development of these bedrock features and allows constraining the environmental conditions in the geological past.

This session shall stimulate discussions concerning the formation of subglacial depressions and that aim at deciphering the sedimentary fill of overdeepenings. Contributions may include investigations based on field observations and/or modelling of modern, Quaternary and pre-Quaternary glacial settings. Possible topics cover: (a) glacial and interglacial stratigraphic successions preserved in overdeepenings, (b) subglacial erosion and deposition, (c) glaciation chronology, and (d) landscape evolution.

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Co-organized by CR4/GM7, co-sponsored by IAS
Convener: Michael SchwenkECSECS | Co-conveners: Marius BuechiECSECS, Thomas BurschilECSECS, Urs Fischer, Bernhard Salcher
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| Mon, 04 May, 10:45–12:30 (CEST)

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Chat time: Monday, 4 May 2020, 10:45–12:30

D1134 |
EGU2020-3124
Aurora Machado Garcia, Bárbara Trzaskos, Fernando Farias Vesely, Eduardo Menozzo Rosa, Emrys R. Phillips, and John L. Isbell

The Itararé Group and time equivalent Aquidauana Formation represent the Late Paleozoic Ice Age in the Paraná Basin. These sedimentary rocks provide an extensive glacial record which contains deformed intervals interpreted as glaciotectonites, with features such as folds, thrusts and subhorizontal shear zones, which could be produced sub- or proglacially. In addition, similar deformation structures have been recognised reflecting different depositional processes in glacial related environments, such as subaqueous mass flows, iceberg keel marks and ice-rafting. Micromorphological analysis of glacially-related soft-sediment structures have been widely applied to Quaternary deposits as a tool to aid in the interpretation of paleoenvironments. However, few studies have approached microscale deformation in glacial and glacially-related deposits from the Paleozoic or even older periods, in which the distinction between subglacial, proglacial and non-glacial products is crucial for deep-time paleoclimatic reconstructions. 

Our work aims to address this issue and correlate the range of microstructures present in thin section to different depositional settings interpreted on the basis on macroscale sedimentary facies analysis. The results are used to critically evaluate the applicability of micromorphology in distinguishing paleoenvironments in the pre-Pleistocene glacial record and how those structures can be modified over time in response to lithification and diagenesis. The study combines field data with detailed micromorphological and microstructural analysis of 40 thin sections from the Itararé Group and the Aquidauana Formation. Samples were collected from a range of lithotypes from different depositional settings including: diamictites, sandstones and mudstones from rain-out, mass transport, subglacial overriding and ice-marginal glaciotectonism. The microstructures present include unistrial plasmic fabrics, glaciotectonic laminations, rotational structures (turbates), microshear zones, sheared clasts, faults, folds, boudins and intraclasts. However, in some well-sorted facies, which contain very little or no matrix, also contain features typically associated with compaction and diagenesis, such as grain crushing, reduction of primary porosity, sutured grain contacts and stylolites. 

Preliminary results show that sediments from a range of different depositional facies may contain a similar assemblage of microstructures. This suggests that microstructures on their own cannot fully characterize the original sedimentary depositional environment in older glacigenic sequences. Furthermore, diagenesis plays a major role when it comes to the preservation of primary sedimentary and soft-sediment deformation features and can lead to the overprinting of these structures.

How to cite: Garcia, A. M., Trzaskos, B., Vesely, F. F., Rosa, E. M., Phillips, E. R., and Isbell, J. L.: Micromorphology as a tool to interpret glacial depositional environments from late Paleozoic glacial rocks in the Paraná Basin, Brazil, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3124, https://doi.org/10.5194/egusphere-egu2020-3124, 2020

D1135 |
EGU2020-21432
Marius W. Buechi, Gaudenz Deplazes, Lukas Gegg, Herfried Madritsch, and Flavio S. Anselmetti

Overdeepened glacial basins are excellent archives for the Quaternary glaciation history and the landscape evolution of the Alps. While they are common large-scale glacial features in many inner-Alpine and foreland settings, most of these basins remain underexplored as challenging drilling operations into 10s to 100s of meters of unconsolidated sediment are required to access the sedimentary record.

