SEDIMENTATION RATES

Age-Depth Model

An age-depth model for Site 1257 was established by combining all available biostratigraphic and magnetostratigraphic datums of Hole 1257A (Fig. F13). 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 on board ship (Table T14) are also plotted. Poor preservation or absence of microfossils limits lithologic dating in parts of the succession. In particular, the Turonian–Albian interval was difficult to date with shipboard samples.

Sedimentation Rates

Calculated linear sedimentation rates (LSRs) for Hole 1257A vary between 5 and 10 m/m.y. (Table T15). These rates are typical for pelagic oozes and chalks, which are the dominant lithology of lithostratigraphic Units II and III (see "Lithostratigraphy"). Based on significant changes in sedimentation rates, it is possible to differentiate the following six intervals with approximately constant sedimentation rates:

  1. Neogene (0.0–2.5 mbsf): poor and ambiguous biostratigraphic data. This interval is not discussed further.
  2. Oligocene (2.5–43.5 mbsf): LSR = ~9 m/m.y.
  3. Middle Eocene (43.5–70.0 mbsf): LSR = ~5 m/m.y.
  4. Early Eocene–late Paleocene (70–140 mbsf): LSR = ~10 m/m.y.
  5. Maastrichtian–Campanian (140–175 mbsf): LSR = ~5 m/m.y.
  6. Santonian–Cenomanian (175–220 mbsf): LSR = ~4 m/m.y.

Mass accumulation rate (MAR) calculations, using the above LSRs and averaged dry bulk density of the major lithologies (see "Physical Properties") are given in Table T15. These calculations may better reflect sedimentation processes because the calculation corrects for the influence of sediment compaction. Oligocene MARs are relatively high, reaching values of 0.8 g/cm2/k.y. The middle Eocene exhibits values of 0.4 g/cm2/k.y. Highest overall values of ~1.2 g/cm2/k.y. were calculated for the lower Eocene–upper Paleocene interval. Rates of 0.7–0.9 g/cm2/k.y. were observed for the Upper Cretaceous intervals, including the black shale facies (Unit IV [175–220 mbsf]).

Hiatuses

The six intervals described above are bounded by abrupt changes in sediment ages, clearly suggesting four distinct hiatuses. Each of these hiatuses (see letters A–D in Fig. F13) spans several million years, based on biostratigraphic dating (see "Biostratigraphy"):

  1. Hiatus A = ~20 m.y. (late Pliocene–early Oligocene),
  2. Hiatus B = ~2–3 m.y. (early Oligocene–middle Eocene),
  3. Hiatus C = ~10 m.y. (middle Eocene–early Eocene), and
  4. Hiatus D = ~10 m.y. (late Paleocene–mid-Maastrichtian; K/T boundary).

These hiatuses may reflect periods of nondeposition or erosional events or a combination of both. Slump and debris flow deposits observed in sediment cores of Holes 1257B and 1257C (see "Lithostratigraphy") closely correlate to these hiatuses and may argue for erosional events.

NEXT