2.7.4. Hyon Formation - 01/2017
Hyon Formation
Authors
Steurbaut and King (1994), Steurbaut (1998, p 115), Geets et al. (2000)
Description
The Hyon Formation has been introduced in the literature by Steurbaut and King (1994) at the occasion of the study of the Mont-Panisel research borehole (Dupuis et al., 1988) and formalised by Steurbaut (1998, p 115). The Hyon Formation was reported in the review by Geets et al. (2000) but not retained in the official NCS stratigraphy by Laga et al. (2001). In addition to the original descriptions in the literature, also the Egem sand unit has been included now as a Member in the Hyon Formation to make a lithological distinction more practical between a sandy Hyon Formation and a clayey Tielt Formation in which the Egem Member was traditionally included.
Description
Fine sand, with dispersed clay or layers of clay, rich in galuconite and including sandstone layers and concretions.
Members
Egem Member, Mont-Panisel Member , Bois-la-Haut Member
Regional occurrence and previous names
The Egem Member of the Hyon Formation occurs over most of the provinces West and East Flanders and part of the Antwerp Campine whilst the Mont-Panisel Member of the Hyon Formation occurs in the Brabant and Hainaut area where its thickness reaches maximum 25m (Steurbaut, 2006); further northwards the Mont-Panisel Member is only locally preserved from erosion (Steurbaut, 2006). The lateral geometrical relationship between the two sandy Members had already been noticed in the classical lithostratigraphic paper by Steurbaut & Nolf (1986), in which the Mont-Panisel Member was indicated as ‘Panisel Sand’. The relationship between the sand members has been interpreted in sequence stratigraphic reconstructions (Vandenberghe et al., 1998, 2004; Steurbaut, 2011).
The introduction of the Hyon Formation arranges the position of the strata in the hills of Bois-la-Haut and Mont-Panisel, located in the village of Hyon southeast of Mons (map figure 1 in Steurbaut & King, 1994), and which are at the origin of the former classic but now obsolete ‘Paniselian’ stage (Steurbaut, 2006). The problematic geometric position of the ‘Panisel sand’ in Brabant and in outliers in the Hainaut area, as it was demonstrated in Steurbaut& Nolf (1986), has been solved by the introduction of the Hyon Formation.
Stratotype
The section between 0 and 21,85m depth in the Mont-Panisel borehole (151E0340) on the topographic sheet 45/7-8 (Mons-Givry) (x=122.300, y= 125.375, z= +102m). This location is an outlier and at this location the Egem Member does not occur.
Biostratigraphy
Upper part of nannoplankton NP12 (Steurbaut,2006).
Egem Member
Authors
Laga et al. (1980); Steurbaut & Nolf (1986); Steurbaut (1988, 1998) ; Geets (1979).
Description
The sediment is a finely laminated, well sorted, mica and glauconite containing and generally fossiliferous fine sand. Lamination is mainly horizontal with in addition cross lamination, hummocky stratification and infilling of broad shallow gullies. Heavy clay layers occur of cm and dm scale often cut by erosive sand-filled channels. The base of the Egem Member is a strongly erosive level with active channelling above the Egemkapel Member. A marked paleoseismite horizon occurs in the middle of the Egem Member exposed in the Ampe/Egem pit (Crepin et al. ,2004). Towards the top, the sediment becomes coarser and more homogeneous with numerous nummulites. A detailed section of the Egem Member in the Ampe/Egem quarry and corresponding grain-size data (Geets, 1991) in the Tielt borehole 068E0169, are shown in Steurbaut (1998, p 117). Subdivisions of the Egem Member can be regionally followed in CPT logs and borehole descriptions (Jacobs et al., 1996a, b). On geophysical logs the base of the Egem Member can generally be recognised by a sharp increase in resistivity as it generally overlies the clay-rich Egemkapel Member.
Regional occurrence and previous names
The Egem Member occurs over most of the provinces of West and East Flanders (Steurbaut & Nolf, 1986; King, 1991 p370) and its extension northeastwards into the Antwerp province has been interpreted in many publications although an identification as Hyon Formation is probably more prudent . On regional profiles the base of the Egem Member is clearly erosive into underlying units (Vandenberghe et al., 1998; King, 1991).
The Ledeberg sand and Evergem sand are synonymous for the Egem Member (Geets et al., 2000).
In the legend of the 1:40 000 maps the Egem Member is representing the Yd and the P1b units and in the stratigraphic register (1929, 1932) the Y1b division (Geets et al., 2000).
Stratotype
The Ampe/Egem extraction pit - 053W0060 in Egem (Pittem) (mapsheet 21/1 Wingene x= 70.150, y= 190.150, z= +44m) between +39.5 m to +19 m T.A.W, between the Egemkapel Member below and the X-sandstone (in this review named the Hooglede Bed) bed underlying the Pittem Member above (Geets et al., 2000).
A detailed description of the Ampe/Egem extraction pit anno 1994-1995, comprising the Egemkapel, Egem and Pittem Members can be found in Willems (1995) and Steurbaut (2015).
