Rupel group
The Rupel Group is a geological stratification of the Belgian Basin that was deposited in the Oligocene . It is the original stratotype for the rupelium level . The 45 to 140 meters thick group is of marine origin.
History and title
The Rupel group was scientifically described for the first time in 1850 by the Belgian geologist André Hubert Dumont and proposed as a stage of the Oligocene. With the establishment of the Oligocene by Heinrich Ernst Beyrich in 1854, this proposal was taken up and the group was established as the Middle Oligocene (Rupelium).
The group was named after the Rupel River , a right branch of the Scheldt south of Antwerp .
Occurrence
The sediments of the Rupel Group were deposited in the Belgian Basin , a relatively narrow southwestern edge of the North Sea Basin . The flat layers of the group can now be found on the surface in two separate occurrences:
- a band about 60 kilometers long and almost 10 kilometers wide, trending east-south-east, starting 20 kilometers west of Antwerp via Sint-Niklaas , Rupelmonde , Boom to Aarschot .
- another 60 kilometers long ribbon that runs eastward and can be up to 15 kilometers wide. It starts from Leuven and extends to the east of Hasselt . In its southern section, the basal Bilzen formation is open.
The group also emerges in the Netherlands ( Zeeuws Vlaanderen around Hulst ).
It was found northeast of Antwerp in a borehole near Weelde , ie the group is here in the deeper subsurface and rests on the western Campine floe .
The Rupel group comes to lie in the north-west (area Mechelen- Boom- Waasland ) after a shift gap on the Zelzate formation , in the southeast (area Leuven-Tongeren) it also follows after a shift gap on various non-marine facies units of the Tongeren group . In the northwest, the Rupel Group is overlaid by the Berchem Formation from the Lower Miocene . Further to the east, the Boom Formation is followed by the Upper Miocene Diest Formation - sediments of a tidal arm , which in some cases has cut very deeply into its base. In northeast Belgium the sediments of the Rupel Group are covered by the Voort Formation of the Chattian (Upper Oligocene). The Voort formation follows a gap in the layers and then advances to the northwest.
stratigraphy
The Rupel Group is made up of three formations (from young to old):
Bilzen formation
The marine Bilzen formation is about 15 meters thick. It can be found in southeastern Brabant and Limburg , further north it is found underground on the western Campine plaice . The formation is made up of three members (from young to old):
- Kerniel member
- Little Spouwen member
- Mountain Member
The mountain member begins after a shift gap with a conglomerate of chert rubble at the base. This is followed by white-yellow, homogeneous, horizontally layered fine sands , which in places contain glauconite . The subsequent sandy clays of the little Spouwen member are brightly colored (brown, green or yellow) and very rich in fossils . The formation is completed by pale fine sands of the Kerniel member, alternating with clay horizons . The formation wedges to the northwest and is there replaced by the two lower strata of the boom formation .
Boom formation
The marine boom formation has a very variable thickness. In East Flanders it thins out to a few meters, whereas in northern Belgium it reaches a hundred meters. It can also be divided into three layers (from hanging to lying):
- Putte members - black clays enriched with organic material
- Terhagen member - gray clays
- Belsele Waas member - silty clays
The formation consists mainly of gray, silty clays in a characteristic, rhythmic alternation with dark (dark gray to black) clays that can contain carbonate-rich horizons. The alternating position in the 50 centimeter range is dependent on the grain size and should correspond to Milankovitch cycles of the Earth's axis eccentricity and inclination. The grain size rhythm is explained on the basis of fluctuations in sea level , which reflect changes in the Arctic and Antarctic ice cover. Individual locations within the boom formation can even be tracked for up to 50 kilometers.
The silty clays of the Belsele Waas member begin with a phosphate horizon . It is increasingly transgressing to the southeast over the underlying Bilzen Formation. The Terhagen member consists mainly of gray clays and contains septary horizons ( diagenetically created concretions ). It transgresses completely to the southeast via the upper Bilzen Formation. The putte member consists of black, organic clays. It also has a septary horizon. In its lower section it has a layer gap . In the upper section, the member then regresses to the southeast.
Eigenbilzen formation
The maximum 25 meters thick marine Eigenbilzen formation is mainly composed of dark green, clayey to medium-grain sands. The sediments contain glaconite and were ransacked by benthos ( bioturbation ). Analogous to the boom formation, it can also show grain size-dependent alternating storage. The formation has not yet been divided into any members. It only occurs in the southeast near Hasselt and in the Maasmechelen area .
