The Pleistocene to recent geological development

of the Ormen Lange area

K. Berga, A. Solheimb,c,*, P. Bryna

aNorsk Hydro ASA, N-0246 Oslo, NorwaybInternational Centre for Geohazards, Norwegian Geotechnical Institute, P.O. Box 3930, Ullevaal Stadion, N-0806 Oslo, Norway

cUniversity Centre in Svalbard, P.O. Box 156, N-9171 Longyearbyen, Norway

Received 11 February 2004; received in revised form 30 June 2004; accepted 15 October 2004

Abstract

A large volume of geological, geophysical and geotechnical data have been acquired over the last decade in order to perform a safe

development of the Ormen Lange gas field, which is located in the scar of the Holocene Storegga Slide, the last in a series of large slides in

this region over the last 500 ky. Based on high resolution seismic data and a series of geotechnical borings, a unified Pleistocene stratigraphy

has been established for the area. Two main modes of deposition prevailed in the Pleistocene. During periods of peak glaciation, when

glaciers extended to the shelf break, basal tills were deposited on the shelf, and glacial debris flows on the continental slope. During the much

longer periods of reduced ice cover, including interglacial periods, normal marine and distal glacial marine deposition prevailed, partly

developed as contourites on the slope. The resulting two sediment types are unsorted glacial diamictons, and fine grained, partly laminated

sediments, respectively. These display distinctly different sedimentological and geotechnical properties, and the failure planes for all the

major slides are found in the fine grained marine deposits.

q 2005 Elsevier Ltd. All rights reserved.

Keywords: Norwegian sea; Pliocene; Quaternary; Glaciomarine deposits; Glacial deposits; Submarine landslides

1. Introduction

Development of the Ormen Lange gas field, located in

the scar of the Holocene Storegga Slide (Fig. 1), has

necessitated intensive geological, geotechnical, and geo-

physical studies of the entire region. The aim has been to

obtain a best possible understanding of the geology of the

area and geological processes responsible for the large scale

sliding. The Storegga Slide is only the last in a series of

large slides, which have occurred in this area during the last

0.5 My (Solheim et al., 2005).

0264-8172/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.marpetgeo.2004.10.009

* Corresponding author. Address: International Centre for Geohazards,

Norwegian Geotechnical Institute, P.O. Box 3930, Ullevaal Stadion,

N-0806 Oslo, Norway.

E-mail address: anders.solheim@ngi.no (A. Solheim).

The objectives of the present study are to:

†

Establish a robust stratigraphy for the Ormen Lange area,

by tying borehole information, including chronology to

the seismic stratigraphy.

†

Tie the Ormen Lange stratigraphy to the Fennoscandian

glacial history.

†

Identify characteristic sedimentological features in the

strata critical for the large slides of the region.

†

Identify the depositional environments and the processes

which were active and important in the Ormen Lange

area during the Pleistocene.

The interpretations presented in this study are mainly

based on high resolution seismic data and shallow borings.

In addition, both 2D and 3D exploration seismic data have

assisted the interpretations. The extensive seismic data base

suffered from a general lack of ‘ground truth’, however,

Marine and Petroleum Geology 22 (2005) 45–56

www.elsevier.com/locate/marpetgeo

Fig. 1. Map of the Storegga Slide area, with the Ormen Lange Field shown in yellow. The red and green dots in the northern part of the slide are boring sites NF

and 28, respectively. Other boring sites are shown in Fig. 2. NT in location map marks the Norwegian Trench, which is the deep area around southern Norway.

Buadjupet and Onadjupet are local transverse shelf troughs directed toward the Storegga Slide.

K. Berg et al. / Marine and Petroleum Geology 22 (2005) 45–5646

until 13 geological/geotechnical boreholes were drilled

during the Ormen Lange Project (Tjelta et al., 2002).

The boreholes were from 30 to 465 m deep (Fig. 2) and

provided new insight into the Plio-Pleistocene geology of

this region. Highly targeted coring was performed during

the geotechnical borings, which primarily aimed at key

horizons and units. As a result, the overall core recovery in

these borings is low (10–20%) (Fig. 2B), and wireline logs

have been of great importance. Extensive work has also

been carried out to correlate the local and regional geology,

Fig. 2. (A) Map showing the inner part of the Storegga Slide scar, with the Ormen

referred to as geoborings. The piezometer measurements at sites 19_2, 20 and 22 ar

shown in Fig. 1. Location of seismic line in figure 4 is shown by the heavy line in t

relative to the seismic stratigraphy of Ormen Lange. The black areas in each col

as studied to the south, east and north of the Ormen Lange

area proper (Hjelstuen et al., 2005; Nygard et al., 2005; Rise

et al., 2005).

An extensive geological, geotechnical and geophysical

database is available, and the present paper is a summary of

the current knowledge of the Ormen Lange area, much of

which has only been available in confidential reports. The

results reported here have been important for the risk

assessment regarding submarine slides in relation to the

field development at Ormen Lange. The result, that

Lange reservoir outlined in orange. The geotechnical/geological borings are

e described by Strout and Tjelta (2005). Note that boring sites NF and 28 are

he upper part of the map. (B) The geotechnical borings shown schematically

umn mark core recovery.

