CHRONOSTRATIGRAPHY

The approach to defining chronostratigraphic units has evolved markedly over the years. The early practice of defining a stage by use of a unit stratotype inevitably led to gaps and overlaps in time as recorded by rocks composing the stage. Current practice is to define only the boundaries between stages by an actual stratotype. The boundary is called the Global-Boundary Stratotype Section and Point (GSSP), and always defines the lower boundary of a stage (which automatically becomes the upper boundary of the underlying stage). In practice, GSSPs are located as closely as possible to traditional stage boundaries. However, priority is not a requirement in chronostratigraphy, and the most important requirement of the GSSP is that it be easily correlatable by at least one distinctive boundary event (such as a fossil marker or magnetic reversal), and preferably by several secondary markers.

The portion of the Neogene recovered during Leg 161 includes the middle Miocene Serravallian Stage, the upper Miocene Tortonian and Messinian Stages, the lower Pliocene Zanclean Stage, the middle Pliocene Piacenzian Stage, and the upper Pliocene Gelasian Stage. All these stages are stratotypified in Italy; formal GSSPs have recently been approved for the Piacenzian and Gelasian Stage boundaries (Cita et al., 1996; Cita, 1996a, 1996b).

Microfossils present in the traditional unit stratotypes have been exhaustively studied, and various schemes for correlation have been proposed. The LAD of Sphenolithus heteromorphus occurs just above the base of the Serravallian stratotype section, and this species (which marks the NN5/NN6 boundary) can be used to approximate the base of the stage (Rio and Fornaciari, 1994; Fornaciari et al., 1996). Rio and Fornaciari (1994) suggested that the base of the Tortonian be defined close to the FAD of the foraminifer Neogloboquadrina acostaensis, which occurs in the lower part of the stratotype section. The FCO of N. acostaensis occurs just before the FAD of Discoaster hamatus (the nannofossil marker for the NN8/NN9 boundary). The lower boundary of the Messinian Stage corresponds to the FAD of the foraminifer Globorotalia conomiozea. The nannofossil bioevent closest to this boundary is the FO of Amaurolithus primus.

The Miocene/Pliocene Series boundary is based on an historic event—the reflooding of the Mediterranean and the return to normal marine conditions following the Messinian salinity crisis (Benson, 1995). By this definition, no single nannofossil biochronological event occurs at this series boundary. Indeed, Benson (1995) pointed out that no one has demonstrated any global stratigraphic event that correlates with the reflooding of the Mediterranean. In this study, we approximate the Miocene/Pliocene boundary using several FOs and LOs. The boundary is in Zone NN12 below the FO of Ceratolithus rugosus, the LO of C. acutus and the LcO of Helicosphaera intermedia, and just above the FO of C. acutus.

The re-establishment of normal marine conditions also defines the base of the lower Pliocene Zanclean Stage. The disappearance of the foraminifer Globorotalia margaritae occurs close to the base of the Piacenzian Stage stratotype. The LO (or sometimes the LCO of G. margaritae) has sometimes been taken as the marker for the lower boundary of the Piacenzian (e.g., Rio et al., 1984; Langereis and Hilgen, 1991). More recently, Rio et al. (1994) have shown the base of the Piacenzian to be at approximately the "LO" (temporary disappearance of Berggren et al. [1995b]) of G. puncticulata. This event occurs in lower NN16A. Using nannofossils, we can therefore only approximate the lower/middle Pliocene boundary at just above the LOs of Reticulofenestra pseudoumbilicus (>7 Ám) and Sphenolithus abies/S. neoabies.

Rio et al. (1994) proposed that the traditional two stage-two subseries Pliocene be expanded into three units, and this proposal has recently been accepted by the IUGS (Cita, 1996a, 1996b). The Zanclean Stage remains equivalent to the lower Pliocene, but the Piacenzian is now the middle Pliocene and a new stage, the Gelasian, is the upper Pliocene. The main rationale for establishing the Gelasian is that a smaller chronostratigraphic unit (i.e., the Gelasian) can be recognized and correlated globally, thus improving stratigraphic resolution for this interval. The base of the Gelasian lies just below the NN17/NN18 boundary (Rio et al., 1994).

The Pliocene/Pleistocene boundary has long been a controversial stratigraphic topic. The location of the best stratotype section, the position of the boundary within the various sections proposed, the events to be used for correlation, and even the very concept of what the "Pleistocene" should be have been debated (e.g., see Berggren, 1971; Berggren and Van Couvering, 1974; Rio et al., 1991; Berggren et al., 1995a). International agreement seems to have been achieved at last by acceptance of the Vrica section in Calabria, Italy, as the boundary stratotype section, and designation of the top of the laminated level "e" unit within this section as the base of the Pleistocene (Aguirre and Pasini, 1985). This GSSP lies approximately at the top of the Olduvai subchron (C2n), and has been dated at 1.83 Ma (Sprovieri, 1993), or 1.81 Ma (Berggren et al., 1995a). The nannofossil marker for the Pliocene/Pleistocene boundary, the FAD of Gephyrocapsa oceanica (>4.0 Ám), is slightly younger (1.75 Ma; Sprovieri, 1993) than the GSSP.

"Neogene" is a term long-used in the geological literature for the Miocene and Pliocene interval. Berggren et al. (1995a) have recently argued that the terms "Tertiary" and "Quaternary" should be discarded and replaced by "Paleogene" and "Neogene." In their treatment, the Paleogene would include the Paleocene, Eocene, and Oligocene Epochs/Series as before, but the Neogene would include the Miocene and Pliocene as well as the Pleistocene and Holocene Epochs/Series. In this study, we continue to use Neogene in its long-understood meaning—the interval comprising the Miocene and Pliocene.

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