Holger Lykke-Andersen

Structure and development of giant carbonate mounds at the SW and SE Rockall Trough margins, NE Atlantic Ocean

Abstract High-resolution seismic reflection profiling carried out in 1997–1999 showed that giant carbonate mounds occur between 500 and 1200 m water depth along both the SE and SW margins of Rockall Trough. The mounds rise 5–300 m above the surrounding seafloor and have diameters at their bases of up to 5 km. Buried mounds, at relatively shallow depths below the seafloor, are also found. Both individual and complex clusters of mounds can be recognized. Smaller and individual, sometimes buried mounds are found at the upper slope. On the SW Rockall Trough margin, higher, steeper and individual mounds are found deeper downslope (900–1100 m). At the middle slope the mounds merge into a complex structure and form complex clusters with a very irregular upper surface and an apparent lack of internal reflectors (600–1000 m depth). Initial results of high-resolution 3D-seismic profiling at 30 m line spacing in August 2000 in two boxes of 750 m×14.5 km covering a small, central part of the SW Rockall Trough mound area indicate two stages of mound development here, the latest probably of Pliocene–Holocene age. This age, based on the presence of a regionally recognizable unconformity below the mounds, is also assumed for the mounds of the SE Rockall Trough (Porcupine Bank) margin.

Cenozoic evolution of the eastern Danish North Sea

This paper provides a review of recent high-resolution and conventional seismic investigations in the eastern Danish North Sea and describes their implications for the development of the eastern North Sea Basin. The results comprise detailed time-structure maps of four major unconformities in the eastern Danish North Sea: the Top Chalk surface (mid-Paleocene), near top Oligocene, the mid-Miocene unconformity, and base Quaternary. The maps show that the eastern Danish North Sea has been affected by faulting and salt diapirism throughout the Cenozoic. Carbonate mounds, erosional valleys and pockmark- or karst-like structures were identified at the top of the Upper Cretaceous–Danian Chalk Group. Strike-parallel erosional features and depositional geometries observed at near top Oligocene and at the mid-Miocene unconformity indicate that these major sequence boundaries can be attributed to large-scale lateral changes in sediment supply directions. Increases in sediment flux to the southeastern North Sea at the Eocene/Oligocene transition and in the post-Middle Miocene appear to correlate with similar events world wide and with long term δ18O increases, indicating forcing by global factors, i.e. eustasy and climate. Stratal geometries observed on the seismic data indicate that the so-called ‘Neogene uplift’ of the eastern Danish North Sea may have been hundreds of metres less than previously suggested. It is argued that late Cenozoic uplift of the basin margin and of mountain peaks in southern Norway may have been caused entirely by isostatic uplift of the crust in response to accelerated late Cenozoic denudation and dissection of topography created in the Paleogene. The late Cenozoic periods of accelerated denudation and incision rates were most likely driven by climatic deterioration and long term eustatic lowering rather than active late Cenozoic tectonics, the cause of which is conjectural. A series of shallow thrust structures and an associated system of deep, buried valleys were mapped. Thrust faulting most likely occurred in response to gravitational loading at the margin of an advancing ice sheet, and it was followed by deep incision due to subglacial melt-water erosion, probably during the Elsterian glaciation.

The Cretaceous–Palaeogene boundary at Stevns Klint, Denmark: inversion tectonics or sea-floor topography?

The Cretaceous–Palaeogene boundary interval is exposed over 12 km in the coastal cliff, Stevns Klint, Denmark. An important lowermost Danian hardground has been interpreted as an originally horizontal marine abrasion surface. Its present elevation varies, from a few metres below, to about 35 m above sea level. This relief has traditionally been considered as resulting from late or post-Danian Laramide folding. New seismic profiles offshore Stevns Klint show, however, that the Base-Chalk reflector is not folded, is remarkably planar and has a gentle northward dip. Thus, the folding hypothesis cannot be upheld. Seismic stratigraphic analysis of the Chalk Group necessitates a fundamental revision of general ideas of chalk deposition. A highly irregular sea-floor topography was formed at many levels, and includes broad valleys, ridges, channels, drifts and mounds. A system of major WNW–ESE-oriented valleys and ridges can be traced into the succession exposed in Stevns Klint and further inland where it corresponds to the relief of the Cretaceous–Palaeogene boundary. The marked topographic elements of the chalk sea floor are elongate, with a WNW orientation roughly parallel to the axis of the Danish Basin and to the Sorgenfrei–Tornquist Zone forming the NE border of the basin. The sea-floor relief undoubtedly reflects the influence of strong contour-parallel bottom currents. The Cretaceous–Palaeogene boundary succession at Stevns Klint was thus developed on an underlying sea-floor topographic relief of about 40 m. Recognition of the highly irregular, current-influenced topography of the late Cretaceous sea floor stands in marked contrast to the conventional picture of quiet pelagic deposition of the chalk.

