Vert Galand v. 2016 01 13

VERT GALAND Formation
 
Compiled by M. Dusar, secretary Cretaceous Subcommission with collaboration of David Lagrou (Flemish region) and Michel Hennebert (Walloon region)
 
Introduction
On 10.6.2014 a discussion document “Towards a regional subdivision of the Turonian in the southern part of West Flanders: Nieuwkerke De Seule and Nieuwkerke Noordhoek members of Vert Galand Formation” by M. Dusar & D. Lagrou was posted on the NCS website. It proposed a regional subdivision of the Vert Galand Formation in West Flanders, in fact long overdue because the terms introduced are already in use on DOV website (Database Subsurface Flanders).
The e-mail message sent to the Cretaceous Subcommission on 9.6.2011mentioned “these new terms are redundant with existing nomenclature but we feel obliged to introduce them as long as the stratigraphic scales in use contains too many equivocal descriptions and correlations.”
 
The rationale behind introducing new members is a fundamental difference in mapping practice between the Flemish and Walloon regions. Nevertheless, the Vert Galand Formation (of Turonian age) is recognised in both the Walloon and Flemish regions and forms a mappable unit. It is subdivided in two members of distinct lithofacies, which are also recognised in both regions. Therefore it would be logical to make use of the same stratigraphical scale. The subdivision in the Walloon region is oldest and has priority. However, the criteria for subdivision and marking the boundaries for the carte géologique de Wallonie are based on outcrop appearance and do not satisfy for the subsurface geology in the Flemish region. Therefore an independent set of criteria was used and the formation was split in two members, uniquely defined for the Flemish region and posted as a discussion note on the NCS website.
 
Meanwhile, the Vert Galand Formation itself has not been formally described in the NCS framework so far. It is in use for the new geological map of Wallonia, following is definition by Hennebert & Doremus (1997a,b), independently of the stratigraphical scale published by NCS (Robaszynski et al., 2001). Geological mapping for the carte géologique de Wallonie started in the Tournai basin, with incomplete but more regular succession, whereas the NCS stratigraphical scale has been established in the Mons basin, with more complex succession due to the peculiar geodynamic origin of this basin.
 
This means that for the time interval concerned (Turonian), two competing systems exist. It is acknowledged that the lithostratigraphic scale in use for the carte géologique de Wallonie has been approved by a follow-up committee, is published and in use on the maps, which are strong arguments for maintaining thi scale..
At the NCS meeting of 8.6.2015 it was decided to accept that different stratigraphic systems can be used (particularly the 2001 NCS scheme and Carte géologique de Wallonie practice) and that the NCS website should reflect this reality. Restraining from any judgment or ranking of both systems, the carte géologique lithostratigraphy - as far as it is different from the 2001 NCS scheme - should be published on the NCS website as an independent system of equal value, but include a correlation table between both systems.
 
At this meeting, it was also requested to solve the discrepancy between Flemish and Walloon practice for a subdivision of the Turonian Vert Galand Formation, by adhering to the system introduced for the carte géologique de Wallonie, thus without introducing new names. This means amending the description of the members in use for the Carte géologique de Wallonie and published in the Notices explicatives (Hennebert & Doremus, 1997a, b; Vanneste & Hennebert, 2005).
 
As a consequence, the original description of the Vert Galand Formation and its Bruyelle and Merlin Members is adapted according to the NCS template and emended to accommodate for the mapping practice in the Flemish region.
 
 
VERT GALAND Formation
 
  1. Name: derived from Vert Galant hamlet, located along the upstream part of Rieu de Barge water course between Ere and Willemeau villages, Tournai municipality, by Hennebert & Doremus (1997a, b).
 
  1. Code carte géologique de Wallonie VEG; DOV: VG
 
  1. Stratotype. No formal stratotype defined for the formation. The 19th century quarries in Tournaisian limestone of the ‘quartier Longuesault – Barges – Vert Galand’ southwest of Tournai allowed observations of the Cretaceous overburden.
Decimal geographical coordinates for Rue du Vert Galant 50,576 – 3, 363; Lambert ’72 coordinates 78750 - 141100.
 
  1. Description. The Vert Galand Formation groups the marly facies of Turonian age, which were known  as “Dièves” and “Fortes Toises”. The Vert Galand Formation (VEG) consists of two distinctive members, the Bruyelle and Merlin Members.
Summary description from Hennebert & Doremus (1997a, b), from top to bottom:
VEG Fm Merlin Member: greenish-grey marls, with fine glauconite; white marl with grey siliceous concretions.
VEG Fm, Bruyelle Member: greenish-grey marl, with phosphatised gravel at the base; pale-grey marl with fine to coarse glauconite grains.
 
  1. Underlying strata: unconformably overlying Cornet, Hainaut or Paleozoic formations depending on position near centre or border of the sedimentary basin.
 
  1. Overlying strata: conformably overlain by Esplechin Formation or disconformably overlain by Cenozoic formations, depending on depth of pre-Cenozoic or Quaternary erosion levels.
 
  1. Area: Mons (Haine) and Tournai basins, transgressing over the Brabant Parautochton, extending into the North of France
 
  1. Thickness: up to 75 m in the Mons basin, 50 m in the Tournai basin.
 
  1. Age: Turonian
 
  1. Discussion.
Sea level rise resulted in ever younger base of the Turonian transgression over the Brabant Parautochton, albeit with the same lithological succession (Marlière, 1954). However, the Turonian subcrop area in the south of West Flanders is not in a more marginal position compared to the Tournaisis area, quite on the contrary.
 
As the Bruyelle and Merlin Members are deposited in regular succession and often are encountered in normal superposition, it seemed logical to group them in one formation. More detailed descriptions are given at the member level for the Bruyelle and Merlin members.
 
A somewhat different approach was applied in the Flemish region. The 3D model of the subsoil in Flanders requires a complete stratigraphic subdivision for all rock units encountered (Matthijs et al., 2013). The Brabant Massif is overlain by uniform white chalks, assigned to the Nevele Formation (Dusar & Lagrou, 2007), equivalent of the white chalks, from Saint-Vaast to Nouvelles Formations in the Mons basin. South of the axial zone of the Brabant Massif, overlying the Brabant Parautochton the subcropping Cretaceous strata become more diversified and connect to the Cretaceous in the Nord and Wallonia. Indeed, the general stratigraphic framework can be applied, and more particularly the connection to the lithostratigraphic scale developed for the new geological map of Wallonia is rather obvious. However, the further subdivision is based on different criteria: observations from exposures in Wallonia, from boreholes with geophysical well logs in Flanders. Initially, because correlations at the member level are not secured, a regional set of lithostratigraphical subdivisions was applied for the 3D geological model of Flanders (Lagrou et al., 2011) by introducing the Nieuwkerke De Seule Member and Nieuwkerke Noordhoek member for the Bruyelle and Merlin members respectively. Today, the correspondence rather than discrepancies are emphasized, unifying therefore the stratigraphical schemes for the Flemish and Walloon regions.
 
 
 
 
 
 
References
 
Cornet, J., 1923. Géologie stratigraphique. Tome IV. Leich, Mons
 
Dusar, M. & Lagrou, D., 2007. Cretaceous flooding of the Brabant Massif and the lithostratigraphic characteristics of its chalk cover in northern Belgium. Geologica Belgica 10/1-2: 27-38.
 
Hennebert, M., 1999. Carte géologique de Wallonie à 1/25.000. Planche Laplaigne - Péruwelz 44/3-4. Notice explicative. Ministère de la Région Wallonne, Direction générale des Ressources Naturelles et de l’Environnement, 44 p.
 
Hennebert, M. & Doremus, P., 1997a. Carte géologique de Wallonie à 1/25.000. Planche Hertain - Tournai 37/5-6. Notice explicative. Ministère de la Région Wallonne, Direction générale des Ressources Naturelles et de l’Environnement, 66 p.
 
Hennebert, M. & Doremus, P., 1997b. Carte géologique de Wallonie à 1/25.000. Planche Antoing - Leuze 37/7-8. Notice explicative. Ministère de la Région Wallonne, Direction générale des Ressources Naturelles et de l’Environnement, 74 p.
 
Lagrou, D.; Dreesen, R. i.s.m. Dusar, M., 2011. Kartering en karakterisering (sedimentpetrografisch en petrofysisch) van de Krijtgesteenten in Vlaanderen. VITO, 2011/SCT/R/099, 96 p. + bijl.
 
Marlière, R., 1954. Crétacé. In: P. Fourmarier, ed. Prodrome d’une description géologique de la Belgique. Société géologique de Belgique: 417-444.
 
Matthijs, J., Lanckacker, T., De Koninck, R., Deckers, J., Lagrou, D., Broothaers, M., 2013. Geologisch 3D lagenmodel van Vlaanderen en het Brussels Hoofdstedelijk Gewest – versie 2, G3Dv2. Studie uitgevoerd door VITO in opdracht van de Vlaamse overheid, Departement Leefmilieu, Natuur en Energie, Afdeling Land en Bodembescherming, Ondergrond, Natuurlijke Rijkdommen, 21p., VITO-rapport 2013/R/ETE/43.
 
Robaszynski, F.; Dhondt, A.V. & Jagt, J.W.M., 2001. Cretaceous lithostratigraphic units (Belgium). Geologica Belgica 4: 121-134.
 
Tourneur, F.; Babin, C.; Bigey, F.; Boulvain, F.; Brice, D.; Coen-Aubert, M.; Dreesen, R.; Dusar, M.; Loboziak, S.; Loy, W. & Streel, M., 1988. Le Dévonien du sondage de Nieuwkerke (Flandre occidentale, Belgique - extrémité occidentale du Synclinorium de Namur). Annales de la Société Géologique du Nord 108: 85-112.
 
Vanneste, C. & Hennebert, M., 2005. Carte géologique de Wallonie à 1/25.000. Planches Mouscron - Zwevegem 29/5-6 et Templeuve - Pecq 3/1-2. Notice explicative. Ministère de la Région Wallonne, Direction générale des Ressources Naturelles et de l’Environnement, 59 p.
 
 

Correlation table for the Turonian deposits between the traditional generic names (Cornet, 1923), the NCS schema (Robaszynski et al., 2001) defined in the Mon s basin, the Carte géologique de Wallonie scheme (Hennebert & Doremus, 1997) defined in the Tournaisis, and the DOV subdivision (Lagrou et al., 2011), defined for West Flanders and now redundant.

 

Appendix: Profile 9 from Lagrou et al., 2011.

 

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