the late cenozoic eridanos delta system in the southern ...overeem/eridanos/bre151.pdf ·...

The Late Cenozoic Eridanos delta system inthe Southern North Sea Basin: a climate signalin sediment supply?I. Overeem,* G. J. Weltje,* C. Bishop-Kay² and S. B. Kroonenberg*

*Department of Applied Earth Sciences, Delft University of

Technology, The Netherlands, PO Box 5028, NL-2600 GA Delft,

The Netherlands

²Woodside Energy Ltd, 1 Adelaide Terrace, Perth WA 6000,

Australia

ABSTRACT

The Eridanos ¯uvio-deltaic system, draining most of north-western Europe, developed during

the Late Cenozoic as a result of simultaneous uplift of the Fennoscandian shield and

accelerated subsidence in the North Sea Basin. This seismo-stratigraphic study aims to

reconstruct the large-scale depositional architecture of the deltaic portion of the basin ®ll and

relate it to external controls. A total of 27 units have been recognized. They comprise over

62r103 km3 in the Southern North Sea Basin alone, and have an average delta surface area of

28r103 km2, which suggests that the size of the drainage area was about 1.1r106 km2. Water

depth in the depocentre is seen to decrease systematically over time. This trend is interrupted

by a deepening phase between 6.5 and 4.5 Ma that can be correlated with the simultaneous

occurrence of increased uplift of the Fennoscandian shield, increased subsidence of the

Southern North Sea Basin, and a long-term eustatic highstand. All these observations point

to a tectonic control on long-term average rates of accommodation and supply. Controls on

short-term variations are inferred from variations in rates of sediment supply and bifurcation

of the delta channel network. Both rates were initially low under warm, moist, relatively stable

climate conditions. The straight wave-dominated delta front gradually developed into a lobate

¯uvial-dominated delta front. Two high-amplitude sea-level falls affected the Pliocene units,

which are characterized by widespread delta-front failures. Changes in relative sea level and

climate became more frequent from the late Pliocene onward, as the system experienced the

effects of glacial±interglacial transitions. Peaks in sedimentation and bifurcation rates were

coeval with cold (glacial) conditions. The positive correlation between rates of supply and

bifurcation on the one hand, and climate proxies (pollen and d18O records) on the other hand

is highly signi®cant. The evidence presented in this study convincingly demonstrates the

control of climate on time-averaged sediment supply and channel-network characteristics,

despite the expected nonuniformity and time lags in system response. The presence of a clearly

discernible climate signal in time-averaged sediment supply illustrates the usefulness of

integrated seismo-stratigraphic studies for basin-wide analysis of delta evolution on geological

time scales.

INTRODUCTION AND OBJECTIVE

During the Late Cenozoic the North Sea Basin was

dominated by an extensive ¯uvial system that drained the

Fennoscandian and Baltic shield through the present

Baltic Sea (Fig. 1). The dimensions of the drainage area

and the thickness of the deltaic deposits in the present

North Sea are comparable to those of the present Orinoco

delta system (Coleman & Roberts, 1989). Its drainage

system has been referred to as the Baltic river (Bijlsma,

1981) and its proto-delta in Poland as the Eridanos delta

(Kosmowska-Ceranowicz, 1988). The latter designation is

based on ancient Greek records (7th century BC) in which

Correspondence: Irina Overeem, Department of Applied EarthSciences, Delft University of Technology, The Netherlands,PO Box 5028, NL-2600 GA Delft, The Netherlands.E-mail: [email protected]

Basin Research (2001) 13, 293±312

# 2001 Blackwell Science Ltd 293

the legendary Eridanos river in northern Europe was men-

tioned as the source of amber, a highly valued commodity

in the Mediterranean world. We propose to adopt the

name Eridanos for the entire ¯uvio-deltaic system in view

of this historical precedent. The geological importance

of the system has been acknowledged in several pre-

vious studies (Streif, 1996; Friis, 1974; Bijlsma, 1981;

Bishop-Kay, 1993; Cameron et al., 1993; Cartwright,

1995; Michelsen et al., 1995; Sùrensen et al., 1997; Liu &

Galloway, 1997). Marginal gas reservoirs were found in

Miocene and Plio-Pleistocene deltaic units, illustrat-

ing their hydrocarbon potential (Sùrensen et al., 1997;

Tigrek, 1998). The stratigraphic architecture of basin ®lls

depends on the ratio of sediment supply over accom-

modation (Jervey, 1988; Posamentier et al., 1988a,b;

Molnar & England, 1990; Mitchum & van Wagoner,

1991; Miall, 1997). Variations in sediment supply are

controlled by changes in uplift rate and climate, and by

the evolution of the drainage network. Accommodation is

controlled by the combined effects of eustasy, thermal

subsidence, loading and compaction, which determine the

space available for deposition. Stratigraphic architecture

is inherently nonunique, since different combinations of

interacting controls can result in similar depositional

patterns (Weltje et al., 1998). Seismic data in general yield

low-resolution information on depositional patterns, but

time control and correlation with independent data on

depositional controls sometimes allow one to make a

choice between several possible geological scenarios.

Our knowledge about controls on long-term basin-wide

sediment supply is limited (Hovius & Leeder, 1998). We

aim to ®ll part of this gap by (1) reconstructing the

morphology and history of the Neogene ¯uvio-deltaic

Eridanos system in the southern North Sea on the basis

of 2-D and 3-D seismic data, and (2) explaining the large-

scale basin-®ll architecture in terms of external forcing by

tectonics, sea-level variations and climate.

GEOLOGICAL SETTING

Tectonic setting

The development of the Eridanos drainage system is

attributable to simultaneous Neogene uplift of the

Fennoscandian Shield and accelerated subsidence of the

North Sea Basin. Domal uplift of the Fennoscandian

Shield started in the Oligocene (Rohrman et al., 1995,

1996). It has been suggested that the uplift rate increased

in the late Miocene (Sales, 1992) and again in the early

Pliocene (Ghazi, 1992; Jordt et al., 1995). However,

timing of these events proved unclear and controversial

( Japsen & Chalmers, 2000). Total uplift amounted to

3000 m in the central part of the dome in northern

Norway and to about 1000±1500 m more to the south

(Riis, 1992, 1996; Sales, 1992; Lidmar-BergstroÈm et al.,2000). The hinge zone along the western Scandinavian

margin was relatively narrow: up to 600 m differential

uplift occurred over a distance of less than 100 km

(Hansen, 1996; Jensen & Schmidt, 1992; Rohrman et al.,1996). Glacio-isostatic rebound possibly started to play

a role from 12.6 Ma onwards (Riis, 1992; Lidmar-

BergstroÈm et al., 2000).

