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