| IODP
Expedition 304-305: |
Oceanic Core Complex Formation, Atlantis Massif
Expedition
304 and 305 Scientific Parties |
| Introduction |
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Figure
1. (a) Tectonic
and morphologic setting of Atlantis Massif.(b)
Basemap of Atlantis Massif showing prior geological
and geophysical data coverage and the location
of IODP drill sites
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The principle objective of Expeditions 304 and 305 was to
determine the conditions under which oceanic core complexes
develop. A total of 3 sites were drilled, two in the hanging
wall (U1310 and 1311) and one in the footwall (U1309) of
a major detachment fault system. The deepest hole, Hole U1309D
is located on the central dome of Atlantis Massif, 15 km
west of the median valley axis of the Mid-Atlantic Ridge,
where the seafloor coincides with a gently sloping, corrugated
detachment fault surface (Figure 1).
Two drill holes at this site (U1309B and U1309D) penetrate
a multiply-intruded crustal section. During Expedition 304,
Hole U1309B (101.8 mbsf) was drilled and Hole U1309D was
spudded using a hammer drill with casing, to provide stable
reentry for a deep hole. Hole U1309D was cored 401.3 mbsf
with excellent recovery. During Expedition 305, Hole U1309D
was deepened to a final depth of 1415 mbsf. It mainly comprises
gabbroic rocks ranging from troctolite, olivine gabbro, gabbro
and gabbronorite to oxide gabbro. In addition, several ultramafic
intervals were recovered in sections ranging from 1 to 20
m thick at various depths between 60 mbsf and 1240 mbsf.
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Logging Operations |
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Figure
2. Detail of the logging operations in U1309B
and U1309D. Red lines indicate Expedition 304 runs,
blue and green lines Expedition 305 runs.
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Wireline logging operations were carried out in two boreholes
(Figure 2). Depths are shown in meters
below seafloor (mbsf). Although Hole U1309B is of shallow
depth it was logged (21.6 - 95 mbsf) to image the structural
variation. Hole U1309D was logged in three separate stages
(covering in total 54 - 1415 mbsf).
Hole U1309B (Expedition 304)
(1) The triple
combo (HNGS [Hostile Environmental Gamma Ray Sonde], APS
[Accelerator Porosity Sonde], HLDT [Hostile Environmental
Lithodensity Sonde], DLL [Dual Laterolog], TAP [Temperaturea/Acceleration/Pressure tool]) tool string was lowered down
to 94.9 mbsf without any problems. Two complete passes
were recorded from open hole up to the seafloor. (2) The
FMS/Sonic (SGT [Scintillation Gamma Ray tool], DSI [Dipole
Sonic Imager], FMS [Formation MicroScanner]) tool string
was lowered to 95.1 mbsf for two passes. (3) A third run
was devoted to the heave compensator tuning with a short
tool string (GPIT [General Purpose Inclinometry Tool],
and DLL insulating tube).
Hole U1309D
First stage, Interval 54-400 mbsf (Expedition
304)
(1) The triple combo (HNGS, APS, HLDT, DLL) tool string
was lowered to 400 mbsf. Tight spots were encountered at
74, 79, 96 mbsf during the first run. A short repeat was
made at the base of the hole. The second tool string was
the FMS/Sonic. No problems were encountered for reaching
the bottom of the hole and two logging passes were accomplished.
After a period when the Schlumberger heave compensator was
being tuned in the open hole, the tool string became stuck
while entering the pipe and it took approximately 30 minutes
to get it free. Any further attempts to log the hole were
cancelled.
Second stage 400-836 mbsf (Expedition 305)
A total of five tool strings were successfully deployed
to the bottom of the hole at 836 mbsf The pipe was set at
170 mbsf to avoid an interval with bad borehole conditions.
(1) Triple combo (HNGS, APS, HLDT, DLL, TAP). Two passes
were made and excellent data recorded, covering the interval
between 836.5-170 mbsf. However, the TAP failed and no data
were recorded. (2) FMS/Sonic (SGT, DSI, FMS). Two passes
were recorded with the first pass covering the interval from
836.4 to 350 mbsf and the second pass logging the entire
open hole up to the pipe. (3) SGT/UBI (Ultrasonic Borehole
Imager). A short first pass was completed from 824 to 724
mbsf to acquire high-resolution images at a speed of 400ft/h.
