Hohengeroldseck formation
The Hohengeroldseck Formation is a lithostratigraphic rock unit in the history of the earth in the Upper Carboniferous ( Pennsylvania ) of the central Black Forest ( Baden-Württemberg ).
Naming and type locality
The name was proposed by Edgar Nitsch and Hubert Zedler in 2009. Older informal names in the geological maps and the older literature are: Coal Deposits from Geroldseck, Upper Coal Mountains from Hohengeroldseck, Unteres Rotliegende and Stefan and Unteres Rotliegende an der Hohengeroldseck. A type locality was not determined, the type area is in the Emersbachtal between the Hohengeroldseck castle ruins and Rebio bei Seelbach (near Lahr) in the Ortenau district in Baden-Württemberg . TK 25-Nr.7613, approximately R 34 24 900, H 53 05 075. The current distribution area extends from Nordrach to Wittelbach (Gem. Seelbach ), Rebio (Gem. Biberach / Baden ), Hohengeroldseck, Oberharmersbach and Langhard (Gem. Oberharmersbach).
Definition, Correlation and Age
The lower limit of the Hohengeroldseck Formation is the superposition of gray to gray-violet arkose conglomerates and arkoses on the Variscan basement. The upper limit is defined by the onset of the Rotliegend volcanic rocks ( Geisberg Formation ). It is made up of dark and light gray to violet-gray arkose conglomerates and arkoses, which are alternated with gray and black-gray siltstones. The latter also contain thin coal seams. The sediments often contain small pyrite concretions that make the rocks look rusty when weathered. In the higher part, the proportion of red-brown, purple and greenish sediments increases. Arkose conglomerates also intervene more frequently. However, there are (still) no quartz porphyry pebbles in the conglomerates. The thickness in the so-called Hohengeroldseck Graben or in the Geroldseck Basin around the Hohengeroldseck castle ruins is up to about 70 m, in other outcrops only 5 to 25 m.
The Hohengeroldseck formation is limited to the small Geroldseck basin. In the Rotliegend, the Geroldseck basin and also the Oppenau basin were included in the development of the Offenburg basin or the two basins each became a sub-basin of the Offenburg basin. The fossil content of the Hohengeroldseck Formation shows a Stefanium A or Stefanium B age for the lower part and a Stefanium B age for the middle part. Also in the upper part (previously attributed to the Rotliegend) the palynomorphic associations still show great similarities with the stefanian floras. Absolute age dating of rocks in the Hohengeroldseck Formation is not available. In the geological maps, the Hohengeroldseck Formation is denoted by the abbreviation coH.
Storage room and fossil contents
These are alluvial to fluvial channel sediments that cut into lake sediments and flood ledges. In bogs, there is an accumulation of plant material, which was later compacted and transformed into the thin coal seams . Stigmarien prove that the plants also grew on the spot. The strata are relatively rich in fossils; plant fossils in particular have been found. The following were described:
- Wedge-leaf family (Sphenophyllales): Sphenophyllum
- Horsetail (Equisetopsida): Kalamites ( Calamites ), Annularia , Asterophyllites , Macrostachya , Palaeostachia and Pinnularia
- Lycopodiopsida (Lycopodiopsida): Shed trees , Sigillaria , Asolanus , Syringodendron
- " Ferns ": Acitheca , Cyathocarpus
- Cordaitales : Cordaites , Rhabdocarpus , Cordaianthus
- Leaflets of uncertain affiliation: Aphlebia , Schizopteris , Sphenopteris , Alloiopteris , Dactylotheca , Pecopteris , Callipteridium , Neuropteris , Reticulopteris , Linopteris and Odontopteris
- Wood scraps
Animal fossils are rare (unspecified wings of cockroaches (Blattodea) and fish remains); a few trace fossils have also been described.
Breakdown
So far, the formation has not been further subdivided formally. It is possible that three fining-upward cycles can be eliminated, each beginning with arcoses several meters thick and ending with shale clays. The lower cycle begins with a two to three meter thick "lower arkose", above which a three meter thick "lower shale" with thin coal seams lies. Above it follows the approximately 20 meter thick "Middle Arkose", which is followed by the approximately eight meter thick "Upper Slate", which in turn contains coal seams ( fat coal ). The third cycle begins with the several meters thick "Upper Arkose", above which 20 to 30 meters of the "hanging arkose slate layers" follow. These "hanging arkose slate layers", an alternating layer of gray, green-gray and violet-gray sandy siltstones with shredded plants with thin gray to brown arcs and individual layers of rubble, were mostly assigned to the Unterrotliegend. Since they do not yet have any quartz porphyry pebbles, Nitsch and Zedler placed them in the new Hohengeroldseck Formation.
Economical meaning
The little thick, regionally narrowly delimited coal seams were mined over three decades in the 18th century. Then dismantling had to be stopped again due to unprofitability.
literature
- Matthias Geyer, Edgar Nitsch and Theo Simon: Geology of Baden-Württemberg. 5th completely revised edition, 627 pages, Schweizerbart, Stuttgart 2011 ISBN 978-3-510-65267-9
- Edgar Nitsch and Hubert Zedler: Upper Carboniferous and Permian in Baden-Württemberg. State Office for Geology, Raw Materials and Mining, Information, 22: 7-102, Freiburg 2009.