Insights into the development of major rift-related unconformities from geologically constrained...

Insights into the development ofmajor rift-relatedunconformities fromgeologically constrainedsubsidencemodelling:HaltenTerrace, offshoremidNorwayR. E. Bell,* C. A-L. Jackson,* G. M. Elliott,*,1 R. L. Gawthorpe,† Ian R Sharp 2 andLisa Michelsen 3

*Basins Research Group (BRG) Department of Earth Science & Engineering, Imperial College London,London, SW7 2AZ, UK†Department of Earth Sciences, University of Bergen, P.O. Box 7803, Bergen, Norway1 Present address LUKOIL Overseas UK Limited, London, SW1Y 4LR, UK2 Statoil Research Centre, Sandsliveien 90, Bergen, Norway3 Statoil ASA, Mølnholtet 42, Harstad, Norway

ABSTRACT

Due to the effects of sediment compaction, thermal subsidence and ‘post-rift’ fault reactivation, the

present-day geometry of buried, ancient rift basins may not accurately reflect the geometry of the

basin at any stage of its syn-rift evolution. An understanding of the geometry of a rift basin through

time is crucial for resolving the dynamics of continental rifting and in assessing the hydrocarbon

prospectivity of such basins. In this study, we have restored the Late Jurassic–Early Cretaceousgeometry of the southern Halten Terrace, offshore mid Norway, using a combination of well log-

and core-derived, sedimentological and stratigraphic data, seismic-stratigraphic observations and

reverse subsidence modelling. This integrated geological and geophysical approach has allowed the

large number of input parameters involved in flexural backstripping and post-rift thermal subsidence

modelling to be constrained. We have thus been able to determine the regional structure of the basin

at the end of the Late Jurassic–Early Cretaceous rift phase and the associated amount of crustal

stretching. Our basin geometry reconstructions reveal that, during the latest syn-rift period in the

Late Jurassic–Early Cretaceous, the Halten Terrace was characterized by a series of isolated depo-

centres, located between footwall islands, which were not connected into a single depocentre until

the Late Cretaceous (Coniacian). We show that two major unconformities, which are now vertically

offset by ca. 2 km and located ca. 60 km apart, formed at similar subaerial elevations in the Late

Jurassic–Early Cretaceous and were subsequently vertically offset by thermally induced tilting of the

basin margin. Cretaceous sediments were deposited in a single, relatively unconfined basin in water

depths of 1–1.5 km. The b profile that best restores palaeobathymetry to match our geological con-

straints is the same as that derived from summing visible post-Late Triassic heave on faults plus 25–60% additional extension to account for sub-seismic deformation. This indicates that, at least in the

southern part of the Halten Terrace, the amount of upper-crustal stretching during the Late Juras-

sic–Early Cretaceous rift phase is comparable to the total amount of lithospheric stretching, support-

ing a uniform pure-shear stretching model.

INTRODUCTION

Due to the effects of sediment compaction, thermal subsi-

dence and ‘post-rift’ fault reactivation, the present-day

geometry of buried, ancient rift basins may not accurately

reflect the geometry of the basin at any stage of its evolu-

tion. Imaging the final geometry and understanding the

sequential development of rift basins is critical, however,

because they can provide fundamental information

regarding rift evolution and lithospheric extension mech-

anisms (e.g. Shillington et al., 2008; Roberts et al., 2009).Furthermore, an understanding of the geometry of a basin

through time is crucial for the hydrocarbon industry to

determine the timing and location of source rock matura-

tion, the distribution of reservoir rocks, trap geometry at

the time of hydrocarbon migration and the timing and

location of hydrocarbon migration (e.g. Roberts et al.,2009).

Correspondence: R. E. Bell, Basins Research Group (BRG)Department of Earth Science & Engineering, Imperial CollegeLondon, Prince Consort Rd, London, SW7 2BP, UK. E-mail:[email protected]

© 2014 The AuthorsBasin Research © 2014 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists 203

Basin Research (2014) 26, 203–224, doi: 10.1111/bre.12049

EAGE

At the kilometre to tens-of-kilometre (i.e. ‘seismic’)

scale, estimates of palaeo-basin structure can be derived

from seismic-stratigraphic analysis of seismic reflection

data (e.g. Vail et al., 1977). Seismic-stratigraphic indica-

tors of palaeo-water depth, such as coastal onlap, clino-

forms and unconformities, are commonly observed at the

margins of sedimentary basins; however, there are often

fewer useful seismic-stratigraphic indicators of palaeo-

water depth towards the deeper-water basin axis (e.g.

Kjennerud & Vergara, 2005; Bell et al., 2008). Sedimen-

tological and biostratigraphic analysis of well data can also

be used to determine palaeobathymetry; however, these

well-based methods can only typically provide estimates

of palaeo-water depth at a single location (e.g. Van Hinte,

1978; Gradstein & Backstrom, 1996). Therefore, both

seismic-stratigraphic and well data methods have limita-

tions in terms of the spatial constraints they can provide

on palaeo-water depth.

An alternative method to estimate palaeo-basin

structure at the margin- or regional-scale involves ‘back-

stripping’ stratigraphy and comparing the results to geo-

dynamic basin models (e.g. Kusznir et al., 2004;

Shillington et al., 2008; Roberts et al., 2009). However,

flexural backstripping and reverse post-rift thermal subsi-

dence modelling methods require knowledge of a number

of often unknown or hard-to-determine parameters, such

as the effective elastic thickness of the lithosphere (Te)

and the amount of extension across the rift, which is often

expressed as the beta (b) stretching factor (McKenzie,

1978). Such reverse subsidence modelling studies are

often conducted at the margin scale, and tens-of-kilome-

tre scale features, such as individual fault blocks, are often

smoothed-out in the analysis and ignored as ‘noise’ in

favour of understanding larger scale lithospheric pro-

cesses (e.g. Roberts et al., 2009). To reconstruct basin

structure at the scale of individual fault blocks the input

parameters involved in reverse subsidence modelling need

to be constrained to a finer level of detail than is often

required for margin-scale studies.

In this contribution, we integrate geological (sedimen-

tology and stratigraphy) and geophysical datasets (seismic

reflection and refraction) to resolve the magnitude and

distribution of extension that occurred during Late Juras-

sic–Early Cretaceous rifting in the Halten Terrace area,

offshore Mid Norway (Fig. 1). The mid-Norwegian

margin has experienced multiple rifting events, and the

geometry of the basin following rifting in the Late Juras-

sic–Early Cretaceous is currently poorly understood. In

particular, the age and processes associated with the gen-

eration of major unconformities in the region are not well

constrained and we propose that basin geometry recon-

structions will allow us to determine not only the eleva-

tions and/or water depth at which they developed, but

also when they formed. However, a key issue related to

this approach is that the distribution and amount of litho-

spheric extension that occurred during the Late Jurassic–Early Cretaceous rift event is also unknown. This study

aims to: i) constrain the detailed, Late Jurassic–Early

Cretaceous structure of the Halten Terrace by integrating

palaeo-water depth information derived from geological

data with the results of reverse subsidence modelling

based on the interpretation of high-quality 2D seismic

reflection data, ii) determine whether major Late

Jurassic–Early Cretaceous unconformities developed in a

subaerial or submarine environment, iii) assess the post-

formation tectonic processes that have resulted in the

unconformities occurring at their current burial depths;

and to iv) determine the likely lithospheric extension

mechanism operating in the Late Jurassic–Early Creta-

ceous rift event. Furthermore, we outline a systematic

workflow that may be applicable to other rifted margins

along which complementary geophysical and geological

data are available.

REGIONAL SETTING

Tectonic background

The Halten Terrace is located between 64° and 65°30′Non the mid-Norwegian continental shelf; together with

the Trøndelag Platform, which lies to the east, the Halten

Terrace represents the eastern margin of the Vøring Basin

(Fig. 1a). The area has a complex tectonic history, which

is characterized by multiple rift events that likely initiated

in the Devonian (Blystad et al., 1995; Dor�e et al., 1997).Permo–Triassic rifting has affected some parts of the

mid-Norwegian shelf, particularly the Trøndelag

Platform and Froan Basin, however, the true extent and

magnitude of Permo–Triassic rifting is now difficult to

resolve as the associated structures and sediments are now

deeply buried, poorly imaged on seismic data and not

penetrated by wells (Blystad et al., 1995; Scheck-Wende-

roth et al., 2007).The post-Triassic structural evolution of the northern

part of the Halten Terrace has been strongly influenced

by a thick sequence of Triassic evaporites, which have

resulted in the formation of a range of complex, rift- and

halokinesis-related structural styles (Pascoe et al., 1999;Corfield & Sharp, 2000; Marsh et al., 2010; Elliott et al.,2012). The Triassic evaporites are thin and largely immo-

bile in the southern part of the Halten Terrace and thick-

skinned, basement-involved faulting dominates the

structural style and results in the formation of a large,

east-dipping half-graben (Figs 1b and 2). Major, although

as yet unquantified, rifting occurred in the Halten Terrace

area in the Late Jurassic to Early Cretaceous (ca. 140 Ma,

Roberts et al., 2009), resulting in the formation of large

normal faults and rotated fault blocks (Figs 1b and 2).

The most significant fault complexes in the study area

include the west-dipping Vingleia fault complex (VFC) in

the east, and the west-dipping Klakk Fault Complex

(KFC) in the west, which separates the Halten Terrace

from the deeper R�as Basin (Figs 1c and 2). Highly reflec-

tive, planar surfaces are present in the footwalls of both

the Vingleia and KFCs, and the structures capped by

these features are known as the Frøya High and the Skl-

© 2014 The AuthorsBasin Research © 2014 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists204

R. E. Bell et al.

inna Ridge respectively (Fig. 2). Both erosional surfaces

truncate Jurassic stratigraphy, although the Sklinna Ridge

is now buried 2 km deeper than the Frøya High (Fig. 2).

The absolute elevation of these surfaces at the time of

their formation, and therefore the processes responsible

for their creation are unknown.

The mid-Norwegian margin experienced further rif-

ting during the Early Tertiary (ca. 65 Ma; Roberts et al.,2009). This rift phase largely influenced the region to the

west of the Vøring and Møre basins, and eventually led to

continental break-up and the opening of the NE Atlantic

Ocean (e.g. Dor�e, 1991; Scheck-Wenderoth et al., 2007;Fig. 1b). Away from the rift axis, this rift event is also

responsible for basin inversion and the formation of large

domes such as the Helland-Hansen Arch (e.g. Blystad

et al., 1995). In the Halten Terrace, however, there is no

observable evidence that early Tertiary extensional fault-

ing or inversion occurred and this is supported by calcula-

tions of b stretching factor by Roberts et al. (2009), whichshow values of b = 1. Westward thickening of Cenozoic

(a) (c)

(b)

Fig. 1. (a) Simplified structural elements map of the mid-Norwegian Shelf (after Elliott et al., 2012). Profile 3-03 is an ocean bottomseismometer profile from Breivik et al. (2011). (b) Regional cross section across the Halten Terrace and Vøring margin simplified from

Faleide et al. (2010). b factor information across the profile is taken from Roberts et al. (2009). (c) Map displaying the depth to the

Base Cretaceous Unconformity in the Halten Terrace region, together with fault polygons mapped by Elliott et al., 2012. Seismic

reflection profiles and wells used in this study are labelled. Hydrocarbon fields are shown as white polygons. Selected fields for refer-

ence; 1 = Draugen, 2 = Mikkel, and 3 = �Asgard. KFC, Klakk Fault Complex; BFC, Bremstein Fault Complex; VFC, Vingleia Fault

Complex; SR, Sklinna Ridge; FH, Frøya High.


Geologically constrained subsidence modelling

sediments across the Halten Terrace may be related to

flexure associated with Early Tertiary stretching focused

further to the west (Fig. 2), or it may be related to gentle

thermally driven subsidence of the crust following the

Late Jurassic to Early Cretaceous rift event (Figs 1c and

2, Dor�e et al., 2008).

Stratigraphy

Evaporites and non-marine clastics were deposited across

the Halten Terrace during the Triassic (Fig. 3; Dalland

et al., 1988; Richardson et al., 2005). Early to early-Mid-

dle Jurassic stratigraphy in the southern Halten Terrace is

dominated by a series of paralic-to-shallow-marine, sand-

stone (Garn and Ile formations) and mudstone-rich (e.g.

Not Formation) formations (Fig. 3; Dalland et al., 1988).Drowning of the Halten Terrace during the Late Jurassic

was associated with the deposition of the deep marine,

mudstone-dominated, Melke and Spekk formations,

which form a semi-continuous drape over the underlying

rift-related relief across much of the basin (Fig. 3;

Dalland et al., 1988; Elliott et al., 2012). Seismic and well

data suggest that some of the largest, most prominent

basement highs, such as the Frøya High and Sklinna

Ridge, may, however, have remained subaerial during the

Late Jurassic and represented a source of coarse clastic

material (Fig. 2). For example, well data indicate that,

during the Late Jurassic, a sand-rich, shallow marine

system developed on the Frøya High, in the immediate

footwall of the VFC (Rogn Formation; Dalland et al.,1988; Provan, 1992).

The Base Cretaceous Unconformity (BCU) is a promi-

nent stratigraphic horizon and seismic reflection that

separates Jurassic from Cretaceous sediments across the

Halten Terrace (Figs 1c, 2 and 3). The Cretaceous Cro-

mer Knoll and Shetland groups overlie the BCU and are

primarily composed of claystones, which contain thin

beds of carbonate and sandstone that were deposited in a

deep water marine environment (Dalland et al., 1988).The Early Cretaceous (Beriasian to Cenomanian; Dalland

1

2

3

4

5

6

7

8

TWT

(s)

0 6406/8-1 6407/7-5 6407/8-1 6407/9-6 6407/9-3

10 km

Vingleia FaultComplex

Sklinna Ridge

Klakk Fault Complex

Frøya High

F7

F1F2

F3

F4

F5F6

W E

1

2

3

4

5

8

TWT

(s)

0

Draugen(a)

6406/8-1 6407/7-5 6407/8-1 6407/9-6 6407/9-3

10 km

10080604020 120Distance along profile (km)

Halten Terrace

(b)

6

7

Nordland Group

Hordaland GroupRogaland Group

Upper Shetland Group

Lower Shetland Group

Cromer Knoll

Jurassic

Pre-Jurassic

Rås Basin

*

* See Fig. 4 for detailed Cretaceous stratigraphy in footwall of Vingleia Fault Complex

Top Triassic

BCU

Fig. 2. (a) Uninterpreted and (b) interpreted seismic reflection profile MNR07-7135 across the Halten Terrace (location shown in

Fig. 1c). Interpreted horizons are colour coded as shown in the stratigraphic column presented in Fig. 3. The labelled distances along

the profile correspond with distances along the profile used for reverse subsidence modelling (e.g. Figs 5 and 6). There is no correlat-

able reflector across the profile which may be the base Triassic, however, the thickness of Triassic sediments are discussed further in

the text using information from wells away from this profile.


R. E. Bell et al.

et al., 1988) Lange Formation onlaps the BCU on the

eastern flank of the Sklinna Ridge, and is restricted to the

hanging wall of the KFC and the main Halten Terrace de-

pocentre. The Late Cretaceous Kvintos Formation (Tu-

ronian–Coniacian; Dalland et al., 1988) forms a

continuous drape across the Sklinna Ridge (Fig. 2). The

Kvintos Formation also drapes the Frøya High (Fig. 4;

6407/9-9), although the Lange formation has been

penetrated on the dip slope of the Frøya High, where it

thickens southwards from 27 m (6407/9-4) to 40 m

(6407/9-5).

The Rogaland Group consists of claystones, which

contain thin siltstone beds and, within the Tare Forma-

tion, tuffs (Dalland et al., 1988). The Tertiary Hordaland

Cre

tace

ous

Jura

ssic

Tria

ssic

Upp

erLo

wer

Mid

dle

Low

erU

pper

Group Lithostratigraphy

Base CretaceousUnconformity

Intra-MelkeUnconformity

ILE

GARNNOT

TOFTE RORTILJE

ÅRE

GREY/RED BEDSUPPER SALT

LOWER SALT

RED BEDS

MELKE

SPEKK

LYR

LANGE

LYSING

BÅ

TFA

NG

ST

VIK

ING

GR

OU

PC

RO

ME

R K

NO

LL

ROGN

Upp

er

Period Epoch

KVITNOS

NISE

SPRINGAR

TANG

Base CenozoicUnconformity

SH

ETL

AN

D

Seismic StratigraphicFramework

Top Springar

Top Lange

BCU

Top GarnTop NotTop Ile

Mudstone dominated Sandstone dominated Evaporites

Top Triassic

Pal

eoge

neN

eo-

ROGA-LAND

TARE

Q

HO

RD

A-

LAN

D

gene

Cen

ozoi

c NO

RD

-LA

ND

BRYGGE

KAI

NAUST

Top Hordaland

Top Rogaland

Mid-Kvintos

Åre coal

Tectonics

Syn

-Rift

Pos

t-Rift

‘Pre

-RIft

’

Stage

Pliocene

Miocene

Oligocene

Eocene

Palaeocene

Maastricht.

Campanian

Santonian

Coniacian

Albian

Aptian

Barremian

Beriasian

Turonian

Cenomanian

Hauterivian

Valanginian

Tithonian

Kimmeridg.

Oxfordian

Callovian

Bathonian

Bajocian

Aalenian

Toarcian

Pliensbach.

Sinemurian

Hettangian

Rheatian

Norian

Carnian

Age(Ma)

23

56

65

140

165

175

201

Red Beds

Global sea-level Local sea-level

95

Local sea-levelbasins structural highs

0 100 200-100risefall

estimate (m)marine

continental

Dee

p m

arin

e

Shal

low

mar

ine

marinecontinental

2

1

Nordland Group

Hordland Group

Rogaland Group

U. Shetland Group

L. Shetland Group

Cromer Knoll

Jurassic

Pre-Jurassic

?

177.325 mm

Fig. 3. Stratigraphic column for the Halten Terrace, modified after Dalland et al., 1988 and Elliott et al., 2012. The ‘Global sea-level

estimate’ column provides a smoothed version of the Haq et al. (1987) eustatic sea level. The ‘Local sea-level basins’ column provides

an estimate of relative sea level in the Halten Terrace and R�as Basins determined from sedimentary facies information. The curve in

the ‘Local sea-level structural highs’ column shows an estimate of relative sea level in the vicinity of the Frøya High and Sklinna Ridge

determined from sedimentary and seismic facies (see text for details). The blue stars indicate the following constraints on palaeo water

depth in the Frøya High and Sklinna Ridge areas: star 1 = the Draugen field area must be in shallow water depths in the Kimmerid-

gian and star 2 = the Sklinna Ridge must be submarine by the Turonian–Coniacian.



Group is composed of claystones, which contain thin beds

of sandstone. The Tertiary–Quaternary Nordland Group

is composed of claystones, siltstone and sandstone

(Dalland et al., 1988).

Previousmid-Norwegianmargin geometryreconstructions

Several studies have attempted to reconstruct the struc-

ture of the Vøring and Møre basins associated with the

Early Tertiary rift phase (ca. 65 Ma) (Kusznir et al.,2004; Ceramicola et al., 2005; Kjennerud & Vergara,

2005; Roberts et al., 2009). These studies have high-

lighted an apparent discrepancy between the estimates of

palaeobathymetry derived from geodynamic models and

those provided by geological data (Kusznir et al., 2004,2005; Ceramicola et al., 2005), which cannot be explained

by varying the values of input parameters in the geody-

namic modelling within realistic limits. This discrepancy

has been used to support a depth-dependent stretching

model for the Early Tertiary rift phase in the Møre and

Vøring basins (Kusznir et al., 2005).Although a substantial amount of work has focused on

restoring basin structure associated with Early Tertiary

rifting, variations in basin geometry associated with the

earlier, Late Jurassic–Early Cretaceous rift phase in the

Halten Terrace area have received considerably less atten-

tion. Using regional maps of sediment thickness, Scheck-

Wenderoth et al. (2007) propose that the rift axis of LateJurassic–Early Cretaceous rifting was focused in the Møre

and Vøring Basin areas, with an estimated b stretching

factor of 1.7–2. They also propose that an apparent lack ofpre-Cretaceous age syn-rift sediments beneath the Vøring

and Møre Basins points to minimal upper-crustal exten-

sion during the Late Jurassic–Early Cretaceous rift phase,and they thus favour a depth-dependent stretching

model.

Roberts et al. (2009) suggest that Late Jurassic–EarlyCretaceous rifting occurred by uniform stretching across

the rift, with an average beta stretching factor of 1.4

(Fig. 1b), but recognize it was probably closer to 1.1 on

the eastern basin margin and 2 towards the western rift

axis. A more detailed understanding of the distribution

and amount of Late Jurassic–Early Cretaceous rift phase

extension will provide us with improved restorations of

basin geometry and will allow us to investigate the syn-

chroneity between and causes of major unconformity

development in the region. Furthermore, the results of

this study will shed light on the dominant lithospheric

stretching mechanism (i.e. depth dependent vs. uniform)

that drove Late Jurassic–Early Cretaceous normal faulting

and subsidence.