We currently investigate some of the most prominent overdeepened glacial basins between the Aare and Rhine Rivers in Northern Switzerland with a multi-method approach –  including geophysical methods and scientific drilling – to characterise the geometry, sedimentary infill and age of these glacial basins. The focus of this research is on 1) extracting and refining the Middle to Late Pleistocene glaciation history of Northern Switzerland, and 2) identifying the mechanisms and controls of overdeepening subglacial erosion by characterizing former ice-contacts in the basin fills using micro- to macroscale sedimentological tools.

How to cite: Buechi, M. W., Deplazes, G., Gegg, L., Madritsch, H., and Anselmetti, F. S.: Overdeepened glacial basins as archives for Pleistocene glaciation history and subglacial processes: Results from scientific drilling in the Northern Alpine Foreland, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21432, https://doi.org/10.5194/egusphere-egu2020-21432, 2020

D1136 |
EGU2020-5214
Michael A. Schwenk, Dimitri Bandou, Patrick Schläfli, Guilhem A. Douillett, and Fritz Schlunegger

Overdeepenings are common bedrock features that were formed within the Alps and their forelands during repeated glaciations. Here, we present a total of 208.5 m of sediment core recovered during a scientific drilling campaign in spring 2019. The drilling is located on the Swiss Plateau (CH) close to the terminus of the overdeepening that was formed by the Alpine Aare glacier. The drilling reached the Molasse bedrock at an elevation of 362.5 m a.s.l. This is the first time in the middle Aare Valley that a scientific drilling fully recovered the sedimentary fill of the overdeepening.

The retrieved material comprises an alternation of silt- and sandbeds, and massive as well as cross-bedded gravel layers. We divide the suite into two packages reflecting sedimentation within a fluvio-glacial environment. The first package starts with a glacial till, c. 15 m thick, which is overlain by an 89 m-thick succession of cross-bedded sandy and gravelly layers and mud interbeds reflecting deposition at the toe of a prograding delta. At c. 104 m depth, the succession is interrupted by a till sequence, c. 20 m thick, which forms the base of the second package. This package records a fining-upward trend, starting with massive sand beds at the base and ends with massive silt layers at the top. Mapping has shown that the core section is overlain by a sand layer with soft-sediment deformation structures reflecting the occurrence of a mouth bar environment. The sequence ends with fluvial gravels. The entire sediment core thus records a shallowing-up sequence that is interrupted by a glacial till.

Optical stimulated luminescence dating (IRSL; Buylaert et al., 2009) yielded a minimum age of c. 200 ka for the mouth bar deposits, which is in accordance with reported Late-Middle Pleistocene ages of corresponding sequences in the region (Preusser et al., 2005). We therefore assign the upper package in our drilling to the MIS 7/8 glacial-interglacial cycle, and to the MIS 9/10 for the basal package. The sediments in our drilling thus record a crucial time interval when glacial erosion in the Alps (Haeuselmann et al., 2007) and in the northern foreland (Schlüchter, 2004) occurred at the highest rates.

Buylaert, J.P., Murray, A.S., Thomsen, K.J., Jain, M., 2009. Testing the potential of an elevated temperature IRSL signal from K-feldspar. Radiat. Meas. 44, 560–565.

Haeuselmann, P., Granger, D.E., Jeannin, P.Y., Lauritzen, S.E., 2007. Abrupt glacial valley incision at 0.8 Ma dated from cave deposits in Switzerland. Geology 35, 143–146.

Preusser, F., Drescher-Schneider, R., Fiebig, M., Schlüchter, C., 2005. Re-interpretation of the Meikirch pollen record, Swiss Alpine Foreland, and implications for Middle Pleistocene chronostratigraphy. J. Quat. Sci. 20, 607–620.

Schlüchter, C., 2004. The Swiss glacial record - a schematic summary. Dev. Quat. Sci. 2, 413–418.

How to cite: Schwenk, M. A., Bandou, D., Schläfli, P., Douillett, G. A., and Schlunegger, F.: Drilling into mid-Pleistocene sediments in the overdeepened Aare Valley, CH, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5214, https://doi.org/10.5194/egusphere-egu2020-5214, 2020

D1137 |
EGU2020-13149
| solicited
Gilles Antoniazza and Stuart N. Lane

Glaciers are major agents of landscape erosion. By their erosive power, glaciers are also considered as a sediment production “factory”. During glacial cycles, glacially-conditioned sediment is either transported by the flowing ice and the meltwaters towards terminal distal sinks (e.g. oceans), or stored for shorter or longer periods of time inland. In terms of the long-term disposal of radioactive waste (at timescales of 1 Ma) in areas affected by glaciations, scenarios for the potential of excavation of repositories by glaciers have to be calculated, so as the chance for dispersion of contaminated sediment into the environment. The question as to whether glacially-conditioned sediment contaminated by radioactive waste will be stored inland or dispersed across larger scales towards terminal sinks is therefore of great importance, and glacial overdeepenings are considered as one of the best candidates to store sediment through multiple glacial cycles.

