Abstract A number of rock samples were collected from two dredge positions on the Lomonosov Ridge at water depths of 2–3.5 km. The dredge samples are dominated by sediments deformed and metamorphosed under greenschist-facies conditions 470 myr ago according to 40 Ar/ 39 Ar dating of metamorphic muscovite. This shows that the Lomonosov Ridge was involved in a major Mid-Ordovician orogenic event that correlates with early arc–terrane accretion observed in northern Ellesmere Island, Svalbard, and other parts of the Caledonian belt. Detrital zircon age spectra of these metasediments span the Mesoproterozoic–Palaeoproterozoic with a main peak at around 1.6 Ga, and a pattern similar to that known from Caledonian metasedimentary rocks in East Greenland and northern Norway, as well as from Cambrian sediments in Estonia and Palaeozoic sediments on Novaya Zemlya. A second population of dredge samples comprises undeformed, non-metamorphic sandstones and siltstones. Detrital zircons in these sediments span the Palaeoproterozoic with a few Archaean zircons. Both rock types are covered by an up to 8 Ma ferromanganese crust and are evaluated to represent outcrop, and apatite fission-track data from three of the rock samples indicate that exposure at the seabed corresponds to a regional event of uplift and erosion that affected the Arctic in the Late Miocene. The data from the Lomonosov Ridge suggest that the 470 Ma orogenic event extended from Scotland and northern Scandinavia into the Arctic, including Svalbard, the Pearya Terrane and the Chukchi Borderlands. Supplementary material: Detrital zircon age data and details of the thermal history constraints are available at: https://doi.org/10.6084/m9.figshare.c.3852151

Abstract We present observations that demonstrate that the elevated passive margins around the North Atlantic were formed by episodic, post-rift uplift movements that are manifest in the high-lying peneplains that characterize the coastal mountains, in the unconformities in the adjacent sedimentary basins and in accelerated subsidence in the basin centres. Results from West Greenland show that subsidence of the rifted margin took place for c . 25 Myr after rifting and breakup in the Paleocene, as predicted by classical rift theory, but that this development was reversed by a series of uplift movements (starting at c . 35, 10 and 5 Ma) that remain unexplained. East Greenland and Scandinavia seem to have had a similar evolution of post-rift subsidence followed by uplift starting at c . 35 Ma. There was no notable fall in sea-level at this time, so the subsiding basins must have been inverted by tectonic forces. We speculate that the forces causing this phase were related to the plate boundary reorganization in the North Atlantic around Chron 13 time. One feature that these areas have in common is that uplift took place along the edges of cratons where the thickness of the crust and lithosphere changes substantially over a short distance. It may be that the lateral contrasts in the properties of the stretched and unstretched lithosphere make the margins of the cratons unstable long after rifting. These vertical movements have profound influence on hydrocarbon systems, not only in frontier areas such as West and East Greenland, where Mesozoic basins are deeply truncated and exposed onshore, but also for the understanding of near-shore hydrocarbon deposits in mature areas such as the North Sea Basin, where low-angular unconformities may represent episodes of deposition and removal of significant sedimentary sections.

Abstract Changes in seismic response caused by variation in degree of compaction and fluid content in North Sea chalk reservoirs away from a wellbore are investigated by forward modelling. The investigated seismic response encompasses reflectivity, AVO and acoustic impedance. Synthetic seismic data are calculated on the basis of well data from the South-Arne and Dan fields, Danish North Sea and compared to field records. Seismic response predictions are based on three main tools: (1) saturation modelling, (2) compaction/decompaction modelling and (3) rock physics. Hydrocarbon saturation in North Sea chalk is strongly affected by capillary forces and transition zones in the order of 50 m are common. Advanced saturation height modelling is applied, which has proved robust for the prediction of saturation profiles in Danish chalk. Compaction modelling relies on exponential decay of porosity with depth, where abnormal fluid pressures are accounted for. A new set of compaction parameters is presented based on a normal velocity-depth trend and a velocity-porosity transform for North Sea chalk. The parameters appear to allow fairly precise predictions of abnormal fluid pressures from observed average porosity. Based on this, the relative contribution to porosity preservation by abnormal fluid pressure and early hydrocarbon invasion may be estimated. Rock physics theory is applied to obtain all necessary parameters for the complete set of elastic parameters for seismic modelling. Modelling results of importance in the search for subtle traps include: (1) correlation of reflectivity with porosity; (2) primary sensitivity of acoustic impedance is to porosity variation rather than hydrocarbon saturation; (3) the Poisson ratio is very sensitive to hydrocarbon saturation at high porosity, depending on fluid density contrasts. In addition, compaction modelling shows a clear effect of porosity preservation by hydrocarbons in the South-Arne Field, whereas this effect is negligible in the Dan Field. In both fields seismic signatures in field records originating from fluid changes are identified.

Abstract The development of methods of seismic imaging beneath basalts is still hindered by a lack of knowledge about the elastic properties of basaltic sequences and the degree of three-dimensional heterogeneity. The SeiFaBa project (2002–2005) is funded by the Sindri Group as part of the programmes for licensees within the Faroese area and will attempt to address these issues. The Glyvursnes-1 well was drilled by SeiFaBa through the Upper Basalt Formation outside Tórshavn in 2002. A full core and numerous wireline logs were acquired from the 700 m deep well. During the same operations, the existing 660 m deep Vestmanna-1 well drilled mainly into the Middle Basalt Formation was reamed and logged. The two wells are central to a number of closely co-ordinated experiments, which are all targeted at creating models for seismic wave propagation through a succession of basalt by combining detailed analysis at core, log and seismic scales. Data from these two wells, in combination with the data for the Lopra-1 well drilled into the Lower Basalt Formation, will give new stratigraphic and petrophysical control of the Lower, Middle and Upper Basalt formations on the Faroes. The seismic programme was initiated in 2002 and the main acquisition was carried out during 2003. The well site at Glyvursnes gives optimal conditions for combining VSP, offset-VSP and surface seismic experiments both onshore and offshore and the seismic effects of a nearby near-vertical shear zone can be studied in detail. Preliminary analysis of log data from the Lopra-1 well suggests that the acoustic properties of these basalt flows are mainly controlled by porosity of a stiff matrix filled with clay minerals and water. Further studies will allow for explanations of the sonic response of basalt in terms of physical and compositional properties and a better understanding of the seismic signatures of flood basalt successions.