SITE 1149 PRINCIPAL RESULTS
Site 1149: Early Cretaceous Seafloor Subducting at the Izu Bonin Trench
Hole 1149A
Latitude: 31°20.519'N
Longitude: 143°21.078'E
Water depth (m): 5818
Total cored section (mbsf): 0-191.2
Core recovery (%): 91
Hole 1149B
Latitude: 31°20.532'N
Longitude: 143°21.060'E
Water depth (m): 5818
Total cored section (mbsf): 160.6-445.2
Core recovery (%): 12
Hole 1149C
Latitude: 31°20.550'N
Longitude: 143°21.060'E
Water depth (m): 5818
Total cored section (mbsf): 283.6-322 and 388.2-426.7
Core recovery (%): 8.3
Hole 1149D
Latitude: 31°18.792'N
Longitude: 143°24.024'E
Water depth (m): 5867
Total cored section (mbsf): 272.2-440.4
Core recovery (%): 17
Introduction
Although previous drilling was plagued by hole stability problems in
chert horizons, Site 1149 was the first to be continuously cored through
sediment to basement in the Nadezhda Basin, a ~1000 km x 1000 km
region seaward of the Izu-Bonin Trench. The sediment/basement
contact was cored in three Holes (1149B, 1149C, and 1149D), and a
total of 133 m of basement penetration was achieved in Hole 1149D.
Thus, the site objectives to core and log the entire sedimentary
sequence and to core the upper oxidative alteration zone in
basement were successfully carried out after 20 days of operation.
A presite seismic survey produced an excellent seismic
stratigraphy that will enable correlation of sedimentary units at
Site 1149 across the Nadezhda Basin and along the Izu-Bonin
Trench.
APC coring recovered ~100% of the uppermost 160 m in Hole 1149A,
whereas a combination of XCB, MDCB, and RCB systems completed
coring in the 410-m-thick sedimentary section in Holes 1149A and
1149B, albeit with much lower recovery rates (<15%). An excellent
set of geophysical geochemical logs from Hole 1149B will enable
reconstruction of the sedimentary strata not recovered during
drilling. Holes 1149C and 1149D were drilled primarily to
penetrate as much basement as time allowed. Only 26 m of basement
was cored in Hole 1149C before chronic sticking problems prompted
abandoning the hole. A thinner sedimentary section and basement
high was located on seismic line C2005 a few miles to the
southeast of Holes 1149A, 1149B, and 1149C, where conditions were
more promising for basement drilling. Despite some sticking and
hole collapse problems, which slowed drilling and prevented
logging of the basement section, 133 m of basement were
successfully cored at Hole 1149D with 17% recovery. Site 1149 is
one of only six other ODP/DSDP sites with significant (>100 m)
penetration of basement into Mesozoic oceanic crust.
Site 1149 Geophysics
Site 1149 is located on the Pacific plate in the Nadezhda Basin
southeast of Japan. It is on a slight bathymetric high ~100 km
east of the Izu-Bonin Trench where the Pacific plate is flexed
upward before it enters the subduction zone. Although Nakanishi et
al. (1988) identified magnetic Anomaly M12 in the vicinity of Site
1149, close inspection of the lineation pattern suggests instead
that Site 1149 lies on the older portion of magnetic lineation
M11. This stratigraphic position has a paleontological age of late
Valanginian and a radiometric age of ~132 Ma on the Channell et
al. (1995) time scale. A model consistent with other portions of
the magnetic lineation pattern of this age gives a constant
spreading rate of 51 km/m.y. for this portion of the seafloor.
These rates are comparable to the lower end of the range of
spreading rates for the modern East Pacific Rise.
The seismic stratigraphy at Site 1149 is based on two 1976
vintage multichannel seismic lines and sonobuoy data obtained
during cruise C2005 of the Robert Conrad, as well as a short, pre
site single-channel seismic survey conducted aboard the JOIDES
Resolution. Site 1149 acoustic stratigraphy conforms to that
originally described by Ewing et al. (1968) for large portions of
the western Pacific, consisting of (1) an upper transparent layer
(weakly reflective), (2) an upper opaque layer (highly reflective
or stratified), and (3) acoustic basement, or Horizon B. Early
DSDP investigations (e.g., Legs 6, 7, 17, and 20) found that the
upper transparent layer corresponds to a variety of lithofacies:
pelagic clay with ash in the west Pacific, pelagic clay in the
central Pacific, turbidite sequences in the north and east, and
biogenic oozes along the equator. At Site 1149, this top acoustic
layer corresponds to ash and diatom/radiolarian-bearing clay. The
upper opaque seismic layer has been correlated to the uppermost
abundant chert in much of the North Pacific. Drilling at Site 1149
encountered this chert at ~180 mbsf, where it corresponds to a
continuous, high-amplitude reflection at ~0.2 seconds below
seafloor (sbsf). Within the opaque layer, there is a change in
seismic character at ~0.28 sbsf to discontinuous, chaotic to
hummocky reflections, which corresponds to a change from
chert/clay to chert/chalk (+ marl) at Site 1149.
It was not until Legs 129 and 185 that material below "Horizon"
B was sampled in the Mariana, Pigafetta, and Nadezhda Basins.
Horizon B in large areas of the Nauru, Pigafetta, and East Mariana
Basins correlates to middle Cretaceous volcanic material (sills,
flows, volcanogenic turbidites), and it was not clear if this
material would be encountered at Site 1149 below Horizon B at
~0.42 sbsf, or whether this would correspond to true oceanic
basement. Weak and discontinuous reflections at ~0.7 sbsf in the
original multiseismic measurement (MSM) air-gun records at
preliminary site BON-10 also prompted concern that basement could
lie as deep as 700 mbsf. Drilling in Holes 1149B and 1149C,
however, encountered oceanic basement at 410 and 401 mbsf,
respectively, thus demonstrating that Horizon B in the Nadezhda
Basin corresponds to true oceanic crust. Based on the few records
available, the seismic stratigraphy of the Nadezhda Basin is
laterally continuous and correlates well with the
lithostratigraphy developed for Site 1149.
Site 1149 Sediments
The sedimentary section (between 0 and ~410 mbsf) recovered at
Site 1149 consists of carbonate-free clays with variable
admixtures of volcanic ash and siliceous microfossils, cherts
porcellanites, and calcareous nannofossil chalks or marls. On the
basis of the distribution of these lithologies, the sedimentary
column above the basaltic crust has been divided from top to
bottom into five lithologic units (Fig. 14):
Unit I (0-118.20 mbsf) consists of carbonate-free ash and
diatom/radiolarian-bearing clay. Volcanic ash is present as both
discrete ash layers and disseminated throughout the clay
sequences. Thickness of the discrete ash layers typically varies
from a few millimeters to 5 cm; thicker layers (20-45 cm),
however, are also present. This unit has been dated late Miocene
(i.e., 6.5 Ma) to late Pleistocene, based on an excellent
magnetostratigraphic record (Fig. 15). Sedimentation rates during
deposition of Unit I as derived from the shipboard
magnetostratigraphy were on the order of 18 m/m.y. (Fig. 16). The
abundant assemblage of siliceous plankton, mainly diatoms,
silicoflagellates, and radiolarians yield a preliminary age of
Pliocene for Core 185-1149A-9H, in agreement with the
paleomagnetic data.
Unit II (118.20-179.1 mbsf in Hole 1149A and 160.6-180 mbsf in
Hole 1149B) consists of undated dark brown pelagic clay with
several discrete ash layers present only in the upper 30 m of the
unit (Unit IIa) and notable palygorskite in the lower half (Unit
IIb). Unit II clays are also noted by a change in porosity, pore
water chemistry, and bulk sediment chemistry. Clays from Unit II
are barren of siliceous or calcareous microfossils, and thus their
ages at this point remain undetermined. These clays, however,
contain ichthyolith assemblages that increase in abundance
downhole, probably as a result of a decreased sedimentation rate.
Shore-based analyses of the ichthyolith assemblages should allow
for relative ages for this unit to be determined. Clays in Units I
and II indicate pelagic deposition below the CCD.
Unit III (180-191.2 mbsf in Hole 1149A; 180-282.30 mbsf in Hole
1149B). The top of this unit is marked by the first occurrence of
indurated siliceous lithologies (i.e., chert and porcellanites).
This unit is characterized by very low recoveries (<5%). The
recovered sediments consist of radiolarian chert, porcellanite,
and zeolite-bearing clay, whose age is undetermined at this point.
These sediments are typical of a predominantly siliceous
depositional environment.
