Glaciotectonic Complex Research Articles

Descriptive text to the Geological map of Denmark, 1:50 000, Møn 1511 I, 1511 IV and 1512 II

Descriptive text to the Geological map of Denmark, 1:50 000, Møn 1511 I, 1511 IV and 1512 II

The geological framework for Hvideklint, south-east Denmark, using glaciodynamic sequence stratigraphy

Glaciodynamic sequence stratigraphy provides a practical model for grouping and classifying complex geological data to aid interpretation of past climatic and environmental development in Quaternary successions. The principles of glaciodynamic sequence stratigraphy are applied here to summarise the complex glacial geological framework of Hvideklint on the island of Møn, south-east Denmark. The framework of the superimposed deformed Hvideklint is presented in a reconstructed geological cross-section of Hvideklint. For the construction of the architecture of the glaciotectonic complex, the interpretation of structures below sea level was based on a detailed new survey of the cliff section combined with construction of successive approximation balanced cross-sections. The new description is supported by drill hole data from the Jupiter database. Where chalk is not glaciotectonically deformed, the constructed depth to the top-chalk-surface is generally located about 30 m below sea level. In Hvideklint, thrust sheets with chalk are exposed 20 m above sea level, and the balanced cross-section constructions indicate that the décollement surface for a Hvideklint glaciotectonic complex is located about 80 m below sea level. Between the décollement level and the top of the complex, two or more thrust-fault flat-levels and connecting ramps add to the complex architecture of Hvideklint.

Architecture of tunnel valleys in the southeastern North Sea: new insights from high‐resolution seismic imaging

ABSTRACTTunnel valleys are assumed to form near the margin of ice sheets. Hence, they can be used to reconstruct the dynamics of former ice margins. The detailed formation and infill of tunnel valleys, however, are still not well understood. Here, we present a dense grid of high‐resolution 2D multi‐channel reflection seismic data from the German sector of the southeastern North Sea imaging tunnel valleys in very great detail. Three tunnel valley systems were traced over distances ranging between 11 and 21 km. All tunnel valleys are completely filled and buried. They differ in incision depth, incision width and number of incisions. The tunnel valleys cut 130–380 m deep into Neogene, Palaeogene and Cretaceous sediments; they show a lower V‐shaped and an upper U‐shaped morphology. For individual tunnel valleys, the overall incision direction ranges from east–west to northeast–southwest. Two tunnel valleys intersect at an oblique angle without reuse of the thalweg. These valleys incise into a pre‐existing glaciotectonic complex consisting of thrust sheets in the northwest of the study area. The analysis of the glaciotectonic complex and the tunnel valleys leads us to assume that we identified several marginal positions of (pre‐)Elsterian ice lobes in the southeastern North Sea.

Wrench-fault structures superimposed by glaciotectonic complexes, interpreted from high-resolution reflection-seismic sections and boreholes along the western bank of Esrum Sø, north-east Sjælland, Denmark

Wrench-fault structures below Danian limestone and Palaeogene marl, and an overlying structural framework of Quaternary glacial deposits in north-east Sjælland, Denmark, are interpreted from two vibro-seismic sections recorded to 600 msec TWT depth. The main seismic section is 6.3 km long, N–S oriented, and intersected by a 0.7 km long, E–W oriented satellite seismic section. In addition, boreholes in the vicinity of the seismic profile are used for the interpretation. The sections were acquired in 2014 along the western shoreline of the lake Esrum Sø in the Gribskov area. In the lower part of the seismic section (the interval 100–300 msec TWT), parallel-bedded geological layers occur along most of the profile apart from six locations, where six wrench-fault structures displace the upper part of the Chalk Group and lower Palaeogene marl. The northernmost of the six wrench-fault locations correlates to the eastern slope of the buried Esrum–Alnarp valley, which suggests that the valley is an inherited tectonic feature. The location of the wrench- fault structures supports the outline of faults related to the Sorgenfrei-Tornquist Zone on previous geological maps, which had almost no seismic data from the area. Above the stratigraphic level presented by the Danian limestone and lower Palaeogene marl, a composite glaciotectonic complex comprising two glaciodynamic sequences is recognized by e.g. thrust-fault structures and the lithostratigraphy of glacial successions recorded in the wells. In parts of the seismic sections, the lowermost level of the glaciotectonic complex inherited the wrench-tectonic fault structures, most significantly seen in the northern segment. The advance of the Scandinavian ice sheet caused the glaciotectonic structures displayed in the seismic section. The two sequences represent events related to the Norwegian and the Swedish glacial advances. From the interpretation of the seismic section it is found that the glaciotectonic complex conceals the wrench-tectonic flower structures.

