Middle Eocene to early Oligocene calcareous nannofossil stratigraphy at Leg 177 Site 1090

Abstract

Semi-quantitative and quantitative analyses on nannofossil assemblages were carried out on the middle Eocene to early Oligocene interval of Site 1090 Hole B; several biozones were recognized according to the schemes of Martini (1971), Okada and Bukry (1980) and Wei and Wise (1990a), between the FO of Reticulofenestra reticulata and the LO of Reticulofenestra umbilica. Based on nannofossil biostratigraphy, our proposed interpretation of shipboard magnetic inclination data recognizes magnetic chrons from C12 to C19, according to the geomagnetic polarity time scale of Berggren et al. (1995). The Eocene/Oligocene boundary is placed between the last occurrences of Reticulofenestra oamaruensis and Discoaster saipanensis, within Chron C13r. On the basis of the correlation between magnetostratigraphy and biostratigraphy, a new biochronology is proposed for some nannofossil events. The age of FO of Reticulofenestra circus was estimated at 33.39 Ma; no previous biochronology is available in the literature for this event. An age–depth curve is constructed on the basis of the correlation between nannofossil biostratigraphy and the GPTS of Berggren et al. (1995). A very low value of sedimentation rate (0.5 m/Ma) characterizes the upper part of the study site, from the last occurrence of Ericsonia formosa to the last occurrence of R. umbilica. The presence of a hiatus or a condensed interval may occur at this level. It may be related to the mid-Oligocene Marshall Paraconformity and the inception of the Antarctic Circumpolar Current, which contributed to the modification of the nannoflora composition and sedimentation in this sector of the South Ocean.