Introduction. Chapter 1. Chapter General introduction

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Chapter 1 Introduction 1.1 General introduction The Cenozoic orth Sea Basin (Fig. 1.1a) is an intracratonic, saucer-shaped depression, straddling the Mesozoic orth Sea Rift System (P.A.

1 Chapter 1 Introduction 1.1 General introduction The Cenozoic orth Sea Basin (Fig. 1.1a) is an intracratonic, saucer-shaped depression, straddling the Mesozoic orth Sea Rift System (P.A. Ziegler, 1990). The basin was formed by isostatic adjustment 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 orth Sea Basin became apparent during the Mid- to Late Palaeocene. Roughly funnel-shaped, the present orth Sea spans from 3 W to 7 E and from 50 to 60. The basin is bordered by the European mainland in the East, Fennoscandia in the northeast and the British Islands in the West. orth of the British Isles, and via the British Channel, the orth Sea is connected to the orth Atlantic Ocean (Fig. 1.1a). Fig. 1.1 a) The orth Sea and the outline of the landmasses surrounding it. The outline of the orth Sea Rift System is indicated. b) Outline of the study area (shaded) within the Dutch orth Sea sector. The outline of the Mesozoic Broad Fourteens Basin (BFB) is indicated with a dotted line. 9

2 Introduction From the Late Palaeocene to the end of the Oligocene, the southern part of the orth 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 onlaps 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 orth 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 orth Sea are mainly alternations of clays and sandy silts. The Cenozoic lithostratigraphic subdivision of the etherlands (Van Adrichem Boogaert and Kouwe, 1997) is based on these alternations, and the occurrence of regional unconformities.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. ote the lack of a clear shelf break and the semi-parallel deposition of the sedimentary units. The orth Sea area has been extensively studied since the discovery of significant hydrocarbon reserves 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 supplemented with onshore outcrop data. Lithostratigraphic frameworks ( and RGD, 1980; Isaksen and Tonstad, 1989; Knox and Cordey, 1992; Marechal, 1993; Van Adrichem Boogaert and Kouwe, ), 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 orth Sea Rift System (e.g. Van Wijhe, 1987a; Badley et al., 1989; Dronkers and Mrozek, 1991; Williams, 1993; Huyghe and Mugnier, 1994; 1995). umerical and analogue modelling yielded additional information about the structural and thermal evolution of the orth Sea Rift System (Kooi and Cloetingh, 1989; Kooi et al., 1989; Brun and alpas, 1996; Van Wees and Cloetingh, 1996; Van Balen et al., 2000; ielsen 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; alpas 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

3 al., 1995; Jacobs and De Batist, 1996; eal, 1996; Hardenbol et al., 1998; Michelsen et al., 1998; Vandenberghe et al., 1998). Many investigations in the orth 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 orth Sea often scarcely mentions the siliciclastic succession of the Palaeogene. This is regrettable; two of the main inversion phases affecting the orth 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 orth 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). Depth (km) SW E Cenozoic Mesozoic Palaeozoic and Basement 0 50 km Fig. 1.3 Regional transect across the Dutch orth 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 orth 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 sediments in the southern Dutch orth 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 orth 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, improved quantification of Palaeogene burial and uplift and the reconstruction of the geometry of the Palaeogene inversion zone aid in reservoir prediction and production. 11

4 Introduction 1.2 Geological setting The fractured Palaeozoic basement underlying the orth Sea Basin shows two dominant fault directions, E-SW and W-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; Oudmayer and De Jager, 1993; Huyghe and Mugnier, 1994). During the Permian, rifting started in the orth 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-orth Sea / Ringkøbing-Fyn High (Fig. 1.4a). Rifting in the orth Atlantic intensified during the Triassic. The orth 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 orth 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 orth Atlantic resulted in the formation of oceanic crust. After that, the main displacement between the orth American and Eurasian plates occurred in the orth 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 Armorican Massif BFB Broad Fourteens Basin BG Bresse Graben BM Bohemian massif CBH Cleaver Bank High CG Central Graben DP Danish-Polish Trough EG Eger Graben ESP East Shetland Platform Fenno-Scandian High FT Faeroe Trough Great Glen Fault GG Glückstadt Graben Horn Graben LBM London-Brabant Massif LG Limagne graben LRE Lower Rhine Embayment LSB Lower Saxony Basin MC Massif Central MFB Moray Firth Basin MSH Mid orth Sea High Oslo Graben PB Paris Basin PH Pennine High PMB Piemont Basin PYR Pyrenees RFH Ringkøbing-Fyn High RG Rhône Graben Rhenish Massif RVG Roer Valley Graben SP Sole Pit Basin SVP Silver pit Basin Tornquist Line URG Upper Rhine Graben VG Viking Graben WB West etherlands Basin WSP West Shetland Platform Alpine orogeny Palaeozoic massif Fault-bounded graben inverted graben and areas of compression Platform Major fault (zone) Coast line Variscan front Compression Extension Fig. 1.4 (opposite page) Schematic tectonic evolution of W 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 orth Sea Rift System developed. c) Early Cretaceous. Rifting had propagated to the South. When oceanic crust was formed in the orth Atlantic, rifting started to abate in the orth Sea Rift System. d) Late Cretaceous. The sub-hercynean tectonic phase resulted in inversion of the southern basins of the orth Sea Rift System. 12

5 a) Permian b) L. Permian-Triassic Atlantic Rift Atlantic Rift VG PH northern salt basin MSH RFH southern salt basin MFB MSH PH SP CG RFH GG DP LBM BM LBM BM variscan orogeny Palaeo-Tethys o o o o o c) Early Cretaceous o o o o o d) Late Cretaceous FT cont. VG FT VG MFB MFB PH MSH RFH CG DP PH MSH RFH CG DP SP BFB GG SP GG cont. LBM WB RVG LRE cont. BM BFB WB LSB RVG compression LRE BM 45 o? MC Indentation 45 o MC Indentation o o o o o o o o o 13

