BIOSTRATIGRAPHY

Sediments recovered from Hole 900A provide a discontinuous record for Pleistocene through the upper Paleocene. Calcareous nannofossils generally are abundant to very abundant, and preservation varies throughout the recovered successions. Planktonic foraminifers generally are common to abundant in the upper sections of Hole 900A, but samples from the deeper sections contained fewer specimens.

Calcareous Nannofossils

Site 900 is located near the eastern edge of the Iberia Abyssal Plain at a water depth of 5037 m. The calcareous nannofossils define four major stratigraphic successions within the Cenozoic sediments; one succession from the upper Pleistocene (Zone NN21 of Martini, 1971) to the upper lower Pliocene (Zone NN15), a second from the upper-most Miocene (base Zone NN12) to the upper Miocene (Zone NN11), a third from the middle Miocene (NN7) to the middle Eocene (NP14), and a fourth restricted to the upper Paleocene (NP9). Two major hiatuses, representing most of the lower Pliocene and the lower upper Miocene, correspond to those observed at Sites 897, 898, and 899.

In the interval from the upper Pleistocene to the upper lower Pliocene, calcareous nannofossils are abundant and well preserved. Nannofossils are generally abundant and moderately well preserved from the Miocene to the Oligocene. The Eocene succession is characterized by fewer, moderately preserved calcareous nannofossils, or by barren samples. From Core 149-900A-78R, upper Paleocene sediments were recovered that contain abundant and well-preserved nannofossils. No sediments were recovered below Core 149-900A-79R. Ages of calcareous nannofossils are summarized in Figure 16.

Pleistocene

Section 149-900A-1R contains rare to few Emiliania huxleyi and very abundant small Gephyrocapsa spp. (<2.5 µm), indicating the lower part of Zone NN21. Samples 149-900A-2R-1, 28 cm, and -2R-CC were assigned to Zone NN20 by the absence of Emiliania huxleyi and Pseudoemiliania lacunosa. Intervals 149-900A-3R-1, 83 cm, highest occurrence (HO) of Pseudoemiliania lacunosa, to -6R-4, 15 cm, lowest occurrence (LO) of Gephyrocapsa caribbeanica (>4.0 µm), were placed in Zone NN19. Mediterranean subzones of Rio, Fornaciari et al. (1990) were used within this succession. Samples 149-900A-3R-1, 83 cm, and -3R-CC include Pseudoemiliania lacunosa and Reticulofenestra sp. A (>6.5 µm), indicating Zone NN19F. The LO of small Gephyrocapsa omega (>3.8 µm) in Sample 149-900A-3R-CC indicates the lower part of Zone NN19F. The presence of very abundant small Gephyrocapsa spp. (<2.5 µm) and common Reticulofenestra sp. A (>6.5 µm) place Sample 149-900A-4R-3, 80 cm, in Zone NN19E. Sample 149-900A-5R-2, 17 cm, contains few Helicosphaera sellii and common Gephyrocapsa spp. (>5.5 µm), indicating the upper part of Subzone NN19C. The absence of large Gephyrocapsa spp. (>5.5 µm) in Sample 149-900A-5R-CC indicates the lower part of Zone NN19C. Sample 149-900A-6R-4, 15 cm, contains the HO of Calcidiscus macintyrei (cir.; >10.0 µm), few Gephyrocapsa oceanica (>4.0 µm) and Gephyrocapsa caribbeanica (>4.0 µm). This assemblage indicates Zone NN19B.

Pliocene

The Pliocene/Pleistocene boundary was placed between Samples 149-900A-6R-4, 15 cm (LO of Gephyrocapsa oceanica [>4.0 µm]) and 149-900A-6R-6, 6 cm (HOs of Discoaster brouweri and Discoaster triradiatus). A few Discoaster brouweri and Discoaster triradiatus co-occur in Samples 149-900A-6R-6, 6 cm, and -6R-CC, indicating the upper part of Zone NN18. Intervals 149-900A-7R-CC to -8R-CC contain only rare Discoaster brouweri and Discoaster triradiatus. The co-occurrence of Discoaster pentaradiatus and Discoaster surculus in Sample 149-900A-9R-1, 99 cm, defines the top of Zone NN16. Discoaster tamalis appears in Sample 149-900A-9R-CC. The HO of Reticulofenestra pseudoumbilica was observed in Sample 149-900A-10R-CC, together with abundant Discoaster asymmetricus, indicating Zone NN15. The upper/lower Pliocene boundary, equivalent to the zonal boundary NN15/NN16, lies between Samples 149-900A-10R-4, 128 cm, and -10R-CC. Most of the lower Pliocene succession is missing in Hole 900A.

