Geological Survey of Denmark and Greenland Bulletin 33, 2015, 17-20 17 Thrust-fault architecture of glaciotectonic complexes in Denmark Stig A. Schack Pedersen and Lars Ole Boldreel Cross sections of glaciotectonic complexes are exposed in coastal cliff s 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. Th e décollement zone constitutes the more or less horizontal surface that separates undeformed bedrock from the dis- placed thrust-sheet units along the décollement level. One of the most famous exposures of glaciotectonic deformations in Denmark is the Møns Klint Glaciotectonic Complex. Th e 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 c omplex aff ecting comparable rock units, such as the chalk at Møns Klint, was recently recognised in seismic sections from Jammerbugten in the North Sea (Fig. 1). Th ese sections provide an excellent opportunity for com- parable 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. Architecture of thrust-fault deformations in glaciotectonic complexes In contrast to fold-belt ranges, glaciotectonic complexes are relatively small and therefore easier to study. For these struc- tural complexes, an architectural classifi cation was defi ned based on description and ordering of surfaces and their re- lations (Pedersen 2014). It is emphasised that the creation of constructions comprising surfaces is the basic element of their architecture. In geology the use of architectural analysis is well known in investigations of sedimentary deposits. A concept for ordering of bounding surfaces in the architecture of aeolian dunes was suggested by Brookfi eld (1977), and a similar concept was suggested by Miall (1985) for facies anal- ysis of fl uvial deposits. For the analysis of glacial architecture and construction of 3D geological models of glaciotectonic complexes the clas- sifi cation of a hierarchy of bounding surfaces comprises four orders of surfaces (Pedersen 2014). Th e décollement surface is defi ned as a fi rst-order surface (Fig. 2). Th e décollement surface is the ‘base’ of the complex, and therefore the top of the complex also has to be defi ned as a fi rst-order surface. Th is second fi rst-order surface is the topographic top of the tectonic complex, or alternatively, a truncating unconform- ity, above which post-deformational units occur. Th e internal framework of a tectonic complex comprises thrust sheets. Th ese are bounded by thrust faults, which are defi ned as second-order surfaces (Fig. 2). Th e thrust faults are diff erentiated into ramps and fl ats, where a ramp cross-cuts the bedding, whereas the fl at is more or less bedding-parallel. When two or more thrust sheets are bounded by ramps and fl ats they form duplexes. Th ese generally form imbricate com- plexes or may be stacked so they form complex repetitions of the geological units (as exemplifi ed by Pedersen 2005). Th e folded beds comprise third-order surfaces. Th ese are diff erentiated into anticlines, synclines, recumbent folds and monoclinal bends. Folds may further be classifi ed from the © 2015 GEUS. Geological Survey of Denmark and Greenland Bulletin 33, 17–20. Open access: www.geus.dk/publications/bull Weichselian maximum Saalian maximum Baltic Sea North Sea JB MK FK 300 km Fig 1. Extent of ice sheets during the two last glaciations and the location of the three glaciotectonic complexes mentioned in this paper. JB: The Jammerbugt Glaciotectonic Complex was formed by an ice advance from central Scandinavia during the Saalian. FK: The Fur Knudeklint Glacio- tectonic Complex was formed during an ice advance from Norway during the Late Weichselian. MK: The Møns Klint Glaciotectonic Complex is exposed in a coastal cliff by the Baltic Sea, and it was formed during the latest part of the Weichselian. 1818 orientation of their axial surface, the angle of their limbs and the inclination of their fold axes. Fourth-order surfaces include all small-scale structures such as faults with small displacements; such faults are im- portant for the understanding of the dynamic development. Joints and anastomosing joints indicate early deformation impact, and the zone axis of conjugate faults indicates the direction of compaction. Th e asymmetry of small- and mes- oscale folds and the sense of displacement on faults as indi- cated by groove marks can be used to reconstruct the kin- ematics of deformation. For the macro-scale identifi cation of the head and tail of glaciotectonic complexes, a distal, a central and a proximal domain are defi ned. Th e domain nearest to the foreland (the head end) is regarded as the distal part, which is commonly limited by the trace of the last thrust fault displaced towards the undeformed foreland (the tipline). Th e central domain displays the bulk architecture of the complex. Th e proximal domain comprises the deepest level of deformation with the most complicated structural relationships, potentially in- cluding superimposed deformation and mud diapirism. Situ- ated at the tail end, the proximal domain is delimited by the contact to the hinterland of the complex. For glaciotectonic complexes, the hinterland contact is the boundary between the hill and the hole in a ‘hill/hole pair’. At the time of dis- location, it formed the contact between the pushing ice and the dislocated geological units. The distal domain In the distal part of a glaciotectonic complex, the dip of the thrust fault ramps is gentle and the thrust fault fl ats are al- most horizontal (Fig. 3). Th e thrust sheets are thinner than in the central domain, which is a consequence of the décolle- ment surface that rises from the deepest level in the trailing end to the topographic surface in the foreland. One of the most surprising features of distal thrust sheets is their length. In the seismic section from the Jammerbugt Glaciotectonic Complex the length of a thrust sheet exceeds 1 km, and the thrust sheets in the northern part of Møns Klint are more than 500 m long (Fig. 3). Such long thrust sheets are sur- prising when their thickness is taken into account. At Møns Klint the chalk sheets in the distal part of the glaciotectonic structure are only c. 25 m thick and one would expect that the forces pushing the thrust sheets would break them up into fragments. Th e explanation for this missing fragmenta- tion is that high porewater pressure along the thrust faults carries the unbroken thrust sheets. The central domain When a long and relatively thin thrust sheet is created in the distal part of a complex it is easy to understand that, when the thrusting propagates, the distal domain will move to the central domain during the formation of a new distal do- main next to the foreland. Two marked types of structures may form during this development: (1) the thrust sheets are broken into shorter segments creating imbricate fans along steeper-dipping thrust faults (Pedersen 2005), or (2) super- 3 1 3 2 1 1 1 2 3 3 2 2 2 1 Sea level A B 50 m Glacial deposits Røsnæs Clay Formation Stolleklint Clay Holmehus Formation Ash layers +1 to +13 Ash layers –13 to +1 Ash layers –33 to –13 Fur Formation North SouthC Fig. 2. The Fur Knudeklint Glaciotectonic Complex with ash layers in the Eocene diatomite of the Fur Formation that was deformed by the Norwegian Ice Advance in the Late Weichselian. A: An anticline, a syncline and steeply dipping layers. B: Imbricate duplexes. C: Schematic section. 1: First order surfaces, the décollement surfaces at the base and the glaciotectonic un- conformity at the top. 2: Second-order surfaces, the thrust faults that divide the glaciotectonic complex into thrust-sheet segments. 3: Third- order surfaces, the fold structures. To illustrate the typical third-order surfaces the hanging-wall anticlines have been extended above ground. Fourth-order surfaces are too small to be illus- trated, but are documented in Pedersen (2014). The imbricate duplexes in the southern part of the complex are also seen in B. 19 posed thrust sheets are displaced together and passing over new, more deeply seated ramps. During this translation an antiformal stack is created, which is the explanation for the impressive structure at Dronningestolen at Møns Klint (Pedersen 2000, 2014; Pedersen & Gravesen 2009). A simi- lar structure has been identifi ed in a seismic section from the central domain of the Jammerbugt Glaciotectonic Complex. The proximal domain Th e proximal part of a thin-skinned thrust-fault complex is characterised by an increasing number of thrust fault ramps and fl ats, imbricate thrust sheets and duplex segments (Ped- ersen 2005). In Fig. 4 this is illustrated by a section from the proximal part of the Jammerbugt Glaciotectonic Com- plex and the southernmost imbricate thrust sheets at Møns Klint. Th e thrust sheets at Møns Klint are c. 60 m thick, and the dips of the thrust faults are close to the maximum angle of fracturing (< 45º). Th e thrusting probably includes superimposed tilting on deeper thrust