Site U1378 | Site U1379 | Site U1380
IODP Expedition 334:
Costa Rica Seismogenesis Project (CRISP)
Site U1381 Summary
PDF file is available for download.
Background and Objectives
One of the primary objectives of
Expedition 334 was to determine the characteristics of the down going plate
entering the Costa Rica Subduction Zone. Fundamental to this objective is an
understanding of the nature and the hydrologic system of the igneous section
entering the zone. In this context Site U1381 serves as a reference site. Site
U1381 is located along BGR99 Line 7, offshore Osa Peninsula and Ca–o Island on
a basement relative high at CMP 5750 (Lat. 8° 25.7150' N, Long. 84° 9.4690' W)
at 2067 m below sea level. This location is critical because basement relative
highs are thought to act as major fluid discharge areas and have the best
potential to record traces of vigorous fluid flow. The seismic section is
showing a 120-m-thick sediment section on a high reflective basement
interpreted as Cocos Ridge igneous crust. Paleomagnetic data constrain the age
of this portion of the Cocos Ridge to be 14 Ma (Barckhausen et al., 2001). The
sedimentary section has been interpreted as formed by pelagic and hemipelagic
sediments. The sediment thickness along the seismic transect (SW to NE
direction) is variable. This is in accordance with the relief of the Cocos
Ridge (basement). This relief can be partly correlated with normal faulting,
even though faulting is not a particularly strong charactertistic of the
Site U1381 is a key to characterize the
material input into the subduction zone, considering both the composition and
mass of the sediment cover and basement rocks as well as the fluids. In erosive
margins fluids are thought to trigger the hydro-fracturing processes of the
upper plate and to control the strength of the plate boundary. According to
existing models of erosive margins, sediments are inferred to play a minor role
in the input to the seismogenic zone. The role of the igneous basement of the
incoming plate is less well established. Here we target the fluids circulating
in the upper oceanic crust, since a vigorous hydrologic system was discovered
offshore Nicoya during ODP Legs 170 and 205 (Kimura et al., 1997; Morris et
al., 2003). The importance of the hydrological activity in the subducting
oceanic plate is just beginning to be recognized (Silver et al., 2000). The
igneous crust of the incoming Cocos plate has a significant variability along
trench strike related to different origin, either EPR or CNS, and is definitely
different is the hydrologic system. Heat flux measurements revealed high heat
flow in the Cocos Ridge area in contrast with the low values of the Nicoya
smooth crust (Fisher et al., 2003)(Grevemeyer, unpublished data). Cocos Ridge
upper crust is well layered and probably very porous (von Huene et al., 2000),
the contribution from the lower plate to the fluid circulation could be
significant and its definition has important consequences for the fluid system
at the aseismic-seismic boundary that will be investigated during CRISP Program
Two holes were drilled at Site U1381.
Hole U1381A was drilled with the plan to recover as much from the sediment
cover and the basement rock as possible in the specified time window. Hole
U1381B was drilled to retrieve the first 30 m of sediment for detailed
geochemical sampling and to take 5 in situ temperature measurements to
determine the geothermal gradient and the heat flux at this site. Overall 54.15
m of sediment and 35.69 m of basement were retrieved at this site with an
average recovery rate of 42.3 and 54.8%, respectively.
The material cored at Hole U1381A can be divided into three lithostratigraphic units. Unit I, about 46.14 m thick, consists
mainly of light greenish gray, soft, clay sediments with minor layers of silty
clay, and 3 tephra layers, ranging in thickness from 2 to 4 cm thick. In
general, Unit I is massive with minor changes in the proportions of clay and silt.
Biogenic components, especially nannofossils and diatoms are abundant
throughout the unit. Foraminifers, spicules, and radiolarians are present in
trace abundances. Smear slide investigations show that the main accessory
components observed in this unit are silt-sized grains of feldspar, chert,
chlorite, pyroxene, amphibole, opaque minerals, calcite, glauconite, fragments
of radiolarians, foraminifers, sponge spicules, glass and rare quartz. Unit II
is about 49.64 m thick and consists of mainly dark grayish to yellowish brown,
soft to hardened clay/(stone) with abundant, intercalated tephra layers. The base
of the sediment section is partly silicified. Unit II is clearly distinguishable
from Unit I by its abundant biogenic components and by an abrupt color change. Unit
II sediments are >70% composed of spicules, diatoms, radiolarians, and
nannofossils. The dominantly felsic tephra layers range in thickness from 0.5
to 35 cm, from massive to soft, show a normal gradation and are well sorted.
