Geologic Magnetic Data Processing

ODP logging contractor: LDEO-BRG

Hole: 998-B

Leg: 165

Location: Cayman Rise (Caribbean Sea)

Latitude: 19° 29.387' N

Longitude: 82° 56.160' W

Logging date: December, 1995

Bottom felt: 3190.7 mbrf (used for depth shift to sea floor)

Total penetration: 904.8 mbsf

Total core recovered: 287.9 m (83.1 %)

GHMT Logging Runs

Two passes were recorded. Processing was performed on pass 1, open-hole section (203-425.3 mbsf).

Wireline heave compensator was used to counter ship heave.

Hole conditions

The hole diameter in the logged section varies from 12 to 14 inches. The raw susceptibility has been corrected for hole size variations using the FMS caliper.

Depth Shift

The GHMT logs have been interactively depth shifted with reference to NGT from DIT/SDT/HLDT/CNTG/NGT run and to the sea floor (- 3190.7 m). The program used is an interactive, graphical depth-match program that 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) from the NGT tool 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 repeatability of both magnetic susceptibility and total induction is excellent even if the level of both signals is weak. The pipe effect is visible on the total induction signal over its whole length (short record). There is a slight offset of the total induction (8 nT), between the 2 passes but the high-frequency contents are similar. ). This offset indicates the presence of a low-frequency transient component during logging. The high-frequency content of the second pass, however, correlates well with pass 1.

The total induction signal (MAGB) has been corrected for the presence of drill pipe.

Lithology

The logged section corresponds to lithologic unit III, which consists of nannofossil chalk mixed with varying amount of clays interbedded with foraminifers chalk, clay and altered volcanic ash layers. The weak magnetic signals are due to the presence of carbonate (more than 80%). Volcaniclastic ash layers are an important feature of Unit III. They occur in very thin to medium bedded layers and are derived from eolian transport of pyroclastic material from explosive eruptions. This material occurs as discrete layers, bioturbated patches, or dispersed ash in clay. Some ash was also reworked together with carbonates as turbidites layers. Numerous turbidite layers are present in the upper part of the logged section (from top to about 258 mbsf).

Age information

The age of the logged section is from late Oligocene to early to middle Miocene. Biostratigraphic studies on both nannofossils and foraminifers allow one to establish a clear zonation along the section despite the poor average core recovery (60%). From top to bottom, foraminiferal zones N9 to P22 and nannofossil biozones CN3-4 to CP19b can be inferred.

Proposed interpretation of GHMT data

The upper part of the logged section (from top to about 250 mbsf) mainly consists of turbidites; their presence affects the remanent signal and prevents from any interpretation. The following magnetochrons may be inferred:

- C6n down to 258 mbsf (the top is undetermined because of the presence of turbidites)

- C6A1n, C6An.2n and C6A.An around 262, 268, and 277 mbsf, respectively ;

- C6Bn from 289 to 307 mbsf ;

- C6Cn.1n, C6Cn.2n and C6Cn.3n around 327, 337, and 344 mbsf, respectively.

Below 344 mbsf the determination of magnetochrons is not clear : chrons C7n and C7An may be present at about 380 and 400 mbsf, respectively.

The top of chron C8n may be located at 410 mbsf.

Additional information about the logs can be found in the "Explanatory Notes" and Site Chapter, ODP IR volume 165. For further questions about the logs, please contact:

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