K. Berg et al. / Marine and Petroleum Geology 22 (2005) 45–56 47

the stability is high in the area and that field development

activities will not affect this situation (Kvalstad et al., 2002,

2005; Bryn et al., 2002), is founded in the geological

development model presented in this paper.

2. Physiographic setting

2.1. Glaciations

Major glaciations of Fennoscandia began around 2.6 Ma

(e.g. Jansen and Sjøholm, 1991). The early period of

glaciations was generally a time of moderate glaciations

with ice caps over Scandinavia, and only occasionally did

ice sheets carry detritus to the contemporary shelf break.

The first widespread shelf glaciation off Norway is thought

to have been the ‘Fedje Glaciation’ at 1.1 Ma. There was

apparently no further shelf glaciation until about 0.5 Ma

(Haflidason et al., 1991; Sejrup et al., 2000).

At least four shelf-edge glaciations occurred between

500 and 130 ka. During the Weichselian (the last 100 ky),

the south-western part of the Fennoscandian ice sheet

experienced three advances, between 80 and 60, 50 and 37,

and 28 and 22 ka, respectively, of which the last, the Late

Weichselian, was the largest (Sejrup et al., 2000). The most

likely mode of glacial sediment transport was through

deformable till beds carrying fast-flowing ice streams

(Nesje et al., 1987, 1988; Alley et al., 1989; Nesje and

Dahl, 1992). Ottesen et al. (2000) have mapped seafloor

morphological characteristics strongly indicative of ice

streams in the transverse troughs which cross the continental

shelf at several locations, the most prominent being the

Norwegian Trench (Fig. 1).

2.2. Palaeoceanography

Plio- and Pleistocene sedimentation on the NW European

margin was primarily controlled by glacial—interglacial

variability and the thermo-haline circulation system (Stoker,

1999). The Norwegian continental margin is heavily

influenced by the warm and saline Norwegian Atlantic

Current, which dominates the upper water column down to a

strong thermocline, which fluctuates at water depths of

500–700 m, where the water temperature drops from 5 to

6 8C to less than 0 8C (Alendal et al., 2005).

The circulation pattern in the North Atlantic became

similar to that of the present-day around 0.6 Ma, and in the

warmest interglacials (marine isotope stages (MIS) 11 and

5e) there was broad inflow of North Atlantic water with

maximum carbonate deposition in oxygenated deep water

(Henrich and Baumann, 1994). The Norwegian Atlantic

Current was a main transport agent for fine-grained

sediments from the south into the study area. Although

stronger during interglacial/interstadial periods, the north-

ward flowing currents along the margin have probably been

present through the entire Plio- Pleistocene, including

glacial periods. This is demonstrated by open-water

foraminifers and fragments of chert from the North Sea

found in Late Weichselian deposits west of Svalbard

(Hebbeln et al., 1994; Solheim et al., 1998). Deposition of

the suspended sediment load took place in response to

reduced current velocities, either climatically induced or

caused by topography. Large accumulations, exceeding

100 m thickness, commonly found in a mounded, climbing

geometry on the mid- to lower continental slope, are

probably formed by deposition of current-transported

sediments. These accumulations are referred to as contour-

ites in the present paper.

2.3. Mass wasting

The study area has been subject to a number of large

slides during the last 0.5 My, i.e. during the main period of

continental shelf glaciations (Evans et al., 1996; Bryn et al.,

2003; Solheim et al., 2005). The Storegga Slide, which

occurred at about 8150 calendar years B.P. was the last of

the slides, which appears to have occurred at semi-regular

intervals, following the glacial-interglacial cyclicity. This

predisposition for sliding has two primary reasons. Firstly

the region is situated between the bedrock highs formed by

volcanic rocks of the Faeroe-Shetland Escarpment and the

Vøring Escarpment- and Plateau (Berndt et al., 2001).

The Storegga Slide itself was delimited to the south by the

glacigenic deposits of the North Sea Fan and the Vøring

Plateau to the north (Fig. 1). Secondly, the region forms a

regional embayment in the strike of the margin, and thereby

appears to be a preferred location for deposition of fine

grained sediments which are transported by the Norwegian

Atlantic Current from the south. These fine grained

contouritic deposits comprise the slip planes of the slides

in the region.

3. Ormen Lange Pleistocene stratigraphy

The last 3 My are comprised by the Naust Formation,

which in the Ormen Lange area is subdivided into 5 main

units, Naust Units W, U, S, R and O (McNeill et al., 1998)

(Fig. 3). These form regional seismic sequences, which are

linked to the main glaciations. Each sequence may,

however, contain more than one glaciation. The ice sheet

did not reach the shelf break until near the end of Unit U

time, and horizon TNU is correlated to the initial ‘Fedje

Glaciation’ at about 1.1 Ma (Haflidason et al., 1991). This

stratigraphy covers the strata that were affected by the

Storegga Slide, and for which ground truth has been

acquired during the geotechnical borings (Fig. 2). The

correlation across the main Storegga Slide scar is also tied to

exploration wells of the Ormen Lange field. The seismic

stratigraphy coincides mostly with the lithostratigraphy, as

well as with the geotechnical stratigraphy.

Fig. 3. Seismic stratigraphy of the Ormen Lange area, and a north-south running regional seismic section through the northern part of the Storegga Slide. DF

and CD in the interpreted section mark debris flow deposits and contourite deposits, respectively.

K. Berg et al. / Marine and Petroleum Geology 22 (2005) 45–5648

3.1. Seismic facies

The Naust units W, U, S, R, and O are subdivided into a

number of sub-units (Fig. 3, Table 1). In general, the seismic

facies of the investigated sequences vary between homo-

geneous/chaotic and stratified, where the homogeneous

subunits represent glacial sediments (tills and debris flows),

and slide deposits. The stratified deposits, on the other hand,

are shown by the borings to represent fine grained, normal

marine to distal glacial marine sediments.

The lowermost Naust units, W to U, were deposited

during the Late Pliocene to Mid Pleistocene times, roughly

2.6–1.1 Ma. During this period the Fennoscandian ice cap

evolved and at the same time, the main uplift of the

Norwegian mainland occurred. Following a distinct break in

sedimentation evidenced by the Base Naust unconformity

(Fig. 3), Naust Unit W marks the transition from fine-

grained biogenic deposition to predominantly clastic

deposition and is widely distributed. The unit occurs near

the seabed in parts of the Storegga Slide scar. The seismic

character is homogeneous and largely transparent, creating a

strong contrast to the bedded and faulted appearance of the

underlying Kai Fm (Fig. 3).

Naust Unit U is also widely distributed and is present

under the entire Ormen Lange license area, whereas it

downlaps onto Naust Unit W westwards. The unit is

typically homogeneous but may display bedding in its more

distal facies (Fig. 3, Table 1). Most likely, the unit was

deposited in response to the Fedje Glaciation (King et al.,

1996).

The main glacial part of the stratigraphy in the Ormen

Lange area consists of Naust Units S, R and O. The basal

reflectors, TNU, TNS and TNR (Fig. 3, Table 1) are

all regionally significant unconformities, which can be

identified over the entire mid-Norwegian continental margin.

Naust Unit S, of Early to Mid Pleistocene age

(roughly 1.1–0.4 Ma) (Table 1), is about 300 ms TWT

(milliseconds two-way travel time) thick over large areas.

It shows a variable seismic character indicating a range

of depositional processes. Horizon TNU forms the top of

Naust U and is considered the lowermost seismic horizon

reflecting continental shelf glaciation. The horizon forms

a downlap surface for overlying reflectors. Down-hole

logs in exploration wells indicate lower density/higher

porosity sediments below TNU than above. Naust S

consists of 6 subunits, S6 to S1, bounded by internal

horizons INS5 to INS1, and horizons TNU and TNS at

the base and top, respectively (Fig. 3, Table 1). Sub-units

S6/S5 form the first glacial fan built up of debris flow

diamictons on the slope. Of particular importance are

horizons INS4 and INS2, both of which appear to form

detachment levels for rotational faults related to the

Storegga Slide. In particular sub-unit S2 shows a seismic

character indicative of contouritic deposition (Fig. 4).

Naust S sub-units were eroded and they are therefore

mostly non-existent on the inner shelf.

Table 1

Summery table for the Naust Formation in the Ormen Lange region

Fm. Unit Sub-unit Hor. Seismic facies Lithology Phys. Props. Dep. Env. Age (ka)

Naust O O1–O2 INO2 Generally transparent, occasional

weak internal reflections. Largely

removed by Storegga Slide. Present

and but displaced and partly disturbed

at Site 22

Unsorted silty, sandy, gravely clay

(Diamicton), with shell fragments

Clay: 20–40%, S&G:

10–25%, Wc: 14–27%,

UW: 19.7–21.1 g/cm3,

Ip: 16–34%

Basal till and/or deformation till,

deposited beneath fast-flowing ice

stream

30–15

(MIS 1–2)

O3 INO3 Stratified, Mounded, climbing charac-

ter on the slope, with thickness

increase down-slope. Largely eroded

on the shelf

Silty clay with sand, scattered gravel

and shell fragments. Increasing clay

content in lower part of the sub-unit

Clay: 40–65%, S&G:

5–15%, Wc: 30–34%,

UW: 18.5–19 g/cm3,

Ip: 30–32%

Major part is glacial marine, variable

glacial proximity. Lower 5–10 m rep-

resent interglacial marine hemipelagic

deposits. Contouritic deposition

130–30 (MIS

5e–3) (Lower

part is

Eemian)

O4–O7 TNR Homogeneous, transparent, w/ some

discontinuous reflections. Fan-shaped

distribution on the slope, cut by the

Storegga Slide. Some internal high

relief formed by INO5

Diamicton with shell fragments and

variable content of gravel and larger

clasts. In general a upwards-fining

character of O4

Clay: 20–40%, S&G:

10–25%, Wc: 13–20%,

UW: 20.8–22.2 g/cm3,

Ip: 15–20%

Primarily glacial. Tills on the shelf,

debris flows on the slope. O4 is

transitional in character

200–130

(MIS 6)

R R1–R2 INR2 Stratified, with partly mounded,

climbing character (R2) filling in

underlying topography. R1 only lim-

ited extent as a wedge–shaped deposit

near Site 99. TNR is a main horizon

and forms a main failure plane for the

Storegga Slide

Silty clay (w60% clay) w/ little sand

and gravel. Lower part of R2 shows

gradual change to the more diamictic

R3 unit

Clay: 35–70%, S&G:

3–20%, Wc: 20–31%,

UW: 18.9–21.5 g/cm3,

Ip: 27–31%

Glacial marine to normal marine

conditions, in an ice-distal to inter-

glacial setting. Largely affected by

contour currents, i.e. contouritic

deposition

330–200

(MIS 9–7)

R3 TNS Less stratified than R2, but some

internal reflections of variable strength

Increasing sand and gravel compared

to R2

Clay: 31–66%, S&G: 4–

40%, Wc: 16–18%, UW:

21.4–22.0/cm3, Ip: 19–

20%, (Poorly sampled)

Primarily glacial marine, but more ice-

proximal than R2. Probably also some

glacial debris flow deposits on the

slope

380–330

(MIS 10)

S S1–S2 INS2 Stratified, partly mounded, contouritic,

filling in old slide scars in the INS2

surface

Poorly sampled. Silty clays with

variable sand, gravel and shell frag-

ments. Primarily fine grained, as also

supported by downhole logs

Clay: 26–66%, S&G:

2–30%, Wc: 30–40%,

UW: 18.1–19.8 g/cm3,

Ip: 25–45%

Distal glacial marine to normal mar-

ine, hemipelagic deposition

420–380

(MIS 11)

S3–S6 TNU Varies between homogeneous, chaotic

and stratified. S5 and S6 have an

opaque character and limited lateral

extent. S4 partly stratified and

mounded

Poorly sampled. Silty clays with

variable content of sand, gravel and

shell fragments. Thin sand layers.

Increased content of coarse com-

ponents relative to Unit R

Clay: 30–60%, S&G:

3–20%, Wc: 17–30%,

UW: 19.3–21.5 g/cm3,

Ip: 20–28%

Glacial marine deposits, deposited

under variable glacial conditions. The

sequence represents one or more

glaciations

S3–S5: 550–

420 (MIS

14–12), S6:

1000–550(?)

U U1–U2 TNW Present under the entire Ormen Lange

license area. Mainly homogeneous, but

with some stratification in distal parts

Unsampled Clay: S&G: Wc: UW: Ip: U1 represnts distal glacial marine to

normal marine deposits from initial

glacial advance (‘Fedje glaciation’).

U2 is probably debris flow or slump

deposits

w1700–1100

W W1–W3 BNa-

ust

Present under the entire Ormen Lange

license area. Massive, transparent

character. Strong contrast to the

underlying stratified, faulted biogenic

Kai Fm. Deposits

Undiffentiated Unit W sampled spar-

sely at Storegga North Flank (Site 28);

Silty clay with relatively high biogenic

content (foraminifera)

Clay: S&G: Wc: UW: Ip: Mainly normal marine, hemipelagic

deposits, but with some IRD from

mainly land-based glaciers

w2600–1700

Lithology and physical properties are from analyses of core samples. The depositional environment has been interpreted from sediment samples and seismic facies combined, whereas the age is inferred from all

available information, as shown in Fig. 6 for the last 1 My.

K.

Berg

eta

l./

Ma

rine

an

dP

etroleu

mG

eolo

gy

22

(20

05

)4

5–

56

49

Fig. 4. Interpreted seismic section across the shelf break and the upper continental slope in the northern part of the Storegga Slide. Main horizons and sub-units

are indicated. The two blue sub-units (R2 and S1–S2) are contourites, as shown by their monded, climbing seismic character, whereas the green sub-units (O4–

O7, R3, and S3), are all glacial debris flow diamictons. See Fig. 2 for location.

K. Berg et al. / Marine and Petroleum Geology 22 (2005) 45–5650

The following Mid Pleistocene unit, Naust R, (roughly

0.4–0.2 Ma) (Table 1) reaches 450 ms thickness locally

beneath the flank of the Storegga Slide. The unit shows a

bipartite seismic character, with a bedded upper part and a

more structureless lower part. It is largely eroded by the

Storegga Slide (Fig. 5). Three sub-units, R3, R2, and R1 are

bounded by internal horizons INR2 and INR1, and horizons

TNS and TNR at the base and top, respectively. Sub-unit R2

has a regional distribution. It resembles unit S2 in being

relatively thin and stratified on the outer continental shelf,

and thick, with a mounded, contouritic character on the

continental slope (Fig. 4). Sub-unit R1 seems to be only

locally developed, as a wedge shaped body behind the lower

slide scarp (Fig. 2). Naust R sub-units are also eroded and

therefore non-existent on the inner shelf. Horizon TNR

forms one of the main detachment levels for the Storegga

Slide.

The upper part of the Naust Formation, unit O, has been

divided into sub-units O7–O1 by the internal horizons

INO6-INO1 above the basal reflector, horizon TNR (Fig. 3,

Table 1). As TNR forms the base for a large part of the

Storegga Slide, Naust unit O was removed or severely

disturbed in the central slide scar, in which the Ormen

Lange field is located. Units O1 and O2 are seismically

transparent, with occasional weak internal reflectors.

Beyond the shelf edge, they were remoulded and mostly

removed by the Storegga Slide. Sub-unit O3 displays a

seismically stratified character and generally thickens

down-slope. On the shelf, the sub-unit has mostly been

eroded, but is present and cored at Site 20 (Fig. 2). The

lower sub-units of Naust unit O mainly display a

homogeneous to chaotic seismic character, with few and

mostly discontinuous internal reflecting horizons. Sub-units

O5–O7 stack up to form a fan-shaped lateral distribution,

cut by the Storegga Slide. Relief in horizon INO5 on the

shelf may represent an original till topography.

The upper disrupted deposits within the Storegga Slide

scar are termed the ‘Storegga Slide Sediments’. These

sediments are distinguished by their seismically structure-

less character and are commonly associated with headwall

Fig. 5. (A) Regional time structure map of seismic horizon INR2, which is the base of the contouritic sub-unit R2. The horizon and overlying strata are largely

eroded by the Storegga Slide, which forms an indent in which new contouritic deposits can accumulate. Note similar indents in the contours to the north of the

Storegga Slide. These were eroded by previous slides and formed local depocentres for contouritic deposits of sub-unit R2. (B) Isopach map of Naust sub-units

O7–O4, representing the Saalian glaciation, reconstructed to pre-Storegga Slide topography. Despite the uncertainties of the reconstruction, the outline of the

depocenter shows a fan-shaped sediment volume outside the Buadjupet Trough, which most likely was the location of ice streams, both during the Saalian and

other glaciations. Thickness contours are in meters, the Ormen Lange field is outlined in yellow.

K. Berg et al. / Marine and Petroleum Geology 22 (2005) 45–56 51

or sidewall scarps. Due to their completely disturbed nature,

their provenance in the original seismic stratigraphy is most

commonly unrecognisable.

3.2. Lithology and physical properties

Units U and W are only sparsely sampled at the distal site

28 (Fig. 2B), whereas unit U is drilled and logged, but not

sampled, at site 27. Consequently, the interpretation of these

units relies largely on down-hole logs and seismic

interpretation. Both units consist of fine grained marine to

distal glacial marine deposits. The IRD content is sparse in

the sampled deposits of unit W, as seen at site 28, but is

assumed to increase slightly as the glacial influence

increased during the time represented by unit U.

The sampled sediments of unit S are generally silty clays

with variable content of sand and gravel. Indications of

current variations are seen in silty, sandy lamina, which is in

agreement with a mounded, contouritic seismic character.

SEM studies support a fine grained, laminated structure in

particularly sub-unit S2. Internally there is a change across

horizon INS2, to a lower unit weight and higher water

content and plasticity above the horizon (Table 1). This is

also supported by the down-hole logging results, which

show a change to lower density and velocity from S3 to S2,

across horizon INS2.

The transition into Unit R, across TNS is marked by

an increase in unit weight and a corresponding decrease

in water content and plasticity. This change implies a

transition into more coarse-grained and unsorted (dia-

mictic) deposits of sub-unit R3 (Fig. 4, Table 1). There

is a gradual change from R3 into the clay-rich sub-units

R2 and R1. The two latter units show higher clay

content, higher water content and plasticity and lower

unit weight than R3 (Table 1), a trend which is

confirmed by the down-hole logs.

Naust Unit O is the main unit drilled at sites 19_2, 20 and

22 on the outer continental shelf (Fig. 2), and a large amount

of information is therefore acquired from this unit. The

transition across horizon TNR into the lower sub-units of

Naust O is sharp. Sub-units O7–O4 consist of silty, sandy,

gravely clay, with shell fragments and larger clasts, where

the sub-unit O4 appears to form a transitional zone. Water

content increases and unit weight decreases steadily upward

K. Berg et al. / Marine and Petroleum Geology 22 (2005) 45–5652

in the unit. Sub-unit O3 is clay-rich, but with a variable

content of silt, sand and scattered gravel, showing the

glacial influence. In the lower part of the sub-unit at site 20,

as well as at site 22, the clay content exceeds 60%, the

coarse content decreases, and thin silt layers appear. The O1

and O2 sub-units consist of firm to hard clay with variable

contents of silt, sand, gravel and larger clasts, as well as

shell fragments.

3.3. Chronostratigraphy and depositional environment

The chronostratigraphic control is based on analyses of

core samples, in combination with seismic ties to other areas

with dated boreholes. The youngest parts of Naust O (O1,

O2 and most of O3) have been dated by AMS 14C analyses

(Fig. 6), whereas the remaining parts of the section rely

Fig. 6. Chronostratigraphy for the last 1 My of the Ormen Lange area. Sub-

units of Naust S, R and O are placed relative to the marine isotope stages

(MIS, small numbers marking highs and lows on the isotope curve). This is

based on interpretation of depositional environments, with additional age

information from AMS and amino acid analyses. The AMS analyses (on

foraminifera) from sites 20 and 22 are shown vs. depth below sea floor

(mbsf) in the column to the right of the isotope curve. The table shows the

suggested chronostratigraphy. See text for further discussion. The oxygen

isotope curve is based on data from the Pacific cores V1930 (Shackleton

and Pisias, 1985), ODP 677 (Shackleton et al., 1990) and ODP846

(Shackleton et al., 1995).

much on amino acid analyses and biostratigraphic corre-

lations. Due to highly discontinuous coring (Fig. 2),

paleomagnetic analyses have limited value, although both

these (at site 99 only) and oxygen isotope analyses have

been used for additional support. On the unit level, the two

early (prior to continental shelf glaciations) glacial units,

Naust W and U, have been assigned ages of 2.6–1.7 and

1.7–1.1 Ma, respectively. Horizon TNU is correlated with

the Fedje Glaciation (1.1 Ma) (Haflidason et al., 1991). The

main glacial units, Naust S, R and O were probably

deposited in the age windows 1.1–0.38, 0.38–0.2, and

0.2–0.0 Ma, respectively (Fig. 3).

A more detailed chronostratigraphy has been obtained

by adopting the oxygen isotope stratigraphy and assigning

individual Naust sub-units to the various marine isotope

stages (MIS) based on the interpreted depositional

environments (Fig. 6). The age record obtained this way

has been incorporated into the sub-unit stratigraphic

scheme of Table 1.

The lower two units, Naust W and U, represent

conditions during which the Fennoscandian ice sheet

grew, but did not reach significantly beyond the con-

temporary coastline. Hence, the two units represent mainly

normal marine and distal glacial marine conditions, marking

a transition from the biogenic-rich marine deposition of the

Kai Formation into the more heavily glaciated Pleistocene

conditions.

The remaining Naust units S, R and O represent all

different stages between peak glacial and peak interglacial

conditions (Table 1). The glacial influence is evident in the

content of sand and gravel in most sub-units, but the relative

content reflects the proximity to the ice front. Current

influence is evident in several of the marine and glacial

marine sub-units, which are developed as contourites.

Broadly, the deposits of the Naust units can be divided in

two main groups, diamictons representing peak glacial

deposits, and marine clays, which comprise normal marine

sediments and distal glacial marine deposits. The deposi-

tional model is further discussed below.

3.4. Slip planes and failure layers

Large, translational slides are characteristic features of

the Mid-Norwegian margin. The slides have slip planes

parallel to the slope stratigraphy. Failures in the study area

have occurred at different times and stratigraphic levels

(Evans et al., 1996; Solheim et al., 2005). As a rule, the slip

planes or failure zones are closely connected to sub-units of

hemipelagic fine-grained marine or distal glacial marine

sediments. These have distinctly different physical proper-

ties and show different geotechnical behaviour than

the glacial tills and debris flows (diamictic sediments).

One particular difference is that the fine-grained marine

clays develop clear shear planes and reach peak strength

upon triaxial testing (contractant behaviour) (Table 2).

Table 2

Physical property characteristics for the two main sediment types in the

study area

Property Diamicton Marine clay

Clay content 30–40% 50–60%

Water content 10–20% 25–35%

Unit weight 20–22 kN/m3 18–19 kN/m3

Sensitivity Lower Higher

Plasticity 15–25 30–35

Geotechn. behaviour Dilatant Contractant

K. Berg et al. / Marine and Petroleum Geology 22 (2005) 45–56 53

4. Discussion

4.1. The depositional model

The depositional model may, in its simplest form, be

divided into two main stages as a response to the climate

cyclicity:

Stage 1: Periods of peak glaciation.

Stage 2: Periods with the ice front in a retreated position,

including interstadials and interglacials.

Of these, stage 2 was significantly longer than stage 1.

Based on a study from the margin west of Svalbard, Hooke

and Elverhøi (1996) estimated that the ice front may have

been at the shelf break as little as 10% of the Weichselian

glacial period. The background for differentiating between

the two stages is the contrasting depositional processes.

Whereas till deposition and glacial debris flow deposition

are the main processes on the outer shelf and slope during

stage 1, glacial marine and normal marine processes

prevailed in the outer continental margin during stage 2.

The degree of glacial marine and current influence varied

with climate and position of the ice front, whereas the

overall suite of processes remained the same.

Because the depositional facies varied with both climate

and distance from the ice margin, different depositional

processes may have been active at different locations on the

margin at each time period. Marine or distal glacial marine

deposition, influenced by currents to a variable degree, has

occurred in the lower slope settings while tills were

deposited on the outer continental shelf. Because of the

complexity of the glacial depositional systems, any deposi-

tional model has to imply a simplification.

