Quaternary seismic stratigraphy of the North Sea Fan: glacially-fed gravity flow aprons, hemipelagic sediments, and large submarine slides

y 1000 km of high resolution sleeve-gun array transects on the North Sza Fan, located at tbe mouth orwegian Channel, reveal three domina tyles of sedimentation within a thick umerous sequences. large scale tran s, and aprons of glaci construction. Four large, buried translational slides preceded the similarly dimensioned ’ hemipelagic deposits apparently repr indicates fan input via the Norwegian m) aprons comprising stacked lensoid and/or lobate thickness. They characterize debris fl ice stream reaching the shelf edge. glaciation cycles represented by sheet erosion-bounded tills and progradational units. uch of the shelf/slope transitions has been preserved, allowing a preliminary chronology of the fan sequences through correlation with borehole sediments in the Norwegian Channel. Debris flows, which signal the initial shelf-edge glaciation, are not ret from the initial glaciation in the Channel (> 1 .l Myr) but are associated with a Middle Pleistocene and all glacial erosion surfaces (GES) in the outer Norwegian Channel. This was followed by six further sequences, probably totalling over 13,000 km3 of sediment. At least four of these were shelf-edge ice-maximum events the last of which was Late Weichselian age (14C AMS). Considering earlier glaciation-related hemipelagic sedimentation, material since removed by the large slides, and extensive unmapped areas, total Quaternary fan sedimentation was in the vicinity of 20,000 kln3.

002%3227/96/$15.00 0 1996 ElsevierScienceB.V.All rights reserved SSDI 002%3227(95)00168-9

uction The submarine fan situated in the northeastern part of the North Sea, at the mouth of the Norwegian Channel (Fig. 1) has acted as a depocentre for Late Cenozoic sediment shed from

occurrences, the North argi sizable prograding of the Norwegian Sea, much of which has taken place in Quaternary times. The bathymetric contours

Fig. 1. B~thymctryof the oukr NorwcpianChn~mzland North Sea Fnn ( 100 no contour interval) showing seismic coverage, location of sCrate@aic gravity and piston cores (” + “) where degluciation and glaciogenic debris flow sediments were identified, location of the Troll boreholc (insert) to which outer Channel and Fan sequencesare correlated for chronologic control, and the location of geologic sL%tionsA-MY-C and D-B’--E ( Figs. 2 .md 3) as well as the seismic illustrations. Location of the major exposed and buried slide scurps are also shown, as is the buried basaltic Faeroe-Shetland Escarpment (FSE) which formed a sediment barrier during early fi%ndevelopment. (Storegga Slide from Bugge, 1983; Miller Slide from Stevenson, 1991; and FSE from Talwani ??

f Ap~roxinlately 1 reconnaissance seismic lines were ( Fig. 1 ), representing the lution data across the fan other than conventional he seismic source was an arra nstruments 40 in3 sleeve-guns fire a pressure of 120 kg/cm2 at a four se ata were received via a enthos model 2515 single channel, 50 element streamer, bandpas tcred from 70 to 700 k ms rate) and prcsentcd on an graphic recorder, These data provide upwards of one second penetration on the fan (less in the Norwegian Channel ) with a resolution of about 10 m, A small grid of older air- and watergun profiles covering a portion of the western flank, acquired in 1974,was made available by the British Geological Survey. In addition, the Geological Survey of Denmark together with the Netherlands Institute of Sea Research provided a recent seismic line extending along the fan axis. These data were shot with the same sleeve-gun array but recorded digitally via a 144 m, 24 channel streamer and presented as a filtered stack. Seismic ties with a borehole in the Norwegian Channel were aided by a grid of high resolution sparker profiles (ca, 0.5 s penetration, 5-10 m resolution) supplied by the Continental Shelf Institute (IKU).

the shelf break has

initial large and smaller pr

adational units con-

seismic character an.d bounding prominent reflectors. Numbering is from the top, as the lower limit of study is governed by attenuation of the seismic signal rather than a natural “basement”. correlation of the fan sequences with the Norwegian Channel deposits awaits further mapping in the Channel. Typically the unconformities are either translational slide planes or they occur proximal to the shelf break, associated with deltalike tops or glacial sheet-erosional surfaces. The fan correlatives are well to poorly defined reflectors which can be “traced” throughout the fan body.

Slide heodwall

Early‘Plelstocene Apron plnchout

-I---

Q0

F

Apron plnchout Apron plnchout

&

together wi& the fun&

Prs-Slide Seabed Reconstructlon

recsnstrucisd pre-slide scabcd.

within the fm are the result sf minor, unresslv tween the major sequences, or alternaoustic impedance contrast can arise from current induced, thin lag deposits located ces. Given the slope setting phic regime (currents, lags, ts), both situations are considered equally possible, Approximate extent and volume has been calculated for most sequences with the aim of comparing relative magnitude of events (Fig. 4). Volume calculations and estimates for each of the upper nine sequences are presented . 5, These are necessarily crude, given the ions of data covera and in some cases estimates are provided where significant volumes fall outside the survey area. The echo-sounder/ morphology map of Damuth ( 19X& for example, tars

@

indicates that a seabed morphology typical of the upper fan sequences locally extends well beyond the survey limits to 2500 m water depth and as far as 2’W. The seismic sequences of the fan body are interpreted to comprise three major depositional styles, each displaying a unique seismic character. These are ( 1) thick, even blankets with a structureless to slightly laminated character, often mimicking underlying topography and interpreted as Terrigenous Hemipelagic deposits, (2) shelf-edge and slope aprons comprising constructional, stacked lensoid (in cross-section), conduit ar!d/or lobe deposits displaying structureless internal seismic character (also on penetration echosounder profiles) and interpreted to have formed through gravity flow, and termed Debris Flow Aprons, and (3) nearly continuous blankets,

T

en Sli

internally structureless with smooth lower and highly undulating upper surfaces interpreted as partially or entirely Disturbed Slide Sediments on a relatively flat-lying glideplane associated with high, steep head and sidewall scarps, similar in most respects to the exposed Storegga Slides (Bugge et al., 1987) immediately northeast of the fan. 3. I. Terrigenoushemipehgic deposits

Hemipelagic deposits, represented by seismic Sequences 5, 8, and 10, display a moderately transparent, often structureless internal character (Fig. 6). Occasional discontinuous, coherent reflections are generally more common in the prox-

Ie=--Western ^.

store 30 -1

llmlt of Slorfaggo~

1

Q - 1 Glide plene

I

imal fan areas while out near t reflectors are entirely absent. remarkably uniform in thickn tances in a proximal to distal sense, however they thin gradually towards the east. espite a thickness reaching nearly 100 m, the upper surface of the hemipelagic sequences mimic top over which they were deposited. obvious in the case of rough, u of the disturbed slide sediments. This draping character is evidence of sedimentation from suspension where current activity was weak. Greater coherency of internal reflectors in the proximal part of the fan, together with lesser topo mimicking may reflect greater variability in the near-shelf sedimentation environment. The distri-

62’

63

62’

Sequence 7: Debris Flow Apron

20-40

Sequence 6: l’ampen Slide, Parental

Sequence 8: Terrigenoue-Hemlpslagic

in at iBOO m/s

Th$ic;eei

and Disturbed

.-

k

0

Volume estimnte includtnQthat

S

243 Volume’present in survey erea

Volumes in km’

6

4800*

Volumeswithin tht? the outerlimit of uences9 and 6, Mste and Tampa Slides n the construction surfacesshownin of slidfWansportcdsedimentate evrtntsnortheastof the Fan. Tha: Squtxces 7, 16, and 4 naally deposit&between1000and 200 km3on the Fan.

bution of the l~we~~st hemipelagic sequence 10) is difficult to assess as it occurs near the tration, Thou y considerably larger than Sequence 8. Fig. IB shows approximate distribution and thickness of

Sequence 8 Its western a steep sliffe sidewall a NE, and probably thins c reaches the Faeroe/Shetland centre lies slightly east of t between 120 and 140 m thick over most of the fan body, and the bulk of its volume (nearly 1500 lies within the survey area. Seismic Sequence 5 is the youngest hemipelagic unit of significant thickness (Fig. 4E). Its depocentre lies central fan axis and its western boundary is als by a slide sidewal s 400 km3, it represents a ~~~~~1~ influx than its older counterpart. distributior pattern indicates a source fro Norwegian Cannel, yet the control exerted by the ” of slide scarps may reflect a degree of “tra contour current-transported sediment. The presence of an uppermost, relatively uniform, suticial hemipelagic blanketing unit has been demonstrated in numerous cores from the fan (Jansen et al., 1983; Sejrup et al., 1984; Karpuz and Jansen, 1992) but with a thick usually under 3 m, it is not registered on seismic profiles and it is not included in the seismtc sequence designations. The blanket thins gradually to only centimetres in 2000 m water depth and comprises a fairly co s reword of the last aciation, be inning immediately after the last ial maximu . A thin, overlying blanket of olocene sediments with a high biogenic cornpoW nent is typically 10 to 30 cm thick. As total Holocene thickness is negligible in comparison with the thick hemipelagic sequences (Seqs. 5, 8, and lo), then there is simply not the time available for these hemipelagic sediments to have deposited under analogous interglacial conditions. Even under depositional conditions corresponding to the suticial deglacial blanket, Sequence 5 (last main hemipelagic phase) would require lOO-150 kyr of continuous input and Sequence 8 in the vicinity of 200 kyr. Sejrup et al. (1995) recognized -lived Early to Middle Pleistocene nonglacial depositional phase in the Norwegian Channel at the Troll production field and one of these hemipelagic episodes may be its fan correlative.

e~~es~~tative seismic profile in dip direction showing the character of the various sequences and their bounding reflectors ig. 1). The debris lenses are not particularly discernible in this direction in comparison to the strike direction (Fig. 8). Local slumping is evident in Sequence 3. Note the apparent lack of significant erosion on the smooth glide planes.

