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doi:10.2204/iodp.proc.313.105.2010

Chronology

The Pleistocene to upper middle Miocene section was spot-cored in Hole M0029A, and the hole bottomed in the lower Miocene. Chronology for the lower middle to lower Miocene in Hole M0029A was obtained by integrating the following on an age-depth diagram (Fig. F18):

1. Biostratigraphy provided by calcareous nannofossil, planktonic foraminifer, and dinocyst data. Zonal durations are plotted using tables provided in "Paleontology," along with select datum levels (first and last occurrences).

2. Sr isotopic ages and associated age errors.

3. Placement of sequence boundaries and other stratal surfaces (Table T13).

4. The timescale of Berggren et al. (1995; BKSA95).

Surfaces were picked as seismic sequence boundaries in two-way traveltime (Table T13) and converted to depth and predicted core (Table T13) using a velocity function (see "Stratigraphic correlation" in the "Methods" chapter for discussion of criteria used to define seismic sequence boundaries and the derivation of the time to depth conversion of seismic data). In figures, features are presented as follows:

• If unsampled by cores, the reflector is indicated with a gray shaded zone.

• If identified in a core, the surface identified by the science party was plotted as a solid red line for sequence boundaries and a dashed line where uncertain or where there was more than one possible core surface for a given seismic sequence boundary (see "Stratigraphic correlation").

• A few prominent (but not all) features interpreted as MFSs identified in the cores (see "Stratigraphic correlation") are indicated with a green line.

• A visual best fit sedimentation rate line (uncorrected for postdepositional compaction) was estimated for each sequence and an age error assigned to each.

Pleistocene sequences in Hole M0029A were identified on Geopulse seismic profiles and interpreted as representing MIC 3 (Sheridan et al., 2000; Carey et al., 2005). Calcareous nannofossils suggest that the uppermost Pleistocene sequence (above 14.63 mbsf) is upper Pleistocene (Zone NN21; < 250 ka). There are no other age constraints on ?Pleistocene sequences above ~156 mbsf.

There are no age constraints on seismic sequence boundaries m1 (156.24 mbsf), m3 (193.17 mbsf), and m4 (231 mbsf) in Hole M0029A except regional correlations (11.5, 12.8, and 13.6 Ma, respectively) (Miller et al., 1998). A tentative seismic sequence termed m4.2 (231–341.11 mbsf) is poorly dated. It is assigned to calcareous nannoplankton Zones NN6–NN7, with definite Zone NN6 (12.6–13.6 Ma) at its base. It is assigned to dinocyst Zones DN6–DN8 (younger than ~13.2 Ma) and planktonic foraminifer Zone N14 or older (>11.4 Ma). Sr isotope ages for this section show considerable scatter (11.6–14.2 Ma), which is not surprising considering the large age error for this portion of the Sr age regression (±1.17 m.y.). The average value of the five points in this sequence is 12.8 Ma (with an error of better than ±0.8 m.y. considering number of analyses q > 3) (Oslick et al., 1994), which is concordant with a basal age of 13.2 Ma based on nannofossils and a possible extrapolated age as old as 13.6 Ma (Fig. F18). The errors on the age of this sequence are large, and further studies should refine them.

The underlying tentative seismic sequence termed m4.5 (343.24–449.1 msbf) is also poorly dated. It is assigned to dinocyst Zones DN6–DN8 (<13.2 Ma) at the top and DN5 (13.2–15.1 Ma) below 349.70 mbsf and to Zone NN6 at the top (11.8–13.6 Ma) and Zone NN5 (13.6–15.6 Ma) below 360.88 mbsf. The highest occurrence of F. peripheroronda in Sample 313-M0029A-81R-CC, 0–2 cm (371.83 mbsf), helps constrain the minimum estimate for this sequence; this datum level is ~13.8 Ma on BKSA95 and the 2004 geologic timescale (Gradstein, 2004), although Shackleton et al. (1999) suggest that this taxon ranges to Zone N10 (M7). Sr isotope values show considerable scatter in this sequence with ages of 13.3–16.4 Ma. Considering the tight grouping of Sr isotopes ages >15 Ma in underlying sequences, biostratigraphic constraints, and superposition, ages older than 15 Ma are interpreted as reworked and indicated with red on Figure F18. The remaining five Sr isotope ages, which are from the upper part of the sequence, average 13.8 Ma ± 0.8 m.y. Thus, data are consistent with an age estimate of 13.5–14.6 Ma for the sequence.

