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Cartography | Subcrop map - Brabant Massif

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Belgium was the first country to be completely covered by geological maps at a detailed scale. These maps were completed in 1905, and are still regularly updated. After finishing this first series of maps, geologist turned to a more challenging exercise: the subcrop maps.

A subcrop map shows what an area would look like if the topmost deposits would be removed. It is therefore very useful for people such as well drillers, who are curious to know which rocks are present at a certain depth, instead of at the surface.

In the Flanders Region mostly Quaternary and Tertiary deposits are found near ground level. Depending on the location of a drilling site, these may be underlain by Cretaceous and/or Devonian deposits. But eventually everything rests on the lower Paleozoic Brabant Massif.

The lower Paleozoic comprises the Cambrian, Ordovician and Silurian
- PDF - international stratigraphic chart (552Kb, English pdf file, original location : http://www.stratigraphy.org/chus.pdf, will open in a new window)
- HTML - international stratigraphic chart (will open in a new window)
These were deformed and metamorphosed during the Brabantian Deformation Phase, which possibly lasted from the Silurian to the Middle Devonian. As such the Brabant Massif forms a distinct and compact unit.

The rocks of the Brabant Massif can be studied in a few outcrops in the northern part of the Walloon Region, and only on two sites in the southern part of the Flemish Region. Towards the north, the roof of the Brabant Massif is found at increasingly larger depths: in Brussels at about 100 m, in Antwerp at approximately 550 m, and at Loenhout, which is the most northern drilling, at over 1600 m.

The Brabant Massif

The first subcrop maps were rather schematic, and based mainly on data gathered in the outcrop zones. P. Fourmarier made such a map in 1920, based to a large extent on the first set of geological maps at a scale of 1/40 000.

The first author who extensively used the data gathered from drillings, was R. Legrand. His map was published in 1968 at a scale of 1/300 000. The lower Paleozoic rocks of the Brabant Massif contain very few macrofossils. The age he attributed to the different formations, was therefore largely based on lithological similarities.

Timeline
Click on the text of the 4 maps from 1920 to 2004 to get an illustration !!

This problem was partly overcome in later years by studying microfossils, mainly acritarchs and chitinozoa. This method was especially successful for the Silurian and Ordovician, and showed that the assumptions of Legrand were not always correct. This information, and the information from recent drillings, were combined into a new version of the geological map in 1992 which was puiblished one year later (De Vos et al., 1993). The map was scaled to A4-size for the publication.

This map shows a number of fault zones. Most of these were inferred from lineaments visible on the aeromagnetic map.

Aeromagnetic map

Since then, the structure of the Brabant Massif has been studied in much greater detail. This, combined with the continuing efforts to gather more detailed stratigraphic data, has put the map of 1993 out of date. We are currently working on a new map that will be finished by the end of 2004.

Although the Geological Survey of Belgium is continuously involved in research regarding the Brabant Massif, this particular project is financed by the Flemish Region. Therefore the new map will be limited to Flanders and an adjacent fringe area. A remapping of the Brabant Massif in the Walloon Region is also underway.

 
Last modified : August 18, 2009