One notable exception is a 35 cm thick silicified mafic tephra layer (Mary) in
Interval U1381A-7R-1, 92 cm to 7R-2, 9 cm) that shows parallel and cross
lamination. With the exception of one tephra layer (Interval U1381A 5R-5, 29 to
34 cm) all other tephras show devitrification structures within the glass
shards and severe signs of alteration. Grain size ranges from very fine to
coarse ash (up to mm size). The mineral assemblages observed in the tephra
layers consist of plagioclase, pyroxene (hypersthene, augite), hornblende, and
biotite. Bedding dips, identified at compositional boundaries or grain size
differences, of the entire cover sediment sequence are almost horizontal (mostly
≤ 5°). Unit III, the basement unit starting at a depth of about 103 mbsf, is
composed of very dense, tough basalt ranging from almost aphyric to moderately
phyric. Identified phenocrysts are plagioclase and pyroxene. The phenocrysts
are euhedral to subhedral and of variable size (up to 5 mm in diameter). This
phenocryst assemblage exhibits a number of changes with depth ranging from
plagioclase dominated to pyroxene dominated. The groundmass of the basalt is
fine grained and varies from light to dark gray color. A crosscutting vein
network characterizes the majority of the observed basalts. The veins are
preferentially straight with some irregularities. Offsets along the vein
boundaries indicate that they were emplaced along faults (fault veins). Some of
the veins do not show any displacement parallel to the vein boundaries
indicating that some of them were emplaced in mode I fractures. In the
recovered core fragments subvertical green veins of unknown mineralogy
represent the oldest generation followed by subhorizontal white veins and high
angle white veins. Alteration is restricted to halos along fractures, veins and
around the phenocryst/groundmass interface. The secondary mineral assemblage,
most likely a result of hydrothermal processes, is composed of clay minerals,
zeolites and pyrite.
The physical property data obtained on
the cored material are quite variable, consistent with the different
lithologies we have cored at this site. Wet-bulk densities determined from
whole round gamma-ray attenuation (GRA) measurements are relatively constant
throughout the cored sediment section at this site, with a mean density value
of 1.40 ± 0.14 g/cm3. GRA derived bulk densities of the basement are
highly variable due to variable filling of the core liner, having a maximum
value of 2.3 g/cm3. Grain densities determined by mass/volume
measurements on discrete samples of the cored sediment, although showing a
large scatter, generally decrease with depth from approximately 2.7 to 2.5 g/cm3.
Porosities, obtained by mass/volume measurements on discrete samples are
relatively constant through the cored sediment interval with a value of 76%.
Generally, porosity is expected to decrease with depth, the observed constant
values could be an artifact caused by the RCB coring system. The magnetic susceptibility
measured in the sedimentary sequence is low, with a mean value of 0.009 ± 0.016
SI. The magnetic susceptibility measured in the basement rocks is generally
higher (maximum values varying between 1 and 2 SI), increasing from the
sediment/basement interface down to a depth of 140 mbsf followed by a slow
decrease towards the bottom of the hole. The thermal conductivity is relatively
constant throughout the cored interval, with a mean of 0.79 ± 0.08 W/m.K and 1.45 ± 0.07 W/m.K in sediment and basement, respectively.
These values are quite low for basalt and might be an artifact of the samples
not being water saturated before measurement because of time constraints.
Downhole equilibrium temperatures acquired using the SET tool increase linearly
with depth, and give a least-squares geothermal gradient, coupled with the
average bottom water temperature, of 222°C/km. The heat flow calculated using
the mean thermal conductivity of 0.8 W/m.K is 178 mW/m2. This value is
significantly larger than the half space prediction for a 15 Ma old crust (130
mW/m2) and than the observed global average heat flow for crust of
this age (77 mW/m2; Stein and Stein, 1992). This high heat flow
value is an indicator for significant fluid flow within the underlying crust.
The demagnetization experiments made on
the sediments cored at Site U1381 are in general agreement with the physical
property data. The mean NRM (natural remanent magnetization) intensity of the
cored sedimentary sequence is about 10–3 A/m and decreases slightly with
depth. The depth interval between ~30 and 35 mbsf shows higher NRM intensity
values. NRM intensities of the cored basement range from ~1 to 8 A/m. NRM
inclination and intensity show a correlated down hole variation. The relatively
fresh samples from the top of the basalt section (Cores U1381A-12R to 16R)
frequently show shallow inclinations (consistent with the low latitude) and are
characterized by strong NRM intensities. The basalts below Core U1381A-16R are
relatively altered and show NRM with an inclination of ~50°, (indicating a
stronger effect from drilling-induced demagnetization) and lower intensities.
