2.7.2. Mons-en-Pévèle Formation
Mons-en-Pévèle Formation
Authors
King (1991), Steurbaut & Nolf (1986), Steurbaut & King (1994, p180), Steurbaut (1998)
On the formation status
Although the Formation status is given to the Mons-en-Pévèle sand unit , arguments could be forwarded to consider it as a Member of the Kortrijk Formation. A member status could logically reflect the lateral transition zone with vertically alternating sandy layers and clay layers (such as e.g. in the Mouscron borehole in fig.10 as an undifferentiated Kortrijk Formation in King (1991)). The Mons-en-Pévèle unit is not included in the Kortrijk Formation because of the sandy nature of the former in contrast with the dominantly clay nature of the latter. Also the Mons-en Pévèle sand unit can be properly mapped with considerable thickness in Hainaut where it links up with the Cuise Sand of the Paris Basin; such a map unit usually gets the formation status. Also, the 1:25 000 mapping in Wallonia uses the status ‘Formation de Mons-en- Pévèle’. Therefore in the present review it has been chosen to rank the Mons-en-Pévèle sandy unit as a Formation.
Description
Succession of one or a few m thick laminated packages of pure very fine sand (60-80µm), often cross stratified, and strongly bioturbated clayey sand; the latter are more important closer to the base. The sand is micaceous with commonly very fine glauconite. Several coarser beds are packed with Nummulites, appearing for the first time in the basin in the Mons-en-Pévèle Formation. Locally cemented layers occur, a.o. nummulitic limestone beds.
Regional occurrence and previous names
The Mons-en Pévèle Formation is occurring southeast of a line through Lille (North France) (see map in King, 1991), from Mons-en- Pévèle (North France) to Tournai and Ronse and further eastwards. Mons-en-Pévèle is a locality south of Lille in North France and the name ‘Sables de Mons-en-Pévèle‘ was introduced by Ortlieb & Chellonneix (1870, p 27).
Towards the east in Brabant, the Ieper Group thins and a typical clayey basal part is distinguished from an upper fine sandy unit. The basal clay corresponds to the Orchies Member of the Kortrijk Formation whilst the sand has been given a lithostratigraphic name, the Vorst/Forest sand. It was shown by King (1991) that these fine sands are equivalent to the Mons-en-Pévèle Member. Logically therefore the Bierbeek sand above the Orchies Member in the Leuven area (geological map 1:50 000 sheet 32 Leuven, Vandenberghe & Gullentops, 2001) can be considered as a decalcified sand of the Mons-en-Pévèle Member, in a similar way as the sands above a thin clay unit in north Brabant (Rillaar) and Limburg (e.g. Veldhoven, Beringen) as figured by Gulinck (1967) and discussed by Fobe (1989a).
From a nomenclature point of view, in the transition zone of laterally interfingering units such as the Roubaix and Mons-en Pévèle units, the ICS Stratigraphic Guide recommends that a somewhat arbitrary boundary should be chosen in mapping and borehole description, obviously accompanied by an appropriate explanation in the legend or description. In the case of the Roubaix/Mons-en-Pévèle limit the present review suggest that if the unit consists of over 50 to 60% sand layers, the unit should be named Mons-en-Pévèle Fm and otherwise the unit should be classified as Roubaix Member of the Kortrijk Formation. For example the 368-407 m section in the Mol SCK borehole 031W0237 is mainly described as fine sand with minor clay layers (Gulinck & Laga, 1975) and is therefore to be named Mons-en-Pévèle Fm. Localities with Mons-en-Pévèle sand are listed in Steurbaut & Nolf (1991, Fig.3) and appear systematically between Ronse and Brussels. According to the lithological description (sand/clay proportion) the log signature in the borehole Merchtem 072E0229 should be classified as the Roubaix Mbr and in the Kester borehole 101W0079 as Mons-en-Pévèle Fm. However an inspection of shape of the RES and GR logs allows to consider the boreholes Merchtem (compendium), Meise (see profile Gent-Zemst and Meise-Rotselaar), as transitional signatures between Roubaix and Mons-en-Pévèle.
