GEOLOGICA ULTRAIECTINA
Mededelingen van de Faculteit Geowetenschappen Universiteit Utrecht
No. 270
Stratigraphical and structural setting of the Palaeogene siliciclastic sediments in the Dutch part of the North Sea Basin
Iwan de Lugt
1 Cover illustration: a well log from the North Sea Basin
This research was carried out at the Stratigraphy-Paleontology Group, Faculty of Geosciences, Utrecht University and was financed by the Netherlands Institute of Applied Geoscience, TNO-NITG.
Address: Budapestlaan 4 3584 CD Utrecht The Netherlands
Internet site: www.geo.uu.nl
ISBN-10: 90-5744-135-7 ISBN-13: 978-90-5744-135-6
2
Stratigraphical and structural setting of the Palaeogene siliciclastic sediments in the Dutch part of the North Sea Basin
Stratigrafie en tektoniek van de Palaeogene siliciklastische sedimenten in het Nederlandse gedeelte van het Noordzeebekken
(met een samenvatting in het Nederlands)
PROEFSCHRIFT
ter verkrijging van de graad van doctor aan de Universiteit Utrecht op gezag van de rector magnificus, prof. dr. W.H. Gispen ingevolge het besluit van het college voor promoties in het openbaar te verdedigen op maandag 15 januari 2007 des middags te 12.45 uur.
door Iwan Rommert de Lugt
geboren op 31 maart 1975 te Leiderdorp, Nederland
3 Promotores:
Prof. Dr. Th.E. Wong Prof. Dr. J.E. Meulenkamp
4
... a good idea stated within an insufficient theoretical frame loses its explanatory power and is forgotten.
Hans Reichenbach (1957)
They are grubby little creatures of the sea floor 530 million years old, but we greet them with awe because they are the Old Ones, and they are trying to tell us something.
Stephen Jay Gould – Wonderful life
Hora est
pedel Universiteit Utrecht
Voor Madelon, natuurlijk.
5 Table of contents
Chapter 1 - Introduction
1.1 General introduction 9 1.2 Geological setting 12 1.3 Aim and outline of this thesis 17 1.4 Revised Palaeogene nomenclature 18
Chapter 2 - The tectonic evolution of the southern Dutch North Sea during the Palaeogene: basin inversion in distinct pulses.
Abstract 21 1. Introduction 21 2. Data and methods 22 2.1 Data 22 2.2 Seismic stratigraphy 22 2.3 Construction of depth and thickness maps 24 2.4 Quantitative subsidence analysis 26 3. Geological development of the Broad Fourteens Basin 30 3.1 Mesozoic setting 30 3.2 Palaeocene 31 3.3 Eocene 33 3.4 Pyrenean inversion 35 3.5 Oligocene 36 3.6 Neogene 37 4. Discussion 37 4.1 Subsidence patterns 37 4.2 Basin inversion 38 5. Conclusions 40 Acknowledgements 41
Chapter 3 – Reconstruction of the Late Palaeogene tectonic activity of the southern Dutch North Sea, based on a sequence stratigraphic interpretation of log correlations.
Abstract 43 1. Introduction 43 1.1 Lithostratigraphic framework 43 1.2 Geological setting and previous studies 45 2. Data and methods 47 2.1 Data 47 2.2 Wireline log correlations and sedimentary sequence interpretation 51 2.3 Age dating of the sequences 51 3. Sequence stratigraphic development 54
6
3.1 Large scale geometry 54 3.2 Detailed sequence correlation 55 3.2.1 Landen Formation (Thanetian) and Basal Dongen members (Early Ypresian) 55 3.2.2 Ieper Member (Ypresian) 55 3.2.3 Brussels members (Lutetian) 57 3.2.4 Asse Member (Bartonian) 60 3.3 Late Eocene tectonic uplift 61 4. Discussion 61 5. Conclusions 62
Chapter 4 – On the occurrence of a Early Eocene (Late Ypresian) tectonic pulse in the southern Dutch North Sea Basin.
Abstract 63 1. Introduction 63 2. Data and methods 63 3. Observations 69 3.1 Seismic geometry observations 69 3.2 Sedimentological observations 69 3.3 Subsidence analysis 71 4. Discussion 73 5. Conclusions 76
Chapter 5 –The Cenozoic evolution of the Broad Fourteens Basin and Roer Valley Graben in the context of the development of the West European Rift System.
Abstract 77 1. Introduction 77 2. Geological development 79 2.1 West European Rift System 79 2.2 Broad Fourteens Basin and Roer Valley Graben 82 3. Discussion 83 3.1. Stress propagation 85 3.2. Tectonic activity 85 4. Conclusions 89
Chapter 6 – Synthesis 91
References 93 Appendices 98
Samenvatting 107 Dankwoord 109 Curriculum Vitae 111
7 8 Chapter 1
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.
9 Introduction
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.
sea level
coarse sand fine sand silt
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.
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
10 Chapter 1 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).
SW NE 0
2
4 Depth (km) 6 Cenozoic 0 50 km Mesozoic 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.
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.
11 Introduction
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,
Legend
AFB Alpine Foreland Basin MC Massif Central Alpine orogeny AM Armorican Massif MFB Moray Firth Basin Palaeozoic massif BFB Broad Fourteens Basin MNSH Mid North Sea High BG Bresse Graben OG Oslo Graben Fault-bounded graben BM Bohemian massif PB Paris Basin CBH Cleaver Bank High PH Pennine High inverted graben and areas of compression CG Central Graben PMB Piemont Basin Platform DP Danish-Polish Trough PYR Pyrenees EG Eger Graben RFH Ringkøbing-Fyn High Major fault (zone) ESP East Shetland Platform RG Rhône Graben Coast line FSH Fenno-Scandian High RM Rhenish Massif Variscan front FT Faeroe Trough RVG Roer Valley Graben GF Great Glen Fault SP Sole Pit Basin GG Glückstadt Graben SVP Silver pit Basin Compression Extension HG Horn Graben TL Tornquist Line LBM London-Brabant Massif URG Upper Rhine Graben LG Limagne graben VG Viking Graben LRE Lower Rhine Embayment WNB West Netherlands Basin LSB Lower Saxony Basin WSP West Shetland Platform
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.
12 Chapter 1
a) Permian b) L. Permian-Triassic
N N
Atlantic Rift VG GF FSH Atlantic RiftGF FSH o o 60 60
OG MFB OG northern salt basin TL RFH TL DP HG MNSH RFH MNSH HG PH PH o o CG 55 55 southern salt basin SP GG
LBM LBM RM RM o BM o BM 50 50
AM variscan orogeny AM Palaeo-Tethys
o o o o o o o o o o -5 0 5 10 15 -5 0 5 10 15 0 400 km 0 400 km c) Early Cretaceous d) Late Cretaceous
N N FT FT VG VG GF FSH GF FSH o o 60 60 cont.
MFB OG MFB OG
TL HG TL HG DP DP MNSH RFH MNSH RFH PH PH o CG o CG 55 55
GG GG SP BFB SP BFB cont. WNB WNB LSB RVG RVG compression LBM LRE LRE o cont. o 50 RM BM 50 RM BM
AM AM
Indentation o ? MC o MC 45 Indentation 45
o o o o o o o o o -5 0 5 10 15 -5 0 5 10 15 0 400 km 0 400 km
13 Introduction
Early- Middle Palaeocene
N FT VG GF FSH o 60
MFB OG
HG TL MNSH RFH DP PH o CG 55
SP GG BFB continental LSB compression WNB RVG LRE o BM RM 50 Fig. 1.5 Early-Middle Palaeocene PB AM continental tectonic elements map of NW Europe. Due to the Laramide Eo-Alpine tectonic pulse, rifting occurred compression in the Viking Graben and Moray o MC Firth Basin, and inversion 45 PMB occurred in the Dutch Central Graben, the Sole Pit Basin and o o o o the Broad Fourteens Basin/West -5 0 5 10 15 Netherlands Basin. For legend, 0 400 km see Fig. 1.4.
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
14 Chapter 1
Eocene
N FT VG GF FSH o 60
MFB OG
TL HG DP MNSH RFH o CG 55
GG SP BFB
WNB LSB RVG LRE EG o 50 RM
AM URG
LG Continent collision Fig. 1.6 o MC 45 Eocene tectonic elements map of NW Europe. The North Sea rift became tectonically inactive. Rift- o o o o o induced subsidence of the Rhine -5 0 5 10 15 Graben was initiated. For legend, 0 400 km see Fig. 1.4 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-
15 Introduction
Fig. 1.7 Late 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'. 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
16 Chapter 1
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-
17 Introduction 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).
18 Chapter 1
Van Adrichem Boogaert and Weerts et al., 2003 Kouwe, 1997
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.
19 20 Chapter 2
Chapter 2 The tectonic evolution of the southern Dutch North Sea during the Palaeogene: basin inversion in distinct pulses.
(Published in Tectonophysics 373 (2003), 141-159)
I.R. de Lugt, J.D. van Wees1, Th. E. Wong1,2 1 Netherlands Institute of Applied Geoscience TNO – National Geological Survey, Princetonlaan 6, 3584 CB Utrecht, The Netherlands 2 Faculty of Earth Sciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, The Netherlands
Abstract
In this chapter, the response of a sedimentary basin to compressional reactivation of extensional faults in distinct pulses is discussed. The inversion of the Broad Fourteens Basin in the southern Dutch North Sea is used as a case study. The first pulse of inversion occurred during the Late Mesozoic as a result of the sub-Hercynean phase. A second pulse occurred during the Mid-Palae- ocene Laramide phase. A final inversion pulse occurred around the Eocene-Oligocene boundary and coincided with the Pyrenean tectonic event. We reconstructed the stratigraphic and structural development of the area during the Palaeogene in detail. Basic data are two 2D-seismic surveys, supported by 74 geophysical wireline logs, on which newly constructed depth and thickness maps are based. The reconstruction is aided by a quantitative subsidence analysis. Cessation of compression resulted in relaxation, and tensional reactivation of faults caused dif- ferential subsidence. Inheritance of geological structures is a key element in the tectonic activity in the Broad Fourteens Basin during the Palaeogene; tectonic movement during the Palaeogene was always accommodated by reactivation of Palaeozoic-Mesozoic faults. The intensity of inversion in the southern North Sea Basin during the Pyrenean phase appears much larger than previously noticed. Moreover, the area affected by the Pyrenean inversion is much wider than during previous phases. Minor differences in the direction of the local stress field between the tectonic phases and the deposition of a thick sedimentary succession during the periods of tectonic quiescence between the inversion phases can account for this.
Keywords: North Sea Basin; Broad Fourteens Basin; Palaeogene; basin inversion
1. Introduction
Sedimentary basin inversion occurs when basin-controlling extensional faults are reversed as a re- sult of compression tectonics. This leads to the uplift and erosion of the former basin fill (Williams et al., 1989; Dronkers and Mrozek, 1991; Huyghe and Mugnier, 1994; 1995; Nielsen and Hansen, 2000). Basin inversion has been observed by many authors and in many different tectonic settings
21 Palaeogene tectonic evolution of the southern Dutch North Sea
(e.g. Daly et al., 1989; Hayward and Graham, 1989; P.A. Ziegler, 1989; 1990; Coward et al., 1991; Buchanan and Buchanan, 1995). We address the questions to which extent variations in stress field direction and overburden thickness affect the character of inversion tectonics during pulsed inversions, whether inherited structures of previous (extensional) tectonic phases are of relevance during the inversion and whether a sedimentary basin responds in a similar manner to each pulse of inversion. The Broad Fourteens Basin in the southern Dutch North Sea (Fig. 1.1) developed in response to multiple Late Palaeozoic and Early Mesozoic rifting events (P.A. Ziegler, 1990; Oudmayer and De Jager, 1993; Huyghe and Mugnier, 1994). The basin was inverted in three distinct pulses, which resulted in an alternation of periods of tectonic activity and uplift and periods of subsidence and accumulation of thick layers of sediment. The first two periods of inversion occurred during the Late Cretaceous and Mid-Palaeocene as a result of sub-Hercynean and Laramide compression tectonics, respectively (Fig. 2.1) (e.g. Letsch and Sissingh, 1983; Van Wijhe, 1987a; 1987b; P.A. Ziegler, 1990; Dronkers and Mrozek, 1991). Around the Eocene-Oligocene boundary, a final in- version pulse, related to the Pyrenean orogenic phase (Fig. 2.1), took place (Letsch and Sissingh, 1983; Van Wijhe, 1987a; 1987b). Only limited attention has been given to the effect of the Pyrene- an phase on the temporal and spatial tectonic evolution of the area. Because it is the latest in a se- ries of pulses of inversion, however, it provides an excellent opportunity to study the influence of inheritance on the geological development, as well as of the influence of tectonic quiescence and sedimentation between pulses. We present a quantitative reconstruction of the three-dimensional stratigraphic and structural development of the area (Fig. 1.1) based on seismic and geophysical wireline log interpretation, new depth and thickness maps and quantitative subsidence analysis.
2. Data and methods
2.1 Data
This study is based on a new interpretation of two regional 2D-seismic surveys (NOPEC SNST-83 and NOPEC SNSTI-87) and the detailed wireline log analysis of 74 wells (Appendix A), of which 18 wells were tied directly to the seismic data (Fig. 2.2, Table 2.1). The seismic lines and well loca- tions cover a dense grid throughout the study area (mean line spacing is about 20 km). The seismic line density increases towards the North (Fig. 2.2). For 10 of the 74 wells, foraminifer age datings were available (TNO-NITG internal reports; Doppert, 1974-1984, Van Leeuwen, 1985; Table 2.1). Six of the dated wells belong to the selection of 18 wells that were tied to the seismic data (Table 2.1). The log suites usually consist of gamma ray and sonic logs (Appendix A). Some suites also contain resistivity, conductivity or self-potential logs. Additional lithologic descriptions, based on cuttings, are available for most wells.
2.2 Seismic stratigraphy
The Palaeogene sedimentary succession in the study area consists of three formations, comprising 10 lithostratigraphic members (Van Adrichem Boogaert and Kouwe, 1997). These formations and members are recognised on well logs and are indicated in the litho-chronostratigraphic chart of Figure 2.1. The seismic character of the succession allows a division into five seismic stratigraphic units (Figs. 2.1 and 2.3). These have been mapped. Seismic data provides a vertical resolution of
22 Chapter 2 s t i Tect. phase Eustatic curves
n Group Period Epoch Age SW NE Fm. Hardenbol et al., 1998 u . s i e
s 200 150 100 50(m) Long- rise Miocene Aquita- nian term fall Ch 4/Aq 1 Time (Ma) ren et al., 1995 Hiatus Ch 3
Bergg Chattian Ch 2 Oligocene Ru 4/Ch 1 Short-term 30 Rupel Clay Member Rupelian Rupel 5 Ru3 th Sea Middle Nor Pr 4/Ru1 Priabo- Pyrenean� Pr 3 Vessem Member nian Hiatus Pr 2 phase Pr 1 Bartonian Asse Member Bart 1 40 4
e Brussels Lu 4 n Lu 3
e Sand Member Eocene Lutetian g Brussels Lu 2 o
Dongen th Sea
e Marl 3 Lu 1 a Member l er Nor a Yp 8 Yp 10 50 w
P Yp 7
Lo Yp 6 Ypresian Ieper Member 2 Yp 5 Yp 4 Dongen Sand and Tuffite Mbs. Yp 3 Yp 1 Landen Clay Member Landen 1 Thanetian Laramide Palaeocene Heers Member Se 2 Selandian Hiatus Se 1 60 phase Danian
Fig. 2.1 Palaeogene litho-chronostratigraphic chart for the southern Dutch North Sea, indicating seismic-stratigraphic units 1 to 5 and the Laramide and Pyrenean tectonic phases. The litho-chro- nostratigraphic chart is modified after the division of Van Adrichem Boogaert and Kouwe (1997), and recalibrated to the timescale of Berggren et al. (1995). Long-term and short-term eustatic cur- ves (Haq et al. (1987), recalibrated to Gradstein et al. (1994), and Berggren et al. (1995), in: Har- denbol et al., 1998) are added.
10-20 m, which is not sufficient to recognise members with a limited thickness. These members are the Heers Member, the Basal Dongen Sand Member, the Basal Dongen Tuffite Member and the Vessem Member (Fig. 2.1). The seismic stratigraphic division is based on seismic facies char- acteristics (internal reflection patterns) and geometry (truncation of seismic reflectors). There is a good correlation between the seismic stratigraphic boundaries in this study and the main bounda- ries of the lithostratigraphic framework of The Netherlands of Van Adrichem Boogaert and Kouwe (1997) (Fig. 2.1). This shows that the stratigraphic nomenclature is mainly based on regional un- conformities and variations in lithology, which are reflected in the seismic response. Different seismic sequence stratigraphic divisions of Palaeogene sedimentary successions in the North Sea were proposed by Michelsen et al. (1998), based on the Danish offshore, and De Batist
23 Palaeogene tectonic evolution of the southern Dutch North Sea and Henriet (1995), based on the Belgian part of the southern North Sea. The Danish sediments are mainly derived from northern and western directions. Therefore, they are not genetically equiva- lent to the sediments of the southern Dutch North Sea. In contrast, the seismic stratigraphic units of De Batist and Henriet (1995) largely coincide with our units. The Belgian deposits are buried at relatively shallow depth in comparison to the burial depth in the study area, resulting in a marked difference in seismic facies. Further, due to the more proximal position with respect to the paleo- coastline, the depositional water depths were shallower than in our study area. Consequently, the Belgian succession displays a more sensitive response to relative sea level variations and shows a frequent occurrence of erosional truncations and channel incisions. Therefore, De Batist and Hen- riet (1995) were able to construct a more detailed seismic stratigraphic subdivision than we could obtain from the Dutch North Sea.
2.3 Construction of depth and thickness maps
We compiled detailed maps of the distribution of the Palaeogene sediments in the area (Figs. 2.4, 2.5 and 2.6) using the seismic and wireline log data. For mapping purposes, the interpreted seismic horizons were converted to 3D grids. The horizons were interpreted in two-way travel time (TWT) and after gridding they were time-depth converted. Finally, the grids were contoured. Interpretation of 2D seismic data allows fault aliasing to occur, resulting in the mapping of very long, continuous faults. Oudmayer and De Jager (1993) showed, using 3D seismics, that these inferred continuous faults often consist of an elongated zone of discontinuous faults, usually ar- ranged in an en echelon pattern. Therefore, the main fault zones were indicated on the compiled maps, instead of individual faults. The applied time-depth conversion technique is based on the assumption that the sonic velocity increases linearly with depth and does not vary laterally (the V0-k method).
V=V0 + k* Z or stated differently: [Vint-V0]/k = Zmid
V0 and k values for the Cenozoic were adopted from the TNO-NITG regional velocity model project SNET (‘SNelheidsmodel Eigen Territoir’). This model is based on an extensive database of approximately 600 wells of the Dutch on- and offshore. The interval velocities (Vint) of the lithostratigraphic units in the wells were calculated using sonic logs, well shoots and calibrated sonic logs. For each unit mid-depths (Zmid) were calculated. Substituting mid-depth values and corresponding Vint values of differing depths of a well in the formula yielded potential values for k.
With a known k, the value of V0 of each well could be calculated. Clustering of the results, incor-
porating the geographical locations of the wells, resulted in mean values for the V0 and k for the SW Netherlands offshore:
V0 = 1696, k = 0.465 (Doornenbal, pers. comm., 2002).
Fig. 2.2 (opposite page) The dataset used in this study (see also Table 2.1 and Appendix A) a) 2D-seismic dataset and the locations of all used wells. b) The location of the wells, which were directly tied to the seismic data. c) The location of the wells used in the backstrip-analysis. d) The location of the wells dated using foraminifers (TNO-NITG internal reports).
24 Chapter 2
N N
o o 55 55
o o 54 54
o o 53 53
o o 52 52
o o 51 51
o o o o o o o o o o 3 4 5 6 7 3 4 5 6 7 a) Well location b) Location of wells tied to seismics Seismic line Outline study area
N N
o o 55 55
o o 54 54
o o 53 53
o o 52 52
o o 51 51
o o o o o o o o o o 3 4 5 6 7 3 4 5 6 7 c) Location of backstripped wells d) Location of dated wells Outline study area Outline study area
25 Palaeogene tectonic evolution of the southern Dutch North Sea
��������� ������������������������ ���������������� ���������������������
��� ��� ���� � ��� ��� � � ��� ��� � � ��� ��� ���� � � ��� ��� ���� � � ��� ��� ��������� � � ��� ��� � ��� ��� � � ��� ��� � � ��� ��� � � ������ � � ������ �������������� � ������ � ������ � � ������ � � ������ ���� � � ������ � ������ � � ������ � � �������� ���� � � ������ ���� � ������ � � ������ ���� � � ������ ���� ������ � �
Table 2.1: Wells with biostratigraphic control (TNO-NITG internal report numbers are indicated), tied to seismic sections and used in backstrip-analysis. See Fig. 2.2 for the well locations.
The Cenozoic interval was not divided in separate units for the V0-k calculations. However, com- paring the calculated depth of the seismic horizons with the actual depths of these horizons in the
18 wells, which were tied to the seismic data, shows that application of the V0 and k values of the SNET project results in an local depth error that never exceeds 10%.