The Late Tertiary Northwest European Basin com-

prised two connected subbasins, the North Sea Basin and

the East-German Polish Basin (Bijlsma, 1981; Glennie,

1990; Ziegler, 1990). The North Sea Basin extended

over the offshore and onshore parts of the Netherlands,

Germany and Denmark (Fig. 1). In the south the

London-Brabant High separated the North Sea Basin

Fig. 1. North-west European Basin

showing the Eridanos ¯uvio-deltaic

system and coastlines at 25 Ma and

15 Ma (after Bijlsma, 1981; Vinken,

1988; Ziegler, 1990; Cartwright, 1995).

The inset indicates the study area.

LBH=London±Brabant High,

CG=Central Graben, VG=Viking

Graben.

I. Overeem et al.

294 # 2001 Blackwell Science Ltd, Basin Research, 13, 293±312

from the Atlantic Ocean, although at some periods a

connection through the English Channel may have

existed (Wood et al., 1993; Funnell, 1996). While Early

Tertiary subsidence in the southern North Sea was relat-

ively slow, it accelerated at least an order of magnitude

in the Neogene (Kooi et al., 1989; Joy, 1992; Liu &

Galloway, 1997). Isopach maps of Quaternary deposits

suggest that the pre-existing Viking Graben and Central

Graben acted as depocentres (Caston, 1977; Cameron,

1993). However, the overall saucer-shaped geometry

of the southern North Sea Basin indicates that the major

faults have not been actively controlling sediment

distribution (Huuse, 2000). Differential movements,

i.e. rapid subsidence in the centre of the basin and less

subsidence or even uplift along the margins, have been

attributed to intraplate stresses (Kooi et al., 1991;

Cloetingh et al., 1992).

Development of the ¯uvial drainage system

The drainage basin in its earliest form was already

present during the late Oligocene ± early Miocene, as

evidenced by the amber-bearing deltaic deposits in the

Polish basin (Kosmowska-Czeranowicz, 1988). Rivers

that drained to the SE had started to strip the deeply

weathered Fennoscandian Shield (Lidmar-BergstroÈm,

1996). Coarse sediments were supplied from southern

Norway (Michelsen et al., 1995, 1998) and sedimentation

shifted clockwise around the eastern margin of the basin

in the Oligo-Miocene (Clausen et al., 1999; Huuse, 2000).

The depocentres of the system migrated westwards

through north-western Europe, as demonstrated by

deposition of lignite-bearing sediments in Poland and

Eastern Germany in the early Miocene, and in western

Germany and Denmark in the middle and late Miocene

(Vinken, 1988). The sediments exposed onshore have a

distinctive gravel assemblage (Baltic Gravel Assemblage)

traceable to source areas in Fennoscandinavia, the present

northern Baltic Sea and the present Skagerrak (Bijlsma,

1981). The proto-Elbe and proto-Weser formed tribut-

aries to the main system, as deduced from the heavy

mineral composition of the ¯uvial deposits (Bijlsma, 1981;

Streif, 1996).

Early Pliocene coarse sands and gravels, known as

the Kaolin Sands, were deposited in a narrow zone in

Germany and The Netherlands. During the late Pliocene

and Praetiglian (early Pleistocene) deposition of coarse

sands continued in NW Germany, while ®ne sands were

deposited in the Netherlands. During the Tiglian braided

rivers developed in The Netherlands, re¯ecting rapid

westward expansion of the system. During the Waalian,

these rivers covered a large part of the Netherlands.

German rivers were probably still tributaries to the

Eridanos river system and in the southern part the Rhine±

Meuse system started to join in at this time (Bijlsma,

1981; Gibbard, 1988). Around 1.0 Ma the Baltic Gravel

Assemblage started to mix with gravel and sands

originating from the Variscan Massif (Zandstra, 1971).

The Menapian ice sheet most likely destroyed the upper

course of the river system, because the Baltic Gravel

Assemblage disappeared at that time. The North German

rivers continued to ¯ow into the Eastern Netherlands

until the middle Pleistocene (Gibbard, 1988).

DATA AND METHODS

Subsurface data base

Seismic data, both 2-D and 3-D, represent the main

source of information for this study. 2-D seismic lines of

different NOPEC surveys (SNSTI-86 and SNSTI-UK-

87 in the British sector, SNSTI-NL-87 in the Dutch

sector, GR-86 in the German sector and CGD-85 in the

Fig. 2. Study area with NOPEC 2-D

seismic lines and 3-D data sets in the

M09, L08 and F09 blocks. Two

cross-sections, indicated with dotted

lines, are used to illustrate the regional

seismics: 87-06 (Fig. 6) and 87-26

(Fig. 7). Black squares are locations

of wells discussed in the text.

The Cenozoic Eridanos delta, southern North Sea

# 2001 Blackwell Science Ltd, Basin Research, 13, 293±312 295

Danish sector) were used (Fig. 2). These lines have a

60-fold coverage. The uppermost interval (sea bottom to

300±400 ms) of the data is of poor quality. Vertical

resolution estimates, based on interval velocities and

frequencies used by NOPEC during processing, are about

11.5 m for the interval 400±900 ms and about 13 m for

the interval 900±1700 ms. A total of 85 commercial wells

studied by Bishop-Kay (1993) contained little information

on the shallow Cenozoic deposits. The low information

content of these commercial wells is due to the limited

number of samples analysed and the possibility of

contamination associated with the drilling technology

used (Gradstein et al., 1992). The available gamma-ray

logs covering the intervals of interest failed to provide

straightforward grain-size information, because the sands

are in general rich in mica and glauconite (Sha et al., 1996;

Streif, 1996). However, data from nine boreholes drilled

by the Southern North Sea Project did contain

sedimentological, biostratigraphic and magnetostrati-

graphic information to support our interpretations of

the uppermost intervals of the studied delta system (Sha

et al., 1996; Streif, 1996). Three 3-D seismic surveys,

located in the M09, L08 and F09 blocks, were used for

more detailed interpretation (Fig. 2). Line spacing in

these blocks is 12.5 m at best and the sampling rate is

4 ms. Furthermore, these data sets have been time-

migrated to enhance image quality. Vertical and lateral

resolution is estimated to be about 10 m and 100±150 m,

respectively. Gamma-ray and sonic logs in the 3-D blocks

(M09-1, M09-2, L08-4, L08-10, L08-G-01, L08-H-01,

F09-1 and F09-2) were used for time±depth conversion.