4.1.1. Stage 1: Peak glaciation

After a significant period of climatic deterioration, the

onset of continental shelf glaciation involved a major glacial

advance, forming a glacial erosion surface across the

continental shelf. As the sediment source, i.e. the ice

front, approached the shelf edge, depositional rates on the

outer shelf and upper slope increased, and at the same time,

the glacial marine sedimentation increased in the deep

ocean. Fast flowing ice streams developed in topographic

lows across the shelf, and this caused a dramatic increase in

sediment supply to the outer shelf and upper slope, as well

as to the IRD component in the deep sea, although main IRD

peaks probably occurred during early deglacial periods.

Sediment transport to the shelf break through a sub-

glacial layer of deformation till (e.g. Nesje et al., 1987,

1988; Alley et al., 1989; Nesje and Dahl, 1992; Hooke and

Elverhøi, 1996) led to rapid build-up of unstable sediment

configurations in front of the ice streams, followed by

frequent small-scale failures and glacial debris flows on the

continental slope. This prevailed through the period of peak

glaciation, and was also the main process by which the shelf

edge prograded. Recent models show how release of glacial

debris flows in front of a fast flowing ice stream leads to

both shelf edge progradation and aggradation (Dimakis

et al., 2000). Palaeogeographic reconstructions suggest that

the Ormen Lange area was fed by local ice streams in

‘Buadjupet’ and ‘Onadjupet’ east of the slide area (Fig. 1).

The greatest ice stream however, was situated in the

Norwegian Trench, feeding the North Sea Fan (Fig. 1).

Sediments were also delivered to the margin with melt water

and incorporated in floating ice, but these processes are

generally less efficient than the ice stream transport of

deformation till.

A main aspect of stage 1 deposition is the rapid

emplacement of the debris flow deposits. Each flow may

be up to several 10 s of meters thick, and they are deposited

as instantaneous events. Averaged over time intervals of a

few thousand years, rates of several 10 s of m/ky can result,

and thick debris flow aprons may cover large areas of fine

grained deposits from the preceding stage 2 deposition

(below).

Following climatic amelioration and a rise in sea level,

the marine based ice sheet retreated rapidly with calving as

the most important decay process. Lodgement till was

probably deposited on the outermost continental shelf

during the last glacial phase, before lift-off and rapid retreat

took place. During retreat, glacial marine deposition

increased in front of the retreating ice front. Sedimentation

rates of up to 20 m/ky from sediment loaded meltwater

plumes during initial Late Weichselian deglaciation have

been documented from areas to the north of the Ormen

Lange field (Hjelstuen et al., 2004).

4.1.2. Stage 2: Periods of reduced ice cover

Stage 2 comprises most (possibly 80–90%) of each

glacial—interglacial cycle and also quite variable climatic

conditions, including true interglacial periods. Distal glacial

marine and normal marine hemipelagic processes were

predominant over the entire margin during stage 2

sedimentation. Although glacial marine sediments have a

low preservation potential on the continental shelf, glacial

marine sediments of stage 2 are found as sub-units O3 and

R2 close to the shelf edge east of the Ormen Lange area.

During initial glacial retreat across the shelf, the

production of icebergs and, consequently the IRD (ice

rafted detritus) supply increased. During the following

K. Berg et al. / Marine and Petroleum Geology 22 (2005) 45–5654

period of retreated ice front (interglacial, interstadial or even

a limited stadial), normal marine deposition resumed on

the slope and in the deep ocean, partly influenced by

contour-following currents, as the oceanic polar front

moved northwards and the general circulation pattern was

re-vitalized. This led to increased biogenic production as the

terrigeneous sediment supply decreased.

The currents must periodically have been powerful

enough to transport and erode fine grained sediment,

which were re-deposited in areas of reduced current

velocity, such as in the scars left by earlier slides (Solheim

et al., 2005). These palaeo-slide scars trapped sediments

moving northwards along the slope and the thickest

contouritic drift sequences, such as sub-units R2 and S2,

are recorded within the largest slide scars. Where the

palaeo-slope prior to contourite deposition is even, without

local topography such as slide scars, these deposits are

relatively thin.

Although the age control is problematic, relatively long

time spans seem necessary to build up O100 m of

sediment drifts such as the stage 2 deposits of sub-units

R1/R2 and S1/S2. The sediment supply may have been

episodic, and extreme local sedimentation rates in excess

of 20 m/ky provide a likely model although these

conditions probably prevailed only for limited periods

and only for limited parts of the slope. Hence, large output

of terrigeneous sediments during early deglaciation from

areas in the vicinity and south of Ormen Lange are the

most likely sediment sources. In particular the Norwegian

Trench Ice Stream would provide a significant sediment

supply. Consequently, this important deglaciation phase

forms the transition between stage 1 and stage 2 deposition.

During true interglacials, when the ice probably

disappeared from most of the inland areas, conditions

may periodically have been like the present. Large

submarine slides, such as the Holocene Storegga Slide as

well as older slides recorded deeper in the stratigraphy

seem to have occurred during early parts of stage 2

deposition (Solheim et al., 2005). The detachment levels

for the failures seem to follow the fine grained marine,

partly contouritic layers. Hence, the sliding process is

apparently strongly controlled by the glacial—interglacial

sedimentary cyclicity, and the resulting differences in

depositional processes, rates, and physical properties

between stages 1 and 2 sediments (Tables 1 and 2) (Bryn

et al., 2005). During late stage 2 climatic deterioration, ice

again accumulated in inland alpine areas, and a new glacial

period was initiated, which subsequently led to a new

period of stage 1 deposition.

