Geologic Magnetoc Data Processing

ODP logging contractor: LDEO-BRG

Hole: 1050C

Leg: 171B

Location: Blake Nose (NW Atlantic Ocean)

Latitude: 30° 5.995' N

Longitude: 76° 14.099' W

Logging date: February, 1997

Bottom felt: 2308 mbrf

Total penetration: 606 mbsf

Total core recovered: 200.2 m (69.4 %)

GHMT Logging Runs

Two passes (main and repeat) were recorded. Processing was performed on the main pass, open-hole section (109.8-598.4 mbsf).

Wireline heave compensator was not available on Leg 171B. Sea conditions were moderate, with sea swells in the order of 1.5 m.

Hole conditions

The FMS calipers show a slightly elliptical hole from the top of the logged section to 300 mbsf. The raw susceptibility has been corrected for hole diameter variations using the FMS caliper.

Depth shift

The GHMT logs have been interactively depth shifted with reference to HNGS from DIT/HLDS/APS/HNGS run and to the sea floor (- 2303 m). The amount of depth shift to the sea floor corresponds to the water depth as seen on the logs and differs 5 m from the drillers' "bottom felt" depth. 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 HNGS/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 total induction signal (MAGB) has been corrected for the presence of drill pipe. Numerous spikes are present on the total induction signal and are correlated to low values of the NMRT outer voltage.

Lithology

The GHMT was recorded through lithologic units II to VB (see Holes 1050A-B-C):

- Unit II (327.1-343.48 to mbsf): Clayey siliceous chalk with nannofossils and chert (early Paleocene).

- Unit IIB (Hole 1050A: top of the logged section to 304.9 mbsf): Siliceous nannofossil chalk (late Paleocene to early Eocene).

- Unit III (Hole 1050A: 304.9-320 mbsf): Diatomaceous nannofossils (late Paleocene).

- Unit III (343.4-491.4 mbsf): Nannofossil claystone (early Paleocene to Late Campanian).

- Unit IV (491.39-501.7 mbsf): Hard grounds and sediments (late Campanian to Turonian).

- Unit VA (501.7-553.94 mbsf): Nannofossil chalk or limestone with variable amounts of clay and claystone with variable amounts of nannofossils (early Cenomanian).

- Unit VB (501.7 mbsf to bottom of the logged section): Nannofossil claystone, claystone with nannofossils, calcareous claystone with nannofossils and foraminifers, and nannofossil claystone with organic debris and feldspar (early Cenomanian to late Albian).

Age information

The logged section corresponds to biozones CP13a to CC9a, as inferred onboard from nannofossil data.

Proposed interpretation of GHMT data

Paleomagnetic chrons C24n to C34n have been determined from comparison with core measurements of the paleo-inclination made on board.

The numerous spikes present in the data recorded in the lower part of the hole (below 405 mbsf) lead to artifacts in the computation of the correlation slope.

Additional information about the logs can be found in the "Explanatory Notes" and Site Chapter, ODP IR volume 171B. For further questions about the logs, please contact:

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