Calcareous nannofossils across the Cretaceous/Tertiary boundary, Site 761, northwest Australian margin

A biostratigraphically complete (all nannofossil biozones present) Cretaceous/Tertiary boundary was recovered on the Wombat Plateau. A quantitative study of calcareous nannofossils on closely spaced samples across the boundary reveals a rapid change in assemblages in a similar fashion to other Cretaceous/Tertiary boundary sites. Nannofossil species were placed into three categories: Tertiary, Cretaceous, and "survivors'. The rapid sequential turnover in these assemblages is as follows: Cretaceous species are abruptly replaced at the boundary by opportunistic survivor species, which in turn are abruptly replaced by newly evolved Tertiary taxa. The change from a survivor- to Tertiary-dominated assemblage is coincident with the CP1a/CP1b nannofossil subzonal boundary, which is marked by the simultaneous first occurrence of several species including Cruciplacolithus tenuis and C. primus. The latter is found to first occur below C. tenuis in the most complete Cretaceous/Tertiary sections. A hiatus between Subzones CP1a and CP1b is interpreted to explain this discrepancy. -from Authors