The majority of the measured basalts have a NRM with a positive inclination;
however, several blocks reveal a NRM with negative inclinations, possibly
reflecting prolonged igneous activity at this site. The mean carriers of the
magnetic signal in the retrieved basement rocks seems to be titano-magnetite
with a low Ti-content as deduced from thermal demagnetization experiments on
discrete samples. Generally, the remanence of the recovered material is too
weak for the shipboard experiments to determine characteristic magnetization.
Thus, establishment of the magnetic stratigraphy for Site U1381 will be done
during shore-based studies.
Even though a magnetic stratigraphy could
not be established for Site U1381 on the ship, the nannofossil and foraminifera
community observed at this site provided a significant biostratigraphic control
of the cored sediment sequence above the basaltic basement of the Cocos Ridge. Based
on microfossil biostratigraphy, the sedimentary layers are tentatively divided
into an upper part of Pleistocene age, and a lower part of Middle Miocene age.
Thus, the sediments just above the basement are tentatively estimated to be of
Middle Miocene age. The sediments just above basement basalt would be younger
than 16 Ma. The zonation of planktonic foraminifers is approximately concordant
with that of the calcareous nannofossils.
Two different environments, based on
species, abundance and preservation, are represented in the cored sediments at
Site U1381. The upper interval (3.69 to 49.62 mbsf) represents a hemipelagic
environment mixed with terrigenous material, whereas the lower interval (53.86
to 95.5 mbsf), a silicic to calcareous ooze, represents a pelagic environment.
The nannofossil assemblage observed in the upper 31.9 mbsf is characteristic of
the Early Pleistocene Zones NN20-NN19 and contains Geophryocapsa oceanica,
G. caribbeanica, Helicosphaera carteri, and Calcidiscus leptoporus. However, due to poor preservation and
the lack of zonal markers this interval cannot be biostratigraphically zoned.
The interval up to a depth of 41.5 mbsf is tentatively assigned to nannofossil
Zone NN19 based on the occurrence of Pseudoemiliana lacunosa and the absence of Discoaster brouweri. However, the
top boundary, defined by the last occurrence (LO) of Pseudoemiliana lacunosa is undetermined. Sample U1381A-6R-CC
(49.62 mbsf) contains a diverse nannofossil assemblage of mixed ages, ranging
from the Pleistocene Zone NN19 into the Late to Middle Miocene. This diverse
assemblage is dominated by Pleistocene species including Geophryocapsa
oceanica, G. caribbeanica, Helicosphaera carteri, and Calcidiscus leptoporus. Also present, but rare to a few in
abundance, are Miocene species including Discoaster bellus, D. exilis, D.
quinqueramus, D. variabilis, and unidentifiable 5- and 6-rayed discoasters. The discoasters are poorly to moderately preserved,
whereas the placoliths exhibit moderate to good preservation. The condition of the discoasters and the rarity or lack of
biostratigraphic markers prevents the further delineation of the Pliocene and
The depth interval between 53.86 and 95.5
mbsf is assigned to the Middle Miocene Zone NN5 based on the occurrence of Helicosphaera heteromorphus and the absence of H. ampliaperta.
The top and bottom of this zone cannot be constrained due to the uncertainty of
the last occurrences (LOs) of the biostratigraphic markers. Typical species
found in the samples include Sphenolithus heteromorphus, S. moriformis,
Calcidiscus leptoporus, Cyclicargolithus floridanus, Coccolithus miopelagicus,
Discoaster exilis, D. variabilis, D. deflandrei, and Reticulofenestra pseudoumbilicus.
Planktonic foraminifers were analyzed in
9 core catcher samples. Foraminifera are abundant to common in the sediments of
Hole U1381A. The preservation of the foraminifers is good to moderate.
Fragmentation of foraminifera caused by carbonate dissolution is observed in the
samples of sediments coming from lower bathyal depths. Planktonic foraminifers,
being abundant to common in this hole, are much more abundant than benthic
foraminifers. These trends are quite different from the trends observed in the
cored sediments of the other drilling sites. Similar to the observed
nannofossil communities in this hole, the foraminiferal assemblages of the
upper part of the sediment sequence are quite different from those of the lower
parts. This is either caused by a hiatus or by very low sedimentation rates.
