Geisberg formation

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Hohengeroldseck castle ruins in the Schönberg district of the Seelbach community (near Lahr) in the Ortenau district in Baden-Württemberg . The castle ruins stand on the Brandeck quartz porphyry of the Geisberg formation. This superimposes here arcs and conglomerates of the Hohengeroldseck Formation , which has its type locality here
Grünberg quartz porphyry of the Geisberg formation on the Karlsruhe ridge near Ottenhöfen

The Geisberg Formation is a lithostratigraphic rock unit in the history of the earth in the Rotliegend ( Unterperm ) of Baden-Württemberg. It is a volcanic area and its deposits, including intrusions, which originally extended over an area of ​​almost 2000 km² in the central Black Forest . Today it is heavily eroded and only preserved in a few places and exposed or covered by younger sediments.

Naming and type locality

The name was proposed by Edgar Nitsch and Hubert Zedler in 2009. Older informal names in the geological maps are: Mittelrotliegend or Rotliegend volcanic rocks. The type locality is on the Hohen Geisberg near Schuttertal (TK 25 No. 7713: R 34 35 530, H 53 44 440).

Definition, Correlation and Age

The lower limit of the Geisberg Formation is defined with the onset and deposition of tuffs , tuffites or quartz porphyry on layers of the Upper Carboniferous or basement. The upper limit is marked by the onset of the Rotliegend sediments without pyroclastics or even younger layers. At the Hohen Geisberg she has a max. Thickness of 120 m, in Wachendorf (Starzach) Wachendorf, a bore drilled 150 m of quartz porphyry. In stick and passage-like occurrences, diameters or passage widths of up to 600 m are known. The extent of the Geisberg formation was probably around 2000 km² in an area in the Middle Black Forest that is bounded by the cities of Oberkirch , Triberg and Horb . It follows the Hohengeroldseck and Tierstein Formations and is followed by the Schramberg Formation , the Rehberg Formation and the Ibenbach Sediments (informal name).

Storage room

The formation represents the beginning of pyroclastic or epiklastischen sedimentation over the arkose deposits of the Upper Carboniferous in the southern German Permo-Carboniferous basin. It consists of tuffs , Tuffiten , ignimbrites , lavas and gear and floor intrusions of gray, purple, red and white quartz porphyries primary rhyolite to rhyodacite composition and from thin sediments with quartz porphyry rubble. However, it is only preserved in small relics of erosion or covered by younger sediments. The quartz porphyries are often hydrothermally - metasomatically changed. This led to Serizitisierung , silicification , bleaching, formation of ore minerals, carbonation, phosphatizing with transitions to silicification . Folded flow structures and columnar solidification can often be observed in the duct and cane-like deposits. Locally, the deposits can also be brecciated and mixed with secondary rocks. The Ignimbrite are often massive to banky; In the banks there are still occasional flame structures and bubbles to be seen. The tuffs and lapillite tuffs were completely decomposed locally and are preserved as red-brown crystal clay stones. Some dolomite stone benches and dolomite stone knobs are also switched on. Tuffs, lapillite tuffs, lapilli stones are locally silicified.

Due to the strong change ( metamorphosis ) of the rocks, the apparent age values ​​that were carried out on the quartz porphyries scatter very strongly. The least disturbed absolute age data ranges from 296 ± 5 million years to 291 ± 5 million years. This corresponds to a chronostratigraphic lower Permian age ( Asselium and Sakmarium ). A start in the highest upper carbon is not excluded. A higher age is, however, unlikely due to the spatial relationships, as the chimneys of the Grünberg Quatzporphyry penetrate the Upper Carboniferous Oppenau Formation ; But there are no quartz porphyry pebbles in it. It also overlies the Hohengeroldseck Formation and the Tierstein Formation (both Gzhelium ), also here without boulders. However, it delivers pebbles to the Ibenbach sediments (informal name) and the Rehberg formation . The Geisberg formation is divided into the following sub-formations:

  • Geisberg formation
    • White moss subformation, non-ignimbritic pyroclastic sediments
    • Brandeck quartz porphyry (subformation), ignimbrite, lavas with quartz and feldspar fragments
    • Grünberg quartz porphyry (subformation), often silicified ignimbrite without speckles
    • Mooswald quartz porphyry (subformation), heavily seritized and bleached quartz porphyries

Economical meaning

The Grünberg quartz porphyry is mined near Ottenhöfen . The rock is used as aggregate for asphalt and concrete as well as track ballast. Mixtures of grain for road construction are also produced.

The Karlsruhe ridge

The Karlsruher Grat is a 400 meter long rock ridge near Ottenhöfen, which consists of Grünberg quartz porphyry. It is located in the middle of the nature reserve Gottschlätzeal - Karlsruher Grat and has partly given it its name. The forest-free rock areas of the Karlsruhe ridge and the dry scree slopes below are biotopes in which many endangered plant species grow. The Karlsruher Grat is also part of a popular hiking area.

supporting documents

Individual evidence

  1. ↑ Delivery program - quartz porphyry quarry, precast concrete plant Wilhelm Bohnert GmbH & Co. KG Ottenhöfen

literature

  • Matthias Geyer, Edgar Nitsch, Theo Simon: Geology of Baden-Württemberg. 5th completely revised edition. Schweizerbart, Stuttgart 2011, ISBN 978-3-510-65267-9 , 627 pp.
  • Edgar Nitsch, Hubert Zedler: Upper Carboniferous and Permian in Baden-Württemberg. State Office for Geology, Raw Materials and Mining, Information, 22: 7-102, Freiburg 2009.

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