Buried sedimentary mounds in the Nyk High area, Mid ... · more, in the case of the Vigrid diapir...

IntroductionDeep-water mounds may encompass features of varyinggenetic origin e.g. mud mounds/diapirs, mud or asphaltvolcanoes, coral/sponge build-ups and other bioherms,and fluid/gas blisters. Out-runner blocks from sub-marine mass movements may also exhibit a mound-likeappearance in geophysical datasets. In the deep waterarea off Mid-Norway, diapirs are found on the VemaDome (Vema diapir field), on the Vøring marginal high(Vivian diapir field) and on the southern Vøring Pla-teau (Vigrid diapir field) (Fig. 1; Hjelstuen et al. 1997;Vogt et al. 1999). However, all these features may notnecessarily have formed through pure diapirism, as thedriving mechanism may also include venting of hydro-carbon-bearing fluids (Hovland et al. 1998). Further-more, in the case of the Vigrid diapir field, the moundedfeatures identified as diapirs based on 2D-seismic data(Hjelstuen et al. 1997) may in fact reflect sedimentblocks within debris lobes generated from mass failuresupslope (Riis et al. 2005). Also, sparse minor mudmounds of uncertain origin appear on the sea floor inthe area, cf. Fig. 3 in Dahlgren & Vorren (2003).

Deep water coral reefs or mounds, primarily built bythe cold-water coral species Lophelia pertusa, arecommon features along the NW European continentalmargin (see compilations in Hovland & Mortensen1999; De Mol et al. 2002; Roberts et al. 2003). On theNorwegian continental margin they are found from the

shelf break to the fjords where they inhabit watersdown to approximately 500 m depth (Fig. 1). Thereefs/mounds were established subsequent to the lastdeglaciation (Hovland & Mortensen 1999), possibly inresponse to the northward penetration of warmerwaters as ocean circulation shifted from glacial into thepresent interglacial mode (cf. Freiwald et al. 1999). Inthe Atlantic Ocean, mounds with abundant Lopheliapertusa have been found down to depths of 1500 m (seecompilations in De Mol et al. 2002; Roberts et al. 2003).Examples of older buried mounds have also beenpresented from the Porcupine Basin, offshore Irelande.g. the ‘Magellan’ and ‘Belgica’ mounds (Huvenne et al.2003; Van Rooij et al. 2003). To date, occurrences ofburied coral mounds have not been reported along theNorwegian continental margin.

In this study buried sediment mounds on the flanks ofthe Nyk High in the deep-water area off Mid-Norway aredescribed. These are compared with modern deep-watersediment mounds in order to discuss their nature, and toarrive at a plausible explanation for their origin.

Geological settingThe mid-Norwegian margin is characterised by an upto 200-km-wide continental shelf with a gentle slopedown to a marginal plateau, the Vøring Plateau, at

NORWEGIAN JOURNAL OF GEOLOGY Are they Miocene-Pliocene cold-water coral mounds? 295

Buried sedimentary mounds in the Nyk High area, Mid-Norwegian margin: Are they Miocene-Pliocenecold-water coral mounds?

Karl Ivar Torbjørn Dahlgren & Bjørn Lindberg

Dahlgren, K.I.T. & Lindberg, B.: Buried sedimentary mounds in the Nyk High area, Mid-Norwegian margin: Are they Miocene-Pliocene cold-watercoral mounds? Norwegian Journal of Geology, Vol. 85, pp. 295-304. Trondheim 2005, ISSN 029-196X.

Buried sedimentary mounds in the Nyk High area, Mid-Norwegian continental margin, are described from a 3D-seismic dataset and discussed inrelation to modern deep-water sedimentary mounds. The mounds are circular to elongate in shape and their dimensions are: 20-50 m high, 50-400m wide and up to 600 m long. Internally, the mounds exhibit relatively low acoustic reflectivity. They occur on the lower flanks of the Nyk Highwhich at the time of deposition protruded some 250 m above the surrounding sea-floor. Further basinward the mound province makes way for alower energy ooze/mud-dominated environment, characterized by polygonal faulting. The mounds’ location and relationship to under- andoverlying sediments, and the fact that they compare favourably in size and geometry to present day Lophelia-mounds suggests that they are buriedLophelia-mounds of Miocene-Pliocene age.

