Unconformities and changes in sedimentation rate in Hole 1208A are illustrated in a plot of calcareous microfossil datum ages (first and last occurrences) vs. depth (Fig. F21). These rates rely on major calcareous nannofossil and planktonic foraminiferal datums presented in Tables T3 and T4. The Pleistocene-middle Albian section cored at Site 1208 is punctuated by three condensed intervals and at least four unconformities. Condensed intervals in the lower Miocene to lower Oligocene and uppermost Eocene and in the lower Eocene to uppermost Paleocene accumulated at rates of ~0.08-0.09 m/m.y (Fig. F22)
An expanded view of the Neogene (Fig. F23) shows that the upper lower Miocene-lower upper Miocene part of the section accumulated at an average rate of 5.9 m/m.y., increasing to 22.3 m/m.y. in the upper Miocene-basal Pliocene, and 42.4 m/m.y. in the lower Pliocene-Pleistocene interval. Dark-colored, clay-rich sediments in Section 198-1208A-35X-4 and in the upper part of Section 35X-5 represent a condensed interval spanning the lower Miocene to lower Oligocene (Fig. F24). Calcareous nannofossil data suggest the presence of an unconformity separating the condensed lower Miocene-lower Oligocene interval and basal Oligocene nannofossil ooze (Fig. F24). The basal Oligocene ooze accumulated at an average rate of 4.2 m/m.y.
Dark-colored sediments in interval 198-1208A-36X-2, 10 cm, through 36X-CC, 25 cm, contain a condensed uppermost Eocene sequence, supported by the occurrence of the benthic foraminifer Nuttallides truempyi in interval 198-1208A-36X-2, 18-20 cm, unconformably overlying a condensed section of lower Eocene-uppermost Paleocene. A major Eocene unconformity occurs within this condensed section between Samples 198-1208A-36X-2, 42 and 43 cm (see Fig. F17).
A major unconformity at Sample 198-1208A-36X-CC, 25 cm, separates the condensed lower Eocene-uppermost Paleocene interval from mid-Campanian nannofossil ooze. The mid- to lower Campanian part of the section accumulated at a nearly constant rate of ~10.4 m/m.y. (Fig. F25). Another unconformity separates the lower Campanian and middle Albian in Core 198-1208A-42X. The middle Albian chalk recovered in Sample 198-1208A-42X-CC is from the upper Ticinella primula Zone, suggesting that Hole 1208A terminated in strata older than 105 Ma and probably not older than 107-108 Ma (see "Biostratigraphy").
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 three linear sedimentation rate segments in the Neogene (Fig. F23). These segments were chosen to reflect the major changes in sedimentation rate. The three segments are (1) 0-166.66 mbsf (lower Pliocene-Pleistocene), (2) 166.66-256.71 mbsf (upper Miocene-lower Pliocene), and (3) 256.71-314.17 mbsf (middle Miocene-upper Miocene).
Accumulation rates in the lowermost interval examined (256.1-314.17 mbsf; middle Miocene-upper Miocene) are relatively low and show little temporal change (Fig. F26). Bulk sediment accumulation rates average 0.54 g/cm2/k.y. Average accumulation rates of the carbonate (0.3 g/cm2/k.y.) and noncarbonate (0.2 g/cm2/k.y.) fractions suggest that the contribution from each of these fractions was nearly equal throughout this interval.
Increased accumulation rates are associated with a higher average sedimentation rate between 3.82 and 8.28 Ma (166.66-256.71 mbsf). The upper Miocene-lower Pliocene bulk sediment accumulation rates average 1.8 g/cm2/k.y. The average rates at which the carbonate and noncarbonate fractions accumulated during this interval are 1.2 and 0.6 g/cm2/k.y., respectively, indicating an increased contribution to the sediment column from the carbonate fraction.
The bulk sediment accumulation rates of the lower Pliocene-Pleistocene interval average 3.11 g/cm2/k.y. (Fig. F26) but decrease overall from ~3.2 g/cm2/k.y. (lower Pliocene) to ~2.4 g/cm2/k.y (Pleistocene). Although quite variable, average accumulation rates for the carbonate and noncarbonate fractions (1.5 and 1.4 g/cm2/k.y., respectively) suggest that the overall contribution of each of these fractions to the bulk sediment was approximately equal throughout late early Pliocene-Pleistocene time (Fig. F26). Because the accumulation rate for the carbonate fraction shows relatively little change from the underlying interval, the data suggest that the increase in sedimentation rates beginning in late early Pliocene time are related primarily to an increase in the contribution of the noncarbonate fraction. Although there is an increase in the biogenic silica component, possibly indicating an increase in biologic productivity, the major noncarbonate constituent through this interval is detrital clay and silt (see "Lithostratigraphy"). These observations are consistent with the character of seismic reflection profiles over the Central High of Shatsky Rise, which suggest that the Neogene sequence penetrated at Site 1208 is a drift deposit.