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IODP Expedition 324:
Shatsky Rise Formation
Site U1346 Summary
PDF file is available for download.
29 September 2009
Site U1346 (Prospectus Site SRNH-2)
on Shirshov Massif was the first site to be drilled on Shatsky Rise Expedition
324. The site is located on the north rim of the summit of Shirshov Massif (or
"Northern High"), the northernmost large edifice of Shatsky Rise. Magnetic
lineations imply that Shatsky Rise formed from along the trace of a triple
junction (Sager et al., 1999; Nakanishi et al., 1999) and volcanism appears to have
followed the track of the triple junction (or vice versa), but the amount of
volcanism appears to have diminished over time. Shirshov Massif appears to be
one of two intermediate volcanic edifices between the initial eruptions of Tamu
Massif, on the southwest end of the rise, and the waning volcanic trail of
Papanin Ridge on the northeast end of the rise. Thus, Shirshov Massif is
important in the study of Shatsky Rise because it represents the transitional,
waning magmatic output of the rise volcanism. In the parlance of the plume head
hypothesis, Shirshov Massif is in the transition between plume head and tail.
After transit from Yokohama, Japan,
the JOIDES Resolution set position on
Site U1346 (Prospectus Site SRNH-2) at 0930 hr on 14 September and the rotary
core barrel drilling assembly was prepared. Seafloor was tagged at 3630.0 m DRF
and Hole U1346A was advanced without coring to a depth of 100.5 m CSF-A. Rotary
coring was initiated at a depth of 100.5 m and deepened the hole to a total
depth of 191.8 m by 0915 hr on 18 September. Basaltic basement was contacted at
a depth of 140 m (total penetration into basement at this site was 51.8 m). The
average penetration rate for basement coring was 1.3 m/hr with an average
recovery of 38.7%. Because of the slow penetration rate combined with the
successful recovery of material suitable to address most of the objectives for
this site, it was decided to end coring at this depth and prepare the hole for
logging operations. An additional factor in the decision to cease coring
operations at this site was the approach of Typhoon Choi-Wan. The hole was
logged with the triple combo on 18 September. The poor hole conditions,
however, did not warrant deployment of the FMS-sonic tool string. The vessel departed
to Prospectus Site SRSH-3B at 1800 hr on 19 September. Because of the proximity
of Sites SRCH-5 and SRSH-6 to the projected track of Typhoon Choi-Wan, it was
decided to proceed directly to the more southerly and higher priority site.
Since the primary goal of drilling
Hole U1346A was to recover the igneous basement (estimated to reside below ~140
m CFS-A) sediment coring began only ~70 m above the suspected basement/sediment
interface. Despite difficult drilling conditions, approximately 4 m of
sediments were recovered in the first six cores prior to entering into basaltic
basement. The recovered sedimentary material represents various lithologies and
depositional environments, with an average recovery of 10.2% between 100.5 and
141.7 m CSF-A. The uppermost sedimentary interval, Unit I, (Cores U1346A-1W to
-3R) yielded only small isolated pieces of dark-colored chert fragments.
Recovery in Cores U1346A-4R to -6R improved, due to a reduction in the amount
of chert in the formation. This short sequence of lithified sediments included
an intriguing sequence of intermingled basalt and limestone in Section
U1346A-4R-1, interpreted as a debris flow (Unit II). In this unit soft sediment deformation occurs around the larger volcanic
clasts indicating that the clasts impacted the sediment, either rolling down
slope from its eruption source or as a mass flow deposit generated through
post-eruptive erosion of the volcanic edifice. Section U1346A-4R-2,
yielded a series of laminated volcaniclastic sequences, with grading from very
coarse sand to clay, which are interpreted as turbiditic in origin (Unit III).
The remaining sediments (Unit IV), from the base of Section U1346A-4R-2 to the
top of Section U1346A-6R-1, are composed of clay-bearing limestones and
calcareous mudstones containing abundant shell fragments and other biogenic
components, along with glauconite and altered volcaniclastics. Taken together
these components are suggestive of a relatively shallow marine depositional
environment in close proximity to a volcanic material source.
Calcareous nannofossils in
the recovered sediments are rare to abundant, and moderately to poorly
preserved. The age of four samples from Cores U1346A-4R and -5R is assignable
to the Berriasian to Hauterivian biostratigraphic stages. Within the
foraminiferal assemblage obtained from Section U1346A-4R-CC, the planktonic
group is completely absent. Benthic foraminifera are well-preserved and
diverse, comprising the neriticupper bathyal assemblage (estimated paleodepth
²500 m). Various biogenic sedimentary components were observed in the samples
examined for biostratigraphic analyses; these were dominantly radiolaria with
lesser amounts of ostracoda, inoceramid prisms, echinoid plates, sponge
spicules, bryozoan, and carbonaceous fragments.
Basement coring at Site U1346 on Shirshov Massif, established that a
stack of highly vesicular basaltic pillow lavas or lava "inflation units"
(defined as stratigraphic Unit V) occurs beneath the succession of pelagic
nannofossil-bearing chalks and cherts, volcanogenic silts and sands, and larger
volcaniclastic debris materials. Within the lava stack, individual pillow (or
inflation) units were readily identified by the presence of chilled glassy
margins, upper and lower chill zones, characteristic pillow vesicle patterns,
and crystal grain size variations. In total 40 individual "lava cooling units"
were recognized in the recovered cores of Unit V and interpreted to represent a
single eruptive event.
The pillow basalts are generally vesicular in nature and have zones that
are moderately vesicular (30%-50% vesicles). Although they appear
macroscopically aphyric, a closer inspection reveals that micro-phenocrysts of
olivine and pyroxene were originally present but are now totally replaced by
calcite. All samples contain large proportions of less altered, very
fine-grained plagioclase laths set in a variolitic matrix.
