IODP

doi:10.14379/iodp.pr.351.2015

Principal science results

Lithostratigraphy

The lithostratigraphic record at Site U1438 is composed of sediments, sedimentary rocks, and igneous rocks recovered in Holes U1438A, U1438B, U1438D, and U1438E (Fig. F12). Sediments and sedimentary rocks at Site U1438 were sampled from the seafloor to 1461 meters below seafloor (mbsf) and are divided into four lithostratigraphic units. Unit I is 160.3 m thick and is Holocene to latest Oligocene in age (Pleistocene in Hole U1438A). Unit I sediments are primarily terrigenous, biogenic, and volcaniclastic mud and ooze with interspersed discrete ash layers. Unit II is 139.4 m thick and is Oligocene in age. Unit II sedimentary rocks are tuffaceous mudstone, siltstone, and fine sandstone, with localized slumping-induced deformation features. The mudstone to sandstone intervals are typically normally graded beds with sharp lower boundaries to the sandstone bases, as well as moderately to strongly bioturbated mudstone caps. Unit III is 1046.4 m thick and is Oligocene to Eocene in age. Unit III sedimentary rocks are on average coarser grained than those of Unit II (Fig. F12) and include tuffaceous mudstone, tuffaceous sandstone, tuffaceous medium to coarse sandstone with gravel, and tuffaceous breccia-conglomerate with volcanic and sedimentary clasts commonly up to pebble and rarely cobble size. At the largest scale, Unit III comprises five intervals of coarser clastic sedimentary rocks, separated by intervening mudstone-dominant intervals lacking discrete breccia-conglomerate beds. Unit IV is 99.7 m thick and is Eocene in age. Unit IV is composed of radiolarian-bearing mudstone underlain by medium to coarse sandstone, breccia-conglomerate, and tuffaceous siltstone and mudstone. Unit IV is underlain at 1461 mbsf by igneous basement rocks that comprise Unit 1. Within Unit 1, several cooling contacts were recognized, but substantial drilling disturbance made consistent recognition of individual volcanic unit boundaries and estimation of thicknesses impractical, so all igneous basement rocks are grouped together in a single unit. Unit 1 is 150.0 m thick and is presumably Eocene in age. Unit 1 basalts are sparsely vesicular to nonvesicular and microcrystalline to fine grained. Most samples are aphyric, but some contain phenocrysts of plagioclase and/or olivine up to 4 mm in their long dimension. Groundmass textures are variable from subophitic (holocrystalline) to intersertal (hyalophitic) and glassy. Macroscopic alteration of phenocrysts and groundmass to chlorite, oxides, and carbonate pervades rocks of Unit 1, which are also cut by chlorite + carbonate ± sulfide and oxide veins.

Mineralogy and alteration

Downhole changes in mineralogy at Site U1438 are documented from smear slide, X-ray diffraction (XRD), and thin section observations and show the dominantly fresh volcanogenic minerals and glass that occur within Unit II (shallower than 300 mbsf) are transformed into mixtures of dominantly clay and zeolite minerals in Unit III (deeper than 500 mbsf).

Plagioclase, pyroxene, and calcite are the dominant minerals in Unit II, with minor occurrences of amphibole, based on smear slide observations and XRD patterns. The identification of foraminifers and nannofossils in many smear slides from Unit II indicate that calcite occurs primarily as a sedimentary component. Smear slide observations indicate that the silicate minerals and associated volcanic glass in Unit II are largely free of hydrous alteration effects.

In the upper part of Unit III, at 300–500 mbsf, zeolite minerals observed in trace quantities in Unit II become common and their abundance increases relative to fresh, volcanogenic components. XRD data for Unit III show that calcic plagioclase and pyroxene are still abundant minerals, as they are in Unit II. Calcite is also commonly present, as in Unit II, but thin section observations confirm the presence of some authigenic cement. Most Unit III samples at 300–500 mbsf also contain one or more zeolite group minerals in combination with sheet silicate and other minerals that appear to be diagenetic.

Deeper than 500 mbsf in Unit III and to the bottom of Hole U1438D, many samples are dominated by zeolite and related framework and sheet silicate minerals, which are formed by diagenetic processes. In turn, the primary volcanogenic components, which dominate Unit II, diminish in importance in Unit III. Thin section and XRD data show that anhydrite may be present locally, where it appears together with zeolite minerals in replacement of pumice lapilli and in millimeter-scale veins. Calcite is less common in Unit III lithologies deeper than 500 mbsf but persists throughout most of Hole U1438D.

An additional style of diagenetic alteration is evident in Hole U1438D deeper than 600 mbsf, where a red to reddish brown stain has overprinted the dominantly grayish green, green, and black colors found in the diagenetically altered rocks from overlying Units II and III. It is likely that the red coloration reflects a change in the oxidation state of iron and the formation of trace amounts of hematite at the expense of magnetite in the fine-grained sediments.

Additional downhole changes in mineralogy are evident in the bottom third of Unit III, in the transition to Unit IV. These changes are most clearly expressed in the appearance of quartz in the XRD data for most Unit III samples deeper than 1120 mbsf, compared to shallower parts of Unit III, where quartz is rarely or only occasionally present. Other minerals that appear sporadically in lower Unit III and in Unit IV and are not present in the upper part of Unit III include K-feldspar, prehnite, serpentine, and hematite.

Structural geology

Structural features are limited to a few intervals within Site U1438 cores. Smaller scale features, such as faulted and dipping beds, are restricted to the middle part of Unit II, around the overlap zone between Holes U1438B and U1438D. The structures consist of moderately dipping (40°–65°) planar structures of normal and reverse (thrust) geometry that offset the inclined bedding. The zone of deformation is 50–70 m wide, and correlation between Holes U1438B and U1438D suggests northwesterly dip at a moderate angle (40°–60°) for this structural zone. A few inclined beds were noted at widely spaced locations in Unit III throughout Hole U1438D, but they probably resulted from localized slumping and loading. Toward the bottom of Hole U1438D, bedding orientation changes rapidly, suggesting chaotic small-scale folding, and small reverse offsets on moderately dipping bedding planes are ubiquitous.

Biostratigraphy and micropaleontology

A summary of the age-depth plot derived from biostratigraphy is depicted in Figure F13. Calcareous nannofossils generally range from medium to low abundance, although many samples are barren, particularly at the top and toward the bottom of Site U1438. Nannofossil marker species for Zones NP 25 through NP 19/NP 20 are present. The base of Zone NP 25 (26.84 Ma) was assigned between 180.65 and 189.44 mbsf (Hole U1438B) based on the last occurrence (LO) of Sphenolithus distentus. Between 269.84 and 280.66 mbsf (Hole U1438D), material was constrained to the base of Zone NP 24 (29.62 Ma) based on the first occurrence (FO) of Sphenolithus ciperoensis. The LO of Reticulofenestra umbilica, encountered between 548.7 and 555.71 mbsf, identified the base of Zone NP 23 (32.02 Ma). Cores recovered between 565.7 and 576.79 mbsf were assigned to the base of Zone NP 22 (32.92 Ma) based on the LO of Coccolithus formosus. The base of Zone NP 21 (34.44 Ma) was identified between 729.58 and 733.07 mbsf based on the LO of Discoaster saipanensis. The interval 733.07–809.11 mbsf was assigned to Zone NP 19/NP 20 (34.44–36.97 Ma) based on the presence of D. saipanensis, Discoaster barbadiensis, and Isthmolithus recurvus. One final interval in Hole U1438E (~1181 mbsf) was constrained to Zones NP 20–NP 17 (top) (34.44–38.25 Ma) based on the occurrences of D. saipanensis, D. barbadiensis, and Reticulofenestra bisecta. Deeper samples are barren of nannofossils.

Planktonic foraminifers are barren in the majority of samples, although many samples do contain age-diagnostic species and so are able to significantly contribute to the age model for Site U1438. Samples from 0 to ~177 mbsf (Hole U1438B) are largely barren, apart from several foraminifer oozes at ~8, 12.5, 16, and 27 mbsf. These contain rich assemblages of typical Pleistocene species, which include the age-diagnostic Globorotalia tosaensis, Globorotalia hessi, and Globorotalia inflata, constraining the samples to Zones PL4 through Pt1 (<3.6 Ma). The interval between 223 and 235.2 mbsf is constrained to the base of Zone O6 (26.93 Ma) based on the presence of the species Globoquadrina proedehiscens and Paragloborotalia opima. The interval at ~483 mbsf (Hole U1438D) is constrained to Zones O2 through E15 (30.28–36.18 Ma) based on the occurrence of Turborotalia ampliapertura, and the sample at ~576 mbsf is constrained to Zones O2 through E15 (30.28–32.1 Ma) based on the high abundance of T. ampliapertura. The only other age-diagnostic planktonic foraminifer was found at ~1449 mbsf (Hole U1438E): a poorly preserved species of the genus Acarinina. It is likely to be either Acarinina soldadensis or Acarinina bullbrooki, which occur in Zones P4c through E11 (57.79–40.49 Ma).

