Boek Iwan.Indd

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 between 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 calculated. 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 interpretation (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 geometry 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 during 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) tectonicpulse ofanEarlyEocene(LateYpresian) The occurence 68 Fig. 4.4 Detail of seismic section SNST-83-02, showing 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 the Ypresian Ieper Member (Dongen Fm.). The seismic units IE1-IE8 are indicated. The section was rescaled for clarity of display. The positions of the virtual wells Virt1 and Virt2 are indicated. According to interpretation of well L17-02 (see Fig. 4.6), the Basal Dongen Tuffite Member (Dongen Formation) is present in the area. Due to its limited thickness (<20 m) it does not appear as a recognisable unit on the seismic section. 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 inverted 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.

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