New Zealand and Southwest Pacific Late Eocene and Oligocene Calcareous Nannofossils

Abstract

Eighty species of calcareous nannoplankton fossils are described from 42 Late-Eocene and Oligocene stratigraphic sections in New Zealand and the Southwest Pacific. Their stratigraphic and geographic distributions are established and the biostratigraphic zones based on calcareous nannofossils proposed by Edwards (1971a) are modified. Seven biostratigraphic zones are recognised in the intervals represented by the Bortonian to early Waitakian stages in New Zealand. They are correlated with nannofossil zones recognised in other parts of the world, and with zones in New Zealand based on other fossil groups (especially the planktonic foraminifera). Correlation with European stages (made with particular reference to the nannofossil zones at their localities) indicate two likely horizons for the Eocene-Oligocene boundary in New Zealand and the Southwest Pacific. The two horizons are the top and the base of the Coccolithus formosus Subzone, within the lower part of the Whaingaroan Stage and upper part of the Runangan Stage respectively. The paleotemperature decrease widely associated with the Eocene-Oligocene boundary reaches its minimum within the Coccolithus formosus Subzone. The general biostratigraphic pattern for the Late Eocene-Oligocene interval is a cosmopolitan distribution of species and a lack of differentiation of assemblages. The well documented and dramatic oxygen isotope change across the Eocene-Oligocene boundary is reflected by a decrease in nannofossil diversity (partially attributable to changes in environmental conditions) and a decrease in Discoaster – Chiasmolithus rations. Particular attention is paid to the nannofossil and planktonic foraminiferal biostratigraphy immediately above and below the Marshall Paraconformity, a Middle Oligocene sedimentary hiatus that is well defined in the Oamaru District of New Zealand. The data are limited because of poor preservation, but indicate that the amount of of time represented by the hiatus varies from place to place. There is a common interval of time not represented by sedimentation, which suggests that the Marshall Paraconformity is the result of a single widespread eustatic, tectonic or paleo- oceanographic event.