Hole 1128B was drilled at 34°23.4706´S, 127°5.4455´E at a water depth of 3874.6 m. The Oligocene (Cores 182-1128B-18X through 26X) consists of clay with nannofossils. The biosiliceous component consists of sponge spicules, radiolarians, diatoms, and silicoflagellates. The calcareous microfossils show signs of dissolution. Three samples per core were investigated for radiolarians. Their abundance is rare throughout, and preservation is moderately good. Sponge spicules are abundant. Samples 182-1128B-18X-2, 51-53 cm (157.41 mbsf); 19X-1, 60-62 cm (165.70 mbsf); 20X-1, 60-62 cm (175.30 mbsf); and 22X-2, 82-84 cm (196.22 mbsf) contain abundant unidentified actinommids, rare spyrids, abundant Siphocampe acephala group, Cyclampterium fragments, and the rare presence of Aphetocyrtis rossi, Cornutella profunda, Siphocampe nodosaria, and Stylodictya targaeformis. Samples from Cores 22X through 26X contain, in addition to the above assemblage, rare unidentified eucyrtids, pterocorythids, and carpocaniids. Sponge spicules are abundant throughout, and diatoms are present in rare to common abundance.
Hole 1128C was drilled at 34°23.4623´S, 127°35.4619´E at a water depth of 3874.3 m. Eocene, Oligocene, and Miocene sediments were recovered in Hole 1128C (Tables T1, T2). The Oligocene and Eocene deposits are represented mostly by claystone, except for a brief interval in the lower Oligocene of a pelagic ooze. Cores 182-1128C-8H and 9H (65.0-83.8 mbsf) consist of nannofossil ooze with foraminifers; Cores 10H to 15H (84.0-138.1 mbsf), clayey nannofossil ooze with radiolarians and diatoms; and Cores 16X to 26X (138.3-240.2 mbsf), greenish clay with nannofossil ooze and sponge spicules.
Three samples per core (Cores 182-1128C-8H through 26X) from the thick Oligocene to Miocene sequence recovered on the continental slope were examined for radiolarians. In Cores 182-1128C-8H through 15H, the radiolarians are fragmented and very rare (<300 radiolarians per slide) or absent; siliceous sponge spicules are common, and diatoms vary from trace abundance to few. In Cores 182-1128C-16X through 26X, radiolarians vary in abundance from rare to common (300-3000 per slide), and the preservation is moderately good. Sponge spicules are abundant, and diatoms are present in very rare to moderate abundance. Radiolarian abundance and preservation increases in the lower part of Cores 182-1128C-24X through 26X. The diversity is low throughout the Oligocene (Sections 182-1128C-9H-1 through 26X-CC), but it is difficult to judge whether this is due to the high abundance of sponge spicules diluting the radiolarians or low radiolarian diversity at this locality in the Oligocene. The fauna consists of deep-living forms such as cornutellids and artostrobiids, warm-water forms of pterocorythids, phacodiscids, spongurids, botryoids, eucyrtids, carpocaniids, and litheliids, and shallow living forms represented by cosmopolitan species of spyrids and lophophaenids. The sporadic presence of reworked Cretaceous dictyomitrids have been observed throughout the sequence. The stratigraphic zonation of the sediments containing radiolarians is difficult to determine because index species are absent. However, based on the concurrent presence of A. rossi, Eucyrtidium spinosum, Lophocyrtis (Paralampterium) dumitricai, Periphaena decora, P. heliastericus, and Prunopyle hayesi, the age represented in Samples 182-1128C-9H-1, 25-27 cm (74.75 mbsf), to 26X-CC, 20-22 cm (240.21 mbsf), is interpreted to approximate the Axoprunum (?)irregularis to E. spinosum Zones in the sense of Takemura and Ling (1997) and Hollis et al. (1997). The Oligocene/Miocene boundary between Cores 182-1128C-8H and 9H cannot be placed using radiolarians.
Diatoms were either absent or rare and poorly preserved in the samples examined between Cores 182-1128C-9H and 21X. Zonal markers for both the low-latitude and the high southern-latitude diatom zonations for the Oligocene (i.e., Bogorovia veniamini, Rocella gelida, Rhizosolenia antarctica, and Rhizosolenia gravida) (see Fenner, 1985) are absent, and precise correlation of this interval is impossible. On the contrary, samples examined in Core 182-1128C-8H and in Cores 22X through 26X are richer in diatoms, allowing for some stratigraphic and paleoenvironmental interpretation.
The diatom assemblage in Sample 182-1128C-8H-4, 130-132 cm, contains a combination of oceanic diatoms (e.g., Coscinodiscus, Cestodiscus, Asteromphalus, etc.) and neritic diatoms (e.g., Paralia, Actinoptychus, Arachnoidiscus, other neritic pennate diatoms, etc.). Stratigraphically significant diatoms in this sample include R. vigilans and C. rhombicus. The presence of C. rhombicus suggests that this sample is not older than late Oligocene (Fenner, 1985). The presence of R. vigilans in the earliest Miocene is reported by Fenner (1985) from low-latitude DSDP Hole 366A; however, neither Fenner (1985) nor Harwood and Maruyama (1992) report R. vigilans above the Oligocene/Miocene boundary in southern latitudes. There is no other evidence to support or reject an earliest Miocene age.
The presence of C. reticulatus and C. convexus in samples examined between Cores 182-1128C-22X and 26X are indicative of an early Oligocene age (Fenner, 1981, 1985). The presence of Azpeitia oligocenica, Distephanosira architecturalis, Pseudotriceratium radiosoreticulatum, and Pyxilla sp. is consistent with this interpretation. The assemblages consist of a combination of oceanic and neritic species. In Sample 182-1128C-25X-2, 65-67 cm, the oceanic diatoms (Azpeitia, Coscinodiscus, Cestodiscus, and Hemiaulus) are dominant over the neritic diatoms (Paralia, Actinoptychus, etc.). In Sample 182-1128C-26X-3, 115-117 cm, however, the neritic species are more numerous, although not a major component of the assemblage.
Hole 1128D was drilled at 34°23.4563´S, 127°35.4554´E a water depth of 3874.3 m. The Eocene sequence in Cores 182-1128D-3R to 23R is represented by claystone in its uppermost part (Cores 2R to 6R) and siltstone in its lower part (Cores 7R to 23R). Three samples per core were examined for radiolarians. Radiolarians are few and rather well preserved in the early Oligocene (Samples 182-1128D-1R-1, 100-102 cm [232.20 mbsf], and 1R-2, 100-102 cm [233.70 mbsf]). They are absent or present only in trace amounts throughout the remainder of the Eocene (Core 182-1128C-2R [240.80 mbsf] to 23R [445.90 mbsf]). Diatoms are mostly absent, and when they are present, they are very fragmented. Sponge spicules vary in abundance from trace amounts to rare and are weakly silicified. Preservation deteriorates rapidly in the upper Eocene section and most of the lower cores. Sample 182-1128C-12R-CC is barren of calcareous nannofossils (Shipboard Scientific Party, 2000b).
The Eocene/Oligocene boundary between Cores 182-1128C-2R (240.80 mbsf) and 3R (250.40 mbsf) falls in an interval barren of radiolarians and was determined on the basis of calcareous nannofossils. Biostratigraphic interpretation using radiolarians in the Eocene interval is currently not possible. Sample 182-1128D-10R-3, 50-52 cm, contains Dictyoprora mongolfieri, D. urceolus, Lithocyclia ocellus, Lychnocanoma amphitrite, Periphaena decora, and Siphocampe quadrata.