LITHOSTRATIGRAPHY

Site 1138 is located southeast of Heard and McDonald Islands in 1141 m of water on the CKP. Hole 1138A was rotary cored continuously to a depth of 842.7 mbsf. Sediments were recovered down to 698 mbsf. Basalts overlain by volcaniclastic and very minor sedimentary rock layers were recovered below 698 mbsf from the lower 144 m of the hole (Fig. F4). The sedimentary section above igneous basement consists of ~655 m of pelagic ooze, chalk, and calcareous claystone that overlies ~43 m of glauconitic calcareous sandstone and silty clay interbedded with sandstone and conglomerate (Fig. F4; Table T3). The sedimentary section appears to rest unconformably on igneous basement. We recognize six sedimentary lithologic units (I-VI) in the upper part (0-698 mbsf) of Hole 1138A. The basement volcaniclastic rocks and basalts are designated lithologic Unit VII and are subdivided into basement Units 1-22 (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 1138A (Fig. F4).

Unit I is predominantly foraminifer-bearing diatom clay with interbedded foraminifer-bearing diatom ooze (Fig. F4; Table T3). Much of the sediment in the top three cores is highly deformed and contains soupy intervals because of severe drilling disturbance; thus, we could not determine accurately the true thicknesses and stratigraphic positions of individual ooze and clay beds. Most of the diatom clay ranges in color from light gray to dark gray and dark greenish gray. The interbedded diatom ooze ranges from tan to very light gray. The carbonate content ranges from 4 to 46 wt% CaCO₃ with an average of 23 wt%. Organic carbon contents are 0.2% in the upper part and 0.1% in the lower part of the unit (Table T4). X-ray diffraction (XRD) analyses show clay minerals (smectites?) in the diatom clay and amorphous silica in the diatom ooze (Table T4). Alkali-feldspar, quartz, and minor amounts of pyrite and glauconite are found mainly in the diatom clay. Opal-CT and quartz are very rare or absent in the diatom ooze.

Pebbles as long as 6 cm at the tops or in the uppermost intervals of a few cores (e.g., gabbro pebble in interval 183-1138A-4R-1, 0-6 cm) represent ice-rafted debris that has apparently fallen down the hole from higher stratigraphic levels during drilling. A few intervals of Unit I contain volcanic ash. Interval 183-1138A-8R-2, 71-72 cm, contains brown ash, which has been extensively burrowed. A brown ash layer is present in interval 183-1138A-9R-1, 0-8 cm. Brown ash is disseminated through interval 183-1138A-10R-3, 107-127 cm. A pumice fragment is present in interval 183-1138A-10R-2, 46 cm, and a 2-mm-thick lamina of coarse-sand-sized pumice crystals is present in interval 183-1138A-8R-CC, 6-7 cm.

Core 183-1138A-8R contains an interval (183-1138A-8R-3, 0 cm, to 8R-7, 64 cm) with rounded and deformed mud clasts of variegated colors, layers contorted by flowage, and steeply dipping, truncated layers that may represent soft-sediment deformation. The sediment has also suffered from drilling disturbance so the exact interval(s) of primary soft-sediment deformation is uncertain.

The gray color of Unit I reflects a terrigenous component derived from the vicinity of Heard Island (see "Discussion"). The lower contact with Subunit IIA is transitional.

Age: late to early Miocene

Unit II is composed of foraminifer-bearing nannofossil clay underlain by foraminifer-bearing nannofossil ooze. A higher carbonate/silica ratio distinguishes Unit II from Unit I. We divided Unit II into two subunits based on clay content and color (Fig. F4; Table T3).

Subunit IIA (interval 183-1138A-13R-1, 0 cm, to 16R-CC, 18 cm; 112.00-150.50 mbsf) is light gray foraminifer-bearing nannofossil clay of late Miocene age (Fig. F4; Table T3). The carbonate content of sediments in Subunit IIA ranges from 24 to 76 wt% CaCO₃ and averages 55 wt% (Table T4). The XRD analyses show the presence of alkali feldspar, amorphous silica (opal-A), and minor amounts of clay minerals in Cores 183-1138A-13R to 16R. We detected only minor amounts of feldspar in Cores 183-1138A-16R and 17R.

The sediments are homogeneous, but some intervals have fine disseminated black silt- and sand-sized grains of basalt. A sandy layer is present in interval 183-1138A-14R-3, 126-128 cm. A dark gray tephra layer is present in interval 183-1138A-15R-1, 71-73 cm. The predominantly gray color of this subunit may reflect a terrigenous component from the vicinity of Heard Island (see "Discussion").

