Chapter 1

9

Chapter 1

Introduction

1.1 General introduction

The Cenozoic North Sea Basin (Fig. 1.1a) is an intracratonic, saucer-shaped depression, straddling the Mesozoic North Sea Rift System (P.A. Ziegler, 1990). The basin was formed by isostatic adjust-ment due to post-rift thermal subsidence of the lithosphere, which was accentuated by sediment loading (P.A. Ziegler, 1990; Huuse, 2000). The present-day configuration of the North Sea Basin became apparent during the Mid- to Late Palaeocene. Roughly funnel-shaped, the present North Sea spans from –3°W to 7°E and from 50°N to 60°N. The basin is bordered by the European mainland in the East, Fennoscandia in the northeast and the British Islands in the West. North of the British Isles, and via the British Channel, the North Sea is connected to the North Atlantic Ocean (Fig. 1.1a).

Fig. 1.1 a) The North Sea and the outline of the landmasses surrounding it. The outline of the North Sea Rift

System is indicated.

b) Outline of the study area (shaded) within the Dutch North Sea sector. The outline of the Mesozoic Broad

Fourteens Basin (BFB) is indicated with a dotted line.

10

From the Late Palaeocene to the end of the Oligocene, the southern part of the North Sea was a ramp-type margin (Jacobs and De Batist, 1996) on which siliciclastic sediments were deposited. This depositional setting is characterized by a less than one-degree gradient of the basin floor and the lack of a clear shelf break (Fig. 1.2). Seismic clinoforms are difficult to resolve and coastal on-laps are often not present. On a ramp-type margin, sedimentary units are deposited semi-parallel in continuous horizontal layers. Units can be recognised over a very large area (e.g. Vandenberghe et al., 2001). The North Sea Basin experienced open marine conditions during most of the Palaeogene, interrupted by periods of uplift during which large parts of the area were sub-aerially exposed. The sediments that were deposited in the southern Dutch North Sea are mainly alternations of clays and sandy silts. The Cenozoic lithostratigraphic subdivision of the Netherlands (Van Adrichem Boogaert and Kouwe, 1997) is based on these alternations, and the occurrence of regional uncon-formities.Towards the basin centre, the contrast between the lithological units is minimized as the siliciclastic grain size decreases with increased distance from continental source areas.

The North Sea area has been extensively studied since the discovery of significant hydrocarbon re-serves during the 1960’s. The basin has been densely covered by seismic surveys, exploration and production wells, and a wealth of cores and samples was collected. This information was supple-mented with onshore outcrop data. Lithostratigraphic frameworks (NAM and RGD, 1980; Isaksen and Tonstad, 1989; Knox and Cordey, 1992; Marechal, 1993; Van Adrichem Boogaert and Kouwe, 1993-1997), geological maps and regional syntheses (e.g. Heybroek, 1974; 1975; P.A. Ziegler, 1975; 1978; 1990; 1994; W.H. Ziegler, 1975) have been published. Several studies focussed on the tectono-stratigraphic evolution of the North Sea Rift System (e.g. Van Wijhe, 1987a; Badley et al., 1989; Dronkers and Mrozek, 1991; Williams, 1993; Huyghe and Mugnier, 1994; 1995). Nu-merical and analogue modelling yielded additional information about the structural and thermal evolution of the North Sea Rift System (Kooi and Cloetingh, 1989; Kooi et al., 1989; Brun and Nalpas, 1996; Van Wees and Cloetingh, 1996; Van Balen et al., 2000; Nielsen and Hansen, 2000) and provided information about the mechanisms involved in rifting and inversion (e.g. Koopman et al., 1987; McClay, 1989; Huyghe and Mugnier, 1994; Eisenstadt and Withjack, 1995; Nalpas et al., 1995). During recent years, sequence stratigraphic studies of seismic and well data have increased the temporal resolution of interpretations (e.g. De Batist and Henriet 1995; Laursen et

sea level

siltfine sandcoarse sand

Fig. 1.2 Schematic cross-section of a ramp-type continental shelf. Note the lack of a clear shelf break and the

semi-parallel deposition of the sedimentary units.

Introduction

Chapter 1

11

al., 1995; Jacobs and De Batist, 1996; Neal, 1996; Hardenbol et al., 1998; Michelsen et al., 1998; Vandenberghe et al., 1998).Many investigations in the North Sea Basin were based on industrial data sets. These data focus on commercially interesting stratigraphic intervals and areas. As a result, information is limited for several stratigraphic intervals. For instance, research devoted to the evolution of the Dutch part of the North Sea often scarcely mentions the siliciclastic succession of the Palaeogene. This is regrettable; two of the main inversion phases affecting the North Sea Rift System occurred during the Palaeogene, but no detailed temporal and spatial tectono-stratigraphic reconstructions of the Palaeogene evolution of the Dutch part of the North Sea Basin have been published. Only general occurrence maps and lithostratigraphic descriptions are available (e.g. Keizer and Letsch, 1963; Van Staalduinen et al., 1979; Letsch and Sissingh, 1983; Zagwijn, 1989; Van Adrichem Boogaert and Kouwe, 1997; Vinken, 1998).

The geometry of the Late Palaeocene-Oligocene sediments in the Dutch part of the North Sea Basin seems to be straightforward on the large scale at which regional geological transects usually are shown (Fig. 1.3). The severely folded and faulted character of the underlying Mesozoic strata is a sharp contrast to the relatively undisturbed Cenozoic geometry. However, the Palaeogene sedi-ments in the southern Dutch North Sea were significantly influenced by inversion tectonics, salt tectonics, relative sea level fluctuations and post-depositional erosion (Letsch and Sissingh, 1983; Remmelts, 1995). The Cenozoic tectono-stratigraphic evolution of the southern Dutch North Sea had a pronounced effect on Mesozoic reservoir development in the area (Dronkers and Mrozek, 1991; Kockel, 2003). The deposition of the thick succession of Cenozoic sediments resulted in deep burial and charge of source rocks. Palaeogene inversion is also of importance. Strata that were deeply buried prior to inversion make poor reservoirs with low porosity and permeability. Migration of hydrocarbons may have preceded the formation of stratigraphic traps. Therefore, im-proved quantification of Palaeogene burial and uplift and the reconstruction of the geometry of the Palaeogene inversion zone aid in reservoir prediction and production.

4

0

2

6

Dep

th (k

m)

SW NE

Cenozoic

Mesozoic50 km0

Palaeozoic and Basement

Fig. 1.3 Regional transect across the Dutch North Sea (redrawn after Dronkers and Mrozek, 1991), illustrating

the generalized view of the Cenozoic interval obtained at such a scale.

12

1.2 Geological setting

The fractured Palaeozoic basement underlying the North Sea Basin shows two dominant fault directions, NE-SW and NW-SE, associated with the Caledonian and Variscan orogenic phases, respectively. Many of these deep faults were reactivated during subsequent tectonic phases (P.A. Ziegler, 1975; 1990; W.H. Ziegler, 1975; Van Wijhe, 1987a; Dronkers and Mrozek, 1991; Oud-mayer and De Jager, 1993; Huyghe and Mugnier, 1994). During the Permian, rifting started in the North Atlantic domain (P.A. Ziegler, 1975; 1978, 1985). The intracratonic northern and southern Permian Salt Basins developed (Fig. 1.4a), in which a thick sequence of clastics and evaporites was deposited (P.A. Ziegler, 1975; 1978). The basins were divided by the Mid-North Sea / Ringkø-bing-Fyn High (Fig. 1.4a). Rifting in the North Atlantic intensified during the Triassic. The North Sea Rift System started to form (Fig. 1.4b). An arrangement of grabens.developed, of which the major elements were the Viking Graben and Central Graben (P.A. Ziegler, 1975; 1978). During the Early Mesozoic, multiple phases of rifting occurred in the North Sea Rift System (P.A. Ziegler, 1978; 1990; Van Hoorn, 1987; Oudmayer and De Jager, 1993; Huyghe and Mugnier, 1994). During the Early Cretaceous (Fig. 1.4c), the continued rifting in the North Atlantic resulted in the formation of oceanic crust. After that, the main displacement between the North American and Eurasian plates occurred in the North Atlantic Rift Zone (P.A. Ziegler, 1975; 1978; 1990; Van Hoorn, 1987; Van Wijhe 1987a; 1987b; Dronkers and Mrozek, 1991; Oudmayer and De Jager,

AFB Alpine Foreland BasinAM Armorican MassifBFB Broad Fourteens BasinBG Bresse GrabenBM Bohemian massifCBH Cleaver Bank HighCG Central GrabenDP Danish-Polish TroughEG Eger GrabenESP East Shetland PlatformFSH Fenno-Scandian HighFT Faeroe TroughGF Great Glen FaultGG Glückstadt GrabenHG Horn GrabenLBM London-Brabant MassifLG Limagne grabenLRE Lower Rhine EmbaymentLSB Lower Saxony Basin

Palaeozoic massif

Fault-bounded graben

inverted graben and areas of compression

Alpine orogeny

Platform

Major fault (zone)

Coast line

Compression Extension

Variscan front

MC Massif CentralMFB Moray Firth BasinMNSH Mid North Sea HighOG Oslo GrabenPB Paris BasinPH Pennine HighPMB Piemont BasinPYR PyreneesRFH Ringkøbing-Fyn HighRG Rhône GrabenRM Rhenish MassifRVG Roer Valley GrabenSP Sole Pit BasinSVP Silver pit BasinTL Tornquist LineURG Upper Rhine GrabenVG Viking GrabenWNB West Netherlands BasinWSP West Shetland Platform

Legend

Fig. 1.4 (opposite page) Schematic tectonic evolution of NW Europe during the Palaeozoic and Mesozoic.

The maps are a compilation of literature results, to which is referred in the text.

a) Permian. The intracratonic Permian Salt Basin was formed.

b) Late Permian to Triassic. The North Sea Rift System developed.

c) Early Cretaceous. Rifting had propagated to the South. When oceanic crust was formed

in the North Atlantic, rifting started to abate in the North Sea Rift System.

d) Late Cretaceous. The sub-Hercynean tectonic phase resulted in inversion

of the southern basins of the North Sea Rift System.

Introduction

Chapter 1

13

FSH

OG

GF

RFHMNSH HG

LBMBMRM

TL

AM

Atlanti

c Rift

PH55

o

60o

50o

-5 0 5 10 15o o o o o

0 400 km

N

Palaeo-Tethys

northern salt basin

southern salt basin

a) Permian

variscan orogeny

VG FSH

MFB

GG

OG

GF

CG

RFHMNSH H

G

SP

LBMBMRM

TLDP

AM

Atlanti

c Rift

PH55

o

60o

50o

-5 0 5 10 15o o o o o

0 400 km

N

b) L. Permian-Triassic

VG

MNSH RFHCG

MFB

SP BFB

WNB

Indentation

OG

FSH

HG

BM

AM

?

DPTL

FT

GF

MC

PH

cont.

cont.

cont.

LBM

RM

GG

55o

60o

50o

45o

-5 0 5 10 15o o o o o

0 400 km

N

c) Early Cretaceous

RVGLRE

CG

SP BFBWNB

MNSH RFH

FSH

HG

VGFT

GF

MFB

DPTL

GG

compression

MC

PH

OG

AM

RM BM

55o

60o

50o

45o

-5 0 5 10 15o o o o

0 400 km

N

d) Late Cretaceous

RVGLRE

LSB

Indentation

14

1993; Huyghe and Mugnier, 1995). Africa started to rotate anticlockwise and northwards towards the European plate, when the South Atlantic Ocean started to open (Illies and Greiner, 1978; P.A. Ziegler, 1978; Dercourt et al., 2000). The European stress pattern changed from extension to com-pression and at the beginning of the Late Cretaceous, major rifting in the southern North Sea Rift System abated (Fig. 1.4d). Rifting continued in the North Atlantic.The Late Cretaceous and Palaeogene development of the North Sea Basin was characterized by periods of basin subsidence and sedimentation, alternating with distinct periods of tectonic activity. Three major compressive tectonic phases have been recognised and named in the North Sea Rift System (P.A. Ziegler, 1987; De Jager, 2003). These compressive phases are the Late Cretaceous sub-Hercynean phase, the Early Palaeocene Laramide phase and the Eocene-Oligocene Pyrenean

Fig. 1.5

Early-Middle Palaeocene

tectonic elements map of NW

Europe. Due to the Laramide

tectonic pulse, rifting occurred

in the Viking Graben and Moray

Firth Basin, and inversion

occurred in the Dutch Central

Graben, the Sole Pit Basin and

the Broad Fourteens Basin/West

Netherlands Basin. For legend,

see Fig. 1.4.

Introduction

CG

SPBFBWNB

Eo-Alpinecompression

OG

MNSH RFH

FSH

BM

HG

AM

FTGFGF

DPTL

GG

PB

VG

MFB

PMB

continental

compression

MC

PH

continental

RM

RVGLRE

LSB

55o

60o

50o

45o

-5 0 5 10 15o o o o

0 400 km

N

Early- Middle Palaeocene

Chapter 1

15

phase (Figs. 1.4 – 1.7). As a result, the majority of the grabens of the North Sea Rift System were inverted. The timing of periods of inversion differs between the grabens (Cooper et al., 1989; Rob-erts, 1989; Roberts et al., 1990; P.A. Ziegler, 1990; Oudmeyer and De Jager, 1993; Williams, 1993; Huyghe and Mugnier, 1995). During the sub-Hercynean tectonic phase, the Dutch Central Graben, the Sole Pit Basin, the Broad Fourteens Basin and the West Netherlands Basin were inverted (Fig. 1.4d). The sub-Hercynean phase has been associated with the onset of Alpine compression in the South (P.A. Ziegler, 1975; 1978; 1990; Van Hoorn, 1987; Van Wijhe 1987a; 1987b; Dronkers and Mrozek, 1991; Oudmayer and De Jager, 1993; Huyghe and Mugnier, 1995).In the northern North Atlantic and in the Norwegian-Greenland Sea, a renewed phase of sea-floor spreading occurred during the Palaeocene (P.A. Ziegler, 1975; 1978; 1990; Srivastava and Tap-

Fig. 1.6

Eocene tectonic elements map of

NW Europe. The North Sea rift

became tectonically inactive. Rift-

induced subsidence of the Rhine

Graben was initiated. For legend,

see Fig. 1.4

CG

SP BFB

WNB

Continent collision

OG

MNSH RFH

FSH

LSB

HG

AM

RM

FT

GF

DPTL

GG

VG

MFBUR

G

MCLG

EGRVGLRE

55o

60o

50o

45o

-5 0 5 10 15o o o o o

0 400 km

N

Eocene

16

scott, 1986). The Laramide phase, during the Mid-Palaeocene (Fig. 1.5), resulted in compression of the whole European platform (Michon et al., 2003). Rifting continued in the Viking Graben and Moray Firth Basin (P.A. Ziegler, 1975; 1978; 1990; Srivastava and Tapscott, 1986). In the southern North Sea, compression resulted in the inversion of the Dutch Central Graben, the Sole Pit Basin, the Broad Fourteens Basin and the West Netherlands Basin (P.A. Ziegler, 1978; 1990; Van Wijhe, 1987a; 1987b; Oudmayer and De Jager, 1993; Brun and Nalpas, 1996; Van Balen et al., 2000). The Laramide phase terminated during the Late Palaeocene. Inversion in the grabens of the south-ern North Sea Basin halted, as well as the rifting of the Viking Graben. Thermal subsidence in the North Sea area resulted in the formation of a saucer-shaped basin. The surrounding landmasses

Fig. 1.7Late Eocene to Early Oligocene tectonic elements map of NW Europe. Pyrenean compres-sion resulted in uplift in the Broad Fourteens Basin/West Netherlands Basin and Roer Valley Graben. For legend, see Fig. 1.4. The Rhenish Triple Junction is indica-ted with the letter 'r'.

Introduction

Chapter 1

17

emerged above sea level during the Late Palaeocene and became the main sources of the siliciclas-tic sediments that were deposited in the basin during the remainder of the Cenozoic (P.A. Ziegler, 1990; Huuse, 2000).Near the end of the Eocene (Fig. 1.7), tectonics associated with the Pyrenean orogenic phase re-sulted in renewed uplift in the grabens of the southern North Sea (Letsch and Sissingh, 1983; Van Hoorn, 1987; Van Wijhe 1987a; 1987b; Geluk, 1990; P.A. Ziegler, 1990; 1994; Geluk et al., 1994; Huyghe and Mugnier, 1995). This caused the sub-aerial exposure of large parts of the North Sea Basin, resulting in erosion of previously deposited sediments. Most of the exposed Palaeocene and Eocene sediments were only loosely consolidated, and were easily reworked and transported deeper into the basin. The Pyrenean phase ended during the Oligocene, after which marine sedi-mentation resumed. The ‘Savian’ phase of low global sea level occurred during the Miocene. This event is associated with the ‘Mid-Miocene unconformity’, a sequence boundary visible on seis-mic data throughout the North Sea Basin (Letsch and Sissingh, 1983; Van Wijhe 1987a; Cameron et al., 1993; Oudmayer and De Jager, 1993; Kuhlmann, 2004). During the Neogene, subsidence continued in the North Sea Basin. A very thick succession of delta sediments with well-developed clinoforms was deposited in the basin (Van Wijhe, 1987b; Cameron et al., 1993; Overeem et al., 2001; Kuhlmann, 2004). No major tectonic movements have been observed in the southern North Sea Basin since the Miocene.