Seismo-stratigraphic interpretation

Interpretation of seismic lines is based on the assump-

tion that re¯ectors follow bedding planes, that bedding

planes are isochronous and hence that seismic re¯ectors

approximate time lines (Payton, 1977; Emery & Myers,

1996). Depositional sequences are bounded by re¯ectors

and their mutual relations are determined by re¯ector

terminations (Payton, 1977; Posamentier et al., 1988a,b;

Van Wagoner et al., 1988; Emery & Myers, 1996).

Intrasequence re¯ector con®guration has been studied to

assess depositional facies. The con®guration of the

re¯ectors depends on the depositional regime (Payton,

1977; Mitchum & Van Wagoner, 1991) because sequence

geometry is a function of the ratio of sediment supply

over accommodation. Signi®cant basinward shifts of

coastal onlap were used to identify sequence boundaries.

Re¯ectors considered sequence boundaries are character-

ized by consistent onlap terminations of the overlying

deposits. Galloway (1989) has questioned the validity of

using unconformities as sequence boundaries, since the

hiatus represented by an unconformity may vary signi-

®cantly. Marine ¯ooding surfaces would be a better (iso-

chronous) alternative. However, identi®cation of ¯ooding

surfaces relies on well interpretations and these were

found to be of limited usefulness. Salt tectonics and

postdepositional tectonic movements locally complicated

seismic interpretation. In addition, gas occurrences

`blanked' the signal in some areas.

A 3-D framework was constructed by interpolation of

the re¯ectors interpreted from the 2-D lines. Interpola-

tion on a 12.5r12.5-km grid was carried out by means of

a weighted least squares contouring algorithm in the

software package ZMAP. Palaeosurfaces, isopachs and

channel network maps were constructed from the

interpolated data. No correction for loading or compac-

tion was made, so that the actual depth of the seismic

surfaces is shown. Volumes and surface areas of the

deltaic units were calculated from the isopach maps.

Mapping of the delta channel network was based on

interpreted channels and further inferred from contour

bulges in the isopach maps of each unit. Since the vertical

resolution of the data is approximately 13 m and the

lateral resolution 100±150 m, only larger channels could

be identi®ed. Wherever possible, the 3-D seismic data

have been used to investigate the channels in more detail

(van de Bilt, 2000; Steeghs et al., 2001).

Chronostratigraphic control

A chronostratigraphic framework was provided by

combining various types of stratigraphic data from

wells in the study area (Fig. 2). Biostratigraphic data

comprise foraminifera (wells Kim-1, Cleo-1, M10:

Knudsen, 1985; Knudsen & Asbjùrndottir, 1991;

Konradi, 1995, 1996), Azolla species (wells A-1 and

A-2: Bertelsen, 1972) and molluscs (wells R-1 and 89/5:

Streif, 1996). In addition, we used magnetostratigraphic

datings (wells 89/2 and 89/3: Sha et al., 1996; Streif,

1996) and lithostratigraphic correlations based on Dutch

Geological Survey gamma-ray log interpretations (wells

A16-1 and A12-1). Age data for the studied deposits are

sparse. Furthermore, the seismic data were interpolated

over 12.5r12.5-km blocks and depth-converted with the

stacking velocities of the NOPEC data with a few sonic

logs only, hampering an exact correlation of unit

boundaries with the age data. The correlation problems

are illustrated in a cross-section comprising three wells

(Fig. 3). Matching the interpolated unit boundaries to the

boreholes yields detailed sedimentological and strati-

graphic information about speci®c units. However, any

correlation of the borehole data with these particular units

should take into account the combined uncertainties of

the interpolation and the stratigraphic age dating. The

most evident example is well 89/4, where the borehole

data suggest a much younger age than our seismo-

stratigraphic correlations, on the basis of which we

identi®ed units D8 to D10. The uncertainties of inter-

polation are reduced in cases where unit boundaries can

be correlated across several adjacent seismic lines. We

therefore believe that our 3-D seismo-stratigraphic

framework is more reliable than the local stratigraphic

I. Overeem et al.


evidence obtained in isolated wells. Consequently, a

scatter in unit ages is unavoidable.

The Eridanos delta deposits are bounded below by a

major unconformity discernible on all seismic pro®les, the

Mid-Miocene Unconformity (MMU). The age of down-

lap onto the MMU becomes gradually younger towards

the central part of the North Sea Basin. This introduces a

range of uncertainty in the onset of deposition, which has

been estimated as 12.4 Ma in the Danish sector (Sùrensen

et al., 1997; Michelsen et al., 1998) and 10.7 Ma in the

German sector (Streif, 1996; Breiner, 1999). A best-®t

negative exponential curve through the scattered age data

(AM-0) seems to underestimate the onset of deposition of

the deltaic units (Fig. 4). We therefore constrained two

age models to comply with the ages reported above. The

®rst age model (AM-1) is based on an age of onset equal to

10.7 Ma, the second (AM-2) uses an age of 12.4 Ma

(Fig. 4). We will use AM-1 in all subsequent calculations,

and use the second only to examine the degree of

robustness of our correlation with external forcing

mechanisms. The interpolated AM-1 ages of the deltaic

units are summarized and compared to the sequence

stratigraphy of the Eastern North Sea (Michelsen et al.,1998; Huuse, 2000) in Fig. 5.

DELTA ARCHITECTURE

Unit boundaries and 2-D architecture

The Eridanos units overlying the MMU have a maximum

thickness of 1500 m. Twenty-eight unit boundaries were

distinguished (Bishop-Kay, 1993). Complete stacking of

all 27 units does not occur due to delta progradation and

lobe switching (Figs 6 and 7). The most complete stacks

of units are located in the centre of the basin. In the

western part of the basin only the youngest units are

represented. Along both margins the unit boundaries

show signs of postdepositional uplift. Most units

terminate updip in an onlap con®guration. Unit bound-

aries extending over larger areas are associated with

aggradational units. Downdip the unit boundaries

terminate against the MMU in a downlap con®guration

indicative of progradation. These prograding units are

overlain by distinctive units 100±300 m in thickness,

representing the delta top facies. A comparable large-scale

shallowing-upward sequence has been described from the

central and south-eastern North Sea (Michelsen et al.,

1995, 1998; Sùrensen et al., 1997). Truncation of

re¯ectors associated with erosional features or channels

Fig. 3. Three boreholes forming a SE±NW cross-section through the Dutch and German sectors matched with the nearest

data points from the seismo-stratigraphic data base. Core interpretations and gamma-ray log adapted from Sha et al. (1996) and

Streif (1996).



at the top of units along the delta front has been observed

(Fig. 8).

The lowermost units are dominated by oblique

clinoforms with dips of up to 10u. Distinct packages of

oblique clinoforms have been recognized, representing

either parasequences or lateral shifts of the system. The

estimated water depth, based on the assumption that the

of¯ap break coincides approximately with sea level, is up

to 300 m in the centre of the basin. This type of

architecture indicates a high-energy regime with a high

rate of sediment supply and a relatively low rate of

accommodation (suggesting a fairly stable sea level and

limited subsidence). Near the margins of the basin, coeval

shingled clinoforms with planar, gently dipping, pro-

gradational geometry have been identi®ed. Hummocky

clinoforms with irregular, discontinuous and subparallel

re¯ectors occur toward the landward limit of the units

dominated by high-angle oblique clinoforms. These

smaller units are interpreted as small interdeltaic lobes

or channel-levee systems in the ¯uvial part of the delta.

Typical dips of such hummocky clinoforms are 2.5u as

observed in the 3-D seismic data of the L08 block

(Steeghs et al., 2001). The limited thickness of stacked

beds (10 m) suggests that this type of clinoform is typical

of deltas that prograde in shallow water, which is

corroborated by the thickness of stacked units deduced

from the gamma-ray log of the L08 block (Fig. 8).

Characteristic chaotic re¯ector con®gurations have been

observed in units D10±D14. This seismic facies has

been interpreted as slump deposits, i.e. a recorder of

synsedimentary deformation due to slope instability

Fig. 4. Age models for the

seismo-stratigraphic units based on all

available stratigraphic data. AM-0 is

the unconstrained least-squares best ®t,

predicting an age of onset of

deposition onto the Mid Miocene

Unconformity (MMU) of 8 Ma. Two

reported ages of onset of deposition on

the MMU have been used to establish

two constrained time scenarios

(AM-1: 10.7 Ma; AM-2: 12.4 Ma).

Fig. 5. Chronostratigraphic scheme (after Berggren et al.,

1995; Harland et al., 1990). The sequence-stratigraphic

scheme of the Southern North Sea Basin proposed in this

paper is compared to the scheme of the Eastern North Sea

(Michelsen et al., 1998).

I. Overeem et al.


(Cameron et al., 1993; Sùrensen et al., 1997; Overeem

et al., 2001). In units D15 and higher, gently dipping

sigmoidal clinoforms are the dominant seismic facies in

the depocentre. Examples of such sigmoidal clinoforms

encountered in the L08 block are shown in Fig. 8. Water

depth estimated from heights of these sigmoidal clino-

form reaches a maximum of 130 m in the Central Graben

area. Parallel re¯ectors, bounding thinner units, dominate

Fig. 6. Regional NE±SW cross-section 87-06, extending about 280 km (NOPEC lines Jccg-92-9, SNSTNL-87-30 and 87-28).

Lower Pliocene compact units (for example D9) prograde from the northern margin of the basin. Progradation into the deeper

parts of the basin of units D11 and D13 can be seen. Units D6, D17 and D23 are widespread and of more uniform thickness.

Fig. 7. Regional SE±NW cross-section 87-26, extending about 330 km (NOPEC lines SNSTI 87-10, 87-12 and 87-14). It shows

the upper Pliocene and lower Pleistocene units (D18±D27) prograding from the south.



Fig. 8. Example of interpretation of seismo-stratigraphic units and speci®c features in 2D seismic sections from the L08 block.

Lengths of cross-lines are 10.7 km; inline is 17.7 km long. Two gamma-ray logs (L08-10 and L08-g1) are shown on the inline.

A: low-angle clinoforms, indicating progradation into shallow water. B: sigmoidal clinoforms. C: truncated re¯ector associated

with channel and slump deposit (sequences D13±D14). D: postdepositional uplift due to salt tectonics.

Fig. 9.

I. Overeem et al.


the uppermost units. These are generally dif®cult to

correlate regionally due to poor data quality in the upper

part of the seismic pro®les.

Morphological evolution of theEridanos delta system

Straight delta front (upper Miocene, units D1±D8)

Palaeosurface maps show a gradual shift of the depo-

centres towards the west. A straight N±S-orientated delta

front was located in the German and Danish sectors

during the earliest units D1±D4 (Fig. 9; Sùrensen et al.,1997; Clausen et al., 1999). The Eridanos river system

probably comprised two active branches that together

acted as a line source. The northward ¯owing North-

German rivers were either tributaries of the Eridanos

river system or discharged into the Southern North Sea

Basin separately. The locus of deposition was limited to

the area between the delta front and the foot of the delta

slope. All channels were relatively straight, whereas only

a few isolated distributaries developed a branching net-

work. The predominance of single-thread channels,

indicative for steep offshore slopes and high wave

energy, in conjunction with the straight delta front,

point to a wave-dominated delta regime. The delta

migrated slowly westward during unit D5 and D6.

Development of a delta plain with bifurcating channels is

con®rmed by sedimentological interpretation of borehole

data from the German sector (Fig. 3, borehole 89/4). The

locus of deposition shifted to the north during unit D7,

but during unit D8 deposition returned to the previous

site, suggesting reoccupation of the channel belt. The

oldest unit boundaries are therefore not necessarily

coupled with relative sea-level changes.

Lobate delta front (lower Pliocene, unit D9-D11)

Westward progradation continued and several large lobes

started to develop. The lobate morphology indicates a

shift in depositional regime from a wave- to a ¯uvial-

dominated delta. Rivers coming from the north-east

Fig. 9. Delta development in the Southern North Sea Basin from late Miocene to middle Pleistocene, illustrated by depth

contours of unit boundaries with superimposed channel belts and slumps. Tops D1 and D3: development of delta-channel belt

originating from the E, with N±S-orientated straight delta front. Top D8: delta-channel network was fully developed, with lobate

delta front. Top D13: delta-front slump scars and associated deposits at base of delta slope. Top D15: re-establishment of

delta-channel network. Top D19: fully developed channel network, sourced from the S and E. Tops D22 and D27: few channel

belts, over®lled basin, SW±NE-orientated delta front.



continued to be the main sources. During unit D10 and

D11 the lobes prograded further. From unit D11 onwards

sediment accumulated mainly at the base of the delta

slope. Mass movements were important, because chaotic

re¯ectors within these units are common, especially in the

centre of the basin.

