Offshore Western Norway Research Articles

Contamination of 8.2 ka cold climate records by the Storegga tsunami in the Nordic Seas

The 8200-year BP cooling event is reconstructed in part from sediments in the Norwegian and North Seas. Here we show that these sediments have been reworked by the Storegga tsunami – dated to the coldest decades of the 8.2 ka event. We simulate the maximum tsunami flow velocity to be 2–5 m/s on the shelf offshore western Norway and in the shallower North Sea, and up to about 1 m/s down to a water depth of 1000 m. We re-investigate sediment core MD95-2011 and found the cold-water foraminifera in the 8.2 ka layer to be re-deposited and 11,000 years of age. Oxygen isotopes of the recycled foraminifera might have led to an interpretation of a too large and dramatic climate cooling. Our simulations imply that large parts of the sea floor in the Norwegian and North Seas probably were reworked by currents during the Storegga tsunami.

Background Seismicity Monitoring to Prepare for Large-Scale CO2 Storage Offshore Norway

AbstractPrior to planned CO2 injection startup in the Horda platform offshore western Norway, in 2024, the Horda Network project has taken several measures to assess the potential of seismic hazard in the area. A study of the fault-plane solutions in the Horda platform region confirms that the direction of maximum horizontal stress is dominantly northwest–southeast to east–west over the entire area. The relative stress ratio is higher in the southeast near the Norwegian craton and lower in the northwest. Analysis of the catalog of seismicity (in the period of 2001–2021) in the Horda platform region suggests a moderate rate of seismicity with a b-value of ∼1. The magnitude of completeness is 1.5 (ML). One of the main challenges in monitoring offshore earthquakes in the Norwegian continental shelf (NCS) is the lack of azimuthal coverage when using the onshore permanent seismic stations from the Norwegian National Seismic Network (NNSN), located to the east of offshore events. To improve the azimuthal coverage, we integrated a limited number of offshore geophones from permanent reservoir monitoring systems of selected oil and gas fields (Grane and Oseberg on NCS) with the onshore NNSN seismic stations. This integration is challenging because of the level of ambient noise in the offshore geophones. To further improve the detection and location capability, we deployed a nine-element onshore array of broadband seismometers (HNAR) on Holnsnøy island to the east of the Horda platform. By incorporating array processing methods on HNAR, the signal-to-noise ratio is improved, and several previously uncataloged earthquakes could be detected. Offshore sensors are often subject to correlated noise from seismic interferences and platform or shipping noise sources, so we also incorporated array processing for selected geophones from offshore deployments, which greatly reduced such noise and hence improved the event detection.

From Caledonian Collapse to North Sea Rift: The Extended History of a Metamorphic Core Complex

AbstractExtensional systems evolve through different stages due to changes in the rheological state of the lithosphere. It is crucial to distinguish ductile structures formed before and during rifting, as both cases have important but contrasting bearings on the structural evolution. To address this issue, we present the illustrative ductile‐to‐brittle structural history of a metamorphic core complex (MCC) onshore and offshore western Norway. Combining geological field mapping with newly acquired 3‐D seismic reflection data, we correlate two distinct onshore basement units (BU1 and BU2) to corresponding offshore basement seismic facies (SF1 and SF2). Our interpretation reveals two 40 km wide domes (one onshore and one offshore), which both show characteristic kilometer‐scale, westward plunging upright folds. The gneiss domes fill antiformal culminations in the footwall of a >100 km long, shallowly west dipping, extensional detachment. Overlying Caledonian nappes and Devonian supradetachment basins occupy saddles of the hyperbolic detachment surface. Devonian collapse of the Caledonian orogen formed dome and detachment geometries. During North Sea rifting, brittle reactivation of the MCC resulted in complex fault patterns deviating from N‐S strike dominant at the eastern margin of the rift. Around 61°N, only minor N‐S faults (<100 m throw) cut through the core of the MCC. Major rift faults (≤5 km throw), on the other hand, reactivated the detachment and follow the steep flanks of the MCC. This highlights that inherited ductile structures can locally alter the orientation of brittle faults formed during rifting.