Subunit IIB (interval 183-1138A-17R-1, 0 cm, to 28R-CC, 10 cm; 150.50-265.90 mbsf) is white foraminifer-bearing nannofossil ooze of late to early Miocene age (Fig. F4; Table T3). The carbonate content of sediments in Subunit IIB ranges from 69 to 95 wt% CaCO₃. The average is 90 wt%, which is significantly higher than in Subunit IIA (Table T4). XRD analysis shows almost pure calcite. Minor amounts of feldspar are present in interval 183-1138A-17R-1, 90-92 cm, and interval 183-1138A-20R-1, 90-92 cm, has traces of feldspar, clay, and amorphous silica (Table T4). The organic carbon content is very low (0.02%). As in Subunit IIA, the sediments of Subunit IIB are essentially homogeneous. Disseminated black silt- and sand-sized grains are common in a few intervals of the upper portion of the subunit. Rare black (basalt?) pebbles are present. Gray volcanic ash is disseminated in interval 183-1138A-19R-1, 110-111 cm. The nannofossil ooze grades to semilithified nannofossil chalk in Core 183-1138A-28R.

Age: late Oligocene to mid-Campanian

Unit III consists of white to light gray and light greenish gray foraminifer-bearing nannofossil chalk (Fig. F4; Table T3). We subdivided Unit III into two subunits based on the presence or absence of identifiable nannofossils in smear slides.

Subunit IIIA (interval 183-1138A-29R-1, 0 cm, to 52R-CC, 26 cm; 265.90 to 496.40 mbsf) is white to light gray or light greenish gray foraminifer-bearing nannofossil chalk of late Oligocene to late Maastrichtian age (Fig. F4; Table T3). Chert nodules are present in Core 183-1138A-41R and below. The carbonate content ranges from 72 to 96 wt% CaCO₃ (Table T4), with an average of 90 wt%. Organic carbon content is 0.1%. The XRD analysis shows minor amounts of clay and amorphous silica in Cores 183-1138A-33 to 35. We detected traces of clinoptilolite in intervals 183-1138A-37R-2, 64-65 cm, and 49R-1, 90-91 cm (Table T4).

Cyclic color variations reflect the abundance (<1%) of disseminated black fine silt-sized particles, which are probably basaltic rock fragments. Gray intervals typically display relatively sharp basal contacts and gradational, bioturbated tops and may reflect a greater abundance of terrigenous material with a source in the vicinity of Heard Island. In Core 183-1138A-36R, the foraminifers in light greenish gray chalk are filled with a green material, presumably glauconite. The sediment is extensively burrowed, and some intervals show multiple generations of burrows.

Core 183-1138A-52R contains the Cretaceous/Tertiary (K/T) boundary (see "Biostratigraphy"). We did not observe any unusual lithologic changes (e.g., erosional contact in Core 183-1135A-28R) in this core. The sediment shows cyclic color variations from white to greenish gray, and some of these intervals have sharp, well-burrowed bases. Similar color cycles, however, are present in many cores, both above and below this interval.

Subunit IIIB (interval 183-1138A-53R-1, 0 cm, to 63R-CC, 18 cm; 496.40 to 601.80 mbsf) is predominantly white to very light gray foraminifer-bearing chalk of late Maastrichtian to mid-Campanian age. The carbonate content ranges from 92 to 96 wt% CaCO₃ (Table T4) with an average of 94 wt%. XRD analysis shows pure calcite throughout Subunit IIIB. Organic carbon ranges from 0.02% to 0.2%. Cores 183-1138A-57R through 63R contain two or three chert nodules per core. The contact at the base of Unit III is gradational.

Age: mid-Campanian to Turonian

Unit IV consists of cyclic alternations of light gray foraminifer-bearing chalk with gray through greenish gray to black intervals of nannofossil claystone (Figs. F4, F5; Table T4). The dark gray to black nannofossil claystone beds are increasingly prominent in the lower portion of Core 183-1138A-67R and are increasingly black, clay-rich, and organic carbon-rich toward the base of the unit (Figs. F5, F6). In Cores 183-1138A-68R and 69R, these light and dark cycles are ~20-30 cm thick (i.e., ~3/m). The dark gray to black intervals are typically 5 cm thick, with gradational tops and sharp bases, which mark the bases of the cycles (Fig. F6A, F6B).

The carbonate content ranges from 93 to 96 wt% CaCO₃ near the top, but drops to 1 wt% at the bottom of Unit IV. The light gray chalk has a very low organic carbon content of 0.04% (Table T4). The XRD analyses of chalk (CaCO₃ contents between 58 and 77 wt%) (Table T4), located close to gray chert (porcellanite) nodules, show high amounts of opal-A and a distinct opal-CT peak. A significant amount of quartz is present in only one sample.

Unit IV is extensively burrowed (except for the basal black claystone) (Fig. F6). Chondrites burrows are below some of the dark intervals. Inoceramid bivalve fragments are present in Cores 183-1138A-65R and 66R. Gray chert nodules are present (two to three per core) in the upper part of the unit.

The deepest dark interval in Unit IV is a bed of black claystone at the base of the unit (interval 183-1138A-69R-5, 21-112 cm) (Figs. F5, F6C). This bed contains two nodules of massive, fine-grained pyrite (intervals 183-1138A-69R-5, 54-56 cm, and 69R-5, 84 cm). Although the top few centimeters of this black interval are moderately burrowed, most of the bed displays undisturbed faint horizontal laminations (Fig. F6C). Sample 183-1138A-69R-5, 89-90 cm, in the lower part of this bed contains only 1 wt% CaCO₃, but has a very high organic carbon content of 2.24 wt%. XRD analysis of this layer shows high amounts of clay minerals, clinoptilolite, quartz, and pyrite.

Age: Santonian to Turonian

Unit V consists predominantly of glauconite-bearing to glauconitic calcareous sandstone (Fig. F4; Table T3). This unit is predominantly reddish to rusty brown but also includes dark green to black (glauconite rich) and light gray to tannish brown (carbonate rich) intervals (Figs. F5, F7, F8). The uppermost interval (interval 183-1138A-69R-5, 112 cm, through 69R-6, 118 cm) is very dark green to black glauconitic sandy claystone. The matrix is light tannish gray; however, fine- to medium-sized glauconite sand grains are so abundant that this light color is obscured. The carbonate content in this dark green sediment is only 7 wt% CaCO₃ (Sample 183-1138A-69R-6, 88-89 cm) (Table T4). XRD analysis shows glauconite, calcite, and clay minerals. Light gray to tannish gray glauconite-bearing calcareous sandstone (Sample 183-1138A-69R-6, 118-128 cm) underlies this darker glauconitic unit.

Reddish to orange-brown glauconite-bearing calcareous sandstone constitutes most of the remainder of Unit V (interval 183-1138A-70R-1, 0 cm, to 71R-2, 138 cm) (Figs. F5, F7, F8). The carbonate content is high (55 wt% CaCO₃ in Sample 183-1138A-70R-1, 93-96 cm) (Table T4) in the upper part of this interval. The organic carbon content is low (0.1%). XRD analysis shows calcite, opal-CT, opal-A, and clinoptilolite.

The glauconite content increases downward (Fig. F7B) such that interval 183-1138A-71R-1, 100 cm, to 71R-2, 50 cm, consists of glauconitic sandstone with low carbonate content (12 wt% in Sample 183-1138A-71R-2, 15-16 cm) (Table T4). XRD analysis shows the presence of calcite, glauconite, and clinoptilolite.

Abundant white serpulid worm tubes (as much as 0.5 cm in diameter) are present throughout interval 183-1138A-69R-6, 118 cm, to 71R-1, 130 cm (Figs. F7A, F8). Large pectinid bivalve fragments (as much as 2 cm long) are present in interval 183-1138A-71R-2, 27-50 cm. Scattered smaller shell fragments are rare to common throughout the unit. A bioclastic packstone (Sample 183-1138A-70R-2, 46-50 cm) (Table T5) contains 1%-2% glauconite, 1% opaque grains, and 1% brown translucent grains. Bioclasts include bivalves, crinoid columnals, echinoid spines and plate fragments, benthic foraminifers, ostracodes, and bryozoans. Many of the glauconite grains have brown oxidized rims. The echinoderms have syntaxial overgrowths.

