All the principal biostratigraphies, plus an extensive set of over 60 paleomagnetic reversals, are defined in Holes 1220A, 1220B, and 1220C (Tables T10, T11). Paleomagnetic reversals are used to calculate the average linear sedimentation rates (LSRs) for Site 1220 through most of the section. The first reliable paleomagnetic reversals are found in Core 199-1220A-3H, along with the youngest radiolarian events, and extend through the section recovered by APC methods. The age of the base of the hole is based on the identification of the BEE in Section 199-1220B-20X-2 (Table T11).
Calcareous nannofossils and radiolarians are present in the uppermost part of the fossiliferous section in the lower Miocene (Fig. F15). From the lower Miocene to the base of the Oligocene, both nannofossils and radiolarians were useful in establishing age control. In most of the Eocene, only radiolarians are present (Fig. F15), although both nannofossils and foraminifers are found in the chalks at the very base of the section of lithologic Unit IV.
Based on a simple linear interpolation from the sediment surface (assumed to be zero age) and the first identifiable paleomagnetic reversal (Tables T10, T11), the clays of lithologic Unit I (see "Lithostratigraphy") have an LSR near 1 m/m.y. Piston Core EW9709-13P (Lyle, 2000), taken in the survey area, can be correlated to the density records of Site 1220A (Fig. F16). By mapping the density record of the upper part of Hole 1220A (Tables T10, T11) onto the density record of piston Core EW9709-12P, we can determine the LSR of the piston core relative to the assumed constant sedimentation rate of Hole 1220A. This comparison indicates an interval of relatively rapid sedimentation in the piston core near 15 Ma (Figs. F16, F17). Above this interval, the LSR of the piston core is less than half that of Hole 1220A. Below the relative peak in LSR, the piston core has about the same accumulation rate as assumed for the upper part of Hole 1220A (Fig. F17).
The LSR at Site 1220 in the alternating siliceous and calcareous clays of lithologic Units II and III reaches ~5.5 m/m.y. in the Oligocene part of the section (Tables T10, T11; Fig. F15). This sedimentation rate continues through the E/O boundary into the upper Eocene with no significant change, despite the disappearance of carbonate from the Eocene sediments. A second maximum in LSR of ~8.3 m/m.y. is present in the silica-rich middle Eocene radiolarian ooze of lithologic Unit IV (Fig. F15). The LSR apparently drops slightly (to ~6.6 m/m.y.) near the top of the lower Eocene with the appearance of the first significant chert layers.
LSR data may be combined with the dry bulk density (DBD) data (see "Physical Properties") (Table T16) to determine the bulk mass accumulation rates (MARs) of the sediments (Table T12). Sediment with an LSR of 1.0 cm/k.y. and a DBD of 1.0 g/cm3 will have an MAR value of 1.0 g/cm2/k.y. The observed values are rarely this high, so we report the data as milligrams per square centimeter per thousand years (mg/cm2/k.y.).
MAR values are low in lithologic Unit I, generally <75 mg/cm2/k.y. (Fig. F18). Lithologic Unit II, dominated by siliceous ooze, accumulates at flux rates up to 100 mg/cm2/k.y. Maximum flux rates of 400-500 mg/cm2/k.y. occur in lithologic Unit III, the very light brown nannofossil ooze of early Oligocene age. Unit IV, dominated by radiolarian ooze and radiolarite, has much lower MARs of ~160 mg/cm2/k.y. in the upper portion to ~300 mg/cm2/k.y. in the period between ~40 and 46 Ma. The basal chalk of Unit V accumulated at rates of ~400-600 mg/cm2/k.y.