LITHOSTRATIGRAPHY

Site 1140 is located north of the Kerguelen Archipelago in 2394 m of water on the northern edge of the Kerguelen Plateau. Hole 1140A was rotary cored continuously to a depth of 321.9 mbsf. Sediments were recovered from 0 to 234.52 mbsf. Pillow basalts and three minor sedimentary beds were recovered from the lower 87.38 m of the hole (Fig. F4; Table T3). The sedimentary section above igneous basement consists entirely of pelagic oozes and chalks and appears to rest unconformably on the underlying submarine basalt flows. We recognize only one sedimentary lithologic unit in the upper part (0-234.52 mbsf) of Hole 1140A. The basement volcanic rocks are designated lithologic Unit II and are subdivided into basement Units 1-6 (Fig. F4; Table T3) (see "Physical Volcanology," "Igneous Petrology and Geochemistry," and "Alteration and Weathering" for description). Core recovery was generally poor throughout the sedimentary section of Hole 1140A, except for the topmost and lower portions (Fig. F4).

Age: middle Miocene to early Oligocene

Unit I predominantly consists of light greenish gray foraminifer-bearing nannofossil ooze and nannofossil chalk. We divide this unit into two subunits (Subunits IA and IB) based on the presence of diatom ooze in the uppermost part of the unit (Fig. F4; Table T3).

Subunit IA (interval 183-1140A-1R-1, 0 cm, to 2R-1, 61 cm; 0-10.01 mbsf) consists of predominantly white diatom nannofossil ooze with interbeds of dark brown silty diatom ooze (intervals 183-1140A-1R-1, 0-25 cm and 40-65 cm; 1R-2, 33-89 cm; 1R-3, 73-80 cm; 2R-1, 0-9 cm), light brown silty foraminifer-bearing diatom ooze (intervals 183-1140A-1R-1, 25-40 cm; 1R-3, 80-89 cm; 1R-5, 0-10 cm) and yellowish brown nannofossil-bearing diatom ooze (interval 183-1140A-2R-1, 29-54 cm) of middle Miocene age (Fig. F4; Table T3). The CaCO₃ content is only 2 wt% in the dark brown layer in interval 183-1140A-1R-1, 0-25 cm, but ranges from 23 wt% to 53 wt% in the white layers below (Table T4). X-ray diffraction (XRD) analysis shows opal-A, feldspar, quartz, clay minerals, and maghemite as major components of the dark brown layers and calcite and opal-A with only traces of feldspar and clay minerals in the white layers. The organic carbon content varies from 0.03 wt% in Sample 183-1140A-1R-1, 11-12 cm, to 0.51 wt% in Sample 183-1140A-2R-1, 38-39 cm. Cores 183-1140A-1R and 2R contain a few rock pebbles up to 4 cm long scattered throughout the sediments. Much of the sediment in the top core is highly disturbed to soupy from drilling; thus, true thicknesses and stratigraphic positions of some individual beds are uncertain.

Subunit IB (interval 183-1140A-2R-1, 61 cm, to 25R-5, 92 cm; 10.01-234.52 mbsf) comprises most of the sedimentary section at Site 1140 and is predominantly light greenish gray foraminifer-bearing nannofossil ooze and nannofossil chalk of early Oligocene to middle Miocene age (Fig. F4; Table T3). The sediments become progressively stiffer downhole below ~75 mbsf (Core 183-1140A-9R) and are semilithified by 120 mbsf (Core 183-1140A-14R); below this depth the sediments are classified as chalk. The upper part of Subunit IB (interval 183-1140A-2R-1, 61 cm, to 5R-CC, 50 cm) is slightly burrowed light greenish gray diatom-bearing nannofossil ooze. White intervals are present at 183-1140A-20R-1, 34-51 cm, and 23R-1, 0-89 cm. Interval 183-1140A-23R-4, 0 cm, to 23R-CC, 29 cm, consists of white nannofossil chalk. A thin layer of brown diatom-bearing nannofossil chalk is present in interval 183-1140A-21R-1, 45-50 cm. A dark greenish gray volcanic ash layer, with vesicular basaltic glass shards, is present in interval 183-1140A-18R-6, 102-107 cm. The basal portion of Subunit IB (interval 183-1140A-24R-1, 0 cm, to 25R-5, 92 cm) is light pale brown because of disseminated yellow orange altered lithic grains (basalt?).

Clear rhombic dolomite crystals are disseminated throughout the sediments at the base of Subunit IB in Sections 183-1140A-25R-4 and 25R-5. XRD analyses indicate that the dolomite content increases from below detection limit in Section 183-1140A-24R-1 to about 5% in Section 25R-1, 10% in Sample 183-1140A-25R-5, 37-38 cm, and >95% in Sample 183-1140A-25R-5, 90-92 cm (Table T4). The position of the major dolomite XRD peak at 2.901 Å suggests an iron-rich composition.

The carbonate content in Subunit IB ranges from 51 wt% to 91 wt% CaCO₃ (average = 77%). The organic carbon content is very low (0.00 to 0.09 wt%) and averages 0.03 wt% (Table T4). XRD analyses show calcite and some opal-A throughout most of Subunit IB. Sample 183-1140A-14R-1, 90-92 cm (CaCO₃ = 51 wt%) also contains some clay minerals and K-feldspar. Clay minerals were also detected in Core 183-1140A-25R-1 (Table T4).

Most sediments of Subunit IB are extensively burrowed; burrows include Zoophycos and Chondrites. Scattered black silt- and rare sand-sized particles of volcanic material are disseminated in some intervals. Steeply dipping faults are present in intervals 183-1140A-21R-1, 45-50 cm, and 23R-4, 110-125 cm. The presence of slickensides argues for faults, as opposed to drilling disturbance.

Lithologic Unit II is composed of pillow basalts with three thin sedimentary beds (Fig. F4; Table T3). Unit II is subdivided into six basement units, which are described in "Physical Volcanology," "Igneous Petrology and Geochemistry," and "Alteration and Weathering". The top of basement Unit 3 (interval 183-1140A-31R-1, 134-140 cm) is a thin bed of well burrowed greenish white nannofossil chalk of latest Eocene age.

Basement Unit 4 is a sedimentary bed (interval 183-1140A-32R-3, 41-115 cm). Intervals 183-1140A-32R-3, 41-91 cm, and 32R-3, 91-115 cm, at the top and bottom of this bed are composed of rusty brown to rusty orange dolomite, which consists of cemented medium sand-sized dolomite crystals. The dolomite content of these beds (Sample 183-1140A-32R-3, 83-84 cm) (Table T4) is close to 100%; no calcite was detected and clay is common. These dolomite beds are separated by well-burrowed, very pale brown dolomitic nannofossil chalk. Some burrows are filled with black manganese oxides. The top interval of basement Unit 6 (interval 183-1140A-34R-4, 0-6 cm) is composed of a thin bed of rusty brown dolomite similar in appearance to that of basement Unit 4.

Submarine pillow basalts with thin interbeds of dolomitized nannofossil chalk in Unit II indicate that during the late Eocene(?) into the early Oligocene Site 1140 was in bathyal water depths and that pelagic deposition accompanied extrusion of the lava flows. Pelagic deposition in a bathyal environment continued uninterrupted from early Oligocene to at least middle Miocene time. The site most likely remained at bathyal depths until the present. A seismic profile across this drill site shows that a substantial portion of the uppermost strata of the seafloor has been removed by erosion.