2.4 Quantitative subsidence analysis
The well data were used as a basis for backstripping the Palaeogene basin subsidence history. Backstripping analysis allows a quantitative assessment of the tectonic evolution of the area. It provides valuable information about the timing and magnitude of Late Eocene uplift and erosion. The method also aids in distinguishing between the factors contributing to subsidence and uplift of the area, which are tectonic forcing, isostatic subsidence and differential compaction of sediments. In this study the backstripping techniques of Van Wees et al. (1998) were applied. The backstrip-model first calculated air-loaded tectonic subsidence assuming local isostasy, which
26 Chapter 2
Fig. 2.3 Seismic panel illustrating the seismic facies and the division in five seismic stratigraphic units. The unit boun- daries are indicated by black lines on the left side of the seismic panel. In the grey column (1) the schematic seismic response is indicated (black lines are positive-amplitude reflectors, white lines are negative-amplitude reflectors). For clarity, the right side of the panel is left uninterpreted. corrected for the effect of sediment loading. Compaction was taken into account using porosity- depth relationships on the basis of the observed lithologies, applying standard mean exponential relations and material parameters (cf. Sclater and Christie, 1980). The paleobathymetry (depositional water depth) was taken into account for calculation of the wa- ter-loaded tectonic subsidence. Estimated paleobathymetric values are mainly based on sedimen- tary facies, applying analysis of borehole cuttings and lithology descriptions on well logs, sup- plemented with facies descriptions of Letsch and Sissingh (1983) and Van Adrichem Boogaert and Kouwe (1997).
27 Palaeogene tectonic evolution of the southern Dutch North Sea
The study area is located on a ramp-type continental shelf (Jacobs and De Batist, 1996), typically a setting with a low gradient. Deposition occurred under relatively shallow marine conditions; we suggest that water depth never exceeded 150 m in the study area. The errors in depositional water depth are estimated to be around 50 m at maximum. For the Neogene, an average depositional wa- ter depth of 75 m was applied. Relative water depths fluctuated severely during this period, but for the modelling calculations, this average value is sufficient. The application does not incorporate changes in eustatic sea level. This does not affect the basic model calculations; the calculated rock column is assumed to be in isostatic equilibrium with respect to the geoid at each time step and the depositional water depth is already included in the calculations. Present-day sea level is taken as the vertical datum (depth = 0 m). Periods of uplift causing an erosional hiatus result in an underestimation of the associated tectonic subsidence and subsequent uplift. This is due to the assumption of the model that partly eroded units were deposited during their maximum defined time-period, as indicated by the preset unit o N 5 active fault zone 450 disturbance due to salt tectonics 350 o 4 250 50 150 kness (m) 50 o thic 3 o o o 53 54 52 b) direction of transgression o 5 N 1500 1300 1100 o
700 4 900
1100 700 900 50 km 500 depth (m) o 3 0 o o o 53 54 52 a) Fig. 2.4 Landen Formation (unit 1), Late Palaeocene. a) Depth of the base of the Landen Formation. On all contour maps the main active fault zones indicated are with dark lines. grey b) Thickness of the Landen Formation. The of direction the and transgression the disturbance due to salt tectonics is indicated.
28 Chapter 2 boundary ages. Unfortunately we cannot estimate with reasonable accuracy the stratigraphic in- tervals lost due to erosion. This is due to the generally insufficient detail of the available biostrati- graphic dating. For clarity of display, the tectonic subsidence curves are only shown for three well locations (Fig. 2.7). These wells are representative for the northern, central and southern part of the study area. The locations of all analysed wells are given in Figure 2.2. Appendix B shows all results of the subsidence analysis. o 5 N o 250 4
erosion BFB 100 150 kness (m) 50 thic o 3 o o o 53 54 52 c) o 5 N . o
BFB 4 250 l Mb
50 erosion 150 . ussels Mar Br kness (m) 50 ussels o thic Br Sand Mb 3 o o o 53 54 52 b) o 550 5 N 450 active fault zone 350
o 100 4 250 150 250 50 km kness (m) 50 thic o 3 0 o o o 53 54 52 a) Fig. 2.5 Dongen Formation, Early-Late Eocene. a) Thickness of the Basal Dongen Sand Members and and Tuffite the Ieper Member (unit 2). b) Thickness of the Brussel Sand and Marl Members (unit 3). c) Thickness of the Asse Member indicate (unit the in 4). difference the To width of the zone of maximum inversion between the phase Pyrenean and the phases, previous the outline of the Mesozoic Fourteens Broad basin (BFB) is indicated with a dotted line in b Figures and c.
29 Palaeogene tectonic evolution of the southern Dutch North Sea
3. Geological development of the Broad Fourteens Basin
3.1 Mesozoic setting
The Palaeogene siliciclastic sedimentary succession in the southern Dutch North Sea covers vari- ous Mesozoic structures, one of which is the Broad Fourteens Basin (Fig. 2.8). This narrow NW- SE trending basin developed as a result of multiple rifting events, the most important of which was the Late Jurassic-Early Cretaceous extension phase (Huyghe and Mugnier, 1994). However, the o 5 N 900 o 4 700
400 500 600 300 depth (m) o 3 o o o 53 54 52 c) o 5 N ero- sion o 4 250 150
erosion 100 kness (m) 50 50 o thic 3 erosion erosion o o o 53 54 52 b) o N 5 1100 ero- sion 900 o 4 700
800 BFB 500
erosion 50 km 300 depth (m) o 3 erosion erosion Rupel Formation, Oligocene.
0 o o o 53 54 52 a) Fig. 2.6 a) Depth of the base of the Rupel Formation. The outline of the Mesozoic Fourteens Broad basin (BFB) is indicated with a dotted white line. b) Thickness of the Rupel Formation (unit 5), displaying the partial of erosion the outer rim of the that area was uplifted during the phase. Pyrenean c) Depth of the base of the Neogene, displaying the in depocentre the uplifted previously area.
30 Chapter 2
NW-SE structural grain of the study area is believed to have originated in pre-Mesozoic times (P.A. Ziegler, 1990; Oudmayer and De Jager, 1993; Huyghe and Mugnier, 1994). Rifting activity abated during the Late Cretaceous after which Cenomanian and Turonian Chalk sediments were deposited. During the Late Santonian to Early Campanian, the Broad Fourteens Basin was structurally inverted. The inversion movements are thought to be the result of transpres- sive stresses related to the sub-Hercynean tectonic phase (e.g. Letsch and Sissingh, 1983; Van Wijhe, 1987a; 1987b; P.A. Ziegler, 1990; Dronkers and Mrozek, 1991; Oudmayer and De Jager, 1993). Uplift was accommodated by the selective reactivation of preexisting faults. Most of the reactivated faults have a dominant NW-SE orientation and originated in Caledonian times (Van Wijhe, 1987a; P.A. Ziegler, 1990; Dronkers and Mrozek, 1991; Oudmayer and De Jager, 1993). Locally, more than 3000 m of sediment was eroded (Van Wijhe, 1987b; Dronkers and Mrozek, 1991; Huyghe and Mugnier, 1994; Hooper et al., 1995). The horizontal shortening related to the sub-Hercynean phase was approximately 10% (Hayward and Graham, 1989; Huyghe and Mugnier, 1994; Hooper et al., 1995). Shortening of the cover sequence was mainly accommodated by the backward expulsion of the halfgraben fill along a series of newly formed reverse faults and listric thrusts. These faults have detached mostly in the Permian Zechstein evaporite sequence (Hayward and Graham, 1989). Transgression during a period of tectonic quiescence resulted in the deposi- tion of Maastrichtian Chalks in the area.
3.2 Palaeocene
Deposition of Chalk sediments continued into the Early Palaeocene Danian. Inversion movements in the Broad Fourteens Basin resumed in the Mid-Palaeocene during the Laramide tectonic phase (Letsch and Sissingh, 1983; Van Wijhe, 1987a; 1987b; P.A. Ziegler, 1990). A contemporary global sea level lowstand (Hardenbol et al., 1998) resulted in exposure and peneplanation of the entire southern North Sea. The zone that was most severely inverted during the sub-Hercynean and Lara- mide phases was relatively narrow (Burgers and Mulder, 1991), as indicated in Figure 2.8. The width of the inversion zone also becomes apparent by the upwarping of the base of the Upper Mesozoic Chalk succession (layer C, Fig. 2.9). During the Thanetian (Late Palaeocene), a transgression resulted in deposition of the clastic sedi- ments of the Landen Formation, which consists of the Heers Member (only locally present) and the Landen Clay Member (seismic stratigraphic unit 1, Figs. 2.1 and 2.4a) (Letsch and Sissingh, 1983; Van Wijhe, 1987a; Wong et al. 2001). On seismic profiles, the lower boundary of unit 1 is visible as a pronounced high-amplitude reflector. This is due to a sharp decrease in sonic veloc- ity at the boundary between Mesozoic and Cenozoic sediments, which is also distinctly visible on well logs. This seismic reflector truncates underlying reflectors. Onlap on the basal surface is sporadically observed. The internal seismic configuration of the unit is an interval with a few parallel reflections (Fig. 2.3). The transgression resulted first in flooding of low-lying depocentres in the WNW and NW of the study area (Fig. 2.4b). These areas were fault-bounded depressions at the time of deposition, a result of extensional reactivation of faults in the northwestern part of the inverted Broad Fourteens Basin after the Laramide phase ended and subsidence resumed (Nalpas et al., 1995). Subsequently, the whole area was flooded and a succession of shallow marine clays with a thickness between 50 and 100 m was deposited. Mean basement subsidence rate was about 4 cm/ky, as indicated by backstripping analysis (Fig. 2.7). A short regressive phase ended the depo- sition of the Landen Formation (Letsch and Sissingh, 1983).
31 Palaeogene tectonic evolution of the southern Dutch North Sea
age (ma) 60 50 40 30 20 10 0
50 depth (m)
100
150 a) Paleobathymetry age (ma) 60 50 40 30 20 10 0 N 0 o 200 55
400
600 depth (m)
800 o Pyrenean phase 54 1000 erosion Savian 1200 gene Neo- unit 2 unit 3 unit 5 unit 4 unit 1 1400 o 1600 53 b) North, K06-01 age (ma) 60 50 40 30 20 10 0 0 o 52 200
400
600 depth (m)
o 800
Pyrenean phase 51 Savian erosion 1000 o o o o o
Neogene 3 4 5 6 7 1200 hiatus unit 2 unit 3 unit 5 unit 1 1400
1600 c) Centre, P06-02 e) Location map age (ma) 60 50 40 30 20 10 0 0
200
400 paleobathymetry depth (m) 600 air-loaded tectonic subsidence 800 water-loaded tectonic subsidence Pyrenean phase
Savian erosion basement subsidence, not com- 1000 pensated for compaction
Neogene 1200 basement subsidence unit 2 unit 3 unit 5 unit 4 unit 1 incomplete succession 1400 Legend 1600 d) South, S05-01
Fig. 2.7 Basement subsidence curves for 3 key wells. a) Paleobathymetric curve used in the backstrip model. b) Well K06-01, representative of the part of the study area North of the area inverted during the Pyrenean phase. c) Well P06-02, representative of the area of maximum inversion during the Pyrenean phase. d) Well S05-01, representative of the part of the study area South of the area inverted during the Pyrenean phase. e) Location map indicating the position of the three wells in the study area (encircled).
32 Chapter 2
Figure 2.4b shows that the deposits in the northern to northeastern parts of the study area locally have been disturbed by salt domes and associated peripheral sag basins.
3.3 Eocene
During the Eocene, a thick succession of clays and silts of the Dongen Formation was depos- ited. Three seismic stratigraphic units occur within the Dongen Formatio (seismic stratigraphic units 2, 3 and 4, Fig. 2.1). Seismic stratigraphic unit 2 contains the lowermost Members of the Dongen Formation: the Basal Dongen Sand Member, the Basal Dongen Tuffite Member and the Ieper Member (Fig. 2.1). The lower boundary of the unit is a distinct high-amplitude reflector, concordant on the underlying unit. This reflector is tied to the lithologic base of the Dongen Sand and Tuffite Members using the distinct velocity contrast displayed on the sonic velocity logs. The internal seismic configuration shows, from bottom to top of the unit, an interval of discontinuous, low-contrast and low-amplitude parallel reflectors. Towards the top, a band of higher amplitude parallel reflectors, affected by intraformational deformation, is present. Sporadically, the top of the unit displays again low-contrast and low-amplitude parallel reflectors (Fig. 2.3).
N
o 55
o 54
B o 53
Mesozoic BFB
o 52 A
o 51
o o o o o 3 4 5 6 7
o 100 km
Legend Fig. 2.8 Map of the series sub-cropping below the base
Upper Cretaceous present of the siliciclastic Palaeogene (modified after Burgers Upper Cretaceous eroded and Mulder, 1991). The Mesozoic Broad Fourteens Basin, Complete Cretaceous eroded the outline of the study area and the position of SW-NE Jurassic and Cretaceous eroded transect A-B (Fig. 2.9) are indicated. It is clearly visible A B line of transect (Figs. 2.9 and 2.10) that the inverted Broad Fourteens Basin is a relatively outline study area narrow structure.
33 Palaeogene tectonic evolution of the southern Dutch North Sea B (NE) Unit 1 Unit 2 C Unit 5 Unit 3 teens Basin our ing the Pyrenean phase Unit 5 ted dur er v iginal width of the Broad F or width of the area in ault F C Unit 3 Unit 2 Unit 2 Unit 3 Unit 1 Unit 1 Unit 4 Unit 4 Unit 5 Unit 5 0
0
750 500 250
2500 2250 2000 1750 1500 1250 1000
750 500 250 2500 2250 2000 1750 1500 1250 1000 TWT (ms) TWT TWT (ms) TWT A (SW)
Fig. 2.9 SW-NE seismic section A-B (SNST-83-02), showing the distribution of the 5 seismic stratigraphic units. The position of the transect is indicated on Fig. 2.8. Vertical scale is in two-way time (ms). Indicated are the approximal width of the original Broad Fourteens basin, and the width of the area inverted during the Pyrenean phase. The base of the Upper Cretaceous Chalk Series is indicated with (C).
34 Chapter 2
In the centre of the study area, the top of this unit has been erosionally truncated. Units 3 and 4 are absent there. The Basal Dongen Sand/Tuffite Member and the Ieper Member (combined into unit 2, Figs. 2.1 and 2.5a) were deposited during at least two, but probably more, cycles of transgres- sion and subsequent highstand during the Ypresian (Wong et al., 2001). Unit 2 was deposited under inner- to outer-neritic conditions. Backstripping analysis indicated that the mean basement subsid- ence rate in the study area increased to a mean value of about 6.6 cm/ky (Fig. 2.7). The sedimenta- tion rate kept up with the continuous creation of new accommodation space. As a result, a thick succession of sediments was deposited during the Ypresian. A Mid-Eocene (Ypresian-Lutetian) regression phase, possibly forced by the decrease of mean base- ment subsidence rate to about 0.5 cm/ky (Fig. 2.7), resulted in the deposition of the inner-neritic to near-shore Brussels Sand Member in the southern part of the study area and the laterally equivalent outer-neritic Brussels Marl Member in the northern part of the study area (Van Adrichem Boogaert and Kouwe, 1997). These two Members are combined into seismic stratigraphic unit 3. Their seis- mic appearance is similar. The lower boundary of unit 3 is a very pronounced negative-amplitude reflector, concordant on unit 2. This reflector was tied to the base of the Brussels Sand Member using the sonic velocity response on well logs. The internal seismic facies of the unit consists of medium-amplitude parallel reflectors (Fig. 2.3). Locally, the top of this unit has been erosionally truncated. Unit 4 is absent then. During the following tectonic inversion period, the originally grading contact between the two members was lost due to erosion (Fig. 2.5b). The uppermost part of the Dongen Formation, the open marine Asse Member (unit 4, Fig. 2.5c), was deposited during the following transgressive interval (Lutetian-Bartonian). The lower boundary of unit 4 is a dis- tinct, negative, but not very high-amplitude reflector, concordant on unit 3. Low-contrast or very low-amplitude parallel reflectors generally exemplify the internal seismic characteristics of the unit. The upper boundary of the unit is a surface of reflector termination through truncation (Fig. 2.3). This boundary has been erosionally truncated everywhere, although only locally at an angle which exceeds a few degrees. The maximum thickness of the Dongen Formation exceeds 700 m.
3.4 Pyrenean inversion
Around the Eocene-Oligocene boundary, the onset of a final inversion pulse terminated sedimenta- tion and resulted in a regional unconformity (Fig. 2.9). The tectonic movements were probably caused by the Pyrenean orogenic phase (Letsch and Sissingh, 1983; Van Wijhe, 1987a; 1987b). This phase is sometimes stated to have been of minor importance in the research area (Van Wi- jhe, 1987b) and is in some publications even fully neglected (e.g. Oudmayer and De Jager, 1993; Hooper et al., 1995; Huyghe and Mugnier, 1995). However, fault activity (Fig. 2.9) and subsidence analysis (Fig. 2.7) indicate that the tectonic uplift in the centre of the area was severe: the total offset was about 170 to 200 m (Figs. 2.5 and 2.7c). This caused tilting and truncation of Eocene sediments. The isostatic response due to erosional unloading resulted in a total exhumation of 200 to 250 m. The vertical tectonic movements are illustrated in a schematic reconstruction along the SW-NE transect A-B (Figs. 2.10a and 2.10b). Large quantities of sediment in the uplifted area were erod- ed. In a wide central zone the Brussels (seismic stratigraphic unit 3) and Asse (unit 4) Members were completely removed (Figs. 2.5b and 2.5c and Figs. 2.9, 2.10a and 2.10b). The underlying Ieper Member (unit 2) was deeply incised (Figs. 2.5a and 2.9). The width of the area, which was uplifted during this phase, is much larger than the width of the area affected most severely by the
35 Palaeogene tectonic evolution of the southern Dutch North Sea
A (SW) B (NE) 0 Neogene unit 5 unit 4 unit 3 400 unit 2 Depth (m) unit 1 800 d)
unit 5 Fig. 2.10 unit 4 unit 3 Schematic geological reconstruction unit 2 along the SW-NE transect A-B in Fig. 2.9 (Palaeogene-Recent). Palaeogene seismic unit 1 stratigraphic units 1 to 5 are indicated, as well as the major unconformities (dashed c) lines). For clarity, only the relative ver- tical movements along the transect are unit 4 unit 3 indicated, components of strike-slip are omitted. unit 2 a) Late Eocene, after deposition of seismic stratigraphic units 1 to 4, prior to the unit 1 Pyrenean tectonic phase of inversion. b) Early Oligocene, during the Pyrenean b) tectonic phase of inversion. The centre of the area is uplifted and eroded. unit 4 c) Late Oligocene to Early Miocene, unit 3 during the 'Savian' phase of erosion.
unit 2 Differential subsidence due to tensional reactivation of faults had started. unit 1 d) Present. During the Neogene, a large depocentre formed in the previously uplif- a) ted centre of the study area.
Late Cretaceous sub-Hercynean and Mid-Palaeocene Laramide inversion of the Broad Fourteens Basin (compare Figure 2.5 with Figure 2.8). This shows that during the Late Eocene-Early Oli- gocene Pyrenean tectonic phase, a different set of faults was active than during the previous sub- Hercynean and Laramide tectonic phases. This resulted in overstepping of the original boundaries of the inverted area. The active faults are, without exception, reactivated Palaeozoic and Mesozoic faults, possibly inherited from Caledonian times (Oudmayer and De Jager, 1993). This indicates that inheritance of geological structures played an important role in the tectonic development of the area.
3.5 Oligocene
After cessation of the Pyrenean tectonic phase, during the Rupelian, transgressive sediments of the Rupel Formation (unit 5, Fig. 2.1) were deposited upon the Palaeocene and Eocene strata. The low- er boundary of unit 5 is a discordant, distinct, positive-amplitude reflector, truncating the underly- ing units. The internal seismic facies of the unit usually shows a few parallel, often discontinuous, reflections of medium amplitude (Fig. 2.3). A relative sea level lowstand terminated Oligocene
36 Chapter 2 deposition and resulted in erosion. Letsch and Sissingh (1983) and Wong et al. (2001) associated this event with the ‘Savian’ phase of low global sea level. The top of the unit has been erosionally truncated everywhere and is overlain by a downlap surface. Mean basement subsidence rate was low, about 1.5 cm/ky (Fig. 2.7). Due to the Savian erosion, only part of the Rupel Formation has been preserved. In the centre of the study area (Fig. 2.6b), the thickness of the formation ranges from several metres up to a few tens of metres. The outer rim of the area that was uplifted during the Pyrenean phase was in sev- eral places completely removed by erosion (Figs. 2.6b, 2.10c). This indicates that a local depres- sion developed in the centre of the previously uplifted area after cessation of the Pyrenean phase as a result of increased local subsidence. In the non-inverted northern and southern parts of the study area, up to a hundred metres of Rupel Formation sediments have been preserved.
3.6 Neogene
Neogene sediments disconformably overlie the Palaeogene. A local depocentre developed in the centre of the area as the result of continuing differential regional subsidence during this period (Figs. 2.6c and 2.7). Sediments were deposited in large-scale delta foresets (Fig. 2.9) (Van Wijhe, 1987b). The thickest succession of Neogene sediments is present in the centre of the study area, overlying the thinnest remaining succession of Palaeogene sediments (Figs. 2.6c and 2.10d).
4. Discussion
We have observed an increase in the extent of the area of maximum uplift between the Pyrenean phase and the previous inversion phases (compare Fig. 2.8 with Fig. 2.5, also see Fig. 2.9). Basin inversion in pulses is fairly common, but no comparable change in inversion geometry through time has been reported in the literature for other basins. Recently, a similar widening trend, dur- ing the same time interval, has been observed in the Dutch Roer Valley Graben by Michon (pers. comm., 2001), therefore it might have occurred more often than previously thought. In the follow- ing discussion, we will address the observed inversion geometry, structural inheritance and pos- sible causes of the subsidence and uplift patterns.
4.1 Subsidence patterns
The tectonic subsidence patterns during phases of tectonic quiescence are generally the same eve- rywhere in the study area (Fig. 2.7). After cessation of the Laramide phase, a first phase of rapid subsidence was followed by a period of decreasing subsidence rates. The subsidence was interrupt- ed by the Pyrenean phase of inversion, which resulted in differential uplift. Subsidence resumed when this phase ceased. Differential subsidence occurred during all periods of tectonic quiescence and is the result of two factors: (1) Differential compaction of the thick succession of Mesozoic sediments through the loading of the Palaeogene succession caused increased subsidence in the non-inverted northern and southern part of the study area. In the centre of the study area the Mesozoic sediments became overcompacted as the result of uplift during the sub-Hercynean and Laramide inversion phases and remained overcompacted ever since. Additional compaction of these sediments due to the loading of Palaeogene sediments can therefore be neglected. This resulted in a distinct contrast
37 Palaeogene tectonic evolution of the southern Dutch North Sea in total subsidence between the centre and the edges of the study area. The difference in total subsidence is a few tens of meters during the Palaeogene (Fig. 2.7). (2) The local depocentre that developed in the northwestern part of the Broad Fourteens Basin at the time of the deposition of the Landen Formation (Fig. 2.4) was probably the result of tensional reactivation of faults due to stress relaxation after cessation of the Laramide phase. This relaxation effect also accounts for the increased subsidence rates after cessation of the Pyrenean compression forces as observed in the centre of the area that was uplifted around the Eocene-Oligocene boundary (Fig. 2.10c). The differential regional subsidence continued during the Neogene (Fig. 2.10d). A few faults extend into the Neogene and show normal fault movement during this period. The result was an evident Neogene depocentre in the centre of the study area (Figs. 2.6c and 2.7). The resulting total subsid- ence has probably increased due to Neogene sediment loading. It is unlikely that local (thermal) sagging of the lithosphere resulted in the marked differences in local subsidence, because of the small size of the area affected.
4.2 Basin inversion
Seismic interpretation shows that in the research area the compressive tectonic movements during the Pyrenean phase were, without exception, accommodated by reactivation of relatively steep Mesozoic faults (Fig. 2.9). The formation of new, low-angle reverse faults was not observed. Most active faults have dominant NW-SE trends (between 140º and 160º) (Fig. 2.5), which was prob- ably inherited from Caledonian times (Oudmayer and De Jager, 1993). The Pyrenean inversion phase has been suggested to have been induced by a roughly N-S (~170º) oriented compression (Nalpas et al., 1995; Michon et al., 2003). The principal compressive stress during inversion thus acted at an oblique angle of 10º-30º on the major fault zones. This implies that existing faults have been reactivated with an oblique-slip component. This can be illustrated as follows: when a principal compressive stress is directed perpendicular to the strike of preexisting normal faults, these existing faults have a tendency to lock (Daly et al., 1989; McClay, 1989; Nalpas et al., 1995). Analogue experiments show limited inverse reactivation of normal faults during initial compression. Upon increased compression, many fault planes rotate to higher dip angles (>60º dip) and cease to be active. Instead, lower angle (back-) thrust faults de- velop that take over the principal displacement (Koopman et al., 1987; McClay, 1989; Eisenstadt and Withjack, 1995). However, fault reactivation can occur under stress levels lower than needed for creating new faults (Krantz, 1991). High-angle normal faults are more easily reactivated in a strike-slip sense than in a reverse dip-slip sense if the faults have a strike which is oriented at an angle of less than 30º with respect to the compressive stress field (Etheridge, 1986, in Daly et al., 1989). Increasing obliquity of the main compressive stress direction with respect to the preexisting normal fault orientation therefore favours inversion of existing faults with a strike-slip or oblique- slip component over the formation of new, low-angle reverse faults (Daly et al., 1989; Huyghe and Mugnier, 1994; Hooper et al., 1995; Nalpas et al., 1995; Nieuwland and Nijman, 2001). The geometry (cf. Christie-Blick and Biddle, 1985) of the reactivated faults also clearly indicates oblique-slip movements. This is illustrated with a series of 8 transects across one aliased fault trace (Fig. 2.11). Along the trace of the fault often upward splays, or flower structures, are present. The flowers show varying magnitude and sense of separation for different horizons offset by the same fault. In successive profiles, the fault zone shows inconsistent, sub-vertical dip directions. At least one pull-apart basin is present due to a stepover of the main fault trace, indicating that the shear
38 Chapter 2
SW 1 NE
N
o 55
87-10 o 54 SW NE 1
o 53
o 52 83-06
o SW 1 NE 51 o o o o o 3 4 5 6 7
0 100 km 87-10 83-06 87-11 87-11 83-05 SW 1 NE 87-25 87-12 83-04 83-03 sections 87-14
83-05 fault trace SW 1 2 NE
NW 2 SE
87-12 SW 1 2 NE
87-25 section parallel to 83-04 fault trace SW 2 NE
1 Main fault trace North of extensional 83-03 duplex Fig. 2.11 SW NE Cross-sections along a fault zone, dis- 2 2 Main fault trace playing flower structures, which indi- South of extensional cate strike-slip. The position of the duplex fault trace and the seismic cross-sec- tions are indicated on a location map. An extensional duplex is interpreted 87-14 and indicates sinistral shear.
39 Palaeogene tectonic evolution of the southern Dutch North Sea sense of this particular fault zone was sinistral (Fig. 2.11). The several phases of deformation observed in the study area were attributed to a combination of North Atlantic rifting and the compressive stresses generated by the Europe-Africa collision (P.A. Ziegler, 1975; 1989; 1990; Dronkers and Mrozek, 1991; Gölke and Coblentz, 1996). Mi- chon et al. (2003) note that the individual inversion phases in the southern North Sea correspond with the onset of different phases of the continent-continent collision in the Alpine evolution. This contemporaneity suggests that the tectonic activity of the southern North Sea was the result of phases of stress induced by the Alpine orogeny, which were directly propagated through the European plate. Small angular variations in the stress field are sufficient to change the state of a fault from non- reactivation to reactivation (Huyghe and Mugnier, 1994). This might be the cause of the observed difference in the response of the Broad Fourteens Basin to the two inversion phases. Michon et al. (2003) observe that the Laramide phase affected the whole European plate, whereas the Pyrenean phase resulted in deformation in the southern North Sea basins and the Channel area only. Al- though the principal compressive stress during both the Laramide and Pyrenean phase was N-S oriented in the southern North Sea, the local stress field probably differed slightly between phases. This might have resulted in the widening of the area of inversion in the Broad Fourteens Basin. Nalpas et al. (1995) suggested that spatial differences in the structural styles resulting from the Late Cretaceous oblique compression of the Broad Fourteens Basin were mainly caused by the effects of cover thickness variations. They experimentally showed that a thick Mesozoic sedimen- tary cover hampers reactivation of normal faults during oblique compression, causing strike-slip faults to develop within the graben fill. Where the cover is thin, normal faults are reactivated and low-angle thrust faults develop above the graben margins. A similar experimental result was ob- tained by Dubois et al. (2002). The period of tectonic quiescence and associated subsidence after the end of the Laramide phase resulted in the accumulation of a thick (up to 700m) succession of siliciclastic sediments. Because the thickness of the sediment cover affected by compression ap- parently plays an important part in the type of reactivation, this may be another reason why the area that was severely affected by the tectonic compression was wider during the Pyrenean phase than during previous inversion phases.
5. Conclusions
The southern Dutch North Sea experienced a distinct pulse of tectonic inversion, associated with the Pyrenean orogenic phase, during the Late Palaeogene. Our study shows that the intensity of this phase of inversion is much larger than previously assumed. The centre of the study area was tectonically uplifted up to 200 m. The main stress orientation during the Pyrenean phase was N-S, at an oblique angle to the preexisting NW-SE structural grain, resulting in compressive reactiva- tion of faults with an oblique-slip component. During the Pyrenean phase of inversion, original basin boundaries were overstepped and a much wider area then during previous inversion pulses was uplifted. Our conclusion is that this must have been caused by a difference in the direction of the Pyrenean compressive stress field with respect to the main stress directions of the previous inversion pulses. In addition, the accumulation of a thick Palaeocene-Eocene sediment succession possibly contributed to the increase in the width of the uplifted area with respect to previous inver- sion phases. Tensional reactivation of faults, caused by relaxation after cessation of compressive forces, re- sulted in differential subsidence patterns in the area during periods of tectonic quiescence. This re-
40 Chapter 2 sulted in the development of local depocentres. The compaction of Mesozoic sediments resulted in increased subsidence in the non-inverted northern and southern part of the study area. The contrast in total subsidence totals a few tens of meters.
Acknowledgements M. Huuse (Cardiff University, Wales, UK) and L. Gemmer (University of Aarhus, Denmark) are gratefully acknowledged for their constructive comments and useful suggestions on improving the manuscript of this chapter before publication.
41 42 Chapter 3
Chapter 3 Reconstruction of the Late Palaeogene tectonic activity of the southern Dutch North Sea, based on a sequence stratigraphic interpretation of log correlations.
Abstract
The existing stratigraphic framework of the Palaeogene in the Netherlands shows limited temporal resolution. Using this framework, it is difficult to unravel the influence of local tectonic move- ments and eustatic sea level changes on sedimentation. The precise dating of the onset of the Late Eocene - Early Oligocene Pyrenean orogenic phase, which caused inversion and uplift in the southern Dutch offshore, is also dubious as a result of the limited resolution. To improve the correlations and dating of the Palaeocene to Oligocene successions in the south- ern Dutch North Sea, a new sequence stratigraphic interpretation, based on correlated wireline logs, is presented. The method is based on the reconstruction of local sea level cycles, interpreted from differences in grain size distribution, as derived from gamma ray log response. The sea level signal in the study area is then calibrated with biostratigraphically defined sea level cycles from onshore Belgium. This reconstruction method enables a high-resolution correlation between the Late Palaeocene to Eocene tectono-stratigraphic evolution of the southern Dutch North Sea and the standard eustatic cycle chart. The correlation demonstrates that the Pyrenean phase, which started during the Early Priabonian (circa 37 Ma), was preceded by an earlier period of tectonic activity during the Middle to Late Eocene. The tectonic overprinting of eustatic sequences in the southern Dutch North Sea started in the beginning of the Lutetian (circa 48 Ma). The tectonic events in the study area can be correlated to time-equivalent tectonic uplift in the Brabant and Artois Blocks in Belgium, which suggests that the processes causing the tectonic activity are of regional importance
1. Introduction
1.1 Lithostratigraphic framework
The stratigraphic framework of the Palaeogene of the Netherlands (Van Adrichem Boogaert and Kouwe, 1997) shows low temporal resolution (Figs. 2.1 and 3.1), as a result of the limited data available for this interval. The data from the Dutch offshore are seismic surveys, wireline logs and lithologic cuttings from industrial boreholes. Detailed outcrop information is only available onshore, in more proximal depositional settings. The Palaeogene succession has been biostrati- graphically dated in a limited number of wells (Table 2.1, Fig. 2.2d). Most datings are based on foraminifers, derived from cutting samples, providing a low biostratigraphic resolution (Fig. 3.2). As a result of the limited stratigraphic resolution, it is difficult to unravel the relative influence of local tectonics and eustatic sea level variations on the Palaeogene sedimentation in the southern Dutch North Sea. The aim of this study is to introduce a new sequence stratigraphic interpretation of existing well
43 Sequence stratigraphic interpretation of log correlations and seismic data. The interpretation provides a higher-resolution reconstruction of the Late Pal- aeogene tectono-stratigraphic evolution of the southern part of the Dutch North Sea than currently available. The detailed reconstruction of local relative sea level movements, correlated to the glo- bal eustatic sea level chart, assists in the recognition and dating of local tectonic activity. Landen Ieper Zenne Rupel Voort Tongeren Group Gent Fm. Fm. Tienen Fm. Bilzen Fm. Eigenbilzen Fm. Lede Fm. Hanut Fm. Heers Fm. Maldegem Fm. Voort Fm. Brussel Fm. Kortrijk Fm. Zelzate Fm. Boom Fm. Aalter Fm. Tielt Fm. 3 M-P Kortemark Buisp.2 Zomergem Berg Sand Gelinden . Orp (Roubaix) Egem watervliet Aalbeke Wemmel top Belgium Brussels Sand Boom Clay Onderdijke Asse Ursel waterschei Lede Onderdale Eigenbilzen Varangeville-Mt. Heribu Dieppe-Grandgliese Aalter (Orchies) Tienen Fm Bassevelde Sands Bassevelde Sands 1 Halen Ruisbroek Buisputten 1 Bassevelde Sands 2 Vlierzele Maaseik Asse Mb. Fm. Steensel Mb. Marl Mb. Marl Gelinden
Houthem Someren Mb. Reussel Mb. Netherlands SE
Klimmen Mb. Voort Mb. Goudsberg Mb. RVG Veldhoven Clay Mb. Heers Mb Sand Mb. Brussels Sand Mb. Asse Mb. Vessem Mb.
Rupel Clay Mb. Ieper Clay Mb. Basal Dongen Landen Clay Mb. Netherlands Offshore to SW Tuff Mb.
Fm.
Rupel
Landen Dongen Sea
th Sea th Nor wer Lo th Nor
Group Middle tonian iabo- Age Danian Aquita- nian Pr nian Lutetian Chattian Rupelian Ypresian Bar Thanetian Selandian
Epoch Miocene
alaeocene Oligocene Eocene
P alaeogene P
iod
r
Pe 1995 al., et Berggren
Age (Ma) 50 60 40 30
Fig. 3.1 Simplified lithologic correlation between the Netherlands offshore, Netherlands southern onshore and Roer Valley Graben (RVG). Modified after Marechal (1993), Van Adrichem Boogaert and Kouwe (1997) and Vandenberghe et al. (1998).
44 Chapter 3
1.2 Geological setting and previous studies
The geometry of the Late Palaeocene to Oligocene marine succession of the southern Dutch North Sea area (Fig. 1.1) was significantly influenced by relative sea level movements, Oligocene inver- sion tectonics, salt tectonics and post-depositional erosion (Letsch and Sissingh, 1983; Remmelts, 1995). Nevertheless, the lithologic composition of the southern Dutch North Sea succession is relatively uniform, as the largest part of the area occupied a distal position with respect to the
King (1989), King (1989), Doppert and Neele (1983), biostratigraphic zones North Sea North Sea Dutch North Benthics Plankton Sea
Early NSB 9 FE2 Miocene
Neogene NSB 8c FE2 Late NSB 8b Oligocene NSB 8a FE3
NSB 7b FF
Early NSB 7a FF Oligocene
NSB 6b NSP 9a FHa
NSB 6a Late Eocene FHb e NSB 5c NSP 8 n e
g NSB 5b o e
a Middle NSP 7 FHc
l NSB 5a Eocene a P
NSB 4 NSP 6 FI
Early b Eocene NSB 3 NSP 5 FI a
NSB 2 NSP 4 FI
Late c Palaeocene NSB 1 FJ b Fig. 3.2 Early NSB 1a Correlation scheme of different Palaeocene FK biostratigraphic zonations.
45 Sequence stratigraphic interpretation of log correlations
a) b) N N
o o 55 55
K06-01 K06-01 o o 54 54 K07-02 L02-04 L02-04 K12-01 K12-01 o o L13-01 53 53 Q01-01 L13-01 P05-03 P15-02 o o 52 52 S02-01 S05-01 S02-02 S05-01
o o 51 Knokke 51
o o o o o o o o o o 3 4 5 6 7 3 4 5 6 7
Location of wells in correlation Location of wells in correlation panels of Figs. 3.6 and 3.8 panels of Fig. 3.7 Outline study area Outline study area
Fig. 3.3 a) The locations of the wells in the correlation panels of Figs. 3.6a, b and 3.8. b) The locations of the wells in the correlation panels of Figs. 3.7a and b.
paleocoastline during most of the Palaeogene (cf. Huuse, 2000). Most sedimentary units consist of silty clays deposited in an open marine shelf setting. Only a few units are sandy, indicating coastal settings (Fig. 2.1). No Middle Eocene tectonic activity has been reported in the study area. Furthermore, accurate dating of the onset of the Pyrenean orogenic phase, which caused inversion and uplift in the study area during the Late Eocene to Early Oligocene, is not possible without additional data. Until now, it was assumed that tectonic movements in the area started close to the end of the Eocene (Letsch and Sissingh, 1983; Van Wijhe, 1987a; 1987b). This broad age range is partly due to the wide- spread erosion of Late Eocene sediments in the study area (Fig. 2.5). As a consequence, it has been difficult to correlate the Late Palaeogene tectonic development of the Netherlands offshore to the other parts of the North Sea. Previously, there has been little effort to construct a sequence stratigraphic framework for the Dutch part of the Palaeogene North Sea Basin. The stratigraphic nomenclature of the Palaeogene
46 Chapter 3
(Van Adrichem Boogaert and Kouwe, 1997) only mentions sequence stratigraphic interpretations when the transgressive or regressive nature of deposits could be unambiguously linked with the ‘Vail curve’ (Haq et al., 1988). Wong et al. (2001) presented a sequence stratigraphic reconstruction based on the analysis of a well in the Broad Fourteens Basin area. This well lacks a significant part of the Eocene interval due to post-depositional erosion during the Pyrenean phase. In contrast to the limited studies in the Netherlands, various high-resolution sequence stratigraphic studies of the on- and offshore Palaeogene in the areas surrounding the Dutch North Sea have been published (e.g. Jacobs and Sevens, 1993; Michelsen, 1994; De Batist and Henriet, 1995; Laursen et al., 1995; Jacobs and De Batist, 1996; Neal, 1996; Michelsen et al., 1998; Vandenberghe et al., 1998; 2004). An attempt can be made to correlate the relative sea level changes in the Neth- erlands offshore to these surrounding areas. Assuming that sequence stratigraphic boundaries are isochronous over long distances, such a correlation can be used for age dating. From the Late Palaeocene to the Late Oligocene, the southern North Sea Basin was a ramp-type continental shelf (Jacobs and De Batist, 1996). On the shelf, depositional angles were general- ly less than one degree. Distinct seismic clinoforms or coastal onlaps, which would facilitate a ‘classic’ sequence stratigraphic interpretation (sensu Vail et al., 1977; Posamentier and Vail, 1988; Posamentier et al., 1988), are rare or absent within the succession. However, Vandenberghe et al. (1998) have shown conclusively that eustatic sea level fluctuations, based on grain size variations, can be recognized in an open marine ramp-type margin setting.
2. Data and methods
To provide a sequence stratigraphic interpretation of the limited data, the available wells are first interpreted based on their log signature. The succession is subsequently correlated between wells and divided into sedimentary sequences. A local sea level curve is constructed. Subsequently, the Dutch succession is correlated with the Belgian lithostratigraphic succession, which is biostrati- graphically and sequence stratigraphically well calibrated, and correlated with the global eustatic cycle chart (Haq et al., 1988). This correlation enables an indirect age assessment of the Dutch suc- cession. Local deviations in sea level from the eustatic sea level chart in distinct parts of the Dutch territory are related to local tectonic activity.
2.1 Data
The correlations in this study are based on the interpretation of gamma ray (GR) and sonic (DT) logs from 74 wells (Fig. 2.2a, Appendix A). Of these 74 wells, 11 are presented in log-correla- tion panels in this chapter (Figs. 3.3a and b). The other wells were discarded for various reasons, such as poor log quality (e.g. logs run through casing), the availability of only a gamma ray log, the absence of lithology descriptions, or their position on the flank of salt domes. Lithology de- scriptions based on cutting samples were available for all 11 presented wells. The descriptions provide information on sediment composition and grain size, sediment colour and the occurrence of constituents such as glauconite, pyrite and fossils. Industrial biostratigraphy reports, based on foraminifers from cutting samples, were available for ten of the 74 wells (Fig. 2.2d, Table 2.1). Several 2D-seismic lines aided the reconstruction. The seismic panels illustrate the large-scale sedimentary geometry of the Palaeogene (Fig. 3.4).
47 Sequence stratigraphic interpretation of log correlations
SNSTI-NL-87-11 A A'
200
400 Neogene 600
800
1000
1200 Palaeogene TWT (ms) 1400
1600 Salt dome
1800 Cretaceous
2000
2200 Salt dome
2400
0 10 km B SNST-NL-83-01 B'
200 Neogene
400
600
Palaeogene 800
1000
1200 London-Brabant
TWT (ms) Massif Cretaceous 1400
1600
1800
2000
0 10 km
Fig. 3.4 a) Seismic section A-A’, showing post-depositional deformation of the Palaeogene sedimentary succession due to salt-tectonics. In the inset, the Palaeogene succession is indicated in grey. b) Seismic section B-B’, showing the depositional setting of the Palaeogene. Note the parallel internal layering and the absence of clinoforms.
48 Chapter 3
C SNSTI-NL-87-25 C'
200
400 BFB Margin Neogene 600
800
TWT (ms) Palaeogene 1000
1200
1400
Broad Fourteens Basin
0 10 km
D SNSTI-NL-87-14 D'
200
400 Neogene BFB Margin
600
800 TWT (ms) 1000 Palaeogene
1200
Broad Fourteens Basin N
10 km o 0 55
o 54 A' A C D'
o 53
D C' B' o 52 B
o 51
o o o o o 3 4 5 6 7 c) Seismic section C-C’, illustrating post-depositional erosion of Upper Eocene (Palaeogene) sediments within the inverted Broad Fourteens Basin (BFB) and the geometry of the basin margin. d) Seismic section D-D’, illustrating post-depositional erosion of Upper Eocene (Palaeogene) sediments within the inverted Broad Fourteens Basin (BFB) and the geometry of the basin margin.
49 Sequence stratigraphic interpretation of log correlations
Mb. Mb. litho- borehole Fm. with stratigraphic well-logs boundary Fm.
Data available: Formations and - Log response members - Lithology indicated on logs - Biostratigraphy
sequence boundary
Correlation panels Higher-order sequences constructed, linking defined, using grain wells in the study size trends and distinct area log markers
Belgian succession: - High-resolution biostratigraphy - Correlated to eus- tatic cycle chart
Panels correlated to Belgian lithostratigraphic framework, using grain size trends and biostratigraphic data.
FigFig. 3.5 3.5 Flow diagram of the method of interpretation of the wells used in this study.
