Inden Formation v012016
Inden Formation (new), revised discussion text M. Dusar, 4.1.2016
Name: commune Inden, located between Aachen and Jülich (Nordrhein-Westfalen, Germany) in the vicinity of opencast lignite quarries Inden and Tagebau Hambach. Name Inden Schichten proposed by Hager (1966), more recently classified as formation (Schäfer et al., 2004, 2005).
Stratotype:
German stratotype: composite stratotype located in active quarries Inden and Hambach.
Dutch parastratotype: boring B58D0688 te Herten, traject 366,50-439,30 m (Menkovic & Westerhoff, 2010).
Belgian parastratotype: Exploration borehole Maaseik – Jagersborg, drilled 1980; GeoDoc 049W0220; kb18d49w‐B220 [https://dov.vlaanderen.be/dovintra/rapportservlet?connection=dov&proefid... Lambert 72 coordinates x 246636, y 200835, ground level (= zero level) +33,14 m TAW. Depth interval: 166 – 192,70 m (according to Vandenberghe et al, 2005) or 164 – 190,50 m (according to log, Fig. 2).
Description:
The sediment of the Inden Formation predominantly consists of sand with some clay and lignite admixture. The sands are generally medium coarse to very coarse (210 – 2000 µm), rarely gravelly. The sand of fluvial origin is poorly sorted and mixed with silt and humic matter, displaying clay films around the grains. The sand contains large quantities of detrital lignite (reduced to the size of the quartz grains) and larger wood fragments. These impurities cause a dusty (hygroscopic) appearance and dark grey colour (almost black when fresh, pale grey when dried) of the sand and are characteristic for this formation (Fig. 1).
Description of the main lithology is in agreement with the Dutch stratigraphical nomenclator (Menkovic & Westerhoff, 2010). It is the lithological correspondence between the lithofacies and the formal Dutch description that is decisive for attribution to the Inden Formation, similar to the practice in The Netherlands.
The heavy minerals association is characterised by stable minerals (B association with tourmaline and staurolite); zircon content is negatively correlated to grain size (Menkovic & Westerhoff, 2010); no data are available on Belgian sediments.
Clay layers may occur and a persistent clayey layer marks the boundary with the overlying Waubach sands. The clays are moderately sandy to silty, with thin sand straticulations; they are humic and contain lignite layers (Menkovic & Westerhoff, 2010). Although this level is encountered in Belgium as well, is it not distinctive in the cuttings and probably has no aquiclude properties.
Geophysical well logs: Gamma-ray values over the Inden Formation are consistently higher than in the overlying quartz-rich sands of the Waubach Member of Kieseloolite Formation; they are consistently lower than in the underlying green sands of the Breda / Diest Formation. Although showing an overall coarsening upward trend towards the clayey top layer, similar to the overlying Waubach sand unit, resistivity values are at the low end of the sand range and are indicative for low hydraulic conductivity (no water production from Inden Formation) (Fig. 2).
Hydrogeological units, introduced by H3O project (Deckers et al., 2014): IE-z-1, IE-k-1, IE-z-2, IE-k-2, IE-z-3. In Belgium: undifferentiated sand unit IE-z-2.
The sands of the Inden Formation are assigned a hydrostratigraphical S&T code 7 (Schneider & Thiele, 1965).
Underlying strata:
The fluvial deposits of the Inden Formation rest on grey-green glauconiferous marine deposits of the Breda / Diest Formation. In borehole Maaseik – Jagersborg the top of the latter unit consists of pale yellow micaceous fine sand devoid of (micro)fossils, named Formation X in Vandenberghe et al. (2005). Due to the high mica content this unit corresponds to a peak in the natural gamma-ray, despite the absence of clay in this interval. This unit attains a thickness of 5 m in borehole 049W0220 (Fig. 3) and is present in the surrounding area but much reduced in thickness (on metric scale till beyond resolution of samples or well logs). This micaceous sand facies terminating the marine deposits in the eastern part of the Rur Valley Graben is possibly altered under influence of meteoric weathering and dissolution.
Notwithstanding this peculiar horizon, the boundary between both formations is sharp and immediately recognised by drillers when the drilling mud turns green. This property of the drilling mud is indicative for the economic basement while drilling for water; hence it is also used as a guiding principle for lithostratigraphical subdivision.