Manifestation of Tectonic and Climatic Perturbations in Deep-Time Stratigraphy – An Example From the Paleocene Succession Offshore Western Norway

Sediment routing systems may be subjected to different external controls that can modulate long and short-term sediment delivery to nearby basins. Here we investigate a Paleocene depositional system offshore western Norway that was subjected to long-term (~10 Myrs) tectonic perturbation and significant hinterland erosion. Superimposed on this long-term uplift, the system was also subjected to a short-lived climatic perturbation, which lasted ~200 Kyrs. Regional 3D seismic reflection data is integrated with high resolution well and biostratigraphic data to map the stratigraphic responses to these different scales of perturbations on the Paleocene system. The initiation of the tectonic perturbation is marked by an angular unconformity in seismic data. A rapid increase in sediment flux followed, causing initial progradation of a shelf-slope wedge. Calculations indicate that the tectonic uplift caused an order of magnitude increase in sediment flux to the basin, which peaked in the latest Paleocene. This period coincided with the Paleocene-Eocene Thermal Maximum (PETM), which is documented by biostratigraphic data as a discrete event within the overall regressive system. Although the PETM often is characterised by increased continental runoff, no increase in sediment supply can be demonstrated from seismic data. This study shows that the system response to tectonic perturbations may vary along strike, depending on the size of the routing systems and the antecedent topography prior to hinterland uplift. A low supply system may produce a tectonically-linked wedge that is of similar thickness as a climatically-linked wedge in a high supply system. This study documents how the same routing system responded to perturbations operating at different spatial and temporal scales and may help recognise similar process-response relationships in other areas.

Origin and significance of intra-basement seismic reflections offshore western Norway

We use 3D seismic data to image a series of enigmatic, SW-dipping reflection packets within pre-Mesozoic crystalline basement offshore western Norway. Based on their low-angle dip and complex reflection wave-train our preferred interpretation is that the reflection packets are the seismic expression of mylonitic zones generated by nappe emplacement during the Caledonian orogeny. Late Jurassic faults truncate and offset these reflection packets by several hundred metres, suggesting that these faults did not exploit pre-existing basement weaknesses. Our observations suggest that older basement fabrics may not always play a significant role in determining the geometry of later fault systems. Supplementary materials: Details of the time-depth relationships from wells that have been used to depth-convert interpretations (S1), a supplementary figure showing an uninterpreted version of the seismic profile presented in Fig. 2 (S2) and a throw v. distance plot for fault F1 at the structural levels of the basement reflection and intra-basement reflection 1 are available at www.geolsoc.org.uk/SUP18683 .

A Double Gas‐Hydrate Related Bottom Simulating Reflector at the Norwegian Continental Margin

Abstract: An unusual pattern of two bottom simulating reflections (BSRs) has been observed on seismic profiles from the continental margin offshore Western Norway. One of these reflections (BSR1) extends over large areas and has the characteristics of the classical BSR, that is a phase‐reversed reflection from the base of the gas‐hydrate stability zone. The second BSR (BSR0) occurs at approximately 70 ms two‐way travel time beneath BSR1 and is here called a double BSR. The distribution of BSR0 is more local than that of BSR1 and it does not show the phase‐reversal relative to the sea floor reflection that is characteristic for a BSR at the gas hydrate‐free gas boundary. Results from an industrial borehole, from full waveform inversion of multichannel seismic data, from high‐frequency ocean bottom hydrophones, and interpretation of seismic profiles, clearly indicate that BSR1 is reflected from the base of the methane hydrate equilibrium field. Results from full waveform inversion indicate that BSR0 corresponds to a 16–20 m zone where the velocity drops from about 1.8 km/s to a minimum of 1.4 km/s and then increases again. The low velocity of 1.4 km/s suggests the presence of free gas. The results support the hypothesis that BSR0 is a reflection from the base of gas hydrates containing hydrocarbons with a heavier molecular weight in addition to methane gas. Interference of reflections from the top and base of the low‐velocity zone associated with BSR0 explain why BSR0 is not phase‐reversed.

