NW European Shelf Research Articles

3D Anatomy of Late Neogene Contourite Drifts and Associated Mass Flows in the Faroe-Shetland Basin

Abstract We have combined 3D mapping of key reflectors with seismic profiles to describe the geometry and anatomy of contourite drifts formed by deep waters passing through the oceanic gateway of the Faroe-Shetland Channel. The West Shetland Drift complex is characterized by sheeted-mounded drift units, and upslope migrating sediment waves accreting over an early Pliocene unconformity. The basin section is constructed by a series of asymmetric depositional units of early Pliocene-Pleistocene age, interlayered by three mega-debrite sequences that extend into the basin. The Pliocene drift surface display an enhanced topography of bifurcating moat-channels that tend to branch out in a southwest direction. Along the lower slope a succession of upslope migrating sediment waves has accumulated from the Pliocene drift topography. These features extend to the present sea bed at water depths of 700-1000 m where they appear as a series of linear, bifurcating ridges. The high accumulation rates of the West Shetland Drift since the early Pliocene transition and the formation of upslope migrating sediment waves is related to a sustained flow of Norwegian Sea deep waters and cross-slope transport of fine-grained sediments from the NW European shelf.

Seismic stratigraphy of the West Shetland Drift: Implications for late Neogene paleocirculation in the Faeroe‐Shetland gateway

The morphology and anatomy of a late Neogene contourite drift on the British margin of the Faeroe‐Shetland Channel is investigated in unprecedented detail using industrial two‐dimensional and three‐dimensional seismic data. The West Shetland Drift is constructed by a succession of mounded, asymmetric strata that in the basin and on the midslope has accumulated on a regional unconformity of early Pliocene age. The midslope component of the drift system forms a 400 m thick convex, elongate sedimentary body that can be traced from the outlet of the Norwegian Channel and 250 km southwest along the Shetland margin. Contourite drifts occupying the basin are evident as a succession of sheeted‐mounded and wavy migrating units intercalated by three mega‐debrite sequences. The seismic architecture of the West Shetland Drift and tentative age estimates of its bounding surfaces suggests that it formed rapidly (∼5–10 cm kyr−1) by alongslope transport of hemipelagic sediments derived from the NW European shelf. The change from erosion/nondeposition to enhanced contourite drift accumulation during the early Pliocene suggests that a moderate thermohaline current regime prevailed in the Faeroe‐Shetland Channel prior to the main phase of Northern Hemisphere glaciation at 3.0–2.5 Ma. A reduction in meridional heat transport associated with a decrease in the flux of North Atlantic deep waters may have been an important factor for the growth of Northern Hemisphere ice sheets.

Paleogene volcanic ash layers in the Danish Basin: compositions and source areas in the North Atlantic Igneous Province

The Paleogene mo clay basin in Denmark contains ca. 200 layers of mostly well-preserved volcanic ash. Of these, ca. 77 layers have been analysed for major and trace elements by X-ray fluorescence spectrometry (XRF), 22 for rare-earth elements (REE), and 11 for Sr and Nd isotopes. The ‘negative’ ash series (layers −39 to −1) comprise tholeiitic basalt, crustally contaminated trachyte, and rhyolite; alkaline basalt, trachybasalt, trachyte, and rhyolite; and Ti-rich nephelinite and phonolite. The ‘positive’ ash series (layers +1 to +140) comprise enriched tholeiitic ferrobasalt and two rhyolite layers. The ferrobasalts form one comagmatic group; however, oscillations are seen up-section between less-enriched and more-enriched compositions, as indicated by variations in Zr/Nb, REE contents, and isotope compositions, suggesting heterogeneities in the mantle source. Two samples of Eocene ash from Greifswalder Oie in northern Germany are identical to the positive series ashes. By comparison of the ash compositions with other rocks from the North Atlantic Igneous Province, probable source areas can be identified. Four stages of deposition can be distinguished. In stage 1 (layers −39 to −22), basalts and rhyolites were sourced from centres on the NW European shelf. In stage 2 (layers −21b to −15), trachytes and rhyolites were still sourced from centres on the shelf, whereas the strongly alkaline layers could all have originated in the Gardiner Complex in East Greenland. In stage 3, alkali basalts (layers −13 to −11) may be the products of a failed or propagating part of the opening oceanic rift. In stage 4 (layers +1 to +140), we suggest that the comagmatic suite of voluminous ferrobasalts were sourced from a gigantic volcanic system representing the nascent Proto-Iceland within the opening ocean. The cataclysmic character of stage 4 can be understood if the areas of extremely high magma production associated with the Proto-Icelandic mantle plume, which had until then produced the large subaerial flood basalts in Greenland, at this time moved away from the continent and into the sea-covered opening rift, thus switching the bulk of volcanism from effusive to explosive. When Proto-Iceland finally emerged, the explosive activity abated again.