50 Chapter 3
2.2 Wireline log correlations and sedimentary sequence interpretation
For each well, the Palaeogene interval is subdivided using the log response and lithology descrip- tions. The lithostratigraphic interpretation is based on grain size variations and coarsening upward or fining upward trends (see Fig. 3.5 for a flow chart). The lithostratigraphic framework of the Netherlands (Van Adrichem Boogaert and Kouwe, 1997) is applied. Based on the subdivision, a low-resolution correlation between the boreholes is constructed (Fig. 3.5). Within the coarser correlation framework of formations and members, a higher-order correlation is made, which is based on trends in the gamma ray and sonic logs. Changes between fining-upward and coarsening-upward trends and distinct log markers are noted and form the boundaries between high-resolution sedimentary sequences within the limits of the coarse lithostratigraphic framework. Between the different wells, similar trends are correlated. This enables a detailed correlation of sedimentary sequences between boreholes in the area (Fig. 3.5). Although most trends correlate well, there are exceptions within some members. The results of the correlation are presented in four correlation panels (Figs. 3.6 and 3.7).
2.3 Age dating of the sequences
The sedimentary sequences in the study area cannot be correlated directly to the global eustatic sea level curve, due to the poor biostratigraphic control on the Dutch Palaeogene succession. There- fore, the sedimentary sequences are correlated first to the biostratigraphically calibrated sequences in Belgium (Fig. 3.8). The lithostratigraphic framework of Belgium (Marechal, 1993; Vanden- berghe et al., 1998; 2001) shows more detail than the Dutch Palaeogene lithostratigraphy (Van Adrichem Boogaert and Kouwe, 1997), as is illustrated in Fig. 3.1. The Belgian succession was deposited closer to the Palaeogene coastline of the North Sea Basin, compared to the more distal deposits of the Dutch offshore. Hence, in the Belgian succession, the sea level fluctuations are more clearly visible. Compared to the Dutch succession, it shows more evidence of erosion, trun- cation, channel incision and coastal onlaps (Jacobs and De Batist, 1996), as well as an alternation of marine and continental strata (e.g. Gullentops et al., 1988; Marechal, 1993). Sea level variations have been interpreted in detail from outcrops. A detailed sequence stratigraphic framework, with good biostratigraphic control, was constructed by Marechal (1993) and Vandenberghe et al. (1998, 2001, 2004). The recognition of eustasy-controlled sequences (based on well logs) in the study area, correlated to the lithostratigraphic framework of Belgium and the detailed sequence stratigraphic framework of Vandenberghe et al. (1998, 2001), allows an indirect correlation of the Palaeogene succession of the southern Dutch North Sea to the standard eustatic cycle chart of Haq et al. (1988). The Haq eustatic cycle chart was recalibrated by Hardenbol et al. (1998). A detailed age model for the sequence stratigraphic correlation is shown in Fig. 3.8. This Figure shows the Ypresian interval of well K06-01 and well Knokke in the Belgian offshore. The sediments in the wells were not influenced by the Oligocene tectonic movements and contain a continuous Late Palaeocene to Late Eocene succession. Ypresian sea level variations at both well locations, which can be recon- structed from the gamma ray log (K06-01) and grain size log (well Knokke, Fig. 3.8) respectively, are therefore likely to be of eustatic nature. Although the wells are situated at positions about 280 km from each other (Fig. 3.3a), their sedimentary successions can be correlated successfully. Through the correlation with well K06-01, other wells in the study area can be tied to the sequence
51 Sequence stratigraphic interpretation of log correlations 0 40 80 South Fig. 3.3a. ieper sandy interval 120 ansit time (us/ft) Tr 160 200 4 5 7 NLFFY NLLFC NLFFD NLFFS NLFFB NU NMRFV NMRFC 160 Yp Yp 8? Yp Yp 6? Yp 4b Yp ? T ? ? TS ? TST LST? HST HST ? TST S05-01 120 TST 0 Ieper Mb highstand systems tract transgressive systems tract lowstand systems tract Gamma Ray Sonic Lithostratigraphy NU Upper North Sea Group NMRFC Rupel Clay Mb. NLFFB Asse Mb. NLFFM/S Brussels Marl/Sand Mb. NLFFY ite/Sand Mb. ff NLFFT/D Basal Dongen Tu NLLFC Landen Clay Mb. 3 sequence boundary Yp TST HST LST Foraminifer Zones FF Early-Middle Oligocene FH Middle-Late Eocene (FH a, FHb, FHc-subzones) FI Early Eocene FJ Palaeocene Legend Gamma ray (API-Units) 0 4 0 8 available as a separate enclosure 0 50 00 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950 1000 1050 11 11 1200 1250 1300 1350 1400 1450 1500 1550 1600 1650 1700 120 140 160 ansit time (us/ft) Tr 180 NU 4 3 5 NLFFY NMRFC NLFFM NLLFC 7? 8? 6 4b Yp 200 Yp Yp Yp Yp Yp Yp T 100 NLFFT TST TST ? TS HST TST HST ? ? TST HST HST HST? HST? TST? 0 L13-01 A larger version of thid figure is A Gamma Ray Sonic Gamma ray (API-Units) 0 2 0 4 0 6 0 8 0 50 00 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950 1000 1050 11 11 1200 1250 1300 1350 1400 1450 1500 1550 1600 1650 1700 ? ? 80 FI FJ FF FHc FHb FHa? 120 160 ansit time (us/ft) Tr 4 NU 3 5 7 8 4b 6 NLFFY NLLFC ? NLFFM NMRFC 200 Yp Yp Yp Yp Yp Yp Yp NLFFT TST? TST HST? LST TST TST 70 HST HST ? HST HST TST TST 60 K12-01 50 40 Gamma ray (API-Units) 30 Gamma Ray Sonic 20 0 50 00 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950 1000 1050 11 1200 1250 1300 1350 1400 1450 1500 1550 1600 1650 1700 11 40 80 120 160 ansit time (us/ft) Tr 200 NU 240 NLFFY NLLFC NLFFB 6 4 5 NLFFM 7 3 NMRFC 8 4b Yp Yp Yp Yp Yp Yp Yp TST HST 0 TST TST LST HST HST HST HST? HST? TST? HST? TST HST TST K06-01 NLFFT Gamma ray (API-Units) Gamma Ray Sonic 0 2 0 4 00 6 8 0 50 00 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950 1000 1050 11 1200 1250 1300 1350 1400 1450 1500 1550 1600 1650 1700 11 40 80 120 ansit time (us/ft) Tr 160 200 4 7? 6 3 8? 5? 4b NLFFY NMRFC NLFFB NLFFM NLLFC ? Yp Yp Yp Yp Yp Yp Yp NU 100 HST? HST? L02-04 TST HST HST TST TST HST HST HST TST HST 0 NLFFT 8 60 4 0 Gamma Ray Sonic 20 Gamma ray (API-Units) Figure 3.6a 0 0 50 00 50
100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950
11 1000 1050 11 1200 1250 1300 1350 1400 1450 1500 1550 1600 1650 1700 North (m) depth Fig. 3.6 Fourteens Basin. wells in the Dutch North Sea, located outside Mesozoic Broad between five of well correlations a) Sequence stratigraphic interpretation indicated in The positions of the wells are indicate sea level trends. Arrows sea level. the local relative The sedimentary composition of the sequences reflects
52 Chapter 3 stratigraphic framework of Vandenberghe et al. (1998, 2001). This, in turn, allows the recogni- tion of local tectonic activity, erosion or non-deposition in the study area. Sequence stratigraphic interpretation of the correlated wells in the study area generally yields good results. However, ex- ceptions occur within individual cycles in some wells, which is illustrated by deviating grain size trends. 100 120 3 4 Yp Yp 140 South 160 ansit time (us/ft) Tr 180 200 NU 160 NLFFY NLLFC NLFFT/D TST HST TST 120 P15-02 available as a separate enclosure 0 Gamma ray (API-Units) Gamma Ray Sonic 0 4 0 8 0 50 00 50
100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950
1000 1050 11 1200 1250 11 depth (m) depth 4 Yp 3 Fig. 3.3a. Yp 30 A larger version of thid figure is A 40 ansit time (us/ft) Tr 50 NU NLFFY NLFFT NLLFC 120 P05-03 TST HST TST 0 8 4 0 Gamma Ray Sonic Gamma ray (API-Units) 0 0 50 00 50
100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950
1000 1050 11 1200 1250 11 depth (m) depth 0 20 4 3 4b Yp Yp Yp 40 60 80 ansit time (us/ft) FJ Tr FF FI? 100 120 NU 160 NLLFC NLFFY NMRFC Q01-01 TST 120 HST TST NLFFT 0 0 8 Gamma Ray Sonic Gamma ray (API-Units) 0 4 North 0 50 00 50
100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950
1000 1050 11 1200 1250 11 depth (m) depth Fig. 3.6 (continued) wells in the Dutch North Sea, located within between three of well correlations b) Sequence stratigraphic interpretation indicated in Fourteens Basin. The positions of the wells are the Mesozoic Broad
53 Sequence stratigraphic interpretation of log correlations
3. Sequence stratigraphic development
3.1 Large scale geometry
Seismic data show that the original geometry of the siliciclastic Palaeogene succession was af- fected by post-depositional tectonic movements and erosion (Fig. 3.4). Oligocene uplift in the centre of the study area resulted in severe erosion of Late Eocene sediments (Letsch and Sissingh, . 120 140 South 160 ransit time (us/ft) T Ieper Mb Lithostratigraphy NU Upper North Sea Group NMRFC Rupel Clay Mb. NLFFB Asse Mb. NLFFM/S Brussels Marl/Sand Mb NLFFY 3 sequence boundary Yp transgressive interval T R regressive interval aggrading interval A Legend 180 100 NLLFY NLFFM NMRFC 100 L13-01 available as a separate enclosure 120 T A-R T 80 140 60 ransit time (us/ft) T 160 Gamma Ray Sonic 40 Gamma ray (API-Units) 180 20 650 750 850 600 700 800 900 NU 160 NLLFY NLFFB NLFFM NMRFC L02-04 120 120 T A-T R R 130 80 A larger version of thid figure is A 140 Gamma Ray Sonic Gamma ray (API-Units) 40 150 ransit time (us/ft) T 160 0 00 950 170 1000 1050 11 50 11 1200 1250 NU 80 NLLFY 80 NLFFM NMRFC K12-01 T R T 120 60 160 ransit time (us/ft) T 40 Gamma Ray Sonic 200 Gamma ray (API-Units) 240 20 550 600 650 700 750 800 850 100 NLLFY NLFFB NLFFM K06-01 NMRFC 80 120 T A-T T R T R 60 140 40 Gamma Ray Sonic Gamma ray (API-Units) 160 20 ransit time (us/ft) 0 T 180 0 00 800 850 900 950 100 1050 11 200 North NLLFY NLFFB NMRFC NLFFM ? T 60 T A-R T K07-02 50 1 7 1 3 2 4 2 1 3 10
50 40 Lu Pr Bart Pr Yp Yp Ru3 Lu Pr Lu Lu 8
Pr 4/Ru1 Yp T R T T 100 30 Gamma Ray Sonic Gamma ray (API-Units) 150 200 Eustatic curves Hardenbol et al., 1998 20 est 550 600 650 700 750 800 850 500 Fig. 3.7a) Sequence stratigraphic interpretation of well correlations of the Brussels and Asse members (Dongen Fm.) between five wells, five between Fm.) (Dongen members Asse and Brussels the of correlations well of interpretation stratigraphic Sequence 3.7a) Fig. Fourteens Basin. The position of the wells is indicated in Fig. 3.3b. located North of the Mesozoic Broad W
54 Chapter 3
1983; Van Wijhe, 1987a; 1987b). The uplift was accommodated by large fault zones bordering the inversion zone (Figs. 3.4c and 3.4d). In the North of the study area, the Palaeogene sediments were deformed by salt tectonics (Fig. 3.4a), which resulted in piercing of the succession and the forma- tion of associated rim synclines (Remmelts, 1995).
3.2 Detailed sequence correlation
3.2.1 Landen Formation (Thanetian) and Basal Dongen members (Early Ypresian)
The wells located outside the inverted Broad Fourteens Basin (Fig. 3.3a) comprise a relatively complete Palaeogene succession (Fig. 3.6a). The correlation of the Thanetian Landen Formation and the Early Ypresian Basal Dongen Sand and Tuffite Members is based on wireline log signature. The main body of the Landen Formation is a highstand systems tract, which is suggested by ag- grading to prograding (coarsening upward) log trends. The Basal Dongen Sand and Tuffite mem- bers are separated from the Landen Formation by a sharp gamma ray log boundary (Fig. 3.6a). The Dongen Sand and Tuffite members show a distinct low gamma ray response and ‘erratic’ sonic velocity with spikes (clearly visible in L13-01 and S05-01). The sonic spikes result from intermix- ing of tuffaceous ashes, derived from volcanic events further to the North (Jacque and Thouvenin, 1975; Knox and Morton, 1988). According to the log signature, the Dongen members were depos- ited during a short regressive phase, which was followed by the deposition of transgressive sandy clays (Letsch and Sissingh, 1983). The Landen Formation and Basal Dongen members (Fig. 2.1) cover sequences Se2-Yp2 (Van Adrichem Boogaert and Kouwe, 1997; recalibrated to Hardenbol et al., 1998).
3.2.2 Ieper Member (Ypresian)
Outside the Broad Fourteens Basin area, the Ieper Member has not been affected by post-deposi- tional erosion. This is indicated by the occurrence of the Brussels Sand or Brussels Marl Member on top of the Ieper Member (Fig. 2.5a, b). Within the Broad Fourteens Basin area, post-deposition- al erosion linked to the Pyrenean inversion phase resulted in the removal of most of the Ypresian sedimentary succession (Figs. 2.5a, b, 3.4). Only the lowermost Ypresian sequences are preserved (Fig. 3.6b). The sequence correlation of the Ypresian succession is based on wells in which the Ieper Member is complete. The sediments of the Ieper member were deposited in an open marine environment with water depths of tens to hundreds of meters (van Adrichem Boogaert and Kouwe, 1997). The lower half of the Ypresian succession is an aggrading interval of silty clays (Fig. 3.6a), in which small-scale (~50 m) transgressive and highstand sequences occur. The transgressive and highstand sequences are recognized by coarsening upward and fining upward grain size trends. Abrupt changes in log response typically indicate sequence boundaries, more pronounced in the gamma ray than in the sonic logs. The grain sizes inferred from the gamma ray logs (Figs. 3.6a, 3.8) indicate that the fluctuations in local sea level during the deposition of this part of the Ieper Member were relatively minor. Biostratigraphic results from well K12-01 (Fig. 3.6a) indicate that this section of the Ypre- sian succession spans the Dutch North Sea Plankton Zone FI (Early Eocene, Fig 3.2). The local sea level trend, inferred from the grain sizes in the succession, correlates well to the Belgian sequence stratigraphic framework (Fig. 3.8) and indicates that this part of the Ieper Member covers the inter-
55 Sequence stratigraphic interpretation of log correlations 40 80 120 South 160 0 ansit time (us/ft) 20 Tr 240 280 NLFFB NLFFS NLFFY NMRFC available as a separate enclosure S05-01 160 120 T R T R R 80 sandy interval Gamma Ray Sonic Gamma ray (API-Units) 40 0 0 0 0 0 0 30 35 400 45 500 55 600 65 ? 40 A larger version of thid figure is A 80 120 160 ransit time (us/ft) T . 200 240 NU Fig. 3.3b NLFFB NLFFS NLFFY NMRFC 80 S02-02 T R A 60 T sandy interval 40 Gamma Ray Sonic Gamma ray (API-Units) 20 0 350 450 550 650 400 500 600 80 120 160 ransit time (us/ft) T 200 240 NLFFY NLFFB NLFFS NMRFC S02-01 80 R R T T A-R 60 sandy interval 40 Gamma Ray Sonic Gamma ray (API-Units) 20 0 450 500 550 600 650 700 750 North 1 7 1 3 2 4 2 1 3 10
50 Lu Pr Bart Pr Yp Yp Ru3 Lu Pr Lu Lu 8
Pr 4/Ru1 Yp T T R T 100 150 200 Eustatic curves Hardenbol et al., 1998 b) Sequence stratigraphic interpretation of well correlations of the Brussels and Asse members (Dongen Fm.) in five wells, located of the Brussels and of well correlations Fig. 3.7 b) Sequence stratigraphic interpretation Fourteens Basin. The position of the wells is indicated in South of the Mesozoic Broad
56 Chapter 3 val Yp3 to Yp7 (54.6-51.6 Ma) of the sequence chart of Hardenbol et al. (1998). The upper part of the Ypresian succession (Fig. 3.6a) shows a gradual increase in mean grain size, which changes from slightly silty to very silty clay (wells K06-01 and K12-01 in the North). In well S05-01 in the South, a distinct sand layer is deposited at the base of the silty interval. The grain size increase in this part of the succession reflects a lowering of the local sea level. In well L02-04, relative fine grain sizes during the interval point to a higher local sea level, although the interval in this well also shows a gradual increase in mean grain size. Only in well L13-01, the gamma ray log trend indicates a continuous local sea level rise in the up- per part of the Ypresian succession (Fig. 3.6a). The local transgression indicates the formation of a local depression, as a result of tectonic activity during the Late Ypresian. This Ypresian tectonic pulse is the first of a series of short tectonic pulses, which finally culminate in the main Pyrenean tectonic pulse. The Ypresian tectonic pulse will be discussed in detail in Chapter 4. The subsequent pulses have a different character, and are discussed below. The sequence stratigraphic interpretation of the Upper Ypresian succession is not straightforward. Although generally the construction of a sequence stratigraphic framework works very well, with- in individual sedimentary cycles local deviations in grain size trends can be observed. In wells K06-01 and K12-01, the interval is interpreted as a Lowstand Systems Tract, which is suggested by gamma ray values indicating relatively coarse sediments. In well L02-04, the occurrence of much finer sediments, accompanied by a coarsening upward response of the gamma ray log, sug- gests that the interval comprises one or two Highstand Systems Tracts (Fig. 3.6a). This suggests that these sedimentary cycles are not exactly time equivalent. According to the biostratigraphic report of well K12-01 (Figs. 3.6a, 3.8), the upper interval of the Ypresian succession spans Dutch North Sea Plankton Zones FHc and Fhb, which would indicate a Middle to Late Eocene age (Fig. 3.2). However, the report mentions that the boundary between FH and the FI zones has been drawn rather arbitrarily in this well. A few benthic foraminiferal species considered characteristic for the lower part of the FH zone and the FI zone are found higher in the well, in samples interpreted to belong to the FHb subzone. Therefore, these samples might be of Early Eocene age, too. These ambiguous results make these biostratigraphic data inadequate for our research goals. Correlation of well K06-01 to the Belgian sequence stratigraphic framework (Fig. 3.8), based on local sea level trends inferred from grain size analysis, indicates that the upper part of the Ieper Member covers the interval Yp7 and Yp8, which is a period of long-term eustatic sea level fall.
3.2.3 Brussels members (Lutetian)
The Lutetian Brussels Sand Member was deposited in the South of the study area. It is a non-cal- careous, silty to sandy clay with local intercalations of siltstone or very fine sandstone beds. To the North, the member grades into calcareous clays of its more distal equivalent, the Brussels Marl Member. The Brussels members are coarser grained than the silty top of the Ieper Member (Fig. 3.6a). In a wide area, the Brussels members were partly or completely eroded in response to the Pyrenean inversion (Fig. 2.5b). A complete succession of the Brussels Members is found only in small parts of the study area. This is indicated by the occurrence of the Asse Member, which was deposited concordantly on top of the Brussels members (Fig. 2.5c). The log signature of the Brussels Members is less homogenous than the log signature of the Landen Formation and the Ieper Member. This reflects a more dynamic environment during deposition of
57 Sequence stratigraphic interpretation of log correlations ies (ma) sequence Yp 5 (53.1) Yp 7 (51.6) boundar Yp 4 (53.6) Yp 6 (52.1) Yp 3 (54.6) Yp 1 (54.9) Yp 10 (50.0) Yp 8 (51.0) Yp 2 (54.8) fall (1998) 100 el (m) v 150 e sea le el and sequences v 200 rise Hardenbol et al. relativ (1987)
2.5 2.9 2.8 2.4 2.7 2.6
A 2 A T ces Eustatic sea le
Haq et al. sequen-
Asteria Zone Asteria
NP13 Biostr. NP11
* W. Top grain size grain 100% 2um 8um e 0 TS TS
100 1
5
4
2
3 6 Knokk HST TST TST HST TST TST HST HST HST TST LSW HST LSW HST 50 GR (API) Vandenberghe et al. (1998)
0
250
150 200 100 (m) Depth 40 ? FI FJ FHc FHa FHb 80 well K12-01 Biostratigraphy 120 160 200 DT (microsec/ft) TST HST HST HST HST LST TST HST LST HST TST TST 80 60 K06-01 40 20 GR (API)
0
. Mb Ieper . Mb l
. Mar
ussel Br (1) Asse Mb Landen Fm.