The tool string was lowered again to make the full main pass
at normal speed (~800 ft/h) but no reasonable results were
acquired because the software could not find a consistent
signal to define the travel time window. Consequently, only
a depth interval of particular interest and good borehole
conditions (between 700 and 500 mbsf) was logged at the slow
speed. (4) WST-3 (Well Seismic Tool, three components) Following
the IODP marine mammal protocol, the WST-3 was lowered. Nine
stations obtained viable interval velocities, seven of which
were in line with sonic velocities. The other two stations
were adjacent to each other and one gave a high Interval
velocity (>7.5 km/s) and the other a somewhat low value
(5.0 km/s), relative to the corresponding sonic measurement.
(5) Third-party magnetometer (GBM, Goettingen Borehole Magnetometer).
The tool was initialized, taken to the rig floor, connected
to the wireline and oriented along the ship-axes. Down- and
up-going passes were recorded in real-time without problems.
Third stage 836-1414.5 mbsf (Expedition 305)
(1) Triple combo (HNGS, APS, HLDT, DLL, TAP). The first
pass covered the interval from the bottom of the hole at
1415 mbsf to the pipe (194 mbsf). For data quality check
a short repeat pass was run in an interval of low core recovery
(1270-1096 mbsf). (2) FMS/Sonic (SGT, DSI, FMS). The FMS/sonic
tool was lowered to the bottom of the hole, but telemetry
problems with the lower part of the tool were encountered.
A broken isolation joint between transmitter and receiver
section in the DSI was identified when the tool string was
pulled back to the rig-floor, and the DSI was subsequently
removed. The remaining SGT, GPIT, and FMS tools were lowered
back into the hole. A successful first pass was recorded
from TD to 734 mbsf and a second pass was run from TD to
629 mbsf. (3) The WST-3 failed after reaching the bottom
of the hole and it was replaced by the WST-1. During the
WST-1 descent, weather conditions deteriorated and logging
operations were terminated. The third-party GBM magnetometer
tool was not deployed because the borehole temperatures (>80°C)
were above the safe operating range of the instument electronics.
Results
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Figure
3. Results of selected logging measurements from
Hole U1309B and their correlation with discrete core
measurements. |
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| Figures
4a, b and c. Results of
selected logging measurements from Hole U1309D. |
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| Figures
5. Example of the excellent borehole wall coverage
by the FMS passes.
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Figures
6. Detailed FMS and
UBI image displaying A: the transition from a patchy
looking coarse-grained olivine gabbro to an olivine
gabbro, and B: a steep fracture indicated by low
resistivity (dark)
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| Figures
7. Detailed Formation MicroScanner (FMS) image
displaying an oxide-rich layer (192-195 mbsf). |
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| Figures
8. Dips measured in Holes U1309B and D between
50 and 400 mbsf, and 400 to 830 mbsf. |
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| Figures
9. Comparison of GBM and GPIT vertical components. |
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| Figures
10. Temperature profile recorded by the TAP tool
while final stage logging (Legend, see figure 4a). |
Overall, the logging data expand upon core-basedobservations
and provide in situ measurements at Site 1301. The triple
combo was the only tool string that provided reliable
data of the entire borehole revealing ideal places for Packer
experiments and allowing for the interpretation of several
logging units that correlated to physical and lithological
changes identified from core-based observations. The
downhole coverage obtained with the other tool string deployments
consisted of only 1/4 of the borehole’s total
depth because of a borehole obstruction. The VSP experiment
obtained the best results of any subsequent tool deployment
allowing for the estimation of the shallow basement velocity
profile.