The morphometric variation of Actinomma boreale (Radiolaria) in Atlantic boreal waters

A morphometric study on the polycystine radiolarian species Actinomma boreale (Cleve) from ten trigger weight core-tops from the Norwegian-Iceland Seas, three piston cores taken offshore western Norway and three surface sediment samples from Lygrepollen, Sogndalsfjord and Høyangsfjord (western Norwegian fjords) shows a variation in morphology that groups A. boreale into three distinct clusters, interpreted to be related to different oceanographic settings. The largest specimens of A. boreale are found in the western Norwegian fjords, the smallest in the Iceland Sea, giving an apparent positive correlation with temperature. Down core studies in piston cores from the Norwegian Sea demonstrate a considerable size variation of the cortical shell of A. boreale. In the eastern Norwegian Sea, the climatically cold Younger Dryas had a population of A. boreale that was characterized by large cortical shells, while the climatically warm Holocene population was dominated by small sized cortical shells, showing a negative correlation with temperature. We suggest that the large sized conical shell population of A. boreale in the Younger Dryas is not reflecting precisely the sea-surface water temperature. Another factor must play the dominant role here, probably nutrients.

Light hydrocarbon gases in shallow sediments in the northern North Sea

Large amounts of data on hydrocarbons in shallow marine sediments (0–6 m below sea floor) have been gathered on the Norwegian sector in conjunction with surface exploration for petroleum seepage. In this paper some results from a re-investigation of five surveys is presented. The surveys include a total of 2136 shallow cores and cover mainly the Stord Basin and the Viking Graben areas offshore Western Norway. Adsorbed gases, interstitial gases, headspace gases, grain size, and inductively coupled argon plasma elemental data have been investigated (Table 1). The adsorbed methane ranges from 10 to 5000 ppb. It has a uni-modal distribution with a median concentration of 504 ppb (weight/weight). The interstitial methane ranges from less than 1 to 500 ppb. It has a uni-modal distribution with a median concentration of 13.4 ppb. There are two classes of headspace methane samples. A “normal” class comprising about 90% of the samples lies in the range 0.1 to 500 ppb with a median of 2.1 ppb. The second class lies in the range 500 to more than 100,000 ppb with a median of 6990 ppb. The ratio of unsaturated to saturated alkane homologues in the interstitial and headspace gasses is on average equal to one and show positive correlation. This rules out thermogenic hydrocarbons as a direct source for the headspace and interstitial gasses. The proportion of unsaturated hydrocarbons in the adsorbed gas is much lower (< 1%), but also here a significant positive correlation between unsaturated and saturated alkanes is observed. The highest interstitial methane contents are found in sediments with high contents of coarse material. Principal component analysis show that the yields, dryness, and isotopic composition of adsorbed methane correlate positively with the contents of calcium and strontium, the Sr/Ca ratio in the sediments and the proportion of silt size material in the sediments. We infer from this that a major component in the adsorbed gas is cogenetic with carbonates in the fine fraction, probably resulting from in situ bacterial degradation of organic matter. Distinct and unquestionable regional headspace gas anomalies are found within the Quaternary Kleppe Senior (KSE) formation of soft, silty marine/glaciomarine clays. Outside of KSE only background levels of headspace methane is observed. Some of the most prominent headspace gas anomalies correlate very well with a shallow gas accumulation in a sand body just 10–20 m below sea floor. We conclude that gas found at the surface has migrated upwards from this sand body. The 1/3C 1/2C isotopic ratio for the anomalous methane (−51 to −100) ppt and its extremely dry characteristics are typical for a biogenic origin. We speculate that shallow biogenic gas may be a secondary form of carbon related to leakage from petroleum deposits at depth in this very prolific area.