1.3 Aim and outline of this thesis

In this thesis, a detailed tectonic and stratigraphic reconstruction of the development of the south-ern part of the Late Palaeocene - Oligocene Dutch North Sea Basin (Fig. 1.1b), is presented. The multidisciplinary research concentrates on fault geometry and sedimentary architecture in response to tectonic activity. The aim of this research is to gain an improved insight in the mechanics of inver-sion tectonics, when multiple inversion phases occurred in separate pulses. The research is based on seismic and well data, which cover a significant part of the Dutch offshore territory. Tectonic subsidence and uplift are quantified, and sources of lithospheric stress identified. Sequence strati-graphic correlation improves the temporal resolution of reconstructions. In Chapter 2, the Palaeogene tectono-stratigraphic evolution of the Broad Fourteens Basin in the southern Dutch North Sea Basin is reconstructed. The reconstruction is a case study of the re-sponse of a sedimentary basin to compressional reactivation of extensional faults in distinct pulses. In the chapter, new depth and thickness maps are presented, based on interpretation of two 2D-seismic surveys and 74 well logs. The reconstruction of the tectonic development is aided by a quantitative subsidence analysis.In Chapter 3, a sequence stratigraphic interpretation of log correlations of Late Palaeocene and Eocene successions in the southern North Sea is presented. The method enables a detailed corre-lation between the observed stratigraphy and the standard eustatic cycle chart of Hardenbol et al. (1998). This sequence stratigraphic correlation increases the resolution of the lithostratigraphic framework of Van Adrichem Boogaert and Kouwe (1997) and helps to unravel the influences of

-local tectonics and eustatic sea level variations on sedimentation in the study area.In Chapter 4, a Late Ypresian compressive tectonic phase in the Broad Fourteens Basin area is dis-cussed. This tectonic pulse is indicated by tilted strata and a Late Ypresian sedimentary sequence onlapping on a topographical relief of unconsolidated Lower Ypresian deposits along the north-eastern margin of the inverted Broad Fourteens Basin. The interpretation is aided by a high-reso-

18

lution quantitative subsidence analysis. Most evidence for the tectonic activity was subsequently removed by widespread erosion during the Pyrenean tectonic phase. In Chapter 5, the Cenozoic tectonic history of the Broad Fourteens Basin is compared with the Roer Valley Graben. These two structural elements are located close to each other in the South of the North Sea Rift System. The Roer Valley Graben is also the northwestern termination of the West European Rift System. The tectonic development of the Broad Fourteens Basin and the Roer Valley Graben is comparable during most of the Mesozoic and Palaeogene. During the Late Oli-gocene, however, the evolution of the two basins started to diverge. The possible controls on the difference in tectonic development between both structural elements are investigated.

1.4 Revised Palaeogene nomenclature

The lithostratigraphic framework of the Palaeogene of the Netherlands (Van Adrichem Boogaert and Kouwe, 1997) is not exclusively based on macroscopically recognizable sediment character-istics. The lithostratigraphic classification is partly based on biostratigraphic and sediment-petro-logic data, which does not conform to the International Stratigraphic Guide (Salvador, 1994). Additionally, the names of several lithostratigraphic units do not formally comply with rules of the ISG. Currently, a working group on the Tertiary of the Netherlands Institute of Applied Geoscience TNO is revising the Palaeogene nomenclature (Weerts et al., 2003). In the new lithostratigraphic subdivision, most of the formations and members are not changed, but several are renamed, and some units previously defined as members will be given the status of formations. Because the re-sults of the working group have not been published as yet, this thesis applies the lithostratigraphic framework of Van Adrichem Boogaert and Kouwe (1997), which is currently the standard.In Table 1.1, the lithostratigraphic units mentioned in this thesis are compared with the lithostrati-graphic units informally proposed by Weerts et al. (2003).

Introduction

Chapter 1

19

Van Adrichem Boogaert and Kouwe, 1997

Weerts et al., 2003

Rupel Fm. Rupel Subgroup

Rupel Clay Mb. Boom Fm.

Vessem Mb. Zelzate Fm. and Bilzen Fm.

Dongen Fm. Dongen Fm.

Asse Mb. Asse Mb.

Brussels Sand Mb. Brussel Mb.

Brussels Marl Mb. not described

Ieper Mb. Ieper Mb.

Basal Dongen Sand Mb. Oosteind Mb.

Basal Dongen Tuffite Mb. Layer within Oosteind Mb.

Landen Fm. Landen Fm.

Reusel Mb. Reusel Mb.

Landen Clay Mb. Liessel Mb.

Gelinden Marl Mb. Gelinden Mb.

Heers Mb. Orp Mb.

Table 1.1: The proposed names of the working group on the Tertiary from the Netherlands Institute of Applied Geoscience TNO (Weerts et al., 2003) set against the nomenclature of Van Adrichem Boogaert and Kouwe (1997). Only the lithostratigraphic units presented in this thesis are covered.