The North Sea, an can, as note

divided into a probable pre-glacial and a dominantly glacial accumulation phase. It is the debris flow aprons which appear to represent the shelfedge glacial events, The earliest examples of these aprons are buried about 600 m (350 m in the distal fan). This glacial-related style of deposition thereafter constituted approximately 80% of the fan deposition, represented by five of the seven uppermost seismic sequences (Seqs. l-4 and 7). The debris flow apron sedimentary style is best exhibited in Sequence 1, partly due to enhanced seismic resolution at the top, but the characteristic pattern is also easily recognized in the deeply buried sequences. The apron is a broad depositional sheet, generally of an even thickness in the strike (shelf-edge parallel) direction, but exhibiting a basinward

or, at its distal end, only is too sparse to establi

( 1978) on echosounder profiles and interpreted as gravity-controlled mass flows. heir geometry in the North Sea Fan area is not yet well understood. An E-W seismic transect (Fig. 3) shows that these lenses often have strong top and basal (single event) reflectors and an internal homogeneous, incoherent reflection character. The lenses vary greatly in size, with apparent widths (on strike lines) from 2 to 40 km and thicknesses up to 60 m. The larger of these are composite forms and the “unit” lens is more typically 5 km across with a thickness between 15 and 30 m. They have a

concave base and convex upper surface imparting a constructional form. An internally structureless body and only minor surficial micro-relief preclude significant “brittle” remoulding (e.g. block flow, cks and shear), typical of slide failure oleman, 1979) and they are interpreted to form more from flowage than slab-type translation. The lenses are usually stacked such that succeeding lenses are centrally situated at the lowest topographic osition between two preexisting lenses. In some cases a strong upper and lower reflector define a large, generally lensoid body which comprises several weaker and discontinuous reflectors. Another feature of the lenses is occasional basal indentations, presumably troughs, indicating the possibility of minor erosion at the base. Also of note are occasional minor indentations on the upper surface of the lenses which may have a trough-like geometry. Both the basal r troughs are more common in the smaller lenses, located in deeper water depths. Preliminary investigation of piston/gravity cores indicates that the lenses are comprised of extremely eous, poorly sorted sandy mud with 38% silt, and 28% sand and usually less than ravel, includin rare pebbles of land and re (Mesozoic) erivation. It is remarkably consistent in texture, both with d

WKW 7 represents the first clear debris flow ase. Individual lenses are at ter, developed than the other sequences (Fig. 3). The isopach map (Fig. K) shows a clear depocentre at the immediate mouth of the Norwegian Channel. ickness reaches 150 m but thins to less than 20 . Qne interesting feature is to the northwest, possibly 8 separate depositional event or routing 8, Volume (Fig. 5) slightly exceeds the fan body. This phase was en Slide event and subse-

quent hemipel a multi-phase

stacked lenses, usu also deposited ten

Slide sidewall. Sm the proximal reaches of the sequence. veneers extend o

the slide scar (western fan flank) where the sequence appears more uniform (less lensoid), perhaps reflecting a greater degree of normal suspension settling. Sequences 2 and 3 are differentiated primarily on the basis of a nearly continuous re develops into a marked glacial erosion surface with an angular unconformity in t Norwegian Channel. This horizon can be followed clearly in the dip direction (Fig. 6), but the complex stacking nd discontinuity of lens reflectors in the strike ( --IV ) direction precludes de correlation without a tighter seismic grid. these two sequences were considered in combina.4C, Compared to the other debris flow sequences, the lensoid character in these is not as clear. Qccasional individual lenses are well developed, yet most reflectors lack continuity across both the base and top. Together the two sequences blanket most of the fan, with over 180 m, and locally over 220 m, thick deposits. It thins gradually basinward, but quite abruptly on the eastern fan flank. Together the sequences make up over 3000 km3, plus a considerable unmapped amount north of the FSE and west of the main fan body (probably totalling 4000 km3). While the eastward thinning may be a primary depositional attribute, considerable transport and disturbance of these sequences is associated with the first and largest Storegga slide (Fig. 3). A minimum age of ca. 30 kyr B.P. was proposed (extrapolated below l*S curve, Jansen et al., 1987) for the “First Storegga

