PRINCIPAL RESULTS

Sites 1135 and 1136

Sites 1135 and 1136 (water depths of 1567 and 1931 m, respectively) are on the southern Kerguelen Plateau, approximately midway between two ODP Sites (738 and 750) where basaltic basement has previously been recovered. Sites 1135 and 1136 are ~350 km north of Leg 119 Site 738 and 300 km south of Leg 120 Site 750 (Figs. F3, F4). Major objectives of drilling on the southern Kerguelen Plateau were to obtain 150 m of igneous basement to characterize the age, petrography, and compositions of the lavas, the physical characteristics of the lava flows, and the environment of the eruption (subaerial or submarine). A specific goal was to evaluate the areal extent of the continental lithosphere component that has been recognized in the Site 738 lavas using trace element and isotope geochemistry; such a component is not present in the more northerly Site 750 lavas (Figs. F9, F10, F11, F12, F13). Sedimentary objectives at Sites 1135 and 1136 were to determine sequence facies, to define the ages of seismic sequence boundaries, to estimate the duration of possible subaerial and shallow-marine environments, and to obtain minimum estimates for basement age. Hole instability forced us to abandon Site 1135 after drilling to ~70 m above acoustic basement (Fig. F16), but we were able to accomplish some of our basement-oriented objectives at Site 1136 (Fig. F17), located ~30 km east of Site 1135. In particular, we penetrated 33.3 m of basaltic basement that included three flows; two are apparently inflated pahoehoe flows characterized by massive, relatively unaltered interiors. These rocks provide excellent samples for radiometric dating and geochemical analyses.

The 526-m-thick upper Pliocene to Upper Cretaceous sedimentary sequence recovered at Site 1135 is almost entirely pelagic calcareous ooze and chalk (Fig. F18). Compressional wave velocities increase gradually with depth, from 1510 to 1780 m/s in the ooze and from 2000 to 2800 m/s in consolidated chalks. Porosity decreases from 62% to 55% in the ooze and from 55% to 36% in the chalk. Grain density varies little downhole, averaging 2.7 g/cm³. Chert nodules are common from ~140 m below seafloor (mbsf) to the bottom of the hole. We recovered an expanded (238 m thick) middle Eocene to latest Paleocene nannofossil ooze section, an interval not well represented during previous coring on the Kerguelen Plateau or elsewhere in the Southern Ocean at these high latitudes (~60°S). The study of this section will improve high-latitude biostratigraphic correlations. Furthermore, a Cretaceous/Tertiary boundary section at ~260 mbsf is possibly marked by a bed of light greenish gray calcareous clay with an irregular upper contact and scattered well-rounded clasts of white nannofossil ooze. These features suggest an erosional or mass-wasting event. We have tentatively identified Chron C29n above and Chron C29r below the boundary, respectively. Near the boundary, velocity, magnetic susceptibility, and natural gamma-ray intensity change significantly; in addition, water content, porosity, and carbonate content decrease below the boundary. Sedimentation rates were high in the Paleogene ooze (as much as 15 m/m.y.) and Cretaceous chalks (8-10 m/m.y.); the Paleocene section, however, is abbreviated by hiatuses.

The 128-m-thick sedimentary sequence recovered at Site 1136 (Fig. F19) includes an expanded upper lower Eocene to lower middle Eocene section of pelagic calcareous ooze and chalk (Unit II) that is not well represented in other drill holes on the Kerguelen Plateau or in any other southern high-latitude sites. Study of these sediments will refine high-latitude middle Eocene biostratigraphic zonations. Porosity averages 56.2% in Unit II, and compressional wave velocities are typically <1800 m/s. Grain density averages 2.70 g/cm³. Underlying these pelagic sediments is calcareous zeolitic volcanic clayey sand (Unit IV), probably deposited in a high-energy neritic (shelf) environment, and a carbonate-bearing zeolitic silty clay (Unit V). Fossil debris in Unit V is common and suggests deposition at shallow paleodepths (upper bathyal to outer neritic) in more tranquil conditions than prevailed during deposition of the overlying clayey sand. The sands and clays overlying basement basalt contain sparse but relatively well-preserved micro- and nannofossil faunas of middle Albian age, thereby providing a minimum age for the unit and underlying basalts of ~105-107 Ma. Albian marine sediments were not recovered during previous drilling on the Kerguelen Plateau, but microfossils in the sands and clays recovered in Hole 1136A resemble those from Albian sediment drilled on the Falkland Plateau. The sands and clays may correspond to nonmarine, palynomorph-bearing Albian sediment found in silt and claystone cored at Site 750 on the SKP. Albian and Late Cretaceous nannoplankton, foraminifers, and pollen assemblages will provide information on regional paleoceanographic conditions during those times. The epiclastic succession (Units IV and V) and overlying calcareous sediments reflect increasing water depths with time concomitant with a decreasing volcaniclastic component in the sediments. Basaltic volcanic components in the epiclastic sediments at this site are probably derived from erosion of the basaltic plateau.

At Site 1136, from 128.1 to 161.4 mbsf, we cored three normally magnetized tholeiitic basalt flows (55% recovery; Fig. F19). We infer that the vesicular tops of the two upper flows were not recovered. With increasing depth, basalt from the uppermost flow (6.2 m recovered from an ~10-m-thick flow) changes from a moderately altered, massive interior to a fine-grained, vesicle-rich (~10% clay-filled vesicles) and oxidized base. Horizontal vesicle sheets and the distribution of vesicles within the massive interior and lower crust of the upper flow suggest that it formed as an inflated pahoehoe flow. Basalt from the middle flow (13.3 m recovered from an ~20-m-thick flow) varies downward from a massive interior to a fine-grained, vesicle-rich (10%-15%) base. This flow is also probably an inflated, large-volume pahoehoe flow. We only recovered 53 cm of a vesicular basaltic breccia that forms the rubbly flow top of the lower flow. Although we cannot unambiguously determine the eruption environment of these flows, the inference that they are inflated pahoehoe flows and the absence of features indicating submarine volcanism (e.g., pillows and quenched glassy margins) suggest subaerial eruption.

All lavas are sparsely to moderately phyric basalts containing phenocrysts of plagioclase with lesser amounts of clinopyroxene and olivine. Phenocrysts are found as either isolated grains or as two texturally distinct types of glomerocrysts. Corroded plagioclase cores in one glomerocryst type resemble those in small (~1 cm) microgabbro xenoliths. Vesicle-rich segregations (1-2 cm wide) contain 10%-30% vesicles in a nonporphyritic, fine- to medium-grained matrix rich in glass and titanomagnetite. The basaltic rocks are slightly to completely altered to low temperature secondary phases that partly replace primary minerals, completely replace mesostasis, fill veins, and partly to completely fill vesicles. The most common secondary minerals are clays (Mg-saponite and celadonite), calcite, and zeolites. In general, clay minerals abound at all depths, whereas the abundances of calcite and zeolites exhibit more pronounced downhole variations. The wide variation of K and Rb contents in the lavas analyzed by XRF reflects formation of these secondary phases.

The four least altered samples (loss on ignition [LOI] = 0.9 to 2.1 wt%) from the upper and middle flows have 50.0-51.0 wt% SiO₂, 6.4-6.7 wt% MgO, and 1.60-1.76 wt% TiO₂. Both flows are quartz normative tholeiitic basalts (Fig. F20) with relatively low MgO and Ni contents and low Mg numbers; they are similar to basement rocks from other parts of the Kerguelen Plateau. In detail, the upper flow has marginally lower Ti, Nb, Zr, Y, and Ce, distinctly lower V, and higher Cr abundances than the middle flow. Primitive mantle-normalized abundances of highly incompatible trace elements (Ba, Nb, and Ce) are only slightly greater than those of less incompatible elements (Ti and Y). Site 1136 lavas do not have the anomalously low Nb/Ce and high Zr/Ti ratios that have been used in conjunction with isotope data to infer a continental lithospheric component in basalt from Site 738 on the southern plateau (Fig. F21). In many geochemical characteristics, Site 1136 lavas are similar to the low Al₂O₃ group at Site 749 (Storey et al., 1992). No evidence indicates that these lavas contain a component derived from continental lithosphere.

Major results of drilling at Sites 1135 and 1136 on the SKP include the following:

1. Expanded middle Eocene to uppermost Paleocene (Site 1135) and upper lower Eocene to lower middle Eocene (Site 1136) pelagic calcareous sediment sections not previously recovered at any Southern Ocean sites, which will improve our understanding of high-latitude paleoceanography at critical times of Paleogene cooling as well as aid high-latitude biostratigraphic correlations.
2. Albian and Late Cretaceous nannoplankton, foraminifers, and pollen assemblages, which will provide information on regional paleoceanographic conditions during those times of high relative sea level and high global temperatures.
3. Paleoenvironments of volcanic rock and overlying sediment range from subaerial (basalt) to neritic (clay and sand) to pelagic (chalk and ooze), documenting subsidence of the Kerguelen Plateau since Early Cretaceous time.
4. The >105-Ma basement basalts at Site 1136 are 10- to 20-m-thick inflated pahoehoe flows similar to continental flood basalts, such as the Columbia River Basalt; there is no geochemical evidence for the continental lithosphere component present in Site 738 basalts.

Site 1137

Site 1137 lies on Elan Bank, a large western salient of the main Kerguelen Plateau, at a water depth of 1004 m (Figs. F3, F4). Elan Bank, flanked on three sides by oceanic crust of the Enderby Basin, had not been sampled before our drilling at Site 1137; therefore, the age and geochemistry of its igneous crust, as well as its relationship to the contiguous central and southern Kerguelen Plateau, were completely unknown. Site 1137 lies on the eastern portion of the crest of Elan Bank; we chose the location as representative of the entire Elan Bank on the basis of its relatively simple structural setting, thin sedimentary section, and the presence of intrabasement seismic reflections (Fig. F22). The major objective at Site 1137 was to obtain igneous basement to characterize the age, petrography, and compositions of the lavas, the physical characteristics of the lava flows, and the environment of eruption (subaerial or submarine). We were especially interested in constraining the age of the uppermost igneous basement at Elan Bank for comparison with the proposed ~110 and ~85 Ma volcanic pulses on the southern and central Kerguelen Plateau, respectively (Fig. F7). Sedimentary objectives at Site 1137 were to determine sequence facies, to define the ages of seismic sequence boundaries, to estimate the duration of possible subaerial and shallow-marine environments, to obtain minimum estimates for basement age, and to determine the paleoceanographic history of this high-latitude site. As discussed below, we largely achieved our goals at Site 1137. We cored basaltic basement and interbedded sediment from 219.5 to 371.2 mbsf. One of the most significant and unexpected results of the leg was the discovery of garnet gneiss clasts in a fluvial conglomerate interbedded with basaltic basement at this site. This provides unequivocal evidence of continental crust in Elan Bank. In addition, the geochemical characteristics of these basalts clearly indicate a continental crustal component.

We recognize three sedimentary lithologic units (I-III) in the upper 219.5 mbsf (Fig. F23). They rest unconformably on basaltic basement (Unit IV). Unit I (0-9.5 mbsf) consists of Pleistocene foraminifer-bearing diatom ooze with interpreted ice-rafted sand and pebbles. Unit II (9.5-199.5 mbsf) is Miocene to uppermost Eocene white nannofossil ooze with rare chert. Some intervals contain diatoms or foraminifers. Units I and II represent marine pelagic deposition and are characterized by compressional wave velocities of 1564 to 1785 m/s that show little scatter. Porosity in the upper two units clusters between 50% and 63%. Unit III (199.5-219.5 mbsf) is a 20-m-thick sequence of glauconite-bearing sandy packstone with abundant shell fragments that was probably deposited in a neritic environment. In the core overlying basaltic basement, the packstone contains well-preserved upper Campanian (72-76 Ma) foraminifers, calcareous nannofossils, and dinoflagellates. Compressional wave velocities in Unit III vary considerably from 3120 and 4340 m/s, and porosity, which ranges between 4.7% and 24.9%, averages 11.7%. Natural gamma-ray intensities are relatively high in the glauconitic sand.

Basalt and interbedded volcaniclastic sediment comprise the 151.7-m basement sequence in Hole 1137A (Unit IV; 219.5-371.2 mbsf), which we subdivide into basement Units 1-10 (Fig. F23). The 10 units include seven basaltic lava flows, totaling ~90 m in thickness, and three sedimentary/volcaniclastic units. All basement units are clearly distinguishable in downhole logging data. Relatively good core recovery and high quality logs enable us to constrain true thicknesses of the basement units through core-log integration. All basalts are normally magnetized; in light of biostratigraphic ages in Unit III and thickening of this unit to the east, the basalts likely acquired their magnetization during the long Cretaceous Normal Superchron prior to 83 Ma. Inclinations after thermal demagnetization of basalts from the seven flows range from -55° to -72°, with a mean of -66°. We calculate a paleolatitude of ~48°S, which is 8.5° north of Site 1137. Compressional wave velocities in basement rocks range from 2648 to 6565 m/s, averaging ~4650 m/s. Velocities within lava flows correlate inversely with the degree of alteration estimated from visual inspection.

The excellent core recovery and logging data show that the seven basaltic flows were temporally distinct subaerial eruptions. The flows show oxidation zones and morphologies consistent with subaerial emplacement. The flows are 7 to 27 m thick, and we recovered three flow-top breccias, a pahoehoe surface, and two basal contacts where lava apparently baked the underlying units. Flow-top breccias appear to have formed by breaking up of small pahoehoe-like fingers that are repeatedly intruded into older breccia. This style of autobrecciation is atypical of pahoehoe, aa, or Hawaiian transitional lava flows, but is common in western United States flood basalts. The three (or possibly four) pahoehoe flows in Hole 1137A are inflated; morphologically, they resemble large-volume lava flows forming continental flood basalts. Multiple horizontal vesicular zones within the 27-m-thick flow suggest a complicated inflation history of perhaps four separate pulses of lava. Sediments intercalated with the lowermost flow top breccia indicate that either the lava intruded a small volume of wet sediment or that the flow top was reworked by sedimentary processes.

Basement Units 1-4 are aphyric to moderately plagioclase ± clinopyroxene ± olivine-phyric basalt, whereas Units 7, 8, and 10 are moderately to highly plagioclase ± clinopyroxene-phyric basalt. Like all other Cretaceous basement basalt recovered from the Kerguelen Plateau, Site 1137 basalts are tholeiitic to transitional in composition (Fig. F20); they have 50.4 to 52.7 wt% SiO₂ and 4.4 to 7.3 wt% MgO. However, abundances of incompatible minor and trace elements (Fig. F24), as well as the ratio of more incompatible to less incompatible elements (e.g., Zr/Y; see Fig. F21), are higher than in other Cretaceous basement tholeiites recovered from the plateau. Consequently, Site 1137 basalts form a distinct geochemical group that may reflect a relatively lower extent of melting or a source more enriched in incompatible elements. A continental lithospheric component, probably crust, in Site 1137 basalts is indicated by their relatively low Nb/Ce and high Zr/Ti ratios, as well as their trend in Nb/Y vs. Zr/Y (Figs. F21, F25). Such a component is also present in Kerguelen Plateau basalts from Sites 738 and 747, but not in basalts from Sites 749, 750, and 1136 (Figs. F21, F25).

Lava flow interiors at Site 1137, as at Site 1136, are relatively unaltered compared to Cretaceous basement lavas previously recovered from the Kerguelen Plateau. For example, LOI for least-altered samples is <2.2 wt% and averages 1.2 wt%. The mobility of Rb and K during postmagmatic alteration shows in the poor correlation of Rb and Nb abundances (Fig. F24) and the wide abundance range of K and Rb compared to other incompatible elements (Fig. F26). Basement rocks vary from slightly to completely altered with low-temperature secondary phases replacing primary minerals and mesostasis and filling veins, fractures, and vesicles. Clay minerals are the dominant secondary minerals in all basement units. More permeable horizons (such as brecciated and/or vesicular flow tops and bases, and zones with high vein and fracture densities) exhibit higher degrees of alteration and more diverse secondary phases that include calcite, zeolite, quartz, and amorphous silica. In downhole logs, highly altered flow tops are characterized by higher potassium content caused by the increased abundance of clay minerals. Alteration of Hole 1137A lavas likely results from both weathering and low-temperature alteration. Unlike postmagmatic submarine alteration of typical oceanic crust, subaerial weathering and low-temperature interaction of basalts and interbedded sediments with groundwater at Site 1137 preceded submarine alteration.

Basement Units 5, 6, and 9 consist of volcaniclastic sedimentary rocks. Basement Unit 5 (286.7-291.0 mbsf) is a succession of interbedded crystal-lithic volcanic siltstones and sandstones. Many beds are normally graded, and others show parallel laminations. These sediments overlie the volcanic conglomerate of basement Unit 6 (291.0-317.2 mbsf). Clasts in the conglomerate range from well-rounded granules to small boulders. Most intervals are clast supported, but matrix-supported intervals also are present. The depositional environment of basement Units 5 and 6 appears fluvial, perhaps associated with a braided river. These units represent a significant hiatus of unknown duration between eruptions of basaltic flow Units 1-4 and 7 and 8. Furthermore, fluvial facies of Units 5 and 6 corroborate our interpretation of subaerial lava effusion.