DATASET

The data used in this study include high-quality, time-

migrated, 2D seismic reflection profiles that were

acquired by TGS-Nopec in 2005 and 2007 (Figs 1c and

2). These data have a record length of 0–8 s two-way time

Fig. 4. Stratigraphic correlation across five wells in the region of the Draugen oil field. The stratigraphic correlation has been

flattened on the Base Cretaceous Unconformity.


R. E. Bell et al.

(TWT) and provide good quality imaging down to lower

Mesozoic (i.e. Triassic) levels. Interpretation of these

seismic reflection profiles has been aided by the availabil-

ity of stratigraphic data from nine key wells, which have

been tied to the seismic data through synthetic seismo-

grams (Fig. 1c). In this contribution, we focus on line

MNR07-7135 from the southern Halten Terrace, and in

‘Geological constraints on palaeo-water depth’ section

discuss the information these datasets provide on water

depths in the area through time.

METHODOLOGY

Eight key horizons have been interpreted along the seis-

mic profiles (Seabed, top Hordaland Group, top Rogaland

Group, top Springar Formation, intra-Kvintos Forma-

tion, top Lange Formation, BCU and top Triassic; Figs 2

and 3). The tops of the Garn, Not, Ile and�Are formations

have also been locally interpreted (Fig. 2). The thickness

of the underlying Triassic unit in the immediate vicinity

of MNR07-7135 is unknown because it has not been fully

penetrated by wells and there is no obvious candidate

base-Triassic reflection on seismic data (Fig. 2). Lithol-

ogy information in wells was provided by final well

reports and wireline log data, and this allowed us to deter-

mine the likely depositional environments for the various

stratigraphic units. However, biostratigraphic data are

lacking in the Upper Jurassic to Lower Cretaceous suc-

cession, thus we cannot directly constrain palaeo-water

depths in the corresponding time periods. The lack of

these data was a primary motivation for this study.

In this study, we reconstruct the basin geometry along

line MNR07-7135 using reverse-basin modelling tech-

niques, and compare the basin reconstructions with infor-

mation on palaeo-water depths derived from well and

seismic data. Reverse-basin models that can reconstruct

the basin geometry to meet the geological constraints on

palaeo-water depth are deemed viable solutions, and those

that do not are not considered further. In this way we

develop a suite of viable basin reconstructions for the

study area.

The reconstruction of Late Jurassic–Early Cretaceous

basin structure in the southern Halten Terrace requires

the removal of the effects of post-BCU sediment loading

and post-rift thermal subsidence (Fig. 2). The depth to

the BCU, after the removal of sediment loading and

tectonic subsidence, will provide an estimate of the

bathymetry and basin structure at the end of the Late

Jurassic–Early Cretaceous rift event. To remove basin

subsidence caused by sediment loading we undertake

sequential 2D flexural backstripping and decompaction

(Watts et al., 1982; Roberts et al., 1993, 1998; Kusznir

et al., 1995; Nadin & Kusznir, 1995). The ‘backstripping’

procedure involves first removing the youngest sedimen-

tary layer and allowing the lithosphere to isostatically

rebound in response to the load removal. In addition, the

structure of the basin is also modified by the decompac-

tion of underlying sediments in response to the removal

of this load. After these adjustments have been made the

next sedimentary layer can be removed, and so on, until

the target horizon, the BCU, is reached.

The subsidence history of the Halten Terrace is not

only controlled by sediment loading, but also by thermal

subsidence following rifting. To accurately reproduce

palaeobathymetry, the thermal subsidence occurring dur-

ing the deposition of each sedimentary layer must also be

considered. With the removal of each sedimentary layer

in the backstripping procedure, a ‘thermal uplift correc-

tion’ must also be applied to correct for thermal subsi-

dence. The value of this thermal correction is determined

from lithospheric stretching models, such as that pro-

posed by McKenzie (1978), and depends on the time

since rifting ceased and the magnitude, in terms of total

stretching, of the rift event.

The modelling software we have chosen to use in the

study is Flex-Decompaction developed by Badley Geosci-

ence Ltd and details of the methodology are provided by

Roberts et al. (1997, 1998, 2009) and Ceramicola et al.(2005). In summary, the workflow involves: i) removal of

the water layer and calculation of the flexural-isostatic

response to this removal; ii) removal of the shallowest

stratigraphic unit and decompaction of the remaining sed-

iment units. Decompaction is conducted by calculating

near-surface porosity, decay constants and matrix densi-

ties for lithologies observed in wells using relationships

from Sclater & Christie (1980) (see Table 1); iii) calcula-

tion of the flexural-isostatic response to the removal of the

sediment load; iv) modelling of thermal uplift using the

McKenzie (1978) post-rift thermal subsidence model;

Table 1. Relative palaeo-water depths at different time periods within Halten Terrace basins and across structural highs, estimated

from sedimentology and seismic stratigraphy. See Fig. 3 for approximate relative sea-level curves

Period Epoch Basins (R�as, Halten, Trøndelag) Structural highs (Sklinna Ridge, Frøya high)

Cenozoic Deep marine Deep marine

Cretaceous Upper Deep marine Sklinna Ridge and Frøya High submarine by Turonian/Coniacian

(ca. 95 Ma)

Lower Deep marine No Lower Cretaceous sediment is observed on top of the Sklinna

Ridge or Frøya High

Jurassic Upper Draugen field area in shallow

marine conditions (0–100 m)

Sklinna Ridge unknown water depth. Jurassic stratigraphy eroded

over a width of ca. 16 km



and v) a correction for long-term eustatic variations in sea

level before the model is water loaded, using flexural isos-

tasy to compute the isostatic response. The workflow is

then repeated for the removal of other sediment units

down to the target horizon.

The use of 2D flexural backstripping, rather than 3D

flexural backstripping, is only permissive if there are lim-

ited lateral variations in sediment thickness. In the case of

seismic profile MNR07-7135, the thickness of sediments

deposited since the earliest Cretaceous is relatively consis-

tent over length scales of ca. 15 km to the north and south

of the profile (Fig. 1c), thus justifying the 2D flexural

backstripping method used here.

Reverse subsidence modelling requires a number of

sediment and crustal input parameters to be specified: i) a

velocity model; ii) lithology information (i.e. age, surface

porosity, decompaction coefficient, and sediment density

of sedimentary units); iii) depth to notional basement; iv)

lithospheric effective elastic thickness (Te); and v) stretch-

ing history (i.e. age of rift events and stretching factors).

Eustatic sea-level variations are also required in the

reverse-basin modelling and, following the work of

Ceramicola et al. (2005) and Roberts et al. (2009), weassume that the sea-level curve presented by Haq et al.(1987) is appropriate for the Halten Terrace. Eustatic sea

level is, however, still a controversial topic. M€uller et al.(2008) presented an update to the eustatic sea-level curve

of Haq et al. (1987), taking into account ocean basin

dynamics. This sea-level curve is within error of the Haq

et al. (1987) curve for pre-Tertiary time periods (i.e.

>65 Ma), which is the time period we primarily focus on

in this study. We have therefore elected to use the Haq

et al. (1987) curve in our reconstruction. In the ‘Sensitiv-

ity testing’ section we undertake a series of sensitivity

tests to investigate the likely range of error that may be

introduced into our analysis as a function of realistic vari-

ations in these input parameters. In the section ‘Refining

estimates of crustal input parameters’ we then justify a

preferred set of input parameters by comparing recon-

structed basin geometry with the estimates of palaeo-

water depths suggested by geological data (Geological

constraints on palaeo-water depth section and Table 1).

GEOLOGICALCONSTRAINTS ONPALAEO-WATERDEPTH

The HaltenTerraceand R�as Basin

Sedimentary facies information from wells drilled on the

Halten Terrace and in the adjacent R�as Basin to the NW

provide constraints on water depth within these basins

from the Triassic to the present day (see ‘Local sea-level

basins’ column in Fig. 3 and Table 1). No palaeo-water

depth information from biostratigraphy was available for

this study (see above), although the sedimentary succes-

sions present within these basins clearly indicate that

relative sea level has risen consistently since the Triassic;

this resulted in the deposition of marginal marine-to-shal-

low marine sediments in the Early–Middle Jurassic, and

deep water marine sediments in the Late Jurassic, Creta-

ceous and Cenozoic. This overall stratigraphic succession

broadly reflects the general pattern of global (eustatic)

sea-level change during this time (Fig. 3; Haq et al.,1987). An additional important constraint on palaeo-water

depths in the Late Jurassic–Early Cretaceous is provided

by well 6407/7-5, which is located in the south-east of the

Halten Terrace. This well terminates in the Lower Juras-

sic �Are Formation and penetrates a complete Jurassic–Cretaceous stratigraphic succession (Figs 1c and 2). This

suggests that this location was submarine throughout the

Jurassic and Early Cretaceous, although we cannot quan-

tify the water depth from the available well data.

The Frøya High

The Frøya High forms the footwall block to the west-

ward-dipping VFC, which forms the eastern margin of

the Halten Terrace (Fig. 2). The top of the Frøya High is

defined by a flat, very gently westward-dipping surface,

which is ca. 5 km wide. Seismic data indicate that Upper

Cretaceous (Turonian to Coniacian; Kvintos Formation)

strata directly overlie a thin (ca. 10 m), Upper Jurassic

(Beriasian; Spekk Formation) interval across this surface;

the Upper Jurassic interval itself unconformably overlies

a severely truncated, Middle Jurassic (Bathonian; Melke

Formation) succession (Fig. 3). Several wells have been

drilled to the east of the crest of the Frøya High, downdip

of this surface, and they penetrated a more-or-less com-

plete, relatively thick (150–200 m), Middle to Upper

Jurassic succession, which is overlain by Lower Creta-

ceous strata (Fig. 4; Lange Formation, Berriasian–Albian). Downdip to the east of the Frøya High, the

Upper Jurassic succession contains the shallow marine

Rogn Formation, which is absent on the crest of the updip

structure (i.e. in wells 6407/8-4S and 6407/9-9) and

which thickens south–south-eastwards to 40 m (i.e.

6407/9-5, Fig. 4). Well data from 6407/8-4S indicate

that the Rogn Formation was not deposited in the hang-

ing wall of the VFC (Fig. 4). Provan (1992) suggests that

the Rogn Formation was deposited in an offshore sand

bar in a shallow marine environment. These observations

collectively suggest that uplift and broadly eastward tilt-

ing of the Frøya High occurred during the Middle–LateJurassic, and that this resulted in erosion of the upper,

sand-rich part of the Middle Jurassic succession. This

erosion delivered sand to the hanging wall dipslope and

caused the deposition of the fully marine Rogn Formation

in shallow water depths that likely ranged from 0 to

100 m. Flooding of the Frøya High and draping of the

lower Middle Jurassic succession by a thin interval of

deep marine mudstone then occurred during the Late

Jurassic. This was followed by sediment deposition down-

dip of the crest of the Frøya High in the Early Cretaceous,

and eventual flooding of the Frøya High and draping by

deep marine deposits in the Turonian to Coniacian

(Fig. 4).