Storage room of the sediments
The evaluation of benthic foraminifera suggests normal shelf conditions for the lower part of the Rupel group (boom formation) , with a water depth of around 100 meters. There were temporary connections to the open sea. The upper part of the group (Eigenbilzen formation) flattens noticeably and consequently a narrowed, poorly ventilated peripheral area is created. The fauna communities found in the Rupel group indicate cool, temperate climatic conditions at this time. The discordant sediments of the Chattium (Voort formation) with their tropical to subtropical foraminifera benthos document a drastic climate change in the North Sea basin.
paleontology
Macrofossils in the Rupel group include mussels (bivalvia), decapods and fish . Foraminifera, ostracods , dinoflagellates , pollen and spores should be mentioned among the microfossils .
Age
Absolute age dates are not available for the Rupel group. The nannofossil zones NP 23 with Wetzeliella gochtii and NP 24 , the planktonic foraminiferous zones P 18 to P 21 as well as the geomagnetic level C 12 were verified . The limit C 12n and C 12r coincides with the member limit Terhagen / Putte. The first transgressive fine sands of the Chattian are dated 26.7 million years BP . According to Van Simaeys, the discordance between the Rupel group and the Chattium can be traced back to around 500,000 years of glaciation with an associated drop in sea level in the period from 27.5 to 27.0 million years. Conclusion: The Rupel group covers approximately the period 33.0 to 27.5 million years BP .
Paleogeography and tectonics
During the Oligocene, the North Sea Basin was a large epicontinental basin stretching north-south. The surrounding high areas were the Baltic Shield in the northeast, the Central European land mass in the south and the British Isles in the west. Access to the Atlantic via the English Channel was interrupted by the Weald-Artois uplift . In the center of the basin sediments were deposited during the Oligocene up to the 1000 meters, in the Lower Rhine Basin in the south (above all in the Central European rift system belonging Roer Trench ) still remember the 550 meters. In terms of tectonics , the rupelium in the southern North Sea region was a relatively quiet period and so clay sediments could settle in the Lower Rhine Bay. The lack of the Eigenbilzen formation in the north-west of the Rupel Group is attributed to slight movements on north-north-west-south-south-east oriented blocks / clods in the upper Rupelium. The uplift seems to have gone hand in hand with sinking movements in the Roer trench.
literature
- Paleogene and Neogene lithostratigraphic units (Belgium) . In: P. Laga, S. Louwye, S. Geets (Eds.): Geologica Belgica . tape 4 , no. 1–2 , 2001, pp. 135-152 .
- S. Van Simaeys, N. Vandenberghe: Rupelian . In: Geologica Belgica . tape 9 , no. 1–2 , 2006, pp. 95-101 .
Web links
Individual evidence
- ↑ E. Van Echelpoel, GP Weedon: Milankovitch cyclicity and the Boom Clay Formation: at Oligocene siliciclastic shelf sequence in Belgium . In: Geological Magazine . tape 127 , no. 6 , 1990, pp. 599-604 .
- ↑ N. Vandenberghe include: Cyclostratigraphy and climatic eustasy. Example of the Rupelian stratotype . In: Comptes Rendus de l'Académie des Sciences . tape 325 , 1997, pp. 305-315 .
- ↑ S. Van Simaeys et al: Stratigraphic and palaeoenvironmental analysis of the Rupelian-Chattian transition in the type region: evidence from dinoflagellate cysts, foraminifera and calcareous nannofossils . In: Palaeogeography, Palaeoclimatology, Palaeoecology . tape 208 , 2004, p. 31-58 .
- ↑ D. Lagrou et al: Magnetostratigraphy and rock magnetism of the Boom Clay (Rupelian stratotype) in Belgium . In: Netherlands Journal of Geosciences . tape 83 , no. 3 , 2004, p. 209-225 .
- ↑ a b S. Van Simaeys: The Rupelian chat Tian boundary in the North Sea Basin and its calibration to the international time-scale . In: Netherlands Journal of Geosciences . tape 83 , no. 3 , 2004, p. 241-248 (b).
- ^ PA Ziegler: Geological atlas of western and central Europe . Ed .: Shell Internationale Petroleum Maatschappij BV Den Haag 1990, p. 1-239 .