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