Collapse structures on seismic in Flanders

krispiessens Geo-energy Theme Posts and Events, GeoConnect3d Posts

This post is part of the GeoConnect³d blog.

The eastern part of Flanders, also known as the Campine Basin, is covered by a dense network of 2D seismic lines. On some of the seismic lines in the southwestern part of the Campine Basin, collapse structures can be observed in Carboniferous, Upper Cretaceous and Paleogene strata (Dreesen et al., 1987; De Batist & Versteeg, 1999). These collapse structures manifest themselves as depressions on seismic images, consisting mostly of symmetric sag structures and sharply downwards dipping reflections towards their centre.

Figure 1: Seismic section showing three collapse structures affecting Paleogene layers. Base Oligocene, base Eocene and base Paleogene are indicated by dark green, purple and light green lines, respectively. Base Upper Cretaceous and top Dinantian could not be interpreted on this line as it only sufficiently images the Cenozoic layers. These structures were first noted in the study of De Batist & Versteeg. (1999).

Within the framework of GeoConnect³d, a total of 45 of these structures were mapped. Most of them are clustered in an area where the Dinantian is overlain by several hundred meters of Namurian and Westphalian strata below the Upper Cretaceous/Cenozoic coverage. Further towards the northeast, the Namurian and Westphalian strata thicken, and the number of collapse structures rapidly decreases. Interestingly, many of the collapse structures can also be detected on gravimetric maps (Debacker et al., 2018). The size of the collapse structures strongly varies, with maximal vertical offsets of over 200 m, and maximum horizontal extents of the circular to ellipsoid structures of 1500 by 5000 m.

The collapse structures are generally located near or on top of faults. These are all normal faults that were dominantly active during a Jurassic extensional phase, some of which were (re)activated during the Cenozoic. The collapse structures are often aligned along the dominant NNW-SSE and WNW-ESE fault strike direction of the area, but interestingly, also a perpendicular (W)SW-(E)NE alignment of a number of collapse structures can be observed.

Figure 2: Overview of the mapped collapse structures in red and green based on 2D seismic data. Green polygons mark collapse structures that reach into the Paleogene. Red polygons reflect collapse structures in the Carboniferous that are vertically delimited by the base Cretaceous unconformity. The vertical throw of the collapse structures is indicated by the thickness of the contour lines: the thicker the line the larger the throw. Orange lines are major faults that were mapped for the Flemish 3D geological model (G3Dv3) at the top Dinantian level.

Most probably, the observed collapse structures can be related to evaporite dissolution, as was argued by Dreesen et al. (1987). Indeed, the association of the collapse structures with faults suggest that these faults provided fluid pathways that enabled the dissolution and ultimately collapse of the Dinantian rock material in the vicinity and along these faults. The hypothesis that these rocks consist of evaporites is supported by several indirect observations:

  1. The significant horizontal and vertical extent of the collapse structures suggest the disappearance of thick masses of Dinantian rocks. Evaporites are a good candidate for such process, since they are much more soluble than the Dinantian carbonates.
  2. The delimited geographical occurrence of the collapse structures, between a reefal belt further north and the high Brabant Massif further south. Such setting could provide a periodically landlocked sea during sea level lowstands, with evaporite deposition.
  3. Thick evaporites were also deposited during the Dinantian in a similar setting along the southern flank of the Brabant Massif (De Putter et al., 1994).
  4. In the Mons Basin along this southern flank of the Brabant Massif, the dissolution of the Dinantian evaporites caused very similar collapse structures (Rouchy et al., 1986).