Diverse clasts within the conglomerate (Unit 6), and to a lesser extent similar lithic clasts within the underlying crystal-vitric tuff (Unit 9), constitute a greater variety of rock types than usually recovered from a single drill hole into igneous basement. In particular, the predominant clast lithologies in Units 6 and 9 include porphyritic trachyte, flow-banded rhyolite, plagioclase phyric basalt, and a variety of small, highly altered, sparsely phyric and aphyric basalts. The most unexpected clasts, however, are rounded cobbles of garnet-biotite-gneiss and granitoid. We also find single grains of garnet and perthitic alkali feldspar, presumably weathered from sources similar to those of the gneiss and granitoid cobbles, in the sand fraction of Unit 5 and as xenocrysts in parts of the Unit 9 crystal-vitric tuff. It is difficult to imagine anything but an originally continental source for such material.

Basement Unit 9 (344.0-360.7 mbsf) consists of altered crystal-vitric tuff composed of ~40% coarse (1-2 mm) angular crystals of sanidine and quartz and <5% lithic clasts enclosed within a light to dark green dense matrix. Cuspate and tricuspate glass shards, now partially to completely altered to clay minerals, and sanidine and less abundant quartz phenocrysts (<5 mm) are the principal components of the tuff; minor components include amphibole, plagioclase, and opaques. Broken bubble-wall shards and abundant embayed and broken crystals indicate that the tuff formed in an explosive volcanic eruption. Suspended within the tuff are 1%-2% subangular to rounded, granule- to pebble-sized lithic clasts of varying lithologies, dominantly basalt. The coarse grain size of both the enclosed pebbles and the primary sanidine and quartz crystals precludes formation of this unit as a primary air-fall deposit. Although a pyroclastic flow is a possible emplacement mechanism, we infer that the material was reworked because of the absence of (1) internal stratification, (2) a basal breccia or fine flow top, or (3) normal grading of lithics and crystals. The even distribution of crystals and pebbles throughout the tuff and the massive internal texture of the deposit strongly suggest mass flow redeposition of these sediments to their present locations. Small fault zones with offsets of 5 cm are highly altered and contain locally abundant (5%) pyrite and possibly chlorite. Native copper surrounds lithic fragments in more intense alteration zones (<1 cm wide).

In summary, significant results bearing on the origin and evolution of Elan Bank include the following:

1. Subsidence of the Kerguelen Plateau is recorded by the paleoenvironments of volcanic rocks and sediment that range from subaerial or fluvial (basalt and interbedded sediment) to neritic (packstone) to pelagic (ooze).
2. The volcanic conglomerate contains clasts of trachyte, rhyolite, granitoid, and garnet-biotite gneiss; the garnet-biotite gneiss, in particular, indicates continental crust at this south Indian Ocean location.
3. The sanidine-quartz-bearing crystal-vitric tuff, as well as the trachyte and rhyolite clasts in the conglomerate, indicates that highly evolved magmas erupted, in some cases explosively, during the final stages of the volcanism that formed Elan Bank.
4. The seven basement flows erupted in a subaerial environment. These inflated pahoehoe and transitional rubbly flows are typical of continental flood basalts, such as the Columbia River Basalt.
5. Like other Cretaceous igneous basement rocks of the Kerguelen Plateau, the seven basement lava flows are tholeiitic to transitional basalts; however, Site 1137 basalts are more enriched in incompatible elements, perhaps a result of lower extents of partial melting or derivation from a source more enriched in incompatible elements. Also, we infer a continental crust component in Site 1137 basalts from their less-than-primitive mantle ratios of Nb/Ce and Zr/Ti and their Nb/Y vs. Zr/Y trends.

Site 1138

Site 1138 lies on the CKP ~150 km north-northwest of Site 747 (Leg 120) and 180 km east-southeast of Heard Island (Figs. F3, F4). In the vicinity of Site 1138, geological structure and seismic stratigraphy are relatively simple, and interpreted igneous basement contains some internal reflections (Fig. F27). Basalts at Site 747 erupted at ~85-88 Ma, as determined from ⁴⁰Ar/³⁹Ar data and from the biostratigraphy of the overlying sediments. In contrast, Heard Island is dominated by Quaternary volcanism. A major objective at Site 1138 was to determine if the uppermost basaltic crust of the CKP is ~85 Ma at more than one location. Also, geochemical characteristics of Site 747 basalts indicate a continental crust component, possibly Archean granulite, which differs from the continental component in basalt from the SKP at Site 738. During continental breakup, continental lithosphere along the conjugate Antarctic and Indian margins may have been fragmented and incorporated into embryonic Indian Ocean mantle. Subsequently, in localized areas this continental material may have interacted with basaltic magmas forming the Kerguelen Plateau. Therefore, we were especially interested in comparing the petrology and geochemistry of basaltic basement from this second CKP drill site with basalt from the southern, northern, and Elan Bank domains, as well as Heard Island and the Kerguelen Archipelago. Additional basement objectives were to determine the physical characteristics of the lava flows and the environment of the eruption (subaerial or submarine). The sedimentary objectives at Site 1138 were to determine sequence facies, to define the ages of seismic sequence boundaries, to estimate the duration of possible subaerial and shallow-marine environments, to obtain minimum estimates for basement age, and to determine the paleoceanographic history of the CKP. At Site 1138 our objectives were achieved by coring ~144 m of volcanic basement and ~698 m of overlying sediment.

We recovered Upper Cretaceous through Pleistocene sediment from the upper 698 mbsf of Hole 1138A, whereas the lower 144 m of the hole yielded multiple ~5-m-thick basalt flows overlain by volcaniclastic and minor sedimentary rocks (Fig. F28). We recognized seven lithologic units in Hole 1138A; Units I-VI are sedimentary rocks resting unconformably on the volcanic basement (Unit VII). The upper 650 m of sediment is biosiliceous and carbonate pelagic ooze, of which the top 110-m section comprises a relatively complete and expanded sequence of Quaternary and Pliocene biosiliceous sediments. The lower ~50 m of the sedimentary section consists of Upper Cretaceous shallow-marine and terrestrial sediments.

Unit I (0-112.0 mbsf) consists of foraminifer-bearing diatom clay with interbedded foraminifer-bearing diatom ooze in the upper portion. We found a few thin volcanic ash layers in this upper Pleistocene to upper Miocene unit. Grain density averages 2.38 g/cm³; porosity, 77%; and compressional wave velocity, 1568 m/s in Unit I.

Unit II (112.0-265.9 mbsf) is composed of foraminifer-bearing nannofossil clay (Subunit IIA) that overlies foraminifer-bearing nannofossil ooze (Subunit IIB). The carbonate/silica ratio of the 153.9-m-thick Miocene Unit II is much higher than that of Unit I. Volcanic material is disseminated in the sediment as well as in rare distinct tephra layers. In Unit II, grain density averages 2.61 g/cm³, porosity 60%, and compressional wave velocity 1672 m/s. Unit III (265.9-601.8 mbsf) is late Oligocene to mid-Campanian in age. It consists of foraminifer-bearing chalk and contains scattered chert nodules in its lower part. Cyclic color variations (white to greenish gray) are common. The Cretaceous/Tertiary boundary near the base of Subunit IIIA (Core 183-1138A-52R) is possibly complete, but lithologies do not change across it. In Unit III, grain density averages 2.70 g/cm³, porosity averages 48%, and compressional wave velocity averages 2310 m/s.

Unit IV (601.8-655.6 mbsf), of mid-Campanian to Cenomanian(?) age, consists of cyclic alternations of light gray foraminifer-bearing chalk with gray through greenish gray to black intervals of nannofossil claystone. The dark gray to black beds become prominent and increase in clay content in the lower portion. Chert nodules are present in the upper part of the unit. Grain density averages 2.67 g/cm³, porosity 35%, and compressional wave velocity 2665 m/s. A bed of black, faintly laminated (unburrowed) claystone with high organic carbon content (2.22%) is at the base of Unit IV. Units I through IV represent deep-marine pelagic sedimentation; however, the relatively high clay content of sediments in Unit I and Subunit IIA suggests terrigenous input from overbank flow of turbidity currents moving down a submarine canyon ~45 km west-northwest of Site 1138. The black claystone at the base of Unit IV reflects an oxygen-starved environment that may be the oceanic anoxic event marking the Cenomanian/Turonian boundary.

