Site 1157 is located in Zone A, ~55 km east of the ~127°E fracture zone that bounds the eastern Australian Antarctic Discordance (AAD) and 230 km south of Site 1153, which is on approximately the same flow line. It is ~100 km southeast of Site 1156, on seafloor of the same age (~22 Ma). The purpose of drilling at this site, together with Site 1158, and the other western Zone A sites was to test for occurrences of Indian-type mantle beneath Zone A.
Hole 1157A was spudded in ~5069 m water depth and was washed through ~200 m of dark to medium brown siliceous clay. Rotary drilling continued 16.4 m into basement, recovering 2.9 m (~17.8%) of rubble made up of basalt and basalt-carbonate breccia. Both aphyric and moderately plagioclase-olivine phyric basalts are present. The breccia is composed of angular fragments of basalt (3 to >8 cm), basaltic glass, and palagonite in a carbonate matrix. Low-temperature alteration halos rim many clasts, and some terminate at broken clast surfaces, indicating that alteration of these clasts preceded the final brecciation.
After Hole 1157A was abandoned because of drilling problems, Hole 1157B was spudded 200 m to the west and was washed through 130.6 m of sediments with no recovery. The hole continued 40.4 m into basement, recovering 11.7 m (29%) of sparsely to moderately plagioclase-olivine phyric pillow basalt with a moderate degree of low-temperature alteration.
Six glass and whole-rock samples range in MgO content from 8.4 to 6.6 wt%. As in all previous sites, the whole rocks have lower MgO contents than the glasses. At Site 1157, Fe2O3, TiO2, Zr, and Y contents are low relative to the near-axis dredge samples from Zone A, whereas at Site 1153, which is 5 m.y. older, these elements are high. These temporal fluctuations indicate a complex spectrum of accretionary processes near the eastern margin of the depth anomaly and the isotopic boundary. Ba and Zr contents suggest an Indian-type mantle source for lavas at Site 1157. This was the first Indian trace element signature that we observed in Zone A.
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