2.7.3. Tielt Formation - 01/2017
Tielt Formation
Authors
Geets (1988b), Steurbaut (1998).
Description
This marine unit consists in general of a very fine sandy, coarse silt and clay.
Stratotype: at its base the formation is defined by the top of the Aalbeke Member (see Aalbeke Member). In the compendium this boundary is placed at 48.5m in the Tielt 053E0061 borehole (see also Aalbeke Member). Steurbaut (1998) defined a boundary in the Tielt 068E0169 borehole in which the Aalbeke top was located at 71 m (see also Geets, 2000) which is different from the present definition (see Aalbeke Member). Steurbaut (1998) correlated the in-the-present-text defined top of the Aalbeke Member with the top of his unit D in the Kortemark silt Member (sensu Steurbaut 1998) of the Tielt 068E0169 borehole located at 46.7 m ( Sheet 21/6 (Wakken). Co-ordinates: x =76439, y = 187576, z = +48 m).
The upper boundary is placed at the base of the Egem Mbr in the "Ampe/Egem" quarry - 053W0060 (see Steurbaut, 1998 Figs 5,11). Sheet 21/1 (Wingene). Co-ordinates: x = 70.150, y = 190.150, z = +44 m.
Area
The western and northern part of Belgium. The formation outcrops in the north of Hainaut, the south and the centre of East- and West-Flanders and the western and southwestern part of Brabant. Outliers occur in the Mons Basin and south of the river Sambre. The regional distribution map of the Tielt Formation is figured in Maréchal, R. (1993, p 222), Walstra et al. (2014) and in https://dov.vlaanderen.be
Thickness
Maximum 25 m in the centre of the outcrop area. It decreases to the south and the east, and probably to the north.
Members
The formation is subdivided into the Kortemark Mbr and the Egemkapel Mbr.
Age
Middle to Late Ypresian.
Remarks
The formation is also discussed by De Coninck (1973), De Moor & Geets (1973), Geets (1979), Laga et al. (1980), Maréchal (1993), Steurbaut (1988), Steurbaut & Nolf (1986).
On the position of the Egem Member
The Egem Member, traditionally ranked into the Tielt Formation (see a.o. also 1:50 000 map legend), has in this review been ranked in the Hyon Formation. The Hyon Formation has been introduced in the literature by Steurbaut (1998, p 115) and described in the review by Geets et al. (2000) but was not retained in the official NCS stratigraphy by Laga et al., (2001). The grouping of the Egem Member in the Hyon Formation has been suggested by Steurbaut (2011) applying the logic to group the sandy deposits, like the Egem Member, in the Hyon Formation and the clayey deposits like the Kortemark and Egemkapel Members in the Tielt Formation. This definition is also practical when no distinction can be made between the sand members (Egem and Mont-Panisel) of the Hyon Fm, as is the case in the subsurface occurrence in northeast Belgium.
Kortemark Member
Authors
Steurbaut & Nolf (1986), Steurbaut (1998), Geets (1988), Geets et al. (2000).
Description
A grey silty clay unit with sandy layers of several dm thickness have been observed near the base. The presence of silt and sand is distributed in layers of cm to dm. Several subunits can be distinguished as proposed by Jacobs et al. (1996a, b) and Steurbaut (1998). The Kortemark Member occurs between heavy clay units: the Aalbeke Member below and the Egemkapel Member above. The maximal thickness is about 25 m (Geets et al., 2000).
In the top of the underlying Aalbeke Member a gradual coarsening upwards occurs, ended by a sharp coarsening that marks the start of coarser sediments in the Kortemark Member (see analyses from Geets (1991) and interpreted in Steurbaut (1998)). In geophysical log patterns the start of the coarsening upwards interval in the Aalbeke Mbr above its very clay-rich main part, as well as the sharp coarse shift at the top of the coarsening upwards part of the Aalbeke Mbr which marks the position of a fine sand layer, can be observed fairly consistently (e.g. Torhout 052E0195, Tielt 053E0061, Gent 55W1020, On-Kallo 1 014E0355). The formal boundary between the Aalbeke and Kortemark Members is drawn at the position of the major grain-size shift and the income of the first fine-sand layer (correlation profiles in Welkenhuysen and De Ceukelaire, 2009). This boundary definition at the base of the lowest fine-sand layer has the advantage to correspond to an observable horizon with water outflow in the upper part of the Desimpel clay pit in Kortemark (Steurbaut, 1998, Fig.5).
Detailed lithological analyses of the Kortemark Member sections in the Tielt borehole 068E0169 and the Kortemark and Egem extraction pits, are figured in Geets (1991) and Steurbaut (1998, p 117). Details in the geophysical well log signature in the Kortemark Member can be correlated between wells, especially the significant higher values of the resistivity, standing for more sandy layers, can be correlated between the different logs.
Regional occurrence and previous names
The Kortemark Member occurs north of Kortrijk and in particular in the west of Flanders where it can reach 25 m thickness. It is also known towards the east and northeast of Flanders (Antwerp Province) where it becomes thinner.
In the southeast of East Flanders and the neighbouring eastern Brabant provinces, the Mont-Panisel Member overlies the Aalbeke clay Member in Kerksken (086E0340) and Kattem (087W0479) (Geological Service Company, 2003), implying the disappearance towards the east of the Kortemark Member and the Egemkapel Member (see also Mont-Panisel Member). 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.
In the 1:40 000 geological map legend the Kortemark Member is included in the Yc unit and in the Stratigraphic Register of the Conseil Géologique(1929) and the Aardkundige Raad (1932) in the Y1a division.
In the Bolle & Jacobs (1993) nomenclature the unit Yd1c unit is tentatively correlated to the Kortemark Member. In the present review the Yd2 unit of these authors, a 5 m densily packed fine glauconitic sand underlying the Egemkapel clay Member, is also included in the Kortemark Member, notwithstanding its resemblance to the Egem Sand above. (see also Egemkapel Member)
Stratotype
Steurbaut (1998) has proposed the level of about 71m below surface in the Tielt borehole (068E0169); map sheet 21/6 x=76.439; y=187.576; z=48) for the lower boundary with the underlying Aalbeke Member. However, in-the-present-text the base of Kortemark has been replaced at a level in the Kortemark Desimpel quarry corresponding to the level at 48 m depth in the Tielt borehole according to the log interpretation Tielt 053E0061 in the compendium. Indeed because the top of the Aalbeke Member gradually becomes siltier upwards (see analyses in Steurbaut, 1998 fig. 5) it has been argued in-the-present-synthesis that the first marked sandy layer in the Desimpel clay pit in Kortemark (marked as ‘ sharp junction waterflow’ at the base of subunit C in Steurbaut 1998 p 117) (map sheet 20/3-4 Kortemark-Torhout, x= 57.050,y= 190.400, z= +16m) is a more easily recognisable lithostratigraphical horizon to mark the base of the Kortemark Member. In the present review this level is chosen as the formal boundary between the Aalbeke and Kortemark Members (see discussion in Description above).
The top of the Kortemark Member has during many years (80’s and 90’s) been exposed in the classical Egem extraction pit – 053W0060 (map sheet 21/1, x= 70.150, y= 190.150) as an erosive contact with the overlying Egemkapel (see Steurbaut, 1998, p 117).
Geophysical borehole references
Reference boreholes with geophysical log pattern of the Kortemark Mbr between the Aalbeke and Egemkapel clay Mbrs are in the outcrop area of the unit : Tielt - 053E0061, Kruishoutem – 084E1412, Gent – 055W1020, Torhout 052E0195, Pittem -053W0073 and also Knokke – 011E0138, Rijkevorsel – 007E0200, Kallo – 014E0355.
Egemkapel Member
Authors
Steurbaut (1998), Geets et al. (2000).
Description
A thin heavy clay unit of about 6m thick, contrasting with underlying silty to sandy clays of the Kortemark Member and the sandy overlying deposits of the Egem Member. The unit is thinner than the Aalbeke Member. In the Egem Quarry the unit has an erosive basis with a characteristic lag deposit of fossils, mainly fish remains but also snake vertebrae and bird bones and even a rare mammal tooth (Steurbaut, 1998; Smith & Smith, 2003,2013); also, a thin transgressive sandy layer, less than 1m thick, occurs just overlying the erosive basis and well expressed on some borehole logs. This thin basal lag sand is different from and should not be confused with the underlying sandy top of the Kortemark Member (the Yd2 unit, Jacobs et al., 1996a,b). Also the upper boundary with the Egem Member is erosive. The Egemkapel Member is a clay-rich unit, contrasting sharply with the more silty and sandy unit below (Kortemark Member) and above (Egem Member) as shown in core descriptions (see e.g. unit Yd3 in Jacobs et al., 1996a fig. 9), grain-size analysis (see Steurbaut, 1998 fig. 5;) and in the geophysical well pattern (see compendium).
Regional occurrence and previous names
In the legend of the 1:40 000 maps it was included in the top of the Yc unit.
Steurbaut & Nolf (1986) included the Egemkapel clay in the top of the Kortemark silt unit and Jacobs et al. (1996 a, b) in the Egem Member.
As a thin unit, the Egemkapel was only individualised as a separate Member when its consistent occurrence over the whole central Flanders north of the Mons area became obvious (see e.g. Walstra et al., 2014). The unit disappears towards the east of the East Flanders and Brabant but is still recognised in the Kallo wells 027E0148 & 014E0355 north of Antwerp and in the Rijkevorsel well – 007E0200.
Stratotype
The name Egemkapel refers to the hamlet where the Ampe – 053W0060 or Ampe/Egem extraction pit is located (map sheet 21/1, x= 70.150, y= 190.150). The clay unit has been exposed in this pit during a long time in the 80’s and 90’s, occurring between two erosive horizons: at its base with the underlying Kortemark Member and at its top with the strongly erosive base of the Egem Member of the Hyon Formation.
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
To define the Egemkapel Member on the geophysical logs the reference must be the Egem quarry section correlated by Steurbaut (1998,Fig.5) to the close-by Tielt borehole using grain size and log signature: the thickness is 4,5m with sharp boundaries and a very clay-rich core of 2-3 m; sharp boundaries are confirmed in geotechnical logs (Bolle & Jacobs, 1993;Jacobs et al., 1996a,b) an by grain size data in the BGD- Kallo borehole (Geets, 1990).
This logic was followed to identify the pattern in the reference boreholes, a thin marked GR and RES excursion, exemplary expressed in boreholes : Tielt - 053E0061, Kruishoutem – 084E1412, Gent – 055W1020, Rijkevorsel – 007E0200, Brugge – 023W0454, Torhout 052E0195, Pittem- 053W0073.
Mohammad (2009) and Van Marcke et al. (2005) included additional clay layers from the underlying Kortemark Member in their identified Egemkapel Member.