Interaction between bottom currents and slope failure in the Late Cretaceous of the southern Danish Central Graben, North Sea

The NW European Chalk Group was deposited in a deep epicontinental sea traditionally conceived as a quiet depositional setting that was only affected by redeposition along structural highs. The ‘chalk sea’ was, however, locally affected by powerful bottom currents during some periods. Three-dimensional seismic reflection data from the southern Central Graben show abundant intra-chalk morphological elements concentrated at two stratigraphic levels within the Turonian–Campanian sequence. Both levels are topped by extensive unconformities. The geometry of the elements produced by alongslope processes is interpreted as caused by SE-directed bottom currents flowing between the Bo-Jens and Adda ridges. The highest current intensities led to the formation of the two unconformities, judged by the occurrence of the largest current-formed channels and drifts at these two stratigraphic levels. This sea-floor topography was locally modified by large-scale downslope mass transport. Stratigraphical coincidence of the largest slumps and bottom-current erosion suggests a coupling between downslope and alongslope processes. Current erosion impinged upon the slopes during periods of highest current speeds, destabilizing the slope sediments and triggering slumping. The smoothing of the bathymetry following local relaxation of the tectonic inversion accounted for the coeval deactivation of alongslope and downslope processes during the succeeding Maastrichtian deposition.

The post-Triassic evolution of the Sorgenfrei–Tornquist Zone — results from thermo-mechanical modelling

Abstract The Sorgenfrei–Tornquist Zone (STZ) is part of the Fennoscandian Border Zone separating the Danish Basin from the Fennoscandian Shield. The STZ as a structural element is of Palaeozoic origin and represents the north-westerly segment of the Tornquist–Teisseyre Zone, which separates the younger West European crust from the older East European Platform and extends from the Black Sea to the eastern North Sea area. The STZ was reactivated in Triassic–Jurassic extension and Late Cretaceous and Paleogene compression. This paper investigates the regional geological consequences of the reactivations by quantitative modelling along a profile across the STZ in the Danish area. The numerical model invokes elastic, viscous and plastic deformations of the lithosphere as well as surface processes governed by erosion, sedimentation and lateral transport under the influence of eustatic sea level variations and regional isostatic compensation. Surface processes and lithospheric mechanics are coupled through thermal blanketing effects and loading. The results, in general, address the regional geological consequences of the existence of intracontinental zones of structural weakness. More specifically the results show that the Late Cretaceous and Paleogene chalk depocentres in the Danish Basin are a direct consequence of the inversion of the STZ, and that the STZ inversion together with falling sea level in Cenozoic time are amongst the principal controlling factors in the geological evolution in the eastern North Sea area.

Ridge and valley systems in the Upper Cretaceous chalk of the Danish Basin: contourites in an epeiric sea

Abstract Extensive low-lying parts of the NW European craton were flooded during the Late Cretaceous transgression, creating a relatively deep epeiric sea with reduced supply of siliciclastic material and insignificant coastal upwelling. The chalk, essentially an oceanic sediment type, was deposited as a pelagic rain of mainly coccolith debris and with local redeposition along structural highs. The study area is located in the eastern part of the Danish Basin, where the bordering Ringkøbing-Fyn High and the inverted Sorgenfrei-Tornquist Zone converge. Multichannel seismic reflection lines show the Chalk Group to be far from the expected flat-lying pelagic succession. A multitude of features of considerable relief, comprising an extensive unconformity, sediment waves, drifts and moats, are recognized. At least two episodes of widespread drift deposition are identified, one in the Santonian-Late Campanian and one in the Maastrichtian, separated by a Top Campanian Unconformity. The structures were formed by strong bottom currents flowing northwestward through the basin parallel to bathymetric contours. A lateral northeastward change, from more depositional to more erosional architecture, indicates a positive current velocity gradient towards the inversion zone, probably as a result of the Coriolis force. The strong similarity between the chalk drifts and modem contourite deposits supports the proposal that the oceanographic conditions linked to continental margins were extended into the Late Cretaceous epeiric sea of NW Europe.

