A 316.5-mcd-thick (280.9 mbsf) lower Oligocene (~31 Ma) to Pleistocene pelagic sediment sequence was recovered at Site 1264. A total of 92 biostratigraphic datums and 6 magnetostratigraphic polarity reversals (Table T15) were used to construct an age-depth model for this site (Table T16; Fig. F29). Linear sedimentation rates (LSRs), total mass accumulation rates (MARs), and carbonate MARs were calculated at 1-m.y. intervals (see "Age Model and Mass Accumulation Rates" in the "Explanatory Notes").
The main objective of Site 1264 was to recover a complete and well-resolved Neogene section representative of the Walvis Ridge transect. The sediment section at Site 1264 is fairly complete, although the interval from 17.5 to 14 Ma (204–186 mcd) is condensed and an unconformity exists in the uppermost Miocene and lowermost Pliocene (5.9–5.2 Ma; ~77 mcd) (see "Biostratigraphy"). Biostratigraphic and magnetic reversal datums are in general agreement for most of the section (Fig. F29). The most significant discrepancies between nannofossil and planktonic foraminiferal datums exist in the Pliocene–Pleistocene, and we relied on calcareous nannofossils and magnetostratigraphy to define the age model in that interval.
LSRs range between <1 and 30 m/m.y., and total MARs range from <0.1 to 3.9 g/cm2/k.y. Total MAR fluctuations essentially represent variations in carbonate MAR. Our 1-m.y. resolution record shows large fluctuations in LSRs and carbonate MARs with little overall trend. Low to moderate and relatively constant rates were calculated for the Oligocene; the lower part of the lower Miocene has significantly higher rates, followed by near-zero accumulation rates based on a planktonic foraminifer–based age-depth model. The LSRs and MARS remained low through the middle Miocene and increased significantly in the late Miocene to early Pliocene, punctuated by a 0.6-m.y hiatus not resolved in this age model (see "Biostratigraphy"). The rates declined steadily in the Pliocene–Pleistocene to low values for the top of the Site 1264 sediment section.
In general, averaged noncarbonate MARs are low (<0.2 g/cm2/k.y.) throughout the section, and the small fluctuations reported here are probably representative of the analytical uncertainty rather than environmental changes.