Benthic foraminifers exhibit a similar occurrence pattern to planktonic foraminifers, but several intervals of moderate recovery allow the definition of four assemblage zones. Assemblage 1 (0–28 mbsf) ranges from ~0 to 1.6 Ma and is characterized by a relatively high diversity of bathyal to abyssal benthics, including typical Neogene species Uvigerina peregrina, Cibicidoides mundulus, and Planulina weullerstorfi. These species occur only in the foraminifer ooze horizons, indicating that for the majority of this interval the site was below the CCD. The core top “mudline” sample contained agglutinated species including Rhizammina sp., Reophax sp., and Saccammina sp., confirming that Site U1438 (4700 m water depth) is below the CCD today. A barren interval follows, which passes into Assemblage 2 (166–245 mbsf; Hole U1438B), ranging from 25.5 to 29 Ma. It is characterized by sparse samples that include Cibicidoides spp., Nodosaria spp., Globocassidulina moluccensis, and Gyroidina sp. The species are typically found at lower bathyal to abyssal depths, and the generally low abundance and lack of many planktonic foraminifers indicates the site may have been close to the CCD at this time. Assemblage 3 (257–430 mbsf; Hole U1438D) ranges from ~29 to 30.2 Ma and is characterized by very low abundances of Amphistegina spp. and Lepidocyclina spp. These larger foraminifers, occurring exclusively in sandy intervals, were transported from the photic zone in shallow water. Assemblage 4 (520–587 mbsf) ranges from 31 to 33.2 Ma and is characterized by low abundances of benthic species including bathyal to abyssal Cibicidoides havanensis and Stilostomella spp. One sample includes Sigmavirgulina tortuosa and Cibicidoides pachyderma that may be transported from neritic to upper bathyal depths. The interval ~597–1460 mbsf is barren of benthic foraminifers.

In general, most of the samples recovered from Site U1438 are barren of radiolarians or only contain very low diversity and poorly preserved radiolarian assemblages. However, some samples yielded moderately preserved radiolarian faunas, which provide some biostratigraphic control in the Pleistocene–Holocene, early middle Miocene and latest Paleocene–middle Eocene. The interval 0–15.3 mbsf is constrained to Zones RN13–RN17 (<1.26 Ma) based on the presence of the species Lamprocyrtis nigriniae. The interval 121.3–127.7 mbsf (Hole U1438B) is constrained to Zones RN2–RN5 (20.05–12.6 Ma) based on the presence of the species Stichocorys delmontensis. At about 127.8 mbsf, the radiolarian age is constrained to Zones RN4–RN5 (17.59–12.6 Ma) based on the presence of the species Calocycletta costata. At about 1387.6 mbsf (Hole U1438E), the radiolarian age is constrained to Zones RP12–RP14 (46.21–40.65 Ma) based on the occurrence of Eusyringium fistuligerum, Lithocyclia ocellus, Periphaena tripyramis, Phormocyrtis striata striata, Sethochyrtis cf. triconiscus, Thyrsocyrtis rhizodon, and Thyrsocyrtis triacantha. Critically, at about 1419.7 mbsf, the radiolarian age is constrained to Zone RP8 (53.35–50.05 Ma) based on the occurrence of Buryella tetradica, Calocycloma castum, Lamptonium fabaeforme, Phormocyrtis cf. striata exquisita, Theocotyle cryptocephala, Theocotyle ficus, and Theocotyle nigrinae. In the last age-diagnostic sample (~1420.3 mbsf), the radiolarian age is constrained to Zones RP7 (top) through RP9 (56.83–48.57 Ma) based on the occurrence of Buryella spp., Phormocyrtis spp., Podocyrtis spp., and Theocotylissa spp.

