All core archive halves from Holes 1201A, 1201B, 1201C, and 1201D were measured on the shipboard pass-through superconducting rock magnetometer. Natural remanent magnetization (NRM) and remanent magnetization after 5-, 10-, 15-, and 20-mT alternating-field (AF) demagnetization steps were measured at 5-cm intervals. In situ orientation data were collected with the Tensor tool for all APC cores from below 30 mbsf.
A total of 311 oriented discrete samples (standard 8-cm3 plastic cubes from Hole 1201B and 8-cm3 cubes cut with a nonmagnetic parallel saw from Hole 1201D) were collected from the working halves at a resolution of one to two samples per core section for progressive AF and thermal demagnetization and for rock magnetic studies. Samples from the turbidite sequence from Hole 1201D were taken from the fine-grained parts of the turbidites (see "Lithostratigraphy"). The basaltic basement (509-600 mbsf) was sampled at a frequency of one per section and was complemented with samples taken at the same frequency for the physical properties measurement program. All discrete samples were demagnetized at successive peak fields of 5, 10, 15, 20, 25, 30, 35, 40, 50, 60, 70, and 80 mT to verify the reliability of the split core measurements. Two hundred forty-six of the 273 sediment samples yielded good principal component analyses (PCAs) (Kirschvink, 1980) with fits having an average maximum angular deviation (MAD) of 4.4°. The PCA of all 38 basalt samples yielded excellent results with an average MAD of 1.8°. Twelve discrete samples were also progressively thermally demagnetized at temperatures of 100°, 200°, 300°, 350°, 400°, 450°, 500°, 550°, 570°, 580°, 590°, 600°, and 640°C to assess the effectiveness of thermal vs. AF demagnetization and for mineral magnetic analyses.
A composite paleomagnetic record was constructed for Site 1201 using data from Holes 1201B (0-81 mbsf) and 1201C (80-509 mbsf). Data from Hole 1201C repeat the APC section of the record for Hole 1201B and for this reason are not incorporated in the composite record presented in this site report.
Both magnetic declination and inclination were used when possible for the magnetostratigraphic interpretation at this site. The geomagnetic field at the latitude of Site 1201 (19.28°) has an inclination of 35°, assuming a geocentric axial dipole model, which is sufficiently steep to determine magnetic polarity in APC, XCB, and RCB cores that lack a horizontal orientation.
Mineral magnetic analyses were performed on discrete samples after AF demagnetization. The low-field magnetic susceptibility and the anhysteretic remanent magnetization (imparted using a 100-mT alternating field and a 0.05-mT bias field) were routinely measured on all samples. On selected samples, we measured the acquisition of an isothermal remanent magnetization (IRM) up to 1.0 T and the coercivity of backfield remanence (Bcr).
The magnetic susceptibility and anhysteric remanent magnetization (ARM) (Fig. F51) are concentration-dependent measurements that are sensitive to the amount of magnetic material present. The ARM is more sensitive to the fine-grained magnetic grains, whereas the magnetic susceptibility measures coarser-grained magnetic particles. The lowest values and the least variability of these parameters are recorded in the upper 50 mbsf of the core and are directly related to lithologic variation (brown clays of lithostratigraphic Unit I) (see "Lithostratigraphy"). Between 50 and 509 mbsf, the base of the sedimentary section, the variability of both parameters is dominated by the lithologic variations within the turbidite sequences (lithostratigraphic Unit II). With the exception of an interval of higher magnetic concentration, recorded by both magnetic susceptibility and ARM between 100 and 120 mbsf, both parameters appear to be related to small- and large-scale turbidite cycles. The ARM/k ratio is a concentration-independent parameter that provides an estimate of relative magnetic grain-size changes (Thompson and Oldfield, 1986; Verosub and Roberts, 1995). The smallest magnetic grain sizes are found in the brown pelagic clays of lithostratigraphic Unit I (Fig. F51), with peaks within the top few meters and at 35 mbsf. The turbidite sequence is characterized by alternating intervals of very coarse grained magnetic minerals (low ARM/k ratio) with finer-grained material.
Figure F52 shows representative curves of acquired IRM to saturation IRM (SIRM) and backfield SIRM. Samples from both lithostratigraphic units and the underlying pillow basalt exhibited very uniform behavior, were saturated by 300-500 mT, and had Bcr values of 8-24 mT. AF demagnetization of these samples (Fig. F53) demonstrated moderate to soft magnetization, with ~50%-90% of the intensity of magnetization lost by the 20- to 30-mT demagnetization step. Samples from lithostratigraphic Unit II had a harder magnetization, in general.