The foraminifera community of the upper sediment sequence (3.69 mbsf to 49.62
mbsf) is characterized by tropical fauna (Globigerinoides quadrilobatus
(Globigerinoides sacculifer), Globigerinoides ruber, Orbulina universa,
Globorotalia menardii, and Neogloboquadrina dutertrei).
Sample U1381A-3R-CC (13.34 mbsf) contains pink G. ruber and is assigned to the Pleistocene (older than 0.12 Ma). Sample U1381A-6R-CC (49.64 mbsf) contains sinistral coiling Pulletiatina
and is assigned to be older than 0.8 Ma but younger than 4 Ma. From 53.86 to
95.55 mbsf the planktonic foraminiferal assemblages are composed of Dentoglobigerina
altispira, Globigerinoides quadrilobatus, Globigerinoides obliquus,
Globoquadrina dehiscens, Globorotalia peripheronda, Globorotalia
peripheroacuta, Paragloborotalia siakensis, Orbulina suturalis. This sequence is tentatively assigned to planktonic foraminiferal zone M7 (14 Ma). However, the occurrence of Praeorbulina circularis at a depth of
about 95.55 mbsf may be a sign that the sediments just above the basement
basalt are much older (either zone M5 or M6).
The geochemical trends displayed by the
analyzed pore water samples (17 whole rounds) are generally controlled by the
following processes: (1) organic carbon cycling, (2) volcanic ash alteration,
and, to a lesser extent, (3) clay-ion exchange reactions in the upper 40 m of
the cored sediments and (4) by ongoing ash alteration, (5) ion exchange
reactions and (6) diffusive exchange with a basement fluid below this depth. Salinity
is lower than the seawater value (SW = 35) between 13 and 24 mbsf, but increases
gradually below this depth to the base of the sediment column. Chloride
concentrations are slightly below modern seawater value. A similar dilution of
Cl concentrations of about 2.5% lower than modern seawater was observed in the
upper 100 m of the sediment column cored at the reference site (ODP Site 1039)
offshore the Nicoya Peninsula (Kimura et al., 1997). Sodium concentrations are
below the seawater value throughout the cored section and reach a minimum of at
35 mbsf. K and Mg show a similar decrease at this depth, suggesting local
alteration of volcanic ash, which is consistent with the lithostratigraphic
observations made. Sodium concentrations are relatively constant below this
depth. Sulfate concentrations decrease to a minimum at 23 mbsf and then
increase nearly linearly with depth. The alkalinity concentration-depth profile
is a mirror image of the sulfate profile reaching a maximum at 23 mbsf and then
decreasing towards the base of this hole. Organic matter diagenesis in the
upper part of the sediment section is also observed in the ammonium profile,
which reaches a maximum value at 23 mbsf. Ammonium concentrations remain nearly
constant to 35 mbsf, and then decrease nearly linearly. Calcium concentrations
reach a minimum value just below the sulfate minimum at 23 mbsf, suggesting
precipitation of authigenic carbonates in the zone of active sulfate reduction
and alkalinity production. Below this depth Ca concentrations increase
gradually towards the base of the cored sediment. This increase with depth most
likely reflects both ash alteration in the sediment column and diffusive
exchange with a basement fluid.
Potassium and boron concentrations
decrease gradually with depth reaching minimum values at the base of the sediment
cover, whereas dissolved Mn concentrations increase with depth reaching maximum
values at the base. Magnesium concentrations exhibit a minimum at 35 mbsf, consistent with alteration of the volcanic tephra that are abundant in this interval. Magnesium
concentrations remain relatively constant between 45 and 81 mbsf, and then
decrease again at the sediment/basement interface. Dissolved Si concentrations
are elevated throughout the cored section and the profile is primarily
controlled by lithology. Si concentrations increase slightly from 13 to 46
mbsf, then increase abruptly from 46 to 68 mbsf reflecting the change from the
clay dominated terrigeneous sediments in the uppermost sediment column to the
hemipelagic sediments dominated by calcareous nannofossils and diatoms below.
Silica concentrations remain elevated and constant throughout the hemipelagic
nannofossil ooze section. Strontium concentrations increase slightly with depth
reaching a maximum at the base of the sediment section. The only slight
increase in Sr concentrations in the calcareous nannofossil rich sediments
suggests they are relatively unaltered and have not undergone significant diagenetic
modification. This interpretation is corroborated by the pristine appearance of
the nannofossils within these sediments.
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