Extensive low temperature
water-rock alteration has left a
marked impression on all igneous rocks recovered at this site, resulting
in near complete replacement of pyroxene, olivine and glassy pillow rinds and
complete replacement of glassy mesostasis. In contrast, plagioclase shows only
slight to moderate alteration. Based on rock color and mineralogy, three types
of alteration have been determined at Hole U1346A: a green alteration,
recovered only in volcaniclastic debris interspersed with sediment at the top
of the hole (Unit II; interval U1346A-4R-1, 0 cm to U1346A-4R-1, 39 cm), a dark
gray alteration and a brown alteration. In the basement pillow lavas, the dark
gray alteration is most abundant and is interspersed with the brown alteration
throughout the hole. The most abundant secondary minerals observed in the
basaltic rocks are clay minerals, and the nature of clay minerals changes with
respect to alteration type, from predominantly green clays, including
nontronite, in the green alteration, to green and brown clays in the gray
alteration, including saponite, to mainly brown clays in the brown alteration.
Calcite is also abundant in all alteration types, replacing pyroxene and
olivine, rarely the groundmass, and filling vesicles and veins. Sulfide
minerals are present only in Unit II and the upper portions of basement pillow
lavas in Sections U1346A-7R-1 and U1346A-8R-1. These mineral assemblages
suggest that the basaltic rocks from Hole U1346A have extensively interacted
with seawater-derived and CO2-rich fluids at low temperature, and
more locally with S-rich hydrothermal fluids.
Despite the moderate to complete
alteration, shipboard analysis of
major and several trace elements by inductively coupled plasma atomic
emission spectroscopy (ICP-AES) of lava samples recovered from lithologic Unit
V, reveals that the rocks are tholeiitic basalt. The concentrations of many elements,
including K, Si, Ca, P, Sr, Ba, and Ni, were modified significantly by the
alteration. However, several elements, including Ti, Zr, Y, Cr, V, and Sc,
appear to have been affected relatively little. Relationships among these
elements indicate a strong similarity with the approximately 5 m.y.
older basalts recovered from ODP Site 1213, which lies approximately 870 km to
the southwest of Site U1346.
There are two kinds of structures that can be distinguished within the
igneous complex penetrated at Hole 1346A; i.e. syn- and post-magmatic
structures. The main syn-magmatic structural features are represented by
amygdaloid structures, pillow structures, irregular vein networks or curved
veins, and breccias. Post-magmatic structures include conjugate veins and
joints. Dip angles of the veins in the hole from top to bottom become gradually
steeper; however, joint dips are generally low. The structures observed in the
entire Unit V are consistent with the interpretation as a pile of stacked
pillows, whose sizes differ from ~20-200 cm.
Attenuation (GRA) measurements of the basement sections reveal that there is
low variability in bulk density despite changes in the style and degree of
alteration. Magnetic susceptibility (MS) appeared to be a more sensitive tracer
of alteration, and several regions were identified where MS co-varied with
changes in alteration style. Thermal conductivity measurements averaged
1.52 W/(mK), with no significant relationship with depth.
P-wave velocity measurements of 24 discrete samples showed no
appreciable anisotropy or depth relationship and averaged 4.5 km/s throughout
the core with maximums of ~5.6 km/s and minimums of ~3.5 km/s. Moisture and
density measurements of bulk, dry, and grain density of the discrete samples confirmed
indications from the GRA density (measured on whole cores) that there was
little systematic change in the density of the basaltic sequence with depth.
Porosity was high and ranged from 8% to 33% with the highest values closer to
the top of the hole. As expected, P-wave velocity followed expected relations
with bulk density determined on discrete samples.
In total, 20 discrete samples
obtained from Hole U1346A were measured to investigate paleomagnetic remanence
of the upper part of the Shirshov Massif basement. Most of samples show a
stable component between 300475° C steps and have fairly low unblocking temperatures, around
400450°C, which is characteristic of titanomagnetite (-maghemite). AF
demagnetizations show that the magnetization is stable after 10-15 mT
demagnetization. These samples from Hole 1346A are characterized by shallow
negative inclinations (arithmetic mean = -20.3°±5.3°).
Downhole logging data obtained from
Hole U1346A included natural and spectral gamma ray, density, photoelectric
factor, and electrical resistivity measurements from three depths of
investigation. Interpretations of gamma ray and electrical resistivity downhole
logs were used to identify a total of 14 logging units in Hole U1346A with
three in the section covered by the bottom hole assembly (BHA), four in the
sedimentary sequences in the open hole interval, and seven in the basaltic
basement. The sedimentary sequence shows several prominent gamma ray anomalies
associated with uranium enrichment. The most prominent anomaly is found at the
sediment/basement interface and may be indicative of focused hydrothermal fluid
flow. Shallower anomalies recorded through the BHA may represent oceanic anoxic
events previously interpreted in this area. Electrical resistivity measurements
in the basaltic basement show four distinctive massive zones characterized by
higher resistivity values, which may represent individual thick lava flows.
Relatively high potassium content in the basement section also indicates a high
degree of hydrothermal alteration.
Sager, W. W., Kim, J., Klaus, K., Nakanishi, M., and Khankishieva, L. M.,
1999. Bathymetry of Shatsky Rise,
northwest Pacific Ocean: Implications for ocean plateau development at a triple
junction. J. Geophys. Res., 104:7557-7576.
Nakanishi, M., Sager, W. W., and Klaus, A., 1999. Magnetic lineations
within Shatsky Rise, northwest Pacific Ocean: Implications for hot spot-triple
junction interaction and oceanic plateau formation. J. Geophys. Res., 104:7539-7556.