Overlying strata:
The sands of the Inden Formation are covered by Waubach sands, constituting the lower unit of the Kieseloolite Formation. The Waubach sands are easily distinguished (in Belgium) from the Inden Sands by their more whitish colour and the absence of silt/lignite admixture within the sand layers (Fig. 3). While drilling, the boundary is not so conspicious, because of coarsening upwards sequences in the Waubach sands, making the boundary seem more gradual. East of the Meuse river, in The Netherlands, the boundary is marked by a clay layer on top of the Inden Formation.
Area: southeastern part of the Rur Valley Graben, crossing the Meuse near Maaseik (Fig. 4). The typical Inden lithofacies seems restricted to an area in Belgium parallel to the Meuse river and north of the Feldbiss fault system, marking the southern boundary of the Graben.
Thickness: The thickness of the Inden Formation varies between 30 m to over 100 m in the Dutch subcrop area across the boundary with Belgium. The Belgian part of the subcrop area constitutes a more marginal position with more limited thickness (max. 30 m) and absence of intervening clay layers.
Laterally equivalent beds provisionally correlated to the Kasterlee sand Formation further north in the Rur Valley Graben show thickness variations from 20 to 70 m (Fig. 6).
Age: Late Miocene, Tortonian, according to the Dutch stratigraphical nomenclature (Menkovic & Westerhoff, 2010, based on Schäfer et al., 2004, 2005 for the Lower Rhine Embayment).
Inferred to be Late-Miocene, late Tortonian (Vandenberghe et al., 2014) to Messinian (‘Susterian’, Vandenberghe et al., 2005 - Fig. 5).
Besides indicative ages based on palynofacies evolution, no real datations are available, which makes that lithological characteristics are preponderant in identifying this formation. Proposed ages refer to datations in the Lower Rhine embayment by Schäfer et al. (2004) or to lateral transition to better dated marginal marine deposits (Louwye et al., 1999, 2007; Munsterman & Brinkhuis, 2004).
Lateral equivalent beds in northwestern direction correlate with the Kasterlee Sands Formation of Late Miocene age (Fig. 6) but may laterally correspond to the younger deposits of the Diest Formation as well (Louwye & Laga, 2008; Vandenberghe et al., 2014).
The age of the Inden Formation is thus constrained as late Miocene from late Tortonian to early Messinian.
In the Lower Rhine Embayment the boundary between the Inden Formation and the Hauptkies (= Waubach Sand Member of Kieseloolite Formation) is considered as a major sequence boundary (Schäfer et al., 2004) – (Fig. 7). In Belgium and the adjoining Netherlands the major change in depositional environment occurs at the transition from marine to fluvial depositional environment, corresponding to the boundary between the Diest – Kasterlee Clay and the Inden Formations.
Discussion:
The current lithostratigraphical subdivion introduced in Belgium by the H3O Roerdalslenk project (Deckers et al., 2014) separates the former Kieseloolite Formation s.l. (cf. Laga et al., 2001) into three formations, from bottom to top Inden, Kieseloolite s.s., Stramproy. They share their fluvial origin, which distinguishes these units from underlying deposits of marine (glauconiferous) origin. Their composition dominated by quartz derived from erosion of the Cenozoic cover of the Ardennes-Rhine Massif distinguishes them from overlying coarser-grained deposits, containing lithic fragments derived from river incision into the old massifs.
The Formation of Inden represents the earliest fluvial deposits in the Belgian and Dutch parts of the Rur Valley Graben, overlying the latest marine incursion of Miocene (Tortonian) age assigned to the Diest Formation.
Further east, in the central part of the Lower Rhine Graben (Germany), detrital fluvial sediments deposited by meandering rivers surround a lignite deposit splitting into three seams, named Friesheim, Kirchberg and Schophoven. These lateral deposits correspond to periods of base-level rise and are grouped in the Upper Seam group of the Ville Formation (Schäfer et al. 2004, 2005). The Ville Formation is long-lived, being also equivalent to the Kikbeek Lignite bed of the Bolderberg Formation. As a consequence the Inden Formation in the Netherlands, and certainly in Belgium, may represent the upper part of the Inden Formation in Germany.
In the Maaseik-Jagersborg well the coarse sand interval at 166-193 m (surmounted by a clayey sand layer at 165 – 166 m) was interpreted by Vandenberghe et al. (2005) as a braidplain deposit. The fine fraction present in the sand is held responsible for the lower resistivity values. This sedimentary facies differs from the meandering river deposits surrounding the lignite seams incorporated in the upper part of the Ville Formation. However, as there is no established time equivalence, changing sedimentation models (in time and/or space) must be envisaged.