Combined Use of Altimetry and in Situ Gravity Data for Coastal Dynamics Studies

Accurate local geoids derived from in situ gravity data will be valuable in the validation of GOCE results. In addition it will be a challenge to use GOCE data in an optimal way, in combination with in situ gravity, to produce better local geoid solutions. This paper discusses the derivation of a new geoid over the NW European shelf, and its comparison with both tide gauge and altimetric sea level data, and with data from ocean models. It is hoped that over the next few years local geoid methods such as these can be extended to cover larger areas and to incorporate both in situ and satellite measured gravity data.

Palaeoecology of calcareous dinoflagellate cysts in the mid-Cenomanian Boreal Realm: implications for the reconstruction of palaeoceanography of the NW European shelf sea

A transect from the bathyal to proximal shelf facies of the Boreal Realm was investigated to compare spatial and temporal distribution changes of calcareous dinoflagellate cysts (c-dinocysts) throughout the mid-Cenomanian in order to gain information on the ecology of these organisms. Pithonelloideae dominated the cyst assemblages to more than 95% on the shelf, a prevalence that can be observed throughout most of the Upper Cretaceous. The affinity of this group with the dinoflagellates, which is still controversially discussed, can be confirmed, based on evidence from morphological features and distribution patterns. The consistent prevalence of Pithonella sphaerica and P. ovalis in c-dinocyst assemblages throughout the Upper Cretaceous indicates that they were produced more frequently than cysts of the other species and might, therefore, represent a vegetative dinoflagellate life stage. P. sphaerica and P. ovalis are interpreted as eutrophic species. P. sphaerica is the main species in a marginal-shelf upwelling area, offshore Fennoscandia. Here, sedimentary cyclicity appears to have been reduced to the strongest light/dark changes, while in the outer shelf sediments, light/dark cycles are well-developed and show pronounced temporal assemblage changes. Cyclic fluctuations in the P. sphaerica / P. ovalis ratio reflect shifts of the preferred facies zones and indicate changes in surface mixing patterns. During periods of enhanced surface mixing most parts of the shelf were well-ventilated, and nutrient-enriched surface waters led to high productivity and dominance of the Pithonelloideae. These conditions on the shelf contrasted with those in the open ocean, where more oligotrophic and probably stratified waters prevailed, and an assemblage with very few Pithonelloideae and dominance of Cubodinellum renei and Orthopithonella ? gustafsonii was characteristic. While orbitally-forced light/dark sedimentary cyclicity of the shelf sections was mainly related to surface-water carbonate productivity changes, no cyclic modulation of productivity was observed in the oceanic profile. Therefore, dark layer formation in the open ocean was predominantly controlled by the cyclic establishment of anoxic bottom water conditions. Orbitally-forced interruptions in mixing on the shelf resulted in cyclic periods of stratification and oligotrophy in the surface waters, an expansion of oceanic species to the outer shelf, and a shelfward shift of pithonelloid-facies zones, which were probably related to shelfward directed oceanic ingressions.

Present and past influence of the Iceland Plume on sedimentation

Abstract The Cenozoic development of the North Atlantic province has been dramatically influenced by the behaviour of the Iceland Plume, whose striking dominance is manifest by long-wavelength free-air gravity anomalies and by oceanic bathymetric anomalies. Here, we use these anomalies to estimate the amplitude and wavelength of present-day dynamic uplift associated with this plume. Maximum dynamic support in the North Atlantic is 1.5–2 km at Iceland itself. Most of Greenland is currently experiencing dynamic support of 0.5–1 km, whereas the NW European shelf is generally supported by <0.5 km. The proto-Iceland Plume had an equally dramatic effect on the Early Cenozoic palaeogeography of the North Atlantic margins, as we illustrate with a study of plume-related uplift, denudation and sedimentation on the continental shelf encompassing Britain and Ireland. We infer that during Paleocene time a hot subvertical sheet of asthenosphere welled up beneath an axis running from the Faroes through the Irish Sea towards Lundy, generating a welt of magmatic underplating of the crust which is known to exist beneath this axis. Transient and permanent uplift associated with this magmatic injection caused regional denudation, and consequently large amounts of clastic sediment have been shed into surrounding basins during Cenozoic time. Mass balance calculations indicate agreement between the volume of denuded material and the volume of Cenozoic sediments deposited offshore in the northern North Sea Basin and the Rockall Trough. The volume of material denuded from Britain and Ireland is probably insufficient to account for the sediment in the Faroe-Shetland Basin and an excess of sediment has been supplied to the Porcupine Basin. We emphasize the value of combining observations from both oceanic and continental realms to elucidate the evolution of the Iceland Plume through space and time.