i et al. reason that a at the immediate shelf break (up to 40 m) and has a very home eneous internal character. continuity of the apron as a whole t marginal moruinic build-up at the Channel mouth (Figs. 2 and 3) suggests that the bulk of the material was deposited from the latest ice sheet as the last major depositional event. Total volume of Sequence 1 in the survey area is about 700 km3, while minor additional amounts must be present, especially to the west (total less than 1000 km3)* In summary, the lensoid deposits are interpreted as debris flows, probably initiating at the shelf break and comprising the “building blocks” of the aprons. Similar lensoid features were described in a comparable glaciated fan setting on the Island Trough Mouth Fan in the ‘Barents Sea (Vogt et al., 1993) and were likewise interpreted as debris flows. Laberg and Vorren (1995) identified a digital pattern to these flows and associated them with periodic build-up and failure during

aprons, one comprising at least 3 ca. 1000 m water depth). Volumes o debris flow sequences probably rea but the last and smallest is about half Future field work will help define geometry and relationships of both the aprons and lensoid features at which time further speculation on their genesis will be more fruitful. 3.3. Disturbed sedimerzts: large scale translational mass-movement

The North Sea Fan is flanked in the east by one of the largest known exposed submarine slides, the

Storegga Slide (Bugge, 1983; Bugge et al., 1987) and in the west by the Miller Slide (Long and Bone, 1990;Stevenson, 1991) ( Fig. 1). Four buried slide events have been identified from the recently collected seismic data. These are designated North Fan slide-l (NSFS-1 ), Vigra Slide ( Evans et al., 1996this issue), Marre Slide, and Tampen Slide in chronological order. The three subsequent Storegga Slide events are designated Storegga-1, -2 and -3. The North Sea Fan slides are comparable to the Storegga Slide in size, indicating that this phenomenon is a volumetrically important mechanism for mass transport of terrigenous sediment to the deep sea. The earliest slide (NSFS-1) occurs at such depth (700-1000 ni below seabed) that it cannot be well mapped with our data and is identified primarily through analogy of its rough, disturbed surface and homogeneous acoustic character, with the overlying slide events. Limited coverage of conventional seismic profiles indicates that this may have been a much larger event than the following ones. 8% all the slides, the disturbed portion is thickest in NSFS=l (Seq. 1 1), but a volume measurement WM mt possible. The outermost NE-SW seismic

traverse revealed soft sedime at great depth (max. relief o has affected sediments a Sequence 2. This activity c tive affect on the glide pl overlying Vigra Slide is unknown, but tance (several tens of km), large scale ( 10 100 m thick), low angle (0.5”) block sliding is evident (Evans et al., 1996~th The two overlying slides ( exhibit high and steep headsharp lower in ctions. In a seaw failure planes develop into s (0.5”) expansive d&colleme plane horizons across w disturbed sediments ha The sediment remaini highly undulatory up exceeding 40 m. The strata that remained intact (behind the head- and sidewalls) are distinguished from the disturbed sediments, though both are assigned to one seismic sequence, and th referred to here as “parental” material. illustrates the headwall of the Fig, 8 shows both the Msre and Tampen sidewalls.

Ifmedge,Moralnlc mound \

Prsgradlng Unlta \ \

ie profilein the No an Channelto North Sea Fan transitionarea (location, Fig. I) showing the outer shelf Glacial 8 (GES-A through E), boundingtill units, the till delta (illustratedin Fig. 9), and the progradingunits, transitional tly sloping fan sequences.Sequences 1 through4 and 7 comprise knsoid debris flows (illustrated in Fig. 8). Occasional ons in the prograding units indicate small scale failures. The Tampen SIide headwall and subsequent draping are as is a possible candidate for the Msre Slide headwall.

Fig. 8. E-W seismic profile showing Tampen and disturbed/translated sediment. The debris

ore Slide sidewalls and the near vertical transition ed. (Location.

ble exception of the easter ide, the entire s nt removal portion of the slides; that is, an assumed pre-slide ~e~o~str~~tio~s lies above the to of the remainin sediments. No associated netdepositional areas or lobes are recognized and it is assumed that these sediments lie the FSE, The Store ga events cut the glide and disturbed sedi nts of the Tam the full original eastward extent

from parent to thick

re~ollstru~tioil as

esent a complex involvi Slide does not have a readily identifiable headwall, probably because of its position below the seabed multiple on the seismic profiles. Its western sidewall is upwards of 200 m high (Fig. 8), yet identified on the seismic profile more by near vertical truncation of strata (greater than spatial resolution of about 30”) than by an actual slide surface. Some of this relief may be due to an “overbank” build-up. A partial eastern sidewall near the eastern extremity of Fig. 3 illustrates that the glide plane “hopped” from one major stratigraphic horizon to a much higher one on the eastern fan margin. Fig. 4A shows the position of the slide head- and sidewall together with disturbed

and disturbed portion (Seq. 6) is thinner, but this may reflect more efficient transport and removal from the fan. This sequence thins gradually basinward, but it is not clear if this reflects more complete removal or an original depositional thinning, A steep and deep sidewall is recognized well north of the top of the FSE on the western fan. Also, the characteristic rough slide topography is recognizable at this horizon on an interpretation of a deep seismic line (Sundvor et al., 1974, thei section B2-C4) in the eastern flank of the where it is shown to extend north to the

Thus, major sediment removal occurred all across the fan and north to the FSE in the Tampen Slide event, Within the survey area, great thicknesses of the undisturbed parental material of Sequence 6 remain, and probably continue well to the west of survey coverage. This part displays occasional ionally continuous reflectors with intervening terial possessing a more chaotic pattern. Some lens&i style is recognizable, but much of it exhibits only short, discontinuous reflectors. Thus, the original Tampen Slide material, like that of the More Slide, appears to be a more complex assembly of both gravity flow and hemipelagic deposits. Disturbed sediment remaining within the fan body measures 1000 km3, but with a reconstructed preslide surface as shown in Figs. 2 and 3, 2500 km3 can have been removed to the ocean basin (Fig. 5). Correlation with shelf units indicates, as discussed later, that this slide event occurred well into the Middle Pleistocene. Subsequent, much smaller scale, yet considerable (ie. 25 m failure scarp) mass failure is also observed locally within Sequence 1, but these exhibit a more rotational slide character with no clear translational plane. At the extreme eastern end of the E-W profile .3) the positions of two of the Storegga slide sidewalls are shown (see also F 1). The first and ly constructional uch of its western edge, especially in the r portions, Th disturbance effect of this slide nulation and destruction of most of ic strata throu out the entire 150 m thickness of Sequences 2 and 3, A N-S seismic profile along the western Storegga-1 slide scar also shows complete disturbance above the glideplane. h headwall cutting into both Sequences as also observed on one profile, The lide plane occurs on top of the disturbed Tampen eposits (at the Seq. 3/Seq. 6 contact) and its atypically smooth upper surface probably resulted from some erosion during Storegga-1 overriding, The sidewall of the Stor -2 and/or -3 events cut both the Store material and well pen Slide material. Thus in this part of the eastern North Sea Fan nearly the entire upper 400 m of the section comprise

disturbed material of stacke Storegga slide events. The entire glide plane of bot Tampen Slides, cover areas of within the survey area alone, an stratigraphic horizons across long distances. Apart from the head- and sidewalls no erosion is apparent. Neither is there any gas shadowing, or gas hydrates recognized on the szis anywhere on the fan. Perhaps these glideplane horizons initi water content or provi which allowe upon deformation/dewat motion. A “tra explain the long translational distances of this type of slide. Lack of coherent internal reflectors is consistent with its interpreted disturbed or remoulded genesis, yet the rough upper surface results in numerous hyperbolae which may mask weak internal structure. Local zones of build-up and increased relief contrast with neighbouring smoother, thin sectors possibly reflecting tensional versus compressional flow regimes. A “plug flow” type of slide mechanism with a thin deforming zone or plane beneath a relatively non-deforming body in motion is commonly envisaged for translational slides (cf. Norem et al., 1990). failure include interstitial gas, gas hydrates, porewater overpressures, wave loading, sedimentational and tectonic oversteepening and earthquake loading (Hampton et al., 1978). Though gas enhanced seismic horizons and gas blanki common in the Channel, there z~re,as stir gas features observed on the fan. Likewise, water depths at the headwalls are too great for appreciable wave loading. The greatest loading by sedimentation is clearly associated with the debris flows of the glacial maxima (shelf-edge grounding conditions) yet many sequences, including the latter few, remain intact despite subsequent failure of the non-debris flow deposits associated with the Storegga Slide (Evans et al., 1996-this issue) The absolute timing of the fan slides is vague, but immediately ensuing hemipelagic sedimentation sts that they occurred after glacial maxima. This holds for the Storegga slides also. Perhaps

ungum et all., 1991)

utions of the fan sequences show that the llal~i~e1was a conduit for rnuc ajor advances are presently bein made in understanding the ocesses and chrono of the thick sediment ence in the Cl~a~~i~e ng cl al., 1995; Sejru al., 1995, in press). The dominantly flat-lying sediment package overlying a marked seismically defined angular uncon-1, equivalent of Level A of ngsland, 1983), present over all but the outermost Norwegian Channel, has been shown to comprise Quaternary age sediments (Sejrup et al., 1995). This confirmed the inference by Sellevoll and Sundvor ( 1973) from seismic alone and corroborates findings from foraminiferal analysis of well cuttings from several petroleum exploration wells in the outer Norwegian Channel and More Plateau by Eidvin et al. ( 1991) and Eidvin and Riis ( 1992). Despite quite active shelf-edge slide failure many of the continental shelf equivalents to the seismic sequences in the fan body have been preserved in the outer Norwegian Channel. This presents a

e interpret them as ome of the ~vid~l~~~for this comes from correlation of outer shelf units and horizcns with a orehole at the field barehole, west of insert). Other evident project of mapping multiple ma units from a tight seismic grid in the northern Norwegian Channel ( g et al., 1995). The geometry of these surfaces ng et al., 1993, 1995,and work in progress) is reminiscent of large scale glacial fluting (channel-parallel orientation), similar in scale to “mega-scale glacial lineations” discovered from Landsat images in Canada (Clark, 1994). On a larger scale, broad “U”-shaped moulding of the Norwegian Channel e cates a dominant northerly ice flow direction

the exception of the fjords of western Norway and to some degree a marginal trough on the eastern side of the Channel, clear overdeepening is only evident in the Skagerrak, over 500 km up-ice from the shelf margin. Three of the shelf-edge units bounded by a GES in the outer Channel are intersected at the Troll borehole. This provides both lithologic and chronologic control for the Channel seismostratigraphy, Hiatus-bounded units with this seismic character were found to be comprised of glacial diamict, most likely deposited sub-glacially (Andersen et al., 1995; Sejrup et al., 1995). Units identified as till in the borehole display a constructional geometry (morainic mounds and ridges) across the Channel floor, a phenomenon displayed at numerous stratigraphic horizons. Corresponding to the hiatus horizons in the borehole are the GES reflectors on the seismic grid. Seismic correlations between the Troll borehole and the shelf edge are complicated by local erosional bevelling of younger unconformities down to older horizons and can only be overcome by detailed mapping (in progress) of the various units and correlation of “dated” units, rather than unconformable surfaces alone. It should be noted that most of the seismic correlations and relationships in the outer Channel are intricate enough would not be ima d on conventional R-l reflector, for smic profiles and t could easily be overlooked for later,

tots R-1 and the * 1.1 Myr Lit rup et al. ( 1995). A thick, continuous glacioand marine deposit probably spanning thousand years overlies the till n the Early to Middle Pleistocene t al., 1995). It is present in the outer Channel but only east of the Fig, 2, A-B profile. The outer shelf unit overlying GES-A correlates directly (seismicall with the Middle Pleistocene L4 till, and the up till, above GES-E corresponds to Sejrup et al,% L.2, V!eichselage till Weiehsel in upper part). It is via this corr&tion that tht: chronology established at Troll (Sejrup et al., 1995) is linked with the fan sequences.

The shelf-to-fan transition from t flow aprons is usually relatively (seismically) with no significant inter change. The physical properties are similar as the tills are believed t sourced the debris flows and both depositional processes involve homogenization. This contrasts with a less common shelf-break phenomenon where a relatively steep (up facies, also truncated present. Two of these pa slumping (deposition on ing packages appear t subglacially-fed sediment to a sharply steepened by failure at the shelf break. A much smaller scale prograding feature is present at the distal end of the uppermost debris flow apron (Fig. 9). apron represents the last phase of Sequence 1 shelf-edge glacier related slope sedimentation. The progradational features, both large and small, resemble a form of “‘till deltas”, described by Alley and his colleagues (e.g. Alley et al., 1989) to form in a decoupling zone, roximal to the grounding line, below an Anta tic ice stream concurrent with proximal and “topset” sheets of laterally and vertically aggrading “deforming till”. The delta provides a platform over which the er advance. t is not clear roundi~~glick can if both the large and small delta forms seen here represent till deltas sensu stricti where till and progradation are la ly coeval [such as those nized by Anderson and Bartek ( 1992) in the Ross Sea and termed sub-glacial deltas]. The pertinent question is whether or not the material below the recognized erosion surfaces at their outermost margin is associated with the earlier, advance phase of that glaciation or if it originates from a previous glacial phase. (In an up-ice direction the GES clearly separates two glacial phases.) In both cases the prograding foresets are blanketed with till from the maximum and/or retreat phase. Erosion along the GES can have been either penecontemporaneous with progradation (a till delta) or the GES can represent a higher order erosion event separating entirely different glacial phases. The former case may have parallels with the work of Ring (1993) on the mid Norwegian