K.I Torbjørn Dahlgren1 & Bjørn Lindberg2, Department of Geology, University of Tromsø, NO-9037 Tromsø, Norway. 1Present address: Statoil ASA,Postboks 40, 9481 Harstad, Norway ([email protected]). 2Present address: Statoil ASA, Forusbreen 50, NO-4035 Stavanger, Norway ([email protected]).

Fig. 1. A) Map of the Norwegian continental margin showing the distribution of known Lophelia pertusa-reefs and diapirs, modified from Lindberg(2004) and including diapir-occurences from Hjelstuen et al. (1997). The area shown in detail in B is marked by the rectangle. B) Bathymetry of thestudy area and the surroundings (combination of colour and shaded relief). Map is based on the seabed pick of a dense grid of 2D-seismic data, refe-rence grid is ED50-UTM zone 32. The box and the outlined area shows the extent of the horizon shown in C. C) Time-structure map of the mound-horizon (green pick in Figs. 2 & 3) displayed as a coloured shaded relief map 3× vertical exaggeration). Note the mounds on the lower flanks of theNyk High and the polygonal faulting pattern in the basinal area. Also shown are the locations of the seismic profiles A-A’ and B-B’ (Figs. 2 & 3).

K. I. T. Dahlgren & B. Lindberg NORWEGIAN JOURNAL OF GEOLOGY296

1200-1400 m water depth (Fig. 1A). The Vøring Plateauslopes gently into the Lofoten Basin northwards andsouthwestwards into the Norwegian Basin. The shelfbreak in the area is situated at water depths rangingbetween 250 to 400 m (Fig. 1B).

The Nyk High (Fig. 1C) represents a large rotated anduplifted fault block bordering the Hel graben of the nor-thern Vøring Basin. The high is largely composed of Cre-taceous sedimentary rocks (Kittilsen et al. 1999) but withPaleocene sediments onlapping the lower flanks of thehigh (Brekke 2000; Lundin & Doré 2002). The top of thehigh represents an unconformity which has been looselyconstrained to the Oligocene-Miocene (Lundin & Doré2002). During the westward progradation of the thickUpper Pliocene to Pleistocene wedge the high was finallyburied by sediments (Brekke 2000; Dahlgren et al. 2005).

In Middle Miocene time, a regional compressionalphase affected most of the NW European margin resul-ting in the growth of inversion domes (e.g. the VemaDome) and arches along the margin (Brekke 2000;Lundin & Dore 2002; Stoker et al., 2005), and possiblyfurther elevating the Nyk High (Brekke 2000). It hasalso been speculated that this compressional phaseaffected the sill areas of the Greenland-Scotland ridge,allowing it to subside and permit a more vigorous oce-

anic exchange between the Greenland-Iceland-Norwe-gian (GIN) Seas and the North Atlantic Ocean (Stokeret al. 2005). Coeval to the compressional event is also awidespread hiatus on the Norwegian continental shelfwhich Eidvin et al. (1998) related to a possible fall inrelative sea level.

In Early and Middle Miocene times, theNorwegian–Greenland Sea was characterized by biogenicopal deposition related to enhanced chemical weathe-ring in a warm and humid climate with run-off watersupplying silica to the ocean (Bohrmann et al. 1990).The modern thermohaline ocean circulation patternwas probably not yet established at the time (Bohrmannet al. 1990). In the Late Miocene to Early Pliocene,shifting dominance between opal- and carbonate-richsediments is recognised (Bohrmann et al. 1990). In thelate Early to early Late Pliocene large parts of theNorwegian continental shelf were characterized by nodeposition or erosion, possibly due to sea-levellowering (Eidvin et al. 2000). In the Late Pliocene,accumulation and progradation of glacigenicsediments from the Fennoscandian mainland wasinitiated, building an extensive and voluminousprograding wedge off Mid-Norway (Eidvin & Riis1991; Henriksen & Vorren 1996; Stuevold & Eldholm1996; Hjelstuen et al. 1999a; Dahlgren et al. 2005).

