Ole Rønø Clausen

Small-Scale Mantle Convection Produces Stratigraphic Sequences in Sedimentary Basins

Changes in the Rocks Changing sea level or major tectonic events, such as continental collisions, shift stratigraphic sequences by changing the depositional environment where certain rock types form. For example, a deep marine environment where limestone formation is favored may shift relatively quickly to a near-shore environment favoring sandstone formation because the relative sea level has dropped several meters. Petersen et al. (p. 827; see the Perspective by Müller), however, suggest that small-scale convection in the mantle may also induce appreciable changes in the sequence of sedimentary deposits. Using a modeling approach, they found that this is possible on a small scale (that is, just a few hundreds of kilometers) over variable time scales. Thus, while the co-occurrence of sedimentary deposit sequences at regional and global scales can allow sedimentary rocks to serve as markers of marine environments, it should be kept in mind that local changes in surface movements may also manifest themselves in the rock record. Sedimentary stratigraphic sequences may result from sea-level changes that were induced by small-scale mantle convection. Cyclic sedimentary deposits link stratigraphic sequences that are now geographically distant but were once part of the same depositional environment. Some of these sequences occur at periods of 2 to 20 million years, and eustatic sea-level variations or regional tectonic events are likely causes of their formation. Using numerical modeling, we demonstrate that small-scale mantle convection can also cause the development of stratigraphic sequences through recurrent local and regional vertical surface movements. Small-scale convection-driven stratigraphic sequences occur at periods of 2 to 20 million years and correlate only at distances up to a few hundred kilometers. These results suggest that previous sequence stratigraphic analyses may contain erroneous conclusions regarding eustatic sea-level variations.

Factors controlling the Cenozoic sequence development in the eastern parts of the North Sea

The causal relationship between the Cenozoic sequence development in the southeastern North Sea Basin and sea-level changes, climatic fluctuations and tectonic events is unravelled by relating variations in the relative sea level and base level, based on interpretations of seismic surveys, to published δ18O variations and eustatic changes. The latter curve is based on the Earths orbital forcing, and here informally termed as the GSI curve. The analysis shows that the Cenozoic sequence development in the southeastern North Sea was influenced by climatically and tectonically induced sea-level changes. The major Cenozoic sequence stratigraphic boundaries (lower order) are highly influenced by tectonic events, e.g. uplift of Fennoscandia and increased subsidence rates in the basin centre. Reactivation of Mesozoic fault zones controlled the deposition of minor sand bodies transported to the centre of the basin during the Late Palaeocene by mass flows. The location of an Oligocene mound structure, which constitutes part of a sequence, is controlled by the overall palaeotopography of the basin and local fault-related depressions. Correlation between (i) the ages of our sequences and the δ 18O variations in the Oligocene succession, and (ii) the GSI curve and the base-level fluctuations of the late Miocene and younger sequences, show that the generation of the higher order sequence boundaries were driven by glacio-eustatic sea-level changes. A climatic control of the sequence formation due to glacio-eustatic sea level changes is therefore suggested for the Oligocene and Pliocene sequences, and probably also for the Upper Miocene sequences.

The mechanics of clay smearing along faults

A clay- or shale-rich fault gouge can significantly reduce fault permeability. Therefore, predictions of the volume of clay or shale that may be smeared along a fault trace are important for estimating the fluid connectivity of groundwater and hydrocarbon reservoir systems. Here, we show how fault smears develop spontaneously in layered soil systems with varying friction coefficients, and we present a quantitative dynamic model for such behavior. The model is based on Mohr-Coulomb failure theory, and using discrete element computations, we demonstrate how the model framework can predict the fault smear potential from soil friction angles and layer thicknesses.