950
1200 1300 1150 1250 1350 1000 1100 1050 1400 1450 1500 Depth (m) Depth
58 Chapter 3 these shallow marine sediments. Wells K06-01 and L02-04 are located far away from faults that were active during the Palaeocene to Oligocene (cf. Fig 2.4, 3.3). These wells contain a tectonically undisturbed Middle to Late Eocene sequence, which reflects the eustatic sea level history (Fig. 3.7a). Wells K07-02, K12-01 and L13-01 are located close to the margin of the Mesozoic Broad Fourteens Basin. The Brussels Marl Member sediments in wells K12-01 and L13-01 were affected by post-depositional erosion, associated with the Pyrenean tectonic phase. Correlation of the sonic logs of wells K12-01 and L13-01 with wells K06-01 and L02-04 suggests that only a small part of the Brussels Marl Mem- ber was eroded (Fig. 3.7a). The Brussels Marl Member North of the Mesozoic Broad Fourteens Basin can be divided into three sedimentary sequences (Fig. 3.7a). The Brussels Marl Member contains a transgressive low- er unit, which is characterized by a fining upward trend. The upper boundary of the sequence is a maximum flooding surface, indicated by a gamma ray maximum, often followed by a change in the sonic log signature. The middle sequence shows an aggrading to slightly fining upward trend in wells K06-01 and L02-04 (possibly a Transgressive Systems tract), and an aggrading to slightly coarsening upward trend in well K07-02 (possibly a Highstand Systems Tract). In wells K12-01 and L13-01, closer to the Broad Fourteens Basin, the unit shows a coarsening upward trend (High- stand Systems Tract). The uppermost sequence of the Brussels Marl Member in wells K06-01 and L02-04 displays lower gamma ray values than the underlying sequence. This indicates relatively coarse grain sizes. In wells K07-02, K12-01 and L13-01, the gamma ray values indicate that the uppermost sequence starts with a grain size maximum. The gamma ray response indicates increas- ing grain sizes in the northernmost wells (K06-01 and L02-04), and fining upward grain size trends in the wells closer to the Mesozoic Broad Fourteens Basin. South of the Broad Fourteens Basin (S02-01, S02-02, S05-01), the Brussels Sand Member shows a coarsening upward trend (Fig. 3.7b). These opposing trends are examples of the deviations, which occur within individual cycles. Biostratigraphic results for well K12-01 (Fig. 3.6a) indicate that the Brussels Marl Member spans Dutch North Sea Plankton Zone FH (Eocene, Fig. 3.2). The Brussels Sand Member spans nan- noplankton zones NP13-15 (van Adrichem Boogaert and Kouwe, 1997), and therefore eustatic sequences Yp9-Lu2 (Vandenberghe et al., 2004). The sea level curve of Hardenbol et al. (1998) indicates that the fluctuations in eustatic sea level were much larger during deposition of the Brus- sels members, than during deposition of the Ieper Member (Fig. 2.1). The base of sequence Yp10 is marked by an abrupt fall in eustatic sea level, which possibly resulted in the end of open marine deposition of the Ieper Member clays. This sea level drop was followed by two transgressive in- tervals, Yp10 and Lu1. During deposition of sequence Lu2, eustatic sea level was close to the high
Fig. 3.8 (opposite page) Sequence stratigraphic correlation between the gamma ray and sonic logs of well K06-01 in the Dutch North Sea and the gamma ray log of well Knokke in Belgium (redrawn after Vandenberghe et al. (1998), their fig. 7). The position of the wells is indicated in Fig. 3.3a. Vandenberghe et al. compared the sequence boundaries 1-6 (encircled) of the Ypresian deposits of Belgium to the standard sequence chart of Haq et al. (1988), which was recalibrated in the same volume (Hardenbol et al., 1998). The interval numbered (1) in well K06-01 is the Basal Dongen Tuffite Member. Biostratigraphic results of well Knokke are shown. Correlation to other Belgian wells increased the accuracy of these results (Vandenberghe et al., 1998). The * in the biostratigraphic results from well Knokke indicates the first major planktonic influx of foraminifers and nannofossils into the basin, base biochron C24BN.
59 Sequence stratigraphic interpretation of log correlations
b) cu ? fu _ Lu2 ? Lu1 Yp10 BFB
tectonic uplift?
possible winnowing _ a) + of sediments
cu cu a fu Lu1 BFB Yp10 Fig. 3.9 a) Interpretation of the sequence architecture of Yp10 Lutetian sequence Lu1 BFB in the Brussels Sand and South North Marl Member (Dongen Fm.).
BFB = Area overlying the Mesozoic Broad Fourteens basin b) Interpretation of the sequence architecture of Schematic log response Lutetian sequence Lu2
Schematic grain size in the Brussels Marl Member (Dongen Fm.). level of Lu1, although it slowly decreased. The eustatic sea level chart correlates well with the sequences which were interpreted in wells K06-01 and L02-04 (Fig. 3.7a), which are thought to be unaffected by tectonic activity and to reflect the eustatic sea level.
3.2.4 Asse Member (Bartonian)
In the central part of the study area, the Asse Member was completely removed by Eocene-Oli- gocene erosion. The unit is only present in the far North and South of the study area (Fig. 2.5c). The top of the member is a regional erosional unconformity. As the available data is limited, a detailed correlation of the Asse Member in the study area to the sequence chart of Hardenbol et al. (1998) is tentative. The gamma ray log response of the Asse Member (in the North in wells L02-04, K06-01 and K07-02, Fig. 3.7a, in the South S02-01, S02-02 and S05-01, Fig. 3.7b) shows a coars- ening upward megatrend. Exception is well K07-02, which shows overall fining upward grain sizes (Fig. 3.7a). The sediments of the Asse Member can tentatively be divided into two, possibly three sequences (Fig. 3.7b). In the South (wells S02-02 and S05-01), the lower sequence of the
60 Chapter 3
Asse member starts with a fining upward interval. This transgressive sequence is very thin close to the southern margin of the Broad Fourteens Basin. The transgressive sequence is followed by a (coarsening upward) highstand systems tract. The second sequence shows a fining upward trend (Fig. 3.7b). Biostratigraphic control of the member is poor, but the interval is thought to cover nannoplankton zones NP16 and NP17 (Van Adrichem Boogaert and Kouwe, 1997). This would suggest that the member covers the eustatic sequences Lu3, Lu4 and Bart1. The long-term eustatic sea level movements during deposition of sequences Lu3, Lu4 and Bart1 are regressive (Fig. 2.1). This long-term trend seems to be reflected by the succession.
3.3 Late Eocene tectonic uplift
The onset of tectonic activity associated with the Pyrenean phase was dated Late Eocene (Letsch and Sissingh, 1983; Van Wijhe, 1987a; 1987b). This dating was probably based on the age of the youngest sediments (Asse Member) below the regional erosional hiatus resulting from the uplift. It shows a maximum age of 37.0 Ma (coinciding with the base of the Priabonian), based on an esti- mated age of 43.4-37.0 Ma of the Asse Member (biozone NP16 and possibly NP17, Van Adrichem Boogaert and Kouwe, 1997). No other pulses of Middle to Late Eocene tectonic activity have been reported from the study area.
4. Discussion
The sequence stratigraphic interpretation of the well log correlations suggests that tectonic activity could have started at the beginning of the Lutetian. The northernmost wells (K06-01 and L02-04) were not affected by erosion and uplift during deposition of the Lutetian Brussels Marl Member. The internal grain size variations of the Brussels Marl Member in these wells closely reflect the eustatic sea level curve (Fig. 3.7a). In contrast, in all other analysed wells, the Brussels members display grain size trends, which deviate from those of the two northernmost wells. The occurrence of these local variations in grain size is interpreted to be caused by local tectonic activity during deposition of the Brussels and Asse members (Fig. 3.7). Alternatively, the grain size variations might indicate that these sequences are diachronous. There is, however, no indication of erosion or non-deposition within the succession. Moving from the North towards the Broad Fourteens Basin area, the grain size trend of the second sequence (Lu1) of the Brussels Member changes progressively from a fining upward, to constant, to a coarsening upward trend (Figs. 3.7a, 3.9a). Additionally, the wells in the South of the study area (S02-01, S02-02 and S05-01) display a distinct regressive, coarsening upward grain size trend in the Brussels Sand Member. This suggests a local relative sea level drop, and a reduction in ac- commodation space, during deposition of the second sequence (Lu1) of the Brussels Marl Member in the wells closest to the margin of the Mesozoic Broad Fourteens Basin (K12-01, L13-01 and the wells in the South). Because the eustatic sea level curve (Hardenbol et al., 1998) and the north- ernmost wells do not indicate a eustasy-controlled regression during this period, but a significant sea level rise, the regressive signal in the other wells is probably the result of tectonic forcing. The tectonic activity possibly occurred in the form of uplift of the centre of the study area, possibly re- stricted to a small area lying over the Mesozoic Broad Fourteens Basin only (Fig. 3.9a). The uplift could have been fault-bounded or accommodated by flexure. The uplift resulted in a reduction of the local accommodation space and therefore in a regressive signal in the wells close to the centre
61 Sequence stratigraphic interpretation of log correlations of the study area. Associated winnowing or erosion of the unconsolidated Middle to Upper Eocene sediments in the centre of the uplifted area would also result in a regressive signal (Fig. 3.9a). The third sequence (Lu2) of the Brussels Member is regressive in wells K06-01 and L02-04, which is conform the eustatic sea level trend (Fig. 3.7a). In contrast, sequence Lu2 is transgressive in the wells closer to the northern margin of the Mesozoic Broad Fourteens Basin area, which points at a local increase in relative sea level (Figs. 3.7a, 3.9b). This might be the result of formation of a lo- cal depression after the previous phase of compression, due to the relaxation effect discussed in the previous chapter (Fig. 3.9b). The variability in sequence architecture of the remaining sediments of the Bartonian Asse Member suggests that the tectonic activity continued during the deposition of the Asse Member. The tectonic activity during the Lutetian and Bartonian demonstrates that the onset of the Pyrenean tectonic pulse in the southern Dutch North Sea was preceded by other episodes of small-scale tectonic activity. The tectonic pulses in the Dutch offshore are time-equivalent to Lutetian and Bartonian phases of uplift of the Brabant and Artois Blocks in Belgium, which have been reported by Vandenberghe et al. (2004). This would suggest that the inferred tectonic activity is of regional importance. The Lutetian-Bartonian tectonic pulses in the southern Dutch North Sea are only perceptible as de- viations of the local relative sea level from the eustatic sea level curve. The tectonic activity caused local changes in subsidence rates and possibly even uplift. The local sea level deviations are re- flected in the Middle to Upper Eocene sedimentary succession, such as observed for the Lutetian Brussels members. Local tectonic overprinting of eustatic sequences started at the beginning of the Lutetian (Lu1, circa 48 Ma), which is much earlier than the previously assumed first period of Late Eocene tectonic activity, which started during the Early Priabonian (circa 37 Ma) with the onset of the Pyrenean phase. Even before the Lutetian, during the Late Ypresian, tectonic activity occurred in the area. This period of tectonic activity is, however, of a different character. It will be discussed in Chapter 4.
5. Conclusions
Sequence stratigraphic interpretation based on well logs provides a high-resolution correlation of the siliciclastic sediments of the Palaeogene in the southern Dutch North Sea. Although such inter- pretations have to be applied with proper care, they appear to be a fairly reliable tool to discrimi- nate between eustatic sea level variations and local tectonic movements and to date the latter. The Late Eocene tectonic activity started much earlier than previously assumed. Previously, it was thought that the area was tectonically quiet until the onset of the Pyrenean phase, which started during the Early Priabonian (circa 37 Ma). However, based on the present study, it can be con- cluded that tectonic activity in the study area already started during the Early Middle Eocene (be- ginning of the Lutetian, circa 48 Ma). Moreover, there are indications that tectonic activity started even earlier, during the late Ypresian. The episode of tectonic activity can be correlated to tectonic uplift in the Brabant and Artois Blocks in Belgium, which suggests that it might be of regional significance. The results shown in this Chapter are an illustration of the occurrence of small-scale tectonic activ- ity during periods, which were previously thought to be tectonically calm. In Chapter 4, the Ypre- sian episode of tectonism preceding the Lutetian, which is also such an example of small-scale tectonism, is elaborated on.
62 Chapter 3
Chapter 4 On the occurrence of an Early Eocene (Late Ypresian) tectonic pulse in the southern Dutch North Sea Basin.
Abstract
The occurrence of onlap of Upper Ypresian (Lower Eocene) sediments on underlying Ypresian deposits along the northeastern edge of the area straddling the Mesozoic Broad Fourteens Basin indicates a period of compressive tectonic activity in the area. The tectonic event resulted in local differential subsidence, which is indicated by tilted and slightly flexured strata. The tectonic event shows that in addition to the major phases of Mesozoic-Cenozoic inversion, which affected the Mesozoic Broad Fourteens Basin, at least one other episode of tectonic activity occurred. In the area of the onlaps, no indication of erosion (e.g. truncation of strata, channel incision) is found, indicating that at the time of the event the margin itself was not sub-aerially exposed. In contrast, the central part of the basin area may well have become emerged. However, the later Pyrenean inversion phase may have removed possible evidence of sub-aerial exposure during the Late Ypresian tectonic phase.
1. Introduction
In this chapter, a pulse of Late Ypresian (Early Eocene) compressive tectonic activity is investigat- ed, which affected the area straddling the Mesozoic Broad Fourteens Basin (Figs. 4.1a, 4.2). The event was mentioned in the previous chapter, and is discussed in more detail here. Previously, no tectonic activity during the Late Ypresian has been recognized in this area, but closer to the edge of the Palaeogene North Sea Basin, Late Ypresian tectonic activity influenced the stratigraphic archi- tecture of the basin (Vandenberghe et al., 2004). This tectonic activity was associated by the latter authors with the uplift of the Brabant and Artois Blocks. The effects of the Late Ypresian tectonic event were noticed near the northeastern edge of the inverted Mesozoic Broad Fourteens Basin (Figs. 4.1a, b), where Late Ypresian sediments are dis- cordantly deposited on underlying Ypresian sediments. The recognition of tectonic activity be- tween the major tectonic phases indicates that the tectono-stratigraphic evolution of the North Sea is even more dynamic than previously thought.
2. Data and methods
To reconstruct the Late Ypresian tectono-stratigraphic development of the area, a seismic section showing onlaps within the Ypresian succession is studied in detail. Seismic horizons on the panel are dated. The seismic section is backstripped, after which local subsidence rates have been calcu- lated. The study is based on the detailed interpretation of about 200 km of 2D-seismic lines (Figs. 4.1a, b), log data of well L17-02, and quantitative subsidence analysis. The method is illustrated in Fig. 4.3. Seismic onlaps within the Ieper Member have been identified (Fig. 4.3a) on several
63 The occurence of an Early Eocene (Late Ypresian) tectonic pulse
Fig. 4.1 a) Outline of the study area within the North Sea (black square). The outline of the Mesozoic Broad Fourteens Basin (BFB) and the position of seismic line SNST-83-02 (Fig. 4.2) are indicated. b) Detailed map of the study area, showing the seismic data, the onlap ‘front’ (dotted line), the edge of the Broad Fourteens Basin (BFB, dashed line) and the positions of well L17-02 and virtual wells Virt1 and Virt2. lines of 2D-seismic surveys SNST-83 and SNSTI-87 (Figs. 4.1b, 4.4 and 4.5). The Ieper Mem- ber has been subdivided into 8 seismic stratigraphic units (IE1-IE8), the boundaries of which are formed by distinct seismic horizons (Figs. 4.3, 4.4). Seismic stratigraphic units IE5 and IE6 onlap on unit IE4 (Fig. 4.4). Well L17-02 is situated at a distance of 7 km from the seismic transect SNST-83-02 (Fig. 4.1b). The lithostratigraphic composition of the well is interpreted using its lithologic description, gamma ray and sonic logs (Fig. 4.6b). The eustatic sequences within the Ieper Member in well L17-02 were correlated to the wells in figure 3.6a. The high-resolution correlation is used for an age interpreta- tion (Fig. 4.6b, c), which is possible because the wells in Chapter 3 were dated using the standard eustatic cycle chart of Hardenbol et al. (1998). Not every eustatic sequence boundary (Yp) could be recognized. By tying the stratigraphic units of well L17-02 with the seismic data (Fig. 4.3b), the age of several seismic horizons could be inferred (Table 4.1, Fig. 4.6). Subsidence analysis allows a quantitative assessment of the tectonic evolution of the area. It pro- vides information on the factors contributing to subsidence and uplift in the area, i.e., tectonic forcing and isostatic subsidence, as well as the influence of differential compaction. To assess the local factors resulting in the observed onlaps, a location (Virt1) on seismic transect SNST-83-02, with a complete Ypresian succession (seismic units IE1 to IE8), is compared to a location (Virt2) at a distance of about 10 km (Fig. 4.1b). Location Virt2 is positioned West of the onlaps. Thus, seismic units IE5 and IE6 are absent at location Virt2 (Figs. 4.1b, 4.4). Locations Virt1 and Virt2 are treated as virtual wells (Fig. 4.3c). At both locations, the depth of the seismic boundaries were
marked (Fig 4.4) and time-depth converted, using the V0-k method of Chapter 2. Both virtual wells
64 Chapter 4
Fig. 4.2 Seismic cross-section SNST-83-02. The locations of virtual wells Virt1 and Virt2 and the detailed section of Fig. 4.4 are indicated.
65 The occurence of an Early Eocene (Late Ypresian) tectonic pulse
Table 4.1 Well data of L17-02, Virt1 and Virt2 (TWT and m) with ages. The Dongen Tuffite Member is below the seismic resolution (bsr); it does not appear as a recognisable unit in the seismic section and is therefore not included in the virtual wells. The Dongen Tuffite Member is thus incorporated in Unit Y1 of the Ieper Member in the virtual wells. were backstripped to reconstruct the local subsidence history (Figs. 4.3d, 4.7). The backstripping method of Van Wees et al. (1998) is used. The basement subsidence rates were calculated applying the model results (Table 4.2). Although well L17-02 comprises a complete Ypresian section, it is not used for subsidence analysis, as it is positioned too far from the seismic transect. Locations Virt1 and Virt2, positioned on the seismic transect, illustrate more clearly which part of the geom- etry is analysed. It must be mentioned that when constructing subsidence curves using the backstripping program of Van Wees et al. (1998), there is a problem if sedimentary units are partly removed (e.g. due to uplift), resulting in an erosional hiatus. The program unfortunately cannot apply estimates of the local stratigraphic interval lost due to erosion. The program only gives output values for the
Table 4.2 Tectonic subsidence rates (Rsubs) of wells Virt1 and Virt2, as well as the average southern North Sea tectonic subsidence rates (Av. Rsubs) calculated in Chapter 2.
66 Chapter 4 base of the incomplete interval and the base of the overlying unit, which was deposited above the erosional hiatus. The graphic representation and subsidence rate calculations interpolate between these values. As a consequence, this results in an underestimation of the tectonic subsidence dur- ing deposition of a subsequently partially eroded unit. In some cases, the interpolation might even result in apparent uplift during deposition of the unit. This is for instance the case for the Brussels Member in the virtual wells (Table 4.2). The intervals with an incomplete stratigraphic succession
Fig. 4.3 Flowchart depicting the followed method. Note that this is not an actual interpretation of a seismic section, but an example of the working method.
67 The occurence of an Early Eocene (Late Ypresian) tectonic pulse Fig. 4.4 Detail of seismic section showing SNST-83-02, the stratigraphic geometry of Palaeogene sediments at the NE margin of the inverted Mesozoic Broad Fourteens Basin (BFB) and the onlap of a sequence within Ieper the Member Ypresian (Dongen Fm.). The seismic units IE1-IE8 are indicated. The section was rescaled for display. clarity The of positions of the virtual wells Virt1 and Virt2 are indicated. According to interpretation of well unit on the seismic section. L17-02 Due to its limited thickness (<20 m) it does not appear as a recognisable in the area. (see (Dongen Formation) is present Fig. 4.6), the Basal Dongen Tuffite Member
68 Chapter 4 are illustrated with a dashed line (Fig. 4.7a, b). A dotted line gives an impression of the inferred realistic subsidence, based on the average values (0.5 cm/ka) for the southern North Sea from Chapter 2.
3. Observations
3.1 Seismic geometry observations
Seismic section SNST-83-02 is a NE-SW transect of the inverted Mesozoic Broad Fourteens Basin and the overlying Cenozoic succession (Fig. 4.2). The detail of section SNST-83-02 (Fig. 4.4) shows the Palaeogene and Neogene sedimentary succession at the northeastern margin of the in- verted Broad Fourteens Basin. The succession is slightly flexured. Upper Ypresian sediments (units IE5 and IE6) onlap to the Southwest onto older Ieper Member units (IE1 to IE4). The onlaps are visible in several seismic sections near SNST-83-02; e.g. SNSTI-87-14, SNST-83-19 and SNSTI- 87-26 (Fig. 4.5). Figure 4.1b shows, in map view, the onlap ‘front’ and its position with respect to the Mesozoic Broad Fourteens Basin. No indication of Late Ypresian erosion of the underlying Ypresian sediments, for instance by truncation of strata or channel incisions, is found in the area next to the Mesozoic Broad Fourteens Basin (Fig. 4.4). This shows that no, or only very minor, aerial exposure occurred in the area during the Late Ypresian. The absence of channel incisions may suggest that the observed onlaps are not the seismic expression of a submarine lowstand fan at the base of an existing slope. Two seismic units (IE7 and IE8) were deposited concordantly on top of the succession and overlie the complete area (Fig. 4.4). The extensional faults in the middle of the seismic section (Fig. 4.4) were formed during the Early Neogene.
3.2 Sedimentological observations
The lithologic composition of the Ieper Member is relatively uniform, consisting of a succession of silty clays, which is illustrated by well L17-02 (Fig. 4.6). Variations in silt content are reflected in fining (fu) and coarsening upward (cu) trends in the gamma ray response. The Ypresian succession starts with an aggrading interval in which small-scale intervals, each consisting of a fining-coarsen- ing pair, suggest transgressive and highstand sequences (seismic stratigraphic unit IE1). This interval can be correlated with eustatic sequences Yp3-Yp6 (Fig. 4.6). Seismic units IE2/IE3 and IE4 cover eustatic sequence Yp7, which starts at 51.6ma. Seismic units IE2/IE3 and IE4 consist of a relatively thick coarsening upward sequence, followed by a fining upward interval. This indicates a regression, followed by a transgression. The low gamma ray values imply that the grain sizes of seismic units IE2/IE3 and IE4 are large in comparison to those of the under- and overlying sediments, suggesting a low relative sea level. Seismic units IE5 and IE6 correlate to eustatic sequence Yp8, which starts at 50.0 ma. The base of this interval is the onlap surface (Fig. 4.6). In seismic units IE5/IE6, the gamma ray response indi- cates a continued fining-upward trend. The interval is topped by a gamma ray maximum, interpreted as a maximum flooding surface (sensu Emery and Myers, 1996). This suggests that the onlapping sediments were deposited during a transgressive interval (Transgressive Systems Tract). In seismic units IE7 and IE8 (the upper part of sequence Yp8) the gamma ray response shows a coarsening up- ward trend. This trend suggests a progradation (Highstand Systems Tract). The Brussels Marl Mem- ber, in the study area consisting of silty clays, is deposited concordantly on the Ypresian sediments.