Seismic interpretation and structural restoration of the Heligoland glaciotectonic thrust-fault complex: Implications for multiple deformation during (pre-)Elsterian to Warthian ice advances into the southern North Sea Basin

Despite a long history of research, the locations of former ice-margins in the North Sea Basin are still uncertain. In this study, we present new palaeogeographic reconstructions of (pre-) Elsterian and Warthian ice-margins in the southeastern North Sea Basin, which were previously unknown. The reconstructions are based on the integration of palaeo-ice flow data derived from glaciotectonic thrusts, tunnel valleys and mega-scale glacial lineations. We focus on a huge glaciotectonic thrust complex located about 10 km north of Heligoland and 50 km west of the North Frisian coast of Schleswig-Holstein (Northern Germany). Multi-channel high-resolution 2D seismic reflection data show a thrust-fault complex in the upper 300 ms TWT (ca. 240 m) of seismic data. This thrust-fault complex consists of mainly Neogene delta sediments, covers an area of 350 km2, and forms part of a large belt of glaciotectonic complexes that stretches from offshore Denmark via northern Germany to Poland. The deformation front of the Heligoland glaciotectonic complex trends approximately NNE-SSW. The total length of the glaciotectonic thrust complex is approximately 15 km. The thrust faults share a common detachment surface, located at a depth of 250–300 ms (TWT) (200–240 m) below sea level. The detachment surface most probably formed at a pronounced rheological boundary between Upper Miocene fine-grained pro-delta deposits and coarser-grained delta-front deposits, although we cannot rule out that deep permafrost in the glacier foreland played a role for the location of this detachment surface. Restored cross-sections reveal the shortening of the complex along the detachment to have been on average 23% (ranging from ca. 16%–50%). The determined ice movement direction from east-southeast to southeast suggests deformation by an ice advance from the Baltic region. The chronospatial relationship of the thrust-fault complex and adjacent northwest-southeast to northeast-southwest trending Elsterian tunnel valleys implies a pre-Elsterian (MIS 16?) age of the glaciotectonic complex. However, the age of these Elsterian tunnel valleys is poorly constrained and the glaciotectonic complex of Heligoland may also have been formed during an early Elsterian ice advance into the southeastern North Sea Basin. The glaciotectonic complex underwent further shortening and the Elsterian tunnel-valley fills that were incised into the glaciotectonic complex were partly deformed during the Saalian Drenthe and Warthe (1) ice advances.

Chalk-glacitectonite, an important lithology in former glaciated terrains covering chalk and limestone bedrock

A glacitectonite is defined as a brecciated sediment or a cataclastic sedimentary rock formed by glaciotectonic deformation (Pedersen 1988). The term tectonite was initially introduced by Sander (1912), mainly for tectonically brecciated metamorphic rocks in the Alps. In the classic work on cataclastic rocks, Higgins (1971) stated that the term covered all rocks with fabric displaying coordinated geometric features related to continuous flow during deformation.Therefore brecciated lithologies formed by glaciotectonic deformations can be termed tectonites. Banham (1977) suggested the prefix glaci- to clarify the relation to glacial dynamics. Furthermore, Pedersen (1988) suggested the application of the bedrock prefix. Thus, a chalk-glacitectonite is a brecciated chalk formed by shear deformation during a glacial advance over an exposed bedrock surface of chalk (Fig. 1). Hence the term describes a sedimentary rock in which the primary structures are so disturbed that they cannot be continuously traced, and a glacitectonic fabric developed as joint fractures or shear surfaces superimposed on the lithology. The significance of recognising chalk-glacitectonite from chalk and limestone bedrock is the difference in textural properties, which is fundamental in geological modelling. In areas dominated by glaciotectonic complexes, which include thrust sheets of pre-glacial sedimentary rocks, the sheets are subject to shearing and dragged along the sole of the ice during its movement over the glaciotectonic complex. Due to truncation and shear-drag, the glacitectonite forms at the base of the deformational layer in a lodgement till. From the source area, which typically is a detachment anticline, the glacitectonite thins out in the direction of transport from 1–2 m (Fig. 2) to a thin shear zone only a few centimetres thick over a distance of one to a few kilometres (Pedersen 1996). Moreover, brecciation of thrust sheets displaced by glacial thrusting occurs within glaciotectonic complexes. The deformation ranges from initially anastomosing jointing (Figs 1, 3) to brecciation with bedrock clasts in crushed bedrock matrix (Fig. 4). The tectonic breccia distributed from the décollement zone at the base to the truncating glacial unconformity at the top may additionally be termed glacitectonites. Here we describe the occurrence and identification of chalk-tectonites.