Plate-wide stress relaxation explains European Palaeocene basin inversions

During Late Cretaceous and Cenozoic times, many Palaeozoic and Mesozoic rifts and basin structures in the interior of the European continent underwent several phases of inversion (the process of shortening a previously extensional basin). The main phases occurred during the Late Cretaceous and Middle Palaeocene, and have been previously explained by pulses of compression, mainly from the Alpine orogen. Here we show that the main phases differed both in structural style and cause. The Cretaceous phase was characterized by narrow uplift zones, reverse activation of faults, crustal shortening, and the formation of asymmetric marginal troughs. In contrast, the Middle Palaeocene phase was characterized by dome-like uplift of a wider area with only mild fault movements, and formation of more distal and shallow marginal troughs. A simple flexural model explains how domal, secondary inversion follows inevitably from primary, convergence-related inversion on relaxation of the in-plane tectonic stress. The onset of relaxation inversions was plate-wide and simultaneous, and may have been triggered by stress changes caused by elevation of the North Atlantic lithosphere by the Iceland plume or the drop in the north–south convergence rate between Africa and Europe.

Paleocene initiation of Cenozoic uplift in Norway

Abstract The timing of Cenozoic surface uplift in NW Europe relies on the assumption that the sedimentary response in basins is synchronous with tectonic processes in the source areas. However, many of the phenomena commonly used to infer recent uplift may as well be a consequence of climate change and sea-level fall. The timing of surface uplift therefore remains unconstrained from the sedimentary record alone, and it becomes necessary to consider the constraints imposed by physically and geologically plausible tectonic mechanisms, which have a causal relation to an initiating agent. The gradual reversal of the regional stress field following the break-up produced minor perturbations to the thermal subsidence on the Norwegian Shelf and in the North Sea. Pulses of increased compression cannot be the cause of Cenozoic land surface uplift and accelerated Neogene basin subsidence. Virtually deformation-free regional vertical movements could have been caused by changes in the density column of the lithosphere and asthenosphere following the emplacement of the Iceland plume. A transient uplift component was produced as the plume displaced denser asthenosphere at the base of the lithosphere. This component decayed as the plume material cooled. Permanent uplift as a result of igneous underplating occurred in areas of a thin lithosphere (some Palaeozoic and Mesozoic basins) or for lithosphere under extension at the time of plume emplacement (the ocean-continent boundary). In areas of a thicker lithosphere (East Greenland, Scotland and Norway) plume emplacement may have triggered a Rayleigh-Taylor instability, causing partial lithospheric delamination and associated transient surface uplift at a decreasing rate throughout Cenozoic time. A possible uplift history for the adjacent land areas hence reads: initial transient surface uplift around the break-up time at 53 Ma caused by plume emplacement, and permanent tectonic uplift caused by lithospheric delamination and associated lithospheric heating. The permanent tectonic uplift increased through Cenozoic time at a decreasing rate. Denudation acted on this evolving topography and reduced the average surface elevation, but significantly increased the elevation of the summit envelope. The marked variations in the sedimentary response in the basins were caused by climatic variations and the generally falling eustatic level. This scenario bridges the gap between the ideas of Paleocene-Eocene uplift versus repeated Cenozoic tectonic activity: the tectonic uplift history was initiated by the emplacement of the Iceland plume, but continued throughout Cenozoic time as a consequence of early plume emplacement, with climatic and eustatic control on denudation. The mechanism is consistent with topography, heat flow, crustal structure, and the Bouguer gravity of Norway, and may be applicable also to East Greenland.

A new strategy for discrete element numerical models: 2. Sandbox applications

[1] Here we present a series of numerical experiments using a new formulation of the discrete element method (DEM) that improves performance in modeling faults and shear zones. In the new method, named the stress-based discrete element method (SDEM), which is introduced in the companion paper by Egholm, stress tensors are stored at each circular particle. Further, SDEM includes rotational resistivity of particles and elastoplastic constitutive rules for governing particle deformation. When combining these new features, the SDEM is capable of reproducing the friction properties of rocks and soils, without the need for the ad hoc calibration routines normally associated with DEM. In contrast to the conventional DEM, the friction properties of a SDEM particle system are in agreement with the Mohr-Coulomb constitutive model with friction angles specified on a particle level. ‘‘Benchmark’’ sandbox models show that unlike most commonly used numerical methods, SDEM faults and shear zones develop at angles in agreement with general observations from structural geology and analogue modeling studies. Citation: Egholm, D. L., M. Sandiford, O. R. Clausen, and S. B. Nielsen (2007), A new strategy for discrete element numerical models: 2. Sandbox applications, J. Geophys. Res., 112, B05204, doi:10.1029/2006JB004558.

