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IODP Expedition 324:
Shatsky Rise Formation
Week 4 Report (27 September 3 October 2009)
PDF file is available for download.
4 October 2009
exchanging the drill bit, Hole U1347A was successfully reentered at 2250 hr on
26 September. The drill string was advanced to the bottom of the hole where the
driller found only one meter of soft fill. From 1530 hr on 27 September to 1230
hr on 30 September rotary coring deepened the hole from 242.7 m CSF-A to a
final depth of 317.5 m CSF-A with no hole problems. Operations were concluded
when the last core was only able to advance one meter in three hours, possibly
due to either a worn bit, an extremely hard formation, or a combination of
total penetration into basement in Hole U1347A was 159.5 m cored at an average
rate of penetration (ROP) of 1.5 m/hr. While coring basement, the ROP for
individual cores ranged from a lethargic 0.7 m/hr to a more energetic 4.1
m/hr. The average recovery for basement coring was 62.3%.
was prepared for logging, the bit was released at the bottom and the bore was
displaced with 83.5 barrels of 10.5 ppg mud. The drill string was pulled back
in the hole and placed at the logging depth of 131.5 m DSF. The first logging
run was made with the triple combo and succeeded in reaching within two meters
of the bottom of the hole. A preliminary analysis of the results of the first
logging run indicated that the hole was in good condition and suitable for the
additional measurement runs planned for this site. The second tool deployed was
the FMS-sonic, which was also successfully run (two complete passes). The third
logging tool suite included the ultrasonic borehole imager and had to be
cancelled because of hardware problems.
the logging equipment was secured, the drill string and the beacon were
retrieved in routine fashion. Once the drilling equipment was secured, the
vessel departed for the next site (SRSH-6) at 0400 hr on 2 October. The total
time on Site U1347 was 10.4 days.
vessel was positioning on the new site at 1515 hr on 2 October after a voyage
of 115 nm at an average speed of 10.8 knots. The 4-stand RCB bottom hole
assembly was quickly deployed with a new C-4 bit and mechanical bit release.
With the bit at a depth of 1716 m DRF, the pipe trip in had to be suspended for
12.5 hours to repair the low-speed clutch on the drawworks. By 0830 hr on 3
October, the tripping of the drill string was resumed.
the driller tagged seafloor at 3275.0 m DRF, Hole U1348A was spudded at 1245 hr
on 3 October. The hole was washed ahead with a wash barrel in place to 84.2 m
DSF where rotary coring was initiated.
the bit change operation, nine additional cores (324-U1347A-21R to -29R) were
successfully retrieved from Hole U1347A. These cores are predominantly composed
of igneous rocks, a mixture of pillow basalts and massive basaltic flows. A
total of ~50 cm of small sedimentary interbeds, ranging from ~5 to 10 cm in
thickness, were recovered in Cores U1347A-21R, -22R, -24R and -26R. These
interbeds suggest the possibility of hiatuses in the emplacement of igneous
units, during which sedimentation of mixed volcaniclastics and nearshore marine
material could occur. The small intervals of interbedded sediment consist
predominantly of altered radiolarian-bearing, sandy siltstones with a minor
calcitic component and exhibit a strong volcanogenic influence. Many of these
sediments, however, are highly altered and show signs of compaction, fritting,
and baking where they had presumably come into contact with still molten
basalt. One piece of altered volcaniclastic limestone was recovered in Core
324-U1347A-22R, indicating a possible increase in the amount of carbonate
relative to detrital volcanic material during this interval.
investigation of the calcareous nannofossil assemblage suggests that the
sediments above the igneous basement (Cores U1347A-2R to -11R) are ascribed to
the Berriasian to Valanginian stages. For benthic foraminifera, a single
specimen of taxon indicative of the neritic setting is recognized in Core
U1347A-8R. The thorough micropaleontological survey has been extended to the
several fine-grained sedimentary layers interbedded in the underlying basement
basaltic units, yet both calcareous nannofossil and foraminfer taxa are nearly
barren in these intervals.
The volcanic succession of the basement can be
described in terms of three broad groups based upon volcanic characteristics
and associations. Group (1): an uppermost volcanic succession consisting of
four major basalt lava units (~8 - 19 m thick); Group (2): a more complex
intervening ~75 m volcanic stack consisting of a mixture of pillow units and
larger (typically 1-2 m thick) inflation units, which themselves are
interspersed with the up to ~5 m thick sedimentary intercalations (described
above) and instances of much thicker homogenous flows 3 to 6 m in thickness;
and Group (3): a lower set of two, particularly massive lava flows consisting
of a very thick (~23 m) homogenous flow overlying a unit of similar character
at the bottom of the hole (the last Core U1347A-29R recovered only 1.6 m
from its top). In many instances, the high recovery rate for these volcanic
units yielded well-preserved lower- and upper-contact zones (i.e. glass,
chilled margins, baked sediment contacts, rubbly and/or pahoehoe-type flow
tops), or else the intercalated sediment intervals.
