The Oligocene to Miocene calcareous nannofossil assemblages recovered during Leg 149 are presented in detail on the range charts (Table 3, Table 5, Table 7, Table 9). A short discussion for each hole gives a synthesis of the age and major problems encountered, followed by a description of Martini's standard zonation (1971) and a description of the succession of biohorizons considered important for biostratigraphic interpretations and paleoenvironmental correlations. Martini's index fossils are used to describe the age and zone assignments of the Oligocene/Miocene interval except two zonal markers: the last occurrence (LO) of Helicosphaera recta used to define the Zone NP25/ NN1 boundary (LO observed in Zone NN2) and the first occurrence (FO) of Catinaster coalitus (not recorded in this study).
This site is located on the eastern edge of the Iberia Abyssal Plain (Fig. 1, Table 1). The nannofossil zonal marker events and the important biohorizons are given in Figure 2; the sample intervals and average depths of these events are listed in Table 2 and the stratigraphic distribution of calcareous nannofossil are indicated in Table 3. Nannofossils are rare to very abundant and moderately well-preserved throughout the Oligocene-Miocene sequence. Two intervals barren of calcareous nannofossils were observed: a short one in the lower Miocene Zone NN2, and a longer interval (425-443 m), intercalated with samples yielding poorly preserved nannofossils. We place the Zone NP25/NN1 boundary at the LO of Sphenolithus ciperoensis (Fig. 3), which was observed in Sample 149-897C-37R-5, 27-28 cm; in the same sample the LO of Reticulofenestra bisecta was detected. The barren samples did not allow a better resolution of Zone NP25. Nevertheless, we placed the Zone NP25/NP24 boundary at Sample 149-897C-42R-1, 41-42 cm, based on the LO of Sphenolithus predistentus, just below the barren interval, although the LO of Sphenolithus distentus was observed one sample below, in Sample 149-897C-42R-3, 61-62 cm. This assignment was inferred assuming that S. predistentus, as described for Site 900, is considered a good alternative marker for the top of Zone NP24, as it was never found in Zone NP25. The FO of S. ciperoensis, detected in Sample 149-897C-45R-5, 4-5 cm, marks the base of Zone NP 24. The Zone NP23/NP22 boundary was identified in Sample 149-897C-49R-1, 128-129 cm with the LO of Reticulofenestra umbilicus and the LO of Isthmolithus recurvus. The LO of Ericsonia formosa detected in the same sample is considered unreliable and is ascribed to reworking. Yet the presence of a short Zone NP21 is arguable: the Zone NP21/NP22 boundary could be inferred to be at the drop in abundance of E. formosa, between Sample 149-897C-49R-3, 123-124 cm and Sample 149-897C-49R-5, 16-17 cm (Fig. 3). The FO of Sphenolithus akropodus in Sample 149-897C-49R-3, 123-124 cm could confirm this hypothesis (see "Site 900").
Site 898 is located on the eastern edge of the Iberia Abyssal Plain (Fig. 1, Table 1). The purpose of drilling this site was to elucidate the nature of the basement at the ocean/continent transition. About 870 m of sediment overlie the basement that has been defined by seismic interpretation. Hole 898A recovered the upper 340 m of sediment, consisting of turbidite and contourite sequences. The nannofossil zonal marker events and the important biohorizons are given in Figure 4. The sample intervals and average depths of these events are listed in Table 4. The stratigraphic distribution of calcareous nannofossil are indicated in Table 5.
The Oligocene-Miocene interval extends from Samples 149-898A-18X-4, 105-106 cm, to 36X-CC and ranges from the middle Miocene Zone NN7 to the late Oligocene Zone NP25. Medium to well-preserved, common to very abundant nannofossils are found in most of the samples. Only the hemipelagic or pelagic intervals were sampled in order to have better material and avoid as much as possible reworking problems. Nevertheless, in the pelagic intervals, consistent, rare reworked Cretaceous species were found, as well as rare, reworked Cenozoic species. The stratigraphic distribution of the main Helicosphaera and Sphenolithus species across the Oligocene/ Miocene boundary is given in Figure 5.
The upper Miocene Zones NN8 to NN12 are not present and a major unconformity is placed between Samples 149-898A-18X-4, 58-59 cm (Pliocene), and Samples 149-898A-18X-4, 134 cm. Samples 149-898A-18X-4, 105-106 cm and 149-898A-18X-4, 142 cm are red clay layers barren of nannofossils, which may be related to a temporary drop of the CCD. The presence of Discoaster kugleri and the absence of Cyclicargolithus abisectus (>10 µm) in Sample 149-898A-18X-4, 134 cm indicate middle Miocene age in the upper part of Zone NN7. The Miocene interval extends down to Sample 149-898A-28X-2, 49-50 cm based on the LO of S. ciperoensis in Sample 149-898A-28X-3, 145-146 cm (Fig. 5). The Oligocene interval extends down to Sample 149-898A-36X-CC. The presence of S. ciperoensis and the absence of S. distentus indicate that the Oligocene interval recovered is represented only by Zone NP25. On the presence of Helicosphaera obliqua in Sample 149-898A-36R-1, 108-109 cm, we assume an age not younger than lower part of Zone NP25 for the base of Hole 898A. Zones NN2 and NP25 appear to be very thick. In the Martini's zonal scheme, Zone NN2 represents 4.0 Ma based on the Discoaster druggii FO at the top of chronozone C6C (23.3 Ma) Gartner (1992). It is one of the longest Cenozoic nannofossil zones, and sixteen biohorizons are recognized in Zone NN2 in Hole 898A (see discussion of biohorizons).
