Site 1135 was drilled in 1578 m of water on the southernmost Kerguelen Plateau near the western margin of the Labuan Basin. This site was chosen to sample the basement rocks at a location where seismic reflection profiles showed the overlying sediments to be ~600 m thick. Hole 1135A was drilled and continuously rotary cored to a depth of 526 mbsf. The hole collapsed and had to be abandoned before penetrating basement (see "Operations"). Core recovery was good to poor. The recovered cores consist almost entirely of pelagic calcareous ooze and chalk (Fig. F4). Most notably, a core (Core 183-1135A-28R) containing the Cretaceous/Tertiary (K/T) boundary was recovered between 257.0 and 267.l mbsf. We recognize three lithologic units in Hole 1135A based mainly on subtle lithologic changes.
Unit I consists of coarse sand, granules, pebbles, and very minor amounts of pelagic ooze (Fig. F4; Table T3). All material recovered from Unit I has been highly disturbed by drilling, and thus the true composition of the sediment of this unit is uncertain. Pebbles up to 4 cm in diameter are composed of granitic, metamorphic, and basaltic lithologies. These pebbles are semi- to well-rounded and are apparently ice-rafted debris (IRD) derived from the Antarctic continent. They were probably dispersed throughout the pelagic ooze and then concentrated by drilling disturbance. Section 183-1135A-1R-1 contains very minor amounts of radiolarian-bearing diatom ooze, which appears to be the only in situ sediment in this unit. The underlying sediments in interval 183-1135A-2R-1, 0-52 cm, are also highly disturbed by drilling and contain diatom-bearing foraminifer ooze with minor sand and radiolarians. Unit I overlies Eocene strata; thus, the contact between Units I and II must represent a major unconformity.
Unit II consists of white nannofossil ooze. We subdivide this unit into two subunits based on the abundance of foraminifers, which are much more abundant in Subunit IIA (Fig. F4; Table T3). In addition, chert nodules are common in Subunit IIB, but are rare in Subunit IIA. The calcium carbonate content throughout this unit is uniformly very high (94-96 wt%) (Fig. F4). Unit II was deposited in a pelagic environment.
Subunit IIA comprises the interval 183-1135A-2R-CC, 0 cm, to 15R-CC, 18 cm (10.02-141.90 mbsf), and is composed entirely of middle to lower Eocene, homogeneous white foraminifer-bearing nannofossil ooze (Fig. F4). Bioturbation is rare. Coring disturbance appears minimal; however, several cores contain one or more rock pebbles at their tops, which we assume are younger IRD that has fallen down the hole from higher stratigraphic levels during drilling. One thin interval (interval 183-1135A-10R-4, 94-120 cm) has faint disturbed black stains (manganese oxide?) that show the sediments are contorted. Whether this disturbance results from drilling disturbance or soft-sediment deformation (e.g., slumping) is uncertain. Contorted black stains of pyrite in Section 183-1135A-12R-6 (Table T4) indicate disturbance by drilling. A large light gray chert nodule is in interval 183-1135A-14R-1, 0-13 cm, and is the stratigraphically highest occurrence of chert nodules downhole; however, this nodule is at the top of the core so it may have fallen from a higher stratigraphic level. Core 183-1135A-15R contains scattered small black patches of pyrite. In addition, interval 183-1135A-15R-1, 17-20 cm, includes a bed of greenish gray foraminifer-bearing nannofossil clay that contains minor glauconite (Table T4).
Subunit IIB comprises the interval 183-1135A-16R-1, 0 cm, to 26R-CC, 22 cm (141.90-247.70 mbsf), and is composed entirely of middle Eocene to upper Paleocene homogeneous white nannofossil ooze. Foraminifers are present, but less common (<10%) than in Subunit IIA. Rare nodules of light to dark gray chert, as large as 8 cm in diameter, are scattered frequently throughout this subunit (see the "Core Description" contents list). Bioturbation is slight. Rare fossils are present, including a "tooth"-shaped shell ~2 cm long in Section 183-1135A-20R-2, 133 cm, crinoid stalk segments and a single columnalia in interval 183-1135A-21R-6, 120-140 cm, and a bivalve shell in interval 183-1135A-23R-1, 81-82 cm. The sediments appear largely undisturbed by drilling. The sediments are slightly fractured from Core 183-11335A-20R to the base of the subunit, indicating a gradual transition to the lithified chalks in Unit III below.
Unit III consists of nannofossil chalk and calcareous chalk (Fig. F4; Table T3). We divide this unit into three subunits based on lithology and color. Subunit IIIA is white nannofossil chalk, whereas Subunits IIIB and IIIC comprise sections of white and light green calcareous chalk, respectively. Subunit IIIA appears to contain the K/T boundary in Core 183-1135A-28R (see "Biostratigraphy"). The carbonate content of this unit is more variable than that of Unit II, with values ranging from 78 to 97 wt% CaCO3. Most variation is found in Subunit IIIC (Fig. F4).