297NORWEGIAN JOURNAL OF GEOLOGY Are they Miocene-Pliocene cold-water coral mounds?

Fig. 2. Seismic line A-A’ (see Figs. 1C & 4 for location) showing the mounded surface morphology of unit B. Overlying the mounds areglacigenic sediments of unit C (the Naust Formation). The inset shows the seismic character of the mounds in greater detail with no verticalexaggeration. Interpreted reflections/horizons are marked (L-light blue; G-green; Y-yellow; Unc.-unconformity).

Data and methodsA time-migrated 3D-seismic dataset (Fig. 1B), providedby the Norwegian Deepwater Programme's SeabedProject, and covering the Vema/Nyk area was used. Thedataset is composed of two merged surveys, onecovering the Vema Dome area and one over the NykHigh area. Slight variations in reflection amplitudes areevident between the datasets. The data were interpretedand manipulated on a GeoQuest Charisma worksta-tion, using the GeoViz application for the presentationof the horizons. The horizontal resolution of the data is25 m and the vertical resolution is better than 20 m atthe studied interval, assuming a maximum acousticvelocity in the sediments of 2000 ms-1.

For the delineation and mapping of seismic units, weselected a negative maximum amplitude for the lowerbounding horizon (Y) of unit B, and an upper zero-crossing for its upper bounding horizon (G; Figs. 2 &3). In addition, a horizon (L) within the overlying sedi-ment package (unit C) was mapped by its negativemaximum amplitude. The selection of reflections wasbased on the contrasting internal acoustic character ofthe sediments and the continuity of the reflections.Apart from the three horizons upon which the results ofthis study are built, a large number of additional hori-zons were interpreted and analyzed. These stratigraphi-cally shallower and deeper horizons provide additionalsupport for the interpretation presented below.

ResultsThe stratigraphic interval we focused on in this study(unit B) is coincident with a general shift fromacoustically laminated sediments (unit A) below, toacoustically massive transparent sediments above (unitC), see Figs. 2 & 3.

Stratigraphy

Sandwiched between two units (A & C) of markedlydifferent acoustic character is a less than 100 ms TWTthick (equals <100 m assuming a sediment velocity of2000 ms-1) unit (B) with a relatively low and chaoticacoustic reflectivity (Figs. 2 & 3). The upper boundingreflection (G) of unit B displays a mounded topo-graphy around the flanks of the Nyk High (Fig. 1C),against which the unit pinches out (Figs. 2 & 4A).Basinward of the mounds, unit B is dominated by poly-gonal faulting (Fig. 1C). The lateral distribution of unitB (within the 3D-dataset) is limited south and west ofthe Nyk High (Figs. 2 & 4A), but displays two maindepocentres (Fig. 4A). One occurs on the southern towestern flanks of the Nyk High and is associated with

many smaller accumulations (mounds) (Figs. 1C & 4A-C). The second depocentre occurs along the centreof the basin, within the area with a pattern of polygonalfaulting (Fig. 4A).

The underlying unit (A) is acoustically laminated andhas relatively low reflectivity. Sediments of this unitonlap the flanks of the Nyk High and eventually pinchout (Figs. 2 & 3). In the basinal areas they are disturbedby numerous faults which laterally connect to form apolygonal pattern (Fig. 3).

Unit C is characterized by alternating intervals ofacoustic sub-parallel lamination and intervals displayingmore transparent acoustic facies. The transparent faciesmay show weak internal reflections exhibiting astacked-lensoid body configuration (Figs. 2 & 3).A reflection amplitude image from the light blue (L)horizon (Fig. 4D), situated some 30-50 ms TWT abovethe upper bounding reflection (G) of unit B, shows apattern of subtle channels feeding into shiftingdepositional lobes (Fig. 4 D).

