Off-shore map of the Brabant Massif

Transcription

1 Extended abstract made available online through ResearchGate in 2015 to complement the abstract published in the proceedings of GB2016. Please follow this publication to stay informed of more official publications. Off-shore map of the Brabant Massif Kris Piessens Geological Survey of Belgium, Jennerstreet 13, B-1000 Brussels The on-shore map of the Brabant Massif (Piessens et al., 2005) is structurally consistent in 3 dimensions, based primarily on actual observations, and complemented by mainly aeromagnetic data. No borehole information is available off-shore (North-Sea), but the on-shore map can be extrapolated into this area using the structural concepts demonstrated on-shore, as well as aeromagnetic and gravimetric data. The lithostratigraphic resolution is however strongly reduced and grouped into one Cambrian, one Ordovician and one Silurian unit. Introduction The Brabant Massif, named after its outcrop position in the former province of Brabant, constitutes the Lower Palaeozoic basement of the northern part of Belgium. It is essentially a subcrop unit with an area of km², consisting of Lower Cambrian to Upper Silurian rocks. Its cartographic boundaries are set where the massif is truncated by the Middle Devonian erosional surface, but in the northwest, the North Sea is classically the observational limit. Nevertheless it is known that below the North Sea these Lower Palaeozoic rocks continue to the northwest, towards the East-Anglian subcrop area (UK) and both subcrop areas form part of a larger unit, referred to as the Anglo-Brabant Deformation Belt (Lee et al., 1993; De Vos et al., 1993; Van Grootel et al., 1997; Sintubin, 1999). The rocks of the Brabant Massif, and probably the entire Anglo-Brabant Deformation Belt, were deformed by the longlasting, Silurian to Middle Devonian Brabantian Deformation event, mainly resulting in folding and cleavage development (Van Grootel et al. 1997; Debacker 2001; cf. Michot 1979). On-shore data and subcrop maps Because of its largely concealed nature, maps of the Brabant Massif to a large extent rely on borehole data. Over 1000 drillings, of which about 170 cored, penetrate the Brabant Massif. Borehole information is limited in the northern part of the massif, where the thickness of the Upper Palaeozoic and younger overburden exceeds 1000 m near the Dutch-Belgian border (fig. 4). More data are available towards the South, where the basement becomes increasingly shallower, and is locally exposed in river valleys. These data show the Cambrian units in the central parts of the Brabant Massif. These units represent as such a Cambrian core, flanked by Ordovician and Silurian deposits. Cartographically, this gives the impression of a large anticlinal structure (cf. Fourmarier 1920). The large-scale architecture, however, remains disputed, and current structural interpretations range from traditional anticlinal concepts to hypotheses of fault bounded, internally ductile deformed structural units (Sintubin 1999; Sintubin & Everaerts 2002; cf. Mortelmans 1955). Three subcrop maps were constructed of the Brabant Massif (Legrand 1968; De Vos et al.

2 1993; K. Piessens et al. 2005), of which the last one was based on a full review of the available data and a thorough evaluation of the new insights gained during a decade of intense research on the Brabant Massif which had led to to several, sometimes controversial hypotheses on the large-scale architecture of the Brabant Massif (e.g. Mansy et al. 1999; Sintubin 1999; Sintubin & Everaerts 2002; Debacker 2001; Debacker et al and references therein). Contrary to the previous subcrop maps, the new map strongly relies on demonstrated structural concepts. Since 2005 over 30 new drillings have penetrated the Brabant Massif. These borehole data have allowed testing the accuracy of the map produced by Piessens et al. (2005) and the validity of its integrated concepts. Geophysical methods provide important additional information for concealed geological terrains, and may as such support and complete the information obtained through borehole data. The information is however by nature indirect and stratigraphic or structural interpretations have to be carefully motivated. The most useful source of information is the aeromagnetic survey of The NS fly-lines are 1 km apart, and in the central zone of the Brabant Massif only 0.5km. Flying height was 120m above ground level, and EW tie-lines were flown at an interdistance of 10km. The aeromagnetic maps reveal a lot of detail on the subsurface, especially where the partly magnetic Tubize Formation is present. Also gravimetric data were collected during different surveys between 1985 and 2002, leading to one measurement per square kilometre for a large part of the country. The gravimetric data provides information on mainly deeper and larger structures than the aeromagnetic data. The BELCORP seismic profile, that was shot in 1986 and traverses the Brabant Massif in an NE-SW direction, was targeted at localising the depth of the MOHO. No details were revealed about the structure of the basement. Off-shore map The map of Piessens et al. (2005) is used as the basis for extrapolating the off-shore map for Belgium. Part of the original on-shore map is shown in figure 1. This map is originally drawn at a scale of 1/ and distinguishes 15 non-magmatic stratigraphic formations. Direct information from drillings is not available for the off-shore region, and it is therefore not possible to draw this map at the same stratigraphic resolution. The formations are therefore grouped into Cambrian, Ordovician and Silurian units. Magnetic susceptibility is higher for the Cambrian (Ordovician and Silurian both being low), which allows tracing their continuation from on-shore to off-shore (fig. 2). The extrapolation assumes that the overall strike of the formations does not change in the offshore region. The axis of the Brabant Massif lies approximately below Brussels and passes a little south of Bruges (Brugge). The formations at subcrop level along this axis are on-shore Cambrian in age, but decrease in age in a WNW direction (see fig. 1). Also the magnetic pattern becomes less intense, likely corresponding to an increasing depth (or diminishing thickness) of the Cambrian unit. This trend continues off-shore, and it can be readily assumed that overall the Cambrian axis plunges further in this direction. The anomaly I in figure 2 probably marks the reappearance of the Tubize Formation, after which the Cambrian formations disappear at subcrop level towards the WNW. A secondary Cambrian axis near Diksmuide is also on-shore less continuous. Along its trace uplifts of Cambrian units are separated by

