Open Session - SWISS GEOSCIENCE MEETINGs

6
Symposium 1: Structural Geology, Tectonics and Geodynamics
1. 6
Volcanic ash layers in OSM sediments: clues to their origin and the postsedimentary
tectonic history
Rahn Meinert 1 , Selbekk Rune 2 , Spikings Richard 3 , Zaugg Alfred 4 & Burkhalter Reto 5
1Mineralogisch-Geochemisches Institut, University of Freiburg i. Brsg., Germany (meinert.rahn@hsk.ch)
2Natural History Museum, Geology, University of Oslo, Norway
3Départment de Minéralogie, Rue de Maraîchaire 13, CH-1205 Genève
4 CSD Ingenieure und Geologen AG, Zürcherstrasse 34, CH-8500 Frauenfeld
5Bundesamt für Landestopographie, Seftigenstrasse 264, CH-3084 Wabern
Layers of volcanic origin have been reported from within Upper Freshwater Molasse (OSM) sediments (e.g. Pavoni 1958,
Hofmann 1958, 1959, 1961, 1975, Hofmann et al. 1975, Pavoni and Schindler 1981). These layers are bentonite horizons and
volcanic tuff layers of a few cm to several m thickness. While the source area of these intercalations is clear for many layers
close to the Hegau Volcano area in OSM sediments of NE Switzerland and southernmost Germany, they are less obvious for
e.g. the widely distributed ash layers in the Randen area (Hofmann 1958) or completely unknown for occurrences in the
Tabular Jura (Hofmann 1961), with some occurrences being closer to the Kaiserstuhl volcano than to the closest volcanic
cones of the Hegau.
Despite strong weathering, some layers provide fresh volcanogenic minerals that can be dated (e.g. Gubler et al. 1992, Rahn
and Selbekk 2007). The radiometric ages can be compared to mammalian fossil stratigraphy (e.g. Bolliger 1998, Kälin 1993).
Radiometric ages include U/Pb data from zircon (in bentonites only), Ar/Ar data from sanidine, biotite and hornblende (for
a compilation, see Schreiner 1992) and apatite FT data (Rahn and Selbekk 2007).
Investigations on volcanic minerals do not only provide answers to their stratigraphic age and the bracketing of otherwise
weakly constrained sedimentary events (e.g. the horizon of the “Appenzell granite”). In this contribution, we illustrate that
age data of the volcanic horizons also serve to tightly estimate vertical offsets along faults that are only vaguely constrained
by field mapping. In addition, the age data in combination with the composition of the dated minerals allow clarifying their
origin.
We have separated apatite from ash layers from within the Heliciden marls of the Randen area, and from the Tabular Jura
to determine their origin and the time period of volcanic activity. In addition, hornblende and apatites have been separated