Mound character

The distribution of mounds within the study area isshown in Fig. 1C. The mounds are 20-50 ms TWT high(equals 20-50 m assuming an average sediment velocity of2000 ms-1), 50-400 m wide and up to 600 m long (Figs.1C & 4A-C). They are usually elongated, and on a localscale, their orientations appear to be sub-parallel (Fig. 4A& B). Some mounds are complex and appear as partlymerged (Figs. 4A & B). The dip of the mound flanksreaches up to 3.75 msm-1 (Fig. 4B). Assuming an averageacoustic velocity of 2000 ms-1, this equals a dip of 75˚.

Below some of the mounds, there is a marked depres-sion of the lower bounding reflection (Y) as well as theunderlying sediments (Fig. 2). This may indicatedifferential loading and compaction and/or an acousticvelocity ‘push-down’ effect. An integrated amplitudemap of the upper 32 ms TWT of unit B (Fig. 4C), indi-cates that this unit has a somewhat higher acousticreflectivity around the flanks of the Nyk High thanelsewhere. However, when compared to the over- andunderlying sediments, the mound unit has a relativelylow internal acoustic reflectivity (Figs. 2 & 3). Thecentral parts of individual mounds generally show hig-her acoustic amplitudes than the lower flanks (Fig. 4C).

There seems to be no relationship between the locationof the mounds and underlying fluid or gas escapefeatures such as pipes and faults (Figs. 2 & 3). The poly-gonal fault pattern which is well developed in thebasinal areas gradually dies out towards the Nyk High,and the largest concentration of mounds is found outsidethe occurrence of polygonal faults (Figs. 1C & 4A-C).

298 K. I. T. Dahlgren & B. Lindberg NORWEGIAN JOURNAL OF GEOLOGY

DiscussionStratigraphy and chronology

No firm chronological constraints exist from boreholedata for this particular period in the study area. DSDPsite 341 located in the Vema Diapir Field (Fig. 1B) isdifficult to correlate with the study area, and this alsoprecludes a firm age assignment for the investigatedinterval. However, the acoustic character of the under-and overlying sediments do provide some constraintson the age of the mounds as does regional seismo-stratigraphic investigations (e.g. McNeill et al. 1998;Bugge et al. 2004).

The acoustic character of unit C and the recognition offeatures identical to glacigenic debris flows within theunit (Fig. 4D; cf. Vogt et al 1993; Nygård et al. 2002)support a correlation of unit C with the NaustFormation (cf. Mc Neill et al. 1998, Dahlgren et al.2002). The Naust Formation (Dalland et al. 1988) haspreviously been shown to range from Late Pliocene tothe Pleistocene (Eidvin et al. 1998). Consequently, themounds most likely predate the Late Pliocene. A LatePliocene age cannot be excluded, however, as they mayhave developed coeval with the early phase of

glacigenic wedge progradation, which during earlyNaust time was limited to inner shelf areas (cf.Dahlgren et al. 2005). If the mounds were built by reef-forming corals, a Late Pliocene age seems unlikely inthe light of the carbonate-undersaturated water masseswhich characterised the Norwegian Sea at that time (cf.Jansen et al. 1990; Henrich & Baumann 1994). Bottom-water temperature estimates from oxygen isotopes inbenthic foraminifera shells further suggest relativelycooler bottom waters after 4 Ma (Jansen et al. 1990).The warmer (albeit fluctuating) bottom waters prior to4 Ma may have been more favourable for coral growth.

