Overview
We present here a summary of the logging data
and some highlights for each site visited.
Because of hole instability in the shallower
sediments, wireline data are usually not
recorded in the upper ~80 m.
Site U1339
|
Figure 3.
Summary
of the logging data recorded in Hole
U1339D. |
The primary objective of drilling at Site U1339
was to study high-resolution paleoceanography in
the easternmost part of the Bering Sea. The
average core recovery in the four holes drilled
was 103%, ensuring a full record of the
deposition history and a complete integration
with the log data. The caliper log shows that
hole size rarely exceeded the nominal bit size,
indicating data of high quality (See Figure 3).
Comparison with the gamma ray and density
measurements made on the recovered core shows
that logs and core measurements are in good
aggreement, allowing for reliable core log
integration. Some trends can be detected in the
logs that express the changes in the
sedimentation history at this site, such as the
parallel decrease with depth in gamma ray,
density and resistivity from 86 to 102 WMSF,
which is typical of a retrograding fining upward
sequence. Similar sequences, although less
clearly defined, seem to define the general
trend of the logs down the hole. The FMS images
illustrate in particular the steep dip to the
West of the deepest strata recovered in this
site.
Site U1341
|
Figure 4.
Summary
of the logging data recorded in Hole
U1341B. |
The primary objective of drilling at Site U1341
was to study high-resolution paleoceanography in
the southern part of the Bering Sea, on the
western flank of Bowers Ridge. The average core
recovery in the three holes drilled was 102%,
ensuring a full record of the deposition history
and a complete integration with the log data.
The caliper logs recorded by the density sonde
and the FMS in Hole U1341B indicated that the
borehole was irregular and significantly larger
than the bit size, but that the tools maintained
contact with the formation over most of the
interval logged, ensuring the overall good
quality of the data (See Figure 4).
The combined analysis of the logs allows to
define four logging units characterized by
specific trends. Logging unit 1 (80-220 m WMSF)
is characterized mainly by decreasing trends
with depth in gamma ray and resistivity,
interrupted by several highs indicative of
authigenic carbonate. This unit coincides mostly
with Lithologic Unit I, made of diatom ooze and
diatom silt. Logging Unit 2 (220-350 mbsf) is
defined by slightly increasing trends with depth
in gamma ray and density, while resistivity
mostly decreases. Several peaks in gamma ray can
also be observed in this unit, generally due to
higher uranium content and associated with
authigenic carbonate observed in the core.
The top of Logging Unit 3 (350-425 m WMSF) is
defined by a sharp drop in density at ~350 m
WSMF, and by similar changes in gamma ray and
resistivity likely associated with the strong
reflector that can be observed in Seismic Line
Stk5-1 at 3340 ms two-way travel times. This
reflector can be reproduced, although with a
lesser amplitude, by a synthetic seismogram
generated from the density and Vp logs. This
Logging Unit is characterized by generally
higher uranium values that can be associated
with a higher higher Total Organic Carbon (TOC)
measured on core samples from this interval.
The top of Logging unit 4 (425-600 m WMSF) is
defined by a drop in resistivity, followed by a
decrease with depth over the entire unit. Gamma
ray values are generally lower than in the
overlying logging unit and display several peaks
due to higher uranium content associated with
subtle resistivity peaks and the occurence of
authigenic carbonate and dolostones.
Site U1343
|
Figure 5. Summary of the logging
data recorded in Hole
U1343E.
|
Site U1343 was drilled on a topographic high
near the Bering shelf to study high-resolution
Pliocene-Pleistocene paleoceanography at a
location proximal to the gateway to the Arctic
Ocean. Its proximity to the current seasonal sea
ice limit and the 2 km water depth were also
likely to provide information regarding the
history of mid-depth water in the Aleutian
Basin. The average core recovery in the five
holes drilled was 96.5%, ensuring a full record
of the deposition history and a complete
integration with the log data. The caliper log
in Hole U1343E shows that the borehole was
irregular and significantly larger than the bit
size above ~430 m WMSF, but that the tools
maintained contact with the formation over most
of the interval logged, ensuring the overall
good quality of the data (See Figure 5).
The combined analysis of the different logs
allows to identify three logging units. Due to
the uniformity of the sediments at this site,
these units are mostly defined by changes in
trends and correlations rather than indications
of significant changes in the formation. As the
Vp log also allows correlations with the seismic
stratigraphy, it was the primary guide in the
delimitation of the units.
Logging Unit 1 (100-330 m WMSF) is
characterized mainly by a steady increase with
depth in velocity, while the other log data
remain mostly uniform despite some variability
in the gamma ray. The velocity increase at the
bottom of logging unit 1 is likely responsible
for the strong reflector that can be observed at
2.86 s two-way travel times in Seismic Line
Stk-1 crossing site U1343. While it was
speculated that this reflector might indicate
the existence of gas hydrate overlying free gas,
the possible indications from the logs
supporting the occurrence of gas hydrate were
not conclusive. However, slightly higher
velocity and resistivity trends and lower dipole
waveform amplitudes above the reflector, as well
a lower chlorinity values measured on several
pore water samples, suggest that some amount of
gas hydrate might be present.
Logging Unit 2 (330-510 m WMSF) is defined by
slightly decreasing trends with depth in
resistivity, and by gamma ray values slightly
higher than the shallower and deeper units. The
top of Logging Unit 3 (510-745 m WMSF) is
defined by a drop in gamma ray, an increase in
Vp and a general change in the trends of all the
logs. Below this depth, all logs display a
variability with depth of wider amplitude and
lower frequency than in the upper units,
suggesting a significant change in the
deposition history and rates.
Site U1344
|
Figure 6. Summary of the logging
data recorded in Hole
U1344A.
|
The objective of drilling at Site U1344 was to
study high-resolution Pliocene-Pleistocene
paleoceanography at a proximal gateway location
to the Arctic Ocean at the deepest water depth
of Expedition 323. The average core recovery in
the five holes drilled was 92%, but only 87% in
Hole U1344A, where significant gaps in recovery
occurred between ~615 and 655 CSF. The caliper
log in Hole U1344A indicated that the borehole
was irregular and significantly larger than the
bit size above ~300 m WMSF, but the generally
good agreement between the gamma ray and density
logs and the MAD and GRA core measurements
suggest that the data are of good quality (See Figure 6).
The combined analysis of the logs allows the
identification of four logging units. Due to the
uniformity of the sediments at this site, these
units are also mostly defined by changes in
trends and correlations rather than indications
of significant changes in the formation. As the
Vp log allows correlations with the seismic
stratigraphy, it was the primary guide in the
delineation of the units.
Logging Unit 1 (100-330 m WMSF) is
characterized mainly by a steady increase with
depth in Vp and Vs, with some variability in the
gamma ray suggesting some sandy intervals (e.g.,
140-150 m WMSF). The bottom of this unit is
defined by a noticeable drop in Vp, Vs, gamma
ray, density and resistivity, immediately above
a sharp peak in these measurements, particularly
in Vs and resistivity, indicating a fine stiff
layer that was apparently nor recovered. The low
gamma ray suggests a more sandy layer, often
prone to poor recovery. Using the preliminary Vp
and density logs to generate a synthetic
seismogram, we were able to match this interval
with a strong reflector at 4.67 s two-way travel
times (twtt) on Seismic line Stk 3-7 crossing
Site U1344.
Logging Unit 2 (330-460 m WMSF) is almost
uniquely defined by the Vp and Vs log, both
increasing steadily through the unit. A drop in
Vp and Vs at 380 m WMSF can be correlated with a
reflector at 4.73 s twtt which can be extended
updip to high amplitudes that have been
interpreted as free gas trapped at the bottom of
the gas hydrate stability field. As in Site
U1343, there were no conclusive indications from
the logs to support the occurrence of gas
hydrate. However, slightly higher velocity and
resistivity trends and lower dipole waveform
amplitudes above the reflector, as well as lower
chlorinity values measured on several pore water
samples, suggest that some amount of gas hydrate
might be present.
The top of Logging Unit 3 (460-620 m WMSF) is
defined by an inflection in the velocity
profiles which, combined with a decreasing trend
in density, generates the reflector at 4.83 s
twtt. The variability with depth in gamma ray
and in most logs displays a cyclicity more
clearly defined than in the upper units.
Finally, the top of Logging Unit 4 (620-745 m
WMSF) is defined by a sharp increase in Vp, Vs,
gamma ray and density, as well as a significant
change in the trends of all the logs. As in the
deepest unit of Site U1343, mot logs display a
variability with depth of wider amplitude and
lower frequency than in the upper units,
suggesting a significant change in the
deposition history and rates.