Holes 1115B and 1115C were drilled in a water depth of 1150 m. Hole 1115B was cored using the advanced piston corer to a depth of 216 meters below seafloor (mbsf), followed by extended core barrel to a depth of 293 mbsf. Hole 1115C was drilled using the rotary core bit from a depth of 283 to hole termination at 802.5 mbsf. The sediments consist mostly of calcareous silty clay and claystone with abundant volcaniclastic ash and sand layers down to ~415 mbsf. Calcareous sandstones and siltstones extend downward from that level to below the base of the Pliocene (inferred to be at 504 mbsf at this site).
The nannofossil biostratigraphy for the two holes, using the zonation of Martini (1971) with modifications from Rio et al. (1990a), is given in Figures F2 and F3. Several zonal conventions and assignments warrant comment. The Pliocene/Pleistocene boundary is within Subzone NN19a. The top (1.77 Ma) of the Olduvai Chron was determined to be at 90.5 mbsf and the base (1.95 Ma) between 102.5 and 103.5 mbsf in Hole 1115B (Takahashi et al., this volume), which places the Pliocene/Pleistocene boundary (taken to be 1.81 Ma) (Berggren et at., 1995) at 93.5 mbsf by interpolation. The NN16b/NN17 boundary may be somewhat higher than shown on Figure F2. The last occurrence (LO) of Discoaster surculus defines this boundary. The LO of D. surculus normally occurs closely below the LO of Discoaster pentaradiatus, which defines the NN17/NN18 boundary. In this hole, however, the apparent LO of D. surculus is 31 m below the LO of D. pentaradiatus, even though the last appearance datums (LADs) of the two species are separated by only 0.03 m.y. D. surculus is rather rare in these cores, and I believe its stratigraphic range is incomplete here. Paleomagnetic data seem to bear this out. The Gauss/Matuyama boundary (2.58 Ma) is inferred to be at 162.0 mbsf in Hole 1115B (Takahashi et al., this volume). The LAD for D. surculus has been shown as 2.55 Ma (Berggren et al., 1995). Thus, the 2.55-Ma LAD between 175.7 and 178.6 mbsf for D. surculus and the 2.58-Ma age at 162.0 mbsf for the Gauss/Matuyama boundary are inconsistent.
The Miocene/Pliocene boundary (5.32 Ma) lacks a recognizable biostratigraphic event, being defined by a historic event—the reflooding of the Mediterranean following the Messinian salinity crisis. Reflooding occurred within Zone NN12 (5.00-5.54 Ma), which spans the Miocene/Pliocene boundary. The top of Zone NN12 is marked by the first occurrence (FO) of Ceratolithus rugosus (or approximated by the LO of Ceratolithus acutus) (Perch-Nielsen, 1985) and the base of the LO of Discoaster quinqueramus. The LO of D. quinqueramus between 514.3 and 516.4 mbsf is an easily recognizable event, but confident recognition of the actual FO of C. rugosus and/or LO of C. acutus is difficult because of the rarity of ceratoliths in this section (Taylor, Huchon, Klaus, et al., 1999). Relying mostly on the LAD of D. quinqueramus at 5.54 Ma and the estimated sedimentation rate for this interval (Takahashi et al., this volume), I estimate the Miocene/Pliocene boundary to be at ~504 mbsf in this hole.
A detailed summary of the sedimentology, lithostratigraphy, biostratigraphy, and other characteristics of the sediments at this site may be found in the Leg 180 Initial Reports volume (Taylor, Huchon, Klaus, et al., 1999).