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OUT OF DISCUSSION : Stramproy Formation


Stramproy Formation - Michiel Dusar et al (on website 23/02/2015) -> removed and moved to subcommission Quaternary

Stramproy Formation (new for Belgian lithostratigraphy)
Authors: Dusar, M.; Deckers, J.; Matthijs, J.; Walstra, J.; Westerhoff, W.E.
Posted as discussion document for Paleogene-Neogene Subcommission at NCS website on 23.02.2014.
Name: The name is derived from the Dutch village of Stramproy (commune of Weert, Dutch province of Limburg), located close to the Belgian – Dutch border (de Lang & Weerts, 2003).
Note that Stramproy often is spelled Stamproy in older documents and on Belgian maps and signposts.
The Stramproy Formation replaces the sandy part of the Jagersborg Member of the Kieseloolite Formation sensu Lithostratigraphic scale of Belgium (Laga et al., 2001) but there are differences in the concept, hence the stratigraphic level, for distinguishing between Stramproy – Kieseloolite Formations vs. Jagersborg – Brunssum I Members of the Kieseloolite Formation (see discussion).
Holostratotype: Borehole 57H0058 in Stramproy, interval 46,30 – 136,80 m below the surface level of 34,07 m NAP. This borehole is located near the Belgian village of Molenbeersel (commune Kinrooi).
Belgian parastratotype: Exploration borehole Maaseik – Jagersborg, drilled 1980, published in 2005 by Vandenberghe et al.; GeoDoc 049W0220; Lambert coordinates x 246636, y 200835, ground level (= zero level) +33,14 m TAW. Depth interval: 22 – 62 m (according to Vandenberghe et al, 2005 after correcting for revision of the lithostratigraphic subdivision, cf. Kieseloolite Formation) or 22-63,20 m (according to log, in annex).
This formation is not outcropping.
Quartz-rich sand with very low gravel content. The sand is medium fine to medium coarse (150 – 300 μm), light grey to chocolate brown or bleached; with thin lignite beds and thin layers of brown to black humic to loamy sand and clay (de Lang & Weerts, 2003). The heavy mineral content is predominantly stable with tourmaline, metamorphic minerals including staurolite (Westerhoff et al., 2008).
On geophysical well logs the Stramproy Formation can be singled out by very low gamma-ray values in the quartz-rich sands, interrupted by higher values in more clayey intervals; especially in the lower part of the formation several clayey beds can occur at close intervals. Also the resistivity displays broad peaks, interrupted on the clayey intervals.
The formation consists of fluvial deposits by ‘Belgian rivers’, depositing reworked Cenozoic sediments in the graben from small-scale delta fans across the graben boundary faults (Westerhoff et al., 2008).
Notwithstanding the varying depositional environment the resulting deposit presents common characteristics and comparable thicknesses over the Belgian part of the Rur Valley Graben and adjoining areas in the Netherlands.
Further subdivision of the Stramproy Formation is based on the eventual occurrence of clayey intercalations. Due to bifurcations, wedging out and great thickness variations of the intervening sand lenses correlation over the whole Rur Valley Graben becomes inconsistent. Nevertheless, they influence hydrogeological conditions on a local scale. Therefore these are not assigned lithostratigraphical member status but are retained as hydrostratigraphical units, which do not imply that these everywhere represent exactly the same interval (Deckers et al., 2014, cf table).
Underlying strata:
The lower boundary consists of one or several erosion surfaces, resulting in varying lithological contacts. In the Belgian part of the Rur Valley Graben the Kieseloolite Formation is always underlying the Stramproy Formation; across the border with the Netherlands and further to the northwest the Stramproy Formation may both interfinger with and be underlain by clay bed of the Waalre Formation.
Tracing the boundary is not obvious when sand deposits of the Kieseloolite Formation are directly overlain by sand deposits of the Stramproy Formation, although the granulometry is more variable and tends to be coarser grained in the Kieseloolite Formation. In drilling practice the boundary may be placed on top of the first ‘floodplain’ clay layer encountered, a stiff clay often associated with wood-rich lignite, and traditionally assigned to the Reuver Clay, e.g. in the area Bree - Maaseik where this layer consists of slightly clayey lignite, followed by very permeable (high resistivity peak) bleached sand which is more coarse and heterogeneous than the Stramproy sands. This combination is diagnostic for the transition from Stramproy to Kieseloolite formations.
The top clay layers of the Kieseloolite Formation are distinguished by their lower resistivity values, bottoming out against the resistivity minima encountered on the logs. Gamma-ray peaks are less outspoken because the upper clay layers of the Kieseloolite Formation are rich in lignite. Clay layers, which are not seldom in the basal part of the Stramproy Formation, are less compact, more sandy/silty and not associated with in-situ lignite deposits; they do not function as hydrogeological barriers (Dusar et al., 2014).
Overlying strata:
The lower boundary of the overlying formations is always erosive, with an abrupt transition from whitish or brown-stained mostly fine-grained sands to the more reddish yellow-coloured gravels and coarse grained deposits of the overlying Beegden or Sterksel Formations. The transition to Sterksel sands is marked by a sharp upwards increase in gamma-ray value, in response to the less pure quartzic character of the sands in the Sterksel Formation. The geophysical well log boundary with the Beegden Formation is less clear, but the occurrence of a basal gravel cannot be missed while drilling.
Area: The entire Belgian part of the Rur Valley Graben, north of the Heerlerheide to Reppel Faults (Feldbiss fault system). 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 Stramproy Formation in Maaseik – Jagersborg borehole approaches 40 m. Time of deposition coincided with climate change and reorganization of the drainage system, under continued high subsidence of the graben, creating accommodation space for sedimentation. The thickness varies between 35 m and 80 m in the Belgian part of the Rur Valley Graben, to reach a maximum of 110 m on the Netherlands territory.
Age: essentially Lower Pleistocene (cf. chronostratigraphic correlation scheme in Kieseloolite Formation), deposited after the Gauss-Matuyama magnetostratigraphic boundary (Kemna, 2008).
Disused units: Tiglian, Eburonian, Waalian, Menapian, Bavelian, Lower-Cromerian.
The time of deposition of the Stramproy Formation is rather long but punctuated by periods of non-deposition resulting in hiatuses of various time span; due to graben subsidence the formation thickness is nevertheless considerable.
Pollen spectra are used to define the transition from warm to cold vegetation patterns in the clay beds at the transition from Kieseloolite to Stramproy Formations (Vanhoorne, 1963, 1979). Borehole Maaseik – Jagersborg is a clear illustration: a transition between two palynozones is located by Vandenberghe et al. (2005) at 57,6 m below a lignite layer (corresponding to hydrostratigraphic layer SY-k-3 on log in attachment). The lower palynozone represents a densely forested ‘warm’ landscape, the upper palynozone an open ‘boreal’ landscape with mires. The clayey beds occurring in the lower part of the Stramproy Formation (but in the middle of the Jagersborg Member) were henceforth correlated with the Reuver Clay bed, solely on biostratigraphic arguments. The underlying clay beds, assigned to the Brunssum Member of the Kiezeloolite Formation were biostratigraphically correlated with the lignite of De Maat member in the middle of the Mol Formation, and assigned a Pliocene age (Vanhoorne, 1963). Similar palynostratigraphical successions were described by Vanhoorne (1979) for boreholes in the area around and north of Maaseik, coming to the conclusion that the strata which are now interpreted as top Kieseloolite Formation possess a Pliocene pollen spectrum, whereas the strata which are now interpreted as base Stramproy Formation possess the boreal spectrum of the Praetiglian, indicative for a Pleistocene age. It can be concluded that the lithostratigraphical boundary coincides with a change in vegetation. It seems logical that sediment transport processes differ and that peat building and lacustrine sediments are more likely to occur in a forested environment than under ‘boreal’ circumstances.
As stated also for the Kieseloolite Formation, the chronostratigraphic value of the palynozonation is questioned (Kemna & Westerhoff, 2007; Donders et al., 2007).
The Stramproy Formation forms the terminal part of an as yet informal lithostratigraphic Group of the continental Neogene. This group further includes the Inden, Kieseloolite and Mol Formations.
In the Netherlands, the Stramproy Formation is part of the Upper North Sea Group.
The Stramproy Formation is newly introduced in Belgium and split off the Kieseloolite Formation, following the Dutch stratigraphic scheme (Deckers et al., in press). Belgian and Dutch practice in selecting the criteria for defining this lithostratigraphic boundary was different: in Belgian practice the base of the Jagersborg member was placed on top of the Brunssum I clay member, whereas Dutch practice placed the lower boundary of the Stramproy Formation consequently at the top of the first stiff clay layer (as observed in the sense of drilling), irrespective of lateral correlations.  This way, the Jagersborg Member has a dual lithological composition, encompassing strata of different depositional environment, sediment provenance, whereas the Dutch practice is more easily applicable, not only on the basis of scientific reasoning but also on the basis of describing cuttings and interpreting geophysical well logs.