Unit V (655.57-671.88 mbsf) consists predominantly of glauconitic calcareous sandstone of Turonian-Cenomanian age deposited in a neritic environment. Serpulid worm tubes and large bivalve fragments are common. Grain density averages 2.71 g/cm³, porosity averages 42%, and compressional wave velocity averages 2719 m/s in Unit V. The gradual transition from neritic oxidized sediment (Unit V) to interbedded black claystone and chalk (Unit IV) to pelagic sediments (Unit III) supports the postulated major transgression causing the Cenomanian-Turonian oceanic anoxic event. This hypothesis will be tested by shore-based studies.

Unit VI (671.88-698.23 mbsf) consists of Upper Cretaceous fossil-rich, dark brown silty claystone with interbedded sandstone of fluvial or shallow-marine origin. The silty claystone contains many wood fragments, possible sporangias, a seed, and fossil spores and pollen. The sandstone beds contain well-rounded pebbles and sand grains of volcanic material. At the bottom of Unit VI, silty claystone rests upon volcanic basement rocks (Unit VII). Large rounded pebbles of weathered basalt close to the base of Unit VI suggest a regolith formed by weathering of volcanic basement. In Unit VI, grain density averages 2.72 g/cm³, porosity averages 37%, and compressional wave velocity averages 2328 m/s, the latter defining a pronounced velocity inversion from overlying Unit V. The seismic sequence containing the deepest marine sediments cored at Site 1138 thickens to the northeast, suggesting that basaltic basement rocks could be significantly older than the minimum age indicated by biostratigraphy.

We recognize 22 units within the 144 m of igneous basement (Unit VII) drilled at Site 1138 (Fig. F28). Basement Unit 1 includes rounded cobbles of flow-banded, aphyric to sparsely sanidine-phyric dacite. Unit 2 is a complex succession of volcaniclastic rocks overlying basalt lava flows—Units 3 through 22. The 20-m-thick volcaniclastic succession comprising Unit 2 includes six variably oxidized and altered pumice lithic breccias. Our preferred interpretation is that these are minimally reworked and unwelded, subaerial pyroclastic flow deposits. The pumice clasts are typically aphyric, and the bulk composition of a pumice-rich sample is trachytic (Fig. F29). The volcaniclastic sequence also includes pumice beds, reworked volcaniclastic sediments, and highly altered ash deposits that contain accretionary lapilli.

Basement Units 3-22 are ~5-m-thick subaerial basaltic lava flows that range from inflated pahoehoe to classic aa. Several boundaries are oxidized, suggesting subaerial weathering between eruptions. The relatively thin flows at Site 1138 resemble Hawaiian lavas and contrast with the generally thicker flows drilled at Sites 1136 and 1137. Most flows have flow-top breccias that are not easily classified. Some breccias contain slabs of pahoehoe mixed with aa clinker; others are a jumble of pahoehoe lobes. The breccias contain varying amounts of sediment; some appear reworked, most likely in a fluvial environment. Most flows probably erupted on a moderate slope (1° to 4°) under conditions of high shear resulting from a high eruption rate or topographic confinement. Several observations indicate that these are near vent flows: (1) aa and slab pahoehoe flows rarely travel more than a few tens of kilometers from vents; (2) abundant small vesicles indicate that the lavas did not flow far enough for vesicles, which formed at vents, to coalesce; and (3) clasts in some of the welded basal breccias appear to be spatter, which only forms close to vents. There are also suggestions of explosive interaction between the lava and groundwater. The complex relationships between the flows at Site 1138 do not allow us to identify the products of individual eruptions, but the flows and the eruptions that fed them were considerably smaller than for typical continental flood basalts.

All basalts show normal magnetic inclinations. We calculated a mean inclination of -60.8°, which corresponds to a paleolatitude of 46.4°S, assuming a geocentric dipole field. The paleolatitude is thus 7° north of Site 1138. This southward shift in latitude since Late Cretaceous time is consistent with the 8.5° difference we found at Site 1137 on Elan Bank. The basalts have average grain densities of 2.90 g/cm³ (range of 2.44-3.1 g/cm³), porosities of 25% (range of 9%-55%), and compressional wave velocities of 4014 m/s (range of 1884-7491 m/s).

The massive parts of flow Units 3-22 are slightly to locally highly altered, whereas alteration ranges from high to complete in the brecciated zones. Rubbly flow tops are partly to completely altered to clay minerals, and abundant euhedral zeolites form the matrix, fill veins, and partially fill vesicles and large voids. Lava clasts are commonly completely altered to brown clay minerals. Multiple generations of zeolite (clinoptilolite) exhibit many crystal shapes, predominantly equant and prismatic, but fluffy forms frequently fill fissures. Sediment filling breccia void space is variably indurated, perhaps caused by silicification. Calcium carbonate is absent except from the uppermost basalts directly underlying the volcaniclastic sequence.

Most of the basalts are moderately to highly vesicular and aphyric to sparsely plagioclase-phyric tholeiites (Fig. F21, F29). Units 9 and 19 contain clinopyroxene phenocrysts, and Units 5-16 and 19 contain 1%-5% olivine microphenocrysts, now completely replaced by secondary clays. The relatively unaltered (LOI of only 0.5 to 2 wt%) massive parts of these basaltic flows have similar major element compositions (e.g., MgO contents vary only from 4.5 to 7 wt%). However, with increasing depth, Mg/Fe, Ni, and Cr contents decrease, and abundances of most incompatible elements (Sr is an exception) increase by nearly a factor of two (Fig. F30), thereby defining a trend to Fe- and Ti-rich basalt. This systematic downhole trend is consistent with extensive fractionation of the phenocryst phases, plagioclase, olivine, and clinopyroxene. Basalts from the two drill sites on the CKP (Sites 747 and 1138) overlap in a Nb/Y vs. Zr/Y plot (Fig. F31).

The major results of drilling at Site 1138 on the CKP include

1. Paleoenvironments of the Cenomanian/Turonian boundary event appear to be preserved in the transition from oxidized neritic sediment to black claystone (shale) to pelagic sediment, which may enable testing of the hypothesis that a major transgression caused this oceanic anoxic event.
2. The sedimentary sequence overlying basement contains Upper Cretaceous shallow-marine and terrestrial sediments. Turonian (and older?) silty claystone contains well-preserved wood fragments, a seed, spores, and pollen, documenting for the first time that the CKP was subaerial after volcanism ceased.
3. The inferred minimum basement age, Turonian (89-93 Ma), is older than the 85-88 Ma proposed for Site 747, only 150 km to the southeast.
4. Volcanic growth of the Kerguelen Plateau at this site on the CKP terminated with eruptions of highly evolved magma that include dacitic lavas and pyroclastic flow deposits of trachytic composition.
5. Basaltic basement underlying evolved rocks is represented by ~20 thin, Hawaiian-scale subaerial flows that erupted onto moderate slopes of 1° to 4°. These are tholeiitic basalt flows whose compositions define a systematic downhole trend to Fe- and Ti-rich basalt. Such highly evolved basalts have not been recovered previously from the Kerguelen Plateau; incompatible element abundance ratios, such as Nb/Zr and Nb/Y, of Site 1138 basalts, however, overlap with the field defined by basalts from Site 747, the other drill site on the CKP.

Site 1139

Site 1139 lies on Skiff Bank (Leclaire Rise), a bathymetric and gravimetric high ~350 km west-southwest of the Kerguelen Archipelago (Figs. F3, F4). Flanked to the south and west by Cretaceous oceanic crust of the Enderby Basin, Skiff Bank appears to be structurally related to, and bathymetrically continuous with, the NKP. At least two major faults, however, offset interpreted igneous basement between Skiff Bank and the large massif containing the Kerguelen Archipelago. Skiff Bank has been proposed to be the current site of the Kerguelen hot spot (Figs. F1, F2), but hundreds of meters of sediment on parts of the elevated feature argue against Skiff Bank originating entirely by recent volcanism. Both Skiff Bank and Elan Bank trend east-west, approximately perpendicular to the trends of fracture zones in the Enderby Basin and thus parallel to the axis of breakup between Antarctica and India. The free-air gravity signatures of the two features are also similar; pronounced negative anomalies flank their southern margins, but not their northern margins (Fig. F4). Many rock types, including both aphyric basalt and plutonic rocks such as alkali granite, were recovered in a single dredge haul from Skiff Bank, quite close to Site 1139 (Weis et al., 1998a). The plutonic rocks were interpreted as ice-rafted debris. Hence, the age and composition of Skiff Bank's igneous crust and its relationship to the contiguous northern Kerguelen Plateau are not established. The NKP is commonly believed to have formed since ~40 Ma, when the SEIR separated Broken Ridge and the CKP (Fig. F2), but submarine igneous basement of the NKP has never been drilled.