Large erosional features (upper Pliocene, unit D12±D14)

A signi®cant southward shift in depocentre location of

150 km occurred during unit D12. Deposits of that stage

represent the ®rst Eridanos sediments in the M09 block,

where south to westward progradation was mapped. Two

major lobes now dominated the system. The south-

eastern lobe became the main depocentre from unit D13

onwards. Depositional environments in the eastern part of

the basin have been characterized as low-gradient deltaic

or estuarine (Fig. 3, borehole 89/9). Distributary channel

networks could not be identi®ed, but features interpreted

as lowstand wedges consisting of slump deposits and

slump scars have been observed. Interpretation of the

slump scars was facilitated by the presence of small feeder

channels testifying to headward erosion of the scars. The

slump fans and associated feeder systems are clearly

visible in the 3-D data (Fig. 10, Steeghs et al., 2001;

Overeem et al., 2001). Widespread mass movements

occurred in several phases and appear to be related to a

high rate of progradation and a persistent prodelta current

regime (Cartwright, 1995). The isopach map of unit D14

shows a large delta lobe with the apex pointing towards

the east. The combination of erosional features and the

fan-shaped geometry of the delta lobe are attributed to a

sea-level fall and subsequent lowstand.

Re-establishment of channel network(upper Pliocene ± lower Pleistocene, unit D15±D18)

During unit D15 and D16 deposition resumed at the

N±S-orientated delta front, and the distributary network

started to re-establish itself. During deposition of unit

D16 the delta front split into an upper segment

dominated by lobe features and a lower segment

formed by a base of slope fan. The N±S-orientated zone

of maximum sediment accumulation was narrow and

almost ribbon-like. The architecture of unit D17 seems

to vary laterally from aggradational in borehole 89/2 to

progradational (CU) in borehole 89/9 (Fig. 3). Sediment

input from the south increased signi®cantly during unit

D18, suggesting that the proto-Weser system became

part of the ¯uvial channel network (Gibbard, 1988). The

N±S-orientated depocentres during deposition of units

D15±D18 were located preferentially above the Central

Graben area, suggesting that local accommodation was the

principal control on delta architecture.

In®ll of isolated depressions(lower Pleistocene, units D19±D27)

Deposition became restricted to small isolated depres-

sions during unit D19. Lateral thickness variations on a

local scale, suggesting deposition in a ¯uvial environment,

are clearly visible in the M09 and L08 blocks. Unit D20 is

thin and exceptionally widespread. It represents a phase

of aggradation of the delta plain, most likely in response to

sea-level rise. Sediment was also supplied from the British

mainland during units D19±D21 (Fig. 9). The main

direction of progradation was de¯ected towards the

north-west in the later units (D22±D27), due to the

Fig. 10. Examples of delta-front failure in the 3-D seismic data of the L08 block (units D13±D14). A: slump-fan deposit

(T=724 ms). B: slump scar and feeder channel (T=644 ms).

I. Overeem et al.


increasing importance of the proto-Weser and Elbe

source, in conjunction with an increasing subsidence

rate of the Central Graben. Units D22±D27 show a

generally convex delta front. Borehole 89/2 (Fig. 3)

shows a vertical alternation of delta-top facies (channels)

and glacial tills in these units. The base of the tills may

form the dominant re¯ectors identi®ed as unit bound-

aries. It is thought that the proto-Weser and Rhine±

Meuse system became the dominant sediment sources in

the ®nal units. Only two major channel belts crossing the

delta plain were identi®ed. One of these originates from

the south-west, draining through the present Netherlands

onshore. On the local 3-D seismics these features have

been recognized as well. A single channel belt was

encountered in the L08 block (Steeghs et al., 2001) as well

as in the F09 block (Fig. 11, van de Bilt, 2000). According

to Zagwijn (1989), the Rhine started to contribute

sediment to the Eridanos delta only in the Late Tiglian

(about 1.7 million years BP, which corresponds to unit

D21). Earlier, deposition of Rhine sediment was restricted

to the Lower Rhine Graben. In addition, a southward-

¯owing distributary of the Eridanos river and an ancient

¯uvial system draining the British High continued to

supply sediment.

ANALYSIS OF DELTA PARAMETERS

De®nitions

Quantitative parameters have been extracted from the

seismic data to investigate the possible effects of

autocyclic and allocyclic controls on the formation and

stacking patterns of the 27 units. The information carried

by each of these parameters partly re¯ects the spatial and

temporal resolution of the data. Secular trends of

parameter values may therefore be more informative

than their absolute values. The following primary

parameters were measured and calculated for each unit:

V: volume of sediments between successive unit

boundaries (km3);

At: total delta plain area of the upper unit boundary (km2);

Ar: reduced delta plain area of upper unit boundary (km2);

N: number of coexisting channels in each unit (±);

L: total length of the channels in each unit (km).

The reduced delta plain area Ar is de®ned as the newly

formed surface in the time interval corresponding to

a unit, which may be much smaller than At for units

without appreciable aggradation. The reduced area is

calculated by subtracting the area where sediment

thickness is below seismic resolution (20 m) from At.

The average reduction in unit volume resulting from this

operation is 1.5%, which is well within the measurement

error of the volumes. The following parameters were

derived from the above:

D=LrAtx1: drainage density (kmx1);

H=VrArx1: average clinoform height (km).

The average clinoform height H is a measure of average

water depth in the depocentre. We believe that this

method of estimating water depth is more robust than the

conventional method, which is based on estimation of

local clinoform heights between toeset and of¯ap break.

The latter may be dif®cult to locate, and its palaeobathy-

metry is not well constrained (Huuse, 2000).

By introducing the age models shown in Fig. 4, the

following parameters were de®ned as a function of time

T (kyr):

S=VrTx1: net sediment accumulation rate

(km3 kyrx1);

F=NrTx1: bifurcation rate (kyrx1);

P=ArrTx1: rate of progradation (km2 kyrx1).

Fig. 11. Examples of the interpretation and characterization of channels based on 3-D data of the F09 block. A: distributary

channels in unit D24 (T=350 ms); B: meandering channel in one of the latest units (T=116 ms).



The bifurcation rate F is a crude measure of the rate at

which delta distributaries have formed in the time interval

corresponding to a unit. Unfortunately, our data do

not allow an assessment of the extent of inheritance of

delta distributaries from previous units. The rate of pro-

gradation P represents the average rate of growth of

delta surface area within each unit. This differs from the

term progradation as used in the conventional sequence-

stratigraphic context, which is cast in terms of distance

perpendicular to the shoreline. Moreover, the resolution

of our data does not allow us to study intra-unit

architecture, so we cannot distinguish aggradation from

progradation. The time-averaged rate of progradation

of a unit, represented by P, thus underestimates the

`true' rate of progradation in cases where progradation

alternated with aggradation.

Volumes, areas and growth rates

The total volume of the delta deposits is estimated as

62r103 km3. Volumes of individual units vary between

3.7r103 and 0.8r103 km3 (Fig. 12A). A general

decrease in unit volume can be observed during the life

span of the delta, as shown by the volumes of the Miocene

and lower Pliocene units, which are signi®cantly larger

than those of the upper Pliocene and Pleistocene units.

Some departures from this general trend are also obvious:

an abrupt decrease in unit volumes can be observed from

unit D15 onward. Only the anomalously large Pleistocene

units D21 and D26 depart from this trend. In all

probability, the total volume of sediments supplied to the

delta system has not been deposited in the delta itself.

Coeval sediments, attributed to the same system, have

been reported from the Northern North Sea (Michelsen

et al., 1995; Sùrensen et al., 1997) and from the present

onshore (Friis, 1974; Bijlsma, 1981; Streif, 1996). In

addition, signi®cant volumes are located in the Viking

Graben (Jordt et al., 1995).

The overall average net sediment accumulation rate in

the deltaic units was calculated as 6.4 km3 kyrx1, which

compares favourably to the rate of 6.9 km3 kyrx1 given

by Sùrensen et al. (1997). The net sediment accumulation

rate for the upper Miocene (units D1±D9) is approxi-

mately twice as high as the value calculated by Liu &

Galloway (1997) for the Northern North Sea in the same

period, indicating that at most 30% of the sediment

bypassed the delta. The true percentage of bypassed

sediments was probably much smaller, in view of the

likely presence of other ¯uvial sources in that area.

The average total surface area of the delta plain was

approximately 44r103 km2 (Fig. 12B). The gradual

increase of total surface area during the life span of

the delta clearly demonstrates the bias introduced by

depocentre migration due to progradation. This bias can

be removed by considering only the areas added in the

course of each unit, i.e. the reduced delta-plain areas,

that had an average size of 28r103 km2 (Fig. 12C).

The rate of sediment accumulation S increased over

time (Fig. 13A). The average clinoform height H, which

is a proxy for water depth in the depocentre, decreased in

the course of time from about 150 m to 50 m (Fig. 13B).

The massive increase in the rate of progradation P(Fig. 13C) is attributable to the combined effects of

increasing accumulation rate and decreasing water depth.

Drainage area and sediment load

The calculated volumes and surface areas have been

used to quantify drainage basin and river system para-

meters through empirical relationships based on recent

¯uvio-deltaic systems (Coleman & Roberts, 1989;

Milliman & Syvitski, 1992). The reduced delta-plain

area derived from our measurements can be directly

compared to the delta-plain areas of these modern

systems, which comprise a single Holocene prograda-

tional phase. Delta-plain areas are related to drainage

areas by the following equation (recalculated from Fig. 3

of Coleman & Roberts, 1989):

Ad=arArb

Fig. 12. Basic delta-unit parameters calculated from the

seismo-stratigraphic data base. A: volume, B: total surface

area, C: reduced surface area.

I. Overeem et al.


where Ar is the delta area in km2, Ad is the drainage

area in km2, a=1.15r103 and b=0.669. The average

drainage area resulting from this calculation is

1.1r106 km2, comparable in size to the Niger, Ganges

and Volga drainage basins (Coleman & Roberts, 1989).

For comparison, the drainage basin of the Rhine is only

0.18r106 km2, almost an order of magnitude smaller

(Middelkoop, 1998).

The drainage area has been related to the sediment load

by the following equation (Milliman & Syvitski, 1992):

Qs=crAdd

where Qs is the sediment load in 106 t yrx1. The values of

c and d vary with the maximum topographic elevation

in the drainage basin. In basins with a maximum topo-

graphic elevation of less than 1000 m, c=12 and d=0.42.

In basins with a maximum topographic elevation in excess

of 1000 m, c=50 and d=0.73. As the actual maximum

topographic elevation in the Eridanos drainage basin is

unknown, we calculated the predicted sediment load for

both scenarios (Fig. 14). According to the ®rst scenario,

the predicted sediment load ranges from 7r106 t yrx1

to 22r106 t yrx1, comparable to that of the Volga

(19r106 t yrx1). These estimations closely match the

sediment accumulation rates calculated from the unit

volumes. According to the second scenario, which is not

unlikely given the total uplift and present-day relief

of Fennoscandinavia, predicted sediment loads range

from 25r106 t yrx1 to 145r106 t yrx1, comparable to

those of the Niger (40r106 t yrx1) and Orinoco

(79r106 t yrx1). The latter estimates are clearly too

high. The anomalously large volumes of the Pleistocene

units D21 and D26 are probably attributable to a large

in¯ux of glacially derived material.

EXTERNAL CONTROLS ONDELTA EVOLUTION

Rank-correlation tests

We have assessed the in¯uence of external controls on

delta evolution by correlation of delta parameters with

various curves compiled from the literature (Figs 16±18

and Table 1). Relations between delta parameters have

been quanti®ed with Spearman's rank correlation coef®-

cient, which is a distribution-free and robust version of

Pearson's linear product-moment correlation coef®cient

(Davis, 1986; Press et al., 1992). The procedure consists

of replacing the absolute values of the parameters of

interest by their relative values or ranks. The ranks are

uniformly distributed, so that the presence of a monotone

relation between two sets of ranks can be statistically

evaluated through calculation of Pearson's linear correla-

tion coef®cient. Correlations exceeding the critical limit

for Student's t at a 95% con®dence level are considered

statistically signi®cant. The results for the most relevant

delta parameters and external factors are shown in

Table 1.

