LITHOSTRATIGRAPHY

Site 1139 is located west of Kerguelen Island in 1415 m of water on Skiff Bank (Leclaire Rise) on the western flank of the Kerguelen Plateau. Hole 1139A was rotary cored continuously to a depth of 694.2 mbsf. Sediments were recovered from 0 to 461 mbsf. Extensively altered volcaniclastic rocks, basalts, and one minor sedimentary bed were recovered from the lower 233 m of the hole (Fig. F4). The sedimentary section above igneous basement consists of ~383 m of predominantly calcareous claystones and chalks with relatively thin intervals of calcareous ooze and calcareous chalk at the top and base, respectively. These sediments overlie a thin interval (<10 m) of sandy packstones and ~77 m of predominantly grainstones (Fig. F4; Table T3). The sedimentary section rests unconformably on igneous basement. We recognize five sedimentary lithologic units (Units I-V) in the upper part (0-461.7 mbsf) of Hole 1139A. The basement volcanic rocks are designated lithologic Unit VI and are subdivided into basement Units 1-19 (Fig. F4) (see "Physical Volcanology," "Igneous Petrology," and "Alteration and Weathering" for descriptions). Core recovery varied from good to poor throughout the sedimentary sections of Hole 1139A (Fig. F4).

Unit I is calcareous ooze. In the top three cores, much of the sediment is highly disturbed or soupy from drilling; thus, we could not accurately determine the true thicknesses and stratigraphic positions of individual beds. We have divided Unit I into two subunits based on the abundance of diatoms (Fig. F4; Table T3). The lithology changes between Cores 183-1139A-2R and 3R, corresponding to a major hiatus (early Pleistocene to middle Miocene; see "Biostratigraphy").

Subunit IA (interval 183-1139A-1R-1, 0 cm, to 2R-CC, 19 cm; 0-19.00 mbsf) is predominantly tan to light gray foraminifer-bearing, diatom-bearing nannofossil ooze of middle to early Pleistocene age. X-ray diffraction (XRD) analysis shows calcite and amorphous silica. The carbonate content ranges from 64 to 75 wt% CaCO₃ (Table T4). In addition, thin intervals of deformed white foraminifer-bearing nannofossil ooze (intervals 183-1139A-1R-3, 48-95 cm; 2R-1, 28-53 cm; 2R-1, 98-136 cm; and 2R-2, 47-85 cm) are present within this unit, but we cannot determine if these intervals represent in situ mass-transport deposits (e.g., slump) or normal pelagic deposits disrupted by the drilling. Scattered basalt sand grains and rare small pebbles are disseminated throughout the subunit. We identified traces of pumice in smear slides. A major hiatus marks the base of Subunit IA.

Subunit IB (interval 183-1139A-3R-1, 0 cm, to 5R-CC, 25 cm; 19.00-47.50 mbsf) is predominantly foraminifer-bearing nannofossil ooze of middle to early Miocene age, which grades downward in color from white through very light gray and very pale brown to light greenish gray. Core 183-1139A-3R is composed of foraminifer-bearing, diatom-bearing nannofossil ooze. The carbonate content of the subunit ranges from 74 to 83 wt% CaCO₃ (Table T4), with a mean of 78 wt%. The organic carbon content in Sample 183-1139A-4R-1, 92 cm, is 0.3%. The XRD analyses show the presence of calcite, traces of alkali feldspar, and traces of opal-A in Core 183-1139A-3R (Table T4). Clay minerals are present in Core 183-1139A-5R, and sponge spicules are abundant. A few black pebbles of basalt and other volcanic material are present in Section 183-1139A-3R-1. Green mottles, most likely from traces of glauconite, are present around and within burrows scattered through Cores 183-1139A-4R and 5R. The basal contact of Subunit IB is gradational.

Unit II comprises most of the sedimentary section at Site 1139 and is a thick (333 m) section of interbedded gray to greenish gray nannofossil-bearing clay and claystone and nannofossil-bearing ooze and chalk (Fig. F4; Table T3). The color ranges from dark greenish gray to greenish gray in the upper portion of the unit (47.5-170 mbsf, Cores 183-1139A-6R through 18R) and is primarily gray with several thin intervals of light gray in the lower portion (170-380.72 mbsf, Cores 183-1139A-18R through 40R). The sediments become progressively stiffer downhole and are sufficiently indurated by 100-110 mbsf (Cores 183-1139A-11R and 12R) to be classified as claystone and chalk. Most of the sediments of Unit II are extensively burrowed, including many good examples of Zoophycos and Chondrites.