Site 1139 lies at a depth of 1415 m on Skiff Bank's southwestern terrace, which is >1000 m lower than the crest, located <50 km to the northeast (Fig. F3). The top of acoustic basement is flat lying, and basement is overlain by a sediment sequence ~500 m thick (Fig. F32). The fault scarp marking the boundary between Skiff Bank and the Enderby Basin lies ~20 km southwest of Site 1139 and offsets the basement by more than 2700 m.

The major objectives at Site 1139 were to obtain igneous basement to characterize the ages, petrography, and compositions of lavas, the physical characteristics of the lava flows, and the environments of eruption (subaerial or submarine). We were especially interested in testing the hypothesis that the age of the uppermost igneous basement at Skiff Bank is <40 Ma. The sedimentary objectives at Site 1139 were to determine sequence facies, to define the ages of seismic sequence boundaries, to estimate the duration of possible subaerial and shallow-marine environments, to obtain minimum estimates for basement age, and to determine the paleoceanographic history of this high latitude site. As discussed below, we largely achieved our goals at Site 1139. We drilled 233 m into igneous basement that is overlain by lower Oligocene shallow-marine sediments.

Sediments were recovered from the upper 462 mbsf of Hole 1139A, whereas extensively altered felsic volcaniclastic rocks and mafic to intermediate composition lava flows were recovered from the lower 233 m of the hole (Fig. F33). We recognize six lithologic units. Units I-V are sediment and sedimentary rock resting on volcanic basement (Unit VI). Unit I (0 to 47.5 mbsf) consists of foraminifer-bearing diatom-bearing nannofossil ooze (Subunit IA) of Quaternary age and foraminifer-bearing nannofossil ooze (Subunit IB). Scattered basaltic sand grains and rare pebbles as well as traces of pumice are present in Subunit IA. Unit II (47.5 to 380.7 mbsf) consists of nannofossil-bearing clay and claystone with interbedded nannofossil-bearing ooze and chalk of early late Miocene to mid-Oligocene age. Trace fossils are very common. Unit II records a substantial influx of terrigenous clay from an adjacent volcanic landmass. In Subunit IB and the upper portion of Unit II (to ~107 mbsf) compressional wave velocity averages 1822 m/s, bulk density ranges from 1.5 g/cm³ to 1.7 g/ cm³, grain density ranges between 2.6 and 2.8 g/cm³, and porosity changes from 60% to 74%. Sediments become semilithified by 100-110 mbsf. An unusual nannofossil (Braarudosphaera) bloom in late Oligocene time, reported previously on the SKP, may have been synchronous with other occurrences in the Atlantic and Indian Oceans. Minimum sedimentation rates were ~16 m/m.y. in the Miocene and ~20 m/m.y. in the Oligocene. We observed very rare tephra layers and disseminated volcanic ash, locally concentrated in burrows. Chert nodules appear only at the base of Unit II. We correlate normal and reverse magnetic polarities between ~100 and ~380 mbsf to early Miocene to early Oligocene geomagnetic Chrons C5D to C12 (or C13). From 108.9 mbsf to the base of Unit II at 380.7 mbsf, velocities increase linearly with depth, from 1785 to 4331 m/s. Within this depth interval, three volcanic ash layers have high compressional wave velocities. In the same interval, bulk density increases from 1.3 g/cm³ to 2.1 g/cm³ with a mean of 1.7 g/cm³, and porosity decreases from a maximum of 75% to 42%. Grain density maintains a nearly constant value of ~2.8 g/cm³. Unit III (380.7 to 383.5 mbsf) is foraminifer nannofossil chalk of anomalous brownish to reddish yellow color. The compressional wave velocity averages 3616 m/s. Units I-III represent deep-marine pelagic sedimentation. The base of the pelagic section is earliest Oligocene in age.

Unit IV (383.5 to 384.9 mbsf) consists of dusky red to greenish pink sandy packstone with rare planktonic foraminifers and bivalve shell fragments (Fig. F33). Index properties change significantly near the boundary between Units III and IV, from 381.4 to 384.4 mbsf. The bulk density in this zone averages 2.0 g/cm³, grain density averages 2.8 g/cm³, and porosity ranges between ~50% and ~31%. Grains are predominantly highly altered volcanic lithic fragments. Compressional wave velocity averages 3616 m/s. Unit V (384.9 to 461.7 mbsf) consists of interlaminated grainstone and sandstone with some thin interbeds of rudstone and cross-bedded intervals. Bryozoans, bivalves, and echinoids are the major biogenic components. Units IV and V were deposited in a shallow-marine neritic environment in low-energy and very high energy (near shore) settings, respectively. Well-rounded cobbles at the top of the basement suggest a beach deposit at the base of the sedimentary succession.

At Site 1139, we drilled 232.5 m into igneous basement with 37.4% recovery (Fig. F34). We identify 19 basement units; an upper succession of variably welded trachytic to rhyolitic volcanic and volcaniclastic rocks (Units 1-5) is underlain by a series of 14 lava flows (Units 6-19). All basement units are highly altered and fractured. The high degree of alteration and poor core recovery in Units 1-5 make it difficult to identify physical volcanic features and interpret modes of emplacement. However, these units can be distinguished in the natural gamma-ray logs. Rocks in Units 1-5 have compressional wave velocities varying from 2577 to 4770 m/s, with a mean value of 3616 m/s. Their bulk densities average 2.3 g/cm³, grain densities range from 2.6 to 2.9 g/cm³, and porosities average ~25%. The underlying 14 subaerial lava units (Units 6-19) have compressional wave velocities that are typically >3000 m/s, with a mean of 4416 m/s. Bulk densities vary widely, with a mean of 2.4 g/cm³; mean grain density is 2.8 g/cm³ and decreases slightly with depth; and porosity varies widely, from 65% to 3%. All basement units have positive magnetic inclinations, corresponding to reversed polarity.

Unit 1, which had poor recovery (57 m thick; 5.3 m recovered), contains a variety of felsic volcanic and volcaniclastic rocks (Figs. F34). Subunit 1A consists of rounded, massive to flow-banded rhyolite cobbles. Subunit 1B is a thin lens of bioclastic sandstone that resembles the grainstone at the base of the sedimentary section. Beneath this, a thin felsic pumice breccia (Subunit 1C) overlies a zone of altered, perlitic felsic glass that contains lithic fragments (Subunit 1D). We interpret the glassy zone to be the densely welded core of a pyroclastic flow deposit. The base of Subunit 1D is a silicified basal breccia with lithic fragments and pumice. Beneath this are highly sheared and altered, clay-rich volcaniclastic sediments (Subunit 1E) that we interpret as a fault zone. Within Subunits 1C through 1E, both clasts and the matrix commonly display cataclastic fabrics, and slickensides are ubiquitous on broken clay-rich surfaces. Unit 2 (10.5 m thick; 1.35 m recovered) consists of dark red (oxidized) rhyolite with ~10% sanidine and minor quartz phenocrysts. Flattening and agglutination textures suggest that this is a welded pyroclastic flow deposit. Unit 3 (9.7 m thick; 4.6 m recovered) is a green, highly altered crystal-vitric tuff. It contains abundant sanidine phenocrysts, minor quartz, and lithic clasts, in a perlitic glassy matrix that is locally banded. As with Subunit 1C, we interpret Unit 3 to be the densely welded core of a pyroclastic flow deposit. Unit 4 (30.1 m thick; 5.9 m recovered) contains massive to brecciated, dark red (oxidized) rhyolite that is similar to Unit 2. Unit 5 (17.4 m thick; 4.2 m recovered) is highly altered sanidine-phyric trachyte that consists of a massive central zone bounded by a brecciated top and base; this unit is probably a lava flow.

Basement Units 6-17 (65.7 m drilled; 41.4 m recovered) consist of aphyric to sparsely plagioclase-phyric volcanic rocks ranging in composition from trachybasalt to trachyandesite (Fig. F35). We subdivide this sequence on the basis of brecciated zones and vesicularity patterns, but intense alteration commonly precludes identifying contacts between flows. Although defined flow units vary widely in thickness, most flows are ~5 m thick. Unit 10 consists of small pahoehoe lobes, Unit 11 is an aa flow, and the other flow units have brecciated margins of indeterminate character. The breccias are highly altered and sheared, with both matrix and clasts nearly completely altered to clay minerals. Breccia clasts are oxidized and cemented by calcite and siderite(?) as well as clay minerals. The relatively thin massive portions of the flows have many moderately to steeply dipping fractures and pronounced streaks of mesostasis, now altered to green clay minerals. Thin veins, commonly containing carbonate, pervade the basalt units. Rarely, the rock has a pale gray hue, and the groundmass is bleached because of the replacement of igneous minerals by secondary calcite.

Basement Units 18 and 19 are highly to completely altered highly sanidine-phyric trachyandesite and trachyte, respectively. Alteration consists of either intense red hematitic staining or white/pink bleaching. The minerals in the bleached rocks include quartz, sanidine, and siderite. The latter mineral is a common phase that cements groundmass, replaces primary phases, and fills veins and vesicles. The veins within the bleached intervals have prominent red alteration halos (hematite) and are filled with hematite, quartz, siderite, and calcite. The alteration of these rocks probably results from interaction of felsic igneous rocks with large volumes of CO₂-rich fluids in a hydrothermal (subaerial?) system.

Although their compositions were affected by posteruption alteration, the major element compositions of the volcanic and volcaniclastic rocks comprising the basement at Site 1139 clearly form a series from trachybasalt to trachyte and quartz-bearing rhyolite (Fig. F35A). These lavas are significantly more alkaline than the dominantly transitional to tholeiitic basement lavas recovered from all other Kerguelen Plateau drill sites (Fig. F35B). However, with the exception of the rhyolites, the alkaline Skiff Bank lavas are quite similar to alkaline lava series erupted in the Southeast Province of the Kerguelen Archipelago in the early Miocene and again in the Pliocene and Pleistocene (cf. Figs. F8A, F35A).

In summary, significant results bearing on the origin and evolution of Skiff Bank (Site 1139) are

1. Subsidence of the NKP is recorded by the paleoenvironments of volcanic rocks and overlying sediments; since Eocene time, environments have changed from subaerial (volcanic and volcaniclastic rocks) to intertidal (beach deposits) to very high energy, nearshore (grainstone and sandstone) to low-energy, offshore (packstone) to bathyal pelagic (ooze).
2. The oldest sediments overlying igneous basement are earliest Oligocene; this minimum age for basement is consistent with a <40-Ma age for the uppermost igneous basement of Skiff Bank.
3. The 233 m of igneous basement consists of an uppermost 124-m succession of variably welded trachytic to rhyolitic volcanic and volcaniclastic rocks; in addition, the two lowermost lava flow units comprise 33 m of sanidine-phyric trachyandesite and rhyolite. As at Elan Bank (Site 1137), highly evolved magmas erupted, in some cases explosively, during the final stages of volcanism that formed Skiff Bank.
4. The volcanic basement at Skiff Bank, with a minimum age of earliest Oligocene, includes an alkaline lava series ranging from trachybasalt to trachyte and rhyolite. Similar alkaline lavas have erupted in the Kerguelen Archipelago in Miocene and Pliocene/Pleistocene time; therefore, Skiff Bank, which crests <500 m below sea level, may have been a somewhat older island analogous to the Kerguelen Archipelago 350 km to the east-northeast.

Site 1140

Site 1140 lies on the northernmost Kerguelen Plateau ~270 km north of the Kerguelen Archipelago (Figs. F3, F4). Flanked to the north and east by Eocene and younger oceanic crust of the Australian-Antarctic Basin, and to the west by Cretaceous oceanic crust of the Crozet and Enderby basins, the NKP is believed to have formed since 40 Ma via Kerguelen hot spot magmatism (Royer and Sandwell, 1989; Royer and Coffin, 1992). The boundary between the northern Kerguelen Plateau and the Australia-Antarctic Basin lies ~5 km north of Site 1140 and offsets basement by ~400 m.

The Kerguelen Archipelago is part of the NKP; its igneous rocks yield dates from 39 Ma to recent (Nicolaysen et al., 1996, in press). However, submarine igneous basement of the NKP has never been sampled, so its age and composition, as well as its relationship to the central and southern plateau sectors and to Skiff and Elan banks, are unknown. Site 1140 lies at a depth of 2394 m on the northern flank of the NKP. We chose this location as representative of the NKP on the basis of its relatively simple structural setting and thin sedimentary section (Fig. F36). The top of acoustic basement is flat lying, and the overlying basement is a sediment sequence ~350 m thick.

The major objectives at Site 1140 were to obtain igneous basement to characterize the ages, petrography, and compositions of the lavas and the environments of eruption (subaerial or submarine). We were especially interested in (1) testing the hypothesis that at least the uppermost igneous basement of the NKP is <40 Ma and (2) comparing the submarine NKP lavas with the subaerial lavas forming the Kerguelen Archipelago. The sedimentary objectives at Site 1140 were to determine sequence facies, to define the ages of seismic sequence boundaries, to estimate the duration of possible subaerial and shallow-marine environments, to obtain minimum estimates for basement age, and to determine the paleoceanographic history of this moderate latitude site. As discussed below, we largely achieved our goals at Site 1140. We drilled 88 m into pillow basalt flows that are intercalated with thin chalk beds containing late Eocene nannofossils and foraminifers.

The sedimentary section above igneous basement consists entirely of pelagic ooze and chalk and appears to rest unconformably on the underlying submarine basalt flows. We recognize only one sedimentary unit (lithologic Unit I) overlying volcanic basement rocks (Fig. F37). Unit I (0 to 234.5 mbsf) consists predominantly of light greenish gray foraminifer-bearing nannofossil ooze and nannofossil chalk. Biostratigraphic data, as well as preliminary interpretation of reversed and normal magnetic Chrons, indicate that lithologic Unit I is middle Miocene to early Oligocene or latest Eocene in age. We divide this unit into two subunits (IA and IB) based on the presence of diatom ooze in the uppermost part of the unit. Subunit IA (0 to 10.0 mbsf) consists of white diatom nannofossil ooze with interbeds of dark brown silty diatom ooze, light brown silty foraminifer-bearing diatom ooze, and yellowish brown nannofossil-bearing diatom ooze. Subunit IB (10.0 to 234.5 mbsf) comprises most of the sedimentary section and is predominantly light greenish gray foraminifer-bearing nannofossil ooze, which contains middle Miocene nannofossil and planktonic foraminifer species of warm-water affinity not found elsewhere on the Kerguelen Plateau. Physical properties in Subunit IA and the upper part of Subunit IB (0-180 mbsf) vary only slightly; bulk density ranges from 1.4 to 1.7 g/cm³, grain densities range between 2.1 and 2.8 g/cm³, and porosity changes from 57% to 76%. Compressional wave velocities show little scatter, ranging from 1491 to 1852 m/s. As the ooze becomes semilithifed nannofossil chalk downhole (~180-234 mbsf), bulk density gradually increases from 1.5 to 2.0 g/cm³ (mean = 1.7 g/cm³), with porosity decreasing from a maximum value of 74% to 44% (mean = 60%). Grain density is nearly constant at ~2.7 g/cm³ throughout this interval, and compressional wave velocities increase from 1578 to 2018 m/s. At the base of Subunit IB, just above igneous basement, transparent rhombic dolomite crystals are disseminated throughout the sediments. Nannofossils and planktonic foraminifers in the ooze directly overlying igneous basement indicate a minimum basement age of early Oligocene (30.0-34.3 Ma). All physical properties change abruptly at the sediment/basalt boundary. From 235 to 250 mbsf, porosity decreases sharply from a mean of 60% in lithologic Subunit IB to 6% in basalt flows, and grain density increases from 2.7 to 2.9 g/cm³. Compressional wave velocity varies from 5484 to 6859 m/s.