Based on the acoustic signature and internal configura-tion of the underlying unit A, the mounds (unit B)could be interpreted to belong to part(s) of the EarlyMiocene-Early Pliocene Kai Formation or the upperpart of the Eocene to Oligocene Brygge Formation (cf.McNeill et al. 1998; Hjelstuen et al. 1999a; Hjelstuen etal. 2005). If the underlying sediments (unit A) belongto the Brygge Formation, unit B must record a longperiod of condensation corresponding to the KaiFormation. In this case, no more than 100 m ofsediments were deposited in ~21 m.y. This equalsaverage sedimentation rates of less than 4.76 m/m.y.


Fig. 3. Seismic line B-B’ (see Figs. 1C & 4 for location) showing the mounded sediment package (unit B) and its continuation into the basinalarea where mounds are absent. Polygonal faults which pass through the underlying unit A (the Brygge Formation) are dominant features in thebasinal area. Interpreted reflections/horizons are marked (L-light blue; G-green; Y-yellow; Unc.-unconformity).


Fig. 4. Attribute maps A.) Thickness map of unit B, note thickness maxima in the mound province at the flanks of the Nyk High as wells as inthe basinal area characterised by polygonal faulting. The map is superimposed on a shaded relief of the mound horizon with no verticalexaggeration. B.) Dip map of the G-horizon showing in detail the outline of individual mounds. The map is superimposed on a shaded reliefwith 3× vertical exaggeration. C.) Integrated seismic amplitude of the upper 32 ms of unit B. The map is superimposed on a shaded relief of theG-horizon with 3× vertical exaggeration. D.) Seismic amplitude of the L-horizon immediately overlying the mounds within unit A (the NaustFormation). Note the characteristic pattern of lobes indicative of glacigenic debris flows (GDFs). The map is superimposed on a shaded relief ofL-horizon with 3× vertical exaggeration.

Support for this alternative is the chronology of deposi-tion at DSDP site 341, where Smalley at al. (1986)found low depositional rates throughout the Miocene.They estimated a depositional rate of ~15 m/m.y. forthe Early to early Middle Miocene period and a rate of~0.2 m/m.y. for the remainder of the Miocene. In con-trast, the overlying Pliocene sediments had a mean rateof deposition of 45 m/m.y. (Smalley at al. 1986). Alsoregional seismo-stratigraphic interpretations (cf. McNeillet al. 1998) suggest that the mounds belong to the KaiFormation, the base of the Kai Formation being close tothe base of unit B. The calibration of the regionalseismo-stratigraphic framework relies on only a fewwells, and mapping of the relevant boundaries is nostraightforward task due to units pinching out, on- andofflap relationships and the occurrence of disruptivefeatures such as diapirs. Nevertheless, in the light of thecombined information, we regard it as most likely thatunit B is equivalent to the Kai Formation, and thereforeis of Miocene-Early Pliocene age.

This age estimate would further imply that unit A canbe correlated with the Eocene-Oligocene BryggeFormation (Dalland et al. 1988). The moat featuresdeveloped in unit A, around the Nyk High (Fig. 2), canbe regarded as evidence for bottom current control onsedimentation in the area already during Eocene-Oligocene times. In support of this, Laberg et al. (2005)recently presented evidence for similar Eocene-Oligocene bottom current-controlled features from thenearby Vøring Marginal High.

Comparison with modern deep water mounds

The variety of mounds which are directly related to, orconstructed by subsurface venting of fluids, gas or mudseem to represent poor analogues to the buriedmounds observed on the flanks of the Nyk High. Thelack of indications of possible subsurface fluid/gasmigration paths (e.g. Hovland et al. 1998; Hjelstuen etal. 1999b; Svensen et al. 2003; 2004), disruption ofunderlying strata and/or acoustic blanking (e.g.Hovland et al. 1998; Pinheiro et al. 2003) argue againstan origin of the observed mounds being linked to thesubsurface geology. Another argument centres on themorphology of the mounds which do not display thecommonly occurring central vent depression (crater) ofmounds built by active venting (e.g. Vogt et al. 1999).Nor can any sediment flow features sourced from themounds be observed, this is in strong contrast to forexample mud volcanoes (Vogt et al. 1999; Pinheiro etal. 2003). The nearby example of the Vema diapirs(observed within the same 3D-seismic dataset) thoughtto be related to a combination of mud-diapirism andhydrocarbon discharge (Hovland et al. 1998) showsmarked diffeences to the observed mounds. Thesecontrasts include: 1) size; diapirs are an order of

magnitude larger, 2) diapirs show ample evidence ofdisruption of subsurface strata, 3) sediment flowfeatures are evident around diapirs piercing the(paleo)seabed.

