CLAY-WITH-FINTS Formation v2 20160226

Discussion text: CLAY-WITH-FINTS Formation

Compiled by M. Dusar, secretary Cretaceous Subcommission, with the input of J.L. Pingot; first draft deposited 14.1.2016, revised 26.2.2016

Introduction

The Clay-with-flints Formation is a residual deposit covering chalk deposits or remaining after their dissolution, known under different names. It is an informal unit not mentioned on the stratigraphical scale, as is the case with all other residual weathering deposits or alterites. Nevertheless, it is a mappable unit, indicated by different symbols on old maps, e.g. in the Hautes Fagnes region or on new – yet unpublished – geological maps of the Hesbaye region, showing at least their areal extent. Moreover, this unit has its proper geological history and distinctive geomechanical and hydrological properties compared to underlying and overlying strata.

Therefore it is proposed to formalise the status of the Clay-with-Flints unit as a proper formation. A formation status is required to include the totally dissolved Cretaceous deposits on the Hautes Fagnes. Otherwise, a bed status could seem sufficient in the Hesbaye, Pays de Herve or Haine regions.

On the new geological map of the Walloon region, especially in the Waremme-Momalle area, the Clay-with-flints is considered as a formation, being a consequence of weathering and dissolution of the Gulpen Formation principally (Pingot, submitted). The thickness of the Clay-with-flints deposit varies between 0 and 12 meters (Calembert, 1947). This layer is formed by angular, never rounded corroded flints with a greyish-yellow colour (Lohest, 1884). The space between the flints is predominantly filled by silt. The flints are macrofossil-bearing in the upper part of the deposit, displaying belemnites echinoids (Indeherberge et al., 1993).

As the thickness of the overlying cover composed of loess/loam and Cenozoic marine sands is sometimes very important in the Hesbaye region, the Clay-with-flints is known only by borings, with the exception of (former) quarries (cf the Lixhe – Halembaye quarries). From these observations it appears that the Clay-with-flints deposit is nearly contiguous, covering to great extent the Gulpen Formation. In the neighbourhood of the Geer River valley where the cover strata are thin, flints and cretaceous fossils are found in the ploughing fields.

The time of the accumulation of flints must start soon after regression of the sea cover, presumably during the Danian. So far, no fossils indicative for the Danian Houthem Formation have been found in the Clay-with-Flints Formation (Indeherberge et al., 1993). Climatic conditions at the beginning of the early Palaeogene certainly favoured the weathering of the chalk in an uplifted but still near coastal setting: with abundant precipitation and warm climate, the vegetation was important, so the chemical aggressiveness was severe (Luterbacher et al., 2004).  

Saprolitic alterite (or regolith) deposits are known to cover Paleozoic basement rocks in the Ardennes (Dupuis et al., 1996; Quesnel et al., 2006; Thiry et al., 2006), on the Lower Paleozoic subcrop of the Brabant Massif (Legrand, 1968; Mees & Stoops, 1999) or the Upper Paleozoic deposits of the Visé – Maastricht basin east of the Brabant Massif (Bless et al., 1981), but occur all over. Saprolites on the Brabant Massif and the Pays de Herve may reach a preserved thickness of ca 30 m (Laloux et al., 2000). These saprolites differ from the Clay-with flints in that they precede the Cretaceous sediments and develop in non-calcareous (or silicified) strata. Saprolites and Clays-with-flint are both categorised as ‘substractive alteration’ (Wyns et al., 2014) and a link with ‘ghost rock karstification’ processes is obvious (Quinif, 2010).

On the other hand, silicifications are widespread alterites, resulting in peculiar rock types, e.g. tauw on top (or instead) of the Maastricht Formation in the Mergelland of SE Belgian Limbourg or Dutch South Limburg (Dusar et al., 2011) but result from replacement rather than massive dissolution and can be categorised as ‘additive alteration’ (Wyns et al., 2014). Intra-Cretaceous alterites already have received bed status, such as the Saint Denis Microbialite in the Haine Basin (Baele, 2010).

Weathering and decalcification are affecting a wide span of other superficial formations, the results of which are mostly incorporated in soils or in unspecified colluvial Quaternary deposits. Deep decalcification of the Brussels Formation has not been mapped as a singular unit, although it considerably alters the geomechanical properties of the formation.

