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Journal of the Geological Society; 2003; v. 160; issue.2; p. 271-284;
DOI: 10.1144/0016-764902-029
© 2003 Geological Society of London

Original Article

Mid-Triassic integrated U–Pb geochronology and ammonoid biochronology from the Balaton Highland (Hungary)

JÓzsef PÁlfy1, Randall R. Parrish2,3, Karine David2,4 & Attila VÖrÖs1

1 1Department of Geology and Palaeontology, Hungarian Natural History Museum, POB 137, Budapest, H-1431 Hungary (e-mail: palfy@paleo.nhmus.hu)
2 2NERC Isotope Geosciences Laboratory, Kingsley Durham Centre, Keyworth NG12 5GG, UK
3 3Department of Geology, University of Leicester, Leicester LE1 7RH, Leicester, UK
4 4Present address: Laboratoire Magmas et Volcans, CNRS UMR 6524, 5, rue Kessler 63038, Clermont-Ferrand, France

Ladinian (Middle Triassic) strata of the Balaton Highland (west–central Hungary) comprise interbedded marine carbonate and volcaniclastic rocks. The sediments are noted for their rich ammonoid faunas, which allow detailed biostratigraphic subdivision and correlation. For the first time, isotopic dating of the tuff layers was carried out to calibrate the age of ammonoid zones and subzones. Four successive horizons were dated from the Felsouu"örs section, a candidate stratotype for the base of the Ladinian stage. Within the Reitzi Zone, which is interpreted here as the basal Ladinian unit, the following biostratigraphically tightly constrained U–Pb zircon ages were obtained: Felsoeoersensis Subzone 241.1 ± 0.5 Ma; Liepoldti Subzone 241.2 ± 0.4 Ma; Reitzi Subzone 240.5 ± 0.5 and 240.4 ± 0.4 Ma. A redeposited tuff from the Gredleri Zone at Litér yielded an additional U–Pb age of 238.7 ± 0.6 Ma. The new isotopic ages are in agreement with published U–Pb dates from the Southern Alps. Together, they allow estimation of the start of the Ladinian at 241.5 Ma and of the end of the stage at 237 Ma. A duration of <5 Ma for the stage strengthens the argument that cyclicity of the Ladinian Latemar platform (Southern Alps) is not exclusively of Milankovitch origin but forcing with shorter, sub-Milankovitch periods was also involved. The Ladinian, which started 10 Ma after the latest-Permian mass extinction, was a time of radiation and rapid evolutionary turnover, hence empirically established ammonoid biozones are of short duration, averaging 0.75 Ma.

Key Words: Middle Triassic • U–Pb geochronology • biochronology • time scale




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