Post-mid-Cretaceous eastern North Sea evolution inferred from 3D thermo-mechanical modelling

Abstract The Late Cretaceous–Cenozoic evolution of the eastern North Sea region is investigated by 3D thermo-mechanical modelling. The model quantifies the integrated effects on basin evolution of large-scale lithospheric processes, rheology, strength heterogeneities, tectonics, eustasy, sedimentation and erosion. The evolution of the area is influenced by a number of factors: (1) thermal subsidence centred in the central North Sea providing accommodation space for thick sediment deposits; (2) ∼250-m eustatic fall from the Late Cretaceous to present, which causes exhumation of the North Sea Basin margins; (3) varying sediment supply; (4) isostatic adjustments following erosion and sedimentation; (5) Late Cretaceous–early Cenozoic Alpine compressional phases causing tectonic inversion of the Sorgenfrei–Tornquist Zone (STZ) and other weak zones. The stress field and the lateral variations in lithospheric strength control lithospheric deformation under compression. The lithosphere is relatively weak in areas where Moho is deep and the upper mantle warm and weak. In these areas the lithosphere is thickened during compression producing surface uplift and erosion (e.g., at the Ringkobing–Fyn High and in the southern part of Sweden). Observed late Cretaceous–early Cenozoic shallow water depths at the Ringkobing–Fyn High as well as Cenozoic surface uplift in southern Sweden (the South Swedish Dome (SSD)) are explained by this mechanism. The STZ is a prominent crustal structural weakness zone. Under compression, this zone is inverted and its surface uplifted and eroded. Contemporaneously, marginal depositional troughs develop. Post-compressional relaxation causes a regional uplift of this zone. The model predicts sediment distributions and paleo-water depths in accordance with observations. Sediment truncation and exhumation at the North Sea Basin margins are explained by fall in global sea level, isostatic adjustments to exhumation, and uplift of the inverted STZ. This underlines the importance of the mechanisms dealt with in this paper for the evolution of intra-cratonic sedimentary basins.

Large-scale glaciotectonic thrust structures in the eastern Danish North Sea

Abstract The distribution of shallow, detached thrust fault complexes in the eastern Danish North Sea has been mapped based on 6400 km of high-resolution multichannel seismic data. Individual thrust segments are 200–1000 m long and 100–250 m thick with up to 200 m of horizontal displacement. Thrusting mainly affected upper Middle Miocene to lower Pleistocene strata and predated an extensive system of now buried subglacial valleys. The thrust structures are located along a 200 km long NW-SE trend. The general thrust direction is towards the SW, parallel to inferred directions of ice movement during the Elsterian and Saalian glaciations. This direction is, however, also parallel to the dip of the underlying detachment surface. The detachment surface corresponds to the mid-Miocene unconformity in the SE and the base of the Quaternary to the northwest. Detachment is generally located at depths of 130–270 m below sea level. The structures are interpreted as glaciotectonic thrust structures of Elsterian and/or Saalian age. The driving mechanism is interpreted to be gravity spreading in front of glaciers advancing from the north, NE and east. Thrusting was facilitated by the presence of SW-dipping detachment surfaces.

Optically stimulated luminescence dates for Late Pleistocene Sediments from Stensnæs, Northern Jutland, Denmark

Abstract Quartz and feldspars from Late Pleistocene deposits have been studied using the single-aliquot regenerative protocol. Eleven samples were collected from a single deposit on the north-west coast of Jutland, Denmark. From geological evidence, we interpret this sedimentary sequence to be glaciofluvial (three samples), glaciolacustrine (six samples) and glaciomarine (two samples). Feldspar fading was measured after storage at elevated temperature, and was undetectable for five samples. In the remainder, fading was between 5 and 9%. Nevertheless, the overall agreement between the quartz and feldspar dates was very good. The conventional interpretation of the nine freshwater samples is that they were deposited by the Late Weichselian ice. Our luminescence ages are significantly older than the time at which this event is believed to have started in this region, and they are independently supported by one 14 C age.

Investigating the structural evolution of the western Baltic

The western Baltic Sea, located along the northern margin of the Central European Basin System (CEBS), is a world-class site for investigating the dynamics and stratigraphic evolution of a continental basin with marine geophysical data acquisition techniques. The universities of Aarhus and Hamburg have joined forces to investigate the post-Permianto-recent structural evolution of the western Baltic, with special emphasis on neotectonic re-activation along major structural lineaments. Deep crustal structures of the CEBS are well established from previous studies. However, no systematic and localized research has yet been carried out to investigate the neotectonic activity in this region. In fact, the limited seismic resolution of previously available data prevented detailed research on Mesozoic and Cenozoic evolution or neotectonics.