LITHOSTRATIGRAPHY

Site 1137 is located in 1005 m of water on Elan Bank, a submarine promontory of the western margin of the Kerguelen Plateau. Hole 1137A was rotary cored continuously to a depth of 371 mbsf. Sediments were recovered from 0 to 224 mbsf, and basalts with interbedded sedimentary and volcaniclastic rock layers were recovered from the lower 147.2 m of the hole (Fig. F4). The sedimentary section above basaltic basement consists of ~200 m of pelagic ooze that overlies about 20 m of glauconitic sandy packstone (Fig. F5; Table T3). This packstone rests unconformably on basaltic basement. We recognize three sedimentary lithologic units (I-III) in the upper part (0-224 mbsf) of Hole 1137A (Figs. F4, F5). The basement basalts and their interbedded sediments are designated lithologic Unit IV and are subdivided into basement Units 1-10 (Fig. F4). Various aspects of these basement units are described in "Igneous Petrology," "Physical Volcanology," and "Alteration and Weathering". In this section, we describe the sedimentary and volcaniclastic rocks of Unit IV (basement Units 5, 6, and 9) (Fig. F6; Table T3). Core recovery varied from good to poor throughout the sedimentary sections of Hole 1137A (Fig. F4).

Unit I is foraminifer-bearing diatom ooze (Fig. F5; Table T3). Radiolarians and silicoflagellates are present as minor components. Foraminifers decrease in abundance downcore. Most of the unit is very light gray; however, the top of the unit (interval 183-1137A-1R-1, 0-123 cm) is very pale brown. The ooze has a carbonate content of 51% (Sample 183-1137-1R-1, 90 cm). X-ray diffraction (XRD) data show opal-A with minor traces of opal-CT and very minor quartz. Sand- to granule-sized grains of basalt are dispersed throughout Core 183-1137A-1R. An angular pebble-sized fragment of basalt is found in interval 183-1137A-1R-4, 70-71 cm, an angular pebble-sized fragment of pumice is found in interval 183-1137A-1R-2, 70-71 cm, and an angular granule of quartz is found in interval 183-1137A-1R-5, 18-19 cm. All sections of Core 183-1137A-1R are highly disturbed (soupy); thus, these coarse materials were probably dispersed in the ooze by drilling disturbances. Interval 183-1137A-1R-2, 68-120 cm, contains angular 2- to 7-cm-sized pieces of white ooze dispersed in an ooze matrix. In Sections 183-1137A-1R-3 through 1R-5, similar white clasts are deformed into clumps and streaks. Again, these dispersed, deformed fragments probably result from drilling disturbance, rather than primary mass-wasting (e.g., debris flow) processes.

Unit II is white nannofossil ooze (Fig. F5; Table T3). Most of the ooze is homogeneous, and bioturbation is not apparent. In the upper part of the hole, diatoms are common to abundant in the sediments of Cores 183-1137A-2R through 9R. Some intervals of Cores 183-1137A-2R, 6R, 7R, 9R, 13R, 17R, and 20R are foraminifer-bearing nannofossil ooze. Radiolarians are present in Cores 183-1137A-2R-1 through 12R-1. Benthic foraminifers were observed in the lower part of Unit II (Cores 183-1137A-14R, 15R, 17R, and 19R) (i.e., mainly in the Oligocene sediments). The carbonate content of sediments in Unit II ranges from 88 to 96 wt% CaCO₃ (Fig. F5; Table T4) with the lower values in Cores 183-1137A-2R through 7R. These lower values probably reflect the presence of diatoms (amorphous silica), as the XRD analyses show no significant amount of minerals other than calcite throughout Unit II.

Silt-sized black pyrite nodules are disseminated through intervals 183-1137A-13R-6, 99-101 cm; 13R-6, 137-138 cm; 16R-1, 135-142 cm; 17R-1, 90-115 cm; and 19R-1, 8-9 cm. Pebbles and sand-sized grains of basalt are scattered through Core 183-1137A-2R. Interval 183-1137A-6R-1, 85-94 cm, contains disseminated silt-sized shards of yellow palagonitized glass and a pebble of black basalt; however, the pebble, which is found along the edge of the core, may not be in place. Intervals 183-1137A-15R-3, 25-44 cm, and 16R-1, 12-25 cm, contain slightly darker clay-rich layers with sparse fragments of volcanic glass. A short interval (interval 183-1137A-19R-2, 0-27 cm) of light greenish gray (5G8/1), indurated nannofossil chalk is highly burrowed. Dark silt-sized grains are sparsely disseminated through intervals 183-1137A-21R-1, 70-126 cm, and 21R-CC, 1-8 cm. A major unconformity apparently separates Units II and III (see "Biostratigraphy"); however, the lower contact of Unit II was not recovered.

Unit III is predominantly glauconite-bearing sandy packstone (Fig. F5; Table T3). The upper 25 cm of this unit (interval 183-1137A-22R-1, 0-25 cm) consists of light gray, well-cemented packstone with abundant bioclasts and minor dispersed grains of glauconite. A concentration of large (as much as 1 cm) shell fossils, including an inoceramid, is found in interval 183-1137A-22R-1, 10 to 12 cm (Fig. F7). This uppermost interval grades into very light gray to green ("salt and pepper") glauconite-bearing sandy packstone, which grades downward from fine to medium sand-sized grains. A thin section from near the top of the unit (Sample 183-1137A-22R-1, 26-33 cm) (Table T5) reveals the composition to be 30% clay-sized carbonate matrix, 25% quartz sand, 25% brachiopods, bivalves and ostracodes, 15% glauconite, 5% benthic foraminifers with rare planktonic foraminifers, and rare sponge spicules. Many intraparticle pores are filled with chalcedony cement. However, many dissolution pores of bivalve shells (aragonite) are still open and undeformed, which suggests that dissolution has occurred after lithification, probably in deep water below the aragonite compensation depth. Carbonate content is high (75-77 wt%) throughout most of Unit III but abruptly drops to only 3 wt% CaCO₃ (Sample 183-1137A-24R-1, 40-41 cm) near the base (Fig. F5; Table T4). Petrographic study of a thin section from near the base of the unit (Sample 183-1137A-24R-1, 35-38 cm) (Table T5) has 40% glauconite, 40% brown silicified matrix, 15% pore-filling opal, and 5% bioclasts and mineral grains, including shell fragments, benthic and planktonic foraminifers, a possible ooid, and brown hornblende. XRD analysis of this sample shows the presence of quartz, opal-A or glass, opal-CT, glauconite, calcite, pyrite, hematite(?), and marcasite(?). Hence, the packstone grades into a glauconite-bearing sandstone just above the base of the unit. Unit III rests unconformably on massive basalt (basement Unit 1 of Unit IV). The absence of a brecciated top on this lava flow suggests a period of erosion before deposition of Unit III began (see "Physical Volcanology").

Lithologic Unit IV consists of basalt flows and interbedded volcaniclastic and sedimentary rocks (Figs. F4, F6; Table T3). Unit IV is subdivided into 10 basement units, which include seven basalt flows and three intervals of volcaniclastic sedimentary rocks. The volcanic basement units are described in "Physical Volcanology," "Igneous Petrology," and "Alteration and Weathering". In this section, we describe the sedimentary features of the volcaniclastic and sedimentary rocks of basement Units 5, 6, and 9. Additional description of other features of basement Units 5, 6, and 9 (e.g., composition of clasts) may be found in "Physical Volcanology," "Igneous Petrology," and "Alteration and Weathering".

Basement Unit 5 (interval 183-1137A-33R-1, 122 cm, to 33R-4, 121 cm; 286.72-291.01 mbsf) is a 4.3-m-thick succession of interbedded dark greenish gray crystal-lithic volcanic siltstones and light gray crystal-lithic volcanic sandstones (Fig. F6; Table T3). A thin section (Sample 183-1137A-33R-3, 50-53 cm) (Table T5) shows angular, well-sorted medium sand composed of mainly volcanic lithic grains, minor feldspars and quartz, and traces of garnet, hornblende, and biotite (see "Igneous Petrology" for additional description of grain compositions). Beds are thin to medium in thickness. Several beds are normally graded (e.g., from silt- to medium-sand-sized in interval 183-1137A-33R-3, 42-53 cm) (Fig. F8A). Many sandstone beds show sharp, irregular bases. Some sandstone beds have gradational tops, but others have sharp, irregular tops. A few sandstone beds show subtle, low-angle cross-stratification (e.g., interval 183-1137A-33R-3, 82-90 cm) (Fig. F8B). Some siltstone and sandstone beds show parallel laminations. Burrows are rare. Thin black flakes (1 mm × 2 cm) that appear to be fragments of coal are observed within at least one sandstone bed (interval 183-1137A-33R-4, 102-106 cm). The beds are locally broken along small normal faults with ~1-cm offsets and a few small areas of ductilely deformed sediment suggest minor soft-sediment deformation (see "Structural Geology"). The uppermost sediment of this basement unit (interval 183-1137A-33R-1, 123-129 cm) is black and dense and may have been metamorphosed by the overlying lava flow.

Basement Unit 6 (interval 183-1137A-33R-4, 121 cm, to 37R-1, 0 cm; 291.01-322.80 mbsf) is a thick (31.8 m) unit of mainly gray lithic volcanic conglomerate (Figs. F6, F9; Table T3). In cores, stratification is not apparent except for one sand layer. The downhole logs clearly show other fine-grained layers that separate thick conglomerate beds (see "Downhole Measurements"). Grain sizes within the conglomerate range from granules to small boulders. Grains are well rounded and most are equant. Colors are light gray to rusty brown. Most intervals are clast supported, but matrix-supported intervals also are present. The matrix is mostly coarse, well-sorted sand with calcareous cement. Pebbles and cobbles in interval 183-1137A-34R-3, 20-100 cm, appear to display imbricate structure with the long axes of pebbles oriented at angles of 35°-50° from horizontal (Fig. F9C). The conglomerate is slightly polymictic. The clasts are mainly volcanic rocks but also include rare granitoid and garnet-biofite gneiss (see "Igneous Petrology"). Many of the pebbles and cobbles display concentric weathering rinds (see Fig. F9).