Seismic sequence m5 (449.1–478.61 mbsf) is assigned to calcareous nannofossil Zone NN5 (13.6–15.6 Ma) and to dinocyst Zone DN5 (13.2–15.1 Ma). The last occurrence of P. sicana in Sample 313-M0029A-108R-CC, 0–5 cm (451.22 mbsf), has an age of 14.8 Ma according to BKSA95 (this taxon may range into Zone N10 according to Shackleton et al. [1999], who provide a GTS2004 calibration of 13.73 Ma [13.7 Ma on BKSA95]). Five Sr isotope ages >15 Ma are rejected; the remaining three (14.4, 13.5, and 14.7 Ma ± 1.17 m.y.), together with the biostratigraphic constraints, are consistent with an age assignment of seismic sequence m5 to 14.6–15.4 Ma, with a possible basal age of 15.0–15.4 Ma. However, the calibration of P. sicanus would allow this sequence to be significantly younger (~14 Ma).

The age of seismic sequence m5.2 (478.61–602.25 mbsf) is well constrained as 15.6–16.1/16.2 Ma. This sequence has excellent agreement among nannofossil (Zones NN4 and NN5), dinocyst (Zones DN4–DN5), and planktonic foraminifer (Zone N8/M5) zones. The tightly constrained Sr isotopic age estimates (12 points between 15.5 and 16.3 Ma, with two potential outliers of 16.7 and 16.6 Ma) are in excellent agreement with the biostratigraphic age estimates. Of note is the placement of the Zone NN4/NN5 boundary (15.6 Ma) in the middle of this sequence in this hole and in Hole M0028A, but its placement in Sequence m5.3 at Hole M0027A suggests a premature/depressed last occurrence in the updip site.

The age of seismic sequence m5.3 (602.25–640.51 mbsf) is only broadly constrained by calcareous nannofossils and dinocysts to Zone NN4 and upper Zones DN3–DN4, respectively, and one Sr isotope age of 16.9 Ma ± 0.6 m.y. However, the last common occurrence of nannofossil D. deflandrei (16.2 Ma) can be used as a datum level, suggesting that this sequence is ~16.2–16.9 Ma.

The ages of seismic sequences m5.4, m5.45, and m5.47 cannot be precisely estimated because they are assigned to Zone NN4 below the last common occurrence of D. deflandrei (16.2–18.3 Ma) and dinocyst Zone DN3 or older (>16.7 Ma). There are no current Sr isotope age estimates, but subsequent work should constrain the ages of these sequences.

Seismic sequence m5.6 (687.87–707.56/710.16 mbsf) has reasonable age constraints in Hole M0029A versus the updip sites. Here, for the only time, Zone NN3 (18.3–19.6 Ma; note that the BKSA95 timescale dashed the base of the zone at 19.1 Ma, but it should be 19.6 Ma) was identified, remarkably consistent with the sole Sr isotope age of 18.3 Ma ± 0.6 m.y. The sequence also contains S. belemnos, which further constrains its age to 18.3–19.2 Ma. Further Sr isotope studies should constrain the basal age of the sequence (?18.5 Ma based on extrapolation of sedimentation rates).

Seismic sequence m5.7 (707.56/710.16–728.56 mbsf) is assigned to calcareous nannofossil Zone NN2 (19.6–23.2 Ma), although it is likely younger than 21.5 Ma based on nannofossil datum levels (ratio of H. carteri/H. euphratis). It is assigned to the younger part of Zone DN2 (19 to ~20.2 Ma). The best estimate is thus 19.6–20.2 Ma. Further work should help define the precise age of this sequence.

Seismic sequence m5.8 (728.56–746/750 mbsf) is assigned to the mid-upper part of Zone NN2 (older than 19.6 and younger than 21.5 Ma) and dinocyst zone DN2 (19–22.2 Ma). It has a Sr isotope age estimate of 21.0 Ma ± 0.6 m.y., suggesting that the sequence is 20.4–21.6 Ma. Further work should help define the precise age of this sequence.

We may have penetrated seismic sequence m6 below 746/750–752.8/753.22 mbsf. Dinocysts suggest that the base of Hole M0029A is in Zone DN1 (>22.2 Ma); however, nannofossils suggest that the base of Hole M0029A was younger than 21.5 Ma, remarkably consistent with a Sr isotope age of 21.3 Ma ± 0.6 m.y. These ages are more consistent with assignment to sequence m5.8 based on regional correlations.

Sedimentation rates are difficult to estimate in Hole M0029A from the preliminary age constraints. Average sedimentation rates shown on Figure F18 are ~80 m/m.y. Sedimentation rates during deposition of the targeted m5.2 sequence in a position near its greatest thickness were higher: the ~123 m of this sequence was deposited in 0.7 m.y., with sedimentation rates of ~176 m/m.y. Sedimentation rates in the m4.5 and m4.2 sequences were high (>80 m/m.y) and may have approached the sedimentation rates in sequence m5.2.

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