69 The occurence of an Early Eocene (Late Ypresian) tectonic pulse
70 Chapter 4
Fig. 4.5 Seismic sections SNSTI-87-14, SNST-83-19 and SNSTI-87-26, showing onlaps within the Ypresian Ieper Member and the position of the border faults of the Mesozoic Broad Fourteens Basin. The location of each seismic section is indicated in Figure 4.1b.
3.3 Subsidence analysis
In Chapter 2, the basement subsidence rates were calculated for 24 wells in a larger part of the southern Dutch North Sea. The average subsidence rate of the Landen Formation is 4.0 cm/ka, of the Ieper Member 6.6 cm/ka and of the Brussels Member 0.5 cm/ka (Table 4.2). Subsidence analy- sis of wells Virt1 and Virt2 indicates that during deposition of the Landen Formation (Thanetian), the tectonic subsidence rate in the study area approximates the average value found in the southern North Sea, ~4 cm/ka (Table 4.2, Fig. 4.7). During deposition of the Dongen Formation (Ypresian), the local basement subsidence rate started to divert from the average of the southern North Sea area. In wells Virt1 and Virt2, mean subsidence rates during the deposition of the Early Ypresian seismic units IE1-IE4 are 11.1 cm/ka and 8.8 cm/ka respectively (Table 4.2, Figs. 4.7d), versus 6.6 cm/ka for the southern North Sea as a whole. In well Virt1, the basement subsidence rates re- mained high during deposition of onlapping seismic units IE5 and IE6 (10.4-10.9 cm/ka). In well Virt2, the tectonic subsidence halted during this period (Fig. 4.7d, Table 4.2). This indicates that the onlaps are the result of a tectonic event between 50.0 and 49.5 ma. This conclusion is in line with the observations from the seismic geometry. The event resulted in tilting of the sea floor. Dur- ing deposition of seismic unit IE7, the subsidence rates in wells Virt1 and Virt2 were 10.3 cm/ka
71
The occurence of an Early Eocene (Late Ypresian) tectonic pulse
ren et al., 1995 al., et ren Bergg Time (Ma) Time 50 60 40 30 iabo- tonian nian Burdi- galian Age Aquita- Pr nian Danian Lutetian Chattian Rupelian Ypresian Selandian Bar Thanetian Epoch Eocene Miocene
alaeocene Oligocene
P
th Sea th Nor th Sea th Nor Lo th Sea th Nor er w
Upper Middle Middle
Group . Dongen
Fm.
l Mb l Mar . Mb .
.
Asse Mb Asse .
ussels Br Rupel Ieper Undiff Landen Hiatus Hiatus Hiatus (1) Mb Neogene 50 m es t-ter v 100 Shor 150 m
Long- ter 200 Eustatic cur Hardenbol et al., 1998
d) age (ma) age
53.6 53.1 51.1 51.6 52.1
boundar y sequence Yp4 Yp7 Yp6 Yp5 Hardenbol et al., 1998 Yp8? c) 80 ace 50.0? 120 160 = mfs = onlap surf DT (us) * 200 240 unit IE4 top unit IE1 100 units IE2 + IE3 units IE5 + IE6 units IE7 + IE8 ell L17-02 W 80 * 60 40 GR (API) fu fu fu fu cu cu cu cu 20 fu-cu
0 .
. Mb
oic Mesoz
Fm. Neogene . Ieper (1) Landen ussels Mb Mb Rupel Fm. Br b)
0 50
100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950 1000 1050 1100 1150 1200 1250 1300 Depth (m) Depth 7km a) Seismic section (detail)
72 Chapter 4 and 8.6 cm/ka, respectively. In Virt1 the subsidence rate remained high (10.0 cm/ka) during depo- sition of seismic unit IE8, but in Virt2 it decreased to 4.3 cm/ka (Fig. 4.7d). Due to the reduction in tectonic subsidence in Virt2 between 50.0 and 49.5 ma, the total tectonic subsidence of the base- ment at the end of the Ypresian was about 190 m less in well Virt2 than in well Virt1 (Fig. 4.7d). Along the margin of the Broad Fourteens Basin, no Late Ypresian uplift occurred and no Ypresian fault activity is observed (Fig. 4.4). Post-Ypresian subsidence rates were similar for both well sites and approximate the average values calculated for the southern North Sea (Fig. 4.7a,b).
4. Discussion
The onlap of sediments (Fig. 4.4) and the differential subsidence (Fig. 4.7) near the margin of the Mesozoic Broad Fourteens Basin are considered indicative of an episode of tectonic activity dur- ing the Late Ypresian (50.0-49.5 ma). A model of the episode is shown in Fig. 4.8a. The reduced subsidence rate at location Virt2 is thought to reflect the compressive nature of the tectonic event. The compression caused tilting of the sea floor and slight flexuring of sediments (Figs. 4.4, 4.8a). The onlapping strata (seismic units IE5 and IE6 in Virt1) were deposited during a relative sea level rise. This is indicated by the continuous decrease in grain size over this interval in well L17-02 (Fig. 4.6). The Late Ypresian tectonic activity had a significant effect on the sedimentation patterns in the southern Dutch North Sea. The onlaps show that between 50.0 and 49.5 ma, no deposition took place in the area straddling the Mesozoic Broad Fourteens Basin. Subsidence in the Broad Four- teens Basin area was reduced, or even changed into uplift. Possibly, sub-aerial exposure occurred (Fig. 4.8a) after which previously deposited sediments were eroded. However, possible evidence for Late Ypresian sub-aerial exposure of the Broad Fourteens Basin area was obscured due to the subsequent Eocene to Oligocene Pyrenean inversion phase. Renewed and more severe uplift and erosion associated with the Pyrenean phase removed the largest part of the Eocene succession in the area (Fig. 4.2). Reactivation of faults during the Pyrenean inversion obscured evidence of Ypresian fault activity (Fig. 4.8b, c). This tectonic overprint explains why the Ypresian tectonic pulse was hitherto unnoticed. It is significant that the Brabant Massif was uplifted during the Late Ypresian, leading to the sepa- ration of the Paris Basin from the North Sea Basin (Vandenberghe et al., 2004). This uplift was simultaneous with the tectonic event in the southern Dutch North Sea. Andsbjerg (pers. comm., 2003) detected Late Ypresian onlaps in the Danish North Sea sector, too. This suggests that the Late Ypresian compressive phase inferred from the record of the southern Dutch North Sea was not a local phenomenon, but probably of regional importance. The observation of Late Ypresian tectonic activity has some practical implications for basin mod- elling studies and geological reconstructions. The Ypresian tectonic activity shows that in addition to the known major phases of Mesozoic-Cenozoic tectonic inversion, which affected the Broad
Fig. 4.6 (opposite page) Correlation scheme. Seismic units (a) are correlated to the gamma ray (GR) and sonic (DT) logs of well L17-02 (b). The well is correlated to (c) the sequence-stratigraphic framework of Hardenbol et al. (1998) and (d) the Dutch lithostratigraphic framework of Van Adrichem Boogaert & Kouwe (1997). In well L17-02 (b) fining upward (fu) and coarsening upward (cu) intervals are indicated. In (b) and (d), the notation (1) indicates the position of the Basal Dongen Tuffite Member in Well L17-01 and the lithostratigraphic framework.
73 The occurence of an Early Eocene (Late Ypresian) tectonic pulse
age (ma) age (ma) 60 55 50 45 40 35 30 25 20 15 10 5 0 60 55 50 45 40 35 30 25 20 15 10 5 0
0 0
100 100
200 200
300 300
400 400
500 500
600 600
700 700 depth (m) depth (m) 800 800
900 900
1000 1000
1100 1100
1200 1200
1300 1300 Savian erosion Brussels Mb. Landen Fm. Ieper Mb. hiatus Neogene Rupel Fm. Pyrenean phase Savian erosion Brussels Mb. Landen Fm. Ieper Mb. hiatus Neogene Rupel Fm. 1400 1400 Pyrenean phase
Subair Subair Subwat Subwat BHC (tectonic subsidence, no comp) BHC (tectonic subsidence, no comp) BackSed (total tectonic subsidence) BackSed (total tectonic subsidence) a) Virt1 backstrip results b) Virt 2 backstrip results
Age (ma) age (ma) 60 55 50 45 40 35 30 25 20 15 10 5 0 56 55 54 53 52 51 50 49 48
0 0
100 100
200 200
300 300 onlap interval
400 400
500 500
600 600
700 700 depth (m) depth (m) 800 800
900 900
1000 1000
1100 1100 Ieper Member 1200 1200
1300 1300 Landen Fm. Brussels Mb. Savian erosion Brussels Mb. Landen Fm. Ieper Mb. hiatus Neogene Rupel Fm. 1400 Pyrenean phase 1400 IE1 IE2 IE3 IE4 IE5 IE6 IE7 IE8
Virt2 total tectonic subsidence (m) Virt2 total tectonic subsidence (m) Virt1 total tectonic subsidence (m) Virt1 total tectonic subsidence (m) c) Total tectonic subsidence of d) Close-up of tectonic subsidence Virt1 and Virt2 compared of Virt1 and Virt2 during Ieper Mb. deposition
Fig. 4.7 Subsidence analysis results. Dashed intervals comprise partly eroded units and their subsequent hiatus. In intervals where erosion occurred, uplift and subsidence values are underestimated. a) subsidence history well Virt1. b) subsidence history well Virt2. c) comparison between the tectonic subsidence history of Virt1 and Virt2. d) close-up of Ypresian tectonic subsidence history of Virt1 and Virt2.
74 Chapter 4
c) Neogene-Present SW Neogene subsidence NE sea-level
Neogene
Rupel Fm.
Rupel Fm. Brussels Mb. IE8 IE7 Ieper Mb. IE6 IE5 BFB Ieper Mb. IE2-4 IE1
b) Pyrenean inversion renewed uplift, sub-aerial exposure, severe erosion of Ypresian sediments
sea-level Ieper Mb. Brussels Mb. IE8 IE7 IE6 IE5 BFB Ieper Mb. IE2-4 IE1
a) Ypresian tectonic phase Zone of reduced tilted sea floor subsidence next to the zone of uplift non-deposition and/or non-deposition, continuous sub-aerial exposure no sub-aerial exposure deposition might have been possible. sea-level
Ieper Mb.
IE6 ? IE5 Ieper Mb. IE2-4 BFB IE1
0 5km Fig. 4.8 a) Schematic reconstruction of Ypresian tectonic activity and inferred vertical movements of the Broad Fourteens Basin (BFB), resulting in tilting, onlaps, and possible erosion. Vertical scale is exaggerated. b) Schematic reconstruction of the Pyrenean tectonic activity and inferred vertical movements of the BFB, resulting in severe erosion in the centre of the inversion zone. c) Schematic reconstruction of the Neogene-Present geometry of the BFB.
75 The occurence of an Early Eocene (Late Ypresian) tectonic pulse
Fourteens Basin, more tectonic pulses occurred. Although the effects of the Late Ypresian phase were relatively small and mostly overprinted by the subsequent Pyrenean phase, burial history and compaction of sediments are influenced. Similar small pulses may well have occurred in the southern North Sea during other stratigraphic intervals. However, if so, they have so far escaped attention. This is illustrated, for example, by the Lutetian tectonic activity observed in Chapter 3. Although often unnoticed on the scale on which basin modelling is performed, the combined ef- fect of several pulses might be significant. As a result of the Late Ypresian tectonic activity alone, pre-Cenozoic sediments in the Broad Fourteens Basin area experienced a total tectonic subsidence that was at least 190 m less than the subsidence of the surrounding areas, which affects estimates of compaction of sediments.
5. Conclusions
The onlapping succession of sediments, slight flexuring and differential subsidence near the mar- gin of the Broad Fourteens Basin indicates an episode of tectonic activity during the Late Ypresian (50.0-49.5 ma). The observed locally reduced subsidence is an expression of the compressive na- ture of the tectonic event. The Late Ypresian compressive phase is not a local phenomenon, but has been of influence in a much larger part of the North Sea. The Ypresian pulse was previously unnoticed in the study area. Nonetheless, the tectonic activity had a significant effect on the sedi- mentation patterns in the southern Dutch North Sea, and although often unnoticed on the scale on which basin modelling is performed, the combined effect of several pulses on subsidence patterns might have been significant.
76 Chapter 5
Chapter 5 The Cenozoic evolution of the Broad Fourteens Basin and Roer Valley Graben in the context of the development of the West European Rift System.
Abstract
The Broad Fourteens Basin and the Roer Valley Graben are located in the South of the North Sea Rift System. The Roer Valley Graben is also the northwestern termination of the West Eu- ropean Rift System. Both grabens experienced a largely comparable geologic evolution until the Oligocene, and have a similar structural composition. During the Late Oligocene, the Roer Valley Graben started to subside in response to extension, associated with the development of the West European Rift System. The Broad Fourteens Basin area was not tectonically reactivated. The local stress conditions and existing fault directions were similar in both areas, so there has to be another explanation for the difference in tectonic activity. In this chapter it will be discussed that the divergence in tectonic evolution might be a result of the difference in the thickness of Palaeogene overburden present in both areas. The preserved Palaeogene sediment succession is much thicker in the southern North Sea Basin area than in the Roer Valley Graben. In contrast to views in literature, it is assumed that this thick sediment cover prevented the Late Oligocene to Recent reactivation of faults in the North Sea Basin area. As a consequence, the area remained tectonically inactive during the Neogene.
1. Introduction
The Broad Fourteens Basin, its eastward extension the West Netherlands Basin, and the Roer Valley Graben are located in the South of the North Sea Rift System (Fig 5.1). The Roer Val- ley Graben is also the northwestern termination of the West European Rift System (Fig 5.1). The Broad Fourteens Basin and the Roer Valley Graben have a similar structural composition and the tectono-stratigraphic history of both grabens was comparable until the end of the Oligocene. After the Oligocene, the evolution of the grabens started to diverge. In this chapter, the causes for the differences between both areas are investigated. Although the North Sea Rift System and West European Rift System seem to be connected in one semi-continuous fault system (Fig. 5.1a), major differences in tectonic development, both tempo- rally and in tectonic style, point to different controlling mechanisms. The North Sea Rift System started to develop during the Late Palaeozoic. The tectonic activity in the North Sea Rift System propagated from North to South. Processes of basin inversion started to affect in particularly the southern parts of the North Sea Rift System during the Late Mesozoic and the Early to Middle Cenozoic (P.A. Ziegler, 1975; 1978; 1994; W.H. Ziegler, 1975). The West European Rift System (P.A. Ziegler, 1990) came into existence during the Middle Eocene, in response to the Alpine orog- eny, which started during the Mesozoic. The tectonic activity in the West European Rift System propagated from South to North (Illies and Greiner, 1978; P.A. Ziegler, 1990; 1994; Huyghe and Mugnier, 1995). The rifting and inversion history of the Broad Fourteens Basin is closely linked to the geological
77 The Cenozoic evolution of the North Sea and the West European Rift System
N FT N
GF VG FSH o o WSP 55 60 ESP CG
OG MFB North Sea
o TL 54 MNSH RFH DP HG PH o CG 55 SVP CBH SP GG o SP BFB 53 BFB LSB WNB LSB LBM RVG LRE EG o 50 RM BM WNB o PB 52 AM URG
AFB R LG BG Alps V G MC o RG PMB 45 o 51
o o o o o o o o o o -5 0 5 10 15 3 4 5 6 7 a) b) 0 400 km 0 100 km
Fig. 5.1 a) Simplified structural map of NW Europe. The inset in the lower left corner shows a simplified outline of the structural elements making up the North Sea Rift System and the West European Rift System. For legend, see Fig. 1.4. b) Simplified structural map of the Dutch Central Graben (CG), Broad Fourteens Basin (BFB), West Netherlands Basin (WNB) and the Roer Valley Graben (RVG). development of the larger North Sea Rift System. Tectonic activity in the Broad Fourteens Basin area ceased during the Oligocene, after the Pyrenean phase had come to an end (Van Wijhe, 1987a; 1987b). In the previous chapters, the Palaeogene tectonic and stratigraphic development of the southern Dutch North Sea Basin has been discussed in detail. The Mesozoic to Palaeogene evolu- tion of the Roer Valley Graben was part of the development of the North Sea Rift System. In con- trast to the Broad Fourteens Basin area, the Roer Valley Graben area was also tectonically active in the Oligocene to Recent period (Fig. 5.2). Fault bounded subsidence resulted in the deposition of more than 1700 m of sediments in this area. This activity was related to the development of the West European Rift System (Zagwijn, 1989; Geluk 1990; Geluk et al., 1994). The Cenozoic geological evolution of the Roer Valley Graben (Fig. 5.1b) was assessed on a European scale by Michon et al. (2003), focussing on the grabens’ response to the development of the Alpine fore- land. In that study, no detailed correlation to the geological history of the North Sea Rift System was made. This correlation is provided in this chapter. Possible causes for the differences in the Oligocene to Recent tectono-stratigraphic history of the Broad Fourteens Basin area and Roer Valley Graben area are discussed in this chapter. The Ceno-
78 Chapter 5 zoic evolution of the Broad Fourteens Basin and the Roer Valley Graben is compared in detail, and the factors controlling their respective development are identified. A comparison between the Broad Fourteens Basin area and the Roer Valley Graben area may provide a better insight into the interaction between the North Sea and West European Rift Systems during the Cenozoic.
2. Geological development
2.1 West European Rift System
Although the Roer Valley Graben is a Mesozoic structure that was formed as part of the North Sea Rift System, it was tectonically reactivated in response to the Cenozoic development of the West European Rift System. In the following paragraph, therefore, a short introduction of the geological development of the West European Rift System is presented. The West European Rift System came into existence during the Middle Eocene (Fig. 1.6) (e.g. P.A. Ziegler, 1975; 1990; 1994; Illies, 1977; Dercourt et al., 1986; 2000; Meulenkamp and Sissingh, 2000; Michon, 2003; Sissingh, 2003a; 2003b). The evolution of the West European Rift System is governed by the interaction of the Eurasian and African plates, and by resulting changes in stress regime (Kooi et al., 1989; P.A. Ziegler, 1994; Sissingh, 2006a; 2006b). The onset and end of dif- ferent phases of graben formation in the West European Rift System were marked by significant tectonic events in the Alpine chain (Sissingh, 2006a). Similar to the North Sea Rift System, the location of the West European Rift System seems to have been controlled by a crustal weakness zone, inherited from Palaeozoic (Permo-Carboniferous) times. During the evolution of the Ceno- zoic West European Rift System, preexisting faults were repeatedly reactivated (Illies, 1977; Illies and Greiner, 1978; Sissingh, 2006b). Doming of the mantle below the southern Rhine Graben is considered an additional factor controlling the extension of the Upper Rhine Graben (Illies, 1977; Illies and Greiner, 1978; Prodehl, 1981; Schumacher, 2002; Sissingh 2006b). During the Priabonian and Early Rupelian, rifting propagated northward from the Upper Rhine Graben toward the Hessian depression, and also breached the Rhenish shield towards the Lower Rhine Embayment. This resulted in the development of the Rhenish Triple Junction (Fig. 1.7). Close to the Eocene-Oligocene (Priabonian-Rupelian) boundary, the Lower Rhine Embayment started to subside, probably in extensional response to NNW-SSE directed local compressional stress (Figs. 1.7, 5.3). From the Late Oligocene to Early Miocene, rapid subsidence occurred in the Rhine Graben (Fig. 5.4). The tectonic activity was characterized by a dextral strike-slip regime, due to a local NE-SW direction of compression. Simultaneously, rapid subsidence took place in the Roer Valley Graben (Illies, 1977; Illies and Greiner, 1978; P.A. Ziegler, 1990; 1994; Geluk, 1990; Geluk et al., 1994; Schumacher, 2002; Michon et al., 2003). Extension in the Rhine Graben ceased during the Middle Miocene, when a transtensional stress regime was established, associ- ated with a rotation of the maximum horizontal compression to NW-SE (Fig. 5.5). This stress direction resulted in continued NE-SW extension in the Roer Valley Graben during the Miocene to Quaternary (Fig. 5.5). The present-day stress field in NW Europe shows a NW-SE to NNW-SSE maximum compressive stress. Under the present stress regime, NE-SW extension continues in the Roer Valley Graben and the northern Rhine Graben. Most other parts of the Rhine Graben are subjected to transpres- sional and transtensional stresses, inducing left-lateral displacement in the South and central Rhine Graben (e.g. Illies, 1977; Illies and Greiner, 1978; P.A. Ziegler, 1994; Michon et al., 2003).
79 The Cenozoic evolution of the North Sea and the West European Rift System
CENOZOIC Netherlands RVG and SE Tectonic Period Epoch Age Offshore to SW Netherlands phase Quater- Holocene nary Pleistocene Calabrian Piacenzian (Ma) Pliocene Zanclean Age ren et al., 1995 Messinian
Bergg Tortonian 10
Serra- ? ? Miocene vallian Langhian
Burdi- NEOGENE galian 20 Someren Mb.
Aquita- Rifting and subsidence RVG Veldhoven Clay Mb. nian
Voort Mb. Chattian Oligocene Steensel Mb. 30 Rupel Clay Mb. Rupelian Goudsberg Mb.
Priabo- Klimmen Mb. nian Vessem Mb. inversion Pyrenean
Bartonian BFB and RVG 40 Asse Mb. Asse Mb.
Eocene Lutetian Brussels Sand Mb.
50 ALAEOGENE P Ypresian Ieper Clay Mb. Reussel Mb. Tuff Mb. Basal Dongen Sand Mb.
Thanetian Landen Clay Mb. Palaeocene Selandian Gelinden 60 Heers Mb Marl Mb. inversion Danian Laramide
Houthem BFB and RVG Fm.