4.2. Age constraints

Age control is a key problem in unravelling the

depositional history for the Ormen Lange area. Results

of AMS 14C dating show that most of the upper section of

sub-units O1, O2 and O3 consist of sediments deposited

during the Late Weichselian, i.e. the last approximately

30 ky. Seismic horizon INO2 at site 22 has an age between

23 and 24 ka (Fig. 6).

Horizon INO3 is important because it forms the

shallowest and laterally greatest of the Storegga Slide

slip planes. The age of 33 ka 5 m above INO3 (Fig. 6)

shows that the horizon was not created by the Late

Weichselian glaciation, which reached the shelf break

later than 30 ka. The horizon is a distinct, regional

unconformity and is most likely created by a regional

glaciation reaching the shelf break. According to Sejrup

et al. (2000), the Early and Mid Weichselian ice

advances did not reach the shelf break and were probably

quite moderate in extent. Hence, the most likely

glaciation to create the INO3 erosional unconformity

was the Saalian. This is in accordance with the dating

results, but implies a period of low sedimentation rate on

the shelf between the Saalian retreat and the Late

Weichselian advance. Possibly this may also include

periods of non-deposition or erosion, but our temporal

resolution is not sufficient to resolve this. An Eemian age

is therefore inferred for the lowermost part of sub-unit

O3, which also clearly represents normal marine

deposition and is the finest grained part of the sub-unit.

Horizon INO3 hence represents the final Saalian retreat.

This also supports a Saalian age for the lower glacial

sub-units (O4–O7) of Naust Unit O, as interpreted based

on amino acid analyses. The reconstructed isopach map

of O7–O4 (Fig. 5B) supports a glacial fan build-out from

the east.

The peak glacial periods represented by tills on the

outermost shelf, and glacial debris flow units on the upper

slope are only short intervals during each glacial period.

This may introduce an error to the ages based on the marine

isotope stratigraphy (Fig. 6). In fact, the periods assigned

interglacial or interstadial may be considerably longer and

the periods assigned glacial shorter. The ‘interglacial to

interstadial’ periods may therefore represent periods during

which the ice cover varied from none to an ice margin at a

mid-shelf position.

The ice sheets reached the shelf break from MIS 12

(Fig. 6). The thick and extensive glacial debris flow units R3

and S5 are assigned glacial stages 10 and 12, respectively,

although the amino acid analyses also open for a possible

stage 14 age for S5 (University of Bergen, 2002). R1–R2 is

assigned stages 7–9. Since R1–R2 are essentially marine to

glacial marine deposits, this implies that stage 8 glaciation

probably was less extensive than stages 6, 10 and 12, and

did not manifest itself with significant debris flow deposits

on the continental slope.

The long interval represented by sub-unit S6 is a concern

with regards to the above discussion. A younger age than

1.1 Ma for horizon TNU cannot be excluded. However,

continuous coring and detailed isotope- as well as

magnetostratigraphic investigations work would be necess-

ary to clarify this.

K. Berg et al. / Marine and Petroleum Geology 22 (2005) 45–56 55

5. Conclusions

†

Pleistocene sedimentation in the Ormen Lange area was

strongly controlled by glacial—interglacial climatic

variability, which also controlled the development of

sedimentary sequences.

†

Two distinctly different depositional modes were

responsible for the Pleistocene section in the area;

(1) peak glaciation with ice reaching the shelf break, and

(2) periods of retreated ice front.

†

The two broad classes of sediments resulting from the

depositional stages are (a) glacial diamictons, consisting

of tills and debris flow deposits, and (b) fine grained

normal marine and glacial marine clays, with variable

content of coarser IRD and biogenic components.

†

Deposition was most rapid during periods of peak

glaciation, when fast-flowing ice streams supplied glacial

diamictons through a system of subglacial deformation

tills directly to the outer shelf and upper slope. Frequent

debris flows moved these glacial sediments further out on

the continental slope, thereby giving rise to rapid,

periodic shelf progradation and debris flow aprons

(fans) on the slope.

†

Under the influence of contour currents, deposition

during periods of a retreating or retreated ice cover

may also locally reach significant thickness, in particular

when filling in older slide scars.

†

The two sediment types show distinctly different

geotechnical properties, and this is interpreted to be a

main controlling factor for the development and the

geometry of the large submarine slides, which took place

in this area, particularly during the last 0.5 My.

†

Slip occurred in the fine grained marine clays and slide

detachment levels follow the stratal geometry.

Acknowledgements

Norsk Hydro is acknowledged for the open access to the

data related to the Storegga Slide. The manuscript was

critically reviewed by Drs Atle Nesje and David Long. This

is publication number 54 of the International Centre for

Geohazards (ICG).

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