The criterion will need further refinement and the study of more wells. Also it has to be recognised that the descriptions of the boreholes, especially if destructive, are often not accurate enough to reliably decide on the number of sand layers. Also if grain-size data are available, it will be needed to define how exactly to apply the criterion; e.g. at first glance the amount of >62µm fraction in the borehole Kattem (087W0479) south of Aalst (Geological Service Company, 2003) is high enough to describe the unit below the Aalbeke Member as the Mons-en-Pévèle Member. The same holds for the application of the GR/RES log values in determining how much sand layers occur in the interval. Maybe the 50-60% sand layer boundary will need to be changed or maybe it will appear practical to introduce a new transitional lithological unit.
Lithological trends and paleogeography
Paleogeographically, from central Flanders towards the east and the south, several clay enriched facies of the Kortrijk Formation are replaced by more sandy deposits (maps in Steurbaut, 2006). The lateral transitions are well documented and figured in King (1991). The Roubaix Member laterally interfingers with the Mons-en-Pévèle Fm. More southwards to the Paris basin the closer to the base of the Ieper Group starts the occurrence of the sand unit (profiles in King, 1991). The sands are known as the Mons-en- Pévèle Member and grade into the ‘Cuisian‘ sands in the Paris Basin. The Aalbeke Clay extends over the Mons-en-Pévèle Sand into the Paris basin where it corresponds to the Laon clay (King, 1991). Where the ‘argilite de Morlanwelz’ is a lateral equivalent of the Orchies Member (Steurbaut, 1991) more sandy facies in southern direction in the Hainaut province with specific names such as the Godarville sand and the Peissant sand (Steurbaut & Nolf, 1986) are included, without a specific stratigraphic status in the Mons-en-Pévèle Member. The Morlanwelz Sand, with a Formation status, is figured in Steurbaut (1998 p 145; Steurbaut et al., 2003 p 11) as a lateral equivalent of the Roubaix Member but as a separate unit underlying the Mons-en-Pévèle Sand Formation (see also Steurbaut, 1998 p 110); however this subdivision is not retained in the present review due to a lack of precise data.
It should be noticed that the reverse lithological trend logically is also present in the north direction leading King (1991, p 361, 370) to introduce the name Flanders member for the homogeneous Ieper Group clays beneath the Egem Member in the Knokke well. In this review this suggestion is not followed as these very clay rich sections can still be subdivided using existing nomenclature such as ‘the Tielt and Kortrijk Formations ‘ (see Welkenhuysen and De Ceukelaire, 2009 fig. 16) (see also Knokke well in the compendium) and as the geophysical well log divisions of these clay-rich sections can even be recognised further north in the Netherlands (de Lugt, 2007).
Stratotype
No formal stratotype has been designated. Logically the Mons-en-Pévèle hill south of Lille and north of Douai in North France is the preferred reference area (see Steurbaut, 1998 p 116); also the Waaienberge (Wayenberghe) railway section near Ronse ( described in King 1988 (1990) p 359 and figured in Steurbaut & Nolf 1988 (1990) p 328) is a potential stratotype section.
Geophysical borehole references
The following borehole logs in the reference compendium have a Mons-en-Pévèle signature Zemst-Hofstade – 073E0397, and are confirmed by analysis in the Mol – 031W0237, Kester- 101W0079 wells.
Typical Roubaix Mbr log response confirmed by clay dominated lithology can be observed in Kallo - 014E0355, Knokke - 011E0138.
The signature in the Merchtem–072E0229 borehole is somewhat intermediate but according to the borehole description sand-layers represent only 24 % of the interval and therefore the interval is classified as Roubaix Mbr base on the 50-60% sand layer criterion (see above).