Evidence for a grounded ice sheet in the central North Sea during the early Middle Pleistocene Donian Glaciation

Interpretation of 3D seismic data from the central North Sea yields evidence of a pre-MIS (Marine Isotope Stage) 12 grounded glaciation. The glaciotectonic complex shows buried push moraines resulting from the thrusting of multiple ice advance phases with horizontal shortening of 35 – 50%. The earliest feature observed within the complex, a hill–hole pair, represents the initial glaciation of the area. This is overlain and deformed by multiple thrust units with numerous inferred ice-flow directions. The thrust deformation observed shares characteristics with kinematic processes, push moraines and static gravity processes, seen as gravity spreading and contraction. The glaciotectonic complex in its entirety is interpreted to correlate to a pre-Elsterian glaciation, becaue of its stratigraphic position below central North Sea tunnel valleys, estimated to be Elsterian in age (MIS 12; 450 ka). The study proposes that the thrust complex correlates to the Donian glaciation in Russia (MIS 16; 600 ka) with ice sourced from Norway. The complex therefore represents a glaciation where a significant area of the central North Sea was covered by an ice sheet, 200 kyr prior to the Elsterian. This study highlights the fragmentary record of pre-Elsterian glaciations and the importance of incorporating offshore sedimentary archives and regional frameworks when reconstructing Pleistocene climate change.

Glaciotectonic deformations in the Jammerbugt and glaciodynamic development in the eastern North Sea

ABSTRACTThe Quaternary geology in the eastern part of the North Sea is characterized by large‐scale glaciotectonic complexes. The northernmost complex is the Jammerbugt Glaciotectonic Complex here addressed, which occupies an area of more than 300 km2. It was recently recognized during an investigation of conventional seismic profiles located about 10 km offshore the west coast of northern Denmark. The deformed bedrock includes the main part of the Cretaceous Chalk Group in the North Sea. In the northern part of the complex the detachment surface is situated in Lower Cretaceous greensand about 400 m below sea level. In the central part of the complex, the thrusting ramps the strong reflectors at the base of the Chalk Group and the detachment surface continues in the lower part of the Upper Cretaceous chalk. In the tectonic depression north of the complex, marine and glaciomarine deposits represent the Eemian–Early Weichselian Skærumhede Group, indicating a Saalian age of the glaciotectonic deformation. The balancing of the thrust fault structures shows that the thrust sheets in the tailing end of the complex had their source in the Skagerrak sea. The hole caused by displacement contributed to formation of the Skagerrak depression, i.e. early stage of the Norwegian Trench.

Thrust-fault architecture of glaciotectonic complexes in Denmark

Cross sections of glaciotectonic complexes are exposed in coastal cliffs in Denmark, which allow structural studies of the architecture of thin-skinned thrust-fault deformation (Pedersen 2014). However, the basal part of the thrust-fault complex is never exposed, because it is located 50 to 100 m below sea level. It is in the basal part the most important structure – the décollement zone – of the complex is found. The décollement zone constitutes the more or less horizontal surface that separates undeformed bedrock from the displaced thrust-sheet units along the décollement level. One of the most famous exposures of glaciotectonic deformations in Denmark is the Møns Klint Glaciotectonic Complex. The structures above sea level are well documented, whereas the structures below sea level down to the décollement level are poorly known. Modelling of deep structures was carried out by Pedersen (2000) but still needs documentation. A glaciotectonic complex affecting comparable rock units, such as the chalk at Møns Klint, was recently recognised in seismic sections from Jammerbugten in the North Sea (Fig. 1). These sections provide an excellent opportunity for comparable studies of the upper and lower structural levels in thin-skinned thrust-fault deformation, which is discussed in this paper with examples from three major glaciotectonic complexes.

Combining 3D geological modelling techniques to address variations in geology, data type and density – An example from Southern Denmark

The very complex near-surface geology in Denmark is a big challenge when constructing 3D geological models. Borehole data alone are normally insufficient for proper 3D modelling because data are too widespread. Therefore, Airborne ElectroMagnetic (AEM) techniques are often used to obtain supplementary information on the spatial distribution and composition of the geology.A large-scale AEM survey and high-resolution seismic data along with both new and existing borehole data and seismic data from hydrocarbon exploration were available for the construction of a detailed 3D geological model in our study area. The data are unevenly distributed, and only part of the study area was covered by the AEM survey. Cross-cutting tunnel valleys, erosional unconformities, delta units and a large glaciotectonic complex are among the geological features identified in the area. The geological complexity varies significantly across the model area.A broad geological overview and understanding of the area was obtained by joint cognitive interpretation of the geophysical and the geological data. To address the geological complexity and the very high level of detail gained from the AEM data, the model was constructed as a voxel model with lithofacies attributes supplemented by a number of bounding surfaces. In areas where the geology is not too detailed and complex, the model was constructed manually, whereas automated methods were used to populate voxels in areas with a high complexity. The automated methods comprised clay fraction modelling, which was used where AEM data are available, and stochastic modelling, which was used outside the area covered by AEM data.Our study shows that it is advantageous to combine several modelling methods in areas with varying geological complexity and data density. The choice of modelling methods should depend on the character and coverage of available data and on variations in geology throughout the model area.

Architecture of Glaciotectonic Complexes

Glaciotectonic studies are an integrated part of the Quaternary geological research carried out by the Danish geological survey. Almost all the hilly areas in Denmark were created or affected by glaciotectonic deformations, and the features are included in the mapping of surface near deposits. For the mapping and support of constructing 3D geological models a classification of architecture of glaciotectonic complexes is suggested. The important elements for classification of architecture are the surfaces. Four orders of surfaces are defined for glaciotectonic complexes: first-order surfaces are décollement surfaces and glaciotectonic unconformities; second-order surfaces are ramps and flats—the thrust faults; third-order surfaces are folded beds—anticlines and synclines; and fourth-order surfaces are small scale folds and faults—kink bands, conjugate faults, box folds, etc. The most important first-order surface is the décollement surface. This surface limits the glaciotectonic complex at its base and controls the extent of glaciotectonic disturbances. Below this surface, ordinary flat lying planar bedding occurs, whereas above the surface a number of structures are present characteristic of second- to fourth-order elements in the glaciotectonic architecture. The other first-order surface is the topographic top of the glaciotectonic complex, which eventually may be replaced by a truncating unconformity.

Genesis of the glaciotectonic thrust-fault complex at Halk Hoved, southern Denmark.

The coastal cliff of Halk Hoved, southern Jutland, Denmark, is a major glaciotectonic complex formed by proglacial deformation of the North-East (NE) advance from the Scandinavian Ice Sheet in Late Weichselian. We describe and interpret the pre-, syn- and post-tectonic sedimentary successions and macro-scale architecture of this complex. Initially, the Lillebælt Till Formation (unit 1) and the overlying glaciofluvial sediments (unit 2) were deposited during the Warthe glaciation in Late Saalian. During the NE advance towards the Main Stationary Line (MSL) in Late Weichselian, these sediments were pushed along a décollement surface whereby a thrust-fault complex was formed. In a cross section the complex extends for more than 900 m and consists of eighteen c. 15–20 m thick thrust sheets stacked by piggyback thrusting. Accumulated displacement amounts to at least 235 m along thrust faults dipping at 30–40° towards N-NE, resulting in at least 24% glaciotectonic shortening of the complex. Deformation was presumably facilitated by elevated pore-water pressure in the Lillebælt Till Formation. As the compressive stress exceeded the shear strength of the weakened till, failure occurred, and a décollement horizon formed along the lithological boundary between the Lillebælt Till Formation and the underlying aquifer. During deformation, piggyback basins formed wherein sediments of hyperconcentrated flow (unit 3) and glaciolacustrine diamicton (unit 4) were deposited. The whole thrust-fault complex and the intervening sediments were truncated subglacially as the NE advance finally overrode the complex. Following the retreat of the NE advance, a succession of glaciofluvial sediments (unit 5) and finally the East Jylland Till Formation (unit 6) were deposited during the advance of the Young Baltic Ice Sheet. The Halk Hoved thrust-fault complex is a prominent example of glaciotectonism at the southern fringe of the Scandinavian Ice Sheet.