Basement Unit 9 (interval 183-1137A-41R-1, 103 cm, to 44R-4, 49 cm; 344.03-360.67 mbsf) is a 16.6-m-thick succession of crystal-vitric tuff (Fig. F6; Table T3). The tuff is composed of ~40% coarse (1-2 mm) angular crystals of white sanidine and <5% lithic clasts enclosed within a light to dark green dense matrix. The distribution of white crystals in a green matrix produces a "salt and pepper" appearance. The dense matrix is composed of green clay and felsic glass shards (see "Igneous Petrology" and "Physical Volcanology"). Dark green blebs (~1 mm in length) are common in this matrix; in some intervals, they are elongate and define a subtle horizontal fabric. Otherwise, the entire unit is massive and without visible sedimentary structures. The lithic clasts, dispersed through most intervals, include well-rounded to angular granules and pebbles as much as 5 cm in diameter. These clasts are composed predominantly of basalt and felsic materials (Fig. F10). At least one ragged, flattened pumice fragment is present within this unit. A rounded pebble (2 cm × 1 cm) of garnet-biotite gneiss is present near the base of the tuff (Sample 183-1137A-44R-4, 44-46 cm). A 1-cm-thick layer of gray siltstone is at the base of the tuff (interval 183-1137A-44R-4, 48-49 cm), and the top centimeter of the tuff (interval 183-1137A-41R-1, 103-104 cm) is brown (oxidized) and contains only sparse K-feldspar crystals (interval 183-1137A-41R-1, 112-113 cm). Interval 183-1137A-41R-1, 104-112 cm, contains several pieces of black dense rock with disseminated small feldspar crystals.

The basaltic lava flows and interbedded volcaniclastic sediments, recovered from the uppermost basement rocks (Unit IV) of Elan Bank (Site 1137) formed before late Campanian time (~75 Ma; see "Biostratigraphy"). Silt and sand accumulated in association with the conglomerates but also intermingled with the lava flows. For example, silts and sands are observed to infiltrate into lava-flow Units 3, 7, and 10 (see "Physical Volcanology"). Siltstone and sandstone constitute a discrete fine-grained unit (basement Unit 5) on top of the conglomerates (basement Unit 6).

Our observations cannot constrain the depositional environment of the crystal-vitric tuff (basement Unit 9). Larger pebbles dispersed (i.e., "floating") throughout the matrix indicate that if deposition was by gravity-controlled processes, then this sediment was deposited by a plastic (laminar) mass flow, such as a sandy debris flow.

The conglomerates of basement Unit 6 clearly indicate that Site 1137 was proximal to a region of high relief. The absence of marine microfossils, the probable occurrence of coal in basement Unit 5 (Core 183-1137A-33R-4, 102-105 cm), and oxidized tops on basaltic lava flows strongly suggest that the environment was nonmarine. Within an interstice of a lava flow near the basement Unit 3/Unit 4 contact (interval 183-1137A-29R-2, 57-64 cm), mud drapings, graded beds, and "mud chip" sand (resedimented desiccated mud drapings) indicate deposition by flowing ground water in a vadose environment. Also, baked siltstones beneath lava flows of basement Units 4 and 8 indicate a nonmarine environment (see "Physical Volcanology").

Sedimentary features of basement Units 5 and 6 are consistent with a high-energy fluvial (braided stream) environment. The moderate size sorting and an interval of apparently imbricated pebbles in the conglomerates of Unit 6 (Fig. F9C) suggest fluvial deposition. The imbricated pebbles have apparent dips of 35°-50°, which is consistent with a braided river system (Reineck and Singh, 1980). In the overlying silts and sands of basement Unit 5, normally graded beds, subtle cross-stratification, and paucity of burrows are also consistent with fluvial deposition.

Following cessation of basaltic volcanism, at least minor subaerial erosion occurred (Unit IV-Unit III unconformity) before the area subsided into a quiet shallow marine setting during late Campanian time (~75 Ma). Littoral facies were not observed at Site 1137. The glauconite-bearing sandy packstones of Unit III suggest that carbonate and quartz sand were transported to a quiet marine environment where interstices were filled with micrite and glauconite was formed.

A major hiatus exists from Late Cretaceous to late Eocene time at this drill site. Subsequently, the pelagic oozes of Unit II accumulated in a deep marine environment from latest Eocene through late Miocene time (as Broken Ridge and the Kerguelen Plateau separated along the Southeast Indian Ridge).

Diatom ooze records the movement of a high productivity zone (Polar Frontal Zone) into the region sometime before Pleistocene time (Mackensen et al., 1992), and the paucity of calcareous nannofossils suggests decreased sea-surface temperature (Wei and Wise, 1992). The basalt sand in Unit I could be ice-rafted debris from Antarctica; however, the predominantly volcanic material points to a local source.