R. E. Bell et al.

Not only do these observations provide important

insights into the general tectono-stratigraphic develop-

ment of the Frøya High, but also they provide an excel-

lent palaeobathymetric constraint. More specifically, they

suggest that the Draugen field area, which is located

100 km along MNR07-7135 in the footwall of the VFC

(Fig. 2), and ca. 5 km to the east of the Frøya High ero-

sional unconformity, was in 0–100 m of water in the Late

Jurassic (Table 1). Furthermore, the crest of the Frøya

High must have been submarine by the Turonian–Conia-cian to allow the deposition of deep marine deposits of the

Kvintos Formation (Fig. 4). The lack of sediment deposi-

tion on the Frøya High between the Late Jurassic and

Late Cretaceous (i.e. Berriasian to Turonian/Coniacian)

suggests that it may have been subaerial or close to sea

level at this time. We do not use this inference as a con-

straint in our modelling, but instead investigate if the

Frøya High is restored to be subaerial at this time.

The Sklinna Ridge

The Sklinna Ridge, now buried to a depth of ca. 5 km,

has not been penetrated by boreholes in this area of inter-

est, thus seismic-stratigraphic observations are used to

constrain the likely palaeo-water depth and/or elevation

of this structure through time. The seismic expression of

the Sklinna Ridge is similar to that of the Frøya High; it

too is defined at BCU level by a prominent planar surface,

which is tilted gently to the west (seismic profile MNR07-

7135; Fig. 2). This ca. 16 km wide erosion surface trun-

cates eastward-tilted, Early–Middle Jurassic stratigraphy

(Fig. 2). Due to a lack of well data, it is uncertain if this

erosion occurred in a shallow marine realm in response to

the action of waves and/or tides, and/or if the erosion

occurred subaerially due to fluvial processes.

The eastern margin of the Sklinna Ridge is onlapped

by Early Cretaceous strata (Lange Formation) and the

structure is capped by Turonian to Coniacian age (Kvin-

tos Formation) strata (Fig. 2). This observation provides

another important palaeobathymetric constraint: by at

least ca. 95 Ma, the Sklinna Ridge was submerged

(Table 1; Fig. 3).

SENSITIVITY TESTING

In this study, we attempt to constrain the input parame-

ters for reverse subsidence modelling using observations

from geological and geophysical datasets. Inevitably, there

will still be a significant range in the possible values for

each of these parameters and we have therefore conducted

sensitivity tests to assess the role that each parameter

plays in controlling the restored palaeobathymetry. In this

section, we simply explore what effect variations in each

input parameter have on the restored palaeo-water

depths, without attempting to fit the geological con-

straints described in the section ‘Geological constraints

on palaeo-water depth’. In the section ‘Refining estimates

of crustal input parameters’ we then consider which com-

bination of input parameter values can best recreate a

basin structure that satisfies the water depth constraints

from sedimentological data and seismic-stratigraphic

observations (Table 1).

Parameter 1:Velocitymodel for depthconversion

Five wells that contain check-shot data lie within 10 km

of the modelled seismic profile (MNR07-7135; Fig. 1c).

The check-shot data from each of these wells are plotted

in the inset in Fig. 5 and they reveal very similar time-

depth relationships. A best-fit, second-order polynomial

relationship can be fitted to these time-depth curves such

that, for a particular TWT value, the depth in all of the

wells is within 5% of this best-fit polynomial (grey enve-

lope in Fig. 5).

The best-fit, second-order polynomial relationship has

been used to depth-convert our seismic interpretation and

they are shown together with estimates of the likely maxi-

mum and minimum depth to the horizons based on

potential �5% errors (Fig. 5b). Wells 6407/9-3 and

6407/9-6, which lie directly on MNR07-7135, reveal that

the depth conversion produces a very good fit to true

depth data (the best-fit polynomial relationship recreates

the correct depth to within 0–5% error, Fig. 5).

We have reconstructed the Late Jurassic–Early Creta-

ceous palaeobathymetry (i.e. back to the BCU surface)

using the minimum, maximum and preferred depth con-

versions presented in Fig. 5. In this sensitivity test, which

focuses solely on the impact of depth conversion on the

restoration, all other parameters shown in Fig. 6a have

been held constant. For this sensitivity analysis we choose

an arbitrary Te of 3 km, assume that there has been no

thermal subsidence since the creation of the BCU surface,

and that the Top Triassic horizon represents the notional

basement (Fig. 6a).

This model shows that variations in the resulting Late

Jurassic–Early Cretaceous palaeobathymetry due to varia-

tions in our depth conversion approach are minimal. The

depth to the Frøya High and Sklinna High in each of the

three reconstructions is almost identical (<20 m varia-

tion), and although there is some variation in water depth

on the Halten Terrace and in the R�as Basin, these are

<200 m.

We conclude that likely errors introduced into the pal-

aeobathymetry restorations due to inaccuracies in depth

conversion are minimal, particularly in the vicinity of

palaeo structural highs. We select the best-fit second-

order polynomial relationship between time and depth to

produce a preferred depth-converted model that will be

used throughout this study (i.e. Fig. 5a).

Parameter 2:Lithology

The post-rift stratigraphy of the study area has been

divided into six units that are sequentially backstripped in



the restoration (Cromer Knoll, lower Shetland, upper

Shetland, Rogaland, Hordaland and Nordland; Figs 2

and 3). The ages of the horizons that bound these units

have been determined from data provided by the Norwe-

gian Petroleum Directorate and are shown in Table 2 and

Fig. 3 (http://www.npd.no/en/).

The lithology of each of the six post-rift units has been

determined from an analysis of gamma ray logs from five

wells located within 10 km of MNR07-7135 (Fig. 1c),

and core descriptions from the Norwegian Petroleum

Directorate (http://www.npd.no/en/) and Dalland et al.(1988). The percentage of sand, shale and limestone in

each unit is summarized in Table 2. Generally, the Ter-

tiary succession is shale-dominated, the Cretaceous sedi-

ments have a small carbonate component that has been

estimated at 10%, and pre-BCU Jurassic and Triassic

sediments have a slightly higher (20%) sand component

(Table 2). Compaction parameters for each of the six

post-rift units with these given lithologies have been cal-

culated using Flex-Decompaction’s sediment properties

calculator, which uses the porosity-depth relationships of

Sclater & Christie (1980).

In addition to these six post-rift units, we have also

interpreted, where possible, seismic horizons that corre-

spond to the tops of the Garn, Not, Ile and �Are forma-

tions; these are used as marker beds, to investigate how

their structure changes during the restoration (Fig. 2).

The Flex-Decompaction software assumes that the

lithology of each sedimentary layer is laterally continuous

across the basin. This software-imposed assumption is

potentially valid in some stratigraphic units, but demon-

strably not valid in others, thus we investigate the sensi-

tivity of the restored Late Jurassic–Early Cretaceous

basin structure to variations in lithology, while keeping all

other parameters constant (Fig. 6b).

Changes in lithology, which vary between 100% sand

and 100% shale, result in 150 m variations in restored

palaeobathymetry in the Frøya High area, and 500 m

variations in the vicinity of the Sklinna Ridge (Fig. 6b);

more specifically, deeper water depths are predicted by a

basin-fill succession that is composed of 100% sand. Well

data indicate that the lithology does not vary laterally by

such large extents across the study area and our preferred

lithology estimates, which are presented in Table 2, result

in a restored Late Jurassic–Early Cretaceous palaeobathy-metry that is very similar to the 100% shale case. Varia-

tions in lithology in the order of 10–20%, which is the

maximum variation permissible from well data along

MNR07-7135, will therefore not introduce significant

variations in restored basin structure and we are confident

that lateral variations in facies will only have a minimal

effect on the restored palaeobathymetry. We use the

lithology information provided in Table 2 throughout the

rest of the study.

Parameter 3:Depth to thenotional basement

The reverse subsidence modelling method requires the

insertion of a notional basement, below which no sedi-

ment compaction occurs. Ideally, the depth to the base-

ment used in the model should be the depth to

crystalline basement; in the southern Halten Terrace,

however, due to a lack of well penetrations, it is not pos-

sible to constrain the depth to top crystalline basement.

There is also no obvious seismic reflection candidate for

the crystalline basement (Fig. 2), however, estimates of

the depth to basement in the region have been provided

by seismic refraction experiments (e.g. Scheck-Wende-

roth et al., 2007; Breivik et al., 2011; Fig. 7a). In the

sensitivity analyses described in ‘Parameter 1: Velocity

model for depth conversion’ and in ‘Parameter 2:

Lithology’ sections we have used the Top Triassic

reflection as the notional basement and have implicitly

assumed that the Triassic strata does not decompact

10 km

1

2

3

4

5

6

7

8

9

Top Triassic

BCU

Top Lange

Top SpringarTop Rogaland

Top Hordaland

Seabed

Dep

th (k

m)

0

TWT (ms)500 1500 2500 3500 4500

1000

2000

3000

4000

5000

6000

Dep

th (m

)

770901.0790581.0055.9 ++−= xxEy 2

6407/9-36407/9-6

6407/7-56407/7-26408/8-1

W EDraugen

10080604020 120

Distance along profile (km)

R2=0.9981

0

Best fit time-depth relationshipBest fit time-depth relationship + 5%

Best fit time-depth relationship - 5% Likely range of horizon depthsdue to errors in depth conversion

(a)

(b)

Fig. 5. (a) Check-shot information for five wells within 10 km of seismic profile MNR07-7135 (see Fig. 1c for location). The depth-

time relationships for each of these wells are very similar and a best-fit second-order polynomial curve can be constructed through the

data. All time-depth information from these wells can be fit within 5% of this best-fit curve. (b) Each of the eight interpreted horizons

in this study have been depth converted three times using the best-fit second-order polynomial relationship, and relationships where

depths deviate by +5% and -5% from this best-fit polynomial. This gives maximum and minimum bounds on the likely depth to the

horizon. Only seven horizons are shown here for clarity in this figure.


R. E. Bell et al.

during reverse-basin modelling. In this subsection, we

explore what influence the depth to the notional base-

ment has on the restored Late Jurassic–Early Cretaceous

basin geometry by testing notional basements defined by

the Top Triassic, and also notional top basement depths

located 1 km, 2 km and 3 km deeper than our assumed

Top Triassic reflection. The properties of Triassic

sediments used in these reconstructions are shown in

Table 2.

Deeper notional basements result in shallower recon-

structed palaeo-water depths, due to the decompaction of

the Triassic succession. Although the difference in recon-

structed palaeo-water depths resulting from a 0 m thick

and a 1 km thick Triassic succession are up to 250 m, the

difference in reconstructed palaeo-water depths for a

2 km and 3 km thick Triassic succession is less (<100 m;

Fig. 6C).