The logging data reflect the overall variability of the
drilled lithologies comprising diverse kinds of gabbroic
rocks, diabase, and dunitic troctolites. Figures 3 and 4
present the results of selected logging measurements from
Holes U1309B and D. In the gabbroic rock intervals, log bulk
density varies between 2.8 and 3.2 g/cm3, resistivity ranges
from 50 to 2000 Ohm.m. In general, the PEF is below 4 barns/e-
and it averages around 3.1 barns/e-. The compressional velocity
log ranges between 5.5 and 6.5 km/s. Most intervals of oxide
gabbro, as identified in the visual core descriptions, can
be recognized in the logging data. They are generally characterized
by elevated values of density (3.0-3.2 g/cm3), PEF (4-8 barns/e-),
Sigma (>30 cu) and low electrical resistivity (<100
Ohm.m).
Logging data also reflect structural changes and alteration
modes. Structural features like discrete, open faults and
fracture zones are portrayed by enlarged borehole diameter
(> 11 in), which causes sudden apparent drops in density
(1.5-2 g/cm3), resistivity (10-50 Ohm.m), and velocity (4-5
km/s) and an increase in neutron porosity. FMS images show
structural variations as well as textural variations of gabbroic
rocks. In most intervals the coverage of the borehole wall
by the FMS is excellent and is in limited intervals complemented
by the UBI images (Figures 2, 5).
FMS sections with patchy appearance correspond to intervals
of coarse-grained gabbro (resistive patches) (Figure
6) or oxide-rich gabbro (conductive patches) (Figure
7).
There is not only a good correlation of logging data with
cataclasis and vein occurrence but also with alteration intensity.
Alteration most strongly affects the neutron porosity. Most
olivine-rich rocks, such as troctolite, dunitic troctolite
or olivine gabbro show high levels of serpentinization and
they contain more structurally bound H2O than olivine-poor
gabbros. Based on this relation, intervals with neutron porosities
of less than 5% as the least altered gabbro. In concert with
low neutron porosity are high resistivities (>500 Ohm.m).
The dunitic troctolites at 689-691, 1092-1170 and 1185-1195
mbsf are highly altered and chemical analyses on core samples
indicate H2O contents of around 8%. For these intervals,
neutron porosity is on average 20% and electrical resistivity
decreases to below 100 Ohm.m. In Hole U1309B, within the
interval 57.6 to 61.5 mbsf, high porosity values correspond
to interval where serpentinized peridotite was recovered.
High neutron porosity in this particular interval could be
explained by the high content of bound water in the serpentine
minerals (10% H2O).
The continuous structural information gained from the FMS
images with respect to dip and azimuth of conductive fractures
is a crucial contribution to the understanding of the tectonic
evolution of the Atlantis Massif. Structural analyses of
FMS images indicates a change in direction of the dominant
azimuth for conductive features from the upper 400 m to the
lower depth interval between 400-800 mbsf (Figure
8). The dominant azimuth changes from preferentially
southeast dipping structures to a combination of north dipping
shallow structures and south dipping steep structures.
Magnetic field intensity and direction were recorded by
the GBM and GPIT (Figure 9). The
vertical field component z shows a high level of repeatability
for the downhole and uphole logs. In addition to the GBM
fluxgate sensors, the angular rate of the GBM tool around
the x, y, and z spin axes was measured using three fiber
optic gyros. Rotation data will be used for reorientation
of the magnetic data, the processing is still in progress.
During the final logging run the temperature of Hole U1309D
was recorded using the TAP tool. Log curves show a slight
change in the temperature gradient below the 375 mbsf, at
720 mbsf, and 1100 mbsf (Figure 10).
These changes are recorded in each pass. The depth intervals
coincide with changes in lithology (occurrence of dunitic
troctolites) or structural features (fault zone). The maximum
recorded borehole temperature is 118.9°C at 1415 mbsf;
it is a minimum temperature as the borehole fluid was not
in full equilibrium so shortly after the drilling operation
had finished.
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Florence Einaudi: Logging Staff Scientist, Expedition
304:, LGHF, Université de Montpellier II, France
Email: Florence Einaudi
Heike Delius : Logging Staff Scientist, Expedition
305:, Department of Geology, University of Leicester, United
Kingdom
Email: Heike Delius
Margarete Linek: Logging Trainee, Angewandte Geophysik
Rheinisch-Westfälischen Technischen Hochschule, Aachen,
Germany
Email: Margarete Linek
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Additional Expedition-Related
Publications:
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