Interval 183-1138A-71R-2, 68-91 cm, is light tannish brown because of higher carbonate content. Interval 183-1138A-71R-2, 91-104 cm, is a distinctive orange brown (ferruginous), well-laminated bed of glauconite-bearing fine-grained sandstone (Figs. F5, F7C). Some laminae appear to converge slightly and may indicate the presence of subtle cross-stratification. A thin section (Sample 183-1138A-71R-2, 93-95 cm) (Table T5) shows abundant brown ferruginous matrix obscuring dominantly nonbiogenic grains. Glauconite is common, and many grains have brown oxidized rims. Brown translucent sand-sized, rounded grains may be oxidized glauconite pellets. Iron ooids are present, including some with magnetite(?) crystal cores. A 1-mm-long iron-coated brown pyroxene(?) crystal and several granule-sized, rounded grains of highly altered plagioclase-phyric lavas are present. The bioclasts include ostracodes, pectinid bivalve shells, benthic foraminifers, echinoderms, and bryozoans. Most bioclasts are reworked, partially rounded, and broken fragments. Echinoderm grains are recrystallized and do not show uniform extinction in polarized light.

Below this bed, the calcareous sandstone is mostly fine grained, but scattered pebbles (as large as 0.5 cm in diameter) are common to the base of the unit (Fig. F7C, F7D). Sharp, wavy contacts at the tops of several tannish gray color bands (interval 183-1138A-71R-2, 121-138 cm) may mark diagenetic fronts. We placed the bottom of Unit V at a prominent sharp, undulating contact (possibly caused by loading) between this lithology and underlying dark brown sediments of Unit VI (Fig. F7D).

Age: Late Cretaceous

Unit VI consists predominantly of dark brown silty claystone with interbedded sandstone and conglomerate (Figs. F4, F5, F9, F10, F11; Table T3). The carbonate content is uniformly low and ranges from 1 to 14 wt%. The XRD analyses show that all the carbonate is FeCO₃; no calcite was detected (Table T4). The upper part of Unit VI (interval 183-1138A-71R-2, 138 cm, to 73R-1, 13 cm) is predominantly dark brown silty claystone. The uppermost bed (interval 183-1138A-71R-2, 138-141 cm) is brown, very fine-grained sandstone with some glauconite grains (Fig. F7D). The brown sandy claystone (interval 183-1138A-71R-2, 146 cm, to CC, 9 cm) contains scattered small pebbles, shell fragments (as long as 2 mm), and small wood fragments. The sandy clayey siltstone (interval 183-1138A-72R-1, 0-50 cm) contains abundant wood fragments as much as a few millimeters long. A 2-cm-long wood fragment is present at the gradational lower contact of this bed (Fig. F9A, F9B). Dark brown silty claystone extends throughout the rest of the upper part of Unit VI to Sample 183-1138A-73R-1, 13 cm (Figs. F5, F9C, F9D). Small wood fragments, a few of which are 1-2 cm long, are common throughout most of this claystone. Small white shell fragments and other bioclastic material also are present in this bed. The organic carbon content of Sample 183-1138A-72R-2, 56-60 cm, is 5.6 wt% and reflects the abundant wood fragments in this interval. The XRD analyses show siderite, kaolinite, pyrite, and possibly gibbsite. Pyrite nodules are rare and generally <1 cm in diameter, except for one 5-cm-long nodule or layer (interval 183-1138A-72R-1, 120-122 cm). A 1-cm-thick coarse sandstone bed (interval 183-1138A-72R-3, 6-7 cm) is cemented with pyrite.

Coarse sandstone comprises interval 183-1138A-73R-1, 13-81 cm, and contains abundant granules and pebbles (as much as 1 cm in diameter) that are highly concentrated in several zones (intervals 183-1138A-73R-1, 13-17 cm; 73R-1, 25-28 cm; 73R-1, 31-33 cm; and 73R-1, 43-47 cm) (Figs. F5, F10A). The variegated grains and pebbles, dark red and blue through tan, produce an overall brown color. The pebbles and coarse sand grains (Sample 183-1138A-73R-1, 24-28 cm) (Table T5) are volcanic and are mostly from lavas with abundant lath-shaped plagioclase microlites. The pebbles are predominantly well rounded, but some of the smaller grains are subangular. This sandstone is moderately well sorted and cemented with carbonate.

Interval 183-1138A-73R-2, 0 cm, to 73R-3, 40 cm, is dark brown silty claystone that contains fine laminated zones. Abundant, very small wood fragments are oriented along many of these laminae and also are found throughout the nonlaminated zones. The carbonate content of this interval is about 14 wt% FeCO₃ (Table T4). XRD analysis shows siderite, kaolinite, possible gibbsite, and minor pyrite.

