2.7.5. Gentbrugge Formation - 01/2017
Gentbrugge Formation
Authors
See also Geets (1988) and Steurbaut (1998).
The formation is also discussed by de Heinzelin & Glibert (1957), De Moor & Geets (1973), De Moor & Germis (1971), Fobe (1986), Geets (1979), Gulinck (1967), Gulinck & Hacquaert (1954), Kaasschieter (1961), Maréchal (1993), Steurbaut & Nolf (1986) and Wouters & Vandenberghe (1994).
Description
This formation of marine origin consists at the base of a very fine silty clay or clayey, very fine silt. To the south and upwards, it is followed by an alternation of layers of glauconiferous, clayey silty, very fine sand and clayey sandy, coarse silt, disturbed by bioturbation. The clayey members are covered by fine sand, clearly horizontally bedded or cross bedded. The sediments contain different layers of sandstones.
Stratotype
Stratotypes have only been designated for the members.
Area
The formation mainly outcrops in the centre of East- and West-Flanders and on the hills in the southern part of East- and West-Flanders. It occurs also in the subsoil of the province of Antwerp and northwest Belgium. Some outliers can be observed to the south till northern Hainaut and eastwards from the Senne River.
The regional distribution map of the Gentbrugge Formation is figured in Maréchal (1993, p 222) as understood at that time; the extension mapped on the 1:50 000 geological maps can be consulted on https://dov.vlaanderen.be/dovweb/html/geologie.html.
Thickness
Maximum 50 m in the north and decreasing to the south and the east.
Members
The formation is subdivided into the Kwatrecht, Merelbeke, Pittem and Vlierzele Members. Note that in the present review the now more generally recognised, Kwatrecht Member is ranked in the Gentbrugge Formation because of its more clayey nature compared to the sediments in the Hyon Formation.
The original and present significances of the term Kwatrecht Member are explained in the full description of this member below. Whereas originally in sections the poorly defined alternating clay and sand Kwatrecht unit has been omitted or integrated as a local variation in the top of the Egem sand Member; the recognition of a more systematic occurrence of clayey sand, supposedly related to the originally described Kwatrecht unit, has justified the formal individualization of this unit with a Member status. The clayey nature of the sediment is the only reason to group the Kwatrecht Member in the Gentbrugge Formation in contrast to the Egem sand Member which is incorporated in the sand dominated Hyon Formation.
The Vlierzele Member has been traditionally included in the Gentbrugge Formatie of the Ieper Group. It could be argued that the Vlierzele unit as a sand unit would be better ranged in the sandy Zenne Group. However it is also argued that the Vlierzele unit also contains clayey parts and therefore should remain in the Ieper Group. However, taking into account the full significance of the clayey parts of the Vlierzele unit led Fobe (1995, 1997) to differentiate different members in the Vlierzele unit and to rank the Vlierzele unit as a Formation (see further/ to be discussed). In the present review the Vlierzele is kept as a Member and ranked in the Gentbrugge Formation (see below).
Age
Late Ypresian.
Remark
the Gentbrugge Fm is called Gent Fm on the 1:50 000 geological maps. The name Gent Fm was changed to Gentbrugge Fm since it was already in use for Quaternary eolian cover-sand deposits in Flanders (Paepe & Vanhoorne 1976, see website NCS Quaternary subcommission).
Kwatrecht Member
Authors
De Moor & Geets (1973)
The occurrence described in the Gent area (Merelbeke) as reported by De Moor & Geets (1973) is only documented in this one section (boreholes DB11&12) as alternating clayey and sandy sediments in contrast to the more well-calibrated fine sand of the Egem Member below. No further sedimentological details can be derived from this description and the suggestion for a fill of erosive channels as figured on the De Moor and Geets (1973) profile is not considered justified by the existing borehole data. An analogue profile in the same area around Gent (Jacobs et al. 1996a fig. 9) omitted the Kwatrecht unit below the Merelbeke Member. Vandenberghe et al. (1998 & 2004) have suggested this Kwatrecht unit could be an erosion remnant as a consequence of intense erosion phases in the late Ypresian and early Lutetian.
However a review of the original documentation and biostratigraphic information (dinoflaggellate data) suggests that this clayey sand unit can be maintained as a separate unit. Furthermore a detailed analysis of the Zemst-Weerde borehole (Steurbaut et al., 2015) seems to confirm a more regional extension of this lithological unit at the same biostratigraphic level. Therefore it is justified to introduce the Kwatrecht Member as a formal lithostratigraphic unit.
Description
A layered complex of greenish glauconitic and micaceous bioturbated sand and sandy clays, without stone beds, originally indicated as the Kwatrecht Complex, has been described underlying the Merelbeke Member and overlying the Egem Member in the Gent area near Merelbeke by De Moor and Geets (1973, see 2.2.3.3).
In regional sections, the Kwatrecht Member is geometrically positioned between the Egem and Merelbeke Members by Steurbaut & Nolf (1986), Steurbaut (1991) and Willems & Moorkens (1991). Based on geometry and biostratigraphy the Kwatrecht Member has been related to the Hyon Formation by Vandenberghe et al. (2004). However more recently the Kwatrecht Complex is ranged in the Gentbrugge Formation by Steurbaut (2006, 2011).
Regional occurrence and stratigraphic position
Original description by De Moor and Geets (1973) in the Gent area. Steurbaut (2006, p 79) has reported the presence of the Kwatrecht Member in the Zemst-Weerde borehole (073E0359, Buffel at al., 2009); according to the description of this borehole in the present review (see Mont-Panisel Member), the about 5 m clayey sand between the Mont-Panisel Member and the Merelbeke Member, are meant as Kwatrecht Member by Steurbaut (2006). Consequently this Kwatrecht Member is now also recognised in the east of the Brabant province (boreholes Kerksken – 086E0340, Kattem – 087W0479, Meise – 073W0394, Merchtem - 072E0229.see Mont-Panisel Member).
Stratotype
The Gent area (Merelbeke) section as described by De Moor & Geets (1973). As data from this stratotype are not easily accessible, the 41-46 m interval in the Zemst-Weerde (073E0359) borehole could be considered as the parastratotype.
Geophysical borehole references
A twofold subdivision of a sand layer between the Aalbeke and Merelbeke Mbrs allows to distinguish an upper Kwatrecht Mbr signature above a Mont-Panisel Mbr signature in the reference boreholes Merchtem – 072E0229, Zemst-Weerde – 073E0359, Kerksken – 086E0340, Wortegem – 084W1475 and in the analysis of the Kattem borehole – 087W0479 (Geological Service Company, 2003) and the published Meise borehole (073W0394) (Welkenhuysen & De Ceukelaire, 2009 Fig. 32). In the Wieze – 072W0159, the interpretation is debatable and therefore in the compendium two alternative are figured.
Merelbeke Member
Authors
De Moor & Germis (1971,p 57), Steurbaut & Nolf (1986, p 128), Geets et al. (2000).
Description
The Merelbeke Member is a compact heavy to silty clay; it is a marine deposit, although some periods with fresh water algae influx have also been observed. Thin sand laminae with organic matter and small pyritic concretions have been described by De Moor & Geets (1974). The Merelbeke Member thickness is generally limited to about 6 to 7 m but exceptionally up to 14 m near Melle in the profile 3 by De Moor & Geets (1973). The thickness variations as figured in the profile by De Moor and Geets (1973) are most probably due to the lack of borehole sample quality although they may also be the result of strong erosion at that stratigraphic interval (Vandenberghe et al. 2004).
Regional occurrence and previous names
The Merelbeke Member occurs in the western part of the Brabant province and in the north of the provinces of East and West Flanders. Its distribution is irregular because of erosion by later Eocene deposits (Vandenberghe et al., 1998, 2004).
Where the Merelbeke Member occurs, it overlies either the Egem Member or the Mont-Panisel Member as in the Ronse-Aalst-Brussel area or the Kwatrecht Member in the east. The Member is overlain by the Pittem Member.
On the 1:40 000 maps the Merelbeke Member is mapped as P1m, a code also often used in borehole descriptions. In the stratigraphic register (1929, 1932) it is part of the Y2 division. In the 1:40 000 mapping, Merelbeke and Aalbeke Members have been confused in the southwest of Flanders.
In the area west of Mechelen (Hombeek, Zemst...), the Merelbeke Member has been confused in some borehole descriptions with the P1n clay (1:40.000 map legend), which is a unit occurring above or in the top of the Vlierzele Member (Buffel et al., 2009). This confusion in North Belgium was already pointed out by Fobe (1995).
Stratotype
The section described between +5,6 and -4,9 m T.A.W. in the borehole Melle (055E0783) (222/E3/SWK/F/DB11), topographic map sheet 22/1-2,Gent-Melle (X= 109.125, Y = 188.775, Z = + 12.6 m) (Geets et al., 2000).
Geophysical borehole references
The Merelbeke Member signature in the reference borehole logs can be observed in many boreholes: Merchtem – 072E0229, Zemst-Weerde – 073E0359(confirmation by biostratigraphy in Buffel et al., 2009), Kerksken – 086E0340, Brugge – 023W0454, Knokke – 011E0138, Kallo – 014E0355, Rijkevorsel – 007E0200, Oosterzele – 070E0237, Kruishoutem – 084E1412, Merksplas – 017W0280.
The Zemst-Hofstade – 073E0397 borehole presents an interesting case. The top clay unit, consisting of two parts on the log, is either entirely the Aalbeke Clay or it might be composed of the Aalbeke Clay overlain directly by the Merelbeke Clay, or with only a very thin remnant about 1m Mont-Panisel Sand in between (interpretation Johan Matthijs); the latter case implies the erosion of the Mont-Panisel, and probably Kwatrecht, units before the deposition of the Merelbeke Member rather than their erosion before deposition of the overlying Zenne Group as would be the case if only Aalbeke clay is present.
It should be noted that in the reference borehole Knokke – 011E0138, and also Mol-SCK15 – 031W0237, also a similar two fold Aalbeke Mbr signature is observed.
Pittem Member
Authors
Geets (1979), Geets et al. (2000), Steurbaut et al. (2003)
Description
The Pittem Member consists of a bedded alternation of thin, dm scale, layers of silty clay and clayey fine glauconitic sand, locally cemented into thin sandstone and siltstone beds which can be microporous after dissolution of sponge spiculae and fossils. Bioturbation is common. Tidal gullies have been reported by Geets et al. (2000). The thickness of the Pittem Member is about 15 to 20 m. Traces of lignite have been reported in the Pittem Member occurring between Knokke and Kruibeke in the north of West and East Flanders by Fobe (1993).
The lower boundary is easily distinguished from either the underlying Merelbeke Member, the Egem Sandstone Bed or the Egem Member. The often reported gradual transition between Pittem Member and the overlying Vlierzele unit in boreholes, is erroneous and due to a confusion between the Pittem Member and clayey parts of the Vlierzele unit sensu Fobe (1995) (Fobe, 1995 p 143).
Also, in typical cases, the limit between the clayey sediment of the Pittem Member and the overlying Vlierzele Sand can be traced with reasonable confidence in the geophysical log correlation profiles by Welkenhuysen and De Ceukelaire (2009).
Fobe (1997) reports that in the subsurface of northwest Belgium the upper part of the Pittem Member is a conspicuous horizon, brown coloured by lignitic material.
Regional occurrence and previous names
The Pittem Member occurs almost continuously in a small zone north of a line Torhout-Tielt-Oudenaarde-Ninove and in West Brabant but subcrops over a larger area north of this line. South of this line it occurs only in the South Flemish hills. Towards the south the Pittem Member becomes more sandy.
On the geological maps 1:40 000 the Pittem Member is represented by the P1c unit and in the stratigraphic register (1929, 1932) as part of the Y2 division. On the 1: 40 000 maps of the Kortrijk area, clayey deposits of the Tielt Formation have been incorrectly interpreted as P1c. The name ‘sandy clay of Anderlecht’ is a synonym.
Stratotype
Geets (1979) considered the now abandoned Claerhout extraction pit in Pittem (topographic map 21/5-6, Izegem-Wakken, X = 74.250, Y = 187.540, Z = + 46 m) as the reference section for the Pittem Member. An identical section is exposed in the Ampe/Egem pit – 053W0060 between +43.5 and +40 m T.A.W (topographic mapsheet 21/1 Wingene x= 70.150, y= 190.150, z= +44m) above the X-stone Bed.
Geophysical borehole references
The Pittem log signature can be observed in the reference boreholes of the type area of central Flanders such as Tielt 053E0061, Brugge – 023W0454, Knokke – 011E0138, Oosterzele – 070E0237, Kruishoutem – 084E1412 but also in the borehole logs of Merchtem – 072E0229, ON-Kallo-1 – 014E0355, Rijkevorsel – 007E0200 and Merksplas – 017W0280.
Hooglede Sandstone Bed
Authors
Bolle & Jacobs (1993), Fobe (1997b), Steurbaut et al. (2003, p 33, 34)
Description
A pale yellowish brown, limonite stained, about 40 cm thick, cemented and originally shelly coarse-grained sandstone layer; most shells have been dissolved and left large voids. Typically, numerous very coarse glauconite grains are dispersed across the sandstone and sometimes glauconite staining occurs in the dissolved shell voids. The fossils in the layer are bivalves, oysters, nautiloids and shark teeth; also phosphatic nodules are reported (Steurbaut, 2006). The sandstone bed overlies the Egem Member and underlies the Pittem Member.
Because of its characteristic aspect, it is preferred to attribute a bed status to the stone bed. It has been named bed X in Steurbaut (1998, fig. 5), bed 22 in Steurbaut (1998) and Steurbaut et al. (2003, p34). It is proposed in this review to name the bed the Hooglede Sandstone Bed of the Pittem Member after Fobe (1997b).
Regional occurrence and previous names
The bed occurs in the classical Ampe/Egem extraction pit – 053W0060 at Egem and is named bed 22 in the classical section of the pit published by Steurbaut (1998). Lithotratigraphically, the Egem Sandstone Bed has generally been considered as the base of the Pittem Member (a.o. Steurbaut, 2003) as several thin and fine-grained sandstone beds also occur in the Pittem Member ; it was tentatively suggested to be a lateral equivalent of the Bois-la-Haut Member by Steurbaut (1998) (reported also in Geets et al., 2000), although in Steurbaut (2011, fig.8 p 255) the Egem Sandstone Bed (X-stone bed) is again included in the base of the Pittem Member.
Stratotype
The Ampe/Egem extraction pit – 053W0060 in Egem (Pittem) (mapsheet 21/1 Wingene x= 70.150, y= 190.150, z= +44m) between the Egem Member and the Pittem Member.
Vlierzele Member
Comment on the stratigraphic ranking
In the recent literature, the Vlierzele Member has been included in the Gentbrugge Formation of the Ieper Group. However it could be argued that the Vlierzele Member as a sand unit better fits in the Zenne Group overlying the clay dominated Ieper Group. This would be partly in line with Fobe (1995) who argues that the Merelbeke and Pittem Members as clayey units should be united in the Gentbrugge Formation and distinguished from the sandy Egem and Vlierzele units, respectively below and above.
As the lower part of the Vlierzele unit can also be clayey (see Jacobs et al., 1996a Fig. 13; Lochristi unit sensu Fobe, 1995 p 142), and also for continuity reasons, the present review keeps the Vlierzele unit in the Gentbrugge Formation as a Member.
Authors
Kaasschieter (1961), Geets et al. (2000), Fobe (1995, 1997)
Description
In the recent literature, based on outcrop observations, the Vlierzele Member is described as consisting of a lower part of mostly bioturbated, slightly clayey, glauconitic sand and an upper section of alternating units of tidal cross-bedding with mud drapes and structureless intercalations; the upper section may contain lignite lumps (Houthuys, 1990).
In the recent literature, based on outcrops, the Vlierzele Member is described as consisting of fine glauconitic green-grey mostly bioturbated sand, finely laminated horizontally and in cross stratification. Towards the base the sands becomes clayey and more homogeneous. Towards the top individualised clay layers occur together with humic intercalations. Macrofossils are very rare. Thin cemented siliceous sandstone beds commonly occur (Geets et al., 2000); irregularly shaped siliceous sandstone concretions are also common. The maximal thickness is about 20 m; in the type locality the cross bedded sand above is 7m thick and the lower homogeneous sand at least 5 m (see sections in Houthuys & Gullentops, 1988 p 142).
Fobe (1995), after reviewing information available from more than 25 localities, considers the ‘traditional Vlierzele sand sensu stricto’ as only one of 5 members in a formation between the Pittem Member and the Aalter Sand in the Zenne Group. Steurbaut (2006) reports erroneous correlations in Fobe’s (op.cit.) subdivisions ; the Beernem sand, traditionally a member of the Aalter Formation of the Zenne Group (Maréchal & Laga,1988 p 120-121; Geets et al., 2000), is included in the Vlierzele unit by Fobe and the existence of a distinct Aalterbrugge unit is refuted by this author. Therefore the present review is not following the interpretations by Fobe (1995, 1997) but recognizes that the clayey basis, a 3-10m very fine clayey sand with mm-thick clay layers, (Lochristi layer sensu Fobe) and locally a thin coarser basal layer (Hijfte layer sensu Fobe) merit a separate mention aside the traditional Vlierzele sand sensu stricto (which according to Fobe 1995,1997 could be named Oosterzele unit).
Regional occurrence and previous names
The Vlierzele Member outcrops in the northern and central parts of the provinces East and West Flanders and in the western part of the Flemish Brabant province. It also occurs as outliers in the top zones of the South-Flemish hills. On a regional scale the base of the Vlierzele is erosive into underlying strata (see also Fobe, 1989b, 1995). In northern Flanders the grain-size properties of the Vlierzele Member seem to be more variable (Laga & Vandenberghe, 1990 p 1; Fobe, 1993, 1995). The boundary between the clayey sediment of the Pittem Member and the overlying Vlierzele Sand can mostly be traced with reasonable confidence in the geophysical log correlation profiles by Welkenhuysen and De Ceukelaire (2009).
On the legend of the 1:40 000 maps the Vlierzele Member is coded P1d and P1n for the upper clayey facies. In the stratigraphic register (1929, 1932) the Vlierzele Member is included in the Y2 division.
The P1n-clay, defined by Rutot (1890) and described as a local top clay in the Vlierzele Sand (Gulinck & Hacqaert, 1954) is believed to correspond in fact to the Merelbeke Clay (Fobe, 1995; Buffel et al., 2009).
Stratotype
The Vlierzele locality is part of the Sint-Lievens-Houtem municipality in the East Flanders province where several extractions have been active in the past. The sand pit, formerly known as the Verlee or Balegem sand pit (at present Balegro sand pit)– 070E0050, is the stratotype; it is located on topographic map sheet 22/7-8, Oordegem-Aalst (X = 116.650, Y = 181.725, Z = + 45 m) (Geets et al., 2000).
However this stratotype is limited to the Vlierzele sand sensu stricto. Following Fobe (1995), as far as the Vlierzele sensu stricto, the Lochristi and Hijfte layers are concerned, the Ursel borehole (039W0212 x= 87.910, y= 204.260, z= + 29 m TAW) shows the Vlierzele Member between 58 and 69,3 m with the Vlierzele sand sensu stricto between 58-63 m, Lochristi layer between 63-66 m and the Hijfte layer between 66-69,3 m.
Geophysical borehole references
The Vlierzele Mbr has been identified on top of the Pittem clay Mbr in the following reference borehole logs: Brugge, Merchtem, Knokke (comprising the Hijfte, Lochristi and Oosterzele units sensu Fobe 1995, 1997), ON-Kallo-1, Rijkevorsel.
Aalterbrugge Member
Authors
Hacquaert (1939); Gulinck & Hacquaert (1954); De Moor & Geets (1973); Fobe (1995); Van Simaeys (1999).
Description
The Bed consists of clays and sand occurring in a complex geometrical relationship like usually encountered in continental deposits; also lignite beds and drift wood, sometimes silicified, occur. It occurs between the Vlierzele Member and the Aalter Formation (Zenne Formation) (Maréchal & Laga, 1988; Steurbaut, 2006), both more homogeneous, glauconitic, marine sediments.
In the synthesis on Belgian geology (P. Fourmarier, 1954, Prodrome d'une description géologique de la Belgique, Soc. Géol. Belg.) Gulinck & Hacquaert (1954) describe in the chapter XIV the Complexe d'Aalterbrugge occurring in the top of the Vlierzele sand unit as follows : " Ces sables prennent souvent dans les zones supérieures, quelquefois aussi dans les parties moyennes, un facies plus grossier, pauvre en galuconie, parfois ligniteux, avec bois flottés percés de tarets et souvent silicifiés. On y rencontre également des niveaux de galets de glaise, spécialement dans la région de Torhout (Rutot [explic.carte géologique 1:40 000]) et d'Aalter (Hacquaert, [1939]).
Les bois flottés sont parfois très volumineux. Leur nature fragmentaire permet rarement une détermination précise, mais on a pu y distinguer une dizaine d'espèces (F. Stockmans [région de Bruxelles])".
The Aalterbrugge unit as represented on the section in De Moor & Geets (1973, fig.4) attains 10m thickness. The temporary exposure described by Jacobs (2015 p 137) was at maximum 3m thick (Steurbaut or. com.). Van Simaeys (1999) shows the presence of the Aalterbrugge Complex in the Hijfte borehole (040E0373) section between 46.1 and 53.1 m depth.
No Aalterbrugge unit is reported in Geets et al. (2000) and also in the explanatory notes of the 1:50 000 map sheet 22 Gent, the Aalterbrugge unit is not reported (Jacobs et al., 1996).
From his extensive data review, Fobe (1995) even concludes that the Aalterbrugge layer does not exist as a separate facies and has been confused with lignitic rich zones occurring at different levels in the Vlierzele unit.
The Aalterbrugge unit is well documented and analysed by Van Simaeys (1999) in the Hijfte borehole 040E0373 in which it is 7m thick. Because of this thickness the member rank is justified for the Aalterbrugge Member. A several meter thick similar unit was also exposed during the construction of the pedestrian bridge over the E40 motorway in Wetteren.
Boundaries
The section in De Moor & Geets (1973, fig.4) suggest an erosive base into the underlying Vlierzele Sand. Also Hacquaert (1939) reports intraformational clasts at the lignite level, suggesting erosion during the complex formation. Also Steurbaut (2015 p132) suggests that with the regression after the Vlierzele Sand deposition gullies were formed in the area of Aalterbrugge. Maréchal & Laga (1988, p 119) attribute a bed status to the Aalterbrugge layer between the Gent Fm and the Aalter (at that time named Knesselare) Formation and note that the transition between the Aalterbrugge member and the overlying marine Aalter Sand is continuous. Jacobs (2015, fig.3.21 p 137) shows the Aalterbrugge unit in Wetteren as a continuous transition between the Vlierzele and Aalterbrugge unit, but reports that the top of the Aalterbrugge unit is eroded.
Regional Occurrence
The Aalterbrugge Member is most often reported between Aalterbrugge and Beernem (Jacobs, 2015). It was also described in the Hijfte borehole - 040E0373 northeast of Gent. A recent outcrop along the E40 in Wetteren also showed the presence of the Aalterbrugge Member.
Roche (1988-1991, p 375) and DeConinck (1988-1991, fig. 9 p 304) report the presence of the Aalterbrugge Complex in the boreholes Kallo - 027E0148 (level 203 m) and Woensdrecht (NL) (level 385 m).
Stratotype
Sections along the Gent-Brugge canal (Hacquaert 1939 section).
Parastratotype in the Hijfte borehole (040E0373) section between 46,1 and 53,1 m depth (Van Simaeys, 1999).
Remark
The ‘Aalterbrugge Member’ of the Hijfte borehole (Van Simaeys, 1999) contains isolated records of the freshwater fern Azolla sp. which occurs massively in the North Sea and even the Atlantic Ocean at the base of chron C21r (Vandenberghe et al., 2004).