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Chronology is provided for the uppermost Eocene? to Pleistocene section in Hole M0027A by integrating the following on an age-depth diagram (Figs. F34, F85):

  1. Biostratigraphy provided by calcareous nannofossil, planktonic foraminifer, and dinocyst data. Zonal durations are plotted using tables provided in "Paleontology."

  2. Sr isotopic ages and associated age errors.

  3. Placement of sequence boundaries and other stratal surfaces (Table T14).

  4. The timescale of Berggren et al. (1995; BKSA95). (Although this timescale is demonstrably incorrect in the earliest Miocene [e.g., correlation of the base of the Miocene as 22.9 ± 0.1 Ma versus 23.8 Ma in Shackleton et al., 2000], Sr isotope regressions and dinocyst zonations have not been calibrated to the GTS2008 [Ogg and Gradstein, 2008.])

Surfaces were picked as seismic sequence boundaries in TWT (Table T14), converted to depth, and tied to cores (Table T14) 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 (e.g., m4 at ~140 m; Fig. F34).

  • If identified in a core, the surface was plotted as a solid red line for candidate sequence boundaries.

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

  • A visual best fit sedimentation rate line was estimated for each sequence and an age error assigned to each (shaded zones; Fig. F34).

  • All estimated sedimentation rates do not include estimates of postdepositional compaction, so they are minimum values.

Pleistocene sequences in Hole M0027A were identified on Geopulse seismic profiles and interpreted as representing MIC3a, MIC3c, and MIC4 (Sheridan et al., 2000). Calcareous nannofossils suggest that the top sequence (above 10.41 mbsf) is upper Pleistocene (<90 ka) because it is within the acme zone of Emiliania huxleyi (Hine and Weaver, 1998). One or more sequences between 10.41 and 26.38 mbsf are estimated as 90–250 ka between the acme and FO of E. huxleyi. A thin Pleistocene (Zone NN19) sequence from 26.4 to 31.9 mbsf appears to be lower Pleistocene based on an Sr isotope age estimate of 1.35 Ma ± 0.35 m.y. We speculate that this may represent MIC30 (~1 Ma). Sedimentation rates are difficult to establish with the age model at this preliminary stage. Pleistocene sedimentation rates appear to be high (>100 m/m.y).

The fluvial–estuarine section above seismic sequence boundary m1 (~96 mbsf) and the poorly sampled section from that depth to 140 mbsf containing seismic sequence boundaries m3 (~115 m) and m4 (~140 mbsf) provide no age constraints at this location. Seismic sequence boundary m2 and all of the sediments that make up the m2 sequence have been removed by erosion. The m1, m3, and m4 seismic sequence boundaries have been dated on the slope as ~11.5, 13.6, and 14.1 Ma, respectively (Miller et al., 1997), which yields minimum sedimentation rates of 50, 25, and 50 m/m.y., respectively, for the sequences in Hole M0027A.

Reflector m4.1 is interpreted as a MFS at this site based on its correlation into the middle of a succession of clay. Seismic reflector m4.1 seismically merges with several older surfaces (m4.2, m4.3, and m4.4 [dated in Hole M0029A]) and must represent a significant hiatus that is not discernable with the available age control. Magnetic susceptibility measurements suggest a break at 195 mbsf in the middle of the clay that may mark this coalesced surface. Seismic sequence boundary m5 apparently merges with the m4.5 surface (218.4 mbsf), and sediments from the overlying sequence(s) are poorly dated. Sr isotope ages of 13.6–13.7 Ma ± 1.2 m.y. in the lower part of the section are consistent with dinocyst assignment to upper Zone DN5 and calcareous nannofossils to Zones NN6–NN7 (~11.2–13.6 Ma).