Glaciodynamic sequence stratigraphy

Abstract The glaciofluvial deposits are by volume and permeability the most important unit in the terrestrial glacial successions, and they are the obvious target for groundwater as well as hydrocarbon reservoir exploration. The dominant glaciofluvial units are related to the proglacial setting in the foreland of an advancing ice margin, which results in a coarsening-upwards sequence with fine-grained beds at the base and glaciofluvial gravel at the top. In a complete sequence a till caps the unit, and at its base a glacitectonite is formed by shearing related to the development of the deformational layer below the ice. The glacial deposits laid down during the same glacial advance represent a glaciodynamic sequence. An important feature added to this is the proglacial glaciotectonic deformation. The glaciotectonic architectural elements comprise thrust faulting, folding of hanging-wall anticlines, thrust-sheet duplexes, hydrodynamic breccias and mud diapirs, the structural style of which define the glaciotectonic complex. The glaciodynamic sequence corresponds to the glaciodynamic event related to one major ice advance. The glaciodynamic processes representing the event comprise deposition as well as deformation, creating a glaciogenic sedimentary succession and a set of glaciotectonic structures. These constitute the elements to be recognized for defining a glaciodynamic sequence.

Structural development of Maglevandsfald: a key to understanding the glaciotectonic architecture of Møns Klint, SE Denmark

The Møns Klint Glaciotectonic Complex (Fig. 1) exposed in the N–S-trending chalk cliff on the east coast of the island of Møn in south-east Denmark is one of the most famous glaciotectonic geosites in the world. People of all nationalities are attracted to the site, which has more than 300 000 visitors per year. Many of them may not realise the uniqueness of the glaciotectonic framework, and are probably more fascinated by the spectacular view of the white cliff and chalk peaks separated by the deep green gorges. However, without the glaciotectonic deformation the cliffs would never have formed. Instead the Cretaceous chalk would still have been resting horizontally below the seabed, covered by glaciofluvial sand, glaciolacustrine clay and clayey till.

Geology of outer Horns Rev, Danish North Sea

In 2006, Dong Energy initiated the development of the Horns Rev II offshore wind farm in the North Sea (Fig. 1). In order to evaluate and map the characteristics of the surface features of the sea bed and to characterise the subsurface in the wind farm area, the Geological Survey of Denmark and Greenland (GEUS) conducted a geophysical survey of the area. The survey utilised a variety of instruments: sparker, side-scan sonar, marine caesium magnetometer and a multibeam echo-sounder. In addition, information on the subsurface sediments was obtained by cone penetration tests (CPT) and by drilling to 30–50 m below the sea bottom. Geological correlation of the CPT results with the other survey results was extremely complicated but was required in order to understand the architecture of the ice marginal glaciotectonic complex. Information on the geology is crucial for evaluation of the geotechnical problems of the region.

Colophon - Abstract - Introduction - Glacial tectonics - Location and construction of cross-section - Geological setting