This site is located on the eastern edge of the Iberia Abyssal Plain (Fig. 1, Table 1). The nannofossil zonal marker events and the important biohorizons are given in Figure 6; the sample intervals and average depths of these events are listed in Table 6; and the stratigraphic distribution of calcareous nannofossil are indicated in Table 7. Two holes were drilled. Miocene sediments were recovered from Hole 899A to the lower Miocene Zone NN2, where drilling was stopped by an abrupt lithological change. Lower Miocene to lower Oligocene sediments were recovered from Hole 899B.
The Zone NN1/NP25 boundary was detected in Sample 149-899B-4R-CC, at the same level the LO of R. bisecta was observed; within Zone NP25 the drop in abundance of S. ciperoensis, indicated as S. ciperoensis last common occurrence (LCO) (Table 6), occurs in Sample 149-899B-6R-1, 68-69 cm. The LO of S. distentus marks the base of Zone NP25 in Sample 149-899B-10R-2, 147-148 cm, revealing an expanded thickness for the latter biozone.
The base of Zone NP24 is placed at Sample 149-899B-11R-3, 90-91 cm, but common S. ciperoensis occur only from Sample 149-899B-10R-2, 147-148 cm, up hole. Since the FCO of S. ciperoensis coincides with the base of Zone NP25, the result is a reduced thickness for Zone NP24 at Site 899 compared to the other sites studied. Therefore, we assume the presence of a hiatus involving the upper part of Zone NP24. The LO of R. umbilicus was observed in Sample 149-899B-14R-2, 15-16 cm, together with the I. recurvus LO. Therefore the Zone NP23/NP22 boundary was positioned at 347.28 m.
This site is located on the eastern edge of the Iberia Abyssal Plain (Fig. 1, Table 1). The nannofossil zonal marker events and the important biohorizons are given in Figure 7; the sample intervals and average depths of these events are listed in Table 8. The Oligocene-Miocene nannofossil assemblages are abundant, well-preserved, and highly diverse, having numerous species of Discoaster, Helicosphaera, Sphenolithus, and holococcoliths. A detailed range chart (Table 9), including more than 220 species, is presented. The LO of Cryptococcolithus mediaperforatus is used to define the uppermost Miocene zone (lower part of Zone NN12) in Sample 149-900A-11R-CC. Part of the lower upper Miocene, Zones NN9 and NN10, is missing. The interval around 120 m includes some numerous thin silt layers and an interval barren of calcareous nannofossils between 113.65 and 112.70 mbsf. The Oligocene/Miocene boundary (top of Zone NP25) is placed at the LO of S. ciperoensis in Sample 149-900A-35R-1, 125-126 cm. Recently Fornaciari et al. (1993) proposed a redefinition of the Zone NP25/NN1 boundary using the LO of S. ciperoensis, because the original definition using the LO of H. recta has not proven reliable in many areas (e.g., Fornaciari et al. 1990; see discussion below in biohorizon description). Just below, at Sample 149-900A-35R-2, 121-122 cm, the LO of R. bisecta was observed. The LCO of S. ciperoensis is easily recognizable in Sample 149-900A-37R-5, 53-54 cm. Its sporadic occurrence above this level is not believed to be due to reworking, since the same distribution pattern has been found at Sites 898 and 899.
The LO of S. distentus was observed in Sample 149-900A-45R-4, 112-113 cm. S. predistentus LO was observed 1 m below, in Sample 149-900A-45R-5, 82-83 cm, and was used to approximate the Zone NP24/NP25 boundary. The FO of S. ciperoensis, detected in Sample 149-900A-49R-3, 113-114 cm, marks the base of Zone NP 24. In its lower range, S. ciperoensis is rare, but becomes more abundant upward within Zone NP24 (Fig. 8). This zone is characterized, in terms of sphenoliths, by peaks in abundance of S. distentus. The interval between Sample 149-900A-49R-3, 113-114 cm, and Sample 149-900A-51R-2, 124-125 cm was assigned to Zone NP23. In the latter Sample, R. umbilicus LO was recognized. The LO of I. recurvus is 1 m below, in Sample 149-900A- 51R-3, 50-51 cm. This form is widely recognized as a useful marker in approximating the top of Zone NP22. S. distentus is present below the LO of R. umbilicus, thus demonstrating that the first occurrence of this taxon is not a reliable marker for the CP17/CP18 boundary (middle of Zone NP23), as already suggested by many authors (e.g., Backman, 1987; Fornaciari et al. 1990; Olafsson and Villa, 1992). In Zone NP23 high abundances of S. predistentus were found in association with S. akropodus. E. formosa underwent a marked decrease in abundance, which we use to tentatively place the Zone NP21/NP22 boundary at the horizon in Sample 149-900A-52R-4, 143-144 cm. The persistence of this taxon above R. umbilicus LO is due to reworking. The lower Oligocene interval at Site 900 is marked by the presence of more reworked species. On the basis of the FO of Ilselithina fusa in Sample 149-900A-52R-3, 65-66 cm (483.99 mbsf), known to occur in the early Oligocene, and the juxtaposition of the FO of S. akropodus (481.50 mbsf) to the LO of E. formosa (481.50 mbsf), we infer the presence of a short Zone NP21 in the interval between these two bioevents; nevertheless it is uncertain if the presence of E. formosa could be ascribed to reworking. In fact in this interval few Discoaster saipanensis and Discoaster barbadiensis are found reworked. Below the Ilselithina fusa FO, the succession is assigned to latest part of the Eocene. This determination is based on the common occurrence of D. barbadiensis and D. saipanensis and the absence of Reticulofenstra reticulata. The Eocene/Oligocene boundary interval is incomplete in this section, as evidenced by the reduced thickness of Zone NP21.
The stratigraphic correlations among Leg 149 sites are presented in Figure 9. Epoch boundaries are drawn from site to site with a major biostratigraphic marker.