Unit IV (282.30-416.40 mbsf) consists of radiolarian chert,
porcellanite, marlstone, and chalk with intercalated calcareous
lithologies. Shipboard biostratigraphy of carbonates recovered in
Cores 185-1149B-16R to 29R yielded well to poorly preserved
calcareous nannofossil assemblages with average high diversity and
good preservation. Preliminary ages are assigned from the
Hauterivian Lithraphidites bolli Zone from 311 to 340 mbsf (Core
185-1149B-18R to 21R). Downhole assemblages are dominated by
Watznaueria barnesae, Cruciellipsis cuvillieri, and Tubodiscus sp.
The first occurrence of T. verenae is in Core 185-1149B-24R and
indicates the lowermost Hauterivian-uppermost Valanginian.
Rucinolithus wisei, a species that may be restricted to the
Valanginian, is present from Core 185-1149B-25R downhole.
Persistence of T. verenae downhole to Core 185-1149B-29R confirms
a Valanginian age for the basement/sediment contact, consistent
with the assigned M11 seafloor magnetic anomaly (132 Ma). The
sedimentation rates during this time interval (i.e., 125.8-~132
Ma), derived from calcareous microfossil biostratigraphy, is ~20
m/m.y. (Fig. 16). Sediments from this lower unit were likely
deposited when the site reached subequatorial paleolatitudes,
which were characterized by high primary productivity.
The relatively simple stratigraphy and sedimentological changes
at Site 1149 provide an exceptional natural laboratory to examine
diagenetic processes operating over a long time scale (~135 m.y.)
in a sequence bounded by basaltic crust and the oceanic reservoir.
Interstitial waters recovered in sediment from the seafloor to the
basement from Holes 1149A and 1149B reflect a low supply of
organic matter, alteration of volcanic ash and authigenic clay
formation at mid-depths, diagenesis of biogenic opal and
carbonate, and on-going basement alteration. Dissolved phosphate
and ammonium have well-defined shallow maxima, although absolute
concentrations are relatively low, reflecting low organic matter
contents. Alkalinity remains <3 mM throughout the entire sequence,
and sulfate is present at all depths (minimum value of 19 mM). The
deep brown pelagic clays of Unit II define a sequence that is
undergoing active authigenesis, acting as a sink for dissolved
silica, strontium, and potassium, and as a source of alkalinity,
ammonium, and lithium. X-ray diffraction (XRD) results and the
concentration profile of dissolved silica together reflect the
diagenetic transformations of opal-A to opal-CT (at ~170 mbsf) and
of opal-CT to diagenetic quartz (at ~310 mbsf). Basement
alteration and the relatively low diffusivity of Units IV and V
lead to strong diffusive gradients in Ca and Mg, with extreme
enrichments in Ca recorded (135 mM) in interstitial waters from a
few meters above the basement contact (407 mbsf in Hole 1149B).
Uptake of potassium and sodium, and high dissolved Cl (638 mM near
basement) reflect hydration and alteration of the oceanic crust.
These dramatic gradients and fluxes in the interstitial waters
record ongoing alteration in the oceanic crust ~135 Ma after it
formed.
Bulk chemical analyses of the sediments at Site 1149 corroborate
several of the processes inferred from the interstitial-water
analytical results, as well as reveal others that are better
recorded in the bulk solid phase. In stratigraphic order, Site
1149 sediments preserve a well-developed metalliferous sedimentary
profile in Fe/Al variations in the lower 130 m of section (280-410
mbsf), which documents clearly the decreasing influence of plume
precipitation with lateral distance from the ridge. The biogenic
rich sediments Units III and IV are high in Ba/Al, signaling an
increase in biological productivity as the site lingers near the
paleoequator. Unit II pelagic clays are highly enriched in K2O (> 4
wt%), recording the complement to the K uptake inferred from the
pore waters, possibly because of authigenic formation of K-rich
zeolites in these sediments. In addition to the discrete ash
layers observed throughout Unit I and Subunit IIa, a depletion in
Nb/Al with respect to average shales indicates a significant
(25%-35%) dispersed ash component in upper sediments. Shore-based
studies will help to identify the source of these ashes, most
probably as either the Izu-Bonin or Japan volcanic arcs.
