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Kieselooliet Formation v012016

Kieseloolite Formation (emended), revised by M. Dusar, 4.1.2016


Name: The name is derived from silicified oolithic limestones outcropping around the Vosges massif in Alsace-Lorraine (France), rarely found as pebbles in the Meuse and Rhine gravels. The name Kieseloolith has been introduced for the Lower Rhine graben by Kaiser (1907), and used in the Netherlands soon after (Tesch, 1908). The Kieseloolite (most common translation for Kiezeloöliet) Formation has been formally described by Westerhoff (2003). In Belgian lithostratigraphy, the ‘Kiezeloöliet Formation’ (Laga et al., 2001 after Sels et al., 2001, not translated) is in need of revision after the H3O project (Deckers et al., 2014).


This formation has a peculiar name. Silicified oolithic limestone pebbles, supposedly originating from Jurassic limestones of the Upper Meuse valley, were described as characteristic (although very rare) for the ‘Onx’ bed, corresponding to the uppermost Meuse river terrace of the ‘Trainée mosane’, renamed Liège Formation (Paepe & Vanhoorne, 1976; Gullentops et al., 2001). Exceptionally, silicified oolithic limestone pebbles are also found in gravelly downstream deposits. A recent find of silicified oolithic limestone of Dinantian age and provenance from the Dinant-Namur Basin in the Zutendaal gravel on the Campine Plateau may require to revise the Jurassic ‘Kieseloolite’ hypothesis (Roland Dreesen, personal communication). The formation name Kieseloolite could be considered ill-chosen but this is considered insufficient reason to abandon it.



In the German Lower Rhine Graben the Erft Block has been designed as the type area for the fluvial facies of the Kieseloolith Schichten (Klostermann, 1992).

In the Netherlands, borehole 60D1033 at Broeksittard, depth interval 19,00 – 201,00 m below ground level has been designed as reference section (van Adrichem Bogaert & Kouwe, 1993), upgraded to lectostratotype by Westerhoff (2003).


Belgian parastratotype: Exploration borehole Maaseik – Jagersborg, drilled 1980, published in 2005  by Vandenberghe et al.; GeoDoc 049W0220; kb18d49w‐B220 [ Lambert coordinates x 246636, y 200835, ground level (= zero level) +33,14 m TAW.

Depth interval: 62 – 166 m (depth according to Vandenberghe et al, 2005) or 63,20 – 164 m (depth according to log, Fig. 1).

Subdivided in two members:

Brunssum Member: 62 – 127 m in borehole Maaseik – Jagersborg (depth according to Vandenberghe et al, 2005) or 63,20 – 125,80 m (depth according to log, Fig. 1);

Waubach Member: 127 – 166 m in borehole Maaseik – Jagersborg (depth according to Vandenberghe et al., 2005) or 125,80-164 m (logging depth).


Note that Van der Sluys (2000), followed by Laga et al. (2001), Vandenberghe et al. (2005) had a different concept of the Kieseloolite Formation, which incorporated the Jagersborg Member (22 – 76 m, Fig. 2), largely equivalent to the Stramproy Formation, and also the Inden Formation (166 – 192,70 m, Fig. 2, or 164 – 190,50 m according to log, Fig. 1) in borehole Maaseik - Jagersborg. In their concept the Kieseloolite Formation sensu lato ranged from 22 to 192,70 m, which would give it group status compared to the proposed subdivision.

Note also that the proposed top of the Kieseloolite Formation sensu stricto does not coincide with the base of the Jagersborg Member, as defined by Van der Sluys (2000) and applied to the Maaseik – Jagersborg borehole by Vandenberghe et al. (2005). These authors included the clay beds at depth 62 – 72 m in the Jagersborg Member whereas in this proposal they form the upper clay unit of the Kieseloolite Formation sensu stricto. According to their interpretation the base of the Jagersborg Member coincides with the top of the Brunssum I clay at depth 76 m (Fig. 1).