Miocene

Sample 149-900A-11R-4, 93 cm, contains Amaurolithus delicatus, Triquetrorhabdulus extensus, but lacks Discoaster quinqueramus. This assemblage indicates the lower part of Zone NN12, which has been placed in the uppermost Miocene. An unconformity, representing most of the lower Pliocene sequence, lies between Samples 149-900A-10R-CC (acme in Discoaster asymmetricus) and -11R-4, 93 cm (presence of Triquetrorhabdulus extensus). Samples 149-900A-11R-CC to -16R-5, 86 cm, contain Discoaster quinqueramus, the total range of which defines Zone NN11. The LO of Amaurolithus delicatus was observed in Sample 149-900A-13R-CC and defines the NN11a/ NN11b boundary. The LO of Discoaster berggrenii can be observed in Sample 149-900A-16R-CC, indicating the lower part of Zone NN11 or the upper part of Zone NN10. The absence of Discoaster bollii, Discoaster hamatus, and Catynaster calyculus between this sample and Sample 149-900A-17R-3, 54 cm (HO of Cyclicargolithus floridanus ) indicates an unconformity representing Zones NN8 to NN10. Samples 149-900A-17R-3, 54 cm, and -17R-CC were placed in Zone NN7 on the basis of the presence of rare Cyclicargolithus floridanus and common Coccolithus miopelagicus and Triquetrorhabdulus rugosus. The NN6/NN7 zonal boundary was difficult to define because of only a rare occurrence of Discoaster kugleri; however, it was placed between Samples 149-900A-17R-CC and -18R-3, 97 cm, on the basis of the LO of Triquetrorhabdulus rugosus. The interval 149-900A-19R-CC to -20R-CC contains rare to few Discoaster deflandrei, few Calcidiscus premacintyrei, and rare Reticulofenestra umbilica, which indicate Zone NN5. The HO of Sphenolithus heteromorphus occurs in Sample 149-900A-20R-2, 89 cm. In the absence of Helicosphaera ampliaperta, the NN4/NN5 boundary may be approximated by the end of the Discoaster deflandrei acme (Rio, Raffi, et al., 1990) (Sample 149-900A-22R-1, 122 cm), by the LOs of Discoaster formosus and Discoaster moorei (Sample 149-900A-21R-1, 94 cm), and by the LO of Discoaster exilis (Sample 149-900A-20R-CC). This boundary was placed above the LO of Discoaster exilis. The LO of Sphenolithus heteromorphus occurs in Sample 149-900A-25R-CC and marks the NN4/NN3 boundary. The HO Triquetrorhabdulus carinatus was ob served in Sample 149-900A-26R-3, 53 cm, and indicates the NN3/ NN2 boundary. From Samples 149-900A-26R-3, 53 cm, to -31R-CC, the nannofossil assemblages include Sphenolithus dissimilis and the S. dissimilis-S. belemnos Intergrade. The LO of the rare Discoaster druggii was difficult to place because of the occurrence of overgrowth specimens, which were slightly smaller than 15 µm, between Sample 149-900A-32R-CC and -34R-CC. Therefore, the NN2/NN1 boundary was placed by the LO of Helicosphaera elongata (Sample 149-900A-34R-CC). The interval from Samples 149-900A-35R-CC to -38R-CC was assigned to Zone NN1. The calcareous nannofossil assemblages in this interval are characterized by rare Triquetrorhabdulus carinatus and few Reticulofenestra bisecta, Clausicoccus fenestratus, Clausicoccus tasmaniae, Helicosphaera perch-nielseniae, Helicosphaera intermedia, Helicosphaera euphratis, Discoaster woodringii, and Zygrhablithus bijugatus. Sphenolithus conicus was present from Samples 149-900A-36R-CC to -37R-CC.