A series of relatively thin (<25 m) sequences (m5.2, unnamed, m5.3, m5.32, m5.4, and unnamed) span the lower/middle Miocene boundary (16.2 Ma BKSA95 and the 2004 geologic timescale. There are no constraints on the age of seismic sequence boundary m5.2 and the overlying sequence other than superposition. The underlying sequence m5.3 is assigned to Zone NN5 (>15.6 Ma) and Sr isotope age estimates of 15.8 and 16.0 Ma ± 0.6 m.y. (below). The sequence overlying seismic sequence boundary m5.32 records the change from Zone NN5 to NN4 (15.6 Ma) with a basal Sr isotope age of 16.3 Ma ± 0.6 m.y. However, correlations to Holes M0028A and M0029A suggest that the base of Zone NN4 may be depressed in Hole M0027A, as the marker for the base of this zone is predicted to range higher in the sandy sediments in Hole M0027A. Sequence m5.4, an unnamed sequence with a base at 295.0 mbsf, and sequences m5.45 and m5.47 all lie within Zone NN4 (15.6–18.4 Ma), with planktonic foraminifer Zone N6/M3 (17.3–18.8 Ma) in the basal 5.47 sequence. Sr isotopes provide some age constraints on the sequences overlying the following seismic sequence boundaries: 17.0 Ma ± 0.6 m.y. (m5.4), 17.2 and 17.8 Ma ± 0.6 m.y. (unnamed; 295 mbsf), and 18.4 Ma ± 0.6 m.y. (m5.45). Age estimates of basal sequence boundary ages (ranging from ~17.3 to 18.4 Ma; Table T14) have fairly large errors because of limited data, and future work should tighten the age estimates. Mean apparent sedimentation rates during deposition of the thin sequence spanning the lower/middle Miocene boundary (sequence m5.2–m5.47) were at least 30–40 m/m.y. Shorebased studies should improve the age resolution for this interval.

A 36.9 m thick sequence m5.45 (295.01–331.90 mbsf) and a thin (4.2 m) sequence m5.47 (331.90–336.06 mbsf) are estimated as ~18.0 and ~18.3 Ma, respectively, and are reasonably well constrained by Praeorbulina sicana and Zone NN4 with ±0.5 m.y. age resolution.

Thin sequences m5.6 and m5.7 (336.1–355.5 and 355.5–361.3 mbsf, respectively; ~18.7–19.6 Ma) have no current age information (barren/non–age diagnostic nannofossils), although they contain shells. Shore-based analyses are expected to produce ages based on dinocysts and Sr isotopes.

A thick (128.1 m) sequence m5.8 (361.3–489.4 mbsf) has a lower seismic sequence boundary dated as 21.1 Ma. Ages in the lower part of the sequence are well constrained by Sr isotopes and foraminifer and dinocyst zones and are assigned to the middle portion of nannofossil Zone NN2. The upper sands of this sequence are poorly dated (20 Ma? by extrapolation of sedimentation rates), although sedimentation rates for this sequence must be high (~120 m/m.y.).

A thin lowermost Miocene sequence m6 (489.4–499.9 mbsf) is dated only by assignment to mid-Zone NN2. Nevertheless, the short duration (<0.5 m.y.) required by the short zone and a thick section assigned to mid-Zone NN2 above suggest a moderate sedimentation rate (>20 m/m.y.).

At least four Oligocene sequences and part of an uppermost Eocene to lowermost Oligocene sequence occur in the bottom of the hole. The Oligocene succession is unusually thick (>129 m) compared to onshore successions (Pekar et al., 1997). A previously unnamed sequence (o.5; 499.8–539.5 mbsf) apparently straddles the Oligocene/Miocene boundary with nannofossil Zone NN1 and a tentative assignment to planktonic foraminifer Zone N4 in the upper part of the sequence and dinocyst Zone DN1 (uppermost Oligocene–lowermost Miocene) throughout. Further work on this boundary interval is needed. A slightly discordant Sr isotope age (25.3 Ma) requires verification. Sedimentation rates can only be roughly estimated as ~40 m/m.y.

The o1 sequence (539.5–585.5 mbsf) is well dated by all fossil groups and Sr isotopes as mid-Oligocene (28.5–29.0 Ma). This sequence has a high sedimentation rate (~92 m/m.y.). The underlying unnamed sequence (585.5–596.3 mbsf) is poorly dated as ~30 Ma. One or more sequences between 596.3 and 628.9 mbsf have conflicting ages, and further study is needed. The base of this interval is assigned to lower Oligocene nannofossil Zone NP22 (Chron C12r). The base of the hole is in lower Zone NP21, suggesting that Chron 13n and some of upper Chron C13r (the Eocene/Oligocene boundary) is represented by a hiatus across a sequence boundary at 628.9 mbsf. This basal Oligocene sequence boundary is most likely equivalent to the global sea level fall associated with isotopic increase Oi1, and further magnetostratigraphic, biostratigraphic, and stable isotopic studies will test the continuity of this section.