3 Ordovician formations. This alternation at subcrop level is reflected in the intensity of the aeromagnetic pattern. The isolated offshore aeromagnetic anomaly (marked II in figure 2) lies along the trace of this secondary axis, and is in analogy with the observations on-shore taken as indicative for the Cambrian unit. The gravimetric map (fig. 3) shows a low gravimetric anomaly that approximately, but not exactly, corresponds to that of the aeromagnetic high (anomaly I). However, there is not necessarily a direct link. The circular shape of this anomaly rather suggests a genetic link with the chain of gravimetric lows that underlie the southern part of the on-shore part of the Brabant Massif (Mansy et al. 1999). These anomalies are caused by rocks with a density approximating that of granite, and may be granitic intrusions. The off-shore anomaly, the roof of which was roughly estimated at a depth of 5 km (Mansy et al. 1999) is not well pronounced and likely out of range of drilling operations. The higher densities (i.e. a gravity high, see figure 3) in the northern part of the off-shore territory confirm the presence of the Silurian unit. A large part of the subcrop off-shore Belgium shown in figure 1 is characterised by the occurrence of the Ordovician unit. This is in contradiction with the assumption of Lee et al. (1993) who hypothesise that the magnetic gradient anomaly that marks the limit of the Cambrian, is a major fault which forms the boundary between Cambrian and Silurian formations. Although one probable fault with similar orientation and throw is known from the onshore Brabant Massif (see fig. 1), it is not considered to be more likely than the hypothesis presented here of a more gradual plunging and deepening of the Cambrian formations towards the off-shore area. The distribution of the Silurian unit is as a result limited to the margins of the off-shore region. These zones form the continuation of the onshore formation boundaries. The validity of the inferred distribution of the stratigraphic units was verified with the structural 3D concept that was developed for the on-shore part of the Brabant Massif. This detailed work is not part of this study, but the main conclusion is that the inferred distribution of the geological units is in agreement with the structural model. The assumed reappearance of the Tubize Formation in anomaly I would for example be expected based on the location of anomaly II. A central element in the structural model is the Asquempont Detachment System. Two possible traces of this fault system are shown in figure 1, only to show that the two possible structural interpretations are both compatible with the assumed stratigraphic distribution. Litho-structural domains The off-shore Brabant Massif is split into three zones, which each have on-shore equivalents: Inner Zone (IZ): comprises the Cambrian core of the Brabant Massif which shows the highest metamorphic grade and the most intense deformation. Outer Zone (OZ): comprises the mainly Ordovician and Silurian formations that show clear evidence of ductile deformation, more specifically the presence of a tectonic cleavage fabric. Peripheral Zone (PZ): comprises mainly Silurian formations that typically do not exceed the diagenetic metamorphic grade and do not show a tectonic cleavage fabric.