Interval 183-1138A-73R-3, 40-126 cm, is mostly brown medium to coarse sandstone with zones of granules and pebbles (Fig. F10B). Near the top of this interval, two beds of brown silty clay are interbedded with this sandstone (intervals 183-1138A-73R-3, 47-53 cm, and 73R-3, 59-61 cm). This interval is highly disturbed by drilling so that exact stratigraphic positions and thicknesses of pebble intervals are uncertain. Most pebbles are <0.5 cm and a few are as much as 1 cm in diameter. One horizon shows alignment of pebbles along bedding planes.

The lowermost portion of Unit VI (interval 183-1138A-73R-3, 126 cm, to 73R-CC, 15 cm) is silty claystone of variegated colors including reds, bluish gray, rust, and greenish gray (Fig. F11). The carbonate content is very low (<1 wt%) (Table T4). XRD analysis shows kaolinite, hematite, quartz, and goethite. Large (2-4 cm) red, rounded pebbles of weathered(?) basalt are present at intervals 183-1138A-73R-3, 138-142 cm; 73R-3, 148-150 cm; and 73R-CC, 0-4 cm. Coarse sand and granules are present in interval 183-1138A-73R-3, 144-146 cm. The material in the core catcher is brecciated and may be weathered basement (i.e., soil) rather than a clastic sedimentary deposit, in which case an unconformity lies somewhere in this interval. We placed the lower contact of Unit VI at the stratigraphically highest occurrence of unambiguous igneous rock in Section 183-1138A-74R-1 at 13 cm. The material in interval 183-1138A-74R-1, 0-13 cm, does not appear to be in the correct stratigraphic position and is probably material that fell downhole during drilling.

Age: Late Cretaceous

Lithologic Unit VII consists of basalt flows capped by volcaniclastic rocks with very minor interbedded sedimentary rocks (Fig. F4; Table T3). Unit VII is subdivided into 22 basement units, which are described in "Physical Volcanology," "Igneous Petrology," and "Alteration and Weathering".

The sediments recovered from Site 1138 preserve a nearly complete transgressive sequence from subaerial volcanic deposits (Unit VII) capped by regolith and fluvial deposits (Unit VI) through shallow marine deposits (Unit V) into bathyal pelagic oozes (Units IV-I). Basaltic lava flows appear to have erupted subaerially and weathered by Cenomanian/Turonian time (~99-89 Ma). A thin bed of dark brown claystone and >20 m of felsic volcaniclastic rocks overlie the basalt flows (see "Physical Volcanology," and "Alteration and Weathering").

At the base of Unit VI, variegated clay with weathered, brecciated volcanic material, which grades upward into sediment with volcanic pebbles, appears to be a regolith. Overlying this regolith, sandstones and interbedded dark brown silty claystones with abundant wood debris were most likely deposited near forested land in a fluvial environment (Fig. F5). The sandstones probably represent channels that migrated through flood plain swamps. Fossiliferous glauconitic calcareous sandstones of Unit V (Fig. F5) indicate that a shallow neritic environment had developed by Cenomanian/Turonian time (~99-89 Ma).

The transition from neritic to pelagic conditions at Site 1138 is marked by a black pyritic claystone and subsequent cyclic dark/light sediments (Fig. F5). At this time, the Kerguelen Plateau was situated in a young, narrow Indian Ocean (Royer and Coffin, 1992), which would have been sensitive to both climatic and paleoceanographic changes. Black pyritic claystone with high organic carbon content at the base of Unit IV implies low oxygen concentrations in waters above the paleoseafloor. The thin dark gray to black nannofossil clay beds interbedded throughout Unit IV most likely represent similar, though increasingly less pronounced, events throughout the Turonian, Coniacian, Santonian, and into the mid-Campanian. The preliminary biostratigraphy permits the possibility of correlating the black pyritic claystone with similar occurrences found worldwide at the Cenomanian/Turonian boundary (Arthur et al., 1987).

Normal bathyal conditions with pelagic deposition of calcareous (Units II and III) and siliceous ooze (Unit I) have existed at Site 1138 since at least mid-Campanian time. Beginning in late Miocene time, these oozes appear to have been diluted by the influx of gray terrigenous clays and sporadic ash derived from a nearby landmass, most likely Heard Island. Seismic data appear to show a large modern submarine canyon with well-developed levees (overbank deposits) located ~45 km to the west-northwest of the drill site. It is possible that the gray terrigenous sediments in Unit I and Subunit IIA represent distal overbank deposits derived from turbidity currents flowing down this canyon since the late Miocene time. The switch from calcareous to siliceous pelagic components, widely observed on the Kerguelen Plateau (Mackensen et al., 1992), reflects movement of the polar front northward through the Site 1138 area during late Miocene time.