Description (from Deckers et al., 2014; Dusar et al., 2014; based on Westerhoff, 2013):

The Kieseloolite Formation consists of fluvial and floodplain sediments, deposited by precursors of Rhine and Meuse. These rivers drained the uplifted Ardenno-Rhenish massifs, eroded and transported the strongly weathered cover deposits of the Tertiary peneplain towards the NW. The sediments therefore predominantly consist of quartz sands with very stable heavy minerals association. Time of deposition coincided with active rift tectonics, characterised by thick sequences of rather coarse sand deposits, interrupted by periods of sediment starvation characterised by widespread deposits of clay with peat in the floodplain.


Two units can be distinguished, which are assigned member status, from bottom to top:

- Waubach Member

Composed of quartz sand, whitish in colour and generally coarse-grained (150 – 2000 µm). The Waubach sand member is quite regular in distribution and thickness, on average 60 m. It corresponds to the Hauptkies in the Lower Rhine Embayment.

Lithified Waubach sand is outcropping in between the Geleen and Heerlerheide fault branches of the Feldbiss fault bundle, south of Neeroeteren, along the lower slope of the High Campine Plateau along the Bergerven road (Janssen & Dreesen, 2010). Waubach sands are poorly exposed in the adjoining former sand pit (now a leisure centre), where is has been extracted as ‘Neeroeteren Sand’ (Geys, 1972).

In geophysical well logs, this unit can be recognised by one or several coarsening upward cycles. On top these are succeeded by a medium-grained sand unit in the 10 to 20 m thickness range without granulometric trends, displaying a facies characteristic for the sand units incorporated in the overlying Brunssum Member. It is remarkable that this top unit, occurring in the interval 127-148 m in borehole Maaseik - Jagersborg has been tentatively named ‘Lower Pey Sand’ by Vandenberghe et al. (2005) - (Fig. 2). It is sedimentologically interpreted as a change from a braided river system to a meandering river system.

Clayey intervals are poorly developed, notwithstanding lowering of the resistivity curves. As a result the Waubach sand member is considered as a single permeable hydrogeological unit, at least in the rather small Belgian part of the Rur Valley Graben, of great importance as a productive and high capacity aquifer (Dusar et al., 2014).


- Brunssum Member

Clay, brownish to black when charged with organic material, slightly sandy to silty, associated with lignite layers, with fine to moderately coarse sand intercalations of rapidly changing thickness, from decimetric to decametric scale (Brunssum Clay, Doppert et al., 1975). The clay is generally compact and stiff, with thicknesses of the metric scale, hence of aquiclude value. Occasionally, light coloured and kaolinitic clay layers are observed.

In geophysical well logs, the Brunssum Member is characterised by the occurrence of clay layers, which constitute the resistivity minima, interspersed with permeable sand layers, with high resistivities. Two clay beds representing a resistivity minimum have been widely recognised in the area Bree – Maaseik, and were given member status, namely the Brunssum I and Brunssum II clay (in the Netherlands named upper and lower Brunssum Clay respectively, with bed status).


In drilling practice it is of utmost importance to recognise these clay layers, especially the lowermost Brunssum II clay bed (hydrostratigraphic code KI-k-3), because these separate the confined from the vadose aquifers (Fig. 3). Productive intercalated sand bodies between the Brunssum I and Brunssum II clay beds are named Pey Sand (Member) [common spelling in Belgium, formal Dutch spelling as Peij]. However, there is limited lateral consistency in these sand intercalations, which may rapidly change in facies and thickness, from 1 to 30 m. Similarly productive but younger sand layers between the Brunssum I clay and overlying clay bed have eroneously been associated with Peij sand, in piezometers north of Maaseik (Fig. 4).

Indeed, on top of the Brunssum I clay beds (hydrostratigraphic code KI-k-2), more clayey layers may be distinguished (KI-k-1 on Fig. 2), but rarely reach the resistivity minima over metric thickness, characteristic for the Brunssum I and Brunssum II beds. The number and/or relative thickness of clay layers increases towards the SE in the Belgian part of the Rur Valley Graben. Especially South of Maaseik, splitting of clay layers and lignite seams makes it difficult to draw consistent boundaries between clay-sand units and use these as a basis for lithostratigraphic subdivision. Therefore the lithostratigraphical splitting of the Brunssum Member has been abandoned, but the valuable information related to the sand – clay succession is retained in the hydrostratigraphic codes (Fig. 5).