Supply and demand of nutrients and dissolved organic matter at and across the NW European shelf break in relation to hydrography and biogeochemical activity

As part of the OMEX I project, nutrient determinations were made on 17 cruises in the region of the Goban Spur and La Chapelle Bank between 46 and 51°N, in all seasons of the year, between 1993–1995. Over this period no change was detectable in the structure of the water masses below the deep winter mixed layer. The N : P (dissolved nitrate-to-phosphate) ratio changed from 16 at 100-m depth to less than 15 at 3300-m depth. At intermediate depths nutrient and oxygen data indicate the presence of Mediterranean Outflow water overlying Labrador Sea Water at its most eastern extension. Estimated maximum levels of production in the spring bloom are the total N-limited new primary production equivalent between 24 and 41 gC m−2, the equivalent maximum diatom production is 11 gC m−2. Measurements during the spring bloom suggest a conversion factor of 1 μM nitrate to 1 μg l−1 chlorophyll, at the shelf break, which is consistent with other recent measurements in European shelf seawaters. Sediment trap data suggest that 80% (5.4 g m−2) of the opal produced in the spring bloom dissolved before reaching the sediment trap at 600 m. A comparison of the winter and summer profiles for dissolved silicon suggests a similar dissolution of 9±3 g opal m−2 above 300-m depth. Measurements of dissolved organic carbon (DOC) in September 1994 show an enrichment of 7 μM-C above the seasonal thermocline relative to the winter values (52±4 μM). In winter dissolved organic nitrogen represents 40% of the pool of total dissolved nitrogen. There is no consistent evidence of an increase in the concentration of DON during summer. Measurements of nitrate in surface waters in January 1994 show that concentrations off-shelf vary with the temperature of the water and are related to the depth of winter mixing. Mixing in surface waters is discontinuous at the shelf break, demonstrating the degree to which exchange across the shelf break is limited even in winter. OMEX winter measurements of nitrate concentrations can be used to estimate the flow of water across the shelf break that would be required to maintain the nitrogen balance in the North Sea at a steady state. The estimate is 0.6 Sv (1 Sv=106 m3 s−1), which is similar to an earlier estimate of a total flow of 1.7 Sv based on salt budgets (cf. Huthnance, Deutsche Hydrographische Zeitschrift, 49 (1997) 153).

Introduction and review

The conference session was originally planned to address aspects of regional and reservoir diagenesis. However, at an early planning stage we decided that basin modelling was going to be the explorationists’ technique of the future and that a theme in the ‘Advances in Technology’ section incorporating basin modelling – but not excluding diagenesis – was more appropriate. How prophetic: some 10 presentations were initially offered for the session, two of which were given in other sessions and two of which had to be withdrawn for reasons of commerciality. Of the six papers presented at the conference and included here, four use 3D migration modelling (all of them integrated with 1D and 2D), one is based on 2D generation and migration modelling and one integrates diagenesis, pressure prediction and fluid migration. The papers encompass frontier exploration on the North Atlantic margin and hydrocarbon systems/diagenetic modelling in the mature exploration areas of the East Irish Sea and the UK and Norwegian Central North Sea. The 3D modelling papers all employ variations of ray-trace (or ray-path) modelling. From the efforts of our trawling and the resulting responses from oil companies and software vendors it would appear that no one has yet applied a full physics, gridded 3D cellular basin model to case studies based in the NW European Shelf – or if they have it is too confidential to be presented. No doubt that dawn is not far away. Ray-trace modelling provides a simple, computationally fast approach to modelling petroleum migration. The technique assumes that petroleum. . .