80 Chapter 5
MESOZOIC Period Epoch Age Netherlands RVG and SE Tectonic Offshore to SW Netherlands phase Maastrichtian
Campanian (Ma)
Santonian
Age Coniacian Turonian
adstein et al., 1995 Cenomanian Late Cretaceous inversion Gr BFB and RVG sub-Hercynean CEOUS Albian
110 A Aptian 120 Barremian ly Cretaceous 130 CRET Hauterivian
Ear Valanginian 140 Berriasian
Tithonian Late Late rifting 150 Kimmeridgian
Jurassic Kimmerian Oxfordian BFB and RVG 160 Callovian Bathonian Middle 170 Jurassic Bajocian Aalenian BFB 180 rifting
Toarcian Middle Kimmerian
JURASSIC Early 190 Pliensbachian Jurassic 200 Sinemurian Hettangian 210 Rhaetian Late Norian Triassic BFB 220 Early Carnian extension Kimmerian 230 Middle Ladinian Triassic 240 Anisian TRIASSIC Early Olenekian Induan PALAEOZOIC Period Epoch Age Netherlands Tectonic Offshore to SW RVG phase Late Tatarian Ufimian-Kazanian 260 Kungurian ? Artinskian ly mian Sakmarian Saalian Ear er P
280 PERMIAN Asselian
Gzelian . ?
S ? Kasimovian
300 erous OUS Asturian
Moscovian . W Late Bashkirian Carbonif 320 Serpukhovian N. erous
ly Visean
340 Ear Tournaisian Carbonif CARBONIFER
81 The Cenozoic evolution of the North Sea and the West European Rift System
Oligocene
N FT VG GF FSH o 60
MFB OG
TL HG MNSH RFH DP o CG 55
GG SP BFB
WNB RVG LRE EG o 50 RM
AM URG
LG BG AFBAlpine orogeny MC o RG 45 Fig. 5.3 Oligocene tectonic elements map of NW Europe, showing the onset of o o o o o -5 0 5 10 15 the Early Oligocene to Quaternary rifting in the Roer Valley Graben 0 400 km and Lower Rhine Embayment.
2.2 Broad Fourteens Basin and Roer Valley Graben
During the Late Palaeozoic and Mesozoic, the Broad Fourteens Basin and the Roer Valley Graben developed in response to the southward progradation of the North Sea Rift System (Fig. 1.4). Both grabens were inverted during the Late Cretaceous sub-Hercynean phase (e.g. P.A. Ziegler, 1975;
Fig. 5.2 (Previous pages) Stratigraphy of the Broad Fourteens Basin and Roer Valley Graben. The Mesozoic chart of the Broad Fourteens Basin is adapted from Van Wijhe (1987a), the Mesozoic chart of the Roer Valley Graben is adapted from Geluk et al. (1994). Note the different vertical scale between the Palaeozoic, Mesozoic and Cenozoic.
82 Chapter 5
Early Miocene
N FT VG GF FSH o 60
MFB OG
TL HG MNSH RFH DP o CG 55
GG SP BFB
WNB RVG LRE EG o 50 RM
AM URG
LG BG Alpine orogeny
MC Fig. 5.4 o RG 45 Early Miocene tectonic elements map of NW Europe. The subsidence in the Rhine Graben was o o o o -5 0 5 10 15 characterized by a dextral strike- slip regime due to a SW-NE main 0 400 km direction of compression.
1978; Letsch and Sissingh, 1983; Van Wijhe, 1987a; 1987b; P.A. Ziegler, 1990; Dronkers and Mrozek, 1991). Subsequent erosion removed most of the syn-rift sediments in the Roer Valley Graben. In the Broad Fourteens Basin, where a thick succession of Early Cretaceous sediments was deposited on the Jurassic basin fill prior to the inversion, Mesozoic sediments were better preserved (Fig. 5.2). A second period of inversion, caused by the Early-Palaeocene Laramide tectonic pulse of the Al- pine Orogen, resulted in renewed uplift in both areas. Uplift was more severe in the Broad Four- teens Basin. After cessation of the Laramide phase, sedimentation resumed. The Late Palaeocene (Thanetian) to Late Eocene (Bartonian) sediments in the vicinity of the Broad Fourteens Basin reached a maximum thickness of more than 700 m. During the Late Eocene, uplift associated with
83 The Cenozoic evolution of the North Sea and the West European Rift System
Late Miocene-Recent
N FT VG GF FSH o 60
MFB OG
HG TL MNSH RFH DP o CG 55
GG SP BFB
WNB RVG LRE EG o 50 RM Fig. 5.5 URG AM Late Miocene-Recent tectonic elements map of NW Europe.
BG LG Alpine orogeny Extension in the Rhine Graben MC ceased during the Middle Miocene. o RG 45 Extension continued in the Roer Valley Graben and the northern part of the Rhine Graben. The o o o o o -5 0 5 10 15 Rhine Graben was reactivated as a sinistral strike-slip from the Upper 0 400 km Pliocene onwards.
the Pyrenean phase caused erosion in the Broad Fourteens Basin and adjacent areas (Chapter 2). In the centre of the Broad Fourteens area, the tectonic uplift was at least 200 m, and a large part of the Eocene succession was removed. In the Roer Valley Graben, only Late Palaeocene sediments (with a maximum thickness of about 150 m) are preserved (Fig. 5.2). Eocene deposits are limited to the graben’s edge (Verbeek et al., 2002). The thickness of the Palaeocene and Eocene sediments originally deposited in the Roer Valley Graben is not known due to severe erosion, which was associated with tectonic uplift during the Pyrenean phase. The uplift is estimated to have varied between 200 and 600 m (Van Balen et al., 2002). Before the onset of the Pyrenean phase, the thick- ness of the Palaeogene succession might have been comparable in both areas. During the Late Oligocene, the structural evolution of the Roer Valley Graben and Broad Fourteens
84 Chapter 5
Basin area started to diverge. Fault bounded subsidence commenced in the Roer Valley Graben (Fig. 5.3). This was a result of the reactivation of the Roer Valley Graben in extensional response to the development of the West European Rift System. The accelerated subsidence continues until the present day, and resulted in the deposition of more than 1700 m of sediments. In contrast, the Broad Fourteens Basin was not tectonically reactivated after cessation of the Pyrenean phase. A siliciclastic succession with a maximum thickness of about 900 m has been deposited since (Fig. 2.6). It is believed that the accommodation space allowing deposition was created by passive ther- mal subsidence. As suggested in Chapter 2, periods of tectonic activity alternated in the area with periods of tectonic quiescence during which passive thermal subsidence prevailed.
3. Discussion
3.1. Stress propagation
The evolution of the North Sea Rift System and West European Rift System, and the formation of grabens therein, is a direct result of variations in the direction and intensity of lithospheric stress, propagated over large distances through the European plate (Kooi and Cloetingh, 1989; Kooi et al., 1989). The main sources of stress in NW Europe since the Carboniferous are the de- velopment and activity of the North Atlantic Rift System and the relative motions between Eura- sia and Africa, which encompasses the stress originating in the developing Alpine orogenic belt (P.A. Ziegler, 1975; 1978; Van Wijhe, 1987a; 1987b; Dronkers and Mrozek, 1991; Oudmayer and De Jager, 1993; Huyghe and Mugnier, 1994; 1995; Gölke and Coblenz, 1996, Sissingh, 2006a; 2006b). Through time, tectonic activity abated in the North Sea Rift System, while the West Euro- pean Rift System became active (Figs. 1.4-1.7, 5.4, 5.5). This shift reflects the growing influence of the Alpine orogenic belt and the diminishing impact of the North Atlantic Rift on the northwest European tectonic evolution. During the Late Mesozoic, extensional tectonics changed to compressional tectonics in the North Sea Rift System. An inferred paleostress-curve of the North Sea Rift System shows a trend from extensional and neutral during the Mesozoic, to compressional during the Cenozoic (Kooi and Cloetingh, 1989; Kooi et al., 1989). Superimposed on this long-term trend are short-term stress fluctuations. The observed periods of tectonic activity in the North Sea Rift System correspond to these short-term fluctuations. Sissingh (2006a) provided a tectono-stratigraphic compilation of the development of the West Eu- ropean Rift System. His conclusions are that the stratigraphic architecture of the Tertiary sedimen- tary fills in the Alpine foreland and the West European Rift System reflects a tectonic episodicity. The occurrence of brief tectonic phases in the Alpine orogenic chain is isochronously recorded in the Alpine foreland basin fills. The sedimentation patterns are tectonically driven; sedimentary sequence boundaries are correlated to coeval changes in the palaeostress regime and relative sea level.
3.2. Tectonic activity
During the Mesozoic and Cenozoic, tectonic movements in the Broad Fourteens Basin and the Roer Valley Graben were accommodated by repeated reactivation of Palaeozoic faults (Oudmayer and De Jager, 1993; Huyghe and Mugnier, 1994; Michon et al., 2003). This indicates that the pre-
85 The Cenozoic evolution of the North Sea and the West European Rift System
CENOZOIC Upper Rhine Hessen Lower Rhine Roer Valley Broad Fourteens Period Epoch Age Graben Depression Embayment Graben Basin Quater- Holocene nary Pleistocene Calabrian Piacenzian Pliocene sinistral shear rifting sinistral shear rifting Zanclean
Age (ma) strong Messinian ? subsidence
Berggren et al., 1995 Tortonian 10 subsidence regional uplift Serra- ? and erosion ? ? Miocene vallian rifting Langhian uplift regional rifting Burdi- Neogene galian 20 passive thermal regional Aquita- subsidence rifting nian regional rifting
uplift main rifting Chattian Oligocene 30 tectonic subsidence Rupelian
main rifting main rifting initial rifting Priabo- nian uplift uplift
Bartonian 40 initial rifting
?
Eocene Lutetian
initial rifting
50 Palaeogene Ypresian
Thanetian
Palaeocene uplift uplift 60 Selandian Danian
Fig. 5.6 Correlation of structural development and sedimentation between the Upper Rhine Graben, Hessen Depression, Lower Rhine Embayment, Roer Valley Graben and Broad Fourteens Basin. Intervals of initial and main rifting are indicated. The dashed sediments in the Roer Valley Graben and the Broad Fourteens Basin were deposited in response to the development of the North Sea Rift System (modified after Sissingh, 2006a). existing structural grain, inherited from the Palaeozoic Caledonian and Variscan orogenies, is the dominant control on the structural style of the grabens. The main faults in the Roer Valley Graben have a NW-SE orientation (Fig. 5.1b), similar to the main fault trends in the Broad Fourteens Ba- sin (Chapter 2). When the regional stress field shifted from NE-SW to N-S/NNW-SSE at the end of the Eocene (Illies, 1977; Schumacher, 2002; Michon et al., 2003; Sissingh, 2003a), a process
86 Chapter 5 associated with the Pyrenean phase, the preexisting fault zones experienced oblique transpression, which resulted in strike-slip movement and uplift in both areas (Fig. 1.7). Starting in the Late Oligocene, the Roer Valley Graben experienced normal faulting again, which resulted in exten- sion and subsidence. The Broad Fourteens Basin area was not tectonically reactivated during this period (Fig. 5.3). This part of the southern North Sea Basin also remained tectonically inactive in response to subsequent Neogene fluctuations of the regional stress field. To our knowledge, there is no evidence for major differences in the lithospheric stress field between both areas after the Late Oligocene, so another explanation is needed to explain the divergence. The development of the West European Rift System is a direct result of the compressional stresses imposed on the European platform by the Alpine orogeny. Propagation of compressional stresses from the South of the European plate to the North started in the mid-Cretaceous (P.A. Ziegler, 1987; Sissingh, 2006b). The intensity of deformations decreases with increasing distance from the Alpine thrust front. Additionally, although the Alpine orogeny started near the end of the Mesozoic, rifting of the West European Rift System only started during the Middle Eocene, and within it a progressive younging of the onset of rift activity to the North can be observed (Sissingh, 2006a; Fig. 5.6). In line with these observations, it might be argued that, although the Roer Valley Graben started to respond with extension and subsidence to the pan-European compressive stress field dur- ing the Late Oligocene, the compressive stress field has not reached the reactivation threshold for the structural elements in the North Sea area, yet. However, there is an argument against this hy- pothesis. Tectonic activity related to the Alpine orogeny commenced in the Lower Rhine Embay- ment and Roer Valley Graben during the Oligocene (Figs. 1.7 and 5.3). The amount of subsidence in the Roer Valley Graben since (accompanied by a total of 1700m of sediment infill) suggests that the strength of the stress field has not diminished since. If the above mechanism was the only ac- tive influence on tectonic (re-) activation in the region, it would be likely that the Broad Fourteens Basin would have been reactivated earlier. Michon et al. (2003) propose another mechanism for the Cenozoic tectonic activity observed on the European Platform. They interpreted the Miocene strike-slip reactivation of the Upper Rhine Graben and the associated NE-SW extension of the Rhine Graben and the Roer Valley Graben (Fig. 5.5) to be a result of the separation of NW Europe into two crustal domains due to the Alpine in- dentation of Apulia into NW Europe (Fig. 5.7). Absorption or deflection of the compressive stress by active basins between the Alpine front and the southern Dutch North Sea could explain why the intensity of deformations during the Cenozoic decreased with distance from the Alpine thrust front (Illies, 1977; P.A. Ziegler, 1990; Sissingh, 2006b) and explain the absence of tectonic reacti- vation of the southern North Sea during the Neogene. The Broad Fourteens Basin is thus not (yet) responsive to the stresses induced by the Alpine orogenic belt, as the Roer Valley Graben takes up the displacement. However, additional to the above two hypotheses, I believe a third mechanism might be responsi- ble for the absence of Alpine tectonic activity in the Broad Fourteens Basin. During the Pyrenean inversion, a different set of preexisting faults had been responsible for the main uplift of the Broad Fourteens area than during the Mesozoic rifting and the first two stages of basin inversion. In Chapter 2, it is suggested that this was probably due to the thick Palaeogene sedimentary overbur- den in the area, a mechanism originally proposed by Nalpas et al. (1995) for spatial differences in structural styles resulting from the Late Cretaceous compression of the Broad Fourteens Basin. In experimental sandbox results, Dubois et al. (2002) showed that the weight of a thick overburden hampers reverse reactivation of normal faults (such as those bordering the Mesozoic Broad Four-
87 The Cenozoic evolution of the North Sea and the West European Rift System
subsidence P assive
Ridge push? N
Fig. 5.7 Major stress directions Stable and schematic tectonic block development during the Neogene. The Alpine collision separates the NW European plate in two crustal domains, causing strike-slip in the Rhine ? Graben and extension in the Lower Rhine Embayment Direction of major and Roer Valley Graben Neogene stress (redrawn from Michon et al., 200 km 2003, figure 16).
teens Basin) during oblique compression, and instead favoured strike-slip faulting. Due to the thick sedimentary cover, the local stress field around faults is altered, resulting in a different response of the tectonic structure to the regional stress field. The Oligocene-Recent difference in tectonic reac- tivation and subsidence between the Broad Fourteens Basin area and the Roer valley Graben might be related to a similar contrast in sedimentary overburden. During the period of tectonic activity associated with the sub-Hercynean phase, erosion removed most of the Mesozoic syn-rift sedi- mentary fill of the Roer Valley Graben (Fig. 5.2). In the Broad Fourteens Basin, this succession was preserved to a larger extend. Additionally, in contrast to the southern North Sea, which was a wide, open marine area in which a thick cover of sediment was deposited during the Palaeogene, the Roer Valley Graben was a narrow rift structure bordered by areas in which thin near-shore and continental sediments were deposited during this period (Van Staalduinen et al., 1979; Letsch and Sissingh, 1983; Geluk 1990; Geluk et al., 1994). After cessation of the Pyrenean phase and associ- ated erosion during the Oligocene, the Palaeogene succession remaining in the southern North Sea Basin was much thicker than in the Roer Valley Graben. The thick cover of preserved sediments in the Broad Fourteens Basin area was apparently more resistant to the Late Oligocene-Recent local stress field, or deflected it in such a way that the reactivation threshold was not overstepped. The Broad Fourteens Basin area therefore remained tectonically inactive during the Neogene. If the Alpine indentation into Europe continues, a renewed phase of compressive tectonic activity in the southern North Sea Basin could occur. Whether this will result in oblique reactivation and thus in a new inversion phase, or in extension and accelerated subsidence, as observed in the Roer Valley Graben, will depend on the local stress conditions at the time of deformation.
88 Chapter 5
4. Conclusions
The main sources of stress in NW Europe since the Carboniferous are the development and activity of the North Atlantic Rift System and the relative motions between Eurasia and Africa. Starting during the Mesozoic, there is a shift in tectonic activity from the North Sea Rift System to the West European Rift System. This reflects the growing influence of the Alpine orogenic belt, and the di- minishing importance of the North Atlantic Rift, on the Northwest European tectonic regime. The difference in the Late Oligocene to Recent tectonic development of the Broad Fourteens basin area and the Roer Valley Graben is not caused by a difference in the local stress directions or the inherited fault grain, which were relatively similar. Instead, diachronity in propagation of tectonic activity away from the Alpine thrust front might have had the result that the reactivation threshold of the Broad Fourteens Basin has not been overstepped yet. Another possible explanation is the separation of two crustal domains due to the Alpine indentation of Apulia into NW Europe. This resulted in absorption or deflection of the compressive stress, generated by the Alpine collision, by the active basins closer to the Alpine front. We speculate that a third possibility might be that the thick Palaeogene sedimentary overburden preserved in the southern Dutch North Sea Basin, which is much thicker than in the Roer Valley Graben, might have prevented the tectonic reactivation.
89 90 Chapter 6
Chapter 6 Synthesis
The Dutch part of the North Sea Basin has experienced distinct pulses of tectonic activity since the Late Palaeozoic. Activity started with rifting, which resulted in the formation of the Broad Four- teens Basin, and continued with tectonic inversion and associated uplift during the Mesozoic and Early Cenozoic. After cessation of the Palaeocene Laramide phase of inversion, the Broad Four- teens Basin ceased to exist as a graben. The graben fill had been partly eroded and the whole of the southern North Sea was levelled. During the Middle to Late Palaeocene, the area was flooded, after which a thick succession of siliciclastic sediments was deposited. Post-Laramide tectonic activity was mild in comparison to the Palaeozoic-Mesozoic activity (Fig. 1.3). Nevertheless, tectonic activity occurred and has had an influence on the sedimentary and stratigraphic evolution of the area. Several episodes of minor tectonic activity have been detected, which occurred from the Late Ypresian to the Bartonian (Figs. 3.9, 4.8). These tectonic pulses are thought to have been of a compressive nature. They resulted in reduced subsidence and maybe even in minor uplift. Their occurrence is mainly deduced from local deviations of relative sea level from the standard eustatic cycle chart (Hardenbol et al., 1998). Unfortunately, later (Early Oligocene) erosion removed much of the Ypresian-Bartonian sediments in the study area, which obscured the evidence of these intervals of tectonic activity. It is therefore not a surprise that this activity has been unnoticed until now. However, these events can be correlated to intervals of tectonic activ- ity in areas surrounding the Netherlands territory, which indicates their regional importance. The combined effect of several of the reported small tectonic pulses on subsidence patterns in the study area might be significant. The accuracy of compaction and burial history reconstructions of deeper sediments will probably increase when these small events are included in the calculations. The method of sequence stratigraphic interpretation of wireline logs is a powerful tool to correlate the Dutch Palaeogene succession to the global eustatic cycle chart of Haq et al. (1988). However, one should realise that this chart was partly based on data retrieved from the North Sea Basin. The method therefore has to be applied with care when it is used for absolute age dating. In response to compressional stress associated with the Late Eocene to Early Oligocene Pyrenean phase, the inverted Broad Fourteens Basin and the southern North Sea were uplifted. The response is of a different nature than during earlier phases of compression. An area more than twice the width of the Mesozoic Broad Fourteens Basin was uplifted and subjected to erosion (Fig. 2.5b). A different set of faults was reactivated than during the earlier periods of tectonic compression. This is probably the result of a difference in the direction of the Pyrenean compressive stress field with respect to earlier pulses of inversion. The thick Palaeocene and Eocene sedimentary cover, which had been deposited in the area after cessation of the Laramide phase, might have enhanced this effect by deflecting the local stress directions even further and by favouring reactivation of normal faults in a strike-slip sense over formation of new, low-angle thrust faults. After the Oligocene, tectonic activity ceased in the study area. In contrast to this, the neighbour- ing Roer Valley Graben, which until the Oligocene experienced a similar tectonic evolution as
91 Synthesis
the Broad Fourteens Basin, was tectonically reactivated in response to stress induced by the Al- pine orogenic belt. The Roer Valley Graben experienced rapid subsidence, which continues until present times. A comparison of both areas shows that this different evolution is not caused by a difference in local stress or existing fault directions. Again, overburden thickness might have been an important factor in prohibiting the reactivation of faults in the southern North Sea. Additionally, absorption of compressive stress emanating from the Alpine belt by the grabens of the West Euro- pean Rift System might result in a decrease in intensity of tectonic activity towards the southern North Sea (Fig. 5.6).