The carbonate content of sediments in Unit II fluctuates widely and ranges from 7 to 72 wt% CaCO₃ (Table T4) with an average of 44 wt%. These fluctuations indicate that the sediments of Unit II are primarily nannofossil clay or claystone with interbeds of nannofossil ooze or chalk, nannofossil-bearing clay or claystone, and rare claystone (Table T4). Carbonate-rich intervals are found above Core 183-1139A-9R (<70 mbsf), between Cores 183-1139A-18R and 25R (160-230 mbsf), and below Core 183-1139A-36R (>340 mbsf). Much of the sediment in interval 183-1139A-8R-1, 0 cm, to 10R-CC, 19 cm (66.5-95.1 mbsf) is diatom-bearing, nannofossil-bearing clay and ooze. Near the base of the unit (interval 183-1139A-39R-1, 0 cm, to 40R-5, 92 cm), the sediment is foraminifer-bearing nannofossil claystone.

The XRD analyses (Table T4) show traces of sanidine throughout Unit II. Quartz is a rare trace component in Cores 183-1139A-13R, 14R, 16R, 28R, and 36R. Clay minerals, maghemite, pyrite, and glauconite are present in most cores. Clinoptilolite is present as a trace component below 250 mbsf. Organic-carbon contents vary from 0.03% to 0.36%, with an average of 0.17%.

Irregular green patches, mottles, and laminae, which we attribute to very fine glauconite, are common throughout many intervals of this unit. XRD analyses confirm the presence of glauconite throughout this unit (Table T4). This glauconite is found as discrete thin laminae (which both crosscut [Core 183-1139A-21R] and are cut by burrows), as diffuse laminae, as halos around burrows, concentrated within burrows, and as discrete dark blebs (possibly burrow cross sections). The laminae, which are generally 1 mm or less in thickness, commonly are in closely spaced groups. Each group is usually 3 to 10 cm thick, and groups of laminae are common (several to more than a dozen per core) in the lower portion of the unit (below 172 mbsf, Core 183-1139A-19R). The crosscutting relationships with burrows clearly indicate that the glauconite is an early diagenetic feature.

Chert is absent from the entire section except for a single nodule near the base of the unit (interval 183-1139A-40R-1, 26-27 cm). Tephra layers are very rare; however, an ash layer of medium thickness is in interval 183-1139A-38R-4, 62-88 cm. In addition, volcanic ash is locally concentrated in burrows and disseminated in thin, presumably bioturbated zones (intervals 183-1139A-11R-1, 107-120 cm; 18R-3, 9-10 cm; 33R-1, 107-115 cm; 38R-3, 0-19 cm, and 39R-2, 119-128 cm). Two minor faults with slickensides in the lower portion of the unit display steep to moderate dips. The slickensides attest to their origin by faulting, rather than drilling disturbance. The lower boundary of Unit II is fairly sharp, though obscured somewhat by burrowing, and color changes noticeably from gray to light brownish gray across the boundary (Fig. F5A).

Unit III is a thin interval of foraminifer nannofossil chalk (Fig. F4; Table T3), which is of strikingly different color than the overlying gray claystones and chalks of Unit II (Fig. F5). This chalk lightens in color from light brownish gray (10YR 6/2; Fig. F5A) in the top through reddish yellow (5YR 6/6; Fig. F5B, F5C) to pink (5YR 7/4) in the lower 20 cm. Groups of darker rusty brown laminae, which are <2 mm thick, are present in intervals 183-1139A-40R-6, 27-40 and 72-82 cm (Fig. F5C). The carbonate content is high throughout the unit and is 94% CaCO₃ near its base (Sample 183-1139A-40R-CC, 12 cm) (Table T4). The organic carbon content is 0.04%. The XRD analysis shows only calcite in Unit III.

The reddish colors of the sediment are not characteristic of deep-sea foraminifer nannofossil oozes and chalks and suggest that iron from the underlying sediments and/or basement rocks has been mobilized, transported upward into this unit, and precipitated during diagenesis. A major hiatus (~33-31 Ma) near or within interval 183-1139A-40R-6, 89-91 cm (see "Biostratigraphy") has no obvious lithologic expression.

Unit IV consists of a thin interval of dusky red to greenish pink sandy packstone (Fig. F4; Table T4). Poorly sorted, subrounded to well-rounded sand-sized grains are predominantly composed of highly altered basaltic glass and volcanic rock fragments (Fig. F6). A thin section (Sample 183-1139A-41R-1, 40-43 cm) (Table T5) shows abundant microsparitic matrix (~30%-50%). Grains are predominantly volcanic lithic (~95%) and include a few vesicular grains. Altered basaltic glass is present and most of the volcanic material is highly altered. A single grain of silt-sized polycrystalline, slightly strained quartz with triple-junction crystal boundaries is present.

Bioclasts are rare (~5%). Benthic foraminifers and a few planktonic foraminifers document an open-marine depositional environment. Accessory bioclasts include echinoderms, ostracodes, and bivalve shell fragments. All bioclasts are strongly recrystallized. Cements are rare and mostly blocky and drusy calcite. Very thin fringes of fibrous calcite are rare. The cements are partially silicified.

