An age-depth model was constructed for Hole 1261A using the estimated age and depth uncertainty for biostratigraphic datums (Table T11) and paleomagnetic data (Table T12). Spot coring in the upper 237 m at this site limited stratigraphic resolution in this part of the hole, and foraminiferal identifications were hampered by poor preservation through much of the section (see "Biostratigraphy"). Further, shipboard paleomagnetic data yielded magnetochron estimates only for the Campanian–Maastrichtian interval (see "Paleomagnetism"). Thus, sedimentation rate calculations depend most heavily on nannofossil datums. Despite these limitations, five intervals of relatively constant sedimentation separated by hiatuses, mass flows, or condensed intervals are apparent in the age-depth plot (Fig. F12).
The upper 368 m of section at Site 1261 is unique among the five sites drilled during Leg 207. The youngest interval is Pleistocene in age and was too thin relative to sampling density for a meaningful sedimentation rate to be calculated. It is separated by a gap of ~3 m.y. from an interval of Pliocene–upper Miocene nannofossil clay and nannofossil ooze, where sedimentation rates (at 65 m/m.y.) were the highest calculated for any interval recovered during Leg 207. This interval of high sedimentation rates sits above a thick series of mass flow deposits, which occupy a stratigraphic position corresponding to a gap in pelagic sediment of >30 m.y.
Below the mass flow deposits, sedimentation patterns are similar to those seen at the other four sites drilled during Leg 207. The middle Eocene is represented by calcareous chalks, and calculated sedimentation rates are close to 9 m/m.y. Chalk deposition ends downcore at a hiatus with a duration of ~4 m.y. Below this hiatus, sediments are richer in clay than above, but early Eocene–early Paleocene sedimentation rates (7.0 m/m.y.) were similar to those in the middle Eocene. This interval ends at another ~4-m.y. hiatus, which includes the basal Danian and K/T boundary interval. Sedimentation rates are markedly lower below this hiatus than above it. At 3.3 m/m.y., the sedimentation rate in this interval is the lowest rate calculated for the Campanian–Maastrichtian interval among the five Demerara Rise sites. Pelagic deposition ends downcore in a hiatus and/or condensed interval of ~11 m.y. that spans the lower Campanian, the Santonian, and perhaps part of the upper Coniacian. Sedimentation rates in the subjacent black shale facies are 8.5 m/m.y., a rate at the high end of those found in this interval during Leg 207. The only datum below the black shale interval yields a maximum age of late Albian (see "Biostratigraphy"), which is consistent with the age of lithostratigraphic Unit V at other sites. If accurate, this age suggests a hiatus of ~6 m.y. separates the black shale from the subjacent sandstones.
Average mass accumulation rates (MARs) were calculated from linear sedimentation rates (LSRs) discussed above and the average of the dry bulk density measured on discrete samples for each interval of apparently constant sedimentation. MAR calculations remove the influence of compaction on LSR calculations and give a better indication of the quantity of accumulating sediment. Comparing MAR rates, though, does not change the conclusion that sedimentation rates were very high during the Pliocene and, hiatuses notwithstanding, slowed progressively from the middle Eocene–Campanian. Finally, MARs were relatively high during deposition of black shales (Table T13).