Drilling at Site 1133 identified two major biostratigraphic units that were dated by nannofossils and planktonic foraminifers as Pleistocene and middle-early Miocene. The Pleistocene section extends down to 21.49 mbsf and includes a slump at the base from 19.9 to 21.4 mbsf. The unit overlies a thin, highly condensed interval with lower Pliocene-upper Miocene assemblages between two firmgrounds at ~23.5 and 29 mbsf. The upper firmground marks a hiatus of ~3 m.y. The firmground at 29 mbsf separates the lower Pliocene-upper Miocene (Zone NN12) from the middle Miocene (Zone NN6), an interval of ~6 m.y. Three main benthic foraminifer assemblages are recognized: a diversified Pleistocene calcareous assemblage, a diversified upper Miocene assemblage, and an impoverished middle-lower Miocene assemblage. All three assemblages indicate middle to lower bathyal paleodepths.
Calcareous nannofossils indicate that the stratigraphic sequence at Site 1133 consists of three main biostratigraphic units: a relatively thin Pleistocene section (~22 m thick) with moderately preserved assemblages (combined Zones NN21-NN20 and Zone NN19); a poorly preserved lower Pliocene-uppermost Miocene assemblage (Zone NN12) from a condensed section (~7 m thick, between 23.48 and 31.36 mbsf); and a relatively thick middle-lower Miocene section (>110 m thick) containing poorly to moderately preserved assemblages (Zone NN6 to combined Zones NN5-NN4).
The disconformity at the base of the Pleistocene at ~22 mbsf represents a hiatus equivalent to most of the Pliocene. Another hiatus of ~6 m.y. is suspected between the lower Pliocene-uppermost Miocene and the middle Miocene.
Sample 182-1133B-1H-CC (2.37 mbsf) yielded an assemblage readily assignable to the combined Zones NN21-NN20. This assemblage contains Gephyrocapsa caribbeanica, Helicosphaera carteri, Calcidiscus leptoporus, small Gephyrocapsa spp., and Umbilicosphaera sibogae. Zone NN19 assemblages occur in Samples 182-1133B-2H-CC (11.8 mbsf) and 4H-1, 62-64 cm (22.02 mbsf). In addition to the key species Pseudoemiliania lacunosa, these assemblages include Braarudosphaera bigelowii, C. leptoporus, Calcidiscus macintyrei, Coccolithus pelagicus, Dictyococcites productus, H. carteri, Reticulofenestra minutula, Reticulofenestra minuta, Rhabdosphaera clavigera, and Scyphosphaera spp.
The Zone NN19 assemblage at 22.02 mbsf (Sample 182-1133B-4H-1, 62-64 cm) is followed downhole by a lower Pliocene-upper Miocene Zone NN12 assemblage at 23.48 mbsf (Sample 182-1133C-3H-CC), indicating a hiatus of ~3.3 m.y. Most of the Pliocene appears to be missing.
The lower Pliocene-uppermost Miocene assemblage of Zone NN12 from Sample 182-1133C-3H-CC (23.48 mbsf) contains Amaurolithus delicatus, Amaurolithus ninae, Discoaster brouweri, Discoaster pentaradiatus, Discoaster surculus, Discoaster variabilis, and Scyphospaera spp., as well as Discoaster barbadiensis recycled from Eocene sediments. Assemblages from Samples 182-1133B-4H-CC (31.36 mbsf) through 7H-CC (50.72 mbsf) contain an association of species suggestive of Zone NN6, including C. macintyrei, Calcidiscus premacintyrei, Reticulofenestra gelida, and Reticulofenestra pseudoumbilicus. These assemblages also contain reworked species from older Miocene and Paleogene sediments (e.g., Cyclicargolithus floridanus, Sphenolithus heteromorphus, and Dictyococcites bisectus). A lithostratigraphic boundary at ~28 mbsf separates the assemblages of Zone NN12 from Zone NN6. A hiatus of ~6 m.y. is suspected at this boundary.