Thermal demagnetization of sediments from both lithostratigraphic units and pillow basalts shows two sharp decreases in magnetization, indicating the presence of at least two carrier minerals of magnetic remanence (Fig. F54). The inflection point at ~450°C suggests that minerals such as goethite or maghemite, which undergo thermal unblocking at these temperatures, contribute to the magnetic signal. At present, we do not have enough evidence to distinguish between these two possibilities. All measured samples display Curie temperatures (Tc) of 570°C, suggesting that the dominant magnetic mineral is magnetite with minor substitution of other elements (e.g., Ti, Al, or Mg), which decreases the Curie temperature from that of pure magnetite (Tc = 585°C). One basalt sample from the deepest part of the section (Section 195-1201D-55R-2, 10 cm) displays a Tc of 585°C and retains a few percent remanence up to 640°C, indicating the presence of impure hematite. Therefore, it is likely that Ti-bearing magnetite is the dominant carrier of magnetic remanence and susceptibility, with varying contributions from goethite(?) or maghemite(?) and hematite.
Average NRM intensities are 0.102 A/m in lithostratigraphic Unit I, 0.058 A/m in the turbidite sequence of Unit II, and 4.76 A/m in the pillow basalts (Fig. F55). The NRM measurements display inclinations ranging between -84° and +89° (mean = 33°). Inclination values after 20-mT demagnetization are skewed toward positive (downcore) values that are consistent with a steeply positive drill string overprint induced during coring. These values show that the 20-mT demagnetization step has not been completely effective in removing the overprint from the split-core archive halves.
The demagnetization behavior of discrete samples that yielded good PCA results from all lithologies is illustrated in Figure F56. Most, but not all, samples displayed a soft magnetic overprint that was removed between 5 and 25 mT. PCA of the entire whole-core record is possible with the four demagnetization steps applied during the paleomagnetic measurements program and will identify where additional cleaning of the paleomagnetic data and demagnetization of discrete samples will aid in elucidating the characteristic remanent magnetization (ChRM) at Site 1201.
The interpretation of magnetic polarity from the composite inclination and declination record for Site 1201 (Fig. F57) is well constrained by key nannofossil datums spanning Biozones NP19/NP20 to NP25 between 29 and 462 mbsf (see "Biostratigraphy;" also see "Biostratigraphy" in the "Explanatory Notes" chapter). The top 29 mbsf is barren of nannofossils, and the preliminary magnetostratigraphic interpretation of this interval (Fig. F57A) is not constrained by any biostratigraphic datums. The inclination record for this interval, when compared with the GPTS (Cande and Kent, 1995; Berggren et al., 1995), assuming constant sedimentation rates, provides a complete record from the Thvera Subchron (C3n.4n) through late and middle Miocene polarity intervals to Subchron C5Bn.1n, close to the base of the middle Miocene (Table T7). This part of the magnetic record is characterized by sharp 180° reversals of the magnetic declinations, which allow the identification of magnetic chrons and subchrons. The magnetic inclination record, on the other hand, is ambiguous and strongly biased toward positive values and can, for the most part, only be interpreted with the help of discrete samples (Fig. F57A). The magnetic polarity reversals and the correlation with the GPTS show that the upper and middle Miocene record at Site 1201 is remarkably complete and possesses strikingly uniform sedimentation rates. The characteristic reversal patterns of the Brunhes, Matuyama, Gilbert, and Gauss Chrons are not discernible at the top of the section, which suggests that the last 5 m.y., consisting of the entire Pleistocene and most of the Pliocene, are missing at Site 1201. This hiatus implies that there is currently no sediment being deposited at Site 1201 or that sediment has been or is being eroded.
The reversed polarity chron below Subchron C5Bn.1n (24-26 mbsf) contains the LO of nannofossils Dictyococcites bisectus (>23.9 Ma) and Spenolithus ciperoensis (24.5 Ma), constraining this interval to Chron C6Cr. Sedimentological evidence (see "Lithostratigraphy") for a hiatus in Section 195-1201B-3H-6, 56 cm (24.8 mbsf), and the absence of continuity in the correlation with the GPTS suggest that the lower part of the normal polarity chron between 24 and 26 mbsf belongs to the late Oligocene Subchron C6Cn.3n. The middle Miocene to late Oligocene unconformity, defined by a combination of magnetostratigraphic, biostratigraphic, and sedimentological evidence spans ~10 m.y.
The normal polarity event between 31.7 and 35.1 mbsf correlates with Subchron C7n.2n. The upper Oligocene record below Chron C7n cannot be interpreted because the magnetic signal is noisy, the correlation between magnetic declination and polarity that guided the magnetostratigraphic interpretation in the top 25 mbsf of the section is missing, the sampling resolution of the discrete samples is not sufficient, and biostratigraphic datums have not been identified. Poor recovery at the bottom of Hole 1201B caused a coring gap between Holes 1201B and 1201D that complicates the magnetostratigraphic interpretation between 60 and 80 mbsf.
The truncation of several nannofossil species at a distinct lithologic change between lithostratigraphic Units I and II (see "Lithostratigraphy") defines a hiatus at 53.4 mbsf within the upper part of nannofossil Zone NP24 (see "Biostratigraphy").