The Rubjerg Knude Glaciotectonic Complex is a thin-skinned thrust-fault complex that was formed during the advance of the Scandinavian Ice Sheet (30 000 – 26 000 B.P.); it is well exposed in a 6 km long coastal profile bordering the North Sea in northern Denmark. The glaciotectonic thrust-fault deformation revealed by this cliff section has been subjected to detailed structural analysis based on photogrammetric measurement and construction of a balanced cross-section. Thirteen sections are differentiated, characterising the distal to proximal structural development of the complex. The deformation affected three stratigraphic units: the Middle Weichselian arctic marine Stortorn Formation, the mainly glaciolacustrine Lønstrup Klint Formation and the dominantly fluvial Rubjerg Knude Formation; these three formations are formally defined herein, together with the Skærumhede Group which includes the Stortorn and Lønstrup Klint Formations. The Rubjerg Knude Formation was deposited on a regional unconformity that caps the Lønstrup Klint Formation and separates pre-tectonic deposits below from syntectonic deposits above.
 In the distal part of the complex, the thrust-fault architecture is characterised by thin flatlying thrust sheets displaced over the footwall flat of the foreland for a distance of more than 500 m. Towards the proximal part of the complex, the dip of the thrust faults increases, and over long stretches they are over-steepened to an upright position. The lowest décollement zone is about 40 m below sea level in the proximal part of the system, and shows a systematic step-wise change to higher levels in a distal (southwards) direction. The structural elements are ramps and flats related to hanging-wall and footwall positions. Above upper ramp-hinges, hanging-wall anticlines developed; footwall synclines are typically related to growth-fault sedimentation in syntectonic piggyback basins, represented by the Rubjerg Knude Formation. Blocks and slump-sheets constituting parts of the Lønstrup Klint Formation were derived from the tips of up-thrusted thrust sheets and slumped into the basins. Mud diapirs are a prominent element in the thrust-fault complex, resulting from mud mobilisation mainly at hanging-wall flats and ramps.
 Shortening during thrust-fault deformation has been calculated as 50%. Only about 11% of the initial stratigraphic units subjected to thrust faulting has been lost due to erosion. The thrust-fault deformation was caused by gravity spreading of an advancing ice sheet. Overpressured mud-fluid played an important role in stress transmission. The average velocity of thrust-fault displacement is estimated at 2 m per year, which led to compression of a 12 km stretch of flat-lying sediments, c. 40 m in thickness, into a thrust-fault complex 6 km in length. The thrust-fault complex is truncated by a glaciotectonic unconformity, formed when the advancing ice sheet finally overrode the complex. When this ice sheet melted away, a hilland-hole pair was formed, and meltwater deposits derived from a new ice-advance (NE-Ice) filled the depression. The NE-Ice overran the complex during its advance to the main stationary line situated in the North Sea. When this ice in turn melted away (c. 19 000 – 15 000 B.P.), the glacial landscape was draped by arctic marine deposits of the Vendsyssel Formation (new formation defined herein).

Structural analysis of the Rubjerg Knude Glaciotectonic Complex, Vendsyssel, northern Denmark

Pedersen, S.A.S. 2005: Structural analysis of the Rubjerg Knude Glaciotectonic Complex, Vendsyssel, northern Denmark. Geological Survey of Denmark and Greenland Bulletin 8, 192 pp.
 The Rubjerg Knude Glaciotectonic Complex is a thin-skinned thrust-fault complex that was formed during the advance of the Scandinavian Ice Sheet (30 000 – 26 000 B.P.); it is well exposed in a 6 km long coastal profile bordering the North Sea in northern Denmark. The glaciotectonic thrust-fault deformation revealed by this cliff section has been subjected to detailed structural analysis based on photogrammetric measurement and construction of a balanced cross-section. Thirteen sections are differentiated, characterising the distal to proximal structural development of the complex. The deformation affected three stratigraphic units: the Middle Weichselian arctic marine Stortorn Formation, the mainly glaciolacustrine Lønstrup Klint Formation and the dominantly fluvial Rubjerg Knude Formation; these three formations are formally defined herein, together with the Skærumhede Group which includes the Stortorn and Lønstrup Klint Formations. The Rubjerg Knude Formation was deposited on a regional unconformity that caps the Lønstrup Klint Formation and separates pre-tectonic deposits below from syntectonic deposits above.
 In the distal part of the complex, the thrust-fault architecture is characterised by thin flatlying thrust sheets displaced over the footwall flat of the foreland for a distance of more than 500 m. Towards the proximal part of the complex, the dip of the thrust faults increases, and over long stretches they are over-steepened to an upright position. The lowest décollement zone is about 40 m below sea level in the proximal part of the system, and shows a systematic step-wise change to higher levels in a distal (southwards) direction. The structural elements are ramps and flats related to hanging-wall and footwall positions. Above upper ramp-hinges, hanging-wall anticlines developed; footwall synclines are typically related to growth-fault sedimentation in syntectonic piggyback basins, represented by the Rubjerg Knude Formation. Blocks and slump-sheets constituting parts of the Lønstrup Klint Formation were derived from the tips of up-thrusted thrust sheets and slumped into the basins. Mud diapirs are a prominent element in the thrust-fault complex, resulting from mud mobilisation mainly at hanging-wall flats and ramps.
 Shortening during thrust-fault deformation has been calculated as 50%. Only about 11% of the initial stratigraphic units subjected to thrust faulting has been lost due to erosion. The thrust-fault deformation was caused by gravity spreading of an advancing ice sheet. Overpressured mud-fluid played an important role in stress transmission. The average velocity of thrust-fault displacement is estimated at 2 m per year, which led to compression of a 12 km stretch of flat-lying sediments, c. 40 m in thickness, into a thrust-fault complex 6 km in length. The thrust-fault complex is truncated by a glaciotectonic unconformity, formed when the advancing ice sheet finally overrode the complex. When this ice sheet melted away, a hilland- hole pair was formed, and meltwater deposits derived from a new ice-advance (NE-Ice) filled the depression. The NE-Ice overran the complex during its advance to the main stationary line situated in the North Sea. When this ice in turn melted away (c. 19 000 – 15 000 B.P.), the glacial landscape was draped by arctic marine deposits of the Vendsyssel Formation (new formation defined herein).