6 Introduction Early- Middle Palaeocene FT VG MFB PH MSH RFH CG DP 45 o continental SP PB MC BFB continental GG LSB compression WB RVG LRE BM Eo-Alpine compression PMB o o o o Fig. 1.5 Early-Middle Palaeocene tectonic elements map of W Europe. Due to the Laramide tectonic pulse, rifting occurred in the Viking Graben and Moray Firth Basin, and inversion occurred in the Dutch Central Graben, the Sole Pit Basin and the Broad Fourteens Basin/West etherlands Basin. For legend, see Fig ; 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 compression and at the beginning of the Late Cretaceous, major rifting in the southern orth Sea Rift System abated (Fig. 1.4d). Rifting continued in the orth Atlantic. The Late Cretaceous and Palaeogene development of the orth 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 orth 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

7 Eocene FT VG MFB MSH RFH CG DP SP BFB GG WB LSB RVG LRE EG URG 45 o LG MC Continent collision o o o o o Fig. 1.6 Eocene tectonic elements map of W Europe. The orth Sea rift became tectonically inactive. Riftinduced subsidence of the Rhine Graben was initiated. For legend, see Fig. 1.4 phase (Figs ). As a result, the majority of the grabens of the orth Sea Rift System were inverted. The timing of periods of inversion differs between the grabens (Cooper et al., 1989; Roberts, 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 etherlands 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 orth Atlantic and in the orwegian-greenland Sea, a renewed phase of sea-floor spreading occurred during the Palaeocene (P.A. Ziegler, 1975; 1978; 1990; Srivastava and Tap- 15

8 Introduction Fig. 1.7 Late Eocene to Early Oligocene tectonic elements map of W Europe. Pyrenean compression resulted in uplift in the Broad Fourteens Basin/West etherlands Basin and Roer Valley Graben. For legend, see Fig The Rhenish Triple Junction is indicated 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 orth Sea, compression resulted in the inversion of the Dutch Central Graben, the Sole Pit Basin, the Broad Fourteens Basin and the West etherlands Basin (P.A. Ziegler, 1978; 1990; Van Wijhe, 1987a; 1987b; Oudmayer and De Jager, 1993; Brun and alpas, 1996; Van Balen et al., 2000). The Laramide phase terminated during the Late Palaeocene. Inversion in the grabens of the southern orth Sea Basin halted, as well as the rifting of the Viking Graben. Thermal subsidence in the orth Sea area resulted in the formation of a saucer-shaped basin. The surrounding landmasses 16

9 emerged above sea level during the Late Palaeocene and became the main sources of the siliciclastic sediments that were deposited in the basin during the remainder of the Cenozoic (P.A. Ziegler, 1990; Huuse, 2000). ear the end of the Eocene (Fig. 1.7), tectonics associated with the Pyrenean orogenic phase resulted in renewed uplift in the grabens of the southern orth 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 orth 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 sedimentation 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 seismic data throughout the orth Sea Basin (Letsch and Sissingh, 1983; Van Wijhe 1987a; Cameron et al., 1993; Oudmayer and De Jager, 1993; Kuhlmann, 2004). During the eogene, subsidence continued in the orth 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). o major tectonic movements have been observed in the southern orth 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 southern part of the Late Palaeocene - Oligocene Dutch orth 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 inversion 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 stratigraphic correlation improves the temporal resolution of reconstructions. In Chapter 2, the Palaeogene tectono-stratigraphic evolution of the Broad Fourteens Basin in the southern Dutch orth Sea Basin is reconstructed. The reconstruction is a case study of the response 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 2Dseismic 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 orth Sea is presented. The method enables a detailed correlation 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 discussed. 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 northeastern margin of the inverted Broad Fourteens Basin. The interpretation is aided by a high-reso- 17

10 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 orth 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 Oligocene, 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 etherlands (Van Adrichem Boogaert and Kouwe, 1997) is not exclusively based on macroscopically recognizable sediment characteristics. The lithostratigraphic classification is partly based on biostratigraphic and sediment-petrologic 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 etherlands Institute of Applied Geoscience TO 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 results 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 lithostratigraphic units informally proposed by Weerts et al. (2003). 18

11 Van Adrichem Boogaert and Kouwe, 1997 Rupel Fm. Rupel Clay Mb. Vessem Mb. Dongen Fm. Asse Mb. Brussels Sand Mb. Brussels Marl Mb. Ieper Mb. Basal Dongen Sand Mb. Basal Dongen Tuffite Mb. Landen Fm. Reusel Mb. Landen Clay Mb. Gelinden Marl Mb. Heers Mb. Weerts et al., 2003 Rupel Subgroup Boom Fm. Zelzate Fm. and Bilzen Fm. Dongen Fm. Asse Mb. Brussel Mb. not described Ieper Mb. Oosteind Mb. Layer within Oosteind Mb. Landen Fm. Reusel Mb. Liessel Mb. Gelinden Mb. Orp Mb. Table 1.1: The proposed names of the working group on the Tertiary from the etherlands Institute of Applied Geoscience TO (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

Role of the Alpine belt in the Cenozoic lithospheric deformation of Europe: sedimentologic/stratigraphic/tectonic synthesis Laurent MICHON (1)

Role of the Alpine belt in the Cenozoic lithospheric deformation of Europe: sedimentologic/stratigraphic/tectonic synthesis Laurent MICHON (1) Géologie de la France, 2003, n 1, 115-119, 2 fig. Key words: