Kortrijk Formation

Authors

Geets (1988), Steurbaut (1998).

Description

The formation is an essentially marine deposit, consisting mainly of clayey sediments.

A standard sequence contains from bottom to top:

- an alternation of horizontally laminated, glauconiferous clayey sands or sandy clay, and compact, silty clay or clayey silt, locally bioturbated. The base consists of oxidized and indurated clayey sand, with lenses of pure sand (Mont Héribu).

- a homogeneous deposit of very fine silty clay, with some thin intercalations of coarse silty clay or clayey, very fine silt (Orchies Mbr);

- a less homogeneous deposit of clayey, coarse or medium silt, with some sand containing layers; fossil rich layers occur; the whole deposit becomes more sandy to the east and the south (Roubaix Mbr);

- a very fine and compact clay and in addition also silty clay parts (Aalbeke Mbr).

To the east, in the Brabant and the Campine, and towards the Mons basin, the deposits become more sandy.

Stratotype

The formation is defined by boundary stratotypes in Steurbaut (1998). The lower boundary stratotype is placed at 288 m depth in the Knokke borehole (011E0138) at the base of the Het Zoute Mbr. Sheet 5/6 (Westkapelle). Co-ordinates: x = 78.776, y = 226.370, z = +4.91 m. The upper boundary has been placed in the Tielt cored borehole (068E0169) at the top of the Aalbeke Mbr. This cored borehole however has no geophysical well logs; therefore the top of the Aalbeke member is picked at the nearby borehole Tielt 053E0061 with geophysical well logs, shown in the compendium, but unfortunately not cored. This upper boundary is located at 48.5 m in the compendium borehole Tielt (053E0061); in earlier versions (Geets, 2000), the Aalbeke top was mislocated at 71 m. Steurbaut (1998, p117) correlated the in-the-present-text defined top of Aalbeke Member (see also further details under Aalbeke Mbr) with the top of his unit D of the Kortemark silt member (sensu Steurbaut (1998)) in the Tielt borehole located at 46.7 m. Sheet 21/6 (Wakken). Co-ordinates: x =76439, y = 187576, z = +48 m.

Area

the formation is found in the western and central part of Belgium. It outcrops in the north of Hainaut, the southern and central part of West-Flanders, the south of East-Flanders Flanders and the southwest of Brabant. Outliers occur in the Mons Basin and south of the river Sambre.

The regional distribution map of the occurrence of the Kortrijk Formation in Belgium is figured in Maréchal (1993, p 221) and Walstra et al. (2014) and can be consulted at the D.O.V. website (dov.vlaanderen.be).

Thickness

125 m in the northern part of West-Flanders, but the thickness decreases in eastern and southern direction.

Members

The formation is subdivided into the Het Zoute Mbr, Mont Héribu Mbr, Orchies Mbr (Lower, Middle, Upper), Roubaix Mbr and the Aalbeke Mbr.

Age

Early and middle Ypresian.

Remarks

The formation is also discussed by Cornet (1874), De Ceukelaire & Jacobs (1998), De Coninck (1975), De Coninck et al. (1983), de Heinzelin & Glibert (1957), De Moor & Geets (1975), Geets (1990), Gosselet (1874), Gulinck (1965, 1967), Gulinck & Hacquaert (1954), King (1990), Laga & Vandenberghe (1980), Maréchal (1993), Ortlieb & Chelloneix (1870), Steurbaut (1988), Steurbaut & Nolf (1986), Vandenberghe et al. (1991) and Wouters & Vandenberghe (1994).

Het Zoute Member

Authors

Based on King (1990), Steurbaut (1998),Geets et al. (2000).

Description

Silty to sandy clay, bioturbated and with irregular pockets and lenses of very fine silty sand. Fine grained mica, woody debris and glauconite are present in sieve residues throughout the unit (King, 1990). Coarse grained angular to subangular grains are identified as degraded volcanic ash. Pebbles occur in the base of the overlying clay.

Regional occurrence and previous name

The Zoute Member is a thin unit of almost 5 m thickness found at the base of the Ieper Group section in the Knokke borehole (011E0138) at the Zoute hamlet and first described in detail by King (1990, p70) and at that time named Member X by this author. The name Het Zoute Member was proposed by Steurbaut (1998, p110). It was erroneously interpreted as Mont- Héribu Member by Geets & De Geyter (1990, p25).

This unit has no equivalent in other sections of the Ieper Group in Belgium where it corresponds to a hiatus between the Landen and Ieper Groups; this is confirmed in Steurbaut (2006, p77).

The volcanic ash

The indication of volcanic activity is a particular property of this unit. The other indication of volcanic grains in the basal sediments of the Ieper Group clays are the heavy mineral types in the basal clays, identified as Mont-Héribu as reported by Geets (1993).

This volcanic activity is related to the ash series at base of the Eocene in the North Sea Basin and correlates to the A1 Division of the London Clay Formation (King, 1990, p 80).

Stratotype

Knokke borehole 011E0138, interval 288 to 284,1 m depth. Geological Map 5/6 (Westkapelle), coordinates: X = 78.776, Y = 226.370, Z = + 4,91 m.

Mont-Héribu Member

Authors

De Coninck et al. (1983, p 98), Steurbaut and Nolf (1986, p 123), Steurbaut (1998), Geets et al. (2000),

Description

Alternating horizontal laminae of glauconite bearing clayey sand or sandy clays with compact silty clays or clayey silts. Locally burrows are present. The base of the unit consists of cemented clayey sand and lenses of just sand. The unit occurs at the very base of the Ieper Group, except where the Zoute Mbr is present.

The definition of the Mont-Héribu is limited to the sandy base of the Ieper Group. This sandy base is 6 m in the Mons Basin, maximal 10 m southwest of Brussels but in most boreholes it is limited to 1 to 2m and rarely noticed in most boreholes (see for example sections in Gulinck, 1967). Therefore the interpretation of the extension of the Mont-Héribu Member in the 1:50 000 mapping of Flanders is exaggerated and comprises for a large part the overlying Orchies Member. The definition refers to a grain-size distribution with only a limited clay fraction and a coarser fraction that gradually evolves upward over a short distance to the larger clay content of the Orchies Member. This short pattern of rapidly fining upwards is the typical signature on GR and RES logs (see borehole logs ON-Kallo-1 014E0355, Rijkevorsel 007E0200).

On geophysical well logs

In some logs of the compendium, the very short change-over interval from sandy to clay deposit as observed on RES, GR logs could be considered as Mont Héribu Member although generally it is considered too short to be individually marked. In the ON-Kallo well, core control allows to identify 1.30m of silty clay interpreted as Mont-Héribu Member at the base of the Ieper Group overlying the Tienen Formation at 401.35m (core) depth (Mohammad, 2009).

Regional occurrence and previous names

The unit was first reported as ‘Argile de l’ Eribus’ (Cornet, 1874, p 567) at the locality Eribus (‘Mont de l’Heribu’, south of Mons) which geology was studied by Ortlieb & Chelloneix (1870, p 168). In the Mons Basin the unit can reach up to 6m and; its maximum thickness is reported from Bierghes (southwest of Brussels) where it reaches 10m (Geets, 1991, including grain-size data). In many borehole descriptions this unit is not formally recognised as an individual unit, or supposed to be reduced to just a few cm thickness; King (1991) interprets the occurrence of the Mont-Héribu Member in central West Flanders and not in the Knokke well 011E0138.

What is mapped as Yb on the Geological maps 1:40 000 logically corresponds to the Mont-Héribu Member. In the Stratigraphic Register by the Conseil Géologique (1929) and translated by the Aardkundige Raad (1932), it is included in the Lower Ypresian Y1a.

Stratotype

Sand pit at the Mont de l’Héribu south of Mons between +57.5 en +51.4 m topographic height on the geological map 151 Mons-Givry (topographic map 45/7).

Coordinates: X = 119.750, Y = 124.510, Z = + 57,5 m.

Orchies Member

Authors

Gosselet (1874, p 611), Steurbaut (1998), Geets et al. (2000)

Description

Compact and heavy stiff bluish-grey clay occurring at the base of the Ieper Group only separated from the base itself by the underlying sandy Mont-Héribu Member where this latter is present. The Orchies Member is overlain by more sandy or silty clay deposits of the Roubaix Member or Mons-en-Pévèle Formation. The thickness can be up to 40 m. A pebble layer has been reported occasionally at its base (Ya on the 1:40 000 geological maps).

Whereas in the visual description of macroscopic samples, even from cores, it is very hard to see any further lithological subdivision of the Orchies Member, the geophysical log signatures (see reference boreholes in compendium) do show a systematic variability interpreted as grain-size and mineralogical variations.

The top of the very high gamma-ray section at the base of the Orchies Member, about 10 to 15 m thick, is a correlatable surface (see also further Geophysical logs and Subdivisions). It corresponds to the top of the mistakenly named Mont-Héribu Member (KoMh) unit in the correlation figures in Welkenhuysen and De Ceukelaire (2009 figs 12,14,16,18, 20, 22, 24) and approximately to the level 1 in the plates in Vandenberghe et al. 1991).

The top of the Orchies clay Member/ base Roubaix Member or base Mons-en-Pévèle Formation, is defined by the top of the very clay-enriched sediment. This level could be identified with confidence in descriptions of a series of destructive boreholes by G. De Geyter (1990, Archives Belgian Geological Survey) as the transition from silty clay above to heavy clay below. (courtesy Marleen De Ceukelaire).

Regional occurrence and previous names

The Orchies Member consistently occurs where the Ieper Group occurs in Belgium. In the Hainaut area thickness is between 10-16m whilst in central Flanders and north Belgium thickness can be over 40 m. Towards the east in Brabant its thickness is reduced to a few meters.

Originally the name was introduced by Gosselet (1874, p 611) to indicate the compact and stiff clays at the base of what is now known as the Ieper Group sediments; later, as a refinement of the lithostratigraphy, the sandy and silty Mont-Héribu Member at its base was individualised as a separate unit and the name Orchies Member was reserved for the compact heavy clays above the Mont-Héribu Member (Steurbaut, 1998). The later introduced name Saint-Maur Member (Belgian stratotype area, Geets, 1988; Maréchal, 1993), used in the legend of the 1:50 000 mapping in Flanders is a synonym of the Orchies Member although it (Saint-Maur unit) was generally used in a more restrictive way, because the lower part of the Orchies Member was erroneously assigned to the Mont Héribu Member; it is preferred to maintain the original name Orchies, a small locality to the southeast of Roubaix in Northern France.

On maps 1:25 000 of the Brabant Wallon (Nivelles-Genappe, Braine-le-Comte -Féluy) the ‘Formation de Carnières’ is used for a unit ‘close to Orchies’.

On the legend of the geological maps 1:40 000 the Orchies Member was included in the Yc clayey deposits and in the stratigraphic register (1929, 1932) in the Y1a.

The ‘argilite de Morlanwelz’ is a lateral equivalent of the Orchies Member (Steurbaut, 1991).

The Wardrecques and Bailleul members are reported in King (1991). The lower part of the Wardrecques member belongs to the Orchies Member whilst the upper part and the Bailleul member corresponds to the Roubaix Member (King, 1991). This subdivision is not commonly used in the literature but the position of Wardrecques and Bailleul members is well documented in boreholes of the Moeskroen-Kortrijk-Marke-Ooigem area by King (1991, fig. 11). In this area at least 5 glauconiferous beds occur, each less than 15 cm thick (King, 1991).

Stratotype

The Wahagnies clay pit (“Briquetterie de Libercourt”) in northern France, map sheet XXV-5 (Carvin). Ortlieb & Chelloneix (1870, p25) had already used the name ‘Argile de Wahagnies’ to indicate the Orchies Member compact clays (Steurbaut, 1998). In the clay pit, the base is defined by the basal pebble bed below about 8 m of stiff clays. Coordinates: X = 649.250, Y = 310.600, Z = +50 m.

The upper boundary, marking the limit with the overlying Roubaix Member, is proposed in the Kallo well 027E0148 (Gulinck, 1969) at 341m depth (Steurbaut, 1998, p 112) (see also below).

Geophysical borehole references and Subdivisions of the Orchies Member

Several horizons in the Orchies clay Member can be defined and correlated with reasonable confidence across the whole area of occurrence of the Orchies Member.

The base of the Orchies is defined by the appearance of very clay enriched sediments and a corresponding rapid installation of a high GR level.

The top of the Orchies clay Member/ base Roubaix Member or base Mons-en-Pévèle Formation, is defined by the top of the very clay-enriched sediment. This level could be identified with confidence in descriptions of a series of destructive boreholes by G. De Geyter (1990, Archives Belgian Geological Survey) as the transition from silty clay above to heavy clay below. (courtesy Marleen De Ceukelaire). On the geophysical logs, this level can most easily and reproducibly be picked below the base of the first marked sandy excursion on RES logs (labeled 6 in the compendium) and the corresponding sharp drop in GR.

This defined top of the Orchies Member occurs at a short distance above the Orchies/Roubaix boundary defined by Steurbaut (1988) and labeled OR ES88 on the logs in the compendium where appropriate; this OR ES88 boundary was put at 331,5m in the BGD-Kallo borehole. However, this last level is a thin sandy layer systematically occurring close to the now defined top of the Orchies Clay Member, with a corresponding small log signature identifiable in GR and RES. This thin sand layer and corresponding geophysical log signature, can also be traced in the cored Mol SCK15/1974 borehole described by Gulinck & Laga (1975) just below the Mons-en-Pévèle Formation. This observation leads to the conclusion that the base of the Roubaix Member and the Mons-en-Pévèle Formation develop at about the same moment in that part of the basin.