Drilling at Site 1140 penetrated 87.9 m of basement rocks, which we divide into six units, five submarine basaltic flows (Units 1-3, 5, and 6) and an ~1-m-thick layer of dolomitized nannofossil chalk (Unit 4). Two other thin calcareous-dolomitic sedimentary interbeds are between basalt flows at the Unit 2/3 and Unit 5/6 boundaries. We observe a magnetic reversal at the boundary between basement Units 1 and 2, which are separated by two cored intervals from which there was no recovery (Cores 183-1140A-29R and 30R in Fig. F37). Unit 1 is normal polarity, and Units 2 through 6 are reversed. Downhole logs of density, resistivity, and velocity show high values in the interiors of basalt flows and lower values at flow margins and in the interbedded sediments. At the top of basement Unit 3, a thin bed of well-burrowed, greenish white nannofossil chalk is latest Eocene in age. Basement Unit 4 contains a sedimentary bed with a top and bottom composed of rusty orange dolomite separated by a bed of well-burrowed, very pale brown dolomitic nannofossil chalk. Index properties change sharply at the boundary between Units 3 and 4; bulk density decreases from 2.8 to 2.1 g/cm³, grain density decreases to a mean of 2.8 g/cm³, and porosity increases to a mean of 41%. In Units 5 and 6, bulk density ranges from 2.5 to 3.0 g/cm³, porosity changes from 4% to 24%, grain density varies between 2.9 and 3.1 g/cm³, and compressional wave velocity ranges from 5099 to 6829 m/s (mean = 6055 m/s). The top interval of basement Unit 6 is rusty brown dolomite resembling that of basement Unit 4. The interbedded sediments indicate bathyal water depths during late Eocene to early Oligocene extrusion of the lava flows. Pelagic deposition in a bathyal environment continued uninterrupted until at least middle Miocene time.

Basement Units 1 and 6 each contain a ~5-m-thick massive lobe in addition to ~30 small (50 to 100 cm) basaltic pillows. Only <1-m-diameter pillows were recovered from Units 2 and 3. Unit 5 contains similar pillows and an ~10-m-thick massive lobe. Comparison between cores and logging data indicate that these flow units are 4.4 to 23.4 m thick. The thick massive lobes are probably sheet flows. Thick sheet flows and absence of rubbly talus suggest low to moderate slopes. Although small pillows cannot advance far before freezing, the larger sheets could efficiently transport magma from a distant vent. The flows are cryptocrystalline to fine grained and generally only sparsely vesicular. Vesicles are largely restricted to chilled margins. Vesicularity varies within the units but is consistently low, suggesting the deep water corroborated by bathyal sediments.

Pillow margins are fine grained with 1- to 2-cm-wide unaltered glassy rims (Fig. F38). Calcareous sediment or carbonate veins commonly fill sutures between pillows. The fine-grained pillow margins consist of moderately plagioclase ± olivine ± clinopyroxene-phyric basalt, whereas pillow interiors range from plagioclase-phyric to aphanitic. Olivine is a minor phenocryst and groundmass phase in Units 1 and 2 (Fig. F39), but it is rare to absent in the lower basaltic units, in which clinopyroxene is a phenocryst phase. Units 4 and 6 are moderately plagioclase-phyric, whereas the others are essentially aphanitic with <1% phenocrysts in the massive portions of the flows. Groundmass phases are calcic (An_60-70) plagioclase (20%-40%), augite (25%-40%), olivine (0%-5%), titanomagnetite (1%-2%), and altered glass. Textures range from ophitic, intergranular, or intersertal in pillow interiors to glassy at pillow margins.

Alteration of Site 1140 lavas strongly resembles that of young mid-ocean ridge pillows from the uppermost ocean crust (e.g., DSDP/ODP Holes 504B and 896A, located in 5.9-m.y.-old crust in the eastern equatorial Pacific Ocean). Glass at the margins of lava pillows is fresh and isotropic in thin section. These margins are crosscut by numerous calcite and dolomite veins that developed concentrically to the pillow rinds. These veins are generally 2 to 3 mm wide, and the carbonates exhibit dog-tooth, sparry habits. Baked, highly indurated chalk-derived marbles are commonly preserved in the pillow interstices. Rarely, these sediments apparently penetrated the magma, resulting in internal glassy quenched zones in the pillow interiors.

Crystalline interiors of the lavas are slightly to moderately altered. The most common feature of the alteration is development of brown to orange oxidation halos concentric with the pillow rinds, as well as along clay and carbonate veins. Orange brown clays and iron oxyhydroxides pseudomorphically replace groundmass mafic minerals. The gray to greenish gray portions of the basalts are generally fresh, except for the replacement of mesostasis by green clays and the partial filling of rare vesicles with green-blue clay and coarse-grained pyrite. Oxidation halos are less common in the more massive, fine-grained interiors of the thicker lava units, except where these rocks are intercalated with an ~1-m-thick bed of dolomitized and oxidized chalk. The sedimentary rocks may have acted as channels enabling the access of large volumes of seawater-derived fluids into the basement sequences, resulting in the precipitation of abundant, euhedral, colorless dolomite crystals in the chalk and numerous sparry carbonate veins in the pillow lavas.

Compared to other basement basalt recovered from the Kerguelen Plateau during Leg 183, the Site 1140 basalts are distinctive in that they (1) were erupted in a submarine environment, as indicated by their pillowed structure and the intercalated nannofossil-bearing calcareous sediments and (2) are relatively unaltered, as indicated by fresh glass preserved at pillow margins and the lack of alteration in massive interiors of flow units. The five basement flow units at Site 1140 are tholeiitic basalts that are poorer in alkalis than lavas at other locations on the Kerguelen Plateau, except for Site 750 (Fig. F20). Basalts from Site 1140 range to higher MgO (8.1 wt%) and Ni (100 ppm) contents than basalts from other Leg 183 drill sites (Fig. F40). They form two distinctive geochemical groups. Relative to Units 1, 5, and 6, Units 2 and 3 are enriched in highly incompatible elements, such as P, Zr, and Nb, by factors of 2 to 4 (Fig. F40). Units 1, 5, and 6 have near chondritic ratios of Nb/Zr and Zr/Y; in this respect they are similar to the ~110-Ma lavas from Site 749 on the SKP. Despite their eruption in late Eocene time, when the SEIR was <50 km away, Site 1140 lavas are not geochemically similar to depleted MORB. They are geochemically similar to other tholeiitic basalts associated with the Kerguelen plume (Fig. F31). Unlike basalts from Elan Bank (Site 1137) and Site 738 on the SKP, basalts from Site 1140 show no evidence for a component derived from continental lithosphere.

Major results of drilling at Site 1140 on the northern flank of the NKP include

1. Miocene nannofossil and planktonic foraminifer species of warm water affinity that had not been recovered previously by drilling on the Kerguelen Plateau.
2. Five submarine basalt flows with intercalated sediments. The ~1-m-thick dolomitized nannofossil chalk and other thinner chalk interbeds within the submarine basaltic flow units indicate episodic eruptions in a bathyal environment. Nannofossils and foraminifers in these chalks indicate a latest Eocene age, confirming that the uppermost igneous basement of the northernmost Kerguelen Plateau formed at <40 Ma. A magnetic reversal at the boundary between Basement Units 1 (normal) and 2-6 (reversed) corroborates nonvolcanic intervals as the uppermost igneous crust of the NKP formed.
3. The ~4- to 23-m-thick basalt flows are dominantly <1-m pillows with a few massive, 5- to 10-m lobes. The ~1-cm-thick quenched pillow margins are fine grained and contain macroscopically unaltered glass that is isotropic in thin section. Basaltic glass has not been recovered at other drill sites on the Kerguelen Plateau; shore-based studies of this glass will provide geochemical data, especially abundances of H₂O, CO₂, and S, that cannot be obtained from studies of altered crystalline rocks.
4. Five pillow lava flow units consist of tholeiitic basalts that form two distinct geochemical groups; both groups have incompatible element abundance ratios within the range of other tholeiitic basalts associated with the Kerguelen plume. Unlike basalts from Elan Bank (Site 1137), there is no evidence for a continental lithosphere component.

Sites 1141 and 1142

Sites 1141 and 1142 are situated near the crest of Broken Ridge ~350 km east of DSDP/ODP Sites 255, 752, 753, 754, and 755 (Figs. F5, F6). Flanked to the south by Eocene and younger oceanic crust of the Australia-Antarctic Basin and to the north by Cretaceous oceanic crust of the Wharton Basin, Broken Ridge appears to have formed during Late Cretaceous time as a result of Kerguelen hot spot magmatism (Duncan, 1991; Duncan and Storey, 1992). Subsequently, Broken Ridge and the Kerguelen Plateau began to separate along the westward-propagating SEIR at ~40 Ma. Igneous basement of Broken Ridge had not been sampled previously by drilling; dredge samples from three locations along the feature's southern, faulted boundary yield dates of ~62, ~83, and 88-89 Ma (Duncan, 1991). Because of the scatter in ages of the dredged rocks and the absence of in situ basement samples from Broken Ridge, knowledge of Broken Ridge's age and composition remains extremely limited. We located Sites 1141 and 1142 on the JOIDES Resolution single-channel seismic Line JR183-101. Sites 1141 and 1142 lie at depths of 1197 m and 1201 m, respectively, ~3-4 km north of the crest of Broken Ridge. We chose this location primarily on the basis of its thin sedimentary section (Fig. F41). The top of acoustic basement has an apparent dip to the north-northeast of 0° at Site 1141 and 2.5° at Site 1142. An ~100-m-thick sediment sequence overlies igneous basement. Since basement of Broken Ridge had never been drilled, our major objective at Sites 1141 and 1142 was to determine its age and composition. Additional basement objectives were to determine the physical characteristics of the lava flows and the environment of eruption (subaerial or submarine). The sedimentary objectives at Site 1141 were to determine sequence facies, to define the ages of seismic sequence boundaries, to estimate the duration of possible subaerial and shallow-marine environments, to obtain minimum estimates for basement age, and to determine the paleoceanographic history of Broken Ridge. At Sites 1141 and 1142, these objectives were achieved by coring 71 and 51 m of volcanic basement, respectively, and ~113 m of overlying sediment at Site 1141 (Figs. F42, F43). The abrupt termination of Hole 1141A led to an unanticipated experiment whereby we compared two basement sections separated by only 800 m.

