During Leg 183, we penetrated igneous basement at five sites on the Kerguelen Plateau and two sites on Broken Ridge. The site locations (Fig. F1) and drilling results are documented in detail in the site chapters of the Leg 183 Initial Reports volume and other appropriate papers (see Coffin, Frey, Wallace, et al., 2000; Frey et al., 2000). A simplified summary of the drill sites is given in Table T1. The lithology and paleoenvironment of the sites will be briefly mentioned here.
Drilling at Site 1136 in the southern Kerguelen Plateau recovered three inflated pahoehoe basalt flows similar to continental flood basalts (Table T1). Generally, these flows show a vertical zonation with a bubbly vesicular top, a fresh massive interior, and a fine-grained and vesicle-rich base typical of subaerial flows. At Site 1137 on the Elan Bank, drilling recovered 151.7 m of a basement sequence, which contains seven basaltic lava flows and three interbedded volcaniclastic sediment units. The interiors of the seven lava flows from Site 1137 are relatively unaltered and similar to those from Site 1136, but the volcanic sediments are highly altered and weathered. The inflated pahoehoe characteristics of the seven lava flows and the fluvial environment of the volcaniclastic sedimentary rocks strongly indicate subaerial eruption of the basalts. At Site 1138 on the central Kerguelen Plateau, we recognized 22 units within the 144 m of igneous basement drilled. These are tholeiitic basalt flows whose compositions define a systematic downhole trend to Fe- and Ti-rich basalt (Coffin, Frey, Wallace, et al., 2000). Alteration ranges from high to complete in the brecciated zones with flow tops partly and completely altered to clay minerals. Site 1138 was above sea level during its construction. Site 1139 was drilled on Skiff Bank of the northern Kerguelen Plateau. Basement rocks recovered include variably welded trachytic and rhyolitic volcanic and volcaniclastic rocks and lava flows. All basement units are altered and fractured at Site 1139. Site 1139 was also subaerial during its final stages of formation. At Site 1140, which lies on the northernmost Kerguelen Plateau, drilling recovered 49.1 m of basement rocks, which includes five submarine basaltic flows and a layer of dolomitized nannofossil chalk, in contrast with all other basement sites drilled at the Kerguelen Plateau. Alteration of Site 1140 lavas strongly resembles that of young mid-ocean-ridge pillows (Coffin, Frey, Wallace, et al., 2000). It is worth noting that Site 1140 is situated at 2394 m, whereas seafloor depths at all other basement sites are much shallower (between 1000 and 2000 m) (Table T1).
Sites 1141 and 1142 are situated near the crest of Broken Ridge (Fig. F1). Initial interpretation of cores suggests that most of the basaltic lavas at Sites 1141 and 1142 were erupted subaerially, but there is also a hint that Site 1142 possibly erupted in submarine environment (Coffin, Frey, Wallace, et al., 2000). The tops of most volcanic units at Site 1141 have been highly to completely altered to clay. Red flow tops and green to gray flow interiors suggest decreasing oxidation with depth. The lithology and alteration intensity at Site 1142 are heterogeneous. Some units are massive and slightly altered, whereas other volcanic units are variably brecciated by both tectonic and volcanic processes and are highly to completely altered. The difference between igneous basement sections at these two sites may be a result of penetration of different stratigraphic levels or considerable unanticipated lateral variability in volcanism (Coffin, Frey, Wallace, et al., 2000).