92 References
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97 Appendices Appendix A All wells used in this study TI SP, TTI, RILD ic, TTI , SP, SFLU, ILD, LB DT, GR, SN, conductivity GR, DT, SFL, ILD GR, DT GR, LSSL, TTI, ILD GR, BS, CAL, BHC, T GR, DT GR GR GR, LSSL, ILD, TTI GR, LSSL, ILD, TTI GR, FDC, SL, IND, TTI GR, DT, CAL, DTL GR, sonic, resistivity GR, DT, SN, IND GR, DT, SN, IND GR, resistivity, GRSL GR, DT GR, SP, RIL, SFL, LSS, ITT GR, GR, resistivity GR, DT GR, sonic GR, DT, ITT GR, IEL, SWM, sonic GR, SWM, SN, IES, sonic GR, DT GR, sonic GR, sonic GR, sonic GR, DT, drilling rate GR, LSSL,TTI, ILD GR, IEL, GRSL GR, BHC, resistivity GR, BHC, resistivity GR, DT, SN, IND GR, son GR, sonic GR, FDC, LSSL, TTI, IND TOOLS 1323.5 1640 1676 NLLFS 3 1316.5 1555 1418 1726 1630 1665 1673 1674.5 1596 1580 1628 1852 158 1534 1616 1542 1742.5 1142 1079 1178 863 737 1025 1162 1100.5 876 783.5 788 928 1256 883 721 830 1240 1137 1435 1497 1386 NLLFC NLFFD 1280 1514 1400 1680 1591 1645 1656 1649.5 1556 1545 1592 1831 1532 1478 1558 1490 1696.2 1088 1038 754 976 1117.5 1055 801 729 712 903 1229 858 701.5 815 1226 1094 1418.5 1470 1337 NLFFT 537 1261 1495 1375 1659 1572 1626 1635 1609.5 1 1526 1580 1813.5 1524 1470 1547 1478 1682.5 1074 1028 723 700 963.5 1095 1037 783.5 712 695 890 1209.5 837 683.5 806 1204 1065 1409 1453 1309 NLFFY 806 956 1069.5 1125 768.5 712 NLFFS 900 1145 1129 1165 1101 1095 1101 1291.5 1099 1152 1192 1170 1208 746 822 752.5 993 1018 767 NLFFM 794 775 941.5 958 955 1021 1028 1036 1184 1035 1120 1111 1123 1129 620 879.5 920 NLFFB 865 NMRFV 739 739.5 695 918 920 846.5 1022 1041 988 996 981 1110 1007 1083 1077 1075 1070 833.5 804 622 565 720 752 703 575 577 586.5 656 506 455 575 720 535 773.5 858 706.5 NMRFC A: all wells used in this study 731 688 650 859 849 780 900 850 884.5 887 911 1041 955 1004 994 998 948.5 815.5 788 855 591 524 650 728.5 674 551 531 559 570 623 486 424 521 638 475 704.5 770 589 NU 03 01 01 04 01 06 02 01 03 02 01 05 01 10 03 01 02 02 03 01 02 01 05 19 02 01 02 04 02 04 03 02 04 04 03 02 01 01 ------L13 L11 L10 L09 L09 L08 L07 L07 L05 L05 L05 L04 L02 L02 L02 L02 L01 K18 K18 K17 K16 K16 K14 K14 K14 K13 K13 K13 K12 K12 K11 K11 K11 K08 K07 K06 K06 K01 wellname Appendix
98
TI CAL., ILD, SFLU GR, FDC, IND, SL GR, drilling rate, SP, resistivity GR, DT, SP, SFLU, ILD GR, BHC GR, TTI , SP, SFL, ILD, DTL GR, LSSL, LDL, ILD, TTI GR, DT GR, DT, SP, CAL, resistivity,, TTI GR, DT, TTI GR, DT GR, DT GR, ITT GR, DT GR, DT, tension, DTL, T GR, DT, SP, SFLU, RIL, TTI GR, DT GR, DT, IEL, short normal GR, DT GR, DT GR, DT, Rild, Rsfr GR, DT, SP, ILD SFLU GR, DT, SP, TTI, ILD, SFL, DTL GR, DT, GR, DT GR, DT, SP, ISF, ILD GR, DT, SN, IND GR, DT, DTL, ILD, SFLU, tens GR GR GR, SN, IND. GR, DT, ILD, SFL, ILM GR, Sonic GR, DT GR, DT, LLD, ILM, ILD, TTI GR, FDC, LSSL, TTI, IND TOOLS 0 1079 901 1210 882.5 95 781 722.5 797 839 677 805 1009 1004 959 853 1055 864 NLLFS 1270 894.5 1155 1035 859 1184 869 861 937 717 772 707.5 780 833 872 696 665 795 999 991 946 844 1039 850 969 710 850 697 671 492 1057 932 828 NLLFC 1120 919 686.5 749 675 752 806 835 659 650 775 969 948 700 840 688 666 1000 872 780 NLFFD 1242 866 1134 969 805 1111 840 804 915 675 741.5 639 961 963 917.5 835 988 830 941 688.5 678 NLFFT 1223 855 1119 956 777 1092 827 786 898 660 714 667 737 795 804 620 625 746 947.5 953.5 909 816.5 977 820 931.5 678.5 830 666 662 966 842.5 748 NLFFY 764 666 500 550 659 558 536 NLFFS NLFFM 370 603 517 447 NLFFB 946 853.5 635 774 624.5 606 542 467 409.5 317 NMRFV 728.5 652 637 941 794 435 327 525 528 881 834 793 634 898 849 627 769 615 602 537 403 358 273.5 NMRFC (continued) A 697 628 592 871 752 930 565 730 786 338 401.5 628.5 535 675 290 507 550 502 863 827 789.5 620.5 893 684.5 843 621 759.5 610 597 442 534.5 481 257.5 165 NU A A A - - - 01 03 13 18 19 01 03 01 05 02 03 01 01 01 04 06 02 01 06 04 01 03 02 01 01 01 01 07 01 01 02 05 01 02 01 ------Appendix L13 L16 L17 P02 P02 P03 P05 P05 P06 P07 P08 P09 P11 P12 P13 P15 P15 P15 Q01 Q01 Q01 Q01 Q01 Q02 Q04 Q07 Q07 Q10 Q10 Q13 Q14 Q16 S02 S02 S05 wellname
99 Appendix A: legend NU = Base Upper North Sea Group NMRFC = Base Rupel Clay Member (Rupel Fm., Middle North Sea Group) NMRFV = Base Vessem Member (Rupel Fm., Middle North Sea Group) NLFFB = Base Asse Member (Dongen Fm., Lower North Sea Group) NLFFM = Base Brussels Marl Member (Dongen Fm., Lower North Sea Group) NLFFS = Base Brussels Sand Member (Dongen Fm., Lower North Sea Group) NLFFY = Base Ieper Member (Dongen Fm., Lower North Sea Group) NLFFT = Base Basal Dongen Tuffite Member (Dongen Fm., Lower North Sea Group) NLFFD = Base Basal Dongen Sand Member (Dongen Fm., Lower North Sea Group) NLLFC = Base Landen Clay Member (Landen Fm., Lower North Sea Group) NLLFS = Base Heers Member Landen Fm., Lower North Sea Group)
100 Appendix B Basement subsidence curves for all wells analysed.
Appendix B K01-02 K06-01 age (ma) age (ma) 60 50 40 30 20 10 0 60 50 40 30 20 10 0 0 0 200 200 400 400 depth (m) depth (m) 600 600 800 800 Pyrenean phase Pyrenean phase 1000 1000 erosion Savian erosion Savian 1200
gene Neo- 1200 gene Neo- unit 3 unit 4 unit 2 unit 3 unit 5 unit 2 unit 1 unit 5 unit 1 1400 1400 1600 1600
K07-02 K11-01 age (ma) age (ma) 60 50 40 30 20 10 0 60 50 40 30 20 10 0 0 0 200 200 400 400 depth (m) depth (m) 600 600 800 800 Pyrenean phase Pyrenean phase 1000 1000 erosion Savian erosion Savian 1200 gene Neo- 1200 gene Neo- unit 3 unit 3 unit 4 unit 2 unit 2 unit 5 unit 1 unit 5 unit 1 1400 1400 1600 1600
K12-01 K13-02 age (ma) age (ma) 60 50 40 30 20 10 0 60 50 40 30 20 10 0 0 0 200 200 400 400 depth (m) depth (m) 600 600 800 800
Pyrenean phase 1000 1000 erosion Savian erosion Savian 1200 1200 gene Neo- gene Neo- unit 3 unit 2 unit 3 unit 2 unit 1 unit 5 unit 1 1400 1400 1600 1600
101 102
Appendix B (continued) L09-01 L13-01 age (ma) age (ma) 60 50 40 30 20 10 0 60 50 40 30 20 10 0 0 0 200 200 400 400 depth (m) depth (m) 600 600 800 800 Pyrenean phase Pyrenean phase 1000 1000 erosion Savian erosion Savian 1200 1200 gene Neo- gene Neo- unit 3 unit 2 unit 5 unit 3 unit 1 unit 4 unit 2 unit 5 unit 1 1400 1400 1600 1600
P02-01 P05-01 age (ma) age (ma) 60 50 40 30 20 10 0 60 50 40 30 20 10 0 0 0 200 200 400 400 depth (m) depth (m) 600 600 800 800
Pyrenean phase 1000 1000 erosion Savian erosion Savian 1200 1200 gene Neo- gene Neo- unit 2 unit 2 unit 5 unit 1 unit 1 1400 1400 1600 1600
P05-03 P06-02 age (ma) age (ma) 60 50 40 30 20 10 0 60 50 40 30 20 10 0 0 0 200 200 400 400 depth (m) depth (m) 600 600 800 800
1000 Pyrenean phase 1000 erosion Savian erosion Savian 1200 1200 gene Neo- gene Neo- unit 2 unit 2 unit 5 unit 1 unit 1 1400 1400 1600 1600
103 Appendix B (continued)
Appendix B (continued) P07-01 P09-01a S05-01 age (ma) age (ma) age (ma) 60 50 40 30 20 10 0 60 50 40 30 20 10 0 60 50 40 30 20 10 0 0 0 0 200 200 200 400 400 400 depth (m) depth (m) depth (m) 600 600 600 800 800 800
1000 1000 Pyrenean phase 1000 erosion Savian erosion Savian erosion Savian gene Neo- gene Neo- 1200 1200
gene Neo- 1200 unit 2 unit 2 unit 1 unit 1 unit 3 unit 4 unit 2 unit 5 unit 1 1400 1400 1400 1600 1600 1600
P13-01 P15-02 age (ma) age (ma) 60 50 40 30 20 10 0 60 50 40 30 20 10 0 0 0 200 200 400 400 depth (m) depth (m) 600 600 800 800
Pyrenean phase 1000 1000 erosion Savian erosion Savian gene Neo-
1200 gene Neo- 1200 unit 3 unit 4 unit 2 unit 2 unit 5 unit 1 unit 1 1400 1400 1600 1600
Q01-01 Q07-05 age (ma) age (ma) 60 50 40 30 20 10 0 60 50 40 30 20 10 0 0 0 200 200 400 400 depth (m) depth (m) 600 600 800 800 Pyrenean phase 1000 Pyrenean phase 1000 erosion Savian erosion Savian gene Neo- 1200 gene Neo- 1200 unit 2 unit 2 unit 5 unit 5 unit 1 unit 1 1400 1400 1600 1600
104 Appendix B (continued)
Appendix B (continued)
S05-01 age (ma) 60 50 40 30 20 10 0 0 200 400 depth (m) 600 800
Pyrenean phase 1000 erosion Savian
gene Neo- 1200 unit 3 unit 4 unit 2 unit 5 unit 1 1400 1600
S05-01 age (ma) 60 50 40 30 20 10 0 0 200 400 depth (m) 600 800
Pyrenean phase 1000 erosion Savian
gene Neo- 1200 unit 3 unit 4 unit 2 unit 5 unit 1 1400 1600
105 106
Samenvatting
In dit proefschrift wordt de tektonische en stratigrafische ontwikkeling van het Nederlandse deel van de Laat Palaeocene tot Oligocene Noordzee gedetailleerd gereconstrueerd. Seismische data en data uit boringen die een groot deel van het Nederlandse continentale plat bestrijken worden gebruikt. Het onderzoek concentreert zich op breukgeometrie en de sedimentaire architectuur die ontstaan in reactie op tektonische activiteit. Tektonische daling en opheffing worden gekwantifi- ceerd, en oorzaken van lithosferische druk worden geïdentificeerd. Sequentie stratigrafische corre- latie verbetert de resolutie van reconstructies. Het doel is het verkrijgen van een beter inzicht in de oorzaken en gevolgen van bekkeninversie in meerdere fases. In Hoofdstuk 2 wordt de Palaeogene tectono-stratigrafische evolutie van het Breeveertien bekken in de zuidelijke Nederlandse Noordzee gereconstrueerd. Dit is een studie naar de reactie van extensie- breuken in een sedimentair bekken op compressieve reactivatie in fases. De Laat-Mesozoïsche en Palaeogene ontwikkeling van het gebied werd namelijk gekarakteriseerd door bodemdaling en sedi- mentatie in het bekken gedurende periodes van tektonische rust, onderbroken door discrete periodes van inversie. De Laat-Eocene Pyreneeën orogenese veroorzaakte de meest recente fase van inversie en opheffing in het gebied. De Pyreneese tektonische activiteit, waarvan vaak beweerd wordt dat die slechts van minimaal belang is, zorgde voor een opmerkelijk afwijkende reactie van het bekken dan eerdere inversiefases. Een veel wijder gebied dan begrensd door de originele bekkenranden werd opgeheven. Dit werd veroorzaakt door een klein verschil in de richting van het compressieve spanningsveld in vergelijking met de compressierichtingen van eerdere fases. Ook de accumulatie van een dik Palaeoceen-Eoceen sedimentpakket heeft waarschijnlijk bijgedragen tot dit effect. In Hoofdstuk 3 wordt een nieuwe correlatie geïntroduceerd, die gebaseerd is op de sequentiestrati- grafische interpretatie van boorgatmetingen. De methode maakt het mogelijk een gedetailleerde correlatie tussen de geobserveerde stratigrafische successie en de standaard eustatische zeespiegel curve van Hardenbol et al. (1998) te maken. Deze nieuwe interpretatie verhoogt de resolutie van het lithostratigrafische schema van de Nederlandse Palaeogene successie van Van Adrichem Boog- aert en Kouwe (1997) en helpt de relatieve invloed van lokale tektonische activiteit en eustatische zeespiegel variaties op de sedimentatiepatronen in het studiegebied te bepalen. De studie geeft ook aan dat kleinschalige tektonische activiteit ook doorgaat in de rustige periodes tussen tektonische fases. Dit is nog eens benadrukt in Hoofdstuk 4, waarin de aanwezigheid van een compressieve tektonische fase tijdens het Laat-Ypresien bediscussieerd wordt. Deze fase, waarvoor overigens de meeste bewijzen verdwenen zijn als gevolg van grootschalige erosie gedurende de daaropvol- gende Pyreneeën tektonische fase, resulteerde in een korte periode van gedifferentieerde plaatsel- ijke bekkendaling, wat geïllustreerd wordt door de ontwikkeling van een topografisch reliëf waarop nieuwe sedimenten onder een hoek gedeponeerd worden. In Hoofdstuk 5 wordt de Cenozoïsche tektonische geschiedenis van het Breeveertien Bekken verge- leken met die van de Roerdal Slenk. Deze twee structurele elementen liggen dicht bij elkaar in het zuiden van het Noordzee Riftsysteem. De Roerdal Slenk is ook het noordwestelijke eind van de
107 West-Europese Riftzone. Dit maakt ze uitstekende gebieden om de interactie tussen deze tekto- nische systemen te bestuderen. Gedurende het Laat-Oligoceen begon de evolutie van het Breeveert- ien Bekken en de Roerdal Slenk uiteen te lopen. De Roerdal Slenk werd gereactiveerd in reactie op plaatspanningen die de West-Europese Riftzone vormden, terwijl het Breeveertien bekken niet tektonisch gereactiveerd werd, net zoals het Noordzee Riftsysteem. In het hoofdstuk worden de mogelijke oorzaken van dit verschil gepresenteerd.
108
Dankwoord
Liefste Madelon! Jou bedank ik als eerste. Zonder jou zou ik de afgelopen jaren vast meermalen gillend weggerend zijn. Bedankt dat je er altijd voor me was en altijd voor me bent. Ik ben er ook heel trots op dat je mijn paranimf wil zijn.
Mijn promotores Theo Wong en Johan Meulenkamp bedank ik voor hun ondersteuning bij de totstandkoming van dit proefschrift. Daar zijn aardig wat uurtjes in gaan zitten, zeker in de laatste fase toen ik het allemaal op papier moest zetten. Ook gaat mijn dank uit naar Poppe de Boer, Wim Sissingh en Kees van der Zwan, met wie ik ook meermaals van gedachten gewisseld heb over de inhoud van mijn proefschrift en die niet te beroerd waren mijn schrijfsels meermaals te lezen en van commentaar te voorzien. Mijn kamergenoten Allard van der Molen en Gesa Kuhlmann worden heel hartelijk bedankt voor de gezelligheid, het luidkeels delen van allerlei frustraties in niet mis te verstane krachttermen en niet te vergeten de vele hectoliters koffie (“koffietje?”). Ook de bezoekjes aan de Utrechtse horeca waren een cruciaal onderdeel van dit project. Pekingeend sla ik nog even over, dank je. Allard wordt ook heel hartelijk bedankt voor de vele uurtjes printplezier waar ik hem de laatste maanden van zijn proefschrift mee opzadelde. Inmiddels staat de werkkamer “leeg”, omdat we alledrie Utrecht en Nederland verlaten hebben, maar je weet maar nooit waar we elkaar weer tegenkomen. Ook mijn paranimf Quintijn Clevis wil ik graag bedanken voor zijn lichtende voor- beeld.
Ik ben TNO-NITG (tegenwoordig TNO B&O) zeer erkentelijk voor het financieren van het project. Daar begint elk onderzoek natuurlijk mee.
De overige leden van de leescommissie, Noël Vandenberghe, Robert Knox en Oscar Abbink, wil ik hartelijk bedanken voor hun inspanning. Verder ben ik de leden van mijn begeleidingscommissie erg erkentelijk. Ik was het weliswaar regelmatig niet eens met de ideeën, maar ze gaven me, zeker in het begin van mijn project, toen ik nog zoekende was, veel nieuwe inzichten!
De volgende mensen hebben me uitstekend op weg geholpen toen ik net begonnen was met het project. Nora Parker maakte me wegwijs in de stratigrafie van Nederland, log interpretatie en seis- miek. Joost Verbeek gaf een eerste introductie op een seismisch werkstation. Peter Horst schreef een uitstekend doctoraalverslag als pilot voor mijn studie. Later heb ik prettig met Rory Dalman samengewerkt aan een kort project voor zijn doctoraal. Ik hoop dat jij het ook leuk vond?
Alle collega’s en ex-collega’s van de afdelingen GE en DO van TNO-NITG dank ik voor de gezel- ligheid, zeker tijdens de lunches. En de koffie. Jan-Diederik van Wees dank ik speciaal voor het
109 inruimen van tijd om mij de edele kunst van de subsidentie analyse bij te brengen. Ook alle andere professionele hulp die ik van de diverse collega’s mocht ontvangen werd zeer gewaardeerd, vooral als het niet voorafgegaan werd door de vraag wat mijn projectnummer was. Heelco, Paulien, koffie?
Een promotie begint natuurlijk niet pas als je aan een promotie begint. Daarom wil ik al diegenen die mij de liefde voor de wetenschap hebben bijgebracht tijdens mijn studie hartelijk bedanken dat ze mij de inspiratie gaven om er na het behalen van mijn doctoraal nog ruim vijf jaar aan vast te knopen. Bedankt, Bert van der Zwaan, Ivo Duijnstee, Maarten Prins en al die andere toegewijde docenten en begeleiders. Dank, iedereen op de faculteit aardwetenschappen, die mij heerlijk van mijn werk afhield als ik mijn neus er (overigens veel te weinig) liet zien. Ook dank voor de zeer gezellige lunches! In het bijzonder wil ik iedereen met wie ik in het promovendiplatform heb gezeten hartelijk bedanken voor de constante stroom nieuws en het samen aangaan van de ultieme uitdaging: de facultaire politiek. Alle studenten die me tijdens veldexcursies en practica bewezen dat je ook als docent of begeleider nog wel wat kan leren worden ook hartelijk bedankt. Doceren is tweerichtingsverkeer.
Een heel groot bedankje gaat naar mijn vrienden, for keeping me insane en voor het verminderen van mijn productiviteit door het reageren op werkelijk alle e-mail die ik stuurde (kopje cyberkof- fie?). You know who you are! Anne, Alec, Bas, Esther, Harald, Hélène, Marc, Miranda, Pieter-Jan, Ruud, Ragna, dankjulliewel! Hasso, bedankt voor je vriendschap.
Onmetelijke dank gaat uit naar mijn ouders, schoonouders, Yuri, Erika, Xander, Inès en Ronan voor de niet aflatende belangstelling (“waar ging het ook alweer over, dat project?”, “Wat is het nut van jouw onderzoek eigenlijk?”) en steun (“Wanneer studeer je eigenlijk af?”). Na het af- gelopen jaar meerdere keren van mij gehoord te hebben dat het nu dan toch “eindelijk klaar is, ik hoef alleen nog maar…”: nu is het eindelijk klaar! Ik hoef alleen nog maar mijn proefschrift te verdedigen. Hoi Stephan, Daphne en Faya! Een speciale dikke knuffel ook voor Ciska, die braaf de makkelijkste baby ter wereld was, waar- door papa nog een beetje kon doorwerken.
Bert, bedankt voor het opmaken van het proefschrift. En natuurlijk iedereen die hierboven hard op zoek is geweest naar zijn of haar naam, maar die helaas, om geen enkele reden anders dan mijn beroerde geheugen, niet kon vinden: hartelijk bedankt.
Dit proefschrift zou nooit afgeschreven zijn als ik niet met Madelon naar Oman verhuisd was. Wat een heerlijkheid om bij elke hapering van het denkproces een blik naar buiten te kunnen werpen, te zien dat de zon schijnt, en aldus opgevrolijkt weer verder te typen. Oman, thank you very big!
110
Curriculum Vitae
Iwan Rommert de Lugt was born on 31 March 1975 in Leiderdorp, the Netherlands. He attended secondary school (VWO) in Haaksbergen from 1987 to 1993. In 1993 he started his geology stud- ies at Utrecht University. He obtained his M.Sc. (doctoraal examen) in 1998 with sedimentary geology/climatology and marine ecology as his principal subjects. In December 1998, he started the Ph.D. project of which the results are presented in this thesis. In 2002 he married Madelon Nijman, and near the end of 2003 they moved together to the Sultanate of Oman, where the writing of this thesis was completed.
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