Cenozoic Calcareous Nannofossil Biostratigraphy from the Northeastern Atlantic OceanDeep Sea Drilling Project Leg 81

Using calcareous nannofossils, a biostratigraphic analysis of sedimentary sequences drilled during Leg 81 reveals continuous upper Neogene deposition at all sites, except at Site 555 where the Pliocene is missing. The series of Pliocene discoaster disappearances are practically synchronous with their low latitude extinctions. An exception is Discoaster surculus which disappears at the initiation of ice-rafted deposition, 0.1 m.y. prior to its extinction age; D. pentaradiatus disappears at the beginning of the second lowest cycle of ice rafting, very close to its extinction age. The onset of ice rafting is an abrupt event, which reflects the initiation of major northern European glaciation at 2.4 m.y. ago. The lower and middle Miocene sequences are poorly represented, except at Site 555 which shows an expanded and continuous middle Miocene sequence. Site 554 shows the longest Oligocene sequence, but it is condensed (11.5 m) and probably marred by hiatuses. Manganese formation took place during the early Oligocene (Hole 552A) and during the Eocene at Site 554 (overlain by upper Eocene). The upper and middle Eocene intervals are poorly represented; for example, the Chiasmolithus gigas subzone (Subzone CP13b or Zone NPl5) is only present in a single 2-cm thick horizon (Hole 552A). Site 552 shows the most complete record of the Eocene Zones NPl 1 through NP14, whereas Zone NP10 is considerably expanded at Sites 553 and 555. Definite upper Paleocene (Zone NP9) is only recovered at Site 555, where this sequence is interbedded between two basalt piles. INTRODUCTION Eight holes at four sites were drilled at the southwest margin of the Rockall Plateau in the northeast Atlantic Ocean during Leg 81 of the Deep Sea Drilling Project. Site locations are shown in Figure 1. The sediments recovered yielded Pleistocene through upper Paleocene calcareous nannofossils, and their distribution is presented from selected samples in Tables 1-8. Approximately 1200 samples were investigated. The upper Neogene and lower Eocene reflect virtually continuous deposition, whereas only fragments of the intervening intervals are preserved. Therefore the Neogene and Paleogene sequences are presented and discussed separately. The distribution of taxa is presented through qualitatively estimated relative abundances. The Neogene tables show three levels of relative abundance: filled circles represent an abundance in excess of 10% of the total assemblage; open circles represent an abundance between 1 and 10%; crosses represent an abundance less than 1% of the total assemblage. Smear slides made from the lower Eocene and upper Paleocene shelf sediments contain, as a rule, very low abundances of coccoliths. In many samples considerable difficulties were met in finding enough specimens to estimate relative abundances, and in many other samples this estimate turned out to be completely meaningless. Because consistent estimates of relative abundances could not be successfully achieved, Paleogene abundances are shown either as "rare" (open squares) or "more commonly occurring" (filled squares). Oligocene 20 10° W Roberts, D. G. , Schnitker, D. , et a]. , Init. Repts. DSDP, 81: Washington (U.S. Govt. Printing Office). 2 Address: Department of Geology, University of Stockholm, S-106 91 Stockholm, Sweden. Figure 1. Locations of sites drilled during Leg 81 (Sites 552-555), Leg 48 (Sites 403-404), and Leg 12 (Site 117). and upper-middle Eocene assemblages are incorporated in this system for internal consistency in the tables, although the way of representing abundances in the Neogene samples could have been applied at several sample levels. ZONATION Previous studies of calcareous nannofossil biostratigraphy from the Rockall area include those of PerchNielsen (1972) and Müller (1979). These authors, as well as the present one, have predominantly relied on biostratigraphic zonations suggested by Martini (1971), Bukry (1973, 1975), and Okada and Bukry (1980). Their zonal schemes primarily reflect stratigraphic relationships of exits and entries of species in tropical and subtropical areas. As a consequence some stratigraphic in-