Late Neogene biostratigraphic and magnetostratigraphic synthesis, Leg 135

Integration of biostratigraphic and magnetostratigraphic results from Leg 135 sites has given additional information as to the position and reliability of various bioevents compared with previously published results. Two sites (834: Gilbert to Brunhes; and 836: Brunhes) provided excellent magnetic and biostratigraphic data. From these it is suggested that some bioevents are older than previously recorded: the first appearances (FAs) of Emiliania huxleyi (within the Brunhes Chron, at the same level as the FA of Helicosphaera inversa) and Globorotalia (Truncorotalia) truncatulinoides (within the upper Gauss Chron), and the last appearance (LA) of Gr. (Tr.) tosaensis (upper Matuyama Chron). The FA of Gr. (Tr.) crassaformis hessi is variable, but the oldest occurrence is just below the Cobb Mountain Subchron. Other key bioevents, such as the LAs of Discoaster pentaradiatus (just above the Reunion Subchron), D. tamalis (within the lower reversed part of the Matuyama Chron), Sphenolithus (lower Gauss Chron), and Amaurolithus primus (topmost Gilbert Chron) appear higher than previously recorded. Some key biostratigraphic taxa, such as Globigerinoides quadrilobatusfistulosus, Pulleniatinafinalis, P. primalis, and Sphaeroidinella dehiscens, are either rare or their distribution is sporadic to the extent that they are unsuitable for biostratigraphic use in the area studied. Because of the rarity of P. primalis, the FA of Globorotalia (Globorotalia) multicamerata has been used to mark the base of Zone N17B. Though levels are present at most sites in which populations of Pulleniatina are sinistrally coiled, it is difficult to equate these coiling changes with previous records. INTRODUCTION Biostratigraphic events form the basis for the establishment of time scales (such as those of Berggren et al. [1985] and Haq et al. [1988]), when correlated with magnetostratigraphic events and oxygen isotope stages. Studies such as those of Berggren et al. (1985) are based on compilations of studies by many workers. In these compilations, however, the relationship between any bioevent and the magnetostratigraphy may be based on a single record, the reliability of which may not be known. Thus, any additional records of correlations between bioevents and magnetostratigraphy are of potential value for establishing global time scales by providing tests to assess their reliability. This synthesis is based on the magnetostratigraphic and biostratigraphic results (planktonic foraminifers and calcareous nannofossils) obtained from the upper Miocene to the Pleistocene from Ocean Drilling Program (ODP) sites drilled during Leg 135 in the Lau Basin and Tongan Platform (Fig. 1), as reported in this volume (Chaproniere and Nishi; Quinterno; and Styzen, all in this volume) as well as in the shipboard records (Parson, Hawkins, Allan, et al., 1992). Figure 2, which is based on Berggren et al. (1985), summarizes the biostratigraphy, magnetostratigraphy, and geochronology used in this paper. In general, the sites in the Lau Basin provided good biostratigraphic and fair to good magnetostratigraphic results, although core recovery was poor at some levels and abundant ash deposits at some sites made the recognition of magnetochrons difficult. In contrast, the magnetic signature and the biostratigraphic results obtained for the Quaternary and upper Pliocene sediments from Site 840 on the Tongan Platform proved Hawkins, J., Parson, L., Allan, J., et al, 1994. Proc. ODP, Sci. Results, 135: College Station, TX (Ocean Drilling Program). ' Marine Geoscience Program, Australian Geological Survey Organisation, P.O. Box 378, Canberra, A.C.T. 2601, Australia. 3 Shell Offshore Inc., P.O. Box 61933, New Orleans, LA 70161, U.S.A. 4 Department of Oceanography, Texas A&M University, College Station, TX 77843, U.S.A. ' Department of Earth Sciences, Yamagata University, Yamagata, 990, Japan. 6 U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, U.S.A. 7 Geologisk Institut, Aarhus Universitet, Finlandsgade 6, DK-8200 Aarhus N., Denmark. disappointing, to some extent because of the poor core recovery in the upper 260 m, and have not been used in this work. However, those for the upper Miocene were more satisfactory and have been included. Although the magnetostratigraphy from Site 841 can be interpreted back to the middle Miocene, many barren, or at best poorly, fossiliferous intervals yielded very poor biostratigraphic controls. Furthermore, the Miocene and older sections may have been remagnetized by hydrothermal metamorphism (Sager et al., this volume). For these reasons, Site 841 has not been used in this study. Four samples were taken for each of the biostratigraphic groups studied from each core for the Lau Basin sites, with sample intervals being increased over critical intervals. Thus, on average, samples were taken over 2or 3-m intervals. For Site 840, where the sequence was expanded by the volcaniclastic deposits, sampling intervals were greater than for the Lau Basin sites, with two samples per core being the norm, that is, a sample interval of approximately 4 m. This paper is divided into two sections. The first part summarizes the foraminifer and nannofossil biostratigraphic results and the magnetostratigraphic results presented elsewhere in this volume, and then draws the three different studies together, making assessments of the reliability of the magnetic and biostratigraphic events with respect to previously published results, drawing on data presented in the second part, the Appendix. By using this strategy, the reader is spared detail that can be accessed by studying the Appendix. MAGNETOSTRATIGRAPHIC SUMMARY This part of the report is based on the shipboard results following Parson, Hawkins, Allan, et al. (1992). Lau Basin Sites A detailed magnetic polarity record was obtained from both the sediments and the underlying volcanic rocks at Site 834 (Fig. 3), with all polarity chrons and major subchrons being recognized back to the Cochiti Subchron of the Gilbert Chron (Zone N19-20, early Pliocene), a result in agreement with the biostratigraphy. At Site 835, the magnetostratigraphy is a complex pattern of reversals (Fig. 4), not readily G.C.H. CHAPRONIERE ET AL.