Geochemistry

For Site U1438, a total of 67 interstitial water (IW) samples for pH and chemical analyses, 160 headspace samples for hydrocarbon gas analyses, 111 samples for total carbon/total nitrogen and carbonate analysis, and 70 samples for bulk chemical analysis of the solid phase were taken. These samples were collected in Holes U1438A, U1438B, U1438D, and U1438E and span lithostratigraphic Units I, II, III, IV, and 1. Methane concentrations were low in all headspace samples, with an average of 2.35 ppm. Total organic carbon and total nitrogen concentrations were low (<0.52 and <0.06 wt%, respectively) through all the cores. Carbonate content was generally low in Unit I except for a few intervals containing foraminiferal oozes, higher but variable in Unit II, and very low in Units III and IV except in a few intervals.

At Site U1438, IW chemical analyses indicate a pH increase downhole to a maximum of 9.9 and a decrease in alkalinity to a minimum of 0.6 mM. The depletion of ammonium and phosphate deeper than 250 mbsf (bottom of Hole U1438B) may be related to a sharp downhole decline of microbial activity. The downhole increase of salinity is related to the increase of Ca and Cl concentrations. Mg concentrations show an opposite trend with respect to Ca, suggesting an exchange occurs between Mg and Ca during alteration of volcaniclastic sediments. Aqueous concentrations of K and Na also decrease with depth, balancing the gain in Ca. This has been well documented in volcaniclastic settings in Western Pacific marginal basins and is observed in the Izu-Bonin fore arc at Sites 792 and 793, as well as the West Philippine Sea Basin, including Site 1201. Ba concentrations increase downhole, similarly to Ca, again indicating the alteration of volcaniclastic sediments. An additional source is probably the basement, which could release these cations into the IW. The correlation between Li/B and pH suggest that these elements may be released in the upper sediment, where pH is lower, as a result of silicate dissolution and desorption from clay minerals, but are retained in secondary minerals downhole where pH increases.

The downhole increase of Sr concentration in IW across Units I and II is most likely the result of low-temperature alteration of volcaniclastic sediments. Cl and Br concentrations are nearly constant in Unit I and increase with depth in Units II and III, possibly due to a low-permeability layer at the Unit I/II boundary, which acts as a semipermeable “membrane,” as well as mineral hydration processes at greater depth.

Bulk analysis of sediments shows loss on ignition (LOI) ranging from 3.91% to 14.47%. The highest LOI values are from unconsolidated sediments in Unit II. In Unit III, CaO decreases in the sediments from 9.17 wt% (~466 mbsf) to 3.04 wt% (~1260 mbsf) and is inversely correlated with the increase of Ca in the IW. The silica content of the volcaniclastic-dominated sediments is extensive, ranging from levels equivalent to basalt to dacite, but with a majority of bulk compositions in the basaltic andesite to andesite composition. The Si/Mg maximum, observed near conglomerate layers, suggests that during these time intervals the source of volcaniclastic debris was rhyolitic in composition.

Thirty-seven samples from the lava flows of Unit 1 were collected between Cores 351-U1438E-70R and 88R and analyzed for major and trace elements by inductively coupled plasma–atomic emission spectroscopy (ICP-AES) (Fig. F14). The majority of these samples are high-MgO (mostly >8 wt%), low-TiO2 (0.6–1.1 wt%) tholeiitic basalts. Two sills sampled in Unit IV above the basement are Na-rich basaltic andesite in composition.

Paleomagnetism

Paleomagnetic analyses of the sedimentary succession and underlying igneous rocks from Site U1438 have provided a continuous record of the geomagnetic field inclination for the last ~50 My. A total of 87 geomagnetic reversals have been recognized in the studied succession based on the inclination of the paleomagnetic vectors. This allowed precise dating of the cores to 847 mbsf, extending back to ~36 Ma (Fig. F13). Deeper in Hole U1438E, higher magnetic coercivities of some intervals and more extensive overprinting by drilling-induced magnetic components in others prevented reliable identification of geomagnetic reversals, and no ages could be determined based on the shipboard paleomagnetic data alone. Archive-half core remanent inclination data are insufficient to allow accurate determination of changes in paleolatitude of the PSP (as suggested by previous workers; e.g., Yamazaki et al., 2010), as isolation of characteristic remanences in many intervals requires demagnetization to higher levels than can be achieved using shipboard systems. However, discrete samples analyzed during the expedition indicate that shore-based demagnetization of additional discrete samples will allow a robust determination of plate latitudinal motion. Discrete samples obtained from APC cores will provide additional constraints on plate rotation, as these cores were oriented using the FlexIT tool. Finally, postcruise integration of remanence data and analysis of wireline Formation MicroScanner (FMS) logs may potentially allow magnetic declinations to be recovered in some deeper intervals, allowing plate rotation to be documented beyond 25 Ma.