In 1998 Steurbaut (1998) picked another boundary between the Orchies and Roubaix Members in the BGD-Kallo well 027E0148 at 341m instead of at 331,5m as before in Steurbaut (1988). The log signature associated with this 1998 definition, labeled OR ES98, can easily be recognised as it is plotted by Steurbaut (1998, Fig.10) on a series of logs including the Rijkevorsel well 007E0200 also included in the compendium; it occurs at a marked GR low , slightly more than about 10m below the top of the Orchies Member.

Therfore it is agreed to keep this level as a formal subdivision of the Orchies Member, defining the Upper Orchies Member between this level and the top of the Member.

The lower part of the Member with stable very high GR readings can also be reliably delineated by the top of this high GR reading, where the signal recedes to lower readings. This level defines the boundary between the Lower Orchies Mmeber below and the Middle Orchies Member above it. The OR ES98 level , defines the boundary between the Middle and Upper Orchies Member.

Still other horizons have a reliable correlation potential as illustrated in Steurbaut (1998, Fig 10 ), Vandenberghe et al. (1991), Van Marcke et al. (2005) and others. For example a change in GR gradient recognized in the ON-Kallo well 014E0355 in the compendium at 375m (Mohammad, 2009) can be correlated with the 416,5m level in the ON-Doel well, in which it is named the Orchies/Roubaix boundary (Van Marcke et al., 2005). However such horizons are not used to further officially subdivide the Orchies Member.

Roubaix Member

Authors

Gosselet (1874), Steurbaut & Nolf (1986, p 123), Steurbaut (1998), Geets et al. (2000).

Description

In contrast with the underlying (Orchies Member) and overlying (Aalbeke Member) compact heavy clays, the Roubaix Member consists of more silty to fine sandy calcareous clays. The thickness varies from about 40m in south Belgium to 60m in North Belgium. Calcareous fossils like nummulites and molluscs are present. Glauconite-rich horizons occur. The more heterogeneous composition of the sediment is shown by layering (see e.g. Marke quarry in Steurbaut, 2006 fig. 7), also well visible in the geophysical well logs.

Several of these specific layers, labeled 1 to 6 in the log examples in the compendium, can be recognised and correlated between well logs with a reasonable degree of confidence. Based on the correlation of these levels, Welkenhuysen and De Ceukelaire (2009) have selected a specific level as the boundary level between the Orchies and the Roubaix Member which corresponds approximately to the position of the Steurbaut (1988) definition.

On the geophysical logs in the compendium, this level can most easily and reproducibly be picked below the base of the first marked sandy excursion on RES logs (labeled 6 in the compendium) and the corresponding sharp drop in GR.

 Regional occurrence and previous names

The Roubaix Member occurs over northwest France, north Hainaut, east and west Flanders. Towards the south the occurrence of sandy layers becomes more pronounced whilst to the northwest the Member becomes more clayey and hardly distinguishable from the underlying Orchies Member (Geets et al., 2000). Towards the southeast and the east the Roubaix Member evolves into a fine sandy unit, the Formation of Mons-en-Pévèle (see further).

The later introduced Moen Member (Belgian stratotype area, Geets, 1988; Maréchal, 1993) used in the legend of the 1:50 000 mapping of Flanders, is synonymous with the Roubaix Member. Roubaix is a town in North France and was the original reference for this clay type as described by Gosselet (1874) and therefore the name Roubaix Member is retained.

In the 1:40 000 mapping the Roubaix Member was mapped in the Yc unit, however in the Kortrijk area it was erroneously mapped as Yd (Steurbaut & Nolf, 1986; Geets et al., 2000). In the Stratigraphic Register of the Conseil Géologique (1929) and the Aardkundige Raad (1932), the Roubaix Member is included in the Y1a unit.

Stratotype

The Roubaix Member was previously exposed along the Bossuit Canal at Moen (near Kortrijk) (Steurbaut & Nolf, 1986) and in the Marke and Heestert clay pits near Kortrijk. As these outcrops are no longer accessible a reference section for the lower boundary is choosen in the Kallo well (Gulinck, 1969) at 341m depth (see further) whilst an upper boundary with the overlying stiff clays has been visible in the former Kobbe clay pit (DOV kb29d97e-B989) at Aalbeke (x= 68.450, y= 164.300, z= 49 m) Steurbaut (1998).