The LO of S. distentus (27.5 Ma) occurs in the normal polarity chron between 50 and 60 mbsf, which probably correlates this interval with one of the subchrons of Chron 10n. The FO of S. ciperoensis (29.9 Ma) within Core 195-1201B-10X constrains this short normal polarity interval to Subchron C11n.2n.
The magnetic inclination record in the RCB section of the turbidite interval between 100 and 500 mbsf defines several long normal and reversed polarity chrons (Fig. F57B). The normal polarity interval between 90 and 173 mbsf contains the FO of S. distentus (31.5 Ma), which constrains this interval to Chron 12n. The long reversed polarity interval between 173 and 389 mbsf correlates to Chron C12r and is well constrained by the LO of Reticulofenestra umbilicus (32.3 Ma) and Ericsonia formosa (32.8 Ma).
The transition to the upper Eocene occurs at the well-defined polarity transition at 423.4 mbsf, the onset of Chron C13n. Three normal polarity intervals can be distinguished in the upper Eocene strata that are assigned to Chron 15n and the two subchrons of Chron 16n. Two nannofossil events at 462 mbsf, the LO of Discoaster barbadiensis (34.3 Ma) and D. saipanensis (34.2 Ma) constrain Chron C15r between 468 and 458 mbsf. The termination of Subchron 16n.2n at 35.685 Ma, more than 20 m above the basaltic basement, is the oldest and lowermost magnetostratigraphic event that can be identified at Site 1201. The onset of Subchron 16n.2n could not be determined, but the normal polarity continues right to the contact with the basement.
In the 509-m Neogene-Paleogene composite record of Site 1201, 65 subchrons of the GPTS are present. The record is interrupted by three major hiatuses, between 0 and 5 Ma, 14.8 and 24.1 Ma, and in the top section of Biozone NP24 in the uppermost Oligocene (see "Biostratigraphy"). A paleomagnetic age model for Site 1201, listing depths and ages of magnetic polarity reversal events, is given in Table T7.
An age-depth summary for Site 1201, using the GPTS reversal polarity ages given in Berggren et al. (1995) and the nannofossil events (see "Biostratigraphy"), is provided in Figure F58A (entire record) and 58B (0-60 mbsf). The average sedimentation rate for lithostratigraphic Unit I above the Miocene/Oligocene unconformity (0-24.8 mbsf) is 3 m/m.y. Above the unconformity, the average sedimentation rate decreases from 4 m/m.y. in the middle Miocene to 2 m/m.y. in the late Miocene at a well-defined inflection point at 15 mbsf (12.5 Ma). The average sedimentation rates in the interval between the two hiatuses (24.8-51 mbsf) is 8 m/m.y. and increases to >100 m/m.y. in the turbidites between 100 and 420 mbsf. An inflection point at 420 mbsf (33.5 Ma) denotes a change to lower sedimentation rates (35 m/m.y.) between 420 and 485 mbsf. The lack of biostratigraphic and magnetostratigraphic ages directly above the basement precludes the assessment of sedimentation rates for this important part of the section.
The trend to decreasing sedimentation rates from the late Eocene to the early Pliocene reflects the environmental history of the West Philippine Basin in the vicinity of the Palau-Kyushu Ridge with extensive amounts of reworked arc-derived volcaniclastic material deposited from the late Eocene through the early Oligocene and a transition to a deepwater environment with slower pelagic sedimentation in the middle to late Miocene. The onset of a bottom-water current sometime after the early Pliocene could be responsible for the nondeposition or erosion of sediments <5 Ma.
PCA of 38 basalt samples revealed magnetically extremely well-behaved samples (Fig. F59). A soft magnetic overprint was easily removed with AF demagnetization techniques. A comparison between AF and thermal demagnetizations carried out on two basalt samples (Fig. F59) demonstrates that although a small high-temperature and high-coercivity component can be present, the AF technique is more effective in isolating the ChRM. The magnetic inclination record shows a mean inclination of 13.9° between the top of the basement at 509 mbsf and 530 mbsf (Fig. F60). Inclinations between 530 and 550 mbsf are shallow, with both positive and negative polarity. The bottom part of the basalt sequence (550-600 mbsf) is characterized by shallow inclinations with negative polarity. The mean inclination for this part of the section is -12.8°. Although the basalt has been severely altered, the thermoremanent magnetization carried by the (titano) magnetites is not affected.
The age of the basement at Site 1201 was estimated from seafloor anomalies (Chron 21) to be middle Eocene (~48 Ma). The inclination record of the basalts suggests the existence of a magnetic reversal at 550 mbsf. Magnetic inclinations above and below this reversal are shallow (13°-14°) and pass the reversal test by being perfectly antipodal. They indicate a position of the Philippine plate near the equator, at ~7° paleolatitude, during the middle Eocene. This supports the model of a major northward movement of the plate proposed by Hall et al. (1995), based on earlier coring results, to the current position of 19° latitude at Site 1201.