Late Valanginian extinction and turnover of Tethyan brachiopods: A signal of the Weissert Event (Bakony Mountains, Hungary)

摘要

Brachiopods, together with ammonoids, were collected bed-by-bed from several Lower Cretaceous pelagic sections and localities in the Bakony Mountains (Transdanubian Range, Hungary), dated by detailed ammonoid and nannofossil biostratigraphy. These yielded abundant and diverse brachiopod material (1015 specimens, 25 species), and a possibility to determine the stratigraphic ranges of the brachiopod species. Two sections, including the Berriasian to Hauterivian interval, were studied in detail. In the Harskut HK-12 section the early late Valanginian Weissert Event was documented previously by stable isotope investigations and it was biostratigraphically correlated with the Edesviz Key section. The Lower Cretaceous brachiopod record in the Bakony was separated into four assemblages. The abundant early Berriasian brachiopod Assemblage 1 diminished and with a stepwise extinction and partial turnover passed to an impoverished Assemblage 2 in the early Valanginian. These species ultimately disappeared at the early/late Valanginian boundary. This extinction is interpreted as the effect of the global Weissert Event. The extinction horizon is marked by an almost monospecific, dwarf brachiopod fauna (Assemblage 3). In the late Valanginian an abundant and diverse brachiopod fauna appeared, with a complete turnover of species (Assemblage 4). The development of this assemblage in the late Valanginian and Hauterivian was probably controlled by environmental changes, (1) by a switch in palaeoclimate, i.e. by the late Valanginian cooling episode and (2) by the change in the sedimentary environment.