Name: Clay-with-flints, translation of Argile-à-Silex (Pomerol, 1965; Catt, 1986; Quesnel et al, 1996; Quesnel, 1997; Quesnel et al, 2003). In Wallonia also named Silex résiduels, with French equivalent Résidu à Silex or Residual Clay-with-Flints (RS) when occurring under the Quaternary cover or Paleo RS when occurring in between Cretaceous and Paleogene deposits (Quesnel et al., 1996). The name Clay-with-flints is already in use, and its origin well explained since 1906 (Jukes-Brown, 1906).

Clay-with-flints (Argile-à-silex) is to be preferred to Conglomérat à silex résiduels (Marlière, 1954), renamed Formation de l’Argile-à-silex (Pingot, submitted).

Flint eluvium or Vuursteeneluvium is the generally used Dutch word (Indeherberge et al., 1993). The name Clay-with-flints was already used by van den Broek & van der Waals (1967) as the English translation of Vuursteeneluvium.

Also described as ‘chert/terra rossa breccia’ (Knauth, 1994), or ‘residual chert deposit’ (Van der Ham et al., 2007).

Code: Sx on old- geological maps, or PS on the new geological maps of Wallonia. The P stands for the very thin phosphatic level which is included at the base of the “Formation de l’Argile-à-silex”. 

Stratotype. Stable exposure on upper faces of the old Hallembaye quarry is proposed as a new stratotype (see photographs).

Hockai railway section serves as parastratotype for the completely decalcified Hautes Fagnes region (Bless & Felder, 1989).

Upper reaches of abandoned Hallembaye quarry, exposing Clay-with-flints (photos M. Dusar).

Description: The Clay-with-flints deposit covers the eroded top of the Cretaceous strata and is the result of dissolution of the carbonate matrix, leaving a more or less stratiform concentration of flint embedded in clay-rich impurities, becoming more sandy towards its base or towards the edges of the Cretaceous depositional basin. In detail, the Clay-with-flints deposit may be deformed by epikarstic dissolution on the top chalk.

Volume and thickness of the Clay-with-flints unit depends on the depth of dissolution of the Cretaceous carbonates and their flint content. Without residual flints accumulating on a (karstic) dissolution surface no Clay-with-flints deposit can form. Due to the weathering of a stratified carbonate deposit, with residual concentration of its detrital admixture (clay, sand, locally gravel) or diagenetic (flint, locally phosphate pebbles), a superposition can be observed in the Clay-with-flints deposit as well, from bottom to top mostly black flints from the Zeven Wegen Member of the Gulpen Formation (covering a sandy to gravelly base corresponding to the Vaals Formation, which is strongly reduced to metric scale on the Brabant Massif), followed by impure greyish flints (and possibly also increased sand/silt concentrations) from the Vijlen Member, followed by larger variegated irregular ‘cavernous’ flints from the Lixhe Member, passing into grey flints of still larger dimensions from the Lanaye Member of the Gulpen Formation, and finally the coarser grained flints of the Maastricht Formation, in which flints gradually disappear upward. Hence, preservation of the upper part of the Maastricht Formation strongly reduces the potential volume of the Clay-with-flints layer. When occurring on top of the Maastricht Formation the Clay-with-flints bed rarely attains 1 m in thickness and contains less clay, based on the low impurities content of the calcarenites composing this formation. The unit may be predominantly composed of silicified calcarenite (‘tauw’), which strictly speaking is not belonging to the Clay-with-flints formation, but which is rarely distinguished in borehole descriptions.

The most typical and clay-rich Clay-with-flints facies (as exposed in Hallembaye) is found where this bed occurs on top of the chalk facies of the Gulpen Formation. Stratigraphic assignment of the flint to their original position within the Cretaceous sequence is mostly based on silicified micro- or macrofossils (Bless & Felder, 1989; van der Ham et al., 2006).

Upper reaches of CBR quarry Hallembaye, Clay-with-flints covering epikarstic surface overlying fine calcarenites with flint layers of Lanaye Member of Gulpen Formation, hence locally consisting of flints derived from top of Lanaye member and eroded Maastricht Formation (photo M. Dusar).

Ruthy et al. (2014) describe two types of dissolution residue in the Pays de Herve, but this distinction can be applied elsewhere too: Conglomérat à silex or Argile à silex, depending on the removal of the matrix by later karstic drainage or by erosion below overlying transgressive surfaces sealed by Oligocene sands. Clay or sand content of the matrix between flint and silicified chalks is variable in composition and colour.

