Geologic Magnetic Data Processing
ODP logging contractor: LDEO-BRG
Well name: 1166A
Leg: 188
Location: Prydz Bay (Antarctic Ocean)
Latitude: 67° 41.8' S
Longitude: 74° 47.2' E
Logging date: February 20-21, 2000
Bottom felt: 480 mbrf
Total penetration: 381.3 mbsf
Total core recovered: 18.6%
GHMT
Logging Runs
Two passes of the GHMT were recorded open
hole. Processing was performed on pass two, from 40 to 385 mbsf.
The Wireline Heave Compensator was used to
counter ship heave.
Hole
conditions
According to the FMS calipers, the hole size
varies from 11 to 19 inches. The raw susceptibility has been corrected for hole
diameter using an average value of 12 inches.
Depth
Shift
The GHMT logs were depth matched to the HNGS
from the DIT/APS/HLDS/HNGS run and then shifted to the sea floor (-480 m).
Log
Quality
The total magnetic induction (MAGB) is
generally of good quality except the presence of a few very locally spiky zones
that were removed manually. The pipe effect is visible on the total induction
signal down to about 120 m and is corrected as well as a linear drift of 0.13
nT/m.
The magnetic susceptibility signal is of good
quality except at 277 mbsf, where a spiky zone has been removed manually. Due
to local saturation of the MAGS signal, the RMGS signal was used instead.
Lithology
The GHMT crosses lithologic units Ib to V,
which consist of a succession of poorly sorted sands, fine-grained sediments,
and diamict.
Age
information
The processed section corresponds to
Quaternary (top of the section) to Early Cretaceous (bottom of the section)
periods, as inferred onboard from biostratigraphic data. There is a major
unconformity observed on cores at 135 mbsf, separating upper Pliocene sediment
above from lower Oligocene sediments below.
Because of the very poor core recovery, the
shipboard magnetostratigraphy obtained from core is very poor. Normal polarity
intervals are observed at 115 mbsf (C2An.1n), and between 126 and 160 mbsf
(C12n / C13n ?).
Proposed
interpretation of GHMT data
Because of the very poor core recovery and
the intervals barren of fossils, it is not possible to propose any precise
magnetostratigraphic interpretation from GHMT data. Nevertheless, the few
normal zones observed on cores are also observed on the GHMT data. The GHMT
data also indicates the presence of normal polarity zones between about 270 and
350 mbsf.
Additional information about the logs can be
found in the "Explanatory Notes" and Site Chapter, ODP IR volume 188.
For further questions about the logs, please contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia