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Limestone with palaeokarst; vertical 2D slice of a CT scan at ~112.5 m depth (left). The limestone can be distinguished well from the palaeokarst infill (middle). Note fragments of limestone embedded within the Bolus Clay (red outline). These fragments can be pieced back together and fit into breakouts of the wall rock (right), giving an impression of the void wall prior to brecciation.
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Water pressures at the base of active glacial overdeepenings are known to fluctuate strongly on various time scales. Rapid peaks in basal water pressure can lead to fracturing of the glacier bed, a process that has been described at numerous sites around the world, mostly based on large hydrofracture systems. This article presents drill‐cores from...
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... Samples of veins considered representative for the latest contractional deformation phase of this structure were obtained from the nearby Nagra exploration borehole BUL1 (Figure 10). This borehole penetrated a several meter-thick karstified zone close to the top of the Mesozoic sequence with clear evidence for tectonic deformation (dark blue line in the borehole section of BUL-1-1 in Figure 10; Gegg et al., 2020;Nagra, 2019bNagra, , 2021a. The calcite veins sampled some 10 m below this deformation zone (Figure 6e) are further discussed in Section 5.2. ...
... The compressively overprinted BIH north of the Lägern Anticline was not accessible for direct sampling. The veins and slickenfibres sampled in the nearby borehole BUL1-1 are taken into account as indication here given their proximity to a tectonically deformed zone (Gegg et al., 2020;Nagra, 2021a; Figure 6e) previously interpreted as splay thrust of the BIH (Nagra, 2019b). Unfortunately, most of our dating attempts of small-scale structures around this deformation zone remained unsuccessful (see Supporting Information S1). ...
This case‐study from the Jura Mountains in the foreland of the European Alps demonstrates how the coupling of subsurface analysis and U‐Pb carbonate dating can provide absolute timing constraints and shortening rate estimates of fold‐and‐thrust belts. It is confirmed that the initial Late Cenozoic foreland deformation driving the formation of the easternmost Jura Mountains in Switzerland was predominately thin‐skinned with contractional deformation largely restricted to the Mesozoic succession above a sub‐horizontal basal décollement. Thereby, the localization and structural style of related deformation structures was strongly guided by the characteristics of underlying Late Paleozoic half grabens. The main thin‐skinned thrust front formed at ∼12 Ma, followed by further deformation in the hinterland and locally continued foreland‐directed thrust propagation. The major deformation zones exposed at surface were established at ∼8 Ma but shortening continued until at least ∼4 Ma. Thick‐skinned contraction associated with the inversion of basement structures only played a subordinate role during the latest deformation phase after 8 Ma. Based on cumulative shortening values derived from balanced cross sections, our U‐Pb ages of syn‐tectonic calcite slickenfibres allow to estimate thin‐skinned deformation rates for the easternmost Jura Mountains between ∼0.9 and ∼0.1 mm/year, decreasing toward the eastern tip of the arcuate belt. Moreover, deformation rates seemingly decreased over time with rates of initial thin‐skinned thrusting being significantly higher than the later deformation north of the main thrust front. These new findings from a classical foreland setting highlight the potential of integrating U‐Pb dating in regional fold‐and‐thrust belt investigations elsewhere.
... Palynological and luminescence data suggest that aggradation phases and filling of the overdeepened valleys began no later than 450 ka (MIS 12;Preusser et al., 2010;Reitner et al., 2010;Gegg et al., 2023;Schwenk et al., 2022), possibly related to erosion-driven flexural uplift of the Alps (Scardia et al., 2012). Recent studies ascribed the overdeepening in the Bern area/Aare Valley to MIS 12 (Gegg et al., 2020;Schwenk et al., 2022). Recently, Dieleman et al. (2022) studied the till layer (Fig. 3D), which is exposed in a gravel pit at Möhlin (Canton of Basel) and attributed it to the Möhlin glaciation. ...
