Changes in sedimentation rate in Holes 1207A and 1207B are illustrated in a plot of calcareous microfossil datum ages (first and last occurrences) vs. depth (Fig. F31). These rates rely on major calcareous nannofossil and planktonic foraminiferal datums presented in Tables T5 and T6. A hiatus of nearly 60 m.y. separates the Neogene and Cretaceous sediments at Site 1207. An expanded view of the Neogene (Fig. F32) shows that the lower middle Miocene is condensed (~0.5 m/m.y.), followed by increasing rates of sedimentation: 4.3 m/m.y. in the upper middle Miocene, 8.5 m/m.y. in the upper Miocene, and 18.4 m/m.y. in the uppermost Miocene-Pleistocene interval. The Barremian-Campanian part of the section (Fig. F33) accumulated at a nearly constant rate of ~8.7 m/m.y.
Mass accumulation rates for the bulk sediment, carbonate, and noncarbonate fractions were calculated using dry bulk density (see "Index Properties" in "Physical Properties") and carbonate concentration (see "Organic Geochemistry") data through two linear sedimentation rate segments in the Neogene. Two segments are defined: (1) 0 to 107.41 mbsf (uppermost Miocene-Pleistocene) and (2) 107.41 to 150.00 mbsf (upper Miocene). These segments were chosen to reflect the major changes in sedimentation rate shown in Figure F32. Because of poor recovery through chert-rich intervals, it was not possible to calculate reliable mass accumulation rates for the Cretaceous section recovered at Site 1207.
Bulk sediment accumulation rates for the upper Miocene section average 0.8 g/cm2/k.y., and average carbonate and noncarbonate fraction accumulation rates are 0.6 and 0.2 g/cm2/k.y., respectively. Both bulk sediment and carbonate accumulation rates decrease gradually through late Miocene time (9.7-5.8 Ma). The average accumulation rate of the noncarbonate fraction remains uniform. A significant increase in accumulation rates coincides with the increase in sedimentation rates at 107.41 mbsf (5.82 Ma). Uppermost Miocene-Pleistocene bulk sediment accumulation rates average 1.4 g/cm2/k.y. Average accumulation rates for the carbonate and noncarbonate fractions during the same time period are 0.9 and 0.5 g/cm2/k.y., respectively. Although, on average, bulk accumulation rates remained relatively constant during latest Miocene to Pleistocene time, there were significant changes in the relative rates at which carbonate and noncarbonate fractions accumulated (Fig. F34). Overall, the decrease of carbonate accumulation rate is balanced by an increase of noncarbonate material, reflecting the increase in abundance of ash and siliceous microfossils, especially in the Pliocene-Pleistocene section (see "Lithostratigraphy").