PRINCIPAL RESULTS


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. 3, 4). In the vicinity of Site 1138, geological structure and seismic stratigraphy are relatively simple, and interpreted igneous basement contains some internal reflections (Fig. 27). 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. 28). 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 late Pleistocene to late Miocene unit. Grain density averages 2.38 g/cm3; porosity, 77%; and P-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 P-wave velocity 1672 m/s. Unit III (265.9-601.8 mbsf) is late Oligocene to middle 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 P-wave velocity averages 2310 m/s.
Unit IV (601.8-655.6 mbsf), of middle 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 P-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 P-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 P-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. 28). 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 contains six variably oxidized and altered pumice lithic breccias. We interpret these as unwelded, subaerial pyroclastic flow deposits. The pumice clasts are typically aphyric, and the bulk composition of a pumice-rich sample is trachytic. 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 unique flow top breccias, which 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 may be reworked, perhaps 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; specifically, aa and slab pahoehoe flows rarely travel more than a few tens of kilometers from vents; abundant small vesicles indicate that the lavas did not flow far enough for vesicles, which formed at vents, to coalesce; and clasts in some of the welded basal breccias appear to be spatter, which only forms close to vents.
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°N 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 (range) grain densities of 2.90 g/cm3 (2.44-3.13), porosities of 25% (9-55), and P-wave velocities of 4014 m/s (1884-7491).
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. 19, 29). 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. 30), 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. 31).
The major results of drilling 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 trachyte.
5. Basaltic basement underlying evolved rocks is represented by 20 thin, Hawaii-like subaerial flows that erupted onto moderate slopes of 1° to 4°. These are tholeiitic basalt flows whose compositions define a systematic downhole trend to FeTi-rich basalt. Such highly evolved basalts have not been previously recovered 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.

Leg 183 Principal Results - Site 1139
Leg 183 Table of Contents