This question of persistence of glacially-conditioned sediment within glacial overdeepenings through multiple phases of glacier advance and retreat has been investigated through a literature review. The study focuses on the Plateau of Northern Switzerland, an area that has been subject to glaciations and which is characterized by many glacial overdeepenings. Results of the literature review show that under some circumstances, glacially-conditioned sediment could persist within glacial overdeepenings across the Plateau of Northern Switzerland during multiple glacial cycles. In the meantime, the conditions required for persistence, as well as the proportions of sediment stored as compared to the sediment transferred to terminal sinks, are much less clear.

How to cite: Antoniazza, G. and Lane, S. N.: Chance for glacially-conditioned sediment to persist within glacial overdeepenings through multiple glacial cycles, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13149, https://doi.org/10.5194/egusphere-egu2020-13149, 2020

D1138 |
EGU2020-7445
Johannes Miocic, Ruth Drescher-Schneider, Hans Rudolf Graf, Marlu Kühn, Frank Preusser, Werner H. Schoch, Nigel Thew, Lucia Wick, and Heinz Furrer

Quaternary deposits within the glacially overdeepened trough of the Wehntal Valley in Northern Switzerland, record glacial and interglacial conditions from the Beringen Glaciation (MIS 6) through to the Holocene. The area is well known for the Niederweningen site, with its rich Late Pleistocene mammal remains found in a buried peat deposit. In addition to this famous “mammoth peat”, more deeply buried peat layers, part of which have previously been attributed to the final part of the Last Interglacial, also include a wealth of environmental data.

Here, we present the first results of an investigation, including sedimentology, geochemistry, palaeobotany (pollen, wood and plant macroremains), malacology, and luminescence dating, of two 16 meter drill cores taken close to the Niederweningen site. The analysed sedimentary successions in both cores show a transition from a series of laminated silts typical of a lake environment to a several meter-thick succession of well-developed organic silts, tufaceous silts and peat layers characteristic of near shore and shore conditions. The presence of dropstones and a lack of organic material in the lower part of the lake sediments indicate glacial conditions, while the peat-rich succession formed during a relatively warm period followed by a time of fluctuating climate. Preliminary results indicate that the organic-rich units represent the Last Interglacial, followed by warm interstadials during the early part of the Last Glacial period. The "mammoth peat" appears to be missing from the studied cores. Erosive surfaces within the peaty succession impede a straightforward interpretation.

How to cite: Miocic, J., Drescher-Schneider, R., Graf, H. R., Kühn, M., Preusser, F., Schoch, W. H., Thew, N., Wick, L., and Furrer, H.: Environmental changes in the Wehntal Valley in Northern Switzerland , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7445, https://doi.org/10.5194/egusphere-egu2020-7445, 2020

D1139 |
EGU2020-8287
Patrick Schläfli, Erika Gobet, Willy Tinner, Jacqueline van Leeuwen, Elisa Vescovi, Guilhem Douillet, Michael Schwenk, Dimitri Bandou, and Fritz Schlunegger