Oligocene

The Oligocene/Miocene boundary, situated in the middle of Zone NN1, lies between Samples 149-900A-36R-CC and -37R-CC and was identified by the occurrence of common Reticulofenestra bisecta, Zygrhablithus bijugatus, and by the HO of Pontosphaera enormis in Sample 149-900A-37R-CC. The HO of Sphenolithus ciperoensis in Sample 149-900A-39R-3, 147 cm, defines the top of Zone NP25. The interval from this sample to Sample 149-900A-44R-CC was placed in Zone NP25. Sample 149-900A-39R-CC contains common Sphenolithus ciperoensis and very rare Sphenolithus distentus; the absolute HO of Sphenolithus distentus was not used at Site 900 to define the top of Zone NP24, because of its very rare, sporadic occurrence through out Zone NP25. Instead, the zonal NP24/NP25 boundary was defined by the highest common occurrence of Sphenolithus distentus in Sample 149-900A-44R-CC. The NP23/NP24 boundary was placed between Samples 149-900A-47R-CC and -48R-CC on the basis of the change in dominance from common Sphenolithus distentus to common Sphenolithus predistentus. This event was substituted for the LO of Sphenolithus ciperoensis (Sample 149-900A-44R-CC), the common marker for this boundary. The LO of Sphenolithus distentus was observed in Sample 149-900A-48R-CC. The interval below, down to Sample 149-900A-51R-4, 143 cm, was assigned to Zone NP23 on the presence of a few, large Helicosphaera compacta, common Sphenolithus predistentus, and on the absence of Reticulofenestra umbilica. The LO of Coronocyclus nitescens occurs in Sample 149-900A-49R-CC . Zones NP22 and NP21 from the lower Oligocene were not observed, and an unconformity was interpreted as existing between Samples 149-900A-51R-4, 143 cm (Zone NP23), and -51R-CC at 7 cm (Zone NP19/20).

Eocene

The interval 149-900A-51R-CC, 7 cm, to -54R-CC was assigned to Zone NP19/20, as indicated by the highest co-occurrence of rare Discoaster saipanensis, very rare Discoaster barbadiensis, Cyclococcolithus formosus, Reticulofenestra hillae, Reticulofenestra umbilica, and by the acme of Clausicoccus subdistichus. The HOs of Discoaster saipanensis and Discoaster barbadiensis were used to define the Eocene/Oligocene boundary. Zones NP19 and NP20 were not separated because the LO of Sphenolithus pseudoradians, the zonal marker for the base of NP20, could not be used at Hole 900A because it was also observed in Zone NP17. Sphenolithus pseudoradians was not observed at other sites during Leg 149. The LO of Isthmolithus recurvus occurs in Sample 149-900A-54R-CC and marks the NP18/NP19 boundary. The NP17/NP18 boundary was defined by the HO of Sphenolithus obtusus in Core 149-900A-54R-CC. The LO of large Reticulofenestra bisecta occurs in Sample 149-900A-57R-CC . Chiasmolithus solitus, Sphenolithus spiniger, and Clausicoccus vanheckae appear in Sample 149-900A-59R-CC and indicate the top of Zone NP16. Sample 149-900A-60R-CC contains the HOs of Neococcolithes dubius, Pseudotriquetrorhabdulus inversus, Pyrocyclus inversus, and numerous species of genera Micrantholithus, Pemma, and Braarudospohaera. The HOs of Nannotetrina alata and Discoaster wemmelensis were observed in Sample 149-900A-67R-CC. Sample 149-900A-68R-CC was assigned to Zone NP15, as indicated by the absence of Reticulofenestra umbilica. Chiasmolithus gigas occurs in Sample 149-900A-69R-CC. The HO of Discoaster lodoensis in Sample 149-900A-75R-CC defines the top of Zone NP14.

The early Eocene Zones NP13 to NP10 have not yet been observed at Site 900. The sediments in Cores 149-900A-76R and-77R are composed mostly of noncalcareous brown claystone layers separated by rare calcareous claystones that contain few-to-common calcareous nannofossils.

