[1 Location] [2 Geological Setting] [3 Biota (3.1 Invertebrates) (3.2 Vertebrates) (3.3 Flora) (3.4 Trace fossils)]
[4 Taphonomy] [5 References] [6 Links] [7 Glossary]
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The world- famous Posidonienschiefer or ‘Posidonia Oil-Shale’ is known from localities around Holzmaden and Dotternhausen, small villages located on the eastern foreland of the Schwäbische Alb, Southwestern Germany (fig.1).
This conservation Lagerstatte also yields fossils in abundance and is named after the bivalve informally known as 'Posidonia', abundant in some horizons. The oil-shale is an important resource used for the production of cement, and electricity.
Fig 1. Map of southern Germany indicating outcrops of Lower Toarcian deposits (black), and the location of the two major fossil bearing localities, Dotternhausen and Holzmaden.
2. GEOLOGICAL AGE AND SETTING
The 9 m thick oil shale (fig 2) is Toarcian (Lower Jurassic) in age, that’s approximately 185 million years old. During the Jurassic period Europe formed an archipelago of islands separated by warm shallow seas resulting from transgression. The ancient ocean of Tethys was situated to the south and east. It was in such an epicontinental marine basinthat the fine-grained posidonia oil-shale was deposited (fig 3. Geography during the Toarcian). This well lithified shale has laminations 1mm or less thick representing alternations in hydrocarbon richness and clay richness. These span many kilometres and the shale splits easily along the bedding planes. The dark grey oil-shale derives its colour from both the high concentration of organic material and the diffusely distributed pyrite. Intercalated with the shale are three layers of concretional limestone. Round concretions also occur between 30-50 cm. The dip and strike of the strata is negligible, which is part of the reason for the perfect condition of the fossils.
The main reason for both the high hydrocarbon content and excellent preservation is lack of oxygen on the sea-bottom. These anoxic conditions prevented the decomposition of organic material by aerobic bacteria. Because the sea was rich in nutrients, blooms of algae grew and subsequently accumulated as masses of organic matter. The biota was only abundant in the water column; these organisms were preserved when they fell to the sea-bed. Benthic organisms are rare but storms at the surface occasionally agitated the water enough to supply oxygen and temporary allow life on the sea floor. Alternatively, the interface between the anoxic/oxic conditions (redox boundary) may have fluctuated naturally in relation to changing sea level. In oxygenated conditions, bivalves could dig into the mud and worms could cause bioturbation reducing the potential for exquisite preservation. Chondrites for example has also been documented. Generally this did not occur, favouring the typically exceptional preservation.
High Organic Content
Anaerobic bacteria were not able to fully decompose the quantities of organic matter being produced but managing to transform it into kerogene. This organic material occurs in some beds in concentrations up to 16% indicating persistent stagnation and slow sedimentation. Disc-like pyrite nodules also indicate the anoxic condition because it's formation involves anaerobic bacteria. These bacteria are not dependant upon oxygen they reduce sulphates within the sediment releasing hydrogen sulphide as a by-product. Further breakdown by bacteria releases sulphur and it is this that combines with iron minerals to form pyrite. Such a process is inhibited where oxygen is present.
In addition, the seabed was a low energy environment, still waters with only very weak currents, again favouring exceptional preservation. Tree trunks for example were calcified or silicified inside, whereas their surface is black 'jet' or 'gagat' as it is called in Germany.
By compression into just one twentieth to one thirtieth of its original thickness, the majority of the fossils in the oil shale are totally flattened, especially evident in the ammonites. During compaction, the pore water dissolved the aragonite and much of the calcite, also indicating acidity. The proteinaceous periostracum has persisted however and the siphuncle is often preserved white or in some cases as brilliant blue phosphorus. The presence of draping laminations around the concretions is also evidence for compaction and that the limestone layers are diagenetic in origin. As the limestone is only slightly compressed, the fossils enclosed are preserved in three dimensions unlike the flattened fossils from the oil-shale. Indeed, they often originated from decaying organic material. Unfortunately, extraction is a difficult job.