Bless & Felder (1989) describe as ‘flint eluvium’ their units 8-14 of the Hockai section (Hautes Fagnes, covering 2m of weathered to saprolised surface of Cambrian (Revinian) strata of the Stavelot Massif and 2 m of sandy layers remaining of the Vaals Formation, which is not chalk-or flint-bearing. The residual chalk deposits consist of angular silicified chalk and nodular flints in a matrix of loamy sand with quartz and quartzite pebbles. The former testify of the original presence of pure chalks, the latter of clastic admixture. The gravel content is characteristic for localities close to the graben boundary faults, site of tectonic inversion. The sand-rich nature of the matrix also indicates a more proximal position with respect to the source of detrital sediment. Composition of the residual deposits and presumed thickness of the strata before dissolution is in the range of the deposits observed along the Meuse between Visé and Maastricht.

Hockai railway section, Hautes Fagnes, of Clay-with-flints (= vuursteeneluvium) residual deposit, recognizing original decalcified Cretaceous stratalog published by Felder & Bosch, 2000

Underlying strata: Development of a Clay-with-flints deposit is generated from the top of flint-bearing Cretaceous strata and gradually expands at the expense of the underlying strata being dissolved. Along the northern and western margin of the Hesbaye plateau (provinces of Belgian Limburg and Brabant) the Clay-with-flints Formation is thin, enriched in silicified calcarenite and covering the Maastricht Formation. Over the major part of the Hesbaye plateau and also on the hills of the Pays de Herve the Clay-with-flints Formation is overlying chalks of the Gulpen Formation. In the outliers of the Hautes Fagnes and Thudinie, were all chalks have been dissolved, the Clay-with-flints is directly overlying weathered of karstified Paleozoic formations, eventually with a basal layer composed of decalcified glauconitic chalk without flint.

Overlying strata: Clays-with-flints are covered by transgressive Cenozoic sands with basal gravel of rounded flints. In the Hesbaye, Pays de Herve and Hautes Fagnes areas these sands are of presumed Oligocene age. Along the northern and western edges of the Clays-with-flints occurrence they might be covered by the Formations of Heers or Hannut but are less developed there and barely distinguished from the regular top flint layers of the underlying Cretaceous. In most cases and over the largest part of the Hesbaye plateau the Clay-with-flints is stripped of its Cenozoic cover and directly occurring beneath the Pleistocene loess – loam mantle. On slopes Clay-with-flints may get mixed up with other superficial or fractured deposits into colluvium.

Area: Hesbaye region in the province of Liège, adjacent part of the province of Belgian Limbourg belonging to the Geer hydrographic basin, on hilltops of the Pays de Herve including its morphological extension in Belgian and Dutch provinces of Limbourg, residual Cretaceous deposits in the Hautes Fagnes (on the north flank of the Stavelot Massif).

Geographical extension of Vuursteeneluvium (Clay-with-flints) in and around Dutch South Limburg. The Clay-with-flints seems to be largely absent where the Cretaceous is crossed by the early Pleistocene Meuse (draining towards the Lower Rhine Graben in the NE) mapped by Felder & Bosch, 2000.

Possibly, the residual deposits of the ‘Cour-sur-Heure’ massif in Thudinie (Entre-Sambre-et-Meuse) composed of decalcified Turonian to Maastrichtian strata (Marlière, 1954) can also be assigned to the Clay-with-flints Formation.

Thickness: from less than 1 m to about 10 m (Brouyère et al., 2004; Orban et al., 2014; Ruthy et al., 2014). In case of complete decalcification >8 m in the Hockai section (Hautes Fagnes), top not exposed (Bless & Felder, 1989; Felder & Bosch, 2000). In the SE part of Dutch South Limburg (Vylenerbosch) the thickness may locally attain 15 m according to Felder & Bosch (2000).

Representative cross-section in Geer (Jeker) basin showing position of Clay-with-flints (= flint conglomerate), from Brouyère et al., 2004

Age: Cenozoic. The Clay-with-flints formation originated from the uplift terminating the Cretaceous depositional sequence. This may have started during the Maastrichtian, in areas where the top units of the Maastricht Formation were not deposited, such as in the SW part of the occurrence area, near the culminating axis of the Brabant Massif. In the Maastricht type region however, uplift and dissolution started after deposition of the Houthem Formation of Danian age (Dusar & Lagrou, 2007). Their formation ended with the first Cenozoic marine transgression which is Selandian (Heers Formation) in the northwestern part of the Hesbaye region or Thanetian (Hannut Formation) in the very northwest of the occurrence area, but presumably Oligocene elsewhere. Also in the proposed type area of Hallembaye the Clay-with-flints is covered by Oligocene sands, resting on a basal gravel of rounded flint pebbles. Nevertheless, the Clay-with-flints deposits could undergo further evolution as a result of karstification or river incision accompanied by lowering of groundwater table.