As for their timing, the collapse structures display different ages: some die out just above the top of the Dinantian, whereas others continue into the Paleogene. The upper limits of the structures are not random, but bounded by some of the major unconformities in the area which coincide with the main tectonic phases. We identified three main surfaces by which the collapse structures are topped and linked these with the tectonic history of the area:

  1. Collapse structures limited to the lowermost part of the Namurian. They were probably formed during an episode of dissolution in the top of the Dinantian when it was aerially exposed just after deposition.
  2. Collapse structures topped by the base of the Upper Cretaceous. Providing a timing for this dissolution phase is complex because of a large hiatus between the Upper Carboniferous and Upper Cretaceous in Flanders, but an established dissolution phase in the Mons area (Quinif et al.,2006) and analysis of a vein in a deep borehole in the western Campine Basin (Swennen et al. 2021) make a case for a major dissolution phase in the Campine area during the late Early Cretaceous.
  3. Collapse structures that reach into the Upper Cretaceous or Paleogene. Most of these are topped by the base of the Oligocene. Since the region experienced large wavelength deformation by the Pyrenean tectonic phase just before the onset of the Oligocene (Deckers et al., 2016), it is likely that dissolution and renewed collapse was related to this phase. The earlier, middle Paleocene Laramide tectonic phase shares similar dynamics as the Pyrenean tectonic phase (Deckers & van der Voet, 2018) and could therefore also have contributed to this Paleogene collapse episode, especially since fracture-filled veins of this age were detected in a drilled section of the Dinantian in the region by Swennen et al. (2021)

As this blog shows, these collapse structures do not only act as particularities on seismic images, but can also provide insights in the tectonic history of the area. Especially when linked to faults, these insights can help understanding the pathways of deep fluid flow. Therefore, they are a nice example of geomanifestations, which are a key concept in the GeoConnect³d project.



Bernd Rombaut, Katrijn Dirix and Jef Deckers
VITO, Vlaams Instituut voor Technologisch Onderzoek – Belgium



References

De Batist M. & Versteeg W.H., 1999. Seismic stratigraphy of the Mesozoic and Cenozoic in northern Belgium: main results of a high-resolution reflection seismic survey along rivers and canals. Geologie en Mijnbouw, 77: 17-37.

De Putter, T., Rouchy, J.-M., Herbosch, A., Keppens, E., Pierre, C. & Groessens, E., 1994. Sedimentology and  Palaeoenvironment of the Upper Visean anhydrite of the Franco-Belgian Carboniferous Basin (Saint-Ghislain borehole, Southern Belgium). Sedimentary Geology, 90, 77–93.

Debacker, T., Deckers, J., Rombaut, B., Broothaers, M., Ferket, H. & Williamson, P., 2018. Subsurface mapping using gravity data during construction of the new 3D model of the Flemish subsurface. Abstract from 6th International Geologica Belgica meeting, 2018. Theme 3: New developments in geology and their role in future research, 7-8.

Deckers, J., Vandenberghe, N., Lanckacker, T. & De Koninck, R., 2016. The Pyrenean inversion phase in northern Belgium: a relaxation inversion phase? International Journal of Earth Sciences, 105, 583-593.

Deckers, J. & van der Voet, E., 2018. A review on the structural styles of deformation during Late Cretaceous and Paleocene tectonic phases in the southern North Sea area. J. Geodyn. 115: 1-9.

Delmer, A., 1977. Le bassin du Hainaut et le sondage de St Ghislain. Service Géologique de Belgique, Professional Paper, 1977, 6, 1-12.

Dreesen, R., Bouckaert, J., Dusar, M., Soille, J. & Vandenberghe, N., 1987. Subsurface structural analysis of the Late-Dinantian carbonate shelf at the northern flank of the Brabant Massif (Campine Basin, N.-Belgium). Toelicht. Verhand. Geologische en Mijnkaarten van België 21, 37 pp.

Quinif, Y., Meon, H. & Yans, J., 2006. Nature and dating of karstic filling in the Hainaut Province (Belgium). Karstic, geodynamic and paleogeographic implications. Geodinamica Acta, 19/2, 73-85.

Rouchy, J.-M., Pierre, C., Groessens, E., Monty, C., Laumondais, A. & Moine, B., 1986. Les évaporites pré-permiennes du segment varisque franco-belge : Aspects paléogéographiques et structuraux. Bulletin de la Société Belge de Géologie, 95, 139–149.

Swennen, R., van der Voet, E., Wei, W. & Muchez, P., 2021. Lower carboniferous fractured carbonates of the Campine Basin (NE-Belgium) as potential geothermal reservoir: Age and origin of open carbonate veins. Geothermics 96, 15 pp.







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