It is the presence of compact clay beds in the strata overlying the Brunssum I clay, which obviously is not a characteristic of the Stramproy Formation, which correctly induced Vandenberghe et al. (2005) to avoid using the name Stramproy and instead refer to Jagersborg (personal communication N. Vandenberghe). Jagersborg had already been introduced by Vanhoorne et al. (1999) to describe the sequence of clay-rich sands overlying the Brunssum I clay bed (member), in a comparable fashion as the informal Dutch Schinveld beds. This practice was generalised by Sels et al (2001) for the whole interval surmounting the Brunssum I Clay on the new ‘Tertiary geological map’ in the graben area, and accepted with member status by the NCS (Laga et al., 2001).
The dual nature of the Jagersborg Member has already led to wrong stratigraphic interpretations, labeling the first clay bed of the Kieseloolite Formation as Brunssum I Member and the intercalated sand (H3O hydrostratigraphic unit KI-z-2) as Pey sand (H3O hydrostratigraphic unit KI-z-3). In other cases, the Brunssum I clay Member was interpreted to include all overlying clay beds. Assigning these to the Kieseloolite Formation leads to a clear and more coherent stratigraphic subdivision. Such modification implies abandoning Jagersborg and accepting a Stramproy – Kieseloolite succession according to the Dutch stratigraphic nomenclature. Moreover, the HCOV code 0211 does not take into account the lower clayey part of the Jagersborg Member, already assuming a subdivision as proposed here.
Previous interpretation of borehole logs therefore described the Stramproy Formation as sandy part of the Jagersborg Member, as sandy upper part of Kieseloolite Formation, but also as Kedichem Formation (Demyttenaere & Laga, 1988; Van der Sluys, 2000, after Doppert et al., 1975) or as Maatheide Member of Mol Formation (VMW, in the Neerpelt area).
Equivalent strata are:
- the Maatheide Member of Mol Formation, based on geophysical well log correlation,
- the Beerse Member, eolian sand unit with cryoturbations, intercalated between the estuarine/lagoonal clay beds of the Rijkevorsel and Turnhout Members – themselves equivalent of the Waalre Formation - of the Weelde Formation (previously known as Kempen Formation or Kempen Complex),
- the Ravels Formation north of the Kempen Clay Complex in the province of Antwerp (Bogemans, 2005)
- the coarse-grained to gravelly Holzweiler Formation, deposited by the Eastern Meuse in the southeastern part of the Rur Valley Graben (mainly in Germany) – (Kemna, 2008).
Bogemans, F., 2005, Toelichting bij de Quartairgeologische Kaart van Vlaanderen. Turnhout-Meerle kaartbladen 2 & 8 schaal 1:50.000. Ministerie van de Vlaamse Gemeenschap, Administratie Economie, Afdeling Natuurlijke Rijkdommen en Energie.
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Doppert, J.W.Chr., G.H.J Ruegg, C.J. van Staalduinen, W.H. Zagwijn & J.G.Zandstra, 1975. Formaties van het Kwartair en Boven-Tertiair in Nederland. In: Zagwijn, W.H. & C.J. van Staalduinen (red.), Toelichting bij geologische overzichtskaarten van Nederland. Rijks
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Kemna, H.A., 2008. A revised stratigraphy for the Pliocene and Lower Pleistocene deposits of the Lower Rhine Embayment. Netherlands Journal of Geosciences/Geologie en Mijnbouw 87: 91: 106.
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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.
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Geophysical well log interpretation of borehole 57H58 Stramproy, type locality of the Stramproy Formation, type section 46,30 - 136,80 m. On the left and central the resistivity logs, on the right the gamma ray log.

Belgian parastratotype borehole Maaseik – Jagersborg, GeoDoc 049W0220: Stramproy Formation at interval 22 – 63,20 m, with hydrostratigraphic subdivision.

Boreholes 48E0321 and 48E0322 in Bree: natural gamma-ray (scale 0-30 API for borehole 48E0321 and 0-100 API for borehole 48E0322) and double resistivity (scale 0-500 Ωm for borehole 48E0321 and 0-1000 Ωm for borehole 48E0322); vertical scale in meters. At top Beegden Formation, characterised by higher natural gamma-ray because of its loamy character, but without clear boundary on the basis of geophysical well logs alone. Marked thickness variations within the Stramproy Formation, nevertheless displaying characteristics of permeable sands. Contact with Kieseloolite Formation is marked by transition to very low resistivity values on clay with lignite beds.


West-East profile though Belgian part of Rur Valley Graben: log correlation showing stratigraphic formations and their hydrostratigraphic subdivision.
Geo-hydrogeological model, applicable to the Belgian part of the Rur Valley Graben,after the H3O project (Deckers et al., in press).