The key species for the combined Zones NN5-NN4, S. heteromorphus, consistently occurs from Sample 182-1133B-8H-CC (55.91 mbsf) down to the lowest section obtained from Hole 1133B (Sample 182-1133B-19X-H-CC at 142.56 mbsf). In addition to S. heteromorphus, assemblages from this thick section (~85 m) include B. bigelowii, Dictyococcites antarcticus, C. premacintyrei, C. floridanus, H. carteri, Helicosphaera euphratis, and Sphenolithus moriformis.
Sediments recovered at Site 1133 contain Pleistocene-middle Mio-cene planktonic foraminifer assemblages. The Pleistocene section extends down to ~21 mbsf and overlies a thin, highly condensed lower Pliocene-upper Miocene unit (22-28 mbsf). The disconformities at ~22 and ~28 mbsf represent hiatuses of ~3 and ~6 m.y., respectively. The underlying Miocene sediments are mainly of middle Miocene age, although poor preservation hampered the recognition of biozones.
Well-preserved planktonic foraminifers of Pleistocene Zone Pt1 were found to 21.33 mbsf (Sample 182-1133B-3H-CC, 13-16 cm). Common species include Globorotalia truncatulinoides, Globorotalia inflata, Globigerinoides ruber, Zeaglobigerina rubescens, Globigerina bulloides, Globigerina falconensis, Globigerina quinqueloba, Neogloboquadrina pachyderma, and Orbulina universa. The entire interval probably represents mainly Subzone Pt1b, because the Subzone Pt1a index species Globorotalia tosaensis occurred only at 21.33 mbsf and immediately below, in Sample 182-1133B-4H-1, 4-6 cm (21.44 mbsf). The stratigraphic interpretation of this unit is complicated by a slump from 19.9 to 21.4 mbsf.
The Pliocene Zone Pl1 assemblage is mainly composed of Globorotalia crassaformis, Globorotalia puncticulata, Globorotalia margaritae, and Zeaglobigerina nepenthes, and Zeaglobigerina woodi, as well as G. falconensis, Globigerinita glutinata, and G. ruber. This assemblage occurs between 21.44 and 23.48 mbsf in Samples 182-1133B-4H-1, 4-6 cm; 4H-1, 62-64 cm; and 182-1133C-3H-CC, 20-23 cm. The latter sample also contains Globorotalia sphericomiozea, Globorotalia conomiozea, and Globigerinoides extremus, indicating a latest Miocene age. The occurrence of this Pliocene-upper Miocene assemblage in such a thin interval suggests that a large interval of time is not represented. The Pliocene-Pleistocene disconformity is ~3 m.y. above the first occurrence of G. truncatulinoides and the first appearance datum of G. crassaformis (<2-4.5 Ma).
Poorly preserved planktonic foraminifers, indicating mainly middle Miocene age, were recorded in the interval from ~31 to 123 mbsf (Samples 182-1133B-4H-CC through 17X-CC). This biofacies change coincides with the lithostratigraphic Unit II/Unit III boundary at ~28 mbsf (see "Lithostratigraphy"). A middle Miocene age is indicated by the association of Globoquadrina dehiscens, Globoconella conoidea, G. panda, Neogloboquadrina nympha, O. universa, and Fohsella periphero-ronda. However, the occurrence of upper Miocene species Globorotalia conomiozea, Globorotalia menardii, and Globorotalia plesiotumida (Samples 182-1133B-4H-CC, 11-14 cm, and 5H-CC, 8-11 cm) is puzzling. These younger taxa are probably downhole contamination. If this is the case, the disconformity at ~28 mbsf, between this middle Miocene section and the overlying thin Pliocene-upper Miocene unit, represents a hiatus of ~6 m.y. with most of the lower part of the upper Miocene missing. Postcruise studies, however, are needed to clarify this assertion. Further downhole, the two chert samples from the base of Hole 1133B (Samples 182-1133B-18X-CC, 30-33 cm, and 19X-CC, 6-9 cm) contain no diagnostic planktonic foraminifers; however, the samples do contain nannofossils of the combined Zones NN4-NN5 of the early-middle Miocene (see "Calcareous Nannofossils") (Fig. F6).