faults below sea level. According to Surlyk (1984) the stratigraphic level of the Maastrichtian chalk is lower in the thrust sheets shown in Fig. 4 than the chalk exposed in the distal domain in Fig. 3. Th us the thrusting and hence also the position of the décol- lement surface have shift ed to a deeper level in the proximal domain. Th is relationship is also seen in the thrust-fault ar- chitecture of the seismic section from Jammerbugten (Fig. 4). In the distal and central domains the décollement surface is situated above the base of the Chalk Group (BC in Fig. 4). In the proximal domain, the décollement surface drops down to the lower part of the marked refl ectors representing the base of the Chalk Group. Th e marked BC refl ectors are present in the thrust sheets of the tailing part of the proxi- mal domain. Th e thrust sheets in the Jammerbugt Complex are about twice as thick as the thrust sheets at Møns Klint. Th is refl ects that at Møns Klint only the frontal parts of the wedge-shaped thrust sheets are exposed, whereas in the seis- mic section the deeper, thicker parts of the thrust sheets can be recognised. Conclusion Th e architecture of thin-skinned thrust-fault deformation is described on the basis of three glaciotectonic complexes. Th e thrust-fault architecture of thrust-fault belts and of glacio- tectonic complexes is fairly similar even though the former are related to compressional regimes in plate-tectonic set- tings, the latter to compression caused by gravitational ex- pansion of ice sheets. Glaciotectonic thrust-fault complexes are divided into proximal (nearest to the source of force), 50 m B 500 m 200 m A BC Fig. 3. Thrust-fault architecture in the distal domains of two complexes. A: Seismic section from the Jammerbugt Glaciotectonic Complex. The strong ref lectors are interpreted as the base of the Chalk Group (BC) in the North Sea. This implies that the main parts of the thrust sheets comprise Upper Cretaceous chalk. B: Thin, gently dipping thrust sheets in the northern part of Møns Klint. The chalk at Møns Klint is of Maastrichtian age. 2020 central and distal domains (farthest away from the source of force). Th e distal domain includes the foreland boundary of the thrust-fault complex, and it is characterised by long and thin, gently dipping thrust sheets. Th e central domain is characterised by sequentially superimposed folding of thrust sheets formed in the distal domain. Imbricate thrust- fault segments are formed when the sheets break. Th e proxi- mal domain is characterised by the shift of the décollement surface down to the deepest level, thicker thrust sheets and stacking of thrust-fault duplexes. References Brookfi eld, M.E. 1977: Th e origin of bounding surfaces in ancient aeolian sandstones. Sedimentolog y 24, 303–332. Miall, A.D. 1985: Architectural-element analysis: a new method of facies analysis applied to fl uvial deposits. Earth-Science Reviews 22, 261–308. Pedersen, S.A.S. 2000: Superimposed deformation in glaciotectonics. Bulletin of the Geological Society of Denmark 46, 125–144. Pedersen, S.A.S. 2005: Structural analysis of the Rubjerg Knude Glacio- tectonic Complex, Vendsyssel, northern Denmark. Geological Survey of Denmark and Greenland Bulletin 8, 192 pp. Pedersen, S.A.S. 2014: Architecture of glaciotectonic complexes. Geo- sciences 4, 269–296. Pedersen, S.A.S. & Gravesen, P. 2009: Structural development of Mag- levandsfald: a key to understanding the glaciotectonic architecture of Møns Klint, SE Denmark. Geological Survey of Denmark and Green- land Bulletin 17, 29–32. Surlyk, F. 1984: Th e Maastrichtian Stage in NW Europe, and its bra- chiopod zonation. Bulletin of the Geological Society of Denmark 33, 217–223. Author’s addresses S.A.S.P., Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. E-mail: sasp@geus.dk L.O.B., Department of Geosciences and Natural Resource Management, University of Copenhagen, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. 500 m BC 200 m 50 m A B Fig. 4. Two examples showing thrust-fault architecture in the proximal parts of glaciotectonic complexes. A: Seismic section from the Jammerbugt Glacio- tectonic Complex. The décollement surface is stepping down to the lower part of the marked ref lectors that represent the base of the Chalk Group (BC). B: The oldest chalk (Surlyk 1984) at Møns Klint is found in the centre of the photograph, which shows the southernmost imbricate thrust sheets in the Møns Klint Glaciotectonic Complex.