Paleocene

Upper Paleocene sediments from Zone NP9 occur in Sample 149-900A-78R-CC and represent the oldest Cenozoic sediments containing calcareous nannofossils that were recovered during Leg 149. The assemblage contains abundant and well-preserved calcareous nannofossils. The presence of common Discoaster multiradiatus, few Fasciculithus bobii, F. hayii, Prinsius martinii, and Campylo sphaera eodela (>7.0 µm) indicates the middle of Zone NP9.

Foraminifers

Planktonic foraminifers in core-catcher samples from Hole 900A were examined to establish preliminary ages for the sediments (Fig. 16). The number of planktonic foraminifers was generally high in the upper sections of the hole; most of the samples yielded a high-diversity planktonic foraminiferal assemblage having good to moderate preservation (Table 6). Fewer, and more poorly preserved, planktonic foraminifers were present in the lower sections of Hole 900A. Similar patterns were observed in the benthic foraminiferal assemblages.

Hole 900A

Samples 149-900A-1R-CC and -2R-CC are characterized by the presence of Globorotalia truncatulinoides and can be assigned to Zone N23, which is of Pleistocene to Holocene age. The co-occurrence of Globorotalia truncatulinoides and Globorotalia tosaensis in the interval 149-900A-3R-CC to -6R-CC enables us to place it in Zone N22 or the lower part of Zone N23, which is of latest Pliocene to early Pleistocene age. The interval 149-900A-7R-CC to -9R-CC is characterized by the presence of Globorotalia inflata and the absence of Globorotalia tosaensis. It can be assigned to the interval from the top of Zone N19 to Zone N21, which is of late Pliocene age. Sample 149-900A-10R-CC is characterized by the presence of Globorotalia crassaformis crassaformis and Sphaeroidinellopsis paenedehiscens and the absence of Globorotalia inflata and Globigerina nepenthes; this sample can be assigned to the upper part of Zone N19, which is of late early to early late Pliocene age. Abyssal and reworked shallow-water benthic foraminifers were present in all these samples.

The interval 149-900A-11R-CC to -16R-CC contains Globigerina nepenthes and Neogloboquadrina pachyderma, while every species variant of the Globorotalia crassaformis group is absent. This interval can be assigned to Zones N16 to N17, which are of late Miocene age. A hiatus was interpreted between Samples 149-900A-10R-CC and -11R-CC, where the latest Miocene to early Pliocene age sediments (Zone N18 and the early part of Zone N19) are missing.

Sample 149-900A-17R-CC contains only the marker species Globigerina nepenthes, which ranges from Zone N14 to the lower part of N19. As Zone N17 had already been reached in Sample 149-900A-11R-CC, Sample 149-900A-17R-CC was assigned to Zones N14 to N17, which are of late middle to late Miocene age. Sample 149-900A-18R-CC is characterized by the presence of Globorotalia acrostoma, Globorotalia siakensis, and Globorotalia mayeri and the absence of Praeorbulina spp. It can be assigned to Zones N9-N11, which are of middle Miocene age. Although Zones N12 and N13 were not identified in core-catcher samples, calcareous nannofossil evidence does not sup port an unconformity between Samples 149-900A-17R-CC and -18R-CC. Sample 149-900A-19R-CC is characterized by the presence of Praeorbulina glomerosa curva and P. glomerosa glomerosa and can be assigned to Zone N8, which is of early middle Miocene age.

The co-occurrence of Praeorbulina sicana and Catasydrax stainforthi and the absence of Praeorbulina glomerosa glomerosa in Sample 149-900A-20R-CC allow us to assign this sample to the upper part of Zone N7, which is of early Miocene age. Samples 149-900A-21R-CC and -22R-CC do not contain Praeorbulina sicana, thus these samples were placed in the lower part of Zone N7. The interval 149-900A-23R-CC to -27R-CC is marked by the presence of Catapsydrax dissimilis and Catapsydrax unicavus, thereby allowing us to assign it to Zones N5 to N6, which are of early Miocene age. Within this interval, Sample 149-900A-24R-CC is barren of planktonic foraminifers.