L'“oolithe de Balin” (Pologne méridionale).Nouvelles observations et interprétations stratigraphiques

Abstract

The unusualness and provisional outcropings of the “Balin Oolithe” (Krakow area) have led authors to propose, either a Late Bathonian, or an Early Callovian age for this formation. More than 120 years later than the monographs of Laube, Neumayr, Szajanocha, Teisseyre and Siemiradzki, new collected faunas owe to precise that the sandstones, underlaying the ferrugineous deposits, begin in the Middle Bathonian and reach the Upper Bathonian (Retrocostatum Zone). The famous “Balin Oolithe” is dated from Late Bathonian (Retrocostatum (=Hodsoni pars.. sup.-Orbis) and Discus Zones) to Early (Macrocephalus/Bullatus (=Herveyi pars)) and Gracilis (=Koenigi-Calloviense Zones) and Middle Callovian (Jason Zone). considering the special conditions of sedimentation of this ferruginous facies corresponding to a transgressive period and the subsequently action of cryoturbation and solifluxion during the Quaternary, it is obvious that the use of this formation, to specify the biostratigraphy of the Bathonian-Callovian boundary, should lead to confusion and unfruitfull discussions. The sedimentation is reduced, discontinuous and the fauna is different in each profile, even if the typical ferruginous facies is widespread; one can notice that, in the new section, no Callovian Proplanulites have been collected within the ammonite fauna, although Teisseyre have created his genera on specimens coming from neighbouring sections showing this formation. In comparison with other European areas, these faunas, collected in thin and lenticular sediments, typical of the global transgressive Late Bathonian-Lower/Middle Callovian, must be considered only for fundamental problems in paleontology (variability, systematic, biogegraphy, etc…) but not as reliable data for a refined biozonation to a better understanding of the boundary between these two Middle Jurassic stages.