Although seismic data alone do not allow us to con-

strain the thickness of Triassic sediments in the southern

Halten Terrace region, well 6407/10-3 (Fig. 1) indicates

that a 1.1 km thick Triassic succession is present above a

granitic basement. Well 6507/12-2, which is located fur-

ther to the north on the Trøndelag Platform, penetrates a

Triassic succession that is at least 2.4 km thick. Seismic

refraction studies suggest the basement depth in the

Halten Terrace and R�as Basin could vary from 7–15 km,

leading to Triassic sediment thicknesses of 2 km in the

Halten Terrace, and potentially as great as 5 km in the

R�as Basin (Breivik et al., 2011; compare Fig. 7a and the

interpretation presented in Fig. 5b). In our restorations

of basin geometry, in the section ‘Refining estimates of

crustal input parameters’ we opt for a notional basement

depth 2 km beneath the Top Triassic horizon (i.e.

Triassic stratigraphic thickness of 2 km), although we are

Best-fit 2nd order polynomialBest-fit 2nd order polynomial + 5% depthBest-fit 2nd order polynomial - 5% depth

100% sand100% shale50% sand, 50% shaleLithology from Table 2

Te = 0 kmTe = 1.5 kmTe = 3 kmTe = 10 km

β determined from crustal thinning (Fig. 7)β determined from fault heave (Fig. 7)β profile D (Fig. 7)

0.0

1.0

2.0

3.0

0.0

1.0

2.0

3.0

0.0

1.0

2.0

3.0

0.0

1.0

2.0

3.0

0 20 40 60 80 100 120 140

Dep

th b

elow

sea

leve

l (km

)D

epth

bel

ow s

ea le

vel (

km)

Dep

th b

elow

sea

leve

l (km

)D

epth

bel

ow s

ea le

vel (

km)

Distance along MNR07-7135 (km)

Draugen

−1.0(a)

(b)

(c)

(d)

(e)

Depth conversion = variableLithology = from Table 2Depth to notional basement = Top TriassicTe = 3 kmRift history= no rift event

Depth conversion = best-fit 2nd order polynomialLithology = variableDepth to notional basement = Top TriassicTe = 3 kmRift history= no rift event

Depth conversion = best-fit 2nd order polynomialLithology = from Table 2Depth to notional basement = Top TriassicTe = variableRift history= no rift event

Depth conversion = best-fit 2nd order polynomialLithology = from Table 2Depth to notional basement = Top TriassicTe = 3 kmRift history= variable

Sklinna Ridge Halten TerraceRås Basin Frøya High Trøndelag Platform

VFCKFC

W E

0.0

1.0

2.0

3.0

Dep

th b

elow

sea

leve

l (km

)

Top Triassic Top Triassic + 1 km Top Triassic + 2 km Top Triassic + 3 km

Depth conversion = best-fit 2nd order polynomialLithology = variableDepth to notional basement = variableTe = 3 kmRift history= no rift event

Fig. 6. (a) Sensitivity test to investigate

the effect that the use of maximum, mini-

mum or best-fit depth-conversion rela-

tionships (presented in Fig. 5) has on the

restored Late Jurassic–Early Cretaceoussea floor geometry. All other parameters

are held constant and are described

within Fig. 6a. Sensitivity tests to inves-

tigate the effect (b) lithology, (c) depth to

notional basement, (d) effective elastic

thickness (Te), and (e) lithospheric

stretching factor (b) have on recon-structed Late Jurassic–Early Cretaceoussea floor geometry. VFC, Vingleia Fault

Complex, KFC, Klakk Fault Complex.



aware that errors in reconstructed palaeo-water depth, of

the order of 100 m, may occur due to unknown Triassic

sediment thickness.

Parameter 4:Effective elastic thickness

The strength of the lithosphere controls how much iso-

static rebound will occur as the result of the removal of a

sediment load. The flexural-isostatic response of the lith-

osphere is determined by the flexural rigidity, which is

commonly expressed as the effective elastic thickness or

Te (Watts, 1992, 2001). If the lithosphere has zero or little

strength (Te < 5 km) loads are supported locally (Airy

isostasy), whereas if the lithosphere is strong (Te > 5 km)

loads are supported regionally over longer wavelengths

(Watts, 1992, 2001).

The long-term strength of extended continental litho-

sphere remains controversial. Some studies have sug-

gested that extended continental crust is weakened

during rifting, and remains weak after rifting (e.g.

Watts, 1992). Other studies suggest that the lithosphere

may regain its strength after rifting (e.g. England, 1983;

Bertotti et al., 1997; Manatschal & Bernoulli, 1999; van

Wijk & Cloetingh, 2002; Close et al., 2009). We may

therefore expect Te to vary laterally between the rift axis

and margin (e.g. Watts, 1992; Bertotti et al., 1997; Re-emst & Cloetingh, 2000; P�erez-Gussiny�e et al., 2009;

Ferraccioli et al., 2011), and also with time (e.g. Eng-

land, 1983; Karner et al., 1983; Karner & Watts, 1983;

e.g. Bertotti et al., 1997; Manatschal & Bernoulli, 1999;

van Wijk & Cloetingh, 2002; Close et al., 2009). Elasticthickness is typically calculated using two methods:

i) forward modelling of lithospheric deformation using

geological and gravity data (e.g. Karner & Watts, 1983;

Stewart & Watts, 1997); and ii) determining the ‘coher-

ence function’ by cross-correlating topography and grav-

ity (e.g. McKenzie, 2003; P�erez-Gussiny�e et al., 2004).These two methods have been found to give very differ-

ent estimates of the elastic thickness for the same mar-

gin (reviewed by Sacek & Ussami, 2009).

The elastic thickness in the vicinity of the Vøring and

Møre basins is often taken as a low but finite value of

1–5 km (e.g. Roberts et al., 1998, 2009; Ceramicola et al.,2005), which is based on 2D forward flexural-cantilever

modelling (Kusznir et al., 2005). Roberts et al. (2009)

justify the use of a low Te value even when considering

the restoration of Tertiary stratigraphy, because the wave-

length of Tertiary sediment loads in the Vøring Basin is

large and shows little sensitivity to Te. Reynisson et al.(2010), in contrast, through the use of forward gravity

modelling, determine that a Te of 5 km during rifting,

increasing to 25 km during the Tertiary post-rift period,

is more appropriate to recreate the structure of the mid-

Norwegian margin. Reemst & Cloetingh (2000) have also

argued for higher values of Te in the Vøring and Møre

basin areas, opting to use a value of Te equal to the depth

to the 400 °C isotherm (ranging from 10 to 35 km),

following the work of van der Beek et al. (1994). van der

Beek (1997) argues that flexural-cantilever models will

underestimate Te, and pure-shear necking and pure-

shear/simple-shear detachment models are therefore

more appropriate to estimate Te.

Variations in the value of Te used in the reverse subsi-

dence modelling impact reconstructed palaeo-water

Table 2. Summary of lithological parameters used in flexural backstripping

Horizon

age (Ma) Horizon name Unit name Lithology

Near-surface

porosity (%)

Decay constant

(1/km)

Matrix

density (g/cc)

0 Seabed

Nordland Group 90% Shale

10% Sand

61.6 0.49 2.71

~20 Top Hordaland

Hordaland Group 100% Shale 63 0.51 2.72

~55 Top Rogaland

Rogaland Group 100% Shale 63 0.51 2.72

~65 Top Springar

Upper Shetland Group 90% Shale

10% Limest

63.7 0.53 2.72

~88 Mid-Kvintos

Lower Shetland Group 90% Shale

10% Limest

63.7 0.53 2.72

~95 Top Lange

Cromer Knoll 90% Shale

10% Limest

63.7 0.53 2.72

~140 Base Cretaceous Unconformity

Jurassic 80% Shale

20% Sand

60.2 0.46 2.71

~200~250

Top Triassic

Notional Basement

Triassic 80% Shale

20% Sand

60.2 0.46 2.71


R. E. Bell et al.

depths significantly (Fig. 6d). Low values of Te

(0–1.5 km) effectively ‘iron-out’ relief on the buried hori-

zons when the palaeobathymetry is restored; for example,

the Halten Terrace is defined by a westward-dipping

slope and the relief of the Sklinna Ridge and Frøya High

are eliminated (Fig. 6d). This restoration clearly does not

fit the seismic-stratigraphic observation that Early Creta-

ceous sediments onlap against the Sklinna Ridge, indicat-

ing significant topography must have existed in the Early

Cretaceous (Fig. 2).

Larger values of Te (>5 km) produce restored palaeo-

bathymetry that maintains the rift-related relief and indi-

cate that the Frøya High and Sklinna Ridge were

significant, positive structural features in the Late Juras-

sic–Early Cretaceous (Fig. 6d). When a Te of 10 km is

applied, the Sklinna Ridge and Frøya High are restored to

the same elevation. Despite the controversy regarding the

correct elastic thickness to use along the Norwegian mar-

gin, our initial sensitivity testing suggests higher Te values

(>5 km) may be more appropriate for Late Jurassic–EarlyCretaceous palaeobathymetry restorations in the Halten

Terrace.

Parameter 5:Stretchinghistory

The southern Halten Terrace experienced rifting in the

Permo-Triassic and Late Jurassic–Early Cretaceous, and

there is no evidence for active faulting during the Tertiary

(e.g. Blystad et al., 1995; Dor�e et al., 1997). Both the

Permo–Triassic and Late Jurassic–Early Cretaceous rift

episodes would have been followed by periods of thermal

subsidence. Thermal subsidence, as defined by McKenzie

(1978), decays as the rift cools, in an exponential manner

with a time constant of ca. 65 Myr. This means that ther-

mal subsidence occurs during the ca. 150–200 Myr after

the cessation of rifting. The timing of initiation and the

duration of the Permo–Triassic rift episode is still contro-versial, although Reemst & Cloetingh (2000) suggest it

began in the Early Permian (290 Ma) and ended in the

Late Triassic (235 Ma). Therefore, depending on when

the major phase of rifting ended there has been a duration

of 95–150 Myr between the end of the Permo–Triassicand the onset of the Late Jurassic–Early Cretaceous rift

phase. In this case, the magnitude of thermal subsidence

due to Permo–Triassic rifting at the onset of Late Juras-

sic–Early Cretaceous rifting 140 Ma may be negligible to

moderate. The amount of stretching that occurred during

the Permo–Triassic is difficult to directly determine

within the study area because Permian and Triassic strata

are too deep to be well imaged; this problem becomes

even more significant further west, towards the rift axis,

where these units are even more deeply buried. In our

reconstructions, we therefore ignore the effects of thermal

subsidence related to the earlier Permo-Triassic rift phase

on the overall amount of Late Jurassic–Early Cretaceous

subsidence, however, we consider the implications of this

assumption further in the section ‘Refining estimates of

crustal input parameters’.

