HORNUNG, T. (2002)

Kondensierte Ammonitenbänke im Beckenbereich des basalen Kimmeridge der Nördlichen Frankenalb
(Alter Steinbruch Ludwag / Ofr.)

Condensed ammonite horizons in the basalmost Kimmeridgian of the Northern Franconian Switzerland
(Lower Ludwag quarry / Upper Franconia)

Abstract

The Kimmeridgian in the Southern Germany is known for its extreme richness in ammonites and accompanying well developed shallow-water macrofauna (molluscs, crinoids, echinoderms, vertebrates, etc.). Its lower ammonite zone (platynota ammonite zone) and in turn, its basalmost ammonite subzone (polygyratus ammonite subzone) yields condensed ammonite layers, traceable in Upper Jurassic series of both the Swabian and the Franconian area. The question about their possible contemporaneity is not treated in this paper and matter of continuative studies. This article deals, in fact, with a very detailed local investigation and correlation settled in the Northern Franconian Switzerland.
Due to intensive karst formation of the carbonate cap of the Franconian Switzerland and its persistent covering with insoluble loamy soils, the only possibility to get meaningful multistratigraphical and palecological datasets, is to sample in big quarries or greater recent road cuts. Only few of the Franconian quarries, however, have the correct stratigraphical age and are settled mostly in the reefal facies due to major content on pure carbonates. Moreover, as a result of the excavation-modus in high walls and treating the marl-dominated lower Kimmeridgian interbeds as rock waste, the absolute number of "quarry-candidates" was reduced to the "Drügendorf quarry" near Forchheim and the "Ludwag quarries" near Bamberg (both in Upper Franconia). The study presented herein was prepared in the disused Lower Ludwag quarry.
There are a lot of descriptive studies about the polygyratus ammonite subzone in Franconia (authors), but actually, only Schairer (1967 and 1974) established a qualitative and quantitative study of the basal condensed ammonite horizons. The recent paper is based on a diploma-thesis (Hornung 2002a) and a subsequent excavation under the direction of the museum of natural history Bamberg (Hornung 2002b).
The basic concept of this work was a plane bed-to-bed-excavation (about four centiars) of the complete polygyratus ammonite subzone in the marly "basinal" as well as in the reef-influenced detrital facies to get funded information about the faunal composition and the quantitative compilation during a short-termed succession of a few 100 ka within the Lower Kimmerigian. In addition, the planar exposure of the condensed ammonite beds in both facies spaces should give information about possible "regularities" like paleo-currents, possible (?local) genesis and, at least, their correlation.

References

HORNUNG, T. (2002a): Die polygyratus-Subzone (Unterstes Unterkimmeridge) in eng verzahnten Faziesbereichen der "Ludwager Bucht" unter besonderer Berücksichtigung der Genese und Taphonomie zweier Ammonitenbänke (Alter Steinbruch Ludwag / Ofr. / Nördliche Frankenalb). - unveröff. Dipl. Arb., 122 S., Erlangen.

HORNUNG, T. (2002b): Eine Fossilbank in der Riffschuttfazies des Untersten Unterkimmeridge Ludwags (Oberfranken). - Geol. Bl. NO-Bayern 52, 1-4: 69-84, Erlangen.

SCHAIRER, G. (1967): Biometrische Untersuchungen an Perisphinctes, Ataxioceras, Lithacoceras der Zone der Sutneria platynota (Reinecke) (unterstes Unterkimmeridge) der Fränkischen Alb. - Diss (Fotodruck), 131 S., München.

SCHAIRER, G. (1974): Quantitaive Untersuchungen an Perisphinctidae (Ammonoidea) des untersten Unterkimmeridgium der Fränkischen Alb (Bayern). - Zitteliana 3: 37-124, München.