Late Cretaceous–Cenozoic intraplate contractional deformation at the Norwegian continental shelf: timing, magnitude and regional implications

Contractional structures (large anticlines and synclines, reverse faults and inverted centres of deposition) of assumed Late Cretaceous–Cenozoic age are common in Cretaceous–Tertiary basins of the northwestern European margin. The similarities in style, orientation and timing of these structures are striking. The present detailed analysis of one anticline (the Ormen Lange Dome) of the mid-Norwegian continental shelf indicates that the total contraction is moderate (less than 2–3%), and that the analysed anticline has been growing almost continuously since its initiation in Eocene till Present. Inversion in the Barents Sea started already in the Late Cretaceous. This episode is suggested to be related to far-field effects of active plate-margin processes, and transfer of stresses across the plate as a consequence of the sub Hercynian and Paleocene `Laramide' event of the Alpine Orogeny. The development of co-axial structures was facilitated by stress focusing along pre-existing, high-relief N–S- and NE–SW-trending fault complexes. Far-field plate tectonic stresses originating mainly from the Alpine Orogeny seem to have been the most important cause of contractional deformation on the NW European shelf. In addition, ridge push from the North Atlantic spreading may have contributed significantly, particularly during the Neogene.

Biogeography of marine armoured dinoflagellates and dinocysts in the NE Atlantic and North Sea

The distribution patterns of 250 species of armoured planktonic dinoflagellate, 28 species of armoured planktonic and potentially cyst-forming dinoflagellate and 22 types of dinocyst have been analysed by multivariate clustering and ordination techniques. The area of the study consisted of the NE Atlantic, bounded by 20°S and 70°N latitude and 25°W longitude, and the larger part of the North Sea. The overall distribution patterns were examined by cluster analysis ( Twinspan) which gave a separation into four biogeographic zones clearly relating to the prevailing oceanographic conditions. The zones defined are: (1) Sub-boreal, which consists of the area north of 60°N; (2) NW European Shelf, which comprises the North Sea and the shelf area around the British Isles and France; (3) Temperate Oceanic, the area of the Atlantic between 40 and 60°N which is bounded on the east by zone 2; (4) Warm Temperate, the southern portion of the area sampled which is mainly oceanic but within which is situated the localised area of the NW African upwelling which relates more closely with zone 2. The Twinspan analyses of cyst-forming dinoflagellates and the dinocysts clearly show that the NW European neritic shelf sea is a quite distinct biogeographic zone from the more oceanic areas where relatively few dinocyst types apart from those of Impagidinium are found. Since they are the most important groups for the formation of cysts the 21 gonyaulacoid species and the 50 protoperidinioid species of thecate dinoflagellates were separately analysed. The gonyaulacoids showed a clear influence of temperature on distribution, and the cyst-forming species all seem to share similar requirements since they form a tight cluster in the ordination. The protoperidinioids formed a very strong cluster which included most of the cyst-forming forming species but excluded the rare warm-water species. Analysis of the 22 dinocyst types revealed no clear difference between the majority of the gonyaulacoid or peridinioid cysts. However, Nematosphaeropsis labyrinthus, one of the five cysts in the analysis linked to the thecate dinoflagellate Gonyaulax spinifera, and the five Impagidinium species occupied a distinct position on both analyses.

Early carbonate diagenesis within Phanerozoic shales and sandstones of the NW European Shelf

AbstractContinuous nodular limestone beds in north-west European shale sequences developed during early diagenesis via anaerobic, bacterial catabolysis of organic matter which yielded both carbonate and sulphide. The high carbonate content of the nodular limestones (50–90%) demonstrates that they developed in the upper tens of metres of the sediment column. In addition, each of the nodular limestones displays a centrifugal decrease in carbonate, indicating that growth was synchronous with compaction of the host ooze. The chemistry of the carbonate changed with time, becoming more rich in magnesium and iron and reflecting a progressive change in the porewater chemistry from its initially marine composition. The earliest precipitate was calcite containing < 1 mole% MgO; later precipitates contained several percent MgO and ultimately dolomite was precipitated. Similarly, iron content of the carbonate increased as precipitation continued and sulphate reduction, and thus pyrite production, declined. The early diagenetic cementation of many marine sandstones followed a pattern similar to that of nodular limestones and much of the carbonate was also derived from organic matter via bacterial oxidation. Thus, early diagenetic bacterial oxidation of organic matter may not only reduce the source potential of shales but also reduce the reservoir potential of associated sandstones, or those more distant which lie along compactional-fluid pathways.

NW european continental shelf oil

NW european continental shelf oil