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

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.