7. Site 1139¹

Shipboard Scientific Party²

BACKGROUND AND OBJECTIVES

Site 1139 lies on Skiff Bank (Leclaire Rise), a bathymetric and gravimetric high ~350 km west-southwest of the Kerguelen Archipelago (Fig. F1). 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 northern Kerguelen Plateau (NKP) (Schlich et al., 1998). At least two major faults, however, offset interpreted igneous basement between Skiff Bank and the NKP massif (Coffin et al., 1986) containing the Kerguelen Archipelago. Skiff Bank has been proposed to be the current site of the Kerguelen hot spot (e.g., Duncan and Storey, 1992; Müller et al., 1993), but hundreds of meters of sediment cover in places argue against Skiff Bank originating entirely by Holocene volcanism (Figs. F2, F3). Both Skiff Bank and Elan Bank trend east-west (Fig. F1), approximately perpendicular to the trends of fracture zones in the Enderby Basin and, thus, parallel to the axis of breakup between Antarctica and India (Royer and Coffin, 1992). 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. F1). It is thus possible that Skiff and Elan (Site 1137) Banks originated via similar processes and are composed of basaltic and continental rocks. 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 (Figs. F1, F3). These plutonic rocks were interpreted as ice-rafted debris (Weis et al., 1998). Hence, the age and composition of Skiff Bank's igneous crust, and its relationship to the contiguous NKP are not established. Although the age of the NKP is commonly believed to be Cenozoic, having largely formed since ~40 Ma when the Southeast Indian Ridge separated Broken Ridge and the central Kerguelen Plateau (Royer and Coffin, 1992), submarine igneous basement of the NKP has never been drilled. We located Site 1139 on Marion Dufresne multichannel seismic (MCS) line MD109-05 (Fig. F2). Site 1139 lies at a depth of 1427 m on Skiff Bank's southwestern terrace, which lies >1000 m lower than the crest, located <50 km to the northeast. We chose this location primarily because of its thin sedimentary section (Fig. F3). The top of acoustic basement is flat lying, and overlying basement is a sediment sequence ~500 m thick. The fault scarp marking the boundary between Skiff Bank and the Enderby Basin lies ~20 km southwest of Site 1139, and it offsets basement by >2700 m.

The main objectives at this site were to

1. Characterize the petrography and compositions of the lavas;
2. Determine the age of the lavas, testing the hypothesis that the uppermost igneous basement of Skiff Bank is ~40 Ma, the age of the oldest known igneous rock from the Kerguelen Archipelago (Giret and Lameyre, 1983; Giret and Beaux, 1984), or younger (e.g., Duncan and Storey, 1992; Müller et al., 1993);
3. Determine the physical characteristics of the lava flows;
4. Identify the environment of eruption (subaerial or submarine);
5. Obtain minimum estimates for the age of basement from overlying sediment;
6. Estimate the duration of possible subaerial and shallow marine environments from the sedimentary and igneous record;
7. Determine the facies of the seismic stratigraphic sequences;
8. Define the ages of seismic sequence boundaries; and
9. Determine the paleoceanographic history of this high-latitude site.