Subunit IIIA comprises the interval 183-1135A-27R-1, 0 cm, to 33R-CC, 10 cm (247.7-315.1 mbsf), and is primarily white nannofossil chalk of late Paleocene to Maastrichtian age. The transition from ooze to chalk is gradational and the boundary between Units II and III is somewhat arbitrary. Scattered brown to black chert nodules up to 7 cm long are in intervals 183-1135A-29R-3, 37-42 cm, and 108-110 cm; 30R-2, 22-26 cm; 31R-1, 96-120 cm; and 33R-2, 42-45 cm. Thin (<1 mm) gray laminae are in intervals 183-1135A-32R-1, 10-90 cm, and 33R-3, 137 cm, to 33R-4, 10 cm. Laminae in the latter interval display normal faulting. Interval 183-1135A-33-1, 13-26 cm, also has a small normal fault. Burrowing is rare to slight throughout most of the subunit. Most sediments are moderately to highly fractured, presumably by drilling. Shell fragments are rare. Calcispheres were observed in smear slides from Sections 183-1135A-29R-1, 29R-2, 31R-1, 31R-5, 31R-7, and 32R-2.
Core 183-1135A-28R appears to contain the K/T boundary (see "Biostratigraphy"), located within interval 183-1135A-28R-2, 124-150 cm (Fig. F5). The sediment in this interval is light greenish gray calcareous clay (in contrast to the white ooze above and below), and the upper contact is irregular. The clay is moderately burrowed and contains scattered, well-rounded clasts of white nannofossil ooze. These features suggest an erosional or mass-wasting event at or very near the K/T boundary.
Subunit IIIB comprises interval 183-1135A-34R-1, 0 cm, to 39R-CC, 10 cm (315.1-372.6 mbsf), and consists of white to light gray calcareous chalk of Maastrichtian to Campanian age. In contrast to the nannofossil chalk of Subunit IIIA, smear slides show that calcareous nannofossils are only rarely preserved, and calcareous micrite is dominant. The sediment is slightly to moderately burrowed. A few burrows are filled with pyrite. Gray chert nodules are found in intervals 183-1135A-37R-1, 0-17 cm, and 39-60 cm; 37R-CC, 0-13 cm; 38R-CC, 16-24 cm; 39R-1, 0-12 cm, and 39R-2, 12-18 cm. Rare laminae of clay-bearing chalk are in most of the cores and are slightly darker than the background chalk. Recovery from Subunit IIIB was poor, and the core is slightly to severely fractured by drilling.
Subunit IIIC comprises interval 183-1135A-40R-1, 0 cm, to 55R-CC, 5 cm (372.6-526.0 mbsf) and consists of light greenish to gray chalk with moderate amounts of chert and silicified sediment, which is apparently in transition to chert. The age of this subunit is ~Turonian to Campanian. Gray to black chert beds and nodules are frequent within Subunit IIIC (see the "Core Description" contents list). The entire section is moderately to highly burrowed. Inoceramid bivalve shell fragments are common in Core 183-1135A-41R. Other bivalve shells are rare throughout the subunit. Lenses and layers of sand-sized bioclasts were found in intervals 183-1135A-43R-1, 58-59 cm, and 46R-1, 25 cm. A thin layer of laminated chalk with fenestral pores, filled by chert, is found in interval 183-1135A-46R-1, 145-150 cm. Two slightly mineralized firmgrounds are found in Sections 183-1135A-46R-1, 38 cm, and 46R-1, 89 cm. Recovery was poor and the chalk is moderately to highly fractured in most places.
Bulk samples from most cores from Hole 1135A were analyzed by X-ray diffraction (XRD) (Table T4). Most samples are nearly pure calcium carbonate (calcite). Quartz and feldspar are present in Unit I (Core 2R). In Unit II, traces of pyrite are found in Section 183-1135A-12R-6, 90 cm (faint black stains). Traces of glauconite are in Section 183-1135A-15R-1, 18 cm (light green clayey layer). Only calcite was identified by XRD in this clayey layer (Section 183-1135A-15R-1, 18 cm), even though elongate, lathlike mineral grains as much as 0.025 mm in length were observed in smear slides. Traces of clinoptilolite and glauconite are common from the base of Subunit IIIB downward (Table T4). Quartz and feldspar are of minor importance, except for quartz in chert nodules. Two chert samples from Subunit IIIC (Sections 183-1135A-46R-1, 46 cm, and 52R-3, 15 cm) show a strong broad peak of opal-CT and a sharp quartz peak (Fig. F6). These samples are, therefore, immature chert.
Unit I provides a scant record of latest Quaternary sedimentation and paleoceanographic conditions south of the Antarctic polar front. The diatom ooze recovered indicates moderate productivity. The ice-rafted pebbles and sand reflect the influence of the Antarctic ice sheet. The absence of thick early Neogene through late Paleogene strata suggests a long period of nondeposition and/or erosion, perhaps related to the development of the Antarctic Circumpolar Current, which became active during the Miocene (Kennett et al., 1975). As much as a few hundred meters of sediment may have been eroded from the plateau during this period. The Eocene through Santonian(?) section indicates pelagic deposition dominated throughout that time interval; only the oldest subunit suggests a hemipelagic influence (clays and inoceramid and other bivalve fragments). This succession is consistent with progressive subsidence through the Late Cretaceous and early Paleogene.