Tübinger Geowissenschaftliche Arbeiten, Series A, Vol. 52, p. pp. 201 - 202.
Abstracts of the 4th Workshop on Alpine Geological Studies, Tübingen 21-24 Sept. 1999
Tübinger Geowissenschaftliche Arbeiten, Series A, Vol. 52, p. pp. 201 - 202.
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Detlef Schneider* 1, Johann Genser 1, Robert Handler 1, Franz Neubauer 1
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Institut für Geologie und Paläontologie, Universität Salzburg, Austria |
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Correspondence: Hellbrunner Str. 34, A-5020 Salzburg, Austria (Detlef.Schneider@sbg.ac.at) |
The investigation is focused on 40Ar/39Ar multi- and single-grain dating of detrital white mica from the northern Alpine Molasse basin. The studied area is situated between Salzburg in the south and river Inn in the north. 20 multi- and 3 single-grain samples have been measured up to now with a VG-Isotech NG3600 mass spectrometer linked with a Merchantek IR-laser ablation unit at the University of Salzburg.
Detrital white mica were taken from sandstone samples covering the whole Molasse sequences from the upper Eocene (Limnic beds and Limestone-sandstone Formation) to the Pannonian (Kobernaußer Schotter) . The northern Alpine Molasse Basin along the northern boundary of the Alpine Mountain Belt is part of the peripherial foreland basin formed during the Alpine continent-continent collision. These sediments are underlain by incomplete Jurassic and Cretaceous sedimentary rocks and by the metamorphic basement of the Bohemian Massif.
The sediment input into the Molasse basin originated from two main source areas: In the Upper Eocene (Priabonian) sediments derived from the Variscan Bohemian Massif in the north (Wagner 1980). In Eggerian sediment input from the rising Alpine mountain belt in the south started and increased rapidly in time.
Precisely datable detrital minerals in well known sedimentary basins could be a powerful tool to reconstruct the timing of geodynamic evolution in the hinterland. 40Ar/39Ar dating of detrital white mica gives strong constraints for uplift and exhumation in mountain belts.
The following major conclusions can be drawn:
a) Multi-grain samples show mixed ages (Fig. d + e, Tab. b). This indicates that mica within one sample are derived from different source rocks. So multi-grain samples are insufficient for precise age determination in this rocks due to the mixture of variable proportions of grains from different sources (Schneider et al., 1998).
b) In the units (Priabonian) close to the base, there are uniform Variscan single-grain (Tab. c) and multi-grain ages with no influence of younger detrital mica (Fig. f + g).
c) In post-Priabonian sediments we observe late Variscan ages (270-285 Ma; Tab. a +b) which are well known in Lower Austroalpine Units (Frank et al. 1987)
d) Minimum single grain ages of detrital mica decrease in age from Priabonian (Variscan ages: 300-320 Ma; Tab. c) to Egerian (early Alpine ages: 80-90 Ma; Tab. b). These ages reflect the change of main sediment source from the Bohemian Massif in the north to the Alpine Mountain Belt in the south.
e) The time interval between cooling through the closure temperature (350-420°C) of muscovite and the deposition of the detrital mica decreases rapidly from the Priabonian (up to 250 Ma) to the Egerian (c. 50 Ma).
Frank W. , Kralik, M. , Scharbert, S. , Thoeni, M. , 1987, Geochronological Data from the Eastern Alps.. in Flügel, H.W. , Faupl, P., Eds., Geodynamics of the Eastern Alps. pp. 272-281.
Schneider D. et al. , 1998, A comparison between sandstones of two peripheral foreland basins: the Alpine Molasse basin versus the Variscan Moravo-Silesian basin.., Abstract
Wagner L. , 1980, Geologische Charakteristik der wichtigsten Erdöl- und Erdgasträger der oberösterreichischen Molasse.. Erdoel-Erdgas-Zeitschrift, 96:338-346.