4 Inner zone (IZ) The Inner Zone corresponds to the central axis of the Brabant Massif which consists of mainly Cambrian formations. It is characterised by tectonised, steeply dipping bedding planes (in general 60 ). If folds occur, then these will be close and almost upright. Folds with near vertical fold axes are well known from the Brabant Massif, but it is not known if they occur in the subcrop part extending to the offshore region. The locally coarse grained and permeable Tubize Formation is likely to be present at subcrop and deeper levels. Fine grained geological units, such as the thick and relatively homogeneous Oisquercq Formation, are quite dominant in the IZ. In spite of the fine grained composition, significant permeability is observed at least near the top of the bedrock. Outer Zone (OZ) The Outer Zone contains formations of Ordovician and Silurian age that were subject to ductile deformation processes, identified by the presence of a tectonic cleavage. The Ordovician and especially the Silurian formations have in general fine grained lithologies (rarely exceeding the grain size of silts and fine sands). Permeability may be higher where the rocks are naturally fractured (cf. the open fractures in the top of the Brabant Massif that allows for groundwater production). The underlying Cambrian has at least one formation with a higher permeability (Tubize Formation), but this is not the one directly underlying the Silurian/Ordovician and it may therefore be located at depths exceeding 5 km. Brittle deformation structures may be relatively abundant in the OZ. The Lower Palaeozoic Brabant Massif is cross cut by several faults and fracture zones. Their presence is well demonstrated onshore by the higher production of ground water in narrow zones and by the regional distribution of trace and major elements (Sterpin & De Vos 1996). Most of the active or open fractures are assumed to be steeply dipping, near vertical structures. The vertical extent of individual fracture systems may be extensive, especially where they correspond with faults (e.g. late orogenic horst-and-graben faults). Open fracture systems are sufficiently abundant to consider the whole top of the Brabant Massif as an aquifer. The metamorphic grade of the tectonised area is in general anchizonal, resulting in a less favourable mineral composition of the sealing formations due to the absence of smectite. This mineral is indeed only rarely found in fresh rock samples (Larangé 2002). Peripheral Zone (PZ) The rocks in the Peripheral Zone in general do not exceed the diagenetic metamorphic grade, they occur in a more gently dipping and undulating configuration, and faults may be less abundant. The formations at subcrop level in this zone are mainly of Silurian age. Ordovician formations are probably present at depth (and look similar to the rocks in the OZ). This lithology of the Silurian formations is mainly siltstone to claystone that, given the low metamorphic grade. Permeable faults and fractures are known to occur in the on-shore part of the Brabant Massif in the PZ. These can also be expected off-shore. References De Vos, W. et al., A new geological map of the Brabant Massif, Belgium. Geological Magazine, 130(5), Debacker, T., Palaeozoic deformation of the Brabant Massif within eastern Avalonia: how, when and why? PhD thesis. Ghent: Ghent University.

5 Debacker, T. et al., Timing and duration of the progressive deformatino of the Brabant Massif, Belgium. Geologica Belgica, 8, Fourmarier, P., La tectonique du Brabant et des régions voisines, Extrait des Mémoires, Classe des sciences de l'académie royale de Belgique. Larangé, F., Low-grade metamorphism and geotectonic setting of the Brabant Massif and the Medio-occidental part of the Ardenne, Belgium: application of white mica crystallinity, b cell dimension and transmission electron microscope studies. PhD thesis. Louvain-la- Neuve: Université Catholique de Louvain. Lee, M. et al., Evidence of the deep structure of the Anglo-Brabant Massif from gravity and magnetic data. Geological Magazine, 130(5), Legrand, R., Le Massif du Brabant, Mémoire, Service Géologique de Belgique. Mansy, J., Everaerts, M. & De Vos, W., Structural analysis of the adjacent Acadian and Variscan fold belts in Belgium and northern France from geophysical and geological evidence. Tectonophysics, 309, Michot, P., La faille mosane et la phase hyporogénique bollandienne, d'âge emsien dans le rameau calédonien condruso-brabançon. Annales de la Société Géologique de Belgique, 101, Mortelmans, G., Considérations sur la structure tectonique et la stratigraphie du Massif du Brabant. Bulletin de la Société belge de Géologie, de Paléontologie et d'hydrologie, 64, Piessens, K., Vancampenhout, P. & De Vos, W., Geologische subcropkaart van het Massief van Brabant in Vlaanderen. Sintubin, M., Arcuate fold and cleavage patterns in the southeast part of the Anglo- Brabant Fold Belt (Belgium): tectonic implications. Tectonophysics, 309, Sintubin, M. & Everaerts, M., A compressional wedge model for the Lower Palaeozoic Anglo-Brabant Belt (Belgium) based on potential field data. Geological Society, London, Special Publications, 201, Sterpin, M. & De Vos, W., Onderzoek naar metallische mineralisaties in de Paleozoïsche sokkel van Vlaanderen. Eindverslag project VLA/94-3.5, Univ. Gent en ANRE. Van Grootel, G. et al., Timing of magmatism, foreland basin development, metamorphism and inversion in the Anglo-Brabant fold belt. Geological Magazine, 134, Verniers, J. et al., Cambrian-Ordovician- Silurian lithostratigraphical units (Belgium). Geologica Belgica, 4(1-2), 5-38.

6 Figure 1: Geological map of the Brabant Massif onshore, extrapolated to off-shore. The dotted line delimits the Outer, Inner and Peripheral Zones. The double trace of the Asquempont Detachment System is shown to demonstrate that the extrapolated distribution of the subcrop data is compatible with different structural interpretations. The geological map shows only regional fault and fold structures. Also much smaller structures are important for reservoir properties and are as such discussed in the text. 1

7 Figure 2: Extrapolation of onshore data to offshore based on aeromagnetic data. I and II refer to the two aeromagnetic highs that are discussed in the text. The aeromagnetic map is shown as background and is overlain by the geologic map. Yellow and red colours represent high values (see also colour legend). 2

8 Figure 3: Extrapolation of onshore data to offshore compared with gravimetric data. The low gravimetric anomalies associated with deep-seated rocks with granitic composition are contoured. The gravimetric map is shown as background and is overlain by the geologic map. Green and blue represent low values (see also colour legend), indicating values of low density at depth. 3

9 Figure 4: Depth to the Lower Palaeozoic basement expressed in meters below TAW (negative values for depth below sea level. Depths increase from 250m in the south to 650m in the north. 4