To apply the appropriate thermal correction in our res-

toration, the amount of stretching that occurred in the

Late Jurassic–Early Cretaceous rift phase must be quanti-

fied, and expressed in terms of the b stretching factor

(McKenzie, 1978). Extension may be quantified in three

ways: i) upper-crustal extension from faulting; ii) whole-

crustal extension from crustal thinning; and iii) litho-

spheric extension from post-rift thermal subsidence

(reviewed in Bell et al., 2011). Calculating extension from

the degree of post-rift thermal subsidence assumes that

the palaeobathymetry is known. In our case, this is the

parameter we are attempting to restore, thus we do not

apply method (iii).

Upper-crustal extension can be estimated by summing

fault heave, which assumes that all upper-crustal

0 50 100 150

1

2

β

W EDistance along model profile (km)

β from crustal thinning

β from fault heave + 25, 40 and 60% for sub-seismic scale faulting


VFCKFC

FE

D

C

B

A

–35

–30

–25

–20

–15

–10

–5

Dep

th (k

m)

0 50 100 150 200 250 300Distance (km)

Loca

tion

of th

e B

rem

stei

n Fa

ult C

ompl

ex

Basement

Moho

Crust

0

1

2

3

4

5

6

Froan BasinBFC

Halten TerraceRås Basin

KFC

0

β

β from crustal thinning

(a)

(b)

extent of profile shown in 7b

Fig. 7. (a) Basement and Moho depth along profile 3-03 from

Breivik et al. (2011) (see Fig. 1a for location). The b profile

displayed has been calculated assuming an initial crustal thick-

ness of 35 km. KFC, Klakk Fault complex, BFC, Bremstein

Fault Complex. (b) Summary of the b profiles determined from

a measure of crustal thickness and fault heave along

MNR07-7135 (see text for details). The dashed, dotted and solid

grey lines show b profiles calculated from fault heave plus an

extra 25, 40 and 60% extension, respectively, to account for

sub-seismic deformation. KFC, Klakk Fault Complex, VFC,

Vingleia Fault Complex.



extension is accommodated on planar faults that are

observed in the seismic section. Due to the difficulty in

picking fault planes and hence constraining fault dips on

seismic sections, it is typically more accurate to measure

vertical throw from seismic sections and assess fault heave

for a range of fault dips (Lamarche et al., 2006; Bell et al.,2011). Using this method, the extension related to differ-

ent rift episodes can be isolated, by only considering

throw related to the rift event of interest.

In this study, we have measured throw on each of the

major faults offsetting the Top Triassic horizon (Fig. 2)

and these values are presented in Table 3. Using a range

of fault dips (40–60°) we calculate minimum and maxi-

mum estimates of fault heave from these throw values,

which provide extension estimates of 5–11 km. These

estimates, however, do not include sub-seismic scale

faulting, which may add as much as an additional 25–60%to estimates of extension (Marrett & Allmendinger,

1992). If this extra 25–60% extension is applied to the

heave values we have determined, this provides a revised

estimate of 6.25–17.6 km for upper-crustal extension. We

convert our estimates of fault heave to a b profile using

Badley Geoscience Ltd’s forward modelling software

‘Stretch’, which allows the heave on individual faults to

be input and pure-shear stretching envelopes for each

fault to be summed to create a b profile (Roberts et al.,2009). This has been conducted for the maximum fault

heave values (based on the assumption of 40° fault dip,

deemed favourable from depth conversion in Fig. 5) plus

25%, 40% and 60% extra extension, uniformly distrib-

uted along the profile to account for sub-seismic deforma-

tion. This results in b profiles ranging from b = ~1 on

the Trøndelag Platform, to b = ~1.2 in the R�as Basin

(Table 3 and Fig. 7b). The addition of 25%, 40% and

60% additional extension results in variations in the bprofiles of only�0.03 (Fig. 7b).

Total crustal extension can also be derived from the

magnitude of crustal thinning. Crustal thinning-derived

estimates of extension will, however, provide the cumula-

tive extension for all the rift episodes that have occurred

since basin formation started. In the case of the Halten

Terrace, crustal thinning is a function of the cumulative

effects of the Devonian, Permo–Triassic, Jurassic and,

potentially, Tertiary rift events. b stretching factors

determined from crustal thinning will therefore be an

overestimate for the amount of rifting that was exclusively

associated with the Late Jurassic–Early Cretaceous rift

phase.

Crustal structure studies across the Halten Terrace

have used seismic and potential field data, and have inter-

preted the depth to the basement and Moho along profiles

close to MNR07-7135 (Breivik et al., 2011; Fig. 7a). Byassuming an initial crustal thickness of 35 km, the crustal

thickness across profile 3-03 can be used to determine a bprofile across the Halten Terrace (Fig. 7a). This b profile

ranges from values of 1 on the eastern margin to 6 in the

western R�as Basin. The VFC occurs around 185 km

along profile 3-03 and, if we position profile MNR07-

7135 with the VFC at the same location, we can use these

data to provide another estimate of b along our line of sec-

tion (Fig. 7b). We expect this b profile to overestimate

the amount of stretching that occurred in the Late Juras-

sic–Early Cretaceous rift event; this profile, therefore,

represents maximum b values.

The fault heave and crustal thickness derived b profiles

have both been used to reconstruct palaeobathymetry,

using the assumption that the Late Jurassic–Early Creta-

ceous rift event ended at ca. 140 Ma (Roberts et al., 2009;Fig. 6e). These reconstructions, which now correct for

thermal subsidence, restore the Frøya High to water

depths of 100–200 m and come closer to fitting one of our

key geological constraints (Table 1). The restoration that

uses the b profile derived from crustal thinning predicts

that the Sklinna Ridge was subaerial in the Late Jurassic–Early Cretaceous by 400 m above sea level. In contrast,

the restoration that uses the b profile derived from fault

heave predicts that the Sklinna Ridge would have been

submarine in the Late Jurassic–Early Cretaceous in water

depths of 300 m.

The preceding sensitivity analysis has demonstrated

that uncertainty in velocity and lithology values deter-

mined from well data will not introduce large variations in

the restored palaeo- water depths (Fig. 6a, b). The uncer-

tainty in the thickness of Triassic sediments will likely

introduce errors in the range of �100 m. Although we

have been able to constrain a maximum and likely mini-

mum estimate of b stretching factor across MNR07-7135,

the variation in restored Late Jurassic–Early Cretaceous

palaeo water depths for this range of b values is significant

(up to 350 m variation), and allows the Sklinna Ridge to

be submarine or subaerial in the Late Jurassic–EarlyCretaceous for the same estimate of Te (Fig. 6e). The use

of crustal thickness- or fault heave-determined b profiles

results in differences in the restored Late Jurassic depth

to the Frøya High of 200 m. Most significantly, estimates

of Te are not well constrained in the Halten Terrace and

this results in extreme variability in restored Late

Jurassic–Early Cretaceous basin geometry (Fig. 6d).

Therefore, Te and b stretching factor are still relatively

poorly constrained and require further refinement.

Table 3. Summary of fault throw and heave values across the

Top Triassic reflector for the numbered faults in Fig. 2

Fault

number Throw (m)

Max. Heave

for 40o

faults (m)

Min. Heave

for 60°faults (m)

1 222 265 128

2 270 321 156

3 3320 3957 1917

4 263 313 152

5 421 502 243

6 500 596 289

7 4434 5285 2560

Total heave 11239 5445

Total heave + 40% 15734 7623


R. E. Bell et al.

REFINING ESTIMATESOF CRUSTALINPUT PARAMETERS

In this section, we consider which pairings of Te and bprofile can restore palaeo-water depths to meet our key

geological constraints (Table 1), namely: i) the Draugen

field area must have resided in 0–100 m water depth in

the Late Jurassic–Early Cretaceous (ca. 140 Ma); ii) the

Sklinna Ridge was submarine in the Early Cretaceous (ca.95 Ma); and iii) the site of well 6407/7-5 must have been

submarine at ca. 140 Myr. Combinations of Te and b pro-

file that do not restore palaeo-water depths to meet these

criteria can be disregarded. We then attempt to refine our

parameter estimates further by considering the other ‘sec-

ondary’ water depth constraints shown in Table 1. We

have performed 54 reverse subsidence models for various

combinations of Te and b profile values. In our analysis

we have varied Te from 1 to 25 km, and we have used six

b profiles that follow the trend but lie within of the b pro-

file ‘envelope’ derived from fault heave and crustal thin-

ning (Fig. 7b).

Figure 8a shows the restored water depth for the Drau-

gen area (100 km along MNR07-7135) at ca. 140 Ma

(backstripped to the BCU) for each of the 54 b profile and

Te pairings. Only b and Te pairs that can restore the

Draugen area to within 0–100 m water depth are permis-

sible. This geological constraint significantly reduces the

possible combinations of b and Te values to those that fall

between the 0 and 100 m water depth contours (bound by

thick lines in Fig. 8a). The fact that we have identified

other palaeo-water depth constraints (Table 1) means

that we can attempt to constrain b and Te values further

still. Figure 8b shows the shallowest restored depth of the

Sklinna Ridge in the Turonian (ca. 95 Ma) and Fig. 8c

shows the depth to the site of well 6407/7-5 (73 km along

the profile, west of the VFC, Fig. 2) in the latest Jurassic

(ca. 140 Ma). Only b and Te pairings that can restore the

Sklinna Ridge and well site 6407/7-5 so that they are sub-

marine during these time periods are permissible (area

below the thick lines in Fig. 8b, c).