Geophysical borehole references

The base of the Roubaix Mbr has been defined in the literature in 2 different ways by Steurbaut (1988, 1998). In the Kallo log (014E0355) (Steurbaut, 1988) the boundary between the Orchies and Roubaix Members is put at the top of heavy clay at 331,5m whilst in the 1998 definition the boundary is put 10m lower at 341m. The correlation between the Kallo well 027E0148 (without geophysical logs) and the ON-Kallo-1 014E0355 with geophysical logs (courtesy Peter Stassen) allows to identify the log signatures associated with the two definitions. The 1998 definition is also plotted on a series of logs by Steurbaut (1998, Fig.10) located in West-Flanders but also on the Rijkevorsel well – 007E0200. Therefore the two boundaries can systematically be indicated as OR ES 88 and OR ES 98 on ON-Kallo-1 – 014E0355 and Rijkevorsel – 007E0200 and on many other borehole logs in the compendium (see also Mons-en-Pévèle Fm).

The boundary level between the Orchies and Roubaix Members as defined above (see Description) corresponds to the level selected by Welkenhuysen and De Ceukelaire (2009, e.g. in the Merchtem and the Gent boreholes).

Aalbeke Member

Authors

De Moor & Geets (1975), Steurbaut & Nolf (1986), King (1991), Steurbaut (1998)

Description

A very compact heavy clay without sand fraction of some 10 m thickness sharply contrasting with more silty to fine sandy overlying (Tielt or Hyon Formations) and underlying units (Roubaix Member or Mons-en-Pévèle Formation). The Aalbeke Member is mostly non calcareous. Small pale brown to yellow phosphate nodules are common in the Aalbeke Member.

It can be pointed out that this pure clay unit is relatively thin and therefore can be mistaken for other even thinner clay units above, namely the Egemkapel and the Merelbeke units. To unequivocally identify these different clay-rich layers, a complete vertical succession is often required or support by micropaleontological characterisation.

In most geophysical log responses the lower boundary of the Aalbeke Member is sharply marked; at present there is no field outcrop of the contact between the Roubaix and Aalbeke Members.

It is strongly suspected that the top of the Aalbeke Member is an erosive contactas it is overlain by different lithological units in different areas: in clay pits in Aalbeke, it is overlain by the Mont-Panisel Member of the Hyon Formation, in central Flanders by the Kortemark Member, and in SE Flanders and Brabant by the Hyon Formation. Also, at the base of the overlying Kortemark Member in the De Simpel clay-pit erosion can be observed (Steurbaut, 1998; Vandenbergh et al., 1998). The upper boundary can be sharp (e.g. Kerksen borehole 086E0340 in compendium, data Geological Service Company; Brugge - 023W0454) or more generally the upper part of the clay unit shows a gradual coarsening upward. In the latter case, the upper boundary of the Aalbeke Member in contact with the Kortemark Member is put at the top of this coarsening upwards section.

Regional occurrence and previous names

The Aalbeke Member is exposed in the hills around Kortrijk, where also the type locality Aalbeke is located, and in the adjacent border area of north France where it corresponds to the ‘argile de Roncq’ (see De Coninck, 1991 fig.9). It occurs in the subsurface of the whole Flanders and has an average thickness of about 10 m varying between 5 and 15 m.

On the geological maps 1:40 000 the Aalbeke Member was part of the Yc unit and in the Stratigraphic Register of the Conseil Géologique (1929) and Aarkundige Raad (1932) it is part of Y1a. In the Kortrijk area, on the 1:40000 sheets it was mapped erroneously as the ‘P1m’ unit (Merelbeke Member of the Gentbrugge Formation).

Stratotype

Several clay pits exist in Aalbeke and the De Witte clay pit, the extension of the now filled-up Kobbe clay pit – DOV kb29d97e-B989 (X = 68.450, Y = 164.300, Z = + 49 m), designated as stratotype by Steurbaut (1998) (map sheet 29/5-6 (Mouscron - Zwevegem), is the logical new unit stratotype locality.

Geophysical borehole references

Exemplary log signatures with the identification of a base and a top of the Aalbeke Member are the boreholes logs of Gent 055W1020, Kallo 014E0355, Merchtem 072E0229, Pittem 053W0073, Rijkevorsel 007E0200, Torhout 052E0195, Wieze 072W0159.

‘Pink silt’ bed

Within the Aalbeke Member outcrops in the Kortrijk area a pronounced pinkish silty layer of some dm thickness occurs. It might serve as a stratigraphic marker bed. However the bed is not given an official bed status as it is not yet established that only one such layer occurs in a complete Aalbeke Member section.