ODP logging contractor: LDEO-BRG

Well name: 1084A

Leg: 175

Location: Cape Basin (tropical SE Atlantic)

Latitude: 25° 30.8345' S

Longitude: 13° 1.6668' E

Logging date: September, 1997

Bottom felt: 2003.5 mbrf (used for depth shift to sea floor)

Total penetration: 605 mbsf

Total core recovered: 511.6 m (91.4 %)

GHMT Logging Runs

Two passes (main and repeat) were recorded. Processing was performed on the main pass, open-hole section (58.8-599.3 mbsf).

Wireline heave compensator was used to counter ship heave.

Hole conditions

The hole diameter ranges from 10.5 to 15 inches above 150 mbsf. The raw susceptibility is corrected assuming a mean diameter of 12.5 inches.

Depth Shift

The GHMT logs have been interactively depth shifted with reference to NGT from APS/HLDS/HNGS run and to the sea floor (- 2003.5 m). The program used is an interactive, graphical depth-match program, which allows to visually correlate logs and to define appropriate shifts. The reference and match channels are displayed on the screen, with vectors connecting old (reference curve) and new (match curve) shift depths. The total gamma ray curve (SGR or HSGR) from the NGT or HNGS tools run on each logging string is used to correlate the logging runs most often. In general, the reference curve is chosen on the basis of constant, low cable tension and high cable speed (tools run at faster speeds are less likely to stick and are less susceptible to data degradation caused by ship heave). Other factors, however, such as the length of the logged interval, the presence of drill pipe, and the statistical quality of the collected data (better statistics is obtained at lower logging speeds) are also considered in the selection. A list of the amount of differential depth shifts applied at this hole is available upon request.

Log quality

The logging data have been corrected for the drill pipe effect visible on the total induction signal as well as for a linear drift of 0.05 nT/m.

Spikes are present in the total induction signal (MAGB) at 387, 510, 575, and 586 mbsf, which correspond to low values of the NMR outer voltage. Any correlation analysis is meaningless at these depths because the spikes create artifacts.

Lithology

The GHMT encompasses lithologic units I to IV:

- Unit IA (58.8-107.5 mbsf): nannofossil clay and nannofossil ooze

- Unit IB (107.5-325.9 mbsf): diatom-bearing clay to nannofossil-rich diatomaceous clay

- Unit IC (325.9-393.3 mbsf): diatomaceous clay

- Unit II (393.3-498.4 mbsf): clay-rich diatomaceous ooze to clay-rich nannofossil ooze

- Unit III (498.4-547.3 mbsf): nannofossil clay

- Unit IV (547.3-599.3 mbsf): nannofossil ooze

Age information

The logged section corresponds to biozones CN15 to CN11, as inferred onboard from nannofossil data.

Proposed interpretation of GHMT data

Comparison of the GHMT results with core paleomagnetic measurements allows one to determine the following magnetochrons:

- chron C1n (Brunhes) from 70 mbsf) to 144 mbsf

- chron C1r (Matayuma) from 144 to 430 mbsf with the Jaramillo normal zone between 190 and 203 mbsf, and the Olduvai normal zone (C2n) between 283 and 308 mbsf

- top of chron C2An (Gauss) at 430 mbsf.

The onset of chron C2An cannot be clearly identified from the GHMT data.

The polarity sequence determined from the GHMT data is in good agreement with the one determined from the core measurements.

Additional information about the logs can be found in the "Explanatory Notes" and Site Chapter, ODP IR volume 175. For further questions about the logs, please contact:

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