Geophysical borehole references
In the central West Flanders type area of the Egem Mbr several boreholes can be used as reference for the Egem Mbr and its Yd4, 5, 6 subdivisions: Tielt 053E0061, Gent – 055W1020, Brugge – 023W0454, Torhout – 052E0195, Oosterzele – 070E0237, Kruishoutem – 084E1412.
Mont-Panisel Member and the Bois-la-Haut Member
Authors
d’Omalius d’Halloy (1862, p 536 & 625), Steurbaut & Nolf (1986), Steurbaut & King (1994), Steurbaut (1998), Geets et al. (2000).
Name
The Mont-Panisel hill is the twin hill of Bois-la-Haut in the village of Hyon, near Mons (map figure 1 Steurbaut & King, 1994).
Description
Poorly sorted, faintly laminated, prominently glauconitic and highly bioturbated clayey fine sand occur in the reference borehole section of this unit at the Mont-Panisel (151E0340) (Steubaut & King, 1994). A separate 3,6 m thick layer at the base of the section in the Mont-Panisel borehole (151E0340, between 18 and 21,58m), is highly glauconitic, highly bioturbated, rather well-sorted fine to medium sand with clayey patches in contrast to the finer and less-sorted sand above (see section in Steurbaut and King, 1994 fig.3). The lower part is named the Bois‐la‐Haut Member and the main upper part is called the Mont-Panisel Member. The latter contains also numerous irregularly shaped siliceous sandstone concretions and locally poorly cemented nummulite-bearing sandstones occur (Steurbaut, 2006). Maximal thickness is 20 m.
Geets et al. (2000) report that somewhat coarser glauconite-rich sand in boreholes between Aalst and Brussels could correspond to the Bois-la-Haut Member. The X-stone bed , named Hooglede Bed in this review, underlying the Pittem Member in the Ampe/Egem quarry has also been tentatively suggested to be a lateral equivalent of the Bois-la-Haut Member by Steurbaut (1998) although in Steurbaut (2011, fig.8 p 255) the X-stone bed is again included in the base of the Pittem Member. Although the Bois-la-Haut Member is until now only clearly identified in the Mont Panisel borehole, it is ranked as a member seen its thickness of several meter, more than the normal thickness for a lithostratigraphic bed.
Regional occurrence and previous names
These deposits were originally described by d’Omalius d’Halloy (1862) as ‘psammites, sables et argiles du Mont-Panisel’ at the Mont-Panisel near Mons. The Mont-Panisel Member occurs in the area Gent-Brussel-Mons-Kortrijk. The Mont-Panisel Member overlies the Aalbeke Clay in clay pits around Kortrijk (e.g. Mulier clay pit) (Steurbaut 2006).
The sands correspond to the previously used unit ‘Panisel sand’ in Steurbaut & Nolf (1986) and this Member corresponds to the ‘Unnamed Sand member’ in the top of the Mouscron borehole and the Kortrijk outcrops of King (1991 p 365). It also occurs in the hills of North France. It corresponds to the term ‘Paniselien’ used by Gulinck in his profiles around Brussels (Archives Belgian Geological Survey, MG/00/250-329-547; MG/53/327; MG/55/335; MG/56/176-177-313-316; MG/58/249).
Whereas in the Gent (055W1020) area the Egem Member subdivisions Yd4, Yd5, Yd6 (sensu Bolle & Jacobs, 1993) can be recognised between the Egemkapel (Yd3 unit sensu Bolle & Jacobs,1993) and the Merelbeke Clay, such identification becomes difficult to the east near the boundary with the Brabant province. It seems that in this latter area and more to the east, the Mont-Panisel Member replaces the Egem Member. Jacobs et al. (1996a p 28) have reported that the Egem Member becomes more clayey to the south.
In the southeast of East Flanders and the neighbouring eastern Brabant province, about 6 to 11 m of glauconitic sand occurs containing sandstone layers and overlies the Aalbeke clay Member in Kerksken (086E0340) and Kattem (087W0479) boreholes; its description corresponds to the Mont-Panisel Member (Geological Service Company, 2003). The typical Mont-Panisel sand in the borehole is overlain by a clayey sand of about 11 m which in its turn is capped by the Merelbeke clay Member. The lithostratigraphic position of this clayey sand unit overlying the Mont-Panisel sand is further discussed under the Kwatrecht Member.
The implication of this succession is also that towards the east, the Kortemark Member and the Egemkapel Member have disappeared. Also on the map sheet 23 Mechelen, Buffel et al. (2009) note that the Kortemark Member disappears to the east and is only present in the western part of the map.
More northwards in the Brabant province (east of Aalst), the Merchtem borehole (072E0229) (Buffel et al., 2009) shows above the Aalbeke Member and below the Merelbeke Member, the same twofold borehole geophysical log signature and thickness as the Kerksken – 086E0340 and Kattem – 087W0479 boreholes, with the lower part being the typical Mont-Panisel Member below a more clayey glauconitic sand without sandstones (see Kwatrecht Member). This pattern can also be observed further west and north-westwards in geophysical logs (Meise borehole 073W0394 in Welkenhuysen & De Ceukelaire, 2009) and in grain-size analysis of the Zemst-Weerde borehole (073E0359) (Buffel et al. 2009). The presence of Merelbeke clay in the Zemst-Weerde - 073E0359 borehole was confirmed by micropaleontological data (Buffel et al., 2009). Note that in the Zemst-Weerde- 073E0359 interpretation by Buffel et al. (2009) these two units together were named Egem Member, an interpretation not followed in the present review. Also, just north of Brussels in Vilvoorde, Gulinck described in his profile MG 00/504 ‘Paniselien’ above a clay rich top of the ‘Ypresian’ and below the Brussel and Lede Formations; this ‘Paniselien’ is characterised by stone layers in its lower part.
