Unconformities and changes in sedimentation rate at Site 1212 are illustrated in a plot of calcareous microfossil datum ages (first and last occurrences) vs. depth (Fig. F14). These rates rely on major calcareous nannofossil and planktonic foraminiferal datums presented in Tables T2 and T3 (see "Biostratigraphy"). The Pleistocene-upper Albian section cored at Site 1212 is punctuated by at least four unconformities: lower middle Miocene/lower middle Eocene, mid-Campanian/lower Santonian-Coniacian, and Coniacian/basal Cenomanian. In addition, a fourth unconformity or condensed interval separates the uppermost Miocene and lower middle Miocene.
An expanded view of the Neogene (Fig. F15) shows that the uppermost Miocene-Pleistocene section accumulated at rates of ~8.7 m/m.y., although discrete intervals accumulated at rates that varied from 4.1 to 16.3 m/m.y. Dark-colored, clay-rich sediments in Sections 198-1212A-8H-1 and 8H-2 and Sections 198-1212B-7H-3 and 7H-4 may contain an unconformity or condensed interval(s) between uppermost Miocene and lower middle Miocene sediments. The gradational color change between Sections 198-1212A-8H-3 and 8H-4 and 198-1212B-7H-6 and 7H-7 marks the bioturbated unconformity separating the lower middle Miocene and the lower middle Eocene (Figs. F15, F16).
Sedimentation rates in the Eocene range between 1.5 and 2.0 m/m.y. (Fig. F16). The upper Paleocene shows sedimentation rates of ~2.5 m/m.y. decreasing to 1.8 m/m.y. through the mid- and lower Paleocene (Fig. F17). A diastem was detected in the uppermost Paleocene, at the base of the PETM, in Hole 1212B (see "Biostratigraphy"). Shipboard biostratigraphy indicates that the K/T boundary interval is paleontologically complete.
Sedimentation rates for the Campanian-Maastrichtian interval were significantly higher than the Paleogene with rates of ~8.5-9.2 m/m.y. (Fig. F17). A 20- to 25-cm-thick interval of intact chalk in Section 198-1212B-24H-6 in an otherwise highly disturbed core indicates an age of early Santonian-Coniacian. Two major unconformities occur in the interval between Sections 198-1212B-23H-CC and 25H-1. Lowermost Cenomanian-uppermost Albian sediments accumulated at an average rate of 8.8 m/m.y. (Fig. F18).
Mass accumulation rates for the bulk sediment, carbonate, and noncarbonate fractions were calculated using dry bulk density and carbonate content (see "Organic Geochemistry") data through eight linear sedimentation rate segments in the Cretaceous-Neogene section recovered at Site 1212 (Table T8). These segments were chosen to reflect the major changes in sedimentation rate as shown in Figure F14. Mass accumulation rate data are not available for sections of core immediately adjacent to intervals of stratigraphic importance, such as the PETM and the K/T boundary.
As observed at Sites 1209-1211 on the Southern High of Shatsky Rise, sedimentation during Campanian and Maastrichtian was carbonate dominated and marked by high mass accumulation rates, averaging 1.0 g/cm2/k.y. (Fig. F19). Beginning in the earliest Paleocene, the rate of bulk sediment accumulation dropped dramatically to an average of 0.2 g/cm2/k.y. The same low rate of accumulation prevailed through early middle Eocene time (Fig. F19). Bulk sediment accumulation rates remained low during middle Miocene and late Miocene to early Pliocene time, averaging 0.2 and 0.4 g/cm2/k.y., respectively. As at Sites 1207-1211, there was a shift to higher rates of bulk sediment accumulation accompanied by an increased proportion of noncarbonate sediment in the late Pliocene (Fig. F19). These conditions have prevailed through to the present time.