Unconformities and changes in sedimentation rate at Site 1209 are illustrated in a plot of calcareous microfossil datum ages (first and last occurrences) vs. depth (Fig. F22). These rates rely on major calcareous nannofossil and planktonic foraminiferal datums presented in Tables T2 and T3. The Pleistocene-Maastrichtian section cored at Site 1209 is punctuated by one major unconformity between the lower Miocene and lower Oligocene, as well as an interval of diastems and/or multiple condensed intervals in the upper Miocene to lower Miocene (Figs. F23, F24).
An expanded view of the Nogene (Fig. F23) shows that the upper Miocene-Pleistocene part of the section accumulated at an average rate of 13.4 m/m.y. Dark-colored, clay-rich sediments in Cores 198-1209A-10H and 11H and Core 198-1209B-10H and 11H may represent multiple condensed intervals and/or diastems in the upper Miocene to lower Miocene interval. The dark-colored layers in Sections 198-1209A-12H-4 through 12H-6 and Sections 198-1209B-12H-6 through 12H-CC and 13H-1 likely correspond to condensed section and/or major unconformity separating the lower Miocene and the lower Oligocene (Fig. F24).
Sedimentation rates in the lower Oligocene-lower Paleocene interval range between 1.5 and 5.2 m/m.y., with the lowest rates occurring in the upper middle Eocene to upper Eocene and in the basal Paleocene (Figs. F25, F26). Rates for the Maastrichtian were significantly higher, ranging from 8.1 to 23.9 m/m.y. (Fig. F26).
Mass accumulation rates for the bulk sediment, carbonate, and noncarbonate fractions were calculated using dry bulk density (see "Physical Properties") and carbonate concentration (see "Organic Geochemistry") data through 12 linear sedimentation rate segments in the Cretaceous-Neogene section recovered at Site 1209 (Table T8). These segments were chosen to reflect the major changes in sedimentation rate. Physical properties and carbonate content analyses were not performed within certain intervals of stratigraphic importance. Mass accumulation rate data are not available for these intervals (Table T8).
During the Maastrichtian, the average bulk sediment accumulation rate decreased from 2.7 to 1.0 g/cm2/k.y. Further decrease in the average rate of bulk sediment accumulation to 0.5 g/cm2/k.y. coincided with the K/T boundary (Fig. F27). With the exception of an interval during the middle to late Eocene (~44-34 Ma) when the average decreased to 0.2 g/cm2/k.y., the bulk sediment accumulation rate remained relatively constant from early Paleocene to early Oligocene time (Fig. F25). Age-depth relationships for segments 2 and 3 (Table T8), which lie between middle Miocene and lower Miocene-upper Oligocene unconformities, indicate that sedimentation rates were 18.9 and 4.4 m/m.y., respectively. The average bulk sediment accumulation rates that correspond to these segments are 0.5 g/cm2/k.y. (segment 3) and 1.9 g/cm2/k.y. (segment 2). By contrast, the upper Miocene-Pleistocene was characterized by continuous sedimentation (Fig. F23), although the bulk accumulation rate decreased slightly through this interval, from 1.3 g/cm2/k.y. at 5.8 Ma to ~1.0 g/cm2/k.y. in the youngest sediments recovered. The carbonate accumulation rate approximates that of the bulk sediment in segments 2-12 (Table T8). The noncarbonate fraction, composed largely of siliceous biogenic material, clay, and volcanic ash, became more significant during the last 6 m.y., making up an average of 27 ± 15 wt% of the accumulating sediment.