Pollen analysis has shown that at regional to subcontinental scales interglacial vegetation successions usually follow similar patterns throughout the Middle and Late Quaternary. Yet, relevant compositional and species-specific differences among interglacials exist and contribute to their characteristic biostratigraphic fingerprint. We use these divergences to indirectly date palaeolake deposits from Spiezberg (SW margin of overdeepened lake Thun, Switzerland), which have been assigned to either MIS 5e (last interglacial) or MIS 7a (penultimate interglacial) according to the average IRSL age of 164 ± 16 ka. For this purpose, we analyzed the fossil pollen record of the Spiezberg palaeolake deposits and applied optimal partitioning and the broken stick model for zonation. Furthermore, we re-assessed local pollen assemblage zones (LPAZ) of two physically dated reference records (Beerenmösli: MIS 5e; Meikirch: MIS 7a) from the study area by using the same zonation approaches. LPAZ of all three records were then compared optically and numerically (PCA and significance tests) to test which of the two reference records is more similar to Spiezberg. Pollen data show that the major part of the Spiezberg record was sedimented during full interglacial conditions. The irrelevance of Fagus points towards an MIS 5e age for the sediments. This is supported by PCA axis 2, which shows a statistically significant similarity of the Spiezberg record to the MIS 5e reference. This outcome is explained by Fagus playing an important role in the PCA axis 2 gradient, which is probably determined by competition for light. PCA axis 1 is not taken into consideration for the correlation since it represents a climate gradient (e.g. from boreal to temperate forests) and explains most of the intra-locality but less of the inter-locality variance. We thus assign the Spiezberg record to the last interglacial and show that climate-driven compositional differences between MIS 5e and MIS 7a are not diagnostic. In contrast, distinct differences in single-species abundances (e.g. Fagus) are strong enough to significantly distinguish between MIS 5e and MIS 7a records. We conclude that variability across various interglacial vegetation successions (e.g. MIS 5e vs. 7a, PCA axis 2) is smaller than local vegetation variability within a full interglacial succession at the same site (PCA axis 1). This implies a very high vegetation resilience to glacial-interglacial climatic variability, the biotic properties involved might be migration capacity, survival in refugia and adjustments to rapidly changing Quaternary environments.

How to cite: Schläfli, P., Gobet, E., Tinner, W., van Leeuwen, J., Vescovi, E., Douillet, G., Schwenk, M., Bandou, D., and Schlunegger, F.: Biostratigraphic dating of palaeolake deposits from an overdeepening in the Swiss Northern Alpine Foreland by numerical assessments of vegetation composition and the role of species dynamics, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8287, https://doi.org/10.5194/egusphere-egu2020-8287, 2020

D1140 |
EGU2020-15807
Dimitri Bandou, Patrick Schläfli, Michael Schwenk, Guilhem Douillet, Edi Kissling, and Fritz Schlunegger

The processes and mechanisms resulting in overdeepenings, valleys carved deeper than today’s rivers base level during glaciations, are still a matter of debate. Whether or not these valleys formation is due to glacial or fluvio-glacial processes or through fluvial down cutting in the geological past is difficult to affirm, as the depressions are filled with sediment or host lakes (Cook and Swift, 2012). In order to bypass this limitation, we use precise gravimetric data, GNSS data and borehole data, which we combine within a 3D forward modelling code, Gravi3D. We particularly aim at reconstructing the geometry of overdeepened valleys’ walls, which bear information on the erosional mechanism leading to the formation of these troughs. We proceed through the building of models for a given geometry to reproduce the Bouguer gravity that we measured in the field along sections and on a grid of stations. We constrain our models by using precise density values, determined by gravimetry, along with borehole data.

We apply this technique to overdeepenings located in the Alpine foreland (Belpberg area, Central Switzerland) because this area hosts multiple overdeepenings from the past glaciations. The region is characterized by three hill ranges made up of Molasse bedrock with c. 300 m-deep and c. 1 km-wide valleys in-between, where overdeepenings with a Quaternary infill are expected. The results of gravity data collection, accomplished over a section with stations spaced between 100 and 300 m and after standard corrections yield a Bouguer anomaly for the Belpberg region that ranges from c. -99 to -106 mgal. We infer this large range to the regional trend (c. 2 mgal over 8 km) and to the effect of the overdeepening infill (2-4 mgal over 1 km), disclosing a sharp anomaly pattern over the inferred overdeeping. The subsequent three steps include: (i) the removal of the regional trend, (ii) the use of the Nettleton method for the quantification of an accurate density contrast between the Molasse bedrock and the Quaternary infill, and (iii) the configuration of Gravi3D for the Belpberg situation, will yield further information on the morphology of the overdeeping. We thus conclude that Gravi3D, within this framework, is a useful tool to determine the geometry of overdeepings in particular, and shallow subsurface bodies and structures in general.

Reference:

Cook, S.J., Swift, D.A., 2012. Subglacial basins: Their origin and importance in glacial systems and landscapes. Earth-Science Reviews 115, 332–372.