Only Te and b combinations that satisfy all of these pri-

mary geological constraints are permissable, thus by com-

bining the results presented in Fig. 8a–c we have

drastically reduced the range of possible Te and b values;

Te values can only range between 3 and 13 km, and the

true b profile must lie between D–F. The possible rangeof Late Jurassic–Early Cretaceous basin geometries and,

more specifically, water depths that are produced by this

range of parameter values, are presented in Fig. 9. All of

these parameter combinations also satisfy the constraints

on Halten Terrace basin water depth from the Late Juras-

sic to the present day (see Table 1). We find that all resto-

rations using permissible combinations of Te and bindicate that the Sklinna Ridge was subaerial during the

Late Jurassic–Early Cretaceous.The range of Te and b parameter values that satisfy

constraints 1, 2 and 3 all produce similar elevations of

the Frøya High and Sklinna Ridge, although in detail

they vary in the extent to which the Sklinna Ridge was

subaerial. The Sklinna Ridge unconformity ca. 16 km

wide (Fig. 2), suggesting that the Sklinna Ridge was

exposed to prolonged subaerial erosion processes only

over an extent of ca. 16 km at this location. We, there-

fore, may have more confidence in b profile and Te

parameter pairings that uplift the Sklinna Ridge such

that it was only subaerial over a width of around 16 km

(Table 1). Figure 10a illustrates the width of the Skl-

inna Ridge that was shallower than 50 m below sea level

(effectively subaerial or exposed to wave-base erosion

processes) for the total range of Te and b pairings. The

application of this constraint further restricts the possi-

ble range of Te and b pairings to Te values between 4

B

C

D

E

F

Water depth in Draugen area ~140 Ma (km)

Te

β pr

ofile

(Fig

. 7)

Incr

easi

ng s

tretc

hing

0 10 20 Te

Depth of Sklinna Ridge by ~95 Ma (km)

Increasing lithosphere strength Increasing lithosphere strength

Incr

easi

ng s

tretc

hing

A

B

C

D

E

F

A

(a) (b)

subaerialsubmarine

0 10 20

Constraint 1: Draugen field area must be in 0 - 100 m water depth ~140 Ma

Constraint 2: The Sklinna Ridge must be submarine by ~95 Ma

subaerialsubmarine

−0.7−0.6

−0.5

−0.4

−0.3−0.2

−0.1

0

−0.2

−0.1

0

0

0.1

0.1

0.2

0.2

0.3

0.3

0.4

2 4 6 8 10 12 14 16 18 20 22 24

(c)

B

C

D

E

F

A

Te

Incr

easi

ng s

tretc

hing

Increasing lithosphere strength

β pr

ofile

(Fig

. 7)

0 10 20

Constraint 3: Well site 6407/7-5 must besubmarine ~140 Myr

Water depth in well site 6407/7-5 ~140 Ma (km)

subaerialsubmarine

−0.1

0

0

0.1

0.1

0.2

0.2

0.3

0.4

0.5

0.6 0.4 0.2 0.0 –0.2 –0.4 –0.6 0.8 0.6 0.4 0.2 0 –0.2 –0.40.6 0.4 0.2 0 –0.2 –0.4 –0.6

Fig. 8. (a) Restored Late Jurassic (ca. 140 Ma) water depth in the Draugen oil field area, (b) shallowest depth of the Sklinna Ridge in

the Early Cretaceous (ca. 95 Ma) and (c) restored Late Jurassic (ca. 140 Myr) water depth at the location of well 6407/7-5. In all of

these reconstructions the preferred time-depth relationship presented in Fig. 5 has been used for depth conversion, the lithology

values provided in Table 2 and a notional basement 2 km below the Top Triassic horizon.



and 9 km and b profiles E and F (hatched area in

Fig. 10b). We suggest that a Te of 8 km and b profile F

may be the most representative of the true values of

these crustal parameters, as these parameters bring the

subaerial width of the Sklinna Ridge closest to 16 km

(Fig. 10b). However, we stress that this study only con-

siders the elastic thickness of the rift in the Halten Ter-

race region, and the Te values suggested from our

analysis will likely only be appropriate for the rift mar-

gin area. The full restoration for Te = 8 km and b pro-

file F is shown in Fig. 11.

In this study, we have interrogated the key parameters

that are required for reverse subsidence modelling of

time-migrated seismic reflection data: velocity, lithology,

depth to notional basement, Te and b. Using four geologi-

cal constraints on palaeo-water depth we have been able

to constrain a preferred set of b and Te values, which,

although non-unique, all do result in basin restorations

which have similar geometries (Fig. 9). There are, how-

ever, a number of parameters that could plausibly impact

the restoration but have not been fully explored in this

study, these are: i) the choice of eustatic sea-level curve,

−1.0

0.0

1.0

2.0

3.0

0 20 40 60 80 100 120 140

β profile D, Te = 3 kmβ profile E, Te = 6 kmβ profile F, Te = 7 km

Dep

th b

elow

sea

leve

l (km

)

Distance along MNR07-7135 (km)

Depth conversion = best-fit 2nd order polynomialLithology = from Table 2Depth to notional basement = 2 km below Top TriassicTe = variableRift history= variable

Sklinna ridgeFrøya high

β profile F, Te = 13 km

Fig. 9. Reconstructed Late Jurassic–Early Cretaceous (ca. 140 Ma) seafloor

geometries for the full range of possible

Te and b profile values that satisfy geo-

logical constraints 1, 2 and 3 (from

Fig. 8).

30

22

24

26

28

3030


Incr

easi

ng s

tretc

hing

0 10 20

Constraint 1 satisfied




Constraints 1,2 & 3 satisfied

Constraints 1,2,3& 4 satisfied

Width of Sklinna Ridge shallower than 50 m b.s.l by ~140 Ma (km)

2 4 6 8 10 12 14 16 18 20 22 24

(a)

B

C

D

E

F

A

Te

Incr

easi

ng s

tretc

hing


β pr

ofile

(Fig

. 7)

0 10 20

Constraint 4: The width of the Sklinna Ridge thatis subaerial ~96 Ma must be between 16 - 20 km

0 4 8 12 16 20 24 28 32

(b)

Fig 10. (a) Width of the Sklinna Ridge that is subaerial in the Late Jurassic (ca. 140 Ma), for various combinations of Te and bprofile.(b) The Te and b combinations that satisfy each of the geological constraints presented in Fig. 8 are shaded. The pink, blue,

yellow and green areas show the range of Te and b profiles where constraints 1, 2, 3 and 4 are satisfied respectively. The region marked

with diagonal lines indicates parameter pairings that can satisfy constraints 1, 2 and 3, and the hatched area shows parameter values

that are capable of satisfying all four of the geological constraints.


R. E. Bell et al.

ii) the effect of Permo-Triassic thermal subsidence and

iii) the role of erosion.

M€uller et al. (2008) review a number of sea-level

curves generated using a range of methods. Their com-

parison reveals that the Haq et al. (1987) curve, which we

used in this study, generally predicts higher sea level than

many other curves by as much as ca. 200 m (c.f. Miller

et al., 2005). If the global eustatic sea level during the

Late Jurassic–Early Cretaceous (140 Ma) was lower than

that suggested by Haq et al. (1987), this would mean

lower Te values and lower b factors would be required to

restore the Draugen area to a water depth of 0–100 m

(Fig. 8a). However, this study has shown that a b profile

derived from fault heave observed on seismic profiles,

plus an additional 25% to account for sub-seismic defor-

mation (i.e. b profile F), can restore the Draugen area to

the correct depth. We cannot have less stretching across

the area than that observed on seismic sections, therefore

it may be unlikely that sea level in the Late Jurassic–Early

Cretaceous was much less than that suggested by Haq

et al. (1987).In the restoration presented here it has been assumed

that thermal subsidence following the Permo–Triassic riftphase by 140 Ma was negligible. However, if a compo-

nent of thermal subsidence from this earlier rift phase did

still affect the Halten Terrace during the Late Jurassic–Early Cretaceous, our estimates of b would be an overesti-

mate, simply because some of the subsidence we are mod-

elling and attributing to the Late Jurassic–EarlyCretaceous rift phase is actually driven by thermal subsi-

dence from the earlier rift phase. Again, due to the fact we

already only require extension observed on post-Triassic

faults plus 25% for sub-seismic deformation; that is bprofile F to restore the area to the correct depth at this

time, may indicate that thermal subsidence following the

Permo–Triassic rift phase was indeed negligible.Finally, Cenozoic strata along the eastern basin margin

have been partly eroded (Fig. 2), and the true magnitude

0

2

4

6

8

10

Dep

th (k

m)

–2

0 20W

40 60Distance (km)

80 100 120E


VFCKFC

Jurassic

Cromer KnollLower Shetland GroupUpper Shetland Group

Hordaland GroupNordland Group

0 Ma

0

2

4

6

8

10

Dep

th (k

m)

–2

~65 Ma

0

2

4

6

8

10

Dep

th (k

m)

–2

~95 Ma

Lower Cretaceous sediments deposited in deep marine conditionsSklinna Ridge submarine

Dep

th (k

m)

–2

0

2

4

6

8

10 ~140 Ma

Upper Cretaceous sediments deposited in deep marine conditions in basins and above palaeo-structrual highs

Draugen area in shallowmarine conditions (20 m)

Triassic

Notional basement

VFCKFC

VFCKFC

VFCKFC

VFCKFC

(a)

(b)

(c)

(d)

Ther

mal

Subs

iden

ce(k

m)

00.20.40.6

Ther

mal

Subs

iden

ce(k

m)

00.20.40.6

Ther

mal

Subs

iden

ce(k

m)

00.20.40.6

Thermal subsidence between 65 - 0 Ma



water

Fig. 11. Restoration of line MNR07-

7135 back to the Late Jurassic–EarlyCretaceous syn-rift geometry using the

preferred reverse subsidence modelling

input parameters described in the text. In

this restoration the best-fit polynomial

relationship has been used for depth

conversion (Fig. 5), and the lithology

information derived from wells described

in Table 2 is used. A Te value of 8 km

and b profile F, which increases from 1 to

1.2 from east to west across the profile

has been applied (Fig. 8e). KFC, Klakk

Fault Complex, VFC, Vingleia Fault

Complex. The inset graphs in Fig. 11a, b

and c show the magnitude of thermal

subsidence during the period

140–95 Ma, 95–65 Ma and 65–0 Ma

respectively. See Fig. 3 for legend of

stratigraphic units.



of subsidence at the eastern end of the model may have

therefore been underestimated (i.e. subsidence associated

with this now-missing load has not been taken into

account). Therefore, the estimates of b at the eastern end

of the modelled profile should be considered as minimum

values.

INTERPRETATION AND DISCUSSION

HaltenTerrace rift-basin geometry since theLate Jurassic

Figure 11 shows the full basin structure reconstruction

for seismic profile MNR07-7135 based on our preferred

parameter values of Te 8 km and b profile F. This restora-

tion satisfies all of the palaeo-water depth constraints that

have been derived from sedimentological and seismic-

stratigraphic methods (Table 1).

Rift Geometry ca. 140 Ma

The restoration indicates that, at the end of the Late

Jurassic–Early Cretaceous rift phase, the Sklinna Ridge

and Frøya High were both subaerial footwall islands,

confirming that the high-amplitude, planar reflections

observed in the footwalls of the Vingleia and Klakk Fault

systems are likely the result of subaerial erosion

(Fig. 11a). The restorations also suggest that the Sklinna

Ridge may have reached a greater elevation (i.e. above sea

level) than the Frøya High. At this time, the footwall

islands represented by the Sklinna Ridge and Frøya High

were surrounded by marine basins that had maximum

water depths of 1–2 km (Fig. 11a).


In the Early Cretaceous (ca. 95 Ma), the Sklinna Ridge

and Frøya High were located at water depths of 0.2–0.25 km due to post-rift thermal subsidence following the

Late Jurassic–Early Cretaceous rift phase (Fig. 11b). Sed-iment deposition at this time was still restricted to isolated

depocentres between the footwall islands, which were

sites of non-deposition and/or erosion.


