An age-depth model was established for Site 1258 by combining all available biostratigraphic and magnetostratigraphic datums of Hole 1258A (Fig. F17). 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) are also plotted.
Moderate to poor preservation and even absence of the microfossils limit the biostratigraphic age estimates in parts of the succession investigated. In particular, the Turonian interval was difficult to date using shipboard samples. Reliable magnetostratigraphic datums are restricted to the middle Eocene–early Paleocene (Table T14).
Linear sedimentation rates (LSRs) in Hole 1258A varied between 10 and 15 m/m.y (Table T15). These rates are typical for pelagic chalks and oozes, lithologies that characterize the Eocene–Campanian interval (lithostratigraphic Units II and III) (see "Lithostratigraphy"). Hiatuses separate the sequences into four segments with approximately constant sedimentation rates (Fig. F17). The Neogene (0–5 mbsf), with few biostratigraphic datums, is not included in the subsequent discussion:
Mass accumulation rates (MARs) were calculated from LSRs and averaged dry bulk density data of the four intervals discussed above (see "Physical Properties") (Table T15). These MAR calculations may allow for better assessment of the sedimentation processes because the influence of sediment compaction has been taken into account. Eocene–late Paleocene MAR values are 1.7 g/cm2/k.y., the highest values calculated at this site. Paleocene values are 1.5 g/cm2/k.y., whereas values of 1.6 g/cm2/k.y. were calculated for the Late Cretaceous interval excluding the black shale facies (Unit IV [249–442 mbsf]).
The succession is characterized by intervals of approximately constant sedimentation rates separated by apparent breaks. These breaks suggest five distinct hiatuses, periods of slow deposition, erosion events, or a combination of these (see letters A–E in Fig. F17). Each of these hiatuses spans at least 1 m.y., based on biostratigraphic dating (see "Biostratigraphy"). Hiatus A covers ~10 m.y. (Pleistocene–late Miocene), Hiatus B spans ~32 m.y. (middle Miocene–late Eocene), Hiatus C covers ~1 m.y. (early Paleocene; nannofossil Zone NP 3), Hiatus D spans 1 m.y. (earliest Paleocene), and Hiatus E spans 15 m.y. (early Campanian–Turonian).