BIOSTRATIGRAPHIC ZONATION

The irreversible nature of organic evolution produces a series of nonrepetitive "bioevents." Geologic time, as recorded in marine sediments, is conveniently ordered by stacking these bioevents, which are usually the first occurrence (FO) or last occurrence (LO) of species, into biozones. The bioevents of planktonic organisms are most suitable for long-range biostratigraphic correlation because, as floating organisms when alive, they had widespread geographic distribution. Biostratigraphy allows a stacking of relatively older and younger biozones, but does not give absolute ages (in years) for the biozones.

Biochronology refers to the accurate dating of the evolutionary first appearance or extinction of a species using ages calibrated by radiometric methods alone, or by interpolation between radiometrically calibrated magnetic reversals, stable isotope zones, or astronomically tuned events. These dated levels are known as first appearance datums (FADs) and last appearance datums (LADs) and they are considered to be essentially globally synchronous. The FO or LO of a species in a particular region may, however, be controlled by environmental factors and not correspond to its actual FAD or LAD. Environmental changes may cause migration of a species into or out of a region, or cause marked changes in the relative abundance of the species within the region.

The Pliocene-Pleistocene record in the Mediterranean area is a good example of a time interval and region where FOs and LOs of calcareous nannofossils do not necessarily correspond with those of extra-Mediterranean regions and where certain traditional zonal marker species may be almost absent (Rio et al., 1984; Rio et al., 1990a). These discrepancies are probably the result of the marked climatic-oceanographic changes that occurred in the Mediterranean during the Neogene. Nevertheless, Rio et al. (1990a) produced a high-resolution nannofossil biozonation based on a detailed analysis of terrestrial sections and oceanic cores. The Rio et al. (1990a) zonation is modified after the zonation of Martini (1971; Fig. 2).

The nannofossil event that most closely approximates the Pliocene/Pleistocene boundary (see next section) is the FO of Gephyrocapsa oceanica (>4 Ám). The FO of this species also marks the MNN19A/MNN19B subzonal boundary in the Rio et al. (1990a) zonation. The boundaries of Subzone MNN18 are defined by the same bioevents that Martini (1971) used for Zone NN18, that is, the LO of Discoaster brouweri for the top and the LO of D. pentaradiatus for the base of the zone.

Martini's Zone NN17 is defined as the interval between the LO of D. surculus and the LO of D. pentaradiatus. These LO events are very closely spaced, however, and Zone NN17 is often combined, either with Zone NN18 (e.g., Siesser and Bralower, 1992) or NN16 (e.g., Rio et al., 1990a). The high sedimentation rate in the western Mediterranean allowed the separation of these two LO events in this study, and thus the recognition of a distinct Zone MNN17.

Martini's (1971) Zone NN16 is subdivided by Rio et al. (1990a) into MNN16B and MNN16A by the LO of D. tamalis, a distinctive event in the Mediterranean area. The LO of Reticulofenestra pseudoumbilicus marks the Zone NN15/NN16 boundary. Martini's (1971) Zones NN14 and NN15 were combined by Rio et al. (1990a) because of the rarity of the Zone NN14/NN15 bioevent, the LO of Amaurolithus tricorniculatus, in the Mediterranean region. We also found that A. tricorniculatus was too rare to use as a boundary marker species. Just above the level where the apparent LO of A. tricorniculatus does occur, however, and coincident with the last common occurrence (LCO) of Globorotalia margaritae, we consistently found the LCO of Reticulofenestra pseudoumbilicus (>7 Ám). We have therefore used this nannofossil event to approximate the MNN14/MNN15 boundary. Zone MNN13 has the same top as NN13 (the FO [or FCO in the Mediterranean] of D. asymmetricus). The FO of Ceratolithus rugosus formally marks the lower boundary of Zone NN13, and the LO of C. acutus, a secondary marker, occurs at or near the boundary, but, again, these species occur too infrequently in the Mediterranean to be used for reliable biostratigraphy. There is not, therefore, a good nannofossil marker for the Zone NN12/NN13 boundary in the study area. The last consistent occurrence (LcO) of Helicosphaera intermedia occurs just below the NN12/NN13 boundary in the Mediterranean and this LcO can be used for a rough approximation of the zonal boundary. In the Mediterranean region, the FO of small Pseudoemiliania lacunosa is also found in the upper part of Zone MNN13, and this species can be used as a secondary maker for the zone.

The Rio et al. (1990a) modifications to Martini's (1971) zonation stop with MMN12/NN12. We have subdivided Zone NN12 into Subzones NN12a, NN12b, and NN12c. Subzone NN12a is defined here as the interval from the LO of D. quinqueramus to the FO of C. acutus; Subzone NN12b is the interval from the FO of C. acutus to the LcO of Helicosphaera intermedia; Subzone NN12c is the interval from the LcO of H. intermedia to the LO of C. acutus. The total range of D. quinqueramus defines Zone NN11 in Martini's (1971) nannofossil zonation. This species is always very rare or absent in Mediterranean strata (Rio et al., 1984). We therefore recognized the approximate NN11/MNN12 boundary by using the LCO of Helicosphaera stalis, H. orientalis, Cryptococcolithus mediaperforatus, and Coccolithus miopelagicus. These species range slightly into Zone NN12, but decline markedly in numbers relative to Zone NN11. Zone NN11 can sometimes be subdivided by using the C. pelagicus, Amaurolithus primus, and Reticulofenestra rotaria Subzones of Theodoridis (1984). Martini's standard zonal markers were used for zoning the rest of the upper and middle Miocene. For convenience, and following the practice of Young (1991), we shall simply use "NN," rather than "MNN" for the Martini (1971) zones emended by Rio et al. (1990a; Fig. 2).

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