By the end of the Cretaceous (ca. 65 Ma), the Sklinna

Ridge was buried by sediments and the R�as Basin,

Sklinna Ridge and Halten Terrace area had become a

single, interconnected depocentre (Fig. 11c). Due to the

eastward decrease in thermal subsidence caused by

lower levels of precursor, rift-related extension to the

east, the Frøya High was not flooded and buried until

the Coniacian (Fig. 11c). At this time, the entire south-

ern Halten Terrace became a single depocentre and Late

Cretaceous sediments were deposited in water depths of

1–1.5 km (Fig. 11c). This confirms the hypothesis

presented in a number of publications that Cretaceous

sedimentation in this area occurred in a deep marine

environment (Færseth & Lien, 2002; Scheck-Wenderoth

et al., 2007).

Rift Geometry 0 Ma

Despite the fact that the Sklinna Ridge is currently buried

>2 km deeper than the Frøya High (Fig. 11d), our recon-

structions indicate that, at the end of the Late Jurassic–Early Cretaceous rift event, they were at similar elevations

and were both subaerially exposed (Figs 9 and 11). This

is due, primarily, to the greater levels of post-rift thermal

subsidence that occurred in the R�as Basin-Sklinna Ridgearea compared to the Frøya High-Trøndelag Platform,

related to the westward increase in b stretching factor

during the Late Jurassic–Early Cretaceous (see inset

graphs that show the thermal subsidence component in

Fig. 11a–c).The results of our restorations of the geometry of the

Halten Terrace and bounding fault complexes reveal that

the present-day geometry of an ancient rift basin may not

accurately reflect the palaeostructure of the basin. Simple

geometric flattening techniques will not reproduce realis-

tic ancient fault block geometries; sediment compaction,

flexural loading and thermal subsidence must also be

accounted for.

Late Jurassic–Early Cretaceous rift eventstretchingmechanism

Previous studies in the Vøring and Møre basins have

found that palaeobathymetric estimates derived from

reverse-basin modelling using upper-crustal-derived bfactors for the Early Tertiary rift event often underesti-

mate the palaeo-water depth implied by geological evi-

dence, in some cases by up to 1 km (Ceramicola et al.,2005). These studies account for the missing tectonic

subsidence by invoking depth-dependent stretching

models for the Early Tertiary rift event, whereby the

lithosphere stretches more than the upper crust (Cerami-

cola et al., 2005; Kusznir et al., 2005). Prior to our study,Late Jurassic–Early Cretaceous lithospheric extension

was unquantified, although some authors have suggested

that this rift event too may have been controlled by

depth-dependent stretching (Scheck-Wenderoth et al.,2007).

To the contrary, our study supports the hypothesis that

the Late Jurassic–Early Cretaceous rift event, at least on

the inboard, proximal part of the margin, was controlled

by uniform, pure-shear stretching. We have found that

the amount and distribution of extension that can best

restore palaeo-water depths in the southern Halten Ter-

race and match the available geological constraints is very

similar to the b profile suggested from summing extension

on 40° dipping upper-crustal faults, with the addition of

25–60% extra extension to account for sub-seismic fault-

ing. This indicates that upper-crustal stretching factors

are adequate to explain the degree of post-rift thermal


R. E. Bell et al.

subsidence associated with the Late Jurassic–Early Creta-ceous rift event in the southern Halten Terrace. Further-

more, we have demonstrated that the larger b factors

determined from crustal thinning (Fig. 7a) are too large

to correctly restore palaeobathymetry in the Halten Ter-

race (Fig. 8), principally because this value also includes

extension associated with the Devonian and Permo-Trias-

sic rift events.

The results of the basin reconstruction presented in

this contribution show that geologically derived estimates

of palaeo-water depth can only be reconstructed using a

variable b profile which ranges from values of 1 in the

east, to 1.2–1.6 (depending on the Te value employed) in

the west. Although in detail this b profile is affected by

the location of fault blocks (Fig. 7b), in general it shows

increasing stretching to the west, indicating significant

Late Jurassic–Early Cretaceous rifting occurred further

west in the Møre and Vøring basins, an observation that,

is supported by the great thickness of Cretaceous sedi-

ments preserved within these basins (Scheck-Wenderoth

et al., 2007). The apparent lack of pre-Cretaceous stratig-raphy or direct evidence of Late Jurassic–Early Creta-

ceous fault activity in the axis of the Møre and Vøring

basins (Scheck-Wenderoth et al., 2007), is likely to be theresult of poor seismic reflection imaging below recording

times of 8 s, and lack of well control in determining the

depth to the BCU and basement, rather than the true

absence of structures and stratigraphy of this age.

In summary, the Late Jurassic–Early Cretaceous rift

event may have had a different lithospheric stretching

mechanism to the Early Tertiary rift event that led to

the opening of the NE Atlantic. This study supports a

growing body of evidence that mildly extended conti-

nental rifts (b < 2) tend to have equal crustal and litho-

sphere extension, implying rifting prior to break-up

occurs by largely uniform, pure-shear thinning of the

continental lithosphere (e.g. the North Sea, White,

1990; Gulf of Corinth, Bell et al., 2011; Beibu Gulf

Basin, South China, Clift & Lin, 2001). It has been sug-

gested that uniform pure-shear stretching may be com-

mon to the early stage of rift margin development, with

a transition to non-uniform simple-shear extension dur-

ing advanced rifting and seafloor spreading (Manatschal

& Bernoulli, 1999; Whitmarsh et al., 2001). Subsidenceanalysis in the Black Sea, where stretching factors up to

5 have been estimated, also reveal pure-shear stretching

mechanisms may be applicable (Shillington et al., 2008).Other recent studies have suggested that break-up

between rift margins may similarly be the result of uni-

form stretching between the upper and lower crust, but

with complex brittle deformation, related to sequential

multi-phase faulting, accounting for the apparent dis-

crepancy between upper and lower crustal extension

(Reston, 2009; Ranero & Perez-Gussinye, 2010). The

depth-dependent extension model, which is often pre-

scribed to the Early Tertiary break-up rift phase off-

shore mid Norway, may also need to be reassessed in

the light of the new results presented here.

IMPLICATIONSAND CONCLUSIONS

In this study, we have restored the Late Jurassic–EarlyCretaceous geometry of the southern Halten Terrace

along 2D seismic reflection profiles using a combination

of sedimentology, seismic stratigraphy and reverse post-

rift thermal subsidence modelling. We have investigated

the influence that seismic velocity, lithology, depth to

basement, effective elastic thickness and rift history have

on fault block-scale reconstructions of rift basin geometry

and have demonstrated that, despite the wide range of

possible input parameter values that could be used in

reverse subsidence modelling, the availability of four

major palaeo-water depth constraints from geological data

allows the range of viable combinations of input parame-

ter values to be significantly reduced (Fig. 10b). The vari-

ation in reconstructed basin geometry brought about by

this restricted range of possible parameter values is low

(Fig. 9), and we suggest that the fully integrated geophys-

ical and geological approach described here can be used to

reconstruct ancient rift-basin geometries with confidence

in other settings.

Late Jurassic–Early Cretaceous rift phase stretching

factors across the southern Halten Terrace derived from

fault heave range from b = 1 to 1.25 from east to west,

and stretching factors derived from crustal thinning range

from b = 1 to 2.1. These estimates provide minimum and

maximum bounds, respectively, for Late Jurassic–EarlyCretaceous stretching in the southern Halten Terrace.

Halten Terrace lithospheric elastic thickness since the

Late Jurassic–Early Cretaceous is largely unconstrained

from previous studies and may range from 1 to 25 km.

The variation in Late Jurassic and Early Cretaceous palae-

o-water depths brought about by these variations in b and

Te is extreme.

Sedimentological data from wells and seismic-strati-

graphic observations provide important constraints on

palaeobathymetry in the Late Jurassic and Early Creta-

ceous. b and Te parameter pairings that can restore palae-

obathymetry to comply with all of these constraints are

restricted between Te’s of 3–13 km and b profiles ranging

from b = 1 to 1.2 and 1 to 1.6 from east to west. The

availability of more palaeo-water depth constraints may

allow these parameters to be more tightly constrained.

Alternatively, independent estimates of Te could be made

using stratigraphic forward modelling (e.g. Kusznir et al.,1995) or process-orientated gravity modelling (e.g. Watts

& Fairhead, 1999).

For the full range of possible Te and b values the

Sklinna Ridge and Frøya High are restored to subaerial

footwall islands at the end of the Late Jurassic–EarlyCretaceous rift phase (Fig. 9), despite the Sklinna Ridge

presently residing over 2 km deeper than the Frøya High

(Fig. 2). Therefore, the present-day geometry of fault

blocks can be misleading when considering the geometry

of ancient fault blocks. To restore late syn-rift geometry

at the ‘seismic-line’ scale with confidence sediment com-

paction, flexural loading and thermal subsidence should



all be considered, rather than the use of simple horizon

flattening techniques. The late syn-rift geometry of the

Halten Terrace in the Late Jurassic–Early Cretaceous

involved a series of isolated depocentres between footwall

islands that were not connected into a single subsiding de-

pocentre until the Coniacian. Cretaceous sediments were

deposited in water depths of 1–1.5 km.

The b profile that best restores palaeobathymetry to

match geological constraints is the same as that derived

from summing post-Late Jurassic heave on faults. This

indicates that in the southern Halten Terrace the amount

of upper-crustal stretching during the Late Jurassic–EarlyCretaceous rift phase is comparable to the degree of litho-

spheric stretching, supporting a uniform pure-shear

stretching model.

ACKNOWLEDGEMENTS

Statoil ASA is thanked for providing funding and data for

the Salt Influenced Rift Basins project at Imperial College

London, University of Manchester and the University of

Bergen. In addition, we would like to thank the partners

and licence holders of PL585, 107, 348 and 93 for permis-

sion to publish this study along with Fugro Multi Client

Services for permission to use and to publish the seismic

data illustrated in Fig. 2. We thank, in particular, Paul

Wilson, Alan Roberts, Graham Yielding and Nick Kusz-

nir for useful discussion. We also thank Badley Geosci-

ence Ltd for kindly providing FlexDecompaction and

Stretch and Schlumberger for providing Petrel to Impe-

rial College London and the University of Bergen. We are

grateful to the Akademia agreement between the Univer-

sity of Bergen and Statoil ASA for visiting researcher

funds for R. Bell. The manuscript has benefitted from

detailed reviews by Tony Dor�e and an anonymous

reviewer, as well as useful comments from journal editor

Peter van der Beek.

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Manuscript received 25 April 2013; In revised form 20August 2013; Manuscript accepted 05 September 2013.


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