Tectonics

Uplift of the Fennoscandian Shield

Variations in sediment accumulation rates (Fig. 15A)

have been compared with a curve of relative uplift rates

compiled from various sources (Jensen & Schmidt, 1992;

Rohrman et al., 1995, 1996; Hansen, 1996; Stuevold &

Eldholm, 1996). As yet, few reliable data are available to

constrain the uplift rate of Fennoscandinavia (Japsen &

Chalmers, 2000). We have been careful not to include

estimates of uplift rates based on offshore sediment

volumes in our compilation (Fig. 15B), as this would lead

to circular reasoning. Figure 15A,B show that net

sediment accumulation rates approximately follow the

uplift pattern of Fennoscandinavia over the late Cenozoic,

apart from the anomalously high rates corresponding to

Pleistocene units D21 and D26 (1.8 and 1.0 Ma). It seems

likely that the high rate of uplift of the Fennoscandian

shield ensured a high rate of sediment supply to the

Eridanos delta, similar to the situation at the Norwegian

margin (Evans et al., 2000). Because tectonic uplift is the

ultimate driving force of erosion, these results are not

surprising. However, Clausen et al. (1999) could not

Fig. 13. Derived delta-unit parameters. A: sediment

accumulation rate; B: average clinoform height, representing

water depth in depocentre, C: rate of progradation.



demonstrate a direct impact of uplift on sedimentation

rate from the Miocene onwards.

Subsidence of the Southern North Sea Basin

Correlation of our seismo-stratigraphic data with sub-

sidence records turned out to be problematic for two

reasons. The ®rst is that subsidence records with

suf®ciently detailed time control are not available.

According to various authors, the average Miocene

subsidence rate of 5 m Myrx1 increased dramatically to

about 100 m Myrx1 during the Pliocene (Kooi et al.,1989, 1991; Joy, 1992). The available data on subsidence

rates in the study area do not allow the onset of this

acceleration to be clearly recognized (Fig. 15D). If

subsidence would have been the dominant control on

accommodation, one expects the increase of subsidence

rate at the Mio-Pliocene transition to be re¯ected in H(Fig. 15C), which essentially captures the variation of

water depth in the depocentre. The overall shallowing

trend indicates that supply exceeded accommodation.

However, there is a clear indication of deepening between

6.5 and 4.5 Ma. The second problem is that subsidence

in the North Sea Basin was not spatially uniform (Jordt

et al., 1995). High rates of subsidence in the basin centre

tended to coincide with low rates of subsidence or even

uplift along the basin margins (Cloetingh et al., 1992).

The phase of deepening in the depocentre (Fig. 15C) also

coincides with the peak in uplift of Fennoscandinavia

(Fig. 15B), testifying to the existence of this spatial

pattern of vertical movements. The comparison of our

data with the published tectonic curves thus supports

the idea that long-term average rates of supply and

accommodation are controlled by tectonics.

Sea level and climate

Introduction

The overall trend in Neogene climate is one of

deterioration, attributable to the feedback between

global tectonics, circulation patterns and growth of ice

sheets, modulated by changes in the Earth's orbital

parameters. The global climate deterioration culminated

Fig. 14. Sediment load (Qs)

calculations based on inferred drainage

area for two topographic scenarios

compared to sediment-load estimates

from unit volumes. Present-day

sediment loads for rivers with drainage

areas of comparable dimensions are

also indicated.

Table 1. Rank-correlation tests of delta parameters and external controls (using AM-1). Signi®cant correlations

(95% con®dence level) are indicated by bold type.

External controls (AM-1)

Sea level

(Haq, 1991)

Climate

(pollen)

Climate

(d18O)

D Channel network density 0.01 0.28 0.02

H Palaeo water depth 0.36 0.55 x0.70

P Rate of progradation 0.02 x0.54 0.75

S Sediment accumulation rate 0.19 x0.30 0.65

F Rate of bifurcation x0.31 x0.67 0.86

I. Overeem et al.


in the formation of extensive northern hemisphere ice

sheets from about 5 Ma onwards (Jansen et al., 1990;

Zubakov & Borzenkova, 1990; Eyles, 1993). The onset of

southern hemisphere glaciations was much earlier (Miller

et al., 1987), implying that global sea level has been

in¯uenced by glacio-eustasy throughout the Neogene. A

correlation between global sea level and climate proxies

such as oxygen-isotope records is thus to be expected, and

has indeed been reported (Eyles, 1993). Nevertheless,

unravelling of eustatic and climatic signatures in Neogene

basin ®lls may be possible if useful measures of

accommodation and supply can be extracted from the

seismo-stratigraphic data. Below we will compare two

accommodation proxies, i.e. the rate of progradation (P)

and palaeo water depth (H ) to global sea level.

Furthermore, we will compare the rate of sediment

accumulation (S) and the parameters of the delta-channel

network, which are closely related to sediment supply, to

climate proxies.

Eustatic sea-level change

The in¯uence of eustatic sea-level changes on unit

architecture was investigated by comparing our accom-

modation proxies P and H to the global sea-level curve

published by Haq (1991), as shown in Fig. 16. In spite

of the low correlation coef®cients between sea level

and delta parameters (Table 1), some remarkable simi-

larities between our data and the sea-level signal deserve

to be mentioned. The rate of progradation (Fig. 16A)

shows two distinct peaks at 1.7 and 1.0 Ma, which corre-

spond well with high-amplitude Pleistocene sea-level

falls (Fig. 16C). These same peaks are also visible in

Fig. 13(A) as intervals with anomalously high rates of

sediment accumulation. The long-term sea-level high-

stand between 6 and 3 Ma is clearly visible as a distinct

`bulge' on the water-depth curve, superimposed on the

overall gradual shallowing over time (Fig. 16B). The

plausibility of a tectonic control on this deepening phase,

which seems to have coincided with increased uplift

and sediment supply, suggests that the long-term eustatic

highstand also represents a tectonic signal. Other

signi®cant in¯uence of relative sea-level variations is

provided by the Pliocene phases of widespread mass

movement (units D12 and D14), which correlate well

with the two high-amplitude sea-level falls at 4.0 and

3.0 Ma (indicated by arrows on the water-depth curve).

Clausen et al. (1999) concluded that relative sea-level

variations correlate well with the ratio of sediment supply

over accommodation in the eastern part of the North Sea

Basin. Our data do not allow such assessments to be made,

because time control and seismic resolution are insuf®-

cient to couple intrasequence architecture to variations in

the rate of sea-level change.

Climate change

In addition to S, the rate of sediment accumulation,

several parameters of the delta-channel network have

been compared to climate proxies in order to detect a

possible climate control on the sedimentation pattern in

the Eridanos delta. The basic delta-channel parameters

are shown in Fig. 17(A,B). The channel network density

D is shown in Fig. 17(C). A general increase of channel

network density can be observed, starting from single

thread channels in the early units and culminating in unit

Fig. 15. Tectonic control on sediment supply and accommodation. A: sediment accumulation rate. B: estimated rate of uplift of

the Fennoscandian shield (based on various sources, see text for discussion). C: water depth in depocentre. D: estimated total

subsidence in the Southern North Sea Basin (after Kooi et al., 1989, 1991).



