Current studies in the southeastern part of Cologne focus on a 60 km² area in the east of the Cologne/Bonn airport and the districts of Porz and Rodenkirchen in the west. A 3 dimensional model of the cenozoic stratigraphic sequence was established with the GSI3D methodology and the software tool GSI3D ver. 1.5. About 4,000 borelogs, ranging from one meter to 400 meter depth, have been used for the cross-section construction.

Fig. 1: Location of the work area (bluegreen) in southeast Cologne.

Initial cross-sections have been based on borehols deeper than 100 m covering the geological units from Devonian, Tertiary and Quaternary. These sections are in general orientated in north to south and east to west direction. Surface near boreholes were used in addition sections to reconstruct the quaternary sediments in detail. Altogether 11 sections have been constructed to cover the deeper surface down to the Devonian, 12 cross-sections for a detailed 3D mapping of the Quaternary units and 10 supplementary sections with an emphasis on the Tertiary sediments.

Geological Setting

The area under consideration is located on the Cologne Block in the southeast of the Lower Rhine Basin, bordered in the east by the Rhenish Massif. The basement of the Cologne Block is of Lower- and Middle Devonian sediments. The Tertiary strata are composed of the Bergisch Gladbach-Formation (upper Rupelian) and the Cologne Formation (Chattian to Aquitanian). The tectonic structures of the eastern Rhenish Massif continue below the cenozoic sediments of the Lower Rhine Basin, e.g. the Bergisch Gladbach-Paffrath Syncline continues in the subsurface of Cologne up to the Ville, the latter forming the western margin of the Cologne Block.

The continental Bergisch Gladbach-Formation occures in the whole area at the base of the oligocene sediments in argillaceous-lignitic and sandy facies. The clays and lignites are exceptionally thick in former sink-holes in the limestones of the Bergisch Gladbach-Paffrath Syncline. The Cologne Formation in the area under consideration is represented by the horizons 05 up to 3 (nomenclature after Schneider & Thiele, 1965) with marine fine and middle sands originating from transgressions of the “North Sea” in the Lower Rhine Basin during the upper Oligocene. The intercalated clays and lignites are of continental origin and represent deposits of marsh and lagoonal environments.

The transition from the Rhenish Massif to the Lower Rhine Basin in the east of the working area occures over a short lateral distance with some minor faults in northwest-southeast direction. The maximum thickness of Tertiary deposits is 365 meters in Rodenkirchen, in the western part of the working area. The Cologne Formation forms a gently syncline in the area of Cologne, dipping in westward direction with less than 1°.

The clays and sands of the Tertiary are overlain by the fluvial sediments of the Quaternary. The Quaternary is represented by the Younger Middle Terrace (MT IIIa and MT IIIb after Brunnacker et al., 1978), the Older and Younger Lower Terrace and Early and Late Holocene overbank sediments. The distribution and thickness of individual terrace bodies have been mapped in detail.

The here performed 3D mapping of the MT IIIa (“Rinnenschotter”) clearly shows, that the distribution of these fluvial deposits is far more extensive than expected from prior literature studies. The MT IIIa was expected to have been eroded by the Older Lower Terrace sediments. But remnants of Holstein Age silts and clays at the base of the Older Lower Terrace show, that the gravel below them must be older than the Holstein-Interglacial. In this case it can be only the gravel from the MT IIIa (see also table at the end of the text). The MT IIIa is overlain by silts and clays from the Holstein-Interglacial in the eastern parts of the working area, itself covered by the MT IIIb deposits. In the western parts silts and clays occure below the Lower Terrace sediments. These fine grained deposits are in the same elevation as the Holstein-Interglacial in the east, characterising them as relics of the Holstein-Interglacial.

Fig. 2: Block model of the Cologne and Bergisch Gladbach-Formation in the work area.

The gravel of the Older Lower Terrace coveres the area east of the recent Rhine. In the area west of the Rhine the gravel of the Younger Lower Terrace cuts into the Older Lower Terrace. The inner banks of the Rhine are built up by Holocene overbank sediments, which cut into the gravel of the Younger Lower Terrace.

With GSI3D it is possible to calculate volumes of the geological units, giving maps of the individual distribution and thickness as well the elevation of the base and top of the deposits, listed in the table below.

The established 3d structure model and the calculated volume bodies of aquivers and aquitards e.g. are the basement for further analysis as groundwater flow modelling and are also used in the management of groundwater catchment areas.

age		unit	base (m a.s.l.)	top (m a.s.l.)	thickness (m)
Holocene		Late Holocene overbank sediments	38-40	44-45	4.5-5.5
		Early Holocene overbank sediments	42-44	45-46	3-3.5
Pleistocene	Weichsel-Glacial	Younger Lower Terrace	40-41	47-49	7-9
		Older Lower Terrace	30-32	50-52	6-20
	Eem-Interglacial
	Saale-Glacial	MT IIIb	44-46	61-65	17-19
	Holstein-Interglacial	Holstein-Interglacial	29-35	44-46	13-15
	Elster-Glacial	MT IIIa („Rinnenschotter“)	12-31	30-32	3-15

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References:

Brunnacker, K.; Boenigk, W.; Dolezalek, B; Kempf, E.K.; Koči, A.; Mentzen, H.; Rad, M.R. & Winter, K.-P. (1978): Die Mittelterrassen am Niederrhein zwischen Köln und Mönchengladbach. – Fortschr. Geol. Rheinld. u. Westf. 28: 277-324; Krefeld.

Schneider, H. & Thiele, S. (1965): Geohydrologie des Erftgebietes. Ministerium für Ernährung, Landwirtschaft und Forsten des Landes Nordrhein-Westfalen, 185 S.; Düsseldorf.

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Alexander Kunz, Dipl. Geol
Institute for Geology and Mineralogy
University of Cologne

Zülpicher Straße 49 a
50674 Köln

E-mail:  kunz777@gmx.net
Web:     www.uni-koeln.de