Lower Cretaceous nannofossil biostratigraphy off northwestern Australia (Leg 123)

Abstract

Moderately to sparsely nannofossiliferous Neocomian siliciclastics and rich Aptian-Albian nannofossil chalks were cored at two Leg 123 sites on the abyssal plains off northwestern Australia. At Site 765, the basal 70 m of cored section yields questionable Tithonian and Berriasian to early Hauterivian assemblages of moderate diversity. The overlying Hauterivian-lower Aptian is represented by 140 m of sediments barren of nannofossils. Above this, the remaining 80 m of the Lower Cretaceous section has been assigned to the Rhagodiscus angustus Zone (late Aptian-early Albian) and the Prediscosphaera columnata Zone (middle-late Albian). At Site 766 the Neocomian is represented by 200 m of sediment and yields four species. Within the overlying Aptian-Albian sequence of 80 m, the Rhagodiscus angustus and P. columnata zones were recognized. The palaeobiogeographic patterns and implications are discussed as are biostratigraphically important species. Their occurrence in the Indian Ocean is compared with one from the Tethys and Boreal realms. Two new species are described. -from Author