BIOSTRATIGRAPHY AND PALEOBIOGEOGRAPHY OF EARLY CRETACEOUS CALCAREOUS NANNOFOSSILS

Nineteen nannofossil zones are recognizable in the Lower Cretaceous of north-west Europe. Some of these can be found in other boreal regions, but correlation with the Tethys and the Indo-Pacific is not possible before the Aptian. The nannofossil assemblages of the Early Cretaceous show a distinctive palaeobiogeographic distribution pattern for the intervals Berriasian-Barremian on the one hand and for the Aptian-Albian on the other. The Berriasian-Barremian period is characterized by provinciality on the generic level and it is possible to differentiate between a Boreal Realm and a Tethyan Realm. The Tethyan Realm is subdivided into a Mediterranean Province and an Indo-Pacific Province. The Aptian-Albian was marked by a nannofloral (and faunal) turnover, causing a worldwide homogenization of nannofloras. The changes of the distribution patterns are best explained by (1) increased sea floor spreading; (2) a sea-level highstand in the Aptian; (3) temperature control of some species. © 1992.