The Alps experienced extensive glaciations during many Pleistocene cold stages. New stratigraphic and geochrono-logical data gathered in the last decade depict the Early Pleistocene glaciations and their record is continuously updated. The onset of major glaciations since the late Matuyama Chron (MIS 22-20) is better recognized in many end moraine systems along the southern side of the Alps. The updated chronology of the Middle Pleistocene phases indicates an improvement of the knowledge about the penultimate glaciation (MIS 6) and the evidence that every sector has had its own most extensive glaciation in a different time span. The dissimilar architecture of the end moraine systems suggests a different behavior of the glaciers from one cold stage to the others. The development of the largest glacier networks with associated piedmont lobes (i.e., Adige, Adda and likely Inn) required abundant snow supply promoted by the southerly circulation, like in the LGM. For the systems with the highest accumulation areas (i.e., Valais, Dora Baltea, Rhine-Reuss and Ticino-Toce) a larger number of glacial units was recorded likely because these were more sensitive to every circulation regime impacting the Alps, whether northwest or south dominated. The Alps remain the most studied mountain range with respect to Quaternary glaciations, thereby providing a unique and valuable resource.
... Such processes may have interacted to dilate rock fractures (Leith et al. 2014), brecciate parts of the rock mass (Gegg et al. 2020) and loosen blocks in response to fluctuating groundwater pressures in hydraulically transmissive fracture zones (Follin et al. 2013). The reduced resistance of the rock surface to drag from sliding glacier ice or tractive forces from fast meltwater flow may then allow extraction of closely brecciated rock (Anderson 2014, Leith et al. 2014. ...
Glacial erosion in the Öregrund archipelago has led to the formation of rock trenches, valleys and basins, now submerged between the low-lying islands. The depths of these topographic depressions
are 10–50 m, greater than in areas to the West and South. This study explores the likely reasons for enhanced glacial erosion in the Öregrund archipelago and the potential for future extension of
topographic depressions ENE towards Forsmark.
The present basement topography of Uppland is inherited from the sub-Cambrian basement unconformity. Previous work has reconstructed its form in summit envelope surfaces as a smooth, block faulted surface. The elevations of the floors of bedrock depressions represent depths of glacial erosion below
the re-exposed unconformity surface found on neighbouring, upstanding basement fault blocks. In the Öregrund archipelago, the presence of Proterozoic sandstones and Ordovician limestones in down-faulted positions indicates that most glacial erosion was a result of erosion in these relatively soft and closely fractured sedimentary rocks. In the basement gneisses, glacial erosion was generally low across the fault block tops and mainly focussed along the regional deformation zones and faults.
Previous analyses of fracture lengths and spacings at Forsmark and in its surroundings indicate that fractures in the gneissic basement conform to general power-law relationships. Major regional brittle
deformation zones have lengths of 10s of km, spacings of 20 to 40 km but widths which do not normally exceed ~100 m but which vary along strike. Together with regional faults, these major fracture zones
control the locations of long rock trenches which delineate the edges of exhumed fault blocks. Many individual trenches across scales remain segmented by rock thresholds. Comparisons of fracture and
trench network topologies show relatively poor connectivity of trench networks, with many isolated trenches developed on short fractures on fault block tops. Previous studies show that in situ joint-bound
rock block sizes in fracture zones are < 0.1–0.5 m. As in other crystalline rocks, in situ joint-bound rock-block size can be regarded as the dominant control on patterns and rates of glacial erosion in
lowland gneissic terrain beneath the Fennoscandian Ice Sheet.
The faulted surface of the sub-Cambrian unconformity provides a reference surface for estimating depths of denudation, including glacial erosion. The onset of deep glacial erosion around the Baltic
Sea basin was at 1.2 Ma. Assuming a similar timing for the Öregrund archipelago, rates of glacial erosion for past 100 ka glacial cycles can be derived. Minimum rates of erosion in sedimentary cover
were 3–4 m/100 ka. Average erosion rates in basement were lower at 1.8–2.0 m/100 ka. Terrestrial cosmogenic nuclide inventories at Forsmark indicate erosion rates for rock surfaces on gneiss bedrock
hills of 1.6–3.5 m/100 ka, similar to estimates of average erosion based on geomorphological criteria. Except for the rim of the Åland Deep, rates of trench deepening were 0.8–4.2 m/100 ka and rates of
trench widening were 13–71 m/100 ka. Thalweg steepening remains largely confined to incision of the western rim of the Åland Deep; in the eastern Öregrund archipelago, headward erosion operated
at 0.4–1.2 km/100 ka.
