A quantitative look on northwestern Tethyan foraminiferal assemblages Campanian Nierental Formation Austria

摘要

Deposits spanning the Radotruncana calcarata Taxon Range Zone at the Postalm section, Northern Calcareous Alps (Austria) are examined quantitatively for foraminiferal assemblages, especially the planktonic group. This study focuses on establishing a high resolution record spanning an 800 ka long stratigraphic interval from the active continental margin of the Penninic Ocean. The Postalm section displays reddish limestone- marl alternations representing precession cycles. For this study, 26 samples were taken bed by bed to allow a “per-precession-cycle” resolution (i.e., a minimum sample distance of ∼20 ka). Samples from limestones as well as from marls were examined for foraminiferal assemblages. Data suggest a typical, open marine Campanian foraminiferal community. The >63 µm fraction is dominated by opportunist taxa, i.e., members of Muricohedbergella and biserial planktic foraminifera. Archaeoglobigerina and “Globigerinelloides” appear frequently and benthic foraminifera are very sparsely found. The share of globotruncanids, representing more complex morphotypes amongst planktonic foraminifera, is recorded with 5–10%. The state of preservation of foraminifera from the Postalm section is moderate to poor. Differences between samples from marls and samples from limestone are evident, but do not reveal evidence that there was an influence on the postdepositional microfossil communities. However, data from microfossils showing moderate to bad preservation can still offer valuable insight into the palaeoenvironment and biostratigraphy. Information gathered on the composition of the planktonic foraminiferal assemblage confirms a low-to-mid-latitude setting for the Postalm section. As well resolved records of Late Cretaceous foraminifera assemblages are rare, the examination of the Radotruncana calcarata Taxon Range Zone provides some insights into variations and short term changes during the very short period of 800 ka.