Superimposed deformation in glaciotectonics

The identification of glaciotectonic structures is an exclusive field for the structural geologist. The structures comprise a series of different types and regimes. The sequential development of the glaciotectonic structures reflects superimposed subglacial and proglacial deformation processes. The glaciotectonic structures may involve earlier formed structures thus superimposed by the glaciotectonics, or the glaciotectonic structures may eventually be overprinted by neotectonic deformations. Four different superimposed settings may be distinguished: 1) glaciotectonic deformation superimposed on pre-Quaternary tectonics, 2) glaciotectonic deformation superimposed on earlier formed glaciotectonic structures (superimposed deformation involving two or more glaciodynamic events), 3) glaciotectonic deformations superimposed sequentially in the same glaciotectonic unit (two or more glaciotectonic phases in the same glaciodynamic event), and finally 4) neotectonic deformation superimposed on glaciotectonic structures. Examples of type 1 are taken from the deformed Palaeogene diatomites with ash layers at Hanklit, Mors, and Hestegården, Fur. Dokumentation of glaciotectonic deformation superimposed on halokinetic structures is demonstrated from Erslev, Mors, and further examplified by structures occurring at Junget on the north side of the Batum salt diapir in Salling. Type 2 is examplified by glaciotectonic structures in the Skarrehage mo-clay pit on Mors. An example of Elsterian glaciotectonics superimposed by Saalian glaciotectonics is recorded from the hilly island Møborg, central part of western Jylland. The classic glaciotectonic site Møns Klint is described as a combination of an imbricate fan and an antiformal stack formed by the Young Baltic ice advance in the Late Weichselian superimposed by a regressional re-advance from the east. Type 3 is exemplified by the glaciotectonic complex at Feggeklit. Type 4 is described from the island of Fur where glaciotectonic structures are cut by neotectonic faults roughly parallel to the main E-W trend of Limfjorden.

The Hanklit Gaciotectonic Thrust Fault Complex, Mors, Denmark

The Hanklit cliff section is a classical glaciotectonic locality where an excellent cross-section through a thrust fault complex is exposed. The thrust imbrications were thrust from north to the south during a Late Weichselian glacier advance. Three thrust sheets with thickness of more than 50 m are involved in the thrust fault complex. The deposits exposed in the cliff-section comprise ca. 39 m diatomite with ash layers of Paleocene-Eocene age (the Fur Formation), overlain by up to 27 m of Quaternary glacigenic sediments. Common structural features related to compressional thrust fault helts, are recognizable in the glaciotectonic complex, which shows a lateral shortening of more than 40%. Structures created by soft sedimentary deformation due to high water pressure occur in relation to the thrust fault zones. The structural framework is described with the terminology of thin-skinned tectonics and flats, ramps and hanging-wall anticlines are identified in the cliff-section. A balanced cross section has been constructed. Based on this construction the structural model is verified and the glaciotectonic development is interpreted. A geoelectrical investigation has been made to support the construction of the 3-dimensional framework of the thrust complex. This demonstrates that the impressive Hanklit thrust sheet extends for more than 1 km along the strike and is displaced more than 200 m to the south. Superimposed extentional structures, situated above the sole of the Hanklit Thrust Sheet, are interpreted to be related to Tertiary subsidence tectonics.