Overall, the 158.5 m volcanic section consists
of plagioclase-pyroxene phyric basalts, whereby the plagioclase and
clinopyroxene cotectically occur as clusters, glomerocrysts and intergrowns.
Olivine phenocrysts occur in the basalt, but always as a trace and typically
completely replaced by clays and calcite. The frequent recovery of (fresh)
glassy rinds from the pillow-unit stack, as well as delicate pillow-pillow
contact and pahoehoe-type structures indicates that alteration was efficiently
buffered in these rocks.
Based on the visual description and on
petrographic observation of thin sections, overall alteration of the basaltic
lavas ranges from slight to, at worst, moderate (from 5% to 50%), with the
majority of the rocks showing ~15% alteration. Generally both the primary mineralogy (with the
exception of extensive olivine replacement) and the finer spherulitic textures
in the interstices between phenocrysts and microcrysts are well preserved. Plagioclase and clinopyroxene are
relatively fresh throughout the hole, either in the groundmass or as
phenocrysts. In contrast, both olivine and groundmass glass are highly altered
to brown clays and calcite (in various proportions). Fresh glass was observed
in some slightly altered basaltic samples, from Section 324-U1347A-15R-2 (187 m
CSF-A) downhole. Fresh glass and plagioclase and pyroxene phenocrysts are also
commonly well preserved at pillow margins, whereas olivine phenocrysts are always
completely altered. Vesicles are filled with very fine-grained clays and
calcite and traces of pyrite.
Structural descriptions of
recovered basement cores continued from last week. Most of the igneous rocks
show the characteristic features for massive lava flows. In general, the
chilled margins between the flow unites and interbedded sediment intervals are sub-horizontal.
Compared to the upper part of the basement, the number of joints increase and
the dip angles steepen in the lower part of Hole U1347A. Veins are widespread along the cores, with an
average of 3 veins/m over the ~160 m of basement lavas, and are predominantly
filled with calcite with or without fine-grained green clays.
The physical properties of the lower igneous
basement cored after the drill bit change are similar to those preceding
it. Magnetic susceptibility remained high (generally over 2500 x 105
SI and ranging up to 3800 x 105 SI in massive flows) and total
counts from the natural gamma ray logger remained low (2-4 cps). Massive
flows in igneous sections gave higher gamma ray attenuation (GRA) densities
compared to smaller pillow lava units. Filtering of whole round (GRA density
and MS) data for Hole U1347A was completed by the end of this week.
Fifty-nine thermal conductivity measurements of
igneous material yielded values from 1.402 W/mK to 1.801 W/mK, which is
slightly higher than found at previous Site U1346. The thermal conductivity of
the ~23 m thick massive flow unit beginning at ~290 m CSF-A is consistently higher
than the rest of the hole, averaging 1.733 ± 0.092 W/mK (n=7). A single sample
of sedimentary material from Section 324-U1347A-17R-1 yielded a lower thermal
conductivity of 1.007 W/mK.
Fifty-seven discrete samples were taken for
measurement of moisture and density characteristics and compressional wave
velocity in three directions. Forty-four have been processed and the remaining
thirteen will complete their circuit within the next day. Bulk densities range
from 2.49 to 2.88 g/cm3, which is higher than seen at Hole U1346A.
The massive igneous flows are distinguishable by their elevated P-wave
velocities (up to 6.982 km/s in the downhole direction), which correlate well
with high density and low porosity (3-5%).
this week, a total of 63 cubic samples were paleomagnetically measured using
alternating field and thermal demagnetization. About one third underwent
alternating field (AF) demagnetization while the other two-third was used for
thermal demagnetization. AF demagnetizations showed evidence of a large
overprint, which was erased around 20 mT, after which it was usually possible
to isolate a stable characteristic remanent magnetization. However, most
samples are characterized by an extremely low coercivity (median destructive
fields as low as 2 mT). Many samples show erratic behavior in thermal
demagnetization steps, mainly because of thermochemical alteration that takes
place at >300°C. According to the measurements carried out so far (about
2/3), samples taken at the top of the hole have a negative shallow inclination,
while the rest of the samples have a positive inclination.
The geochemistry group completed ICP-AES
analysis of 21 lava samples from Site U1347 during this week. Data reduction
and interpretation commenced. Processing of another batch of 19 lava samples
for ICP-AES analysis is underway. Additionally, thirty-seven sedimentary
samples from Site U1347 were analyzed for carbonate and for total carbon.
The week was concluded by logging operations in Hole U1347 with two successful tool deployments.
The triple combo toolstring was run to within two meters of total depth and
collected good density, natural gamma, porosity and resistivity data from the
formation including across the sediment-basement interface. The second
deployment was the FMS-sonic toolstring, which completed two passes to within
two meters of total depth. Shipboard processing show good FMS images in the
basement section. The third planned deployment at the site (UBI) had to be
abandoned after the toolstring failed to activate fully.
Technical Support and HSE Activities
are busy processing samples. A new overhead safety shower was installed at the
hazardous stores lockers. Site U1348 was occupied after a short transit at the
end of this week.