Inden Formation v092014
Stratotype:
Belgian parastratotype: Exploration borehole Maaseik – Jagersborg, drilled 1980; GeoDoc 049W0220; Lambert 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, in annex).
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 fluviatile origin is poorly sorted and mixed with silt, 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 (‘tuffaceous’) appearance and dark grey colour (almost black when fresh, pale grey when dried) of the sand and are characteristic for this formation.
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).
Clay layers may occur and mark 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).
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. Resistivity values are at the low end of the sand range and are indicative for low hydraulic conductivity (no water production from Inden Formation).
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: undifferenciated sand unit IE-z-2.
Underlying strata:
The fluviatile 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), here considered as a weathering phase on top of the marine deposits. This unit which attains a thickness of 5 m in borehole 049W0220 is present in the surrounding area but much reduced in thickness (on metric scale till beyond resolution of samples or well logs). It is therefore considered as a top horizon of the marine Neogene, incorporated in the underlying Breda / Diest Formation.
Apart from this 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. 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.
Thickness: The thickness of the Inden Formation varies between 30 m to over 100 m. The Belgian part of the subcrop area constitutes a more marginal position with limited thickness and absence of intervening clay layers.
Age: Late-Miocene, late Tortonian (Vandenberghe et al., 2014) to Messinian (‘Susterian’, Vandenberghe et al., 2005).
Discussion:
The Formation of Inden represents the earliest, late Miocene, fluviatile deposits in the Belgian and Dutch parts of the Rur Valley Graben. Further east, in the central part of the Lower Rhine Graben, detrital fluviatile sediments deposited by meandering rivers surround a lignite deposit splitting into three seams, namely Friesheim, Kirchberg and Schophoven, corresponding to periods of base-level rise, grouped in the Upper Seam group of the Ville Formation (Schäfer et al. 2004, 2005).
In the Maaseik-Jagersborg well the coarse sand interval at 166-193 m (surmounted by a clayey bed 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. 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 without proper naming of the lithological 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 is not mapped as such because of its heterogenity (mxing of facies characteristics) and the absence of a clayey boundary bed. Reworking at the base of the Kieseloolite Formation may be a likely explanation.
Further to the northwest, the Inden sediments pass laterally into marine deposits, assigned to the Breda Formation (whose top therefore becomes diachronous). 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) consists of reworked fine grained marine, hence glauconite-bearing sediments, occasionally containing ferruginous sandstone clasts. Tectonically controlled indentation between both formations and/or establishment of estuarine conditions is likely, as shown by influx of detrital lignite and wood fragments (e.g. borehole 033W0188). In this western part of the Belgian Rur Valley Graben the equivalent sandy sediment has a speckled nature, caused by glauconite and/or lignite. Also the log signature is differentiated from the underlying green sands, assigned to the Diest Formation. By their stratigraphical position they are also equivalent to the sands of the Kasterlee Formation. As a result, these intermediate deposits (both in vertical and horizontal sense), which are not constrained by datation, are assigned to the Breda Formation (in the Dutch sense of usage), while in the practice of describing cuttings and correlating geophysical well logs they were mostly associated with the overlying Waubach sands (‘Breda post Diest, transitional between Inden and Kasterlee’).
References
Deckers, J., Vernes, R.W.; Doornenbal, H.; Matthijs, J.; Dusar, M.; Walstra, J.; Witmans, N.; Den Dulk, M.; Menkovic, A.; Hummelman, J.; Reindersma, R. & Dabekaussen,W., 2014. Geologisch en hydrogeologisch 3D model van het Cenozoïcum van de Roerdalslenk in Zuidoost-Nederland en Vlaanderen (H3O –Roerdalslenk). Mol/Utrecht: VITO/TNO, Geologische Dienst Nederland (in druk).
Hager, H., 1966. Das Braunkohlenvorkommen. Geologische und bergbauliche Übersicht des rheinischen Braunkohlenreviers. Broschüre Tagung der deutschen Geologischen Gesellschaft in Köln. Krefeld: 11-13.
Menkovic, A. & Westerhoff, W.E., 2010. Beschrijving lithostratigrafische eenheid: Formatie van Inden. Geologische Dienst Nederland van TNO.
Schäfer, A., Utescher, T. & Mörs, T., 2004. Stratigraphy of the Cenozoic Lower Rhine Basin, northwestern Germany. Newsletters in Stratigraphy 40 (1/2): 73-110.
Schäfer, A., Utescher, T., Klett, M. & Valdivia-Manchego, M., 2005. The Cenozoic Lower Rhine Basin – rifting, sedimentation, and cyclic stratigraphy. International Journal Earth Science (Geol. Rundsch.) 94. 621–639.
Sels, O.; Claes, S. & Gullentops, F., 2001. Toelichtingen bij de geologische kaart van België – Vlaams Gewest 1:50.000, Kaartblad 18-10 Maaseik + Beverbeek. Belgische Geologische Dienst en Ministerie van de Vlaamse Gemeenschap, ANRE.
Vandenberghe, N.; Burleigh Harris, W.; Wampler, J.M.; Houthuys, R.; Louwye, S.; Adriaens, R.; Vos; K.; Lanckacker, T.; Matthijs, J.; Deckers, J.; Verhaegen, J.; Laga, P.; Westerhoff, W. & Münsterman, D., 2014. The implications of K-AR glauconite dating of the Diest formation on the paleogeography of the Upper Miocene in Belgium. Geologica Belgica 17/2: 161-174.
Vandenberghe, N.; Laga, P.; Louwye, S.; Vanhoorne, R.; Marquet, R.; De Meuter, F.; Wouters, K.; Hagemann, H.W., 2005. Stratigraphic interpretation of the Neogene marine - continental record in the Maaseik well (49W0220) in the Roer Valley Graben, NE Belgium. Memoirs of the Geological Survey of Belgium 52: 1-39.
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).