How to cite: Bandou, D., Schläfli, P., Schwenk, M., Douillet, G., Kissling, E., and Schlunegger, F.: Overdeepenings modelled with gravimetry-based data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15807, https://doi.org/10.5194/egusphere-egu2020-15807, 2020

D1141 |
EGU2020-7501
Lukas Gegg, Lorenz Keller, Marius W. Buechi, Thomas Spillmann, Gaudenz Deplazes, Herfried Madritsch, and Flavio S. Anselmetti

Subglacial overdeepenings are common features of past and presently glaciated landscapes. In the Northern Alpine Foreland, these troughs occur mostly within the rather soft, poorly lithified sandstones of the Molasse basin. An exceptional setting is the Lower Aare Valley in Northern Switzerland, where a narrow, finger-like overdeepening (Gebenstorf-Stilli Trough) has been incised more than 100 m below the present surface into the fold-and-thrust belt of the Jura Mountains with its diverse Mesozoic lithologies including competent limestone units. Consequently, the morphology of this overdeepening can provide valuable information on lithological and structural controls on subglacial overdeepening erosion.

We investigate the Gebenstorf-Stilli Trough with three scientific boreholes located along the South-North oriented trough axis. In addition, a set of seismic cross-sections has been acquired by a combination of active and passive seismic approaches analysing surface waves, namely passive horizontal-to-vertical spectral ratio (HVSR) measurements, active measurements applying multiple filter analysis of group velocity (MFA), and extended spatial auto correlation of ambient vibration array data (ESAC).

Preliminary results show that the base of the overdeepening can be well imaged using our methodology. In combination with borehole information, surface elevation data and 3D models of the subsurface geology, we see great potential to better constrain the morphology of the Gebenstorf-Stilli Trough, and to assess how different bedrock lithologies and structures influence subglacial overdeepening erosion – an underexplored and poorly understood issue.

How to cite: Gegg, L., Keller, L., Buechi, M. W., Spillmann, T., Deplazes, G., Madritsch, H., and Anselmetti, F. S.: Geological controls on the morphology of an overdeepened canyon investigated by seismic surface waves (Lower Aare Valley, Switzerland), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7501, https://doi.org/10.5194/egusphere-egu2020-7501, 2020

D1142 |
EGU2020-8386
Thomas Burschil, Hermann Buness, Anna-Catharina Brandt, David C. Tanner, and Gerald Gabriel

Seismic imaging is a spiffing tool to explore the shape and stratigraphy of Quaternary structures such as glacially overdeepened valleys. Recent seismic investigations have unlocked some of the secrets of three of such valleys in the European Alps. Their locations range from distal Quaternary overdeepened basins in the Alpine foreland, i.e. Tannwald Basin (TB) in Germany and the Basadingen Valley (BV) in Switzerland, to the proximal, intra-mountainous Lienz Basin (LB) in Austria. All of them are also primary investigation sites of the project Drilling Overdeepened Valleys (DOVE), funded by the International Continental Scientific Drilling Program (ICDP).

Seismic reflections map the shape and the internal structure of the infill of these basins in detail. The LB shows a classical sedimentary succession of only one glacial cycle, i.e. (from bottom to top) tills, lacustrine sediments, and fluvial deposits. Reflections in the bedrock are not visible. This is in contrast to the distal foreland basins that incise Tertiary sediments and possess sediments of more than one glacial cycle. The reflection patterns of the substrata of the TB and BV basins consist of deeper, continuous horizons and a thick sedimentary package that was incised by glacial erosion. However, the internal reflections differ significantly. In the TB, we discriminate (from bottom to top) plucked molasses units, till of an older glacial cycle, lacustrine sediments, younger till and till sequences, as well as fluvial deposits on top. The preliminary result of the BV illustrates several unconformities and a spatial highly variable setting. The latter is supported by trackable horizons that show several laterally-shifted troughs within the basin fill.

The comparison of the seismic results thus reveals the difference of the sedimentary infill of all three glacially overdeepened valleys. Especially, the internal structure of both basins in the foreland shows unexpected variability.

How to cite: Burschil, T., Buness, H., Brandt, A.-C., Tanner, D. C., and Gabriel, G.: Seismic response of three glacially overdeepened valleys – a comparison, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8386, https://doi.org/10.5194/egusphere-egu2020-8386, 2020