D10. Destruction of the delta-channel network during

the period of widespread delta-slope erosion and mass

movements is evident (D12±D14). Re-establishment of

the channel network is recorded by an initial phase

of drainage in a few active channel belts, followed by a

second density peak in the ®nal phases of delta evolution.

Figure 18 shows the rate of sediment accumula-

tion (Fig. 18A) and the bifurcation rate, a derived

delta-channel parameter (Fig. 18B). The climate proxies

consist of a North Atlantic d18O record (Shackleton &

Opdyke, 1977; Shackleton, 1987; Jansen et al., 1988;

Jansen et al., 1990) shown in Fig. 18(C) and a NW

European pollen record (Zagwijn & Doppert, 1987;

Zagwijn & Hager, 1987; Zagwijn, 1989) shown in

Fig. 18(D).

Variations in oxygen isotope composition have been

employed to estimate Neogene temperature changes

(Miller et al., 1987). The palynological data from the

NW European Mio-Pliocene indicate a warm moist

climate with generally low proportions of nonarboreal

pollen (herbs) and varying proportions of evergreens,

deciduous trees and conifers. The proportion of (sub-

tropical) evergreen vegetation, which is regarded as

proportional to palaeotemperature records a stepwise

cooling culminating in the glacial±interglacial ¯uctua-

tions of the late Pleistocene. There is little evidence for

major variations in precipitation during the late Neogene

(Schwarzbach, 1974), which is corroborated by the

widespread occurrence of lignite and the absence of

evaporites in this period. The terrestrial palaeotempera-

tures estimated from pollen assemblages are consistent

with evidence from marine ostracod assemblages (Wood

et al., 1993). Not surprisingly, a highly signi®cant

Fig. 16. Accommodation control on basin-®ll architecture. A: rate of progradation. B: water depth in depocentre.

C: global sea level (Haq, 1991).

Fig. 17. Delta-channel network characteristics of the units.

A: number of coexisting tributaries. B: total channel length.

C: drainage density.

I. Overeem et al.


negative correlation exists between the two climate

records shown in Fig. 18(C,D).

Table 1 shows that signi®cant correlations exist

between the climate records on the one hand, and rates

of sediment accumulation and bifurcation on the other

hand. The same analyses carried out with the alternat-

ive age model (AM-2) provide equally signi®cant cor-

relations. These relations indicate that rates of supply

and bifurcation were relatively low under subtropical and

warm temperate conditions, and relatively high during

cool periods and glacials. This seems logical, as high

sedimentation rates are typical of (cold) arid climate zones

with sparse vegetation (Kirkby, 1994). For instance, the

extremely high sedimentation rate at 1.8 Ma coincides

with the onset of glaciation of the Fennoscandian shield.

The bulldozer mechanism of the glaciers added large

amounts of sediment to the ¯uvial system (cf. Bloom,

1991; Leeder et al., 1998). The increasing bifurcation rate

over time may also be explained in terms of an increase in

the rate of sediment supply combined with the loss of

vegetation cover, because such conditions would have

favoured the development of extensive ¯uvial braid belts

and the repeated switching of delta distributaries.

DISCUSSION AND CONCLUSIONS

This seismo-stratigraphic study has documented the

evolution of the Eridanos delta in the Southern North Sea

Basin on the basis of careful integration of seismics,

logs, cores and stratigraphic age data. Analysis of the

delta-channel network resulted in estimates of channel

bifurcation rate (F ). From volumetrics and surface areas

we obtained the rate of sediment accumulation (S),

the 2-D time-averaged rate of progradation (P) and the

time- and space-averaged palaeobathymetry (H ). The

latter was helpful in identifying tectonically controlled

variations in the rate of subsidence, which seem to coin-

cide with a peak in the rate of uplift of Fennoscandinavia

and long-term eustatic highstand.

Although long-term average rates of supply and

accommodation are clearly controlled by tectonics,

there is still a lot of debate about the dominant controls

on the short-term ¯uctuations. Assessing the effects of

eustatic variations was not feasible in the absence of

suf®cient information to constrain intrasequence archi-

tecture. The most important result of our study is the

inferred climate control on rates of sediment supply and

bifurcation of the delta-channel network. Rates of supply

and bifurcation were relatively low under subtropical and

warm temperate conditions, and relatively high during

cool periods and glacials. The combination of increasing

sediment supply and decreasing vegetation density under

conditions of climate deterioration would have favoured

the development of extensive ¯uvial braid belts and the

repeated switching of delta distributaries.

This straightforward response of the system to climate

change is surprising, but not unlikely. Non-deterministic

and non-linear responses of hill slopes and drainage basins

(Bull, 1991; Weltje et al., 1998; Blum & ToÈrnqvist, 2000)

may cancel out on the large spatial and temporal scales

associated with the Eridanos system. Our study shows

that the seismic architecture of large delta systems can

provide valuable information about basinwide controls

on accommodation and supply. However, accurate

datings of North Sea Neogene sediments are needed to

reconstruct the response of this major delta to internal

Fig. 18. Climate control on delta evolution. A: sediment accumulation rate. B: bifurcation rate. C: oxygen isotope record

(Shackleton & Opdyke, 1977; Shackleton, 1987; Jansen et al., 1988, 1990). D: NW European pollen record

(Zagwijn & Doppert, 1987; Zagwijn & Hager, 1987; Zagwijn, 1989).



and external controls in more detail, and to test the

hypotheses presented above.

ACKNOWLEDGMENTS

This study is a contribution to component 6B of the

NEESDI project (Netherlands Environmental Earth

System Dynamics Initiative). The 3D seismic data

sets and well logs were made available by TNO-NITG

(National Geological Survey of the Netherlands).

Discussions with C. Mesdag, R. J. van Leeuwen and

R. Giessen (TNO-NITG) signi®cantly improved some of

the ideas presented in this paper. Their commitment to

this research is gratefully acknowledged. P. Steeghs,

S. Tigrek (DUT) and B. van de Bilt (VUA) are thanked

for their help with the 3-D seismic data interpretation.

The present paper is a continuation of previous PhD work

of C.B.K. at Edinburgh University under the supervision

of Professor Boulton, Dr Cameron and Dr Fannin. The

British Geological Survey and NOPEC are thanked for

supplying data and for their help in interpretation. We

thank J. Storms, C. van der Zwan and BR reviewers

J. Cartwright and M. Huuse for helpful comments on

an earlier version of the manuscript.

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