Age-Depth Model

An age-depth model was established for Site 1259 by combining all available biostratigraphic and magnetostratigraphic datums from Hole 1259A (Fig. F18). The diagram was constructed by plotting highest and lowest possible ages for selected paleontological samples examined shipboard against the depth of those samples (Table T13). In addition, the age and depth of magnetic reversals recognized shipboard (Table T14) were also plotted.

The lower Miocene–Cenomanian section is punctuated by at least five unconformities or hiatuses and at least one condensed interval that spans the middle Campanian. Sedimentation rates ranged from 0.4 to 13 m/m.y. and are summarized in Table T15. The upper 50 m at Site 1259A is slumped Oligocene sediments sitting on lower Miocene sediments; this interval was not considered in sedimentation rate calculations.

Sedimentation Rates and Hiatuses

The lower Miocene nannofossil and foraminifer oozes accumulated at an average rate of 13 m/m.y. Calcareous microfossil datums indicate an unconformity (see letter B on Fig. F18) separating the lower Miocene and lower Oligocene. A 1.3-m.y. unconformity (see letter C on Fig. F18) is also recognized by nannofossil datums at the E/O boundary. The upper Eocene foraminifer-nannofossil chalks accumulated at a rate of 6 m/m.y. and are separated from the middle Eocene siliceous foraminifer-nannofossil chalks by an unconformity (see letter D on Fig. F18).

The longest interval of apparently continuous sedimentation is the middle Eocene–early Paleocene section of siliceous and foraminifer nannofossil chalks and clayey nannofossil chalks. Calcareous microfossil datums and paleomagnetic data indicate a relatively constant sedimentation rate of 12 m/m.y. through this interval. The absence of foraminiferal Zone P3a suggests an unconformity (see letter E on Fig. F18) in the basal upper Paleocene.

The Maastrichtian and upper Campanian clayey nannofossil chalks are characterized by relatively low accumulation rates of 4.5 m/m.y. A significant unconformity between the upper and lower Campanian (see letter F on Fig. F18) is indicated by calcareous nannofossil data. An expanded view of the lower Paleogene and Upper Cretaceous (Fig. F19) shows the condensed section found in dark brown clay-rich sediments deposited following the middle Campanian unconformity (see letter F on Fig. F18) in Sections 207-1259A-52R-5 and 52R-6. Calcareous nannofossil Zones CC18–CC15 are represented in Sections 207-1259A-52R-5 and 52R-6, indicating a minimum sedimentation rate of 0.4 m/m.y. for the earliest Campanian and Santonian.

Mass Accumulation Rates

Average mass accumulation rates (MARs) were calculated from linear sedimentation rates (LSRs) and averaged dry bulk density data. MARs were not calculated for Units IV and V because reliable dry bulk density data are not available for these units. MARs remove the influence of compaction on LSR calculation and give a better indication of the quantity of accumulating sediment. The lower Miocene had lower MAR values (1.1 g/cm2/k.y.) but higher sedimentation rate (13 m/m.y.) than the middle Eocene (MAR = 1.4 g/cm2/k.y.; LSR = 12 m/m.y.), indicating possible burial compaction and infilling of pore space in the Eocene section. The MAR values (0.8 g/cm2/k.y.) found in the lower Paleocene–upper Campanian section are consistent with a shift to clay-rich deposition and lower carbonate percentages (see "Lithostratigraphy").

MAR and carbonate MAR for the middle Eocene–upper Paleocene are shown in Figure F20. Carbonate MAR was calculated using percentage carbonate analyses that were taken within 4 cm of dry bulk density measurements. Significant gaps in the curve were filled by visual assessment of the locales of carbonate analyses and the nearest dry bulk density measurements with a separation not >30 cm between measurements. If no change in lithology or color was observed, then the data were used for the curve found in Figure F20. The 130- to 430-mbsf interval shows a slight increase in bulk MAR but relatively constant carbonate MAR. This trend is coincident with an increasing clay content downcore, suggesting a relatively constant carbonate production and changing terrigenous input.