Site 1148 recovered the longest sediment record for the leg (~33 Ma), and its chronostratigraphy is based on 74 datum levels: five paleomagnetic (Table T10), 39 nannofossil, 29 planktonic, and one benthic foraminiferal (Table T7) events. Sediment slumping and mass flows occur in some intervals (such as Unit VI, near the Oligocene/Miocene boundary; see "Lithostratigraphy"), but no quantitative estimates have been attempted.
Figure F18 (also given as Synergy Software KaleidaGraph plots and Microsoft Excel data files [see the "Supplementary Materials" contents list]) shows the linear sedimentation rate and mass accumulation rate curves based on calculations described in "Sedimentation and Accumulation Rates" in the "Explanatory Notes" chapter. The resultant average sedimentation rates decreased from a maximum of 68 m/m.y. for the early Oligocene to ~11-17 m/m.y. in the Miocene and have risen again since the late Pliocene, reaching 63 m/m.y. in the Pleistocene (Table T11). When converted to mass accumulation rates (g/cm2/k.y.) and partitioned into the carbonate and noncarbonate components, extremely high accumulation rates occurred in the early Oligocene, with 3.6 g/cm2/k.y. for carbonate and 15.2 g/cm2/k.y. for noncarbonate, mainly terrigenous material. After a hiatus of 1-3 m.y. near the Oligocene/Miocene boundary, accumulation was low and relatively constant during the Miocene, followed by an increase in the Pliocene and Pleistocene. However, the Pleistocene accumulation rates (0.5 g/cm2/k.y. for carbonate and ~5 g/cm2/k.y. for noncarbonate) are much lower than those of the Oligocene (Fig. F18; Table T11). The high accumulation rates of the early Oligocene must be related to the active tectonic regime and paleoenvironmental conditions at the early stage of seafloor spreading of the South China Sea.