MERLIN Member 13/01/2016

MERLIN Member

 

  1. Name. Village in Brunehaut commune, SW of Tournai, selected because of a good temporary outcrop during construction of TGV (High Speed Train), in Hennebert & Doremus (1997a, b).

 

  1. Code carte géologique de Wallonie MER

 

  1. Stratotype. No formal stratotype defined. Nominal decimal geographical coordinates for Merlin 50,553 – 3,401; Lambert ’72 coordinates 81425 – 138500.

Parastratotypes for the Flemish region:

Parastratotype 1: Borehole GeoDoc 095W0153, Nieuwkerke Noordhoek, drilled 1985 by VMW (De Watergroep) as their cored borehole K11 (top of cored interval 190,78 m, continuing into underlying Paleozoic section). Lambert coordinates x = 40880, y = 162250, z (ground level) = 32 m; depth interval: 169 - 192 m. Stratotype selected because of presence of a core in the lower part of this member (no geophysical well logs over this interval).

Parastratotype 2: Borehole GeoDoc 110W0007, Nieuwkerke-De Seule. Lambert coordinates x = 40290, y = 157985, z (ground level) 19,5 m; depth interval 136 – 146 m. Parastratotype selected because of presence of a geophysical well log over the corresponding depth interval.

 

  1. Description. The Merlin Member corresponds to the traditional “Fortes Toises”. In the type area it could be observed in temporary and partial outcrops along the TGV line (Hennebert & Doremus, 1997a, b). It consists , from top to bottom of:

- white chalky marl, with grey silicified concretions;

- grey-white marl with greenish shine, with fine glauconite.

 

In the area transitional to the Mons basin (Péruwelz, Hennebert, 1999a, b) the member is not subdivided in successive units but consists of grey marls to grey-white marly chalk with dull partly silicified concretions (“chailles”), better corresponding to the original definition as “Fortes Toises”.

 

In the parastratotype area of the Flemish region observations are based on cores and geophysical well logs, allowing a less detailed but more consistent description.

Cuttings (95W0153) consist of bright green silty marl in the upper part, mixed with fine glauconitic sand in the lower part.

The core consists from top to bottom of light greenish chalk with oxidised specks of iron sulfide, turning into green silty chalk with clay laminations containing shell debris, with a basal layer of white very coarse-grained and porous chalk.

The gamma-ray log allows to recognize two sequences in the Merlin Member, delineated by beds which show an increase of the gamma ray readings; these probably correspond to the more sandy glauconiferous base of a fining upwards sequence, grading into white chalks. Overall a steady increase in gamma ray level is observed from top to bottom of the Vert Galand Formation.

 

Although the stratigraphic succession observed in boreholes in the Flemish region may be deposited in a more distal part of the sedimentary basin compared to the Tournaisis and does not show the silicifications (which are observed at a higher level and included in the overlying stratigraphic unit) it seems to confirm the subdivision in two units in the type area.

 

  1. Underlying strata: conformably underlain by Bruyelle Member of Vert Galand Formation.

Because of the greater lateral persistence of the Merlin Member compared to the Bruyelle member of the Vert Galand Formation in the southern part of West-Flanders, this unit may unconformably overlap Paleozoic strata (e.g. limestones of the Tournai Formation in borehole 096E0076 in Rekkem, Silurian shales in borehole 097E0865 in Bellegem).

 

  1. Overlying strata: conformably overlain by Esplechin Formation (Hautrage Flint Formation of the NCS 2001 scale), when not truncated by Cenozoic erosion phases.

In boreholes, the transition from Esplechin to Vert Galand Formation immediately appears by the colour change to green of the drilling mud, which is used as a lithostratigraphic indicator in the Flemish region.

 

  1. Area: as for the Vert Galand Formation.

 

  1. Thickness. In the type area from 5 to 10 m.

Parastratotype area of the Flemish region: 23 m in the parastratotype 1 095W0153, 10 m in parastratotype 2 110W0007. The thickness in parastratotype 1 appears slightly anomalous as the average regional thickness is in the order of 10-12 m.

 

  1. Age. Middle to Upper Turonian (from Hennebert & Doremus, 1997a, b); late Turonian (from Robaszynski et al., 2001).

 

  1. Discussion.

As stated before, the Merlin Member corresponds to the ‘Fortes Toises’ of the informal stratigraphic subdivision (Cornet, 1923). On DOV it appears as Nieuwkerke Noordhoek Member of the Vert Galand Formation.

The NCS-stated equivalent to the Merlin Member is the Ville-Pommeroeul Chert Formation (Robaszynski et al., 2001).

 

At this level there appears some ‘noise’ in the correlations between West-Flanders, the Tournai basin (where the Vert Galand Formation has been introduced) and the Mons basin (where the NCS stratigraphic scheme has been defined). The upper part of the Merlin Member would include whitish chalks with silicifications; in the West-Flemish boreholes such divergent lithologies are already assigned to the overlying Esplechin Formation. Some discrepancies in the interpretation will continue to exist as long as these strata are not properly dated in the boreholes from West-Flanders, nor properly described and correlated between the Tournai and Mons basins.

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