Erosion in past glacial cycles at Forsmark involved downwearing of fault-block tops, backwearing of fault-block edges and incision of trenches and basins along fracture zones (involving both deepening
and widening of the trenches); the pattern of erosion in future glacial cycles will likely be similar. Rates of downwearing of rock surfaces during future glaciations are estimated to < 1 m/100 ka based on
modelling of cosmogenic nuclides and of future climate/glacial periods. In the past, fault scarps on block edges elsewhere in Uppland have retreated at 42–125 m/100 ka; future backwearing is unlikely
to extend far behind existing fault block edges over the next 100 ka. The Singö, Eckarfjärden and Forsmark deformation zones carry relatively shallow and segmented trenches at Forsmark when compared to their continuations in the Öregrund archipelago. Block removal by future glacial erosion is likely to excavate rock trenches within the main deformation zones at Forsmark. The limited widening
of trenches beyond underlying fracture zones during the Pleistocene indicates that future trench erosion will remain largely confined within the deformation zones that delineate the Forsmark tectonic lens.
Future trench development can be expected to extend to depths of 10–50 m and widths of < 1 km over the next 1 Ma under assumptions of a similar future total duration of glacial conditions. Thus, headward
erosion of rock trenches from the Öregrund archipelago would require around 1 Ma to reach Forsmark. Recent modelling suggests, however, that glacial cycles over the coming several 100 ka will be short.
Hence, the timescales for headward erosion under this scenario would extend beyond 1 Ma.
... Towards the distal part of the trough, the glaciolacustrine sand interfingers with deltaic gravel. This rather unusual infill pattern is most likely related to the local confluence situation as well as the overdeepening's narrow cross-section combined with large discharge of meltwater (see also Gegg et al., 2020). The Birrfeld basin in the lower Reuss Valley contains a multiphase infill that has been attributed to up to five different ice advances (Graf, 2009;Nitsche et al., 2001). ...
Overdeepened structures occur in formerly and presently
glaciated regions around the earth and are usually referred to as
overdeepenings or tunnel valleys. The existence of such troughs has been
known for more than a century, and they have been attributed to similar
formation processes where subglacial meltwater plays a decisive role. This
comparison highlights that (foreland) overdeepenings and tunnel valleys
further occur in similar dimensions and share many characteristics such as
gently sinuous shapes in plan view, undulating long profiles with terminal
adverse slopes, and varying cross-sectional morphologies. The best explored
examples of overdeepened structures are situated in and around the European
Alps and in the central European lowlands. Especially in the vicinity of
the Alps, some individual troughs are well explored, allowing for a
reconstruction of their infill history, whereas only a few detailed studies,
notably such involving long drill core records, have been presented from
northern central Europe. We suggest that more such studies could
significantly further our understanding of subglacial erosion processes and
the regional glaciation histories and aim to promote more intense exchange
and discussion between the respective scientific communities.
... Funding granted by ICDP and other partners in 2021 allowed the drilling of two sites (5068_1 (TANN) and 5068_2 (BASA)) and the revisiting of three legacy cores previously drilled at two sites in Bavaria (5068_3 (SCHA) and 5068_4 (FREI) and in an inner-Alpine overdeepening (5068_5 (BADA)). Information from these sites will be complemented by similar drilling programs in glacial overdeepenings in northwestern Switzerland ( Fig. 6; QBO -Quaternary drill holes: Gegg et al., 2021;SNF-Bern: Schwenk et al., 2022), and DOVE Phase 1 will eventually comprise more than 20 sites. In addition, ICDP has indicated they will provide funding for the remaining four DOVE sites along the southern transect (DOVE Phase 2), if DOVE Phase 1 is successful and matching funds can be secured. ...
The sedimentary infill of glacially overdeepened valleys (i.e., structures eroded below the fluvial base level) is an excellent but yet underexplored archive with regard to the age, extent, and nature of past glaciations. The ICDP project DOVE (Drilling Overdeepened Alpine Valleys) Phase 1 investigates a series of drill cores from glacially overdeepened troughs at several locations along the northern front of the Alps. All sites will be investigated with regard to several aspects of environmental dynamics during the Quaternary, with focus on the glaciation, vegetation, and landscape history. Geophysical methods (e.g., seismic surveys), for example, will explore the geometry of overdeepened structures to better understand the process of overdeepening. Sedimentological analyses combined with downhole logging, analysis of biological remains, and state-of-the-art geochronological methods, will enable us to reconstruct the erosion and sedimentation history of the overdeepened troughs. This approach is expected to yield significant novel data quantifying the extent and timing of Middle and Late Pleistocene glaciations of the Alps. In a first phase, two sites were drilled in late 2021 into filled overdeepenings below the paleolobe of the Rhine Glacier, and both recovered a trough filling composed of multiphase glacial sequences. Fully cored Hole 5068_1_C reached a depth of 165 m and recovered 10 m molasse bedrock at the base. This hole will be used together with two flush holes (5068_1_A, 5068_1_B) for further geophysical cross-well experiments. Site 5068_2 reached a depth of 255 m and bottomed out near the soft rock–bedrock contact. These two sites are complemented by three legacy drill sites that previously recovered filled overdeepenings below the more eastern Alpine Isar-Loisach, Salzach, and Traun paleoglacier lobes (5068_3, 5068_4, 5068_5). All analysis and interpretations of this DOVE Phase 1 will eventually lay the ground for an upcoming Phase 2 that will complete the pan-Alpine approach. This follow-up phase will investigate overdeepenings formerly occupied by paleoglacier lobes from the western and southern Alpine margins through drilling sites in France, Italy, and Slovenia. Available geological information and infrastructure make the Alps an ideal area to study overdeepened structures; however, the expected results of this study will not be restricted to the Alps. Such features are also known from other formerly glaciated mountain ranges, which are less studied than the Alps and more problematic with regards to drilling logistics. The results of this study will serve as textbook concepts to understand a full range of geological processes relevant to formerly glaciated areas all over our planet.
... Advancing glaciers widened and locally overdeepened the river valleys (e.g. Gegg et al., 2020Gegg et al., , 2021 that progressively lowered over time due to repeated base level drops (Bitterli-Dreher et al., 2007). Today, remnants of ancient, elevated valleys are mainly found in the shape of isolated occurrences of Early ('Deckenschotter'; Graf, 1993Graf, , 2009a, and of Middle Pleistocene gravel deposits ('Hochterrasse' system; Graf, 2009b). ...
While timing and ice extent of the last glacial maximum are generally well known, the courses of earlier glaciations have remained poorly constrained, with one of the main reasons being the scarcity of sedimentary archives. This study introduces a new palaeolake record from a Mid-Pleistocene glaciofluvial channel system in the Lower Aare Valley (Northern Switzerland). The record of Rinikerfeld comprises a >40 m long succession of Quaternary deposits that are targeted by multi-method sedimentological analysis. Sedimentary facies together with geochemical and geotechnical parameters, pollen content, as well as luminescence ages allow the reconstruction of the establishment, evolution and infilling of the early Marine Isotope Stage 6-aged Rinikerfeld Palaeolake. A drastic change in lake sediment composition and structure indicates cessation of the initial glacially derived input, which is explained by landscape modification and drainage rerouting during the Penultimate (Beringen) Glaciation. Geochemical and palynological data further reveal cold, initially periglacial but slightly ameliorating, climate conditions, while the lake was progressively filled up by local runoff, before being buried by periglacial colluvial diamicts, and potentially overridden by ice. It is therefore concluded that the onset of the Beringen Glaciation was an environmentally as well as geomorphically dynamic time period in the Northern Alpine Foreland.
... The maximum trough depth exceeds 110 m below surface and 75 m below the lowest known Pleistocene base level (PBL, 300 m a.s. l.; Graf, 2009;Gegg et al., 2020). Situated entirely outside the LGM (Bini et al., 2009), the GST was presumably incised during the late Middle Pleistocene (Bitterli-Dreher et al., 2007;Graf, 2009). ...
... m in QGVO (corrected after Terzaghi, 1965). Deepreaching, sediment-filled paleokarst predating the Quaternary and exhibiting presumed subglacial hydrofractures was encountered in QGBR (Gegg et al., 2020). The calcareous marls of the Wildegg Fm. have a similar fracture spacing of~1.1 m in QUST, and contain intervals where the rock is softened or granular-disintegrating. ...
... A similar erosion pattern was observed in a seismic study of Lake Neuchâtel (NW Switzerland; Ndiaye et al., 2014), whose overdeepened floor reaches down to, but is not significantly incised into, the Mesozoic strata under~200 m of Molasse cover. In QGBR, deep-reaching sediment-filled paleokarst was encountered in the limestone (Gegg et al., 2020). The fact that even karstified and presumably weakened «Malmkalk» was preserved and not completely eroded by the overdeepening glacier emphasizes its erosional resistance (see also Ndiaye et al., 2014). ...
Subglacial overdeepenings are common elements of mountain forelands and have considerable implications for human infrastructure. Yet, the processes of overdeepening by subglacial erosion and especially the role of bedrock geology are poorly understood. We present a case study of the Gebenstorf-Stilli Trough in northern Switzerland, a foreland overdeepening with a regionally unique, complex underlying bedrock geology: in contrast to other Swiss foreland overdeepenings, it is incised not only into Cenozoic Molasse deposits, but also into the underlying Mesozoic bedrock. In order to constrain the trough morphology in 3D, it was targeted with scientific boreholes as well as with seismic measurements acquired through analysis of surface waves. Our results reveal an unexpectedly complex trough morphology that appears to be closely related to the bedrock geology. Two sub-basins are incised into calcareous marls and Molasse deposits, and are separated by a distinct ridge of Jurassic limestones, indicating strong lithological control on erosional efficiency. We infer generally relatively low glacial erosion efficiency sensu stricto (i.e. quarrying and abrasion) and suggest that the glacier's basal drainage system may have been the main driver of subglacial erosion of the Gebenstorf-Stilli Trough.
Hydrofracture systems have been described in glacial sediments for almost a century and accelerating research since the 2000s, boosted by the advent of micromorphological techniques applied to glacial deposits, led to a significant rise of studies using palaeo-hydrofractures (and their fills) as a new proxy for reconstructing glacial processes and environments. This review covers the great diversity of hydrofracturing context (subglacial, marginal, proglacial) and physical characteristics (at macro- to micro-scale) of hydrofracture systems and their fills based on a compilation of published and unpublished field-based data from both Quaternary and pre-Quaternary glacial sediments.
The text covers (1) the fundamental concepts of hydrofracturing processes including causes and triggers of overpressure in glacial environments as a preamble, (2) the physical characteristics of hydrofracture systems in glacial environments and (3) the parameters controlling these physical characteristics, (4) the characteristics of hydrofracture-fills, (5) the processes of sediment injection inferred from fill characteristics and (6) the wider implications of hydrofracturing and injection processes on palaeoglaciological reconstructions. Future research perspectives, including the need for modelling of hydrofracture network in glacial environments, are finally discussed as it will certainly allow the role of ice thickness, slope and speed, meltwater input and host sediments in governing the architecture of hydrofracture systems to be untangled.
The geometry of glacial overdeepenings on the Swiss Plateau close to Bern was inferred through a combination of gravity data with a 3D gravity modelling software. The target overdeepenings have depths between 155 and > 270 m and widths between 860 and 2400 m. The models show incisions characterized by U-shaped cross-sectional geometries and steep to over-steepened lateral flanks. Existing stratigraphic data reveals that the overdeepenings were formed and then filled during at least two glacial stages, which occurred during the Last Glacial Maximum (LGM) within the Marine Isotope Stage (MIS) 2, and possibly MIS 6 or before. The U-shaped cross-sectional geometries point towards glacial erosion as the main driver for the shaping of the overdeepenings. The combination of the geometries with stratigraphic data suggests that the MIS 6 (or older) glaciers deeply carved the bedrock, whereas the LGM ice sheet only widened the existing valleys but did not further deepen them. We relate this pattern to the different ice thicknesses, where a thicker MIS 6 ice was likely more powerful for wearing down the bedrock than a thinner LGM glacier. Gravity data in combination with forward modelling thus offers robust information on the development of a landscape formed through glaciers.