Another alternative may be that the mounds representout-runner blocks from a large submarine mass failure.Such mass failures are widely known from this portionof the Norwegian margin (Bugge 1983; Laberg &Vorren 2000; Bryn et al. 2003; Evans et al. 2005). Whilethe shape and size of the mounds may be compatiblewith such an explanation, other evidence is less so. Themounds represent only the surficial morphology of awidespread sediment package (unit B), and it is hard toreconcile a mass-movement origin with the lateraldistribution of unit B, i.e. the thickness maximumassociated with the mounds occurs on the sloping flankof the Nyk High. If it was a gravity flow deposit, wewould expect the thickness maximum to be centred inthe basinal areas. However, the thickness maximum inthe basin is not associated with any mounds. Somemounds also occur on the western flank of the NykHigh (Fig. 1C), a location hard to reconcile with agravitational mass movement originating from thecontinental slope to the east.

The buried mounds may represent bioherms i.e. reefbuild-ups formed by corals or large communities ofsponges. This alternative accommodates the lack ofobserved subsurface fluid/gas migration paths,disruption of strata and/or acoustic blanking in thearea of the mounds. The spatially constrictedoccurrence of the mounds makes this an attractivehypothesis. In addition, the observed low acousticvelocity ‘push-down’ or loading-induced compactionbeneath some mounds can be accounted for by thishypothesis. The greater thickness of sediments withinthe mounds can explain the relatively highercompaction of sediments beneath the mounds than intheir surroundings. In this case the compaction is lar-gely syn-depositional. Alternatively, the depressionsobserved beneath the mounds can be explained interms of ‘push-down’ of the reflections due to loweracoustic velocity within the mounds than in thesurrounding sediments. Low acoustic velocities havebeen inferred for modern Lophelia-mounds on theNorwegian margin, where the reef matrix consists ofwater and/or soupy, fine-grained sediments with a lowacoustic contrast to seawater (e.g. Hovland &Mortensen 1999). Furthermore, higher amplitudes inthe central part of the mounds (Fig. 4C), mirror theacoustic back-scatter reflection pattern for modernLophelia-mounds off Norway (cf. Fosså et al. in press).

The Sula Reef Complex (Freiwald et al. 2002), theFugløy Reef (Lindberg et al. in press), and the Trænareef area (Lindberg 2004) are some of the better knownpresent day cold-water reefs on the Norwegian margin.


We note that the buried mounds in this study comparefavourably to modern analogues, especially in terms ofsize to the Fugløy Reef, and in terms of parallelorientation and distribution pattern to the Træna reefarea. The (paleo-) water depths differ, but as is clearlyillustrated by the presence of Lophelia off Ireland(Rogers 1999; Roberts et al. 2003), the coral is notrestricted to certain depth intervals, but rather to watermasses with specific properties. A link between sub-surface seepage and mounds as suggested for somemodern localities (e.g. Hovland & Mortensen 1999) isnot seen for the buried mounds. In the absence ofdrilling data, we can at present only tentatively suggestthat the observed buried mounds may representLophelia-mounds of Late Miocene to Early Plioceneage. A different biogenic origin of the mounds, e.g.sponges, cannot be excluded. However, we deem thisless likely in the light of the many examples of bothliving and fossil Lophelia-mounds on the NW Europeancontinental margin as compared to other biogenicmounds of similar dimensions.