On the geological map of Belgium – Flemish region (Sels et al., 2001), this interval was erroneously interpreted as Kasterlee Formation, presumably because the dark colour was assumed to be caused by glauconite (which is absent). Vandenberghe et al. (2005) retained this unit in the Waubach sands, as was customary practice for lithostratigraphical interpretion of the boreholes, before introduction of the Inden Formation as a new stratigraphical unit.
Borehole 048E0321 (Bree – Opitterkiezel) ca 10 km to the west of 049W0220 displays at the base of the Waubach Member at 219 m depth a quick transition from coarse ash-grey sand to dark grey much finer but bimodal micaceous sand full of detrital lignite fragments. The lower section (219 m till final depth 233 m) is similar in facies to the Inden Formation. However, it has not been mapped as such because of its heterogenity (mixing of facies characteristics) and the absence of a clayey boundary bed. Assignment to the Inden Formation is controlled by lithofacies only, not by datations or geometric continuity. The extent of the Inden Formation is used in a restrictive way.
Further to the northwest in the Belgian part of the Rur Valley Graben, fluvial sediments with facies characteristics of the Inden Formation pass laterally into marginal marine –estuarine deposits. In the area Bree – Bocholt – Neerpelt – Hamont, the lateral equivalent of the Inden Formation (i.e. occupying the interval between ash-grey Waubach sands and grey-green sands assigned to the Diest Formation or to the clayey base of the Kasterlee Formation) consists of reworked fine grained glauconite-bearing sediments, occasionally containing ferruginous sandstone clasts. Indentation between the estuarine and fluvial facies is likely, as shown by localised influx of detrital lignite and wood fragments (e.g. in borehole 033W0188). The speckled nature of the sandy sediment may be caused by either glauconite or lignite. These beds remain distinctive of the underlying green sands , assigned to the Diest Formation and its clayey top, identified as the Kasterlee clay unit. As a consequence the estuarine deposits which form the lateral extension of the Inden Formation are assigned to the Miocene Kasterlee sand unit, and mapped with the marine Neogene in the H3O Roerdalslenk project, causing a diachronous boundary between Inden and Kasterlee/Diest (= Breda equivalent) Formations (Fig. 6).
In the practice of describing cuttings and correlating geophysical well logs these transitional beds were either associated with the overlying Waubach sands or with the Kasterlee Formation. A lithostratigraphic revision of the Kasterlee Formation and its related units is pending.
Figure 1. Characteristic cuttings of sediments assigned to the Inden Formation from borehole Maaseik-Jagersborg, consisting of dark grey of coarse grained ill-sorted sand with dusty appearance due to admixture with silt and with clay films around the quartz grains.
Figure 2. Parastratotype Inden Formation in borehole Maaseik – Jagersborg, interval 164-190.50 m, based on standard geophysical well logs (H3O Roerdalslenk project, interpretation 21/05/2014).
Figure 3. Cuttings samples from borehole 049W0220, Maaseik – Jagersborg, showing clean ash-grey Waubach sands (155,30 m, at left), grey to dark grey ‘dusty’ (silty) Inden Sands (166,35 – 191,50 m), light yellowish ‘Formation X’, interpreted as a weathering surface at the top of the marine sediment (193,60 – 197,50 m) and oxidised dark green Diest sand (198,10 m, at right).
Figure 4. Subcrop map of Inden Formation in the southeastern part of the Rur Valley Graben, around parastratotype borehole Herten (B58D0688), according to Menkovic & Westerhoff, 2010), completed by a stretch along the Belgian side of the river Meuse.
Figure 5. Extract of stratigraphical synthesis for the Maaseik-Jagersborg borehole over the interval assigned to the Inden Formation, according to Vandenberghe et al., 2005.
Figure 6. NW-SE profile through the Belgian part of the Rur Valley Graben, showing lateral transition from Inden Formation based on characteristic lithofacies to Kasterlee Sand Formation based on presence of glauconite. The boundary between Kasterlee Sand equivalent and overlying Waubach sand at base of Kieseloolith Formation is marked by a dark yellow line.
Figure 7. Schematic NW-SE cross-section across Rur Valley Graben - Niederrhein Graben, showing the Inden Formation at the basis of a progradating fluvial sequence (Schäfer et al., 2004).