Underlying strata:

In the area around Maaseik (and across the Meuse in the Netherlands), the Formation of Inden intervenes with comparable depositional environment and main lithological components to the overlying Waubach sands but with different lithofacies, which is dark coloured because of high organic content and a greater portion of fines. In any case, the contact with the marine Neogene deposits, grouped in the Formation of Breda, remains the most important boundary for mapping and hydrological purposes.

Towards the western part of the Belgian Rur Valley Graben, another transitional deposit occurs between the Kieseloolite Formation and the Diest – Kasterlee Clay Formations, which is provisionally assigned to the Kasterlee Sand Formation and laterally correlates with the Inden Formation.

Otherwise deposits of the Kieseloolite Formation are overlying those of the Diest - Kasterlee Clay Formations in Belgium or the Breda Formation in the Netherlands. The boundary is sharp, with a clear contrast between coarse, quartz-rich whitish sands of the Kieseloolite Formation and finer, glauconite-bearing grey-green sands of the Formation of Breda, and can be recognised instantly from the colour of the drilling mud.


Overlying strata:

The Kieseloolite deposits are covered by the deposits of the Stramproy Formation which are mostly sandy over the major Belgian part of the Rur Valley Graben. The sands of the Stramproy Formation are generally finer grained, the clay with lignite intercalations are less extensive or compact than those of the Kieseloolite Formation and do not act as hydrogeological barriers.

In the extreme south, between the Geleen and Heerlerheide faults, the Kieseloolite Formation is outcropping on the margin of the Campine Plateau (Neeroeteren – Bergerven) or is unconformably covered by the Meuse river terraces grouped into the Beegden Formation.


Area: The entire Belgian part of the Rur Valley Graben, north-east of the Heerlerheide to Reppel Faults (Feldbiss fault system) – (Fig. 6). To the west and southwest, on the upthrown side of the faults, contemporaneous deposits are assigned to the Formation of Mol (partim).


Thickness: The thickness of the Kieseloolite Formation in Maaseik – Jagersborg borehole slightly surpasses 100 m. Time of deposition coincided with high subsidence of the graben, creating accommodation space for sedimentation. On average the thickness varies between 75 m and 150 m but may reach 200 m in The Netherlands.


Age: Laga et al. (2001) indicate a wide age range, from Tortonian (late Miocene) to Gelasian (early Pliocene). However, their Kieseloolite Formation s.l. incorporates the Inden Formation, of Tortonian to Messinian age, and the Stramproy Formation, of presumed Gelasian age.

There is no direct age evidence for the Kieseloolite Formation in Belgium. Palynology-based chronostratigraphy (Brunssumian and Reuverian AB) is abandoned (Kemna, 2005; Kemna & Westerhoff, 2007; Donders et al., 2007). Palaeomagnetic dates from the Lower Rhine Embayment allow to constrain the age of the Kieseloolite Formation in Belgium (Fig. 7): Hauptkies is equivalent to Waubach Member, Rotton to Brunssum Member. According to Schäfer et al. (2005) the Hauptkies is considered Messinian (upper Miocene) to Zanclean (Lower Pliocene), the Rotton is Zanclean to Piacenzian (Pliocene). The top of the German Kieseloolith Formation is placed at the base of the Reuver Clay within the Piacenzian. Although likely subject to discussion about details this chronostratigraphic interpretation is robust and differs from the more simplistic assumption that the Kieseloolite Formation covers the entire Pliocene.



The Kieseloolite Formation forms the core of a lithostratigraphic sequence provisionally named ‘the continental Neogene Group’. This group further includes the Inden and Stramproy Formations inside the Rur Valley Graben and the Mol Formation to the west of the Rur Valley Graben in Belgium.


The lithostratigraphic subdivision of the Kieseloolite Formation was based on different principles. Originally, a palynological distinction was made between the clay beds outcropping at Reuver and Brunssum. This formed the basis for a twofold chronostratigraphic subdivision of the Pliocene into Reuverian and Brunssumian, applicable to the Kieseloolite Formation, which spans most of the Pliocene. However, this palynozonation is facies-dependent, even impractical for defining a biostratigraphic boundary. Its chronostratigraphic use is no longer supported (Kemna & Westerhoff, 2007; Donders et al., 2007). To add confusion, the same names are applied as lithostratigraphic beds which do not necessarily correlate with their chronostratigraphic homonyms.

Based on the same palynological interpretation, Vandenberghe et al. (2005) identified in borehole Maaseik – Jagersborg a Reuverian sequence and the Reuver Clay. According to current proposal in line with Dutch and German practice, the beds tentatively correlated with the Reuver Clay by Vandenberghe et al. (2005) are assigned to the Stramproy Formation, the underlying Reuverian is largely assigned to the Kieseloolite Formation.


Note that the Dutch informal unit of the Schinveld Sands encompass the sands in between the Reuver clay bed and the underlying ‘Reuverian’ strata down to the top of the Upper Brunssum Clay (= Brunssum I in former Belgian stratigraphy) - (Westerhoff, 2013). These correspond to the hydrostratigraphic codes SY-k-3 and KI-k-1 respectively on Fig. 1. Typical Schinveld Sand occurs at depth 55.5 – 63.2 m in borehole Maaseik – Jagersborg and is incorportated in the base Stramproy Formation. Note that on Fig. 3 the ‘Reuver clay’ unit (SY-k-3) is absent. A sand bed with typical Schinveld Sand characteristics occurs in borehole 48E0321 in between KI-k-1 and KI-k-2, thus in the top of the Kieseloolite Formation (Fig. 3, left). This discrepancy illustrates the need to abandon the informal classification that depends on palynostratigraphical clues.


Moreover, there is no straightforward correlation possible between the Reuver Clay type area on the eastern Peel horst or the Reuver Formation in Germany and the deposits assigned to the ‘Reuverian’ period in borehole Maaseik – Jagersborg: palynology is especially useful for defining the evolving ecosystem characteristics, suggesting rather than confirming correlations or evolution in time.

In the present proposal, transboundary geophysical well log correlation forms a solid base for connecting the Belgian and Dutch stratigraphy.


The Brunssum Member, as currently defined, contained a series of units which had informal member status in Belgium and bed-status in South Limburg, and which were widely utilised for interpreting water wells (cf Van der Sluys, 2000). These were from bottom to top (Fig. 5):

- Brunssum II clay (lower Brunssum bed in NL)

- Pey sand (also written Pey in Belgium)

- Brunssum I clay (upper Brunssum bed in NL)

The sequence is surmounted by Jagersborg / Schinveld (in NL) sand and clay beds, split between Stramproy and Kieseloolite Formations.


The clay beds in general and the Brunssum clays in particular are composed of several clay layers, with at least one plastic but rather stiff clay layer with minimal thickness of 1 meter able to act as a aquiclude, meaning a boundary layer in a hydrological system. Compact clay layers can be recognised by gamma-ray peaks and especially by the lowest resistivity values observed in the profile (Brunssum I, KI-k-2 or Brunssum II, KI-k-3 on borehole sections) – (Figs. 1, 3, 4).


These units retain their value as hydrostratigraphic units but can no longer be used as lithostratigraphic units because of correlation obstacles due to frequent splitting and wedging out of sand and clay layers. It is rather the more complex sequences which were recognised and interpreted in lithostratigraphic terms, but their boundaries are not consistent over the graben area. Nevertheless in the region of Bree – Maaseik (north of the Elen-Neeroeteren fault) this informal subdivision can be applicable. On the contrary, in the region of Elen – Rotem (between the Elen-Neeroeteren and Heerlerheide faults) at least the Brunssum I clay is easily confounded with the overlying Reuverian / Schinveld beds (southern boreholes on fig. 4).


The lithostratigraphical scale of Belgium established in 2001 (Laga et al., 2001) predates publication of the Memoir on borehole Maaseik – Jagersborg (Vandenberghe et al., 2005); instead it refers to the definition by Doppert et al. (1975) and the Dutch stratigraphical nomenclature (van Adrichem Bogaert & Kouwe, 1997), but deviates from the Dutch nomenclature by including the Jagersborg Member, named after borehole Maaseik – Jagersborg (Sels et al., 2001), which is now largely assigned to the Stramproy Formation. The lower part of the unit named ‘Waubach sand and gravel’ by Vandenberghe et al. (2005) at 166-193 m (surmounted by a clayey bed at 165 – 166 m) has been assigned to the Inden Formation. The whole unit is interpreted as a braidplain deposit; the fine fraction present in the Inden Formation but absent in the Waubach Member is at the basis for the distinction between both units (see ‘description’ for more detail).


The area south of the major graben boundary faults (from Peer to Opitter) contains fine sands which have been tentatively assigned in borehole descriptions either to the Kieseloolite – Waubach or Mol – Donk members. They have the characteristics of eolian sands blown out of the floodplain in the graben and deposited in the adjoining graben shoulder. No name has been introduced for this peculiar lithological unit.


The Mol Formation has been described as a lateral equivalent of the Kieseloolite Formation. As the boundaries of the Kieseloolite Formation have been emended, the correlation with the Mol Formation needs more precision. This correlation is based on geophysical well logs only (Fig. 9). Transition between typical Kieseloolite and Mol is gradual, though enhanced at the graben boundary faults. Indeed, in the region of Neerpelt – Bocholt (in between the Grote Brogel and Bocholt faults) the Brunssum Member already gradually wedges out (cf. Fig. 6 of Inden Formation file). In the Mol – Lommel area it is especially the younger strata which disappear (Fig. 10).


The correspondence between the lithostratigraphical units of the Mol and Kieseloolite Formations is not yet established. The following correlation is tentatively proposed but needs confirmation, from top to bottom:

Mol – Maatheide (MlMh)        Stramproy Formation

Mol – De Maat (MlMa)                       Kieseloolite Formation Brunssum Member

Mol – Donk (MlDo)                Kieseloolite Formation Waubach Member

Kasterlee Sand                        lateral equivalent of Inden Formation


This scheme is based on log interpretation by Verheyen (2003) but slightly emended. According to Verheyen & Vandenberghe (2003) De Maat Member corresponds only to the hydrostratigraphic unit KI-k-1 on Fig. 9, whereas in the present interpretation, it encompasses a larger interval extending downward till KI-k-3.


The proposed correlation accounts for the Plio-Pleistocene transition derived from palynological change between ‘warm’ and ‘cold’ floras, observed both in the Mol and in the Stramproy / Kieseloolite Formations and for the thinning of the Brunssum Member and increasing hiatuses between Kieseloolite and Stramproy Formations towards the west in the Belgian part of the Rur Valley Graben. The boundary between the Donk and Maat Members seems to correspond with the base of the Brunssum II Clay Bed (KI-k-3), supporting the correlation value of the latter unit.

The geographical boundary between Mol and Stramproy/ Kieseloolite Formations coincides with the main graben boundary fault, facilitating stratigraphic interpretation and mapping.



- A distinction must be made between the Kieseloolite Formation sensu stricto, as defined in this proposal, and a Kieseloolite Formation sensu lato, as previously defined by NCS but now split in Inden, Kieseloolite sensu stricto and Stramproy Formations.

- Borehole Maaseik – Jagersborg 49W0220 is chosen as stratotype and is essential for a proper understanding of the Kieseloolite problem.

- The former (informal) stratigraphic units with bed or member status, such as Reuver, Schinveld, Brunssum I, Brunssum II, cannot be retained as lithostratigraphic units because of limited correlation potential and/or palynological constraints. Instead two members have been defined with thicknesses around 50 m, which are the Brunssum and Waubach Members. Clay beds are important, however, because of their impact on the aquifer characteristics; they are retained as hydrostratigraphical units.

- The boundary between Kieseloolite and Stramproy Formations does not correspond to the former boundary between Jagersborg and Brunssum I Members of the Kieseloolite Formation sensu lato but is placed on top of a clay-lignite bed with strong areal persistence.

- West of the Rur Valley Graben, the Kieseloolite Formation is replaced by the Mol Formation – partim, because the upper part of the Mol Formation correlates with the Stramproy Formation.