U

Fig. 9. Processed sleeve-gun profile across possible till delta at distal margin of the latest apron (Late Weichsel age) of Sequence 1.

deposits and sheet erosion surfaces in the Pleistocene, shows that multiple glaciations in the form of ice streams constrained by the Norwegian Channel have reached the shelf edge and contributed to major fan buildup. The recognition that shelf-edge tills correlate with the debris flow aprons helps in discerning the frequency, duration, and magnitude of glacial phases as well as the relative magnitude of shelf-edge glaciation as opposed to other sedimentation processes which build the fan. It also places some constraints on the possibilities of debris flow sediment type and processes of initiation and flow. nalogues to the Norwegian Channel in terms of a trough-mouth setting, $acial erosion-bounded multiple tills, and/or glacially derived, truncated prograding deltas and shelf-edge delta-fan complexes are found in the Ross Sea (e.g. Alonso et al,, 1992; Anderson and Bartek, 1992), Prydz Bay, Antarctica (e.g. Hambrey et al., 1992), the Crary Fan (Kuvaas and Kristoffersen, 1991; Bart et al., 1994) the Bear Island Trough (e.g. Vorren et al., 1989).

The r*,1.1 Myr L6 till unit ( Fedje Glaciation), directly on e R-l unconformity at Troll p et al,, 199 continues nearly to the shelf break. Thus deposits above the Rml horizon repret a minimum Quaternary thickness near the rther out on the r horizons are locally surface which limited icate can be a major slide us, at least the upper ten sequences on the fan post-date the w 1.1 Myr R-l horizon, which, including the since-removed slide-transported portions, is well over 1000 m. There is no particular uniqueness on the upper slope to either the R-l reflector or its bounding unik It remains unclear how much of the prograduence below R-l in the outer Channel is of Likewise, how much pre-1 Myr entation occurred on the Fan is unknown, but clearly what remains today is much less than deposited during the Middle and Late Pleistocene.

The lowermost shelf-edge glacial erosio (GES-A, Fig. 2) displays str Channel. It cuts the Early to marine rrnit (intersected at Channel mouth area. Thus GES-A is not older than Middle Pleistocene. GES-A and its over till represent the first major glaciation reac shelf edge after the m 1.l Myr Fedje Glaciation. It is followed by a minimum of four later major shelf-edge erosion and till deposition cycles. proposed Middle Pleistocene age fo e ac~e~ltuat~o laciations at t like R-l, has been truncated at its outer margil; , Fig. 2) but it is (seaward equivalent of unclear if this was through extensive glacial erosion or massive sliding (Msre Slide?). Apparently any fan equivalents of this lie below Seq. 8 or 9 but no debris flow sequence is recognized. The earliest recognized debris lobe apron (Seq. 7) is not easily linked to its shelf equivalent. However, it is best correlated with one of the earlier glacial erosional surfaces, most likely GES-B. Chronologically, Seq. 7 is then placed well into the Middle Pleistocene as the shelf correlation shows it was preceded by the Middle Pleistocene rup et al., 1995) and marine unit (L5 at Troll, at least one major till unit. as postu~~~ted,debris fiow scque;jces signal exclusively shelf edge glaciation, this indicates that any glacially related fan deposition previous to this was either since removed (by massive sliding) or related to ice sheets which did not develop enough to reach their maximum shelf-ed The latest major fan slide event (Tampen Slide), is also difficult to tie stratigraphically to the Channel tills and GES’s. This is because the crosscutting relationships at the top of the headwall are not fully resolved and because of the possibility of later, retrogressive failure. The Tampen event headwall can have cut the second last till unit in the outer Channel suggesting a late Middle Pleistocene or possibly younger age. However, given a Middle Pleistocene age for the preceding sequence and the thickness and periodicity of all the subsequent fan sequences (including the inferred long-lived hemipelagic sedimentation of

and then a less extensive rea

over 1000 m of

scenarto.

The pronounced shift in the outer Norwegian Channel from progradation to a vertical aggradation pattern has been shown to result from the onset of direct glacial deposition (till). The glacial erosion surfaces associated with each of the glaciations occur progressively deeper with age and demonstrate increasing deflection (seaward dip) from the presumably original flat-lying aspect. Farther south in the Channel there was no “accommodation space” (Sejrup et al., in press) so the numerous glacial erosion surfaces all bevel down on to one “multi-generation Glacial Erosion Surface” (Fig. 2). In the context of a fluvial origin for R-l, Rokoengen and Ronningsland ( 1983) reasoned a tectonic subsidence in the coastal zone of western Norway to explain its persistent eastern dip. This