Benthic foraminifers were studied from all core-catcher samples from Hole 1133B. Additional samples were examined from intervals in which marked lithologic change occurred. Benthic foraminifers are generally abundant in the upper part of Hole 1133B (Cores 1H through 4H). Abundance decreases significantly and preservation deteriorates below Core 182-1133B-4H. Between 100 and 300 benthic foraminifers were picked from the >63-µm fraction, except in samples in which abundance was low. The following benthic foraminifer assemblages, containing mainly cosmopolitan taxa, are recognized in the Cenozoic succession at Site 1133.
This relatively diverse assemblage is characterized by Laticarinina pauperata, Hoeglundina elegans, Martinottiella communis, Plectofrondicularia vaughni, Anomalinoides globulosus, Bulimina aculeata, Eggerella bradyi, Pyrgo murrhina, Sphaeroidina bulloides, Planulina wuellerstorfi, Uvigerina hispidocostata, Sigmoilina obesa, Globocassidulina subglobosa, Sigmoilopsis schlumbergeri, Cibicidoides mundulus, Migros sp., Loxostomoides spp., Nodogenerina spp., Anomalinoides spp., and various nodosariids. Middle to lower bathyal paleodepths are suggested by the presence of the depth indicative species S. schlumbergeri, Eggerella bradyi, P. wuellerstorfi, P. murrhina, and P. vaughni.
This well-preserved, relatively diverse assemblage occurs within Core 182-1133B-4H in a discrete lithostratigraphic unit (see "Lithostratigraphy") representing a condensed interval of early Pliocene-late Miocene age, bounded by two disconformities. The assemblage in Sample 182-1133B-4H-1, 62-64 cm, includes taxa characteristic of Assemblage 1. However, the sample contains a higher abundance of P. wuellerstorfi, lower abundance of Nodogenerina spp., and lacks H. elegans, Migros spp., and Loxostomoides spp. Middle to lower bathyal paleo-depths are indicated by the overall composition of the assemblage.
This poorly preserved, impoverished assemblage is found in an interval of poor core recovery, from which mostly chert fragments were recovered. The assemblage consists mainly of Stilostomella spp., Bolivina spp., and Cibicidoides spp. However, Sample 182-1133B-5H-CC contains a slightly more abundant and diverse assemblage than the core-catcher samples below. The assemblage also comprises S. bulloides, Vulvulina spinosa, Eggerella bradyi, Rectuvigerina striata, A. globulosus, Siphonina tenuicarinata, Trifarina spp., Tritaxia spp., and various nodosariids. A few abraded tests of Patellina corrugata and Elphidium spp. within Sample 182-1133B-5H-CC suggest reworking and redeposition from shallower depths. Middle to lower bathyal paleodepths are tentatively proposed for this interval, which lacks distinctive bathymetric indicators. Further work will clarify the paleobathymetry of this interval.
Sediment accumulation rates were calculated from preliminary bio-stratigraphic and paleomagnetic data from Site 1133 and the results are presented in Figure F7. The datum levels used to calculate sedimentation rates are listed in Table T2. Slow sedimentation rates of ~9-16 m/m.y. are calculated for the Pleistocene, in comparison with the high sedimentation rates averaging between ~240 m/m.y. and ~31 m/m.y. in coeval sections at Sites 1127 and 1126, respectively. A disconformity representing a hiatus of ~3 m.y. separates the Pleistocene from an upper Miocene condensed interval at ~21 mbsf. A second disconformity at 28.55 mbsf separates upper Miocene (Zone NN12) from middle Mio-cene (combined Zones NN5-NN4), a hiatus of ~6 m.y. High sedimentation rates of 50 m/m.y. are indicated for the remainder of the lower-middle Miocene succession. However, sedimentation rates for this site remain tentative and should be treated with caution because of the scarcity of reliable datum levels.