Downhole measurements in Hole 1149B were made after completion
of drilling. Five logging runs were performed, consisting of one
pass with the geophysical tool string, two passes with the
geochemical string, and two passes with the Formation MicroScanner
(FMS) sonic tool string. The geophysical string provided the most
physically complete logging run from ~10 m above the volcanic
basement contact at 400 mbsf (Core 185-1149B-28R) to within the
clay-volcanic ash section at ~65 mbsf (Core 185-1149A-8H). The
subsequent logs were limited by additional fill in the bottom of
the hole and/or a 20-m-thick section of tight hole in the pelagic
clays at 140-160 mbsf (Cores 185-1149A-16H to 18H) just above the
uppermost cherts. Overall, the logging data agree very well with
the lithologic units identified from cored intervals. Geophysical
properties (e.g., resistivity, density, and P-wave velocity)
delineate well the clay/ash layers, and the chert/clay layers. The
radioactive element data identify regions predominantly composed
of pelagic clay. The geochemical log data correlate Si-rich zones
with chert layers, Ca-rich zones with the presence of nannofossil
marls, and Al-rich zones with clay and ash layers. Calcium from
the geochemical log is the best indicator of carbonate-rich
sediment, a feature that the other logs do not distinguish well.
The logging data will be essential to reconstructing the
sedimentary section, which suffered from 6% average recovery in
Units III and IV. Over 90% of Site 1149 was logged or recovered.
In contrast to the East Mariana and Pigafetta Basin sediments
subducting at the Mariana Trench, the Nadezhda Basin sediments
subducting at the Izu-Bonin Trench lack a mid-Cretaceous
volcaniclastic section and contain more siliceous and carbonate
rich biogenic material because of its longer passage beneath zones
of high biological productivity. Shore-based geochemical studies
will demonstrate the extent to which these clear differences in
sedimentary lithologies can be traced to the volcanic output from
the two arc systems.
Site 1149 Basement
The sediment/basement boundary was recovered in three holes: at
410 mbsf in Hole 1149B, at 401 mbsf in Hole 1149C, and at 307 mbsf
in Hole 1149D. Approximately 35 and 26 m of basement was drilled
in Holes 1149B and 1149C, respectively, before hole conditions
halted operations; a thinner sedimentary sequence in Hole 1149D
may have contributed to better hole conditions there, where 133 m
of basement was drilled. The contact zone between the sediments
and basement in Hole 1149B is brecciated and filled with
interfragment sediment. Igneous units in all three holes consist
of aphyric basalt pillows, thin flows, and interpillow breccia.
Cooling units are on average <50 cm thick, in contrast with Hole
801C, where the cooling units averaged >50 cm. The predominance of
these thin, fractured, and brecciated units at Site 1149 may have
contributed to the overall low recovery of basalt (<20%).
Plagioclase and olivine phenocrysts are rare (usually <1%). Most
basaltic glass has been highly altered; fresh glass exists on a
few pieces (~10), most of which are in Core 185-1149D-9R. The
chemical compositions of the least altered lavas in Hole 1149B are
fairly primitive (>7.9 wt% MgO) and low in Fe2O3 (<10 wt%) but
otherwise typical of MORBs from the modern East Pacific Rise.
The volcanic rocks at Site 1149 are spectacularly altered with a
pervasive dusky red alteration, which commonly displays a light
gray to brown mottling. Complex, multicolored alteration halos
also are present along fractures and other surfaces that were
exposed to circulating fluids. The halos are up to 2 cm wide and
range in color from brown to dark green. In addition to the
abundant alteration halos, another striking feature of the basalt
from all three holes is an abundance of reddish fracture surfaces.
In general, the alteration is more intense at Site 1149 than at
Site 801. At Site 1149, alteration halos comprise 34 vol% of the
recovered basalts, as opposed to ~2 vol% at Site 801. Veining is
also more intense, with 35 veins/m at Site 1149 compared to ~25
veins/m at Site 801. Finally, flow breccias and hyaloclastites are
a common feature in the cored interval at Site 1149, whereas
unlike Hole 801C, massive flows and interpillow sediments are
uncommon. The more intense alteration at Site 1149 may lead to
greater overall budgets for K than at Site 801 (e.g., >0.3 wt%
K2O).
Although Hole 1149D was drilled in a basement high, the
recovered basalts are similar in their igneous and alteration
characteristics to Holes 1149B and 1149C basalts. Thus, the
basement topography near Hole 1149D is probably related to near
ridge processes (abyssal high formation?) rather than off-axis
magmatism.