Topography of the Top Chalk surface on- and offshore Denmark

Abstract A time-structure map of the top of the Upper Cretaceous–Danian Chalk Group (the Top Chalk surface) in the entire Danish sector is presented. The present topography of the Top Chalk surface is a result of various erosional and tectonic events taking place during the Cenozoic. The analysis shows that the Cenozoic is characterised by significant reactivation of the Mesozoic structural elements of which the Central Graben, the Ringkobing–Fyn High and the Sorgenfrei–Tornquist Zone are the most important. A tectonically controlled relief caused the generation of widespread sheet like erosion on the Ringkobing–Fyn High and incisions along the margins of Ringkobing–Fyn High during the mid Paleocene. Incisions in the northern part of the Danish sector formed valleys, which probably transported sediment towards the west. The present topography of Top Chalk in the eastern part of the Danish Sector is highly influenced by the late Cenozoic erosion of the uplifted eastern margin of the North Sea Basin. The topography of Top Chalk also reflects the distribution of mobile Zechstein salt as collapse structures follow the pitch-out line of mobile salt onto the Ringkobing–Fyn High, and numerous salt structures deformed the Top Chalk surface.

Detailed stratigraphic subdivision and regional correlation of the southern Danish Triassic succession

This paper provides a regional, log-based lithostratigraphic correlation of the Danish Triassic deposits with deposits known from the southern North Sea Basin, including the northwest German and Dutch onshore areas. A new lithostratigraphic nomenclature, mainly adapted from the Dutch nomenclature, is suggested for the Danish area south of the Ringkobing-Fyn High, replacing the previously used nomenclature. The new nomenclature comprises the Lower Triassic Bunter Shale and Bunter Sandstone Formations, the Lower to Middle Triassic Rot and Muschelkalk Formations, and the uppermost Middle Triassic and Upper Triassic Keuper and Sleen Formations. In total, 22 members are also defined from the southern North Sea into the southern Danish area. This main marine southern facies province is separated from a continental northern facies province by the northern flank of the Ringkobing-Fyn High trend. Units of the southern province only crossed the central and western part of the Ringkobing-Fyn High during the Rot and the Muschelkalk transgressions, whereas the basin north of the eastern part of the high was influenced by the southern facies province during the Early to early Late Triassic.

Late Triassic structural evolution of the southern margin of the Ringkøbing-Fyn High, Denmark

A Late Triassic (Carnian) unconformity is observed on seismic sections and in wells along the southern margin of the Ringkobing-Fyn High. The unconformity is characterised by low angle erosion on the central parts of the eastern Ringkobing-Fyn High (Mon High), and by a significant onlap onto the unconformity surface. Structural reconstruction shows that the unconformity is related to differential subsidence between the Ringkobing-Fyn High and the North German Basin. The differential subsidence initiated movements in the mobile Zechstein Salt, causing deformation of the cover sediments. The variability in differential subsidence observed during the Triassic along the southern margin of the Ringkobing-Fyn High is interpreted to reflect the subdivision of the Ringkobing-Fyn High into a number of structure blocks.

Oligocene sequence stratigraphy and basin development in the Danish North Sea sector based on log interpretations

Abstract Five sequences are defined in the Oligocene succession of the Danish North Sea sector. Two of the sequences, 4.1a and 4.3, have been identified onshore Denmark. Two types of prograding lowstand deposits are recognized. Sand-dominated deposits occur proximally, comprising sharp-based forced regressive deposits covered with prograding low-stand deposits. Clay-dominated prograding lowstand deposits occur distally in the sequences. The highstand deposits are proximally represented by thick prograding sandy deposits and distally by thin and condensed intervals. The main sediment input direction was from the north and the northeast. A succession oif lithofacies, from shallow marine facies dominated by sand to outer shelf facies dominated by clay, is mapped in each of the sequences. An overall southward progradation of the shoreline took place during the Oligocene, interrupted only by minor shoreline retreats.