Over a short distance to the east, between Zemst-Weerde (073E0359) and Zemst-Hofstade (073E0397) the Mont-Panisel sand and the overlying clayey sand have disappeared, except maybe for a very thin remnant, and it appears that the Aalbeke and the Merelbeke clay Members are almost superposed (interpretation Johan Matthijs), although this needs micropaleontological confirmation. This superposition would imply the wedging out of the Mont-Panisel and Kwatrecht units by erosion before the deposition of the Merelbeke Member rather than their later erosion before deposition of the overlying Zenne Group as would be the case if only Aalbeke clay is present (see also Merelbeke Member).
On the other hand in the Kallo wells (027E0148/014E0355), more north-westwards, the Kortemark and Egemkapel Members can be recognised and between the Egemkapel and the Merelbeke clay Members the sandy unit is often interpreted as Egem Member, although identification as Hyon Formation is probably more prudent. The same log signature of the Hyon Formation interval in the Kallo well 014E0355 is also recognised in the Rijkevorsel 007E0200 borehole.
Stratotype
The section between 0 and about 21.58m depth in the borehole of the Mont-Panisel (151E0340) (topographic map sheet 45/7-8 Mons-Givry, (x=122.300, y= 125.375, z= +102m).
As the Mont-Panisel borehole is located in an outlier area of the Mont-Panisel Member, the interval 46-54 m in the borehole Zemst-Weerde (073E0359) can be considered a parastratotype of the Mont-Panisel Member (verslag Zemst, FV Matthijs-Buffel, 2000; Steurbaut et al., 2015).
Geophysical borehole references
Typical log signature of the Mont-Panisel Mbr can be observed in the reference borehole logs: Merchtem - 072E0229, Zemst-Weerde - 073E0359, Wieze - 072W0159, Wortegem – 084W1475, Kerksken – 086E0340, Kester – 101W0079.
Often it is not possible to distinguish Egem and Mont-Panisel Mbrs. In that case the signatures are best described as Hyon Fm such as a prudent interpreter could do in the case in the reference logs Rijkevorsel – 007E0200, Mol – 031W0237, Kallo – 014E0355,...
In the Kallo well – 014E0355, and eventually the Rijkevorsel borehole – 007E0200, the subdivisions Yd4,5,6 are still recognisable and could be assigned to the Egem Mbr as has commonly be done in the Mol well – 031W0237 (M. Gulinck) although in this Mol well, Steurbaut (1988) has differentiated Kortemark and Egem above the Aalbeke Mbr.
The Egem Member in the Knokke well 011E0138 is intriguing as the Yd4,5,6 subdivisions on the geophysical logs are apparently identifiable although only Yd6 is a sandy deposit but Yd4,5 are reliably described as clay deposits in the cores; only the Yd6 interval is therefore considered as the Egem Mbr in the Knokke Memoir (Laga & Vandenberghe, 1990; King, 1990; Welkenhuysen & De Ceukelaire, 2009 p. 72). The profile designed by Van Burm and Bolle (in Jacobs et al, 1996a) indicates the appearance of a clay-layer above the Egemkapel Member; this clay layer called Yd5 is clearly increasing to the west. Apparently in the Brugge-Knokke area the lower part of the Egem Member is developed as a clay whereas it was a sand, subdivided in Yd4-Yd5-Yd6, in the Tielt-Gent area (Jacobs et al, 1996a-b). A provisional informal name could be used for this clay unit in the Brugge-Knokke area, the Hazegraspolder unit referring to the location of the cored Knokke borehole.
Differentiation between the Mont-Panisel and the Egem Members
The Mont-Panisel Formation consists of poorly sorted clayey glauconitic sands in which the glauconite can make up to 15%. The Egem Member consists of well sorted laminated fine sands in which also somewhat thicker clay layers can occur. In this sense, the Mont-Panisel Formation is more homogeneous than the Egem Member.
The Mont-Panisel Member, in contrast to the Egem Member, contains numerous irregularly shaped siliceous sandstone concretions whilst sandstones in Egem are rare.
In the practice of the 1:50 000 mapping, the Egem Member was identified whenever it could be subdivided in the subunits Yd4,5,6 introduced by Jacobs & Bolle (1993) and Jacobs et al. (1996a, b); towards the east, in the neighbourhood of Aalst, the sediment became more clay-enriched and the traditional Egem sand subdivisions could no longer be followed in the mapping; consequently, this more clayey unit in the east, which also contained sandstones, was mapped as the Mont-Panisel Member (see Jacobs et al., 1996, a /Fig.15 showing this transition).