Schamhaupten, an outstanding fossil-Lagerstatte in a silicified Plattenkalk around the Kimmeridgian-Tithonian boundary (Southern Franconian Alb, Bavaria)

摘要

The silicified plattenkalk of Schamhaupten, which was deposited in a basin surrounded by massive facies (now dolomite) and placed around the Kimmeridgian-Tithonian boundary, has proved to be a Foss il-Lagerstatte of great diversity. About 200 taxa can be distinguished so far, and probably there are much more. The most spectacular find is the small theropod Juravenator starki. The abundant pelagic forms (coccolithophorids, radiolarians, cephalopods, the planktonic crinoid Saccocoma, most fishes, and marine reptiles) can be regarded as autochthonous and indicate good connections to the Tethys. All benthic and demersal organisms (foraminifers, sponges, brachiopods, bryozoans, bivalves, gastropods, crustaceans, echinoids, sessile crinoids, and some fishes) as well as terrestrial organisms were allochthonous and washed into the basin. The lithology is characterized by two main rock types: bindstones containing articulated fossils and detrital beds with only shells and other skeletal elements. During diagenesis the limestones underwent a strong silification which took place in at least two phases. If it occurred before the compaction of the sediment, it could lead to excellent three-dimensional preservation of fossils. Due to the warm, semi-arid climate a salinity-density stratification had developed in the basin leading to a hypersaline, dysaerobic bottom zone which was hostile to benthic life. Only microbial mats could thrive there forming bindstones by trapping sedimentary particles. They played an important role in the conservation of articulated fossils. The rims of the basin which now appear as massive dolomite consisted of an arenitic substrate, probably bound by microbial mats and partly settled by sponges. Storm events reworked it and triggered turbulent density currents which deposited the detrital carbonates in the basin. Nearby, probably to the south, an island must have emerged where land plants and terrestrial animals have come from.