Rapid Burial: A Soupy Sediment
The taphonomy has paid a particularly important part in the preservation of vertebrate remains. Localised disarticulation has been interpreted as a result of the soft 'soupy' substrate into which the organism sank fully articulated. This would ensure rapid burial, one of the prerequisites of exceptional preservation. In many cases the underside of the organism sank into the mud, becoming sheltered from destruction, whereas the exposed upper side was prone to disarticulation. Weak currents have been inferred from the uniform alignment of belemnite rostra. Because of this, the underside is usually better preserved and so fossils are prepared from the underside. Large vertebrates can often be found partly disarticulated for this reason, usually the extremities of the body (limbs, tails and necks in plesiosaurs) because these were of insufficient mass for complete submersion. Soft tissue preservation of vertebrates, including famously ichthyosaurs, has been fundamental in our understanding of their soft biology. The soft body and sometimes ink sacs of belemnites, e.g. Passaloteuthis, are also preserved. The organic material is preserved as very thin films and the outlines may be identified.
Allison, P. A. 1990. 3.8.3. Pyrite. in Briggs, D. E. G. and Crowther, P. R. Palaeobiology: a Synthesis. Blackwell Science, London, 583pp.
Jäger, M. 1994. Oil Shale, Cement and Fossils. A Short Guide to the Jurassic Fossils Exhibited in the Werkforum of Rohrbach Zement. Rudolf Rohrbach Kommanditgesellshaft, Dotternhausen, 32pp.
Kauffman, E. G. 1978. Benthic environments and paleoecology of the Posidonienschiefer (Toarcian). Neues Jahrbuch für Geologie und Paläontolpgie, Abhandlungen, 157, 18-36.
Martill, D.M. 1993. Soupy Substrates: A Medium for the Exceptional Preservation of Ichthyosaurs of the Posidonia Shale (Lower Jurassic) of Germany. Kaupia – Darmstädter Beiträge zur Naturgeschichte, 2, 77-97.
O'Keefe, F. R. 2001. A cladistic analysis and taxonomic revision of the Plesiosauria (Reptilia: Sauropterygia). Acta Zoologica Fennica, 213, 1-63.
Riegraf, W. 1985. Mikrofauna, Biostratigraphie und fazies im Unteren Toarcian Südwestdeutschlands und Vergleiche mit benachbarten Gebieten. Tübinger Micropaläontologie, 3, 1-232.
Röhl, H., Schmid-Röhl, A., Oschmann, W., Frimmel, A. and Schwark, L. 2001. The Posidonia Shale (Lower Toarcian) of SW-Germany: an oxygen depleted ecosystem controlled by sea level and palaeoclimate. Palaeogeography, Palaeoclimateology, Palaeoecology, 165, 27-52.
Seilacher, A. 1990. 3.11.1. Overview. in Briggs, D. E. G. and Crowther, P. R. Palaeobiology a Synthesis. Blackwell Science, London, 583pp.
Schmid-Röhl, A., Röhl, H., Oschmann, W., Frimmel, A. and Schwark, L. 2002. Palaeoenvironmental reconstruction of Lower Toarcian epicontinental black shales (Posidonia Shale, SW Germany): Global versus regional control. Geobios, 35, 13-20.
Wild, R. 1990. Section 3.11.6 Holzmaden. In Briggs, D. E. G. and Crowther, P. R. Palaeobiology a Synthesis. Blackwell Science, London, 583pp.
Der Posidonienschiefer: excellent summary of all aspects of the Posidonia Shale
Erwelt-Museum Hauff: An exquisite Posidonian fossil collection on public display
Werkforum: A centre exhibiting beautiful locally discovered fossils, owned by Rohrbach-zement
Rohrbach-zement: A highly efficient and environmentally aware cement works utilising the oil shale
Plesiosaur Directory: About plesiosaurs in general