| Z. dt. Geol. Ges. | 147/2 | S. 169-181, 10 Abb., 1 Tab. | de.Txt. | Stuttgart, März 1996 |
Die Graupensande der süddeutschen Brackwassermolasse[An incised valley-fill following the Ries impact
the Graupensande from the Northern rim of the Alpine foreland basin in Southern Germany.]
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In Southern Germany, the sediments of the last marine ingression into the Alpine foreland basin (Obere Meeresmolasse) are unconformably overlain by fluvial, estuarine and terrestrial deposits (Brackwassermolasse). The unconformity is a palaeo-valley (named Graupensandrinne) which extended along the northern rim of the foreland basin. The fluvial deposits which form the basal part of the valley-fill sequence are called Graupensande because of the strong predominance of granule to pebble-sized fieldspar, granite, gneiss, and chert grains all along the structure. Caliche horizons (Albstein) partly covered flanks and shoulders of the valley; they are underlain by terrestrial mudstones (Heliciden-Mergel) which contain a bentonite layer. The valley-fill ends with the estuarine Kirchberger Schichten. The provenance of the Graupensande has always been linked with basement rocks from the Moldanubian Zone of the Bohemian Massif, despite the fact that nobody could name a process to produce such an amount of coarse grains and a process to transport them over hundreds of kilometers without grading and sorting. A good candidate for a near-by source is the Ries impact, which happened to bring Moldanubian basement to the surface. Consequently, we checked grains from all the above-mentioned formations for signals of impact-induced shock metamorphism and found plenty of unambiguous evidences such as mosaicism and planar deformation features (PDFs). An impact layer at the very base of the valley-fill sequence contains high concentrations in Fe, Ni, Co, Pb, Cu, Zn, Mn, and Cr as well as about 108 % of carbon, which we relate to soot produced by wildfires. | The bentonite layer from the Helidicen-Mergel turned out to be the ash from the initial explosion blast; it contains diaplectic glass and glass grains with 908 % SiO2. In conclusion, we explain the Graupensand as material reworked from the ejecta blankets of the Ries impact and swept into the Graupensand valley. The Ries impact has been dated at an average of 15.0 m.y. The impact layer at the base of the valley-fill sequence and the bentonitic impact blast layer from the Heliciden-Mergel are coeval. This means that the time-stratigraphic position of the lithostratigraphic boundary between Obere Meeresmolasse and Brackwassermolasse must be shifted by 3.5 m.y. from the Ottnangian into the Badenian. Even from a sequence stratigraphic point of view, the new boundary makes more sense that the older one: the Brackwassermolasse sedimentation now starts after a lowstand (and not during a highstand as 3.5 m.y. before). The Graupensand valley is the incised valley which corresponds to this lowstand. The turning point in the sea-level curve coincides with the Ries impact. However, the incoming flood of clastics suppressed the effects of rising sea-level. It was only when the ejecta blankets were torn down that the sea could partly invade the valley; the estuarine Kirchberger Schichten are the product of this episode. |
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