Crustal Stress in the northern North Sea as inferred from borehole breakouts and earthquake focal mechanisms

ABSTRACTThe regional stress field in the northern North Sea (offshore western Norway) has been studied through the acquisition and analysis of directions of maximum horizontal compression (s̀H) as extracted from borehole breakouts and from earthquake focal mechanism solutions.The results indicate that the regional stress field is dominated by NW‐SE compression, with good consistency between shallow borehole breakouts (2–5 km depth) and deeper earthquakes (10–25 km depth). The broad spatial consistency in stress direction indicates that the main stress field is related to factors of primarily plate tectonic origin, and the results are in good agreement with the western Europe trend found in earlier investigations.The Tampen Spur region in the northern North Sea has been subjected to particularly complex deformation, with two dominating fault directions trending NW‐SE and NE‐SW. From Tampen Spur in the west to the Sogn graben in the east an anomalous stress field is indicated, with NE‐SW oriented maximum horizontal compressions. This anomaly is clearly seen both in the borehole breakout data and in the earthquake data. Possible sources for this anomaly are discussed, and include postglacial uplift and/or lateral variations in the physical properties of the crust.

The Moho depth distribution in Fennoscandia and the regional tectonic evolution from Archean to Permian times

From a variety of seismic refraction and reflection profiling results, a new detailed Moho depth map for the Fennoscandian part of the Baltic Shield has been compiled. A complementary crystalline crustal thickness map was compiled for Denmark, the Skagerrak Sea, offshore western Norway and adjacent areas where sedimentary strata reach maximum thicknesses of 10 km and even more. These maps are discussed in relation to tectonic evolution and crustal ages of Fennoscandia, seismic velocities, Bouguer gravity and seismicity. Areas deviating from the proposed age-thickness relationships are the Kola Peninsula (Archean) and the Oslo Rift (Permian). The average crustal thickness in the former area is around 44 km, while the latter area is associated with a Moho elevation of 3–5 km. Maximum crustal thicknesses in excess of 50 km have been found in southwestern and Central Finland and correlate with a crust of Proterozoic age. The Caledonides of western Norway do not exhibit, as elsewhere, a crustal root. Sub-Moho P n and S n seismic velocities are relatively small in that area and Bouguer gravity values are strongly negative. Intracrustal P-velocity variations do not seem to reflect geological age or specific tectonic features. Although the seismicity is moderate in Fennoscandia, most of the earthquake activity and, in particular, the largest earthquakes are confined to areas where the crustal thinning is most pronounced.

Periodicity in seismicity: examination of four catalogs

Seismic activity in four different catalogs is examined for periodicity, both in terms of yearly energy release and number of earthquakes per year. The four catalogs examined cover different areas in different time windows: the Mohns Ridge (a segment of the northern Mid-Atlantic Ridge) (1960–1988), earthquakes felt in Norway (1891–1989), earthquakes offshore Western Norway (1980–1989) and finally seismic energy release from global earthquakes with magnitudes greater then or equal to 7.9 for the years 1898–1977. A period of around 10 years is found in all catalogs and there is a clear correlation between seismicity felt in Norway and global seismic energy release. The correlation in the periodicities of the seismic activity found in these catalogs indicates a global cause for changes in seismicity.

Three recent larger earthquakes offshore Norway

ABSTRACTThree earthquakes of magnitude around 5 occurred offshore western Norway on 5th February, 1986, on 8th August, 1988, and on 23rd January, 1989. These earthquakes, representing the highest seismic activity level in this area for at least 30 years, were all felt by people over most of southern and central Norway. Focal‐mechanism solutions for these earthquakes indicate thrust faulting along N‐S to NNE‐SSW striking fault planes, in response to NW‐SE compressional stress, most probably of plate tectonic origin.A number of high‐quality digital recordings of the ground motions at various distances from these and other recent earthquakes in Norway have shown that source spectral as well as wave attenuation characteristics in this area are reasonably consistent with what has been derived from other intraplate areas.