Hole 1138A was drilled on the central Kerguelen Plateau in a water depth of 1153 m to 842.7 meters below seafloor (mbsf). One hundred forty-four meters of basalts is overlain by 698 m of sediments, of which 343 m was recovered.
In the following, a short description of the lithologies is given (after Coffin, Frey, Wallace, et al., 1999). For a more detailed description, see Coffin, Frey, Wallace, et al. (1999).
Units I-VI of Site 1138 are sedimentary rocks resting unconformably on the volcanic basement (Unit VII). The upper 650 m of sediment is biosiliceous and carbonate pelagic ooze. The lower ~50 m of the sedimentary section consists of 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. Few volcanic ash layers were found. Unit II (112.0-265.9 mbsf) is composed of foraminifer-bearing nannofossil clay (Subunit IA) that overlies foraminifer-bearing nannofossil ooze (Subunit IB). Unit III (265.9-601.8 mbsf) consists of foraminifer-bearing chalk and contains scattered chert nodules in its lower part. Unit IV (601.8-655.6 mbsf) consists of cyclic alternations of foraminifer-bearing chalk with intervals of nannofossil claystone. Unit V (655.6-671.88 mbsf) consists predominantly of glauconitic calcareous sandstone. Unit VI (671.88-698.23 mbsf) consists of fossil-rich silty claystone with interbedded sandstone of fluvial or shallow-marine origin.
At Site 1140 on the northern Kerguelen Plateau, the sedimentary section above igneous basement rocks consists entirely of pelagic ooze and chalk (Coffin, Frey, Wallace, et al., 1999). Only one sedimentary unit (lithologic Unit I) overlying volcanic basement rocks was recognized. Unit I (0-234.5 mbsf) consists predominantly of foraminifer-bearing nannofossil ooze and nannofossil chalk. This unit was divided into two subunits (IA and IB).
Subunit IA (0-10.0 mbsf) consists of white diatom nannofossil ooze with interbeds of dark brown silty diatom ooze. Subunit IB (10.0-234.5 mbsf) comprises most of the sedimentary section and is predominantly nannofossil ooze.