- The chronostratigraphic age of the Kieseloolite Formation (and its lateral equivalents) ranges from late Messinian to late Piacenzian and is therefore Mio-Pliocene.


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Figure 1. The Belgian parastratotype for the Inden, Kieseloolite and Stramproy Formations, based on the stratigraphic study published in 2005 by Vandenberghe et al.: exploration borehole Maaseik – Jagersborg.

Figure 2. Synthesis of stratigraphic results of the Maaseik borehole (49W0220) from Vandenberghe et al. (2005), with indication of proposed lithostratigraphic boundaries (SY for Stramproy, KI for Kieseloolite, IE for Inden Formations) and hydrostratigraphic clay beds with REGIS denomination. Note the higher position of the top Kieseloolite Formation s.s. (boundary SY / KI) compared to the base of the Jagersborg / Schinveld sands, moved from top lignite 3 to top lignite 2. Note also that the Inden Formation was included in the Waubach Sands and that the proposed Waubach Member incorporates the Lower Pey Sand (marked ? on the graph).



Figure 3. Subdivision of the Kieseloolite Formation and boundary with overlying and underlying formations in two reference sections between Bree and Maaseik. The proposed lithostratigraphical subdivision splits off the Stramproy and Inden Formations, and introduces two members, Waubach and Brunssum for the remaining Kieseloolite Formation. High-quality cuttings were examined in detail to determine their aquiclude potential, leading to the recognition of two impervious hydrostratigraphic units, KI-k-2 (corresponding to the Brunssum I clay) and KI-k-3 (corresponding to the Brunssum II clay). They can be recognised by their resistivity minima. The overlying KI-k-1 unit is much richer in lignite.

Figure 4. SW-NE section between Rotem and Kessenich along the river Meuse in Belgium (Dusar et al., 2014 after Deckers et al. 2014).

Figure 5. Hydrogeological units, introduced by H3O project (Deckers et al., 2014). KI-k-1 corresponds to lignite 2 in Jagersborg member of fig. 2, KI-k-2 to former Brunssum I clay member, KI-z-3 to former Pey sand member, KI-k-3 to former Brunssum II member, KI-z-4 to informal Lower Pey sand of fig. 2, KI-z-5 to informal upper Waubach sand of fig. 2.

Figure 6. Subcrop map of Kieseloolite Formation in the Rur Valley Graben in Belgium (Dusar et al., 2014, after Deckers et al., 2014). Note that Mol Formation, occurring to the west of the graben is partially lateral equivalent of the Kieseloolite Formation.

Figure 7. Stratigraphic synthesis of the Lower Rhine Embayment giving chronostratigraphic dates for the successive formations of the Neogene (from Schäfer et al., 2005). The Kieseloolite Formation is marked by a red square and encompasses Hauptkies and Rotton up to the base of the Reuver Formation, to below the Reuver Clay.

Figure 8. Cross-sections across the Lower Rhine Embayment according to chronostratigraphic age, with lithostratigraphic units, lithofacies and Schneider & Thiele hydrostratigraphic codes (Schäfer et al., 2004).

Figure 9 Subdivision of the Mol Formation in the Mol – Lommel area, west of the Rur Valley Graben boundary faults (from H3O De Kempen project, unpublished; equidistance between the horizontal lines is 50 m). The upper part of the Mol Formation, east of the Rauw fault can be correlated to the Stramproy Formation. The middle part of the Mol Formation containing the clay – lignite beds marked KI-k-1 to KI-k-3 is assigned to the Brunssum Member of the Kieseloolite Formation. The lower part of the Mol Formation in between KI-k-3 and Kasterlee clay can be assigned to the Waubach Member of the Kieseloolite Formation, except for the lower low-resistivity unit of ca 10 m thickness where present, which corresponds to the Kasterlee sand formation, in need of lithostratigraphical revision but lateral equivalent of Inden Formation.

Figure 10. Borehole 31W0263 Dessel, showing Mol Formation, Member of Mol – Donk, equivalent to Waubach Member of Kieseloolite Formation, overlying Kasterlee clay  and Diest Formations (H3O De Kempen, unpublished).