Physical properties

Physical properties of recovered core were analyzed to help characterize lithostratigraphic units and provide the basis for linking the lithostratigraphy to the crossing seismic lines. Reflecting the compaction and lithification of sediments, there is an overall reduction in porosity through Units I–IV that fits with an exponential decay typical of that seen for shales, sandstones, and mudstones. There are significant jumps in sonic velocity, grain density, and magnetic susceptibility at the Unit I/II and II/III boundaries and oscillations in sonic velocity and magnetic susceptibility within the top of Unit III indicative of the changing proportions of sands and conglomerates to muds. Higher velocities are correlated to mudstones with clasts, and lower velocities are found in mudstones without clasts. These major changes occur between 160 and ~300 mbsf (Holes U1438B and U1438D) and are likely responsible for the most prominent seismic reflectors seen in the crossing seismic lines of the site. Within Hole U1438E, the apparent anisotropy of sonic velocity was also measured. The azimuthal variation in sonic velocities are small, but the differences between the horizontal and the vertical velocities can be much larger and correlated with lithology; mudstones and fine sandstones with and without apparent bedding planes are generally more anisotropic than the coarser sandstones. There is also a prominent spike in the level of natural gamma radiation within Unit IV, most likely due to elevated concentrations of U, Th, and K.

Successful temperature measurements were made at seven depths using the advanced piston corer temperature tool (APCT-3) from the mudline to 83.2 mbsf, and these give a linear geothermal gradient without any substantial deviation from 77.6°C/km. Together with nearly constant values of thermal conductivity, it is concluded from this observation that the geotherm is undisturbed by local processes, such as sediment compaction, fluid flow within the porous sediments, and internal heat production from radioactive decay. The calculated heat flow is 73.7 mW/m2, implying a thermal age for the underlying lithosphere of 40–60 Ma (Fig. F15).

Downhole measurements

Logging occurred in three holes (U1438D, U1438E, and U1438F) over 1200 m of sediments, with the best interval between 100 and 700 mbsf. It covers an approximate age of 40 Ma. Natural gamma radiation, resistivity, density, porosity, self-potential, magnetic susceptibility, sonic velocities, and magnetic orientation of the sediments were measured. In addition, resistivity images of the borehole were acquired and a seismic experiment was organized.

Three major transitions in the logging data were described. The first one at 160 mbsf corresponds to lithostratigraphic Unit I. This interval is characterized by (1) high gamma ray values (with particularly high thorium and uranium concentrations); (2) mean density of 1.3 g/cm3; (3) very high porosity (>80%); and (4) low magnetic susceptibility, resistivity, velocity (~1800 m/s), and self-potential values. The second interval is located between 160 and 300 mbsf, corresponding to lithostratigraphic Unit II. This interval is characterized by an overall increase in sonic velocity, magnetic susceptibility, density, and self-potential. Gamma radiation becomes weak. Thorium and uranium reach concentration values <1 ppm from that depth. Porosity decreases slightly; however, this may be due to changes in borehole diameter.

The last major interval is 300–1200 mbsf and could be divided into subunits. Overall, this unit is characterized by (1) a decrease of porosity from 80% to 50%, (2) an important decrease in self-potential, (3) low values of gamma ray (around 20 gAPI with a slight increase at 500 mbsf), (4) a large increase in resistivity (from 1 Ωm to an average value of 3.5 Ωm), (5) an increase to velocity values around 2500 m/s, and (6) a large increase in magnetic susceptibility values.

The logging data have been compared with measurements made on cores and with the lithostratigraphic units, where they correlate well. Furthermore, the logging data will fill in the gaps where core recovery is lower (<100%). The FMS images display bedding features that will be reoriented and help characterize the source of mass wasting deposits described in the cores. Finally, downhole sonic velocities agree well with core measurements and will allow the construction of a seismic traveltime-depth relationship for Site U1438 and thus provide characterization of seismic boundaries.