Geologic Magnetic Data Processing

ODP logging contractor: LDEO-BRG

Hole: 883F

Leg: 145

Location: Detroit Sea Mount (NW Pacific)

Latitude: 51° 11.906' N

Longitude: 167° 46.085' E

Logging date: August, 1992

Bottom felt: 2396.5 mbrf (used for depth shift to sea floor)

Total penetration: 894.4 mbsf

Total core recovered: 13.6 m (46.2 %)

SUMT/NMRS Logging Runs

The SUMT and NMRS were run separately: two passes were recorded for each tool. Processing was performed on pass one of each tool from 220 to 720 mbsf.

Wireline heave compensator was used to counter ship heave resulting from the mild sea conditions.

Hole conditions

The FMS calipers vary from 10 to 15 inches. The raw susceptibility has been corrected for hole diameter variations using the FMS caliper down to 650 m, and a constant value of 10.5 inches below.

Depth Shift

Because of the lack of NGT tool on the magnetometer and susceptibility tool strings, no depth match of these logs to the reference run (DIT/SDT/HLDT/CNTG/NGT) could be performed. However, the second pass of each tool has been depth matched to the first pass. The data has been shifted to the sea floor using the "bottom felt" depth given by the drillers (2396.5 mbrf).

Log Quality

The total magnetic induction (MAGB) is of good quality from 220 to 720 mbsf: above 220 mbsf NMRS data are bad because of the presence of magnetic material in the adjacent Hole 883E; below 720 mbsf, NMRS data are too spiky. The pipe effect is visible on the total induction signal down to about 350 mbsf and is corrected for; also, a linear drift of 0.07 nT/m is applied.

The magnetic susceptibility (MAGS) signal from 150 to 830 mbsf is of good quality.

Lithology

The processed SUMT/NMRS logs cross lithologic units II to IVA:

- Unit II (86.9-458 mbsf): diatom ooze

- Unit III (458-652.2 mbsf): calcareous diatom ooze

- Unit IVA (652.2-740 mbsf): nannofossil chalk.

Age information

The processed section is lower Pliocene (top of the section) to Eocene (bottom of the section) in age, as inferred onboard from biostratigraphic data. There is no magnetostratigraphic interpretation proposed from core below 100 mbsf.

Proposed interpretation of NMRT data

No comparison was possible with paleomagnetic analysis performed on core on board. Because of the uncertain depth-match between the NMRS and the SUMT, the proposed interpretation is preliminary. The proposed normal polarity chrons are located in the upper Miocene:

325-337 mbsf: chron C3An

400-410 mbsf: chron C4n

457-472 mbsf: chron C4An

527 552 mbsf: chron C5n

These events are in agreement with the age-depth relationship proposed on board from biostratigraphic results.

Additional information about the logs can be found in the "Explanatory Notes" and Site Chapter, ODP IR and SR volume 145. For further questions about the logs, please contact:

Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia