Site 1140
Site 1140 lies on the northernmost Kerguelen Plateau ~270 km north of the Kerguelen Archipelago (Fig. 4). 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; M.F. Coffin et al., unpubl. data). 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 (K.E. Nicolaysen et al., unpubl. data). 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 2450 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. 36). 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 87 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. 37). Unit I (0 to 234.5 mbsf) predominantly consists 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%. P-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 velocity increases from 1578 to 2018 m/s. At the base of Subunit IB, just above igneous basement, clear 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. P-wave velocity varies from 5484 to 6859 m/s.
Drilling at Site 1140 penetrated 87.4 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. Unit 1 is normal polarity, and Units 2 through 6 are reversed, indicating a hiatus in volcanism as construction of the northernmost Kerguelen Plateau's igneous crust ended. 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 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. 38). 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. 39) 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 (An60-70) 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-Ma-old crust in the eastern equatorial Pacific Ocean). Glass on the margins of lava pillows is fresh and isotropic in thin section. Glassy margins are crosscut by numerous calcite and dolomite veins that developed concentrically to the pillow rinds. These veins are generally wide (2-3 mm), 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 to 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 ~1-m-thick beds 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 intercalcated 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. 19). Basalts from Site 1140 range to higher MgO (8.1%) and Ni (100 ppm) contents than basalts from other Leg 183 drill sites. 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 two to four (Fig. 40). 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. 31). Unlike basalts from Elan Bank (Site 1137), basalts from Site 1140 show no evidence for a component derived from continental lithosphere.
Major results of drilling 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 dolomotized 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. Basalt flows, ~4 to 23 m thick, which 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.
Leg 183 Principal Results - Sites 1141/1142
Leg 183 Table of Contents