A summary of the sequence of events and approximate chronology on the outer Norwegian Channel and North Sea Fan includes: ( 1) Early Pleistocene (pre-1 .1 yr) shelf progradational build-out of an undetermined amount. Likely includes significant coastal glaciation whose associated ocean-bound ice rafted debris was periodically significant (Jansen and Sjoholm, 1991; Henrich and Baumann, 1994). (2) First shelf glaciation for which shelf sedimentary record is preserved: “Fedje Glaciation” ( N 1.l Myr) with major erosion of the Norwegian Channel (forming R-l surface) and till deposition to near the paleo-shelf break. Progradation on the

proximal fan but no recognized debris flow stacking. (3) Large scale sliding event including NSFS- 1 (sq*

11)

Longlived, mainly marine sedimentation in the Norwegian Channel (spanning several glaciation cycles (Sejrup et al., 1995). No direct correlation with Fan sequences because of later, largescale sliding. Fan Sequence 10 is youngest possible equivalent. (5) Middle Pleistocene major glaciation (extensive erosion) to shelf-edge associated with GES-A but fan correlatives unknown because of later truncation ( Mare Slide?). (6) Middle Pleistocene glaciation associated with GES-B closely preceded or followed by More Slide event (Seq. 9) and subsequent hemipelagites (Seq. 8). (7) Qldest existing debris flow deposition on fan (Scq, 7) apparently associated with GES-B (Middle Pleistocene), or later glaciation. (8) Tampen Slide event (Seq. 6) in the late? Middle Pleistocene and subsequent longlived hemiite sedimentation (Seq. 5). Debris flows associated with several separate cial phases including the Late Weichselian. ssible two-stage Late Weichselian advance to shelf break, (4)

resolution ( 10 m) air gun seismic large Quatemary sediment volorth Sea Fan that are derived largely direct shelf-eoge glacial feeding and more indirect, thou h glacially related, us sequences on the fan are thought to comprise three main elements: ( 1) ~e~rigenous-HemipelagicSequences: Distribution within the fan indicates derivation through the Norwegian Channel gateway. Their thickness, in comparison to the Late Weichst31 1 blanket, indicates long depositionai hods, upwards of 200 kyr. They are probably with low sea level and glaciations which did not reach the shelf ed comprising stacked lemoid andfor (2)

lobate Debris Flows:

ive sequences of sea

thinning aprons comprise stat deposits. They contribute great1 tion, both laterally and vertically, r ness up to 700 m and comprising volume since their first occurrence. (3) Large Translational Slides: Four mass ment events are characterized by head- and sidewalls exceeding 100 m, planar glide planes coverin over 15,000km2 beneath thick (> 100 m), uneven, transported and remoulded blankets. They are probably triggered over 3000 km3 younger than 1 predates the Pleistocene event the Storegga Slides (last interstadial and Holocene), but post-dates signals of initial shelfedge glaciation, and occurred much later in the Middle Pleistocene. Several cycles of glaciation are recorded in the mouth of the Norwegian Channel, each with a thick, glacial unconformity bounded, vertical aggradation of sediment (believed to rise mainly of till) reaching the shelf break. The larger glacial events are thought to have developed fastflowing ice streams which both eroded and were steered by the Norwe ian Channel. The debris flow sequences on the fan are associated with glaciations which reached the shelf edge. This type of deposit in a fan/slope setting is apparently a signal of ice maximum events, A maximum age for the first debris flows is Middle Pleistocene and its distribution indicates that the Norwegian Channel functioned as a major conduit for ice, even at this early stage. The latest flow sequence was deposited during the Late Weichselian maximum persisting nearly to 15 kyr B.P. Genesis of the stacked debris flow lenses, in terms of initiation mechanism, flow paths, flow rates, unit flow size, and termination geometry, remains to be investigated. A minimum estimate of Quaternary sediment volume deposited on the fan (Seqs 9 to 1) is 13,000km3. Considering that extensive portions of the fan are unmapped, the total volume could be in the vicinity of 20,000 km3. Estimates of relative magnitude of glaciatious are as yet tenu-

seismic

stratig

1992.Cenozoic glacial history

basin via large slide events.

ta were co~lecte and the Capta m securing the success of the cruise. olar Institute supplied most of ment for seismic p K. Nilsen duly kept it and the sleeveThe Geological Survey of The Netherlands Institute provided the processed seismic section interpreted he gravity core utilized for dating was collected by Hans Schrader as part of the POC (Predicting Ocean Climate) programme. The manuscript was critically reviewed by LB. Anderson and N.H. Kenyon and their comments were helpful and constructive. This work was funded by the European Commission MAST II programme, contract MAS2-CT93-0064, under the auspices of ENAM (European North Atlantic Margin; sediment pathways, processes and fluxes) and by the Research Council of Norway (Norges forskningsrtfd). The support of these people and institutions is gratefully acknowledged.

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2, A high-resolution

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