After measuring the natural remanent magnetization (NRM), all sections of the archive-half of the core were partially demagnetized using AF magnetization at 30 mT at 5-cm intervals to remove magnetic overprints acquired during RCB coring.
Inclinations document early Miocene to Pliocene geomagnetic polarity changes and were used in conjunction with the standard geomagnetic polarity time scale (GPTS) of Cande and Kent (1995) to date the sediments.
After AF demagnetization, the stable depositional remanent magnetization was measured from 300.25 (Section 190-1177A-1R-1, 5 cm) (Fig. F15) to ~832.08 mbsf (Section 56R-3, 100 cm). The inclinations from 300.25 (Section 190-1177A-1R-1, 5 cm) to 450.25 mbsf (Section 16R-5, 35 cm) show a clear record of geomagnetic polarity changes. Below 450 mbsf, however, inclination changes provide limited information on the geomagnetic polarities because of poor core recovery. Rapid declination changes are interpreted to have been caused by rotation of individual pieces of the core during the RCB coring process.
The magnetic intensity shows a rapid repetition in values. There are five high-intensity and four low-intensity anomalies from the top to the bottom of Hole 1177A (Fig. F15). High intensity was continuously observed from the top of Hole 1177A (Section 190-1177A-1R-1, 5 cm) to 473.35 mbsf (Section 19R-1, 55 cm). A distinct low-intensity zone is abruptly present at 473.55 mbsf (Section 190-1177A-19R-1, 75 cm), and the low-intensity values terminate at 521.65 mbsf (Section 24R-1, 95 cm). After this low magnetic intensity, two high-intensity peaks are observed at 521.70-540.05 mbsf (interval 190-1177A-24R-1, 100 cm, through 25R-CC, 20 cm) and 569.35-608.15 mbsf (interval 190-1177A-29R-1, 40 cm, through 33R-1, 65 cm). These are followed by a rapid decrease in intensity values from 608.15 mbsf (Section 190-1177A-33R-1, 60 cm) to 684.85 mbsf (Section 41R-1, 45 cm). Below 684.90 mbsf (Section 190-1177A-41R-1, 45 cm), the intensity changes gradually into higher values, and this continuation of high intensity is terminated at 759.15 mbsf (Section 48R-5, 145 cm). Slightly low intensity is again seen from 759.25 mbsf (Section 190-1177A-48R-6, 5 cm) to 831.00 mbsf (Section 56R-2, 70 cm). The lower part of Hole 1177A consists mainly of volcaniclastics with high magnetic intensities in the range from 10-1 to 1 A/m.
Site 1177 magnetostratigraphy is based on polarity changes determined by measuring the inclination of the archive-half of the core after AF demagnetization at 30 mT. Early Miocene to Pliocene magnetic polarity changes were identified using biostratigraphic datums (calcareous nannofossils; see "Biostratigraphy") and correlated with the GPTS of Cande and Kent (1995) (Fig. F16). The identified chrons and subchrons are given in Table T12.
A magnetic polarity change from reversed to normal at 301.85 mbsf (Section 190-1177A-1R-2, 15 cm) is interpreted as the Reunion Event within the Matuyama Chron and is dated at 2.14 Ma (Cande and Kent, 1995). The boundary between the Matuyama and Gauss Chrons (2.581 Ma) is at 328.55 mbsf (Section 190-1177A-4R-1, 5 cm), and the boundary between the Gauss and Gilbert Chrons (3.58 Ma) is at 384.25 mbsf (Section 9R-6, 15 cm).
The termination of the Gilbert Chron and the beginning of Chron C3A (5.894 Ma) is identified at 427.45 mbsf (Section 190-1177A-14R-2, 145 cm). The boundary at 444.25 mbsf (Section 190-1177A-16R-1, 35 cm) is interpreted as the beginning of Chron C3B (6.935 Ma). A normal polarity inclination at 534.55 mbsf (Section 190-1177A-25R-3, 115 cm) is interpreted as the beginning of Chron C5 (9.74 Ma), although magnetic measurements for this interval are limited because of poor core recovery. Based on the biostratigraphic analysis, the normal inclination at 755.1 mbsf (Section 190-1177A-48R-3, 40 cm) may be correlated with the beginning of Chrons C5C (16.014 Ma).
The relationship between magnetostratigraphy and biostratigraphy is shown in Figure F17. A slight change in sedimentation rate from 6.15 to 2.96 cm/k.y. occurs at 370 mbsf (~3.3 Ma) within the top of the lower Shikoku Basin facies. Below 370 mbsf, the sedimentation rate remains stable in the lower Shikoku Basin facies (Fig. F17).