Paleoenvironment

The protrusion of the Nyk High above the surroundingseabed at the time of mound formation probablygenerated relatively strong bottom currents in itsvicinity, albeit no moats have been observed at thesame stratigraphic level as the mounds. However, thesomewhat higher acoustic amplitudes around the NykHigh (Fig. 4C) might indicate somewhat firmer groundas compared to the basinal areas where polygonalfaulting is developed. The fact that no sediments weredeposited on the upper flanks and the top of the NykHigh throughout the time of mound formation/-deposition also supports the notion of a strong bottomcurrent regime. This may also explain the lack ofmounds on the upper flanks and on top of the NykHigh. Because cold-water corals such as Lopheliapertusa are suspension feeders the most suitablecombination between food supply (nepheloid layer)and a firm substrate is likely to be found on the lowerflanks of the high. On the upper flanks and on the topof the high the substrate was likely well adapted to coralgrowth, but too little sediments were probably availablefor resuspension, limiting the food supply forsuspension feeders.

The top of the Nyk High is presently situated at some2000 ms TWT depth. Flexural backstripping(Ceramicola et al. 2005) shows shallower depths in thearea prior to the deposition of the Upper Pliocene-Pleistocene prograding wedge than the present waterdepths. This indicates that at the time of mound growththe water depth may have been up to hundreds ofmetres less, putting the mound-bearing unit within thedepth-range modern Lophelia-mounds thrive in (see

compilation in De Mol et al. 2002). Even so, the water-mass properties may have a larger influence on Lopheliacolonisation and growth than absolute water depth.

Assuming that the mounds represent Lophelia-mounds, we can infer that the water masses had atemperature and salinity in the range of 4-12 ˚C and32-35 units, respectively. This is based on the range ofenvironmental conditions of present Lophelia-mounds(cf. Hovland & Mortensen 1999). Further more, thebottom-waters were probably not highly undersaturatedwith regard to carbonate. In the light of previous pale-oceanograhic studies (Bohrmann et al. 1990; Jansen etal. 1990; Henrich & Baumann 1994) these conditionsconform well to the environment of the Norwegian Seaduring the Late Miocene Early Pliocene period.

ConclusionsBased on the interpretation of a 3D-seismic dataset wehave reached the following conclusions:1) Buried mounds which are up to 50 m high, 50-400 mwide and 600 m long occur on the flanks of the NykHigh on the Mid-Norwegian margin.2) The mounds form the surface expression of an-up-to100 ms TWT thick sediment package (unit B).Internally the mounds exhibit a relatively low acousticreflectivity. This sediment package (unit B) istentatively correlated with the Kai Formation, which isof Late Miocene to Early Pliocene age.3) Based on their acoustic appearance, spatial location,and by comparison with modern deep-sea mounds, themounds likely represent buried cold-water coral(Lophelia pertusa) mounds. The paleoenvironment atthe time of mound growth was characterised bysignificantly reduced water depths and relatively strongbottom currents.4) Assuming our interpretation that the moundsrepresent Lophelia-mounds is correct, the water massesprobably had a temperature and salinity range of 4-12 ˚Cand 32-35 units, respectively.

Acknowledgements: STATOIL kindly provided doctoral and post-doctoral fellowships to K.I.T. Dahlgren. B. Lindberg was supported byECOMOUND (Environmental Control on Mound Formation on theEuropean Margin), a project of the EC 5th Framework Programme(Contract EVK3-CT-1999-00013). The Norwegian DeepwaterProgramme's Seabed Project (JIP sponsored by Norsk Hydro, BP,Statoil, Conoco, Shell, TFE, Esso, Phillips Petroleum, RWE-DEA andPetoro) kindly made the seismic data available for academic use.Geoquest is acknowledged for providing software and support. StefanBünz and Ståle Schwenke are thanked for database and softwaremaintenance and support. The GMT software (Wessel & Smith, 1991)was used to create the map in Fig. 1. The constructive comments of thejournal reviewers Michael R. Talbot and Arve Lønøy and editor Ole J.Martinsen were much appreciated. Brian MacTiernan helpfullycorrected the language.


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