Unit V consists predominantly of grainstone with some thin interbeds of rudstone (e.g., interval 183-1139A-41R-2, 48 cm, to 42R-1, 9 cm) and packstone (e.g., interval 183-1139A-42R-1, 9-20 cm) (Figs. F4, F7; Table T3). These sediments are white to brownish gray in the upper part of the unit (Figs. F7A, F8) but are rusty yellowish brown in interval 183-1139A-46R-1, 0 cm, to 49R-1, 110 cm (Fig. F7B). White layers of subangular, coarse-grained bioclasts are interlaminated with brownish, siliciclastic, fine- to coarse-grained sands, which are well sorted, subrounded, and iron stained. Some layers (Cores 183-1139A-46R through 48R) include subangular to well-rounded siliciclastic granules and pebbles. Several intervals display well-developed cross-stratification with apparent dips up to 25° (Figs. F7C, F7D, F8A, F8B, F8C). A few burrows are present. The sorting and grain size indicate a high energy environment (e.g., a carbonate shoal).

Two thin sections (Samples 183-1139A-41R-2, 50-54 cm, and 42R-1, 67-70 cm) (Table T5) confirm the very well-sorted nature of this coarse grainstone. In both thin sections, porosity is high (~20%-30%) and approximately half of the pores are filled with calcite cement. Bioclasts are highly fragmented and consist of shell fragments including serpulid worm tubes (50%), bryozoans (10%), echinoderms (<5%), and rare benthic foraminifers. Many shells are micritized. Although the size-sorting is entirely consistent with a high-energy environment, most of the bioclastic grains are angular. Mineral and rock grains make up only ~2% of the total volume. In interval 183-1139A-41R-2, 50-54 cm, they include volcanic fragments with phenocrysts of plagioclase and alkali feldspar. In interval 183-1139A-42R-1, 67-70 cm, the volcanic rock fragments have phenocrysts mainly composed of alkali feldspars.

A thin section (Sample 183-1139A-41R-2, 50-54 cm) (Table T5) shows that irregular dog-tooth calcite cement fills some pores. Syntaxial rim cements on echinoderms and rare blocky calcite cement also are present. The syntaxial cements formed synchronously with the dog-tooth cements. In Sample 183-1139A-42R-1, 67-70 cm, echinoderm syntaxial rims are predominant. Other bioclasts show very thin fringes of dog-tooth cements.

Lithologic Unit VI consists of highly altered volcaniclastic and felsic volcanic rocks, altered basalt flows, and minor sedimentary rock (Fig. F4; Table T3). Unit VI is subdivided into 19 basement units, which are described in the "Physical Volcanology," "Igneous Petrology," and "Alteration and Weathering". A 47-cm-thick bed of bioclast-bearing sandstone, which is very similar to the grainstones of Unit V, is present in the uppermost basement unit (Subunit 1B) (see "Physical Volcanology").

Basaltic lava flows and overlying felsic volcanic rocks (Unit VI) were severely hydrothermally altered (see "Alteration and Weathering") before deposition of a marine sequence commenced in the Eocene. Well-rounded pebbles at the base of the marine sequence in the top of basement Subunit 1A (lithologic Unit VI) suggest beach deposition. Cross-stratified grainstones (Unit V) record a high-energy shallow marine (neritic) environment such as a carbonate shoal. The coarse grain sizes (granules and pebbles) in the terrigenous fraction suggest a nearshore setting. The siliciclastic component of the grainstones records erosion of the felsic volcanic rocks. The grainstone facies differs from Cretaceous neritic facies on the Kerguelen Plateau (Sites 747-750 and 1136-1138), which contain glauconite and micritic matrix and reflect a lower-energy environment than that of Site 1139. This ~80-m-thick grainstone deposit thus constitutes the first sequence of high-energy neritic carbonate facies recovered from the plateau.

With continued subsidence, grainstone deposition gave way to deposition of sandy packstone (Unit IV), which indicates a low-energy neritic environment. The presence of planktonic foraminifers indicates an open-marine setting. However, the predominance of sand-sized terrigenous material still indicates a position close to the shore. The strong recrystallization of the calcitic components of the packstone may be explained by meteoric diagenesis, in accordance with the presence of dog-tooth cements and echinoderm rim cements. Altered volcanic grains and abundant clay minerals suggest a weathered source terrane.

Chalk in Unit III indicates the onset of pelagic conditions in a bathyal environment during latest Eocene to early Oligocene time. From late Oligocene through middle Miocene time, the Site 1139 area received variable influxes of terrigenous (hemipelagic) clay from an adjacent volcanic landmass, and these formed the gray claystone and chalk of Unit II. These fluctuations may be related to sea-level changes, climatic fluctuations, or tectonic activity. Undiluted pelagic calcareous oozes (Subunit IB) were deposited from the later part of middle Miocene time until at least early Pleistocene time. The abundance of diatoms in Subunit IA indicates that during the Pleistocene Site 1139 was subject to cold water masses south of the Polar Front.