Abstract

This paper deals with a time interval of the Jurassic Period where existing time scales vary by as much as 10%. An Oxfordian section of the Late Jurassic in central Europe with a rich ammonite fauna and with authigenic glauconite was subdivided into four ammonite zones and subzones: the Cordatum Subzone, the Densiplicatum Zone, the Antecedens Subzone, and the Parandieri Subzone. The diagnostic ammonites are figured and discussed, and the controversial Oxfordian zonation is explained.

The K-Ar ages of the glauconites range from 149.2 to 145.9 m.y. They are in agreement with the stratigraphic succession of the samples, but there is a possibility that these ages of low-temperature minerals are too young. The resolution of the isotopic age measurements is on the order of one ammonite subzone or of about 1 m.y. This is less than 1% of the age of the samples that approaches 150 m.y. The content of radiogenic strontium and neodymium in belemnites from one of the numerically dated horizons is evidence supporting the K-Ar and the Rb-Sr isotopic ages. Analyses of the isotopic composition of lead in the belemnites gave values typical of carbonates.

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