Distribution of silicifications and ‘fossil soils’ on the Cenozoic peneplains of Dutch South Limburg: 4: Residual flints or silicified chalk, 5: Clay-with-flints exposed, 6: Clay-with-flints under overburden (van den Broek & van der Waals, 1967).

Discussion: Despite a variable composition the unit can generally be recognized unequivocally when the thickness exceeds 1 m. When the unit becomes thinner in most borehole descriptions it will become difficult to distinguish from the top flint layer of the underlying chalk, from in-situ silicified top beds of the Cretaceous or from reworked flint-bearing basal gravels.

Mapping may become difficult in case of deeply karstified surfaces of the underlying chalk, which disturbs the layering and may also separate fine and coarse components, resulting locally in flint accumulations without matrix (especially where this matrix originally was very sandy and/or glauconitic).

Early Cenozoic development of the Clay-with-flints deposit is associated with terra rossa soil development (van den Broek & van der Waals, 1967; Knauth, 1994). However, preservation of original pedogenetic characteristics is not evident because of long-lasting, often repeated exposure after erosion of the Paleogene marine sands, sealing the Clay-with-flints until final coverage by loess-loam under cold climate. Long-term exposure to groundwater percolation and climate change is a cause for facies variability, especially in the finer fraction. The flints tend to be more weathered than in the original chalk – calcarenite deposit, complicating the distinction between nodular flint and silicified chalk (chaille).

Clay-with-flints may be reworked in colluvial deposits. In this case the nodular flint component of the deposit tends to be much lower than in the original Clay-with-flints deposit.

The variable thickness and sand content, the irregular distribution due to later karstification of the chalks make that this clay-rich unit does not function as an aquiclude, exerting a comparable effect on groundwater circulation as the overlying loam.

Cretaceous outliers in the Thudinie region are composed of weathered flint nodules, phosphate conglomerates, compact clays and glauconite accumulations, as the decalcified residues of Turonian to Maastrichtian strata, trapped in karstified Devonian limestones under a Cenozoic cover (Marlière, 1954). It is evident that the mode of formation is similar to the Clay-with-flints in Hesbaye, even if the time range of the Cretaceous deposits and the onset of decalcification may be different.

Exposure of Clay-with-flint derived from Lanaye Member in a ploughed field on leveled landscape, Froidmont, overlooking the Hallembaye quarries (photo M. Dusar).

Conclusions

- Clay-with-flint is defined as a formation in the category of alterite deposits. It cannot be inserted in the stratigraphical scale based on sedimentary strata respecting strict superposition.

- Clay-with-flints consists of residual deposits after partial or total dissolution of the carbonated Cretaceous strata. Its components thus are insolubles (most conspicuously flint) and clayey weathering products derived from the Cretaceous sediments but time of formation is mainly Cenozoic, largely pre-Oligocene transgression.

- Its composition is variable, due to differences in the amount and composition in the original Cretaceous deposits, and to post-depositional weathering and erosion under different groundwater movement and climate regimes. Hence, the fine-grained matrix of the Clay-with-flints formation may be predominantly sandy. In vertical succession the original stratigraphical succession of the dissolved Cretaceous strata can be reconstructed from contained fossils.

- Its main area of occurrence is the Hesbaye plateau, largely corresponding to the Geer hydrographic basin, with discontinuous deposits east of the river Meuse in Pays de Herve and Hautes Fagnes. Outliers consisting of decalcified Cretaceous strata, such as those found in the Entre-Sambre-et-Meuse region, may be assigned to the Clay-with-flints Formation a well.

- Towards the north and west the Clay-with-flints Formation thins and becomes indistinguishable from underlying and/or overlying strata. This constitutes a practical mapping boundary. Towards the south and east, erosion has isolated remnant occurrences of Clay-with-flints formation. In areas currently devoid of Cretaceous strata, such as the High Ardenne or the axis of the Brabant Massif, saprolites may be indicative of former deposition of Cretaceous sediments and their evolution into a Clay-with-flints deposit.

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