At Site 1141, sediments were recovered from 0 to 103.8 mbsf (Fig. F42). We recognize only one sedimentary unit, lithologic Unit I. The basement volcanic rocks are designated lithologic Unit II. Unit I (0-113.5 mbsf) consists of white foraminifer nannofossil ooze of Pleistocene to early Miocene age. Core 183-1141A-1R consists of nannofossil-bearing foraminifer ooze that is predominantly composed of sand-sized foraminifers and displays slight normal size-grading. Traces of aragonite are present in Core 183-1141A-1R. Temperate calcareous microfaunas and floras characterize the current-winnowed Neogene calcareous ooze recovered at Site 1141. They are joined by subtropical index taxa in the Pliocene-Pleistocene section, a result of northward movement of Broken Ridge into warmer, lower latitude waters. The average sedimentation rate of 6 m/m.y. for the entire carbonate ooze section is the lowest Neogene rate for Leg 183. In Cores 183-1141A-8R and 9R, we obtained reliable remanent magnetization and correlated normal and reversed polarities with middle Miocene Chrons C5 to C5AD. Bulk densities in Unit I vary from 1.6 to 1.8 g/cm³, and porosity ranges from 54% to 65%, with a mean of 62%. Compressional wave velocities in Unit I show very little scatter, with a mean value of ~1860 m/s. The base of Unit I consists of a layer of sandy foraminifer limestone with abundant sand- to pebble-sized rock fragments and mineral grains. The limestone, late middle to late Eocene in age (35-38 Ma), postdates rifting and separation of Broken Ridge and the CKP. The pebbles include basalt with ferromanganese crusts. The thickness of this basal layer is uncertain as only two small fragments were recovered. The pelagic sedimentary succession at Site 1141 indicates that Broken Ridge has been at bathyal water depths since at least early Miocene time. Neritic fossils in the basal limestone indicate redeposition from shallow-water areas to a bathyal environment during the Eocene or later. Unit II consists of basalts, which are highly altered in the upper portion of the section. It is subdivided into six basement units. Near the top of Unit II (~114 mbsf), index properties change abruptly. From 115.8 to 116.7 mbsf in the upper part of basement Unit 2, bulk densities increase to a mean value ~2.0 g/cm³, grain densities increase to 2.9 g/cm³, and porosities decrease to 48%.

At Site 1142 (Fig. F43) no sediments were recovered from the drilled interval (0-91 mbsf; Core 183-1142A-1W, except for some small fragments of sandy pebbly foraminifer limestone with Oligocene or Eocene nannofossils.

The six basement units at Site 1141 represent 72.1 m of basement penetration and consist of five mafic lava flows overlain by a poorly sampled, coarse-grained sedimentary deposit consisting of three pebbles of moderately altered, medium-grained, plagioclase-clinopyroxene-olivine gabbro. The lava flows appear to have been erupted subaerially. The tops of most volcanic units in Hole 1141A have been highly to completely altered to clay; in some cases this intense alteration affects entire units. Red flow tops and green to gray flow interiors suggest decreasing oxidation with depth. In many intervals, traces of native copper are in the groundmass, and abundant native copper lines some fracture surfaces. Vesicles are filled with dark green clay, calcite, zeolite, amorphous silica, and quartz and have well-developed colloform textures. Slickensides are present along some fractured surfaces. Perhaps the most noteworthy alteration within Hole 1141A is the spectacular alteration halos associated with quartz veins. In some instances, a single quartz vein extends for >120 cm with multiple, symmetrical alteration halos progressively altering the surrounding wall rock. Common calcite veins are generally <0.5 mm wide and crosscut the quartz-filled veins.

Basement Unit 2 (20 m thick) is fine-grained, aphyric basalt. The least-altered portion of basement Unit 3 (8 m thick) is fine- to medium-grained, sparsely plagioclase-phyric basalt. Basement Unit 4 (20 m thick) is aphyric to moderately plagioclase- or plagioclase-olivine-phyric basalt. Basement Unit 5 (8 m thick) is sparsely olivine-phyric. Basement Unit 6 (15 m thick) ranges from aphyric to moderately olivine-plagioclase-phyric basalt. In addition to olivine phenocrysts, groundmass olivine and minor apatite in the lower part of basement Unit 4, and throughout basement Units 5 and 6, suggest that these basalts are alkalic. Thin sections show that carbonate, clay, and iron oxides completely replace the mafic phases and groundmass glass from the top of Unit 1 to the upper part of Unit 6; the bottom part of Unit 6 retains a large proportion of relatively fresh phenocryst and groundmass olivine. All index properties change markedly near the boundary between basement Units 2 and 3 and reach extremes in basement Unit 6, where bulk densities vary from 2.6 to 2.9 g/cm³ with a mean of 2.7 g/cm³, grain density approaches a mean of 2.8 g/cm³, and porosity varies from 16% to 3%. Compressional wave velocities in basement Unit 6 increase gradually with depth, from 4276 to 6902 m/s.

At Site 1142, 50.9 m of basement penetration recovered six units (Fig. F43). They include a diverse range of lithologies, including olivine-phyric basalt lava flows, possible pillow basalts, subaerial deeply weathered (felsic?) lavas, and volcaniclastic sediments. Alteration and weathering of these basement rocks suggest subaerial exposure. The lithologies and alteration intensity within Hole 1142A are heterogeneous. Some units are relatively massive, slightly altered basalt, whereas other volcanic units are variably brecciated by both volcanic and tectonic processes and have been completely altered to clay. Primary igneous textures are still visible in most units and are typically accentuated by the replacement of feldspar by light green clay and mafic minerals by red-brown clay.

Basement Unit 1 (2 m thick) is a slightly to moderately altered, massive, fine-grained, aphyric to sparsely olivine-plagioclase-phyric basalt; the upper portion has prominent oxidation halos, suggesting a period of exposure and weathering, possibly related to Eocene rifting and breakup between Broken Ridge and the CKP. Basement Unit 2 (1 m thick) consists of a single section containing 20 moderately to completely altered cobble-sized pieces of genetically unrelated rock types, including volcanic breccia, clinopyroxene-phyric, plagioclase-phyric, olivine-phyric, and aphyric basalt, and feldspar- and feldspar-quartz-phyric felsic volcanic rocks. Some of the pieces have abraded, slightly weathered surfaces, suggesting that the unit may be a near-source debris flow or a talus pile. Basement Unit 3 (20 m thick) is a completely altered, aphanitic, aphyric to moderately olivine-plagioclase-phyric basaltic breccia with three subunits defined on the basis of textural characteristics. Basement Unit 4 is a 4-m-thick, well-indurated, normally graded claystone or mudstone with very highly to completely altered, very coarse sand-sized to small granule-sized lithic clasts and crystals of quartz and altered feldspar in a red clay matrix. We interpret this unit to be a mudflow deposit. Basement Unit 5 (9 m thick) is a very highly to completely altered, aphanitic, aphyric volcanic breccia. The only hint of the original rock type is provided by very rare quartz crystals, which suggest an evolved composition. Basement Unit 6 is composed of 10 m of aphyric, nonvesicular, fine-grained to aphanitic basalt. Several alteration features indicate that this unit may be a pillow basalt; specifically, semicircular, red-brown oxidation halos and narrow dark green to black intervals associated with carbonate veins that may be altered pillow margins. However, grain size does not decrease toward these highly altered zones. Thus, evidence is equivocal as to whether these are pillow basalts. The numerous sinuous and semicircular oxidation halos may represent subaerial weathering after the uplift of Broken Ridge that accompanied its breakup with the CKP.